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FORCES ACTING ON RESTORATIONS CONTENTS

 Introduction  Force  Force on dental structure  Stress  Types of stress  Mechanical properties of material  Biomechanics for restorative dentistry  Stress analysis and design of dental structures a) Finite – element stress analysis b) Photoelasticity  Stress in the periodontal membrane  Stress patterns of teeth  On anterior teeth  On posterior teeth  Occlusal considerations in restoring teeth  Forces exerted during occlusion / mastication and their resolution  forces acting on amalgam restorations  Class i  Class ii  Forces acting on inlay restoration  Forces acting on composite restoration  Forces acting on posts  Forces acting on a cast metal and porcelain restorations  Conclusions


FORCES ACTING ON RESTORATIONS INTRODUCTION: Design of any structure requires a means to predict the stress that will develop in the structure under the anticipated applied loads. In many respects the design of structures for the oral environment is among the

most demanding because of the

complexity of the functional and parafunctional loads that must be accommodated and because of esthetic and space limitations. In spite of these special conditions however all dental tissues and structures follow the same laws of physics as any other material or structure. By necessity these studies involve the application of physics and engineering to the oral cavity and its surrounding structures. All structural analysis and design require knowledge of the forces that will be applied and the mechanical properties of the materials that must withstand these forces. Since most restorative materials must withstand forces in service either during mastication or fabrication. Those mechanical properties are important, quantities of force, stress, strain, strength hardness, and others can help identify the properties of a material. FORCE The general concept of force is gained through the muscular action of pushing or pulling on an object. When there is a tendency to change the position of rest as the motion of a mass, it is said that a force is applied.  A force always has a direction and the direction is often characteristic of the type of force.  If the body to which the force is applied remains at rest, the force causes the body to deform.  Units of force are the pound or the kilogram or Newton. FORCES ON DENTAL STRUCTURES : One of the most important applications of physics in dentistry is in the study of forces applied to teeth and dental restorations. There are numerous reports in the dental literature that describe the measurement of biting forces on teeth. The maximum forces reported have ranged form 200 to 2440 N (45 to 550 lb).

2


Numerous instruments have been used to make this measurements, including strain gauges and telemetric devices small enough to be incorporated into dental restorations. NORMAL BITING FORCES : Experiments conducted on adults have shown that the biting force decreases form the molar region to the incisors. Studies have revealed that four patients developed biting forces on the first and second molars that varied form 390 to 800 N (88 to 198 lb), with the average being 565 N (127 lb). The average force on the bicuspids, cupids and incisors was 288, 208 and 155 N (65, 47 and 35 lb) respectively. In a similar investigations of the biting forces in children, 783 boys and girls were studied. Children form 6 to 17 years of age were included, and it was observed that there was an increase in force form 235 to 494 N (53 to 111 lb) as age increased, with the average yearly increase being in the order of 22.2 N (5 lb). The average biting forces in persons with normal and modified occlusion were measured. Data indicate that the when the bite was raised

0.5 mm, the measured

forces were generally higher, approaching twice the values obtained with normal occlusion. This observation may be explained by the fact that the force on teeth are determined by muscular effort, and this effort is controlled by the nervous system. Thus some force – regulating mechanism was operating and it probably exists in case of malocclusion. The maximum force measured will depend on the type of food. FORCES ACTING ON THE TEETH : FORCES AND RESPONSES :  The forces which act on the teeth and cause them to move within their periodontal tissues vary in magnitude, duration, frequency and direction.  The responses by the teeth to the forces depend on such factors as the shape and length of the roots the characteristics of the fluid content of the periodontal space, the composition and orientation of the periodontal fibres and the extent of the alveolar bone.  The responses by the teeth will also depend on the consistency of the bolus being chewed and the muscular forces being used to crush it. This will also apply to parafunctional clenching and chewing with or without a foreign body between the teeth. It is, therefore, difficult to assess what is a normal response to a force on a tooth and what is potentially harmful. As a result of these forces, a tooth can be 3


displaced in one of six directions : - apically, mesiodistally or buccolingually, and each one producing a rotation or a translation.  The result is likely to be a combination of all directions leading to an omnidirectional movement.

The same principle of movement will apply to the opposing tooth

involved. OMNIDIRECTIONAL AND UNIDIRECTIONAL RESPONSES : These omnidirectional tilting and rotations of teeth will reach a limit when an equal and opposite resistance is reached and the periodontal receptors cause a reflex arrest of the muscle force. When the force is removed, the teeth will recover their positions due to the elastic recovery of the compressed periodontal tissues. This is referred to as “replacement” of the teeth. This phenomenon may be modified by 3 factors ; i)

Alveolar bone support

ii)

Adjacent teeth support

iii)

Horizontal muscle activity on both buccal and lingual surfaces of the teeth.

 These 3 variable factors may lead to an unidirectional movement of a tooth or teeth when they will become repositioned. Teeth will continue to move unidirectionally until positions of stability are reached. The opposing forces are then equal to the moving forces.

Thus, maxillary incisors with poor periodontal support and

incompetent lips will drift forwards. This forward drift will continue until the teeth are shortened or are prevented from moving further by an appliance and by treatment of the periodontal breakdown. STRESS • When a force acts on a

body, tending to produce deformation, a resistance is

developed to this external force application. • Stress is the internal reaction to the external force. • Both the applied force and stress are distributed over a given area of the body, and so the stress in a structure is designated as the force per unit area. Force Stress = --------Area

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• Area over which the force acts is an important factor of consideration especially in dental restorations in which areas over which the force applied often are extremely small. Since stress at a constant force is inversely proportional to the area, the smaller the area, the larger the stress. And vice versa. • Technically, stress is the internal resistance of the body in terms of force per unit area and is equal and opposite in direction to the force (external) applied. This external force is also known as load. TYPES OF STRESSES : Depending upon the nature of the force, all stresses can be divided into 3 basic types which are recognized as ; i.

Tension

ii.

Compression and

iii.

Shear

1) Tension : Results in a body when it is subjected to 2 sets of forces that are directed away from each other in the same straight line. F

F 2) Compression : Results when the body is subjected to 2 sets of forces in the same straight lien and directed to each other. F

F 3) Shear : Is a result of 2 forces directly parallel to each other. S

F

F Tensile Stress : -

Is caused by a load that tends to stretch as elongate a body.

-

The molecules making up the body must resist being pulled apart. 5


Compressive Stress : -

Produced by a load that tends to compress the body.

-

The molecules resist being forced more closely together.

Shear Stress : -

A stress that tends to resist a twisting motion, or a sliding of one portion of a body over another.

-

The molecules resist sliding of one body past another.

-

A force applied to a dental restoration may be resolved in the structures as a combination of compressive, tensile and shear stresses.

Complex Stresses : Whenever force is applied over a body, complex as multiple stresses are produced. They may be a combination of tensile, shear or compressive stress. These multiple stresses are called complex stresses. MECHANICAL PROPERTIES OF A MATERIAL : The mechanical properties of a material describe its response to loading. It is common to simply describe the external load in terms of a single dimension (direction) as compression, tension, or shear combination of these can produce Torsion (Twisting) or Flexion (transverse bending). When a load is applied, the structure undergoes deformation as it bonds are compressed, stretched, or sheared. The load deformation characteristics are only useful information if the absolute size and geometry of the structure involved are known. Therefore, it is typical to normalize load and deformation as stress and strain.  Stress is load per unit cross sectional area.  Strain is deformation per unit length. During loading, bonds are generally not compressed as easily as they are stretched.

Therefore, materials resist compression more readily and are said to be

stronger in compression than in tension.

Materials have different properties under

different directions of loading. “It is important to determine what the clinical direction of loading is before assessing the mechanical property of interest”. As loading continues, the structure is deformed. At first this deformation (or strain) is completely reversible (Elastic strain).

However, increased loading finally

produces some irreversible strain as well (plastic strain), which causes permanent deformation. The point of onset of plastic strain is called the elastic limit. Continuing 6


plastic strain ultimately leads to failure by fracture. The highest stress before fracture is the ultimate strength. The total plastic tensile strain at fracture is called the elongation. The slope of the linear portion of the stress strain curve is called the modulus, modulus of elasticity, young’s modulus, or the stiffness of the material. Two of the most useful mechanical properties are the modulus of elasticity and elastic limit. A restorative material generally should be very stiff so that under load, its elastic deformation will be externally small. An exception is Class V composite which should be less stiff to accommodate tooth flexure. If the stress is well beyond the elastic limit, then the resulting deformation is primarily plastic strain and at some point ultimately results in failure. Often it is convenient to determine the elastic limit in a relative manner by comparing the onset of plastic deformation of different materials using scratch or indentation tests, called hardness tests. The energy that a material can absorb before the onset of any plastic deformation is called its resilience, and is described as the area under the stress-strain curve up to the elastic limit. The total energy absorbed to the point of fracture is called the toughness and is related to the entire area under the stress strain curve. Time-dependent responses to stress or strain also occur. Deformation with time in response to a constant stress is called creep (strain relaxation). Deformation overtime in response to a constant strain is called stress relaxation. BIOMECHANICS FOR RESTORATIVE DENTISTRY : Teeth are subjected to many forces during normal use. The interactions between the applied forces, the shape and structure of teeth, the supporting structures, and the mechanical properties of tooth components and restorative materials are all included in the subject of biomechanics. Biomechanical Unit : The standard biomechanical unit involves the 1. Restorative material 2. Tooth structure, and 3. Interface between the restoration and tooth The importance of considering three structures in the biomechanical unit is to detect stresses that may cause unwanted fractures or debonding. The restorative material may be strong enough to resist fracture, but the interface or tooth structure may not be. 7


STRESS TRANSFER : Normal tooth structure transfers external biting loads through enamel into dentin as compression. The concentrated external loads are distributed over a large internal volume of tooth structure and the local stresses are lower. During this process a small amount of dentin deformation may occur which results in tooth flexure. A restored tooth tends to transfer stress differently than an intact tooth. Any force on the restoration produces compression, tension, or shear along the tooth restoration interface. Once enamel is no longer continuous, its resistance is much lower. Therefore, most restorations are designed to distribute stresses onto sound dentin, rather than to enamel. The process of stress transfer to dentin becomes more complicated when the amount of remaining dentin is thin and the restoration must bridge a significant distance to seat onto thicker dentin (Liners or bases). TOOTH FLEXURE : Tooth flexure has been described as either a lateral bending or an axial bending of a tooth during occlusal loading. This flexure produces the maximal strain in the cervical region, and the strain appears to be resolved in tension or compression within local regions, causing the loss of bonded class V restorations in preparations with no relative grooves. Moreover, one current hypothesis is that tensile or compressive strains produce microfractures (called ABFRACTIONS) in the thinnest region of enamel at the CEJ. Such fractures predispose enamel to loss when subjects to tooth brush abrasion and/or chemical erosion. This process may be key in the formation of Class V defects. PRINCIPLES OF BIOMECHANICS : Stress transfer and the resulting deformations of structures are principally governed by : 1. The elastic limit of the materials 2. The ratio of the elastic moduli involved 3. Thickness of the structures Materials with a high elastic modulus transfer stresses without much strain. Lower modulus materials undergo dangerous strains where stresses are concentrated, unless there is adequate thickness.

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STRESS ANALYSIS AND DESIGN OF DENTAL STRUCTURES ďƒ° The mechanical properties of a material used in a dental restoration must be able to withstand the stresses and strains caused by the repetitive forces of mastication. The design of dental restoration is particularly important if the best advantage of a material is to be taken. It is necessary to use designs that do not result in stresses or strains that exceed the strength properties of a material under clinical conditions. ďƒ° Stresses in dental structures have been studied by such techniques as brittle coatings, strain gauges, two and three-dimensional photoelasticity, and finite element analysis. Stress analysis studies of inlays, crowns, bases supporting restorations, fixed bridges, complete dentures, partial dentures, and implants have been reported. a) Two Dimensional Photoelasticity : The procedure for two-dimensional models is to prepare a transparent plastic or other isotropic model of the restoration or appliance. This model is usually larger than the actual size. The material becomes axis atropic when stressed, and so the behaviour of light is affected by the direction it takes. As a result of the applied stress, the plastic model exhibits double refraction because of its an isotropic structure.

The light from the source passes through a

polarizer, which transmits light waves parallel to the polarizing axis, or plane polarized light. The plane polarized light is converted to circularly polarized light by a quarter wave plate, and this polarized beam is split into two components travelling along the direction of principal stress in the model. Depending on the state of stress in the model, the two beams travel at different rates. After the light emerges form the model, it passes through a second quarter – wave plate, which is crossed with respect to the first, and an analyzer that is most frequently perpendicular to the polarizer. The interference pattern may be recorded photographically, which is the isochromatic fringe pattern. These isochromatic fringes, or dark liens, represent locations where the difference in the principal stresses is a constant. The magnitude of the stress can be determined by identification of the order of the isochromatic fringes. The fringe order multiplied by a constant and divided by the thickness of the model gives the value of the differences in the principal stresses. Areas in the model where the fringer are close together are under higher stress gradients than areas where there are fewer fringes, and areas containing fringes of higher order are under higher stress than these having fringes of lower order. 9


A two dimensional photoelastic model of a second molar with a gold crown is analyzed. The elastic modulus of the plastics used to represent the gold, dentin and bone had the same relative values as the actual materials. The crown was luted to the tooth with dental stone, and a layer of silicone rubber, simulating the periodontal membrane, separated the tooth from the bone. A force of 266 N (60 lb) was applied 30 degrees to the axis of the tooth at a single site on the mesial cusp, and the isochromatic fringes were photographed. High stresses are apparent under the contact and in the bone at the tip of the mesial root (seven fringes). Considerably lower stresses occurred in the bone just under the distal root and at the crest of the ridge on the mesial side. The effect of various configurations of the proximal margins was studied by twodimensional photoelasticity on the stress distribution in Class II inlays. Light field isochromatic fringes for rounded shoulder and shoulderless models under a 445 N load were analyzed. The load was applied at 3 other locations : i)

At the groove in the restoration

ii)

On the cusp

iii)

At the junction of the restoration and the tooth The maximum shear stress was determined at nine critical areas, tow in the

restoration, two in the tooth and five at the junction of the restoration and the tooth. The study showed that the chamfer and rounded type of preparations are the optimum designs in proximo-occlusal posterior restorations, since they demonstrated the lowest stress when loaded vertically.

The maximum fringe order for the rounded

shoulder was 10 whereas that for the shoulderless preparation was 17. It was also shown that rounding the axiogingival line angle in the shoulder geometry reduced the stress concentration factor by upto 50%. The gingival area of the proximal shoulder was the area of high stress, and extra retentive features such as pins or grooves should not be placed in this area. FINITE ELEMENT STRESS ANALYSIS : The finite element is a newer method than photoelasticity and offers considerable advantages.

In this method a finite number of discrete structural elements are

interconnected at a finite number of points or nodal points. These finite elements are formed when the original structure is divided into a number of appropriately shaped sections, with the sections retaining the actual properties of the real materials. 10


The information needed to calculate the stresses and displacement in the model is 1) The total number of nodal points and elements. 2) A numbering system for identifying each nodal point and element. 3) The elastic modulus and Poisson’s ratio for the materials associated with each element. 4) The coordinates of each nodal point 5) The type of boundary constraints 6) The evaluation of the forces applied to the external nodes. A first molar with an amalgam restoration was idealized by an axisymmetrical model and analyzed by the two-dimensional finite element method.

The model is

divided into a number of triangles. The smaller triangles are located in areas of greater interest.

The ability of various types and thickness of cement bass to support the

amalgam was studied. The plots of maximum tensile stress start at the centre of the cavity and extend toward the cavity wall. The stress induced in the amalgam restoration was from four to five times higher when the amalgam was supported by 2 mm Zinc Oxide – Eugenol cement base, as compared with an equal thickness of zinc phosphate cement base. When the stresses induced in the amalgam by a zinc phosphate base of 2 mm are considered in relation to those induced by a dentin floor alone one can see that replacement of dentin by zinc phosphate to a depth of 2 mm does not result in any significant increase in the tensile stress induced in the amalgam. The zinc oxide eugenol cement base unlike the zinc phosphate cement bar, does not function as

rigid material and induces a larger

displacement. In comparison with zinc phosphate cement base the zinc oxide eugenol material does not have adequate mechanical properties to support a restoration. Even thin layers (0.5 mm) of zinc oxide eugenol cement caused significant changes in the stress induced n the amalgam. Therefore the study indicates that the fracture of amalgam is influenced more by the modulus of elasticity (Stiffness) of the base material than by the compressive strength of the base. An ideal situation would be to have a cement base with a modulus of elasticity equal to that of the restorative material. Also, a subsequent study found that tensile and shear stresses occurring in the cement base were of sufficient magnitude to exceed the strength of some cements. The stress distribution in porcelain fused to metal and porcelain jacket crowns was conducted using a finite element method. Design parameters of rounding of shoulders, 11


avoidance of sharp notches, minimum thickness of metal copings, and minimum labial bulk of porcelain were incorporated into the model of an upper central incisors. A load of 444 N was applied at the incisal third of the lingual surface and at the middle third of the lingual surface.  Vertical loading and loading 30 degrees to the vertical were used.  Since fracture is probably initiated by tensile failure at the periphery, the tensile stress at the boundary is of special importance.  With vertical loading at the incisal third, the highest tensile stresses were found tat the labial third and on the lingual surface near the load, decreasing toward the incisal edge. Low stresses wee observed at the margin and on the lingual surface below the load. The surface stress was nearly the same whether a gold or Ni-Cr base alloy was used; the use of Ni-Cr caused a slight decrease in surface stress.  When the direction of the loading was changed to 30 degrees from the vertical, high tensile stresses were observed near the lingual margin that would be of sufficient magnitude to fracture the cement in this area. STRESS IN THE PERIODONTAL MEMBRANES : Although limited measurements have been made on the periodontal membrane of animals, the actual stress in the membrane has not been determined experimentally. However, the stress to be expected has been calculated. In one case, it was assumed that the periodontal membrane was incompressible, whereas in another it was assumed to be approximately that of water. In both cases the root of the tooth was assumed to be a cone and the elastic modulus of the membrane was taken as 1.45 MN/m2. When the force was applied to the center o the tooth axis, the stress distribution was uniform with respect to the longitudinal axis of the tooth and the pressure was greatest at the apex. If the loading was transverse, the maximum stress occurred near the apical third of the root on the same side as the compression force.

STRESS PATTERNS OF TEETH

12


Every tooth has its own stress pattern, and every location on a tooth has special stress patterns. Recognizing them is vital prior to designing a restoration without failure potential. A) STRESS

BEARING

AND

STRESS

CONCENTRATION

AREAS

IN

ANTERIOR TEETH : i)

The junction between the clinical crown and clinical root bears shear components of stress, together with tension on the loading side and compression at the non-loading side, during excursive mandibular movements.

ii)

The incisal angles, especially if they are square, are subject t tensile and shear stress in normal occlusion. Massive compressive stresses will be present in edge-to-edge occlusion, and if the incisal angles are involved in a disclusive mechanisms, these stresses are substantially increased.

iii)

The axial angles and lingual marginal ridges will bear concentration shear stresses. In addition, on the loading side, tensile stresses are present, and on the nonloading side, compressive stresses are found.

iv)

The slopes of the cuspid will bear concentrated stresses (3 types), especially fi the cuspid is a protector for the occlusion or part of a group function during mandibular excursions.

v)

The distal surface of a cuspid exhibits a unique stress pattern as a result of the anterior components of force concentrating compressive loading at the junction of the anterior and posterior segments of the dental arch and microlateral displacement of the cuspid during excursive movements. Both of these factors will lead to tremendous stress concentration with resultant abrasive activity there.

vi)

The lingual concavity in upper anterior teeth bears substantial compressive stresses during centric occlusion, in addition to tensile and shear stresses during protrusive mandibular movements.

vii)

The incisal edges of lower anterior teeth are subjected to compressive stresses. In addition, tensile and shear stresses are present during protrusive mandibular movement. The incisal ridges of upper anterior teeth will have these same stresses during the mid-protrusive and sometimes at the protrusive border location of the mandible. 13


B) STRESS

BEARING

AND

STRESS

CONCENTRATION

AREAS

OF

POSTERIOR TEETH : i)

Cusp tips, especially on the functional side, bear compressive stresses.

ii)

Marginal and crossing ridges bear tremendous tensile and compressive stresses.

iii)

Axial angles bear tensile and shear stresses on the non-functional side, and compressive and shear stresses on the functional side.

iv)

The junction between the clinical root and the clinical crown during function (especially lateral excursion) bears tremendous shear stresses, in addition to compression on the occluding contacting side and tension on the non-contracting side.

v)

Any occlusal, facial or lingual concavity will exhibit compressive stress concentration, especially if it has an opposing cuspal element in static or functional occlusal contact with it.

C) WEAK AREAS OF TOOTH : Weak areas in the tooth should be identified and recognized before any restorative attempt, in order to avoid destructive loading. They are ; i)

Bifurcation and trifurcation area.

ii)

Cementum should be eliminated as a component of a cavity wall. The junction between the cementum of the dentin is always irregular, so the dentin surface should be smoothed flat after cementum removal.

iii)

Thin dentin bridges in deep cavity preparations.

iv)

Subpulpal floors in root canal treated teeth. Any stress concentration there may split the tooth interceptally.

v)

Cracks or crazing in enamel and/or dentin. Both should be treated passively in any restorative design. They may act as shear lines leading to further spread. SOME APPLIED MECHANICAL PROPERTIES OF TEETH:

1. Although the following figures are averages, they provide an idea about the principal mechanical properties of tooth structure. It must be understood that these figures can differ from one location on a tooth to another and from one tooth to another. a) Compressive strength of enamel supported by vital dentin is usually 36-42,000 psi. b) Compressive strength of vital dentin is 40-50,000 psi. 14


c) Modulus of resilience of enamel supported by vital dentin is 60-80 inch – lbs/cubic inch. d) Modulus of resilience of vital dentin is 100-140 inch – lbs/cubic inch. e) Modulus of elasticity of enamel supported by vital dentin under compression is 7,000,000 psi. f) Modulus of elasticity of vital dentin is 1,900,000 psi. 2. In general, when enamel loses its support of dentin, it loses more than 85% of its strength properties. 3. Tensile strength of dentin is about 10% less than its compressive strength. 4. Tensile strength and compressive strength of enamel are similar, as long as the enamel is supported by vital dentin. 5. Shear strength of dentin is almost 60% less than its compressive strength, and this is very critical in restorative design. 6. There is minimal shear strength for enamel when it loses its dentin support. 7. When the dentin loses its vitality, there is a drop of almost 40-60% in its strength properties. VALE EXPERIMENT : The original experiment involved preparation of occlusoproximal cavities with different crossing dimensions at the marginal and crossing ridges with a standard depth. The teeth were then subjected to measured occlusal loads. The load that split the tooth was recorded and compared to the control, which was the load that split a round tooth. Later, the same experiment was repeated by several investigators using more sophisticated equipment than that used by vale. The results were consistent. A summary of their findings brought to the closest round figures is as follows : i)

By crossing one marginal ridge at ¼ the intercuspal distance, there is almost 10% loss of a tooth’s resistance to splitting.

ii)

By crossing two marginal ridges at ¼ the intercuspal distance, there is almost 15% loss of a tooth’s resistance to splitting.

iii)

By crossing one marginal ridge at 1/3 the intercuspal distance, there is almost 30% loss of a tooth’s resistance to splitting.

iv)

By crossing two marginal ridges by 1/3 the intercuspal distance, there is almost 35% loss of a tooth’s resistance to splitting. 15


v)

By crossing one marginal ridge at ½ the intercuspal distance, there is almost 40% loss of a tooth’s resistance to splitting.

vi)

By crossing two marginal ridges at ½ the intercuspal distance, there is almost 45% loss of a tooth’s resistance to splitting.

vii)

By crossing a crossing ridge at ¼ the intercuspal distance, there is almost 20% loss of a tooth’s resistance to splitting.

viii)

By crossing a crossing ridge at 1/3 the intercuspal distance, there is almost 35% loss of a tooth’s resistance to splitting.

ix)

By crossing a crossing ridge at ½ the intercuspal distance, there is almost 45% loss of a tooth’s resistance to splitting. OBTAINING RESISTANCE FORM FOR TOOTH STRUCTURES :

1) To best resist masticatory forces, use floors or planes at right angles to the direction of loading to avoid shearing stresses.

2) If possible, walls of preparations should be parallel to the direction of the loading forces, in order to minimize or avoid shearing stresses.

3) Intracoronal and intraradicular cavity preparations can be done in box, or cone or inverted truncated cone shapes.

Thus from the above figures, it is possible to deduce that the inverted truncated cone shapes will have a higher resistance to loading than the box shapes, and the box shapes will have a higher resistance than the cone shapes. Therefore, if conditions and requirements allow, cavity preparations should be prepared in an inverted truncated cone shape. 16


4) Definite floors, walls and surfaces with line and point angles are essential to prevent micromovements of restorations, with concomitant shear stresses on remaining tooth structures.

5) Increasing the bulk of a restorative material or leaving sufficient bulk of tooth structure in critical areas is one of the most practical ways of decreasing stresses per unit volume. Load – A

Load A

10 stress units/mm3 1 stress unit / mm3 6) Designing the outline form with minimal exposure of the restoration surface to occlusal loading will definitely minimize stresses and the possibility of mechanical fracture in the restoration. 7) Junctions between different parts of the preparation, especially those acting as fulcra, should be rounded in order to minimize stress concentration in both tooth structure and restorations and to prevent any such sharp components from acting as shear lines for fracture future. 8) Retentive features must leave sufficient bulk of tooth structure to resist stresses resulting from displacing forces. OCCLUSAL CONSIDERATIONS IN RESTORING TEETH The way we occlude teeth affects the periodontium, the temporomandibular joints, throat muscles, tongue, cheeks, lips, nerves and so son. The occlusion of the restored teeth should hence establish healthy relations between the dentition and rest of the stomatognathic system. A clinician must have adequate knowledge about the principles of occlusion, which enables him to diagnose cases that need modifications / alteration of occlusion with or without the use of various restorative materials. Before initiating any restorative care, thorough occlusal examination should be carried out. The kind of occlusion, a patient should have, must be justified by the principles of physiology. 17


The occlusion affects almost every part of stomatognathic system, mainly : a) The pulp of the tooth is a very sensitive organ. IT reacts immediately to abnormal occlusal forces. Hence, occlusion should not be detrimental to pulp. b) The proximal relations of the occlusion should prevent food impaction between teeth. c) The cusp-fossa relationship should be such that the adequate forces exerted during functional movement, aids in optimum health of the periodontium. Each tooth should be restored following the principles of occlusion, so as to achieve optimum functions of the neuromusculature joints and the supporting structures of the teeth.

New restoration should not introduce any premature contacts and cuspal

interference’s. POSTERIOR RESTORATIONS : All posterior restorations should be planned keeping in mind the basic principles of occlusion.  Prior to cutting a tooth, its opposing occlusal surface should be examined. Malpositioned opposing supporting cusps and ridges should be recontoured in order to achieve optimal occlusal contacts in the restored tooth.  Use articulating paper to register the centric holding spots and excursive contacts so that these marked areas can either be excluded form the outline form or properly restored.  Plunger cusps and over erupted teeth should be reduced, removing all the cuspal interference’s so as to improve the plane of occlusion and decrease the chances of fracture of new restoration as a result of occlusal forces.  When carving for occlusion, attempt to establish stable centric contacts of cusps with opposing surfaces that are perpendicular to occlusal forces should be made.  Occlusal contacts located on a cuspal incline or ridge slope are undesirable because these create a deflective force on the tooth and hence should be adjusted until the resulting contact is stable. i) AMALGAM RESTORATIONS :  Sufficient bulk of amalgam is mandatory when restoring a cavity with amalgam so as to withstand the load of occlusion.  Adequate thickness of amalgam should be provided at the marginal ridges in order to support the opposing supporting cusps. 18


 Amalgam restorations are carved following the cuspal inclines.  In case of large restorations, where there are no cuspal planes to guide carving, the operator should follow a cautious approach : • Buccal and lingual cusp tips should be placed in lines joining those of adjacent teeth. • The level of central fossa and the marginal ridge should be carved similar to that of adjacent teeth. • The bucco-lingual width of the occlusal surface is kept narrower than the original buccolingual width of the tooth. • In case both the working cusps on more than 2 cusps are restored, preferably the occlusal table is kept narrowed. This narrower occlusal table leads to : • Reduction of force : When the occlusal table is made narrower, lesser force is applied over the same to undergo masticatory functions. Force is transmitted to all structures underlying the occlusal table, which include the restoration, the tooth structure and the periodontium. • Reduction of the effect of force : The direction in which the applied force is transmitted is governed by muscular activities and the area on which the force is applied. However, the effect of the force may be modified by altering the surface at which the force is applied, thus altering the direction of resolved components. • Reduction of torque : The tendency to rotate may be reduced by altering the point of application of the force relative to the fulcrum. The point of application of the force may be altered by modifying the occlusal table which indirectly depends upon the design of the cavity and the restoration. ii) CAST METAL RESTORATIONS :  Similar to amalgam restorations, before preparations of any tooth, evaluate the occlusal contacts of the teeth in centric occlusion and in excursive movements. As part of this evaluation decide if the existing occlusal relationships can be improved with the cast metal restorations.  The cuspal interferences are depicted in mandibular working movements, non working movements and protrusive movements.  The opposing occlusal surfaces should be examined an he malpositioned cusps, plunger cusps and over erupted teeth should be recontoured. 19


 Premature contacts or cuspal interferences from the teeth opposing the required restoration should be removed.  The remaining tooth structure and the length of clinical crown dictates us to choose the type of cast restoration. ANTERIOR RESTORATIONS :  The resin composites and the glass ionomer cements are mainly used in anterior restorations. Though these teeth do not come under direct occlusion, however, they do take part in various movements of the mandible. The restoration should be carved and finished, maintaining the contacts and the cervical curvature of these restorations. The lingual area is carved to maintain the anatomy of cingulum and the lingual marginal ridges. Patient is asked to protrude and the interferences are checked and removed.  Similarly, the relationship of lips with the labial surfaces of restored teeth are checked and the over-contouring, if any, is removed.  The gingival extension of the material is taken care of to maintain the gingival health. Role of Contact Areas :  Good restorative dental procedures must reproduce the proper contact areas. Restorations with contact areas which are flat, open, improperly placed, rough or poorly polished will lead to failure.  A slight frictional movement of teeth always occurs between the interproximal surfaces of teeth during physiologic movement; and with time, the contact point becomes broad resulting in a wider contact area. IF the teeth remained in contact with each other merely by contact points, they would eventually be forced out of the dental arch in either a buccal or lingual direction. Whereas with a wider contact between teeth, this is not likely to occur. The opposing interproximal surfaces of restorations must be hard in order not to flow, flatten, wear or become abraded with use.

Relationship between tooth wear and restorative materials :  Occlusal forces lead to wear of enamel. The wear is, however, very slow if occlusal forces are appropriately transmitted to underlying bony tissues. 20


 The pattern of wear varies individually depending upon various factors. Non-uniform ear of opposing teeth is quite common when one teeth is restored with a restorative material whose wear resistance is different as compared to that of enamel. Very rarely, the wear resistance of a restorative material equal the wear resistance of enamel.  At present, no restorative material is available which wears at the same rate as enamel or as enamel and dentin at later stages.  Differential wear can result in localization of occlusal loads with subsequent failure of restorative materials or development of deflective contacts with mandibular repositioning and an effect on a distant tooth.  Hypothetically, if two restorative materials, which wear at a slower rate than the natural teeth, are placed so as to oppose each other in a dentition undergoing wear, the restorations will produce occlusal interferences at a later stage.  Non-wearing materials opposing each other can lead to natural teeth wear during contact in lateral and protrusive movements.  Conversely, if the materials wear faster than the teeth, the opposing cusp might over erupt into the worn material. IN lateral excursion, this cusp might then come in contact with an opposing cusp and if weakened by previous caries can lead to fracture. Compensation for Occlusal Wear :  Occlusal interferences can develop through differential wear patterns and unmatched compensatory mechanisms.  The clinician can shape and regulate the form of occlusal surfaces of teeth and restorations so that he can determine surfaces of teeth and restorations, which contact during activities such as mastication, swallowing and bruxism. The advantage of this approach are : • The direction of stresses through the strongest portions of the restorations an the remaining tooth structure can be arranged. • The effect of occlusal interferences developing from differential wear can be minimized. • It is possible to maintain the partially restored dentition by means of periodic adjustment. 21


 Since wear defects are not repaired automatically, the dentist should replace and maintain the configuration of teeth in accordance with the functional activities. FORCES EXERTED DURING OCCLUSION / MASTICATION AND THEIR RESOLUTION Various types of forces are exerted on teeth during movement of mandible and also during mastication. Since the tooth surfaces are curved or at an incline, these forces are not only vertical but other types of forces may also be exerting n these surfaces. The tooth, in turn, counteracts these forces with the help of periodontal membrane and alveolar bone. If the surfaces are flat and perpendicular to the force of mastication, only vertical forces would take part. But in curved surfaces, other forces are also set up and the resulting forces might not be exerted along the long axis of the tooth. This phenomenon can be understood by studying the resolution of forces on inclined planes. The cuspal planes are taken as inclined planes. When a force acts perpendicular to a fixed horizontal surface, the resolving force reacts perpendicular to the surface with an equal and opposite force. If the surface is tilted at an angle to the horizontal, it still reacts at right angle to the surface. F Surface

F Surface

R

Thus, the reaction force no longer opposes the applied force in direction nor is equal to its magnitude. Hence the forces are not in equilibrium when applied on inclined planes. The equilibrium can be maintained if more than one force is exerted on tooth or the forces are resolved in both directions. Forces acting on inclined planes. AB is a tangent drawn at inclined plane or the contact between 2 cusps. Angle ‘α’ represents the angle made with the horizontal AC by the tangent AB of the cuspal contact. M is the force of mastication and N is the resolving force. M is perpendicular to the horizontal AC and N is perpendicular to the incline plane, tangent AB, H is the horizontal component of the resolving force, which maintains the equilibrium. 22


As the angle ‘α’ decreases, i.e. incline plane decreases, N and H become shorter and finally merge with M i.e. equal to zero. The effect of friction between cusps also plays an important role. Friction is the resistance to a sliding motion of one body over another and the coefficient of friction is the force of friction over normal force. Many a times, two or more inclined surfaces with slopes facing each other of one tooth contact the buccal and lingual cusps of the opposing tooth or the buccal and lingual cusps and marginal ridges. This condition accounts for the proper balance in occlusion and in case the contact is not normal, it may account for displacements of the restoration or the fracture of the teeth. The effect so produced is termed as wedging effect. The horizontal components of the normal force are responsible for this wedging effect. These horizontal components set up by inclines are equal and opposite and tend to push the inclined surfaces apart. FORCES ACTING ON THE TOOTH : A) In centric occlusion a, b, c are forces which acts at 3 contact points.  Rab is the resultant of forces a and b. Rab and c are the 2 adjacent sides of the parallelogram passing through a given point as shown. The resultant is represented by diagonal passing through the same point i.e. Vabc.  Hc is the horizontal component of force c. H ab the horizontal component of force a and b and Hc should be equal for achieving equilibrium that is why R abc and Vabc are equal. B) During Chewing :  When mandible moves form lateral to centric occlusion, the resultant of forces acting is not vertical but inclined medially.  When tough food is compressed or all cusps are in intimate contact at the 3 points, the forces a and b are decreased and c is increased with resultant changes in horizontal and vertical components. Since during chewing, H c is greater than Hab and the net resultant is Habc. So the horizontal component is along the direction of c.  By using triangle of vector addition, the resultant is represented by Rabc.  The resultant Rabc is a thrust inclined palatally on the maxillary teeth and buccally on the mandibular teeth, whose horizontal component is Habc. 23


MECHANICAL FUNCTIONS OF THE MARGINAL RIDGES : Role of Marginal Ridges :  The marginal ridges play an important role in withstanding and dissipating the occlusal stresses.  The correct form of marginal ridge compatible with the adjacent tooth and also with its own surrounding is important during carving of posterior restorations.  The absence of marginal ridge, or marginal ridge with improper height can lead to altered dissipation of forces subsequently damaging the underlying periodontium. 1. Normal Marginal Ridge :  Forces 1 and 2 act on marginal ridges of teeth A and B respectively. The horizontal component of 1, H1 and the horizontal component of 2, H2, counteract each other. The vertical component V1 and V2 are resolved normally by the underlying tissues. 2. No marginal ridge  Tooth B has no marginal ridge.

Force 1 and 2 are acting on tooth a and B

respectively. The horizontal component of 2, H2 is missing in the tooth B, because force 2 is mainly directed towards tooth A.  Horizontal component H2 will drift the tooth A apart and the vertical component V 1 and V2 of both the forces 1 and 2 will help the food impact vertically. The vertical force V2 will be more than required, there may occur slight tilting of the tooth B. This will further deteriorate the resolution of forces and lead to further food impaction. 3. A Marginal Ridge with a wider occlusal embrasure.  Inspite of putting optimal pressure on marginal ridges of tooth A and B, the forces 1 and 2 act on adjacent teeth. The force 2 will put pressure on tooth A and force 1 will put pressure on tooth B. This will lead to drifting of both the teeth. The vertical components of forces will wedge the food is between the two teeth.  Similar effect is seen when one marginal ridge is higher than other.

4. No occlusal embrassure

24


In totality, the vertical component of forces 1 and 2 will be more concentrated than horizontal components. Though there will to be any vertical impaction of food, the continuous impact of higher concentration of vertical component of forces may lead to changes in alveolar bone after sometime. VERTICAL LOADS AND DISTRIBUTION OF STRESSES :  As the load is applied over the teeth, stresses are distributed. i)

Parallel to the long axis and

ii)

Perpendicular to the long axis

 The force or the load is applied at different areas at a time and the stress distribution depends upon various factors. a) If the cross – section of that area is constant, stress distribution is practically uniform. b) If there is variation in cross-section (such areas are normally termed as prisms); here stress varies form point to point, being inversely proportional to area. c) If change of cross-section area is abrupt; greater concentration of stress occurs at that point.  In vertical loading, there will be shearing stresses in prism in any plane. This haring stress increases to a maximum at 45o and then decreases to zero at 90 o. Therefore, materials that are weaker in shear than in compression or tension replace in planes at 45o to the axis.  The modulus of elasticity of the material is an important property and should be taken care of. If a cavity is restored with gold inlay or porcelain, the modulus of elasticity varies between the tooth and the restorative material. With the vertical force exerting on both, the compression will be the same for the restoration and the tooth, but since gold/porcelain is much stiffer, they will be highly stressed, since S = dE. S (Stress) = S (Unit strain) x E (Modulus of elasticity)  When the force is applied perpendicular to the prism axis, the resultant resolution is known as beam. Beam can be supported form both the ends (simple beam) and may be supported form one end (cantilever beam). Example of simple beam : MOD preparation Example of Cantilever beam : MO / DO preparation

25


The retention of the restoration depends upon these beams, although the strength and the deflection of the material also play part. Moment of Force = Force x Perpendicular Distance  The bonding moment is at the axiopulpal line angle, which tends to rotate the restoration out of the cavity.  Gingival retention with a moment equal to F x L is required to counteract this moment. The total retentive force (R) is equal to F x L / l Where l is the depth of the axial wall.  If we take depth of gingival wall (d) into account, then R and d will be in the same direction, so their moment of force is zero. Therefore, the depth of the gingival wall does not take part in retention. In MOD Preparation : In MOD preparation, the force (F) is divided equally on both the sides. The mesio distal distance (L) is also divided into two. The moment of force at the midpoint is : F / 2 x L / 2 = FL / 4 If this moment of force is divided into two (because it is actually acting on both the ends) then the moment of force : FL

1

FL

----- x ----- = ----4

2

8

Since the beam forces a concave downward curvature between the load and the fixed end, therefore, by sign convention, this end moment is taken as negative. By equation R x l = FL / 8 So R = FL / 8 l The negative sign is used only in vector form and in magnitude only positive sign is used.  If we take depth of gingival wall (d) into account, then R and d will be in the same direction, so their moment of force remains zero.  It is presumed in MOD preparations that the length of the axial wall (l) is kept equal on booth the ends. If there is marked discrepancy between the two ends, the end result may not be the same as is described earlier. Therefore, preferably the length of the two axial walls should be the same. 26


In Cervical / Gingival Restorations :  It has been established that certain forces act on the cervical reign, which could destabilize the restoration and even lead to cracks at the cemento-enamel junction.  The forces acting on inclined planes of the occluding cusps consequently lead to transverse stresses. These transverse stresses try to bend the tooth gingivo-occlusally. Since the teeth are firmly held in alveolar socket, these rotations are minimum and counteracted.  In cases where a cavity is cut on the cervical surfaces, depending upon the height of the axial wall, the deflective force is increased. If the restorative materials are not adhesive in nature, a gap can be created at the cervical surface of the restoration on buccal side and occlusal surface on the lingual side.  Force (F) is applied at incline plane perpendicular to the tangent of the planes. The horizontal component (H) acts approximately at the centre of the tooth. The vertical component (V) is constant. The deflection is mainly by the horizontal component, which depends upon the height of the axial wall (L) and the depth of the occlusal (d 1) and cervical walls (d2). APPLICATION OF STRESSES AND THEIR DISTRIBUTION IN INDIVIDUAL RESTORATIONS : 1) Class I restoration a) If restored with amalgam  It is recommended to converge the side walls occlusally and to keep the floor flat.  In case the floor is not kept flat, the forces will rotate the restoration on both the sides. And also, since the remaining dentin will be less at the centre and if the restoration is deep, the forces might fracture the tooth. b) Cast restorations :  Movement / rotation of the cast restoration is easy, if the pulpal floor is not kept flat.  Since the walls are diverging occlusally, the chances of rotation are much more. c) Composites or GIC  These adhesive materials counteracts such rotational forces. 2) Class II restoration  Stresses which tend to rotate the restoration, mostly act on marginal ridges. 27


 Stresses also is more at axiopulpal line angle, hence, this axiopulpal line angle should be well rounded, thereby decreasing stress concentration and increasing the bulk of the material at this point.  In MOD restorations, bending of the occlusal portion is caused by the difference between the total masticatory force and the support given by the pulpal floor of the cavity.  Gingival retention and rounding of the axiopulpal line angles are required as in MO and DO cavity.  In cases where the opposing cusps occlude in such a way that one contact point is on a proximoocclusal restoration while the other is on tooth structure, there is a tendency to wedge the two apart. To prevent this wedging, the occlusal lock is used even though occlusal surface is not involved by caries. 3) Class III and Class IV Restorations :  Since these lesions are not in direct contact with opposing teeth, only transverse stresses play part in dislodging / rotating the restoration  In such restorations, there is tendency to rotate about an axial approximately parallel to the long axis of the tooth. As incisal retention cannot be made due to thin labiolingual size, so lingual lock, is placed on lingual surface. It should be as close to the incisal edge as possible and still be in dentin to reduce the stress in this lingual lock.  In maxillary teeth, force of mastication ahs labial component, which provides the seating effect on the restoration. In case the labial enamel is not intact, the chances of dislodgment of the restoration will increase. In mandibular teeth, the component of the masticatory force is from the labial to the lingual so chances of dislodgement of restoration are more. 4) Class V restorations :  Analysis indicates that physical forces putting on occlusal surfaces could result in displacement of the restoration.  During occlusion, the vertical stresses on the teeth led to transverse stresses and this component of stresses tends to rotate the cervical restoration.  The mandibular teeth bend more than maxillary teeth.  A gap is evident on the cervical / occlusal wall of the cavity and if the depth of these walls is less, the restoration may come out. 28


FORCES ACTING ON AMALGAM RESTORATION CLASS – I : By definition, Class I cavity preparations are placed in pit and fissure lesions that occur in one more of the following locations : A. Occlusal surfaces of molars and premolars B. Occlusal 2/3 of the buccal and lingual surfaces of molars C. Lingual surfaces of the upper anterior teeth (usually the central and lateral incisors) D. Any other usually located pit or fissure involved with decay. Mechanical problems in Class I restoration and their solutions. A. All Class I cavity preparations will have a mortise shape, i.e. each wall and floor is in the form of a flat plane, meeting each other at definite line and point angles. -

The seat of the restoration is placed at a distinct right angle to the direction of stresses.

-

It is advantageous to have a mortise shape preparation in an inverted cone shape to minimize shear stresses that tend to seperate the buccal and lingual cuspid elements i.e. to prevent the splitting of the tooth. So whenever the anatomical and cariological factors allow, the cavity preparation should be an inverted cone shape.

B. When a caries cone penetrates deeply into dentin, removing undermined and decayed tooth structures can lead to a conical (hemispherical in cross-section) cavity preparation. Mechanically, two problems can occur if a restoration is inserted into such a cavity preparation. 1. If the occlusal loading is applied centrically, the restoration may act as a wedge, concentrating forces at the pulpal floor, and leading to dentin bridge cracking, and an increased tendency for tooth splitting. 2. If the occlusal loading is applied eccentrically the restoration will have tendency to rotate laterally, for there would be no lateral locking walls in definite angulation with a floor. Although these lateral movements are microscopic, they occur frequently enough to encourage microleakage around the restoration, predisposing to a recurrence of decay. These movements can also lead to fracture of marginal tooth structure, and even to splitting of lateral walls. To solve these problems, flatten the pulpal end of the cavity preparation. However, if accomplishing this at a deep location incurs increased risk of involving the pulp chamber, pulp horns, or recessional lines containing remnants of pulp tissues, make 29


the pulpal floor at more than one level. One level will be the ideal depth level (1.5 mm) and the others will be the caries cone(s) level(s), dictated by the pulpal extent of the decay. The shallow level creates the flat portion of the pulpal floor at definite angles to the surrounding walls, adequately resisting occlusal forces and laterally locking the restoration, without impinging on pulp tissues. Reiterating, the other level(s) is (are) only necessitated by the caries extent., creating one or more concavities or cones in the pulpal floor. C. When a cavity wall comes in contact with a marginal ridge, the wall should be divergent pulpo-occlusally, making an obtuse angle with the pulpal floor. This design allows for maximum bulk of tooth structure supporting the marginal ridge and avoids undermining of the marginal ridge, creating more mechanical and biological problems. D. If cariogenic conditions do not dictate otherwise, the width of the cavity should be limited to Ÿ to 1/5 the intercuspal distance (not less than 1.5 mm). This minimizes loss of tooth structure in this critical cross-section of the tooth. This width will also facilitate easy carving of the restoration, and minimize the possibilities of occlusal interference’s. E. All cavosurface angles should be right angled to create a butt-joint with the marginal amalgam. This configuration allows marginal amalgam to withstand stresses with the least possibility of failure. F. All line and point angles, or any junction between different details in the cavity preparation, should be rounded but definite. This design has all the advantages of the mortise shape, while avoiding stress concentration in the tooth structure and restorative materials that may occur from sharp angulations. G. Occluding forces will tend to move marginal amalgam and tooth structures from position #1 to position #2. As vital tooth structures are more deformable than set amalgam, the displacement will not be equal thereby creating a gap between them. This places the marginal amalgam under intolerable tensile loading which may lead to amalgam failure if the amalgam is in thin cross sections, i.e. acute angled marginal amalgam will fracture. B, If marginal amalgam is right angled, it can be stand induced stresses from occlusal loading with less possibility of failure, even if the stresses are tensile in nature. CLASS II AMALGAM RESTORATION By definition Class II cavity preparation is proximal preparations of molars and premolars. 30


Resistance Form : The fundamental concept of resistance form is based on the reaction within the restoration and the remaining tooth structure to occlusal loading. The objective of a cavity preparation design is to establish the best possible configuration that can cope with the distribution and magnitude of the stresses in both structure and the restoration without failure. To design such a configuration, one must first comprehend the nature of loading and of resistance to such loading. A. Occlusal Loading and Its Effect : During centric and excursive movements of the mandible both restoration and the tooth structure are periodically loaded both separately and jointly. This brings about different stresses patterns depending on the actual morphology of the occluding area of the both the tooth in question and opposing contacting cuspal elements. For the purpose of this discussion, one can classify these loading situations and their induced stress patterns in the following way. 1) A small cusp contacts the fossa away from the restored proximal surface, in a proximo occlusal restoration at centric closure. As shown due to the elasticity of the dentin, (in young teeth) a restoration will bevel at the axio-pulpal line angle (provided the proximal part of the restoration is selfretained). This creates tensile stresses at the isthmus portion of the restoration, shear stresses at the junction of the main bulk of the proximal part of the restoration and self retained parts and compressive stresses in the underlying dentin. 2) A large cusp contacts the fossa adjacent to the restored proximal surface in a proximo-occlusal restorations at centric closure, either in the early stages of moving out of centric or at the late stages of moving toward it. As shown, the large cusps will tend to separate the proximal part of the restoration from the occlusal part. This crates tensile stresses at the isthmus portion of the restoration even fi the proximal portion is self – retained. This loading situation will deliver compressive forces in the remaining tooth structure, apical to the restoration. 3) Occluding cuspal elements contact facial and lingual tooth structure surrounding a proximo-occlusal or proximo-occluso-proximal restoration, during centric and excursive movements. Concentrated shear stresses will occur at the junction of the surrounding tooth structure and corresponding floors, with a tendency towards failure there. 31

This


loading situation can be unilateral or bilateral, depending on the mandibular movement it is the most deleterious to tooth structure especially on the orbiting side if there is interference during lateral excursion. 4) Occluding facial elements contact facial and lingual parts of the restoration surrounded by tooth structure, during centric and excursive movements. This arrangement will induce tensile and compressive stresses in the restoration which will be transmitted to the surrounding tooth structure. 5) Occluding cuspal elements contact facial or lingual parts of the restoration completely replacing facial or lingual tooth structure during centric or excursive movements. The tensile stresses will be induced at the junction of the occlusal and facial and/or lingual part of the restoration in both occluding situations. 6) Occluding cuspal elements contact a restoration’s marginal ridge(s) or part of a marginal ridge during centric or excursive movements (assuming the restoration is locked occlusally), there will be concentrated tensile stresses at the (junction of the occlusal and facial or lingual parts of the restoration at full intercuspation and to end from that position) at the junction of the marginal ridge and the rest of the restoration. This will be true if its an area of advance contact during mandibular closure. 7) Cuspal elements occlude or disclude via the facial or lingual groove of a restoration. There will be tensile stresses at the junction of the occlusal and facial or lingual parts of the restoration at full intercuspation, and to and from that position. 8) Cusps and crossing ridges are part of the restoration in centric and excursive movement. Both will be subjected to compressive stresses during such positions and movement. Besides tensile stresses could concentrate at their junction with the main restoration, specially during contacting excursive movement. 9) Axial portions of the restoration during centric occlusion and excursive movement contacts: Whenever these portions are in contact with opposing occlusal surfaces, there will be induced compressive and shear stresses when they are not reciprocating (one side not in contact with occluding surfaces while other axial portion). The axial surfaces will be stressed in a slight tensile and shear pattern at their junction with the main bulk of the restoration. 32


10)Restoration is not in occluding contact or is in premature contact during centric occlusion or excursive movement of the mandible.

The first situation is not

conducive to function, insofar as the restoration will not be involved with direct loading from the opposing occluding teeth. After a period of time, however, the tooth will supraerupt, rotate, and or tilt, establishing contact with the opposing cuspal elements.

Usually, this newly acquired location will not be the most favorable

position for the restoration, tooth, or the remainder of the gnatho stomatic system, either mechanically or biologically. It is safer to build the restoration to predetermined contacting areas with opposing teeth which will lead to predictable physiologic stress patterns in the tooth structure and restoration. Conversely, any portion of the restoration occluding prematurely will tremendously exaggerate the same types of stresses normally induced in that area of the restoration. Besides, additional shear components of stress could be precipitated there. This, too, could lead to localized or generalized gnatho stomatic disturbances, with eventual mechanical and biological failures. Needless to say, pre-existing premature contacting areas should be eliminated before restorative treatment. This is done for many reasons, but primarily, because cavity preparation increases the susceptibility of remaining tooth structure to fracture failure.

Besides, the restoration should be built to the predetermined occlusal

position, even fi the preexisting tooth structures were not. Several factors must be accommodated in designing Class II preparations for amalgam. Occlusal loading is dynamic and cyclic in nature, which is a far more destructive type of loading than static loading. Amalgam is least resistant to tensile stress and most resistant to compressive stress. Tooth structure, particularly when interrupted by a cavity preparation, is least resistant to shear stress. Therefore, Class II cavity preparations for amalgam restorations should be designed to resist cyclic loading while minimizing tensile loading in the amalgam and shear loading in the remaining tooth stricture. B. Design features for the protection of the mechanical integrity of the restoration : 1. Isthmus : In the isthmus, i.e. the junction between the occlusal part of a restoration and the proximal, facial or lingual parts, potentially deleterious tensile stresses occur under any type of loading. 33


Most mathematical, mechanical and photoelastic analyses of these stresses reveal three things : 1) The fulcrum of bending occurs at the axio-pulpal line angle 2) Stresses increase closer to the surface of a restoration, away from that fulcrum, and 3) Tensile stresses concentrate at the marginal ridge area of a Class II restoration. Materials tend to fail, therefore, starting from the surface, near the marginal ridge, and proceeding internally, toward the axio-pulpal line angle. These problems may be solved by applying common engineering principles. A theoretical solution might be : 1) to increase amalgam bulk at the axio-pulpal line angle, thereby, placing the surface stresses away from the fulcrum. However, its actually results in increased stresses within the restorative material and a deepened cavity preparation, dangerously close to pulp anatomy. Therefore, such a solution, in and of itself, is wholly unacceptable. 2) Another solution might be to bring the axio-pulpal line angle closer to the surface, in an effort to reduce tensile stresses occurring near the marginal ridge. However, this, too, is unacceptable in that the consequent diminished bulk of amalgam would no longer adequately resist compressive forces. 3) A combination of the two solutions i.e. increasing amalgam bulk near the marginal ridge, while bringing the axio-pulpal line angle away from that stress concentration area and closer to the surface, can be achieved simply by slanting the axial wall toward the pulpal floor. a) The obtuse axial pulpal line angle thereby created not only provides greater amalgam bulk in the marginal ridge area of the restoration, but also reduces tensile stresses per unit area by bringing this critical area of the preparation closer to the surface of the restoration.

Furthermore, this design feature improves

accessibility to the proximal facial and proximal lingual parts of the cavity during preparation procedures. This is the first design feature. b) Secondly, if the axio pulpal line angle is rounded, structural projections or sharp junctions that may concentrate stresses at the isthmus would be avoided. This second feature will also improve the visibility for the facial and lingual gingivoaxial corners of the preparation proximally, as well as increase the amalgam bulk at the fulcrum.

34


c) Thirdly, by slanting the axial wall, bulk is improved by increased depth rather than increased width. Increasing the width at the isthmus portion only increases the surface area receiving deleterious occluding stresses. 4) As a fourth design feature, the pulpal and gingival floors at the isthmus should be perfectly flat in order to resist forces at the most advantageous angulation. 5) The fifth design feature is that every part of the preparation (occlusal, facial, lingual or proximal) should be self-retentive. If every part of the restoration is locked in tooth structure independently from other parts, there will be minimum stresses at the junction of one part with another, i.e. the isthmi. This can be achieved in amalgam preparations by retentive grooves, internal boxes, and undercuts. 6) Sixth, one should avoid, as much as possible, placing or leaving any surface discontinuities, such as carved developmental grooves, scratches, etc at these critical areas in the restoration. These can precipitate and accentuate stresses leading to fatigue failure. Finally, by checking occlusion to eliminate prematurities in the restoration, immediate overloading and failure can be avoided. 2. Margins : Amalgam has good compressive strength when it has sufficient bulk (1.5 mm minimum). However, frail, feather edged margins of amalgam, which will occur when the cavosurface angles of preparations are bevelled, will fracture easily. Occluding forces will cause amalgam at the bevel to bend with maximum tensile stresses, occurring as a result of elastic deformation of the tooth structure beneath the bevel. Marginal excess of amalgam will similarly fracture, leaving a ditch around the restoration that will enhance recurrence of decay. So, for the margins of these p reparations, four design features should be observed ; create butt joint amalgam tooth structure at the margins, leave no frail enamel at the cavosurface margins, remove flashes of amalgam on tooth surface adjacent to amalgam margins, and, as practically as possible, the interface between amalgam and tooth structure should not be at an occluding contact area with opposing teeth either in centric or excursive mandibular movements. 3. Cuspal and Axial angles : The following are the design features for these parts of a restoration. a. Amalgam bulk in all three dimensions should be atleast 1.5 mm 35


b. Each portion of the amalgam should be completely immobilized with retention modes. c. Amalgam should be seated on a flat floor or table in these areas. d. Amalgam replacing cusps or axial angles should have a bulky connection to the main part of the restoration with similar design features as for the isthmus areas. C. Design features for the protection of the physiomechanical integrity of remaining tooth structure : In addition to design features in the restoration, there are also certain design features in the tooth structure, which enhance resistance of the restored tooth to deleterious stresses. Retention from : In order to design a cavity preparation that will hold a restorative material, it is necessary to know the possible displacements that can happen to such a restoration, the forces that can cause them, and the fulcrum of these movements. There are four such displacements for a Class II proximo-occlusal restoration. A. Proximal Displacement of the Entire Restoration : In analyzing the obliquely applied force “A” into a vertical component “V” and a horizontal component “H”., it can be seen that “V” will try to seat the restoration further into the tooth, , but “H” will tend to rotate the restoration proximally around axis “X” at the gingival cavosurface margin. To prevent such displacement, self-retaining facial and lingual grooves proximally are necessary, in addition to an occlusal dovetail. B. Proximal Displacement of the Proximal Portion : If one were to consider the restoration as being L-shaped, with the long arm of the L occlusally and the short arm proximally, when the long arm is loaded by vertical force “V”, it will seat the restoration more into the tooth. This is due to the elasticity of the dentin, especially in young teeth, wherein the pulpal floor will change location from position 1 to position 2. However, since metallic restorations are more rigid than the dentin, the short arm of the L will move proximally, as shown in the figure. The fulcrum of this rotation is the axio-pulpal line angle.

In order to prevent such a

displacement, proximal self-retention in the form of facial, lingual and/or gingival grooves are required. However, shear stresses will be induced at the junction between the amalgam of the main restoration and that in the grooves. Therefore, it is to be 36


understood that these grooves are prepared only when there is complete assurance that there will be sufficient dentinal bulk to accommodate them, and that they will not impinge on the axial angle or on the pulp anatomy. C. Lateral Rotation of the Restoration Around Hemispherical Floors (Pulpal and Gingival) As in Class I cavity preparations this displacement can be prevented by definite point and line angles, and ledges where indicated. D. Occlusal displacement : The can be prevented by directing occlusal loading to seat the restoration and by inverted truncated cone shaping of key parts of the preparation. Although the magnitude of these four displacements is minute, they are repeated thousands of times per day. This can definitely increase microleakage and initiate mechanical and biological failure of the restoration and surrounding tooth structure. Therefore, proper locking of the restoration into the tooth should be exercised to minimize these hazards. To repeat every part of the cavity preparation should be self-retaining, if possible, i.e. independent in its retention from the rest of the cavity.

This minimizes shear

concentration areas at the junctions of different parts of the restoration, with less failure to be expected as a result. FORCES ACTING ON INLAY RESTORATION The cavity should have such retention form that the restorations will be firmly held in place, the cavity should also have resistance form that the restoration will withstand the stress without being dislodged. An understanding of the materials used in constructing an inlay, together with a knowledge of correct manipulation is also an important factor in the success or failure of an inlay (inlay is not only a part of mechanical structure replacing lost teeth, but it is also intimately related to the vital tissues, it is the medium through which mechanical and physical forces are translated into physiological functions and biological reactions in living tissues. The other preparation features that will help solve the mechanical problems of cast restorations are as follows :

37


All the line and point angles should be definite, but not angular, so they can be easily reproduced in a casting and to avoid stress concentration in the casting and the tooth structure. The roundness must be substantial for Class V materials. The axial wall should slant toward the pulpal floor, as part of the taper. This, together with rounding of the axio-pulpal line angle, can reduce stresses at the isthmus area. Reduction of tooth structure should follow the original anatomy of the tooth, to create even reduction, with minimum tooth involvement, and even physiologic distribution of forces applied on the restoration and remaining tooth structure. Maximum reduction should be at the occluding surfaces, especially the parts of the tooth surfaces that are in contact during static and dynamic relations of the mandible. An average of 1 mm should be cleared for metallic casting in the inclined planes of the cusps. This reduction should be 1.5 mm for cast ceramics. The reduction of the occluding inclined planes should be cut in a concave form, to accommodate maximal bulk of the casting where stresses are at their maximum. The internal parts of the cavity preparation should be mortised to preserve the resistance and retention features of the preparation (and to assure one path for the preparation). The internal boxed up portion should occupy the maximum dimensions of the cavity preparation as practically as possible. This will necessitate making the cavity wall in different planes. At least, the internal planes are fixed in their angulation (almost right angle) with the adjacent floors or walls. Since the retention of an inlay and its resistance to displacement are primarily mechanical problems, a group of the principles of retention is based on understanding the forces of mastication and the analysis of strains which are present in the restoration. It has been stated that when a force is applied at right angles to a surface its effectiveness with the direction of force and that is proportional to its magnitude likewise, the opposing forces are equal and opposite in direction. Another law states that if the force is applied at an angle to the surface other than right angle, the magnitude of which depends n the angle of application ad that the reacting force is neither equal nor opposite in direction. Lateral or tangential forces may cause displacement of the restoration unless adequate resistance and retention have been incorporated in the preparation. Frictional retention can be achieved by the action of dentin and enamel walls grasping the restoration (intracoronal retention). 38


Now let us consider the forces applied at right angles to the flat surface of a restoration. Pulpal Floor and Gingival Seal : 1) A typical proximoocclusal cavity will have two such surfaces to vertical forces – the pulpal and gingival walls. If the forces are perpendicular to these surfaces the opposing forces are equal and opposite, then there is no tendency to displace the filling.

Floors positioned

perpendicular to these lines of force absorbs the stress over a broad area of tooth. 2) It is only when the pulpal wall is flat and the two vertical walls are parallel to each other that the maximum retention form is obtained. While these above illustration refer to simple box type cavity preparation, the same principles hold good when the force is applied at right angles to the occlusal surfaces of proximo occlusal inlay. 3) In a tooth weakened by extensive caries, the resistance form is obtained by extracoronal extension of the preparation in the form of extra long reverse bevel in capped cusps or by partial or complete coverage of facial or lingual surfaces. 4) If the dentin of the pulpal wall is compressed elastically under vertical forces, if the compression is conical then the gingival portion of the filling would rotate out of the cavity with the axiopulpal line angle acting as the fulcrum. Because of the added retention obtained by the pulpal extension and if the diagonal force is applied to the casting which is ‘L’ shaped. It will have a tendency to straighten out, so this causes the metal to move out laterally at the gingival area. To resist this lateral spreading, at the gingival wall provision is made for the depression of the wall and creating the gingival groove which restores the retentive form to a certain extent.

Axioproximal Walls (Facial / Lingual) : Compressive forces resulting from vertical pressure have an important bearing on the retention of the inlay.

This bears on the relationship of the buccal and lingual

proximal walls. Now whether they should flare axioproximally or be parallel to each other (that is the part of the wall lying within the dentin). 39


There are 3 different relationships of wall A to wall B in the gingivoocclusal direction. 1) The walls are parallel to each other. 2) Walls are widely divergent. 3) Divergence not exceeding 5o from the vertical plane. When forces are applied at an angle other than right angle, force is resolved in 2 ways, one of which reacts in its effectiveness at right angle to the surface. This force is not opposite in direction, nor is it equally magnitude to the original force. The tendency in a tooth is for the cusp of the opposing tooth to slide down the inclined plane or for an inlay to be pushed out of the cavity in a horizontal plane. When a vertical force is applied to a proximal extension the filling is rotated occlusoproximally out of its cavity.

The rotation point of fulcrum being gingival

marginal wall. These forces are always effective unless counteracted by an opposing movement.

This can be achieved by properly prepared occlusal lock, by proper

preparation of gingival wall, pulpal wall and lastly by the proper contour and contact point. Slice : Slice preparation provides external support of weakened tooth or areas subjected to high stresses during function.

It increases the resistance and retention form by

exposing a larger amount of tooth structure to the frictional grasp of the restoration. Occlusal Dove Tail : Tensile stresses developed by this is one of the strongest means of resisting the displacement of an inlay. Clinical precaution demands that by lingual inclined planes which extend into the isthmus of the occlusal block be on sound cusps with a sufficient amount of supporting dentin. If these are lacking, there is likelihood, of fracture of one or both the cusps whenever inlay is subjected to horizontal forces. Now the buccal and lingual axial walls, instead of flaring from the axial line angles to the cavosurface margin in a continuous plane, are now changed into two narrower but parallel planes and two smaller diverging planes. It is evident that in this type of preparation, it is possible to retain the retentive form of the preparation, even if the walls diverge in a continuous plane, when stress is applied to the occlusal surface, the reaction of the opposite forces will tend to dislodge the filling. So retention in this type of preparation is by placing a gingival groove in the gingival wall and by adding an 40


occlusal lock. Hence effort is made to parallel at least part of the buccal and lingual proximal walls that lie in dentin. Second method of resisting horizontal displacing forces is by the proper preparation of gingival walls. The properly prepared gingival groove assist in preventing the lateral displacement of an inlay. But because of the inherent weakness of the gingival groove the possible fracture to this wall of the tooth structure between the groove an the cavosurface angle, so many operators prefer the inward beveling of the gingival wall, forming an acute angle between the axial and gingival walls. Pulpal Wall : Third method of obtaining opposing movements to horizontal displacing force is by establishing resistance into pulpal wall. The pulp wall which is flat offers no resistance to horizontal displacement when it is prepared with two inclined planes it will prevent the lateral displacement of the inlay. Another modification is placement of grooves parallel to the long axis of the tooth at the axial angles. Such preparation resist horizontal displacement of the inlay. This will also resist rotary displacement because of the frictional resistance of the dentin at this point of the cavity. In addition to increased mechanical retention resulting from slight modification of cavity preparations, it is essential that suitable gold alloys be used and casting made of such alloys be properly heat treated in order that their maximum physical properties are made available. Axiopulpal Line Angle : This line angle is slightly rounded to dissipate the stresses. Gingival Bevel : 30-45o to have sliding lap fit joint, cement tooth interface. Certain forces collectively act on a cemented restoration mainly in the same direction as the path of withdrawal. Some of the factors pertaining to these forces are : 1) Magnitude of the dislodging forces : Forces that tend to remove a cemented restoration along its path of withdrawal are small compared to those that tend to tilt it. Generally exceptionally sticky food stuffs act as a pulling force. The quantum of vertical and oblique forces also tend to dislodge the restoration. The magnitude of the dislodging forces depends on the stickiness of food, occluding and lateral 41


movement forces of the jaws and the surface area and texture of restoration being pulled. 2) Stress Concentration : Stresses are not uniform throughout the cement but are concentrated around the junction of the axial and occlusal surfaces (axio pulpal line angle). This may explain the retentive failure of the cast restoration. The strength of the cement is less than the induced stresses. FORCES ACTING ON DIRECT TOOTH COLOURED RESTORATIONS For any proximal restoration in anterior teeth, there are two possible displacing forces. The first is a horizontal force displacing or rotating the restoration in a labioproximo lingual or linguo proximo labial direction. It has its fulcrum almost parallel to the long axis of the tooth being loaded. The second is a vertical force displacing or rotating the restoration proximally(sometimes facially or lingually). The latter has a loading arrangement similar to occluso-proximal (occluso-facial or occluso-lingual) restorations in posterior teeth. The amount of teeth depends upon the location, extent and type of occluding contacts between the upper and lower teeth during function. The mechanical picture can be summarized as follows : 1. In anterior teeth with normal overbite and overjet during centric closure of the mandible (from centric relation to centric occlusion), mainly the horizontal forces will be in action. Those forces, if loading the proximal restoration directly, would try to move it linguo-proximo labially (for the upper restoration) and labio-proximo lingually (for the lower one). The magnitude of the horizontal force component at this stage of mandibular movement is not very substantial, and the vertical one is almost nil. In protrusive and lateral protrusive movements of the mandible, directly loaded proximal restorations in anterior teeth will be subjected to substantial horizontal as well as vertical displacing forces, especially in restorations replacing the incisal angel. The results of this loading are rotational forces (previously described), as well as forces rotating the restoration labially and proximally (for the upper) or lingually and proximally (for the lower). 2. If anterior teeth meet in edge-to-edge fashion at centric occlusion, loading of the proximal restoration, involving incisal angles (Class IV) will be similar to any Class II proximo-occlusal restorations, i.e. vertical displacing forces with very limited horizontal components. This loading will continue during all centric closures and 42


excursion movements of the mandible. However, if the incisal angle is intact (Class III), these displacing forces will be minimal. 3. If the upper and lower anterior teeth meet such that the lowers are labial to the uppers in centric occlusion (Angle’s Class III), there will be the same type of loading conditions mentioned in (1) except the horizontal loading will tend to rotate or displace restorations labio proximo lingually (for uppers) and linguo-proximo labially (for lowers). During excursive movements, if teeth are in contact and there is a possibility of retrusive mandibular movements, the loading will be much less than that described in (1), with its horizontal displacement capability exactly the reverse to that described in (1). 4. In occlusions with deep anterior overbite and normal or no overjet, the horizontal type of loading will be greatly exaggerated. The vertical displacement, although present, will be minimal by comparison. 5. In occlusion with anterior open bite or severe overjet, or any other condition that creates a no-contact situation between upper and lower anterior teeth during centric occlusion and excursive movements of the mandible, proximal restorations will not be loaded directly either vertically or horizontally. 6. In cases when the proximal restoration of an anterior tooth is a part of a mutually protective occlusion, i.e. an incisor and the adjacent cuspid are involved in an anterior lateral disclusion mechanism, the teeth and restoration will be part of that disclusion mechanism with excessive horizontal and vertical loading forces. This situation should be properly diagnosed, so that the tooth preparation can be designed and prepared accordingly. It should be understood that none of these loading forces work separately. They work together and simultaneously. However, they may differ in magnitude at different stages of mandibular movement. It should be mentioned here that a restoration replacing part or all of the incisal ridges of an anterior tooth will have the same pattern of loading as mentioned in (1) – (6), but with increased intensity. Loss of the incisal angle of a tooth, i.e. conversion from a Class III to a Class IV represents a major complication in the mechanical problems of anterior tooth restorations. This loss will lead to definite direct loading of the restoration, loss of the incisal wall which would normally accommodate one of the two possible main internal retentive modes for the restoration, definite vertical loading with its sequelae, and the placement of margins on the incisal ridge.

This further exposes the

restoration to the maximal loading possible in anterior teeth, and it is with the 43


minimal tooth structure to be used for resistance and/or retention against such loading. The structure of anterior teeth themselves, have a comparatively different stress pattern, as a result of occlusal loading from that of posterior teeth. The unique shape as well as the mechanical structure and function of these teeth is very important to comprehend before designing a cavity and/or tooth preparation for a direct tooth colored restoration. The following is a summary of these unique features : a) Anterior teeth have their maximal bulk gingivally. They taper incisally with the least bulk at the incisal ridge. So resistance to stress fractures will be maximum at the gingival end and decrease incisally. b) Forces are directed horizontally and vertically on anterior teeth as mentioned with the force analyses on restorations for these teeth.

These forces accumulate

maximal shear stresses at the junction of the clinical root with the clinical crown and maximum tensile stresses at the incisal ridges, especially their corners (incisal angles). c) The labial enamel plate is much thicker than the lingual or proximal ones, with maximal thickness of enamel usually at the incisal ridge. d) The incisors may be involved in a disclusion mechanism of the mandible with loading similar to that of the cuspid, but to a much lesser extent. e) Cervical portions of anterior teeth when they are affected with a Class V lesion or cavity preparation will have a stress pattern similar to posterior teeth, and the stress pattern is governed by the same factors as in posterior teeth. In addition, the deeper the overbite is, the more induced the stresses are at these cervical areas. f) AS mentioned previously, loss of an axial angle, incisal angle, or tooth structure at the neck of the tooth will dramatically reduce that tooth’s ability to resist loading without fracture failure. Ideally, a restoration made of tooth colored materials should not be loaded directly, i.e. there should be intervening tooth structure between the occluding tooth and the restoration. This situation can only be achieved by four intact walls surrounding the restoration.

Unfortunately, this is usually not the case.

That is why the clinical

performance of tooth colored materials differs from one situation to another, sometimes dramatically.

44


Anterior teeth have their maximal bulk gingivally. They taper incisally with the lest bulk at the incisal ridge. So resistance to stress fractures will be maximum at the gingival end and decrease incisally. Forces are directed horizontally and vertically on anterior teeth as mentioned with the force analyses on restorations for these teeth. These forces accumulate maximal shear stresses at the junction of the clinical root with the clinical crown and maximum tensile stresses at the incisal ridges, especially their corners (incisal angles). The labial enamel plate is much thicker than the lingual or proximal ones, with maximal thickness of enamel usually at the incisal ridge. The incisor may be involved in a disclusion mechanism of the mandible with loading similar to that of the cuspid, but to a much lesser extent. Ideally, a restoration made of tooth colored materials should not be loaded directly, i.e. there should be intervening tooth structure between the occluding tooth and the restoration. This situation can only be achieved by four intact walls surrounding the restoration.

Unfortunately, this is usually not the case.

That is why the clinical

performance of tooth colored materials differs form the situation to another, sometimes dramatically. FORCES ACTING ON POSTS An endodontically treated tooth has been structurally compromised by caries and its removal, prior restorations, and finally, endodontic preparation and filling. It should be emphasized again that posts are only used for retaining the restorative material in the remaining tooth structures, and by no means will they reinforce or improve the strengths of these tooth structures. Because the retention of posts is accomplished by various means, it might be expected that different stresses are associated with post installation. With posts retained by the cement alone, the main potential for installation induced stresses is the build up of hydrostatic back pressure. This potential with parallel – sided post is circumvented by means of longitudinal vents along the posts, which provide an outlet for the pressure. Tapered post are self-venting, and consequently there is no pressure build up. Endodontic posts provide a protection function through their ability to distribute the forces of mastication to the remaining tooth structure. How well this protection is achieved depends upon post design, embedment depth and diameter. 45


MECHANICAL ASPECTS OF POST-RETAINED RESTORATIONS AND FOUNDATIONS : A. Stressing Capabilities of Posts : The following features and factors of posts and the involved tooth will govern the stress patter induced in the surrounding tooth structures due to the use of posts as retentive means : 1. Type of Posts : Parallel sided posts will have the tendency to evenly distribute the forces it receives at and around its cavity end onto the root canal walls, if these forces are applied parallel (a) to the post axis (vertical occlusal loading. IF the forces applied are at a right angle (b) or oblique (c) to the post axis, the induced stresses in the root canal walls will be unevenly distributed, i.e. there is a great possibility of stress concentration due to uneven thickness of the root canal walls around the post (root taper) while the post remains the same diameter. This leads to a thin sectioned wall at the very apical end of the post. On the contrary, taper sided posts and combination type posts will concentrate stresses due to apical loading (a) in the root canal walls resulting from its wedge shape. Lateral loading on and around cavity ends of the post, however, will induce evenly distributed stresses in the root canal walls for the taper of the post will correspond with the root and root canal taper, leading to an even thickness of walls occlusoapically. 2. Method of Inserting root canal posts : During insertion of a post into the root canals, highly threaded posts can induce ten times the amount and extent of stresses as smooth sided posts. Serrated surface posts will induce about one and a half to two times the stresses that are induced by smooth surfaced posts. This can be explained by the cemented technique utilized by the serrated and smooth surfaced posts. 3. Bulk of dentin in root canal walls : Naturally, the bulkier that the dentin surrounding a root canal post is, the less will be the induced stresses per unit volume during the post insertion and functional use of the post retained restoration. It has been estimated that a minimum of 2 mm of dentinal root canal wall should surround a post, so that the stresses induced there will not lead to dentinal failure in the form of cracks and gross fracture. 4. Length of clinical root involved with the root canal post : 46


Although the tooth to receive a root canal post should be non-vital and endodontically treated, the clinical crown portion of the tooth is much more dehydrated than the clinical root portion as the dentin portion of the root still receives some fluids from the adjacent periodontal ligament. The more dehydration that there is, the less will be the modulus of resilience and elasticity of the dentin, and consequently the less will be the dentin’s ability to absorb and resist stresses without failure. 5. Ferrule or embracing features of the restoration : Post-core and dowel coping foundations for endodontically treated teeth will always induce stresses in the root canal walls and remaining tooth structures which can only be counteracted by embracing the buccal and lingual cuspal elements of the tooth and/or banding (circumferential embracing) the tooth at its most apical part of the clinical crown (i.e. area of maximum stresses). Such bracing is referred to as the Ferrule effect. The Ferrule feature of the restoration should involve at least 2 mm of crown length to counteract stresses induced by the post. Using less than 2 mm of crown tooth structure, the counteracting Ferrule effect will be reduced. The closer this embracing feature is to the junction between the clinical crown and the root, the more effective it will be. This is the major protecting feature against induced stresses in a restoration for endodontically treated teeth. 6. Lateral Locking Mechanisms for the post and restoration : Because most premade posts are rounded in cross-section there is a great tendency for the post and the restoration retained by the post to rotate under torsional forces. This rotational tendency can induce unnecessary stresses in remaining tooth structures. The presence of a method to lock the post and the restoration against such rotation (e.g. a lateral pin, internal boxes, opposing walls, etc) will drastically reduce the effect of torsional forces. 7. Presence of a pulp chamber with pronounced walls : Walls of the pulp chamber, especially if they are opposing each other, will increase the frictional retention of the foundation or restoration, minimizing the retention demands on the pot and thereby minimizing stresses in the root canal walls. 8. Presence of intact marginal or crossing ridges : These ridges will act as a binder between the buccal and lingual cuspal elements, resulting n better distribution and resistance of induced stresses. 9. Proximity of the post to the root canal filling : 47


Root canal fillings should not be involved in the mechanical problems of the posts. For this reason, there should be a space between the apical end of the post and the occlusal end of the root canal filling. IF the post approximates the root canal filling, forces can be transmitted to that filling, which mechanically is made of very weak materials, and lead to profound straining. This can move the post in an undesirable direction, and it may induce unnecessary stresses in the remaining tooth structure. In addition, the direct or indirect loading of the root canal filling may change its relationship to the surrounding walls and apical anatomy, resulting in endodontic failure. 10. Presence of flat planes in the remaining tooth structures, at a right angle to occluding forces : Flat planes, in the form of tables, gingival floors and ledges, etc, which will be able to receive and resist occluding forces before arriving to the post, are the second major feature used to reduce induces stresses in the remaining tooth structure. Besides partially protecting the post from direct loading, these flat planes will protect a very weak subpulpal floor from being directly loaded. 11. Presence of lateral walls in the remaining tooth structure : Extra or intracoronal axial walls, that will receive and resist laterally applied forces on the restoration before they arrive at the post, will drastically reduce stresses I the remaining weakened tooth structure, primarily in the root canal walls. 12. The root post portion relative to the crown post portion : The ideal ratio is to have the root portion of the post twice as long as the crown portion, i.e. a ratio of 2:1. Less than that, especially less than a ratio of 1:1, will definitely concentrate intolerable stresses on the lateral walls of the root canal adjacent to the apical end of the post. 13. Hydraulic pressure during post cementation : If there are no lateral vents in the post, or if the post diameter is very close to that of the post channel diameter, the semi-liquid cement mix, during the cementation of the posts, may exert tremendous amounts of hydraulic pressure that exceed the elastic limit of the surrounding dentin or prevent complete seating of the post. 14. Surface texture and shape of the root end of the post : Greater post surface roughness and/or the presence of a chisel, wedge, or irregular configuration on the root end of the post, increases the possibilities of stress concentration on the root canal walls. The concentration of these stresses will increase with increasing proximity of the pot to the involved root canal anatomy. 48


15. The length of the post relative to the entire length of the root : Generally, speaking the more that the root canal length is involved with a post, the more evenly distributed and the better resisted the stresses will be in the root canal walls. On the other hand, the apical one third of root canals usually have a very limited thickness of dentin walls. By placing the tip of the root pot there, with attendant possibilities of substantial stresses being concentrated at that tip, catastrophic failures become inevitable. As a rule from one half to two thirds of the root canal should encapsulate the post if the forces transmitted by the post are to be adequately dissipated. 16. Shape of the post in cross section relative to the shape of the post channel : A post should have a circumference that coincides with the post channel. Differences, e.g. rounded post in an oval post channel, will concentrate stresses at isolated locations in the root canal wall, possibly exceeding the local breaking point of the dentin. 17. Loose post in the post channel : Unconfined movements of a post within a root canal can exaggerate stresses in the root canal walls upto the fracture point of dentin. 18. Post ending apically at the junction of the clinical root with the clinical crown. This specific location is an area of appreciable stress concentration in normal, sound teeth. With root canal therapy, the strength of the area is decreased. If, in restoring a tooth, the apical end of the root canal post is placed at this junction, a when the clinical crown is far longer than the anatomical crown, three problems will be concentrated at these locations. Less strength than normal (due to a decrease in bulk resulting from the post’s taper) above normal stress concentration resulting from a reduced crown root ratio and maximum stresses from the apical end of the post, as it is, in effect, the end of a level. These stresses may approach the failure level of the dentin. 19. Central Slitting of Posts : Length wise slitting of a post involving one half or more of its length will make the post elastically collapsible in a lateral direction. If such a post is a threaded type, and during threading into the root excessive stresses are induced at the post dentin interface, instead of these stresses being consumed in detrimental deformation of dentin, they may be consumed, in part, to partially close the central slit. The rigidity of the two parts of the post at this area will keep the post engaged in dentin for retention, and their elasticity will reduce the stress concentration in that dentin. 20. Thread numbers and patterns : 49


Continuous threads from one end of a post to another create more stresses than interrupted threading. The greater that the spacing is between threads, the less will be the attendant stresses. The sharper that the threads are, the less will be the stresses. Circumferentially interrupted threading creates less stresses than continuous threading. The wider and more frequent that the interruptions are, the less will be the stresses. Interruptions (cross cuts) further serve to facilitate escape of debris during post insertion. The more extended that the threads are laterally, the more the surface interfacial contact with dentin will be and consequently, the higher the stresses. FORCES ACTING ON A CAST METAL AND PORCELAIN RESTORATIONS BIOMECHANICAL PRINCIPLES OF PREPARATIONS: The design and preparation of a tooth for a cast metal or porcelain restoration are governed by : 1) Preservation of tooth structures. 2) Retention and resistance forms 3) Structural durability of the restoration 4) Marginal integrity 5) Preservation of the periodontium. A restoration can meet its functional, biological and esthetic requirements if it remains firmly attached to the tooth. Its capability for retention and resistance must be great enough to withstand the dislodging forces it will encounter in function.

An

estimate as to the prevailing occlusal forces can be had by noting the degree of wear on the other teeth, firmness of the opposing teeth, thickness of the supporting base and the bulk of masticatory muscles. RETENTION AND RESISTANCE :  If a restoration does not remain firmly attached to the tooth, it cannot meet its functional, biological, and esthetic requirements.  Its capability for retention and resistance must be great enough to withstand the dislodging forces it will encounter in function.  Some estimate of the prevailing occlusal forces in an individual patient can be made by noting the degree of wear on the other teeth, the firmness of the opposing teeth, the thickness of the supporting bone, and the bulk of the masticatory muscles.

50


 It is the geometric form that determines the orientation of the tooth-restorations interfaces to the direction of forces encountered. This in turn determines whether the cement in a given area will be subjected to tension, shear, or compression.  All cements exhibit their greatest strength under compression. They are weakest under tension, with the value for shear strength lying in between.  Where a part of the restoration is pulled directly from the tooth, separation is prevented only by the relatively weak tensile strength and adhesive properties of the cement.  If the applied force is parallel with the cement film, movement at the cement-tooth and cement-metal interfaces is more effectively impeded by the minute projections of cement into the surface irregularities than when the force is tensile in nature.  Movement within the cement film itself is resisted by its relatively greater shear strength.  A force directed at an angle toward the restoration, has one component parallel with and one component perpendicular to the joined surfaces.

Thus the cement is

subjected to a combination of shear and compression, and movement is resisted ore effectively than if the forces were purely tensile or shear in nature.  A compressive force perpendicular to the cement film can produce no movement of the restoration relative to the tooth unless it is great enough to crush the cement or deform the structures. Such forces are seldom encountered in function.  Retention and resistance can be maximized by shaping the preparation so that as much of its surface as possible will experience compression and shear when the restoration is subjected to an unseating force. RETENTION :  It is the ability of the preparation to impede removal of the restoration along its path of insertion.  Under this condition, the cement bond subjected to tension and shear.  A restoration can experience withdrawing forces along its path of insertion during mastication of sticky foods.  There are 4 factors under the control of the operator during tooth preparation which influence retention. i)

Degree of taper

ii)

Total surface area of the cement film 51


iii)

Area of cement under shear

iv)

Roughness of the tooth surface

i) Degree of Taper :  The more nearly parallel the opposing walls of a preparation, the greater will be the retention. Thus retention decreases as taper increases.  However, in order to avoid undercuts and to allow complete seating of the restoration during cementation, the walls must have some taper.  An overall taper or angle of convergence of 6 degrees is considered as appropriate i.e. approximately 3 degrees being produced on each surface, external or internal, by the sides of a tapered instrument. ii) Total Surface Area of Cement Film :  The greater the surface area of cement film or the of the preparation, the greater the retention of the restoration.  The total surface are of preparation is influenced by the size of the tooth, the extent of coverage by the restoration and features such as grooves and boxes that are placed in the preparation. iii) Area under shear :  More important for retention than the total surface area is the area of cement that will experience shearing rather than tensile stress when the restoration is subjected to forces along the path of insertion.  To decrease the failure potential, it is essential to minimize tensile stress.  For the shear strength of the cement to be utilized, the preparation must have opposing walls, i.e. two surfaces of the preparation in separate planes must be nearly parallel with each other an the line of draw.  To obtain the greatest area of cement under shear, the direction in which a restoration can be removed must be limited to essentially one path.  Thus the addition of parallel sided grooves, limits the path of withdrawal to one direction, thereby reducing the possibility of dislodgment.  The length and width of the preparation is an important factors in retention : a long preparation as well as wider preparation has greater retention than does a shorter or a narrower preparation. 52


iv) Surface Roughness :  Adhesion of dental cements depends primarily on projections of the cement into microscopic irregularities on the surfaces to be joined. Therefore prepared tooth surface should not be highly polished. RESISTANCE :  it is the ability of the preparation to present dislodgment of the restoration by forces directed in an apical, oblique, or horizontal direction.  Where there is effective resistance, much of the cement film will be placed under compression, although some parts will be still be subjected to tension and shear.  If the cement film is disrupted by the restorations sliding or tipping on its preparation, the smallest fraction of a millimeter, the restoration is doomed through percolation of fluids, dissolution of the cement, and recurrent caries.  Resistance to sliding and tipping must be designed into a preparation by forming walls to block the anticipated movements. The more nearly perpendicular it lies to the force, the greater is the resistance provided by the supporting surface, because the cement will be compressed, and failures are less likely to occur form compression than shear. Leverage and Resistance :  The strongest forces encountered in function are apically directed and can produce tension and shear in the cement film only through leverage.  Leverage, probably the predominant factor in the dislodgment of cemented reiterations, occurs when the line of action of a force passes outside the supporting tooth structure, or when the structures flex.  If the line of action of force passes within the margin of a restoration, there will be no tipping of restoration. The margin on all sides of the restoration is supported by the preparation. The torque produced merely tends to seat the crown further.  If the line of action of force passes outside the margins of restoration the occlusal table of the restoration is wide, even a vertical force can pass outside the supported margin and produce destructive torque. This can also occur in crowns on tipped teeth.  A force applied to a cemented crown at an oblique angle can also produce a line of action which will pass outside the supporting tooth structure. 53


Preparation Length and Resistance :  The ability of a restoration to resist tipping depends not only on the preparation, but also on the magnitude of the torque  If two crowns of unequal length on two preparations of equal length, are subjected to identical forces, the longer crown is more likely to fail because the force on it acts through a longer lever arm. Resistance and Tooth Width :  A wide preparation has greater retention than a narrower one of equal height. Taper and Resistance :  The resisting area decreases as the preparation taper increases.  The walls of a short, wide preparation must be kept nearly parallel to achieve adequate resistance form. Rotation around a vertical axis :  When a crown is subjected to an eccentric horizontal force, movements of torque occur around a vertical as well as horizontal axis.  It is possible for a full crown on a cylindrical preparation to rotate enough to break the cement bond before may compressive resistance is encountered.  Geometric forms such as grooves or “wings” increase resistance by blocking rotation around a vertical axis.

Path of Insertion :  The path of insertion for posterior full and partial veneer crown is usually parallel with the long axis of the tooth.  A tipped tooth must be handled differently.  If the path of insertion on a tipped tooth parallels the long axis, the crown will be prevented form seating by those parts of the adjacent teeth which protrude into the path of insertion.  The correct path of insertion for such a tooth is perpendicular to the occlusal plane.

54


 All negative taper or undercut, must be eliminated or it will prevent the restoration form seating. Occlusal Reduction :  It should reflect the geometric inclined planes underlying the morphology of the finished crown.  Avoid creating steep planes with sharp angles, since these ca increase stress and hinder complete seating of the casting. To diminish stress, round the angels and avoid deep grooves in the centre of the occlusal surface. FORCES ACTING ON PARTIAL VENEER CROWN Since vertical occlusal forces have horizontal resultants, displacing forces have a tendency to tip or rotate a restoration, usually the tipping is in the lingual direction and rotation occurs mesiolingually or distolingually. Observe the proximal groove of a force ‘P’, directed lingually which is applied at the incisal edge. It will have a tendency to tip the casting out of the cavity, turning on the fulcrum ‘f’. The resistance to the displacement is offered by a rib of ‘Aa, which lies in the axial groove ‘ab’ also by that part of the axial wall lying lingually to the axial groove and encompasses area e and s. It is observed that the lingual wall of the mesial groove does not furnish any resistance to lingual displacement. Since the plane of this wall lies in the tipping path of arc ‘C’ for this reason, the incisal edge is usually prepared in a plane and not with a groove. When force P is applied mesiolingually to the marginal ridge of the upper central, the tendency is to rotate the restoration out of the mesiolabial wall of the cavity, point ‘F’ acting as the centre of rotation.

Obviously then the resistance to this rotational

displacement is furnished by the distoproximal groove and that portion of the proximal surface lying within the arcs R1 and R2. Analogous resting forces are present when acting forces are in a distolingual direction, then the distolabial wall act as to point of rotation, the mesio proximal wall and groove furnish the resistance to displacement. There is a horizontal force ‘P’ applied distally to the incisal area. This has a tendency to tip the casting mesially, rotating on point ‘F’. This displacing force is

55


resisted by the proximodistal groove and that portion of the proximodistal surface lying between R1 and R2. The same general displacing forces are present in the posterior partial crown as in the anterior partial crown, but the ability of the preparation to resist displacement is more favourable than in the anterior teeth. The occlusal forces may be occlusal, horizontal or any one component of force indicated by ‘P’. Minimum problems exerts when the force is vertical, for the resistance is equal and opposite. When the force tends to displace the casting lingually, it does so along the paths R1, R2 and R3 with its rotation center at point F. Resistance in this displacing force is furnished by the ribs of gold lying within the axial groove and by that portion of the proximal and surface extending lingually from the proximal groove and lying within the areas R 1, R2 and R3. In addition, the occlusal surface lying to plane R2 – R3 offers resistance when force ‘P’ is applied in mesiolingual direction, the tendency is to rotate the casting mesiodistally with the rotation centre being point F, the mesiobuccal wall. The resistance is this displacing force is developed by the rib of gold lying into distoproximal groove and by the portion of the casting coming in contact with the proximal surface lying between the areas R 1 and R2.

Additional

resistance to displacement is offered by the occlusal inclined planes R 3, R4 and R5. When force P is applied in the distal direction, the tendency is to rotate the casting occlusally, with its dislodgment along the areas R1 and R2 with F serving as the point of fulcrum. The resistance to this displacing force is furnished by the rib of gold lying in the mesio proximal groove and also by the buccal and mesioproximal walls lying within the areas R1 and R2. FULL COVERAGE CROWN (ANTERIOR PORCELAIN JACKET CROWN) : As mentioned earlier the anticipated forces place don the restoration cannot be taken lightly. Incisal Reduction : In keeping with the rule that planes are placed at right angles to the applied forces, the incisal edges of the upper anterior teeth slopes lingually whereas that of the lower teeth slopes labially. The incisal reduction should be adequate to ensure clearance in protrusive movements of the mandible and permit satisfactory esthetics and enhance optimal function. 56


-2-5o

parallelism is desired with proximal reduction.

After the removal of

enamel, the labial gingival termination is made at or just above the crest of the labial gingiva. Another retention area is immediately below the cingulum. More than any other restoration, porcelain jacket crown depends on its tooth preparation. Tooth support is more critical to fracture resistance of the restoration than is the bulk of porcelain. The crescent moon fracture seen on the labial cervical region is a direct result of inadequate preparation length. Incisal reduction recommended ranges from 1.5 to 2 mm. For esthetic result, it is best to reduce the incisal edge by 2 mm to the level of depth orientation grooves. Any greater reduction will increase the stress on the facial surface which can result on the facial half moon fracture. The plane of the reduced incisal surface should parallel with the former incisal surface and more importantly perpendicular to the forces of mastication. Failure to create this near incisolingual bevel (45 oC) will produce excessive stress at the shoulder. (Shoulder 0.5 – 0.7 mm) Plane of shoulder is perpendicular to long axis of the tooth. If it is at obtuse angle possibilities of fracture at the cervical region is more. The mesioaxial and distoaxial walls are more favourable for developing parallelism to frictional resistance. The buccal and lingual surfaces due to their natural contour, do not afford the same opportunity for paralleling walls. The occlusal planes are reproduced at a lower level.

These planes help considerably to resist stability or

displacement. When necessary and where indicated, additional resistance form may be obtained by placing pins, grooves or boxes in any available surface where the length of this surface is adequate. If an occlusal force is directed ‘P’ buccally, the lingual portion of the crown tends to be dislodged occlusally and buccally with the point of rotation situation at ‘F’, this displacement is resisted by that lingual surface when it lies outside the arc R 1. On the other hand, an occlusal force P2 directed lingually is likely to dislodge the crown lingually, since the buccal wall of the preparation lies within the tipping path of the arc H2. When such a condition prevails either in the buccal or lingual wall, resistance to displacement can be developed by placement of 2 proximal grooves at G in the mesial and distal surface as shown in the figure and it is evident that an occlusal force P directed mesially will not dislodge the crown, since the distal wall of the preparation lies outside the tipping path of arc R1. 57


CONCLUSION Optimal functional capacity and stability of occlusal relationships are major considerations in every phase of restorative dentistry. The first phase objective of a cavity preparation design is to establish the best possible configuration that can cope with the distribution and magnitude of stresses in tooth structure and restoration without failure. To design such a configuration one must first comprehend the nature of loading and resistance to such loading. Restoration not only mechanically replace the lost part but, acts as a medium through which physical and mechanical forces are transmitted to the tooth and investing tissues. Each tooth ahs its own stress patterns. A thorough knowledge in dental materials is necessary to understand the physical properties including their response to stress. Before any restorative procedure, always check location of the tooth in the arch and the patients occlusal relationship. The functional, non functional cuspal elements should be noted bye examining the involved teeth during static and functional mandibular movements The operator can then recognize the nature of stresses that can be expected in the remaining tooth structure especially the occluding ones. From these information’s obtained during the patient evaluation, the operator must envision the restoration replacing lost tooth structure being subjected to functional loading and then try to plan the best tooth preparation to both retain this restoration and make it resistant to these loads.

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