COLLEGE OF DENTAL SCIENCES DEPARTMENT OF CONSERVATIVE DENTISTRY AND ENDODONTICS
“FUNDAMENTALS OF CAVITY PREPARATIONS ”
Presented By : -
Dr. Sharno Mathai Varghese
CONTENTS INTRODUCTION DEFINITION NEED FOR RESTORATIONS OBJECTIVES OF CAVITY PREPARATION FACTORS AFFECTING CAVITY PREPARATION CARIES TERMINOLOGY TOOTH PREPARATION TERMINOLOGY CLASSIFICATION OF TOOTH PREPARATION INITIAL TOOTH PREPARATION STAGE o OUTLINE FORM AND INITIAL DEPTH o PRIMARY RESISTANCE FORM o PRIMARY RETENTION FORM o CONVENIENCE FORM FINAL TOOTH PREPARATION STAGE o REMOVAL OF ANY REMAINING INFECTED DENTIN /OLD RESTORATIVE MATERIAL o PULP PROTECTION o SECONDARY RESISTANCE AND RETENTION FORMS o PROCEDURES FOR FINISHING EXTERNAL WALLS o CLEANING, INSPECTING AND SEALING ADDITIONAL CONCEPTS IN TOOTH PREPARATION o AMALGAM RESTORATIONS o COMPLETE RESTORATIONS CONCLUSION
INTRODUCTION : The basic principles governing the design of cavities and steps in their preparation was first suggested by American Dentist and teacher Dr.G.V.Black in the first decade of the last century. He based these principles on what was known at time about the natural history of caries and the restorative material available. The wisdom of his work was such that it remained unchallenged for more than half a century but now with new materials, a better understanding of caries and research findings into the success of various restorative procedure, his principles have been largely revised. Modification and rearrangement of these original principles have been largely revised. DEFINITION OF CAVITY PREPARATIONS : â€œMechanical alteration of a defective, injured or diseased tooth in order to best receive a restorative material which will reestablish a healthy state for the tooth including esthetic corrections where indicated along with normal form and functionâ€?. NEED FOR RESTORATIONS : Teeth needs restorative intervention for a variety of reasons which are as follows ; 1. Repair of tooth after destruction from carious lesions. 2. Replacement / repair of restorations with serious defects such as improper proximal contacts, gingival excess, poor esthetics etc. 3. Restoration of proper form and function of fractured teeth. 4. Restoration of form and function as a result of congenital malformations 5. To fulfill the esthetic demands 6. Restoration for preventive measures OBJECTIVES OF CAVITY PREPARATIONS : 1. Removal of all the defects and give the necessary protection to the pulp. 2. Location of margins of the restorations as conservative as possible.
3. Form the cavity so that under forces of mastication the tooth or the restoration or both will not fracture and the restoration will not be displaced. 4. Esthetic and functional placement of a restorative material. FACTORS AFFECTING CAVITY PREPARATIONS
1. Diagnosis : 1. Prior to any restorative procedure a complete and through diagnosis must be made assessment of both pulpal and periodontal status will influence the potential treatment of tooth especially in terms of the choice of restorative materials as well as the design of cavity preparation. 2. Assessment of occlusal relationships must be made. 3. Patient concern for esthetics should be considered 4. Other planned treatment should be considered for e.g. such as when tooth is used as an abutment for FPD or RPD, design of restoration is altered to accommodate maximum effectiveness of that prosthesis. 5. Risk assessment find out with dietary habits.
DMFT index and
microbiological examination. 2. Knowledge of Dental Anatomy : Direction of enamel rods, thickness of enamel and dentin, position of pulp relationship of tooth to the investing tissue. PATIENTS FACTORS : 1) Patients knowledge and appreciation of good Dental Health Influences the choice of restorative material. 2) Patients economic status
3) Age of the patient Elderly patient who are physically or medically compromised require special positioning for restorative procedure plus less stressful and shorter appointments. CARIES TERMINOLOGY : Caries can be located according to location, extend and rate. According to Location : 1) Primary caries : Original carious lesion of the tooth. They originate basically at three sites â€“ pit and fissure, smooth surface, root surface. 2) Forward caries : Is seen when the caries cone in enamel is larger as atleast the same size as that in dentin. 3) Backward caries- when spread of caries along DEJ exceeds the caries in contiguous enamel, caries extends into this enamel from the function. 4) Residual caries - Caries that remains in a completed tooth preparation whether by operator intention or by accident. 5) Secondary caries : Occurs at junction of a restoration and tooth and may progress sunder the restoration. According to Extend : 1) Incipient caries (Reversible) : It is the first evidence of caries activity in the enamel. 2) Cavitated caries (Irreversible) : The enamel surface is broken, and usually the lesion has advanced into dentin. According to Rate : 1) Acute caries (Rampant caries) : In this, the disease is rapid in damaging the tooth. 2) Chronic caries (Arrested caries) : The slow rate results form periods when demineralized tooth structure is almost remineralized. Enameloplasty :
It is the removal of a shallow, enamel developmental fissure or patient to create a smooth, saucer shaped surface that is self cleansing or easily cleaned. Prophylactic Odontotomy : It is characterized by minimally preparing and filling with amalgam any pits and fissures to prevent caries originating in these sites. It is no longer advocated as a preventive measure. Affected Dentin And Infected Dentin : Affected dentin has no bacteria, is reversibly denatured, remineralizable and should be preserved. Infected dentin has bacteria present and the collagen is irreversibly denatured. It is not remineralizable and must be removed. TOOTH PREPARATION TERMINOLOGY : 1) Simple, tooth preparation – Only one tooth surface is involved. 2) Compound tooth preparation – Only two tooth surface is involved. 3) Complex tooth preparation – Involves three or more surfaces. CLASSIFICATION
G.V.Black) 1) Class I Restorations : Restoration on occlusal surface of premolars and molars. Restorations on occlusal two thirds of facial and lingual surface of molars. (8 line angles, 4 point angles) Restorations on lingual surface of maxillary incisors (6 point angles, 11 line angles). 2) Class II restorations : Restorations on proximal surface of posterior teeth. 3) Class III restorations : Restorations on proximal surface of anterior teeth that do not involve the incisal angle (6 line angles, 3 point angles).
4) Class IV restorations : Restorations on proximal surface of anterior teeth that involve the incisal angle (11 line angles, 6 point angles). 5) Class V restorations : Restorations on the gingival third of facial or lingual surfaces of all teeth (except pit and fissure lesions) (8 line angles, 4 point angles). Class VI Restorations : Restorations on the incisal edge of anterior teeth or the occlusal cusp heights of posterior teeth. COMPONENTS OF A CAVITY PREPARATION : Cavity Wall
Cavity Preparation Angle
Miscellaneous Component 1. Retention groove
1. Enamel wall 2. Dentin wall
1. Axial 2. Pulpal
3. Bevel-short Long and Full
1. Internal 2. External A) WALLS : 1. Cavity wall External
Floor / Seat One of the enclosing sides of a prepared cavity (it takes the name of the surface of the tooth adjoining the surface involved towards which it is placed). EXTERNAL WALL :
An external wall is a prepared cavity surface that extends to the external tooth surface and such a wall takes the name of the tooth surface that the wall is towards. Enamel Wall : Wall of the prepared cavity that is made up of enamel. Dentin Wall : The portion of the will of a prepared cavity that is made tip of dentin. INTERNAL WALL : An internal wall is a prepared cavity surface that does not extend to the external tooth surface. a) Axial wall : it is an internal wall parallel with the long axis of the tooth. b) Pulpal wall : It is an internal wall that is perpendicular to the long axis of the tooth and occlusal to the pulp. CAVITY PREPARATION ANGLES : The junction of two or more prepared cavity surfaces is referred to as an angle which includes line angles and point angles. a) Line angle : It is the junction of two planar surfaces of different orientation along a line, which is again classified as internal line angle and an external line angle. i)
Internal line angle : Is a line angle whose apex points into the tooth e.g. FP.
External line angle : Is a line angle whose apex points away from the tooth e.g. ap.
b) Point angle : It is a junction of 3 planal surfaces of different orientation e.g. mFP (designated by combining the names of the walls forming angels). c) Cavo surface angle / Cavo surface margins : The cavo surface angle is the angle of the tooth structure formed by the junction of a prepared cavity wall and the external surface of the tooth.
RETENTION AND RESISTANCE FORM : 1) Undercut : Portion of prepared cavity confined by walls which coverage towards the surface. 2) Retention groove : Linear channel within a cavity preparation. 3) Dovetail : Widened or fanned out portion of a prepared cavity established to increase the resistance and retention form. Miscellaneous Components : 1) Margins : Junction of the wall of the cavity with the surface of the tooth 2) Bevel : Inclination that one surface makes with another when not at right angle or in cavity preparation a cut that procedure an angle other than 90o with a cavity wall. a) Short bevel : Bevel involving not more than external one third of a cavity wall. b) Long bevel : Bevel involving more than external 1/3 but not more than external 2/3 of a cavity wall. c) Full bevel : Bevel which involves the entire wall STAGES AND STEPS IN CAVITY PREPARATION : The stages and steps in cavity preparation are as follows ; INITIAL CAVITY PREPARATION STAGE Step-1 Outline form and initial depth
Step-2 Primary Resistance form Step-3 Primary Retention form Step-4 Convenience form
FINAL CAVITY PREPARATION STAGE Step-5 Removal of any remaining enamel pit fissure and or infected dentin and /or old restorative material if indicated. Step-6 Pulp protection Step-7 Secondary resistance and retention form Step-8 Procedure for finishing external walls Step-9 Final procedure; cleaning inspecting; varnishing conditioning
INITIAL CAVITY PREPARATION STAGE : Definition : Initial cavity preparation is the extension and initial design of the external walls of the preparation at a specified, limited depth so as to provide access to the cavity or defect, reach sound tooth structure, resist fracture of the tooth or restorative material form masticatory forces principally directed with long axis of the tooth and retain the restorative material in the tooth. STEP 1 : OUTLINE FORM / INITIAL DEPTH : a) Definition : The outline forms means placing the cavity margins in the position they will occupy in the final preparation except for finishing enamel walls and margins. Initial depth 0.2 â€“ 0.5 mm below D.E. junction (0.5 when restoring with direct gold). 0.2 â€“ 0.8 mm into dentin for smooth surface caries. If there is need for additional deepening of the preparation in the assess of excavation of any remaining faulty tooth structure faulty old restorative or infected dentin it is carried out during the final stage of cavity preparations. b) Principles : There are 3 general principles on which outline from is established regardless of the type of cavity being prepared. 1)
All friable / or weakened enamel should be removed.
All faults should be included
All margins should be placed in a position to afford good finishing of the margins of the restoration i.e. extension of the margins of the preparation far enough on the tooth surface so that the margins of the preparations will 11
be located on finishable, self cleansable area because all the restorations will be surrounded by a microleakage space between itself and adjacent tooth structure ranging between 20-120 microns in width which can accommodate micro organisms and food substance thus predisposing to recurrent decay. Thus this space should be placed in the areas that are easily cleansable by natural / artificial means. This principle is called as “extension for prevention”. c) Factors : * These factors will affect the outline form of the proposed cavity preparation and will dictate the extensions, these are as follows 1. Extent of the carious lesions, defect or faulty old restorations. • Will affect the outline form because the objective is to extend preparation to sound tooth structure except in a pulpal direction. 2. Esthetic considerations • Will not only affect the choice of restorative material but also the design of cavity preparation in an effort to maximize the esthetic result of the restoration. 3. Occlusal relationship • Determines the outline form by avoiding the placement of cavity margins (outline) in an area of heavy occlusal contacts such as centric holding area. 4. Proximal tooth contour 5. Cavosurface margin configurations • Restorative materials which are more effective when having beveled margins will require cavity preparation outline form that must anticipate the final Cavo surface position and form. d) Features :
There are six specific, typical features of establishing outline forms initial depth. There are : 1) Preserving cuspal strength 2) Preserving marginal ridge strength 3) Minimizing the facio-lingual extension 4) Using enameloplasty 5) Connecting two close (less than 0.5 mm apart) faults or cavity preparations 6) Restricting the depth of the preparations into dentin to a maximum of 0.2 mm for pit and fissure caries and 0.2 to 0.8 mm for the axial wall of smooth surface caries. e) Rules : => Rules to follow in establishing the outline form for pit and fissure cavities. 1) Extension of the cavity margin until sound tooth structure is obtained. There should not be any weakened or unsupported enamel. 2) Avoid terminating the margins on extreme eminence such as cusp heights or ridge crest. 3) Consider the cusp capping : Rule for cusp capping, if the extension form a primary groove towards the cusp tip is no more than half the distance then no cusp capping, if this extension is from 1/2 to 2/3 the distance, then consider cusp capping, If
the extension is more than 2/3 the
distance then cusp capping is mandatory. 4) Use of enameloplasty when pit or groove does not penetrate more than 1/3 the thickness of the enamel.
Enameloplasty is a procedure of
reshaping / rounding/saucerization of the enamel surface with suitable rotary cutting instrument (it does not extend the outline form and restorative material is not placed in recontoured area thus the thickness of restorative material at enameloplasty margin is decreased. 5) When pit and fissure involve more than 12 of enamel thickness, extend the cavity margins to include all of them.
6) To be as conservative as possible the preparation of an occlusal surface pit and fissure cavity is first prepared to a depth of 1.5 mm as measured at the central fissure. (Restrict the depth of the preparation to a maximum of 0.2 m into dentin and if gold it should be 0.5 mm). However if the amount of pit/fissure remaining is greater than 50% of the pulpal floor the entire floor is deepened to maximum initial depth of 0.2 mm into dentin. Thus the actual depth of the preparation varies form 1.5 mm depending on thickness of enamel and steepness of cusp inclines. 7) When two pits fissure cavities have less than 0.5 mm of sound tooth structure between them they should be joined to eliminate a weak enamel wall between them. 8) Extension of outline form to provide sufficient access for proper cavity preparation, restoration placement and finishing procedure. Rules Governing Outline form and initial depth for smooth surface cavities : Smooth surface cavities are in two different locations. Proximal surfaces
Gingival portion of the facial and Ig surface
For proximal surface cavities (Class II, III, IV) : 1) Extension of cavity margins until sound tooth structure is obtained, all unsupported to weakened enamel should be removed. 2) Avoid terminating the margins on extreme eminence such as cusp heights or ridge crest. 3) Extent the margins to allow sufficient access for proper manipulative procedures. 4) Restriction of axial wall pulpal depth of proximal preparation when it is in crown 0.5 – 0.6 mm and on root 0.75 – 0.8 mm = (0.2 – 0.8 mm).
5) Extend the gingival margins of the cavities apically of the contact to provide a minimum clearance of 0.5 mm between gingival margins and adjacent tooth and the gingival margins or finish line on the proximal tooth surface should be 0.5 – 1 mm apical to the crest of healthy free gingiva with in the gingival crevices, it should not extend to the bottom of crevice because ; a) The alkalinity of the crevicular fluid can neutralize acids produced from plaque activity an d b) The knife edge relationship of healthy free gingiva to the adjacent tooth surface will discourage food accumulation on adjacent restored surface occlusal to the sulcus for considerable period during after food ingestion. 6) Extension of facial and lingual margins in proximal cavity preparation into the respective embrasures to provide specified clearance between the prepared margins adjacent within order to place the margins in self cleansable area. When extending the proximal surface incisally in Class III preparation it is acceptable to position the incisal margins in the area of contact especially when an esthetic restorative material is used. Gingival Portion of Facial and Lingual Surface : 1) Outline form is governed by the extend of the lesion, except Pulpally so extension mesially, lingually, distally and occlusally is limited to that when sound tooth structure is reached. 2) Depth is no deeper than 0.8 – 1.25 mm Pulpally. • Lesser axial wall depth (0.8 mm) at Gg wall with out an enamel portion (i.e. the margin on root. • The axial wall depth at occlusal wall is 0.5 mm in dentin and remaining in enamel. STEP 2 : PRIMARY RESISTANCE FORM : DEFINITION : 15
Primary resistance form may be defined as that shape and placement of the cavity walls that best enables both the restoration and the tooth to withstand, without fracture, masticatory forces delivered principally in the long axis of the tooth. Principles : The fundamental principles involved in obtaining primary resistance form follow : 1) Box shape with relatively flat floors. Flat floor prevents restoration movement where as rounded pulpal floor is conductive to rocking action of restoration â€“ producing a wedging force, resulting in shearing of tooth structure. 2) Restrict the extension of external walls to allow sufficient dentin support for strong cusp and ridges (resistance against oblique forces and forces in long axis). 3) Straight rounding / coving of internal line angles reduces stress concentration in tooth structure (rounding of internal line angles reduces stress on tooth thus resistance to # of tooth, increased rounding of external faced angle reduces stress on porcelain and amalgam thus resistance to fracture of restoration increases). 4) Consider cusp capping for weak cusp according to rule. 5) Placement of enough thickness of restorative material to prevent its fracture under load. The minimal occlusal thickness for amalgam for appropriate resistance to fracture is 1.5 mm, cast metal = 1-2 mm and porcelain = 2 mm. Factors : The need to develop resistance form in a cavity preparation is a result of several factors, which are as follows : a) Occlusal contact : the greater the occlusal force and contacts, the greater is the potential for future fracture. (The further posterior
the tooth, the greater is the effective masticatory forces since the tooth is closer to the condyle head). b) Amount of remaining tooth structure also impacts the need and type of resistance form. e.g. Very large teeth even though extensively involved with caries or defects may require less resistance from consideration because remaining tooth structure is still bulky and strong enough to resist fracture. c) Type of restorative material used Amalgam : 1.5 mm for adequate strength and longevity. Cast metal : 2 mm Composites : Dimensional needs of composites are more dependent on the occlusal wear potential of the restored area. In posterior teeth thickness requirement is more than the anterior teeth. Features : The design features of cavity preparation that enhances primary resistance form are as following : 1) Relatively flat floors : If large excavation site of infected dentin is present incorporate at least 3 seats on sound dentin so that restorative material will have stable contact with tooth so the occlusal forces directed parallel to the tooth long axis will not cause rocking of restoration. 2) Box shape 3) Inclusion of weakened tooth structure 4) Preservation of cusp and marginal ridges 5) Rounded internal line angles 6) Adequate thickness of restorative materials 7) Seats on sound dentin periphery to excavation site 8) Reduction of cusp for capping when indicated.
STEP 3 : PRIMARY RETENTION FORM : Definition : Primary retention form is that shape or form of the prepared cavity that resists displacement or removal of the restoration from tipping or lifting forces. Principles : Sincere tension needs are related to the restorative material used, the principles of primary retention form may vary depending on the material used. For Amalgam : a) Convergence of external cavity walls occlusally(Class I and Class II) : So once the amalgam is placed in the cavity and hardens, it cannot come out without some type of fracture occurring. b) Occlusal dovetail which aids in preventing the tipping of the restoration by occlusal forces. c) Adhesive systems for bonding amalgam to tooth structure. d) In other preparation for amalgam such as Class III and V, the external wall diverge outward to provide strong enamel margins therefore retention grooves and covers are prepared in dentin. For Composite : Retention by mechanical bond that develops between the material and the conditioned / prepared tooth structure (enamel is etched by an acid and the dentin is conditioned for a dentin bonding agents) so bevel enamel margins are kept to increase the surface area to be etched. For Cast Metals : 1) Parallel, vertical and longitudinal walls to provide retention of the casting in the tooth. 2) Small angle of divergence 2-5o per wall is placed which will enhance the retention form. (Retention is developed by frictional resistance and
mechanical locking of the cement into the minute irregularities of both the casting and the cavity wall to counter act the pull of sticky food). For Gold Foil : Retention is from elastic compression developed in the dentin as a result of condensation of the foil. MEANS OF RETENTION : -
Inverted truncated cone
Frictional Retention : Depends on 4 factors a) Surface area of contact between tooth and restoration More surface area more retention. b) Opposing walls More opposing walls more retention c) Parallelism and non parallelism Higher degree of parallelism increases retention d) Proximity Elastic Deformation of dentin : Changing the position of dentinal walls and floors microscopically by using condensation energy with in dentin limit can add more gripping action by the tooth on the restorative material. This occurs when dentin regains its original position while the restorative material remains rigid, thereby completely obliterating any remaining space in the cavity preparation. STEP 4 : CONVENIENCE FORM : DEFINITION :
Is that shape or form of the cavity that provides for adequate observation, accessibility and ease of operation in preparing and restoring the cavity. Modifications in tooth preparation for convenience form : Modification includes flaring some walls more than otherwise necessary for resistance and retention form such as divergences of vertical walls of cavity preparation for Class II cast restoration. Placement of convenience points for starting the foil condensation. Extending proximal preparations beyond proximal contacts. Separation : Done by wedging of teeth makes interproximal instrumentation convenient. STEP 5 : FINAL CAVITY PREPARATION STAGE : Removal of any remaining enamel pit or fissure and or infected dentin or old restorative materials if indicated. DEFINITION : It is the elimination of any infected carious tooth structure or faulty restorative material left in the tooth after initial cavity preparation. Any old restorative material should be removed if any of the following conditions are present. 1. The old material may affect negatively the esthetic result of the new restoration. 2. The old material may compromise the amount of anticipated needed retention. 3. There is radiographic evidence of caries under the old restorative material. 4. The tooth was symptomatic preoperatively 5. The periphery of the remaining old restorative material is not intake. If none of these conditions are present the operator may elect to leave the remaining old restorative material to serve as a liner or base rather than risk
unnecessary excavation nearer to the pulp which may result in exposure or pulpal irritation. FOR DENTIN : -
The exception to the removal of infected carious dentin is when it has been decided to perform an indirect pulp capping.
Its accepted and appropriate practice to allow affected dentin to remain in a prepared tooth.
It is not acceptable to leave carious dentin at D.E. junction.
Technique : When a pulpal or axial wall has been established at the proper initial cavity preparation position and a small amount of infected carious material remains, only this material should be removed, leaving a rounded, concave area in the wall and floor, thus placing the pulpal floor at more than one level. The first level will be ideal depth of 1.5 mm and other will be at caries cone level. This shallow (initial depth i.e. 1 mm) level will create flat pulpal floor at definite angle to surrounding wall thus resist the occlusal forces and laterally locking the restoration without impinging on pulp this placement of second seat at caries cone level is called as ledge it can be (1) Circumferential, (2) Interrupted or (3) Opposing. CARIES CONTROL TECHNIQUE : When patient is having numerous teeth with extensive caries in one sitting or appointment, infected dentin is removed from several teeth and temporary restorations are placed and then individual teeth are restored as definitively planned. This procedure stops the progress of caries and is often referred to as the caries control technique. If the decayâ€™s soft removal should be done with spoon excavators by flaking up the caries around the periphery of the infected mass and peeling it off in layers.
If the decayed dentin is hard, the excavator may not be sufficient to remove the diseased tissue, so a large round carbide bur revolving slowly should be moved in burnishing strokes from the peripheries of cavity preparation to the center. These strokes should be done with minimal pulpal or axial pressure and with a water coolant in order to minimize thermal irritation to the near by pulp tissue. Removal of remaining old restorative material; when indicated is accomplished with use of a round carbide bur, at slow speed with air or air water coolant. The water spray along with high volume evacuation is used when removing old amalgam material to reduce the amount of mercury vapor. STEP 6 : FROM
INSULTS such as
CHEMICAL 1) Some ingredients of various materials. 2) Thermal changes conducted through restorative material 3) Forces transmitted through materials to the dentin 4) Galvanic shock 5) Ingress of noxious products and bacteria through microleakage. For pulp protection traditional liners or bases are used either to protect the pulp or to aid pulpal recovery or both. When the thickness of remaining dentin is less than 2 mm, heat generated by injudicious cutting can result in a pulpal burn lesion abscess formation death of pulp. Thus a water or air water spray coolant must be used with the high speed rotary instrument.
ď‚§ If the remaining dentin thickness is 1.5 mm or more and the cutting was done atraumatically using high speed with water or air water spray, the pulp is not irritated enough to form replacement odontoblasts and therefore no reparative dentin is formed to seal the pulpal side of dead tracts. Thus it is more important to place a liner or bases to protect the pulp.
Liners : Are those volatile or aqueous suspensions or dispersions of zinc oxide or calcium hydroxide that can be applied to a cavity surface in a relatively thin film and are used to effect a particular pulpal response. Liners Provides : 1. A barrier which protects the dentin from noxious agents from either the restorative material or oral fluids. 2. Electric insulation 3. Thermal protection Bases : Bases are considered to be those cements commonly used in thicker dimensions beneath permanent restoration to provide for mechanical, chemical and thermal protection of the pulp. Example are ; 1. Zinc phosphate 2. Zinc oxide eugenol 3. Calcium hydroxide 4. Polycarboxylate 5. Glass ionomer
If the removal of infected dentin does not extend deeper than 1 mm from initially prepared pulpal or axial wall.
NO LINER IS INDICATED.
excavation extends into or very close to the pulpal tissue, a calcium hydroxide liner is selected in order to stimulate reparative dentin, if the excavation depth is between above two eg. Zn oxide eugenol liner is selected (except for composite restoration where it may impide the polymerization process) to provide a palliative sedative pulpal response. As a general rule it is desirable to have atleast a 0.5 – 0.75 thickness of base dimension of bulk between the pulp and a metallic restorative material. This bulk may include remaining dentin, liner, or bases. The ability of calcium hydroxide to stimulate the formation of reparative dentin when its in contact with pulpal tissue makes it the material of choice for application to very deep excavation and known pulpal exposures. Liners and bases in exposure areas should be applied with out pressure, Atleast a 1 mm thickness of calcium hydroxide is placed over near or an actual exposure which is than over laid with a base. In deep excavation where no exposure of suspension of exposure exist, Zn oxide eugenol is used for its mildly anesthetic effect on the pulp. For composite restorations which are thermal insulators and passively inserted, liner of calcium hydroxide is indicated only when there is a pulpal exposure of the excavation is judged to be within 0.5 mm of the pulp. Cast restorations : In cavity preparation for casting, deeply excavated areas in preparation must be covered with suitable retained liners or bases materials that will withstand the forces. Zn ph. Glass ionomer and polycarboxylate cements serves this purpose. Cavity Varnishes : It is a solution liners which seals most of the dentinal tubules and is placed on all cavity preparation walls for amalgam and on dentinal walls of
cavity preparation for cast gold but not used for composites, (it prevents penetration of materials into the dentin and helps to prevent microleakage and reduces post operative sensitivity by reducing the infiltration of fluids and salivary components at the margins of newly placed restoration. For Amalgam : Two coats should be applied to the prepared surface in shallow excavation is the only material of choice. For cast gold : Two coats on dentin surface reduces pulpal irritation from luting cements. For Composite : Should not be used because solvent in varnish may react with or soften the resin component in the composite and thus affecting polymerization or free monomer of resin may dissolve varnish film and rendering it ineffective. STEP 7 : SECONDARY RESISTANCE AND RETENTION FORMS : Secondary resistance and retention forms are of 2 types : a. Mechanical features b. Cavity wall conditioning features 1. Mechanical features includes : • Retention / locks, grooves, and coves • Longitudinally oriented provides retention to proximal portions of cavity preparations. Locks are for amalgam, they increases retention of the proximal portion against movement proximally due to creep and are believed to increase the resistance form of the restoration against fracture at the junction of the proximal and occlusal portions. • Grooves of cast metal restorations.
• Horizontally oriented grooves – for Class III and Class V amalgam. • Root surface cavity preparation for composites. • These are placed undercuts for the incisal retention of Class III amalgams and for Class V and occasionally for facilitating the start of insertion of certain gold foil restorations. • Retention cones – Undercuts placed for incisal retention of Class III amalgams, occlusal portion of some amalgam restorations, some Class V amalgams. Groove Extensions : Obtained by extending the cavity preparation for molars on to the facial and lingual surface to include the facial and lingual grooves mainly used for cast metal restorations (results in parallel wall retention) also enhances resistance form due to envelopment. Skirts : Mainly used in cast gold restorations. In which extension of preparation is done around all the transitional longitudinal angles of tooth, adds retention form by opposed longitudinal walls and resistance form by enveloping the tooth thus resisting fracture of the remaining tooth from occlusal forces. Beveled Enamel Margins : Used for cast gold or composites. For cast metal retention form is improved when there are opposing bevels and provides better junctional relationship between metal and the tooth. In case of composite restoration, it increase both the surface area of etchable enamel and maximize the effectiveness of the bond by etching more enamel rods.
Pins, Slots, Steps and Amalgam Pins : Used to increase resistance and retention form for amalgam. 2. Cavity Wall conditioning features Enamel Wall Conditioning Conditioned for bonded restorations such as porcelain, composites, amalgam or give (condition consist of etching the enamel by an appropriate acids resulting in microscopic undercuts in which bonding is mechanical bond.
Dentin Wall Conditioning Conditioned for bonded restoration such as porcelain, composites, amalgam or GIC (condition consist of etching the enamel by an appropriate acids resulting in microscopic undercuts in which bonding material is mechanically bonds).
STEP 8 : PROCEEDINGS FOR FINISHING THE EXTERNAL WALLS OF CAVITY PREPARATION : DEFINITION : It is the further development when indicated of a specific cavo surface design and degree of smoothness that produces the maximum effectiveness of the restorative material being used. Objectives : 1. To create he best marginal seal possible between the restorative material and the tooth structure. 2. To afford a smooth marginal junction 3. To provide maximum strength of both the tooth and the restoration at and near the margin. Factors : 1. Direction of enamel rods : Enamel rods radiate from the dentin enamel junction to the external surface of the enamel and are perpendicular to the tooth surface, all rods extends full length from the dentin to the enamel surface. The rods converge towards the center of development grooves i.e. from the dentino enamel 27
junction towards concave enamel surface and diverge towards the height of cusp and ridges i.e. diverge outwardly toward convex surface. In gingival third rods incline slightly apically. Finishing of enamel wall should be such that the cavity should have strongest enamel margin (i.e. margin which is composed of full length of enamel rods that are supported on the cavity side by shorter enamel rods all of which extend to sound dentin) thus increasing the strength of enamel margin. 2. Support enamel rods both at the Dent-E junction and laterally on cavity side. 3. Type of restorative material used 4. Location of margin 5. Degree of smoothness desired Features : There are two primary features to the finishing of the external walls. 1. Design of cavo surface angle. 2. Degree of smoothness of the walls. 1. Design of Cavosurface Angle : It depends on type of restorative material used. For amalgam : Because of low edge strength or friability of amalgam cavosurface of angle of 90o produces maximal strength for both the amalgams tooth and prevents fracture. For cast restorations and composites : Beveling the external walls used for intra coronal cast gold and composite restoration. Beveling can serve 4 useful purposes in the cavity preparation for casting. 1. Produces stronger enamel margins. 2. Permit marginal seal in slightly undersized casting 3. Provides marginal metal that is more easily burnished and adapted.
4. Assists in adaptation of gingival margins of cating that fail to seat by a very slight amount. For casting, bevel should produce a cavosurface angle that will result in 30-40 o marginal metal. Gingival Margin : For amalgam in Class II restoration 15-20 o bevel on the enamel portion of the wall in order to remove unsupported enamel rods (because of gingival orientation of enamel rods). For casting, 30o will result in sliding, lap fit that improves adaptation of metal to this margin. Degree of Smoothness : It is dictated primarily by the restorative material being used. Inlay or onlay preparation requires a smooth surface to permit undistorted impressions and close adaptation of the casting to the enamel surface. With gold foil, amalgam and composite very smooth preparation wall is not as desirable as cast gold. STEP
VARNISHING, CONDITIONING : DEFINITION : Cleaning or debridement is the act of freeing the preparation walls and margins from the objects that may interfere with the proper adaptability and behaviour of the restorative material. There are 3 main objectives for debriding the preparations. a) Freeing of all preparation walls, floors and margins from enamel and dentin chips resulting form excavation and grinding.
b) Drying the preparation walls, floor and margins from any moisture, saliva, blood, exudates. c) Sterilization of preparation walls and floors. Methods : 1. Water, air or combination of air water jets, use of air water syringe or the water and air accompanying any rotary cutting with a high speed handpiece will be efficient in removing gross debris. 2. Dry cotton pellet (best way no chance of desiccation). 3. Cavity cleansers – Solution of low concentration of Citric acid + ascorbic and acetic acid (1-10%) followed by long period of water jet only used in shallow cavities (can irritate P-D organ). 4. Scarping of walls, floors and margins with sharp instruments. Whichever debridement technique used, microscopic layer of dentinal smear will be always present on cut dentin and can be eliminated with a 10o EDTA. ADDITIONAL CONCEPTS IN CAVITY PREPARATIONS FOR AMALGAM RESTORATION : a. Box Only Cavity Preparation : Given for those posterior teeth in which proximal surface requires restoration but occlusal surface does not. Proximal box is prepared without occlusal step. b. Tunnel Cavity Preparations : Preparation joins an occlusal lesion with a proximal lesion by means of prepared tunnel under the involved marginal ridge. Marginal ridge remains intact. c. Bonded amalgam Use of various bonding agents which bonds amalgam to tooth structure. More weakened, remaining tooth structure is retained. For Composite Restorations :
1. Box only type of cavity preparation 2. Tunnel preparations 3. Sandwich technique ď‚§ Use of glass ionomer liner material Advantages are : 1. Glass ionomer bonds both to tooth structure and composite and thus increases in retention. 2. Fluoride content in the GIC reduces caries (recurrent). 3. Provides a better seal when seal when used at non enamel margins.
CONCLUSION : A cavity preparation is determined by many factors and each time a tooth is to be restored each of these factors must be assessed. If the principles of cavity preparation are adhere to, the success of restoration is great increased. The factor that should be considered before initiating a cavity preparation are as follows : 1. Extent of caries 2. Occlusion 3. Pulpal involvement 4. Esthetics 5. Patients age 6. Patients home care 7. Gingival status 8. Anesthesia 9. Bone support 10. Patients desires 11. Operation skill 31
12. Pulp protection The improved ability to bond restoration with tooth structure will likely to alter significantly the entire cavity preparation procedure and thus the emphasis will shift away form cavity preparation to knowledge of restorative materials and dental anatomy.