Introduction In order to ensure the success of the restoration placed in the oral cavity. The physico-mechanics of the forces acting on it has to be understood, by restoring the tooth form, we aim at maintaining the integrity and continuity of dental arch which is very important as far as mastication is concerned. Therefore, the basic aim of cavity preparation design should be to establish the best possible shape that can cope with the distribution of stresses in tooth structure and restoration without failure. for this one should understand the nature of forces acting on it and resistance to such forces. Both resistance and retention form is very important as far as success of restoration is concerned. Resistance form is defined as the architectural form given
to
a
tooth
preparation
which
enables
both
the
restoration and the remaining tooth to resist structural failure from occlusal loading stresses.
1
Building a restoration is similar to building any mechanical structure, in that the stress patterns of the available foundation and the contemplated structure must be predetermined. Accordingly, the following items should be considered. A STRESS PATTERNS OF TEETH 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. I.
STRESS BEARING & STRESS CONCENTRATION AREAS IN ANTERIOR TEETH a. The function between the clinical crown and the 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. b. The Incisal angles, especially if they are square, are subject to tensile and shear stresses in normal
2
occlusion massive compressive stresses will be present in edge-to-edge occlusion, and if the incisal
angles
mechanisms,
are these
involved
in
stresses
are
a
disclusive
substantially
increased. c. The axial angles and lingual marginal ridges will bear concentrated shear stresses. In addition on the loading side tensile stresses are present, and on the non-loading side compressive stresses are found. d. The slopes of the cuspid will bear concentrated stresses, especially if the cuspid is a protector for the occlusion or part of a function during mandibular excursions. e. 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 function of the anterior and posterior
segments
of
the
dental
arch
and
microlateral displacement of the cuspid during excursive movements. Both of these factors will 3
lead
to
stress
concentration
with
resultant
abrasive activity there. f. 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. g. 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 protrusive and sometimes at the protrusive border location of the mandible. II.
STRESS
BEARING
CONCENTRATION
AREAS
AND OF
STRESS POSTERIOR
TEETH a)
Cusp tips, especially on the functional side bear compressive stresses.
4
b)
Marginal
and
crossing
ridges
bear
tremendous tensile and compressive stresses. c)
Axial stresses
on
angles the
bear
tensile
non-functional
and side
shear and
compressive and shear stresses on the functional side. d)
The function 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-contacting side.
e)
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. III.
WEAK AREAS IN THE TOOTH SHOULD BE IDENTIFIED AND RECOGNIZED BEFORE ANY RESTORATIVE
ATTEMPT,
destructive loading they are:
5
in order
to avoid
a. Bi and trifurcation. b. Cementum should be eliminated as a component of a cavity wall. The junction between the cementum and the dentin is always irregular so the dentin surface should be smoothed flat after cementum removal. c. Thin dentin bridges in deep cavity preparation. d. Subpulpal floors in RCT treated teeth. Any stress concentation
there
may
split
the
tooth
interceptally. e. Cracks or crazing in enamel, and / or dentin both should be treated passively in any restoration design. They may act as shear lines leading to further spread. SOME
APPLIED
MECHANICAL
PROPERTIES
OF
TEETH I.
Although the following figures are averages, they provide an idea about the principal mechanical properties of tooth structure. It must be understood
6
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 36-42,000PSI. b. Compressive strength of vital dentin is 4050,000PSI. 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 /inch3. 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
1,90,000PSI. II.
In general, when enamel loses its support of dentin, it loses more than 85% of its strength properties.
III.
Tensile strength of dentin is about 10% less than its compressive strength.
7
IV.
Tensile strength and compressive strength of enamel are similar, as long as the enamel is supported by vital dentin.
V.
Shear strength of dentin is almost 60% less than its compressive strength, and this is very critical in restorative design.
VI.
There is minimal shear strength for enamel when it loses its dentin support.
VII. When the dentin loses it vitality, there is a drop of almost 40-60% in its strength properties. To best resist masticatory forces, use floors or planes at right angles to the direction of loading to avoid shearing stresses. If possible walls of preparations should be parallel to the direction of the loading forces, in order to minimize or avoid shearing stresses. Intracoronal and intraradicular cavity preparations can be done in box, cone or inverted truncated cone shapes.
8
From the drawings, 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. Definite floors, walls and surfaces with line and point angles
are
essential
to
prevent
micromovements
of
restorations with concomitant shear stresses on remaining tooth structure. 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. Designing the outline form with minimal exposure of the restoration surface to occlusal loading will definitely minimize stresses and the possibility of mechanical failure in the restoration. A comparative evaluation of the mechanical properties of the restorative material relative to that of the tooth
9
structure will dictate the preparation and restorative design i.e. if the restorative material is stronger than the tooth structure, the design should be such that the restorative material will support the tooth structure and vice versa if the restorative material is weaker than tooth structure. Junction 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 failure. 1. RETENTION FORM Retention form is defined as that form given to the tooth
preparation,
especially
its
detailed
anatomy
and
general shape which enables the restoration, that it will accommodate, to avoid being dislodged by masticatory loading. Principal means of retention: Frictional retention depends on 4 factors:
10
a.
The
surface
area
of
contact
between
tooth
structure and restorative material. Greater surface area produces a greater frictional component of retention. It is directly proportional to the length, width and depth of the walls and surface involved in the preparation. b.
Opposing walls or surface involved: More opposing walls or surfaces in a tooth preparation
produce greater frictional components of retention and consequently,
a
more
stable
restoration
within
the
preparation. c.
Parallelism and non-parallelism A higher degree of parallelism between opposing walls
produces
greater
frictional
components
of
retention.
Higher convergence of the walls in the intracoronal preparation
and
higher
divergence
of
walls
in
the
extracoronal preparation produce greater locking ability of
the
tooth
preparation
to
irrespective of frictional retention.
11
restorative
material,
d.
Proximity Bringing the restorative material closer to the tooth
structure during insertion will substantially increase the frictional retention. 2. ELASTIC DEFORMATION OF DENTIN Changing
position
of
dentinal
walls
and
floors
microscopically by using condensation energy within the dentin proportional limit. Can add more gripping action by the tooth on the restorative material. This occurs when the dentin regains its original position while the restorative material remains rigid, thereby completely obliterating any remaining space in the cavity preparation.
CLASS I 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. This form is commonly applied in various mechanical structures, so its application here is understandable.
12
It is advantageous to have a mortise shape preparation in an inverted cone shape to minimize shear stresses that tend to separate the buccal and lingual cuspal elements i.e. to prevent the splitting of the tooth. The box shaped mortise is less advantageous and the cone shaped is the least advantageous in this regard. So, whenever the anatomical and cariological factors allow the cavity preparation should be an inverted cone shape. When a caries cone penetrates deeply into dentin, removing undermined and decayed tooth structures can lead to a conical cavity preparation, mechanically, two problems can occur if restoration is inserted into such a cavity preparation. 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 increased tendency for tooth splitting (A). 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 13
they occur frequently enough to encourage microleakage around the restoration, predisposing to a recurrence of decay. These measurements 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 the pulpal floor at more than one level (B) one level will be the ideal depth level (1.5mm) and the others will be the caries cone level 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. The
first
level
should
be
as
pronounced
and
circumferentially continuous as possible. At least it should exist at two opposing locations in the cavity preparation in order to fulfill its objectives. This level is sometimes called
14
“The ledge� and it can be circumferential, interrupted or opposing.
CLASS II During mandible
centric
both
the
and
excursive
restoration
and
movements
of
the
tooth structure
are
periodically loaded both separately and jointly. This brings about different stress patterns, depending upon the actual morphology of the occluding
area of both the tooth in
question and the 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: A small cusp contacts the fossa away from the restored proximal surface, in a proximo-occlusal restoration at centric closure. A)
As
shown
in
mesio-distal
cross
section, due to the elasticity of the dentin, especially in young teeth, a restoration will bend at the axio-pulpal line angle (provided the proximal part of the restoration is self retained). This creates tensile stresses at the isthmus 15
portion of the restoration, shear stresses at the junction of the main bulk of the proximal part of the restoration and its self retained parts, and compressive stresses in the underlying dentin. B)
A
large
cusp
contacts
the
fossa
adjacent to the restored proximal surface in a proximoocclusal restoration at centric closure, either in the early stages of moving out of centric or at the late stages of moving toward it. As the diagram shows, the large cusps will tend to separate the proximal part of the restoration from the occlusal part. This creates tensile stress at the isthmus portion of the restoration even if the proximal portion is self retained. This loading situation will deliver compressive forces in the remaining tooth structure, apical to the restorations. C)
Occluding cuspal elements contact facial and lingual tooth structure surrounding a proximoocclusal or proximo-occluso-proximal restoration during centric and excursion movements.
16
As
shown
in
this
bucco-lingual
cross
section
concentrated shear stresses will occur at the junction of the surrounding tooth structure and corresponding floors, with a tendency toward fracture failure there. This loading situation can be unilateral or bilateral, depending on the direction of mandibular movement, occluding surface morphology, stage of movement, and degree of intercuspation. It is most deleterious to tooth structure, especially on the biting side if there is interference during lateral excursion. D)
Occluding cuspal elements contact facial and lingual parts of the restoration, surrounded by tooth structure during centric and excursive movements. As shown in this bucco-lingual cross section this
arrangement will induce tensile and compressive stresses in the restoration which will be transmitted to the surrounding tooth structure. E)
Occluding cuspal elements contact facial or lingual parts of the restoration, completely replacing facial or lingual tooth structure during centric and excursive movements.
17
As shown in the bucco-lingual cross section the stress pattern will be similar to No. 2 with tensile stresses induced at the junction of the occlusal and facial or lingual part of the restoration in both occluding situation. F)
Occluding cuspal elements contact a restorations marginal ridges or part of a marginal ridge during centric and excursive movements. As shown, in this mesio-distal cross section (assuming
the
restoration
is
locked
occlusally),
there
will
be
concentrated tensile stresses at the junction of the marginal ridge and the rest of the restoration. G)
Cuspal elements occlude or disclude via the facial or lingual groove of a restoration. Assuming, that the restoration is locked occlusally,
there will be tensile stresses at the junction of the occlusal and facial or lingual parts of the restoration at full intercuspation (A) and to and from that position (B) H)
Cusps and crossing ridges are part of the restoration in centric and excursive movement.
18
-
Both compressive
stresses
will
during
be
such
subjected positions
to and
movements. Besides, tensile stresses could concentrate at their junction with the main restoration, especially during contacting excursive movement. I) during
centric
Axial
portions
occlusion
and
of
the
restoration
excursive
movement
contacts. Whenever these portions are in contact with opposing occlusal surfaces, there will be induced compressive and shear stresses whenever they are not reciprocating, the axial surfaces will be stressed in a slight tensile and shear pattern at their junction with the main bulk of the restoration. J)
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, in so far 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 or tilt, establishing
19
contact with the opposing cuspal elements. Usually this newly acquired location will not be the most favourable 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
the
restoration
restoration. occluding
Conversely, prematurely
any
portion
will
of
the
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 could lead to localized or generalized gnatho-stomatic disturbances with eventual mechanical or biological features. Needless to say, pre-existing premature contacting areas should be eliminated before restorative treatment. This is done 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 if the preexisting tooth structures were not.
20
Amalgam is least resistant to tensile stress and more 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 structure (Fig. 13). 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 potentially deleterious tensile stresses occur under any type of loading. Most
mathematical,
mechanical
and
photoelastic
analyses of these stresses reveal three things: 1. The fulcrum of binding occurs at the axio-pulpal line angle. 2. Stresses increase closer to the surface of a restoration, away from that fulcrum and
21
3. Tensile stresses predominate 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. 1) A theoretical solution might be to increase amalgam bulk at the axio-pulpal line angle. Thereby placing the surface stresses away from the fulcrum (fig). However this actually results in increased stresses within the restorative material and a deepened cavity preparation, dangerously lose to pulp anatomy. Therefore such a solution, in and of itself is actually 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 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 22
the
axio-pulpal line
angle
away from
the
stress
concentration area and closer to the surface, can be achieved simply by slanting the axial wall towards the pulpal floor. 1. The obtuse-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. 2. If the axio-pulpal line angle is rounded, structural projections or sharp junctions that may concentrate stresses at the isthmus would be avoided as well as increase the amalgam bulk at the fulcrum. 3. 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
occluding stresses.
23
receiving
deleterious
4. 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 both 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 preparation by retentive grooves internal boxes, and undercuts. 6. One should avoid, as much as possible placing or leaving any surface discontinues 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.
24
MARGINS Frail, feather-edged margins of amalgam, which will occur when the cavo-surface angles of preparations are beveled, will fracture easily. Occluding forces will cause amalgam at the bevel to bend with maximum tensile stress, 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 preparations, four design features should be observed: 1. Create butt joint amalgam tooth structure at the margins. 2. Leave no frail enamel at the cavo-surface margins. 3. Remove flashes of amalgam on tooth surface adjacent to amalgam margins. 4. The interface between amalgam and tooth structure should not be at an occluding contact area with opposing
teeth
either
in
mandibular movements.
25
centric
or
excursive
Cusps 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 1-5mm. 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. RETENTION FORM 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 such displacements for a Class II proximo-occlusal restoration.
26
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 Lshaped with the long arm of the L occlusally and short arm proximally. When the long arm is loaded by vertical force ‘V’, ‘H’ will seat the restoration more into the tooth. This is due to elasticity of the dentin, especially in young teeth wherein the pulpal floor will change location from position 1 to position 2. However, since the metallic restorations are more rigid than the dentin, the short arm of the L will more proximally, the fulcrum of this restoration is the axio-pulpal line angle. In order to prevent such a displacement, proximal 27
self-retention in the form of facial and lingual grooves are required. C)
LATERAL ROTATION OF THE RESTORATION AROUND HEMISPHERICAL FLOORS (PULPAL AND GINGIVAL)
A)
OCCLUSAL DISPLACEMENT This 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 restorations with less failure to be expected as a result. 28
CLASS V Class V restorations confined to one surface and not subjected to direct loading may be thought of as free of any mechanical problems. However, as the mandible moves in lateral excursion, the lingual slopes of the buccal and lingual cusps of maxillary teeth lead to the buccal slopes of the buccal and lingual cusp of mandibular teeth. Assume that we have a facial Class V restoration in the lower molar tooth. As the tooth is firmly seated in bone, the tooth structure of the crown can move from position 1 to position 2, making a V-shape opening at the margin, together with a facial component of force during the restoration facially. Although this opening and the facial component of the force are very minute and may not displace the restoration completely, their repetition, thousands of times per day can create marginal failure and eventually, facial protrusion of the restoration. The same thin can happen for a lingual restoration in lower teeth and a facial or lingual one in upper teeth.
29
To minimize the effects of these displacing forces, grooved occlusal and gingival walls are essential for any Class V cavity preparation for amalgam, in addition to definite surrounding walls, line and point angles. FORCES
ACTING
ON
CAVITY
PREPARATION
FOR
DIRECT TOOTH COLOURED MATERIALS For any proximal restoration in anterior teeth there are two possible displacing forces. The first ‘H’ is a horizontal displacing or rotating the restoration in a labio-proximo-lingual or linguo-proximolateral direction. It has its fulcrum almost parallel to long axis of the tooth being loaded. The second is a vertical forces displacing or rotating the restoration proximally and having a fulcrum at the gingival margin of the preparation. The mechanical picture can be summarized as follows: 1. With
normal
overbite
and
overjet
during
centric
closure of the mandible, mainly the horizontal forces will be in action, these forces would try to move it
30
linguo-proximo-laterally (for the upper restoration) and labio-proximo-lingually (for lower). 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 angle. The results of this loading are rotational forces as well as forces rotating the restoration laterally and proximally (for upper) or lingually and proximally (for the lower). 2. If anterior teeth meet in edge to edge fashion i.e. there will be vertical displacing forces with very limited horizontal components. 3. If the upper and lower anterior teeth meeth such that the lowers are labial to the uppers in centric occlusion (Angle’s Class III), the horizontal loading will tend to rotate or displace restorations labio-proximo-lingually (for uppers) and linguo-proximo-labially (lowers). 4. In occlusions, with deep anterior overbite and normal or no overjet, the horizontal type of loading will be
31
greatly
exaggerated.
The
vertical
displacement
although present will be minimal in comparison. 5. In occlusions 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. It should be understood that none of these loading forces
work
simultaneously.
separately. It
should
They be
work
mentioned
together here
and
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 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 restoration. This loss will lead to definite direct loading of 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.
32
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 disocclusion mechanism, the teeth and the restoration will be part of that disocclusion mechanism with excessive horizontal and vertical loading forces. 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 force intact walls surrounding the restoration, unfortunately, this is usually not the case that is why the clinical performance of tooth colored materials diffuses from one situation to another, sometimes dramatically. CAST PREPARATIONS Cast restorations are usually used for compound or complex displacing
tooth
involvement.
forces,
their
The
fulcra,
possible and
their
loading effects
and on
restorations, together with their effect on remaining tooth structure, have been fully described in the discussions of the 33
different cavity preparations for amalgam and in the general principles of preparation design. The formability of casting materials enables us to use myriad retention and resistance means that are impossible to use with any other materials. INLAY RESTORATIONS Here are 3 illustrations representing the different design of a proximal box cavity
(A)
(B)
(C)
Fig.(A) shows walls parallel to each other where rotational force is applied by means of a bar in a counter clockwise direction. The tendency for ‘x’ to use occlusally on area xy is resisted by dentin lying withing the area xyz. In Fig. (B) same rotational force finds no resistance to point ‘x’ rotating might out of the cavity because of too great divergence of buccal and lingual walls. In Fig. (C) shows gingivo-occlusal divergence of 5° from
vertical
plane
where
the
34
rotational
force
finds
resistance to point x by the bulk of dentin contained in area xyz. Therefore,
while
parallelism
of
the
walls
offers
maximum rotational resistance from clinical standpoint, a slight divergence of 2° to 5° from parallelism will furnish necessary resistance to bucco-lingual torque displacement. This figure shows a proximal view of a MOD inlay not quite seated in the cavity. The width of inlay is ‘N’ at about its vertical center. Contact is assumed to have been made between the inlay and the walls of the cavity. After the contact, the inlay is further forced downwards – an amount ‘dh’. The walls of the cavity make an angle θ with the vertical wall of the restoration. Assuming that the tooth structure is prevented from deformation, the total shortening per unit width of gold is:
35
Σ g = dh tanθ W And the unit stress becomes S = Eg dh tanθ W Eg = modulus of elasticity of gold. The unit stress is not perpendicular to the cavity wall but is parallel to ‘W’ and may be resolved into two components. F Cosθ - perpendicular to cavity wall. And F Sinθ - parallel to the wall. Assuming the coefficient of friction is ‘µ’ between gold of the inlay and tooth structure than µF Cos θ becomes the frictional force between gold and tooth structure which prevents the movement of the two with respect to each other. The component FSin θ parallel to the cavity wall tends to push the inlay back to the cavity wall. Thus the total force of frictional retention tending to hold the inlay in its cavity is: P = µ FCosθ - F Sinθ 36
This shows that as the value of θ increases greatly, the inlay will bounce out of the cavity and this angle is known as the “Critical Angle” θc. In a proximo-occlusal restoration one of the proximal wall is absent and opposite retentive stresses are developed only on the buccal and the lingual surfaces whereas the gripping power has been lost in proximal direction, due to which there is a force tending to push the restoration out through the absent wall. This displacing force can be counteracted by the retaining stresses present in the buccal and lingual walls and can be supplemented clinically by placing a gingival groove in the gingival wall. The occlusal dovetail lock also resists lateral displacement of the key by the additional tensile stresses developed within the lock. With the help of the diagram it is seen that by increasing the angulation to 35°-45° of the gingival bevel the resistance to displacement is offered by that portion of dentin which comes in the path of the arc formed by radius F E and F F with P as the rotation center. Keeping the gingival bevel at 15° won’t serve any purpose as the filling may be
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rotated
out
of
the
cavity
because
no
resistance
to
displacement is offered by either the axial wall CG or DG. Where the buccal and the lingual walls instead of flaring from the axial line angle to the cavo-surface margin in a continuous plane are changed into two narrow-and two smaller diverging planes even with such a modification of the buccal and lingual proximal walls, it is possible to retain the
retentive
stresses
of
a
preparation
since
the
supplementary diverging planes are mostly line angles leaving the balance of the wall in the elastic dentin. If the proximal walls diverge excessively occlusally from the gingival wall. The reacting stresses in the dentin since all forces react as displacing stresses. Hence, such a divergence is not acceptable and every effort should be made to
approach
parallelism
not
exceeding
2째-5째
gingivo-
occlusally. BEVELLING Bevelling plays a very important role in reducing the stresses on the remaining tooth structure, thus maintaining the integrity of both the tooth and the restoration.
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The lower surface bevel helps to seal and protect the margin resulting in a strong enamel margin with an 140°150° angulation. This leaves 30°-40° marginal metal on the inlay. The marginal gold alloy is too thin and weak if its angle is less than 30°, conversely the metal at the margins is too bulky and difficult to burnish, if its angle is greater than 40°. In small teeth such as premolar, the joining of mesial and distal cavity across the occlusal surface results in a considerable weakening of the tooth and an occlusal stress is liable to produce a vertical fracture. When it is thought that there is risk of this occurring the occlusal bevel should be increased so as to extend beyond the summit of the cusps. Occlusal stresses will then be taken entirely by the inlay and transmitted to the flat floor, splitting strains thus being much reduced.
CONCLUSION Tooth is a engineering marvel, which can withstand forces because of resiliency of dentin.
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Increased amount of dentin increases the retention of a restoration and better resistance to forces. An intact tooth can best withstand forces but when lost due to caries, has to be replaced by a restorative material. There are various forces that can act on these restorations hence based on sound principles these restorations should be placed so as to prevent their dislodgement and increase the resistance of tooth as well as restoration to forces.
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FORCES ACTING ON RESTORATIONS CONTENTS
Introduction Retention and Resistance forms in general Forces acting on Class I, Class II, Class V Direct tooth colored restorations Cast restorations i.e. Inlay Conclusion
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