Forces acting on restoration

PRESENTED BY: DR ABHISEK GURIA
DEPT. OF CONSERVATIVE DENTISTRY & ENDODONTICS
Forces Acting on
RESTORATIONS

CONTENTS
 Introduction to biomechanics
 Biomechanical properties of enamel
 Biomechanical properties of dentin
 Force resisting structures in enamel
 Force resisting properties of dentin
 Functional aspects related to forces acting on restorations
 Type of tooth contacts
 Functional cusps
 Non Functional cusps
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forces acting on restoration
 Areas of stress concentration in anterior teeth
 Areas of stress concentrations in posterior teeth
 Weak areas in teeth
 MECHANICAL PROPERTIES OF RESTORATIVE MATERIALS
 Concept of stress and strain
 Modulus of Elasticity and Proportional limit
 Yeild strength and Ultimate strength
 Hardness and Fracture toughness
 Time dependent properties- creep
 BIOMECHANICAL UNIT
 STRESS DISTRIBUTION IN RESTORED TEETH

 Mechanical problems in Class i restorations and their
solutions
 Mechanical problems in Class ii restorations and their
solutions
 Mechanical problems in Class iii @ iv restorations and
their solutions
 Mechanical problems in Class v restorations and their
solutions
 Forces acting on bonded restorations
 Forces acting on Cast restorations
 Forces acting on intraradicular posts
 Conclusion and References

BIOMECHANICS FOR RESTORATIVE DENTISTRY
 Prediction of stress under anticipated applied load
 Biomechanics is the study of loads (or stresses) and deformations (or
occurring in biologic systems.
5

Response of tooth to various forces
 Freedom of displacement in 6 direction
 Omnidirectional movement
 ‘’Replacement’’ of tooth when force is removed
 This depends upon
1. Alveolar bone support
2. Adjacent tooth support
3. Horizontal muscle activity
6

BIOMECHANICAL PROPERTIES OF
DENTAL STRUCTURES

ENAMEL
 Enamel with a high elastic modulus and low tensile
strength, which indicates a rigid structure.
 Hardness of enamel decreases inward, with
hardness lowest at the DEJ.
 The density of enamel also decreases from the
surface to the DEJ.
8

 When enamel loses its support of dentin, it loses more than 85% of its
strength characteristics.
 Tensile strength and compressive strength of enamel are similar, as long
as the enamel is supported by vital dentin.
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 Enamel requires a base of dentin to withstand masticatory forces.
 Enamel rods that fail to possess a dentin base because of caries or improper
preparation design are easily fractured away from neighboring rods.
 For maximal strength in tooth preparation, all enamel rods should be
supported by dentin.
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GNARLED ENAMEL:
 There are groups of enamel rods that may entwine with
adjacent groups of rods, and they follow a curving
irregular path toward the tooth surface.
 These comprise gnarled enamel, which occurs near the
cervical regions and the incisal and occlusal areas.
 Gnarled enamel is not subject to cleavage as is regular
enamel.
 Gnardling of enamel rods provide strength by resisting,
distributing, and dissipating impact forces.
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FORCE RESISTING STRUCTURES IN ENAMEL

HUNTER SCHREGER BANDS:
The changes in direction of enamel prisms
(dextroflexion and sinistroflexion) that
minimize cleavage in the axial direction.
12

ENAMEL LAMELLAE
 Thin, leaflike faults between enamel rod groups that
extend from the enamel surface toward the DEJ,
sometimes extending into the dentin.
 They contain mostly organic material, which is a weak
area predisposing a tooth to the entry of bacteria,
caries or cracks.
13
WEAK AREAS IN ENAMEL:

Biomechanical properties of Dentin
 Dentin is significantly softer than enamel but harder than cementum.
 The hardness of dentin averages one fifth that of enamel, and its hardness near the
DEJ is about three times greater than near the pulp.
 Dentin becomes harder with age, primarily due to increases in mineral content.
 While dentin is a hard, mineralized tissue, it is somewhat flexible, with a modulus of
elasticity of 1.67 X 106 PSI.
14

 Often small "craze lines" are seen in the enamel that indicate minute fractures of
that structure.
 These craze lines usually are not clinically significant unless associated with
cracks in the underlying dentin.
 Dentin is not as prone to cleavage as is the enamel rod structure.
 The tensile strength of dentin is approximately 40 MPa (6000 PSI)
 The compressive strength of dentin is much
higher-266 MPa (40,000 PSI).
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FUNCTIONAL ASPECTS OF DENTITION
TYPES OF TOOTH CONTACTS:
 Cusp to Fossa contact
16

 Surface contact
17

 Ridge Embrasure contact
18

 Ridge Groove contact
19

Posterior cusp characteristics
 Four cusp ridges can be identified as common
features of all cusps.
 The outer incline of a cusp faces either the facial (or
the lingual) surface of the tooth and is named for its
respective surface.
20

 The inner incline cusp ridges are widest at the base and become narrower as
they approach the cusp tip. For this reason, they are termed triangular ridges.
21

 The mesial and distal cusp ridges extend from the cusp tip mesially and distally and
are named for their direction.
22

Supporting cusps
Supporting cusps can be identified by five characteristic
features:
 They contact the opposing tooth in MI.
 They support the vertical dimension of the face.
 They are nearer the faciolingual center of the tooth than nonsupporting
cusps.
 Their outer incline has the potential for contact.
 They have broader, more rounded cusp ridges than nonsupporting cusps.
23

24

25

Non supporting cusps
Features:
 Do not contact opposing tooth in MI
 Keep soft tissue of tongue or cheek off occlusal table.
 Farther from faciolingual center of tooth than supporting cusps
 Outer incline has no potential for contact
 Have sharper cusp ridges than supporting cusps
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Areas of stress concentration in anterior teeth
 Junction between the clinical crown and clinical root
28

 The incisal angles, especially if they are square
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 The axial angles
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 Lingual marginal ridges
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 Slopes of a cuspid
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 Distal surface of cuspid
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 Lingual concavities in the upper anterior teeth
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Areas of stress concentration in posterior teeth
 Cusp tips on the functional side
35

 Marginal and crossing ridges
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 Axial angles
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 Junction between clinical crown and clinical roots
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 Occlusal, facial, lingual concavities
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Weak areas in teeth
 Bifurcations and Trifurcations
40

 Cementum and CDJ
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 Thin dentinal bridges in deep cavities
42

 Sub pulpal floors in root canal treated teeth.
43

 Cracks or crazing in enamel and/or dentin.
44

Biting forces
 Maximum biting forces decrease from the molar to the incisor region.
 The average biting forces on the first and second molars are about 580
Newtons (N)
 The average forces on bicuspids, cuspids, and incisors are about 310,
220, and 180 N, respectively.
45

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) The thickness of the structures.
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Mechanical properties of restorative materials
CONCEPT OF STRESS
 When a force is applied to a material, the material
inherently resists the external force.
 The force is distributed over an area, and the ratio of
the force to the area is called the stress
 Thus, for a given force, the smaller the area over
it is applied, the larger the value of the stress.
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Types of stresses
1. Tension
When subjected to 2 sets of forces directed away from
each other in same straight line.
2. Compression
When subjected to 2 sets of forces in same straight line
directed to each other.
3. Shear
2 forces directly parallel to each other
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 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 (proportional limit,
yield point).
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 Continuing plastic strain ultimately leads to failure by fracture. The highest stress
before fracture is the ultimate strength.
 The slope of the linear portion (constant slope) of the stress-strain curve (from no
stress up to the elastic limit) is called the modulus, modulus of elasticity, Young's
modulus, or the stiffness of the material, and is abbreviated as E.
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51

 A restorative material generally should
be very stiff so that under load, its elastic
deformation will be extremely small.
 An exception is a Class V composite,
which should be less stiff to
accommodate tooth flexure.
52

 Proportional limit and yield strength indicate the stress at which the material no
longer functions as an elastic solid.
 The proportional limit is the stress on the
stress–strain curve when it ceases to
be linear or when the ratio of the
stress to the strain is no longer proportional.
 The yield strength is the stress at some arbitrarily selected value of permanent
strain, and thus is always slightly higher than the proportional limit.
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53

A restoration can be classified as a clinical
failure when a significant amount of
permanent deformation takes place even
though the material does not fracture.
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 The stress at which fracture occurs is called the ultimate strength.
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Some materials do not plastically deform easily.
Such materials are susceptible to cracks and defects.
Fracture toughness is a measure of the energy
required to fracture a material when a crack is
present.
56
Fracture Toughness

 Dental porcelain have a low fracture toughness value
 Metals have high fracture toughness values.
 Many researchers have sought to improve dental composites by
improving their fracture toughness.
57

 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.
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Time dependent responses to intraoral forces
 Deformation over time in response to a constant stress
is called creep.
 Materials that are relatively weak or close to their
melting temperature are more susceptible to creep.
 Traditional amalgam restorations are involved in
intraoral creep.
59

 The biomechanical behavior of restored teeth can be studied
at any level from gross to microscopic.
Examples of situations of interest include:
 The calculation of stress transfer to the margin of an
amalgam restoration
 From the amalgam to tooth structure
 From tooth structure to the periodontal ligament
 From several teeth to bone, and throughout bone.
 The most common analysis focuses on stress transfer at the
interface between a restoration and tooth structure.
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Biomechanical unit
The standard biomechanical unit involves the:
(1) restorative material
(2) tooth structure
(3) interface (interfacial zone)
61

 Different restorative procedures can involve very different
interfaces.
 Composite /enamel interfaces are micromechanically
bonded.
 Amalgam/enamel interfaces are weak and discontinuous
unless a bonding system is used.
 Cemented crown/enamel interfaces are weak but are
continuous.
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 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
resist fracture, but the interface or tooth
may not be.
63

Stress transfer in teeth and restorations
 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 thus local
stresses are lower.
 During this process, a small amount of dentin
deformation may occur that results in tooth flexure.
64

 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
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 Once in dentin the stresses are resolved in a
manner similar to a normal tooth.
 The process of stress transfer to dentin becomes
more complicated when the amount of
remaining dentin is thin
66

VALE’S EXPERIMENT
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68

 To best resist masticatory forces, prepare floors at right angles to the
direction of the loading forces, in order to minimize or avoid shearing
stresses.
69

 If possible, the walls of the preparation should be parallel to
the direction of the loading forces, in order to minimize or
avoid shearing stresses.
70

 Inverted truncated cone shapes will have a higher resistance to loading than
the box shape
 The box shapes will have higher resistance to loading than the cone shapes
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 Definite floors, walls and surfaces with line angles and point angles are
essential to prevent micro movements of restorations with
concomitant shear stresses on remaining tooth structures.
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 Designing the outline form with minimal
exposure of the restoration surface to
occlusal loading.
73

 If the restorative material is stronger than the tooth structure, design should be such
that the restorative material will support the tooth.
74

75

 Junctions between different parts of the
tooth preparation, especially those
acting as fulcra should be rounded in
order to minimize stress concentration
in both the tooth structure and
restorations and to prevent any
such sharp components from
acting as shear lines for fracture
failure.
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 Retentive features must leave sufficient bulk of tooth structure to
resist stresses resulting from displacing forces.
77

Mechanical problems in class I
restorations and their solutions
78

Advantages of a MORTISE SHAPE preparation
Mortise: each wall and floor is in the form of a flat plane, meeting each
other at definite line and point angles.
Advantages:
 Seat of the restoration is at distinct right angle to the direction of
stresses.
 Plastic materials are readily compacted against the smooth flat planed
surfaces of mortise shape.
 A mortise form with two or more opposing walls will facilitate the
gripping and frictional retention of plastic restorative in a cavity
preparation
79

 When a caries cone penetrates deep dentin, removing undermined caries may lead
to a conical (hemispherical) preparation.
80

In this case,
 If the force is applied centrically, the restoration will act
act as a wedge, concenterating forces onto the pulpal
floor (which is not flat) and tends to crack the thin
dentin bridge.
 Increased tendency for tooth splitting.
81

 If the force is applied eccentrically, the restoration will
have the tendency to rotate laterally, for there would be
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.
82

 These movements can also lead to the fracture of the marginal tooth structure, and
even splitting of lateral walls.
83

 Making pulpal floor at more than one level.
 One level: the ideal depth
 Other level: As dictated by dentinal caries
 Shallow level creates a flat portion with definite angles to the
walls, resisting vertical loading (mortise)
 It also locks the restoration laterally preventing movement.
84
SOLUTION:

 Shallow depth level should be as
pronounced as possible.
 Shallow depth level should be as
circumferentially continuous as possible.
 It should exist in at least two opposing
locations in the cavity preparation.
85

 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.
ADVANTAGES
 Maximum bulk of tooth structure supporting marginal
ridge.
 Prevents its undermining
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 When a cavity wall comes in contact with a crossing
ridge, make the wall perpendicular to the pulpal floor.
ADVANTAGES
 Crossing ridges have more bulk of tooth structure.
 Box configuration resists stresses better than the cone
configuration.
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87

 If the cariogenic or anatomical factors demand a
divergent wall around the cusps pulpo occlusally,
 Prepare at least the pulpal half of the walls
perpendicular to the floor.
 Prepare rest of the wall as dictated by caries.
88

89

Occlusal loading on a Class II
preparation and its effects
90

A small cusp contacts the fossa away from the
restored proximal surface, in a proximo occlusal
restoration in centric.
 Tensile stresses at isthmus
 Compressive stresses on the underlying dentin
91

 A large cusp contacts the fossa adjacent to
the restored proximal surface, in a proximo
occlusal restoration in centric.
 Large cusp will tend to seperate the proximal part of
the restoration from the occlusal part.
 Compressive stresses in the remaining tooth structure
apical to the restoration.
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92

 Occluding cuspal elements contact facial and
lingual tooth structure sorrounding a proximo
occlusal restoration, during centric and eccentric.
 Concentrated shear stresses will occur at the junction of
surrounding tooth structure and corresponding floors.
 This situation can be unilateral/bilateral
 Most deleterious to tooth structure.
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 Occluding cuspal elements contact facial
and lingual parts of the restoration
surrounded by tooth structure, during
centric and eccentric.
 Tensile and compressive stresses in the
restoration which will be transmitted to
the surrounding tooth structure
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 Occluding cuspal elements contact facial and lingual
parts of the restoration, completely replacing
facial/lingual tooth structure during centric and
eccentric.
 Cusp will tend to seperate the facial part of the
restoration from the occlusal part.
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95

 Occluding cuspal elements contact a restoration’s
marginal ridge during centric or eccentric.
 Concentrated tensile stresses at the junction of
marginal ridge and the rest of the restoration.
96

 Cuspal elements occlude or disclude via the facial
or lingual groove of a restoration
 Tensile stresses at the junction of occlusal and facial
oir lingual parts of the restoration.
97

In the absence of marginal ridge:
 The force 1 will be directed towards the proximal
ridge of the adjacent tooth
 force 2 is directed on to the faulty tooth
 1H and 2H tend to drive the two teeth away from
each other
 The vertical component 1V and 2V can impact
the food intraorally
98
FORCES ACTING ON MARGINAL RIDGES

Adjacent marginal ridge not compatible with height:
 Force1 (A) on the proximal surface of the restoration
 The horizontal component 1H will drive the restored
tooth away from the contacting tooth
 Vertical component will push debris interproximally
 This happens even in the presence of force 2 (B) with
its horizontal component (BH) acting on the adjacent
marginal ridge
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99

Marginal ridge with no triangular fossae:
 No occlusal planes in the marginal ridges
 so there are no occlusal forces acting 1 and 2
 there are no horizontal component 1H and 2H
to drive the tooth toward each other
 vertical force 1V and 2V will impact the food
interproximally
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100

Single planed marginal ridge in bucco-lingual
direction
 Premature contact during both function and static
occlusion
 One planed marginal ridge increases the depth of
adjacent triangular fossa
 Increasing the stress in this area
 Increases the height of the marginal ridge at the
centre, disturbing the spillway
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101

Biomechanical aspects of composite restorations
 Numerous in vitro studies have demonstrated
that bonded composite restorations increase
resistance to fracture in teeth under axial loads .
 But generate stress areas at the adhesive
interface
 Mechanical properties of the hybrid layer,
adhesive layer, and adhesive tags alter during
function. (Spencer P et al, 2010)
102

WEAR FORCES ON COMPOSITES
 At least five types of composite wear events are based on
the location on the restoration surface:
 Wear by food (contact-free area)
 Impact by tooth contact in centric (occlusal contact area)
 Sliding by tooth contact in function (functional contact
area)
 Rubbing by tooth contact interproximally (proximal
contact area)
 Wear from oral prophylaxis methods (toothbrush or
dentifrice abrasion).
103

 Patient with heavy occlusion such as bruxism or
restoration that provides all the tooth contacts of
antagonist may lead to the failure of the restoration
(Bohaty BS et al.2013, Cavalcanti AN et al. 2007).
104
WEAR FORCES AND COMPOSITE

 The polymerization shrinkage of composites in a
cavity generates stress that can be transmitted via
the adhesive interface to adjacent dental tissues,
producing dental deformation and reduction of the
intercuspal distance.
(Kinomoto Y et al. 2000)
105
POLYMERIZATION SHRINKAGE – A FORCE ACTING ON COMPOSITE
TOOTH INTERFACE

 These forces, in addition to causing cusp flexure,
can cause fracture or crazing of enamel and
fracture in composite material
(Kinomoto Y et al. 2000)
106

 Greater cuspal deflection occurred in teeth with
larger restorations
(Suliman AA et al, 1993)
107

 Versluis et al. demonstrated that Class I restorations (configuration factor of
2.9 to 3) showed a higher degree of stress than did Class II restorations
(configuration factor of 1.2 to 1.8)
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108

 Meredith et al. (1997) reported that
polymerization shrinkage can act as a
preload in restored teeth and weaken the
remaining structure under oblique occlusal
loading. eg, from nonworking side interferences.
109

 The mean inward cuspal movement
produced by polymerization shrinkage is
considerably and significantly greater in
MOD (21.5 μm) than in MO (5.6 μm)
cavities, adding greater pre-stress to the
cuspal deflection produced by the occlusal
load.
(Lopez SG et al. 2007)
110

 Chipping of a material is often easier close to an edge
because lower force is usually required.
 The ability of restorative materials to withstand fracture at
0.5 mm of a thin edge can be described as ‘‘edge-
strength’’.
D.C. Watts et al. 2007
111

FORCES ACTING ON ANTERIOR
RESTORATIONS
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112

UNIQUE FEATURES OF ANTERIOR TEETH
 They have maximum bulk gingivally & least bulk incisally
 The labial enamel plate is much thicker than the lingual
or proximal ones.
 Occluding surfaces of anterior teeth are anterior
determinants of mandibular movements.
 More deeper the overbite, more the stresses on anterior
teeth.
113

Forces acting on anterior restorations
 For any proximal restoration in anterior teeth,
there are two possible displacing forces:
 Horizontal forces displacing the restoration in a
linguo-proximo-labial or labio-proximo-lingual
direction.
 Vertical forces which tend to displace the
restoration proximally.
114

 Mainly horizontal forces will be acting.
 If these forces load the proximal restoration directly, will
cause the restoration to move:
linguo-proximo-labially (upper restoration)
labio-proximo-lingually (lower restoration).
 Magnitude of forces: not very substantial
 Vertical forces: Nil
115
In anterior teeth with normal overjet and overbite during
centric closure..

 Directly loaded proximal restorations will sustain substantial horizontal forces
and also vertical displacing forces, especially in incisally restored teeth.
116
In protrusive and lateral movements..

 Loading of the proximal restoration involving the
incisal angles will be similar to any class II restoration.
 Vertical displacing forces with limited horizontal
component.
 However, if incisal angle is intact, these displacing
forces will be minimal.
117
IF ANTERIOR TEETH MEET IN EDGE TO EDGE
POSITION AT CENTRIC..

 There will be same type of loading conditions as in class I.
 Horizontal forces will tend to displace:
 The upper restroration labio-proximo-lingually
 The lower restoration linguo-proximo-labially
118
IN CLASS III INCISAL SITUATION,

 In occlusions with deep anterior overbite, horizontal
forces will be greatly exaggerated.
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119

 In occlusions with anterior open bite, a no contact situation occurs
during centric and eccentric, and the there will be no horizontal or vertical
loading.
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120

 In cases where the proximal restoration of an
anterior teeth is a part of mutually protected
occlusion..
 The teeth and restoration will be a part of
disclusion mechanism and hence incur massive
horizontal and vertical forces.
 This situation should be diagnosed properly so that
restoration can be planned as such.
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121

 Conversion of a class III situation to a class IV situation represents a major
complication in the mechanical problems of anterior tooth restorations.
 This will lead to definite direct loading of the restoration
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122

 Loss of an axial angle, incisal angle or tooth
structure at the cervical region will dramatically
reduce the tooth’s ability to resist loading.
 Ideally, a restoration made of tooth coloured
materials should not be loaded directly.
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 Amount of force increases as the amount of
steepness of cusps increses.
 Facial protrusion of the restoration.
 Grooved occlusal and gingival walls in addition to
definite surrounding walls and floors
124
Forces acting on Class V restorations

Forces acting on veneers
125
(a) Featheredge preparation model, (b) Incisal bevel preparation
model, (c) Overlapped preparation model

 Featheredge preparation
 Stress distributions on Model 1
a - 0°, b - 60°, c - 120°
126

 Incisal bevel preparation
 Stress distributions on Mode 2
a - 0°, b - 60°, c - 120°
127

 Overlapped preparation
 Stress distributions on Mode 3 a - 0°,
b - 60°, c - 120°
128

BIOMECHANICAL ASPECTS OF POSTS
 Posts affect the distribution of the loads on the dentin.
 Kol et al. showed in a study that posts changed tensions on the
teeth under vertical loads of compression.
 Investigators have shown that the bigger the difference
between the teeth Young modulus and posts, the less
homogeneous the stress distribution on the teeth surface,
this causes areas of stress concentration in the dentin.
129

 A pulpless tooth is not entirely devoid of metabolism, as
has been shown by several authors, who found using
radio phosphorus (P32) that pulpless teeth have a
metabolism, though to a lesser degree than a vital one.
(Volker J et al, 1942)
130
Certain inherent characteristics of pulpless teeth :

 Compared to the coronal part, the metabolism
in the root was found to be higher, probably
conducted through the attachment apparatus.
 It is also recognized that the coronal portion of
a pulpless tooth is more brittle than that of a
vital one. This is attributed to a decrease in
moisture.
131

 In addition, a treated pulpless tooth
generally has lost some of its tooth
substance following caries or traumatic
fracture and access for endodontic
therapy is often difficult.
 Because of these characteristics, the
mechanical resistance of the tooth is
considerably lowered.
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132

 The amount of internal dentin structure has been
directly correlated with the fracture resistance of
endodontically treated teeth.
(Fernandes et al. 2001)
133

 The post and core is intended to provide the
necessary strength as well as the proper retention
form for the final restoration.
 Thus, functional forces acting on the tooth are
transferred from the coronal part, through the core
and post, to the root and the supporting bone.
134

 The post length is an extremely important factor in
achieving adequate resistance
 Tooth fractures occur in most instances diagonally
from the coronal level to the margin of the supporting
bone.
 This is because of the direction of the forces which
depend on the amount of root unsupported by
bone.
5/11/20209:19 AM
135

Hence a relatively short post,
that does not extend
sufficiently beyond the bone
level, will not prevent this
common type of fracture,
while a post having sufficient
extension beyond it will do
much to prevent it.
(Talkov, 1968)
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136

 To avoid a wedge-like action of the post, which,
under extreme conditions, may split the root
completely, the core and post should possess a
positive seat.
 This should be in the shape of a flat surface
perpendicular to the root axis and its function is to
prevent the post from entering the canal beyond the
predetermined limit.
(Goldrich N. 1970)
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137

 It has been reported that the presence of a 2.0-mm
crown ferrule surrounding remaining tooth structure
enhanced fracture resistance of anterior teeth which
were restored with a cast post and core and metal
ceramic crowns.
(Zhi-Yue L et al. 2003)
138

139

 Generally, a tooth restored with a stiff cast-and-post
system was observed to withstand a higher load
before fracturing, but the fracture was more often
catastrophic and resulted in tooth extraction.
(Torbjörner & Fransson, 2004)
 Fiber post systems demonstrated less strength, but
generally resulted in repairable fracture modes.
(Santos-Filho PC et al, 2008)
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140

CONCLUSION
141

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142

Forces acting on restoration

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