This document discusses various biomechanical considerations that are important for restorative dentistry. It covers topics like the forces acting on teeth, stress patterns on different types of teeth, weak areas of teeth, and mechanical properties of dental materials. It emphasizes the importance of understanding stress transfer and how restorations can impact this. It also addresses obtaining resistance form for tooth structures and occlusal considerations when restoring teeth. The overall goal is to design restorations that can withstand biting forces and minimize stress on remaining tooth structures.

2.  Introduction
 Pre-operative analysis
 bite force
 oral habits
 compatibility of materials
 Force
 Force acting on the stomatognathic system
 Normal biting force
 Direction of displacement
 Stress
 types of stress
 Mechanical properties of materials
 Biomechanics for restorative dentistry
 Stress transfer
 Tooth flexure

3. • stress patterns on teeth
• anterior teeth
• posterior teeth
• Weak areas of teeth
• Applied mechanical properties of teeth
• The vale experiment
• Obtaining resistance form for tooth structure
• Occlusal considerations in restoring teeth
• posterior restoration
• anterior restoration
• Occlusal loading and its effect
• Role of contact areas
• Marginal ridges –mechanical function
• Relation between tooth wear & restorative material
• Forces on
• intracoronal restorations
• extracoronal restorations
• CONCLUSION

4. Introduction
• All structural analysis and design require knowledge of the
forces that will be applied and the mechanical properties of the
materials that must withstand these forces.
• Most restorative materials must withstand forces in service
either during mastication or fabrication.
• The mechanical properties, quantities of force, stress, strain,
strength hardness, and others can help identify the properties of
a material.

5. • In many respects designing a structure for the oral
environment is among the most demanding as the
functional and para functional loads that must be
accommodated and the aesthetic and space limitations
makes the designing complicated.
• When we design a structure we try to predict the
stress that will develop in the structure under the
anticipated applied loads.

6. Pre-clinical Analysis

7. Deciding factors for restorative
treatment
o• Extent of caries.
o• Strength of remaining tooth structure.
o• Specific characteristics of the patient’s dentition and
periodontal health.
o• Patient’s oral hygiene and dental caries history.
o• Financial costs of the procedure to the patient.
o• Risks and benefits of the procedure to the patient.
o• Ability of the dentist to perform the procedure.
o• Preferences of the dentist and the prevailing standard of
care.
o• Acceptance by the patient.
• : Shivakumar AT, Kalgeri SH, Dhir S. Clinical considerations in restorative dentistry - A narrative
review. J Int Clin Dent Res Organ

8. Trauma from occlusion
• When occlusal forces
exceed the adaptive
capacity of the tissues,
tissue injury occurs,
mainly periodontium.
Occlusal trauma
• Synonym
• Also called traumatism.
(Carranza)

10. • denture teeth against natural dentition can be severe (Fig. 7). Whereas the
tooth structure loss opposing the complete denture took more than 10 years
• : Shivakumar AT, Kalgeri SH, Dhir S. Clinical considerations in restorative dentistry - A narrative
review. J Int Clin Dent Res Organ

12. • The general concept of force is gained through the
muscular action of pushing or pulling of an object
• A force always has a direction and the direction is
often characteristic of the type of force.
• If the body to which the force is applied remains at
rest, the force causes the body to deform.
• Units of force are the pound or the kilogram or
Newton.
FORCE

13. IN STOMATOGNATHIC
SYSTEM

14. FORCE = MASS OF MANDIBLE x ACCELERATION OF CLOSURE
Four major components that influence the force are ;
1. MUSCLES OF MASTICATION
2. CRANIOMANDIBULAR LIGAMENTS
3. T.M.J
4. TEETH

15. The latest bite force recording devices use load cells (transducers) to
convert force to electrical energy that may be based on one of the
following working principles.
Types of load cells (force transducers):
1.Strain-gauge transducers;
2.Piezoelectric transducers;
3.Pressure transducers.

16. NORMAL
BITING
FORCESMOLARS - 400-
890 N
PREMOLARS - 222-
445 N
CUSPIDS - 133-
334 N
INCISORS - 89-
111 N
WORLD RECORD -
4337 N
NORMAL
BOND
STRENGTH
Micro hybrid composite
: 3.12N
Flowable composite
: 3.44
Resin modified GIC
:1.77N

17. saltwater crocodiles—
slammed their jaws shut with
3,700 pounds per square inch
(psi), or 16,460 newtons,
of bite force.
By contrast, you might tear into a steak
with 150 to 200 psi (890 newtons)

18. Force acting on the teeth
• These forces vary in magnitude, duration,
frequency and direction.
• The responses by the teeth to the forces depend on
factors such as,
• shape and length of the roots,
• the characteristics of the fluid content of the
periodontal space
• the composition and orientation of the periodontal
fibres
• the extent of the alveolar bone

19. • The responses by the teeth will also depend on
• the consistency of the bolus being chewed and
• the muscular forces being used to crush it.
• This will also apply to parafunctional clenching and
chewing with or without a foreign body between
the teeth.

20. Direction of displacement
• A tooth can be displaced in one of
six directions : -
• apically,
• mesiodistally or
• buccolingually,
• each one producing a rotation or a
translation.

21. • The result is likely to be a combination of all
directions leading to an omnidirectional
movement.
• The same principle of movement will apply to the
opposing tooth involved.
• These omnidirectional tilting and rotations of teeth
will reach a limit when an equal and opposite
resistance is reached and the periodontal receptors
cause a reflex arrest of the muscle force.

22. • When the force is removed, the teeth will recover
their positions due to the elastic recovery of the
compressed periodontal tissues. This is referred to
as “replacement” of the teeth.

23. This phenomenon may be modified by 3 factors ;
• Alveolar bone support
• Adjacent teeth support
• Horizontal muscle activity on both buccal and
lingual surfaces of the teeth.

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

25. Stress
Stress is the internal reaction to
the external force.

26. • Area over which the force acts is an important factor
of consideration especially in dental restorations in
which areas over which the force applied often are
extremely small.
•• Since stress at a constant force is inversely
proportional to the area, the smaller the
area, the larger the stress. And vice
versa

27. Types of stress
TENSION
COMPRESSIONSHEAR

28. Tension
• a body when it is subjected to 2 sets of forces that
are directed away from each other in the same
straight line

29. Compression
• the body is subjected to 2 sets of forces in the same
straight line and directed to each other.

30. Shear
• 2 forces directly parallel to each other

31. Complex Stresses :
• Whenever force is applied over a body, complex of
multiple stresses are produced.
• They may be a combination of tensile, shear or
compressive stress. These multiple stresses are
called complex stresses.

32. MECHANICAL PROPERTIES
OF MATERIALS
The mechanical properties of a material describe its response
to loading.

33. • 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”.

34. • As loading continues, the
structure is deformed. At first
this deformation (or strain) is
completely reversible (Elastic
strain).
• However, increased loading
finally produces some
irreversible strain as well
(plastic strain), which causes
permanent deformation.
• The point of onset of plastic
strain is called the elastic
limit. Continuing plastic
strain ultimately leads to
failure by fracture.

35. • Two of the most useful mechanical properties are the
modulus of elasticity and elastic limit.
• A restorative material generally should be very stiff so
that under load, its elastic deformation will be extremely
small.
• An exception is Class V composite which should be
less stiff to accommodate tooth flexure

36. BIOMECHANICS FOR
RESTORATIVE DENTISTRY

37. • Teeth are subjected to many forces during normal
use.
The interactions between the applied forces,
the shape and structure of teeth,
the supporting structures, and
the mechanical properties of tooth components
and restorative materials
-are all included in the subject of biomechanics

38. Biomechanical Unit :
• The standard biomechanical unit involves the
Interface
between the
restoration
and tooth
Tooth
structure,
and
Restorative
material

39. • The importance of considering three structures in
the biomechanical unit is to detect stresses that
may cause unwanted fractures or debonding. The
restorative material may be strong enough to resist
fracture, but the interface or tooth structure may
not be.

40. STRESS TRANSFER :
• Normal tooth structure transfers external biting
loads through enamel into dentin as compression.
• The concentrated external loads are distributed
over a large internal volume of tooth structure and
the local stresses are lower.
• During this process a small amount of dentin
deformation may occur which results in tooth
flexure .

41. 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.

42. • The process of stress transfer to dentin becomes
more complicated when the amount of remaining
dentin is thin and the restoration must bridge a
significant distance to seat onto thicker dentin
(Liners or bases).

43. TOOTH FLEXURE :
• Either a lateral bending or
an axial bending of a tooth
during occlusal loading.
• This flexure produces the
maximal strain in the
cervical region, and the
strain appears to be
resolved in tension or
compression within local
regions, causing the loss of
bonded class V restorations
in preparations with no
relative grooves..

44. • Such fractures predispose enamel to loss
when subjects to tooth brush abrasion
and/or chemical erosion.
• This process may be key in the formation
of Class V defects
Moreover, one current hypothesis is that tensile or compressive
strains produce micro fractures (called ABFRACTIONS) in the
thinnest region of enamel at the CEJ.

45. 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

46. IN ANTERIOR TEETH

47. • The junction 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
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.

48. • The axial angles and lingual marginal ridges will
bear concentrated shear stresses.
• The distal surface of a cuspid exhibits a unique
stress pattern as a result of the anterior
components of force concentrating compressive
loading at the junction of the anterior and posterior
segments of the dental arch and microlateral
displacement of the cuspid during excursive
movements.

49. • Both of these factors will lead to stress
concentration with resultant abrasive activity there.

50. • 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

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

52. In POSTERIOR TEETH

53. • Cusp tips, especially on the functional side bear compressive
stresses.
• Marginal and crossing ridges bear tremendous tensile and
compressive stresses.
• Axial angles bear tensile and shear stresses on the non-functional
side and compressive and shear stresses on the functional side.

54. • 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 .

55. WEAK AREAS IN THE TOOTH SHOULD
BE IDENTIFIED AND RECOGNIZED
BEFORE ANY RESTORATIVE ATTEMPT,
in order to avoid destructive loading
They are,

56. 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

57. SOME APPLIED
MECHANICAL PROPERTIES
OF TEETH:

58. Compressive strength of enamel supported by vital dentin is
usually 36-42,000 psi.
Compressive strength of vital dentin is 40-50,000 psi.
Modulus of resilience of vital dentin is 100-140 inch –
lbs/cubic inch.
Modulus of elasticity of enamel supported by vital dentin
under compression is 7,000,000 psi.
Modulus of elasticity of vital dentin is 1,900,000 psi.

59. 1. In general, when enamel loses its support of dentin, it
loses more than 85% of its strength properties.
2. Tensile strength of dentin is about 10% less than its
compressive strength.
3. Tensile strength and compressive strength of enamel
are similar, as long as the enamel is supported by vital
dentin.

60. 4. Shear strength of dentin is almost 60% less than its
compressive strength, and this is very critical in
restorative design.
5. There is minimal shear strength for enamel when it
loses its dentin support.
6. When the dentin loses its vitality, there is a drop of
almost 40-60% in its strength properties

61. OBTAINING RESISTANCE
FORM FOR TOOTH
STRUCTURES

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

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

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

65. • Definite floors, walls and surfaces with line and point angles
are essential to prevent micro movements of restorations,
with concomitant shear stresses on remaining tooth
structures

66. • 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
• Increasing the bulk of a restorative material or leaving
sufficient bulk of tooth structure in critical areas is one of
the most practical ways of decreasing stresses per unit
volume
Load – A
Load A
1 stress unit / mm3

67. • Designing the outline form with minimal exposure
of the restoration surface to occlusal loading will
definitely minimize stresses and the possibility of
mechanical fracture in the restoration

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

69. OCCLUSAL
CONSIDERATIONS IN
RESTORING TEETH
Before initiating any restorative care, thorough occlusal
examination should be carried out.

70. The way we occlude teeth affects the
periodontium, the temporomandibular joints,
throat muscles, tongue, cheeks, lips, nerves .
A clinician must have adequate knowledge about
the principles of occlusion, which enables him to
diagnose cases that need modifications /
alteration of occlusion with or without the use of
various restorative materials

71. • The kind of occlusion, a patient should have, must be
justified by the principles of physiology.
• The occlusion affects almost every part of
stomatognathic system, mainly :
1) The pulp of the tooth is a very sensitive organ. IT reacts
immediately to abnormal occlusal forces. Hence, occlusion
should not be detrimental to pulp.
2) The proximal relations of the occlusion should prevent food
impaction between teeth.
3) The cusp-fossa relationship should be such that the adequate
forces exerted during functional movement, aids in optimum
health of the periodontium.

72. OCCLUSAL
CONSIDERATIONS IN
RESTORING TEETH
ANTERIOR RESTORATIONS :

73. The resin composites and the glass ionomer
cements are mainly used in anterior
restorations.
Though these teeth do not come under direct
occlusion, however, they do take part in
various movements of the mandible.
The restoration should be carved and finished, maintaining the
contacts and the cervical curvature of these restorations. The lingual
area is carved to maintain the anatomy of cingulum and the lingual
marginal ridges.
Patient is asked to protrude and the
interferences are checked and removed

74. OCCLUSAL
CONSIDERATIONS IN
RESTORING TEETH
POSTERIOR RESTORATIONS :

75. Use articulating paper to register THE CENTRIC HOLDING SPOTS
and excursive contacts so that these marked areas can either be
excluded form the outline form or properly restored.
Prior to cutting a tooth, its opposing occlusal surface should be
examined. Malpositioned opposing supporting cusps and ridges
should be recontoured in order to achieve optimal occlusal
contacts in the restored tooth.
All posterior restorations should be planned keeping in mind
the basic principles of occlusion.

77. Plunger cusps and over erupted teeth should be reduced,
removing all the cuspal interference’s so as to improve the
plane of occlusion and decrease the chances of fracture of
new restoration as a result of occlusal forces.
When carving for occlusion, attempt to establish stable
centric contacts of cusps with opposing surfaces that are
perpendicular to occlusal forces should be made.
Occlusal contacts located on a cuspal incline or ridge slope are
undesirable because these create a deflective force on the tooth
and hence should be adjusted until the resulting contact is stable.

78. Occlusal Loading and
Its Effect

79. • During centric and excursive movements of the
mandible both restoration and the tooth structure
are periodically loaded both separately and jointly.
• This brings about different stresses patterns
depending on the actual morphology of the
occluding area of the both the tooth in question
and opposing contacting cuspal elements.
• For the purpose of this discussion, one can classify
these loading situations and their induced stress
patterns in the following way

80. • A small cusp contacts the fossa away from the restored
proximal surface, in a proximo occlusal restoration at centric
closure.

81. • A large cusp contacts the fossa adjacent to the restored
proximal surface in a proximo-occlusal restorations at centric
closure, either in the early stages of moving out of centric or at
the late stages of moving toward it.

82. • Occluding cuspal elements contact facial and lingual tooth
structure surrounding a proximo-occlusal or proximo-occluso-
proximal restoration, during centric and excursive movements.

83. • Occluding cuspal elements contact facial and lingual parts of
the restoration surrounded by tooth structure, during centric
and excursive movements.

84. • Occluding cuspal elements contact facial or lingual parts of the
restoration completely replacing facial or lingual tooth
structure during centric or excursive movements.

85. • .
• Occluding cuspal elements contact a restoration’s marginal
ridge(s) or part of a marginal ridge during centric or excursive
movements.

86. • Cuspal elements occlude or disocclude via the facial or lingual
groove of a restoration.
• There will be tensile stresses at the junction of the occlusal and
facial or lingual parts of the restoration at full intercuspation,
and to and from that position.

87. • Cusps and crossing ridges are part of the restoration in centric
and excursive movement.
• Both will be subjected to compressive stresses during such
positions and movement.
• Besides tensile stresses could concentrate at their junction
with the main restoration, specially during contacting excursive
movement.

88. • Axial portions of the restoration during centric occlusion and
excursive movement contacts:
• Whenever these portions are in contact with opposing occlusal
surfaces, there will be induced compressive and shear stresses
when they are not reciprocating (one side not in contact with
occluding surfaces while other axial portion). The axial
surfaces will be stressed in a slight tensile and shear pattern at
their junction with the main bulk of the restoration.

89. • Restoration is not in occluding contact or is in
premature contact during centric occlusion or
excursive movement of the mandible.
The first situation is not conducive to function, insofar as the
restoration will not be involved with direct loading from the
opposing occluding teeth.
After a period of time, however, the tooth will supra erupt, rotate,
and/or tilt, establishing contact with the opposing cuspal
elements.
Usually, this newly acquired location will not be the most favorable
position for the restoration, tooth, or the remainder of the gnatho
stomatic system, either mechanically or biologically

90. Role of Contact Areas :
Good restorative dental procedures must reproduce the
proper contact areas. Restorations with contact areas which
are flat, open, improperly placed, rough or poorly polished
will lead to failure.

91. • A slight frictional movement of teeth always occurs
between the interproximal surfaces of teeth during
physiologic movement; and with time, the contact
point becomes broad resulting in a wider contact
area.

92. • IF the teeth remained in contact with each other
merely by contact points, they would eventually be
forced out of the dental arch in either a buccal or
lingual direction.
• Whereas with a wider contact between teeth, this is
not likely to occur. The opposing interproximal surfaces
of restorations must be hard in order not to flow,
flatten, wear or become abraded with use.

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

94. • The pattern of wear varies individually.
• Non-uniform wear of opposing teeth is quite common
when one tooth is restored with a restorative material
whose wear resistance is different as compared to that
of enamel.
• Differential wear can result in localization of occlusal
loads with subsequent failure of restorative materials
or development of deflective contacts with mandibular
repositioning and an effect on a distant tooth.

95. Hypothetically, if two restorative
materials, which wear at a slower
rate than the natural teeth, are
placed so as to oppose each other in
a dentition undergoing wear, the
restorations will produce occlusal
interferences at a later stage.
Non-wearing materials opposing each
other can lead to natural teeth wear
during contact in lateral and protrusive
movements.

96. Conversely, if the materials wear
faster than the teeth, the opposing
cusp might over erupt into the
worn material.
IN lateral excursion, this cusp might
then come in contact with an
opposing cusp and if weakened by
previous caries can lead to fracture.

97. Mn

98. MECHANICAL FUNCTIONS OF THE
MARGINAL RIDGES
The marginal ridges play an important role in withstanding
and dissipating the occlusal stresses

99. Role of Marginal Ridges :
• The correct form of marginal ridge compatible with
the adjacent tooth and also with its own
surrounding is important during carving of
posterior restorations.
• The absence of marginal ridge, or marginal ridge
with improper height can lead to altered dissipation
of forces subsequently damaging the underlying
periodontium.

100. • A marginal ridge should always be formed in two
planes bucco-lingually , meeting at a very obtuse
angle.
• This feature is essential when opposing functional
cusp occludes with the marginal ridge.

101. • The following diagram illustrates how a proper marginal ridge will
perform these functions.
• But, as mentioned, with age, the dimensions of marginal ridges
and occlusal embrassures are reduced, due to vertical occlusal
attrition and proximal flattening of the contact areas.

102. • The following examples will illustrate the consequences
incurred by creation of a faulty marginal ridge :
• Absence of marginal ridge in the restoration.
By the absence of marginal ridge , force 1 will be directed
towards the proximal surface of adjacent tooth.

103. • A marginal ridge with an exaggerated occlusal
embrasure.
• Exaggerating the occlusal embrassure will direct forces 1 and 2
towards the adjacent proximal surfaces , with the horizontal
components , 1H and 2H , separating the teeth and vertical
components , 1Vand 2V , driving debris interproximally.

104. • Adjacent marginal ridges not compatible in height:
• constructing a restoration with a marginal ridge higher
than the adjacent one will allow force A to work on the
proximal surface of restoration .
• The horizontal component AH , will drive the restored tooth
away from the contacting tooth , and vertical component will
drive the debris interproximally.

105. • Even in the presence of force B , with its horizontal component
acting on the adjacent marginal ridge , there will be some
separation of teeth as the surface hold for force B is too small
to counteract that force A.
• By constructing a restoration with a marginal ridge lower
than the adjacent one , the same thing will occur, but the
major movement will be in the non restored tooth.

106. • A marginal ridge with no adjacent triangular fossa :
• In this situation there are no occlusal planes in the marginal
ridges for the occlusal forces to act upon, so there are no
horizontal components to drive the teeth toward each other ,
closing the contact.
• Furthermore the vertical force will tend to impact food
interproximally.

107. • A marginal ridge with no occlusal embrassure:
• In this case , the two adjacent marginal ridges will act like a
pair of tweezers grasping food substance passing over it.
• Although debris may not be forced interproximally , it will be
very difficult to remove once it is thus trapped.

108. • A one planed marginal ridge in bucco-lingual direction
• Usually , the facial and lingual inclines of a marginal ridge are
part of the occluding components of the tooth.
• Therefore making them one planned can create premature
contacts during both functional and static occlusion.

109. • A one planed marginal ridge increases the depth of the
adjacent triangular fossa , magnifying stress in this area.
• Moreover , the one planned marginal ridge could increase
the height of marginal ridge in the center, making it
amenable to adverse effects of horizontal components of
force.
• Like wise a one planed marginal ridge will deflect the food
stream away from normal, proximal embrasure movements
( spill away )

110. • A thin marginal ridge in mesio-distal bulk will be susceptible to
fracture or deformation leading to the problems of previously
mentioned faulty marginal ridge .
• Also, this thinness may leave either shallow or deep adjacent
fossa or bulky occlusal anatomy with their aforementioned
inherent problems.

111. Forces acting on
Intracoronal
restorations
The cavity should have such retention form that the
restorations will be firmly held in place, the cavity should also
have resistance form that the restoration will withstand the
stress without being dislodged.

112. FORCES ACTING ON
POSTS

113. FORCES ACTING ON POSTS
• An endodontically treated tooth has been structurally
compromised by caries and its removal, prior restorations,
and finally, endodontic preparation and filling.
• It should be emphasized again that posts are only used
for retaining the restorative material in the remaining tooth
structures, and by no means will they reinforce or improve
the strengths of these tooth structures

114. • Because the retention of posts is accomplished by various
means, it might be expected that different stresses are
associated with post installation.
• With posts retained by the cement alone, the main
potential for installation induced stresses is the build up of
hydrostatic back pressure.
This potential with parallel – sided post is circumvented by
means of longitudinal vents along the posts, which provide
an outlet for the pressure.
Tapered post are self-venting, and consequently there is no
pressure build up.

115. MECHANICAL ASPECTS OF POST-
RETAINED RESTORATIONS AND
FOUNDATIONS :
• The Stressing Capabilities of Posts :
• The following features and factors of posts and the
involved tooth will govern the stress patter induced in the
surrounding tooth structures due to the use of posts as
retentive means :

116. • Type of Posts :
• Parallel sided posts will have
the tendency to evenly distribute
the forces it receives at and
around its cavity and onto the root
canal walls, if these forces are
applied parallel (a) to the post axis
(vertical occlusal loading. )

117. • IF the forces applied are at a right
angle (b) or oblique (c) to the post
axis, the induced stresses in the root
canal walls will be unevenly
distributed,
• i.e. there is a great possibility of stress
concentration due to uneven thickness
of the root canal walls around the post
(root taper) while the post remains the
same diameter.
• This leads to a thin sectioned wall at
the very apical end of the post.

118. On the contrary, taper sided
posts and combination type
posts will concentrate
stresses due to apical
loading (a) in the root canal
walls resulting from its
wedge shape.
Lateral loading on and
around cavity ends of the
post, however, will induce
evenly distributed stresses in
the root canal walls for the
taper of the post will
correspond with the root
and root canal taper, leading
to an even thickness of walls
occlusoapically.

119. • During insertion of a post into the root canals, highly threaded
posts can induce ten times the amount and extent of stresses
as smooth sided posts.
• Serrated surface posts will induce about one and a half to two
times the stresses that are induced by smooth surfaced posts.
• This can be explained by the cemented technique utilized by
the serrated and smooth surfaced posts.

120. • . Bulk of dentin in root canal walls :
•
Naturally, the bulkier that the dentin surrounding a
root canal post is, the less will be the induced stresses
per unit volume during the post insertion and
functional use of the post retained restoration.
It has been estimated that a minimum of 2 mm of
dentinal root canal wall should surround a post, so that
the stresses induced there will not lead to dentinal
failure in the form of cracks and gross fracture.

121. • Length of clinical root involved with the root canal post :
• Although the tooth to receive a root canal post should be non-vital
and endodontically treated, the clinical crown portion of the tooth is
much more dehydrated than the clinical root portion as the dentin
portion of the root still receives some fluids from the adjacent
periodontal ligament.
• The more dehydration that there is, the less will be the modulus of
resilience and elasticity of the dentin, and consequently the less will
be the dentin’s ability to absorb and resist stresses without failure

122. • Ferrule or embracing features of the restoration :
• Post-core and dowel coping foundations
for endodontically treated teeth will always
induce stresses in the root canal walls and
remaining tooth structures which can only be
counteracted by embracing the buccal and
lingual cuspal elements of the tooth and/or
banding (circumferential embracing) the tooth at
its most apical part of the clinical crown (i.e. area
of maximum stresses).
• Such bracing is referred to as the Ferrule effect.

123. The Ferrule feature of the restoration should
involve at least 2 mm of crown length to
counteract stresses induced by the post.
The closer this embracing feature is to the
junction between the clinical crown and the
root, the more effective it will be.
This is the major protecting feature against
induced stresses in a restoration for
endodontically treated teeth.

124. • . Lateral Locking Mechanisms for the post and restoration :
Because most premade posts are rounded
in cross-section there is a great tendency
for the post and the restoration retained
by the post to rotate under torsional
forces.
This rotational tendency can induce
unnecessary stresses in remaining tooth
structures.
The presence of a method to lock the post
and the restoration against such rotation
(e.g. a lateral pin, internal boxes, opposing
walls, etc) will drastically reduce the effect
of torsional forces.

125. • 9. Proximity of the post to the root canal filling :
IF the post approximates the root canal filling, forces can be
transmitted to that filling, which mechanically is made of very
weak materials, and lead to profound straining.
For this reason, there should be a space between the apical end
of the post and the occlusal end of the root canal filling.
Root canal fillings should not be involved in the mechanical
problems of the posts.

126. In addition, the direct or indirect loading of the root canal filling
may change its relationship to the surrounding walls and apical
anatomy, resulting in endodontic failure.
This can move the post in an undesirable direction, and it may
induce unnecessary stresses in the remaining tooth structure.

127. • . Presence of flat planes in the remaining tooth structures, at a
right angle to occluding forces :
Flat planes, in the form of tables, gingival floors and ledges,
etc, which will be able to receive and resist occluding
forces before arriving to the post, are the second major
feature used to reduce induces stresses in the remaining
tooth structure.
Besides partially protecting the post from direct loading,
these flat planes will protect a very weak subpulpal floor
from being directly loaded

128. • Presence of lateral walls in the remaining tooth structure :
• Extra or intracoronal axial walls, that will receive and
resist laterally applied forces on the restoration before they
arrive at the post, will drastically reduce stresses in the
remaining weakened tooth structure, primarily in the root
canal walls.

129. • . The root post portion relative to the crown post portion :
The ideal ratio is to have the
root portion of the post twice
as long as the crown portion,
i.e. a ratio of 2:1.
Less than that, especially less
than a ratio of 1:1, will
definitely concentrate
intolerable stresses on the
lateral walls of the root canal
adjacent to the apical end of
the post.

130. • . Hydraulic pressure during post cementation :
• If there are no lateral vents in the post, or if the post diameter
is very close to that of the post channel diameter, the semi-
liquid cement mix, during the cementation of the posts, may
exert tremendous amounts of hydraulic pressure that exceed
the elastic limit of the surrounding dentin or prevent complete
seating of the post.

131. • . Surface texture and shape of the root end of the post :
Greater post surface
roughness and/or the
presence of a chisel, wedge, or
irregular configuration on the
root end of the post, increases
the possibilities of stress
concentration on the root
canal walls.
The concentration of these
stresses will increase with
increasing proximity of the
post to the involved root canal
anatomy.

132. By placing the tip of the root post
there, with attendant possibilities of
substantial stresses being
concentrated at that tip, catastrophic
failures become inevitable.
As a rule from one half to two
thirds of the root canal should
encapsulate the post if the forces
transmitted by the post are to be
adequately dissipated.

133. • . Shape of the post in cross section relative to the shape of the
post channel :
A post should have a circumference that coincides with the
post channel.
Differences, e.g. rounded post in an oval post channel, will
concentrate stresses at isolated locations in the root canal
wall, possibly exceeding the local breaking point of the
dentin.

134. • Unconfined movements of a post
within a root canal can exaggerate
stresses in the root canal walls
upto the fracture point of dentin.
. Loose post
in the post
channel :

135. • Thread numbers and patterns :
• Continuous threads from one end of a post to another
create more stresses than interrupted threading.
• The greater that the spacing is between threads, the less will
be the attendant stresses.
• The sharper that the threads are, the less will be the stresses.
• Circumferentially interrupted threading creates less stresses
than continuous threading.

136. • The wider and more frequent that the interruptions are, the less
will be the stresses.
• Interruptions (cross cuts) further serve to facilitate escape of
debris during post insertion.
• The more extended that the threads are laterally, the more the
surface interfacial contact with dentin will be and consequently,
the higher the stresses.

137. FORCES ACTING ON A CAST
METAL AND PORCELAIN
RESTORATIONS

138. •FORCES ACTING ON A CAST METAL AND
PORCELAIN RESTORATIONS
• BIOMECHANICAL PRINCIPLES OF PREPARATIONS:
• The design and preparation of a tooth for a cast metal or
porcelain restoration are governed by :
Preservation of
tooth
structures.
Retention and
resistance
forms
Structural
durability of
the restoration
Marginal
integrity
Preservation of
the
periodontium.

139. • A restoration can meet its functional, biological and esthetic
requirements if it remains firmly attached to the tooth.
• Its capability for retention and resistance must be great enough to
withstand the dislodging forces it will encounter in function.
• An estimate as to the prevailing occlusal forces can be had by noting
the degree of wear on the other teeth, firmness of the opposing
teeth, thickness of the supporting base and the bulk of masticatory
muscles.

140. • There are 4 factors under the control of the operator during
tooth preparation which influence retention.
Degree of
taper
Total surface
area of the
cement film
Area of
cement
under shear
Roughness of
the tooth
surface

141. • Degree of Taper :
• The more nearly parallel the opposing walls of a preparation,
the greater will be the retention. Thus retention decreases as
taper increases.
• However, in order to avoid undercuts and to allow complete
seating of the restoration during cementation, the walls must
have some taper.
• An overall taper or angle of convergence of 6 degrees is
considered as appropriate i.e. approximately 3 degrees being
produced on each surface, external or internal, by the sides of
a tapered instrument.

142. • Total Surface Area of Cement Film :
The greater the surface area of
cement film or the of the
preparation, the greater the
retention of the restoration.
The total surface area of
preparation is influenced by
the size of the tooth, the
extent of coverage by the
restoration and features such
as grooves and boxes that are
placed in the preparation.

143. Area under shear :
• More important for retention than the total surface area is
the area of cement that will experience shearing rather than
tensile stress when the restoration is subjected to forces
along the path of insertion.
• To decrease the failure potential, it is essential to minimize
tensile stress

144. • For the shear strength of the cement to be utilized, the
preparation must have opposing walls, i.e. two surfaces of the
preparation in separate planes must be nearly parallel with
each other and the line of draw.
• To obtain the greatest area of cement under shear, the
direction in which a restoration can be removed must be
limited to essentially one path.
• Thus the addition of parallel sided grooves, limits the path
of withdrawal to one direction, thereby reducing the possibility
of dislodgment.

145. • The length and width of the preparation is an
important factors in retention :
• A long preparation as well as wider preparation has greater retention
than does a shorter or a narrower preparation.
• Surface Roughness :
• Adhesion of dental cements depends primarily on projections of the
cement into microscopic irregularities on the surfaces to be joined.
Therefore prepared tooth surface should not be highly polished.

146. Forces acting on
Extracoronal
restorations

147. FORCES ACTING ON INLAY RESTORATION

148. FORCES ACTING ON INLAY RESTORATION
All the line and point angles should be definite, but not
angular, so they can be easily reproduced in a casting and to
avoid stress concentration in the casting and the tooth
structure.

149. • The axial wall should slant toward the
pulpal floor, as part of the taper. This,
together with rounding of the axio-
pulpal line angle, can reduce stresses at
the isthmus area.
• Reduction of tooth structure should
follow the original anatomy of the
tooth, even reduction, with minimum
tooth involvement

150. • Maximum reduction should be at the occluding
surfaces,
• average of 1 mm should be cleared for metallic
casting in the inclined planes of the cusps. This
reduction should be 1.5 mm for cast ceramics.
• The reduction of the occluding inclined planes
should be cut in a concave form, to accommodate
maximal bulk of the casting where stresses are at
their maximum.

151. • The internal boxed up portion should occupy the
maximum dimensions of the cavity preparation as
practically as possible. This will necessitate making
the cavity wall in different planes.

152. It has been stated that when a force is applied at right angles to
a surface its effectiveness with the direction of force and that is
proportional to its magnitude likewise, the opposing forces are
equal and opposite in direction.
Another law states that if the force is applied at an angle to the
surface other than right angle, the magnitude of which
depends on the angle of application and that the reacting force
is neither equal nor opposite in direction.

153. forces applied at right angles to
the flat surface of a restoration
A typical proximoocclusal
cavity will have two such
surfaces to vertical forces
– the pulpal and gingival
walls.
If the forces are
perpendicular to these
surfaces the opposing
forces are equal and
opposite, then there is
no tendency to displace
the filling. Floors
positioned perpendicular
to these lines of force
absorbs the stress over a
broad area of tooth

154. It is only when the pulpal wall is flat and the two vertical walls
are parallel to each other that the maximum retention form is
obtained.
In a tooth weakened by extensive caries, the resistance form
is obtained by extracoronal extension of the preparation in
the form of extra long reverse bevel in capped cusps or
by partial or complete coverage of facial or lingual surfaces

155. • But because of the inherent weakness of the
gingival groove the possible fracture to this wall of
the tooth structure between the groove and the
cavosurface angle.
• so many operators prefer the inward beveling of
the gingival wall, forming an acute angle between
the axial and gingival walls.

156. • Pulpal Wall : another method of obtaining
opposing movements to horizontal displacing force
is by establishing resistance into pulpal wall.
• The pulp wall which is flat offers no resistance to
horizontal displacement.
• when it is prepared with two inclined planes it will
prevent the lateral displacement of the inlay.
• Another modification is placement of grooves
parallel to the long axis of the tooth at the axial
angles

157. • This line angle is slightly rounded
to dissipate the stresses.
Axiopulpal Line
Angle :
• 30-45o to have sliding lap fit joint,
cement tooth interface.
Gingival Bevel :
Certain forces collectively act on a cemented
restoration mainly in the same direction as the path of
withdrawal.

158. The factors pertaining to these
forces
• Magnitude of the dislodging forces :
• Forces that tend to remove a cemented restoration along its
path of withdrawal are small compared to those that tend
to tilt it.
• depends on the stickiness of food, occluding and lateral
movement forces of the jaws and the surface area and
texture of restoration being pulled.
• Stress Concentration :
• Stresses are not uniform throughout the cement but are
concentrated around the junction of the axial and occlusal
surfaces (axio pulpal line angle). This may explain the
retentive failure of the cast restoration. The strength of the
cement is less than the induced stresses.

159. FORCES ACTING ON DIRECT
TOOTH COLOURED
RESTORATIONS

160. • FORCES ACTING ON DIRECT TOOTH COLOURED
RESTORATIONS
•
For any proximal restoration in
anterior teeth, there are two possible
displacing forces. The first is a
horizontal force displacing or rotating
the restoration in a labio-proximo
lingual or linguo proximo labial
direction. It has its fulcrum almost
parallel to the long axis of the tooth
being loaded.

161. The second is a vertical force
displacing or rotating the
restoration
proximally(sometimes
facially or lingually).
The vertical force has a
loading arrangement similar
to occluso-proximal
(occluso-facial or occluso-
lingual) restorations in
posterior teeth.

162. • The mechanical picture can be summarized as follows :
In anterior teeth with normal overbite
and overjet during centric closure of the
mandible (from centric relation to centric
occlusion), mainly the horizontal forces
will be in action.
Those forces, if loading the proximal
restoration directly, would try to move it
linguo-proximo labially (for the upper
restoration) and labio-proximo lingually (for
the lower one).

163. 3. If the upper and lower anterior teeth meet such that the lowers
are labial to the uppers in centric occlusion (Angle’s Class III),
there will be the same type of loading conditions mentioned in (1)
except the horizontal loading will tend to rotate or displace
restorations labio proximo lingually (for uppers) and linguo-
proximo labially (for lowers).
2. If anterior teeth meet in edge-to-edge fashion at centric
occlusion, loading of the proximal restoration, involving incisal
angles (Class IV) will be similar to any Class II proximo-occlusal
restorations, i.e. vertical displacing forces with very limited
horizontal components.

164. 4. In occlusions with deep
anterior overbite and normal or
no overjet, the horizontal type
of loading will be greatly
exaggerated. The vertical
displacement, although present,
will be minimal by comparison.
5. In occlusion with anterior
open bite or severe overjet, or
any other condition that creates
a no-contact situation between
upper and lower anterior teeth
during centric occlusion and
excursive movements of the
mandible, proximal restorations
will not be loaded directly
either vertically or horizontally.

165. • . Loss of the incisal angle of a tooth, i.e. conversion
from a Class III to a Class IV represents a major
complication in the mechanical problems of
anterior tooth restorations.

166. Anterior teeth have their maximal
bulk gingivally.
They taper incisally with the
least bulk at the incisal ridge. So
resistance to stress fractures will
be maximum at the gingival end
and decrease incisally

167. Forces are directed horizontally and
vertically on anterior teeth.
These forces accumulate maximal
shear stresses at the junction of the
clinical root with the clinical crown
and maximum tensile stresses at the
incisal ridges, especially their corners
(incisal angles).

168. The labial enamel plate is
much thicker than the
lingual or proximal ones,
with maximal thickness of
enamel usually at the incisal
ridge.
The incisor may be involved
in a disclusion mechanism
of the mandible with
loading similar to that of the
cuspid, but to a much lesser
extent.

169. Occluding surfaces of anterior teeth,
especially the lingual surfaces of upper
teeth and incisal ridges of lowers are the
most important anterior determinant of
mandibular movements.

170. The extent and degree of concavities
on these upper teeth lingually and the
inclination and roundness of incisal
ridges of the lower ones , determine to
great extent the amount of loading ,
their directions and the pattern of
mandibular movements anteriorly and
latero-anteriorly.

171. • Cervical portions of anterior teeth when they are affected
with a Class V lesion or cavity preparation will have a stress
pattern similar to posterior teeth, and the stress pattern is
governed by the same factors as in posterior teeth.
• In addition, the deeper the overbite is, the more induced
the stresses are at these cervical areas.

172. Ideally, a restoration made of tooth
colored materials should not be loaded
directly, i.e. there should be intervening
tooth structure between the occluding
tooth and the restoration.
This situation can only be achieved by
four intact walls surrounding the
restoration. This is usually not the case.
That is why the clinical performance of
tooth colored materials differs form the
situation to another, sometimes
dramatically.

173. Crowns

174. • Preparation Length and Resistance :
• The ability of a restoration to resist tipping depends not only
on the preparation, but also on the magnitude of the torque
• If two crowns of unequal length on two preparations of equal
length, are subjected to identical forces, the longer crown is
more likely to fail because the force on it acts through a longer
lever arm.
ADEQUATE PREPRATION SHORT PREPRATION

175. • Leverage and Resistance :
• The strongest forces encountered in function are apically
directed and can produce tension and shear in the cement
film only through leverage.
• If the line of action of force passes within the margin of a
restoration, there will be no tipping of restoration. The
margin on all sides of the restoration is supported by the
preparation. The torque produced merely tends to seat the
crown further.
• If the line of action of force passes outside the margins of
restoration the occlusal table of the restoration is wide, even
a vertical force can pass outside the supported margin and
produce destructive torque. This can also occur in crowns on
tipped teeth

176. • Resistance and Tooth Width :
• A wide preparation has greater retention than a
narrower one of equal height.
•
• Taper and Resistance :
• The resisting area decreases as the preparation
taper increases.
• The walls of a short, wide preparation must be kept
nearly parallel to achieve adequate resistance form.

177. • Rotation around a vertical axis :
• Geometric forms such as grooves or “wings” increase
resistance by blocking rotation around a vertical axis.
•
• Path of Insertion :
• The path of insertion for posterior full and partial
veneer crown is usually parallel with the long axis
of the tooth

178. • Occlusal Reduction :

179. CONCLUSION
• Optimal functional capacity and stability of occlusal
relationships are major considerations in every phase of
restorative dentistry.
• Restoration not only mechanically replace the lost part but,
acts as a medium through which physical and mechanical
forces are transmitted to the tooth and investing tissues. Each
tooth has its own stress patterns. A thorough knowledge in
dental materials is necessary to understand the physical
properties including their response to stress.

180. References
• Textbook of clinical operative dentistry by sturdevant
• Textbook charbeneur
• Textbook marzook
• Textbook of periodontology by Carranza
• Shivakumar AT, Kalgeri SH, Dhir S. Clinical considerations in restorative dentistry
- A narrative review. J Int Clin Dent Res Organ
• Journal of international clinic dental organization
• Journal of Clinical and Diagnostic Research. 2017 Sep, Vol-11(9): ZE01-ZE0
• Journal of Clinical and Diagnostic Research. 2017 Sep, Vol-11(9): ZE01-ZE0
• 0022-3913/79/020143 + 03$00.30/O 0 1979 The C. V. Mosby Co.
• THE JOURNAL OF PROSTHETIC DENTISTRY 143
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