INTRODUCTION
The
restoration of a tooth, be it with gold, amalgam, porcelain, or any other
acceptable material, can be no better than the preparation designed for it.
Mechanics : Webster’s definition is “that science or branch of mathematics
which treats the action of forces in bodies”. It is evident that part of oral
cavity with which the dental surgeon is largely concerned, is a machine, by
means of which forces are applied to food for purpose of comminuting it. The
force is the result of contraction of muscles of mastication and is transmitted
through rigid structures of jaws to food. It follows, therefore, that these
structures are subjected to laws of action of forces on bodies.
ANALYSIS OF FORCES ON
THE TOOTH
The tooth is the immediate agent through which the work of
masticating the food is accomplished. The forces required to bring about the
comminution of food are applied through the bone of mandible and periodontal
membrane to root of the tooth. These forces in turn are determined by the
reacting forces on tooth. If the occlusal and incisal surfaces of teeth were
flat and at right angles to direction of forces applied, the reaction of tooth
would be along its long axis. However, the opposing surfaces are curved, so
that other forces are setup and applied and reacting forces are not along long
axis of teeth. A study of this problem resolves itself into a study of inclined
plane, while the actual cusps themselves are not plane, they may be considered
as such by taking the surface of inclined plane as the slope of tangent at
point of contact of opposing cusps.
When a force acts perpendicular angles to a fixed horizontal
frictionless surface, the surface reacts at right angles to its plane with an
equal and opposing forces. If surface is now titled at an angle to the
horizontal, it still reacts at right angles to its plane as this is the only
direction in which a frictionless surface can react. Its reacting force
therefore no longer opposes the applied force in direction nor it is equal to
it in magnitude, hence the forces are not in equilibrium, unless and third
force enters the picture.
COMPOSITION AND
RESOLUTION OF FORCES
A force can be resolved into a number of component forces usually
two at right angles to each other e.g. an oblique force R in the plane of paper
can be resolved into a vertical component Y and a horizontal component X as
shown. If two forces are applied at a point on a surface and these forces are
in equilibrium with the resisting forces of surface, in other words, if surface
does not move, there must be force of reaction by surface equal in magnitude to
this resultant and in the opposing direction. Such a force would be represented
then by reversing direction of resultant.
FORCES ACTING ON TOOTH
Vertical forces : These are the forces which act along the long axis
of the tooth. They are well tolerated by the underlying periodontium and are
less harmful than the horizontal and oblique forces.
In centric occlusion when
the opposing teeth contact, the resultant of all the forces are in vertical
direction and act axially.
Horizontal forces : Horizontal forces act
perpendicular to the long axis of the tooth. These forces are generally the
horizontal components of oblique forces generated by contact of the opposing
cuspal slopes. When the various horizontal components are in equal and opposing
direction then the resultant force is zero. In the centric occlusion and in
working bite with thin compressable food in between cusp the horizontal forces
are usually balanced if the occlusion is correct.
Oblique forces : Oblique forces act at an angle other than
perpendicular. These are the forces which are usually generated by the contact
of opposing cusps. They are resolved in two components: the horizontal and the
vertical component.
In chewing, the mandible
moves from lateral to centric occlusion under forces whose resultant is oblique
force which is directed medially. When tough foods are compressed the oblique
forces thrust palatal cusp of maxillary teeth and buccal of mandibular teeth.
It is now possible to consider the forces acting at point of contact
between two cusps or forces on cuspal incline planes. AB is a tangent drawn at
incline plane or contact between two cusps. Angle ‘a’ represents the angle made
with horizontal AC by tangent AB of cuspal contact. M is force of mastication and
N is resolving
force. M is perpendicular to horizontal AC and N is perpendicular to
incline plane, tangent AB. H is the lateral force which would counteract the
horizontal component of resolving force and maintain the equilibrium. As angle
‘a’ decreases or as the incline approaches horizontal, N becomes shorter and
finally equal M, and H becomes shorter and finally equal to zero. If ‘a’ is
increased, N becomes greater and finally approaches infinity as does also H.
A) In centric occlusion
In
figure 1, teeth are in centric occlusion under a pure closing effort. In this,
only axial forces are applied to the tooth. Rab gives the magnitude and line of
action of resultant of forces ‘a’ and ‘b’ while Hab is horizontal
component of Rab. Horizontal component of C is Hc which to meet the
condition of equilibrium must equal Hab. Vabc is then the
only force acting on tooth as a whole and is equal to sum of vertical
components of all the applied forces.
B)
Figure II shows the teeth in
lateral relationship or working bite with
no food or a very thin layer between their opposing surfaces. In the absence of
a force ‘c’, there is no horizontal component to balance Hab. A
force ‘c’ may be produced by a thin, compressed layer of food tough enough to
offer resistance to bringing the buccal slope of lingual cusp of maxillary
tooth
into contact with lingual slope of buccal cusp of lower as in Fig.
III, this again approximating a condition of axial loading.
In chewing, the mandible moves from lateral to centric occlusion
under forces whose resultant is not vertical but inclined medially.
When tough foods are compressed or all cusps are in intimate contact
at three points, forces ‘a’ and ‘b’ are decreased and ‘c’ is increased with
resultant changes in their horizontal and vertical components as in Fig. IV.
The resultant Rabc is a thrust inclined lingually on maxillary and
buccally on the mandibular teeth whose horizontal component is Habc.
DESIGN OF TOOTH
- Cusps:
Cusps are rounded eminences on the occlusal surfaces of posterior
teeth. The cusps of lower and upper teeth intercuspate in centric occlusion.
The advantage of having these convex surfaces or cusps rather than flat
surfaces is that these results in point contacts rather than broad areas of
contact. This reduces the overall occlusal load. Another advantage is that they
guide the teeth in correct position on intercuspation .During the working or
protrusive relationship the number of point of contacts further reduce.
The cusps on the occlusal surfaces are divided in two types with
each having certain charteristics which determine how much load it can take.
The two types are :
à Functional cusps : Buccal cusp of Mandibular teeth and palatal cusp of maxillary
teeth.
à Non functional cusps : Lingual cusp of mandibular teeth and buccal cusp of maxillary
teeth.
Functional cusps take up more load and contact 2 opposing cusp in
centric occlusion. Whereas the nonfunctional cusp make contact with only one
cusp in centric occlusion. However the nonfunctional cusp especially of
manibular teeth are more likely to fracture under masticatory load.
Various features of functional and nonfunctional cusp :
Satish C. Khera (1990) gave various anatomic differences between
functional and nonfunctional cusp which determine the fracture potential of
both.
à Buccolingual width : All functional cusps were larger in Buccolingual width than the
nonfunctional cusp, except the maxillary premolars. The smaller width
nonfunctional cusp are thus more likely to fracture. The width of functional
cusps of all teeth except maxillary premolar was approx. 57% of buccolingual
width of tooth (53-67%). For maxillary premolar value was 47% (46%-48%)
à Enamel thickness : Enamel thickness between
functional & nonfunctional cusps except for mandibular 2nd
premolar was significantly different. The difference of 0.126 mm – 0.282 mm was
noted with functional cusp having thicker enamel.
à Cuspal inclination : The results of various surveys show that nonfunctional cusp of
molars and functional cusp of maxillary premolars fracture with greater
frequency. This is consistent with the fact that these cusps have higher
inclination as compared to their counterparts.
Cusp inclines are guiding planes for the lateral masticatory
movements in group function type occlusal relationship. In the canine protected
occlusion these have no part in movement after disoccluding. When centric
contact between inclines of functional cusp in lost due to caries or
restorative procedure, further eruption of teeth occurs. This results in cuspal
incline relationship no longer compatible with lateral excursive movements.
This leads to trauma and fracture of cusps as the forces acting on tooth are not
in equilibrium.
- Transverse ridges on triangular ridges :
These are prominently formed enamel that extends from the cusp tips
towards the center of occlusal surfaces, usually ending in fossae or
developmental grooves. The cusp tips never perfectly fit into the grooves and
sulci because of presence of these convexities. Thus they provide escape spaces
needed for efficient occlusion during mastication.
These ridges also appose the opposing sulci and grooves and create
the guiding paths. The main ridge sulci occlusion are :
à Triangular ridges of buccal cusp of maxillary molar accommodated
into buccal grooves of mandibular molar.
à Distolingual cusp’s triangular ridge fits into lingual groove of
maxillary molar.
- Oblique transverse ridge :
Another important ridge sulci relationship especially during lateral
occlusal movement is of the oblique ridge of maxillary first molar, a
triangular ridge extending from disobuccal cusp to mesiolingual cusp. This
ridge fits into the sulcus formed on the mandibular 1st molar by
junction of distobuccal, central & lingual development groove. During the
sliding contact action, from the most facial contact point to centric
occlusion, the teeth intercuspate and slide over each other in a directional
line approximately parallel with the oblique ridge of the upper first molar.
4. Marginal Ridges
Ø Normal marginal ridge:
Forces 1 and 2 act on marginal ridges of teeth A and B respectively.
The horizontal component of 1, H1 and horizontal component of 2, H2
counteract each other. The vertical component V1 and V2
are resolved normally by underlying tissues.
Ø No marginal ridges:
In this figure, tooth B has no marginal ridge. Force 1 and 2 are
acting on tooth A and B. the horizontal component of 2, H2 is
missing in tooth B, because force 2 is mainly directly towards tooth A.
Horizontal component H2 will drift the tooth A part and vertical
component V1 and V2 of both forces 1 and 2 will help food
impact vertically. The vertical force V2 will be more than required,
there may occur slight tilting of tooth B. This will further deteriorate the
resolution of forces and lead to further food impaction.
Ø A marginal ridge with
wider occlusal embrassure :
Inspite of putting optimal pressure on marginal ridges of tooth A
and B, force 1 and 2 act on adjacent teeth. The force 2 will put pressure on
tooth A and force 1 will put pressure on tooth B. this will lead to drifting of
both the teeth. The vertical component of forces will wedge the food in between
the two teeth.
Ø No occlusal embrasure.
In totality, the vertical component of forces 1 and 2 will be more
concentrated than horizontal components. Though there will not be any vertical
impaction of food, the continuous impact of higher concentration of vertical
component of forces may lead to changes in alveolar bone after sometime.
5. Sulci, grooves and fossae :
Sulci : Sulci are linear depressions between the enamel ridges with
developmental grooves at the bottom of the enamel valley or sulci.
Grooves : Grooves are linear, shallow depressions formed by fusion of
developmental lobes.
Fossae : These are shallow depressions formed at the junction of various
grooves.
Sulci, grooves, fossae, etc. form the concavities on the occlusal
surfaces into which the cusp and ridges interdigitate, without reaching the
bottom of above anatomic features. This forms the escapement spaces that are
needed for efficient occlusion during mastication. The important cusp fossae
relationship are :
à Mesio lingual cusp of maxillary molar fits in major fossae of lower
molar in centric occlusion.
à Distolingual cusp of maxillary molar are in apposition to distal
triangular fossae and marginal ridge.
à Mesiobuccal cusp of mandibular molar are in apposition to distal
fosse and marginal ridge.
à Distobuccal cusps of mandibular molar are accommodated by central
fossae of maxillary molar.
à
STRESSES DUE TO AXIAL
LOADS
A prism or rectangular block of material may be subjected to an
applied force, so that the line of action of the force passed through the
centre of area to which it is applied and is parallel to the axis or may be
perpendicular to this axis as in beams.
The first type which is known as axial loading, is approximated by
class 1 and class II restorations when a vertical load is applied over the
gingival floor. If the cross-section of the prism is constant, stress
distribution is practically uniform along the axis from the point where loads
are applied. If there is a variation in the area, stress varies from point to
point decreasing as the cross-sectional area increases and vice-versa. The unit
stress while no longer uniform across the area may be nearly so, if the change
is gradual. If however, the change in cross-sectional area is sudden, there is
great concentration of stress at the point of change is sudden, there is great
concentration of stress at the point of change as shown below. This could be
important in occlusal locks of class II restoration when these locks are
subjected to tensile forces due to wedging action of cusps.
SHEAR STRESS IN AXIAL
LOADING
In axial loading the applied forces are tensile or compressive but
it can be shown that under these forces there will be shearing stresses in the
prism in any plane neither perpendicular nor parallel to the applied force.
This shearing stress increases to a maximum of 45o and then decreases
to zero at 90o. therefore materials that are weaker in shear than in
compression or tension rupture in planes at 45o to the axis.
STRESSES DUE TO
TRANSVERSE LOADS
When a load is applied to the axis of a prism, the structure is
frequently spoken of as a beam and the mechanical problems are of a different
nature from those due to axial loading.
Beams may be divided into two groups. One group consists of the
simple beam (A) supported at both ends and the cantilever beam (B) (as in the
proxiocclusal restoration) which has one end fixed and other unsupported. The
other group consists of beam with both ends restrained (C) as in
proximocclusoproximal restorations and the beam with one end restrained and
other supported only (D) as in fixed bridge with rests. Simple beam has no
counter part in operative dentistry.
ACTION OF A BEAM
When an originally straight beam is loaded, it forms one or more
curves depending on its type. The simple beam (A) forms single curve concave
upwards. The cantilever beam (B) has one curve concave downward. The beam with
both ends retrained (C) in concave upwards in centre but ends are prevented
from turning up and thus forms two curves concave downwards. The D beam is
concave upward with free and turned up and fixed end concave downward.
Regardless of type, the material on convex side is stretched and
thus under tension while on concave side is shortened and thus under
compression. A longitudinal surface through the centre of beam is neither
lengthened nor shortened and is not stressed. Thus it is known as neutral
surface. So resulting tensile and compressive stresses known as bending
stresses increases from zero at the neutral surface to maximum at the top and
bottom surface of beam. These determine the greatest load a beam can safely
carry.
REACTIONS AND BENDING
MOMENTS
The bending stresses at any point along the beam are determined by
the bending moment at that point. The bending moment at any cross-section is
simply the algebraic sum of moments of all the forces acting on one side of
cross-section. It is necessary to take the moments of forces on only one side
of any imagined section because the sum of moments on either side is equal to
the sum of those on other side, since the beam is in equilibrium.
In case of cantilever beam, the bending moment increases as the
distance from the load increases. In the case of simple beam, it increases from
zero at either end upto the point of application of load.
It is evident from what has preceded that in case of cantilever
beam, the bending moment is greatest at its point of support and in simple beam
at point of application of load, hence the compressive and tensile stresses are
greatest at these points.
STRENGTH OF A BEAM
In order that a beam be not permanently deformed, the maximum bending
stresses i.e. those in the material at the top and bottom of the beam at the
point of greatest bending moment, must not exceed the proportional limit of
material. For a beam of a given shape, the larger the cross-section, the
greater the strength. However the effect of shape on beams of same
cross-sectional area is not obvious. Briefly, an increase in the vertical depth
of beam causes a greater increase in the strength than a corresponding increase
in its width. This knowledge may be of importance in occlusal step of a class
II restoration.
DEFLECTION
The vertical displacement of a point on the neutral surface of a
horizontal beam from the loaded to unloaded position is called deflection of
beam at that point. It has been shown that the bending stresses in a beam
depend on length and manner of loading (bending moment) and shape and size of
the cross-section (section modulus). They are entirely independent of the
material of which the beam is made. Whereas deflection depends upon modulus of
elasticity. Greater the modulus of elasticity lesser the deflection.
It can be shown also that the deflection of a beam increases
directly as the cube of its length, inversely as its width and inversely as the
cube of its depth. The above statement is for rectangular or triangular
cross-section. For a round beam, the relationship is inversely as the fourth
power of the diameter, means if diameter is doubled, the deflection is 1/16 as
great. The deflection of gold inlays and bridge abutments which are luted in
cavity by cement may be a determining factor in the life of restoration. If
deformation or deflection increase certain limit cement bond is broken.
RESTORATIONS AS BEAMS
As mentioned earlier the proximocclusal and proximocclusoproximal
restoration may be thought of as a cantilever beam and a beam restained at both
ends respectively. In structure of two materials, the material with the higher
modulus of elasticity supported the greater part of load. Because some material
in use today have modulus of elasticity several times that of tooth structure,
the occlusal step of class. 11 restoration is not fully supported by the pulpal
floor of the cavity & consequently behaves like a beam.
The cantilever beam has a bending moment at the point of support
equal to the load P times the length L. This moment is transmitted to the
embedded portion of beam which in the proximocclusal restoration is its
proximal part. As a result this part tends to rotate out of the cavity, Gingival
retention with a moment equal to PL is required to prevent this. The retentive
force R must equal PL/l where l is distance from under surface to gingival
floor.
STRESSES AT
DENTINOENAMEL JUNCTION OF HUMAN TEETH
DEJ contour of the functional cusps is dramatically different from that
of non-functional cusp. A recent study reported that this contour is concave in
occlusal third of DEJ in functional cusps of all teeth except maxillary
premolars that were concave in non-functional buccal cusp.
In the study by Vijay K. Goel et
al. (1991), normal & shear stresses in enamel & dentin varied
along the DEJ. Maximum stresses were at occlusal & cervical level. The
extremely cervical enamel demonstrated a high level of normal compressive
stresses, but dentin demonstrated normal stresses to be tensile in character.
This tensile character of stresses in dentin along DEJ, in combination with
compressive character of stresses in enamel in some area, may lead to
separation of enamel from dentin. This separation could also be exhibited as
cervical enamel chipping where it is extremely thin. In addition, cervical area
has a weak mechanical bond between enamel & dentin because of lack of scalloping pattern of the DEJ.
Compressive occlusal stresses in enamel & dentin are high along
the actual contour of DEJ. This has limited clinical significance because in
this region, enamel & dentin are perpendicular to masticatory load &
respond similar.
APPLICATION OF MECHANICAL PRINCIPLES TO OPERATIVE
PROCEDURES
CLASS I RESTORATIONS
The class I restoration is the simplest as to form, of those subject
to heavy masticatory loading. The entire restoration is enclosed in tooth
structure except the occlusal surface. The side walls are usually parallel
except in the case of inlays where there is a slight divergence. The floor is
flat.
If we slope the floor pulpwise toward the centre the following
situations may arrive. Because this type of cavity preparation removes more
tooth structure at centre of dentin bridge over the pulp, this bridge is weaker
and less rigid at the point where bending stresses are greatest. The side walls
lend no support against occlusal loading as in case of parallel or diverging
walls thus taking non of load from the pulpal floor. Because of this increased
deflection, it would be natural to expect failure of margins particularly in
young dentin and where the pulp chamber is large.
Often upon the excavation of decay a rounded pulpal floor remains.
This has the same disadvantage of a weakened dentin bridge over the pulp but
has the further disadvantage of an increase in the unit normal stress in the
deepest part of the cavity. This can be understood by considering a cylindrical
cavity with a hemispherical base of same radius.
Around the periphery at the union of the hemispherical and
cylindrical portion, there is applied load. From this point to the deepest part
of cavity, the unit stress increases to a maximum. If friction is neglected,
the minimum unit stress become double for unit stress for a cylindrical
restoration of same size with a flat base.
A restoration with a rounded base when loaded eccentrically, tends
to rotate as a result of moment produced by the applied force and eccentricity.
This rotation is prevented by side walls of cavity which exerts a horizontal
force H, opposing it. There is an opposing horizontal force ‘H1’
near base of the restoration resisting force ‘H’, in the same manner that there
is vertical force ‘P’ acting on the base opposing the masticatory load. This
follows from the conditions of equilibrium i.e. the forces and their moments
must add up algebraically to zero. The stress on side walls varies inversely as
the square of length of cylinder. This means that if the length of cylindrical
wall is doubled the maximum stress is one fourth as great. It is possible that
in shallow cavities with rounded bases, a compressive stress which results in
the above manner and is maximum at margins of restoration, would be sufficient
to cause a shear failure of restoration (amalgam). The stresses may be reduced
to a minimum by extending the cavity horizontally and cutting a flat ledge
around the periphery at depth of pulpal floor or flat floor at three points so
situated as to render the restoration stable against tipping.
It was pointed out that an eccentric load applied to a prism through
a rigid plate produced tensile stresses in the prism which would cause the
plate the prism to separate in the region of these stresses, if the
eccentricity of load exceeded a certain value. This principle applies
particularly to inlay whose diameter is large compared to depth. Fig. I and II
illustrates this for rectangular and circular cavities. The cross-hatched areas
are safe areas.
Stability may be improved by increasing the depth of cavity and
having the side walls approach as nearly perpendicularly to pulpal floor as
possible. If the restoration approaches the cusp apices these should be capped
so that the compressed side may receive support from enamel on class II cavity,
tipping is prevented by proximal portion. In class I cavity, additional
stability may be gained by buccal and lingual extensions.
In the case of large class I restoration overlying a large pulp
chamber, it fractures under a heavy concentrated load in its centre if the
restoration does not have sufficient thickness. In this case, the yielding
dentin over the pulp would cause most of load to be transferred to dentin of
axial walls of pulp chamber and bending stresses would be introduced. These
stresses would vary much the same as for a beam i.e. directly as radius and
inversely as the square of the depth. From this, it follows that for the larger
restorations in order to the sure of sufficient strength, the depth should be
increased with an increase in the diameter.
CLASS II RESTORATIONS
It has been pointed out that :-
1)
When the modulus of elasticity
of material of a class II restoration is higher than that of tooth structure,
the restoration functions as a beam.
2)
Because of end moments, the
gingival portion of these restorations must be locked mechanically to prevent
their rotation out of cavity.
3)
That in locking gingival
portions, they become curved beams whose bending stresses at the axiopulpal
line angles increase rapidly with a decrease in the radius of curvature of
these angles.
The manner in which the difference in the
moduli of material and tooth structure causes the tendency to rotate may be
seen as :-
When a uniform load 2P is applied, the
dentin prism of height ‘h’ is stressed less because of lower modulus of
elasticity than the corresponding gold prism so that the whole gold structure
tends to rotate counter clockwise. A gingival lock is required to prevent this
rotation by exerting the retentive force ‘R’ which combined with the resistance
‘R’ of axial wall produces a couple which cancels the couple of forces P-D and
Q-P.
What takes place in tooth under a uniformly
distributed load may be represented as by figure following :-
Since the bending stress at axiopulpal line
angle increase rapidly as its radius of curvature decreases, the axiopulpal
line angle should be well rounded for additional at this point.
The proximocclusoproximal restorations
depend on some what different principles. There is no couple tending to rotate
the restoration as a whole since it is supported at both ends. However, moments
are produced at both ends as is true of all because retrained at both ends. In
this case, the proximal portions tend to rotate out of cavity. The centre of
rotation are a the intersection of neutral planes, not at axiopulpal line
angles. The bending of occlusal portion is caused by difference between the
total masticatory force and support given by pulpal floor of cavity. Gingival
retention and rounding of axiopulpal line angles are required as in
proximocclusal cavity. What takes place in tooth may be represented as :
It is obvious that in any class II restoration,
the bending stresses produces and gingival retention required depend on how
much the modulus of elasticity of the material exceeds that of tooth structure.
Casting gold may have a modulus as much as 10 times that of dentin.
Even modulus of amalgam may be higher than
that of deciduous dentin which may account for failure of class II amalgam in
deciduous teeth.
A large pulp chamber would also cause
greater bending stresses than a small one because of less support by the pulpal
floor.
It was pointed out that a wedging action may
be produced by opposing cusps contacting transverse and marginal ridge or cusps
and marginal ridges. When this wedging action takes place in such a way that
one point of contact is on a proximocclusal restoration while the other is on
tooth structure, it follows that there is a tendency to wedge the two apart
with a rotation about the gingival cavosurface angle. It is to prevent this
that the occlusal lock is used even though the occlusal surface is not involved
by caries.
In the actual case, there may be more than
two points of contacts so that these forces would not be halved. Therefore any
force which results from this wedging and is normal to marginal ridge lies in a
perpendicular to axial and gingival wall is represented by N is above Fig. the
moment of this force which tends to cause the rotation about gingival
cavosurface angle is Nd. However for practical purposes, Nd equals Ha since the
vertical component V passes approximately through the centre of rotation 0 and
therefore its moment has a value near zero.
Attempts have frequently been made to
prevent this displacement, which consist of a rotation about the gingival
cavosurface angle by converging buccal and lingual walls from gingival towards
the occlusal. But the convergence of these walls to avoid the use of occlusal
locks i.e. at a great mechanical disadvantage. The obvious solution is the
occlusal lock as given in Fig. In which tension R is approximately equal to horizontal
component of force H since ‘a’ and ‘a’ are nearly equal. The occlusal lock
reduces stress in buccal and lingual walls due to ‘H’ to zero.
CLASS
IV RESTORATION
Of the proximal restorations of an anterior
teeth, only class IV needs to be considered as class III normally is unaffected
by masticatory forces. The problems involved in class IV are not usually
difficult because the applied forces are not great. Where the force usually
difficult because the applied forces are not great. Where the force of
mastication is applied directly to the restoration, it is preferable where
possible, to have the retention in the incisal portion of the tooth. The
function of this retention is however different from that of class II. In the
later, occlusal lock is under tensile and transverse load, while in class IV,
the incisal lock is subjected only to a transverse load, as there is no wedging
action resulting from cusps. There is no tendency of this restorations to
rotate about gingival cavosurface angle as in class II. The tendency is to
rotate about an axis parallel to long axis of tooth. It is frequently
impossible because of the labiolingual thinness of the tooth to put the
retention at incisal edge and it must therefore be placed on lingual surfaces.
In order to reduce the stress in this lingual lock, it should be as close to
the incisal edge as is possible and still be in dentin. Since in maxillary
teeth, the force of mastication has a labial component the lingual lock is more
firmly seated in tooth under load. A lingual lock in mandibular tooth would
have little retention except the shear stress of cement as the component of
masticatory force from labial to lingual.
CLASS V RESTORATION
The class V restoration, being confined to
one surface and not subject to occlusal loading is usually thought of as free
from mechanical problems, the only difficulties encountered being those usually
associated with inaccessibility. Analysis of the problem indicate that
distortion of tooth which may occur under certain conditions could result in
displacement of restoration.
From working bite to occlusion, the lingual
slopes of buccal and lingual cusps of maxillary teeth normally contact the
buccal slopes of buccal and lingual cusps of mandibular teeth. Applying the principles
of inclined plane, there is therefore a component of force acting on the buccal
and lingual cusps of mandibular teeth perpendicular to their axis. Tooth being
more or less firmly held in the alveolus is thus subjected to a transverse
force and consequently would tend to bend in the same manner as a beam loaded similarly.
If a cavity is now cut on the buccal surface
of such a tooth, the dimension of the beam parallel to the direction of transverse
load is reduced. This means that the depth of beam is decreased and
consequently its deflection is increased. In other words, the mandibular tooth
subject to the horizontal lingual force bends more under a given load than
before the cavity was cut. Since this portion of tooth is under tension and
there can be no cohesion between the tooth structure and material, a wedge shaped
gap now opens up between restoration and tooth at occlusal wall.
The
transverse force (shown for the buccal cusp only) and consequently the bending
moment HL would increase with cusp slop and/or lingual inclination of
mandibular tooth. P is force normal to tangent at point of contact and H is
component perpendicular to long axis of tooth. The axial component V will cause
a uniform compressive stress. From the figure, it is evident that a cavity in
buccal surface of maxillary tooth would not open up as this side of tooth is
under compression.
Fracture potential
after cavity preparation
Vale W.A. (1956) conducted a
biomechanical study on class II cavity. He used contralateral pairs of
premolars. One of each pair was an intact control while the other was prepared
class II cavity. The teeth were then fractured by a compressive load applied by
steel ball centered in occlusal fossa.
Vale showed that :
- When the isthmus width was one-quarter the intercuspal distance. The fracture force was the same as intact tooth.
- When the isthmus width was one-third the intercuspal dimension, the fracture force was two-third that of intact controls.
- No difference was evident between fracture strength of empty cavities and those restored with class II amalgam restoration.
- No difference was evident between fracture strength of empty cavities and those restored with class II gold inlays.
- Teeth with gold overlays were twice as strong as unrestored teeth.
Other studies using similar technique have been confirmatory,
contradictory and thus confusing. Thus other more sensitive technique was
developed by Grimaldi J. R. and Hood J.A.A. in 1973. This used strain gauges bonded to buccal and palatal surfaces
to measures cusp movements.
The defections of
buccal and palatal cusps were :
Total defections
11 um – intact tooth
16 um – Minimal width class I
20 um – Minimal width class II
MO cavity
24 um – Minimal width class II MOD cavity
32.5 um – Extended class II MOD cavity
27.5 um – Cavity after pulpotomy
Thus consequences of
Class II restorations are :
-
Lowering in the cavity floor
increases the cusp height and thus more deflection.
-
Increasing the width of isthmus
decreases the width of cusp and leads to more deflection.
Explanation for above consequence is based on the fact that following class II
restorations cusp can be considered behaving as a cantilever beam. If cusp
height doubles as floor drops, then deflection increases eight fold due to L3
factor in formula. Similarly if cusp width decreases by half, deflection will
also increase by eight fold due to the t3 factor.
D = Deflection, L = Length, E = Elastic modulus, I = Moment of inertia and t = thickness.
Under these circumstances fracture occurs as cusp flex and induce
stress at the internal line angles of cavity. Usually clinical fracture starts
at pulpoaxial line angle of isthmus box and passes obliquely downwards to area
of enamel-cementum junction. Stresses can be minimized in these areas by
rounding of internal line angles.
REFERENCES
1.
The American Textbook of
Operative Dentisty, Arthur B. Gabel.
2.
Operative Dentistry, M.A.
Marzouk, First Ed.; p:43, 53-54; 121-157.
3.
Analysis of Amalgam Cavity
Design. Louis G. Terkla, David B Mahler. J.P.D. 29: No.2, 204; 1973.
4.
Biomechanics of Intact,
prepared and restored tooth – Some clinical implications, JAA Hood. Int. Dent J
1991;41:25-32.
5.
Stresses at dentinoenamel
junction of human teeth – A finite element investigation. J.P.D. 1991;66;
451-59, Vijay K. Goel.
6.
Anatomy of cusps of posterior
teeth and their fracture potential. JPD 1990; 64:139-47, Satish C. Khera.
7.
Cavity preparations Vale W.A.,
Ir. Dent. Rev. 1956, 2:33.
8.
Mondelli J. et al, J. Prosth.
Dent. 1980, 43:419.
9.
Grimaldi J.R. and Hood JAA,
J.D.R. 1973, 52:584.
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