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PRINCIPLES OF CAVITY PREPARATIONS
Ass.Prof.Dr.Mohamed ALsakkafAss.Prof.Dr.Mohamed ALsakkafConservative departmentConservative department Faculty of dentistryFaculty of dentistry
University of AdenUniversity of Aden
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3كنزارف س
PRINCIPLES OF CAVITY PREPARATIONS
The principles of modern cavity preparation were first defined in 1896 by Dr. G.V .
Black , a man considered by many to be the " father of modern restorative dentistry"
when his original sequence of cavity was one of the most important considerations of
cavity design .
Ass.Prof.Dr.Mohamed ALsakkaf2
However the degree of cavity extension is considerably less today , because of the
realization that caries , primary or secondary , can be more effectively controlled and
prevented by greater emphasis on oral hygiene by the patient.
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Although techniques have been refined and outlines have been modified , Black's
principles are still used as the basic steps in the process of designing and preparing a
cavity . The adherence to these principles will facilitate the procedure of cavity
preparation and decay removal.
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BIOLOGICAL CONSIDERATION: 'Pulp protection:
)1( Avoid over cutting of dentin and unnecessary deeping of the cavity because the deeper the cavity the
more irritated pulp.)2( Unnecessary application of
pressure in scooping out soft dentin at the bottom of a deep cavity, only gentle pressure, using
sharp excavators.
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)3( Unnecessary or excessive cutting of enamel, in order to expose only a small area of dentin
to irritation.
)4( Avoid heat generation.
)5( Injudicious use of rotary instruments in removing soft dentin at the bottom.
)6( Don’t use chemical irritants in performing the toilet of the cavity. No cavity sterilization
should be done.
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STEPS:
1. Obtain the required outline form.2. Obtain the required resistance form. 3. Obtain the required retention form.4. Obtain the required convenience form. 5. Remove any remaining carious dentin. 6. Finish the enamel wall.7. Cleanse and medicate the cavity
preparation.
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Normally, these principles of cavity preparation are followed in the sequence as Outlined
above. However , occasionally situations arise when the operator may have to deviate from
this sequence.
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In very deep Lesions, where a pulp expose is feared or when there is gross destruction of
the tooth structure because of caries, it is often advisable to remove the carious dentin )Principle #5( resistance and retention from )Principles #2 and #3(, or in extreme cases,
even before obtaining the outline form ))Principle #1((.
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These principles of cavity preparation should be understood on a biomechanical basis .
They are concerned with both the biological structure that complement the physical and
chemical properties of the restorative material .
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The basic biological objectives of cavity preparation procedures are the removal of
a carious lesion , and the preservation of the optimum amount of remaining healthy
tooth structure . Within these parameters the cavity preparation assumes a design
that includes the actual carious lesion and the potential for it's future occurrence .
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This is reflected by the location of the cavity margins in areas easily cleansable by the
patient and readily accessible examination by the dentist.
The mechanical factors included in cavity preparation are basically concerned with
the retention of the restoration and the continued vitality and function of the tooth.
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OUTLINE FROM
Obtaining or establishing the outline from means carrying the margin of the cavity to
the position it will occupy upon completion of the cavity. The outline should encompass the carious lesion and may include portions of caries – susceptible areas on the surface
being restored.
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The outline form should follow a gently, sweeping curve, especially on the occlusal surface. Many factors govern the outline
form of the cavity.
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LOCATION AND EXTENT OF THE CARIOUS LESION
The cavity preparation must be extended sufficiently to include the entire carious
area and the enamel which is not supported by sound dentin. Enamel which is
unsupported is subject to fracture. This conservation of tooth structure is of
paramount importance for the preservation of esthetic, prevention of irritation to the dental pulp, and the maintenance of the
natural relationship between the tooth and the supporting tissues.
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The areas of tooth with caries susceptibility are usually included in the outline form.
This principle is known as "extension for prevention“. The importance of the concept
of conservation of tooth structure must be considered. An occlusal outline form on posterior teeth may include the fissured
primary (developmental) grooves, and in some cases secondary grooves, as well as
buccal and lingual extensions of these grooves.
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Cavity preparation should not automatically extend into all the developmental and
supplementary grooves, especially if : 1) they are not carious, 2)they are not fissured
and are unlikely to become carious
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RELATIONSHIP OF SELF-CLEANING AREAS TO OUTLINE
All cavity preparations normally have their Cavosurface margins located in areas that
are self-cleaning or are easily-cleansable by the patient. Recurrent decay is more likely
to occur in areas of the tooth that are not cleaned naturally by the action of the lips,
cheek and tongue.
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Therefore, the dentist must design the cavity so that all margins are located in areas
easily cleansed by the patient. These areas are found on smooth surfaces above the
height of contour, incisal edges and cusp tips.
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PRESERVATION OF HEALTHY TOOTH STRUCTURE
Healthy tooth structure should not be removed unless justified since tooth structure removed
cannot be put back. However establishing the margins of cavity preparation sometimes
necessitates the removal of some healthy tooth structure. This may conflict with locating the
cavity margins in easily-cleansable areas since it necessitates the removal of healthy tooth
structure. However compromise can be reached.
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RESTORATIVE MATERIAL
Individual properties of each material utilized to restore tooth, demand certain design
factors that must be incorporated into each cavity preparation. For example , the marginal strength ( also called "edge-
strenght") of dental cement and amalgam is relatively weak , so their Cavosurface
margins must be prepared at an angle of approximately "90d" with the tooth surf ace to provide maximum edge strength for both
the restorative material and the enamel. 23
Placing a bevel and etching the enamel at the Cavosurface angle of composite
preparations improves the marginal seal. In some cases, the bevel may also aid in
improving the contour and esthetic qualities of the composite restorations.
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TOOTH MORPHOLOGY
Tooth size and arch position may influence the design of a cavity preparation.
Malpositioned and rotated teeth may require unusual outline form because they
must be restored where they contact the adjacent teeth.
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AREAS OF ENAMEL DECALCIFICATION
These areas may have to be included in the outline form. Areas of decalcified or chalky
enamel are usually included in the outline form. This prevents future marginal
breakdown and recurrent caries. The most common areas for decalcified enamel is in the cervical one-third of the crown, and is
associated with class five cavity preparations.
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HOME CARELack of patient motivation, salivary and
dietary problems may influence the quality of cleaning the tooth. More extension for prevention may be required when these
factors cannot be controlled.
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RESISTANCE FORM
Resistance form is the design given to a cavity preparation to help prevent fracture of the
restorative martial and the tooth. Resistance form is obtained by giving the
cavity "box form" .The factors that contribute to resistance form include:
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WALL ANGULATION
Masticatory forces are directed primary along the long axis of the teeth. Since the teeth and the restorative materials resist
compressive forces best, the cavity preparation should be designed to minimize
tensile and shear stresses.
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This is done by preparing the internal walls of the cavity perpendicular or parallel to the
direction of the force ,which is along the long axis. Thus, pulpal and gingival walls should be horizontal or perpendicular to
the long axis. The other walls, including the axial, are vertical or parallel to the long
axis.
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DEPTH OF THE CAVITY PREPARATION
The depth of the pulpal and axial walls must be sufficient for the restorative material to
have enough bulk to resist fracture. Placing these walls just within dentin is usually
adequate for this purpose. Increasing the width of a cavity will not significantly
increase the resistance of the restoration to fracture but it will decrease the strength of
the tooth.
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The restorative material is more likely to fail if the cavity is too shallow rather if it is too
narrow.
It is important that the pulpal and axial walls be maintained at their ideal depth just
within the dentin if at all possible.
33
The contours of the pulpal and axial walls are important in preserving the interadjacent
dentin. Ideally the pulpal wall is flat and the axial wall is convex, paralleling the
curvature of the surface of the tooth and the dentinoenamel junction. The axial wall
should not be flat or concave.
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INTERNAL LINE ANGLESWell defined internal line angles delineate the
walls of the cavity preparation, aid in establishing uniform depth, and contribute to retention form by preventing rotation of restoration. Line angles such as the pulpo-
axial should be slightly rounded to minimize stress concentration on the
restoration.
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RESTORATIVE MATERIALBrittle material that have low tensile strength, such as amalgam and cement, require good
box form and strict adherence to proper preparation.
Less brittle materials such as composite can be used in shallower or rounded
preparation when other means of retention are used in addition to wall friction
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RETENTION FORM
Retention form refers to the features of a cavity preparation that prevent
dislodgment of the restoration. Retention form is achieved through:
1) WALL FRICTION: Friction between the restorative material and the
walls of the cavity preparation acts to retain all types of restoration. Frictional retention
is improved by increasing the area of the walls (cavity depth) and making opposing
walls parallel (proper box form). 37
Consequently, good resistance form also contributes retention form to the cavity by
enhancing wall friction. Walls are not intentionally roughened to increase
retention, since normal instrumentation leaves the walls sufficiently rough.
2) UNDERCUT POIENTS AND GROOVES: Undercuts are confined to the
dentin to avoid undermining enamel.
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They are usually placed in the facial and lingual walls of occlusal preparation, the
buccal and lingual proximal walls of class 2 preparation, and the incisal and gingival
walls of class 5 preparation. They are not normally placed in the pulpal or axial walls
in order to avoid approaching the pulp.
AUXILIARY RETENTION: They are; Dovetails, Grooves, Pins and etching of
enamel
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CONVENIENCE FORMThis means that the dentist must create
sufficient access to the lesion to facilitate visibility and instrumentation in
preparation of the cavity and insertion of restorative material.
Proper convenience form is a requirement for all operative procedures.
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REMOVAL OF REMAINING CARIOUS DENTIN
Cavity preparations are designed to remove carious tooth structure. Preparations that
are designed according Blacks principles will automatically include those lesions that
have just penetrated the dentin. Moderate or gross amounts of carious tooth structure,
however, are not removed by ideal cavity designs and therefore caries removal in
these instance becomes a separate and significant step.
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FINISHING OF THE ENAMEL WALL
It is necessary to refine the enamel walls of a cavity preparation to ensure the best
possible adaptation between the restorative material and enamel.
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An other objective of this procedure is the removal of any unsupported enamel which
may fracture later, and lead to secondary caries. Adequate planning of all enamel
walls will result in a smooth Cavosurface margin which enhances the condensability
and adaptability of the restorative material, thereby increasing the strength of the
restoration.
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CLEANSE AND MEDICATE
No cavity preparation should be restored that has not been properly cleaned and dried for inspection. It may also be necessary to place
certain medicaments in the cavity preparation.
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