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.

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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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