Current concepts about principles of cavity designsWhat changes have been made to G.V. Black principles of cavity preparation?

What are the factors influencing cavity design?

Principles of cavity design:ReviewEstablishment of outline formEstablishment of resistance formEstablishment of retention formTreatment of residual cariesCorrection of enamel marginsCleaning of the cavity (cavity toilet).

Outline form

Outline form: of the cavity is the shape which the cavosurface line angle of the cavity assumes after preparation.

Outline formAccess: Gain initial access via the most carious part of the tooth.Remove all unsupported enamel to expose the entire extent of caries.

Outline formElimination of the carious lesionIt is the elimination of any infected carious tooth structure left in the tooth after initial cavity preparation.Caries left in the pulpal/ axial floor is excavated thoroughly making the cavity deeper. 0.75- 1 mm of dentin should cover the pulp. All the infected/ soft dentin should be removed.

Outline formPlacement of cavity margins in areas of lessened caries susceptibilityit is not necessary in low caries experience patientsit is advisable to include the pits and fissures in an amalgam cavity in cases of high caries rate patients

Outline formConvenience:Make the cavity wide enough to admit the instruments you are using.Extend proximal cavities buccally and lingually just outside the contact area.

Resistance formResistance form is the design of a cavity in such a way that the remaining tooth substance and the restorative material can withstand masticatory stress.

Resistance formTo achieve this ,the prepared cavity should possess the following 6 attributes: 1.Flat Floor 2.Adequate bulk of the restorative material 3.Absence of weak cusps or marginal ridges 4.Occlusal cavity margins in areas not subjected to excessive occlusal trauma . In practice one-quarter (1/4) of the intercuspal width 5.Flat floor at right angles to the line of stress 6.Walls of the cavity parallel to the direction of the stress

Resistance form

Absence of weak cusps or marginal ridgesRemove any weak cusp and restore with a metallic material.remove any weak marginal ridge specially if it is not supported by sound dentine. This way class I becomes Class II

Resistance formAdequate bulk of the restorationThe depth should be enough to take adequate bulk of the restorative material which can withstand forces of mastication.Amalgam thickness should be 1.5 2 mm at least.

Resistance formOcclusal cavity margins in the areas not subjected to excessive occlusal trauma:Occlusal margins of the cavity should not be subjected to heavy stress. Place them about 1/4 of intercuspal distance.

Resistance formFlat floor at right angles to the line of stress:Make the floor of the prepared cavity flatMake the floor of the cavity at right angles to direction of the occlusal forces.No sound tooth structure should be removed to achieve this.

Resistance formWalls of the cavity parallel to the direction of stress:Make the walls of the cavity parallel to the corresponding surfaces.

Retention form

refers to those aspects of cavity preparation which ensure that the restoration is not dislodged from the prepared cavity.

Retention formUndercuts:Dovetail lockFlat floor and definite internal line angles.Grooves and boxesAcid etch techniqueadhesives

Treatment of residual cariesAmelodentinal junction caries

Pulpal caries

Indirect pulp capping

Correction of enamel marginsNo undermined enamel rodsSmooth enamel surfaceCavosurface angle 90No beveling for amalgamComposite needs a bevel to provide greater enamel surface area for etching and micromechanical retention.

Cleaning of the cavityThe prepared cavity should be free from all debris

No disinfectant should be used to clean the cavity.

Dont desiccate it.

Principles of cavity designDr. G.V. Black Chicago, Illinois 1836-1915"The Grand Old Man of Dentistry" "The professional man has no right to be other than a continuous student."

G.V. BlackThe complete divorcement of dental practice from studies of the pathology of dental caries, that existed in the past, is an anomaly in science that should not continue. It has the apparent tendency plainly to make dentists mechanics only

Critique of his principlesIt does not include root cariesIt does not include secondary cariesIt does not include non-carious lesions which are treated in the same way as carious lesions

The objectives of restoring teethTo remove diseased tissue as necessaryTo restore the integrity of the tooth surfaceTo restore the function of the toothTo restore the appearance of the tooth

What determines cavity design?1. The dental tissues2. The disease3. The properties of the restorative material

I want you to think

1. The dental tissues - EnamelEnamel is the hardest tissue in the body, inelastic and brittle. Tend to split along the line of rodsUnsupported enamel vs. composite and GICUnsupported enamel vs. amalgamUnsupported enamel and tightening a matrix band

1. The dental tissues - dentineDentine is softer than enamel, more porous and sensitive.Dentists can use pinsDentine and pulp are considered one unit.Why?Diseases or operative procedures that affect the dentine may also affect the pulp

1. The dental tissues - pulpPulp size and secondary dentineThe preparation is designed to avoid pulpAvoid physical, chemical or thermal trauma to the pulp during cavity preparation and placing the restoration

1. The dental tissues the gingivaThe cavity design and restoration should have minimal encroachment on the gingivaThe margins of the restoration should be smooth and not retentive to plaqueThe margins should be cleansable as possible

2. The diseaseTwo points to be considered in the spread of caries: pulpal and lateral spread

3. The properties of restorative materials - AmalgamMechanical retentionWeak in thin sectionsIs not tooth colored corrosion forms at the tooth amalgam interfaceNeeds force to be condensed into cavity

Properties - compositeIt is attached physically to the tooth by means of acid etchIt is strong in thin sectionsCan be attached to dentine by chemical bonding or via intermediate layer of GICShrinkage is a problem

Properties - GICChemical adherence to enamel and dentinIt releases fluoride slowlyMargins can be left in areas which are difficult to cleanWeak and not abrasion resistantCermet contains silver to make abrasion resistance and radiopaqueThey destroy the tooth like appearance

New principles of cavity designGain access

Remove caries

Look, think and design

Complete the restoration

Gaining access class IIRemove the marginal ridgeVia the buccal or lingual surfacesTunnel through from the occlusal surface leaving the marginal ridge intactDirect access when the adjacent tooth is missing

Gaining accessAccess cavity

Gaining AccessClass II cavity

Gaining accessMarginal ridge is removed

Gaining access in tunnelInitial access

Prepared cavityConventional cavity

Look, think and designThe final choice of materialRetentionProtection of remaining tooth structureOptimize strengthThe shape and position of cavity margins

Retention in 5 directionsArrows

Design features to protect remaining tooth structureRemove unsupported enamelRemove weakened cuspsSupport the weak structure by bonding it to other parts of the tooth with GICHave it crowned

Design features to optimize the strength of the restorationRemove sufficient tooth tissue to make room for the material taking into consideration its properties

Round the occlusal-proximal line angle

Position of cavity marginsPlace cavity margins supragingivally

Dont extend beyond the contact area unnecessarily

Do not leave sharp edge of an amalgam restorations

Protection of pulpPulpal injury can result due to: 1.Heat generated by injudicious cutting. 2.Restorative material with good thermal conductivity. 3.Chemical from the restorative materials. 4.Galvanic currents 5.Microleakage.

Cavity lining and chemical preparations1. Linings: e.g. calcium hydroxide, zinc oxide eugenol and GIC. These are protective, therapeutic and structural.

2. Varnishes: natural or synthetic resins in a solvent and are applied to cavity walls in an attempt to improve the marginal seal of an amalgam restoration

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