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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?
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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).
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Outline form
Outline form: of the cavity is the shape which the cavosurface
line angle of the cavity assumes after preparation.
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Outline formAccess: Gain initial access via the most carious
part of the tooth.Remove all unsupported enamel to expose the
entire extent of caries.
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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.
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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
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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.
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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.
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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
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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
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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.
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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.
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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.
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Resistance formWalls of the cavity parallel to the direction of
stress:Make the walls of the cavity parallel to the corresponding
surfaces.
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Retention form
refers to those aspects of cavity preparation which ensure that
the restoration is not dislodged from the prepared cavity.
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Retention formUndercuts:Dovetail lockFlat floor and definite
internal line angles.Grooves and boxesAcid etch
techniqueadhesives
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Treatment of residual cariesAmelodentinal junction caries
Pulpal caries
Indirect pulp capping
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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.
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Cleaning of the cavityThe prepared cavity should be free from
all debris
No disinfectant should be used to clean the cavity.
Dont desiccate it.
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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."
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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
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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
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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
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What determines cavity design?1. The dental tissues2. The
disease3. The properties of the restorative material
I want you to think
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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
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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
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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
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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
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2. The diseaseTwo points to be considered in the spread of
caries: pulpal and lateral spread
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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
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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
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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
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New principles of cavity designGain access
Remove caries
Look, think and design
Complete the restoration
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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
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Gaining accessAccess cavity
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Gaining AccessClass II cavity
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Gaining accessMarginal ridge is removed
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Gaining access in tunnelInitial access
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Prepared cavityConventional cavity
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Look, think and designThe final choice of
materialRetentionProtection of remaining tooth structureOptimize
strengthThe shape and position of cavity margins
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Retention in 5 directionsArrows
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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
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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
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Position of cavity marginsPlace cavity margins
supragingivally
Dont extend beyond the contact area unnecessarily
Do not leave sharp edge of an amalgam restorations
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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.
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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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