Principles of cavity preparation 2

Cardinal Steps of tooth preparation

1. outline form and initial depth.

2. Obtaining of the resistance and retention forms.

3. Obtaining of the required convenience form.

4. Removal of all carious dentin.

5. Finishing of the enamel walls and margins.

6. Performing of the toilet of the cavity.

N.B:in case of deep carious lesions, step 4 must

precede step 2

1 .Outline form and initial depth

Definition.Fundamentals.O.L form of :• Pits and fissure caries (class I).• Smooth surface proximal caries (class II).• Smooth surface gingival caries (class V).

Definition.

The shape or form of external boundaries of the completed cavity or the pattern of CSA.

Initial depth should be prepared 0.2-0.5 mm pulpal to DEJ.

Fundamentals.

1. All carious enamel, pits and fissures, should be included in the cavity outline.

2. All unsupported or undermined enamel must be eliminated; otherwise, it will break easily under mastication forces leaving a marginal “ditch”

3. Cavities, which approach closely to each other, should be united into a single cavity to avoid fracture of thin tooth structure during mastication.

4. “Extension for prevention” or “cutting for immunity”.

But

conservation of tooth tissues is now highly recommended

5. Margins of cavities should always be placed in sound enamel at areas, which are, considered to be less susceptible to caries and less stress bearing.

Various Outline forms :

For class I pits and fissures cavities: • Controlled by:

1. extent of caries in enamel, and

2. limits of lateral spread of caries in dentin.

3. Anatomical configuration of pits and fissures

4. Type of restorative material

Various Outline forms :

For proximal portions of classical class II, III, and IV cavities:

• free margins of the cavity from contact with adjacent tooth or restoration

• The junction between occlusal and proximal portions of classical class II (isthmus) is either:

• Straight line• Uniform curve• Reverse curve

Various Outline forms :

III, and IV cavities:

Various Outline forms :

For class V cavities • Gingival by the position of the gingiva • Proximally by the estimate of the operator for

the extent of area of liability to future decay. • Occlusaly by placing the margin where it will

certainly be cleaned in mastication.

Lecture 3

2. Obtaining resistance and retention

Resistance and retention.

These are two distinct but yet, inseparable and

interrelated steps i.e., two faces for a coin

Resistance form.Definition.Factors Influencing Stress Response of Tooth

and Restoration.1. Occlusal loading (magnitude, direction, ccc).

2. Cavity design (box form).

3. Type of the significant or major stress.

(compressive, tensile, shear)

Box (mortise) or modified box form.

cavity which provides resistance for both the tooth and restoration against fracture under masticatory forces.

1. seat of the restoration (pulpal and gingival wall) perpendicular to the direction of functional stresses. neutralization of these stresses.

2. inverted truncated cone shape prevents the wedging action .

3. access to and easy visualization.

4. Improves retention of restoration

so prevents the development

of excessive tensile forces

Retention form.Definition.Examples for the inter-relations between

the resistance and retention.1. Resistance features automatically prepared in the

cavity provide retention (primary retention).

2. Additional retention features can provide resistance (secondary retention).

Retention Resistance Cavity Feature

Frictional retention due to parallel walls.

neutralization of stresses at the cavity seat

1. Box form

Provide retention undercuts in dentine

Prevent wedging action of restoration inside the tooth.

2. inverted truncated cone

augments retention by more friction

Prevent fracture of brittle restoration

3. The bulk

Resistance features automatically prepared provide retention.

Retention classification:

according to the possible direction of displacement into:

1. Axial retention.

2. Lateral retention.

3. particular retentive features

•Axial displacement

• Lateral displacement

1. Axial retention.

Occlusal displacement

Pulpal displacement

1. In amalgam: Retentive undercuts.2. In cast restoration:Frictional parallism

1. Flat pulpal floor. (amalgam & cast rest.)

2. In deep excavated lesions for amalgam rest. Dentine ledge

Undercuts

Parallel walls

1. Lateral retention.

From dentine Modifying the O.L

1. Pin holes and pins.2. Reverse bevel.3. Gripping action of dentine

(direct gold foil rest.)

1. Dove-tail locks .2. Proximal axial grooves.3. Buccal or lingual extensions.4. Skirting the tooth structure

I. From dentine

1. Pin holes and pins

Pins may be cemented, screwed or wedged to dentine

2. Reverse gingival bevel (gingival lock)

•performed in compound class II cavity for cast gold in dentine of the gingival floor.•Action:1. prevent proximal displacement of the restoration.2. produce favorable analysis of forces inwards rather than outwards.3. prevent rotation of the inlay along the axio-pulpal line angle

3. Gripping action of dentin

The dentine resiliency and compressibility enables it to be strained during heavy condensation (such as malleting of direct gold foil), the relaxation of dentine later will grip the hardened restoration preventing its displacement.

II. Modifying the outline form

1 .Dove-tail locks 2. Axial grooves

4. Buccal or lingual extensions

5. Skirting the tooth structure

This is an encirclement of the tooth by grinding of part thickness of enamel in different aspects or planes to provide retention. The prepared enamel will be etched and an adhesive resin applied to retain extensive composite restoration in class IV cavities.

.

3. Particular retentive features

Acid etching Dentin bonding

Interradicular retention

1. Acid etching of enamel micropores that could entangle resin tags from the adhesive resin serves for micromechanical interlocking to retain and improve adaptation of the restoration.

2. Dentin bonding modify the topography of dentine collagen hanging and tubules ready to be infiltrated with low viscosity, resin, a hybrid layer will be formed and micromechanical bonding will be established.

3. Interradicular retention: retention is gained through the root portion a dowel post is inserted into the prepared root canal

Factors controlling the selection of retention features:

1. size of cavity and remaining amount of tooth structure.

2. The number of missing walls.

3. The site of cavity and the occlusal stress.

4. The type of restoration.

5. The pulp vitality.

6. Esthetic requirments.

3. Obtaining of the Convenience Form

Definition:

features in the prepared cavity, which are added for the specific purpose of making it more conveniently seen, approached and/or restored.

Features:

1. Slight more extension.

2. Accentuation of line or point angle.

3. Cavity outline in sweeping curves .

Equipments help:

1. Diagnosis and spotting the decay using caries detection dyes in case of conservative cavity preparations.

2. Use of fiber optic light transmission built in hand pieces.

3. Small sized cutting burs and stones with newly designed shapes.

4.Removal of Carious Dentin

In the average case, most of carious dentin is removed during the previous procedures.

However, the cavity must then be thoroughly inspected with sharp explorers, aided by good illumination and dryness of the operative field.

This inspection may then reveal the cavity floor to be composed of:

Soft discolored(chronic)

Soft dentin(acute)

Hard discolored

dentin.

Hard sound dentin

every bit of decay must be removed even if pulp exposure occurs

Endodontic ttt

must be removed Last layer:(affected and not infected).ccc:1. yellow.2. Semispongy.3. Yield under p.ttt:ID pulp capping.

leave this dentinN.B: in case of anterior teeth where it must be carefully removed for esthetic reasons

no more deepening should be made

“"

Removal by round bur low speed or

excavator

Indirect Pulp capping

2 visit (doubt exists) 1 visit 1st visit:1. Caoh and ZnoE dressing.2nd visit: (4-8 weeks)2. Removal of dressing.3. Reparative dentin is

inspected clinically & radiographically.

4. Restoration inserted.

1. Caoh is applied (alkaline).

2. Restoration insertion.

4. Finishing Of Cavity Walls And Margins.

Objectives:1. Remove undermined enamel.2. Adjust CSA inclination3. Cavity wall correct inclination4. Render the walls smooth for better adaptation.5. Make the cavity outline in sweeping curves.6. Roundation of line and point angles.

Instruments:1. Hand cutting: E hatchet, GMT, chisel and angle

former.2. Rotary inst: fissure bur and abrasive stones.

The strongest enamel wall according to Noyes:

• The enamel rods must have their inner ends resting on sound dentin and the outer ends are covered by a strong restorative material.

• The cavo-surface angle must be in accordance to the physical properties of the restorative material.

(90 with amalgam and beveled with cast restoration).

Types of CS bevels:

Short bevel Full bevel Long bevel

Counter bevel

5. Toilet of the cavity.

Objectives:

1. Elimination of bacteria, debris and saliva.

2. Removal of remnants of dentine chips and temporary restoration.

3. To improve adaptation & stop the recurrence of decay around the restoration.

4. prepare and condition the tissues to receive the entitled restorative material.

Technique: Flush the cavity with water and dry it gently with air.

Smear layer Definition:

Microscopic multi-component layer resulting from cutting the tooth structure. It is present on both cut enamel and dentine.

Thickness : (2-5 microns). Composition: formed of 3zones

1. smear plugs inner most protective.

2. firmly adherent glycoproteins intermediate.

3. externally a loose debris layer containing cut dentine, bacteria and mucin.

Significance:

It was formerly thought that its removal would improve the adaptation of restorations being a contaminant.

It is believed recently that the smear layer has to be modified for bonding resin restorations effectively to dentine and that its removal could endanger the pulp particularly the smear plugs obliterating the tubular apertures.

Thank you