Presentation on theme: "Principles of cavity preparation 2. Cardinal Steps of tooth preparation 1.outline form and initial depth. 2.Obtaining of the resistance and retention."— Presentation transcript:
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 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.
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 stresse s. 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).
RetentionResistanceCavity 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
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..
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 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 even if pulp exposure 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
Objectives: 1. Remove undermined enamel. 2. Adjust CSA inclination 3. Cavity wall correct inclination 4. 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 bevelFull bevelLong 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.