principles of cavity preparation part 2.pdf

Principles of cavity preparation Dr. Ahmed A. Holiel BDS, MSc, PhD Lecturer of Conservative Dentistry Faculty of Dentistry - Alexandria University II – Resistance form: Shaping and placement of the cavity walls that best enable both the restoration and the tooth to withstand the force of masticati...

Principles of cavity preparation Dr. Ahmed A. Holiel BDS, MSc, PhD Lecturer of Conservative Dentistry Faculty of Dentistry - Alexandria University II – Resistance form: Shaping and placement of the cavity walls that best enable both the restoration and the tooth to withstand the force of mastication without fracture. ➢ Flat pulpal floor ▪ resist the restoration movement, if pulpal floor is rounded, so any force exerted on the restoration will produce a wedging action on the tooth cause a splitting or shearing of the remaining tooth structure. Pulpal floor mesio-distally is flat and perpendicular to the long axis ➢ Internal line angles should be slightly curved (rounded). Sharp internal line angles lead to stress concentration at these areas and fracture of tooth structure. ➢ Unsupported enamel should be removed ▪ enamel is brittle non vital structure so unsupported enamel will fracture easily when subjected to force of mastication. ▪ Caries is spread quickly and widely in dentin than enamel because of the difference in their structure. So during cavity preparation if we remove too much from carious dentin this lead to unsupported enamel that should be removed. ➢ Width of the cavity: ▪ restrict the extension of the walls to allow strong cusp and ridge with sufficient dentin support. ▪ The width of the cavity should be 1/4 of the intercuspal distance (the distance between the tips of the apposing cusps in the same tooth) this to preserve sound tooth structure. ▪ Increasing the width of the cavity lead to weak remaining tooth structure. Also wide cavity lead to wide surface of the restoration, so the force of mastication on the large surface area of the restoration will be more, this cause fracture of the restoration. Narrow cavity will interfere with convenient form Rule for cusp capping: • If extension from a primary groove toward the cusp tip is no more than half of ICD, no cusp capping should be done • If the extension is one half to two thirds of the distance, consider cusp capping • If the extension is more than two thirds of the distance , usually cap the cusp ➢ Cavity margins in areas not subjected to excessive occlusal trauma In practice, The cavity should be designed that the occlusal margins of the cavity are in areas not subjected to excessive occlusal trauma, otherwise the enamel wall of the cavity and/or the margins of the restorative material may fracture. ➢ Smooth pulpal floor: irregular pulpal floor created by removing caries form areas other than others so the occlusal force that exerted will be concentrated on deepest areas which will cause fracture of the tooth structure. And the forces concentrated on the elevated areas cause fracture of the restoration. ➢ Mesial and distal walls should be made parallel or slightly diverge because of little amount of dentin supported enamel so any convergence cause unsupported enamel , that will be fractured if subjected to occlusal load. ➢ Axiopulpal line angle should be beveled. If remain sharp this lead to stress concentration at that area which will lead to fracture of the restoration. ➢ Gingival cavosurface line angle should be slightly rounded to prevent the unsupported enamel. ➢ Adequate bulk of the restorative material (1.5-2mm) • Thickness of amalgam: the minimum thickness of amalgam should be 1-1.5 mm. to withstand the load applied without being fractured. • Carving of amalgam: over carving lead to reduce thickness of amalgam which will lead to fracture. Under carving of amalgam lead to stress concentration on one area other than others which will lead to fracture. • Cavosurface line angle should be 90 0 if beveled, a thin layer of amalgam will be present which will be fractured under occlusal load. Less than 90 cause unsupported enamel and this cause fracture of the tooth surface. Advantages of Box Form: • Walls are either parallel or perpendicular to the long axis of the tooth • Walls and floors are flat, smooth and plane • Provides bulk to restoration • Prevents wedging action of the restoration on the tooth III- Retention Form: • Is that shape or form of the prepared cavity that resists displacement or removal of the restoration against tipping or lifting forces. • Aim is to place the restoration in the cavity and prevent it form dislocation against the force of mastication and also against the pull of sticky food Types of Retentive features Primary Retentive Features: • • • • • Mechanical Undercuts through converging walls occlusally. Friction between parallel opposing cavity Gripping action of dentin due to its viscoelasticity Proximity Dovetail(Nevertheless, there is no need to remove sound tooth tissue simply to produce an occlusal dovetail Secondary or Auxiliary Retention  Pins  Axial grooves  Reverse bevel in gold inlays  Cements  Posts in non vital teeth  Etching or acid conditioning ➢ Convergence of buccal and lingual (palatal) walls about 50, because of good bulk of dentin under the cusp supported enamel. Too much convergence by cutting more from dentinal structure leads to unsupported enamel which may fracture under occlusal load Dovetail lock • Extension of the occlusal portion of a Class II cavity into deep and/or carious pits and fissures may result in an occlusal dove tail which resists the displacement of the Class II cavity in proximal direction. (lateral direction) • Nevertheless, there is no need to remove sound tooth tissue simply to produce an occlusal dovetail Dovetail Lock ➢ Extra retention like pins, pinholes, grooves in case of complete destruction of the buccal wall for example. Grooves Undercuts Slots & amalgapins • The recent trends in conservative restorative dentistry reduce the dependence on the mechanical interlocking (retention) of the restorative material into the cavity because it leads to unnecessary tooth cutting and destruction to create dove tails, undercuts, grooves, etc. • Instead, they reduce the cavity size and tooth cutting as much as possible (just remove caries) and depends on the new generation of adhesive materials capable of bonding all restorative materials to tooth structure (enamel and dentin) as for example "Bonded Amalgam Restorations" and "Composite Bonded Restorations" or even "Glass ionomer Restorations (GIC)". • "Adhesion form instead of resistance form and retention form“ By these new concepts the restorative material becomes an integral part of the tooth increasing its overall strength and fracture resistance to masticatory forces. ➢ Acid etching of enamel and applying bonding agent for resin based restorative materials like composite resin. IV- Convenience form • Is that shape and form of the cavity that allow adequate observation, accessibility and ease of operation in preparing and restoring the cavity. • The form of the cavity should allow the operator to distinguish all internal line angles to ensure removing of all caries , also allow the operator to use instrument easily in removing caries , shaping of cavity walls and restoring the tooth . This is influenced by: 1. Armamentarium to be utilized. 2. Location of cavity margins. 3. Accessibility for proper caries removal and pulp protection. 4. Accessibility for proper restoration placement. • Proper finishing of margins of cavity preparation. • Retentive areas. • Convenience (starting) points. 5. Type of restoration to be placed. V- Removing of remaining caries ➢ Caries is removed during outline form and the margins should be placed in sound tooth structure, and the typical depth of the cavity is 1.5 mm ➢ if caries is present in pulpal floor or axial wall or both , it should be removed using spoon excavator or handpiece with large round bur. ➢ The caries dentin is soft, either has the same color of normal dentin which can be detected using sharp probe or most of the time has a different color from normal dentin. ➢ If we have one spot of caries on the pulpal floor, we can remove only this area lead to depression ➢ if we remove all pulpal floor this result in 1) Unnecessary cutting tooth structure 2) Increase the possibility of hitting the pulp. ➢ This depression is small compared with total surface area and is covered by cement to make flat pulpal floor. VI- Finishing of cavity walls: The objectives of finishing the walls are: • To have best marginal seal between the restorative material and tooth structure. • To afford a smooth marginal junction. • To provide maximum strength of both the tooth and the restorative material at and near the margin. Several factors must be considered in the finishing of enamel walls and margins: • • • • • Direction of enamel rods Support of enamel rods Type of the restorative material to be placed in the cavity The location of the margins Degree of the smoothness desired. VII- Toilet of the cavity Toilet of the cavity is cleaning of the cavity from small chips of cutting tooth structure and removing carious lesion, using water – air spray, applied on a cotton pellets then dryness with oil free air. DO NOT DESICCATE DENTIN References • Textbook of Operative Dentistry,3rd edition. Baum, Philips & Lund. Pages 295-304 • Sturdevant’s Art and Science of Operative Dentistry, 5th edition 2006. Pages 711-720, 728-729, 731-734