Complete Cast Crown Preparation PDF

Prof. DR. Shaimaa Ahmed THE COMPLETE CAST CROWN PREPARATION DR. Shaimaa Ahmed Professor of Fixed Prosthodontics Faculty of Dentistry, Mansoura University 1  Prof. DR. Shaim...

Prof. DR. Shaimaa Ahmed THE COMPLETE CAST CROWN PREPARATION DR. Shaimaa Ahmed Professor of Fixed Prosthodontics Faculty of Dentistry, Mansoura University 1  Prof. DR. Shaimaa Ahmed Complete metal cast crown (Full Metal Crown) Def: It is an all-metallic restoration that restores functional properties of the prepared tooth. Requirements (criteria) of full metal crown: 1) Removal of adequate amount of tooth structure. 2) Preservation of remaining tooth structure. 3) Reduction should produce a crown of acceptable strength. Advantages 1. It has greater retention than more conservative restorations (e.g., three-quarter crown) because all axial surfaces of the tooth are included in the preparation. 2. Its preparation has greater resistance form than a partial-coverage restoration. 3. It has superior strength compared to other restorations. 2  Prof. DR. Shaimaa Ahmed 4. It allows the operator to modify the axial tooth contour in specific clinical situations: a. Malaligned teeth. b. Management of furcation involvement to improved patient oral hygiene through re-contouring of buccal and lingual walls. c. Retainers for removable partial dentures (it is often the only restoration that will allow the necessary modifications for the creation of properly shaped survey lines,guide planes, and occlusal rests) 5. It permits easy modification of the occlusion, which is often difficult to accomplish if a more conservative restoration is made. a. Supra-erupted teeth b. when the occlusal plane needs to be re-established. 3  Prof. DR. Shaimaa Ahmed Disadvantages 1. Less conservative compared with partial coverage restorations because all surfaces are involved in the reparation with risk on the pulp and periodontium. 2. It does not allow performance of electric vitality testing of an abutment tooth because the conductivity of the metal interferes with the test. (thermal tests occasionally will yield the necessary information) 3. Patients may object to the display of metal associated with complete cast crowns (in patient with a normal smile line, the restoration may be restricted to maxillary molars and mandibular molars and premolars) Indication: 1. It can be used as a single crown or as a retainer for a fixed partial denture for posterior maxillary molars and mandibular molars and premolars. 2. It is the restoration of choice whenever maximum retention and resistance are needed. 3. It is indicated on teeth that exhibit extensive coronal destruction by caries or trauma. 4. It is indicated on posterior short clinical crowns. 5. It is indicated when high displacement forces are anticipated, such as for the retainer of a long-span FPD. 4  Prof. DR. Shaimaa Ahmed 6. It is indicted when correction of axial contours is not feasible with a more conservative technique. 7. The restoration also may be used to support a removable partial denture. 8. It is indicated on endodontically treated teeth. Its superior strength compensates for the loss of tooth structure that results from previous restorations, carious lesions, and endodontic access. Contraindication: 1. It is contraindicated if treatment objectives can be met with a more conservative restoration. (wherever an intact buccal or lingual wall exists, use of a partial- coverage restoration). 2. If less than maximum retention and resistance are needed (e.g., on a short-span fixed partial denture). 3. If a high esthetic need exists (e.g., anterior teeth), a complete cast crown is contraindicated. Preparation Armamentarium: 5  Prof. DR. Shaimaa Ahmed The sequence of clinical procedure to prepare a tooth for a complete cast crown: 1. Occlusal guiding grooves 2. Occlusal reduction 3. Axial alignment grooves 4. Axial reduction 5. Finishing and evaluation The preparation begun with the occlusal reduction, thus the occluso-gingival length of the preparation can be determined and the potential retention of the preparation can be assessed. Occlusal reduction Aim: ❖ Provide adequate room for the restorative material. ❖ Planner occlusal reduction (follow normal anatomic contours to be as onservative as possible) and reduction parallel opposing triangle ridges. ❖ The minimum occlusal clearance is 1 mm on non-functional (non-centric) cusp, and 1.5 mm on functional (centric) cusp. - Clearance: is the amount of space between the completed preparation and the opposing tooth. - Reduction: is the amount of tooth structure that is removed to establish the desired clearance. Guiding Grooves for Occlusal Reduction Instrument: Tapered carbide or a narrow, tapered diamond. Place guiding grooves in the buccal and lingual developmental grooves and or in each tri-angular ridge extending from the cusp tip to the center of its base. Because the centric or functional cusp is to be protected by an adequate thickness of metal, place a functional cusp bevel to ensure this in the area of contact with the opposing tooth. 6  Prof. DR. Shaimaa Ahmed Correct depth (0.8 mm for the central groove and nonfunctional cusps, 1.3 mm for the functional cusps) is achieved by knowledge of the instruments being used. This will facilitate assessing the adequacy of the reduction in progress. If necessary, a periodontal probe can be used to measure the extent of reduction. Occlusal Reduction Instrument: round-end, tapered diamond Half the occlusal surface is reduced first so that the other half can be maintained as a reference. When the necessary reduction of the first half has been accomplished, reduction of the remaining half can be completed. Functional cusp bevel: 1.5 mm and placed parallel to the opposing triangular ridge. 7  Prof. DR. Shaimaa Ahmed Check that a minimum clearance of 1.5 mm has been established on functional cusps and at least 1.0 mm on nonfunctional cusps. This clearance must be verified in all excursive movements that the patient can make. Methods for evaluation of the occlusal clearance: 1. Utility wax and wax caliper 2. Occlusal reduction gauge 3. Occlusal clearance tab 4. Occlusal reduction ring 5. Rubber base 8  Prof. DR. Shaimaa Ahmed Axial reduction Aim: ❖ 6-degree taper or total convergence angle between opposing axial walls. ❖ Wall height: at least 3 mm. ❖ Margin configuration: 0.5 mm chamfer finish line, following the gingival contour (ideally, supragingival). Alignment Grooves for Axial Reduction Instrument: a narrow, round-end, tapered diamond Three alignment grooves are placed in each buccal and lingual wall. One is placed in the center of the wall, and one in each mesial and distal transitional line angle. The grooves are placed with the shank of the diamond is held parallel to the proposed path of withdrawal of the restoration. This automatically produces a convergence between the axial walls of the alignment grooves that is identical to the taper of the diamond. 9  Prof. DR. Shaimaa Ahmed Do not let the diamond cut into the tooth beyond the point where its tip is buried in tooth structure up to the midpoint; otherwise, a lip of unsupported tooth enamel will be created. Use a periodontal probe to assess the relative parallelism of the alignment grooves. Axial Reduction Instrument: the same narrow, round-tipped diamond The remaining islands of tooth structure between the alignment grooves are removed. As with the occlusal reduction, perform the axial reduction for half the tooth at a time, maintaining the other half as a reference for assessing adequacy of the preparation. To prevent damage to the adjacent teeth, sufficient time must be allowed for the cutting instrument to create its own space. If desired, protect the adjacent teeth by placing a metal matrix band. Cut into the proximal area from both sides until only a few millimeters of interproximal island remain. This area can then be removed and contact broken by using thinner, tapered diamonds. 10  Prof. DR. Shaimaa Ahmed Place the cervical chamfer concurrently with axial reduction. Its width should be approximately 0.5 mm, which will allow adequate bulk of metal at the margin. This chamfer must be smooth and continuous mesio-distally, and a distinct resistance against vertical displacement should be detected when probed with the tip of an explorer. Finishing and evaluation Finishing A smooth surface finish and continuity of all prepared surfaces will aid most phases of fabrication of the restoration. Smooth transitions from occlusal to axial surfaces facilitate impression making, waxing, investing, and casting because bubble formation is reduced. Use a fine-grit diamond or carbide bur of slightly greater diameter for finishing the chamfer margin. Finish all prepared surfaces and slightly round all line angles. Place additional retentive features as needed (e.g., grooves or boxes) with the tapered carbide bur. Evaluation Upon completion, the preparation is evaluated to assess whether all the criteria have been fulfilled. 11  Prof. DR. Shaimaa Ahmed Criteria for complete cast crown preparation  The occlusal reduction must allow adequate room for the restorative material from which the cast crown is to be fabricated.  Minimum recommended occlusal clearance is 1 mm on non-centric cusps and 1.5 mm on centric cusps.  The occlusal reduction should follow normal anatomic contours to remain as conservative of tooth structure as possible.  Axial reduction should parallel the long axis of the tooth while allowing for the recommended 6-degree taper or convergence between opposing axial surfaces.  No over-tapering of the opposing axial walls. This reduces the retention of the completed restoration. If the tooth over reduced and excessive tapering occurred. It should be evaluated to determine how it can be corrected (either by prepare a band of several millimeters with restricted taper of 6-degrees or use groves to enhance the retention).  No undercuts between any opposing axial walls can be accepted. When the diamond is placed against the axial surface of the prepared tooth, parallel to the path of withdrawal, it should be possible to move the instrument around the tooth so the entire height of the preparation is touching the diamond at all times. The tip of the diamond should rest on the chamfer throughout this movement, and no light should be visible between the instrument and the axial surface.  Functional (Centric) Cusp Bevel. This is important for ensuring optimum restoration contour with maximum durability and conservation of tooth structure. The bevel (1.5 mm) must be angled flatter than the external surface and placed at about 45 degrees to the long axis. 12  Prof. DR. Shaimaa Ahmed  Nonfunctional (Non-centric) Cusp Bevel. A minimum of 0.6 mm of clearance is needed here for adequate strength. Maxillary molars in often require an additional reduction bevel in this area. Without it, an over contoured restoration that does not follow normal configuration may result.  The margin should have a chamfer configuration and should ideally be located supra-gingivally. Sometimes crown lengthening is indicated to obtain a supragingival margin. The chamfer should be smooth and distinct and allow for approximately 0.5 mm of metal thickness at the margin. 13