Lec. 3-4 Dr. Saif Alarab Clinical Technique for Class I Amalgam Restorations Class I refers to -Restorations on the occlusal surfaces of posterior teeth, - The occlusal two thirds of facial and lingual surfaces of posterior teeth, - The lingual surfaces of anterior teeth. Tooth Preparation. It is divided into initial and final stages Initial Tooth Preparation. Initial tooth preparation is defined as establishing the outline form by extension of the external walls to sound tooth structure, while maintaining a specified, limited depth and providing resistance and retention forms. The outline form for the Class I occlusal amalgam tooth preparation should include only the faulty, defective occlusal pits and fissures (in a way that sharp angles in the marginal outline are avoided). Extension for prevention has a marked influence on the over-all outline of a cavity preparation. The objective is to prevent the of recurrence of caries adjacent to a restoration. There are three main factors involved when considering extension for prevention: (1) all margins must be placed in sound tooth structure; (2) all continuous non-coalesced pits and fissures must be eliminated; (3) margins must be placed in areas of lessened caries susceptibility Resistance form: the form of the cavity that prevent both fracture of both the tooth and the restoration. Retention form: the form of the cavity that prevent the displacement of the restoration The ideal outline form for a very conservative amalgam restoration (Fig. 17-2, A) incorporates the following resistance form principles that are basic to all amalgam tooth preparations of occlusal surfaces. These principles allow the operator to position margins in areas that are sound and subject to minimal forces, while conserving structure to maintain the strength and health of the tooth. Resistance principles include: Extending around the cusps to conserve tooth structure and prevent the internal line angles from approaching the pulp horns too closely The smaller the area of restoration available the less force that cause fracture for both tooth and restoration Keeping the facial and lingual margin extensions as minimal as possible between the central groove and the cusp tips Minimally extending into the marginal ridges (only enough to include the defect) without removing dentinal support ١
The internal line angle should be rounded to prevent stress concentration on tooth structure Eliminating a weak wall of enamel by joining two outlines that come close together (i.e., less than 0.5 mm apart) Care should be taken not to undermine the marginal ridge. However, when these fissures require extensions of more than a few tenths of a millimeter, consideration should be given to changing to a smaller diameter bur, such as a No. 169L or No. 329. Continue to maintain the bur's orientation and depth and, with intermittent pressure, extend along the central fissure toward the mesial pit, following the DEJ. This may create a flat pulpal floor. However, the pulpal floor should follow the DEJ to maintain a more uniform pulpal floor depth (see Fig. 17-4, E). Fig. 17-4, E The long axis of the bur should be changed to establish a slight occlusal divergence to the mesial wall if the marginal ridge would be otherwise undermined of its dentinal support. Fig. 17-5. Fig. 17-5 The remainder of any occlusal enamel defects is included in the outline, and the facial and lingual walls are extended, if necessary, to remove enamel undermined by caries. The strongest and ideal enamel margin should be made up of full-length enamel rods resting on sound dentin, supported on the preparation side by shorter rods, also resting on sound dentin (Fig. 17-6). ٢
The conservative Class I tooth preparation should have an outline form with gently flowing curves and distinct cavosurface margins. For the conservative Class I preparation a faciolingual width of no more than 1 to 1.5 mm and a depth of 1.5 to 2 mm are considered ideal. The pulpal floor, depending on the enamel thickness, is usually in dentin (see Fig. 17-4, C). Such conservative preparation saves tooth structure, minimizing pulpal irritation and leaving the remaining tooth crown as strong as possible.this completes the initial tooth preparation for the Class I amalgam tooth preparation. To summarize, primary resistance form for both tooth and restoration is provided by: Sufficient area or areas of relatively flat and smooth pulpal floor in sound tooth structure to resist forces directed in the long axis of the tooth and provide a strong, stable seat for the restoration (resistance for both tooth and restoration) Minimal extension of external walls, which reduces weakening the tooth and increase force on restoration(1/4 intercuspal distance), (resistance for both tooth and restoration) Strong, ideal enamel margins (cavo-surface line angle approximately 90-110 ), (resistance for both tooth and restoration) Rounded internal line angle to prevent stress concentration area on tooth(tooth resistance) Mesial and distal walls should be parallel or slightly diverge occlusally to prevent unsupported enamel (tooth resistance ) Sufficient depth (i.e., 1.5 mm) to result in adequate thickness of the restoration, providing resistance to fracture and wear of resroration(restoration resistance) Retention form: The parallelism or slight occlusal convergence of two or more opposing, external walls, flat pulpal floor and 1/4 intercuspal distance provides the primary retention form. Final Tooth Preparation. Final tooth preparation includes: (1) Removal of remaining defective enamel and infected dentin on die pulpal floor; (2) Pulp protection, where indicated; (3) Procedures for finishing external walls; (4) Final procedures of cleaning and inspecting the prepared tooth. The use of desensitizers or bonding systems is considered the first step of the restorative technique. Remaining enamel pit-and-fissure in the pulpal floor should be removed. If several enamel pit and fissure remnants remain in the floor, or if a central fissure remnant extends over most of the floor, deepen the floor with the No. 245 bur to eliminate the fault or faults or to uncover the caries to a maximal-preparation depth of 2 mm (Fig. 17-10). ٣
If the pit-and-fissure remnants are few and small, remove them with a suitably sized, round carbide bur The removal of carious dentin should not further affect resistance form because the periphery will not need further extension. In addition, it should not affect resistance form if the restoration will rest on a flat floor peripheral to the excavated area or areas. The flat floor should be at the previously described initial pulpal floor depth of 1.5 to 2 mm and in sound enamel or dentin. The external walls have already been finished during earlier steps in this conservative tooth preparation for amalgam. An occlusal cavosurface bevel is contraindicated in the tooth preparation for an amalgam restoration. It is important to provide an approximate 90- to 100-degree cavosurface angle,2 which should result in 80- to 90-degree amalgam at the margins (see Fig. 17-21, F). This butt joint margin of enamel and amalgam is the strongest for both. Amalgam is a brittle material with low edge strength and tends to chip under occlusal stress if its angle at the margins is less than 80 degrees. Every completed tooth preparation should be inspected and cleaned before restoration. The tooth preparation should be free of debris after rinsing the tooth with the air-water syringe. Disinfectants are available for cleaning tooth preparations,27'91 but are usually considered unnecessary. A cotton pledget or commercially available applicator tip moistened with water only is generally used. CLASS I OCCLUSOLINGUAL AMALGAM RESTORATIONS Occlusolingual (OL) amalgam restorations may be used on maxillary molars when a lingual fissure connects with the distal oblique fissure and distal pit on the occlusal surface (Fig. 1731). ٤
Initial Clinical Procedures. Tooth Preparation. The initial tooth preparation involves the establishment of the outline, primary resistance, primary retention forms, and initial preparation depth. The accepted principles of outline form (previously presented) are to be observed with special attention to the following: The tooth preparation should be no wider than necessary; ideally the mesiodistal width of the lingual extension should not exceed 1 mm, except for extension necessary to remove carious or undermined enamel or to include unusual Assuring. When indicated, the tooth preparation should be cut more at the expense of the oblique ridge rather than centering over the fissure (weakening the small dis- tolingual cusp). specially on smaller teeth, the occlusal portion may have a slight distal tilt to conserve the dentin support of the distal marginal ridge (Fig. 17-32). The margins should extend as little as possible onto the oblique ridge, distolingual cusp, and distal marginal ridge. These objectives help to conserve the dentinal support and strength of the tooth, and they aid in establishing an enamel cavosurface angle as close as possible to 90 degrees (Fig. 17-33). They also help to minimize marginal deterioration of the restoration by locating the margins away from enamel eminencies where occlusal forces may be concentrated. Using the mirror for indirect vision and the highspeed handpiece with air-water spray, enter the distal pit with the end of the No. 245 bur (Fig. 17-34, A). ٥
The long axis of the bur usually should be parallel to the long axis of the tooth crown. Remember to conserve the dentinal support and strength of the distal marginal ridge and distolingual cusp, which may require directing the bur so that it cuts more of the tooth structure mesial to the pit rather than distal. Penetrate to a depth of 1.5 to 2 mm as measured by the bur on the cut walls (1.5 mm at the fissure and up to 2 mm on the external walls) (see Fig. 17-34, B). This depth the pulpal floor is usually in dentin. Once the entry cut is made (see Fig. 17-34, C), move the bur (maintaining the initial established depth) to include any remaining fissures facial to the point of entry (see Fig. 17-34, D). ٦
At the same depth, move the bur along the fissure toward the lingual surface (see Fig. 17-34, E). As with Class I occlusal preparations, a slight distal inclination of the bur will occasionally be indicated to conserve the dentinal support and strength of the marginal ridge and the distolingual cusp. To ensure adequate strength for the marginal ridge, the distopulpal line angle should not approach the distal surface of the tooth closer than 2 mm. On large molars the bur position should remain parallel to the long axis of the tooth, The mesial and distal walls of the occlusal portion of the preparation should converge occlusally because of the shape of the bur. This convergence provides sufficient retention form to the occlusal portion of the preparation. Next, prepare the lingual portion. This may be accomplished by two techniques. In one technique the lingual surface is prepared with the bur's long axis parallel with the lingual surface (Fig. 17-35, A and B). The tip of the bur should be located at the gingival extent of the lingual fissure. ٧
The facial inclination of the bur must be altered as the cutting progresses to establish the axial wall of the lingual portion at a uniform depth of 0.5 mm inside the DEJ (see Fig. 17-35, C). The axial wall should follow the contour of the lingual surface of the tooth. Keeping the bur perpendicular to the tooth surface, round the axiopulpal line angle (Fig. 1736). Leaving a sharp line angle increases the possibility of fracture of the amalgam because of stress concentration. It also helps to ensure adequate preparation depth and amalgam thickness. The second technique is more difficult. In this case, the No. 245 bur is held perpendicular to the cusp ridge and lingual surface as it extends the preparation from the occlusal surface gingivally (to include the entire fault). This technique also results in opposing preparation walls that converge lingually. Initiate final tooth preparation by removal of remaining caries on the pulpal and axial walls (Fig. 17-37, A and B) with a suitably sized round bur, Fig.17-37 THE CLASS / (BUCCAL PIT) CAVITY PREPARATION FOR AMALGAM (Mandibular right first molar) Using a #330 F.G. carbide bur in a contra-angle turbine handpiece, entry is made through the center of the defective pit. Orientate the long axis of the bur at right angles to the buccal surface of the tooth while preparing the cavity. The outline is extended until sound tooth structure is reached. The outline form is triangular. The apex is towards the occlusal, while the base of the triangle is the gingival wall which is orientated, mesio-distally, approximately parallel with the occlusal plane of the teeth. The mesial and distal walls run from the apex to the respective ends of the gingival wall. The junction of the three walls is rounded by using the #330 bur (Figure 50). ٨
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