CLASS II AMALGAM RESTORATIONS Amalgam restorations that restore one or both of the proximal surfaces of the tooth Initial Tooth Preparation Occlusal outline form (occlusal step).the occlusal outline form of a Class II tooth preparation for amalgam is similar to that for the Class I tooth reparation. enter the pit nearest the involved proximal surface with a punch cut using a No. 245 bur oriented as illustrated in Fig. 17-41, A and B. the long axis of the bur and the long axis of the tooth crown should remain parallel during cutting procedures. Proper depth of the initial entry cut is 1.5 to 2 mm, 1.5 mm as measured at the central fissure, and approximately 2 mm on the prepared external walls. While maintaining the same depth and bur orientation, move the bur to extend the outline to include the central fissure and the opposite pit (see Fig. 17-41, Cand D). For the very conservative preparation, the isthmus width should be as narrow as possible and no wider than one quarter the intercuspal distance. The pulpal floor should be prepared to a uniform depth (and is usually flat). Maintaining the bur parallel to the long axis of the tooth crown creates facial, lingual walls with a slight occlusal convergence, which provides favorable amalgam angles at the margins. It may be necessary to tilt the bur to diverge occlusally at the distal wall if extension of the distal margin would undermine the marginal ridge of its dentinal support (see Fig. 17-41, B and Q. During development of the pit area of the preparation, extension to include any facial and lingual developmental fissures radiating from the pit may be indicated. The pit area provides dovetail retention form, which may prevent displacement of the completed restoration. (see Fig. 17-41, E).
resistance form (occlusal step)is provided by: (1) the pulpal wall being relatively flat and smooth and perpendicular to forces directed with the long axis of the tooth (2) providing enough thickness of restorative material to prevent its fracture under mastication (1.5-2mm depth). (3) slightly rounding the internal line angles to reduce stress concentration in tooth structure (3) restricting the occlusal outline form (where possible) to areas receiving minimal occlusal contact[1/4 inercuspal distance] (4) the proximal wall should be parallel or slightly diverge occlusaly to remove unsupported enamel. (5) cavosurface angles approximately 90 degrees(90-110)º to ensure that no undermined enamel rods remain on the proximal margins and adequate thickness for amalgam retention form (occlusal step)is provided by: 1-the occlusal convergence of facial and lingual walls occlusaly 2- flat pulpal floor
3- restricting the occlusal outline form to areas receiving minimal occlusal contact[1/4 inercuspal distance] Proximal outline form (proximal box). The objectives for extension of proximal margins are to: 1-Include all caries, faults, or existing restorative material. 2- Remove unsupported enamel and Create 90-degree cavosurface margins (i.e., butt joint margins). 3-Establish (ideally) not more than 0.5 mm clearance with the adjacent proximal surface facially, lingually, and gingivally. The initial procedure in preparing the outline form of the proximal box is the isolation of the proximal enamel by the proximal ditch cut. This is a very important procedure in conservative tooth preparation and is, therefore, With the same orientation of the bur, position it over the DEJ in the pulpal floor next to the remaining marginal ridge (Fig. 17-44, A). Allow the end of the bur to cut a ditch gingivally, while the bur is moved facially and lingually along the DEJ. Extend the ditch gingivally just beyond the caries or the proximal contact, thus creating an axial wall that follows the faciolingual contour of the proximal surface (see Fig. 17-44, D). The location of final proximal margins (i.e., facial, lingual, gingival) should be established with hand instruments (i.e., chisels, hatchets, gingival marginal trimmers) in conservative proximal box preparations. Otherwise, these margins may be overextended to achieve 90-degree cavosurface margins with the No. 245 bur (see Fig. 17-44, E). Extending gingival margins into the gingival sulcus should be avoided where possible because subgingival margins are more difficult to restore and may be a contributing factor to periodontal disease. The proximal ditch cut may be converge occlusaly to ensure that the faciolingual dimension at the gingival is greater than at the occlusal (see Fig. 17-44, G). However, the shape of the No. 245 inverted cone bur should provide this converge. The occlusal converge contributes to retention form and
provides for desirable extension of the facial and lingual proximal margins to include defective tooth structure or old restorative material at the gingival level. Proximal margins having cavosurface angles of 90 degrees are indicated. Cavosurface angles of 90 degrees ensure that no undermined enamel rods remain on the proximal margins. In addition, remove the weakened enamel along the gingival wall by using the enamel gingival marginal trimmer in a scraping motion. If the gingival cavosurface margin is in enamel, it will usually require a slight bevel. Ideally the minimal clearance of the completed gingival margin with the adjacent tooth is 0.5 mm.
Final Tooth Preparation Removal of any remaining defective enamel and infected carious dentin. Removing any remaining enamel pit and fissure and infected carious dentin on the pulpal wall in Class II preparations is accomplished in the same manner as in the Class I preparation. The presence of infected carious dentin on a portion of either the pulpal wall (floor) or axial wall does not indicate deepening the entire wall. Infected carious dentin is removed with a slowly revolving round bur of appropriate size or a discoid-type spoon excavator or both. To enhance good resistance form, the occlusal step should have pulpal seats at initial preparation depth, perpendicular to the long axis of the tooth in sound tooth structure (Fig. 17-49). Pulp protection. The reader is referred to this same step in tooth preparation in class I amalgam restoration. Secondary resistance and retention forms. Secondary resistance form in final tooth preparation involves both resistance of the remaining tooth structure against fracture from oblique forces and resistance of restorative material against fracture. Restricting extensions of external walls provides the former; the latter is enhanced by using the gingival margin trimmer to bevel or round the axiopulpal line angle (Fig. 17-53), thereby increasing the bulk of and decreasing the stress concentration within the restorative material. Proximal retention locks in the axiofacial and axiolingual line angles significantly strengthen the isthmus of a Class Il restoration, FIG. 17-53.The occlusal convergence of the facial and lingual walls and the dovetail design provide sufficient retention form to the occlusal portion of the tooth preparation. The occlusal convergence of the mesiofacial and mesiolingual walls offers retention in the proximal portion of the preparation against displacement occlusally. To enhance retention form of
the proximal portion, proximal locks may be indicated to counter proximal displacement. Many operators use proximal locks routinely to ensure that each portion of the tooth preparation is independently retentive. To prepare a retention lock, use a No. 169L bur with air coolant (to improve vision) and reduced speed (to improve tactile "feel" and control). The bur is placed in the properly positioned axiolingual line angle and directed to bisect the angle (Fig. 17-54, A) approximately parallel to the DEJ (Fig. 17-55). This positions the retention lock 0.2 mm inside the DEJ.
Procedure for finishing external walls. The preparation walls and margins should not have unsupported enamel and marginal irregularities (if present, they require correction). It has been demonstrated that less marginal leakage occurs if the margins are straight and smooth. Ideally there should be a 90-degree cavosurface angle at the proximal margin. The occlusal line angle may be 90 to 110 degrees. This angle aids in obtaining a marginal amalgam angle of 90 degrees (no less than 80 degrees). Clinical experience has established that this "butt joint" relationship of enamel and amalgam creates the strongest margin.
Amalgam is a brittle material and may fracture under occlusal stress if its angle at the margin is less than 80 degrees. Final procedures: cleaning, inspecting, desensitizing, and bonding. resistance form (proximal box)is provided by: (1) the gingival wall being relatively flat and smooth and perpendicular to forces directed with the long axis of the tooth (2) cavosurface angles approximately 90 degrees are indicated. to ensure that no undermined enamel rods remain on the proximal margins and adequate thickness for amalgam (3) the buccal and lingual walls shoud be diverge proximally to open contact and remove unsupported enamel. (4) Axiopulpal line angel should be beveled o prevent stress concentration area to restoration (5) slightly rounding the internal line angles to reduce stress concentration in tooth structure (6) gingival cavosurface margin in enamel require a slight bevel using gingival marginal trimmer to remove unsupported enamel. retention form (proximal box)is provided by 1-the occlusal convergence of facial and lingual walls occlusaly 2-the dovetail design of the occlusal step. 3-flat gingival wall. 4- Proximal retention locks in the axiofacial and axiolingual line angles