Design Principles

A. General

1. A distinct path of insertion for the restoration must be created.
2. Sufficient reduction of the enamel must take place to provide resistance form, without tooth overcontouring, to maintain gum health.
3. When seated, the restoration should not be displaced or rocked in any direction by occlusal forces.

B. Posterior Restorations

1. Create distinct path of insertion in an occlusogingival direction by parallel modification of proximal, and then lingual, surfaces of the abutment teeth.
2. Create a proximal resistance form. Alloy framework must extend buccally beyond the distobuccal and mesiobuccal line angles of the respective abutments.
3. Obtain proximal "wrap-around". Alloy framework should be extended to engage the tooth structure at 180o or more of its circumference when viewed from the occlusal.
4. Obtain maximum bonding area (without compromising gingival health or esthetics) on abutments with proximal and lingual modifications (lowers height of contour).
5. Create small but well-defined occlusal rest on each abutment.
6. Create knife-edge gingival margins on posterior abutment teeth.

B. Lingual Segment

1. Create as large a bonding surface as possible without creating periodontal problems, occlusal interferences or plaque traps. Position gingival edge of lingual segment supragingivally. Terminate framework close to the gingival margin (should be definitive finish line and be 0.5 to 1.0 mm away from the gingival crest). Create a distinct but shallow (0.25 to 0.5 mm) gingival chamfer margin. Supragingival finish line prevents overcontouring of the lingual retainer, allows a thicker retainer that will not flex and provides a distinct margin for the laboratory technician.
2. Maintain a 0.5 mm thickness at the critical stress bearing long axis of the bridge adjacent to the pontics (lingual segments in other areas serve primarily as bonding retainers and can be thinned to a minimum of 0.3 mm).
3. Use a tapered diamond bur to reduce height of contour to allow a path of draw and increase the enamel surface for bonding.
4. Use same tapered bur to extend preparation to the proximal surface adjacent to the edentulous area (create minimal extension toward the facial surface to allow for optimal esthetic result). Proximal wrap can be extended further to facial on posterior teeth or in proximal surfaces that are not an esthetic concern. In anterior areas where esthetics are a concern, keep proximal reduction to area just slightly lingual to the projected contact area and employ proximal grooves for retention.
5. Extend preparation to proximal area opposite edentulous ridge (should extend to contact area but not actually "break" contact). On anterior abutment teeth, this portion of the preparation will involve reduction of the marginal ridge. While difficult due to facial/lingual thinness of incisors, retainers must extend to this area of abutment teeth to resist rotational forces from occlusion and possible debonding.

C. Proximal Grooves

1. Place mesial and distal proximal grooves, with 169L carbide bur, in long axis of tooth lingual to the contact area and adjacent to the edentulous area.
2. Place second parallel groove in the marginal ridge (on anterior teeth) or proximal area (on posterior teeth) just lingual to the contact area.
3. To ensure preparation kept in enamel, limit depth of grooves to diameter of bur.
4. Connect mesial and distal grooves with lingual chamfer or cingulum notch to create a "staple" effect for mechanical retention similar to a three-quarter crown.
5. Ensure all grooves are parallel to one another and in line with selected path of insertion (if re-alignment necessary, use a slightly larger 170 tapered fissure bur).

D. Occlusal Clearance and Rest Seats Occlusal clearance must be obtained for all posterior and maxillary bridges (mandibular anterior bridges usually do not require any occlusal modification). Occlusal clearance for maxillary retainers should be a minimum of 0.5 to 1.0 mm (verify in all excursive movements of the mandible). Obtain by reducing abutment tooth (in some cases, may also recontour the occluding surface of the antagonist tooth). Prepare a distinct rest seat on all posterior teeth with a small round diamond or No. 6 round carbide bur. Main function is to dissipate occlusal stresses across adjacent teeth. Posterior rests should be somewhat smaller than those required for a removable partial denture (usually 1.0 mm in diameter, 0.5 mm deep and have distinct vertical walls). Where occlusion is heavy or the abutment is short occlusogingivally, rest seats are necessary on both the mesial and distal portions. When anterior teeth (especially maxillary canines) have large cingula, incorporate a cingulum rest using a full crown preparation diamond with a rounded tip.

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