Assessment of tooth preparation designs for posterior resin-bonded porcelain restorations

Tooth preparation designs advocated for posterior resin-bonded porcelain re storations were evaluated for clinical application. This study examined the design of tooth preparations from stone dies for individual posterior resi n-bonded porcelain onlays and complete porcelain crowns placed in a special ist practice, and compared features that provided retentive and porcelain f racture-resistance with those in the dental literature. Stone dies of tooth preparations for 57 onlay and 46 crown preparations were provided. Measure ments were recorded for the tooth preparations with the aid of interocclusa l silicone impressions. Proximal boxes, grooves and occlusal steps resultin g from removal of amalgam restorations were common. These retentive feature s were more evident with onlays, and appeared larger on the distal than the mesial surfaces. Internal tooth preparation tapers or convergence angles w ere frequently greater than 20 degrees, leading to relatively-wide occlusal isthmus widths, especially for onlays. There were wide variations in occlu sal surface reductions for the central fossae and supporting cusps. Tapers for tooth preparations were frequently more extensive, and occlusal surface reductions were often less than recommended. The implications of these dif ferences for long-term clinical success should be resolved.