Restoring the vertical dimension of mandibular incisors with bonded ceramic restorations

Mandibular incisors are difficult to restore with conventional anterior cro wns due to their small axial diameters. Crown preparation risks pulp exposu re and results in a thin core of dentine. An experiment was designed to det ermine if increasing the thickness of ceramic to restore incisal edges affe cts the load at the point of restoration failure. Forty-eight mandibular in cisors were randomly divided into four groups. The incisal edges of the tee th in three groups were reduced so that the coronal height was 7.5 mm (Grou p A), 6.5 mm (Group B), and 5.5 mm (Group C). Group D was designated as the experimental control. A fifth group (Group E), independent of the random s ample, was introduced to the study for discussion purposes and was characte rized by each of the incisors having an intact incisal edge of enamel. Grou ps A, B and C were prepared in a standard fashion for ceramic veneer restor ations that were constructed to restore the vertical height of each tooth t o 8.5 mm. Prior to cementation, the preparation surfaces were analysed and the relative surface areas of enamel and dentine were calculated. The ceram ic restorations were cemented using a resin luting agent. The teeth were th en thermocycled prior to loading at 135 degrees C until failure. The mean f racture load was 305 N (SD 134 N) for Group A, 403 N (SD 101 N) for Group B , 515 N (SD 296 N) for Group C, 587 N (SD 187 N) for Group D and 395 N (SD 129 N) for Group E. As determined by analysis of variance followed by a She ffe multiple comparison test there was a significant difference (p less tha n or equal to 0.01) between Group A and Group D. There were no significant correlations of load at failure with the percentage of exposed dentine, ena mel surface area, total preparation surface area, and the incisal surface a rea. The patterns of failure of the fractured specimens were also analysed. There were significantly (p less than or equal to 0.05) greater fractures of the ceramic restorations when they were bonded with the minimum incisal thickness of ceramic.