Crown-to-implant ratios of single tooth implant-supported restorations

Statement of problem. The crown-root ratio guidelines used to establish a prognosis for teeth serving as abutments are commonly applied to a dental implant-supported restoration or potential implant site. These guidelines are generally empirical and therefore lack scientific validation. Purpose. The purpose of this study was to first determine the crown-implant ratios of single tooth implant-supported restorations, and then to compare the crown-implant ratios to the guidelines established for the crown-root ratios of natural teeth. Material and methods. A retrospective cohort study design was used. The cohort was composed of subjects (n=294) having 1 or more single tooth implants (Bicon) placed between May 1992 and April 2004. A chart review was conducted to obtain radiographs in which the entire crown and implant were visible. The length of the crown and implant were measured directly from the radiographs using magnification to calculate the crown-to-implant ratio. The length of the crown was divided by the length of the implant to determine the crown-implant ratio. Calculations were made to within 0.1 mm. Implant failure was recorded and defined as removal of the implant for any reason. Data were analyzed with descriptive statistics. Results. A total of 889 single tooth implants from 294 patients were measured and included in the study. The mean (SD) follow-up time was 2.3 (1.7) years, with a range of 0.1 to 7.4 years. Sixteen failures were recorded for a survival rate of 98.2%. The crown-implant ratios ranged from 0.5:1 to 3:1. The mean (SD) crown-implant ratio of implants in function was 1.3:1 (0.34). The mean crown-to-implant ratio of failed implants was 1.4:1 (2.5). Conclusions. The results of this study suggest that the crown-to-root ratio guidelines associated with natural teeth should not be applied to a potential implant site or existing implant restoration. The crown-to-implant ratios of those implants that were in function were similar to those implants that failed.