Excessive crown-implant ratios have
been cited in the literature as being detrimental to long term implant
survival. However, unfavorable crown-implant ratios have not yet been established.
The primary aim of this study was to determine the crown-implant ratios of single-tooth implant-supported restorations on short-length implants in a clinical practice, and to evaluate
the health of these implants via mesial and distal first bone-to-implant
contact levels. Additionally, the relationship between crown-implant ratios and proximal first
bone-to-implant contact levels will be evaluated. In this retrospective
cohort study, the cohort was composed of 194 patients who possessed
at least 1 single 5.7 mm or 6 mm length plateau design implant-supported
restoration that had been surgically placed between February 1997 and December
2005. A chart review was performed to acquire the most recent radiographs in which both the entire crown
and the implant were visible. The length of the crown and implant was measured
directly from the radiographs using consistent magnification to calculate the
crown-implant ratio. Mesial and distal first bone-toimplant contact levels were
measured using 3 times magnification and were mathematically corrected for distortion. The last
available radiograph was used to measure bone levels. Follow-up time was
calculated from the day of implant placement to the date of the last available radiograph.
Statistical analyses with analysis of variance mixed
models were used. Data from 309 single implant-supported fixed restorations
were tabulated and included in the study. The mean (SD) follow-up time was 20.9
(23.2) months, with a range of 15.6 to 122.8 months. The mean crown length (SD)
was 13.4 (2.6) mm, with a range of 6.2 to 21.7 mm. The mean (SD) crown-implant
ratio was 2.0 (0.4) and ranged from 0.9 to 3.2. The average mesial and distal
first bone-to-implant contact levels (SD) measured from the radiographs were
20.2 (0.7) mm and 20.2 (0.9) mm, respectively. No statistically significant relationship
was observed between increasing crown-implant ratios and decreasing mesial and
distal first bone-to-implant contact levels around the implant with P values of
.94 and .57, respectively. In this investigation, mesial and distal first
bone-to-implant contact levels on short-length implants fall within the
established guidelines for success. Also, there are no associations between
crown-implant ratios and first bone-to-implant contact levels.
