Survival Analysis of Orthodontic Micro-Implants: A Retrospective Study on the Effects of Patient-Related Factors on Micro-Implant Success

The purpose of this study was to investigate the effects of patient-related factors such as anteroposterior and vertical skeletal patterns and alveolar bone density on the success rate of micro-implants. Cases of orthodontic micro-implants (n = 404; diameter, 1.6 mm; length, 6 mm) were investigated in 164 patients (127 women, 37 men; mean age, 23.6 ± 5.8 years). Cortical bone thickness and alveolar bone density were measured using diagnostic cone-beam computed tomography to examine their effects on the micro-implant’s survival. Moreover, anteroposterior and vertical facial patterns were considered as independent variables for the success of micro-implants. Marginal survival analysis was performed by analyzing the time from implant placement to the removal of the failed micro-implants, or to the end of treatment for successful micro-implants. Variables including age, sex, implantation side, implantation site, root proximity, and type of loading (immediate vs. delayed) were also assessed. In total, 347 (85.9%) of the 404 micro-implants were successful. The mean loading time was 12.4 ± 4.3 months. Marginal survival analysis showed that the effects of the anteroposterior and vertical facial patterns on the risk of failure were not statistically significant. The factors significantly associated with the micro-implant loading time were cortical bone density, root proximity, and micro-implants replanted in the same site. In conclusion, our findings indicate that anteroposterior and vertical skeletal patterns are not associated with the success of orthodontic micro-implants. Cortical bone density may be associated with the micro-implant’s success.