Factors associated with the stability of mini-implants for orthodontic anchorage: a study of 414 samples in Taiwan

To evaluate failure rates and factors associated with the stability of mini-implants used for orthodontic anchorage.

We enrolled 166 patients (35 male patients and 131 female patients) who had consecutively received mini-implants for orthodontic anchorage at the Section of Orthodontics and Pediatric Dentistry, Taipei Veterans General Hospital (Taipei, Taiwan) from January 2001 to December 2006. A total of 414 mini-implants with diameters ranging from 1.2 to 2.0 mm were evaluated. Clinical variables for analysis were divided into host-related and implant-related factors. Mini-implants that could be maintained for orthodontic anchorage for more than 6 months were considered to be successful. Statistical analysis was used to evaluate the failure rate in our study cohort and to identify possible associated factors.

The overall failure rate was 10.1% (42 of 414 screws) with orthodontic force loading for more than 6 months. Most failures were due to loosening and occurred within the first 2 weeks. Differences in overall failure rates for the maxilla and mandible (9.3% and 16.3%, respectively) were not statistically significant. A lower failure rate was found for the maxilla with implant diameters equal to or less than 1.4 mm (P = .036). The left side had a lower failure rate than the right (6.7% vs 13.9%, P = .019). Length and type of mini-implants, age, and gender were not associated with mini-implant failure.

Use of mini-implants for anchorage is reliable. In our study the overall success rate was 89.9%. Careful diameter selection for different locations is essential. In the maxilla an implant diameter equal to or less than 1.4 mm is recommended. In the mandible an implant diameter larger than 1.4 mm is suggested for better orthodontic anchorage. Hygienic care of implantation sites should also be emphasized for long-term success of mini-implant anchorage.