Effect of cortical bone thickness and implant placement torque on stability of orthodontic mini-implants

Purpose: To examine the relationship between cortical bone thickness, inter-root distance (horizontal space), distance from alveolar crest to the bottom of maxillary sinus (vertical space) at the prepared site, and implant placement torque and the success rate of mini-implants placed for orthodontic anchorage. Materials and Methods: After computerized tomography examination, mini-implants 1.6 mm wide and 8 mm long were placed in the posterior alveolar bone. The mini-implant was judged a success when orthodontic force could be applied for at least 6 months without pain or clinically detectable mobility. The unpaired t test was performed to examine differences between the success and failure groups. The chi-square analysis or Fisher exact probability test was used to compare the implant success according to placement torque, location, and patient gender. P values less than .05 were considered significant. Results: The subjects included 4 males (11 implants) and 28 females (76 implants) who ranged in age from 14.6 to 42.8 years. The success rate of the 87 implants was 87.4%. Cortical bone thickness was significantly greater in the success group (1.42 +/- 0.59 mm vs 0.97 +/- 0.31 mm, P =.015). The success rate was significantly higher in the group with an implant placement torque of 8 to 10 Ncm (100%) as compared to implants with higher or lower placement torques. The odds ratio for failure of the mini-implant was 6.93 (P =.047) when the cortical bone thickness was less than 1.0 mm relative to 1.0 mm or more. Conclusion: A relationship between stability after implant placement and the width and height of the peri-implant bone was not demonstrated. The prepared site should have a cortical bone thickness of at least 1.0 mm, and the placement torque should be controlled up to 10 Ncm.