Pull-out strength of monocortical screws at 6 weeks postinsertion

Abstract

Screws (mini-implants) are increasingly used in orthodontics to provide anchorage for tooth movements. The objective of this study was to determine the biomechanical stability of these screws. We hypothesized that pull-out strengths of screws at 6 weeks (T6) postinsertion (1) vary by location, (2) are directly related to cortical bone thickness, and (3) are different at T6 than at insertion (T0). The T0 time point had been examined in a previous related study. Six skeletally mature male dogs received a total of 88 screws (Synthes USA, Monument, Colo; 2 × 6 or 2 × 8 mm) at predetermined sites in the anterior, middle, posterior, and palatal regions of the jaws. The monocortical screws were not loaded. Seventeen screws became loose or were lost over the healing period. Of the 71 surviving screws, 48 were randomized for mechanical testing and 23 for histology. The harvested bone blocks were prepared for mechanical testing and aligned in a custom-made fixture to record the maximum pull-out force (Bionix 858, MTS, Eden Prairie, Minn). Statistical analyses were conducted with ANOVA and the Tukey-Kramer method. Of the 17 failed screws, 11 were in mandibular anterior sites, 4 in maxillary anterior sites, 1 in a mandibular posterior site, and 1 in the palate. Mean peak pull-out strengths for the various sites ranged from 153.5 ± 37.6 N to 389.3 ± 32.5 N (mean ± SEM) with significantly (P < .05) higher strengths in the mandibular posterior sites versus the maxillary anterior and palatal sites. Pull-out strength was related to cortical bone thickness (r = 0.59, P < .0001). Mean pull-out strengths at T0 and T6 showed no significant (P < .05) differences between the 2 time periods. The results give the clinical orthodontist an estimate of the holding power of these screws for each millimeter of cortical bone purchase. After correction for load orientation, a clinician can expect a static holding power of approximately 122 N (95% CI, 93-150 N) for 1 mm of cortical bone purchase and about 174 N (95% CI, 147-205 N) for 2 mm of cortical bone purchase. Histological analyses of screws placed in the maxillae and mandibles of dogs might give further insight to the early healing response, adaptation physiology, bone contact, and potential reasons for success and failure of the screws.