Pull-out strength of monocortical screws placed in the maxillae and mandibles of dogs

Background:Mini-implants can facilitate orthodontic tooth movement by serving as anchors. The purpose of this investigation was to determine whether the pull-out strength of screws in bone varies depending on the site of insertion in the maxilla or the mandible. Materials: Fifty-six titanium screws (2 mm diameter, 6 mm length, Synthes USA, Monument, Colo) were placed in 4 beagle dogs (14 screws per dog) within 30 minutes after they were killed. The screws were inserted to obtain monocortical anchorage, at predetermined sites in the anterior, middle, and posterior regions of the jaws bilaterally. Two screws were placed in the posterior palate in each dog. The jaws were harvested, and bone blocks, each containing a screw, were prepared for mechanical testing. The bone/screw block was aligned on a custom-made fixture, and the maximum force (Fmax) at pullout was recorded. Cortical bone thickness was measured after extraction of the screw. Statistical analyses to test for differences were conducted with ANOVA and Tukey-Kramer tests. Results: Screws placed in the anterior mandibular region had significantly (P < .05) lower Fmax (134.5 ± 24N, mean ± SE) than those placed in the posterior mandibular region (388.3 ± 23.1N). Regression analyses suggested a weak (r = 0.39, P = .02) but significant correlation between Fmax and cortical bone thickness. Conclusions: The bone supporting monocortical screws would most likely withstand immediate loading and support tooth-moving forces.