Mini-implants (screws) have the potential to facilitate orthodontic tooth movement by serving as anchors. Lack of sufficient teeth to provide anchorage is a major limitation in the treatment planning for orthodontic tooth movements in patients with craniofacial anomalies and in adult patients with compensated malocclusions. Objective: Information on the biomechanical performance of these screws is important to the clinician. We hypothesized that the pull out strengths of screws in bone will vary depending upon the site of insertion in the maxilla or mandible. Methods: Fifty-six (2 mm diameter, 6mm length) titanium screws (Synthes, Monument, CO) were placed in 4 beagle dogs (14 screws/dog) within one hour post-sacrifice. The screws were inserted to obtain mono-cortical anchorage from the buccal plate, at predetermined sites in the anterior, middle and posterior regions of the jaws bilaterally. Two screws were placed in the posterior palate. The jaws were harvested and bone blocks, each containing a screw, were prepared for mechanical testing. The screw-bone block was aligned on a custom made fixture and the maximum force (Fmax) at pull out was recorded (Bionix 858, MTS, Eden Prairie, MN). Statistical analyses to test for differences were conducted using ANOVA and Tukey-Kramer method. Results: Screws placed in the anterior mandibular region had significantly (p < 0.05) lower Fmax (134.5 ± 24N, mean ± SE) than those placed in the posterior mandibular region (388.3 ± 23.1N). The other regions of the maxilla and mandible had intermediate values of Fmax but were not significantly different from each other. Conclusion: Screws placed in the anterior regions of the maxilla and mandible, have lower holding power than screws placed in the posterior regions of the jaws. This is likely related to the thickness of the cortical plate in these regions.

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