Success rate of miniscrews relative to their position to adjacent roots

The purpose of this study was to evaluate, histologically, root contact, proximity to a root, and proximity to marginal bone level as possible risk factors for the failure of mini-screws when inserted between neighbouring teeth. Twenty mini-screws were inserted into the mandible of five beagle dogs. Each dog received two bracket screw bone anchors in each lower quadrant, between the roots of the second and third, and third and fourth premolars. Every six weeks, apical radiographs were taken and vital stains were administered. Twenty-five weeks after insertion of the screws, the dogs were sacrificed and specimens prepared for histological evaluation. The distance between the screw and the roots and between the screw and the marginal ridge level (MRL) were measured on the histological slides. The presence or absence of root contact was evaluated histologically on serial sections. The number of screws was too small to allow for sound statistical analysis of the factors under investigation. During the evaluation period, 11 screws were lost. Six screws were in contact with a tooth root and five of these were lost. In five sites, the distance between the screw and the tooth was less than 1.0 mm, but only one of these screws was lost. The distance between the screw and the marginal bone level was less than 1.0 mm for nine screws and seven of these were lost. The results of this limited study suggest that root contact and marginal position might be major risk factors for screw failure.