Pull-out strength of screws from cortical bone in the maxillo-facial region

Abstract

The fixation of maxillofacial fractures is an important clinical procedure, which may be achieved by the attachment of plates across the fracture. The stability of the fracture will depend on the stiffness of the fracture fixation plates and the security of the fixation screws to the thin maxillofacial cortical bone. The design of screws, manufactured by Champy and AO were tested from both mini-and micro-fixation systems. Pull-out tests were conducted on cortical bone plates, ranging in thickness from 0.6 to 3.5 mm. No significant differences were observed in the ultimate pull-out forces achieved for both mini-systems of 2 mm outer diameter. However, these pull-out forces were generally greater than those obtained for the micro-screws, even at the lower bone thicknesses. Two models were developed which attempted to predict the behaviour of screw pull-out failure. The failure mechanism was primarily dependent on the thickness of the bone, with secondary influences related to the shear strength of the bone and a geometrical factor of the screw.