PULL-OUT STRENGTH OF SCREWS FROM CORTICAL BONE IN THE MAXILLOFACIAL REGION

The fixation of maxillofacial fractures is an important clinical proce dure, which may be achieved by the attachment of plates across the fra cture. The stability of the fracture will depend on the stiffness of t he fracture fixation plates and the security of the fixation screws to the thin maxillofacial cortical bone. The design of screws, manufactu red by Champy and AO were tested from both mini- and micro-fixation sy stems. Pull-out tests were conducted on cortical bone plates, ranging in thickness from 0.6 to 3.5 mm. No significant differences were obser ved in the ultimate pull-out forces achieved for both mini-systems of 2 mm outer diameter. However, these pull-out forces we re generally gr eater 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 primar ily 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.