Comparison of torque measurements between cortical screws and emergency replacement screws in the cadaver mandible.

The so-called emergency screws are designed for use in a drill hole where the conventional screw has failed following a stripped bone thread. It is not known, however, if these screws are as well anchored in bone as cortical screws, nor is it known how well they secure contact to the previously damaged screw bed. On the basis of torque measurements in mandibles of fresh cadavers, emergency screws were compared with cortical screws. Further, the contact between emergency screw and bone was investigated on interfacial sections after embedding in methyl methacrylate resin. The maximum torque reached during insertion of the AO emergency screws was about 70% of the value measured with 2.7-mm conventional cortical screws and 67% of this value when the 3.2-mm emergency screws were used. While the emergency screws had good contact to the bone in the thread crest area, no bone contact was evident at the bottom of thread groove. This experimental situation can not be entirely extrapolated to the clinical situation. However, the study seems to suggest that emergency screws do not fully meet the demands of rigid internal fixation.