CORRELATIONS BETWEEN SCREW HOLE PREPARATION, TORQUE OF INSERTION, ANDPULLOUT STRENGTH FOR SPINAL SCREWS

The bone-screw interface is critical in the use of spinal instrumentat ion. The purpose of these experiments described here was twofold. Firs t, to determine whether a correlation existed between torque generated during screw insertion and the pullout strength. Second, to determine how differing surgical methods of screw hole preparation influenced t orque of insertion and screw pullout strength. A series of experiments were carried out in which screws were inserted into synthetic bone (e xperiment 1) and into calf vertebrae (experiment 2). The method of scr ew hole preparation (i.e., diameter of entrance hole and pilot hole) w as varied while the resulting torque of insertion and the pullout stre ngth of the screw was measured in each case. A torque screwdriver was used to measure the torque of insertion of the screws. Screw pullout s trength was measured using a materials testing machine. Two important results emerged from these experiments. First, a higher torque of inse rtion correlated with a higher screw pullout force. This correlation m ay be useful intraoperatively in evaluating fixation. Second, torque o f insertion and pullout force were more influenced by cortex over-dril l diameter than pilot hole diameter. These experiments show the import ance of the dorsal cortex in pedicle screw fixation.