Tk. Daftari et al., CORRELATIONS BETWEEN SCREW HOLE PREPARATION, TORQUE OF INSERTION, ANDPULLOUT STRENGTH FOR SPINAL SCREWS, Journal of spinal disorders, 7(2), 1994, pp. 139-145
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.