In vitro evaluation of the loosening characteristics of self-tapped and non-self-tapped cortical bone screws.

The heads of self-tapping and non-self-tapping screws in dog femurs were exposed to a cyclic shearing force of 110 N for 200 loading cycles. This cyclic shear loading created a decrease in pull-out strength for both screw types of approximately 11% (p less than 0.01). No statistically significant difference in pull-out strength was found between screw types either before or after cyclic loading. A linear relationship between pull-out force and cortical thickness was observed for both screw types. These tests corroborated past work which showed equal holding power for the self-tapping and non-self-tapping screw. The progressive loosening of the screws with cyclic shear loading was accompanied by increasing load-displacement hysteresis and screw head migration. Greater hysteresis suggested that the non-self-tapping screw might have loosened more than the self-tapping screw from this applied loading schedule. Bone microcracking around screw threads before and after cyclic loading was observed by scanning electron microscopy. Photomicrographs of one non-self-tapping screw type and two self-tapping screw types showed microcracks at the tip of the outer diameter of the screw thread. More microcracks were observed after application of cyclic shear loading.