Possible mechanisms of screw loosening of rigid sternal fixation

The goal of this study was to compare the performance of cancellous and cortical screws for plate and screw fixation of the sternum. Pelvic bone plates were fixated to osteoporotic sternum in both unicortical and bicortical configurations and then loaded transversely (0-50N at 2Hz) for 15,000 cycles. The resulting plate displacement was measured continuously. In the first 10 cycles bicortical screw purchase demonstrated less loosening (p=0.015), while differences between screw type was insignificant. After the 15,000 cycles, cortical screws had less loosening (p=0.039) than cancellous, while the number of cortices fixated was insignificant to loosening. These initial findings indicate that both number of cortices fixated and type of screw are both important in designing and optimized system for sternal fixation. It appears that the cortical shell regions allow more screw purchase than the osteoporotic cancellous regions, while bicortical screw purchase appears to minimize the loosening of the screws.