The Effects of Washers and Staples on Anterior Spinal Instrumentation Stability: Can Plowing be Prevented? A Cadaveric Model

The purpose was to determine if washers and staples used in anterior spinal instrumentation provide a mechanical advantage in eliminating plowing of the screws. Before testing they were separated into 15 single vertebra, potted, and instrumented with a screw, screw-washer, or screw staple equally. A rod was attached and axial compression was applied. Results were compared to ASTM UHMW models. Sample t-test assuming unequal variances yielded that washer vs. screw was not significantly different (p > 0.176), staple vs. screw was not significantly different (p >0.145), and staple vs. washer was not significantly different (p > 0.678). Results were identical to tests using the ASTM model. Washers and staples did not provide a mechanical advantage or make a significant contribution to the stability of anterior scoliosis constructs. Staples actually slightly decreased the stiffness of the construct compared to the screw only construct. Plowing of the screws was not affected by washers or staples.