Technique for the Measurement of Surgical Screw Deflection

Cyclical loading of surgical plating constructs can lead to screw failure through bending loads. In this research, an analytical beam mechanics model was developed to describe the elastic deflection of a cortical screw under a statically applied load. To test the predictive ability of the analytical model, a synthetic bone substitute was used for experiments. The bone substitute was a composite sandwich, consisting of two 6 mm cortical walls separated by a cancellous region. A novel instrument was developed to measure the bending deflection of screws under a radial load applied by a surgical plate at the screw head. A dial indicator and dowel pin arrangement was utilized to measure screw deflection at the screw head and along the length of the screw in order to determine a deflection curve for the screw axis. The analytical model accurately predicted deflection of the screw head at the location of the applied force, but overestimated deflection of the screw body in the cancellous region of the composite bone. Analytical predictions of location of the maximum bending moment matched the location of observed screw fracture in a clinical setting.