Novel Technique for Measurement of Screw Bending in Bone Fixation Constructs

The focus of the present research was to develop a reliable test method and apparatus to measure bending deflection of individual surgical screws during quasi-static loading of a plate construct. It was preferable to measure screw deflection in a non-invasive manner, thus promoting clinical relevance. Significant research has been conducted in the area of axial pullout strength and failure of screws due to torsion, but little exists related to bending deflection of surgical screws and the resultant bending stress. Current test methods to predict screw failure typically require cyclical loading of plate constructs to failure. This is both costly and time consuming. The desire to develop a new method for screw deflection measurement was instigated from observed screw failures postoperative open wedge high tibial osteotomy. Screw failures have been observed prior to healing of the open wedge, resulting in partial collapse and recurring misalignment. It was hypothesized that these failures were due to high bending loads. Engineers continue to design new plate and screw systems to strengthen the tibial plate construct. Most relevant design parameters include materials, plate geometry, number and alignment of screws, and most recently various retention methods of screw heads against the plate. With such a proliferation of designs, there exists a need to develop experimental methods for measurement of screw deflections as a precursor to ultimate cyclic failure. An open wedge, medial, high tibial osteotomy was performed on a composite replica tibia. A servo hydraulic machine was used to cyclically load the tibia in compression, while a Puddu plate construct supported the open wedge osteotomy. Deflection of individual screws was determined by means of laser projection and dial indicators affixed to exposed screw heads and reference points on the bone. During the period of elastic deformation, screw deflection measurements confirmed that each screw in the plate carried a different load. The laser projection method allows engineers to test newly designed plate constructs under relatively few cycles to determine the bending load carried by each screw. This non-invasive method of measuring screw deflection was repeatable with minimal error.