Biomechanical Comparison of Fixed- versus Variable-Angle Locking Screws for Distal Humerus Comminuted Fractures

Background: To compare the stability of fixed- versus variable-angle locking constructs for the comminuted distal humerus fracture (AO/OTA 13-A3). Methods: Eight pairs of complete humeri harvested from eight fresh frozen cadavers were used for the study. We fixed the intact humeri using 2.7-mm/3.5-mm locking VA-LCP stainless steel distal humerus posterolateral (nine-hole) and medial (seven-hole) plates. An oscillating saw was used to cut a 1-cm gap above the olecranon fosse. The specimens were loaded in axial mode with the rate of 1 mm per 10 seconds to failure, and stress-strain curves were compared in each pair. The mode of failure was recorded as well as the load needed for 2- and 4-mm displacement at the lateral end of the gap. Results: The stiffness of the constructs, based on the slope of the stress-strain curve, did not show any difference between the fixed- versus variable-angle constructs. Likewise, there was no difference between the force needed for 2- or 4-mm displacement at the lateral gap between the fixed- and variable-angle constructs. The mode of failure was bending of both plates in all specimens and screw pull-out in four specimen pairs in addition to the plate bending. Conclusions: Our results did not show any difference in the biomechanical stability of the fixed- versus variable-angle constructs. There was not any screw breakage or failure of the plate-screw interface.