Biomechanical Evaluation of Two-Part Surgical Neck Fractures of the Humerus Fixed by an Angular Stable Locked Intramedullary Nail

Objective: The aim of the current study was to see how different interlocking mechanisms would affect construct stability and overall failure in the treatment of two-part surgical neck fractures in the proximal humerus in vitro. Methods: Left and right bones of eight pairs of fresh-frozen human cadaveric humeri were assigned to either a group with conventional or a group with angular stable distal interlocking. The different experimental interlocking mechanisms were used in a surgical neck fracture model of the humerus (Orthopaedic Trauma Association 11-A3) stabilized by a proximal humeral nail. The following variables were evaluated by biomechanical tests: hysteresis width in bending and torsion, stiffness, and fracture gap movement during cyclic axial loading until failure and the overall failure mechanism of the construct. Results: The angular stable group showed significantly less motion in initial bending and torsion and higher bending stiffness throughout the complete deformation cycle compared with the conventional interlocked group. Fracture gap movement was significantly less in the angular stable group. Higher stability was mainly observed in the early phase of the applied loading pattern; however, ultimate failure was not related to distal interlocking but occurred in the proximal fragment in both groups. Conclusions: An experimental angular stable distal interlocking system of proximal humeral nails shows higher construct stability in the early phase of fracture fixation in vitro. In terms of overall failure, loss of fixation in the proximal fragment was crucial and not different between groups.