Purpose: Proximal humeral fractures are a commonplace injury, especially in the elderly population. Management is not always straightforward, and is particularly challenging when bone quality is poor. In recent years, locking plates have become available for the internal fixation of many types of fractures, including those of the proximal humerus, and a growing trend in their use has been noted. This is a randomized biomechanical study to evaluate the mechanical stability in simulated osteoporotic bone of three fixation plates, two locking and one conventional, for unstable two-part proximal humeral fractures.

Method: Eighteen synthetic left humeri were plated with six bones in each of three groups: Synthes Cloverleaf Plate, Synthes Locked Compression Plate Proximal Humerus, and Smith and Nephew Periarticular Locking Plate for Proximal Humerus. Screw holes were overdrilled to simulate osteoporotic purchase. The distal humeral condyles were potted in autobody cement in polyvinylchloride tubes. An eight millimeter osteotomy gap was made at the base of the greater tuberosity to simulate an unstable two-part fracture. Cyclic axial compression testing was done in the vertical plane in 20 degrees of abduction to simulate physiologic loading. Measurements of plastic deformation of the construct were quantified by comparing RSA images taken before and after loading. Following cyclic axial compression testing, quasi-static torsion testing was done in the horizontal plane until construct failure. Failure was defined as the point where the linearity of a load-displacement curve is lost or where visible failure of the fixation occurs.

Results: No plates were loaded to failure. The locked plates were significantly stiffer in axial compression and torsion than the Cloverleaf plate. There was no difference between locked plates. The maximum total point motion seen on the RSA analysis was more than 4 times greater in the Cloverleaf group relative to either locked construct and no difference between the Synthes and Smith and Nephew locked plates was again seen. The majority of the motion in the Cloverleaf construct appeared to be in rotation about the anteroposterior axis (lateral rotation).

Conclusion: This study supports that locked plates, regardless of manufacturer, are stiffer in axial compression and torsion than Cloverleaf plates and result in less displacement in an unstable fracture pattern in an osteoporotic bone model.