Straining of the intact and fractured proximal humerus under physiological-like loading

Surgical treatment of proximal humeral fractures remains challenging in elderly patients, primarily due to insufficient implant fixation. Both bone quality and physiological-like loading conditions are commonly overlooked during pre-clinical in vitro evaluation. However, this knowledge is necessary in order to improve surgical treatment of the proximal humerus and the mechanical behavior of implants, particularly in patients with complex fractures and weak bone stock. We hypothesize that the bone quality has a high influence on the bone straining, independent of the arm position. The goal of this study was to determine the straining of the intact and fractured proximal humerus under physiological-like loading conditions. Furthermore, the impact of augmentation on tissue straining was evaluated. Two representative humeri were selected for this study, one osteoporotic and one reference quality, and scanned using both QCT and DEXA (average DEXA value=0.26 and 0.49 g/cm2 respectively). Subcaptial defects were generated, then stabilized with a plate prior to mechanical stiffness testing. From the QCT data, finite element models were generated and the in vitro stiffness tests analytically simulated. Under physiological-like loading conditions, the straining of the bone and implant were analyzed for 0 degrees, 90 degrees forward flexion, and 90 degrees abduction. Maximal strain values were found for the intact and fractured bone at 90 degrees abduction. This study demonstrates that the straining in a fractured bone of poor quality leads to considerably higher bone strains (up to +30%) than in a more healthy bone. Augmentation of a central void under physiological-like loading with commercial cement led to mechanical failure at the bone-cement interface. New concepts for the surgical treatment of complex fractures of the proximal humerus should take bone distribution into account and thereby allow effective treatment of fractures in osteoporotic patients. The ultimate salvage procedure of augmentation has mechanical limitations as long as current cement materials are used in osteoporotic patients.