The effect of the humeral neck-shaft angle on the biomechanics of a stemless shoulder prosthesis in reverse configuration

Shoulder arthroplasty has shown significant progress over the last few decades, but complications still exist, limiting its success. In order to avoid stem-related problems and preserve the humeral bone stock, stemless prostheses were introduced. The Lima Shoulder Modular Replacement (SMR) stemless device considered here in the context of reverse configuration, has the ball and socket geometry of the shoulder inverted. One variable element is the neck shaft angle (NSA) resection of the humeral head, which enables the use of different sloped liners. The primary aim of this research was to investigate the impact of NSA on primary implant fixation and bone response, by analyzing two different configurations. One configuration featured a 135◦ neck-shaft angle cut and a 7.5◦ sloped liner, while the other had a 150◦ neck-shaft angle cut and a 0◦ liner. 19 right humeri were included in the study and CT scanned using a hydroxyapatite phantom. LimaCorporate Figura system was used to evaluate bone properties using Quantitative Computed Tomography (QCT), and to assess implant-bone micromotions and postoperative bone response. Three-dimensional models of the humeri were imported into ANSYS Workbench software for an implicit nonlinear static structural analysis. The loading scenario analyzed was a 105◦ abduction with no hand weight, identified as the most unfavorable condition based on previous dynamic biomechanical simulations of reverse shoulder arthroplasty implants. From the statistical analyses it emerged that the mean difference for micromotions was equal to 4.48 μm, not enough to infer that one configuration is better than the other one. Nevertheless, in both cases, the values were below the threshold of 150 μm, which allows for bone ingrowth. Regarding bone response, it was observed that the first layer and the lateral quadrant exhibited more evident resorption, while the remodelling is more pronounced distally and in the medial quadrant. The configuration in which the cut is made at an angle of 150◦ resulted to resorb less and remodel better in nearly all direct comparisons between levels and quadrants. These results suggest that under the applied load a more horizontal cut of the humeral head leads to a more satisfactory bone response.