Stress Shielding Effects of Short Stem Alignment and Bone Density in Reverse Shoulder Arthroplasty.

Short-stem humeral implants in reverse total shoulder arthroplasty (rTSA) are increasingly adopted because they preserve bone stock and reduce stress shielding. However, their shorter length raises the risk of malalignment, and outcomes may be further affected by osteoporosis, which is common in rTSA patients. This study investigated how humeral stem alignment (neutral, varus, valgus) and bone mineral density simultaneously influence stress distribution in the proximal humerus using finite element (FE) analysis. FE models were developed from computed tomography scans of one normal and one osteoporotic humerus. A curved short stem was virtually implanted in three alignments, and physiological loading was applied. Von Mises stress was examined in cortical and trabecular bone, subdivided into medial/lateral and proximal/distal regions. Varus alignment produced the largest stress reduction in the proximal-lateral region, particularly in osteoporotic bone (up to -55.5%). Valgus alignment reduced stress primarily in the proximal-medial region (up to -55.9%) but also affected the proximal-lateral region. These results confirm that malalignment induces stress shielding patterns consistent with clinically observed bone resorption. Specifically, varus alignment was associated with shielding in the proximal-lateral region, whereas valgus alignment contributed to reductions in both proximal-medial and lateral regions. In osteoporotic bone, malalignment exacerbated stress shielding, with varus demonstrating the most detrimental effect. Clinical Significance: These findings highlight the importance of stem alignment to minimize proximal stress shielding, especially in osteoporotic patients. Careful intraoperative positioning may reduce adverse mechanical outcomes, though further clinical studies are required to establish definitive surgical guidelines.