An explorative time-elapsed μCT-based cadaveric study on humeral stem stability in reverse shoulder arthroplasty.

Higher complication rates in reverse shoulder arthroplasty are commonly reported in patients receiving smaller humeral implants, potentially due to reduced bone stock and compromised implant stability. While bone-preserving hybrid onlay-Grammont humeral stems are thought to improve cortical engagement, comparative biomechanical evidence remains scarce, particularly in small humeri. A 2 × 2 factorial design was conducted in this exploratory study to evaluate the independent effects of implant size (large vs. small humerus) and implant placement (inlay vs. hybrid onlay-Grammont) on implant stability. Paired humeri from a male (large) and female (small) donor were implanted with inlay and hybrid onlay-Grammont designs. Specimens were tested under controlled physiological and compressive failure loading in a custom-built μCT-compatible rig. Implant stiffness, vertical displacement, and cortical failure were quantified using 3D μCT imaging and force-displacement data. Rigid co-registration and surface mesh segmentation were used to assess implant migration and bone deformation. In the large humerus, the hybrid onlay-Grammont implant showed greater stiffness (288 N/mm) and failure load (2800 N) than the inlay design, consistently causing cortical opening. In the small humerus, the inlay implant exhibited the highest distal migration (-12 mm) without cortical cracking, while the onlay design produced controlled radial cortical failure consistent with the large specimen. These results demonstrate that reduced peri-prosthetic bone stock can prevent the implant from fully loading the cortex causing a shift of the failure mechanism, particularly in the small humerus. They also support further larger studies on patient, surgical, and loading factors concerning implant stability.