Design and Biomechanical Evaluation of a Self-Centering Dual Mobility Concept for Reverse Total Shoulder Arthroplasty

Abstract

Reverse total shoulder arthroplasty (RTSA) remains limited by restricted range of motion, inferior impingement leading to scapular notching, and persistent trade-offs among mobility, constraint, and stability. This thesis investigated whether dual mobility principles established in total hip arthroplasty could be translated to RTSA in a biomechanically coherent manner. The central objective was not simply to introduce a second articulation in pursuit of range of motion gains, but to adapt dual mobility into RTSA in a way that would increase functional range of motion without compromising the established biomechanical benefits of the current RTSA design. For this translation to be mechanically meaningful, motion at the primary glenosphere-liner articulation and the secondary liner-humeral articulation had to be partitioned in a controlled sequential manner, such that the inner articulation remained dominant through mid-range motion while the outer articulation was recruited in a near the end range. This requirement motivated a three-stage methodological approach. First, a standardized computational framework was developed and validated to evaluate how geometric design parameters specific to RTSA influenced impingement-free ROM under controlled and repeatable conditions, thereby enabling consistent comparison of different implant design geometries. Second, structured concept generation and screening methods rooted in classical design frameworks were used to identify a biomechanically coherent dual articulation strategy for an RTSA implant. Third, the selected concept was embodied, computationally evaluated using a full factorial iv parametric study in which compressive load, friction, and radial clearance were varied. The embodied design was then qualitatively assessed experimentally through benchtop testing. The final implant concept employed a deliberate geometric offset (eccentricity) between the centers of rotation of the liner’s inner and outer surfaces, such that the applied joint compressive force generated a restoring moment about the liner’s center of rotation (COR), thus biasing the mobile component toward alignment with the load line and thereby promoting self-centering. The computational framework used a CAD-to Simulink pipeline to prescribe motion, detect impingement, and quantify articulation behavior. The principal embodiment variables were compressive load, inner-articulation friction, outer-articulation friction, and radial clearance at the outer articulation. The evaluated metrics were kinematic surrogates, including liner-shell misalignment and measures of articulation hierarchy via motion contribution metrics. The embodied design incorporated an eccentric liner, a humeral shell, and an inferior end-stop that limited liner excursion. Computational parametric evaluation showed that friction at the primary articulation was the dominant driver of liner-shell misalignment, whereas friction at the outer articulation had a smaller and less consistent effect. Radial clearance further modulated the load-dependent self-centering response: 0 mm clearance favored tighter tracking (better self-centering) under lower loads, 0.5 mm clearance increased geometric freedom but also increased sensitivity to loading, and 0.25 mm clearance exhibited the most balanced overall recentering behavior within the tested design space. Benchtop experiments provided qualitative support for the proposed v mechanism by reproducing the predicted articulation sequence and self-centering tendency under applied compressive load, while also confirming the computationally predicted impingement-free ROM. Outside of the parametric investigation, the embodied DM-RTSA concept demonstrated meaningful improvements in impingement-free ROM relative to a contemporary RTSA configuration. Within the scapular plane of elevation, the DM-RTSA implant increased adduction ROM by approximately 65%, delaying inferior impingement by 32° past the arm-at-side position through activation of the secondary articulation. Experimental evaluation qualitatively reproduced the predicted articulation sequence, self-centering tendency, and delayed inferior impingement behavior observed computationally, supporting the biomechanical feasibility of the proposed mechanism. Within the modeled and experimental scope, the thesis therefore demonstrates biomechanical and kinematic feasibility for a self-centering dual mobility RTSA concept and established a structured basis for future design refinement and preclinical evaluation. More broadly, this work provides a structured biomechanical foundation for the future refinement, preclinical evaluation, and eventual clinical translation of dual mobility principles within shoulder arthroplasty.