A greater understanding of reverse total shoulder arthroplasty (RTSA) in terms of articular contact mechanics and wear is essential for the optimization of current surgical technique and future implant design. Despite the prevalence of RTSA for shoulder reconstruction, there is little information in current literature regarding the effects of changes in implant parameters on articular contact mechanics and wear. The present work describes the use of in-vitro cadaveric studies, a computational model of the articular contact surfaces, and the development and implementation of a wear simulation strategy to assess RTSA contact mechanics and wear.
The articular loading characteristics of RTSA were assessed using an in-vitro shoulder joint motion simulator and a custom instrumented implant, including the effects of changes in implant configuration. Decreasing neck-shaft angle and cup depth was not found to affect joint load or muscle forces. Increasing glenosphere diameter increased adduction range of motion (ROM), but also slightly increased joint load.
The contact mechanics of RTSA were then investigated. The location of the contact patch and peak contact stress was typically in the inferior humeral cup quadrant, coincident with the location of clinical retrieval damage. Reducing neck-shaft angle and decreasing cup depth reduced contact area and increased peak contact stress, which may negatively impact implant performance. Increasing size came at no cost in terms of contact mechanics.
A wear simulation strategy was developed based on the loading and motion characteristics of the cadaveric study, and produced a mean wear rate of 201.1±86.5 mm³/Mc, which was higher than previously published data, and created damage in the cup inferior quadrant. The number of 'cycles' per year for RTSA reconstructed shoulders was estimated between 0.33-1.5 Mc/yr, suggesting a similar order of magnitude as the lower extremities.
The present work advances knowledge regarding RTSA biomechanics and tribology. Specific tradeoffs in terms of ROM and contact mechanics were observed for the reduction of both neck-shaft angle and cup depth, whereby increased motion came at the cost of reduced contact area and increased peak contact stress. Increasing size improved ROM without any negative consequences on contact mechanics.