A gender and size specific evaluation of inlay versus onlay stem designs in achieving lateralization and distalization in reverse shoulder arthroplasty

Abstract Introduction: In reverse shoulder arthroplasty (RSA) new designs enable greater amounts of lateralization to prevent instability and scapular notching and increase range of motion, however, excessive lateralization leads to stress upon the acromion that can result in scapular spine fatigue fractures (SSFF). Aim of this study was to gender- and size-specifically assess the best implant design selection in order to reduce the risk of SSFF and to optimize lateralization, distalization, and bony impingement-free range of motion (ROM) in patients undergoing RSA. Methods: Computed tomography scans from 30 osteoarthritic patients (f:15, m:15) and 20 cuff tear arthropathy patients (f:10, m:10) were used to virtually simulate a RSA. The efficacy of inlay vs onlay stems combined with two glenospheres (36mm vs 42mm) in achieving ROM, lateralization, and distalization was evaluated. Moreover, gender and patients constitution were correlated to humeral size by radiologically measuring the best-fit circle of the humeral head. Results: A different amount of relative lateralization was achieved in both genders using large glenospheres and onlay designs. Latter yielded a higher ROM in all planes for men and women with a 42 mm glenosphere; with the 36mm glenosphere, an increased ROM was observed only in men. In women, this configuration only improved abduction. When adjusting the absolute amount of lateralization to humerus’ size and regardless of implant type, women received greater relative lateralization than men; with a 42mm glenosphere and an onlay system, women lateralized 19% vs 8.0% observed in men. Conclusion: The relative lateralization achieved using onlay design is much higher in women than men. Humeral lateralization using onlay designs should be used cautiously in women, as they may enhance the risk for developing SSFF. Our results point out that glenoid lateralization should be used instead to reduce inferior notching and to improve ROM, especially in small women.