Influence of glenosphere diameter and lateralization on instability of reverse shoulder arthroplasty A biomechanical investigation

Background. Prosthetic replacement of the proximal humerus with reverse shoulder arthroplasty (RSA) is an established means of treatment. Due to its unique biomechanical characteristics, RSA can restore shoulder function to a satisfying level in the case of cuff tear arthropathy, arthritis, and fractures. However, complications are frequent in RSA, one of the more common being implant instability with dislocation. The present study investigated the influence of glenosphere diameter and metaglene lateralization using a unique test setup. Methods. Seven fresh-frozen cadaveric specimens of the shoulder were thawed and dissected. The subscapularis muscle, the infraspinatus, and the three heads of the deltoid muscle were fixed to a pulley system. After implanting an RSA in different configurations (38/42 mm glenosphere with lateralization of +0mm, +5mm, or +10mm), the implants were dislocated using selective muscle pull. The frequency of dislocations depending on the prosthesis configuration was documented. Results. The larger glenosphere diameter of 42 mm showed less dislocations than the diameter of 38mm (39 vs. 46). Lateralization of +0, +5mm, and +10mm showed 26, 29, and 30, dislocations, respectively. Dislocation via pull on the infraspinatus muscle was most frequent. None of the results reached statistical significance. Conclusion. The current investigation used a novel technique for investigating the effect of lateralization and glenosphere diameter on RSA instability. Despite indicating tendencies, the present test setup could not prove the hypothesis that a larger glenosphere diameter and increased lateralization add to stability. The lack of statistical significance could be attributable to the low specimen number. The clinical significance of lateralization and glenosphere diameter should be further assessed in future biomechanical investigations.