Lateralized reverse shoulder arthroplasty shows similar scaption kinematics but greater scapular neck-insert distance during external rotation compared to Grammont prosthesis.

[BACKGROUND]: Biomechanical differences between reverse shoulder arthroplasty (RSA) designs, specifically those with larger neck-shaft angles like the Grammont-type prosthesis and those with smaller neck-shaft angles such as lateralized humerus RSAs, have been analyzed in ex vivo studies. However, there is limited data on the differences in in vivo shoulder kinematics between these designs. The purpose of this study was to analyze in vivo kinematics of lateralized humerus RSA during active scaption and external rotation at the side, and to compare them to previously reported data for Grammont-type RSA. [METHODS]: Twenty shoulders that underwent RSA using a lateralized onlay prosthesis were included in this study. They consisted of 10 males and 5 females with a mean age of 76 years (range, 69-83). Patients underwent fluoroscopy during active scaption and external rotation at the side at or after postoperative 1 year; additionally, computed tomography was performed to create three-dimensional scapular implant models. Using these fluoroscopic images and three-dimensional models, three-dimensional shoulder kinematics were computed using model-image registration techniques. Scaption kinematics and scapular-neck distance were compared with data from previous studies on Grammont-type RSA, which were analyzed using the same techniques as in this study. [RESULTS]: There were no significant differences in scaption kinematics between lateralized humerus and Grammont-type RSA. However, the scapular neck-insert distance during active external rotation at the side was significantly greater in lateralized humerus RSA (3.2-5 mm) than in Grammont-type RSA (approximately 1 mm, P < .001) despite the glenohumeral abduction angles being significantly smaller (P = .03). [CONCLUSION]: Lateralized humerus RSA showed similar scaption kinematics to Grammont-type RSA; however, the scapular neck-insert distance during active external rotation at the side was significantly greater in lateralized humerus RSA than in Grammont-type RSA. The greater neck-insert distance may contribute to a lower incidence of scapular notching.