Design rationale for a posterior/superior offset reverse shoulder prosthesis.

INTRODUCTION: A computer model quantified and compared muscle lengths and moment arms of two different reverse shoulder arthroplasty humeral tray designs during two different motions. METHODS: The computer model simulated internal/ external rotation and abduction in the scapular plane for the normal shoulder, the 38 mm non-offset, and the 38 mm posterior-superior offset reverse shoulders. Muscle lengths were directly measured for seven muscles during each motion. External rotation moment arms were calculated for five muscles during each motion. RESULTS: The offset tray shifted the humerus posteriorly and superiorly relative to the non-offset tray. The more superior humeral position with the offset tray elongated the deltoid 1.0% to 3.8% less and caused each muscle to convert from an adductor to abductor earlier in abduction. The more posterior humeral position with the offset tray better restored the anatomic muscle tension, decreased the internal rotation capability (e.g., moment arm) of the subscapularis and teres major by 7.1 mm and 9.5 mm, and increased the external rotation capability of the posterior deltoid, infraspinatus, and teres minor by 1.3 mm, 8.6 mm, and 7.8 mm. DISCUSSION AND CONCLUSIONS: The offset humeral tray bet- ter restored the anatomic muscle tensioning and increased the external rotation moment arms relative to the non-offset humeral tray, which has positive implications on strength, stability, and motion.