Effect of prosthesis design on muscle length and moment arms in reverse total shoulder arthroplasty.

The purpose of this article is to compare the effect reverse shoulder design philosophy has on the lengths and moment arms of the external rotators of the shoulder (i.e. posterior deltoid, infraspinatus, and teres minor). In this study, a single bone model was used to normalize the origin and insertion of each muscle. Four different commercially available designs were virtually implanted into the bones. The assemblies oriented with the arm at the side (0° of abduction) and articulated to 45° internal rotation and 45° external rotation. The muscle length and moment arm for each muscle were analyzed and compared to the anatomic shoulder. The results indicate that all the external rotators are shortened relative to the anatomic shoulder, but the MGLH design shortened the muscles the least. The moment arm analysis showed the teres minor and infraspinatus moment arms increased relative to the normal shoulder in external rotation for all designs. The moment arms are sensitive to the lateral offset of the stem relative to the center of rotation. The MGLH design had the largest moment arm during the external rotation range of motion due to its large humeral offset.