Radiologic Comparison of Humeral Position according to the Implant Designs Following Reverse Shoulder Arthroplasty: Analysis between Medial Glenoid/Medial Humerus, Lateral Glenoid/Medial Humerus, and Medial Glenoid/Lateral Humerus Designs

Background: The currently available reverse shoulder arthroplasty (RSA) designs can be classified into medial glenoid/medial humerus (MGMH), lateral glenoid/medial humerus (LGMH), and medial glenoid/lateral humerus (MGLH) prosthesis designs. The purpose of this study was to radiologically analyze the effect of different RSA designs on humeral position following RSA. Methods: A total of 50 patients who underwent primary RSA were retrospectively analyzed. Among 50 patients, 33 patients (group A: MGMH) underwent RSA with Aequalis system (Wright, Inc, Bloomington, MN, USA), 6 (group B: LGMH) with Aequalis system using bony increased offset, and 11 (group C: MGLH) with Aequalis Ascend Flex system. The acromiohumeral distance, acromioepiphyseal distance (AED), lateral humeral offset (LHO), LHO from the center of rotation ($LHO^{COR}$), and deltoid length were radiologically measured to quantify the distalization and lateralization of the humerus. Results: The increment in postoperative AED was $19.92{\pm}3.93mm$ in group A, $24.52{\pm}5.25mm$ in group B, and $25.97{\pm}5.29mm$ in group C, respectively (p=0.001). The increment in postoperative LHO was $0.13{\pm}6.30mm$, $8.00{\pm}12.14mm$, and $7.42{\pm}6.88mm$, respectively (p=0.005). The increment in postoperative $LHO^{COR}$ was $20.76{\pm}6.06mm$, $22.04{\pm}5.15mm$, and $28.11{\pm}4.14mm$, respectively (p=0.002). Conclusions: The radiologic analysis of the effect of different RSA designs on humeral position following RSA showed significant differences in the increment in postoperative AED, LHO, and $LHO^{COR}$ between the 3 groups. Therefore, MGLH design seems to be more effective for humeral distalization and lateralization compared to original Grammont design.