Geometric Analysis of the Proximal Humerus in Elderly Japanese Patients: Implications for Implant Selection in Reverse Shoulder Arthroplasty.

The implants used in reverse shoulder arthroplasty were originally designed for white patients; thus, they might not be optimal for smaller Asian patients. The selection of reverse shoulder arthroplasty systems is limited in Japan. The purpose of this study was to measure the geometry of the proximal humerus in elderly Japanese patients to inform implant selection according to humerus size. This study included 155 shoulders from 148 patients 50 years or older who underwent computed tomography for shoulder disorders other than arthritis and trauma. There were 67 male and 81 female patients with mean ages of 68 and 66 years, respectively. The humeral head diameter, head height, neck-shaft angle, neck diameters, and osteotomy diameters for 155 degrees inclination implants were measured using surface models created from computed tomography scans. The mean diameter of head curvature, humeral head height, and neck-shaft angle were 48.2 mm, 19.8 mm, and 136 degrees for male patients and 42.4 mm, 17.1 mm, and 136 degrees for female patients, respectively. The mean mediolateral and anteroposterior neck diameters were 46.9 mm and 43.7 mm for men and 41.6 mm and 38.4 mm for women, respectively. The mean mediolateral and anteroposterior osteotomy diameters were 42.2 mm and 41.2 mm for male patients and 38.6 mm and 36.7 mm for female patients, respectively. There were significant differences between the sexes in all measurements except neck-shaft angle. Humeral implants with 155 degrees inclination will not fit the humerus of smaller Japanese women. Thus, implants with anatomical inclination and an onlay humeral tray may be a better choice for smaller patients.