Scaption kinematics of reverse shoulder arthroplasty do not change after the sixth postoperative month

Background: Changes over time in shoulder kinematics and function after reverse shoulder arthroplasty have not been reported. The purpose of this study was to compare shoulder kinematics and function at 6 months and 1 year after reverse shoulder arthroplasty. Methods: Twenty patients with a mean age of 74 years (range, 63-91 years) were enrolled in this study. Fluoroscopic images during scapular plane elevation were recorded at the mean of 6 months (range, 5-8 months) and 14 months (range, 11-21 months). CT-derived glenosphere models and computer-aided design humeral implant models were matched with the silhouette of the implants in the fluoroscopic images using model-image registration techniques. Glenosphere and humeral implant kinematics during scaption were compared between the two time points. Patients were also clinically examined with active range of motion and Constant score, and postoperative improvement in shoulder function were assessed. Results: Active flexion and Constant score improved after surgery (p < 0.001 for both), but there was no significant improvement after six months. There was no significant improvement in active external rotation at either postoperative exam. There were no significant differences in glenosphere or humeral kinematics between six months and one year. Interpretation: There was no significant additional improvement in either shoulder kinematics during scapular plane elevation or function between the sixth and twelfth postoperative months. We can assess kinematics at six months after reverse shoulder arthroplasty to determine how the shoulder will move. Clinically, treatment in the first six postoperative months should be emphasized to achieve better surgical outcomes.