Three-dimensional kinematics of reverse shoulder arthroplasty: comparison between shoulders with good or poor elevation

Abstract Background Various factors may be related to outcomes of reverse shoulder arthroplasty (RSA) including patient and surgical factors. Differences in shoulder kinematics might be associated with poor function after RSA; however, kinematic differences between shoulders with good or poor elevation have not been elucidated. The purpose of this study was to compare RSA kinematics between shoulders with good or poor elevation. Methods The study included 28 shoulders with minimum 6 months follow-up after reverse shoulder arthroplasty using Grammont-type prostheses. Subjects comprised 17 males and 11 females with the mean age of 75 years (range, 63-91). Subjects underwent fluoroscopy during active scapular plane abduction. Computed tomography of their shoulders was performed to create 3-dimensional scapular implant models. Using model-image registration techniques, poses of 3-dimensional implant models were iteratively adjusted to match their silhouettes with the silhouettes in the fluoroscopic images, and 3-dimensional kinematics of implants were computed. Kinematics and glenosphere orientation were compared between shoulders with good (>90 degree) or poor ( Results Nineteen and 9 shoulders were assigned to the good and poor elevation group, respectively. There were no significant differences between the groups in age, sex, height, weight, preoperative range of motion or Constant score, but body mass index in the poor elevation shoulders was significantly larger than that in the good elevation shoulders. There were no significant differences in glenosphere (upward/downward rotation, anterior/posterior tilt, internal/external rotation) nor glenohumeral (internal/external rotation, abduction/adduction) kinematics between the good or poor elevation shoulders. Scapulohumeral rhythm was significantly higher in the good elevation shoulders than the poor elevation shoulders (p = 0.04). Glenosphere superior tilt was 2.3o ± 4.2o in the good elevation group and 8.1o ± 8.9o in the poor elevation group, and the difference was statistically significant (p = 0.03). Discussion Shoulders with good elevation after RSA demonstrated better scapulohumeral rhythm than those with poor elevation, though there were no significant differences in glenosphere and glenohumeral kinematics. It may be important for better elevation to achieve good glenohumeral motion in shoulders with RSA. Glenosphere orientations may affect postoperative shoulder function.