Purpose: The purpose of this study is to demonstrate that inferior inclination of the glenosphere is a protecting factor from joint dislocation in reverse total shoulder replacement. The hypothesis is that an average of 10° of inferior inclination of the glenoid component would determine a significant inferior rate of dislocation as compared to neutral inclination.
Methods: A retrospective case (dislocation)-control (stability of the implant) study was performed. Inclusion criteria were the homogeneity of the prosthetic model and availability of pre- and postoperative imaging of the shoulder, including antero-posterior and axillary X-ray views. Glenoid and glenosphere inclination were calculated according to standardized methods. Difference in between the angles determined the inferior tilt.
Results: Thirty-three cases fit the inclusion criteria. Glenoid and glenosphere inclination measured, respectively, 74.1° and 83.5°. The average tilt of the glenosphere measured 9.4°. The average tilt in stable patients was 10.2°. Tilt in patients with atraumatic dislocation measured, respectively, -6.9° (superior tilt) and 2.4°, while it was 8.3° for the patient with traumatic instability. The association between the tilt of glenosphere and atraumatic dislocation was significant.
Conclusions: A 10° inferior tilt of the glenoid component in reverse shoulder arthroplasty is associated with a reduced risk of dislocation when compared to neutral tilt.
Keywords: Glenosphere inferior tilt; Instability; Reverse shoulder arthroplasty.