Does the reverse shoulder prosthesis medialize the center of rotation in the glenohumeral joint?

Reverse shoulder arthroplasty is commonly used to improve the function of osteoarthritic shoulders in cases with irreparable refractory rotator cuff-tear arthropathy when conventional prosthesis designs cannot be applied. There is indication that moving the glenohumeral joint center more medially may lead to improved shoulder function by extending the lever arm for the deltoid muscle and facilitating muscle recruitment. However, there is little experimental evidence for this medialization effect. Marker based motion data of pre- and one year postoperative examinations on nine subjects who underwent reverse shoulder arthroplasty were analyzed applying functional methods for joint center estimation. The aim was to determine the location of the functional center of rotation in the operated and the non-operated contralateral side before and after surgery to verify if the joint center of this reverse prosthesis design is located more medially compared to the anatomic situation before surgery. It was shown that the operated shoulders demonstrated a medialization effect of 8.3±4.3mm. For the non-operated side the difference was 0.1±2.3mm, proving the accuracy of measurements.