A method for documenting the change in center of rotation with reverse total shoulder arthroplasty and its application to a consecutive series of 68 shoulders having reconstruction with one of two different reverse prostheses.

BACKGROUND

Reverse shoulder arthroplasty changes the center of rotation (COR) of the glenohumeral joint and in doing so affects the resting tension in the deltoid and residual cuff muscles, as well as their respective moment arms. The purpose of this study was to assess the change in COR from the preoperative to postoperative state in a group of patients undergoing reverse shoulder arthroplasty.

MATERIALS AND METHODS

The position of the COR in relation to a scapular coordinate system was determined for the anteroposterior and axillary radiographs before and after reverse total shoulder arthroplasty for 68 shoulders (63 patients) receiving either a Delta prosthesis or an Encore Reverse Shoulder Prosthesis.

RESULTS

Preoperatively, the COR was superiorly displaced a mean of 9 ± 7 mm from the origin of the coordinate system. For all shoulders, the postoperative COR was inferiorly displaced by 12 mm to a position 3 ± 3 mm below the coordinate origin (P < .001) and medially displaced by 27 ± 4 mm from the coordinate origin (P < .001) in the anteroposterior projection. For the shoulders receiving the Delta prosthesis, the COR was inferiorly displaced by 2 ± 3 mm from the coordinate origin, whereas it was inferiorly displaced by 7 ± 3 mm with the Encore prosthesis (P < .001). The COR was medially displaced by 28 ± 4 mm with the Delta prosthesis and by 19 ± 3 mm with the Encore prosthesis (P < .001).

CONCLUSIONS

The position of the COR relative to the scapula is significantly altered by reverse shoulder arthroplasty and is significantly different for 2 different implant designs.