Shoulder Arthritis / Rotator Cuff Tears: causes of shoulder pain: Reverse total shoulder – the effect of design

Our study in actual patients with cuff tear arthropathy was based on the recognition that 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. We determined the position of the COR in relation to a scapular coordinate system on anteroposterior and axillary radiographs before and after reverse total shoulder arthroplasty for 68 shoulders (63 patients) receiving either a Grammont-style prosthesis (e.g. Delta or Tournier Aequalis) in contrast to an extended glenoid neck prosthesis (e.g. DJO or Encore). 

Preoperatively, the COR of the shoulders with cuff tear arthropathy was superiorly displaced a mean of 9 ± 7 mm from the normal shoulder position. This displacement creates slack in the deltoid which can lead to pseudo paralysis.  For all shoulders, the average postoperative COR was inferiorly displaced by 12 mm to a position 3 ± 3 mm below the normal position (P < .001) and medially displaced by 27 ± 4 mm from the normal position (P < .001) in the anteroposterior projection. The COR was medially displaced by 28 ± 4 mm with the Grammont-style prosthesis and by 19 ± 3 mm with the DJO prosthesis (P < .001). We concluded that 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 in living patients.

The newly published study concerned seven pairs of shoulders that were tested on a biomechanical simulator. Native arms were tested, implanted with a Tornier Grammont-style or DJO Surgical Reverse Shoulder Prosthesis (RSP), and then retested. 

Both systems shifted the center of rotation medially and inferiorly relative to native. As in our in vivo study, medial shifts were greater in the Grammont-style implant (P < .037). In fact the data from the two studies are quite similar.

The authors suggest that these differences may play roles in limiting scapular notching, improving active external rotation by normalizing the residual rotator cuff length, and limiting excessive stress on the deltoid.