Review Question - QID 219991

The patient has rotator cuff arthropathy, which is best treated with a reverse total shoulder arthroplasty (Figure B). The arm position that puts one at most risk for instability in reverse total shoulder instability is extension, adduction, and internal rotation, such as that used when pushing out of a chair (Answer 3; Illustration A).

Reverse total shoulder arthroplasty (RTSA) is a type of shoulder arthroplasty that uses a convex glenoid hemispheric ball and a concave humerus articulating cup to reconstruct the glenohumeral joint. The advantage of an RSA is that the center of rotation (COR) is distalized and medialized, allowing the deltoid muscle to act on a longer fulcrum, thereby providing a greater mechanical advantage to substitute for the deficient rotator cuff muscles in providing shoulder abduction. Though it allows for improved shoulder abduction, it does not restore this function to normal, and has limitations in restoring internal and external rotation. In terms of complications, dislocations represent the most common cause of early failure, with an incidence of around 2-4%, with the position of dislocation most commonly being extension, internal rotation, and adduction.

Gerber et al. provide a review of reverse total shoulder arthroplasty. The authors note that radical changes in prosthetic design in the mid-1980s transformed the historically poorly performing reverse ball and socket total shoulder prosthesis into a highly successful salvage implant for pseudoparalytic, severely rotator cuff–deficient shoulders. They conclude that proper patient selection and attention to technical details are needed to reduce the currently high complication rate.

Cheung et al. reviewed the complications associated with reverse total shoulder arthroplasty. The authors note that increased use of primary reverse total shoulder arthroplasty has led to reports of associated problems unique to the procedure, with the most common complications including neurologic injury, periprosthetic fracture, hematoma, infection, scapular notching, dislocation, mechanical baseplate failure, and acromial fracture. They conclude that further research is required to establish best practices for managing these and other complications associated with RTSA.

Boileau et al. published on the complications and revision of reverse total shoulder arthroplasty. The author notes that the most common causes of revision surgery after RTSA are prosthetic instability (38%), infection (22%), humeral problems (21%), including loosening, unscrewing and fracture, and, lastly, problems of glenoid loosening (13%). He concludes that the functional results of revised RTSA are inferior to those of primary prostheses, and depend on the surgeon’s experience and the number of RTSAs performed, suggesting that patients should be referred to a tertiary center when revision is necessary.

Figure A is an AP radiograph of the shoulder showing the sequelae of a failed rotator cuff repair with retained metallic anchors, evidence of acetabularization of the acromion and femoralization of the humeral head, consistent with rotator cuff arthropathy. Figure B is a post-operative radiograph of the same patient who has undergone a successful reverse total shoulder arthroplasty. Illustration A depicts the position most commonly associated with RTSA instability, namely shoulder adduction, internal rotation, and extension used to lift oneself from a seated position.

Incorrect Answers:
Answer 1: Excessive abduction and external rotation, though the culprit in traditional anterior instability or apprehension of the native shoulder, is not physiologically achieved in the post-op RTSA. The deltoid muscle, which wraps around the prosthesis, provides a "wrap effect" that improves abduction and stability of the prosthesis.
Answer 2: Active external rotation at the side is often limited post-operatively in the setting of RTSA and, thus, does not contribute significantly to post-operative instability.
Answer 4: Internal rotation at 90 degrees of abduction allows a patient to reach their face and mouth for hygiene, but does not contribute significantly to instability. Combining internal rotation with shoulder extension in an adducted arm is what allows the polyethylene tray to separate from the glenosphere.
Answer 5: Resisted forward elevation in the plane of the scapula with the shoulder externally rotated describes the "full can" test (opposite of the "empty can" test).