Purpose: The objectives of this study were:

to determine if the deltoid, conjoint tendon and long head of the triceps provide sufficient soft tissue tension to stabilize a RTSA, and;

The hypotheses were:

that the deltoid, conjoint tendon and long head of the triceps provide sufficient soft tissue tension to stabilize a RTSA, and;

Method: Six cadaveric shoulders had all capsule, rotator cuff, and scapulohumeral muscles removed, leaving only the deltoid, conjoint tendon (i.e. coracobrachialis and short head of biceps) and long head of triceps. A RTSA was then performed. A displacing force was then applied perpendicular to the centerline of the humeral socket and this load was increased until dislocation occurred. The load required to cause a dislocation was recorded for superior, inferior, anterior and posterior load directions. This was repeated to measure the effect of humeral component rotation (neutral, 20 degrees retroversion, 20 degrees anteversion), arm position (0 degrees abduction, 60 degrees flexion, 60 degrees abduction and 60 degrees extension) and polyethylene thickness (3, 6 or 9 mm). Statistical analysis used an ANOVA with Tukey post-hoc tests for multiple comparisons (p< 0.05).

Results: The deltoid, conjoint tendon and long head of the triceps provide sufficient soft tissue tension to stabilize a RTSA. The required dislocation force was increased for an inferior direction of load application (p0.05). The required dislocation force was least in an arm position of 60 degrees abduction, followed by 60 degrees extension, with no difference between 0 degrees abduction and 60 degrees flexion (p0.05).

Conclusion: The deltoid, conjoint tendon and long head of the triceps provide sufficient soft tissue tension to stabilize a RTSA. Stability of a RTSA was greatest for an inferior directed force and an arm position of 0 degrees abduction or 60 degrees flexion. There was no influence of arm rotation or polyethylene thickness on stability of a RTSA. This study indicates that stability of a RTSA can still be achieved despite significant soft tissue loss, as long as key soft tissue structures remain intact. As well, certain loading directions and arm positions lead to an increased risk of instability. However, further in vivo studies are required.

Purpose: Reverse shoulder arthroplasty (RSA) is becoming increasingly common for a variety of indications. The most common postoperative complication is instability. The objectives of this study are to describe causes of RSA instability and preventive and treatment recommendations.

Method: All members of the American Shoulder and Elbow Society were asked for cases of RSA instability. We retrospectively reviewed 52 cases in 52 patients, performed by 13 surgeons. The mean age was 67.5 years (range, 36–83). The mean follow-up was 32.7 months (range, 3–43).

Results: The etiology included: cuff tear arthropathy (14 patients); revision arthroplasty (13 patients); fracture sequelae (6 patients); and other (6 patients). Twenty-four patients had previous shoulder surgery. All patients had a deltopectoral approach. Anterior deltoid deficiency was noted in 3 cases. Subscapularis insufficiency was noted in 27 cases. Humeral component dissociation occurred in 2 patients. The instability was: anterior (27 patients); posterior (5 patients); or inferior (3 patients). Instability occurred: in the first 48 hours (10 cases); between 2 to 30 days (13 cases); or after one month (12 cases). Causes of instability included: inadequate soft-tissue tension (23 patients); a large antero-inferior release (22 patients); subscapularis insufficiency (7 patients); hematoma (2 patients); trauma (2 patients), deltoid insufficiency (2 patients); glenosphere malposition (2 patients); acromion fracture (2 patients); and other (4 patients). Initial treatment included: non-operative (6 patients); closed reduction (23 patients); open reduction (3 patients); and revision arthroplasty (11 patients). Final outcomes included: persistent instability (7 patients); humeral revision (19 patients); glenoid revision (one patient); hemiarthroplasty (3 patients); and resection arthroplasty (2 patients).

Conclusion: Main causes of RSA instability include inadequate soft tissue tension, large anteroinferior release and subscapularis insufficiency. Preventive measures include careful patient selection, restoration of humeral length, minimizing anteroinferior releases, maximizing soft-tissue tension, maintaining or restoring subscapularis integrity, immobilization in the case of subscapularis insufficiency and use of a postoperative drain. Treatment recommendations include restoration of humeral length, soft-tissue retensioning and prolonged immobilization.