Instability after arthroplasty: the shoulder

Shoulder instability after total shoulder arthroplasty may be classified as posterior, anterior, superior, or inferior. Basic causes include malposition of the components, incorrect version of the glenoid or humeral cuts, soft tissue contractures or laxity, and cuff deficiency. These may be present as isolated or as combined deficiencies. As always, avoidance is best, but instability is the commonest complication of shoulder arthroplasty and must be dealt with correctly. Recognition of the anatomic cause is crucial. Proper evaluation with radiographs, computed tomography, and magnetic resonance imaging is crucial to the treatment algorithm. Normally the glenoid is nearly perpendicular to the scapular spine, whereas the humeral head is retroverted about 30 degrees with respect to the transepicondylar axis. After defining the anatomic problem, proper component positioning, elimination of contractures, plication of capsular laxity, and establishing cuff integrity ensure the best opportunity for a stable shoulder.