Instability shoulder arthroplasty is a common complication occurring in up to 18.2% of cases.4 It occurs more frequently here than in knee or hip replacements because of the lack of osseous constraint afforded by the shoulder.

Shoulder instability is classified as posterior, anterior, superior or inferior. Recognition of the anatomic cause is crucial. Proper evaluation with radiographs, computed tomography, and magnetic resonance imaging is critical to the treatment algorithm.

Superior subluxation is the most common, and it is related to failure or dysfunction of the rotator cuff.

Inferior subluxation is most often due to inadequate restoration of proper humeral length during the replacement. This technical error is most common when hemiarthroplasty is performed after a fracture of the proximal portion of the humerus.

Anterior instability is often caused by disruption of the repaired subscapularis and capsule.

Posterior instability is uncommon and is usually caused by excessive retroversion of either the humeral or glenoid components.

Superior subluxation of shoulder hemiarthroplasty in two different patients. Note obliteration of the acromio-humeral distance secondary to chronic rotator tuff tear.

Anterior instability: the humeral component is subluxed anteriorly with the coracoid process (red arrow) now functioning as the anterior portion of the glenoid ("acetabularized glenoid").

Anterior instability: the humeral component is subluxed anteriorly due to tear of the subscapularis. Note atrophied subscapularis (arrow).

Anterior instability: the humeral component is dislocated anteriorly.

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