Glenohumeral instability is defined as an inability to maintain the humeral head centered in the glenoid fossa. This problem is typically caused by either a traumatic rupture of the capsule and ligaments (usually following a complete dislocation or partial dislocation/subluxation), or by generalized laxity of the soft tissue. In the case of post-dislocation instability, the shoulder is unstable in only one plane (namely, in the direction of the dislocation), whereas with generalized laxity, the instability is multidirectional. Patients with glenohumeral instability may suffer repeat episodes of subluxation in which the joint surfaces are damaged. In very broad terms, instability from trauma is amenable to surgical repair, whereas multidirectional is not. Multidirectional instability is treated with physical therapy to strengthen the rotator cuff, but this does not always stabilize the joint sufficiently.
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