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The glenohumeral joint is formed by a unique articulation between a larger and nearly spherical humeral head with a shallow and much smaller glenoid. Minimal bony constraints combined with a unique anatomical architecture and functional arrangements allow the shoulder joint to have the largest range of motion in the body.

Despite its minimally constrained nature, the glenohumeral joint can carry both small and large loads at various speeds of arm motion while maintaining stability due the joint’s tremendous reliance on soft tissue support. Instability is defined as abnormal or painful excessive movement of the humeral head out of the glenoid during active shoulder motion and must be distinguished from laxity, which is asymptomatic instability in both normal and unstable shoulders.

Glenohumeral stability is achieved by the complex interactions between static and dynamic constraints. Static constraints include the capsule, ligaments, and tendons; dynamic constraints are obtained by active muscle contraction. In the middle range of rotation, joint stability is provided by the dynamic action of the rotator cuff and the biceps muscles through compression of the humeral head in the glenoid socket.

The ligamentous structures provide passive restraints at the extremes of rotation, preventing excessive translation of the humeral head on the glenoid. Contraction of the muscles around the shoulder may also have the secondary effect of protecting the smaller ligamentous structures from injury at the end-range positions. Authors have demonstrated differences in rotator cuff and scapular muscle firing patterns between patients with stable and unstable shoulders.

The osseous anatomy, capsuloligamentous structures, negative intra-articular pressure, synovial fluid adhesion-cohesion, the rotator cuff, scapular stabilizers, and biceps tendon all play roles in providing stability. Additional factors that contribute to stability are patient age and gender, capsular integrity, and strength and conditioning of the rotator cuff, and scapular stabilizing muscles of the shoulder complex.

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