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Glenohumeral Joint

The GH joint is a diarthrodial joint with minimal bony constraint, allowing it the largest range of motion of any major diarthrodial joint in the body. The GH joint has been described as being similar to a golf ball on a tee, with a large humeral head balanced on a smaller glenoid.

The GH joint approximates ball-and-socket kinematics, with only one-third of the humeral head being covered by the glenoid in any position of rotation, and the articular surface of the humeral head is threefold that of the glenoid.

In most shoulders, the glenoid and humerus have similar radii of curvature, providing a basically congruent articulation with less than 2 mm of mismatch between the glenoid and the humeral head. This matched concavity–convexity of the articulation provides stability when muscle forces act across the joint. This provides the foundation for the RTC musculature to establish a concavity–compression effect.

The combined version of the glenoid and humeral head results in a retroversion of approximately 30 to 40 degrees. In normal shoulders, the center of the humeral head is usually within 1 mm of the plane of the scapular spine. The spherical humeral articular surface articulates with the spherical concavity of the glenoid, whereas the proximal humeral convexity articulates with the spherical concavity of the coracoacromial arch.

A negative intra-articular pressure exists within the GH joint, creating a vacuumlike effect. The joint acts as a closed compartment with a flexible diaphragm. The weight of the arm tends to pull the joint surfaces away from each other, creating negative pressure. In addition, an adhesion–cohesion effect exists secondary to the viscous and intermolecular properties of the synovial fluid, similar to water keeping two glass surfaces together.

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