Glenohumeral stability. Biomechanical properties of passive and active stabilizers.

The shoulder is characterized foremost by its mobility and large range of motion. The glenohumeral joint is notable for its relative lack of bony constraint, relying heavily on the congruent articulating surfaces and surrounding soft tissue envelope for static and dynamic stability. Effective function in the articulation is achieved by a complex interaction between the various articular and soft tissue restraints. The rotator cuff muscles center the humeral head in the congruent glenoid fossa through the midrange of motion, when the capsuloligamentous structures are lax. However, incongruent joints, especially in positions of loading asymmetry (in external rotation), have larger translations that occur at the extremes of motion. Excessive translations are then effectively restricted by the mechanical properties of the inferior glenohumeral ligament. When the capsule is tightened anteriorly it results in an anterior tether and causes an associated posterior shift in contact on the glenoid. The posterior migration of the humeral head center and glenohumeral contact are again more pronounced in shoulders with reduced congruence. Additional studies of normal motion in different planes, the effects of rotator cuff pathology and dysfunction on the kinematics of the joint, proprioception of the capsule, and biomechanical tests of the inferior glenohumeral ligament and other components of the joint capsule at strain rates associated with injury, need to be conducted to understand the specifics of normal shoulder function and the pathophysiologic processes that occur during shoulder degeneration.