The mechanics of the human glenohumeral joint were studied in order to quantify the relationships between joint loading, kinematics, and contact patterns. A series of related experiments was performed to investigate the biomechanical effects of surgical repair and reconstruction, using normal joints as controls. An experimental model was developed to simultaneously quantify three-dimensional kinematics and joint reaction forces during active elevation in die scapular plane, across a complete range of humeral rotations. Measured humeral head translations were confined to a relatively small area on the glenoid but demonstrated characteristic patterns with rotation. Anterior-posterior and superior-inferior humeral head translations correlated with shear forces in these respective anatomic directions. During scapular plane elevation, it was observed that deltoid muscle forces provided the predominant abduction torque regardless of humeral rotation, due in part to their large moment arms relative to the diameter of the humeral head. The subscapularis and infraspinatus were most efficient in rotating the humerus, although their combined strength contributions to elevation was approximately equivalent to that of the supraspinatus.

The effects of high demand repetitive use of the shoulder were investigated by means of continuous cyclic loading of the inferior glenohumeral ligament to increasing levels of high subfailure strain. Non-recoverable ligament elongations and progressive, dramatic increases lit ligament laxity were consistently observed. Relative to unaltered joints, anterior capsular tightening significantly restricted maximal arm elevation, produced postero-inferior humeral head subluxation, and increased posteriorly-directed joint forces. In contrast, joint mechanics following an inferior capsular shift closely approximated those of the unaltered joint.

Prosthetic replacement of native glenohumeral articular surfaces with perfectly conforming components shifted contact on the glenoid posteriorly both during centered joint loading in the scapular plane, as well as with the humerus in flexion and extension. A new glenoid design featuring a conforming central region and non-conforming periphery appeared to mitigate the effects of glenoid rim loading at the extremes of motion, and in general its performance was intermediate to that of the conforming and nonconforming glenoids.