Traumatic injury and degenerative changes, which affect the glenohumeral (GH) joint, have been reported to alter articular mechanics, forces transmitted through this joint and contact stresses in the cartilage and the underlying bone. High stresses, or alterations in stress distribution resulting from abnormal loading or injury of the articular cartilage, are believed to be responsible for its degeneration and osteoarthritis. However, little is still known about reaction forces and contact mechanics at the GH joint.

Therefore, the objective of this dissertation was to directly measure the reaction forces at the GH joint and to obtain quantitative data on stresses, strains, and contact pressures at this articulation. Another objective was to evaluate the effect of rotator cuff tears and applied muscle forces on the GH reaction forces.

A combined experimental and computational approach was undertaken to achieve these objectives. A Dynamic Shoulder Testing Apparatus, which simulates physiologic GH joint motion, was equipped with a universal force/moment sensor to directly measure the magnitude and direction of the reaction forces at the GH joint. The results of this study showed that reaction forces are dependent on the activity of the supraspinatus muscle, which compresses the humeral head into the glenoid. The extension o f simulated rotator cuff tear beyond the supraspinatus tendon led to a significant decline in reaction force magnitude and change in its direction.

The finite element models were developed to determine the stresses, strains, and pressures for simulated contact between the humeral head and the glenoid. The effect of material composition of the contacting bones and parametric alterations of the material properties of the cartilage and labrum were evaluated. The reaction forces predicted by these models were compared to the experimental results. The results of this study showed that increase in Poisson’s ratio or in Young’s modulus of cartilage led to increase contact pressures, maximum stresses and reaction forces. The axisymmetric model compared well with the experimental results.

The results of this research improve our understanding of the overall function of the GH joint and GH articular mechanics, in particular. They will help surgeons to develop better treatments for shoulder disorders.