Normal kinematics of the unconstrained glenohumeral joint under coupled moment loads

The glenohumeral joint presents difficult experimental challenges, large ranges of motion, coupled motions, and multi-planar motions. Many biomechanical tests performed on the glenohumeral joint constrain it or provide conditions that might not be reproducible within and among specimens or treatments. The goal of this study was to determine a means to test the glenohumeral joint in vitro with coupled moments applied to the unconstrained humerus and recording its "6 degrees of freedom" motion relative to the glenoid. This procedure was achieved with the development of a new testing apparatus. Kinematics were described in abduction-adduction, flexion-extension, and abduction, extension, and external rotation, simulating the cocked phase of throwing. The advantage of the techniques was that only the glenohumeral capsule and articular surface contact control the motion of the humerus in response to the applied loading pattern. Also, the motions were unconstrained by the loading apparatus, allowing coupling of rotations and translations and multiplanar motions. Rotations were found to be coupled with translations. For adduction-abduction and flexion-extension, the humeral head was found to translate first in 1 direction, opposite the direction of increasing rotation, at low moments. Then, with increasing moment and angle of rotation, translations changed direction and moved with the primary motion. This occurrence indicates the existence of a region of joint laxity or neutral zone. Rotations were found to be coupled during motion to the cocked position of throwing, as the humerus adducted with extension from full abduction and then adducted further and extends with external rotation. Changes in these motion patterns with pathologic conditions may provide useful information about the nature of shoulder dysfunction and provide insight into appropriate means of repair.