The role of the supraspinatus and infraspinatus muscles in glenohumeral kinematics of anterior should instability.

To investigate a socket mechanism responsible for controlling the kinematics in the statically positioned glenohumeral joint, a suprascapular nerve block was performed in 13 selected patients, with recurrent anterior instability and defects of the labrum. Kinematics were then documented by roentgenograms in four positions within the horizontal motion plane. Combined paralysis of the supraspinatus and infraspinatus muscles resulted in abnormal anterior translation in only two of 47 roentgenograms. Normal ball-and-socket kinematics were retained in the remaining 45 roentgenograms. The consistent arthroscopic findings were an undamaged glenoid articular surface with a detached or absent labrum. The injury to the labrum reduced the depth of the socket by one-half. The other one-half of the socket provided by the contour of the glenoid remained intact. A balanced muscle envelope was not required to maintain normal kinematics in selected, actively positioned, unstable shoulders. The retained glenoid depth was sufficient to produce the observed ball-and-socket kinematics. Further in vivo study of shoulder kinematics will be needed to clarify the interactive roles of the socket and muscle envelope in maintaining glenohumeral stability during the more demanding stresses during active shoulder motion.