Abstract
The purpose of this study was to simulate the forces in the individual periscapular muscles and investigate the possible biomechanical etiology of posterior erosion of the glenoid. Twelve fresh-frozen human cadaveric shoulders were used with a custom shoulder jig that uses independently controlled pneumatic cylinders to apply forces across the tendon of each muscle. The simulated muscle forces included the rotator cuff, the deltoid, the latissimus dorsi, and the pectoralis major. The shoulders were tested in 12 overhead activity positions. A 6-degree-of-freedom load cell was used to measure the glenohumeral joint forces, and the glenohumeral contact pressures and areas were measured by use of Fuji pressure-sensitive film. There were no significant differences in glenohumeral joint forces between 60° and 90° of vertical abduction or between 60° and 90° of external rotation. At 70° of horizontal adduction from the scapular plane, there was a significant decrease in superior force, a significant increase in posteriorly directed force, and a significant decrease in compression when compared with 30° and 50° of horizontal adduction (P < .05). Similar statistical trends were seen for 60° of vertical adduction. For the glenohumeral contact area and pressure, a significant decrease in contact area was seen between 30° of horizontal adduction and 70° of horizontal adduction for both 60° and 90° of vertical adduction (P < .05). A significant increase in contact pressure was seen at 70° of horizontal adduction compared with 50° and 30° (P < .05). The contribution of the humerothoracic muscles is significant and should be considered for proper restoration of glenohumeral joint biomechanics. Furthermore, asymmetric loading with excessive or repetitive overhead activities may eventually lead to posterior glenoid erosion.