Subacromial pressures in vivo and effects of selective experimental suprascapular nerve block.

Subacromial impingement has been related to increased subacromial pressures. High subacromial pressures may, therefore, have a negative effect on tendon healing after rotator cuff repair, but avoidance of high pressures during healing is only possible if pressures in different positions of the arm are known. The purpose of this study was to determine the subacromial pressures for different positions of active shoulder movement and to determine the effects of supraspinatus and infraspinatus failure on subacromial pressures, as it is currently held that rotator cuff weakness might increase subacromial pressures. Subacromial pressures were statistically significantly altered by arm position (P = .001). There was an increase in pressure from 17.5 mm Hg at rest up to more than 60 mm Hg during abduction and flexion. Subacromial pressure decreased in external rotation and increased in internal rotation. Generally, patients with strong external rotation (arm at the side) had lower subacromial pressures. Conversely, paralysis of the supraspinatus or infraspinatus muscles had no statistically significant effect on subacromial pressures at rest or during active movements of the shoulder. High subacromial pressures can be prevented by avoiding active abduction, flexion, and internal rotation of the arm. External rotation does not need to be limited to reduce subacromial pressure.