Abductors of the Shoulder

Discussion

• (see also: rotator cuff)
• abduction of shoulder is a smooth, integrated motion which has been termed scapulohumeral rhythm by Codman;
• disruption of this smooth motion is important since asymmetrical movement usually indicates a chronic pathological disorder of the shoulder;
  • there is a 2:1 ratio of movement in GHJ & scapulothoracic joints;
• clavicle also plays part in elevation of the arm, w/ about 40 deg of motion occurring at sternoclavicular joint & 20 deg in AC joint during full abduction of the shoulder;

Role of Rotator Cuff in Shoulder Abduction

• w/ suprascapular nerve block (which elminates supraspinatus & infraspinatus function) there is still full abduction of arm although it is about 50% weaker;
• remaining teres minor and subscapularis provide glenohumeral compression and humeral depression;
• as arm is abducted to 90 deg direction of pull of deltoid approximates that of the supraspinatus;
• hence pts w/ large tears of rotator cuff can often actively keep the arm abducted to 90 deg but may not be able to initiate abduction to 90 deg