The shoulder joint, or girdle, is made up the clavicle, scapula and humerus to form a complex series of joints that make it both efficient in performing multiple movements and vulnerable to injury. Whilst there are ligaments in the shoulder and glenohumerral joint and bursae sacs to cushion, it is predominantly the muscles and tendons that give the joint stability. The scapulae rest on the back, attached to the body through the acromioclaviclar joint (linking the scapula to the clavicle). The rotator cuff muscles attach to the humerus but sit on the scapula.
The Rotator Cuff Muscles
The rotator cuff muscles comprise the supraspinatus, infraspinatus, teres minor and subscapularis. They act to stablise the glenohumeral joint (your shoulder’s ball and socket joint where your arm meets the shoulder).
The supraspinatus sits in the fossa above the spine of the scapula attaching at the head of the humerus. It stabilises the humerus and abducts the shoulder (move your arm out to the side to demonstrate this). The supraspinatus is not involved in the movement of shoulder rotation.
The infraspinatus sits in the fossa below the spine of the scapula attaching at the greater tubercle of the humerus. It stabilses the humerus and adducts the shoulder (if your arm is still out to side bring it back to your side). It also laterally rotates your shoulder (rotate your arm outwards behind you).
The teres minor muscle is smaller than the other rotator cuffs. It attaches from the lateral border of the scapular to the greater tubercle of the humerus. It performs the same actions as the infraspinatus to stabilise the humerus, laterally rotate and adduct the shoulder.
The subscapularis is on the inside, anterior side of the scapula and it covers almost the whole surface attaching to the lesser tubercle of the humerus. This means that it is is the only rotator cuff that can’t be easily accessed. It also stabilised the humerus and medially rotates the shoulder (rotate your arm towards the front of your body).
Postural Problem: Protracted scapulae
A common postural problem is protracted shoulders where the shoulders are hunched forward. This can have a big impact on the function of the rotator cuff muscles:
The supraspinatus’ ability to abduct becomes more difficult with protraction because the acromion process may have become mis-aligned. The supraspinatus has to go through this narrow space to reach the humerus and may become compressed leading to impingement syndrome.
Protraction moves the infraspinatus’ is moved to be more towards the top of the shoulder joint. Here it becomes more of an abductor, placing greater demands of strength to work against gravity. This can lead to overuse and muscle trauma.
Protraction means that the teres minor becomes involved in extension of the shoulder when the arm is pulled backwards. Again, this can result in overuse.
Protraction leads the subscapularis to be underused, causing it to become weak.
Shoulder Injuries affecting the rotator cuffs
Any injuries to the shoulder joint will have a dramatic impact on the rotator cuffs because of their location and their attachment across the joint to the humerus. The most obvious injury is a dislocation of the shoulder joint. After manipulation, the arm is immobilised to allow the capsule and ligaments time to heal. The rotator cuffs can be subject to enormous damage and trauma from shoulder dislocation, especially where surgery is involved.
The supraspinatus is the source of most rotator cuff injuries. Partial or complete tears and overuse injuries will have an effect on movements to abduct and hold the arm in an elevated position. Rotator cuff tears are classified by the degree of the tear and the location.
Impingement syndrome injuries can also cause considerable pain when the arm is moved forwards, upwards and internally rotated. The soft tissue becomes compressed with the tendons and bursa rubbing against ligament to cause irritation and pain.
Falls and blows to the shoulder can also affect the bursa located between the supraspinatus and deltoid muscles to become inflamed and painful, resulting in bursitis.