Supraspinatus muscle

The supraspinatus is a relatively small muscle of the upper limb that takes its name from its origin from the supraspinous fossa superior to the spine of the scapula. It is one of the four rotator cuff muscles and also abducts the arm at the shoulder. The spine of the scapula separates the supraspinatus muscle from the infraspinatus muscle, which originates below the spine.

Origin
The supraspinatus muscle arises from the supraspinous fossa, a shallow depression in the body of the scapula above its spine. The supraspinatus muscle tendon passes laterally beneath the cover of the acromion.

Insertion
The supraspinatus tendon is inserted into the most superior facet of the greater tubercle of the humerus.

The distal attachments of the three rotator cuff muscles that insert into the greater tubercle of the humerus can be abbreviated as SIT when viewed from superior to inferior (Supraspinatus, Infraspinatus, and Teres minor).

The much fabled mnemonic SITS regarding the rotator cuff muscles is completed by including the Subscapularis muscle, which unlike the other rotator cuff muscles attaches to the lesser tubercle of the humerus.

Innervation
The supraspinatus muscle is supplied by the suprascapular nerve (C5 and C6), which arises from the superior trunk of the brachial plexus and passes laterally through the posterior triangle of the neck and through the scapular notch on the superior border of the scapula. After supplying fibers to the supraspinatus muscle, it supplies articular branches to the capsule of the shoulder joint.

This nerve can be damaged along its course in fractures of the overlying clavicle, which can reduce the person’s ability to initiate the abduction.

Action
Contraction of the supraspinatus muscle leads to abduction of the arm at the shoulder joint. It is the main agonist muscle for this movement during the first 15 degrees of its arc. Beyond 15 degrees the deltoid muscle becomes increasingly more effective at abducting the arm and becomes the main propagator of this action.

The supraspinatus muscle is one of the musculotendinous support structures called the rotator cuff that surround and enclose the shoulder. It helps to resist the inferior gravitational forces placed across the shoulder joint due to the downward pull from the weight of the upper limb.

The supraspinatus also helps to stabilize the shoulder joint by keeping the head of the humerus firmly pressed medially against the glenoid fossa of the scapula.

Clinicial significance
The supraspinatus muscle tendon is often ruptured in sports involving sudden forceful movements of the upper limb and is the most commonly ruptured rotator cuff muscle. The muscle can also degenerate in the elderly leading to increased instability and loss of function at the shoulder joint.

The supraspinatus tendon can also become inflamed, in persons of any age, leading to a condition called Supraspinatus tendinitis. In this condition initiation of abduction of the shoulder is difficult or impossible, but all other movements are full and pain free. If the therapist support the initial abduction [that is about 15 degree] the further movment is done by the patient himself. The definitive cause of this condition is not clear, although it suspected, without definite evidence, to be an after-effect of minor trauma to the shoulder joint. Treatment may be by anti-inflammatory medication or by simple analgesics, but steroid injection directly into the tendon provides the best chance of early resolution.