Supraspinatus
The supraspinatus is one of the four muscles that form the rotator cuff. It is the most superiorly located rotator cuff muscle and plays a crucial role in initiating shoulder abduction and stabilizing the glenohumeral joint4.
Anatomy
The supraspinatus muscle originates from the medial two-thirds of the supraspinous fossa of the scapula, a concave depression superior to the spine of the scapula. The muscle is covered by the supraspinous fascia and extends laterally beneath the acromion and coracoacromial ligament2.
The muscle fibers converge laterally to form a strong tendon that passes through the subacromial space and inserts onto the superior facet of the greater tuberosity of the humerus. The tendon blends with the shoulder joint capsule, reinforcing it superiorly1.
The supraspinatus has two distinct muscle bellies: an anterior belly with a larger cross-sectional area (140 ± 43 mm²) and a posterior belly (62 ± 25 mm²). The anterior belly is responsible for the majority of contractile force generation5.
The muscle is innervated by the suprascapular nerve, which arises from the upper trunk of the brachial plexus (C5-C6)16.
Function
The primary function of the supraspinatus is to initiate arm abduction at the glenohumeral joint. It begins the first 10-15 degrees of abduction, after which the deltoid takes over to continue the movement14.
In addition to abduction, the supraspinatus depresses the humeral head during arm elevation, keeping it centered on the glenoid cavity. This function allows the deltoid to maintain its mechanical advantage during abduction. Without this depression, the humeral head would migrate superiorly, causing impingement under the acromion3.
The supraspinatus also contributes weakly to external rotation of the humerus and works synergistically with the other rotator cuff muscles—infraspinatus, teres minor, and subscapularis—to stabilize the glenohumeral joint56.
Clinical Significance
Supraspinatus Tendinopathy
The supraspinatus tendon is particularly susceptible to degeneration and injury due to its position in the confined subacromial space beneath the acromion. Supraspinatus tendinopathy is on the spectrum of rotator cuff syndrome and is a common cause of shoulder pain3.
Supraspinatus Tears
The supraspinatus is the most frequently torn of the rotator cuff tendons. Tears can be partial-thickness or full-thickness. A partial-thickness tear involves only one surface (joint or bursal), while a full-thickness tear creates a hole that allows communication between the glenohumeral joint and the subacromial bursa1.
The footprint of the supraspinatus tendon is triangular, measuring approximately 12.7 mm medial-to-lateral and 6.9 mm anterior-to-posterior. The point of maximal susceptibility is along the tendon insertion nearest the greater tuberosity, underneath the acromion3.
Clinical Examination
The supraspinatus is tested using the Jobe test, also known as the empty can test. The patient holds the arm in 90 degrees of abduction, 30 degrees of forward flexion, and maximally internally rotates. The examiner applies downward pressure, and weakness or pain indicates supraspinatus pathology36.
Surgical Considerations
When repairing the supraspinatus tendon, the anterior fibers must be incorporated whenever possible. These fibers are responsible for the primary functions of abduction and humeral head depression, and failing to repair them may result in functional deficits post-surgery4.
Biomechanics
The moment arm of the supraspinatus for abduction measures approximately 2.5 cm in neutral rotation. With the arm in internal rotation, the moment arm decreases to 2.2 cm. With external rotation, it increases to 2.8 cm. This variation affects the muscle's mechanical advantage during different arm positions5.
The supraspinatus tendon has been shown to have an aponeurotic expansion adjacent to the bicipital groove in approximately 50% of individuals. This structure runs parallel to the long head of the biceps tendon and may have clinical significance in surgical repairs4.