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The rotator cuff is a group of four muscles that provide strength and stability to the shoulder complex. These muscles are the subscapularis, supraspinatus, infraspinatus, and teres minor. They are responsible for a wide range of movements in the shoulder, including flexion, abduction, internal rotation, and external rotation. The rotator cuff muscles are also essential in preventing injuries such as impingement, which occurs when the top of the shoulder rubs against the tendon and the bursa, causing irritation to the rotator cuff.
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What You'll Learn
Supraspinatus
The supraspinatus is the smallest of the four muscles that comprise the rotator cuff of the shoulder joint. It is located in the supraspinous fossa of the scapula, superior to the scapular spine. The supraspinatus is the only muscle of the rotator cuff that is not a rotator of the humerus. It is also the most frequently torn rotator cuff muscle.
The supraspinatus muscle performs abduction of the arm, and pulls the head of the humerus medially towards the glenoid cavity. It independently prevents the head of the humerus from slipping inferiorly. The supraspinatus works in cooperation with the deltoid muscle to perform abduction, including when the arm is in an adducted position. Beyond 15 degrees, the deltoid muscle becomes increasingly more effective at abducting the arm and becomes the main propagator of this action.
The supraspinatus helps to resist the gravitational forces that act on the shoulder joint to pull from the weight of the upper limb downward. It also helps to stabilise the shoulder joint by keeping the head of the humerus firmly pressed medially against the glenoid fossa of the scapula. The supraspinatus is commonly thought to be instrumental in the initiation of shoulder abduction.
Calcification of the supraspinatus tendon is a major contributor to shoulder pain in the general population and is often worsened following a supraspinatus tear. The suprascapular nerve which innervates the supraspinatus can be damaged along its course in fractures of the overlying clavicle, which can reduce the person's ability to initiate the abduction.
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Infraspinatus
The infraspinatus is a thick, triangular muscle that is one of the four muscles that make up the rotator cuff. It is a powerful lateral rotator of the humerus and is involved in abducting and externally rotating the arm. The infraspinatus is frequently fused with the teres minor, and together they rotate the head of the humerus outward and assist in carrying the arm backward.
The infraspinatus arises from the scapula and connects to the head of the humerus, forming a cuff around the glenohumeral joint. The muscle fibres converge to a tendon, which inserts into the middle impression on the greater tubercle of the humerus. The infraspinatus is the second-largest muscle of the rotator cuff in terms of its anatomic footprint on the humerus.
The infraspinatus fossa of the scapula is located on the posterior scapula, and the infraspinatus muscle originates from the medial three-quarters of this fossa. The scapular spine divides the posterior aspect of the scapula into the supraspinous and infraspinous fossae. The infraspinatus fascia covers the muscle and separates it from the teres major and teres minor muscles.
The infraspinatus is a rotator cuff muscle that provides glenohumeral stability. It assists in stabilising the shoulder joint and provides the posterior force to balance the movement of the shoulder, with the subscapular muscle providing the anterior force. The infraspinatus also acts on the scapula, allowing the abduction of the inferior angle of the scapula, termed scaption, which makes the scapula more mobile and assists in full shoulder abduction.
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Teres minor
The teres minor is a narrow, elongated muscle of the rotator cuff. It is one of the four muscles that form the rotator cuff, along with the supraspinatus, infraspinatus, and subscapularis. These muscles collectively stabilize the glenohumeral joint and enable a wide range of movements in the shoulder.
The teres minor originates from the lateral border and adjacent posterior surface of the scapula, which is the shoulder blade. Specifically, it arises from the dorsal surface of the axillary border of the scapula for the upper two-thirds of its extent. Its fibres run obliquely upwards and laterally, with the upper fibres culminating in a tendon that inserts into the greater tubercle of the humerus. The lower fibres of the muscle insert directly into the humerus, just below the greater tubercle.
The teres minor is responsible for modulating the action of the deltoid muscle, preventing the head of the humerus from sliding upward as the arm is abducted. It also functions to rotate the humerus laterally or externally. This lateral rotation of the humerus is often done in tandem with the infraspinatus muscle. Additionally, the teres minor assists in adduction and extension of the shoulder.
The teres minor is innervated by the axillary nerve and supplied by the subscapular artery and its branch, the circumflex scapular artery, as well as the posterior circumflex humeral artery. Damage to the axillary nerve fibres innervating the teres minor is clinically significant. This damage can lead to atrophy of the muscle. Ultrasonography and MR imaging are tools used to detect atrophy and other injuries in the teres minor.
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Subscapularis
The subscapularis is a large triangular muscle that fills the subscapular fossa and inserts into the lesser tubercle of the humerus and the front of the capsule of the shoulder joint. The subscapularis muscle is the largest and strongest part of the four rotator cuff muscles. The term "subscapularis" means under (sub) the scapula (wingbone). The subscapularis is the prime mover of medial (internal) rotation of the humerus. It also adducts the humerus, drawing it forward and downward when the arm is raised.
The subscapularis muscle is supplied by the upper and lower subscapular nerves (C5-C6), which are branches of the posterior cord of the brachial plexus. The primary blood supply to the subscapularis muscle is the subscapular artery, a branch of the axillary artery. The subscapular artery follows the inferior margin of the subscapularis muscle and then divides into the circumflex scapular artery and the thoracodorsal artery.
The subscapularis plays an important role in the stabilisation of the shoulder joint. It contributes to the fixation of the proximal humerus during movements of the elbow, wrist, and hand. It is a powerful defence mechanism for the front of the shoulder joint, preventing displacement of the head of the humerus.
The subscapularis may include up to three trigger points, with the two most common occurring near the outside edge of the muscle. Referred pain from trigger points in the subscapularis muscle is concentrated in the posterior shoulder region, with spillover into the shoulder blade region and down the back of the upper arm. Subscapularis tendonitis symptoms include pain when moving the shoulder, especially when the arm is raised above the shoulders.
The lift-off test and the bear hug test are used to evaluate the subscapularis muscle. In the lift-off test, the patient brings their hands around their back to the lumbar region with the palms facing outward. The test is positive if the patient is unable to lift their hands away from their back. In the bear hug test, the patient places their ipsilateral palm on the opposite deltoid and tries to resist the examiner pulling it away anteriorly. Positive results in the bear hug test indicate significant tearing of the subscapularis.
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SITS muscles
The rotator cuff is a group of four muscles that provide strength and stability to the shoulder complex. The muscles are the supraspinatus, infraspinatus, teres minor, and subscapularis, which can be remembered with the mnemonic "SITS".
The rotator cuff muscles originate from the scapula and connect to the head of the humerus, forming a cuff around the glenohumeral joint. They are responsible for stabilising the shoulder joint, allowing a wide range of movement while maintaining the stability of the glenohumeral joint. The muscles also tighten the joint capsule, preventing a pinch during shoulder movements.
The supraspinatus muscle is the only muscle of the rotator cuff that is not a rotator of the humerus. It initiates the abduction of the arm. The infraspinatus is a powerful lateral rotator of the humerus and assists in both abduction and adduction. The teres minor is a long and narrow muscle entirely covered by the deltoid and hardly differentiated from the infraspinatus. It originates from the lateral border of the scapula and is responsible for the external rotation and adduction of the arm.
The subscapularis is the largest component of the posterior wall of the axilla. It is a powerful internal rotator that also supports the arm during abduction and adduction. It arises from the subscapular fossa and inserts on the lesser tubercle of the humerus. The subscapularis muscle can be evaluated using the "lift-off" and "bear hug" tests.
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Frequently asked questions
The rotator cuff is made up of four muscles: the subscapularis, the supraspinatus, the infraspinatus, and the teres minor.
The rotator cuff provides strength and stability during motion to the shoulder complex. It allows for reaching overhead, weightlifting, picking up children, lifting groceries, walking a dog, throwing, swimming, and even pushing up from a chair.
The mnemonic "SITS" can be used to remember the muscles of the rotator cuff, which are the supraspinatus, infraspinatus, teres minor, and subscapularis.
There are several tests that can be used to evaluate the muscles of the rotator cuff. For the subscapularis muscle, the "lift-off" and "bear hug" tests can be used. For the supraspinatus muscle, Jobe's test, commonly known as the "empty can" test, can be used.
