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The rotator cuff is a group of muscles in the shoulder that allow a wide range of movement while maintaining the stability of the glenohumeral joint. The four muscles that make up the rotator cuff are the supraspinatus, infraspinatus, teres minor, and subscapularis. The subscapularis is the largest and strongest of these four muscles and its primary function is the internal rotation of the humerus. The infraspinatus, another muscle in the rotator cuff, is responsible for the external rotation of the humerus.
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What You'll Learn
The subscapularis muscle
The primary function of the subscapularis muscle is the internal rotation of the humerus. It helps in shoulder adduction and extension in certain positions. When the arm is raised, the subscapularis pulls the humerus forward and downward. When the humerus is in a fixed position, the insertion of the subscapularis can act as an origin, producing abduction of the inferior border of the scapula. The subscapularis also helps prevent the anterior dislocation of the humerus during abduction and medially rotates the humerus.
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The infraspinatus muscle
The infraspinatus is frequently fused with the teres minor, and these two muscles work together to rotate the head of the humerus outward (external, or lateral, rotation). When the arm is abducted, the infraspinatus and teres minor work together to oppose the upward pull of the deltoid muscle and to stabilize the humeral head against the glenoid fossa. This prevents the upward displacement of the humeral head as well as impingement on the coracoid process.
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The supraspinatus muscle
The evaluation of the supraspinatus muscle is commonly performed using Jobe's test, also known as the "empty can" test. This test involves positioning the arm at 90 degrees of abduction and internal rotation (thumb pointing down) while applying pressure to the arm. A positive result is indicated by pain or weakness during this movement.
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The teres minor muscle
The muscle is innervated by the posterior branch of the axillary nerve, which forms a pseudoganglion. A pseudoganglion contains no nerve cells, but nerve fibres are present. The blood supply to the teres minor comes from the circumflex scapular artery and the posterior circumflex humeral artery. The circumflex scapular artery is a branch of the subscapular artery, while the posterior circumflex humeral artery stems from the axillary artery.
The primary function of the teres minor is to modulate the action of the deltoid, preventing the humeral head from sliding upward as the arm is abducted. It also functions to rotate the humerus laterally. The teres minor is evaluated using the hornblower's test, which involves the arm being at 90 degrees abduction, the elbow flexed at 90 degrees, and doing a lateral rotation against resistance.
Atrophy of the teres minor muscle is often a consequence of a rotator cuff tear. Selective atrophy of the teres minor muscle has been observed in conjunction with compression of the corresponding axillary nerve branch or posterior humeral circumflex artery. Ultrasonography can be used to detect fatty degenerative atrophy of the teres minor, and MR imaging helps to consolidate the diagnosis of neurogenic muscle atrophy.
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The rotator cuff
The supraspinatus muscle is the only muscle of the rotator cuff that is not a rotator of the humerus. The infraspinatus is a powerful lateral rotator of the humerus, and the subscapularis medially rotates the humerus. The teres minor is a narrow and long muscle entirely covered by the deltoid and is hardly differentiated from the infraspinatus.
Rotator cuff tears are a common cause of shoulder pain and disability among adults. Degeneration, impingement, and tension overload due to trauma may all lead to rotator cuff tears. Tears can cause pain and weakness, with pain worsening during overhead activities and at night when lying on the affected side. Tears can be partial or full-thickness, with partial tears often causing more pain and disability than full-thickness tears.
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Frequently asked questions
The infraspinatus is a powerful lateral rotator of the humerus.
The infraspinatus is a muscle that is part of the rotator cuff. The rotator cuff is a group of muscles in the shoulder that allow a wide range of movement while maintaining the stability of the glenohumeral joint.
The other muscles in the rotator cuff are the supraspinatus, teres minor, and subscapularis.
The subscapularis is the largest and strongest muscle of the rotator cuff. It medially rotates the humerus and adducts it. It also helps in shoulder adduction and extension in certain positions.
