The infraspinatus (IS) is a thick triangular muscle that occupies much of the infraspinous fossa of the scapula. It is a member of the “SITS” muscles (supraspinatus, infraspinatus, teres minor, subscapularis) which contribute to form the glenohumeral joint which aids in shoulder joint motion and stability. The main functions of the IS are assisting in external rotation of the shoulder joint, abduction of the inferior angle of the scapula, and in lateral rotation of the humerus.
The infraspinatus is a portion of the rotator cuff muscles that are composed of the supraspinatus, infraspinatus, teres minor, and subscapularis. These muscles act to stabilize the glenohumeral joint. It originates from the medial three-quarters of the infraspinous fossa and the dorsal part of the scapula inferior to the scapular spine, through tendinous fibers. It also arises from the infraspinous fascia that covers the muscle and creates a distinction between itself and the teres minor and major. It attaches to the middle facet of the greater tuberosity of the humerus, bounded by the attachments of the supraspinatus and teres minor, superiorly and inferiorly respectively. The muscle arises along the infraspinous fossa and runs horizontally to converge to a tendon. The distal tendinous portion runs along the lateral border of the scapular spine then passes across the posterior aspect of the shoulder joint to its insertion on the middle facet of the greater tubercle of the humerus.
Its action on the shoulder is primarily through its function as a rotator cuff muscle providing glenohumeral stability. The rotator cuff applies a concavity compression upon the head of the humerus that allows for stabilization of the humeral head during shoulder abduction. The stabilization allows for the large deltoid muscles to further elevate the arm. If there were a deficiency in the rotator cuff muscles (in other words, a torn infraspinatus tendon), the humeral head would elevate partially out of the glenoid fossa, decreasing the efficiency of the deltoid muscles abduction action.
It also has actions on the head of the humerus. The infraspinatus with its insertion at the middle facet of the glenoid tubercle allows it to exert a lateral or external rotational force. This force is synergistic with the teres minor, as it inserts on the inferior facet of the glenoid tubercle.
It also acts on the scapula. When the shoulder is fixed with its anatomic origin on the medial border of the scapula, it allows abduction of the inferior angle of the scapula; this is termed scaption. Scaption makes the scapula more mobile, which helps the other shoulder muscles in allowing for full shoulder abduction.
The infraspinatus is composed of cross-striated muscles. It is of mesodermal origin. The muscles of the shoulder arise from a common pre-muscle mass that is continuous with the pectoral mass and the common arm sheath known as the upper limb bud. The upper limb bud lies in opposition to somites C4, C5, C6, C7, C8, T1, and T2. In the fifth week of development mesodermal cells from these somites travel into the limb bud and form the posterior and anterior condensation. The posterior condensation allows the creation of the following muscles: deltoid, supraspinatus, infraspinatus, teres minor, teres major, subscapularis, triceps brachii, anconeus, brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis. When the embryo is about 11 mm in size, there is a muscular mass of the deltoid that begins to be transected horizontally as the scapular spine is developed and concurrently the acromion, the origin of the deltoid, forms. These 2 divides start to give rise to a muscle mass that soon becomes the supraspinatus and infraspinatus. This division occurs in an 11-mm to 15-mm embryo. The infraspinatus further separates itself from the teres minor and covers more of the scapula fossa with the development of the embryo, with full coverage of the fossa approximately in a 20-mm embryo.
The infraspinatus is supplied arterially by the suprascapular and circumflex scapular arteries. The suprascapular artery is either a branch of the thyrocervical trunk or a direct branch of the subclavian artery. The artery runs inferolateral to the anterior scalene muscles, phrenic nerve, subclavian artery, and brachial plexus crossing posterolateral and parallel to the clavicle. Then it passes superior to the transverse scapular ligament to the supraspinous fossa. Finally, it courses laterally to the scapular spine (deep to the acromion) to the infraspinous fossa on the posterior surface of the scapula. The subscapular artery, a branch of the third portion of the axillary artery, gives rise to the circumflex scapular artery. The circumflex scapular artery courses around the lateral border of the scapula in its entrance into the infraspinous fossa and anastomoses with the suprascapular artery. The lymphatic drainage is primarily of the subscapular (posterior) nodes. The subscapular nodes consist of 6 or 7 lymph nodes that lie along the fold of the posterior axillary. These lymph nodes run along the course of the subscapular blood vessels. The subscapular, pectoral, and humeral nodes’ efferent drainage passes through the central nodes. The central nodes consist of 3 or 4 nodes placed deep to the pectoralis minor near the base of the axilla, near the second part of the axilla. Efferent drainage from the central nodes passes to the apical nodes, which lie at the apex of the axilla by the medial side of the axillary vein and first part of the axillary artery. Efferent vessels that arise from the apical group runs across the cervicoaxillary canal. These efferent vessels combine to form the subclavian lymphatic trunk. The subclavian lymphatic trunk may join the jugular and bronchomediastinal trunk to form the right lymphatic duct. It may also enter the right venous angle by itself.
The suprascapular nerve (C5-C6) innervates the infraspinatus. It originates at the superior trunk of the brachial plexus. It runs laterally across the lateral cervical region to supply the infraspinatus and also the supraspinatus.
The infraspinatus is most closely associated with the subscapularis, supraspinatus, and teres minor as they all combine around the shoulder to encircle the glenohumeral joint. These muscles act to stabilize and move the glenohumeral joint. The infraspinatus and teres minor act synergistically as external rotators of the humerus.
In current literature, there are not many variants of the infraspinatus muscle, although it has seen to be fused with the teres minor.
The infraspinatus, being a muscle that composes the glenohumeral joint, is susceptible to rotator cuff tears, usually due to anterior dislocations. In anterior dislocations a Hill-Sachs lesion is common. The lesion is on the posterior head of the humerus resulting from the translation of the humeral head back into the glenoid cavity. In the repair of these types of lesions, the infraspinatus is used to fill the defect.
Surgeons use the plane between the infraspinatus and teres minor as the posterior approach for shoulder arthroscopy. This posterior approach is also used for repairs of glenoid fractures and osteotomy.
On physical exam, if there are signs of atrophy of the supraspinatus and infraspinatus muscles, there is usually an associated compression of the suprascapular nerve near the scapular notch. This compression can be seen in overhead throwing motion athletes such as swimmers or baseball players and also in SLAP tears. If there is isolated weakness or atrophy of the infraspinatus muscle, there is usually an associated compression of the nerve at the spinoglenoid notch. Compression of the nerve at the spinoglenoid notch is usually due to a ganglion cyst. Atrophy of the muscle may also be due to tendinous ruptures greater than five centimeters or chronic rotator cuff tears. Weakness in external rotation of a 90-degree flexed arm may be an indication of rotator cuff tears or tendinous rupture of the infraspinatus.