The supraspinatus muscle, the most superiorly located of the rotator cuff muscles, resides in the supraspinous fossa of the scapula, superior to the scapular spine.  The tendon of the muscle extends laterally passing under the acromion process and over the head of the humerus, blending into the glenohumeral joint capsule to insert on the superior facet of the greater tuberosity of the humerus. In conjunction with the other three rotator cuff muscles, infraspinatus, teres minor, and subscapularis, the supraspinatus forms part of the dynamic stabilization for the glenohumeral joint.

These four muscles act in a coordinated fashion to stabilize the head of the humerus on the shallow glenoid fossa and promote structural integrity of the joint. Along with its role as a stabilizer, the actions associated with supraspinatus are abduction of the humerus, and it may contribute weakly to lateral rotation of the humerus. The supraspinatus, as the most frequently torn rotator cuff muscle, has been the subject of extensive research. Much is known, yet much is left to learn about this small but important muscle.

The architecture of the supraspinatus muscle has undergone description in multiple ways from fusiform to circumpennate. A more detailed analysis of the structure through MRI study reveals the complexity of the architecture. In 1993, the supraspinatus was described as including anterior and posterior muscle bellies. Further research has demonstrated that the two bellies differ in function. Roh et al.[1] described the structure of the supraspinatus muscle and found that the tendon of the anterior belly as thicker and more tubular in structure whereas the posterior tendon was flatter and wider in each of the 25 embalmed cadaveric shoulders they examined. They determined the ratio between the mean anterior and posterior muscle belly physiologic cross-sectional area to be 2.45:1. Furthermore, the authors noted the ratio for the tendon cross-sectional area for the anterior and posterior bellies to be 0.9:1. This ratio indicates the greater muscle mass in the anterior belly exerts its force through a smaller cross-sectional area of the tendon in comparison to the posterior belly.

While it is noteworthy that in the literature there is no agreement regarding the determination of tensile load from the physiologic cross-sectional area, the authors assumed a linear relationship between the two variables utilizing ratios rather than absolute values. Based on the ratios noted, the authors concluded that the anterior tendon would experience as much as 288% more stress than the posterior tendon.

Roh et al.[1] defined the structure to the anterior belly as fusiform with an origin entirely from the supraspinous fossa. The anterior belly contained an internal tendon which forms a tendinous core into which the muscle fibers insert.  The internal tendon thickened into a tubular structure as it became extra-muscular.  The tendon from the anterior belly supplies forty percent of the overall width of the external tendon. The posterior belly differed in structure from the anterior belly and is described as unipennate; a structure is less compatible with the generation of large contractile loads.  The posterior belly was more of a strap-like (Vahlensieck) muscle, with an origin from the spine of the scapula and neck of the glenoid and was devoid of an internal tendinous core. 

The muscle fibers of the posterior belly inserted directly into the flatter, thicker external posterior tendon which comprised 60% of the thickness of the supraspinatus tendon. The authors further calculated the tendon stress attributed to the anterior and posterior muscle bellies indicating the anterior tendon is subjected to 2.88 times greater stress than the posterior tendon.  They also noted the histologic intratendinous structural differences between the two portions of the supraspinatus tendon with the anterior portion of the tendon demonstrating a double-layered interwoven pattern of fibers versus thin, dispersed fibers in the posterior division of the tendon. This finding supported the force calculations of the study.

Fallon et al.[2] examined the histological morphology of the supraspinatus tendon and identified four structural subunits within the tendon: tendon proper, attachment fibrocartilage (approx. 2.8 cm in length), rotator cable (extension of the coracohumeral ligament), and capsule. The tendon proper portion extended from the musculotendinous junction to the attachment fibrocartilage. The dissection noted the more tubular, “rope-like” structure of the anterior tendon and the “thin strap-like” posterior portion that together created a wide point of attachment on the greater tuberosity.  The collagen fibers and fascicles were noted to be in a parallel arrangement to the "axis of tension," and histological staining indicated the presence of an abundance of negatively charged glycosaminoglycans which were noted to be areas where the individual fascicles moved against and were easily separated from each other.  The fascicular arrangement in the tendon proper changed to a basket-weave pattern of the attachment fibrocartilage more proximally in the anterior tendon than in the posterior tendon. The attachment fibrocartilage region was histologically similar to fibrocartilage that is subject to compressive forces. The rotator cable was identified as an extension of the coracohumeral ligament that is positioned perpendicular to the axis of the tendon proper located between the tendon and joint capsule. The joint capsule was found to be a composite of thin collagen sheets, with collagen fiber orientation consistent within sheets but varied between layers.  When joined together the sheets create a thin, but strong structure with collagen fibers of varied arrangement. The attachment fibrocartilage and the capsule are inseparable at a point just medial to the supraspinatus tendon insertion on the greater tuberosity.

The supraspinatus muscle and tendon both pass deep to the acromion process to insert on the super facet of the greater tuberosity of the humerus — the inferior portion of the tendon blends with the joint capsule about 1 cm before the attachment on the bone.  The superior portion of the tendon is contiguous with both the coracohumeral and transverse humeral ligaments. This position and location allow the muscle to perform the function of abduction of the humerus.  Along with deltoid, supraspinatus has motor involvement with both the initiation of and continuation of abduction throughout the range of motion. The supraspinatus has also been shown to contribute weakly to lateral rotation of the humerus.  Along with the other rotator cuff muscles, supraspinatus acts to stabilize the head of the humerus on the glenoid fossa during movements of the joint and also acts with the deltoid to limit inferior displacement of the humeral head.[3]

The development of the muscles of the pectoral girdle and upper limb bud have separate sources of origin.  The scapula and muscles that attach to the medial border of the scapula are derived from para-axially located somites whereas the majority of the limb bud structures of the upper limb are derived from lateral plate mesoderm which develops opposite somites 8-10. Additionally, the connective tissue which encompasses the entire pectoral girdle and maintains the integrity of the structure with the axial skeleton is continuous with the connective tissue of the limb and superficial tissues of the thoracic region.[4]  

Upper limb development generally precedes lower limb bud development by several days. Precursor myoblasts of the upper limb migrate from multiple sources to form dorsal and ventral pre-muscle masses of the limb bud in stage 12, approximately week 5 of development. Supraspinatus will arise from the cells in the dorsal pre-muscle mass.  The muscles of the upper limb form as a combination of somatic cells (muscle cells and myosatellite cells), and somatopleuric cells (tendons and other connective tissue structures).  As the embryo reaches approximately 11 mm in length, the spine of the scapula and acromion are forming and at the same time, the cellular mass of the deltoid starts to develop.  Between the 11 mm and 15 mm embryonic stage, the deltoid mass transects horizontally forming a myoblast mass that will become supraspinatus and infraspinatus. This embryologic association is important to the dynamic control of the glenohumeral joint.[5]

The supraspinatus receives its blood supply from the suprascapular and the dorsal scapular arteries.[6]  The suprascapular artery is typically a branch of the thyrocervical trunk, a branch of the subclavian artery, though it may arise directly from the third part of the subclavian artery. The suprascapular artery passes inferolaterally over the anterior scalene and phrenic nerve, deep to the internal jugular vein and sternocleidomastoid then superior to the subclavian artery and brachial plexus.  The artery will ultimately reach the superior border of the scapula where it will generally pass superior to the superior transverse ligament to reach the supraspinous fossa.  It will supply both supraspinatus and infraspinatus. The dorsal scapular artery originates from the subclavian artery, most frequently the third part. It will travel posteriorly through the trunks of the brachial plexus, over the middle scalene muscle then deep to the levator scapula to reach the superior angle of the scapula where it will begin traveling with the dorsal scapular nerve.  This artery will anastomose with the suprascapular as well as the subscapular arteries and supply the muscles in the area.

Superficial lymphatic drainage generally follows the veins, and deep lymphatic drainage typically follows the arteries. Lymphatic drainage of the suprascapular region is primarily through the posterior subscapular nodes which are located along the inferior margin of the posterior axillary fold and are associated with the subscapular vessels. The efferent drainage from these nodes will be to the central axillary nodes, followed by the apical nodes. The efferent vessels combine to form the subclavian lymphatic trunk which will drain either to the jugulosubclavian venous junction, the subclavian vein, the jugular lymphatic trunk, or occasionally a right lymphatic trunk.  The left trunk drains into the thoracic duct.

Supraspinatus receives its nerve supply from the suprascapular nerve (C5-C6), a branch of the upper trunk of the brachial plexus.