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Basic Science-Anatomy and Biology

The rotator cuff consists of the supraspinatus, infraspinatus, teres minor, and subscapularis muscles, all of which arise from the scapula and insert into the proximal humerus. The subscapularis muscle is innervated by the upper and lower scapular nerves, and arises from the anterior surface of the scapula, inserting into the lesser tuberosity.

The nerve supply to the supraspinatus is provided by the suprascapular nerve, and the muscle originates from the supraspinatus fossa, inserting into the greater tuberosity. The infraspinatus muscle is also innervated by the suprascapular nerve after it passes around the spinoglenoid notch.

The muscle arises from the infraspinatus fossa and inserts into the posterolateral aspect of the greater tuberosity. The axillary nerve innervates the teres minor, which originates from the inferior and lateral aspect of the scapula, inserting into the inferior portion of the greater tuberosity.

Another important and under appreciated feature of the rotator cuff is its distinct layered makeup . The first layer is a superficial one extending from the coracoid process to the greater tuberosity following the course of the coracohumeral ligament. The second layer consists of the supraspinatus and infraspinatus tendinous fibers. Layer three is composed of the same muscle groups, but the fibers are oriented obliquely and interconnect with the adjacent rotator cuff fibers including the subscapularis.

The deep fibers of the coracohumeral ligament make up the fourth layer extending into the supraspinatus and infraspinatus junction laterally. Layer five represents the actual capsular layer of the joint. The ability to recognize the complex layered anatomy in addition to the tear configuration is critical if an anatomic repair is to be achieved. Furthermore, collagen organization is more robust on the bursal aspect of the rotator cuff as compared to articular-sided fibers, a pertinent fact when evaluating partial thickness tears.

It is also worth noting that a significant contribution to the vascular environment of the rotator cuff, when considering healing potential, may arise from adjacent subacromial structures including the bursa.

Investigators have further refined the insertional anatomy of the rotator cuff components . Each segment of the rotator cuff has a specific “footprint” that can be quantified. This is particularly important when assessing partial thickness cuff tears. The depth of the tear, as judged by the amount of exposed “footprint,” allows the clinician to not only estimate the severity of the tear, but to also choose the most effective treatment.

The vascular anatomy of the rotator cuff has been well described. The anterior humeral circumflex, the subscapular, and the suprascapular arteries provide the primary blood
supply to the rotator cuff. Lindblom has described an area of avascularity in the supraspinatus tendon proximal to its insertion into the greater tuberosity .

Other authors have reported a dynamic reason for the decreased vascularity within the supraspinatus tendon, citing a “wringing out” effect with supraspinatus tension. Benjamin described the histological transition that takes place from tendon to calcified fibrocartilage to bone, accounting for the vascular differences at the insertion site. The zone of uncalcified fibrocartilage is more avascular with respect to the other zones and may be vulnerable to delayed or incomplete healing when traumatized.

The subacromial arch is defined as the space between the distal clavicle and acromion superiorly and the humeral head inferiorly. This space between the acromion and humeral head averages 8 to 12 millimeters on plain x-ray, and can be further divided into the coracoacromial arch which is formed by the acromion, coracoacromial ligament and the coracoid process.

As the rotator cuff passes beneath this arch, contact between the tendons and the arch can occur, leading to tendon pathology as well as secondary changes to the arch in the form of traction-based ossification within the coracoacromial ligament at the acromial attachment site.

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