A thorough understanding of osteology and the muscular attachments of the scapula is required to grasp the complex movement patterns and function [1, 2]. The anatomy is predicated on the developmental advantages of mobility, such as prehension and overhead use. This is reflected in several primary changes noted through time in the hominid scapula. First, the acromion has broadened and lateralized to allow mechanical advantage for the deltoid muscle. The coracoid enlarged in a manner theorized to assist in the prevention of anterior dislocation at 90° of abduction. Finally, broadening and alteration in the force vector of the infraspinatus and teres minor are postulated to increase both external rotation strength and humeral head depression.

The scapula is a large flat bone which forms from a collection of mesenchymal cells. It shows signs of ossification by the 5th week of embryologic development. The scapula follows a predictable course in descending from the paracervical region to the thorax. Failure of this process leads to Sprengel’s deformity. By the 7th week, the scapula has descended to its final position and the glenoid is easily identified.

Grossly, the scapula is a thin sheet of bone which serves as a critical site of muscle attachment. The blood supply is primarily through a network of periosteal vessels which take origin from muscular insertions. Thickening of the bone is notable at the lateral border, and superior and inferior angles. Ventral concavity creates a smooth articulating surface against the ribs. Small oblique ridges exist ventrally for the tendinous insertions of the subscapularis. Similarly, small fibrous septa are present dorsally to attach and separate the infraspinatus, teres minor, and teres major . The dorsal surface is traversed by the scapular spine which divides two concavities; the supraspinatus and infraspinatus fossas. The medial two-thirds of these fossas give rise to the supraspinatus and infraspinatus muscles. The spine contains two important notches. First, the suprascapular notch at the base of the coracoid contains the suprascapular nerve, and compression at this location will affect both the supraspinatus and infraspinatus muscles. Second, the spinoglenoid notch is present at the lateral border of the spine. Various causes can lead to compression of the suprascapular nerve here as well, producing isolated atrophy of the infraspinatus.

Anatomic interest in the scapula is frequently directed at the coracoid, acromion, or glenoid. The name coracoid derives from the Greek word korakodes meaning “like a crow’s beak.” The bent shape resembles a finger pointed toward the glenoid. From the Greek word akros for point, the acromion is often referred to as the point of the shoulder. The morphology of the acromion is among the most studied in the body. Considerable cadaveric research has been directed at the relative frequency and postulated causes of the different acromion types. However, the relationship between acromial shape and “impingement syndrome” or rotator cuff tear has not borne out in literature. Similarly, the glenoid has been the subject of intensive study in an effort to define bony anatomy in shoulder instability. Average values for size include a height of 35 mm and width of 25 mm, but considerable variability exists. Comparison to the contralateral side may be required to precisely define bone loss. Glenoid version may also range widely. Retroversion, up to 6° is most common, as seen in 75 % of the population, but anteversion up to 2° is reported.

The function of the scapula is dependent on the complex recruitment patterns of the numerous muscular attachments [1, 3, 5, 6]. These muscles can generally be categorized as: axioscapular and scapulohumeral muscles of the upper arm (coracobrachialis , biceps brachii , and triceps brachii ).

The axioscapular muscles serve to anchor the scapula for its role as the foundation of the shoulder. In addition, they guide the scapula through the requisite degrees of freedom. These muscles include the serratus anterior , levator scapula , pectoralis minor , rhomboids, and trapezius . The trapezius is the largest and most superficial axioscapular muscle. The expansive muscle originates from the occiput, nuchal ligament, and spinous processes of C7 through T12. The upper trapezius inserts across the distal third of the clavicle and acromion. The middle trapezius inserts across the scapular spine and the lower portion at the base of the spine. The broad muscle allows complex function in scapular retraction, elevation, and posterior tilting based upon the recruitment pattern [2, 5]. Frequently, the upper and lower trapezius muscles are associated separately. Motor innervation is through cranial nerve 11, the spinal accessory nerve.

The rhomboids are divided into major and minor portions. The rhomboid minor originates from the spinous processes of C7 and T1 and inserts at the medial scapular border at the base of the spine. The rhomboid major begins from T2 through T5 and inserts along the posterior aspect of the medial border from the base of the spine caudally to the inferior angle. This orientation allows an important role in scapular retraction. The dorsal scapular nerve (C5) provides innervation.

The serratus anterior is comprised of three divisions taking origin from the anterolateral aspect of the first through ninth ribs. Innervation of the serratus is provided by the long thoracic nerve. The serratus produces protraction which is a composite of scapular translation and multidirectional rotation. It is not uncommon for upward rotation, posterior tilting, and external rotation of the scapula to occur simultaneously during protraction [9]. The role of the serratus during arm elevation is to provide a critical stabilization function against excessive internal rotation throughout nearly all positions of arm forward flexion and elevation.

The levator scapula is intimately associated with the serratus as a stabilizer but also serves a role to elevate and upwardly/downwardly rotate the scapula. The levator originates from the transverse processes of C1 through C3, and at times C4. Insertion is found upon the superior angle. Innervation comes from the deep branches of C3 and 4.

The pectoralis minor is often overlooked in its role in scapular position. The muscle originates from the second through fifth ribs and courses superolateraly to insert upon the coracoid. It is ideally located to assist the serratus anterior in scapular protraction and anterior tilt. Chronic tightness can contribute to protracted, anteriorly tilted scapular positioning.

The scapulohumeral muscles produce glenohumeral motion and are composed of the deltoid, supraspinatus, infraspinatus, subscapularis, teres minor, and teres major . The deltoid originates broadly across the acromion and scapular spine while inserting on the deltoid tubercle of the humerus. This structure allows it to power elevation in multiple planes. As previously noted, the supraspinatus and infraspinatus originate from the medial two-thirds of their respective fossas while inserting in a complex arrangement on the greater tuberosity. The subscapularis originates from the anterior aspect of the scapula and attaches on the lesser tuberosity. The pennation pattern of the rotator cuff fibers, inserting obliquely to the tendon, allow them to exert a strong compressive force on the humerus, increasing concavity/compression. The teres minor takes origin from the middle section of the lateral scapula and is innervated by the posterior branch of the axillary nerve. The teres major emerges from a more inferior position on the lateral scapula and shares a common tendinous insertion with the latissimus dorsi on the medial aspect of the bicipital groove. It shares innervation from the subscapular nerve and functions in internal rotation, adduction, and extension of the humerus.