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The scapula is a flat, triangular-shaped bone that serves as the articulating surface for the head of the humerus as well as provides areas for 17 muscle attachments. With the arm at the side, the scapula overlaps the dorsal surfaces of the second to seventh ribs. It has three prominent projections: the spine, the coracoid process, and the acromion.

The scapular spine divides the posterior aspect of the scapula into two depressions, the supraspinatus fossa and the infraspinatus fossa. In addition, the spine serves as a site for insertion of the trapezius muscle and as the origin for the posterior third of the deltoid muscle.

The coracoid process is a hooklike projection that curves anteriorly, upward, and outward in front of the glenoid cavity. The base of the coracoid is the attachment site of the coracoclavicular ligaments.

The tip serves as the origin of the short head of the biceps and the coracobrachialis muscles and as the insertion site of the pectoralis minor muscle. The coracoid also serves as the anterior limit of the coracoacromial arch and is a palpable landmark during rotator cuff (RTC) surgery.

The acromion serves as an attachment site for the trapezius muscle above and the deltoid muscle below, and it articulates with the distal clavicle. The acromion usually forms from two or three ossification centers that appear during puberty and that fuse between 18 and 25 years of age. These three ossification centers are described as the preacromion, the mesoacromion, and the meta-acromion.

The os acromiale, an unfused secondary ossification, occurs with an incidence of 1% to 15%, and 60% of cases involve bilaterality. The most common location is at the junction of the meso- and meta-acromion. An os acromiale can be identified on an axillary radiograph or magnetic resonance image (MRI) and should not be mistaken for a fracture.

In addition, the acromion has been classified into three morphologic patterns as viewed on a scapular outlet radiograph: flat (Type I), curved (Type II), and hooked (Type III) (8). A Type III morphology has been implicated in impingement and RTC pathology. Investigators have further classified the acromion based on thickness:

Type A, <8 mm;

Type B, 8–12 mm; and

Type C, >12 mm.

Differences in the size of the acromion have been observed between men and women. Nicholson et al.It observed in men that the average length was 48.5 mm, the average anterior width was 19.5 mm, and the average anterior thickness was 7.7 mm. In women, the acromial length averaged 40.6 mm, the anterior width averaged 18.4 mm, and the anterior thickness averaged 6.7 mm.

That study also determined that basic acromial morphology is a primary anatomical characteristic independent of age and, in contrast, that anterior acromial spurs were dependent on age, because they were present in only 7% of patients<50 years and in 30% of patients >50 years.

The acromion contributes to the coracoacromial arch, or supraspinatus outlet, which consists of the coracoid process, the acromion, and the coracoacromial ligament. This arch marks the superior boundary of the subacromial space. Interest has been focused on the structure and function of the coracoclavicular ligament and the importance of the coracoacromial arch.

Although commonly described as having a “Y”-shaped configuration, other morphological types of the coracoclavicular ligament have been described. Soslowsky et al. It identified four types:

quadrangular (48%);“Y”-shaped,

with a broader lateral band and thinner medial band (42%);

broad banded (8%);

and with multiple bands (2%).

The length of the coracoid attachment averaged 32 mm and the length of the acromial attachment averaged 19 mm. The average midpoint thickness was 1.3 mm. The length of the lateral band was significantly shorter, and the cross-sectional area was significantly larger, in specimens with a tear in the RTC.

The role of the coracoacromial arch as a secondary restraint to anterosuperior migration of the humeral head also has become of interest. The coracoacromial ligament has been shown to provide a static restraint to the GH joint as well as significantly contributing to anterior GH stability at 30 degrees of abduction.

Release of the coracoacromial ligament has been suggested to increase both anterior and superior translation of the humeral head. It is important to maintain the integrity of the coracoacromial arch in the cuff-deficient shoulder, because the arch is the last restraint to anterosuperior migration.

The scapula also gives rise to the glenoid cavity, which is situated laterally, below the acromion. This lateral thickening of the scapula provides the bony articulation with the humeral head. The articular surface of the glenoid is concave and covered with hyaline cartilage, which is thinner in the center and thicker toward the periphery. A bare spot exists in the center of the inferior glenoid, which is equidistant to the anterior, posterior, and inferior glenoid rim when viewed with an arthroscope.

Glenoid version has been examined in several studies. In most, the glenoid displayed from 2 to 10 degrees of retroversion in relation to the long axis of the scapula, with an average of superior tilt of 5 degrees. In a study measuring the glenoid version in relation to the supraspinatus fossa, 40% were retroverted, 38% neutral, and 22% anteverted (32).

In addition, the shape of glenoid also changes from superior to inferior. Inui et al.They have shown that the superior part of glenoid surface is retroverted and that the inferior portion may be anteverted. In an MRI study of 40 subjects, Inui et al.It showed the upper aspect has a large radius of curvature, is convex, and subsequently, becomes flat and then concave in the lower portion with a small radius of curvature.

The shape of the glenoid resembles a pear, being 20% narrower superiorly than inferiorly. The reported average vertical diameter ranges from 33 to 39 mm and the average transverse diameter from 23 to 29 mm. The distance from the anteroinferior margin of the glenoid to the bare area averages 12.8 mm.

The articular surface of the glenoid is one-third to one-fourth the area of the articular surface of the humeral head, whereas the radius of curvature of the glenoid is 2.3 mm greater than the radius of curvature of the humeral head.

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