The base of the acromion is formed from three or four

ossification centers. It is partially formed by an extension

from the spine of the scapula (from the ossification center

of the body), and partially from the two centers of the

acromion (which previously have united to each other). The

tip of the coracoid process may develop a separate ossification center. These various centers join the body by the

twenty-fifth year. Persistence of an ossification center of the

acromion that does not fuse with the others or with the

scapula can present as an accessory bone, the os acromiale.

An os acromiale usually is located at the lateral margin of

the acromion, is of variable size and shape, and usually is

bilateral (25). It also is possible for the os acromiale to exist

as a small accessory ossicle directly above the greater

tuberosity of the humerus. This ossicle is separated from the

acromion by approximately 1 cm, and usually is somewhat

circular in shape.

The superior third of the glenoid cavity may be ossified

from a separate center, or may ossify from an extension of

the center at the base of the coracoid. When ossification is

from a separate center, the center usually ossifies between

the tenth and eleventh years. This superior portion of the

glenoid then joins the rest of the scapula between the sixteenth and eighteenth years. An epiphyseal plate or crescentic epiphysis also may appear for the lower part of the glenoid cavity, which is thicker peripherally. This rim converts

the flat cavity into the gently concave fossa that is present in

the adult glenoid (2,4,5).

1 Skeletal Anatomy 9

FIGURE 1.5. Illustration of right scapula showing several centers of ossification. The scapula may have

seven to eight (or more) ossification centers: one for

the body, two for the coracoid process, two for the

acromion, one for the medial (vertebral) border, and

one for the inferior angle. Additional centers may be

present to help form the inferior and superior portions of the glenoid cavity.

Osteology of the Scapula

The scapula is a large, flat, triangular bone that spans the

dorsal aspect of the second through seventh ribs (Figs. 1.6

to 1.8). Its synovial articulations include those with the

humerus and the clavicle. In addition, the scapula is stabilized to the dorsal surface of the thorax by muscle, forming

the scapulothoracic articulation.

The main processes (acromion, coracoid, and subchondral portions of the glenoid) as well as the thicker portions

of the body contain trabecular bone (see Figs. 1.6 to 1.8).

The remaining portions generally consist of thin cortical

bone. The central portions of the supraspinous fossa and

most of the infraspinous fossa consist of thin cortical bone.

Occasionally the bone is so thin that it may appear translucent or may have areas that are incompletely ossified, being

filled with connective tissue.

Osteology measurements are given in Figure 1.9 and

Table 1.1. The mean length of the scapulae from the superior angle to the inferior angle is 15.5 cm. The width of the

scapula from the medial border to either the superior or

inferior rim of the glenoid is approximately 10.6 cm. The

scapula is significantly larger in men than women (26)

(Table 1.1).

For descriptive osteology, the scapula has two surfaces,

the costal (anterior) and the dorsal (posterior). It contains

the process of the acromion, the coracoid, and the spine. It

has three borders: superior, medial (or vertebral), and lateral

(or axillary). It has three angles: inferior, superior, and lateral (26,27).

10 Systems Anatomy

FIGURE 1.6. Right scapula, anterior surface, showing muscle origins (red) and insertions (blue).

Surfaces of the Scapula

The costal surface forms the large subscapular fossa, a

slightly concave surface for the origin of the subscapularis

(see Fig. 1.6). The medial two-thirds of the subscapular

fossa is roughened, with ridges that course laterally and

superiorly. These ridges give origin to tendinous attachments of the subscapularis. Along the medial border of the

costal surface is a long, thin rim that provides the insertion

of the serratus anterior.