The acromion is a lateral extension of the spine. The

process becomes flattened as it extends laterally, overhanging the glenoid, and forms the most superior portion or

“summit” of the scapula (see Fig. 1.9A–E). The shape is

variable, with a flat configuration in 23%, curved in 63%,

and hook-shaped in 14% (26). The mean length of the

acromion in the anteroposterior plane is 4.8 cm. The mean

width of the acromion in the mediolateral plane is 2.19 cm,

and the mean thickness is 9.4 mm. The narrowest portion

forms a neck, the diameter of which is 1.35 cm (26) (Table

1.1). The acromion is located an average distance of 16 mm

from the glenoid (26). The superior surface is rough and

convex and provides attachment for the thick acromioclavicular ligaments and a portion of the deltoid muscle. The

remaining portions are subcutaneous and smooth. The

inferior surface of the acromion is smooth and concave. The

lateral border is thick and irregular and usually has three or

four tubercles for the tendinous origins of the deltoid muscle. The medial border is shorter than the lateral and concave. In this area, the acromion provides a portion of the

attachment of the trapezius muscle. On this medial border,

there is a small oval area of articular cartilage for articulation

with the acromial end of the clavicle. The apex of the

acromion is a small area where the medial and lateral borders intersect. In this area, the coracoacromial ligaments

form their attachment. Inferiorly, where the lateral border

of the acromion becomes continuous with the lower border

of the crest of the spine, the acromial angle is located. The

acromial angle can be palpated subcutaneously and used as

a landmark.