ULNA

Derivation and Terminology

The ulna derives its name from the Latin word meaning “the

arm” or “the elbow” (1,3). The plural of ulna is ulnae (1).

Ossification Centers and Accessory Bones

The ulna has three ossification centers: one in the shaft

(body), one in the proximal portion (proximal extremity),

and one in the distal end (distal extremity). The mid-portion of the shaft is the first ossification center to appear,

becoming visible at approximately the eighth week of fetal

life (Figs. 1.15 and 1.16). The ossification centers then

extend through the major part of the shaft. At birth, the

distal portions and the major part of the olecranon remain

cartilaginous. Between the fifth and sixth years, a center in

the central portion of the ulnar head appears and soon

extends into the styloid process. At approximately the

tenth year, a center appears in the olecranon near its outer

portion. Most of the ossification of the olecranon actually

develops from proximal extension from the center of the

shaft (2,4,5).

Several accessory bones can be associated with the distal

ulna. These accessory bones, if present, usually are the result

of secondary or additional ossification centers that do not

fuse with the distal ulnar or associated carpal bones. Accessory bones associated with the distal ulna include the os triangulare (os intermedium antebrachii, os triquetrum secundarium), the os ulnostyloideum, and the os pisiforme

secundarium (ulnare antebrachii, metapisoid) (see Fig.

1.27B) (44–46). The os triangulare is located distal to the

head of the ulna, between the ulnar head, lunate, and triquetrum. The os ulnostyloideum is located in the vicinity

of the ulnar styloid. The os pisiforme secundarium is

located between the distal ulna and pisiform, close to the

proximal edge of the pisiform.

1 Skeletal Anatomy 27

FIGURE 1.15. Illustration of ulna, showing the three centers of

ossification. There is one center in the shaft (body), one in the

proximal portion (proximal extremity), and one in the distal end

(distal extremity).

FIGURE 1.16. Illustration of proximal and distal ulna in a young

adult, showing epiphyseal lines.

Accessory bones also can occur from other causes such as

trauma (46) or heterotopic ossification of synovial tags (47).

Therefore, anomalous, irregular ossicles or small, rounded

bones of abnormal size or shape may be encountered that

do not fit a specific described accessory bone or location.

Osteology of the Ulna

The ulna is located in the medial aspect of the forearm lying

parallel to the radius when the forearm is supinated. It is a

true long bone with a triangular cross-section proximally

that becomes rounded distally. The ulna consists of a shaft

with thick cortical bone and a long, narrow medullary canal

(Figs. 1.17 to 1.20). The cortex of the ulna is thickest along

the interosseous border and dorsal surface. In the proximal

and distal ends of the ulna, the cortical bone becomes thinner, and the medullary canal is replaced with cancellous

bone. The cortical bone remains relatively thick along the

posterior portion of the olecranon.

The ulna is anatomically divided into three main portions: the proximal end (proximal portion, proximal

extremity), the shaft (body), and the distal end (distal portion, distal extremity) (Fig. 1.21; see Figs. 1.19 and 1.20).

The proximal end contains the hook-shaped olecranon and

the coronoid process to form the medial hinge-like portion

of the elbow. The shaft consists of the major portion of the

body between the proximal and distal portions. The distal

end consists of the head and styloid process. In general, the

ulna becomes progressively smaller and thinner from proximal to distal.

Proximal Ulna

The proximal end of the ulna consists of the olecranon, the

coronoid process, the trochlear notch, and the radial notch

(see Fig. 1.21A–F).