Of the two bones of the forearm, the ulna is the longer one and is placed medially. It consists of a large proximal end supporting the olecranon and the coronoid processes, a body or shaft tapering distally, and a small rounded distal end called the head.
Proximal End of Ulna
The olecranon process lies in line with the body. Its dorsal surface is more or less triangular in form, smooth, subcutaneous and covered by a bursa.
The coronoid process is a bracket-like process, which projects forwards from the anterior and proximal part of the shaft, and is fused with the olecranon proximally. Of the lateral margins of the coronoid process, the medial is usually the better defined. The smooth medial surface of the coronoid process merges with the olecranon dorsally, and with the medial surface of the body distally.
Body (Shaft) of Ulna:
The body of ulna is nearly straight, or slightly curved, stout and thick proximally, gradually tapering towards its distal extremity. It has three borders and three surfaces. Surfaces are anterior, posterior, and medial while the borders are; interosseous, posterior and anterior.
The interosseous border is crisp and sharp in the proximal three-fourths of the body, but becomes faint and ill-defined in the distal fourth
The posterior border curves laterally above and slightly medially below. It is continuous proximally with the triangular subcutaneous area on the back of the olecranon process. It is well marked above but becomes faint and more rounded below, although it may still be traced distally to the dorsal surface of the base of the styloid process.
The anterior surface corresponds to the front and medial side of the body. Sometimes the anterior and medial surfaces are regarded as two parts of the same surface separated by rounded anterior margin.
The posterior surface of the body lies between the posterior border and the interosseous crest. At its proximal part it is extends on to the lateral side of the olecranon.
Distal End of Ulna:
The distal end of the ulna presents a rounded head, from which, on its medial and posterior aspect, there projects a cylindrical pointed process called the styloid process. The dorsal and medial surface of the styloid process is channeled by a groove which separates it from the dorsal surface of the head.
Nutrient Foramina of Ulna:
A foramen, having a proximal direction, for the nutrient artery of the body opens on the anterior surface of the bone. Vascular canals of large size are seen proximal and posterior to the radial notch. At the distal end of the bone similar openings are seen in the groove between the styloid process and the distal articular surface of the head.
Ossification of Ulna:
The ulna is ossified from one primary and two or more secondary centers. The center for the body appears early in the second month of fetal life. At birth the body and a considerable part of the proximal extremity, including the coronoid process, are ossified, as well as part of the distal extremity. The olecranon and the distal surface of the head and the styloid process are cartilaginous. At About ten years of age a secondary centre appears in the cartilage at the proximal end of the bone, and forms an epiphysis which unites with the body at about sixteen. A scale-like center appears in the cartilage of the head at about the sixth year, from which the distal surface of that part of the bone is developed, and by the extension of which the styloid process is also ossified; this epiphysis does not unite with the shaft till the twentieth or twenty-third years.