Ulna

The ulna lies medial to the radius and is the longer of the two bones. It has a shaft and two ends, of which the superior is the larger presenting as a hook-like projection for articulation with the trochlea of the humerus. The smaller rounded distal end is the head of the ulna. It does not articulate directly with the carpus. The ulna articulates laterally at each end with the radius.

The upper end of the ulna is large with two projecting processes, enclosing a concavity. The olecranon process is the larger of the two processes and forms the proximal part of the bone. It is beak-shaped and points forwards, being continuous inferiorly with the shaft. Posteriorly, it is smooth and subcutaneous while anteriorly it is concave, forming the upper part of the articular surface of the trochlear notch. The borders of the olecranon are thickened and rough.

The coronoid process projects from the front of the shaft and has an upper articular surface which completes the trochlear notch. These two surfaces are often separated by a roughened non-articular area running horizontally across the notch. The trochlear notch is divided by a vertical ridge into a larger medial part and a smaller lateral part, the latter being continuous over its outer edge with the articular surface of the radial notch on the lateral side of the coronoid process. There is a small tubercle where the medial and anterior edges of the articular surface of the coronoid process meet. This gives attachment to the anterior part of the ulnar collateral ligament. The irregular, anterior surface of the coronoid ends inferiorly as the rough tuberosity of the ulna. Both this surface and the tuberosity give attachment to brachialis. At the upper medial part of the coronoid is the small sublime tubercle from which the pronator ridge runs downwards and laterally.

On the lateral side of the coronoid process, the concave radial notch receives the head of the radius. Below this and extending onto the shaft is the triangular supinator fossa. It is bound posteriorly by the distinct supinator crest. The medial border of this area forms a prominent ridge which has a small tubercle at its upper end.

The prominent interosseus border, to which the interosseous membrane attaches, runs down from the apex of the supinator fossa. The anterior border runs down from the medial margin of the coronoid process but is indistinct. The sinuous, subcutaneous posterior border, prominent in its upper part, is continuous with the subcutaneous region of the olecranon and upper part of the shaft. Between these borders are three surfaces, the anterior and medial surfaces being continuous at the rounded anterior border. The lower quarter of the anterior surface is marked by an oblique ridge running downwards and medially. On the posterior surface, an oblique ridge runs downwards and backwards from the radial notch to the posterior border. The remaining posterior surface has faint ridges laterally and is smooth medially.

The lower end of the ulna shows a narrowed neck which expands into a small, rounded head. From the posteromedial part of the head of the ulna, the conical styloid process projects downwards. The head has a smooth articular surface for the radius on its anterior and lateral aspects. The distal surface of the head is smooth and almost flat, and articulates with an articular disc which intervenes between it and the triquetral.

Palpation

At the upper and posterior aspect of the elbow the outline of the olecranon can be identified; it forms the “point” of the elbow seen in flexion. Running downwards from this point the posterior border can be palpated throughout its length. At the lower end, the neck, head and styloid process can be palpated, with the styloid process being the most posterior. When the forearm is fully pronated the rounded head of the ulna stands out from the back of the wrist.

Ossification

A primary ossification centre appears in the shaft during the eighth week in utero. The body, coronoid process and major part of the olecranon ossify from this primary centre. A secondary centre appears in the head during the fifth year and fuses with the shaft between 20 and 22 years. The secondary centre for the remainder of the olecranon appears at about 11 years, with fusion occurring between 16 and 19 years. There may be several secondary centres for the olecranon.