Anatomical Position
Anatomically, the forearm and hand are always considered in the position of strict supination. This means that the human palm faces forward. In this position, the forearm bones do not cross each other and lie parallel.
Latally, the forearm is on the side of the thumb — termed the radial side. Medially, it is on the side of the little finger — termed the ulnar side. Accordingly, the radius (radius) always occupies the lateral position, and the ulna (ulna) occupies the medial position. This basic rule is essential for correctly orienting skeletal specimens in anatomy laboratories.
Anatomy of the Radius
The radius (radius) features a massive distal end because its primary biomechanical function is to transmit weight from the hand to the forearm.
Its proximal epiphysis consists of a small head (caput radii), on the superior surface of which lies the articular facet or articular fovea (fovea articularis). Directly below the head is a constricted portion known as the neck of the radius (collum radii).
The shaft (corpus radii) has a characteristic triangular cross-section. Classically, it presents three surfaces and three borders:
- Anterior border (margo anterior);
- Posterior border (margo posterior);
- Interosseous border (margo interosseus).
A vital landmark on the shaft is the radial tuberosity (tuberositas radii), which serves as the insertion site for the powerful biceps brachii tendon.
On the broad distal epiphysis, there is the radial styloid process (processus styloideus radii), which extends further distally than the ulnar styloid process. The distal end also bears the ulnar notch (incisura ulnaris) for articulation with the head of the ulna and the carpal articular surface (facies articularis carpalis), which forms the joint with the carpal bones.
Anatomy of the Ulna
The ulna (ulna) has a pronounced thickening at its proximal end, as it is primarily responsible for forming the elbow joint.
The proximal epiphysis is formed by two large processes: the massive olecranon (olecranon) posteriorly and the coronoid process (processus coronoideus) anteriorly. Between them lies the deep trochlear notch (incisura trochlearis). Laterally on the proximal end is the radial notch (incisura radialis), and just inferiorly lies the supinator fossa (fossa supinatoria).
The shaft (corpus ulnae), like that of the radius, is triangular. It features three borders (anterior, posterior, and interosseous) and three surfaces formed between these borders:
- Medial surface (facies medialis) — located between the anterior and posterior borders;
- Posterior surface (facies posterior) — bounded by the posterior and interosseous borders;
- Anterior surface (facies anterior) — situated between the anterior and interosseous borders, featuring the ulnar tuberosity (tuberositas ulnae).
The distal epiphysis of the ulna, unlike the radius, is relatively narrow. It presents the head of the ulna (caput ulnae), which bears an articular circumference (circumferentia articularis) for contact with the radius. The epiphysis terminates in the ulnar styloid process (processus styloideus ulnae).
How to Distinguish the Radius from the Ulna
To correctly determine the side of an isolated bone specimen, follow these simple steps:
- Identify the proximal and distal epiphyses. For the radius, the most massive end is distal; for the ulna, it is proximal.
- Keep in mind that the radius is always lateral, and the ulna is medial.
- Examine the anterior surface of the shaft: it bears the tuberosities (tuberositas radii and tuberositas ulnae).
- Evaluate distal landmarks: styloid processes and the presence of carpal articular surfaces.
| Feature | Radius | Ulna |
|---|---|---|
| Location | Lateral | Medial |
| Massive end | Distal | Proximal |
| Distinctive structures | Wide distal end, longer styloid process | Olecranon, trochlear notch |
| Primary function | Transmit weight from the hand | Formation of the elbow joint |