Anatomical position
Anatomically, the forearm and hand are always viewed in the strict position of supination. This means that the human palm is facing forward. It is in this position that the forearm bones do not cross, but lie parallel.
The lateral side of the forearm is on the side of the thumb — it is called the radial side. The medial side is on the side of the little finger and is called the ulnar side. Accordingly, the radius (radius) always occupies a lateral position, and the ulna (ulna) — a medial position. This basic rule is essential for the correct orientation of bone specimens in anatomy classes.
Anatomy of the radius
The radius (radius) features a massive distal end, as its main biomechanical function is to transmit weight from the hand to the forearm.
Its proximal epiphysis is represented by a small head (caput radii), on the upper surface of which lies the articular facet (fovea articularis). Directly below the head is the narrowed part of the bone — 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).
The most important landmark on the shaft is the radial tuberosity (tuberositas radii), which serves as the attachment site for the powerful tendon of the biceps brachii muscle.
On the wide distal epiphysis is the radial styloid process (processus styloideus radii), which is longer than the corresponding ulnar 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 the 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 below it — the supinator fossa (fossa supinatoria).
The shaft (corpus ulnae), like that of the radius, is triangular. It has 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. It bears the ulnar tuberosity (tuberositas ulnae).
The distal epiphysis of the ulna, unlike that of the radius, is rather narrow. It is represented by the head (caput ulnae), which features the 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 a few simple steps:
- Locate the proximal and distal epiphyses. In the radius, the distal end is the most massive, whereas in the ulna, it is the proximal end.
- Keep in mind that the radius is always located laterally, and the ulna medially.
- Examine the anterior surface of the shaft: it bears the tuberosities (tuberositas radii and tuberositas ulnae).
- Assess the distal landmarks: the styloid processes and the presence of articular surfaces for the carpus.
| Sign | Radius | Ulna |
|---|---|---|
| Location | Lateral | Medial |
| Massive end | Distal | Proximal |
| Distinctive structures | Wide distal end, longer styloid process | Olecranon, trochlear notch |
| Main function | Transmission of weight from the hand | Formation of the elbow joint |
Briefly: answers to topical questions
Which surfaces are distinguished on the shaft of the radius?
Three surfaces are distinguished on the shaft of the radius.
- Anterior surface — one of the faces of the bone shaft.
- Posterior surface — the second face of the bone shaft.
- Lateral surface — the third face, characteristic specifically of the radius (unlike the ulna, whose third surface is medial).
The shaft of the radius (corpus radii) has a three-sided shape, and is therefore formed precisely by these three surfaces.
Which carpal bones does the distal epiphysis of the radius articulate with?
The distal epiphysis of the radius participates in the formation of the radiocarpal joint via the carpal articular surface.
- Scaphoid bone (os scaphoideum) — sources indicate proximal articulation with the radius.
- Lunate bone (os lunatum) — sources indicate proximal articulation with the radius and the articular disc.
- Triquetral bone (os triquetrum) — participates in the formation of the radiocarpal joint; connected proximally to the articular disc.
According to sources, the radiocarpal joint is formed by the carpal articular surface of the radius, the distal surface of the articular disc, and the proximal articular surfaces of the scaphoid, lunate, and triquetral bones.
Which muscle attaches to the olecranon of the ulna?
The triceps brachii muscle attaches to the olecranon of the ulna.
- Triceps brachii muscle (m. triceps brachii) attaches to the olecranon (olecranon ulnae) via a strong common tendon.
Near the attachment site of this muscle lie the subtendinous bursa of triceps brachii (bursa subtendinea m. tricipitis brachii) and the subcutaneous olecranon bursa (bursa subcutanea olecrani).
Where are the supinator fossa and the supinator crest located?
The supinator fossa is located on the body of the ulna.
- Supinator fossa (fossa supinatoria or fossa supinatoris) is located on the body of the ulna (corpus ulnae), which has a triangular cross-section, three borders, and three surfaces.
Information on the exact location of the supinator crest is absent in the provided sources, making it impossible to give a complete answer to the question.
To avoid confusing the positions of the forearm bones, remember the first-letter rule: the Radius is always Lateral (words starting with "L").
Class and exam questions
How to correctly orient the forearm bones?
The forearm bones are examined in the supination position, with the palm facing forward. In this position, the bones lie parallel to each other.
Where is the supinator fossa located?
The supinator fossa (fossa supinatoria) is located on the proximal epiphysis of the ulna.
Which bone has a more massive distal end?
The radius has a more massive distal end because its main function is to transmit weight-bearing pressure from the hand to the forearm.
What else to study on this topic
- Orientation of the articular fossa on the radial head
- Formation of the interosseous space of the forearm
- Articulation of the trochlear notch of the ulna with the humerus
- Biomechanics of the radial tuberosity and biceps brachii
- Characteristics of the articular surface for the carpal bones
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