Head of radius
|Bone: Head of radius|
|The radius and ulna of the left forearm, posterior surface. The top is proximal (elbow) and bottom is distal (wrist).|
|Gray's||subject #53 1|
The head of the radius has a cylindrical form, and on its upper surface is a shallow cup or fovea for articulation with the capitulum of the humerus. The circumference of the head is smooth; it is broad medially where it articulates with the radial notch of the ulna, narrow in the rest of its extent, which is embraced by the annular ligament.
The head of the radius is shaped to articulate with a complex of articular surfaces during both flexion-extension at the elbow and supination-pronation in the forearm:
The head's proximal surface is concave and cup-shaped to correspond to the spherical surface of the capitulum of the humerus. The radius can thus glide on the capitulum during elbow flexion-extension while simultaneously rotate about its own main axis during supination-pronation.
The capitulum does not extend to the posterior side of the humerus and, consequently, during full elbow extension only the anterior half of the head articulates with the capitulum. In full flexion the head similarly reaches beyond the capitulum to enter the shallow radial fossa on the anterior side of the humerus.
Proximal radioulnar joint
However, the head of the radius is not perfectly cylindrical but slightly oval. In anatomical position, its major axis (28 mm (1.1 in)) is directed antero-posteriorly and the shorter axis (24 mm (0.94 in)) lateralo-medially. Even though the annular ligament holds the head firmly in place, the ligament is still flexible enough to allow some stretching while the head rotates within it.
During pronation the radius is rotated so that the head's major axis reaches the radial notch on the ulna. This causes a small but significant lateral displacement of the radius' main axis — equal to half the difference between the two axes of the head (2 mm (0.079 in)) — just enough space to accommodate the radial tuberosity as it being moved medially.
- Gray's Anatomy (1918), see infobox
- Kapandji, Ibrahim Adalbert (1982). The Physiology of the Joints: Volume One Upper Limb (5th ed.). New York: Churchill Livingstone. p. 82.
- Kapandji, Ibrahim Adalbert (1982). The Physiology of the Joints: Volume One Upper Limb (5th ed.). New York: Churchill Livingstone. p. 112.