| Head of radius | |
|---|---|
 | |
| Details | |
| Identifiers | |
| Latin | caput radii |
| TA98 | A02.4.05.002 |
| TA2 | 1211 |
| FMA | 33773 |
| Anatomical terms of bone | |
Thehead of theradius has a cylindrical form, and on its upper surface is a shallow cup or fovea for articulation with thecapitulum of thehumerus. The circumference of the head is smooth; it is broad medially where it articulates with theradial notch of theulna, narrow in the rest of its extent, which is embraced by theannular ligament.[1]
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:[2]
The head's proximal surface is concave and cup-shaped to correspond to the spherical surface of thecapitulum 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.[2]
Between the capitulum and thetrochlea of the humerus is the capitulotrochlear groove. A semi-lunar surface around the circumference of head is shaped to articulate continuously with this groove.[2]
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 shallowradial fossa on the anterior side of the humerus.[2]
The head is cylindrical to allow axial rotation of the radius, thus to articulate with theannular ligament and theradial notch on the ulna.[3]
However, the head of the radius is not perfectly cylindrical but slightly oval. Inanatomical 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 theannular ligament holds the head firmly in place, the ligament is still flexible enough to allow some stretching while the head rotates within it.[3]
During pronation the radius is rotated so that the head's major axis reaches theradial notch on theulna. 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 theradial tuberosity as it being moved medially.[3]
This article incorporates text in thepublic domain frompage 1 of the 20th edition ofGray's Anatomy(1918)
