The head of the

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

has a cylindrical form, and on its upper surface is a shallow cup or fovea for articulation with the capitulum of the

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

. The circumference of the head is smooth; it is broad medially where it articulates with the

radial notch The radial notch of the ulna (lesser sigmoid cavity) is a narrow, oblong, articular depression on the lateral side of the coronoid process; it receives the circumferential articular surface of the head of the radius. It is concave from before b ...

of the

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

, narrow in the rest of its extent, which is embraced by the annular ligament.''

Gray's Anatomy ''Gray's Anatomy'' is a reference book of human anatomy written by Henry Gray, illustrated by Henry Vandyke Carter and first published in London in 1858. It has had multiple revised editions, and the current edition, the 42nd (October 2020 ...

'' (1918), see infobox

Articular surfaces

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:

Humeroradial joint

The head's proximal surface is concave and cup-shaped to correspond to the spherical surface of the

capitulum of the humerus In human anatomy of the arm, the capitulum of the humerus is a smooth, rounded eminence on the lateral portion of the distal articular surface of the humerus. It articulates with the cup-shaped depression on the head of the radius, and is limi ...

. The radius can thus glide on the capitulum during elbow flexion-extension while simultaneously rotate about its own main axis during supination-pronation. Between the capitulum and the trochlea of the humerus is the capitulotrochlear groove. A semi-lunar surface around the circumference of head is shaped to articulate continuously with this groove. 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

The head is cylindrical to allow axial rotation of the radius, thus to articulate with the annular ligament and the

on the ulna. However, the head of the radius is not perfectly cylindrical but slightly oval. In

anatomical position Anatomical terminology is a specialized system of terms used by anatomists, zoologists, and health professionals, such as doctors, surgeons, and pharmacists, to describe the structures and functions of the body. This terminology incorpor ...

, its major axis () is directed antero-posteriorly and the shorter axis () 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

on the

. 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 () — just enough space to accommodate the radial tuberosity as it being moved medially.

Additional images

File:Slide1bgbg.JPG, Elbow joint. Deep dissection. Posterior view. File:Slide2bgbg.JPG, Elbow joint. Deep dissection. Posterior view. File:Slide3bgbg.JPG, Elbow joint. Deep dissection. Posterior view.

References

{{Authority control Radius (bone)

Articular surfaces

Humeroradial joint

Proximal radioulnar joint

Additional images

See also

References