The head of the

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

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

capitulum capitulum (plural capitula) may refer to: *the Latin word for chapter ** an index or list of chapters at the head of a gospel manuscript ** a short reading in the Liturgy of the Hours *** derived from which, it is the Latin for the assembly known ...

of the

humerus The humerus (; ) 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 and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

. 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 bac ...

of the

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

, 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 gone through multiple revised editions and the current edition, the 42nd (Octob ...

'' (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 cupshaped depression on the head of the radius, and is limited ...

. 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 In the human arm, the humeral trochlea is the medial portion of the articular surface of the elbow joint which articulates with the trochlear notch on the ulna in the forearm. Structure In humans and apes it is trochleariform (or trochleiform), a ...

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 The radial fossa is a slight depression found on the humerus above the front part of the capitulum. It receives the anterior border of the head of the radius when the forearm is flexed. Structure The joint capsule of the elbow attaches to th ...

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 The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages of animals with r ...

, 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 Beneath the neck of the radius, on the medial side, is an eminence, the radial tuberosity; its surface is divided into: * a ''posterior, rough portion'', for the insertion of the tendon of the biceps brachii. * an ''anterior, smooth portion'', on w ...

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