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.

In full flexion the head similarly reaches beyond the capitulum to enter the shallow radial fossa on the anterior side of the humerus.

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