Circumferentia articularis capitis radii

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.

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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.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.