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. The radius can thus glide on the capitulum during elbow flexion-extension while simultaneously rotate about its own main axis during supination-pronation. However, the head of the radius is not perfectly cylindrical but slightly oval. In
anatomical position, 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
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 () — just enough space to accommodate the
radial tuberosity as it being moved medially. ==Additional images==