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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. [1]
It occurs between the circumference of the head of the radius and the ring formed by the radial notch of the ulna and the annular ligament. [2] The interosseous membrane of the forearm and the annular ligament stabilise the joint. [2] A number of nerves run close to the proximal radioulnar joint, including: median nerve; musculocutaneous nerve ...
The head of the radius is wider than the bone's neck, and, because the annular ligament embraces both, the radial head is "trapped" inside the ligament which thus acts to prevent distal displacement of the radius. [4] It helps to stabilise the proximal radial head, [6] and the radioulnar joint. [7]
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 .
A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces.
The word radius is Latin for "ray". In the context of the radius bone, a ray can be thought of rotating around an axis line extending diagonally [clarification needed] from center of capitulum to the center of distal ulna. While the ulna is the major contributor to the elbow joint, the radius primarily contributes to the wrist joint. [5]
The annular ligament binds the head of the radius to the radial notch of the ulna, preventing any separation of the two bones laterally. Therefore, the humeroradial joint is not functionally a ball and socket joint, although the joint surface in itself allows movement in all directions.
The capitate is the largest carpal bone found within the hand. [2] The capitate is found within the distal row of carpal bones. The capitate lies directly adjacent to the metacarpal of the ring finger on its distal surface, has the hamate on its ulnar surface and trapezoid on its radial surface, and abuts the lunate and scaphoid proximally.