The radius is one of the two long bones that make up the forearm, running parallel to the ulna from the elbow to the wrist. It is a long, prism-shaped bone that provides structural support. The bone is notably shorter and thicker than the ulna, and it is positioned on the thumb side of the forearm when the palm faces forward. Functionally, the radius allows the hand and forearm to rotate (pronation and supination), a movement unique in the skeleton. It serves as a major attachment point for muscles that control hand and wrist movement.
The Proximal Connections at the Elbow
The upper end of the radius, known as the radial head, forms two distinct articulations at the elbow joint. Its smooth, concave superior surface fits against the rounded capitulum of the humerus, the upper arm bone. This connection is part of the elbow’s hinge joint, facilitating the primary movements of flexion and extension of the forearm.
The radial head also articulates with the ulna at the proximal radioulnar joint. Here, the cylindrical edge of the radial head rotates within the radial notch, a shallow socket on the ulna. This pivot joint is responsible for the forearm’s ability to turn the palm up (supination) and down (pronation). A fibrous band called the annular ligament encircles the radial head, maintaining its contact with the ulna while still permitting rotational movement.
The Ulna and Interosseous Membrane Relationship
Beyond the elbow, the shafts of the radius and ulna are connected along their length by a broad, thin sheet of connective tissue called the interosseous membrane. This fibrous tissue spans the space between the two bones, dividing the forearm into anterior and posterior compartments. The membrane’s fibers run obliquely, and this orientation is instrumental in how forces are distributed within the forearm.
When a load is applied to the wrist, such as when pushing, the interosseous membrane actively transfers a portion of the force from the distal radius up to the proximal ulna. This load-sharing mechanism reduces the strain placed directly on the radiocapitellar joint near the elbow. Distally, the radius meets the ulna again at the distal radioulnar joint, where the ulnar notch of the radius rotates around the head of the ulna. This distal pivot point allows the radius to cross over the ulna during pronation, completing the rotational movement initiated at the elbow.
Forming the Wrist Joint Distally
The lower, wider end of the radius is the main weight-bearing bone at the wrist, forming the radiocarpal joint. The inferior surface of the radius has two distinct concave surfaces that articulate directly with two bones in the wrist’s proximal row. Specifically, the radius connects with the scaphoid bone on the thumb side and the lunate bone centrally.
The radius bears approximately 80% of the total load transmitted from the hand and wrist into the forearm. The wrist joint complex also involves the triangular fibrocartilage complex (TFCC), a structure that separates the distal end of the ulna from the carpal bones. The TFCC is a network of ligaments and cartilage that stabilizes the distal radioulnar joint and allows the radius to articulate smoothly with the triquetrum bone.
Common Injuries Related to Radial Articulations
The points where the radius articulates are frequent sites of injury, often resulting from a fall onto an outstretched hand. One common injury is a radial head fracture near the elbow, which typically limits the ability to straighten the elbow or rotate the forearm. Treatment for these fractures is determined by the specific characteristics and severity of the break.
A distal radius fracture, occurring near the wrist, is the most common type of forearm fracture in adults. These breaks include the Colles’ fracture, where the bone fragment is displaced backward, and the less common Smith’s fracture, where it is displaced forward. Another type, the Barton’s fracture, is an intra-articular break that involves a dislocation of the wrist joint surface itself.