The distal radius is the segment of the radius bone located nearest to the wrist and hand, playing a central role in upper limb function. It is one of two bones in the forearm, running parallel to the ulna. The distal end is significantly wider and flared compared to the rest of the bone, which is a structural adaptation for bearing load and forming the wrist joint. This segment acts as a mechanical bridge, transferring forces from the hand to the rest of the arm during movement and impact.
Defining the Location and Structure
The distal radius is situated on the lateral side of the forearm, meaning it is on the side closest to the thumb when the palm faces forward. As the radius extends downward from the elbow, it widens just above the wrist to create a broad surface for articulation. This wider end provides the main bony platform for the intricate mechanics of the hand and wrist.
A distinct and easily felt bony prominence on the thumb side of the wrist is the radial styloid process. This projection is a key anatomical landmark of the distal radius, extending obliquely downward and serving as an anchor point for ligaments and the brachioradialis muscle. The distal radius also features a small concave surface on its medial side called the ulnar notch, which is where it meets the other forearm bone, the ulna.
The distal end of the radius is roughly quadrilateral in shape and contains three concave articular surfaces for joint formation. These surfaces include the scaphoid fossa and the lunate fossa, which articulate with specific small bones of the wrist. The bone’s flared shape and these specialized surfaces allow it to bear approximately 80% of the axial load transmitted from the hand up the arm.
How the Distal Radius Connects to the Hand and Arm
The distal radius forms two separate but functionally linked joints that enable the forearm and hand to move with a wide range of motion. The primary connection to the hand is the radiocarpal joint, formed by the broad distal end of the radius articulating with two proximal carpal bones: the scaphoid and the lunate.
This articulation is a condyloid joint, which permits the hand to move in two planes: flexion and extension, as well as radial and ulnar deviation. The multiple ligaments that bridge from the radius to the carpal bones provide the necessary stability for these complex movements. The distal radius is also connected to the ulna at the distal radioulnar joint (DRUJ).
The DRUJ is formed where the radius’s ulnar notch rotates around the head of the ulna. This pivotal connection is paramount for the rotation of the forearm, allowing the hand to turn the palm up (supination) or down (pronation). These two joints, working in concert, enable the precise placement and manipulation of the hand in space.
Why the Distal Radius is Prone to Injury
The distal radius is the most frequently broken bone in the arm, mainly because of its location and the mechanism of common falls. The most common cause of injury is a “Fall On an Outstretched Hand,” known by the acronym FOOSH. This is the body’s instinctive reaction to break a fall, which results in the entire force of the impact being transmitted directly up the arm and concentrated at the wrist.
When a person falls onto an outstretched hand, the force travels axially through the carpal bones and is suddenly borne by the flared distal radius. The stress of this high-energy impact often exceeds the bone’s structural limits, leading to a fracture. Vulnerability is heightened in older adults, where low-energy falls from a standing position can cause a break due to decreased bone mineral density, or osteoporosis.
Common Fracture Patterns
Specific fracture patterns are named to describe the direction of the break and displacement. A common example is the Colles fracture, which involves a break within two centimeters of the wrist joint that displaces the distal fragment backward toward the back of the hand. In contrast, a Smith fracture (or reverse Colles fracture) displaces the fragment forward toward the palm. The radial styloid process itself can also fracture, known as a Chauffeur’s fracture, when the scaphoid bone is forcefully compressed against it. The high frequency of these injuries highlights the distal radius’s role as the final structural absorber of force.