The radius bone is one of the two long bones that make up the forearm, extending from the elbow to the wrist on the thumb side of the arm. It is slightly shorter and thicker than its parallel partner, the ulna, and bears the majority of the load at the wrist joint. The radius is a dynamic component that enables the hand’s ability to rotate and position itself effectively in space. Its unique shape allows it to serve as a critical link between the upper arm and the hand.
Anatomy and Articulation
The radius is divided into three regions: the proximal end near the elbow, the shaft, and the distal end near the wrist. The proximal end features the radial head, a disc-shaped structure with a shallow, concave surface that fits against the humerus (upper arm bone) to form part of the elbow joint. The smooth circumference of the radial head also articulates with a notch on the ulna, allowing for a pivotal spinning motion.
The shaft, the middle section, is narrower toward the elbow and gradually widens as it approaches the wrist. It features a sharp ridge, known as the interosseous border, which serves as the attachment point for the interosseous membrane. This strong fibrous sheet tethers the radius and ulna together along their lengths, maintaining their relative spacing and providing an anchor for several forearm muscles.
The distal end is much wider and forms the upper surface of the wrist joint. It includes the styloid process, a pointed projection on the outer side of the wrist that can be easily felt and provides attachment for ligaments and tendons. The distal radius also has the ulnar notch, where it articulates with the head of the ulna to form the distal radioulnar joint.
Essential Functions in Arm Movement
The radius’s primary function is to facilitate the rotational movements of the forearm and hand, known as pronation and supination. Pronation is the movement that turns the palm down, such as when placing the hand flat on a table. During this action, the cylindrical radial head pivots at the elbow while the wider distal end of the radius swings across the ulna, causing the two bones to cross over one another.
Supination is the reverse movement, where the palm is turned up, as if scooping soup. This action involves the radius uncrossing from the ulna and returning to a position parallel to it. The ability of the radius to rotate around the relatively stationary ulna makes these movements possible, which are crucial for tasks like turning a doorknob or using a screwdriver.
Beyond rotation, the distal end of the radius forms the main articulating surface of the wrist, known as the radiocarpal joint. This broad articulation with the small carpal bones allows for a wide range of wrist movements, including flexion (bending the hand forward) and extension (bending the hand backward). The radius acts as the main bony connection between the elbow and the hand, allowing for stability, rotation, and fine motor control.
Common Injuries Affecting the Radius
The distal end of the radius is the most common site for fractures in the entire upper extremity, often occurring after a fall onto an outstretched hand. A Colles’ fracture is the most frequent type, resulting from a fall where the wrist is extended, causing the fractured segment to displace toward the back of the hand. The opposite injury is a Smith’s fracture, which occurs when falling onto a flexed wrist, causing the fractured fragment to displace toward the palm. These fractures are common in older adults with osteoporosis, although they can occur from high-energy trauma.
Fractures can also occur at the elbow end of the bone, most commonly affecting the radial head. Radial head fractures typically result from a fall onto an outstretched hand with the elbow slightly bent and the forearm turned inward, or pronated. The force from the fall travels up the forearm and is concentrated at the radial head, which is forced against the humerus. These injuries often impair the ability to fully extend the elbow or rotate the forearm.
Another injury specific to the radius, particularly in young children, is radial head subluxation, commonly known as Nursemaid’s Elbow. This injury occurs when a sudden, sharp pull on the child’s hand or forearm, such as when abruptly lifting a child, causes the radial head to slip partially out of the ligament that holds it in place at the elbow. This dislocation is not a fracture but a soft-tissue injury that prevents the child from using the affected arm.