The forearm, located between the elbow and the wrist, contains two long bones. These bones facilitate movement and provide structural support for the hand and wrist. They are connected by a tough membrane that maintains their relationship and transfers forces. This arrangement allows for a wide range of motion, including complex twisting movements.
The Radius
The radius is the bone located on the thumb side of the forearm. It runs parallel to the ulna, positioned on the pinky finger side. The radius is typically thicker at the wrist than the ulna. In the anatomical position, with palms facing forward, the radius is situated laterally.
Structure and Function
The radius is a long bone with three main parts: a proximal end, a shaft, and a distal end. The proximal end features a disc-shaped head that articulates with the humerus (upper arm bone) at the elbow joint and the ulna. This articulation, along with the annular ligament, forms the proximal radioulnar joint, allowing the radial head to rotate. The shaft is slightly curved and triangular in cross-section, narrowing towards the elbow and broadening towards the wrist.
The distal end of the radius is wider and articulates with the carpal bones of the wrist, forming the radiocarpal joint. This articulation is essential for wrist movements. The radius plays a central role in forearm rotation, specifically pronation (turning the palm downward) and supination (turning the palm upward). This rotation occurs as the radius pivots around the ulna, with its distal end crossing over the ulna during pronation.
Beyond rotation, the radius contributes to elbow stability and facilitates movement at the elbow and wrist joints. It serves as an attachment point for muscles, tendons, and ligaments, supporting forearm strength and flexibility. The radius can also help bear body weight, such as during crawling or push-ups.
Common Conditions
The radius is a frequently injured bone, with fractures often resulting from falls onto an outstretched hand. The location and nature of the break classify different types of radial fractures.
A common injury is a Colles’ fracture, a break in the distal radius (near the wrist) where the bone fragment tilts backward, sometimes creating a “dinner fork” deformity. This fracture is common in older adults, especially those with osteoporosis, often occurring from a fall from standing height. Conversely, a Smith’s fracture, also of the distal radius, involves the fragment displacing forward or towards the palm, often resulting from a fall onto the back of the hand or a flexed wrist.
Radial head fractures, located near the elbow, are another frequent injury. These typically occur after a fall on an outstretched arm, pushing the radial head forcefully against the humerus. Symptoms often include pain, swelling, and limited elbow and forearm movement. In children, nursemaid’s elbow, or radial head subluxation, can occur. This involves a partial dislocation of the radial bone at the elbow joint, often caused by a sudden pull on a child’s arm.