The forearm contains two bones, the radius and the ulna, which extend from the elbow to the wrist. They are fundamental to the mechanics of the upper limb, allowing the hand to be positioned and oriented in countless ways. Understanding their specific location and interaction clarifies how the forearm achieves its remarkable mobility.
Defining the Forearm Bones
The forearm is the anatomical region situated between the humerus and the carpal bones of the wrist. The radius and the ulna run generally parallel to one another, forming the bony scaffolding of the lower arm. They are connected along their length by the interosseous membrane, a strong sheet of connective tissue that transmits forces between them. This arrangement connects the elbow joint, which is formed primarily by the ulna, to the complex joints of the wrist, which are mainly formed by the radius. The dual-bone structure allows the hand to rotate without requiring the entire upper arm to twist.
Identifying Each Bone’s Specific Location and Role
The radius is positioned on the lateral side of the forearm, aligning with the thumb. This bone is the primary component of the wrist joint, articulating directly with the carpal bones to facilitate hand movements. The radius is engineered for movement; its small, rounded head at the elbow allows it to pivot against the humerus and ulna.
The ulna is located on the medial side of the forearm, aligning with the pinky finger. It is slightly longer than the radius and is the main bone forming the hinge joint of the elbow. The proximal end features the olecranon process, a large, C-shaped structure that cups the humerus and creates the bony point of the elbow. While the ulna provides stability at the elbow, its contribution to the wrist joint is much smaller than the radius.
Enabling Essential Arm Movement
The combined action of the radius and ulna allows for a special range of motion called pronation and supination. This movement involves rotating the forearm to turn the palm either down (pronation) or up (supination), an arc that can span approximately 180 degrees.
This rotation is possible because the radius pivots around the ulna at both the elbow and the wrist. During pronation, the radius crosses diagonally over the fixed ulna, changing the alignment of the hand. The ulna remains relatively fixed at the elbow, acting as the central axis for the radius’s rotation. This mechanical relationship allows for everyday tasks like using a screwdriver or turning a doorknob.
Common Injuries
The radius and ulna are frequently involved in injuries, often resulting from a fall onto an outstretched hand. A common injury is the distal radius fracture, sometimes called a Colles’ fracture, which involves a break near the wrist due to high impact forces.
More complex injuries involve both a fracture in one bone and a dislocation of the joint connecting the two bones. A Galeazzi fracture is characterized by a break in the shaft of the radius coupled with a dislocation of the wrist end of the ulna. Conversely, a Monteggia fracture involves a fracture of the ulna shaft with an accompanying dislocation of the head of the radius at the elbow. These specific fracture-dislocations highlight the interconnected nature of the radius and ulna, where an injury to one often compromises the stability of the other.