The Radius and Ulna
The forearm, or antebrachium, is the section of the upper limb extending from the elbow to the wrist. This segment positions the hand and enables its movements. The forearm is composed of two bones that provide both stability and mobility.
The Radius and Ulna
These bones run parallel to each other, with the radius located on the thumb side of the forearm and the ulna on the pinky finger side when the arm is in the anatomical position. Their shapes and features differ, reflecting their specialized roles.
The radius is the shorter and thicker of the two forearm bones. Its proximal end features a disc-shaped head that articulates near the elbow, supported by a narrow neck. Distal to the neck is the radial tuberosity, a bony prominence serving as an attachment point for muscles. The shaft of the radius widens as it extends towards the wrist, where its distal end is broad and features an ulnar notch for articulation with the ulna, along with a styloid process on its lateral side.
The ulna is slightly longer than the radius and is considered the stabilizing bone of the forearm. Its proximal end is notably larger and forms the bony point of the elbow. Key features include the olecranon, a large projection forming the tip of the elbow, and the coronoid process, a projection that, along with the olecranon, forms the trochlear notch. This notch articulates with the upper arm bone. The radial notch, located on the side of the coronoid process, articulates with the head of the radius. The distal end of the ulna is much smaller, terminating in a rounded head and an ulnar styloid process.
How the Forearm Bones Work Together
The radius and ulna articulate at two crucial points: the proximal and distal radioulnar joints. The proximal radioulnar joint is near the elbow, where the head of the radius rotates within the radial notch of the ulna, held by the annular ligament. The distal radioulnar joint is located near the wrist, formed by the articulation between the ulnar notch of the radius and the head of the ulna. These two pivot joints, along with the interosseous membrane connecting the shafts of the radius and ulna, facilitate forearm movement.
These articulations enable pronation and supination, movements that rotate the hand. Pronation involves turning the palm downward, or posteriorly, while supination turns the palm upward, or anteriorly. During pronation, the radius crosses over the ulna, a movement made possible by the rotation of the radial head at the proximal joint and the sliding of the distal radius over the ulnar head. This interaction allows for a rotational range of motion.
Beyond their role in pronation and supination, the forearm bones also articulate with other bones to facilitate broader upper limb movements. The ulna’s trochlear notch and the radius’s head articulate with the humerus at the elbow, forming parts of the elbow joint. This joint allows for flexion (bending) and extension (straightening) of the forearm.
At the wrist, the distal end of the radius articulates directly with the scaphoid and lunate carpal bones, forming the radiocarpal joint. This articulation is responsible for wrist movements such as flexion, extension, and side-to-side deviations. The ulna does not directly articulate with the carpal bones but connects to the radius, contributing to overall wrist stability and function.