The forearm, extending from the elbow to the wrist, contains two long, parallel bones: the radius and the ulna. These bones provide the structural framework for the lower arm and are fundamental to hand mobility. Their positioning allows for the wide range of movements necessary for daily function.
Defining Their Position in the Forearm
The position of the radius and ulna is easiest to describe when the arm is in the standard anatomical position with the palm facing forward. The radius is always situated on the lateral side of the forearm, corresponding to the thumb side. Conversely, the ulna is positioned on the medial side, aligned with the little finger.
The ulna is generally longer and features a prominent end at the elbow joint. In contrast, the radius is slightly shorter and widens dramatically at the wrist, handling the majority of the load transfer from the hand.
Key Articulations at the Elbow and Wrist
The radius and ulna connect with the humerus and the small carpal bones of the hand. At the elbow, the ulna’s C-shaped trochlear notch fits snugly around the trochlea of the humerus, forming the primary hinge joint. The radius contributes to the elbow when its disc-shaped head articulates with the capitulum, a rounded part of the humerus.
The bones also connect with each other at the proximal and distal radio-ulnar joints. At the proximal joint, the head of the radius rotates within a shallow depression on the ulna called the radial notch. At the wrist, the distal end of the radius is the main connection point, articulating directly with the scaphoid and lunate carpal bones. The ulna’s involvement at the wrist joint is minimal, separated from the carpal bones by an articular disc.
Primary Function: Forearm Rotation
The articulation of the radius and ulna enables pronation and supination, complex movements that rotate the hand palm-up or palm-down. Pronation turns the palm backward or down, while supination turns the palm forward or up. This rotation occurs because the radius pivots around the ulna, which remains stable.
During pronation, the radius crosses over the ulna, forming an “X” shape when viewed from the front. The coordinated action of the proximal and distal radio-ulnar joints facilitates this motion. The radius’s head spins at the elbow, and its distal end glides around the ulna at the wrist.