The forearm or lower arm is made up of two bones: the radius and the ulna. These bones extend from the elbow, where they articulate with the humerus, to the wrist, where they articulate with the carpals.
Both the radius and the ulna are relatively long and curved and extend parallel to one another Figure 1. The radius and ulna are attached at the proximal and distal ends as well as through an interosseous membrane along their medial borders. Like the humerus, the primary function of these bones is to facilitate motion and to support the arm.
The radius connects to many muscles such as the biceps, supinator, flexor digitorum superficialis, flexor pollicis longus muscles, extensor ossis metacarpi pollicis, extensor primi internodii pollicis, and the pronator teres muscles. The ulna is also the site of attachment for many muscles including the triceps brachii muscle, supinator muscle, pronator teres muscle, pronator quadratus muscle, and the flexor digitorum superficialis muscle.
The radius is smaller than the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist. It articulates with the capitulum of the humerus, the radial notch, the head of the ulna, and to two carpal bones (scaphoid and lunate). The distal end of the radius has a pointed outgrowth called the styloid process. The corresponding bone in the lower leg is the tibia.
The ulna extends from the medial side of the elbow to the side of the little finger at the wrist and it articulates with the trochlea of the humerus at the olecranon process, a hook-like structure that fits into the olecranon fossa of the humerus. The ulna also articulates with the radius proximally and distally. At the distal end of the ulna is a styloid process. The corresponding bone on the lower leg is the fibula.