The forearm, extending from the elbow to the wrist, contains two long bones: the radius and the ulna. These bones are fundamental for complex upper limb movement, working together to facilitate nearly all actions involving the hand and wrist. They link the humerus (the upper arm bone) to the eight small bones of the wrist, known as the carpals. Their distinct shapes and articulations allow for a combination of strength, flexibility, and dexterity.
Defining the Forearm Bones
The forearm is structured around the radius and ulna, two parallel bones running from the elbow to the wrist. To locate them, place the arm in the standard anatomical position with the palms facing forward. The radius is always located on the lateral side, aligning with the thumb.
The ulna sits on the medial side of the forearm, lining up with the little finger. Although both are long bones, the ulna is typically longer. The two bones cross over each other during specific movements, which is essential for the forearm’s range of motion.
The Ulna and Elbow Stability
The ulna’s primary function is to form the strong, foundational hinge joint of the elbow, providing stability for the entire arm. The proximal end of the ulna connects tightly with the humerus, creating a highly congruent articulation. This connection involves two distinct bony landmarks: the olecranon process and the trochlear notch.
The olecranon process is the large, curved projection that forms the palpable point of the elbow, hooking around the humerus. The trochlear notch is a deep, C-shaped concavity that fits around the trochlea, a pulley-shaped surface on the humerus. This tight fit restricts movement mainly to flexion and extension, operating much like a door hinge. This structure is the main restraint against elbow dislocation, establishing the ulna as the primary stabilizer of the elbow joint.
The Radius and Forearm Rotation
The radius is specialized for dynamic movement, particularly the rotation of the forearm, which allows the hand to turn over. This rotational ability, known as pronation (turning the palm down) and supination (turning the palm up), is achieved because the radius pivots around the ulna at both the elbow and the wrist. The head of the radius is cylindrical, allowing it to spin within the radial notch of the ulna, held in place by the annular ligament.
At the wrist, the radius articulates strongly with the carpal bones, bearing the majority of the force transmitted through the hand. During rotation, the distal end of the radius crosses over the ulna to achieve pronation. This unique mechanical arrangement allows the radius to handle approximately 80% of the axial load transferred from the hand to the arm.