The human forearm is supported by two long bones: the radius and the ulna. These bones run parallel and work together to facilitate a wide range of movements, yet each has a distinct structure and functional role. Recognizing the anatomical differences between the two is fundamental to understanding the mechanics of the arm.
Identifying Location and Orientation
The easiest way to tell the radius and ulna apart is by their consistent anatomical location within the forearm. When the arm is in the standard anatomical position (palms facing forward), the ulna is located on the medial side, closer to the body’s midline. This bone aligns with the little finger. Conversely, the radius is positioned on the lateral side of the forearm, aligning directly with the thumb.
A helpful way to remember this distinction is to associate the radius with the thumb (the radial side of the hand). While the bones lie parallel, they are not fixed. The two bones remain connected by the interosseous membrane, a dense sheet of connective tissue that distributes load between them.
Distinctions at the Elbow Joint
The proximal ends of the radius and ulna, which meet the upper arm bone, display the most pronounced structural differences. The ulna is the dominant bone at the elbow, forming the primary articulation with the humerus. This end features the large, C-shaped trochlear notch, which fits securely around the trochlea of the humerus to create a stable hinge joint for flexion and extension.
Projecting above this notch is the olecranon process, the bony prominence commonly felt as the point of the elbow. This process acts like a stop, preventing the elbow from hyperextending. Just below the trochlear notch is the coronoid process, a projection that contributes to the stability of the elbow joint.
In contrast, the radius has a smaller, disc-shaped structure called the radial head at its proximal end. This head is designed for rotation, not stability, articulating with the capitulum of the humerus and the radial notch of the ulna. This shape allows the radius to spin within its articulation, which is necessary for complex forearm movements. The narrow neck below the head leads to the radial tuberosity, a roughened area where the biceps brachii muscle attaches.
Distinctions at the Wrist Joint
The roles of the two bones reverse at the distal end, where they meet the hand. The radius is the primary articulating bone at the wrist, flaring out to form a broad, rectangular end. This wider surface articulates directly with the carpal bones of the hand, bearing the majority of the force transmitted from the hand to the arm.
The distal end of the radius features the radial styloid process, a pointed projection on the thumb side that extends further down than the corresponding process on the ulna. This longer projection provides stability to the lateral side of the wrist. The ulna, however, tapers to a small, rounded ulnar head at its distal end.
The ulna’s head articulates with a notch on the radius, but it does not directly contact the carpal bones. A disc of cartilage, part of the triangular fibrocartilage complex, separates the ulna from the hand bones. This minimal involvement highlights the ulna’s reduced role in transmitting load across the wrist joint.
Roles in Forearm Movement
The distinct shapes of the two bones are tailored to their roles in the rotational movements of the forearm, known as pronation and supination. These movements allow the hand to turn the palm up (supination) or the palm down (pronation). During these actions, the ulna remains fixed at the elbow, acting as the stable axis of rotation.
The radius performs the dynamic role, rotating around the ulna at both the proximal and distal joints. When the forearm moves from the palm-up (supinated) to the palm-down (pronated) position, the distal end of the radius crosses diagonally over the ulna. This crossing motion allows the hand to rotate nearly 180 degrees. The cylindrical radial head and the broad, pivoting distal end enable this crossing motion, demonstrating how the structure of each bone dictates the range of forearm function.