The ulna is one of the two long bones that make up the forearm, running parallel to the radius from the elbow to the wrist. It connects the upper arm to the hand, acting as an anchor for many movements. The ulna plays a role in the stability and mobility of the upper limb, providing a stable base for the joint mechanics that allow for the arm’s range of motion. Understanding the ulna’s structure and function is important for recognizing when injury may have occurred.
The Ulna’s Anatomical Placement and Structure
The ulna occupies the medial side of the forearm, corresponding with the location of the pinky finger. It is the longer of the two forearm bones and is notably wider at the elbow joint than at the wrist. This shape allows it to form a strong, interlocking connection with the humerus, the bone of the upper arm.
At the elbow, the ulna features a large, C-shaped depression known as the trochlear notch. This notch fits perfectly around the spool-shaped trochlea of the humerus, creating the primary hinge action of the elbow joint. The upper border of this notch extends backward to form the olecranon process, the prominent bony point felt at the back of the elbow. When the arm is straightened, the olecranon process locks into a depression on the humerus, preventing hyperextension.
Just below the olecranon process is the coronoid process, a projection that forms the lower part of the trochlear notch. A shallow depression called the radial notch allows the head of the radius to articulate with the ulna, forming the proximal radioulnar joint. This joint is fundamental for forearm rotation. The triangular shaft of the ulna is connected to the radius by the interosseous membrane, a tough fibrous sheet that helps stabilize the relationship between the two bones.
How the Ulna Facilitates Arm Movement
The ulna’s primary functional role is forming the hinge joint of the elbow, allowing for flexion (bending) and extension (straightening) of the forearm. The deep, interlocking fit of the trochlear notch around the humerus provides the necessary stability for these movements. The triceps brachii muscle, which straightens the arm, attaches directly to the olecranon process, using it as a lever to apply force.
The ulna also plays a passive role in the rotational movements of the forearm: pronation (turning the palm down) and supination (turning the palm up). During these actions, the ulna remains relatively stationary, acting as a fixed anchor. The radius rotates around the ulna at both the elbow and wrist joints to achieve this motion.
Specifically, the head of the radius spins within the radial notch of the ulna at the elbow. This arrangement maintains the ulna’s strong connection at the elbow while allowing the hand and wrist to twist. The ulna also serves as the attachment point for numerous forearm muscles that control the movement of the wrist and fingers.
Common Injuries Involving the Ulna
Because of its exposed position, the ulna is susceptible to several types of fractures and injuries. A fall onto an outstretched hand is a common mechanism resulting in a fracture to the ulna or radius. The specific pattern of injury depends on the location and force of the trauma.
Olecranon Fracture
One of the most frequent breaks is an olecranon fracture, occurring from a direct blow or a fall onto the point of the elbow. The powerful contraction of the triceps muscle, which attaches to the olecranon, can pull the fractured pieces apart, making the injury unstable. Symptoms include immediate, intense pain, swelling, and a limited ability to straighten the elbow due to the loss of the triceps’ lever arm.
Nightstick Fracture
Another common pattern is the isolated fracture of the ulnar shaft, often called a nightstick fracture. This injury derives its name from the mechanism of a direct, forceful blow to the forearm, such as when a person raises their arm to shield themselves. These fractures occur in the mid-section of the bone, presenting with localized tenderness, swelling, and pain when moving the forearm.
Monteggia Fracture-Dislocation
A more complex injury is the Monteggia fracture-dislocation, which involves a fracture of the proximal third of the ulna combined with an accompanying dislocation of the radial head at the elbow. The force that breaks the ulna destabilizes the elbow joint, forcing the radius out of alignment. Patients experience pain and swelling at the elbow, and movement may be limited, especially pronation and supination.
Ulnar Impaction Syndrome
Ulnar impaction syndrome is a chronic condition causing pain on the pinky-finger side of the wrist. This syndrome occurs when the ulna is relatively longer than the radius, leading to excessive pressure and friction at the wrist joint. This chronic impaction can cause degeneration of the cartilage and tears in nearby ligaments. Pain is exacerbated by twisting motions of the wrist or bearing heavy loads with the hand.