The cubital tunnel is a narrow channel located on the inside of the elbow, serving as a protective passageway for a major nerve that travels down the arm. This anatomical space is a common location where a nerve can become irritated or compressed, leading to cubital tunnel syndrome. The area is often referred to as the “funny bone,” describing the shocking sensation felt when the nerve is bumped against the bone. Understanding the location and structures of this tunnel is the first step in recognizing the cause of symptoms affecting the hand and fingers.
Precise Location and Anatomy of the Cubital Tunnel
The cubital tunnel is situated on the posteromedial aspect of the elbow, meaning it is found on the back and inner side of the joint. This fibro-osseous space, constructed by bone and fibrous tissue, is nestled between two prominent bony landmarks.
The inner wall is formed by the medial epicondyle, the large bony bump on the inner side of the humerus (upper arm bone). The outer wall is the olecranon process, the pointed, curved end of the ulna (a forearm bone). The floor of the tunnel is composed of the elbow joint capsule and the medial collateral ligament.
The roof of the tunnel is a band of fibrous tissue known as the arcuate ligament, or Osborne’s ligament. This ligament stretches between the medial epicondyle and the olecranon process. This compact arrangement creates a tight, enclosed space that allows for little margin when the elbow is flexed.
The Ulnar Nerve: What Passes Through the Tunnel
Only one structure passes through the entire length of the cubital tunnel: the ulnar nerve. This nerve is one of the three main nerves running down the arm, originating from the brachial plexus in the neck and shoulder. It courses down the arm and wraps behind the elbow to enter the cubital tunnel on its way into the forearm.
Once it exits the tunnel, the ulnar nerve continues toward the hand, providing both motor and sensory function. The nerve is responsible for sensation in the little finger and the adjacent half of the ring finger. It supplies feeling to both the palm and back surfaces of these fingers.
Beyond sensory function, the ulnar nerve controls the motor function of nearly all the small, intrinsic muscles within the hand. These muscles are responsible for fine movements, such as spreading and bringing the fingers together, and they contribute to grip strength. The ulnar nerve also controls two muscles in the forearm.
Cubital Tunnel Syndrome: Causes and Symptoms
Cubital tunnel syndrome results from the compression or irritation of the ulnar nerve as it passes through the narrow tunnel at the elbow. Because the nerve runs superficially over the bone, it has little protective padding, making it vulnerable to external pressure. This compression causes the nerve fibers to malfunction, leading to a specific set of symptoms.
The most common symptoms involve numbness and tingling (paresthesia), occurring in the ring finger and the little finger. This sensation often presents intermittently but can become constant as the condition progresses. Symptoms often worsen when the elbow is bent for a sustained period, such as when sleeping with the arm curled up or holding a phone.
Mechanisms of Compression
The primary mechanisms causing nerve irritation are direct pressure, prolonged stretching, or repetitive motion. Direct pressure occurs when leaning the elbow on a hard surface for extended periods, like resting an arm on a desk.
Sustained elbow flexion stretches the nerve, which reduces its blood flow and irritates the tissues. The cubital tunnel becomes taut when the elbow is bent past 90 degrees, exacerbating this issue.
Long-Term Effects
Over time, if compression is not relieved, the motor function of the nerve may become impaired. This leads to noticeable weakness in the hand, making fine motor tasks, such as buttoning a shirt or handling small objects, increasingly difficult. In severe cases, the hand muscles supplied by the ulnar nerve may begin to waste away, resulting in a loss of grip strength and hand dexterity. Additional causes include bone spurs, abnormal muscle anatomy, or previous trauma to the elbow joint.