The lunate bone is one of the eight small carpal bones that form the wrist joint, connecting the forearm and the hand. These bones are arranged in two rows. The lunate gets its name from its characteristic crescent or moon-like shape, derived from the Latin word luna. It is a short bone with a deep concavity that contributes significantly to the flexibility and function of the wrist.
Precise Location within the Wrist Anatomy
The lunate bone is situated in the center of the proximal row of carpal bones, which is the row closest to the forearm. It rests directly beneath the bones of the forearm, specifically articulating with the radius, the larger of the two forearm bones. This direct connection makes it a central component of the main wrist joint, or radiocarpal joint.
On its sides, the lunate has two immediate neighbors within the proximal row. The scaphoid bone is located on its lateral (thumb) side, and the triquetrum bone is on its medial (little finger) side. Distally, or toward the hand, the lunate articulates with the capitate bone, the largest of the carpal bones, which sits at the heart of the distal row.
Its crescentic shape allows it to fit snugly into the lunate fossa, a smooth depression on the end of the radius. This positioning means the lunate is a bridge between the forearm and the rest of the wrist. The proximal surface of the bone is smooth and convex, matching the concave surface of the radius to allow for gliding motion.
Role in Wrist Movement and Stability
The lunate bone functions as a dynamic link, guiding the coordinated movement between the forearm and the hand. Its articulation with the radius permits a wide arc of motion, facilitating the complex movements of wrist flexion and extension. It also assists in radial and ulnar deviation, which are the side-to-side movements of the wrist.
As a central carpal bone, the lunate is responsible for transmitting load forces from the hand to the forearm bones. It acts as a pivot point, helping to distribute pressure evenly during activities like gripping or pushing. The stability of the lunate is maintained by strong interosseous ligaments, particularly those connecting it to the scaphoid and the triquetrum. The integrity of these ligaments is necessary for the carpal bones to move smoothly and synchronously.
Common Injuries and Vulnerabilities
Because of its central position and load-bearing function, the lunate is highly susceptible to injury, often leading to specific pathological conditions. One of the most common acute injuries is lunate dislocation, which can occur when a person falls onto an outstretched hand (FOOSH). The force of the impact can push the bone out of its normal position, potentially causing nerve compression and severe pain.
The lunate is also vulnerable to Kienbock’s disease, a chronic condition known as avascular necrosis. This disorder occurs when the blood supply to the bone is interrupted, causing the bone tissue to die and eventually collapse. Trauma or repetitive micro-injuries can disrupt the blood vessels that nourish the bone, initiating deterioration. Anatomical variations, such as a difference in length between the radius and ulna bones, can also increase mechanical stress on the lunate, raising the risk of developing this condition. The disease leads to increasing wrist pain, stiffness, and eventual arthritis if left untreated.