The human wrist is composed of eight small bones known as carpal bones. These bones work in concert, facilitating a wide range of hand and forearm movements. The lunate bone plays an important role in wrist function and integrity.
Locating the Lunate
The lunate bone is one of the eight carpal bones found in the wrist, specifically situated in the center of the proximal row. This row of bones is closest to the forearm, positioned between the scaphoid bone laterally and the triquetrum bone medially. Its name, derived from the Latin word “luna,” meaning moon, aptly describes its crescent or moon-like shape.
The lunate articulates with several surrounding bones, contributing to the wrist’s complex joint structure. Proximally, its convex surface connects with the radius. Distally, it forms connections with the capitate and the hamate bone. Laterally, it articulates with the scaphoid, and medially, with the triquetrum. These articulations allow for coordinated wrist movement.
Role of the Lunate
The lunate bone is central to wrist mechanics, contributing to movement and stability. It acts as a link, effectively transmitting forces from the forearm to the hand. This bone facilitates wrist motions, including flexion, extension, and radial and ulnar deviation.
The lunate also serves as a central pivot point for the wrist joint, helping to maintain proper alignment among the carpal bones. Its articulations, particularly with the radius and capitate, support the wrist’s structural integrity. During activities that involve gripping or weight-bearing, the lunate helps distribute forces evenly across the wrist joint, with approximately 35% of the axial load passing through the radio-lunate joint. This distribution helps prevent excessive stress, reducing injury risk.
Common Lunate Conditions
Due to its central position and functional importance, the lunate bone is susceptible to injuries and conditions. Lunate dislocation often results from high-energy trauma, such as a fall onto an outstretched hand. In a lunate dislocation, the bone is completely displaced from its normal alignment with both the radius and the capitate, often appearing rotated into a “spilled teacup” configuration on imaging. This differs from a perilunate dislocation, where the lunate maintains its alignment with the radius, but the other carpal bones are displaced around it.
Another condition affecting the lunate is Kienböck’s disease, also known as avascular necrosis. This occurs when the lunate bone loses its blood supply, leading to the death and eventual collapse of the bone. While the exact cause is not always clear, factors such as trauma, certain skeletal variations like an uneven length between the radius and ulna, or an irregular lunate shape can contribute to its development. Symptoms often include wrist pain, stiffness, and decreased motion, which can progress to arthritis if left untreated.
Lunate fractures are less common than other carpal bone fractures, accounting for about 0.5% to 6.5% of all carpal fractures. These breaks result from a direct blow to the wrist or a fall on an outstretched hand, where significant force is transmitted. Such fractures can cause localized pain, swelling, and difficulty moving the wrist. Although rare in isolation, lunate fractures can be associated with other carpal injuries or lead to complications like avascular necrosis due to the bone’s limited blood supply.