Contents

LunateEdit

The lunate is a central component of the wrist, one of the eight carpal bones that compose the carpus. Its name, derived from the Latin luna for “moon,” reflects its crescent-shaped, semilunar profile. Nestled in the proximal row of carpal bones, the lunate lies between the scaphoid on the radial (thumb) side and the triquetrum on the ulnar side, forming a key link in both radiocarpal and midcarpal joints. The bone contributes to load transmission across the wrist and serves as an important fulcrum for wrist movements.

The lunate’s position and relationships make it a crucial but often vulnerable element of wrist anatomy. It participates in the radiocarpal joint with the distal end of the radius, and it forms intercarpal articulations with neighboring bones such as the scaphoid laterally and the triquetrum medially, while its distal surface participates with the capitate and, in part, with the scaphoid to facilitate wrist motion. Its shape and central location help distribute axial loads from the hand to the forearm, a function that is especially important during gripping and weight-bearing activities.

Anatomy

  • Position and shape: The lunate is the middle bone of the proximal carpal row, with a smooth articular surface on its proximal aspect that interfaces with the distal radius, and multiple facets on its distal side that articulate with adjacent carpal bones. Its crescent-like shape enables it to contribute to the overall stability of the wrist.
  • Surfaces and facets: Proximally, the lunate forms a radiocarpal articulation with the distal radius; distally, it engages with the capitate, scaphoid, and triquetrum to form parts of the midcarpal joints. The lateral (radial) and medial (ulnar) aspects relate to the neighboring carpal bones, helping maintain carpal alignment during movement.
  • Vasculature and innervation: The lunate receives arterial supply from dorsal and palmar intercarpal vascular networks, with contributions from radiocarpal arteries. Its blood supply is a factor in certain pathological conditions when perfusion is compromised. Nerve considerations are generally related to nearby structures such as the median nerve in the carpal tunnel, rather than a dedicated lunate nerve.
  • Development and variation: The lunate ossifies in childhood as part of the normal maturation of the carpus. Variants such as bipartite or accessory ossicles can occur, but most lunates develop with typical morphology that permits standard wrist function.

Articulations and function

  • Radiocarpal articulation: The lunate participates in the primary wrist joint with the distal radius, permitting flexion, extension, and limited deviation of the hand relative to the forearm.
  • Midcarpal articulations: Distally, the lunate forms joints with neighboring carpal bones, notably with the capitate in the central column and with the scaphoid and triquetrum on the lateral and medial aspects, respectively. This arrangement helps distribute motion and load across the wrist.
  • Mechanical role: As a central carpal element, the lunate acts as a stabilizing link that accommodates changes in carpal alignment during wrist movement, while allowing efficient transfer of forces from the hand to the forearm.

Clinical significance

  • Lunate fractures: Fractures of the lunate are relatively uncommon compared with other carpal injuries but can occur, often accompanying high-energy trauma. Diagnosis may be challenging on plain radiographs and can require computed tomography (CT) or magnetic resonance imaging (MRI) for confirmation.
  • Lunate dislocations and perilunar injuries: Dislocations of the lunate or injuries around the lunate can arise from fall onto an outstretched hand or high-energy impacts. Prompt recognition and management are important to reduce complications such as chronic instability or secondary arthritis.
  • Kienböck’s disease: This condition involves avascular necrosis of the lunate, leading to pain, decreased range of motion, and wrist dysfunction. It is influenced by local vascular supply and mechanical loading, and management ranges from immobilization to surgical procedures aimed at revascularization or joint preservation.
  • Complications and considerations: Because the lunate is relatively small and has a delicate blood supply, injuries to this bone can progress to irreversible changes if not properly diagnosed and treated. Treatment strategies emphasize restoring alignment, stabilizing the carpal row, and addressing underlying factors that influence wrist biomechanics.

Imaging and diagnosis

  • Plain radiography: Initial assessment typically includes posteroanterior and lateral views of the wrist. Specific signs such as misalignment of the lunate, widened intervals between the carpal bones, or secondary changes can indicate injury.
  • Advanced imaging: CT can delineate fracture patterns and fragment displacement, while MRI is particularly useful for assessing bone marrow perfusion (relevance to conditions like Kienböck’s disease) and soft-tissue relationships.
  • Radiographic indicators: In cases of lunate dislocation, radiographs may reveal characteristic patterns such as posterior subluxation of the lunate or the “signet ring” appearance on certain views, reflecting altered lunate contour and alignment.

Development and evolution

  • Ossification: The lunate ossifies in early childhood as part of the normal development of the wrist. By adolescence, its morphology typically resembles the mature carpal anatomy.
  • Evolutionary context: In humans, the carpal bones, including the lunate, support a highly dexterous and versatile wrist. Comparative anatomy shows variations across species in carpal structure, but the lunate’s role as a central stabilizer of wrist motion is a recurring theme in primates.

See also