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Os lunatum – Lunate Bone: Anatomy and Conditions

The os lunatum, or lunate bone, is a crescent-shaped carpal bone located in the wrist. It plays a key role in wrist stability and mobility.

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Things worth knowing about "Os lunatum"

The os lunatum, or lunate bone, is a crescent-shaped carpal bone located in the wrist. It plays a key role in wrist stability and mobility.

What is the Os lunatum?

The os lunatum (Latin: lunate bone or moon bone) is one of the eight carpal bones of the wrist and is positioned in the proximal carpal row. Its name derives from its characteristic crescent or half-moon shape. It is located centrally in the wrist, situated between the scaphoid bone on the thumb side and the triquetrum on the little finger side. The lunate bone forms an important part of the joint connecting the forearm to the hand (radiocarpal joint).

Anatomy and Function

The os lunatum articulates (forms joints) with several neighbouring bones:

Proximally (toward the forearm): with the radius
Distally (toward the hand): with the capitate and hamate bones
Laterally: with the scaphoid and triquetrum

Due to its central position, the lunate bone contributes significantly to wrist stability and enables movements such as flexion, extension, and lateral deviation of the hand. It is surrounded by numerous ligaments that maintain the cohesion of the carpal bones.

Clinical Significance

Lunate Necrosis (Kienbock Disease)

One of the most common and significant conditions affecting the os lunatum is lunate necrosis, also known as Kienbock disease. This condition involves avascular necrosis (death of bone tissue due to insufficient blood supply) of the lunate bone. Contributing factors may include repetitive microtrauma, overuse, or anatomical variations. Typical symptoms include:

Wrist pain, especially with activity
Swelling and tenderness over the back of the wrist
Reduced range of motion in the wrist
Loss of grip strength

Lunate Fracture

The os lunatum can fracture as a result of falls onto an outstretched hand or direct trauma (lunate fracture). Such fractures are relatively uncommon and are often missed due to the bone's deep location. Untreated fractures can lead to avascular necrosis.

Lunate Dislocation (Perilunate Dislocation)

In severe wrist injuries, a perilunate dislocation may occur, in which the lunate bone is displaced from its normal position. This is an orthopaedic emergency requiring immediate medical treatment.

Diagnosis

Various diagnostic techniques are available to assess the os lunatum:

X-ray: First-line imaging for detecting fractures and positional changes
MRI (Magnetic Resonance Imaging): Particularly useful for early detection of lunate necrosis, as changes in bone marrow can be visualised
CT (Computed Tomography): For detailed assessment of fracture patterns
Bone scintigraphy: To evaluate bone perfusion

Treatment

Treatment depends on the specific condition and its severity:

Conservative treatment: Immobilisation with a splint or cast, pain management, and physiotherapy for acute injuries or early-stage necrosis
Surgical treatment: Advanced lunate necrosis or fractures may require surgical intervention, such as revascularisation procedures (to improve blood supply), partial wrist fusion (partial arthrodesis), or, in severe cases, replacement of the lunate bone with an implant

References

Allan, C. H. et al.: Kienbock's disease. Journal of the American Academy of Orthopaedic Surgeons, 9(2): 128-136, 2001. PubMed PMID: 11281636.
Schmitt, R.; Lanz, U.: Diagnostic Imaging of the Hand. Thieme Publishers, 3rd edition, 2014.
Gilula, L. A.; Yin, Y.: Imaging of the Wrist and Hand. W.B. Saunders Company, 1996.

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