The crescent-shaped bone in the middle of the proximal row of the wrist, between the boat-shaped bone and the three-cornered bone
Lunate — the moon-shaped carpal bone.
Anatomy · Bones
Lunate
Crescent-shaped carpal bone
Wrist · proximal row of carpal bones
The lunate is the moon-shaped carpal bone at the centre of the proximal row of the wrist, between the scaphoid and the triquetrum. It sits in the concavity of the end of the radius and cups the head of the capitate. It is the carpal bone most often dislocated, and it can die from loss of blood supply in Kienbock's disease.
How to spot them
- ▸Crescent- or moon-shaped bone in the middle of the proximal row of carpal bones, between the scaphoid and triquetrum
- ▸Its upper surface is convex and fits the radius; its lower surface is a deep concavity that receives the head of the capitate
- ▸Wider on its palmar face than on its back
- ▸Held to its neighbours by the scapholunate and lunotriquetral ligaments
What to know
Named after the Latin for the moon, it is the carpal bone most often dislocated
Lunate — from luna, the moon.
On a side-on X-ray, its dislocation appears as a bone tipped forward like a spilled cup of tea
Lunate — the 'spilled teacup' sign.
When pushed forward into the carpal tunnel it can compress the median nerve
Lunate — palmar dislocation causes acute carpal tunnel syndrome.
Loss of blood supply to this bone leads to collapse, an eponymous condition named after an Austrian radiologist
Lunate — Kienbock's disease (avascular necrosis), after Robert Kienbock, 1910.
A slightly short ulna, a 'negative ulnar variance', is linked with collapse of this bone through changed loading of the wrist
Lunate — a risk factor for Kienbock's disease.
It sits in the hollow of the end of the thumb-side forearm bone and cups the head-shaped bone of the distal row
Lunate — it articulates with the radius proximally and the capitate distally.
Key ideas
- Because it lies at the centre of the proximal row and receives load through the capitate, the lunate is at the heart of wrist stability; its ligaments to the scaphoid and triquetrum are key to the shape of the carpal arch.
- Blood supply from both palmar and dorsal vessels usually keeps it alive, but a few people depend on a single palmar vessel, and repeated or single injury can compromise it.
Quick-fire facts
- •It is one of the eight carpal bones; the name comes from the Latin luna, the moon.
- •It articulates with the radius (and the articular disc) proximally, the capitate and hamate distally, the scaphoid laterally and the triquetrum medially.
- •It is the most commonly dislocated carpal bone; a perilunate dislocation is a related, more severe injury.
- •Kienbock's disease is avascular necrosis of the lunate, first described by Robert Kienbock, an Austrian radiologist, in 1910.
- •The 'spilled teacup' sign of lunate dislocation and the 'piece of pie' sign are seen on lateral and frontal X-rays respectively.
- •It forms the centre of the proximal row between the scaphoid on the thumb side and the triquetrum on the little-finger side.
- •Together with the scaphoid and triquetrum, it takes part in the radiocarpal (wrist) joint.
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Sources
- Kenhub — Lunate bone
- Kenhub — Carpal bones
- OpenStax — Anatomy and Physiology 2e — 8.2 Bones of the Upper Limb