Why was only my wrist lunate bone dislocated when I fell?

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Q: I dislocated the lunate bone of my wrist when I fell off my bike and hit the pavement with my hands. When I looked on-line I saw how this bone is protected by the other bones around it. Nothing else was dislocated so how or why did this one pop out?

A: The lunate carpal (wrist) bone is neatly tucked in between the two bones of the forearm, the two other carpal bones on either side, and another row of carpal bones next to the fingers.

It doesn't dislocate easily and usually only after significant high-energy trauma. Falling off a bike and hitting the pavement could certainly cause this type of injury. Car accidents and sports injuries account for the majority of lunate injuries.

Dislocation of this bone usually means the soft tissues around the bone have been disrupted. The injury is technically referred to as a perilunate injury, meaning "around the lunate." There are four steps or "stages of injury" that occur to force the lunate out of place. It's a bit like dominoes -- once the first one goes, a whole series of events takes place.

In stage one, the carpal bones next to the fingers are forced into a position of extreme extension. The ligaments around those bones pull the scaphoid bone of the wrist (the bone next to the lunate on the thumb side) into a position of extension. The ligament between the scaphoid and lunate tears. The force of the injury continues to transfer through the wrist to the ligaments around the lunate. That is stage two of the sequence.

In stage three, the lunate dislocates and pulls with it the ligament between the lunate bone and the triquetrum (bone on the little finger side of the lunate). And then in stage four, the ligament between the radius bone of the forearm and the lunate tears allowing the lunate to rotate or twist and dislocate.

Of course, all of these events occur in a matter of seconds starting at the moment of impact. The loss of the ligament stability and shift in bone alignment changes the whole structure and dynamics of the wrist. The natural "arcs" or archways formed by the two rows of carpal (wrist) bones is affected. This can put pressure on the nerves that travel through the arcs. And in a perilunate injury, any of the bones around the lunate can be fractured, dislocated, or both.

Reference: Scott R. Hadley, and Steve K. Lee, M.D. Hand and Wrist: Perilunate Injuries. In Current Orthopaedic Practice. July/August 2012. Vol. 23. No. 4. Pp. 318-321.

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