The lunate is the fourth most fractures carpal bone (following the scaphoid, triquetrum, and trapezium). They often are not diagnosed initially and present delayed as lunate osteonecrosis, which is also known as Kienbock disease. The mechanism of injury is typically a fall onto an outstretched hand with a hyperextended wrist or during a forceful push with an extended wrist.
The lunate can be considered a "carpal keystone", since it resides within a protected concavity of the distal radius (lunate fossa). The lunate is attached to the scaphoid and triquetrum via interosseous ligaments. Distally, it is articulates with the convex head of the capitate. It is supplied by a proximal carpal vascular arcade volarly and dorsally. There are three variable intralunate anastomoses.
Lunate fractures can be classified into 5 groups:
- Coronal fractures of the palmar pole with involvement of the palmar nutrient arteries
- Coronal fractures of the dorsal pole
- Transarticular coronal fractures of the body
- Transverse body fractures
- Osteochondral fractures of the proximal articular surface without substantial damage to the nutrient vessels
Patients often present with palpation tenderness on the volar wrist. Wrist range of motion is usually painful.
Standard PA and lateral radiographs of the wrist are often inadequate to detect a lunate fracture because of overlapping radiodensities. Oblique views may be slightly more helpful; however, MRI, CT, or bone scintigraphy is often needed. MRI is also helpful for evaluating healing, vascular injury, and sings of osteonecrosis.
Nondisplaced fractures can be treated with cast immobilization (short or long arm). Follow-up must proceed at close intervals to monitor healing and possible progression to Kienbock disease. Displaced or angulated fractures require surgical apposition to allow healing of the vascular supply to the lunate.
Kienboeck disease is osteonecrosis of the lunate and can lead to devastating advanced collapse and radiocarpal arthrosis. Surgical intervention may be necessary for pain relief if severe. The options for surgical management of Kienbock disease includes radial wedge osteotomy, radial shortening, ulnar lengthening, or salvage procedures such as arthrodesis or proximal row carpectomy.