What Is Scapholunate Dissociation?

Medically Reviewed by Poonam Sachdev on November 22, 2022
5 min read

Eight small bones in your wrist give it flexibility and the necessary range of movements. These bones, arranged in two rows, include the scaphoid and lunate bones in the upper row, close to the forearm bones. Scapholunate dissociation is when their normal position to each other is disturbed. Usually, this happens following forceful trauma, such as a fall or vehicular crash. Early diagnosis and appropriate treatment are essential to recovering full wrist function. 

The scaphoid and lunate bones, and their position relative to each other, are critical for wrist motion. The scapholunate complex is also vital to transmitting force across the wrist. Scapholunate dissociation makes the wrist unstable and weakens your grip.

The scapholunate interosseous ligament holds the scaphoid and lunate bones together. This ligament is a strong fibrous band that allows controlled movements between them while keeping them together. If this ligament is injured, the bones move apart, a condition known as scapholunate dissociation.

Scapholunate dissociation is also known as rotary subluxation of the scaphoid and scapholunate interosseous ligament injury. The two bones, scaphoid and lunate, are placed abnormally relative to each other, as seen on X-rays. The rupture of the scapholunate ligament makes your wrist unstable.

This injury has crucial long-term implications. The ligament holds the bones together and allows them to move in a coordinated manner. If the ligament is injured and does not heal, the wrist movements are disordered, and the joint is damaged. If untreated, a typical pattern of wrist arthritis, scapholunate advanced collapse (SLAC), develops.

An unrepaired ligament allows the scaphoid and lunate bones to move away from each other. This separation alters the distribution of forces along the multiple joints in the wrists. The cartilage between the various bones is subjected to abnormal contact pressures and gradually degenerates. Over time, the capitate bone, typically located in the lower row of wrist bones, moves between the scaphoid and lunate bones. This movement is called the Terry Thomas sign.

The most common cause is trauma. The most frequent cause of scapholunate dissociation is a fall on the outstretched hand. Such falls often cause fractures of the lower end of the radius, one of the two bones in your forearm. Other causes include:

  • Gripping a steering wheel during a car crash
  • Loose ligaments
  • Hereditary spastic paraparesis
  • Rheumatoid arthritis

If you've had a fall on your hand or another injury, watch for these signs that may signal scapholunate dissociation:

  • Weak grip
  • Pain on the back of your wrist when gripping
  • Swelling on the wrist
  • Pain on touching the wrist
  • Pain when moving your wrist backward
  • Clicking or clunking sensations in the wrist

If you feel your wrist is injured, you should consult your doctor. They may refer you to an orthopedic surgeon. Scapholunate dissociation needs careful clinical examination and imaging to diagnose. After examining your wrist for swelling, pain, and range of movement, your doctor may do the following:

Scaphoid shift test. Your doctor tests the stability of your scaphoid bone by putting pressure on it while you move your wrist from side to side. If the ligament has torn, the scaphoid flexes and then snaps back into position when pressure is removed.

X-ray. An x-ray will show the increased gap between the two bones. Typically, the gap is 2 millimeters or less. X-rays from different angles help your doctor judge the alignment of your wrist bones. X-rays will also detect a fracture of the radius if present.

Magnetic resonance imaging (MRI) scan. This imaging method gives more detailed images of your wrist. Your doctor will be able to see the tear in the scapholunate ligament. An MRI scan helps your doctor plan treatment, too.

Arthroscopy. Your doctor gets a view inside the joint by introducing a small tube with a light at the end. They may also be able to carry out procedures to repair any injury.

You should be alert about wrist pain that lasts a long time, especially if it starts after a fall. Untreated scapholunate dissociation has poor outcomes that may not be fully reversible. Your doctor will be alerted if they find a fracture of the lower end of the radius because 10% to 30% of such fractures also have associated scapholunate dissociation.

Your doctor will prescribe medication to relieve your pain and swelling. They'll also apply a splint to keep your wrist from moving. Splinting gives the injured tissues a chance to heal. If the ligament heals, the bones will return to their proper positions. If this doesn't happen, long-term damage may occur, causing:

  • Long-lasting pain
  • Weakened grip
  • Joint inflammation (arthritis)
  • Joint instability
  • Degenerative arthritis, leading to scapholunate advanced collapse (SLAC)

Your doctor will ask for a repeat X-ray after some days of treatment to ensure the healing is progressing as necessary. If not, they will advise surgery to repair the injury.

Your doctor may numb your arm and hand to repair your injury surgically. General anesthesia, making you unconscious, is the other option. After surgery, they'll place a cast on your wrist for a few weeks. You'll then have physical therapy to regain strength in your wrist.

Sometimes, your ligament may not be repairable. Your doctor may use part of the carpi radialis tendon to connect the two bones. This tendon runs from your arm bone to your hand. Another option is arthrodesis — joining the two bones by placing a screw between them. There are several approaches to treating scapholunate dissociation, so your doctor will decide on the best one after carefully studying your X-rays and scans. The result of successful treatment should be a stable painless wrist. 

The lunate and scaphoid bones are crucial. Despite four bones in the upper row of wrist bones, 50% of the wrist load transmits across the radius to the scaphoid, and 35% transfers to the lunate. Disturbances of these bone structures and relative positions can significantly affect the stability of the wrist and hand. Scapholunate ligament injuries are the most frequent cause of wrist instability.

Injuries to the scapholunate ligament must be detected and treated early for the best restoration of function. Untreated scapholunate dissociation leads to long-term complications, including scapholunate advanced collapse (SLAC), the most common degenerative arthritis of the wrist.