Scapholunate dissociation is a separation between two small bones in the wrist, the scaphoid and the lunate, caused by damage to the ligament that holds them together. It’s the most common ligament injury in the wrist and, if left untreated, can lead to progressive arthritis over time. The injury ranges from a mild sprain to a complete tear, and treatment depends heavily on how far the damage has progressed.
How the Scapholunate Joint Works
Your wrist contains eight small carpal bones arranged in two rows. The scaphoid and lunate sit side by side in the row closest to your forearm, and the ligament connecting them (the scapholunate interosseous ligament, or SLIL) is the primary stabilizer of that joint. But stability doesn’t come from one ligament alone. A whole network of secondary stabilizers, including ligaments on the back of the wrist, the palm side, and connections to neighboring bones, work together as a complex to keep the scaphoid and lunate moving in sync.
When the primary ligament tears, these secondary stabilizers can compensate for a while. That’s why some people walk around with partial tears and only mild symptoms. But once enough of the stabilizing complex fails, the scaphoid and lunate begin rotating in opposite directions. The scaphoid tips forward while the lunate tilts backward, a pattern called dorsal intercalated segment instability (DISI). This misalignment changes how forces travel through the wrist and sets the stage for cartilage breakdown.
Common Causes
The classic mechanism is a fall onto an outstretched hand with the wrist extended. This loads the scaphoid and lunate in opposite directions, straining or tearing the ligament between them. The injury frequently accompanies wrist fractures: studies report that scapholunate ligament damage occurs in 7% to 64% of distal radius fractures, with higher rates in fractures that extend into the joint surface. That wide range reflects different methods of detection, since many of these ligament injuries are subtle and missed on initial X-rays.
Repetitive stress can also cause the ligament to degenerate over time, which is why some people develop scapholunate dissociation without a single memorable injury. Athletes who load their wrists heavily, such as gymnasts and weightlifters, are at higher risk.
What It Feels Like
The hallmark symptom is pain on the thumb side of the wrist, roughly in the hollow just beyond the end of the forearm bone. Swelling in that area is common after the initial injury and may persist with activity. Many people notice a clicking or clunking sensation during wrist movements, particularly when gripping or twisting. Grip strength typically drops, and the wrist can feel stiff or weak in ways that don’t improve with rest the way a simple sprain would.
In partial tears, symptoms may come and go. You might feel fine during everyday tasks but notice sharp pain when pushing off a table, opening a jar, or catching yourself during a stumble. That intermittent pattern can delay diagnosis for months or even years.
How It’s Diagnosed
Diagnosis combines a physical exam, standard X-rays, and sometimes advanced imaging. During the exam, your doctor will likely perform the scaphoid shift test: they press on the scaphoid from the palm side while moving your wrist from one side to the other. If the scaphoid shifts out of position and produces a painful clunk when pressure is released, especially compared to the other wrist, it suggests ligament damage. That said, this test has moderate accuracy at best. Studies report its sensitivity at around 50% to 66%, meaning it misses a significant number of injuries, though it’s more reliable for detecting severe tears.
X-ray Findings
On a standard front-to-back X-ray, the gap between the scaphoid and lunate is normally a few millimeters. A gap greater than 5 mm is diagnostic of scapholunate dissociation. This widened space is sometimes called the “Terry Thomas sign,” named after a British comedian known for the gap between his front teeth. On a side-view X-ray, the angle between the scaphoid and lunate is measured. Normal is 30 to 60 degrees. An angle above 60 degrees suggests instability, and above 80 degrees confirms dorsal tilt of the lunate.
Early or partial injuries may not show up on regular X-rays, especially if the secondary stabilizers are still intact. In those cases, MRI can reveal ligament damage directly, and in some situations arthroscopy (inserting a tiny camera into the wrist joint) provides the most detailed assessment.
Grades of Injury
When evaluated arthroscopically, scapholunate injuries are classified into four grades that guide treatment decisions.
- Grade I: The ligament is stretched and bruised but not torn through. The bones still line up normally when viewed from the midcarpal (inner) joint space. These are essentially wrist sprains and typically heal with immobilization alone.
- Grade II: The ligament is more stretched, and a slight step-off appears between the bones, though the gap is still very small. These injuries often benefit from pinning the bones in proper alignment while the ligament heals.
- Grade III: The ligament has progressed from a stretch to an actual tear. A visible gap exists between the scaphoid and lunate from multiple viewing angles, and a small probe can be passed between them. Surgical repair or reduction with pinning is typically needed.
- Grade IV: A complete tear with gross instability. The gap is wide enough that the arthroscope itself can pass between the two bones. This requires open surgical repair.
Treatment Options
For mild injuries (Grade I and some Grade II), immobilization in a cast or splint for several weeks allows the ligament to heal. Anti-inflammatory measures and hand therapy help restore motion and strength afterward.
For more significant tears, surgical options fall into a few categories depending on how much time has passed since the injury and how much arthritis has developed. Acute tears with healthy cartilage can sometimes be repaired directly, with the torn ligament sutured back together and the bones temporarily pinned in place. When the ligament tissue is too damaged for direct repair, reconstruction becomes necessary.
Reconstruction techniques include capsulodesis (tightening the wrist capsule to limit abnormal motion), tenodesis (rerouting a tendon through the scaphoid to act as a new stabilizer, as in the Brunelli procedure), and bone-tissue-bone grafts that replace the torn ligament with donor tissue still attached to small bone blocks. A comparative review found that bone-tissue-bone reconstruction produced excellent functional outcomes in about 65% of cases, compared to 28% for tenodesis and 12% for capsulodesis. Capsulodesis had the highest rate of poor outcomes at around 15%, while tenodesis was closer to 8%. None of these techniques showed significant differences in final range of motion or grip strength, but the quality of functional recovery varied meaningfully.
Recovery After Surgery
Recovery from scapholunate ligament reconstruction is slow and structured. For the first 10 to 14 days, you’ll wear a postoperative splint. That transitions to a cast for 6 to 8 weeks total. During this early phase, you’ll do gentle finger exercises to prevent stiffness but won’t move the wrist itself.
Around 6 to 12 weeks, you’ll begin wearing a removable wrist brace and start supervised range-of-motion exercises. Light seated activities without the brace may begin at 8 to 10 weeks. Resistive strengthening, sustained gripping, and any weight-bearing through the hand are off-limits until at least 12 weeks. Dynamic or demanding activities, like push-ups or lifting heavy objects, are restricted until 16 weeks or later. Return to sports or manual labor depends on the specific procedure and your surgeon’s assessment.
What Happens Without Treatment
Untreated scapholunate dissociation follows a well-documented path toward wrist arthritis called scapholunate advanced collapse, or SLAC wrist. This progresses through predictable stages over months to years.
In Stage I, arthritis develops between the scaphoid and the tip of the radius (the forearm bone), visible as a bony spur called radial styloid beaking. Stage II involves worsening arthritis spreading across the entire joint surface between the scaphoid and radius. By Stage III, the capitate bone (which sits just above the scaphoid-lunate gap) begins migrating upward into the space created by the dissociation, and arthritis develops between the capitate and lunate. Stage IV, which some surgeons consider controversial, involves arthritis throughout the entire wrist.
SLAC wrist is the most common pattern of wrist arthritis and is often the end result of a scapholunate ligament injury that was never identified or treated. The timeline varies considerably from person to person, but the progression is generally irreversible once it begins. Salvage procedures for advanced SLAC wrist, such as partial wrist fusion or removal of arthritic bones, can reduce pain but permanently limit wrist motion. That’s why early detection and appropriate treatment of scapholunate dissociation matter so much for long-term wrist function.