SLAC Wrist Radiology: Radiographic Stages and Key Signs

SLAC wrist, short for scapholunate advanced collapse, is the most common pattern of degenerative arthritis in the wrist, and its diagnosis rests almost entirely on imaging. In a landmark review of 4,000 wrist radiographs, the SLAC pattern accounted for roughly 57 percent of all degenerative wrist arthritis cases, making it the single most frequent finding when other inflammatory and metabolic arthritides were excluded.¹ Because the arthritic changes follow a predictable, stage-by-stage progression visible on standard X-rays, radiologists can often identify the condition and its severity from a single posteroanterior view of the wrist. But the story gets more complicated when it comes to catching early disease, distinguishing SLAC from look-alikes, and deciding which advanced imaging techniques add genuine value.

What Causes SLAC Wrist and Why It Matters

The trouble begins with the scapholunate ligament, the tough band of tissue connecting the scaphoid bone to the lunate bone in the wrist. This is the most commonly injured ligament in the wrist, and when it tears or stretches, the normal relationship between the two bones breaks down.² The scaphoid tips forward while the lunate tilts backward, and the gap between them widens. That abnormal alignment changes how forces distribute across the wrist joint.

MRI-based modeling confirms this in concrete terms: after scapholunate ligament injury, contact pressures and forces increase at the joint surface between the radius and the scaphoid, and the bones shift farther apart than normal.³ Over months to years, that abnormal loading grinds cartilage away in a specific sequence, producing the staged arthritis pattern that defines SLAC wrist. Untreated, the result is worsening pain, stiffness, carpal instability, and eventually significant disability.

The Radiographic Stages

One reason SLAC wrist is so well suited to plain-film diagnosis is its predictable march. The original classification, published in 1984 by Watson and Ballet, described three main stages visible on standard X-rays, and this framework remains the backbone of clinical decision-making today.⁴

Stage I: Arthritis is limited to the radial styloid, the bony prominence on the thumb side of the radius where it meets the scaphoid. Joint-space narrowing and bony spurring appear here first.
Stage II: The degenerative changes spread across the entire joint surface between the scaphoid and the radius. The scaphoid has now been grinding abnormally against the radius long enough to erode the cartilage more broadly.
Stage III: Arthritis extends into the capitolunate joint, the articulation between the capitate (a central carpal bone) and the lunate. By this point the capitate has migrated upward into the space left by the collapsing carpal architecture, and the wrist’s overall alignment is severely disrupted.

A pictorial review in Insights into Imaging emphasizes that recognizing this progressive pattern is the key skill for radiologists evaluating SLAC, because each stage carries different treatment implications.⁵ Missing a Stage I presentation on an early film can mean a lost opportunity for joint-preserving surgery before the disease marches on.

Key Signs on Plain Radiographs

A standard posteroanterior wrist X-ray can reveal several hallmarks of SLAC. In a published case report, radiographs demonstrated scapholunate diastasis (the widened gap between the two bones, sometimes called the Terry Thomas sign after the gap-toothed British actor), arthritis at the radioscaphoid joint, dorsal intercalated segment instability (DISI), radial styloid beaking, and proximal migration of the capitate, all on conventional films.⁶ These features tend to cluster together and reinforce the diagnosis when more than one is present.

The Terry Thomas sign deserves extra attention because it is often the earliest clue. In adults, a gap between the scaphoid and lunate wider than about 2 to 3 millimeters on a standard posteroanterior view raises suspicion for ligament disruption. DISI, meanwhile, is assessed on the lateral view: the lunate tilts dorsally (backward) beyond its normal range, and the scapholunate angle exceeds the upper limit, typically regarded as around 60 to 70 degrees depending on the reference used. Radial styloid beaking, a bony spur that forms where the scaphoid impacts the styloid, is a subtler sign but quite specific for the early stages of SLAC.

The Scaphoid Cortical Ring Sign

One radiographic finding that gets a lot of attention in teaching files is the scaphoid cortical ring sign, a dense oval shadow that appears on the posteroanterior view when the scaphoid is abnormally flexed. In SLAC wrist, the scaphoid tilts forward after losing its ligamentous tether to the lunate, so the cortical ring sign can flag that malalignment. However, the sign is less reliable than it first appears. A study of normal wrist X-rays found that the cortical ring sign can show up in healthy wrists of any age, gender, or hand, depending on the position of the wrist during the exposure.⁷ A separate investigation evaluating over a hundred normal wrist radiographs confirmed there is a range of wrist positions where the cortical ring sign is normally visible, meaning its presence alone does not prove pathology.⁸ In practice, the sign is most useful when combined with other findings like a widened scapholunate gap or radial styloid beaking, rather than as a standalone indicator.

Stress Views and Fluoroscopy

Standard static films sometimes miss the earliest phase of instability, when the ligament is partially torn and the bones shift only under load. Stress views, taken while the patient grips or the examiner pushes the wrist into ulnar deviation, can unmask a widening of the scapholunate interval that stays hidden on a resting film. An interdisciplinary consensus statement concluded that radiographic stress views and dynamic fluoroscopy allow accurate diagnosis of dynamic scapholunate joint instability, the stage before permanent malalignment sets in.⁹ For the radiologist, this means that a normal-looking resting film does not always rule out a clinically significant scapholunate ligament problem.

When Plain Films Are Not Enough

Advanced stages of SLAC wrist are hard to miss on X-ray. Early manifestations are a different story. A review in Seminars in Musculoskeletal Radiology noted that detecting early wrist osteoarthritis frequently requires computed tomography or MRI, sometimes supplemented with arthrography.¹⁰ The question for clinicians and patients is which advanced modality offers the best look at the damaged ligament and the earliest cartilage changes.

A systematic review and meta-analysis pooling data from multiple studies compared three MRI-based approaches for detecting scapholunate interosseous ligament tears. Standard 1.5-Tesla MRI had a sensitivity of only about 46 percent, meaning it missed more than half of confirmed tears. Upgrading to a 3-Tesla scanner improved sensitivity to roughly 76 percent, and MR arthrography, where contrast is injected into the wrist joint before scanning, pushed sensitivity to about 82 percent. Specificity was high across all three, ranging from about 81 percent for 1.5T to over 90 percent for 3T and MR arthrography.¹¹ The practical takeaway is that a 1.5T MRI that looks clean does not confidently exclude a scapholunate ligament tear.

A smaller comparative study reinforced MR arthrography’s advantage, finding it detected all arthroscopically confirmed scapholunate ligament tears, versus only about 38 percent detected by standard MRI sequences.¹² CT arthrography performed similarly well in another study, achieving 100 percent sensitivity and specificity for scapholunate ligament tears, while conventional MRI lagged behind at about 66 percent sensitivity.¹³

Ultrasound offers another option, particularly for dynamic instability. One study concluded that ultrasound has high specificity and accuracy for evaluating dynamic scapholunate instability, but low sensitivity, meaning it rarely gives a false alarm but can easily miss a real injury.¹⁴ Its best role is probably as a supplement to other imaging when dynamic movement of the bones needs to be visualized in real time.

Telling SLAC Apart from SNAC and Other Mimics

SLAC is not the only progressive arthritis pattern in the wrist. Scaphoid nonunion advanced collapse (SNAC) follows a very similar radiographic progression but arises from an old scaphoid fracture that never healed, rather than from ligament injury. Both produce joint-space narrowing and bone spurring in overlapping locations, so the radiologist needs to look carefully for evidence of a scaphoid fracture line or nonunion site, and for the status of the scapholunate interval. In SLAC, the gap between the scaphoid and lunate is widened; in SNAC, the scaphoid itself is broken, and the scapholunate interval may be normal. Distinguishing the two matters because the surgical options differ in their details.

Another condition that occasionally mimics SLAC on X-ray is calcium pyrophosphate deposition disease, sometimes called pseudogout. Because this metabolic arthropathy can affect the wrist, there has been debate about whether it produces a SLAC-like pattern. A study of Japanese patients with confirmed calcium pyrophosphate crystal deposition disease found the incidence of a true SLAC pattern was very low, and no advanced (Stage III) SLAC wrist was found in that population. The researchers concluded that SLAC wrist is not a characteristic feature of the disease and that crystal deposition is unlikely to be a major cause.¹⁵ In practice, the presence of chondrocalcinosis (calcifications within the cartilage) and involvement of joints not typically affected in SLAC can help steer the diagnosis toward a crystal arthropathy instead.

When Imaging and Symptoms Do Not Match

A consistent surprise in the SLAC literature is that some patients with clear radiographic evidence of SLAC wrist have little or no pain. A study titled “Asymptomatic SLAC wrist: Does it exist?” concluded that there are patients, especially older and low-demand individuals, in whom X-ray evidence of arthritis and clinical findings do not correlate.¹⁶ This has real clinical implications. A wrist that looks alarming on X-ray may function well enough for a particular person’s daily life. The finding also means that incidental SLAC-pattern changes on a radiograph obtained for another reason, say a fall, should not automatically trigger aggressive treatment.

Conversely, some patients present with significant wrist pain and grip weakness, yet their plain films look only mildly abnormal. In those cases, the advanced imaging modalities discussed above become essential for catching the early ligament damage before the full radiographic cascade develops.

The Pediatric Problem

Interpreting the scapholunate interval in children and adolescents is a different ball game. In adults, a gap wider than 2 millimeters on a posteroanterior film raises suspicion. In growing wrists, the carpal bones are still ossifying, and the visible gap on X-ray is naturally wider. A study establishing normative values in skeletally immature patients found that the average radiographic scapholunate interval ranged from about 9 millimeters in the youngest children to about 1.6 millimeters as skeletal maturity approached.¹⁷ Applying the adult 2-millimeter threshold to a 6-year-old would flag virtually every normal wrist as abnormal. Pediatric radiologists need age-specific reference ranges to avoid both overdiagnosis and missed injuries in young patients.

AI-Assisted Detection on Radiographs

Artificial intelligence tools are beginning to assist radiologists in measuring the scapholunate interval and associated angles on plain films. A recent study tested an automated system’s ability to detect carpal instability signs. On a large dataset, the system measured the scapholunate joint distance with a mean absolute error of about 0.65 millimeters, the scapholunate angle with an error of about 8 degrees, and the capitolunate angle with an error of about 6 degrees. For detecting disruptions of the smooth carpal arcs (a sign of malalignment), the system achieved a sensitivity of 83 percent and a specificity of 64 percent.¹⁸ Those numbers are promising for distance and angle measurements, where the error margins are small enough to be clinically useful. The arc-disruption detection, however, still has a fairly high false-positive rate, suggesting the technology works better as a screening assistant than as an independent diagnostic tool at this point.

How Imaging Guides Surgical Decisions

The stage of SLAC wrist on imaging is one of the most important inputs into the surgical plan. For Stage I disease, procedures aimed at preserving motion, such as radial styloidectomy or scapholunate ligament reconstruction, may still be on the table. For Stages II and III, the main surgical debate is between proximal row carpectomy (removing the scaphoid, lunate, and triquetrum) and four-corner arthrodesis (fusing the capitate, lunate, triquetrum, and hamate after removing the scaphoid). A comparative study of these two motion-preserving procedures found that proximal row carpectomy showed a high degree of patient satisfaction and was preferred except when advanced capitolunate arthritis was present.¹⁹

That exception is critical, and it highlights a specific imaging task. The radiologist evaluating a SLAC wrist before surgery needs to look carefully at the capitolunate joint, because the presence or absence of arthritis there determines which operation the patient will get. A study of intraoperative assessment found that preoperative radiographs sometimes overstated the degree of capitolunate arthritis, leading surgeons to plan a four-corner fusion for wrists that actually had minimal capitolunate disease and could safely undergo the simpler proximal row carpectomy instead.²⁰ The takeaway for imaging interpretation is that while radiographs are excellent at mapping the overall SLAC stage, they can be unreliable at judging the exact severity of arthritis at the capitolunate articulation. In borderline cases, CT or MRI of that specific joint surface may add useful detail, and some surgeons defer the final decision to direct visualization during the operation.

The Scapholunate Ligament Injury Continuum

SLAC wrist does not appear overnight. It sits at the end of a spectrum of scapholunate ligament injury that can take years, sometimes decades, to reach the advanced collapse stage. Early in the continuum, the ligament is partially torn but the bones maintain their normal alignment at rest. This phase is sometimes called dynamic instability because the widening of the gap shows up only under stress. Standard resting radiographs look entirely normal, and even high-field MRI can miss partial tears. This is the phase where stress views and fluoroscopy become essential, as the consensus statement cited earlier affirmed.

As the ligament fails further, the malalignment becomes visible even on resting films, a phase known as static instability. The Terry Thomas sign, DISI posture, and eventually the early arthritic changes of Stage I SLAC all belong to this static phase. By the time a full SLAC pattern is established, ligament reconstruction alone is usually no longer viable, and the treatment shifts toward salvage procedures. Understanding where a patient sits on this continuum is fundamentally a radiologic question, one that requires matching the right imaging tool to the suspected phase of disease. A normal resting X-ray in a patient with dorsal wrist pain after a fall does not mean the ligament is intact; it means the instability may still be dynamic, and further imaging or stress testing is warranted.