Normal Scapholunate Distance Size on Radiography
The scapholunate distance refers to the gap between the scaphoid and lunate bones as seen on wrist radiography, reflecting the integrity of the scapholunate interosseous ligament (SLIL). Accurate measurement of this interval is essential for detecting carpal instability, one of the most clinically significant and frequently missed wrist injuries.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | <3 mm |
Clinical Significance
A scapholunate distance of 3 mm or greater on a posteroanterior (PA) radiograph is considered abnormal and is often referred to as the Terry Thomas sign (or David Letterman sign), indicating disruption or attenuation of the scapholunate interosseous ligament. This finding is a hallmark of scapholunate dissociation, the most common form of carpal instability.
Untreated scapholunate instability can progress to SLAC wrist (scapholunate advanced collapse), a predictable pattern of degenerative arthritis. Diagnosis can be subtle; stress views with a clenched-fist PA projection may unmask dynamic instability not visible on standard radiographs. Bilateral comparison views are helpful, as inter-individual variation exists.
- Scapholunate dissociation — traumatic SLIL tear, often following a fall on an outstretched hand
- Perilunate dislocation/instability — high-energy injury with widened scapholunate interval
- Rheumatoid arthritis — ligamentous laxity leading to carpal instability
- SLAC wrist — chronic, degenerative sequela of longstanding scapholunate instability
- Pseudo-widening — positioning artefact or overlapping structures; always confirm on true PA view
Reference: Bowden G, McNally M, Thomas S et al. Oxford Handbook of Orthopaedics and Trauma. Oxford University Press. (2010).
Imaging Notes
Measurement is performed on a true posteroanterior radiograph with the wrist in neutral rotation, forearm pronated, and the hand flat on the detector. The scapholunate gap is measured at its widest point between the cortical margins of the scaphoid and lunate. Oblique positioning or rotation significantly affects the apparent interval and can produce false widening or narrowing.
When standard PA views are equivocal, a clenched-fist PA view increases axial load through the carpus and can reveal dynamic instability. Comparison with the contralateral wrist is recommended when measurements are borderline, as ligamentous laxity varies between individuals.