Scapholunate Instability
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Direct answer
A fall on the outstretched hand that leaves the wrist puffy and aching for weeks, with tenderness exactly over the scapholunate interval, may have torn the strongest ligament in the wrist. The scapholunate ligament couples the two bones that govern carpal motion; its dorsal band is the biomechanically strongest part, and once it fails the scaphoid flexes palmar-ward while the lunate tilts dorsally — dorsal intercalated segment instability (DISI). It is the commonest carpal instability pattern. Recognition on plain films — a widened scapholunate gap, an angle above 60 degrees, the cortical ring sign — or on dynamic views and arthroscopy before arthritis develops decides whether a repairable ligament can still be saved.
What you must remember
- Ligament anatomy: the dorsal band of the scapholunate ligament is the strongest, the palmar the weakest, with a proximal fibrocartilaginous membranous portion — the dorsal band's failure drives DISI.
- Numbers that decide: scapholunate gap over 3 mm on the posteroanterior view is the Terry-Thomas (David Letterman) sign; the normal scapholunate angle is 30-60 degrees (mean about 47), and above 60 degrees with a dorsally tilted lunate defines DISI.
- The ring sign: a flexed scaphoid throws a cortical ring (signet-ring) shadow on the PA view — a secondary clue, not a separate lesion.
- Bedside test: Watson scaphoid shift — pressure on the scaphoid tubercle while the wrist moves from ulnar to radial deviation; a painful clunk as the scaphoid reduces is positive.
- Stage ladder: predynamic (normal films, positive shift) → dynamic (gap opens only on clenched-fist PA view) → static (Terry-Thomas plus DISI at rest) → SLAC wrist, the degenerative end state.
- SLAC progression: stage I radial styloid-scaphoid arthritis, stage II scaphocapitate involvement, stage III capitolunate — the radiolunate joint is characteristically spared, which is exactly why proximal row carpectomy and four-corner fusion work.
- Treatment defaults: acute tears do best with early ligament repair and pinning; chronic static instability needs reconstruction or a limited fusion; SLAC wrist gets proximal row carpectomy (early) or scaphoid excision with four-corner fusion (later).
Reading the wrist step by step
A 26-year-old man fell off a motorcycle three months ago; the "sprained wrist" never settled. Tenderness sits just distal and ulnar to Lister's tubercle, in line with the scapholunate interval, and the Watson test clunks. Plain PA and lateral films show a 2 mm gap and a 55-degree angle — borderline. The next step is the clenched-fist PA view: axial load pops the scaphoid away from the lunate and the gap gapes past 4 mm, making dynamic instability static-under-load. Wrist arthroscopy grades the ligament (Geissler grading, I to IV) and confirms which bands are torn.
Predynamic and dynamic tears in a symptomatic wrist justify arthroscopic debridement with or without pinning; a repairable acute tear, ideally within weeks of injury, is repaired directly with bone anchors and temporary K-wires. An established static deformity in a young labourer is treated with a reduction and ligament reconstruction or a scaphotrapeziotrapezoid fusion to unload the scaphoid. The patient who presents years later with a stiff, arthritic wrist follows the SLAC map: intact radiolunate cartilage permits proximal row carpectomy, and pan-midcarpal change shifts the choice to scaphoid excision with four-corner fusion.
Where students slip
Two confusions cost marks. First, the signet-ring sign is nothing but the flexed scaphoid of the same instability — students list it as a separate finding alongside Terry-Thomas as though two diseases coexist. Second, DISI versus VISI: the scapholunate tear tilts the lunate dorsally (DISI, angle over 60), while a lunotriquetral tear tilts it volarly (VISI) — quoting the direction with the ligament is the viva answer. The exam also loves why SLAC surgery works: the spared radiolunate joint makes proximal row carpectomy and four-corner fusion possible. In Indian practice the typical patient is a two-wheeler commuter treated as a sprain for months; a wrist still painful at six weeks deserves at minimum a clenched-fist film.
Frequently asked questions
What is the Terry-Thomas sign?
A scapholunate gap wider than 3 mm on the PA radiograph, named for the comedian's gap-toothed grin; the same finding on a clenched-fist view unmasks dynamic instability.
What scapholunate angle defines DISI?
An angle above 60 degrees between the long axes of scaphoid and lunate, with the lunate tilted dorsally on the lateral film; the normal range is 30-60 degrees.
What is the Watson scaphoid shift test?
Pressure over the scaphoid tubercle during radial deviation blocks scaphoid flexion, and the scaphoid reduces with a painful clunk when pressure is released — positive in scapholunate instability.
Which joint is spared in SLAC wrist and why does it matter?
The radiolunate joint, which is why proximal row carpectomy and four-corner fusion remain salvage options even in stage III disease.
What does a clenched-fist PA view add?
Axial loading forces the scaphoid away from the lunate through the torn ligament, revealing a diastasis that resting films miss in dynamic instability.