Wrist & Hand Imaging: Systematic Interpretation

This patient has clinical signs suspicious for scaphoid fracture - snuffbox tenderness, scaphoid tubercle tenderness, and typical mechanism. Initial X-rays miss 15-20% of scaphoid fractures because the fracture line may not be visible before bone resorption occurs. I would immobilize the wrist in a scaphoid cast or splint and arrange early MRI within 24-48 hours, which is now the preferred investigation. MRI has near 100% sensitivity for scaphoid fractures and can definitively confirm or exclude the diagnosis, allowing early mobilization if negative or appropriate treatment if positive. If MRI is unavailable, CT is an alternative with slightly lower sensitivity for subtle fractures. The traditional approach of repeat X-rays at 10-14 days is now considered inferior as it delays diagnosis and treatment. I would not discharge this patient without a clear follow-up plan, as missed scaphoid fractures can lead to nonunion, avascular necrosis of the proximal pole, and wrist arthritis. If imaging ultimately confirms a fracture, I would assess location (waist, proximal pole, distal) as this affects management and prognosis.

I assess carpal alignment on both PA and lateral views. On the PA view, I draw Gilula's three carpal arcs - smooth curves along the proximal row articular surfaces, distal row articular surfaces, and the proximal capitate/hamate. Disruption indicates dislocation, ligament injury, or fracture. I measure the scapholunate interval, which should be less than 3mm - widening greater than 3mm is the Terry Thomas sign indicating SL ligament injury. On the lateral view, I assess the relationship between radius, lunate, and capitate. Normally, these are co-linear when you draw lines through their long axes. The scapholunate angle is measured between lines through the scaphoid and lunate long axes - normal is 30-60 degrees. DISI, or Dorsal Intercalated Segment Instability, is characterized by: an increased scapholunate angle greater than 70 degrees, dorsal tilt of the lunate, and is typically caused by scapholunate ligament injury. The scaphoid flexes volarly while the lunate, freed from its connection to the scaphoid, tilts dorsally under the influence of the triquetrum. The capitolunate angle also increases. DISI is the more common pattern and is associated with scapholunate ligament tears, while VISI (volar tilt, SL angle less than 30 degrees) is associated with lunotriquetral injury.

Perilunate and lunate dislocations are on a spectrum of the same injury and are differentiated primarily on the lateral radiograph. In a perilunate dislocation, which is more common, the lunate remains in its normal position articulating with the radius, but the rest of the carpus - principally the capitate - is displaced dorsally relative to the lunate. On the lateral view, you see the capitate sitting dorsal to the lunate rather than articulating with it. The PA view may show disruption of Gilula's arcs and the carpus appears to 'crowd' together. In a lunate dislocation, which represents the end-stage of the injury progression, the lunate itself is displaced and characteristically tilts volarly, rotating into the carpal tunnel. On the lateral view, this creates the 'spilled teacup' appearance as the lunate rotates with its concave surface facing anteriorly. The capitate and rest of the carpus maintain their alignment with the radius. Both injuries may have associated fractures - commonly through the scaphoid waist (trans-scaphoid perilunate dislocation). I would also assess for median nerve symptoms as the volarly displaced lunate can compress the median nerve in the carpal tunnel. These injuries are often initially missed, so maintaining high suspicion on the lateral view is essential.