Thread by @RyanChristieMD

Wetread Case 36

Hx: Several weeks of worsening wrist pain

How specific can you be?

Please answer with professionally appropriate gif responses only. No spoilers! wetread.org #WetreadCow #FOAMrad #FOAMed #radres #errad #mskrad #orthorad

Yeah! We have a lot of widened scapholunate distance references, aka:

Terry Thomas Sign
David Letterman Sign
Michael Strahan Sign
Madonna Sign

referring to their diastema (or increased gap between their front teeth)

Answer: Dorsal Intercalated Segment Instability (DISI)

Findings:
1) Widened scapholunate distance from full thickness tear
2) Abnormal scaphoid - old fx perhaps? (m/c carpal fx goes with prior injury) 🤔
3) Dorsal rotation of the lunate on the lateral exam

Here's a normal lateral wrist for comparison. Note how the carpal bones should all line up like bowls stacked on each other. Blue=radius, Red=lunate, Orange=scaphoid, Green=capitate

The carpal bones form a complex ring which includes a 3D twisting/torque component. When the ring is disrupted, it allows the bones to un-torque ("un-twist").

Scaphoid likes to twist to the palm (volar) and the triquetrum/pisiform like to twist to the back (dorsal)

Simply, the lunate twists based upon who it is connected to. 👍

Here we have disruption of the scapholunate ligament so the lunate rotates with the triquetrum, ie dorsally

ie DORSAL intercalated segment instability

And Yes, if the lunotriquetral ligament was torn the lunate would rotate with the scaphoid to the palm (volar) and you would have VOLAR Intercalated Segment Instability (VISI)

Orthopedics like measuring things so you can measure the scapholunate angle (nl 30-60) , the capitolunate angle (nl <30) or radiolunate angle (nl <15).

Here I drew in the bone outlines and measured the capitolunate angle which is >30 degrees (ie the dorsal tilt of the lunate)

Generally these findings can be seen on X-ray. You can measure the S-L distance (nl <2-3) but it does vary upon gender. Generally >3mm abnl

If you want fancy pictures you can do an MRI to look for bone contusions or even an MR Arthrogram to eval the S-L ligament directly.

Clinically:
- Radial/dorsal wrist pain, esp w radial deviation or extension
- weakness/instability

As time goes on and the S-L ligament injury worsens/widens, it can progress to Scapholunate Advanced Collapse (SLAC wrist) where the capitate migrates proximally to fill the gap