What causes the cortical ring sign in the wrist on plain radiographs?
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Answer:
The cortical ring sign in the wrist occurs due to flexion or rotation of the scaphoid bone, most commonly associated with scapholunate ligament injury or scapholunate dissociation. This ligamentous disruption leads to abnormal alignment and palmar flexion of the scaphoid, causing the scaphoid’s cortical outline to appear as a ring on the posteroanterior wrist radiograph. It is an indicator of carpal instability and may be seen in scaphoid fractures as well.
Why is it called so?
It is called the cortical ring sign because the palmar flexion or rotary subluxation of the scaphoid causes the cortical bone of the scaphoid waist to be viewed end-on on radiographs, producing a rounded, tubular, or ring-like cortical density resembling a ring.
Pathophysiology
Normally, the scaphoid lies along its long axis, with the cortical edges seen longitudinally on radiographs. When the scapholunate ligament is disrupted, the distal scaphoid fragment undergoes palmar flexion and internal rotation. This change in orientation causes the cortical bone of the scaphoid waist to be seen in cross-section rather than along its length, producing the distinctive ring appearance. This reflects an abnormal mechanical and kinematic state of the wrist caused by ligamentous instability.
Alternative names:
Signet-ring sign
Other associated named signs:
Widened scapholunate gap (“Terry Thomas sign”), rotary subluxation of the scaphoid
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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