What Causes the Pencil-in-Cup Sign in the Small Joints of Fingers or Toes on X-ray?
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The Pencil-in-Cup sign is caused by severe erosive changes in the small joints, most commonly the distal interphalangeal joints of the fingers and toes, typically seen in advanced psoriatic arthritis, especially the arthritis mutilans subtype. It results from aggressive periarticular bone resorption combined with joint space destruction on X-ray imaging. Though classically associated with psoriatic arthritis, the Pencil-in-Cup sign may also be observed less commonly in systemic sclerosis, rheumatoid arthritis, reactive arthritis, and Behçet’s disease.
Why is it called so?
The name derives from the radiological appearance on plain X-rays, where one bone end adjacent to the joint becomes sharpened and narrowed resembling a pencil tip, while the opposing bone surface is eroded and scooped out in a concave manner resembling a cup. Together, the bones look like a pencil fitting into a cup.
Pathophysiology
The sign develops due to an imbalance between bone resorption and new bone formation in the joint. Chronic inflammation from immune-mediated synovitis triggers osteoclast activation causing aggressive localized bone erosion. The distal bone cortical margins erode to a pointed “pencil” shape, while the adjacent bone surface undergoes central resorption to form a concave “cup.” Over time, this destroys joint architecture, shortens the affected digit, and can lead to joint instability and deformity characteristic of arthritis mutilans.
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