What causes Pencil-in-cup deformity in the peripheral joints on plain radiographs?
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Pencil-in-cup deformity results from periarticular bone erosion and resorption predominantly affecting the distal interphalangeal joints and other small joints of the hands and feet. It is classically associated with psoriatic arthritis (particularly the arthritis mutilans subtype), an inflammatory arthropathy linked to psoriasis, but may also be seen in other conditions such as rheumatoid arthritis, reactive arthritis, systemic sclerosis, and Behçet disease — it is a suggestive but not entirely specific finding. The deformity reflects severe joint damage involving bone loss and remodeling.
Why is it called so?
The name “pencil-in-cup” describes the radiographic appearance where the end of a phalanx is tapered and pointed like a pencil, fitting into a concave erosive lesion on the adjacent bone that resembles a cup. This distinct shape resembles a pencil resting inside a small cup on X-rays.
Pathophysiology
Chronic inflammation in psoriatic arthritis leads to synovial proliferation, osteoclastic bone resorption, and erosions at affected joint margins. Progressive joint destruction causes the distal bone to become tapered and eroded (the pencil), while the adjacent bone margin develops a saucer-shaped erosion (the cup), producing the characteristic interdigitating deformity visible on imaging.
Alternative names: None (also referred to as pencil-in-cup sign)
Other associated named signs: None
References
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Psoriasis
Correct! What other signs are seen in psoriasis?