What causes an ivory phalanx on the hand or foot radiograph?
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Answer:
An ivory phalanx is homogeneous, marked increase in the density of an entire phalanx, most often the distal phalanx of the great toe, produced by combined periosteal and endosteal new bone formation so that the medullary cavity is obliterated and the bone appears uniformly white. It is an uncommon but highly specific manifestation of psoriatic arthritis. It is almost always accompanied by nail disease in the same digit and frequently by dactylitis, reflecting the shared enthesitis of the distal interphalangeal joint, nail bed and periarticular structures that characterises the disease. The important differentials are reactive arthritis, which shares the SpA enthesitis mechanism, and chronic osteomyelitis, which usually shows cortical destruction, sequestrum or a sinus rather than uniform sclerosis; osteoid osteoma, a sclerotic metastasis and melorheostosis are rarer considerations. It should be sought alongside the other radiographic features of psoriatic arthritis: the pencil-in-cup deformity, fluffy periostitis and marginal erosions with adjacent bone proliferation.
Why is it called so?
The uniformly dense phalanx has the smooth, opaque whiteness of ivory.
Pathophysiology
Psoriatic arthritis is an enthesitis-driven disease, and in the distal digit the enthesis complex, the nail apparatus and the distal interphalangeal joint form a single functional unit. Chronic inflammation at that unit stimulates both the periosteum and the endosteum to lay down new bone, so the phalanx thickens from outside while its medullary space fills in from within, until the whole bone is of uniform cortical density. The same bone-forming tendency explains the fluffy periostitis, the enthesophytes and the ankylosis seen elsewhere in psoriatic arthritis, and distinguishes it from the purely erosive, osteopenic pattern of rheumatoid arthritis.
Alternative names: Ivory phalanx sign
Other associated named signs: Pencil-in-cup deformity, sausage digit (dactylitis), fluffy periostitis, and the mouse ear erosions of psoriatic arthritis
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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