Blau syndrome: a rare cause of exuberant granulomatous synovitis of the knee

Blau syndrome (BS) is a rare autosomal dominant familial granulomatous inflammatory disease presenting in early childhood with dermatitis, arthritis and uveitis. Early-onset sarcoidosis represents the sporadic form, and both are characterised by mutations in the CARD15/NOD2 gene on chromosome 16. We describe a 38-year-old man with known BS who presented for orthopaedic review following right-sided patellar dislocation. MRI of the injured knee demonstrated diffuse synovitis and prominent fatty tissue resembling lipoma arborescens with evidence of recent patellar dislocation. Synovectomy was performed and confirmed granulomatous synovitis. Knee imaging findings are described for the first time. Combining distinct morphological bone changes with synovitis which resembles lipoma arborescens and histology which includes sarcoidal-type granulomatous synovitis should lead the radiologist and pathologist to consider the diagnosis of BS.