Knee arthropathy in ochronosis: diagnosis by arthroscopy with ultrastructural features.

Knee arthroscopy in a patient with undiagnosed chronic monoarticular arthritis revealed dark pigmentation of the snyovium; synovial biopsy revealed histologic and ultrastructural features characteristic of ochronosis. Synovial fluid (SF) was non-inflammatory, without pigmented shards of cartilage; calcium pyrophosphate dihydrate crystals were absent in both the SF and biopsy specimen. Homogentisic acid was detectable in the urine by thin layer chromatography, and asymptomatic spondylosis with intervertebral disc calcification was found. The negative family history, lack of mucocutaneous pigmentation and failure of the urine to spontaneously darken obscured the diagnosis, which was easily made by arthroscopy.