Monoarticular chronic synovitis in a child.

Lipoma arborescens is a villous lipomatous proliferation of the synovial membrane characterized by chronic and painless synovial effusion. The aetiology is unknown. It has to be included in the differential diagnosis of chronic monoarticular disease in childhood. Magnetic resonance imaging provides a highly efficient tool for the diagnosis of this very rare condition. This is indeed the fourth paediatric case reported. Rather than resorting to the often inconvenient surgical synovectomy commonly recommended, we chose to treat the knee of this 13-yr-old boy with intra-articular osmic acid.