Treatment of the popliteal cyst in the rheumatoid knee.

THEORY It is axiomatic that active rheumatoid disease may produce a large effusion into the knee, and it has been shown that quadriceps contraction causes a large rise in intracapsular pressure (Jayson and Dixon 1970a). The weakest point of the capsule yields to the rise in pressure: protrusion occurs into the popliteal fossa and may track into the calf. Elimination of the cause of fluid production should, therefore, result in the disappearance of the cyst.