Clinical and arthrographic studies on the valve mechanism in communicating popliteal cysts.

Forty-one knee joints presenting with significant popliteal cysts were examined clinically and arthrographically. The fluid was aspirated from the cysts and replaced by contrast medium of low viscosity. On knee flexion the contrast fluid spontaneously passed over into the joint in 16 cases (no valve mechanism), in 7 small amounts of fluid were forced into the joint when the cysts were manually compressed, and in 18 no fluid passage to the joint was observed on manipulation (valve mechanism). When a valvular action was established, effusion in the joint was rare and the fluid from the cyst was usually viscous. The cysts with a valvular connection were significantly larger than those without a valve mechanism.