Foucher's sign of the Baker's cyst.

We investigated the mechanism of Foucher's sign, the change in pressure in the Baker's cyst with extension and flexion of the knee, by echography, arthrography, and computed tomography. With extension the gastrocnemius and the semimembranosus muscles approximate each other and the joint capsule compressing the cyst against the deep fascia. Opposite effects in flexion allow the cyst to relax.