Baker's cyst

The lump that the patient can feel is usually on the inner aspect at the back of the knee where the knee bands.	The cyst may become tense and rupture, with the fluid tracking into the back of the calf, mimicking a deep vein thrombosis.	Fluid in the Baker's cyst shows up in white in this MRI scan towards the right of the image. You can see its relationship to the gastrocnemius muscle which is just below it.

What causes a Baker's cyst?

A Baker's cyst can appear suddenly and be experienced as a painful lump on the inner aspect of the crease at the back of the knee. A weak area in the bursa (pocket) between the gastrocnemius and semimembranosus tendons of the muscles at the back of the knee can break down, allowing a valve-like flap that allows joint fluid to enter but not escape. It is often associated with damage inside the knee, such as a torn meniscus or osteoarthritis, where there is increased fluid pressure in the joint.