Baker's Cysts and Fullness in the Back of the Knee

Swelling in the back of the knee can be caused by several different things.  The one that we would worry about most is a blood clot in the vein.  This would need to be evaluated and treated by your medical doctor quickly.  Ultrasound and vascular studies are needed to evaluate this possibility. 

In terms of arthritis causes, joint fluid can leak into the back of the knee and form a fluid filled cavity.  Usually this happens in people with large joint effusions. This is called a Baker's Cyst or a Politeal Cyst.  We see these in all forms of arthritis including rheumatoid arthritis and osteoarthritis. The communication between the joint and the cyst is typically a one way valve that lets fluid into the cyst but doesn't let it back into the joint.  Sometimes if these cysts become large, they can break loose and move down the calf and leg, causing pain and bruising.

There are several different options for treatment.  Sometimes the joint itself can be injected with steroids.  This will move into the cyst and help calm down inflammation.  Sometimes the fluid is removed directly from the cyst (and sometimes injected with steroids), but this is usually done with ultrasound or other radiographic imaging to loaclize the fluid and avoid veins. When the cyst breaks down and fluid moves down the leg, surgery is sometimes required.