A Baker's cyst is a fluctuant swelling in the popliteal fossa that sometimes occurs in patients with arthritis of the knee.

The term 'cyst' is a misnomer because it is in fact a synovial sac bulging from the back of the joint. If there is herniation of the sac then it may leak into the calf can cause pain and swelling that is similar to that seen in calf vein thrombosis.

Diagnosis and discrimination from deep vein thrombosis via ultrasound. Note that a Baker's cyst and deep vein thrombosis may co-exist.

Treatment of this condition is directed towards the underlying cause. A short term measure is aspiration and intra-articular injection of corticosteroid which will reduce the effusion and relieve discomfort.