Baker's cyst

A Baker's cyst or a popliteal cyst is a synovial fluid filled swelling in the popliteal fossa (1) that sometimes occurs in patients with arthritis of the knee.

Frequently it occurs between the tendons of the medial head of the gastrocnemius muscle and the tendon of the semimembranosus muscle on the medial side of the popliteal fossa, slightly distal to the centre crease of the knee (1,2)

The term 'cyst' is a misnomer because it is not a true cyst but in fact a distention of the gastrocnemius-semimembranosus bursa bulging from the back of the joint (2). 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.

It can be classified anatomically and clinically as

• primary – there is no communication between the distension of the bursa and the knee joint, no associated knee derangement, majority are seen in children (1)
• secondary – communicates freely between the bursa and the knee joint, almost all popliteal cysts are secondary (1)

The prevalence rate of popliteal cysts in

• adults were between 5 - 19% (according to a large series of knee MRI)
• children were 6.3% (according to a study of child knee MRI) (1)

There are two age-incidence peaks observed in patients with popliteal cysts - the first one from 4 to 7 years and the other from 35 to 70 years (2).

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.

Reference:

1. Fritschy D et al. The popliteal cyst. Knee Surg Sports Traumatol Arthrosc. 2006;14(7):623-8
2. Handy JR. Popliteal cysts in adults: a review. Semin Arthritis Rheum. 2001;31(2):108-18