Introduction

A Baker cyst (also known as popliteal cyst) is swelling caused by fluid from the knee joint protruding to the back of the knee. It is a benign condition without malignant tendency.

 

Anatomy

The lubricating fluid (synovial fluid) produced by the knee joint passes in and out of various tissue pouches (bursa) throughout the knee. For certain reason, if the knee produces too much fluid, some of it may seep out or push through the lining, forming a sac. Baker cyst is the sac that forms between two muscles called gastrocnemius and semimembranosus around the back of knee.

 

Cause

Any cause that irritates the knee joint to produce excessive synovial fluid may result in Baker’s cyst formation. These include:

  • degenerative osteoarthritis
  • inflammatory arthritis, e.g. rheumatoid arthritis
  • trauma e.g. meniscal tear
  • gout, pseudogout

 

Symptoms

  • A swelling behind the knee (the commonest presentation)
  • A sensation of pressure behind the knee which can go down into the calf muscle.
  • Difficult to bend the joint.
  • Aching and tenderness after exercise.

If the cyst ruptures, it may cause severe pain around the leg, mimic a condition called deep vein thrombosis (a blood clot in the leg’s vein)

 

Diagnosis

The diagnosis of Baker’s cyst can usually be done by doctor’s examination. One typical test is to transilluminate the lump with a torch. We should see a red glow around the lump, indicating that it is filled with fluid. Ultrasound and MRI can further confirm the diagnosis.

Non-surgical treatment

If the cyst does not cause any problem, the doctor may prefer to observe. But if it is symptomatic, there are several options of treatment:

Physiotherapy

Ice, a compression wrap may help reduce pain and swelling. Gentle mobilisation and strengthening exercises for the muscles around the knee may reduce the symptoms and preserve knee function.

Fluid drainage

The fluid from the knee joint can be drained by a needle. This is called needle aspiration and can be performed with or without the help of ultrasound

Steroid injection

Injecting steroid into the area of the cyst may reduce swelling and pain, and it can be done after needle aspiration

Nonsurgical treatments are usually effective when the underlying cause of the cyst is addressed. In other words, the cause of arthritis, gout, or the intra-articular pathology of the knee need to be tackled.

Surgical treatment

If the nonsurgical treatment fails, patient may consider to have their cyst excised. This can be done either by open surgery or arthroscopic excision

Arthroscopic

The surgeon can put in the camera and the tools through very small wounds made over the knee, such that the communication between the cyst and the knee joint can be enlarged to prevent the one way valve effect. However, it may be difficult to identify the communication.

Open surgical excision

It is indicated if conservative measures or arthroscopic intervention fail. An incision in the skin over the cyst, at the back of the knee is made. The stalk leading from the cyst down to the joint can often be located and sutured over or cauterised, after which the cyst can be removed

The most common complication is recurrence of the cyst. The other complication is the neurovascular injuries due to the close proximity to the usual site of baker’s cyst.

 

Rehabilitation

After surgery, daily activities and work can be resumed if the patient can tolerate. We advise patient to elevate the knee for several days in order to avoid swelling. Vigorous exercise should be avoided for the initial few weeks. Physical therapy can help to mobilise the knee joint and strengthen the muscles.