A Baker's cyst is a swelling on the back of the knee caused by the build-up of fluid inside sacs called bursa between the two heads of the gastrocnemius (calf muscle). The symptoms are usually mild, except in the rare situation when the cyst bursts or extends down into the calf muscles. A Baker’s cyst is also known as a popliteal cyst.
The knee is a hinge joint between the thigh bone and shin bones. The entire joint is enclosed inside a tough capsule lined with a membrane and filled with lubricating synovial fluid. Extra capsules or sacs of fluid, known as bursa, cushion the joint and help reduce the friction between tissues caused by movement.
When the knee produces too much synovial fluid, the excess fluid causes the bursa behind the knee to expand and bulge. Common causes of Baker's cyst include arthritis, infection, torn knee cartilage and other knee injuries.
Symptoms of a Baker's cyst
Often there are no symptoms and the cyst can remain unnoticed. If symptoms do occur, they can include:
- a pronounced soft lump or swelling on the back of the knee that looks most obvious when the person is standing
- a sensation of pressure in the back of the knee joint
- persistent pain or aching
- restricted mobility of the knee joint
- a sensation of tightness at the back of the knee when the leg is straightened.
Causes of a Baker's cyst
Some of the common causes of a Baker's cyst include:
- injury – trauma or injury to the knee that can cause a build-up of fluid (effusion), which triggers a Baker's cyst
- torn cartilage – usually affecting the cartilages (known as menisci) that bolster the knee joint on both sides
- arthritis – particularly rheumatoid arthritis and osteoarthritis of the knee joint
- infection – which locally can cause fluid retention around the knee joint
- unknown causes – Baker's cysts can sometimes develop in children for no apparent reason.
Complications of a Baker's cyst
A person may be less inclined to seek medical help for a Baker's cyst if the symptoms are mild – which they generally are. However, if left untreated, complications can develop that may include:
- the cyst may continue to grow, causing the symptoms to worsen
- the cyst may extend down into the calf muscles (dissection)
- the cyst may burst (rupture) and cause bruising on the ankle of the affected leg, due to leaked fluid.
The symptoms of calf dissection and cyst rupture are similar to those caused by inflammation of veins, which may make diagnosis difficult and delay treatment. It is important to seek medical advice, as more serious but less common problems can present in a similar way. These may include a tumour, deep vein thrombosis or popliteal artery aneurysm.
Diagnosis of a Baker's cyst
A Baker's cyst is diagnosed using a number of tests, including:
- physical examination
- medical history
- joint x-ray – this will not show the cyst, but can show the presence of arthritis in the knee joint, which may be causing the problem
- shining a light through the cyst (transillumination) – this can determine that the mass is filled with fluid
- magnetic imaging resonance (MRI)
Treatment for a Baker's cyst
Baker's cysts don’t always require active treatment as they can resolve on their own. Sometimes they will only require observation over time by the treating doctor.
If treatment is required, options for treatment can include:
- treatment of the underlying cause, such as medication for arthritis or surgery for torn knee cartilage
- temporarily avoiding activities that aggravate the knee joint
- physiotherapy, which may comprise of activity modification advice, heat or ice treatment, the use of crutches, and exercises to maintain mobility and strength of the knee
- cortisone injections to reduce inflammation in the knee joint
- inserting a needle into the cyst and draining off the fluid, if the cyst is large enough
- in severe cases, surgery to remove the cyst entirely if conservative treatments are ineffective.
A conservative approach of watching and waiting is recommended with children, as the condition commonly subsides on its own without active treatment.
Most people with a Baker's cyst will be able to continue going to work or school.
Prevention of a Baker's cyst
Knee joints are prone to injury during sporting activities. Preventing knee trauma can reduce the risk of a Baker's cyst developing in the first place or recurring after treatment.
- Warm up the knee joints and soft tissue by gently going through the motions of your sport or activity, and stretch your muscles before commencing your sport or activity.
- Wear supportive footwear appropriate to your activity.
- Try to turn on the balls of your feet, rather than through your knees, to help avoid knee injuries.
- Cool down after sport by performing gentle and sustained stretches.
- If you injure your knee, stop your activity immediately, apply ice packs to treat the swelling and seek medical advice.
Where to get help
- Your doctor
- Sports medicine professional
- Australian Physiotherapy Association Tel. 1300 306 622 (diverts to local branch)
- Arthritis and Osteoporosis Victoria, Tel. (03) 8531 8000 or 1800 263 265
Things to remember
- The knee contains sacs of fluid, called bursa, that help to cushion the joint and reduce friction between the structures around it.
- A Baker's cyst is a pronounced swelling on the back of the knee, caused by the abnormal collection of fluid inside the bursa.
- Treatment may include physiotherapy, drawing off the fluid with a needle, cortisone injections, treating the underlying cause, or surgery to remove the cyst.
This page has been produced in consultation with and approved by:
Arthritis Victoria incorporating Osteoporosis Victoria
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