A Baker's cyst is a collection of fluid in the back of the knee and can cause pain and discomfort with bending the knee. So what is it caused by, how do we treat it, and how do we prevent it from coming back?
A Baker's cyst is also known as a popliteal cyst. It’s named after Dr. William Baker, a British surgeon who first identified this condition. It’s caused by the accumulation of fluid in the popliteal fossa, which is the space located at the back of the knee.
The symptoms of a Baker's cyst can vary. Some people have a visible lump that causes swelling at the back of the knee. Others notice a sensation of fullness every time they bend their knee. In most cases though, people with a Baker’s cyst experience no symptoms. Usually their cyst is identified incidentally on an MRI scan.
Now, it’s important to understand that a Baker's cyst usually develops as a secondary symptom. What I mean by this is, there is something inside the knee joint that is causing swelling. And this fluid flows towards the back of the knee, leading to the formation of the cyst. The problem with this is that the flow of fluid is unidirectional. So once the fluid collects in the popliteal fossa, it's difficult for the body to reabsorb it.
And by far the most common cause of a Baker’s cyst is arthritis, particularly osteoarthritis of the knee. Grinding, catching, and rubbing associated with arthritis leads to inflammation and fluid buildup. Meniscus tears, particularly posterior meniscus tears, can also contribute to the development of a Baker’s cyst. Tears in the meniscus disrupt the normal function of the knee. The abnormal mechanics then lead to irritation, inflammation, and fluid accumulation.
A physical exam is helpful in evaluating for both a Baker's cyst and its potential cause. With that said, if we are specifically looking for a Baker’s cyst, an ultrasound evaluation is the easiest and quickest method to confirm the diagnosis. The anatomical landmarks of the cyst are the semimembranosus tendon and the medial head of the gastrocnemius muscle.
It’s also worth mentioning that not all posterior knee pain is caused by a Baker’s cyst. This is especially true if there is no noticeable swelling. Other causes of posterior knee pain include tendinopathy of the popliteus tendon as well as myofascial trigger points within the medial and lateral heads of the gastrocnemius. Trigger points in the muscles surrounding the knee are commonly overlooked as a cause of symptoms. They often contribute to both pain and functional limitations.
Treatment options for Baker's cysts depend on their size and the presence of symptoms. Small, asymptomatic cysts usually do not need any treatment and we can get by with watching it. Again most of the time, these are found incidentally on MRI scans.
Those with mild symptoms can try applying ice to the back of the knee to reduce swelling. A compression bandage or neoprene knee brace can also help. Some people may benefit from anti-inflammatory medications such as ibuprofen or naproxen. Physical therapy and rehabilitation exercises can also help manage symptoms and prevent recurrence.
And what about larger Baker’s cysts? These tend to cause more discomfort and can hinder physical activity. Because of this, they are much more likely to need interventions. Ultrasound-guided aspiration is a fast, safe, and effective procedure. A physician can drain the cyst and then inject it with medications to try to close the cyst. Oftentimes I also inject medications into the knee joint to reduce inflammation. Because ultrasound guided therapies are so effective, surgical intervention is rarely needed.
Now there is one potential complication of a Baker’s cyst and that’s a cyst rupture. Too much pressure in the cyst can cause it to burst. This results in sudden pain and swelling that extends down into the calf. Prompt evaluation to confirm the diagnosis and then aspirate the cyst is usually necessary in order to get symptom control.
Lastly, I want to reiterate that treating the underlying cause of the Baker's cyst is crucial. Doing so will prevent the Baker’s cyst from getting larger and make sure that the cyst doesn’t reform. Osteoarthritis of the knee and meniscus tears are by far the two most common causes.