Noticed a soft lump or a feeling of fullness behind your knee? It is unsettling, and it sends a lot of people straight to a late-night search. The most common explanation is a Baker's cyst, also called a popliteal cyst, which is a pocket of joint fluid that has collected at the back of the knee.
Here is the part that changes how you think about it. In adults, a Baker's cyst is usually a consequence of something else going on inside the knee rather than a problem that starts on its own. It is the smoke rather than the fire. Here is what a Baker's cyst is, why it forms, what tends to help, and when it is worth getting your knee assessed.
In this article
What is a Baker's cyst?
A Baker's cyst is a fluid-filled swelling that sits at the back of the knee, in the hollow behind the joint. The fluid is the same fluid that normally lubricates the inside of your knee.
The knee joint connects with a small sac behind it, and that connection can behave a little like a one-way valve. When the knee produces extra fluid, some of it tracks backward into the sac and does not easily return, so the sac fills and becomes noticeable as a lump or a tight, full feeling behind the knee.
Why does a Baker's cyst form?
The cyst itself is a symptom. It appears because the knee is producing more fluid than usual, and the knee produces more fluid when something inside it is being irritated. Common contributors in adults include:
- Knee osteoarthritis, which is the most frequently reported association
- A cartilage or meniscal irritation or tear
- Inflammatory joint conditions
- A previous knee injury or a period of overload where the knee has become swollen
This is why simply removing the fluid, without addressing what is irritating the knee, often leads to the cyst returning. The more useful question is what the knee is reacting to.
How common are Baker's cysts?
Common enough that finding one is far from unusual, particularly if your knee has been sore or swollen.
Linked to what is happening inside the knee
In the ultrasound study of people with knee pain, the presence of a cyst was associated with sonographic features of osteoarthritis and with joint effusion, supporting the idea that the cyst reflects irritation within the joint rather than being a standalone problem.
What does it feel like?
Baker's cysts vary a lot. Some cause no symptoms at all and are found incidentally on a scan. When they do cause symptoms, people commonly describe:
- A soft lump or fullness behind the knee, often more obvious when standing with the knee straight
- Tightness at the back of the knee, particularly when bending it fully or kneeling
- An ache that is worse after activity or after long periods on your feet
- Some restriction in how far the knee will bend
Occasionally a cyst can leak or burst, which can cause sudden pain, swelling and sometimes bruising down the calf. That can look similar to other conditions, so it should be assessed promptly rather than waited out.
What helps a Baker's cyst?
Because the cyst usually reflects what is happening inside the joint, management focuses on settling the knee and addressing the underlying driver. A typical plan can include:
- An assessment to work out what is irritating the knee, with imaging arranged where it will change the plan
- Settling the current flare, including relative rest from the specific aggravating loads and advice on managing swelling
- Progressive strengthening for the quadriceps, hamstrings, calf and hip so the knee is better supported
- Restoring knee range and control, and improving how you move through squatting, stairs and walking
- A graded return to activity so load is rebuilt rather than spiked
- Onward referral for drainage, injection or a specialist opinion where symptoms persist or the underlying cause needs it
Many people find their symptoms settle as the knee itself settles. If you would like your knee assessed, our team provides physio in Mount Gravatt for people across Brisbane's southside, including hydrotherapy for those who find land-based loading uncomfortable early on.
When should you seek help?
Consider getting assessed if
You have noticed a lump or fullness behind your knee, the knee is repeatedly swelling, or the tightness is limiting how far you can bend. Seek prompt medical care if you develop sudden severe calf pain and swelling, redness or warmth in the calf, fever, or you cannot bear weight, as these need urgent assessment to rule out other conditions.
Helping You Perform at Your Best
A lump behind the knee is usually the knee telling you something rather than a problem of its own. At ACE Sports Medicine in Upper Mount Gravatt, our physiotherapists assess what is irritating the joint, settle the flare, and build the strength and load plan that keeps it settled, for people right across Brisbane's southside.