The Lump Behind Your Knee: What a Baker's Cyst Is Telling You

A soft, full feeling behind the knee usually gets noticed when kneeling or squatting becomes awkward. The lump itself is rarely the thing that needs fixing.

Here is where the cyst sits, why it forms, what else a lump behind the knee can be, and the calf symptoms that mean you stop reading and call someone.

Where the swelling actually sits

The soft hollow at the back of the knee is called the popliteal fossa. A Baker's cyst forms there, in a specific spot between the semimembranosus tendon on the inner side and the medial head of gastrocnemius, one of the two heads of the calf muscle.

That location is not random. There is a natural connection in that area between the knee joint and a small fluid-filled sac called a bursa. In many adults that connection is open, which gives joint fluid a route out of the knee and into the space behind it.

The StatPearls overview of Baker's cyst notes that prevalence rises with age. In adults, the condition shows up most commonly between 35 and 70 years of age. In children, the common range is much younger, 4 to 7 years, and the mechanism in children is generally different from the adult pattern.

A 2014 study published in Arthritis Research and Therapy looked at 900 subjects, 49.7 percent female, with a mean age of 62.3 years. Popliteal cysts were present in 11.7 percent. A related finding, bursitis under the calf muscle, was present in 12.7 percent. The two often appeared together: among people with a popliteal cyst, 36.2 percent also had subgastrocnemius bursitis, compared with 9.7 percent of those without a cyst.

The one-way valve

The mechanism in adults is worth understanding, because it explains why a cyst keeps coming back when only the cyst gets treated.

A meniscal tear can act as a one-way valve. Synovial fluid is pushed out of the joint space and into the cyst, and the tear does not let it travel back. Over time the trapped fluid consolidates into a gel-like material.

Two forces keep the process going. The first is the valve action itself, driven by the knee bending and straightening through the day. Every step works the pump. The second is the pressure difference between the joint space and the cyst, which favours flow in one direction.

That model has a practical consequence. If the source of the extra fluid stays active, the cyst has a supply. This is why the clinical literature is clear that treating the underlying joint disease is essential, and it is the same reason the cyst on its own is a poor treatment target.

What is going on inside the joint

The list of conditions associated with a popliteal cyst is mostly a list of things that irritate the joint lining and make it produce more fluid.

Degenerative meniscal tears sit near the top. These are the gradual, wear-related tears rather than the sudden twisting injuries, and they behave quite differently in terms of treatment, something we cover in more detail in our article on degenerative meniscus tears compared with traumatic tears.

Osteoarthritis is another common association. So are inflammatory conditions such as rheumatoid arthritis, infectious arthritis, and pigmented villonodular synovitis, a less common condition affecting the joint lining.

Because so many of these involve the joint surface and the meniscus together, the assessment usually looks beyond the lump. Range of motion, joint line tenderness, how the knee handles load, and what the person can and cannot do without pain all feed into the picture. For osteoarthritic knees specifically, exercise remains a first-line treatment, which we discuss in our article on knee osteoarthritis and exercise.

How it is examined and imaged

One clinical check has a name. Foucher's sign describes how the swelling reduces or disappears when the knee is flexed to about 45 degrees, and becomes firm again as the knee straightens. A clinician uses that as supporting evidence rather than proof.

MRI is the preferred imaging method, particularly when surgery is being considered. It shows the cyst clearly and, more usefully, shows what is happening inside the joint that might be feeding it.

Ultrasound is also used in practice and has the advantage of being quick and dynamic. Whichever method is used, the goal is the same: identify the cyst and identify the joint problem behind it.

The lump that is not a cyst

A swelling behind the knee deserves a proper look, because several other things can produce one.

The clinical differential includes deep vein thrombosis, a ganglion cyst, a haemangioma, a haematoma, a lipoma, enlarged lymph nodes, and malignancy including fibrosarcoma and liposarcoma. Most swellings behind the knee turn out to be simple popliteal cysts. The value of an assessment lies in the small number that are something else, where finding out early changes the outcome.

Deep vein thrombosis is the one with the tightest time pressure, and it is covered in its own section below because the symptoms overlap with a ruptured cyst.

Treatment, and why the source matters

Standard treatment options described in the literature include rest, anti-inflammatory medication, physical therapy, ultrasound-guided aspiration combined with a steroid injection, and surgery where needed.

Aspiration removes the fluid directly. It is useful, and it has a known limitation: recurrence is reported as higher in older patients and in those with degenerative meniscal tears. That fits the valve model. Emptying a container that is still being filled produces a temporary result.

Physiotherapy focuses on the knee as a whole. That generally means restoring range of motion, building strength around the joint so it tolerates load better, and managing the activities that provoke swelling. Where the underlying driver is a degenerative meniscus or osteoarthritis, the exercise plan is aimed at that condition, and the cyst is monitored as one of several signs of how the joint is doing. Progress is usually judged on function, along the lines described in our article on how long physiotherapy takes.

Recovery time depends on what is driving the cyst, how long it has been present, and how the joint responds to loading, so a general timeline is not particularly useful here.

When to get it looked at

Book an assessment for any new lump behind the knee, for a knee that keeps swelling after activity, or for a knee that locks, catches, or gives way.

Some symptoms need medical attention the same day rather than a physiotherapy appointment. Calf swelling, redness, warmth, or pain in the calf can indicate a deep vein thrombosis, which is a blood clot and a medical emergency. A ruptured Baker's cyst produces a very similar picture: sharp pain in the knee and calf, swelling or redness in the calf, and sometimes a sensation of water running down the calf. In either case, contact your physician or go to Lions Gate Hospital. Do not massage the calf, and do not stretch it, until a clinician has ruled out a clot.

Also see a physician promptly for a lump that is hard, growing, or does not change with knee position, for fever alongside a hot swollen knee, or for a knee that suddenly cannot bear weight.

If the swelling behind your knee has been there a while and nobody has looked inside the joint, book a 30-minute assessment and we will work out what is feeding it. You can see what we treat or reach the clinic here.

This article is general information, not personal medical advice. A regulated practitioner can confirm whether the patterns described apply to you.

Sources

• Baker's Cyst, StatPearls, NCBI Bookshelf
• Cao Y et al., Popliteal cysts and subgastrocnemius bursitis are associated with knee symptoms and structural abnormalities in older adults, Arthritis Research and Therapy, 2014
• College of Health and Care Professionals of BC public registry

This article is for general information only and does not constitute medical advice, diagnosis, or treatment. Individual presentations vary, and assessment findings and treatment plans differ from person to person. If you are experiencing severe symptoms, neurological changes (numbness, weakness, bowel or bladder changes), or a significant trauma, contact your physician or emergency services. Care at Medstar Sport Physio & Health is provided by practitioners registered with their respective British Columbia regulatory colleges.