Pain in the back of your knee most commonly comes from a Baker’s cyst, a meniscus tear, or a muscle/tendon strain in the area behind the joint. Less often, it signals a ligament injury or nerve compression. The cause usually depends on whether the pain came on suddenly or built up gradually, and whether you notice swelling, catching, or instability along with it.
Baker’s Cyst: The Most Common Cause
A Baker’s cyst (also called a popliteal cyst) is a fluid-filled sac that forms in the hollow behind your knee. It develops when excess joint fluid gets pushed through the back of the knee capsule or collects in a small pouch between two muscles. The fluid essentially gets trapped there because of a valve-like effect: when you bend your knee, pressure pushes fluid toward the back, but it can’t easily flow back into the joint when you straighten up.
The hallmark sensation is tightness, discomfort, or an aching pressure behind the knee. You might notice visible swelling that’s more obvious when you’re standing with your leg fully straight. That swelling often shrinks or disappears when you bend your knee to about 45 degrees, because flexing relieves tension on the cyst.
Baker’s cysts rarely appear out of nowhere. They’re usually a downstream effect of something else going on inside the joint: arthritis, a meniscus tear, or ligament damage that increases fluid production. This is why draining the cyst alone often doesn’t fix the problem permanently. The cyst tends to refill unless the underlying issue is addressed. When a meniscus tear is involved, the torn tissue can act as a one-way valve, pushing fluid out of the joint where it pools and thickens into a gel-like mass.
Occasionally a Baker’s cyst ruptures, sending fluid down into the calf. This causes sudden pain and swelling in the lower leg that can look almost identical to a blood clot, which is why imaging is important if your symptoms shift from behind the knee to the calf (more on that below).
Meniscus Tears
Each knee has two crescent-shaped cartilage pads (menisci) that cushion the joint. A tear in the back portion of the inner meniscus, called a posterior horn tear, can produce pain specifically behind the knee. About 85% of people with this type of tear recall a specific moment it started: a click, a sudden shock-like sensation, or a giving-way feeling followed by sharp pain. Swelling in the joint develops in the majority of cases.
You might also notice a catching or locking sensation when you bend or straighten your leg. Pain often flares during deep squatting or twisting movements. Posterior horn tears are worth knowing about because they’re easy to miss, even on imaging. The area is difficult to visualize during arthroscopic surgery, so they can be overlooked if no one is specifically looking for them.
Muscle and Tendon Strains
Several muscles converge at the back of the knee, and irritation to any of them can cause localized pain. The two most common culprits are the hamstrings (which attach just below the back of the knee) and the calf muscles (which originate just above it). Overuse, sudden increases in activity, or insufficient warm-up can strain these tendons where they cross the joint.
A smaller, deeper muscle called the popliteus sits directly behind the knee joint and helps control rotation. When this muscle’s tendon gets inflamed, you’ll typically feel tenderness along the outer-back part of the knee, and pain worsens with downhill walking or running. The pain also increases if you try to rotate your lower leg outward against resistance.
Ligament Injuries
The posterior cruciate ligament (PCL) runs through the center of your knee and prevents the shinbone from sliding backward. PCL injuries classically happen in car accidents when the knee hits the dashboard, or from falling forward onto a bent knee. In sports, football, skiing, soccer, and baseball carry the highest risk.
A PCL tear causes pain behind the knee along with a sense of instability, as if the knee might give out. One telltale sign is a visible “sag” where the shinbone drops backward relative to the thighbone when you’re lying down with the knee bent. Injuries to the structures on the outer-back corner of the knee often happen alongside PCL tears rather than in isolation.
Recovery depends on severity. With conservative treatment (rehab and bracing, no surgery), you can typically return to sport-specific exercises within 2 to 3 months, with full clearance for athletics at 4 to 6 months. Non-contact athletes sometimes return even sooner, around 6 to 8 weeks with focused rehabilitation. If surgery is needed, the timeline extends to 9 to 12 months. The good news: 80 to 91% of people treated without surgery return to their sport.
Nerve Compression
The tibial nerve runs through the back of the knee on its way to the foot. In some people, this nerve gets pinched where it passes under a band of tissue at the top of the calf muscle. The result is severe pain and tenderness directly in the hollow behind the knee, often with numbness or tingling radiating down the calf or into the foot. Tapping on the spot where the nerve is compressed reproduces the pain and tingling.
This is less common than the other causes on this list, but worth considering if your pain has a burning, electric, or shooting quality rather than the dull ache of a cyst or the mechanical catching of a meniscus tear. Some cases resolve on their own. When they don’t, releasing the tight band of tissue surgically tends to produce a full recovery.
When Back-of-Knee Pain Could Be a Blood Clot
Deep vein thrombosis (DVT), a blood clot in the vein behind the knee, can produce pain, swelling, and warmth that mimics many of the conditions above. In one study of over 3,000 patients who had ultrasound imaging, about 3% had Baker’s cysts, and 7 of those 95 patients also had a coexistent blood clot. Some cysts even compressed the vein behind the knee, which itself raises clot risk.
The overlap in symptoms is significant enough that clinical examination alone can’t reliably distinguish a Baker’s cyst from a DVT. If your posterior knee pain comes with calf swelling, redness, warmth, or if the pain worsens when you flex your foot upward, seek prompt evaluation. This is especially important if you have risk factors like recent surgery, prolonged immobility, or a history of clotting disorders.
How to Narrow Down Your Cause
A few patterns can help you sort through the possibilities before you see a provider:
- Gradual onset with visible swelling behind the knee: most likely a Baker’s cyst, especially if the lump shrinks when you bend your knee.
- Sudden onset with a click or pop, plus catching or locking: suggests a meniscus tear, particularly if squatting or twisting makes it worse.
- Pain after a direct blow or fall onto a bent knee: raises concern for a PCL injury, especially with a feeling of instability.
- Burning or tingling that radiates into the calf or foot: points toward nerve compression rather than a joint or muscle problem.
- Pain that worsens going downhill: characteristic of popliteus tendon irritation.
- Calf swelling, warmth, or redness along with knee pain: needs evaluation to rule out a blood clot.
Ultrasound is typically the first imaging tool used for posterior knee pain because it can identify cysts, fluid collections, and blood clots in a single exam. MRI becomes useful when a meniscus tear or ligament injury is suspected, since it shows soft tissue structures in detail. For most soft tissue strains and uncomplicated Baker’s cysts, initial treatment involves rest, ice, compression, and addressing any underlying joint problem driving the symptoms.