Pain behind the knee has several common causes, ranging from fluid-filled cysts and meniscus tears to arthritis and, less commonly, blood clots. The location narrows things down more than you might expect. Unlike general knee pain, discomfort specifically in the back (posterior) area points to a shorter list of possibilities, and most of them are treatable without surgery.
Baker’s Cyst
A Baker’s cyst (also called a popliteal cyst) is one of the most recognizable causes of pain behind the knee. It’s a small, fluid-filled bump that forms in the hollow at the back of your knee joint. These cysts are benign and are never a sign of cancer.
Baker’s cysts don’t usually appear on their own. They develop when something else damages the knee joint and causes excess fluid to accumulate. The two most common underlying causes are knee arthritis and knee injuries. The cyst itself may feel like a tight, water-balloon-like lump behind your knee that gets worse when you fully straighten or bend the leg. Some people notice increased stiffness after sitting for a long time.
Treatment focuses on the underlying problem rather than the cyst itself. Anti-inflammatory medications can reduce swelling, and a cortisone injection may help in more persistent cases. Physical therapy strengthens the muscles around the knee to reduce stress on the joint. Surgery to remove the cyst is very rare.
Posterior Meniscus Tears
Each knee has two crescent-shaped pieces of cartilage (menisci) that act as shock absorbers between your thighbone and shinbone. Tears in the back portion of the meniscus, particularly the medial (inner) side, are a frequent source of posterior knee pain. These tears can happen suddenly during a twisting motion or develop gradually from wear over time.
The telltale symptoms of a posterior meniscus tear include pain in the back of the knee during deep squatting, a catching or locking sensation, and an inability to fully straighten the leg. Swelling and stiffness tend to build gradually over hours to days after the initial injury rather than appearing instantly. You may also feel a sense of instability, as if the knee could give way.
Small tears sometimes heal with rest, ice, and physical therapy. Larger tears, especially those that cause locking, may need a minimally invasive procedure to trim or repair the damaged cartilage.
Knee Arthritis
Osteoarthritis doesn’t just cause pain in the front of the knee. Research published in Osteoarthritis and Cartilage found that among people with medial knee osteoarthritis, about 11% had a combined medial-posterior pain pattern, and posterior pain was one of the three most commonly affected regions overall. The most frequent pattern (41% of patients) was diffuse pain spreading across multiple areas, which often includes the back of the knee.
Arthritis-related posterior pain typically feels worse after periods of inactivity and improves somewhat with gentle movement. It tends to worsen over weeks and months rather than appearing overnight. If you’re over 50 and the pain has been creeping up gradually without a specific injury, arthritis is a strong possibility.
Hamstring and Calf Muscle Strain
The hamstring tendons attach just behind the knee, and the two heads of the calf muscle (gastrocnemius) originate there as well. Tightness or strain in either muscle group can produce aching or sharp pain in the back of the knee, particularly during walking, running, or climbing stairs. This is especially common in people who’ve recently increased their activity level or spent long periods sitting with their knees bent.
Unlike joint problems, muscle-related pain tends to feel better with gentle stretching and worse with sudden movements. It also typically improves within a few days to a couple of weeks with rest and gradual return to activity.
PCL Injuries
The posterior cruciate ligament (PCL) sits deep inside the knee and prevents the shinbone from sliding too far backward. PCL injuries are less common than ACL tears but can cause significant pain and instability behind the knee. They most often happen from a direct blow to the front of the bent knee, like hitting a dashboard in a car accident or landing hard on a bent knee during sports.
A PCL injury typically causes immediate pain in the back of the knee, swelling, and a feeling that the knee is unstable or “loose.” Walking downhill or down stairs often makes it worse. Mild PCL sprains can heal with bracing and physical therapy, while complete tears sometimes require surgical reconstruction.
When Back-of-Knee Pain Signals Something Serious
Most causes of posterior knee pain are orthopedic and manageable, but one condition deserves urgent attention: deep vein thrombosis (DVT). A blood clot in the popliteal vein, which runs directly behind the knee, can mimic musculoskeletal pain. The key differences are that DVT pain is typically accompanied by visible swelling in the calf or entire leg, warmth and redness over the affected area, and pitting edema (when pressing the swollen skin leaves a temporary dent).
Your risk is higher if you’ve been immobile for extended periods, recently had surgery, have active cancer, or have a history of blood clots. If one calf is noticeably more swollen than the other (a difference of more than 3 centimeters is a clinical red flag), or if the pain came on without any injury and your leg looks swollen and discolored, seek medical evaluation promptly. Untreated DVT can become dangerous if a clot breaks loose and travels to the lungs.
Exercises That Help
For most non-traumatic causes of posterior knee pain, strengthening the muscles that support the knee can make a significant difference. Physical therapists at the Hospital for Special Surgery recommend targeting not just the knee itself but also the core and glutes, since weakness in those areas shifts extra load onto the knee joint.
A few effective exercises you can start at home:
- Hamstring stretch: Lie on your back, grasp behind one thigh with both hands, and raise the leg with your foot flexed. Straighten the leg as much as you can without locking the knee and hold the stretch. This directly addresses tightness in the muscles that attach behind the knee.
- Side leg raise: Lie on your side with legs straight. Slowly lift the top leg toward the ceiling, pause, then lower it. This strengthens the gluteal muscles that stabilize the hip and reduce knee strain.
- Single-leg lift: Lie on your back with both legs straight. Flex one foot slightly and lift the leg into the air, pause, then lower slowly. This builds quadriceps strength without bending the knee.
- Quadriceps stretch: Stand with feet shoulder-width apart. Bend one knee and bring your heel toward your buttock, holding your foot with your hand. Hold the stretch, then lower. Tight quads pull on the kneecap and change how forces distribute through the joint.
Start with a few repetitions of each and increase gradually. If any exercise makes your pain sharply worse (not just mildly uncomfortable from the stretch), stop and get the knee evaluated before continuing. Pain that persists beyond two to three weeks, wakes you at night, or comes with swelling, locking, or giving way is worth getting assessed with imaging to rule out structural damage.