Pain behind the knee has several possible causes, ranging from fluid-filled cysts and tendon injuries to ligament tears and, less commonly, blood vessel problems. The location is a crossroads of tendons, ligaments, muscles, nerves, and a major artery, so pinpointing the source matters. Here’s what could be going on and how to tell the difference.
Baker’s Cyst
A Baker’s cyst is one of the most common reasons people feel a distinct swelling or tightness behind the knee. It forms when synovial fluid, the natural lubricant inside your knee joint, builds up and spills into a small sac called a bursa at the back of the knee. You can sometimes feel it as a soft, fluid-filled lump that gets more noticeable when you straighten your leg.
A Baker’s cyst is almost always a secondary problem. Something else in the knee, usually arthritis or a cartilage tear, causes the joint to overproduce fluid. That excess fluid has to go somewhere, and it collects behind the knee. Treating the cyst alone without addressing the underlying knee issue often means it comes back. Small cysts may cause no symptoms at all and are discovered incidentally on imaging. Larger ones can create a feeling of pressure, stiffness when bending the knee, or aching that worsens with activity. If a cyst ruptures, fluid can leak into the calf and cause sudden pain and swelling that mimics a blood clot, which is why imaging is important if that happens.
Hamstring Tendonitis
Your hamstring muscles run down the back of your thigh and attach near the knee through tendons. When those tendons become irritated or inflamed, the pain often settles right at the back of the knee or just above it. This is especially common in runners, cyclists, and anyone who repeatedly bends and straightens the knee under load.
The symptoms tend to build gradually rather than appear all at once. You might notice a dull ache around the back of the thigh or knee, stiffness when walking or bending, and weakness in the back of your leg. A sudden overstretch can cause a sharper pain. Swelling is more likely right after an acute injury. Some people also describe tingling that travels from the lower back down the leg, which can overlap with sciatica symptoms and sometimes leads to confusion about the real source of the problem.
Rest, ice, and gradual strengthening exercises are the standard approach. Most cases improve within a few weeks if the aggravating activity is modified early enough. Pushing through the pain tends to turn a short-term irritation into a chronic one.
PCL Injuries
The posterior cruciate ligament (PCL) sits at the back of the knee and prevents the shinbone from sliding too far backward. It’s less commonly injured than the ACL, but a hard blow to the front of the knee (like hitting a dashboard in a car accident) or a fall onto a bent knee can tear it. Athletes in contact sports are also at risk.
A PCL tear often causes pain and swelling behind the knee, along with a feeling of instability, as if the knee might give way. Mild tears may only produce vague discomfort and stiffness, making them easy to dismiss at first.
Recovery depends on severity. Many partial PCL tears heal without surgery using bracing and physical therapy, though it can take a few months to regain full strength. If surgery is needed for a complete tear, expect at least six months of recovery including physical therapy to restore range of motion and stability.
Meniscus Tears
Your knee has two C-shaped pieces of cartilage called menisci that act as shock absorbers between your thighbone and shinbone. A tear to the back portion of a meniscus can send pain specifically to the back of the knee. These tears happen during sudden twisting movements, whether from a sports pivot or an everyday slip on a wet floor. In older adults, the cartilage weakens over time, and even a minor awkward movement can cause a tear.
Along with pain, you might notice swelling, a catching or locking sensation when bending the knee, and difficulty fully straightening the leg. Small tears sometimes heal with rest and physical therapy. Larger or mechanically problematic tears may need arthroscopic surgery to trim or repair the damaged cartilage.
Arthritis
Both osteoarthritis and rheumatoid arthritis can produce pain behind the knee. Osteoarthritis develops as cartilage wears down over years, and the resulting inflammation and bone changes can affect any part of the joint, including the back. Rheumatoid arthritis is an autoimmune condition where the immune system attacks joint tissue, causing swelling, stiffness, and pain that can radiate to the back of the knee.
Arthritis-related posterior knee pain typically comes with morning stiffness that eases as you move, gradual worsening over months or years, and flare-ups during cold weather or after prolonged inactivity. It’s also one of the most common triggers for Baker’s cysts, so the two conditions frequently appear together.
Calf and Leg Muscle Strains
The calf muscles attach just above the back of the knee, and a strain in the upper calf can feel like it’s coming from the knee itself. Similarly, a pulled hamstring lower on the thigh can send pain into the area behind the knee. These injuries usually follow a sudden sprint, jump, or change of direction, and you’ll often remember the exact moment it happened.
Muscle strains typically cause sharp pain during the injury, followed by soreness, bruising, and difficulty putting weight on the leg. Mild strains resolve in two to four weeks with rest and gradual stretching. More severe tears that involve significant muscle fiber damage can take considerably longer.
Nerve Compression
The tibial nerve passes through the back of the knee in an area called the popliteal fossa. If it gets compressed or entrapped, you may feel pain behind the knee along with numbness or tingling in the sole of your foot. In more significant cases, there can be weakness in foot muscles, particularly difficulty pointing your toes downward or curling them.
Nerve-related knee pain is less common than the musculoskeletal causes above, but it’s worth considering if your symptoms include burning, tingling, or numbness rather than the dull ache or sharp mechanical pain typical of tendon and ligament injuries.
Blood Vessel Problems
Two vascular conditions can cause pain behind the knee, and while both are uncommon, they deserve attention because the consequences of missing them can be serious.
A popliteal artery aneurysm is a widening of the main artery behind the knee. Many people have no symptoms at first. When symptoms do appear, they often start as pain in the lower leg during walking, along with knee pain, swelling behind the knee, or a pulsing sensation you can feel with your fingers. The real danger is blood clots forming inside the aneurysm and cutting off circulation to the lower leg. Warning signs of that complication include skin color changes, cold skin, numbness, loss of a pulse behind the knee, and inability to move the foot. This is a medical emergency.
Deep vein thrombosis (DVT), a blood clot in one of the deep veins of the leg, can also cause pain behind the knee along with swelling, warmth, and redness in the calf. Risk factors include recent surgery, long periods of immobility (like a long flight), pregnancy, and certain medications. If you have sudden swelling in one leg with pain behind the knee and no obvious injury to explain it, getting evaluated quickly is important because clots can travel to the lungs.
How the Cause Gets Identified
Your doctor will start with a physical exam, pressing on the muscles and tendons behind the knee to check for tenderness and swelling. They’ll ask you to bend, straighten, and rotate the leg in specific ways. For suspected ligament injuries, one key test involves pushing the shinbone backward while the knee is bent to 90 degrees. If the bone slides back more than it should, that points to a PCL injury.
Imaging usually comes next. An X-ray can rule out fractures and show arthritis. An MRI gives a detailed look at soft tissues like tendons, menisci, ligaments, and cysts. If a vascular problem is suspected, ultrasound is the go-to tool for checking blood flow and spotting clots or aneurysms.
The combination of where exactly the pain is, how it started (sudden injury versus gradual onset), what makes it worse, and whether there are additional symptoms like swelling, locking, numbness, or skin changes usually narrows the diagnosis quickly. Gradual pain with a visible lump often points to a Baker’s cyst. Pain after a twisting injury suggests a meniscus tear. Pain with numbness in the foot raises the question of nerve involvement. And sudden calf swelling without a clear injury warrants urgent evaluation for a clot.