Knee pain has dozens of possible causes, but most cases come down to a handful of common conditions tied to where exactly you feel the pain, how it started, and what makes it worse. The knee is the largest joint in your body, held together by ligaments, cushioned by cartilage, and powered by tendons and muscles that all have their own ways of failing. Pinpointing your pain’s location is the single most useful step in narrowing down what’s going on.
What the Location of Your Pain Tells You
Pain at the front of your knee, around or behind the kneecap, points toward a different set of problems than pain on the inner side, outer side, or back of the knee. This isn’t a perfect diagnostic tool, but it’s a reliable starting point.
Front of the knee (around the kneecap): The most common culprit here is patellofemoral pain syndrome, sometimes called runner’s knee. It can also come from cartilage softening under the kneecap, a tracking problem where the kneecap doesn’t sit properly in its groove, or irritation of the fat pad just beneath the kneecap. Arthritis that specifically affects the underside of the kneecap also causes pain here.
Inner side of the knee: This area is home to the medial collateral ligament (MCL), which stabilizes the joint. A sprain or tear from a blow to the outside of the knee can cause sharp inner pain. Meniscus tears also commonly cause inner knee pain, along with a condition called pes anserine bursitis, which is inflammation of a fluid-filled sac where three hamstring tendons meet the shinbone.
Back of the knee: A Baker’s cyst is one of the more recognizable causes. It’s a fluid-filled sac that forms when the knee produces too much lubricating fluid, usually because of arthritis or a cartilage tear. It creates a visible bulge and a feeling of tightness that gets worse with activity or prolonged standing. Calf tendon problems and cruciate ligament injuries can also cause posterior pain.
Runner’s Knee: The Most Common Cause
Patellofemoral pain syndrome is by far the most frequent reason for knee pain in active people. It typically feels like a dull ache at the front of the knee or along the sides of the kneecap, and it can shift to a sharp or stabbing sensation when you run, jump, squat, or use stairs. Going downhill or down stairs is especially painful.
The pain tends to flare when you bend your knees deeply, stand up after sitting for a long time with bent knees, switch to a new training surface, ramp up your activity level, or wear different shoes during exercise. Sometimes the kneecap simply doesn’t fit well into the groove in the thighbone, which can cause symptoms even without overuse. Weak or tight quadriceps muscles are a major contributor because they can’t properly guide and support the kneecap during movement.
Most people with runner’s knee improve with targeted strengthening of the muscles around the hip and thigh, stretching tight quads and hamstrings, and temporarily reducing the activity that triggered the pain. It doesn’t usually require imaging or surgery.
Meniscus Tears vs. Ligament Tears
If your pain started suddenly during a twist, pivot, or impact, you may be dealing with a structural injury. The two most common are meniscus tears and ACL tears, and they feel noticeably different.
A meniscus tear, which involves the rubbery cartilage that cushions the joint, causes swelling that builds gradually over two to three days. The knee often feels stiff or locked, like it can’t fully straighten or bend. You might feel a catching sensation when you move. An ACL tear, by contrast, causes immediate swelling, often within hours. The knee typically feels unstable, like it might give out, but it doesn’t lock up the way a meniscus tear does.
For degenerative meniscus tears (the kind that develop gradually rather than from a single injury), physical therapy performs just as well as surgery. A randomized trial of 100 patients with meniscus tears found that at two years, knee function scores were identical between the surgery group and the physical therapy group: 78 out of 100 in both. Nearly 60% of patients assigned to physical therapy never needed surgery at all. This doesn’t apply to every tear, but it means surgery isn’t automatically the first step.
Osteoarthritis and Wear-Related Pain
If you’re over 45 and your knee pain has been creeping in over months or years, osteoarthritis is a likely explanation. The cartilage that lines the joint surfaces gradually wears down, leading to bone-on-bone friction. Early on, you may only notice pain after long walks or heavy activity. As it progresses, pain starts showing up at rest and even at night. Stiffness in the morning that loosens up within 20 to 30 minutes is a classic sign.
More advanced arthritis can cause swelling after activity, a grinding or catching sensation, and visible changes in how straight your leg can extend. X-rays can confirm the diagnosis, but many people with arthritis visible on imaging have minimal symptoms, and some people with significant pain have only mild imaging findings. How the knee feels matters more than what it looks like on a scan.
Weight management, regular low-impact exercise (swimming, cycling, walking), and strengthening the muscles around the knee are the foundation of arthritis management. Every pound of body weight translates to roughly three to four pounds of force on the knee joint during walking, so even modest weight loss can meaningfully reduce pain.
Bursitis: The Squishy Swelling
If the front of your knee is visibly swollen and the swelling feels soft or squishy when you press on it, you likely have prepatellar bursitis. The bursa is a small fluid-filled sac that sits in front of the kneecap and acts as a cushion. It becomes inflamed from repetitive kneeling (sometimes called “housemaid’s knee”), a direct blow, or infection.
Some people feel achiness even at rest, while others only notice tenderness when kneeling or bending the knee. Severe cases can limit your ability to fully bend or straighten the joint. Most bursitis resolves with rest, compression, and avoiding the activity that caused it. If the area is red, hot, and you have a fever, the bursa may be infected, which needs prompt medical attention.
When the Problem Isn’t Your Knee at All
One of the more surprising causes of knee pain is a hip problem. Osteoarthritis of the hip commonly sends pain radiating down the thigh and into the knee. If your knee hurts but doesn’t swell, doesn’t feel unstable, and doesn’t respond to typical knee treatments, your hip may be the actual source. This referred pain happens because the hip and knee share overlapping nerve pathways. A physical exam that includes hip range of motion testing can usually sort this out.
How to Handle a New Knee Injury
The traditional advice of rest, ice, compression, and elevation (RICE) has been updated. Sports medicine experts now recommend a two-phase approach. In the first one to three days, protect the knee by limiting movement, elevate it above heart level, use compression with a bandage or sleeve, and avoid anti-inflammatory medications. That last point surprises many people, but inflammation is the body’s natural repair process, and suppressing it early on, especially at high doses, can slow long-term tissue healing.
After those first few days, the focus shifts to gradual loading. Start pain-free movement and light aerobic exercise as soon as symptoms allow. This increases blood flow to the injured area and prevents the stiffness and weakness that come from prolonged rest. Structured exercise is one of the strongest evidence-based treatments for knee injuries, reducing both pain and the risk of reinjury.
Pain That Needs Urgent Attention
Most knee pain is not an emergency, but a few patterns should send you to a doctor quickly. Severe pain that comes on rapidly in a joint that becomes swollen, warm, and possibly discolored, especially if you have a fever, could signal a joint infection. This is a medical emergency that can permanently damage the joint if not treated within hours. Pain after a joint replacement that worsens with use also warrants urgent evaluation. And any injury that leaves the knee completely unable to bear weight or visibly deformed needs immediate care.