Knee pain has dozens of possible causes, ranging from a single awkward twist to years of gradual cartilage wear. Nearly 375 million people worldwide live with knee osteoarthritis alone, and that number doesn’t account for the many other conditions that can make your knee ache, swell, or give out. Understanding the specific cause matters because treatments vary widely, and roughly 1 in 7 people who visit a doctor for knee pain turn out to have a problem that isn’t in the knee at all.
Osteoarthritis: The Most Common Cause
Osteoarthritis is by far the leading reason for chronic knee pain, especially after age 50. It happens when the cartilage that cushions the ends of your bones gradually breaks down, leaving bone surfaces to grind closer together. Global data from 2021 shows the number of cases climbs steeply with age: about 15 million people in their early 40s have it, but that jumps to over 55 million by the late 50s. Women and people over 50 are at highest risk.
The hallmark signs are a crackling or grinding sensation when you bend the knee, stiffness that lasts less than 30 minutes after waking up, bony tenderness or enlargement around the joint, and the absence of warmth or heat over the kneecap. That last point helps distinguish it from inflammatory conditions. The pain typically worsens with activity and improves with rest, and it tends to develop slowly over months or years rather than appearing overnight.
Ligament and Meniscus Injuries
Acute knee injuries often involve the ligaments or the rubbery pads of cartilage (menisci) that sit between your thighbone and shinbone. The ACL (anterior cruciate ligament) and MCL (medial collateral ligament) are the most commonly damaged. These injuries typically happen during a sudden twist, a direct blow to the side of the knee, or a pivoting motion on a planted foot. Many people hear or feel a pop at the moment of injury.
MCL injuries are graded by how much the joint opens up when stress is applied. A mild sprain involves less than 5 millimeters of looseness, a moderate tear falls between 5 and 10 millimeters, and a severe tear exceeds 10 millimeters. ACL tears usually cause immediate swelling and a feeling that the knee is unstable or “giving way.” Meniscus tears produce catching, locking, or clicking sensations, particularly when you squat or twist. When all three structures are damaged together, it’s sometimes called the “unhappy triad,” a combination caused by a forceful sideways hit with rotation.
Runner’s Knee and Kneecap Tracking Problems
Patellofemoral pain syndrome, commonly called runner’s knee, is the most frequent cause of knee pain in younger, active people. The pain sits behind or around the kneecap and gets worse with squatting, climbing stairs, or sitting for long periods. The root problem is usually biomechanical: your kneecap doesn’t glide smoothly in its groove on the thighbone.
Several factors contribute to this. Weakness in the hip muscles that control rotation and side-to-side stability (the abductors and external rotators) is consistently found in people with this condition, particularly women. When these muscles are weak, the thighbone rotates inward during movement, pushing the kneecap off its ideal track and increasing pressure on the outer edge of the joint. A similar chain reaction can start at the foot: excessive flattening of the arch drives the shinbone into internal rotation, which in turn shifts kneecap alignment. Weakness in the inner quadriceps muscle near the kneecap also plays a role, since this muscle is the primary stabilizer that keeps the kneecap from drifting outward.
Tendon Pain and Bursitis
Two conditions that are easy to confuse cause pain in the front of the knee, but in slightly different spots and for different reasons.
Patellar tendonitis (jumper’s knee) produces pain just below the kneecap, where the patellar tendon connects the kneecap to the shinbone. It’s activity-related, flaring up during jumping, running, or heavy squatting. One useful clue: the tenderness is present when the knee is straight but fades when the knee is bent.
Bursitis, on the other hand, involves inflammation of the small fluid-filled sacs that cushion the knee. The most common type, prepatellar bursitis, causes swelling, tenderness, and a visible lump right on top of the kneecap. It’s often triggered by prolonged kneeling, which is why it’s sometimes called “housemaid’s knee.” The swelling can make it painful to kneel or even rest the knee against a surface.
Gout and Crystal Deposits
The knee is one of the most common joints affected by crystal-related arthritis. Two types look similar but involve different substances. Gout is caused by uric acid crystals forming inside the joint, while pseudogout (calcium pyrophosphate disease) involves calcium-based crystals. Both cause sudden, intense pain, swelling, redness, and warmth that can come on within hours.
Gout attacks often strike at night and may follow triggers like alcohol, red meat, or dehydration. Pseudogout tends to affect older adults, particularly those over 65, and can mimic a joint infection because of how dramatically the knee swells. The only definitive way to tell them apart is by examining fluid drawn from the joint under a microscope.
Pain That Comes From Somewhere Else
One of the most overlooked causes of knee pain is a problem somewhere other than the knee. A study of 1,000 patients presenting to an orthopedic clinic for knee pain found that 13.7% had pain referred from the hip or spine rather than an actual knee problem. Among those with hip pathology, the most common diagnoses were osteoarthritis (58%) and femoroacetabular impingement (23%), a condition where the ball and socket of the hip don’t fit together smoothly.
This happens because the hip and knee share overlapping nerve pathways. Hip joint problems can send pain signals down the front of the thigh and into the knee, sometimes without producing any hip pain at all. If your knee pain doesn’t match up with physical findings in the knee itself, or if it persists despite treatment, the hip and lower back are worth investigating.
Septic Arthritis: A Rare but Serious Cause
A joint infection, called septic arthritis, is uncommon but can destroy cartilage within days if untreated. The classic presentation is a knee that becomes acutely red, hot, swollen, and exquisitely tender, often with severely limited range of motion and intense pain when the joint is compressed. However, only about 50% of people with a septic joint actually develop a fever, so the absence of fever doesn’t rule it out.
The challenge is that crystal arthritis (gout or pseudogout) can look nearly identical. No single physical finding can definitively distinguish an infection from a crystal flare. Fluid analysis from the joint is required. If your knee becomes rapidly swollen, hot, and intensely painful over hours to a couple of days, particularly if you have a wound near the knee, a recent surgery, or a weakened immune system, it warrants urgent medical evaluation.
Signs That Suggest a Fracture
After a knee injury, certain findings make a fracture much more likely. Emergency guidelines known as the Ottawa Knee Rules identify five red flags: being 55 or older, tenderness isolated to the kneecap, tenderness at the top of the smaller lower leg bone (the fibula head, on the outer side just below the knee), inability to bend the knee to 90 degrees, and inability to bear weight for four steps both immediately after the injury and at the time of evaluation. If any one of these is present after trauma, imaging is warranted.