Knee pain has dozens of possible causes, ranging from a single awkward twist during a basketball game to years of gradual cartilage wear. Nearly 375 million people worldwide have knee osteoarthritis alone, and that number climbs steadily after age 30. Understanding the specific cause matters because the right response varies dramatically: some knee pain resolves with rest, while other types signal damage that worsens without treatment.
Osteoarthritis: The Most Common Chronic Cause
Osteoarthritis is the slow breakdown of the cartilage that cushions the ends of your bones. As that cushion thins, bone starts grinding closer to bone, producing stiffness, aching, and eventually sharp pain with movement. The burden peaks between ages 80 and 84 in both sexes, but women develop it earlier and more severely than men, with the highest rate of new cases appearing in women between 50 and 54 compared to 60 to 64 in men.
What makes osteoarthritis tricky to catch early is that X-rays often look normal in the beginning. Early cartilage damage simply doesn’t show up on standard imaging. If your knee locks, gives way, or swells without a clear injury, an MRI or fluid sample from the joint may be needed to rule out other conditions like gout or infection.
The strongest risk factors are age, previous knee injury, obesity, and repetitive occupational stress. Excess body weight doesn’t just add load to the joint; fat tissue releases inflammatory chemicals that accelerate cartilage breakdown even in non-weight-bearing joints.
Ligament Injuries
The anterior cruciate ligament (ACL) is the most talked-about knee ligament, and for good reason. Most ACL tears happen without any contact at all. The classic scenario is a sudden pivot or direction change while the knee is slightly bent and angled inward. Landing awkwardly from a jump does the same thing. A direct blow to the outside of the knee can also tear the ACL, but non-contact mechanisms are far more common in athletes.
About 70% of people who tear their ACL experience immediate swelling from bleeding inside the joint, along with a distinct “pop” they can both hear and feel. The knee typically feels unstable afterward, as though it could buckle at any moment. Walking becomes difficult, and straightening the knee fully may be impossible in the first few days.
Other ligaments on the inner side (MCL), outer side (LCL), and back of the knee (PCL) can also tear, each with its own pattern of instability and pain location. MCL sprains, for instance, are common in contact sports when a hit pushes the knee inward.
Meniscus Tears
Each knee has two C-shaped pieces of cartilage called menisci that act as shock absorbers between the thighbone and shinbone. Tears happen when you twist or rotate the knee forcefully, especially while bearing weight. In younger people this usually occurs during sports; in older adults the meniscus can tear from something as minor as an awkward squat, because the tissue becomes more brittle with age.
Unlike ligament injuries, meniscus tears can be sneaky. If the tear is small, pain and swelling may not appear for 24 hours or more. The hallmark symptoms are pain that worsens with twisting, a feeling that the knee is locked in place, and a catching or clicking sensation during movement. The knee may also feel like it’s about to give way.
Physical exams for meniscus tears are moderately reliable at best. One widely used test has a sensitivity of only about 61% for tears on the inner meniscus, meaning it misses roughly four out of ten injuries. MRI is often needed for confirmation.
Patellofemoral Pain (Runner’s Knee)
Pain around or behind the kneecap that gets worse when you climb stairs, squat, or sit for long periods is often patellofemoral pain. It’s one of the most common knee complaints in younger, active people, and its root cause is almost always biomechanical: some combination of muscle imbalance, tightness, or alignment issues changes how the kneecap tracks across the groove in your thighbone.
Weakness in the hip muscles plays a surprisingly large role. Researchers have consistently found that people with this condition, particularly women, have weaker hip abductors and external rotators. That weakness lets the thigh rotate inward during activities like running or squatting, which pushes the kneecap off its normal path and compresses it against the outer edge of the groove.
Problems can also come from below the knee. Excessive foot pronation (flat arches rolling inward) forces the shinbone into internal rotation, which has a similar effect on kneecap tracking. Tightness in the iliotibial band, the thick tissue running along the outer thigh, can pull the kneecap laterally and create compression. Even limited ankle flexibility has been linked to altered knee mechanics that contribute to this pain.
Tendon and Bursa Problems
Tendons connect muscle to bone, and the knee has several that are vulnerable to overuse. Patellar tendinitis (“jumper’s knee”) affects the tendon just below the kneecap and is common in sports involving repeated jumping. The pain is usually sharp and localized to a specific spot you can press on. Quadriceps tendinitis produces similar symptoms just above the kneecap.
Bursae are small fluid-filled sacs that reduce friction around the joint. Prepatellar bursitis, historically called “housemaid’s knee,” develops from prolonged or repeated kneeling. Gardeners, tile installers, clergy, and anyone who spends extended time on their knees are at risk. The front of the knee becomes visibly swollen, warm, red, and tender. If the bursa gets infected, the redness and warmth intensify, and the inflammation can spread along lymphatic channels up the thigh.
Gout and Other Inflammatory Conditions
Gout is caused by uric acid crystals depositing in a joint, triggering sudden, intense inflammation. While the big toe is the classic location, the knee is one of the next most commonly affected joints. Gout attacks almost always strike suddenly, often at night. The pain is severe, the joint becomes red and swollen, and even light pressure (like a bedsheet resting on the knee) can be excruciating. Between attacks, lingering discomfort and reduced range of motion can persist.
Rheumatoid arthritis and other autoimmune conditions can also target the knee. Unlike osteoarthritis, which is driven by mechanical wear, these conditions involve the immune system attacking the joint lining. They tend to affect both knees symmetrically and come with systemic symptoms like fatigue and morning stiffness lasting more than 30 minutes.
Referred Pain From the Hip
Sometimes the knee itself is perfectly healthy, but hip problems send pain signals there. Between 2% and 29% of knee pain cases may actually originate from hip pathology. This happens because the hip and knee share overlapping nerve pathways. Sensory fibers from both joints converge on the same neurons in the spinal cord, so the brain can misinterpret where the signal is coming from.
In children, hip conditions like Legg-Calvé-Perthes disease or a slipped growth plate frequently present as knee pain rather than hip pain. In adults, hip osteoarthritis can masquerade the same way. If your knee hurts but nothing seems wrong on knee imaging, a hip evaluation is a reasonable next step.
Joint Infection: The Emergency Cause
Septic arthritis is a bacterial infection inside the joint that can destroy cartilage within days if untreated. The knee is one of the most commonly affected joints. It typically presents as a rapidly worsening hot, swollen, extremely painful knee, often with fever. Up to 90% of patients have at least a low-grade fever, though a normal temperature does not rule it out.
What separates a joint infection from other causes of knee swelling is the speed and severity. Pain escalates over hours rather than days, and bearing weight becomes nearly impossible. People with diabetes, rheumatoid arthritis, recent joint surgery, or a prosthetic knee are at higher risk. This is one cause of knee pain that genuinely requires urgent medical evaluation, because the longer the infection sits in the joint, the more permanent the damage.
Weight, Activity Level, and Alignment
Many knee pain causes share the same set of underlying risk factors. Carrying extra weight increases the force on your knees by roughly two to four pounds for every one pound of body weight during walking. Weak quadriceps reduce the joint’s ability to absorb shock, shifting stress onto cartilage and ligaments instead. Flat feet, knock knees, and leg-length differences all alter how force distributes across the joint surface.
Sudden increases in training volume are one of the most preventable triggers. The “too much, too soon” pattern, where someone doubles their running mileage or adds heavy squats without a gradual buildup, overwhelms tissues that haven’t had time to adapt. The result can be anything from tendinitis to a stress fracture in the bone just below the joint surface.