Knee joint pain has dozens of possible causes, ranging from a sudden sports injury to years of gradual cartilage wear. The knee is the largest joint in the body and bears forces of up to four times your body weight during everyday activities like climbing stairs, so it’s vulnerable to both acute damage and long-term breakdown. Understanding where your pain is coming from is the first step toward getting it under control.
Osteoarthritis: The Most Common Cause
Osteoarthritis is the single most frequent reason people develop chronic knee pain, especially after age 50. It happens when the cartilage that cushions the ends of your bones gradually breaks down. Healthy cartilage is smooth and slippery, allowing your thighbone and shinbone to glide against each other. In osteoarthritis, that cartilage loses its structure. It absorbs excess water, the proteins that give it strength become disorganized, and enzymes in the joint start digesting the cartilage faster than the body can repair it. Inflammatory signals from the joint lining accelerate the process.
Over time, the space between the bones narrows, bony spurs form along the edges of the joint, and the underlying bone hardens. The result is pain, stiffness (particularly in the morning or after sitting), and a grinding sensation when you bend the knee. Women are hit harder than men. Globally, the disability burden from weight-related knee osteoarthritis is 1.8 times higher in women, and women typically experience more severe pain and greater loss of function.
The global impact is enormous and growing. A 2025 analysis in Frontiers in Public Health projects that disability from weight-related knee osteoarthritis will nearly double by 2050 compared to 2021 levels. Excess body weight is a primary driver: every extra pound you carry translates into several additional pounds of force on your knee with each step, accelerating cartilage loss year after year.
Ligament and Meniscus Injuries
The knee relies on four major ligaments and two C-shaped pads of rubbery cartilage (the menisci) to stay stable and absorb shock. Damage to any of these structures causes pain that can range from a dull ache to an immediate, disabling pop.
The anterior cruciate ligament (ACL) connects the thighbone to the shinbone and prevents the lower leg from sliding forward. It tears most often during sports that involve sudden stops, pivots, or changes in direction, like basketball, soccer, tennis, and volleyball. An ACL tear usually produces a loud pop, rapid swelling, and a feeling that the knee is giving way. It rarely heals on its own and often requires surgical reconstruction, especially in active people.
A meniscus tear happens when the knee twists forcefully while bearing weight. You might feel a catch or lock in the joint, along with swelling that builds over a day or two. In younger people, meniscus tears are usually traumatic. In older adults, the cartilage becomes brittle with age and can tear from something as minor as an awkward squat.
Fractures of the kneecap or the bone ends that form the joint are another source of acute pain. They typically result from falls, car accidents, or direct blows. People with weakened bones from osteoporosis can fracture the knee simply by stepping wrong.
Patellofemoral Pain (Runner’s Knee)
Patellofemoral pain syndrome is one of the most common causes of knee pain in younger, active adults. The pain centers behind or around the kneecap and worsens with squatting, stair climbing, or prolonged sitting. It develops when the kneecap doesn’t track smoothly in the groove at the front of the thighbone.
Several things contribute to poor tracking. Weak quadriceps muscles allow the kneecap to shift sideways under load, and research shows that decreased quadriceps strength is associated with a significantly higher risk of this condition. Tight hamstrings increase the forces acting on the kneecap during exercise. A pattern called dynamic valgus, where the knee collapses inward during movement, pushes the kneecap further off track. Flat feet or excessive inward rolling of the foot can set off that same chain reaction by rotating the shinbone internally.
The good news is that patellofemoral pain generally responds well to targeted strengthening of the quadriceps and hip muscles, along with correction of movement patterns. It’s not a structural injury in most cases, so imaging often looks normal.
Bursitis and Tendinitis
The knee contains small fluid-filled sacs called bursae that cushion the areas where bones, tendons, and skin rub together. When a bursa becomes irritated, it swells with fluid and creates a visible, tender lump. Prepatellar bursitis, sometimes called “housemaid’s knee,” develops from prolonged kneeling and causes swelling directly over the front of the kneecap.
Patellar tendinitis, often called “jumper’s knee,” involves the tendon that connects the kneecap to the shinbone. It’s common in athletes who do a lot of running, jumping, or explosive leg movements. The pain sits just below the kneecap and gets worse with activity. Both conditions share symptoms like swelling, tenderness, a grating feeling during movement, and limited range of motion. Bursitis tends to cause more localized puffiness, while tendinitis pain is more directly tied to loading the tendon during exercise.
Pain That Starts Somewhere Else
Not all knee pain originates in the knee. The hip and lower back are frequent sources of referred pain that shows up at the knee. When the piriformis muscle deep in the buttock becomes tight, it can compress the sciatic nerve. That compression sends pain, numbness, or tingling down the back of the leg and increases stress on the knee joint during walking and other movements.
Poor hip mobility is another overlooked contributor. When the hip lacks adequate internal rotation, the knee compensates by absorbing extra biomechanical stress with every step, squat, and turn. Over time, this compensation pattern raises the risk of strains, sprains, and chronic pain in the knee. If your knee pain doesn’t improve with knee-focused treatment, a problem higher up the chain may be the real cause.
Key Risk Factors
Some risk factors for knee pain are unavoidable, but several of the most important ones are modifiable.
- Excess weight. Obesity is one of the primary causes of knee osteoarthritis. The more weight the joint carries, the faster cartilage breaks down. Research from the University of Sydney identified obesity, prior knee injury, and occupational hazards as the leading drivers of knee osteoarthritis.
- Occupation. Jobs that involve heavy lifting, prolonged kneeling, squatting, or shift work place chronic stress on the knee and significantly raise the risk of joint degeneration over a career.
- Previous injury. A torn ACL or meniscus dramatically increases your chances of developing osteoarthritis in that knee later in life, even after successful surgical repair.
- Sex. Women develop knee osteoarthritis at higher rates than men and tend to have more severe symptoms. Hormonal differences, wider hip angles, and differences in muscle mass all play a role.
- Muscle weakness. Weak quadriceps and hip muscles reduce the knee’s ability to absorb shock and maintain proper alignment during movement.
When Knee Pain Is an Emergency
Most knee pain develops gradually or follows an obvious injury and can be evaluated on a routine timeline. A few situations demand urgent attention. Septic arthritis, a bacterial infection inside the joint, causes severe pain that comes on rapidly, significant swelling, warmth, skin color changes over the joint, and often a fever. Without prompt treatment, it can destroy the joint within days. If you’ve had a knee replacement and develop new pain, swelling, or a feeling that the joint is loosening months or years after surgery, that can also signal an infection in the prosthetic joint.
How Knee Pain Is Evaluated
An X-ray is the standard first step for chronic knee pain. It can reveal narrowed joint space, bone spurs, fractures, and loose fragments. For many people with clear osteoarthritis on X-ray, no further imaging is needed.
When X-rays look normal or show only fluid in the joint but pain persists, an MRI without contrast is the typical next study. MRI is far more sensitive than X-ray for detecting soft-tissue problems like meniscus tears, ligament damage, cartilage injuries, and stress fractures that don’t show up on plain films. It’s also used when a known condition like a prior cartilage repair or a loose body inside the joint needs closer evaluation. If X-rays already explain your symptoms clearly, an MRI usually isn’t necessary unless something doesn’t add up between what the images show and how your knee actually feels.