Knee pain stems from three broad categories: acute injuries, overuse and mechanical problems, and underlying medical conditions like arthritis or gout. Nearly 365 million people worldwide have knee osteoarthritis alone, making it one of the most common joint complaints at any age. Understanding where your pain is located, how it started, and what makes it worse can help narrow down the cause.
Acute Injuries
A sudden injury is one of the most obvious triggers for knee pain. These typically happen during sports, falls, or accidents and often involve a specific moment you can point to when the pain began.
Ligament tears are among the most common acute knee injuries. The ACL (a ligament deep inside your knee that keeps the joint stable) can tear during sudden stops, direction changes, or awkward landings. You’ll often hear or feel a pop at the moment of injury, followed by rapid swelling and difficulty straightening the knee. Pain tends to be worse on the outer side of the knee. The MCL, which runs along the inner side of your knee, typically tears from a blow or force pushing the knee inward. MCL injuries cause pain specifically on the inside of the knee, and even minor movements like catching your foot on a blanket can trigger sharp discomfort.
A torn meniscus, the rubbery disc of cartilage that cushions the space between your thighbone and shinbone, happens when the knee twists while bearing weight. This is common in sports but also occurs during everyday movements, especially as the cartilage weakens with age. Symptoms include swelling, a catching or locking sensation, and difficulty fully bending or straightening the knee.
Fractures of the kneecap or the bones around the joint can result from falls or car accidents. These cause immediate, intense pain and visible swelling.
Overuse and Mechanical Problems
Not all knee pain starts with a single event. Repetitive stress on the joint gradually irritates tendons, bursae (small fluid-filled sacs that reduce friction), or the cartilage beneath the kneecap.
Patellar tendinitis is inflammation of the tendon connecting the kneecap to the shinbone. It’s common in runners, jumpers, and cyclists, and causes a focused ache just below the kneecap that worsens with activity. Knee bursitis develops when the fluid sacs around the joint become inflamed from repeated kneeling on hard surfaces, overuse, or a direct blow. It most often affects the front of the kneecap or the inner side of the knee just below the joint line. The area feels warm, tender, and swollen, and pain can persist even at rest.
Iliotibial band syndrome produces an aching or sharp pain on the outer side of the knee. The iliotibial band is a thick strip of tissue running from your hip to your shin, and when it’s too tight, it rubs against the bony prominence near your knee with every step. Weakness in the hip, glute, or core muscles, worn-out shoes, and differences in leg length all contribute. It’s especially common in runners and cyclists who ramp up training too quickly.
Sometimes a piece of bone or cartilage breaks free from its normal position and floats inside the joint space. These loose bodies can cause intermittent catching, locking, or sharp pain when they drift into a spot where they interfere with movement.
Arthritis and Inflammatory Conditions
Osteoarthritis is the single most common medical cause of knee pain. It develops as the protective cartilage lining the joint wears down over years, eventually allowing bone to grind against bone. Globally, about 5% of the population has knee osteoarthritis, and it’s significantly more common in women (roughly 6%) than men (about 3.8%). Prevalence climbs steeply with age: by the early 60s, around 14% to 17% of people are affected, and that number exceeds 18% to 23% by the early 80s. Pain typically builds gradually, feels stiff in the morning, and worsens after prolonged activity or sitting.
Rheumatoid arthritis is an autoimmune condition where the body’s immune system attacks the joint lining. Unlike osteoarthritis, it often affects both knees symmetrically and may come with fatigue, warmth, and noticeable swelling that flares and subsides over time.
Gout causes sudden, intense flares of pain, swelling, redness, and heat, usually in one joint at a time. It happens when uric acid crystals accumulate inside the joint. Flares can last days to weeks and are often so severe that even light pressure on the knee is unbearable. Over time, repeated flares can form visible lumps called tophi that damage the joint permanently.
Where the Pain Is Tells You a Lot
The location of your knee pain is one of the most useful clues to its cause. Pain at the front of the knee, especially around or just below the kneecap, often points to patellar tendinitis or issues with how the kneecap tracks in its groove. Pain on the inner side suggests an MCL injury or bursitis below the joint line. Pain on the outer side is more typical of iliotibial band syndrome or, after an injury, an ACL tear.
Deep, diffuse aching that’s hard to pinpoint is more characteristic of osteoarthritis or a meniscus tear. Swelling that appears within hours of an injury usually signals a ligament tear or fracture, while swelling that builds over a day or two is more common with meniscus injuries or inflammatory conditions.
How Knee Pain Is Diagnosed
A standard X-ray is the first imaging step for chronic knee pain. It can reveal arthritis, fractures, loose bodies, and changes in joint alignment. If X-rays look normal but pain persists, an MRI is typically the next step. MRI provides detailed images of soft tissues like ligaments, menisci, and cartilage that X-rays can’t show. Interestingly, when X-rays already clearly show osteoarthritis, an MRI usually isn’t needed unless the symptoms don’t match what the X-ray reveals.
For gout, diagnosis often requires a joint fluid sample taken during a flare, since uric acid crystals are easiest to identify when the joint is actively inflamed. Blood tests, imaging, and a review of your medical history round out the picture.
Risk Factors That Increase Your Chances
Carrying extra weight puts significantly more stress on the knee with every step. Each additional pound of body weight adds roughly four pounds of force across the knee joint during walking, which accelerates cartilage breakdown over time. Muscle weakness, particularly in the quadriceps and hips, leaves the knee less supported and more vulnerable to injury. Tight muscles and limited flexibility reduce the joint’s ability to absorb force evenly.
Certain sports carry higher risk. Basketball, soccer, and skiing involve the cutting, pivoting, and landing movements that most commonly tear ligaments. Running and cycling, while lower impact, predispose people to overuse injuries like iliotibial band syndrome and patellar tendinitis, especially with sudden increases in training volume. Previous knee injuries also raise the likelihood of future problems, even after full recovery.
Signs That Need Urgent Attention
Most knee pain improves with rest, ice, and time. But some symptoms indicate something more serious. A knee that looks visibly deformed or bent at an unusual angle, an inability to bear any weight, sudden dramatic swelling, or intense pain after an injury all warrant urgent medical evaluation. A popping sound at the moment of injury is a hallmark of ligament tears and should be assessed promptly.
Outside of injuries, a knee that is red, hot, swollen, and painful alongside a fever could signal an infection inside the joint. Joint infections can cause permanent damage quickly if untreated.