Knee pain is discomfort, soreness, or aching in or around the knee joint, and it is one of the most common musculoskeletal complaints worldwide. The knee alone accounts for 365 million cases of osteoarthritis globally, making it the most frequently affected joint in the body. Knee pain can stem from an acute injury, a degenerative condition, inflammation, or even a problem somewhere else entirely, like the hip or lower back.
Why the Knee Is Vulnerable to Pain
The knee is the largest joint in the body and one of the most mechanically stressed. It bears your full body weight with every step and absorbs forces several times that weight when you run, jump, or climb stairs. The joint relies on a complex web of ligaments, tendons, cartilage pads, fluid-filled sacs, and muscles to stay stable and move smoothly. When any one of those structures is damaged, overloaded, or inflamed, the result is pain.
The knee is densely packed with pain-sensing nerve fibers. These nerves run through the joint lining, the ligaments, and even into the bone beneath the cartilage surface. In conditions like osteoarthritis, these nerve fibers can actually sprout new branches into areas of damage, which is one reason chronic knee pain can intensify over time even when the joint looks the same on an X-ray.
Common Causes of Knee Pain
Injuries
Acute injuries are among the most frequent causes, especially in active people. An ACL tear occurs when one of the four major ligaments connecting the shinbone to the thighbone is torn, typically during sports that involve sudden direction changes like basketball or soccer. A torn meniscus happens when the rubbery cartilage cushion between those same bones is wrenched during a twisting motion under load. Kneecap dislocations, fractures, bursitis (inflammation of the small fluid sacs that reduce friction around the joint), and patellar tendinitis (irritation of the tendon running from the kneecap to the shinbone) round out the most common traumatic causes.
Arthritis
Osteoarthritis is the single most common cause of chronic knee pain, especially after age 50. It develops as the cartilage lining the joint gradually wears down, allowing bone surfaces to grind closer together. Rheumatoid arthritis, an autoimmune condition, can attack the knee’s joint lining and is often more disabling. Gout, caused by uric acid crystals accumulating inside the joint, tends to produce sudden, intense flares of pain and swelling.
Mechanical Problems
Sometimes the issue isn’t tissue damage but how the joint moves. A loose body, a fragment of bone or cartilage floating inside the joint space, can catch during movement like a pebble stuck in a hinge. Pain in the hip or foot can also cause knee pain indirectly: when you change the way you walk to protect a sore hip, the altered gait shifts extra stress onto the knee.
Where It Hurts Matters
The location of your knee pain is one of the most useful clues to its cause. Pain at the front of the knee, around or behind the kneecap, often points to runner’s knee (patellofemoral syndrome), cartilage softening on the underside of the kneecap, or a tracking problem where the kneecap doesn’t glide properly in its groove. In children and adolescents, front-of-knee pain during growth spurts is commonly Osgood-Schlatter disease, an inflammation where the kneecap tendon attaches to the shinbone.
Pain on the inner side of the knee frequently involves the medial collateral ligament or a meniscus tear, and it’s also the most typical spot for osteoarthritis to settle. Pain on the outer side often relates to the iliotibial band or the lateral collateral ligament. Pain behind the knee can signal a Baker’s cyst, a fluid-filled swelling that forms when excess joint fluid pools in the back of the knee, usually as a secondary effect of arthritis or a meniscus injury.
Pain That Starts Somewhere Else
Not all knee pain originates in the knee. The nerves that supply sensation to the knee travel down from the lumbar spine, and problems at certain levels of the lower back, including herniated discs, arthritis, or nerve compression, can produce pain that feels like it’s coming from the knee itself. Hip joint problems can do the same thing. This “referred pain” is one reason knee pain sometimes persists even after the knee itself checks out fine on imaging. If knee treatments aren’t helping, the source of the problem may be higher up the chain.
How Body Weight Affects Knee Pain
Excess body weight is one of the strongest modifiable risk factors for knee pain, particularly from osteoarthritis. The relationship is more dramatic than most people realize. Research published in Osteoarthritis and Cartilage found that for every 1 kilogram (about 2.2 pounds) of body weight lost, the peak force on the knee during walking dropped by 2.2 kilograms. In other words, every pound you lose takes more than twice that load off your knee with each step. In that study, participants who lost an average of about 30 pounds saw a 7% reduction in overall knee joint loading, a meaningful change for a joint that absorbs thousands of loading cycles per day.
What Chronic Knee Pain Does Over Time
When knee pain becomes persistent, the effects extend well beyond the joint itself. Pain and stiffness reduce how much you move, and reduced movement causes the muscles supporting the knee to weaken. That muscle loss makes the joint less stable, which increases pain, creating a cycle that can be difficult to break. The World Health Organization notes that once pain and loss of movement become chronic, people often experience restrictions in daily activities, decreased well-being, and psychological distress. In severe cases, the joint can become so damaged that it causes pain even at rest.
How Knee Pain Is Diagnosed
A physical exam, including testing the knee’s range of motion, stability, and areas of tenderness, is the starting point. When imaging is needed, a standard X-ray is the first step. It can reveal fractures, joint space narrowing from cartilage loss, bone spurs, and loose fragments. X-rays are enough to confirm osteoarthritis in most cases.
An MRI comes next when the X-ray looks normal but pain persists, or when soft tissue damage is suspected. MRI excels at showing ligament tears, meniscus injuries, cartilage damage, and stress fractures that X-rays miss. A CT scan is used in more specific scenarios: evaluating kneecap alignment problems, confirming loose bodies when MRI results are unclear, or checking for gout using a specialized dual-energy technique that can detect crystal deposits.
Signs That Need Urgent Attention
Most knee pain develops gradually and can be managed with rest, activity modification, and strengthening exercises. But certain symptoms after an injury warrant immediate medical care: a knee that looks visibly bent or deformed, a popping sound at the moment of injury, inability to bear any weight on the leg, sudden significant swelling, or intense pain. A fever alongside knee pain and swelling can indicate a joint infection, which requires prompt treatment to prevent permanent damage.
How Knee Pain Is Managed
For mild to moderate knee pain, the initial approach typically involves rest, ice, compression, and elevation in the short term, followed by targeted exercise to strengthen the muscles around the joint. The quadriceps and hamstrings act as the knee’s shock absorbers, and building their strength is one of the most effective ways to reduce pain and protect the joint long term. Low-impact activities like swimming, cycling, and walking are generally well tolerated even when the knee is sore.
Physical therapy plays a central role for many causes of knee pain, from post-injury rehabilitation to managing arthritis. A therapist can identify movement patterns that overload the joint and correct them. For inflammatory conditions like rheumatoid arthritis or gout, treatment focuses on controlling the underlying disease process. When conservative measures fail and pain significantly limits daily life, joint replacement surgery becomes an option, particularly for advanced osteoarthritis. Modern knee replacements typically last 15 to 20 years, and most people return to walking, light sports, and daily activities within a few months of surgery.