What Causes Knee Pain? Common Causes, Symptoms, and When to Worry

Your knee aches when you climb stairs, twinges when you stand up, or swells after a long walk, and you are not sure why. Knee pain is one of the most common reasons people see a doctor, and the good news is that most cases have a clear, treatable explanation.

Knee pain usually comes down to one of a few things: an injury, a mechanical problem inside the joint, or some form of arthritis. Below, we break down the most common causes, how to tell them apart, and the signs that mean it is time to get checked.

The Three Main Categories of Knee Pain

According to the Mayo Clinic, knee pain falls into three broad buckets. Knowing which one you are dealing with is the first step toward relief.

Most everyday knee pain fits into one of these groups:

• Injuries: damage to a ligament, tendon, or cartilage, such as an ACL tear, a meniscus (cartilage) tear, a fracture, bursitis, or patellar tendinitis.
• Mechanical problems: something interfering with how the joint moves, like a loose fragment of bone or cartilage, iliotibial (IT) band syndrome, or a dislocated kneecap.
• Arthritis: ongoing joint disease, including osteoarthritis, rheumatoid arthritis, gout, and septic (infectious) arthritis.

Arthritis: The Most Common Long-Term Cause

If your knee pain has built up slowly over months or years, arthritis is a likely culprit. Osteoarthritis is the most common form of knee arthritis. It happens when the cartilage that cushions the joint gradually breaks down, leaving bones to rub together and producing pain, swelling, and stiffness.

Osteoarthritis is extremely common. About 33 million U.S. adults have osteoarthritis, and the hands, hips, back, and knees are among the joints it affects most, according to the CDC. It most often begins in people 50 and older, though younger people can develop it too. Globally, the knee is the joint most commonly affected by osteoarthritis, according to the Global Burden of Disease Study 2021.

Not all arthritis is wear-and-tear. Rheumatoid arthritis is an autoimmune condition, while gout and pseudogout are caused by crystals building up in the joint. A hot, red, severely painful knee can signal septic arthritis, a joint infection that needs urgent care.

Runner's Knee and Other Front-of-Knee Pain

If the pain sits at the front of your knee, around or under the kneecap, you may have patellofemoral pain syndrome, often called runner's knee. The AAOS describes it as a leading cause of anterior (front) knee pain. It commonly affects athletes, especially females and young adults, but you do not have to be a runner to get it.

Front-of-knee pain often flares with activities that load the kneecap, such as squatting, kneeling, climbing stairs, or sitting with bent knees for a long time. In growing adolescents, a related cause is Osgood-Schlatter disease, which comes from irritation where the patellar tendon attaches to the shin bone.

Injuries and Mechanical Problems

Sudden knee pain, especially after a twist, fall, or sports collision, usually points to an injury. An ACL (anterior cruciate ligament) tear often happens with a quick pivot and may come with a pop and rapid swelling. A meniscus tear damages the cushioning cartilage and can make the knee catch or feel like it is locking.

Other injury-related causes include fractures, patellar tendinitis (irritation of the tendon below the kneecap), and bursitis (inflammation of the small fluid-filled sacs around the joint). Mechanical issues, like a loose body floating in the joint, a dislocated kneecap, or IT band syndrome along the outer knee, can also cause pain, catching, or instability.

How Knee Pain Is Usually Treated

The right treatment depends on the cause, but many minor cases respond well to self-care. The Mayo Clinic notes that rest, physical therapy, and a knee brace can help many people recover without surgery.

For knee osteoarthritis specifically, the Cleveland Clinic lists several nonsurgical options your clinician might consider, including physical therapy and joint injections. General first steps for milder pain often include:

• Rest, ice, and avoiding activities that flare the pain
• Physical therapy to strengthen the muscles that support the knee
• A supportive knee brace when recommended
• Talking to a clinician about injections or other options for arthritis that does not improve

When to See a Doctor About Knee Pain

Mild knee pain that improves with rest can often be watched at home for a few days. But some symptoms mean you should get medical attention promptly, and a few mean you should not wait.

See a clinician if your knee pain does not improve, keeps coming back, or limits your daily activities. Seek urgent or emergency care if you have any of the warning signs below, since some causes (like a fracture, a serious ligament tear, or a joint infection) need fast treatment.

• You cannot bear weight on the knee or it feels like it will give out
• Marked swelling, redness, warmth, or fever (possible infection)
• An obvious deformity, or the knee locks and will not straighten
• Severe pain after a fall, twist, or direct blow
• Pain that lingers or worsens despite rest and self-care