Your knee is a complex piece of equipment, and as a result, many common conditions and injuries can cause knee pain.

"There are a number of different ligaments around and inside the knee, as well as tendons and bursae — little fluid-filled sacs that can get inflamed," says Joel Press, MD, physiatrist-in-chief at the Hospital for Special Surgery and professor of rehabilitation medicine at Weill Cornell Medical College, both in New York City. "There are lots of different structures there, and a lot of these types of pains will overlap.”

In general, if you have fever, warmth of the knee, lack of full range of motion in the knee, or acute weight-bearing pain in the knee, you should see a doctor, says Robert Gotlin, DO, team physician for the Harlem Wizards who also practices physical medicine and rehabilitation in New York City. “It’s important to see a doctor to obtain a precise diagnosis, prevent further knee damage, address the root cause of the pain, and intervene with an appropriate remedy,” he explains.

Common Causes of Knee Pain

Here are some of the more common causes of knee pain, and their typical symptoms:

Bursitis

If you notice that your knee pain is accompanied by sudden swelling, redness, or a warm feeling on the affected area, you should seek a doctor’s care. The bursa — a fluid-filled sac in the knee that helps lubricate the joint — might be infected. Certain factors, like previous joint injuries or a wound on the knee, can increase your risk for this kind of infection. Infections can also cause fever, chills, and nausea, so be on the lookout for those symptoms, too. If you think you have bursitis, it’s best to limit your activity until the knee is evaluated by a physician, says Dr. Gotlin. Most bursitis in the knee does not need surgery and can be treated with remedies such as ice and anti-inflammatories (like ibuprofen).

Sciatica

This condition, which generally refers to pain that travels from the lower back into the lower leg or legs, can be caused by pressure of a disc onto a nerve in the lower back. “If the pain is not localized to the knee only, and involves the lower leg in general, this type of knee pain may be related to sciatica, and not to the knee itself,” explains Gotlin. However, sciatic pain is not usually isolated to just the knee. “If you suffer with leg pain that starts in the lower back and refers to the knee, looking at the low back as a cause of knee pain would be prudent,” adds Gotlin.

Sprains and Torn Cartilage

Ligament sprains of the knee are usually caused by a blow to the knee or a sudden twist of the knee, according to Johns Hopkins Medicine. Common symptoms include feeling a pop, pain, swelling, instability (knee buckles when walking), or difficulty walking, says Gotlin.

Torn cartilage, which can occur with injuries of the knee or with arthritis, is another common cause of knee pain, adds Gotlin. Trauma (or generalized wear and tear) to the knee can tear the menisci, which are cushioning pads of connective tissue that also absorb shock located within the knee joint. Knee buckling, swelling, and pain during specific motions may indicate torn cartilage.

Arthritis

According to Mayo Clinic, the following types of arthritis are most likely to cause knee pain:

Osteoarthritis occurs when the cartilage in your knee deteriorates with use and age. Weight-bearing pain, activity-related swelling, and decreased range of motion may indicate osteoarthritis, says Gotlin.
Rheumatoid arthritis (RA) is an autoimmune condition that can affect the knee joint. Pain, deformities of the knee joint, and swelling may indicate RA, notes Gotlin.
Gout, which occurs when uric acid crystals build up in the joint, has specific symptoms, including a very painful, red, warm-to-the-touch knee joint, per Gotlin.
Pseudogout, which is caused by calcium-containing crystals in the joint fluid, is similar to gout, but tends to be less symptomatic, explains Gotlin.
Septic arthritis, which occurs when your knee joint becomes infected, can lead to swelling, pain, and redness, along with fever and a general feeling of malaise. It's usually not caused by trauma to the knee. Call your doctor immediately if you’re having symptoms of septic arthritis, as it can quickly cause significant damage to the knee cartilage.

Tendonitis

Tendonitis, or inflammation of the tendons, is another common cause of knee pain, according to Johns Hopkins. Tendonitis of the patellar (knee) tendon, known as jumper's knee, frequently occurs in sports like basketball, where the force of hitting the ground after a jump can strain the tendon. Symptoms of tendonitis typically include range of motion pain, local tenderness, and pain-related weakness of the knee, says Gotlin.

Other Causes and Symptoms of Knee Pain

Other injuries, syndromes, and mechanical problems may also cause knee pain. These include:

ACL Injury An ACL injury occurs when you tear your anterior cruciate ligament (ACL), which is one of the four ligaments that connect your shinbone to your thighbone. It's a common injury among people who play sports like basketball and soccer, which require sudden changes in direction. Call your doctor immediately if you experience symptoms of an ACL injury, such as a loud pop or "popping" sensation in the knee, advises Mayo Clinic.

Fractures A knee fracture can occur if the knee bones, including the kneecap (patella), break during incidents like falls or auto accidents. It can also be caused by something as slight as stepping the wrong way if your bones are weaker due to osteoporosis, per Mayo Clinic. Call your doctor right away if you experience symptoms of a knee fracture, which include sudden pain around the kneecap or in the knee itself, possibly with swelling; inability to bend or straighten the knee or to hold the leg out straight; deformed appearance of the knee; or bone protruding from the skin of the knee, per Johns Hopkins.

Patellofemoral Pain Syndrome Patellofemoral pain syndrome, which is sometimes called runner’s knee, refers to pain between the kneecap and the thighbone, per Mayo Clinic. This condition is common in athletes and young adults, and can develop in older adults due to arthritis.

Mechanical Problems Several mechanical problems with the knee can also cause pain, including:

Iliotibial Band Syndrome (ITBS) ITBS occurs when the iliotibial band — the tough band of tissue extending from the outside of your hip to the outside of your knee — becomes so tight that it starts to rub against the outside of your thighbone. The condition commonly affects runners and cyclists. Talk to your doctor if you have symptoms of ITBS, which include hip pain, a clicking sensation on the outside of your knee, knee pain, warmth, or redness, per the Cleveland Clinic.
Dislocated Kneecap A dislocated kneecap, also called a patellar dislocation, happens when the triangular bone that covers the front of your knee slips out of place, explains Mayo Clinic. Call your doctor if you experience symptoms of a dislocated kneecap, such as an audible pop, buckling of the knee, intense pain, sudden swelling, bruising or locking of the knee, inability to walk, or your kneecap is visually out of place, per the Cleveland Clinic.
Loose Body A "loose body" can occur when injury or degeneration of bone or cartilage causes a piece of bone or cartilage to break off and float in the joint space, which can affect knee joint movement, notes Mayo Clinic. This condition can cause a sensation of locking in the joint, or make it difficult to move or fully extend the joint, explains Northwell Health. Call your doctor if you have symptoms of a loose body.
Hip or Foot Pain Hip or foot pain may alter the way you walk, which can cause stress to your knee joint and lead to knee pain, per Mayo Clinic.

Remember, if your knee pain is severe or isn’t going away after a few days or weeks, it’s important to see a doctor, who will perform an exam along with lab tests or imaging to identify the cause of the pain and recommend the appropriate treatment, therapy, or intervention.

“The physician must get a good history to see how this came about, do a physical exam to try to pinpoint these structures, and try to correlate that with any type of imaging studies that are done,” says Dr. Press.