"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 pain will overlap.”

Injuries, caused by sports, overuse, or accidents, can affect different parts of the knee, leading to pain and various symptoms depending on which area of the knee is affected.

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. You may hear a loud pop or feel a popping sensation in your knee, along with pain, swelling, buckling, and loss of range of motion.

Sprains

Torn Cartilage

Torn cartilage, which can occur with injuries of the knee or with osteoarthritis, is another common cause of knee pain. 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.

Fractures

Knee Bursitis

Tendonitis

Symptoms of tendonitis typically include range of motion pain, local tenderness, and pain-related weakness of the knee.

Patellofemoral Pain Syndrome

Both osteoarthritis and inflammatory forms of arthritis like RA can 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. The pain can be dull or sharp and constant or intermittent, and may also occur after sitting or resting for a long time.
• Rheumatoid arthritis (RA) is an autoimmune condition that can affect the knee joint. Pain, deformities of the knee joint, and swelling may indicate RA.
• 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. Pseudogout, which is caused by calcium-containing crystals in the joint fluid, causes similar symptoms in the knee.

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. Symptoms of ITBS include hip pain, a clicking sensation on the outside of your knee, knee pain, warmth, or redness.

• 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.
   Symptoms can include 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.

• 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.
  This condition can cause a sensation of locking in the joint, or make it difficult to move or fully extend the joint.

• 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.

• 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 the sciatic nerve, in the lower back. If the pain involves the lower leg in general, not just your knee, this type of knee pain may be related to sciatica.

• Obesity or Overweight Carrying excess weight increases stress on your knee joints and raises your risk of osteoarthritis.

• Lack of Flexibility or Strength Strong muscle strength helps stabilize and protect your joints, while muscle flexibility helps with range of motion.
• Sports That Stress Knees Downhill skiing, basketball, and running can increase your risk of knee injury, as can contact sports.
• Previous Injury A previous knee injury increases the likelihood that you'll injure your knee again.

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. It’s important to get a proper diagnosis, prevent further knee damage, address what’s causing the pain, and begin the right treatment.

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.

• The knee is a complex joint, where bones, cartilage, ligaments, tendons, muscles, and bursae connect — and any of these could be involved in various causes of knee pain.
• Injury or trauma to the knee, from sports, accidents, or overuse, can cause acute knee pain while different forms of arthritis and mechanical issues in the body may lead to chronic knee pain.
• If you feel acute weight-bearing pain in the knee, hear a popping sound or feel a popping sensation, seek medical attention, as it could signal a fracture or ligament tear. Check in with your doctor, too, if you have fever, warmth of the knee, lack of full range of motion, or persistent pain.
• Proper diagnosis and treatment can relieve the pain and prevent further damage.