Knee pain can be caused by injuries and trauma that affect the mechanics of the knee (e.g., torn ligaments). Other kinds of knee pain are the result of degeneration and wear and tear such as arthritis. Still others may result from inflammation. Some knee pain may be caused by a combination of these causes. For example, athletes frequently develop tendinitis, an inflammation sometimes caused by overuse.

Some of the most common causes of knee pain include:

• Arthritis. Arthritis, a degenerative disease of the joints, causes knee pain in many of its forms, such as rheumatoid arthritis, juvenile arthritis, Reiter’s syndrome or ankylosing spondylitis, although it is most common with osteoarthritis. The pain, swelling and stiffness may affect both knees and cause cartilage to deteriorate.

Gout is a type of arthritis where uric acid that is normally excreted builds up in a joint. It usually affects the big toe, but can also occur in the knee. Knee symptoms can include redness, swelling and intense sudden onset of pain, often occurring at night. Pseudogout, caused by calcium deposits, may also cause inflammation and intermittent pain and swelling in knees.

• Soft tissue injuries. These may affect the ligaments, tendons and muscles. The most common knee ligament injuries are to the anterior cruciate ligament (ACL) across the front of the knee and medial collateral ligament (MCL), which runs vertically along the inside of the knee. Ligaments can be torn or sprained, usually from a twist during sports or a fall. The person may feel a pop and be unable to bear weight on the knee. When the knee is hyperextended, it moves farther forward than it should and may cause an ACL tear. This is a common injury among gymnasts. Injury to the posterior cruciate ligament (PCL) is more common in severe trauma, such as the knee hitting a dashboard in a car accident.

Slight or complete tears of a meniscus are commonly referred to as torn cartilage in the knee. They may occur from twisting knee movements when the foot is fixed, as in tennis or basketball. Meniscus tears or osteoarthritis may also result in a Baker’s cyst, an accumulation of fluid that forms behind the knee.

• Other traumatic injuries. Injuries can cause fractures of the knee bones and sprains or tears to the ligaments and tendons. The patella (kneecap) may be dislocated, causing visible, side-to-side movement of the knee. A person who experiences a dislocated patella once is more susceptible to a recurrence.

• Loose body. Pieces of bone or cartilage may break off and float inside the knee. It can be painful if it interferes with another part of the knee, such as the kneecap. About half of loose bodies in the knee are caused by osteochondritis dissecans, a disorder in which parts of bone and cartilage lose their blood supply and break off.
  
  
• Inflammation. Tendinitis is irritation or inflammation of a tendon. For athletes, tendinitis in the patellar tendon is common and is known as jumper’s knee. Bursitis is inflammation of one or more of the bursae (fluid-filled sacs around the knee) and can cause warmth, swelling, redness, stiffness while walking and pain while kneeling or using stairs. Many people with various forms of arthritis also develop Many people with various forms of arthritis also develop bursitis. Enthesitis is inflammation of entheses, the sites where tendons or ligaments attach to bone.
  
  
• Osgood-Schlatter disease. This inflammation of the patellar tendon is caused by repetitive stress on the top of the tibia (shin bone). It is common among teenagers whose bones are still growing, especially young athletes who run and jump. Its main symptom is pain below the knee joint.
  
  
• Iliotibial band syndrome. The ligament that runs from the outside of the pelvic bone down to the tibia can tighten and rub against the femur, causing knee pain. It produces sharp knee pain or pain along the thigh. The syndrome is most common among runners, particularly those with unequal leg lengths or those with weakened hip abductors who experience some sideways leg motion.
  
  
• Chondromalacia. This is a softening of the cartilage under the kneecap, causing the kneecap to rub against the femur. Trauma may cause chondromalacia when a blow to the kneecap tears off cartilage or bone.
  
  
• Patellofemoral syndrome. This pain in and around the kneecap may occur in one or both knees. Its cause may be related to abnormal movement of the patella when the knee bends. This condition is often called runner’s knee because it is common in runners. It is also sometimes called moviegoer’s knee because prolonged sitting with the knee bent can set off the pain.
  
  
• Patellar tendinopathy (jumper’s knee). Sports that involve jumping or quickly changing direction are a leading cause of damage to the patellar tendon connecting the kneecap and shinbone.
  
  
• Plica syndrome. When the fetal development of knee tissue is incomplete, it leaves several compartments of tissue (plicae) instead of one large cavity. For people who have this interrupted tissue development, injury and overuse can irritate the excess tissue in the knee.
  
  
• Referred nerve pain. A pinched nerve in the back – for example, sciatica resulting from a herniated disc, spinal stenosis or degenerative disc disease - can cause pain radiating down the leg to the knee and beyond.
  
  
• Complex regional pain syndrome. A chronic pain condition in a leg (or arm) that usually occurs after significant trauma to the limb.
  
  
• Gait disturbances. Unstable gait can cause pressure on muscles and the knee joint, causing pain.
  
  
• Bone tumors. In rare cases, some tumors (masses of abnormal tissue) in the bones of the leg may cause knee pain, but more commonly pain occurs closer to the site of the tumor.

Other causes of knee pain include infections. For example, Lyme disease, which is caused by bacteria transmitted by tick bites, frequently causes knee pain.

Fibromyalgia, a condition characterized by fatigue and muscle pain, may cause knee pain, especially in children. Many other conditions ranging from chronic fatigue syndrome to lupus to sickle cell anemia can cause widespread pain and may affect the knees.