Meniscus tear: A tear in the rubbery cushions of the knee joint.
Ligament tears: E.g. anterior cruciate ligament (ACL). The medial collateral ligament (MCL) is the most commonly injured ligament of the knee.
Fractures of the kneecap, lower portion of the femur, or upper part of the tibia or fibula. Knee fractures are most commonly caused by abnormal force, such as a falling on the knee, a severe twisting motion, severe force that bends the knee, or when the knee forcefully hits an object.
Kneecap dislocation. This type of dislocation occurs more frequently in 13- to 18-year-old girls. Pieces of bone or tissue (loose bodies) from a fracture or dislocation may get caught in the joint and interfere with movement.
Knee joint dislocation. This is a rare injury that requires great force. It is a serious injury and requires immediate medical care.
Bursitis: Inflammation of the small sacs of fluid that cushion and lubricate the knee.
Tendinitis: Inflammation of the tendons or small tears in the tendons
Plica syndrome: Thickening or folding of the knee ligaments.
Patellofemoral pain syndrome: Pain in the front of the knee from overuse, injury, excess weight, or problems in the kneecap
Iliotibial band syndrome: Irritation and inflammation of the band of fibrous tissue that runs down the outside of the thigh.
Osteoarthritis (degenerative joint disease) may cause knee pain that is worse in the morning and improves during the day. It often develops at the site of a previous injury. Other types of arthritis, such as rheumatoid arthritis, gout, and lupus, also can cause knee pain, swelling, and stiffness.
Osgood-Schlatter disease causes pain, swelling, and tenderness in the front of the knee below the kneecap. It is especially common in boys ages 11 to 15.
A popliteal (or Baker’s) cyst causes swelling in the back of the knee.
Infection in the skin (cellulitis), joint (infectious arthritis), bone (osteomyelitis), or bursa (septic bursitis) can cause pain and decreased knee movement.
A problem elsewhere in the body, such as a pinched nerve or a problem in the hip, can sometimes cause knee pain.
Osteochondritis dissecans causes pain and decreased movement when a piece of bone or cartilage or both inside the knee joint loses blood supply and dies.