Knee pain can be caused by injuries, mechanical problems, types of arthritis and other problems. A knee injury can affect any of the ligaments, tendons or fluid-filled sacs (bursae) that surround your knee joint as well as the bones, cartilage and ligaments that form the joint itself.
ACL injury: An ACL injury is a tear of the anterior cruciate ligament (ACL) — one of four ligaments that connect your shinbone to your thighbone.
Fractures: The bones of the knee, including the kneecap (patella), can be broken during falls or auto accidents.
Torn meniscus: The meniscus is the tough, rubbery cartilage that acts as a shock absorber between your shinbone and thighbone.
Knee bursitis: Some knee injuries cause inflammation in the bursae, the small sacs of fluid that cushion the outside of your knee joint so that tendons and ligaments glide smoothly over the joint.
Patellar tendinitis: Tendinitis causes irritation and inflammation of one or more tendons — the thick, fibrous tissues that attach muscles to bones. This inflammation can happen when there's an injury to the patellar tendon.
Courtesy: Mayo Clinic