The knee joint is a relatively complex anatomical structure. In addition to a variety of ligaments to maintain stability and the presence of large muscle groups, internally, it is a classic example of a synovial joint. Both the femur and tibia are enclosed in a joint capsule lined with synovial tissue. Between the condyles of the femur and the condylar surface of the tibia are menisci, which serve as shock absorbers for the knee joint, located medially and laterally inside the joint. Between each muscle group are fluid-filled sacs called bursa and the presence of fat bodies named for their location, which reduce friction and lend added protection to the joint capsule.During normal activity such as walking or running, and even for support while standing, the knee will function superbly. It can tolerate moderate stress without significant injury. However, the knee lacks support to withstand many types of injury, especially rotational forces such as those seen in many athletic activities. Knee injuries, even though minor, may require surgery, and if they involve the cartilage, may have delayed healing time due to a lack of blood supply to the cartilage.