The knee joint joins the thigh with the leg and consists of two articulations: one between the femur and tibia, and one between the femur and patella. It is the largest joint in the human body. The knee is a mobile trocho-ginglymus, which permits flexion and extension as well as slight internal and external rotation. The knee joint is vulnerable to both acute injury and the development of osteoarthritis.
It is often grouped into tibiofemoral and patellofemoral components. (The fibular collateral ligament is often considered with tibiofemoral components.)
The knee is a hinge type synovial joint, which is composed of three functional compartments: the femoropatellar articulation, consisting of the patella, or "kneecap", and the patellar groove on the front of the femur through which it slides; and the medial and lateral femorotibial articulations linking the femur, or thigh bone, with the tibia, the main bone of the lower leg. The joint is bathed in synovial fluid which is contained inside the synovial membrane called the joint capsule. The posterolateral corner of the knee is an area that has recently been the subject of renewed scrutiny and research.