The knee joint has three parts. The thigh bone (femur) meets the large shin bone (tibia) forming the main knee joint. This joint has an inner (medial) and an outer (lateral) compartment. The kneecap (patella) joins the femur to form a third joint, called the patellofemoral joint.
A joint capsule surrounds the knee joint with ligaments strapping the inside and outside of the joint (collateral ligaments) as well as crossing within the joint (cruciate ligaments). These ligaments provide stability and strength to the knee joint.
The meniscus is a thickened cartilage pad between the two joints formed by the femur and tibia. The meniscus acts as a smooth surface for the joint to move on. Fluid-filled sacs called bursae, which serve as gliding surfaces that reduce friction of the tendons, surround the knee joint. Below the kneecap, there is a large tendon (patellar tendon) that attaches to the front of the tibia bone. The large muscles of the thigh move the knee. The knee also rotates slightly under guidance of specific muscles of the thigh. The knee functions to allow movement of the leg and is critical to normal walking and is a weight-bearing joint.