How is the knee designed, and what is its function?

The knee is a joint that has three compartments. This joint has an inner (medial) and an outer (lateral) compartment. The kneecap (patella) joins the femur to form a third compartment called the patellofemoral joint. The thigh bone (femur) meets the large shinbone (tibia) forming the main knee joint.
The knee joint is surrounded by a joint capsule with ligaments strapping the inside and outside of the joint ( ligaments) as well as crossing within the joint ( ligaments). These ligaments provide stability and strength to the knee joint.
The  is a thickened cartilage pad between the two joints formed by the femur and tibia. The meniscus acts as a smooth surface for motion and absorbs the load of the body above the knee when standing. The knee joint is surrounded by fluid-filled sacs called  which serve as gliding surfaces that reduce friction of the tendons. Below the kneecap, there is a large tendon () which attaches to the front of the tibia bone. There are large blood vessels passing through the area behind the knee (referred to as the popliteal space). The large muscles of the thigh move the knee. In the front of the thigh, the quadriceps muscles extend the knee joint. In the back of the thigh, the hamstring muscles flex 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 waking. The knee flexes normally to a maximum of 135 degrees and extends to 0 degrees. The bursae, or fluid-filled sacs, serve as gliding surfaces for the tendons to reduce the force of friction as these tendons move. The knee is a weight-bearing joint. Each meniscus serves to evenly load the surface during weight-bearing and also aids in disbursing joint fluid for joint lubrication.

The goal of this article is to give the reader an overview of causes of knee pain. Most of the topics covered in this article are further expanded in detail as separate articles. For example, bursitis, types of artjrotos, , and others are covered in articles devoted to knee pain-related topics that include specifics on diagnosis, tests, treatments, and other details.