The Normal Knee 

The knee is one of the major weightbearing joints of the body. It is a hinge joint, bending and straightening like a hinge itself. The bending motion of the knee is referred to as flexion, and the straightening as extension. These are the main movements but there is significant rotation which can occur at the knee when it is bent.

Knee structure

The knee is made up of two joints, the bigger one between the tibia (shin bone) and the femur (thigh bone), and the smaller one between the patella (kneecap) and the femur.

The major joint is between the large rounded end of the femur and the relatively flat upper surface of the tibia. Obviously this joint does not ‘fit’ like the ball and socket-type joints such as the hip. It looks unstable if the bony parts are looked at alone, but there are important structures which ensure the stability and safety of the knee.

The bony surfaces of the knee joint and kneecap are lined by articular cartilage, a firm slippery material about 3mm thick. This allows painless and effortless movement of the joint even under a load.

“The cartilages”

Inbetween the large round ends of the femur and the flat top of the tibia are the menisci (singular = meniscus) which are often called the knee cartilages. These are almost circular flexible structures which look a little like banked tracks. They fill up some of the gap between the two major knee components but their function is not well understood.

It is these structures which often get damaged in twisting injuries of the knee in sports such as football. In some cases keyhole surgery (arthroscopy) may be performed to assess any damage and trim any loose parts of the meniscus.

The capsule and ligaments

The knee is surrounded by a strong fibrous bag called the joint capsule, which helps hold the joint together. The capsule is lined by the synovial membrane, which secretes synovial fluid. This lubricates the joint and contributes towards the nutrition of the articular cartilage.

The knee has four major ligaments which stabilise the joint and prevent unwanted movement. These are tough fibrous bands attached at each end to the bone. The ligaments are:

• Medial collateral ligament - found on the inside of the knee, it prevents the knee from being forced into the knock-knee position.
• Lateral collateral ligament - found on the outside of the knee, it prevents the knee from being forced into the bow-leg position.
• Anterior cruciate ligament, deep inside the joint, prevents the shin bone from slipping forward from under the thigh bone. If it is ruptured, the knee is unstable.
• Posterior cruciate ligament, acts like the anterior cruciate to prevent the opposite movement.