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How the Knee and Hip Work

Posted: June 1, 2006

The knee is the largest joint in the body
The knee is made up of the lower end of the thigh bone (femur), which rotates on the upper end of the shin bone (tibia), and the knee cap (patella), which slides in a groove on the end of the femur.

Large ligaments attach to the femur and tibia to provide stability. The long thigh muscles give the knee strength.

The joint surfaces where these three bones touch are covered with articular cartilage, a smooth substance that cushions the bones and enables them to move easily.

All remaining surfaces of the knee are covered by a thin, smooth tissue liner called the synovial membrane. This membrane releases a special fluid that lubricates the knee and reduces friction in a healthy knee to almost zero.

Normally, all of these components work in harmony. But disease or injury can disrupt this harmony, resulting in pain, muscle weakness and less function.

The hip is one of your body's largest weight-bearing joints
The hip consists of two main parts: a ball (femoral head) at the top of your thigh bone (femur) that fits into a rounded socket (acetabulum) in your pelvis.

Bands of tissue called ligaments connect the ball to the socket and provide stability to the joint.

The bone surfaces of your ball and socket have a smooth, durable cover of articular cartilage that cushions the ends of the bones and enables them to move easily.

A thin, smooth tissue called synovial membrane covers all remaining surfaces of the hip joint. In a healthy hip, this membrane makes a small amount of fluid that lubricates and almost eliminates friction in your hip joint.

When all these parts work together in a healthy way, your body moves easily and without pain.

Source: American Academy of Orthopaedic Surgeons

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