Show Number 913


How do artificial implants work in the body?

David learns how orthopedic surgeons are replacing diseased joints.


The hip joint is located where the upper leg bone or femur meets the pelvis. At the tip of the femur is a femoral head which is ball-shaped to fit into an opening in the pelvis called the acetabulum. This ball and socket mechanism at the hip must be stable enough to bear the weight of the upper body and rotate freely enough to give the femur a free range of motion. The bones of the acetabulum and the femoral head are separated by both cartilage and a syrup-like lubricant called synovial fluid which surrounds it to cushion the bones and keep them from rubbing together.

When walking and other simple movement requiring rotation of the hip become painful, or when a hip joint is traumatized or dislocated, an orthopedic surgeon will examine the patient and take X-rays of the hip joint. A common cause of joint pain in middle or old age is osteoarthritis which causes deterioration of the cartilage and the growth of bone spurs. When cartilage can no longer cushion the joint and is combined with the additional bone material, the grinding of bone against bone becomes very painful.

The practice of orthopedic surgery has evolved through the centuries from just physical manipulation of the musculoskeletal system to actual replacement of muscles, bones and cartilage with synthetic materials. Total hip replacement has been used in the United States since 1971. Today, hip replacement has become a routine operation with artificial hips designed and manufactured by computer.

During the hip replacement procedure, the head of the femur is removed and replaced with a titanium prosthesis or small metal ball. The new metal femoral head is made stable by attaching it to a metal stem which is inserted into the thigh bone. The acetabulum is then replaced by a metal cup. While the range of motion after recovery from the surgery does not match that of the original hip, the constant pain of bone-to-bone contact is permanently relieved.

Things to Talk About

  1. Why do doctors use titanium for replacements?
  2. What other joints can orthopedic surgeons replace?
  3. Do you anyone who's had a joint replaced?
  4. Would you want this procedure? Why or why not?

Acetabulum--An opening in the pelvis in which the femoral head fits into to create a joint.

Cartilage--A hard, but flexible tissue in the skeleton.

Synovial Fluid--A thick fluid found in movable joints. It nourishes and lubricates the cartilage covering the joints.

Orthopedics--The correction or prevention of skeletal deformities.

Prosthesis--An artificial device to replace a missing part of the body.


Additional Sources of Information

Activity Page

Hip and Seek

By taking a close look at animals' skeletons you will see how your pelvis and femur work together for movement.

Main Activity

Other animals' joints work similarly to ours. By investigating their skeletons we learn about the placement and functions of our own bodies.


1. Place the owl pellet on a paper towel. Gently pull it apart with the tweezers and your fingers.

2. Separate the bones and sort them by size and shape. Look for the ball joints of the hind legs and the sockets of the hip bones. Compare the size of the femur of your pellet with your classmates'.

3. Fit the femur into the socket. Move the leg around in the socket. Add a few drops of vegetable oil and move the ball and socket again. Use the sand paper to sand down the ball end of the femur and try it in the socket again.

4. Look for other joints that connect to form the skeleton.


1. What happened when you added the oil? How did sanding the ball joint change the motion of the femur? What do you think happened to the cartilage?

2. What other joints did you find? Was the movement the same?

3. Could you rebuild the whole skeleton? How could you display it? (See Newton's Apple's "Dinosaurs: Assembly" for extra hints!)

Interview a physical therapist about orthopedic surgery. How do they help hip replacement patients recover? Are there special techniques used to build muscle tone or recover motion?

Look for evidence of man-made joints in common objects. Make a list of all you find. Does the joint restrict or enhance their movements? What human joints are similar to a door hinge?

Find out how much we rely on joints to move. Try walking without bending your knees, eating without bending your wrists and writing with only one bent finger. Can you think of other joint challenges?

Explore the hip's range of movement with a friend. Lie down on the floor face up and bend one of your knees to your chest. Have your friend measure and record the distance from the middle of your kneecap to the end of your nose. Now rotate your knee toward the middle of the body and measure the distance to the other kneecap. Finally, rotate your knee away from your body and measure back to the other kneecap. Repeat these procedures again but this time lift your leg of the floor and stretch it out to the side. Measure from the ends of both big toes. Switch places with your friend so you can compare your measurements.

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Educational materials developed with the National Science Teachers Association.

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