A hip replacement is a medical technology that substitutes a damaged ball-and-socket joint with engineered parts. It is commonly used when arthritis, injury, or wear destroys cartilage and makes walking painful. The goal is to reduce pain, restore smoother motion, and help the body transmit force safely from the pelvis to the femur.
This procedure shows how anatomy, materials science, and mechanics work together in modern medicine.
In a total hip replacement, the femoral head is removed and replaced with a metal or ceramic ball attached to a stem placed inside the femur. The socket side of the pelvis receives a cup and a low-friction liner, often made from polyethylene, ceramic, or metal. The implant must handle repeated loads that can reach several times body weight during walking or climbing stairs.
Good design spreads stress, reduces friction, and allows bone to grow around or bond to the implant.
Key Facts
- A natural hip is a ball-and-socket joint formed by the femoral head and the acetabulum.
- Cartilage reduces friction in a healthy hip, but arthritis can wear it away and expose rough bone.
- A typical hip implant includes a stem, neck, ball, cup, and liner.
- Joint pressure is approximately P = F/A, where F is force and A is contact area.
- Friction force can be modeled as Ff = μN, where μ is the coefficient of friction and N is normal force.
- During walking, the hip joint can experience forces about 3 to 5 times a person's body weight.
Vocabulary
- Acetabulum
- The cup-shaped socket in the pelvis that holds the head of the femur.
- Femoral stem
- The long implant part inserted into the upper femur to support the artificial ball.
- Liner
- The smooth bearing surface inside the artificial socket that allows the ball to move with low friction.
- Osteoarthritis
- A joint disease in which cartilage breaks down, causing pain, stiffness, and rough bone contact.
- Osseointegration
- The process in which living bone grows onto or into an implant surface to help hold it in place.
Common Mistakes to Avoid
- Thinking the implant replaces the whole pelvis, which is wrong because only the joint surfaces and upper femur connection are replaced.
- Ignoring friction, which is wrong because low-friction materials are essential for smooth motion and reduced wear.
- Assuming a stronger metal always makes the best implant, which is wrong because stiffness, wear, corrosion resistance, and compatibility with bone also matter.
- Forgetting that hip forces exceed body weight, which is wrong because muscle forces and body motion can multiply the load across the joint.
Practice Questions
- 1 A patient has a mass of 70 kg. Estimate the force on one hip during walking if the joint force is 4 times body weight. Use g = 9.8 m/s^2.
- 2 An implant contact area is 0.0008 m^2 and the joint force is 2800 N. Calculate the average pressure using P = F/A.
- 3 Explain why a hip replacement uses a smooth liner and rounded ball instead of simply attaching two rough metal surfaces together.