A knee replacement is a medical device system that restores a painful, damaged knee joint by resurfacing the ends of the bones. Instead of replacing the whole knee with one solid piece, surgeons cap the worn surfaces of the femur and tibia with engineered components. This matters because osteoarthritis, injury, or disease can destroy smooth cartilage, making each step painful and unstable.
The goal is to reduce pain, improve motion, and allow the joint to carry body weight more smoothly.
Key Facts
- A total knee replacement resurfaces the distal femur, proximal tibia, and sometimes the back of the patella.
- Typical components include a metal femoral cap, a metal tibial tray, and a polyethylene plastic spacer.
- Pressure = force / area, so spreading body weight over a larger contact area lowers pressure on the joint surfaces.
- Knee joint torque can be estimated with τ = Fd, where F is force and d is the moment arm from the joint center.
- Low-friction materials reduce sliding resistance, described by Ff = μN, where μ is the coefficient of friction and N is the normal force.
- Implants must balance strength, wear resistance, biocompatibility, and accurate alignment with the leg.
Vocabulary
- Femoral component
- The metal cap placed on the end of the thigh bone to replace the damaged joint surface.
- Tibial tray
- The metal platform fixed to the top of the shin bone that supports the plastic spacer.
- Polyethylene spacer
- A durable plastic insert that sits between the metal components and acts as the new low-friction bearing surface.
- Cartilage
- Smooth tissue that covers bone ends in healthy joints and allows them to glide with low friction.
- Biocompatibility
- The ability of a material to function in the body without causing harmful reactions.
Common Mistakes to Avoid
- Thinking a knee replacement replaces the entire leg joint with a hinge, which is wrong because most designs resurface bone ends while preserving controlled motion through shaped bearing surfaces and soft-tissue balance.
- Ignoring alignment of the implant, which is wrong because even strong materials can wear faster or feel unstable if forces do not pass through the joint correctly.
- Assuming the plastic spacer is weak because it is plastic, which is wrong because medical-grade polyethylene is engineered for low friction and wear resistance under repeated loading.
- Confusing reduced pain with unlimited force capacity, which is wrong because implants still experience stress, friction, and wear during high-impact activity.
Practice Questions
- 1 A person exerts a 700 N load through a knee implant. If the effective contact area is 0.002 m2, what is the average pressure on the bearing surface?
- 2 During a squat, the force on the knee is 1800 N and the effective moment arm is 0.04 m. What torque acts about the knee joint?
- 3 Explain why a knee replacement uses both metal components and a plastic spacer rather than making all contacting surfaces from the same metal.