A knee replacement is not one single piece. It is a system of carefully designed components that work together to replace damaged joint surfaces, reduce arthritis-related pain, and help the knee move more smoothly.
This article explains the main parts of a knee replacement, what materials are commonly used, how the components attach to bone, and why recovery still matters even with a well-designed implant.
Quick Answer
A knee replacement is usually made of metal and medical-grade plastic components. The metal parts typically replace the damaged surfaces of the thighbone and shinbone, while a strong plastic spacer acts like artificial cartilage between them.
Some knee replacements also include a plastic component on the underside of the kneecap. The exact implant design, materials, and fixation method depend on your knee, your surgeon’s plan, and the type of replacement being performed.
Free Roadmap
Not sure where you are in knee replacement recovery?
Download the Total Knee Replacement Recovery Roadmap to see what usually matters most at each stage, how to think about swelling and stiffness, and when to progress, hold steady, or modify.
Key Takeaways
- A knee replacement is made of multiple components, not one single artificial knee.
- The main parts are the femoral component, tibial component, plastic spacer, and sometimes a patellar component.
- Metal parts are commonly made from cobalt-chromium or titanium-based alloys.
- The plastic spacer is usually made from medical-grade polyethylene.
- The implant can be fixed to bone with cement, press-fit/cementless fixation, or sometimes a hybrid approach.
- The implant replaces damaged joint surfaces, but recovery still rebuilds strength, motion, confidence, and function.
What Are the Main Parts of a Knee Replacement?
A total knee replacement usually includes several components that work together. These parts are designed so the knee can bend, straighten, and tolerate daily activity with less pain than the damaged arthritic joint.
AAOS explains that up to three bone surfaces may be replaced during a total knee replacement: the lower end of the femur, the top surface of the tibia, and sometimes the back surface of the patella. AAOS knee replacement implant overview
The Femoral Component
The femoral component replaces the damaged surface at the end of the thighbone, also called the femur.
This part is usually made of metal. It is shaped to curve around the end of the femur so the knee can bend and straighten. It also has a groove that helps the kneecap move as the knee moves.
Why the femoral component matters:
- It recreates the smooth surface on the end of the thighbone.
- It works with the plastic spacer to allow smoother motion.
- Its shape helps guide knee movement during bending and straightening.
The Tibial Component
The tibial component replaces the top surface of the shinbone, also called the tibia.
In many knee replacements, this includes a metal tray that sits on top of the tibia. Some designs may also include a stem for added stability. A plastic insert usually sits on top of the tibial component.
Why the tibial component matters:
- It creates a stable base for the new knee joint.
- It supports the plastic spacer.
- It helps distribute load through the shinbone during standing and walking.
The Plastic Spacer
The plastic spacer, also called the polyethylene insert, sits between the metal femoral and tibial components.
This spacer acts like artificial cartilage. It helps the metal surfaces glide against a smooth plastic surface instead of rubbing metal directly on metal.
AAOS notes that knee replacement components are designed so metal interfaces with plastic, which helps provide smoother movement and reduce implant wear. AAOS knee replacement implant overview
Why the plastic spacer matters:
- It helps create a smooth gliding surface.
- It absorbs and distributes force between the metal components.
- It is one of the parts that can wear over time.
The Patellar Component
The patellar component is used when the underside of the kneecap is resurfaced.
Not every knee replacement includes patellar resurfacing. In some cases, the surgeon may leave the kneecap surface alone. In other cases, the underside of the patella is resurfaced with a plastic component.
Why the patellar component matters:
- It can help the kneecap glide against the femoral component.
- It may be used when the kneecap surface is damaged or arthritic.
- The decision depends on the surgeon’s assessment and the condition of the kneecap.
What Materials Are Used in Knee Replacements?
Knee replacement implants are designed to be strong, smooth, and compatible with the body.
Common materials include:
- Cobalt-chromium alloys: Strong metal materials often used for femoral components.
- Titanium-based alloys: Strong, lightweight metals often used in implant designs.
- Medical-grade polyethylene: Durable plastic used for the spacer and sometimes the patellar component.
- Ceramic or ceramic-metal materials: Used in some implant designs depending on the manufacturer and surgeon preference.
AAOS notes that implant metal parts are commonly made from titanium- or cobalt-chromium-based alloys, while plastic parts are made from medical-grade polyethylene. AAOS knee replacement implant overview
How Are Knee Replacement Components Attached?
The components need to stay securely attached to bone. Surgeons commonly use cemented fixation, cementless fixation, or sometimes a hybrid approach.
Cemented fixation
Cemented fixation uses a fast-curing bone cement to secure the implant to the bone. This is a common method and provides immediate fixation.
Cementless fixation
Cementless fixation, sometimes called press-fit fixation, relies on the implant fitting tightly against the bone. The implant surface is designed so bone can grow into or onto it over time.
Hybrid fixation
Some knee replacements may use a combination of cemented and cementless fixation.
The best fixation method depends on factors like bone quality, implant design, age, activity goals, surgeon preference, and the specific situation.
How Long Does a Knee Replacement Last?
Knee replacements are designed to be durable, but they are not indestructible.
Many knee replacements last for many years, but implant lifespan can vary based on the person, implant design, surgical alignment, activity level, body weight, bone quality, and whether complications occur.
Over time, possible reasons for revision surgery can include implant loosening, wear of the plastic spacer, infection, instability, stiffness, fracture, or progression of problems around the implant.
Factors that may influence implant lifespan include:
- Implant design and material.
- Surgical positioning and alignment.
- Activity level and impact exposure.
- Body weight and overall health.
- Bone quality.
- Infection or other complications.
- How the knee is used over time.
This does not mean you need to be afraid to use your knee. It means the goal is to build strength, move well, and choose activity progressions that support long-term function.
Do Different Implants Feel Different?
Different implant designs can feel different, but the implant is only one part of how the knee feels after surgery.
Swelling, stiffness, quad weakness, walking quality, balance, scar sensitivity, expectations, and strength can all influence how “normal” the knee feels during recovery.
That is why two people with similar implants can have different recovery experiences.
Does the Implant Replace the Need for Rehab?
No. The implant replaces damaged joint surfaces, but it does not automatically restore strength, motion, endurance, confidence, or walking quality.
After surgery, your body still has to recover from the operation and rebuild function.
Recovery still needs:
- Swelling management.
- Knee bending and straightening.
- Quad activation.
- Walking practice.
- Strength training.
- Stair progression.
- Balance and confidence.
A useful way to think about it is this: the implant changes the joint surfaces, but rehab rebuilds the function.
Questions to Ask Your Surgeon About the Implant
You do not need to become an implant engineer before surgery, but it is reasonable to ask basic questions.
Useful questions include:
- What type of knee replacement implant do you plan to use?
- Will my implant be cemented, cementless, or hybrid?
- Will the kneecap be resurfaced?
- Why is this implant a good fit for my knee?
- Are there any material considerations based on allergies or sensitivities?
- What activities should I avoid or modify long term?
- What signs would suggest a problem with the implant later?
Common Mistakes
- Thinking a knee replacement is one single part.
- Assuming the implant alone determines how recovery will go.
- Believing the knee should feel completely normal immediately after surgery.
- Thinking a more expensive or newer implant is automatically better.
- Ignoring the role of swelling, motion, strength, and walking quality after surgery.
- Not asking your surgeon what type of implant and fixation method they plan to use.
Related Learning
If you are learning about knee replacement surgery or preparing for recovery, these articles may help:
FAQ
What is a knee replacement made of?
A knee replacement is usually made of metal and medical-grade plastic components. The metal parts commonly replace the damaged bone surfaces, and the plastic spacer acts like artificial cartilage between them.
What are the main components of a knee replacement?
The main components are the femoral component, tibial component, plastic spacer, and sometimes a patellar component on the underside of the kneecap.
What is the plastic spacer in a knee replacement?
The plastic spacer is usually made from medical-grade polyethylene. It sits between the metal components and helps create a smooth gliding surface for knee movement.
Are knee replacement parts cemented in place?
Some knee replacements use cemented fixation, while others use cementless or press-fit fixation. Some may use a hybrid approach. The best method depends on your bone quality, implant design, and surgeon’s plan.
Does the kneecap get replaced in knee replacement?
Sometimes. In some cases, the underside of the kneecap is resurfaced with a plastic component. In other cases, the surgeon may choose not to resurface it.
Does the implant determine how good my recovery will be?
The implant matters, but recovery also depends on swelling, motion, quad activation, strength, walking quality, balance, confidence, and how the knee responds over time.
