The main parts of a knee replacement are metal, plastic, and sometimes cement

A knee replacement uses three types of material: metal alloys for the femoral component (thighbone side) and tibial baseplate (shinbone side), polyethylene plastic for the spacer between them, and sometimes bone cement to hold the pieces in place. The exact composition varies by manufacturer and implant design, but the metals are almost always cobalt-chromium or titanium alloys chosen for strength and resistance to wear. The plastic is ultra-high-molecular-weight polyethylene (UHMWPE), a durable polymer that mimics the cartilage your knee lost.

Your surgeon will choose from several implant brands and designs before surgery. The choice depends on your age, bone quality, activity level, and whether you need a standard replacement or a custom-fit version. Some implants are cemented into place; others are press-fit or have a porous coating that allows bone to grow into them over time. Understanding what your implant is made of helps you know what to expect during recovery and how long it might last.

Key Takeaways

  • Knee replacements use cobalt-chromium or titanium alloys for the metal parts because these metals resist corrosion and wear better than other materials.
  • The plastic spacer is ultra-high-molecular-weight polyethylene, the same type of plastic used in other medical implants, and it can wear down over 15 to 20 years.
  • Bone cement (polymethylmethacrylate) is optional and used only in cemented implants; cementless implants rely on bone growth or friction to stay in place.
  • Your surgeon selects the implant brand and material composition based on your age, bone density, and activity level before surgery.
  • Most modern knee replacements are designed to last 15 to 20 years, though the plastic component may need replacement sooner in very active patients.

Metal alloys: cobalt-chromium and titanium

The metal parts of your knee replacement are almost always either cobalt-chromium or titanium alloy. Cobalt-chromium is the older standard and remains the most common choice. It is harder and more wear-resistant than titanium, which makes it ideal for the femoral component (the part that slides against the plastic). Titanium alloys are lighter and bond more easily to bone, so they are often used for the tibial baseplate when a cementless design is planned.

Both metals are biocompatible, meaning your body does not reject them. They do not rust or corrode in the body's environment the way ordinary steel would. Surgeons sometimes choose one over the other based on your bone quality and the implant system available. Some implants use cobalt-chromium for the femoral component and titanium for the baseplate in the same knee. The choice rarely affects your outcome, but your surgeon will explain which metal your implant uses.

Ultra-high-molecular-weight polyethylene: the plastic spacer

The spacer between the metal parts is made of ultra-high-molecular-weight polyethylene (UHMWPE), a type of plastic engineered to be much stronger and more durable than everyday plastics. It is the same material used in hip replacements and other joint implants. The plastic is molded into a flat or curved shape and sits on top of the tibial baseplate, allowing the femoral component to glide smoothly across it as you bend and straighten your knee.

UHMWPE wears slowly over time. Most implants are designed to last 15 to 20 years before the plastic thins enough to require revision surgery. Younger, more active patients may wear through the plastic faster—sometimes in 10 to 15 years. Newer formulations of UHMWPE, called cross-linked polyethylene, wear more slowly than older versions, so your surgeon may choose a newer implant if you are young or very active. The plastic itself does not cause pain or problems as it wears; you straightforward notice increased looseness or instability in the knee over time.

Bone cement: optional but common in older patients

Bone cement is a fast-setting adhesive made of polymethylmethacrylate (PMMA), a type of acrylic plastic. It is used only in cemented knee replacements, where the surgeon applies it between the metal implant and your bone to lock the pieces in place when ready. The cement hardens in minutes, allowing you to bear weight on the knee within days. Cemented implants have a long track record and are often chosen for older patients or those with weak bone density.

Not all knee replacements use cement. Cementless implants rely on a porous or textured surface that allows your bone to grow into the implant over several months. This process, called osseointegration, creates a biological bond that can last as long as a cemented implant. Cementless designs are often preferred for younger patients because they preserve more bone and may allow for easier revision if needed later. Your surgeon will recommend cemented or cementless based on your age and bone quality.

How implant design affects material choice

Knee replacement implants come in several designs, and the design influences which materials are used. A standard implant replaces the damaged surfaces of the femur and tibia with metal and plastic. A patellofemoral implant replaces only the kneecap and the groove it slides in, using less material overall. A custom implant is made from a CT scan of your knee and may use slightly different proportions or materials to match your anatomy more closely.

Rotating-platform designs use a plastic spacer that can rotate slightly on the baseplate, which some surgeons believe reduces stress on the cement or bone. Fixed-bearing designs keep the plastic locked in place. Both work well; the choice is often based on your surgeon's training and preference. Regardless of design, the core materials remain the same: metal, plastic, and sometimes cement.

Wear and longevity of knee replacement materials

The plastic spacer is the component most likely to wear out first. Modern UHMWPE is much more durable than the plastic used in implants from the 1990s, but it still wears slowly with every step. The rate of wear depends on your weight, activity level, and how well the metal surfaces are polished. Rougher metal surfaces wear the plastic faster. The metal components themselves rarely wear out; they are designed to last decades.

Bone cement can also break down over time, especially if the implant shifts slightly. This is why cemented implants typically last 15 to 20 years before revision becomes necessary. Cementless implants may last longer because they do not rely on cement to stay in place. If you need a second knee replacement later, your surgeon will remove the old implant and cement, then place a new one. This revision surgery is more complex than the first replacement but uses the same materials.

Allergies and sensitivities to implant materials

True allergies to implant metals are rare. Nickel is present in some cobalt-chromium alloys in small amounts, but the amount is so small and the metal is so stable that nickel reactions almost never occur. If you have a documented nickel allergy, tell your surgeon before surgery; they can choose a titanium implant instead, which contains no nickel. Allergies to the plastic or cement are even rarer and are not a practical concern for most patients.

Some patients experience mild inflammation around the implant in the first few weeks after surgery, but this is a normal response to the surgery itself, not an allergy to the materials. If you have concerns about metal sensitivity or chemical sensitivities, discuss them with your surgeon during your pre-surgery consultation. They can review the exact composition of the implant you will receive and address any specific worries.

Frequently Asked Questions

Can I have an MRI after knee replacement?

Yes. Modern knee implants are made of non-magnetic metals and are MRI-safe. Tell the MRI technician that you have a knee implant so they can note it in your file, but you do not need to avoid MRI scans. Older implants from the 1980s and early 1990s may have had magnetic components, but this is extremely rare now.

Will I feel the plastic wearing down in my knee?

No. Plastic wear is silent and painless. You may notice increased looseness or a feeling that the knee is not as stable as it once was, but you will not feel the plastic thinning. Your surgeon can see wear on X-rays during follow-up visits and will discuss revision surgery if wear becomes significant.

What happens if the cement breaks?

If cement breaks or loosens, the implant may shift slightly, causing pain or instability. Your surgeon will see this on X-rays during routine follow-ups. If loosening is mild and you have no symptoms, you may not need surgery right away. If pain develops or the implant becomes very loose, revision surgery to replace the implant and cement is the standard treatment.

Are newer implant materials better than older ones?

Newer polyethylene formulations wear more slowly than older plastic, and newer implant designs are often more anatomically accurate. However, well-made implants from 10 or 15 years ago still function well. Your surgeon will choose an implant based on current evidence and your individual needs, not straightforward because it is the newest option available.

Can I get a metal-on-metal knee replacement?

No. Metal-on-metal designs are used in some hip replacements but not in knees. The knee joint requires a plastic spacer because the forces and motion patterns are different from the hip. All modern knee replacements use metal-on-plastic or ceramic-on-plastic combinations.