A knee replacement is made of metal, plastic, and sometimes bone cement, assembled as four separate components that resurface the damaged joint. The metal parts cap the ends of your thighbone and shinbone, a tough plastic spacer sits between them to mimic cartilage, and an optional plastic button covers the kneecap. Each piece is engineered from specific materials chosen for durability, low friction, and compatibility with living bone.
The Four Parts of a Knee Implant
A total knee replacement doesn’t replace the entire knee. It resurfaces the worn-out ends of the bones that meet at the joint. The implant has four distinct components, each with a specific job.
The femoral component is a curved metal shell that fits over the bottom of your thighbone. It’s shaped to recreate the natural rounded contour of the bone, allowing the knee to bend and straighten smoothly. The tibial component is a flat metal platform that sits on top of the shinbone. It acts as a foundation to hold the plastic spacer above it. Between these two metal pieces sits the polyethylene liner, a dense plastic insert that functions like artificial cartilage, providing a smooth, low-friction gliding surface. Finally, some surgeons add a patellar component, a small plastic disc cemented to the underside of the kneecap so it tracks cleanly against the new metal surface of the thighbone.
Metals Used in Knee Implants
The femoral component is almost always made from cobalt-chromium, an alloy with a long track record of success in joint replacements. It’s extremely hard, resists corrosion inside the body, and polishes to a mirror-smooth finish that reduces friction against the plastic spacer. The tibial tray is made from either titanium or cobalt-chromium. Titanium is lighter and bonds well with bone tissue over time, while cobalt-chromium offers superior hardness. Both deliver excellent long-term results.
For patients with metal sensitivities, particularly allergies to nickel or cobalt, alternative materials exist. One option is oxidized zirconium, sold under the brand name Oxinium. It starts as a zirconium metal alloy that undergoes a manufacturing process to transform its surface into a ceramic-like layer. The result combines the smoothness and wear resistance of ceramics with the strength of metal. Because it contains virtually zero cobalt, chromium, or nickel, it significantly reduces the risk of an allergic reaction to the implant.
That said, the science around hypoallergenic implants is still evolving. A study presented by the American Association of Hip and Knee Surgeons found that patients who received a hypoallergenic implant actually had nickel ion levels in their joint fluid 3.6 times higher than patients with standard cobalt-chromium implants. This counterintuitive finding suggests that choosing the right implant for metal-sensitive patients isn’t as straightforward as swapping one material for another.
The Plastic Spacer
The plastic insert between the metal components is made from ultra-high-molecular-weight polyethylene, often shortened to UHMWPE. This isn’t ordinary plastic. It’s an extremely dense, durable polymer engineered specifically for use inside the body. Its job is to absorb shock and allow the two metal surfaces to glide against each other with minimal friction, much like natural cartilage does in a healthy knee.
Wear of this plastic spacer has historically been the limiting factor in how long a knee replacement lasts. Tiny plastic particles shed over years of use can trigger inflammation and gradually loosen the implant from the bone. To combat this, manufacturers developed highly cross-linked polyethylene, which has a tighter molecular structure and wears down significantly less than the original material.
A newer refinement adds antioxidants, most commonly vitamin E, directly into the polyethylene during manufacturing. This helps the plastic resist oxidation, the slow chemical degradation that weakens it over time. Early and mid-term clinical data show that antioxidant-stabilized polyethylene performs at least as well as previous generations, with excellent resistance to oxidation, no reported cases of bone loss around the implant, and no revisions due to plastic wear. These improvements are one reason modern knee replacements routinely last 20 years or more.
How the Implant Attaches to Bone
Most knee implants are secured to the bone with a special surgical cement called polymethyl methacrylate, or PMMA. It’s a two-part system: a powder and a liquid that the surgical team mixes in the operating room. When combined, the components undergo a chemical reaction that causes the mixture to harden in place within minutes, locking the metal implant firmly against the prepared bone surface. This reaction generates heat as it cures, which is normal and managed during surgery.
Some implants, particularly the tibial component, can be designed for “cementless” fixation instead. These have a porous or textured surface that encourages your bone to grow directly into the implant over the weeks following surgery. Titanium is especially well suited for this approach because bone tissue bonds to it readily. Whether your surgeon uses cement, a cementless design, or a combination of both depends on factors like your bone quality, age, and activity level.
Custom Implants From 3D Printing
Standard knee implants come in a range of preset sizes, and the surgeon selects the closest match during the procedure. But 3D printing has opened the door to fully custom implants designed from CT scans of your specific leg, from hip to ankle. A 3D printer builds the implant layer by layer, matching the exact contours of your anatomy rather than approximating them.
The practical advantage is precision. A custom implant allows the surgeon to remove less bone during the procedure and size the plastic insert more accurately. It also gets the ligaments around the knee closer to the tension they had with the original joint, which can translate to a more natural feeling knee. Companies like Conformis manufacture these patient-specific implants, and some orthopedic centers now use them as a standard option for patients whose anatomy doesn’t fit well into off-the-shelf sizes.
The core materials in custom implants remain the same: cobalt-chromium or titanium for the metal parts, polyethylene for the spacer. What changes is the geometry. Instead of a one-size-fits-most approach, every angle and curve is tailored to your joint.