A total knee replacement is not one single part. It is a precision system of up to four knee replacement implant components, each engineered from a specific material to do a specific job. The femoral component caps the end of your thighbone. The tibial component anchors into your shinbone. A plastic spacer sits between them and acts as your new cartilage. And in many knees, a small patella button resurfaces the back of your kneecap.
Patients ask me about this all the time, and I think it is one of the best questions you can bring to a surgical consultation. When you understand what is going into your body and why, the whole conversation about surgery changes. So here is the same walkthrough I give in my office, component by component.
What Are the Knee Replacement Implant Components?
During knee replacement surgery, I remove the damaged bone and cartilage lining the joint and resurface it with artificial parts. Up to three bone surfaces may be replaced: the end of the femur, the top of the tibia, and the back surface of the patella.
The knee replacement implant components that make this work are:
- Femoral component: a curved metal cap for the end of the thighbone
- Tibial component: a metal tray and stem that anchor into the shinbone
- Spacer: a medical grade plastic insert that serves as the new cartilage
- Patella button: an optional plastic dome for the back of the kneecap
Together, these parts typically weigh between 15 and 20 ounces. That is roughly the weight of a basketball, recreating a joint that carries you through every step of your day.
The Femoral Component
The femoral component curves around the end of the femur, your thighbone. It is most often made from a cobalt chromium alloy. We use cobalt chromium here for a simple reason: it is hard, highly polished, and scratch resistant, and it holds up under decades of repeated load. Titanium is actually too soft for this surface in the knee because it scratches more easily.
Look closely at a femoral component and you will see a groove running down the center. That groove is not decorative. It recreates the natural track that your kneecap glides through every time you bend and straighten your knee. Getting that track right is a big part of getting a knee that feels natural.
The Tibial Component
If the femoral component is the crown of the new knee, the tibial component is the foundation. It is a flat metal platform, usually titanium or cobalt chromium, that sits on the prepared top surface of the tibia, your shinbone. Most designs include a stem that extends down into the center of the bone for added stability.
The tibial tray has one more job: it holds the spacer. The tray is built with a locking surface so the plastic insert seats securely on top of it. Some designs skip the metal tray entirely and attach an all plastic component directly to bone, but the metal backed tray is the most common construct I use.
The Polyethylene Spacer
The spacer is the quiet hero of the whole system. It is a cushion of medical grade polyethylene, a strong plastic, that fits between the femoral component above and the tibial tray below. In a real sense, this is your new cartilage. It is the smooth, low friction surface that the metal femoral component glides across thousands of times a day.
Here is a design principle worth knowing: in a knee replacement, metal never rubs directly against metal. The components are designed so metal always moves against plastic. That pairing gives smoother motion and far less wear over time.
The spacer is also the part most likely to wear out, which is why material science matters so much here. Modern highly cross linked polyethylene resists wear dramatically better than the plastic used a generation ago. That single improvement is one of the biggest reasons today’s knee replacements last as long as they do.
The Patella Button
The fourth component is optional. The patella button is a small dome shaped piece of polyethylene that resurfaces the back of the patella, your kneecap, so it glides smoothly in the groove of the femoral component. It usually has two or three small pegs on the back that fix it to the prepared bone.
Not every knee needs one. Whether I resurface the patella depends on the condition of your kneecap cartilage and your specific anatomy. It is a judgment call made for each patient, and it is a good question to ask your surgeon directly.
What Are Knee Replacement Implant Components Made Of?
Pulling the materials together in one place:
- Metal parts: titanium or cobalt chromium alloys. Cobalt chromium dominates on the moving surfaces because of its hardness and scratch resistance. Titanium is common in the tibial tray because it is light, strong, and friendly to bone.
- Plastic parts: medical grade polyethylene, often highly cross linked for wear resistance. This is the spacer and the patella button.
- Ceramics: some implants use ceramic or ceramic metal materials such as oxidized zirconium. These play a smaller role in knees performed in the United States, but they are an option I discuss with patients who have a known metal sensitivity.
Every material in the system has to meet the same standards. It must be biocompatible, meaning your body accepts it without a rejection response. It must be strong enough to carry your full body weight, flexible enough to bear stress without breaking, and able to move smoothly against its partner surfaces for years.
Patients sometimes worry about their body rejecting the implant. True rejection, the way we think about it with organ transplants, is exceedingly rare with these materials. Even most patients with skin sensitivity to metals tolerate knee implants well, because that kind of allergy involves a different part of the immune system. If you have a metal allergy history, bring it up. We have options, and it is a conversation worth having before surgery.
How Knee Replacement Implant Components Stay Attached to Bone
Materials are only half the story. The components also have to stay fixed to your bone, and there are three ways we do that.
- Cemented fixation: the components are secured with a fast curing bone cement called polymethylmethacrylate. This is the most common method.
- Cementless fixation: the implant is press fit onto the bone. The bone facing surfaces are textured or coated so your own bone actually grows into the implant over time, locking it in place.
- Hybrid fixation: a combination, often cementing the tibial and patellar components while press fitting the femoral component.
The right choice depends on your bone quality, your age, and your activity level. There is no single answer that fits every patient.
How Long Do Knee Replacement Implant Components Last?
Modern implants are expected to last 15 to 20 years, and the majority of patients can anticipate long term success in the range of 80 to 85 percent at 10 to 15 years after surgery. The core materials have been refined rather than reinvented over the past 30 years, and that long track record is a feature, not a flaw. We know how these materials behave over decades.
The spacer remains the component most likely to wear with time. If it does, the plastic can often be exchanged without replacing the entire implant. That is another reason the metal plastic metal design has endured.
What you do matters too. A healthy body weight reduces the load on the implant with every step. Staying active with joint friendly exercise like walking, swimming, and cycling keeps the bone and muscle around your new knee strong without overstressing the bearing surface.
Choosing the Right Implant Design for Your Knee
There are more than 150 knee implant designs on the market today. Some preserve your posterior cruciate ligament. Others substitute for it with a post and cam mechanism built into the components. Some spacers are fixed to the tray, while mobile bearing designs allow a few degrees of rotation.
Here is what I want every patient to hear clearly: there is no strong evidence that one brand or design is superior to the others in function or longevity. What matters is matching the implant to your anatomy, your bone quality, your ligaments, and your goals, and then executing the plan precisely. That philosophy is the same one behind personalized knee replacement, where we tailor alignment and balance to the knee you were born with rather than forcing every patient into the same template.
Key Takeaways
- A total knee replacement uses up to four components: femoral, tibial, spacer, and patella button.
- The metal parts are titanium or cobalt chromium. The spacer and patella button are medical grade polyethylene.
- Metal always glides on plastic, never metal on metal, which protects the joint from wear.
- The polyethylene spacer is your new cartilage and the part most likely to wear over time.
- With more than 150 designs available, the right implant is the one matched to your anatomy and goals, not a particular brand name.
To learn more from trusted patient education sources, see the American Academy of Orthopaedic Surgeons overview of knee replacement implants and the American Association of Hip and Knee Surgeons guide to what knee replacements are made of.
Medical Disclaimer: This information is for educational purposes only and should not replace professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare provider before making decisions about your health. Individual results vary based on personal health circumstances.
Ready to Talk About Your Knee?
If knee pain is limiting your life and you want to understand whether knee replacement is right for you, I would be glad to walk you through your options, including the implant components we would choose for your specific anatomy. Request an appointment at the Bone and Joint Institute of Tennessee to start the conversation.
About the Author Dr. Cory Calendine, MD is a board certified orthopaedic surgeon at the Bone and Joint Institute of Tennessee specializing in hip and knee replacement. His practice focuses on personalized joint replacement, robotic assisted surgery, and patient education.
