What Are Knee Replacements Made Of?
Knee replacement surgery involve replacing the worn-out bone and cartilage lining your knee joint with new implants that are composed of materials such as ceramic, metal and plastic.
Over the last two decades, the number of total knee replacements being performed each year has steadily increased in the United States. Accompanying this boom in surgery has been the release of information via the news, social media, documentaries and internet. If you’re considering knee replacement surgery, it’s best to be informed by reliable sources about what is being placed inside of your body. AAHKS has created this guide to provide all the information and research related to materials used in knee replacement surgery.
Total Knee Replacement Materials
Total knee replacements are made of 4 parts as well (see Figure 1 below):
- Femur (thigh)—top part
- Tibia—(shin) bottom piece
- Liner—serves as your new cartilage
- Patella (kneecap)—this is an optional part of the surgery, and some physicians do not use this as part of the procedure.
The Femur
Typically made from cobalt-chromium as this material is scratch resistant and safe for use in total knee replacements. The majority of total knee femoral components are made out of cobalt-chromium with excellent long-term success rates. Titanium is often too soft (scratches easily) to be a femoral component in the knee, and ceramics have a much smaller role in procedures done in the United States.
The Tibia
Made from either titanium or cobalt-chromium with excellent results for each. This portion of the replacement is meant to hold the plastic liner. Some tibial components are made of all plastic.
The Polyethylene (Plastic) Liner
This essentially is your new cartilage and fills the space between the two metal pieces. This is the “weak link” and is what will possibly wear out with time.
The Patella “Button”
If your surgeon elects to use this, the undersurface of the patella (knee cap) can be covered with a piece of a plastic. This is optional, and not all surgeons routinely use this implant.
The success rate for total knee replacement is outstanding with modern implants being expected to last between 15-20 years. Total knee replacement has been amongst the most successful orthopaedic surgeries in restoring function and quality of life. Total knee replacements have become incredibly popular in the last two decades rising at greater rates than hips (nearly a 2:1 ratio). The basic materials are the same for both hip and knee replacements, although ceramics are not routinely utilized in knee implant materials in the United States.
Long-Term Outlook
As we continue to pursue perfect knee replacements, there are likely to be some designs and materials that perform better than others. The bulk materials used to make knee replacements are quite similar and have not changed drastically in the last 30 years. Reaction and complications directly related to the materials themselves, while possible, are not common as manufacturer guidelines and standards are strictly followed and monitored.
The majority of total knee patients can anticipate long-term success somewhere between 80-85% at 10-15 years after the surgery.
Despite recent concerns for the metals and materials used in knee replacement, the vast majority of these procedures lead to successful outcomes with improvement in patients’ quality of life and function.
Talk to Your Surgeon
AAHKS recommends discussing implant options with your surgeon to gain an understanding of the particular materials to be used in your surgery.
Discuss with your surgeon the risks and benefits of different implants and materials to gain an understanding of the proposed procedure.
Developing a good relationship and level of trust with your surgeon is a key aspect of ensuring a successful outcome.
In the end, your surgeon is looking out for your best interests, and as much as you trust an airline pilot and the flight plan they follow, you should have the same trust in your surgeon.
Frequently Asked Questions about Knee Replacement Materials
What is the Chance of My Body “Rejecting” the Implant?
Unlike an organ transplant, the risk of your body rejecting the artificial knee parts is exceedingly rare. The materials used in the typical replacement surgery are well tolerated by the body and have a long track record of successful implantation. While in rare cases the parts may become loose or infected, this is typically related to other factors and not due to your body “rejecting” the parts.
How Do I Know if I Have a Recalled Implant?
Most modern knee replacement parts have a long history of excellent safety and few known mechanical issues. In recent years, there have been a handful of very specific implants involved in a recall process due to metal reactions, higher than expected failure rates and other unanticipated problems.
Most companies offer numerous models and designs of their implants (like the car makers and the different makes and models they offer). It is important to keep in mind that just because one model has been recalled, the company is likely to have many others that are performing very well.
The vast majority of patients will not experience an issue with a recalled implant. If you are concerned about your particular type of replacement, we recommend you contact your surgeon’s office to ensure that your implant has not been involved in a recall.
I Have a History of Nickel Allergy and/or Break Out in a Rash With Certain Types of Jewelry. How Do I Know if I am Allergic to the Implants?
The metals used in knee replacements are generally well tolerated by the body, even in patients who have skin sensitivity to certain metals (this type of allergy involves a different part of your immune system).
For more detailed information, please see the FAQ on metal allergy and joint replacement.
How Do I Know if I Have Developed Metal Poisoning From the Replacement Parts?
While recent concerns have been raised about the potential for developing metal poisoning (cobalt or chromium toxicity) from knee replacement parts, such cases are thought to be exceedingly rare. In some cases (typically associated with very specific models of hip replacement – most of which are no longer used by surgeons), excessive levels of metal ions may be generated by the implant surface contact points. These metal ions may in turn cause a reaction in the tissue around the joint and can, in isolated cases, lead to tissue and/or bone destruction.
If your implant is functioning well and you have little pain or change in comfort level, chances are you are at very low risk of a metal reaction.
If you are experiencing new or worsening pain, contact your surgeon to be evaluated. Again, cases of metal poisoning from orthopedic implants are very rare and generally occur in the tissues around the knee involved. While in theory knee implants can lead to elevated levels of metal ions in the blood, systemic side effects of metal poisoning from joint replacements (kidney damage, neurologic symptoms, psychosis) are exceedingly rare.
Can I Have a Knee Replacement if I am Allergic to Metal?
Yes, you can still have a joint replacement if you are allergic or sensitive to metal. Metal reactions are very rare. If there is a concern about metal allergy this is an individual discussion to have with your surgeon
