Maya Ellison
Knee replacement surgery involves replacing worn-out bone and cartilage with new implants. The artificial knee joint is usually made of different types of metal and plastic. Metal caps replace the thigh bone and shin bone, while high-density plastic is used to replace damaged cartilage. The most common type of implant features a metal femoral component that rides on a polyethylene plastic spacer attached to the tibial component. While implants made entirely from plastic are not available, the plastic parts can be combined with ceramic or metal.
| Characteristics | Values |
|---|---|
| Materials Used | Metal, Plastic, Ceramic |
| Plastic Type | Polyethylene |
| Metal Type | Cobalt-Chromium, Titanium, Zirconium, Nickel |
| Ceramic Use Case | Patients with metal allergies |
| Bone Cement | Polymethyl methacrylate |
| Surgery Duration | 2 hours |
| Surgery Age Group | 60 and older |
| Surgery Frequency | Increased in the US over the last two decades |
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What You'll Learn
Metal-on-plastic is the most common type of implant
Knee replacement surgery involves replacing worn-out bone and cartilage with new implants made from materials such as ceramic, metal, and plastic. The artificial joint is called a prosthesis. The most common type of implant is metal-on-plastic. This type of implant features a metal femoral component that rides on a polyethylene plastic spacer attached to the tibial component. The metal components are most commonly made from cobalt-chromium or titanium alloys. In some cases, they are coated in zirconium if the patient has a pre-existing metal allergy.
The metal femoral component replaces the end of the thigh bone (the femur). The replacement part is usually made of metal. The underside of the kneecap (the patella) is usually replaced with a strong plastic cap. The top surface of the lower leg (the tibia) is replaced with a metal alloy platform, with a short stem that is anchored into the shinbone. A plastic spacer made of polyethylene is placed between the upper and lower leg components to help the artificial knee joint move smoothly.
The metal used in knee replacements is generally well-tolerated by the body, even in patients with skin sensitivity to certain metals. While cases of metal poisoning from orthopedic implants are very rare, in some cases, excessive levels of metal ions may be generated by the implant surface contact points, leading to a reaction in the tissue around the joint. Plastic particles from implants can also cause an immune reaction, leading to bone breakdown and implant failure. However, advances in manufacturing have greatly reduced the rate of wear in plastics.
The choice of materials in knee replacements can influence the success of the surgery and the longevity of the implant. Polyethylene plastic, which is used in the tibial component, provides a smooth, gliding surface for the metal femoral component, ensuring minimal wear and tear over time. It is important to discuss the risks and benefits of different implants and materials with a surgeon to gain an understanding of the proposed procedure.
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Ceramic-on-plastic is an option for people with nickel allergies
Artificial knee joints are typically made of metal and plastic. The metal alloys commonly found in knee prostheses include cobalt, chromium, titanium, and nickel. While metal allergies are rare, they can cause implant failure. The most common metal allergy is nickel, which is found in very small quantities in knee implants.
If you have a nickel allergy, ceramic-on-plastic knee joints are an option. Ceramic implants include oxidized zirconia (Oxinium), which utilizes a femur coated with oxinium and a tibia made of titanium, thus avoiding nickel allergy issues. Ceramic-coated implants are also available and are made of titanium metal for strength, with a ceramic coating applied to the portions of the implant that are exposed to soft tissues and bearing surfaces. Ceramic-coated implants are safer than fully ceramic implants, which can fracture.
Other options for nickel-allergic patients include adding a coated layer on cobalt-chromium and titanium implants. Most of these implants are designed to minimize allergic reactions to nickel. Titanium implants may also undergo a hardening process that creates a nitrogen-enriched zone on the surface of the implant, which has been shown to significantly reduce wear rates.
It is important to discuss any potential metal allergies with your surgeon prior to surgery. While metal allergies are rare, they can cause implant failure and may necessitate revision surgery.
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Metal-on-metal implants are rarely used now due to health concerns
Metal-on-metal implants are rarely used nowadays due to various health concerns. Metal-on-metal bearings showed early promise, with 1.5 million implanted worldwide as stemmed hip replacements and hip resurfacings. However, these devices experienced unexpectedly high short-term failure rates and have rarely been used since 2012. Metal-on-metal hip implants have a high failure rate and may lead to additional problems. Friction from normal wear produces particles that cause inflammation in the tissues around the joint. Over time, bone erodes and the implant loosens, leading to pain and decreased function. Metal-on-metal hips have raised other concerns, including potential harm from cobalt and chromium ions released into the bloodstream. These are associated with a range of potential health problems, including cancer, neurological difficulties, thyroid disease, heart disease, and cardiac failure secondary to metal ion toxicity.
In January 2013, the FDA published a recommendation to change the requirements for all metal-on-metal hip implants from premarket notification to premarket approval, the most stringent regulatory category of the FDA's oversight for medical devices. Since then, all manufacturers of metal-on-metal hip implants have been required to stop marketing their devices and submit premarket approval applications. As of now, there are no FDA-approved metal-on-metal total hip replacement devices marketed for use in the US.
Surgeons in most countries, including the US, no longer use metal-on-metal implants for total hip replacement. All-metal components are still rarely used in hip resurfacing, a procedure in which the head of the thighbone is reshaped and capped with a metal covering. However, this does not help the estimated 1.5 million people who already have metal-on-metal hips. These patients may require revision surgery, which is riskier, more expensive, and less successful than the original hip replacement due to bone loss.
While metal-on-metal hip implants have fallen out of favour due to health concerns, metal is still used in knee replacement surgery, along with other materials such as ceramic and plastic. The vast majority of these procedures lead to successful outcomes, with improvements in patients' quality of life and function. Cases of metal poisoning from orthopedic implants are very rare, and the metals used in knee replacements are generally well tolerated by the body.
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Ceramic-on-ceramic prostheses can shatter under pressure
Artificial knee joints are made up of several parts, including a metal cap that is placed on the end of the thigh bone, a platform that replaces the top surface of the lower leg, and a plastic spacer made of polyethylene between the upper and lower leg components. While most artificial knees have both metal and plastic parts, some surgeons use different materials, including metal-on-metal, ceramic-on-ceramic, or ceramic-on-plastic.
Ceramic-on-ceramic prostheses are a type of knee implant where both the femoral and tibial components are made of ceramic. Ceramic parts are known to be the least likely to react with the body. However, one potential drawback of ceramic-on-ceramic prostheses is their tendency to shatter under heavy pressure. In rare cases, the ceramic material can shatter into pieces, requiring surgical intervention for removal.
The brittleness of ceramic material contributes to its susceptibility to shattering under pressure. Its hardness makes it more prone to cracking when subjected to excessive force. This characteristic differentiates ceramic from materials like steel, which typically endure indentations rather than cracking under pressure.
To mitigate the risk of shattering, researchers have explored the application of an electric field during the formation of ceramics. This technique, known as flash sintering, introduces defects that make the ceramic more resistant to sudden failure. The electric field treatment slows down the shattering process, providing advance warning of potential cracks and enhancing the safety of ceramic usage.
While advancements have been made to increase the shatter resistance of ceramics, it is important to recognize that under sufficient pressure, ceramics can still crack. This inherent fragility poses challenges in certain applications, such as coating metal parts inside airplane engines, where the sudden shattering of ceramics could have severe consequences. Therefore, when considering the use of ceramic-on-ceramic prostheses, it is crucial to carefully assess whether the material can withstand the anticipated load and the potential presence of heavy impact loads, for which steel or stainless steel bearings may be more suitable alternatives.
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Partial knee replacements are possible if ligaments are intact
A partial knee replacement is a surgery to replace only one part of a damaged knee. It is a common treatment for arthritis in just one section or compartment of the knee. In this procedure, the damaged cartilage and bone in the affected area are removed and replaced with an artificial implant, while the rest of the knee is preserved.
Partial knee replacements are possible if the ligaments are intact. Generally, a patient must have intact knee ligaments to be considered for a partial knee replacement. For example, patients with an untreated torn ACL are often not considered for this procedure. The patient's anterior cruciate ligament (ACL) must be intact for a surgeon to use either a unicondylar fixed-bearing knee replacement or a mobile-bearing unicondylar knee replacement.
The knee is a complex structure made up of bone, muscles, cartilage, and ligaments. Ligaments and tendons connect the three bones of the knee: the femur (thigh bone), tibia (shin bone), and fibula. The ligaments help support the knee joint. In a partial knee replacement, the ligaments that support the knee joint are preserved.
Partial knee replacement surgery is a challenging procedure to perform. It is important to select the right patient to ensure a good functional outcome and longevity for the replacement knee. Improvements in surgical techniques have made partial knee replacement an increasingly viable option for a growing number of patients.
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Frequently asked questions
Yes, most artificial knee joints contain plastic parts. The metal femoral component rides on a plastic spacer attached to the tibial component. The tibial component is often made of polyethylene plastic.
The plastic spacer replaces the joint space and meniscus and helps the artificial knee joint to move smoothly.
Ceramic-on-ceramic and metal-on-metal are alternatives to plastic spacers. Ceramic-on-plastic is also an option.
