Different Types of Knee Replacement Implants
This checklist of factors to finding the best implant may seem overwhelming, but luckily, this decision is usually left in the hands of your orthopedic surgeon. After all, surgeons have preferred implant manufacturers and only a couple different styles they work with. Very rarely do surgeons operate with multiple brands and more than a few styles. Your surgeon should be an expert in the exact type of implant you will receive.
With that said, let’s get you acquainted with all things knee replacement implants, shall we? This way, you can assist your care team in matching you with an implant that best matches your lifestyle, activity level and other requirements.
Here’s What the Ideal Knee Implant Should Have Going For It:
- Allow for normal activities and motion
- Last the expected 15-20 years or more.
- Have a good track record of use in joint replacement recipients (this should be 5-10 years minimum).
- Meets your condition, needs, and any additional requirements (e.g. is biocompatible for those with nickel allergies). Your surgeon will ask about lifestyle, allergies and so forth.
- Be a familiar brand and style to the chosen surgeon.
What is My Knee Implant Made Up Of?
A natural knee joint is composed of three compartments: the medial compartment (the inner side of the knee), the lateral compartment (the outer side of the knee), and the patello-femoral compartment (underneath the kneecap). During a total knee replacement all three compartments of the knee are replaced, while during partial knee replacements only one compartment is replaced. There are four artificial components that compose a total knee replacement implant: a tibial component, femoral component, patellar component, and a plastic spacer.
*Image from http://orthoinfo.aaos.org/topic.cfm?topic=A00712
- Tibial component— The flat part that attaches to the top of the resurfaced shin bone at the front of the leg (tibia). This part is usually made of metal platform with a polyethylene (plastic) insert. This part is usually made of softer metal, like titanium alloy.
- Femoral component— The largest, curved part that attaches to the end of the resurfaced thighbone (femur). This part comes in various metal options or ceramic. Most commonly, this part is made of cobalt-chromium alloys which are most durable since this part engages in the most movement.
- Patellar component— A dome-shaped piece that replaces the damaged knee cap that rubs against thighbone. This part is only used in some knee replacement surgeries and is made of polyethylene (a durable plastic).
- Plastic Spacer— The plastic spacer fits between the tibial and femoral components. As the name suggests, it’s made of plastic (polyethylene). This flexible plastic spacer provides a smooth, gliding surface for your new knee joint to bend and flex.
The Materials: What is a knee joint made of?
Wait, what kind of metal is used in knee replacements?!
Artificial replacement parts can be made of strong plastic, metal, or ceramic. All knee replacements, partial or total, will have parts made of a few different materials (likely metals and plastic). In most cases, each component is built from titanium, cobalt-chromium alloys, or a titanium and cobalt mixed metal. The chosen materials must be durable, allow for some flexibility with movement and be biocompatible (meaning it will not be rejected, corrode nor react with the body).
Cobalt-Chromium Alloys— Chromium alloys are one of the most commonly used materials in implant components. This metal is biocompatible, tough, durable and will not be broken down inside the body. This material is commonly used for the femoral implant since it is very sturdy and the femoral implant has a lot of traction against the plastic spacer during movement. For those with nickel allergies, some particles from this metal can get into the bloodstream and cause a reaction. Please let your surgeon know if you have an allergy as there are alternatives.
Titanium and Titanium Cobalt— Pure titanium is not used nearly as commonly as titanium cobalt. Titanium cobalt is used as frequently as cobalt-chromium alloys in joint replacement components. Titanium alloys are biocompatible, will not corrode, nor change inside the body. Titanium cobalt is more elastic and favorable to the natural bone surrounding the implant. Since this metal is softer, it tends to make up the tibial part of the replacement where the plastic insert locks in. In the tibial component, the metal choice is less important since there is little traction or rubbing during movement.
Polyethylene— This is a type of strong plastic found in the the tibial patellar (kneecap) component and the plastic spacer. The plastic components allow the metal pieces to glide smoothly throughout movement. This material is metal-free.
For Those with Nickel Allergies...
If you have a history of skin reactions to metal jewelry or are unsure if you may be allergic, it might be worth getting formally tested by an allergist. In general, the femoral component is the only artificial component that those with a nickel allergy need to be conscious of when selecting a material and manufacturer. Majority of the femoral parts are made from cobalt-chromium since it is a very tough metal. The tibial component (the other component made of metal) is usually made of softer, titanium alloy, which has very low (almost untraceable) amounts of nickel.
Implant manufacturers, BioMet and Smith & Nephew, both make nickle-free versions of the femoral part.
Another option to eliminate nickel to the nth degree is to pair a ceramic femoral implant with a zirconium alloy tibial component. This new material has zero nickel and is thought to be competitively long-lasting. In saying this, ceramic implants are more costly and can be more susceptible to fracturing since the material is brittle.
Types of Knee Replacement Implants
According to Dr. Trevor North, orthopedic surgeon, “Most surgeons use one style of implant for a significant majority of knee replacement cases. Only about 10% of total cases may require a different style of implant.” He adds, “There are certain nuances and different tools for each implant. You want a surgeon who is very familiar with the style of replacement you are receiving.”
The most common type of knee replacements is called a “single radius knee”. It’s commonly accepted that this type of implant offers the greatest level of extension and flexion for majority of patients, regardless of age, gender, activity level and so forth.
Other variations in the type of replacement received are fixed bearing and mobile bearing. This has to do with how the plastic insert locks into the tibia component. Majority of replacements are fixed bearing, meaning that the plastic insert is locked into the metal tibial plate. In a mobile-bearing knee, the plastic insert in the tibial component can rotate inside the tibial tray. An overwhelming majority of the replacements produced by the various manufacturers are classified as single radius, fixed bearing implants.
Find the Best Knee Replacement Implant: Top Brands
There are more than a dozen manufacturers of artificial knee implants. However, like anything else, some knee implant manufacturers have a larger market share than others. In the US, the most popular knee replacement parts manufacturers are Zimmer (which has merged with Biomet), Stryker, DePuy and Smith & Nephew.
Here are the top brands and styles used by surgeons in the US.
Zimmer is the most popular manufacturer of knee replacements. In fact, according to Zimmer, more than 25% of knee replacements implanted worldwide are a Zimmer product. That’s a whole lotta implants!
- Zimmer Persona Knee— An updated total knee design, based on the patella-femoral designs from Zimmer that have decades of clinical history.
- Gender Solutions Natural-Knee Flex— Different design for men and women, allowing for PCL-retaining and PCL-sacrificing solutions.
- Zimmer Gender Solutions NexGen High-Flex Knee—Two different designs said to better match the size and shape of female and male joint anatomy.
- Zimmer NexGen LPS-Flex Mobile and LPS Mobile Bearing Knees— A mobile bearing option offered that allows for greater motion and flex.
- NexGen Complete Knee Solution Legacy Knee Posterior Stabilized (LPS) LPS-Flex Fixed Bearing Knee— A fixed bearing option that allows for maximum flex, designed for those with increased activity. Zimmer claims that this solution allows for 155 degrees of active flexion.
- Zimmer Gender Solutions Patello-Femoral Joint (PFJ) System— A partial knee replacement solution designed differently for men and women.
Biomet, who has recently merged with Zimmer and is now called “Zimmer Biomet”, offers a nickel-free implant for those with allergies. They also offer an “antioxidant” polyethylene (plastic) material insert component to pair with their tibial and femoral parts. In saything this, there is nothing to prove that antioxidant plastic holds any true benefit. Admittedly, it sounds pretty good though!
- Signature Personalized Patient Care— During surgery, this implant does not require metal instruments to be placed inside bone canal. The claim is that this results in a less invasive surgery.
- Vanguard Complete Knee System— Traditional knee replacement implant that comes in 90 different size combinations for a personalized fit.
- Vanguard XP Preserving Knee— Designed for surgeons who want to keep a patient’s ACL (anterior cruciate ligament) intact. Oxford Partial Knee— Designed to repair only the inside of the knee, removing 75% less bone and cartilage than a total knee replacement.
Among the most popular manufacturers in the US, Stryker offers total, partial, and revision knee replacement systems that are suitable to a wide variety of patients. Stryker implants are used in over 100 countries.
- Triathlon Total Knee— Designed to feel natural, and allow up to 150 degrees of flexion.
- Triathlon Tritanium Total Knee— A cementless total knee replacement option.
- Scorpio NRG— A fixed bearing design that Stryker claims more closely resembles the motion of a mobile bearing design, without sacrificing stability and risking dislocation.
- Scorpio Single Axis— Among the most popular implants with surgeons with more than 500,000 implanted worldwide over the last decade.
- Triathlon PKR— A fixed bearing, partial knee replacement option that preserves more bone.
- AVON Patello-Femoral— A partial replacement option for patients with isolated patello-femoral arthritis.
DePuy offers the widest variety of mobile bearing (also known as rotating platform) models of all the major manufacturers in the US. Only two manufacturers make mobile bearing implants. These types of implants are said to reduce the wear and tear on the tibial implant, but if the ligaments surrounding the implant are not strong enough to support this type of implant, the risk for dislocation is higher. DePuy also produces fixed bearing total knee systems.
- Sigma Total Knee System— Available in fixed and mobile-bearing designs, this knee system covers a wide variety of patient and surgeon preferences.
- Attune Knee System— A new, fixed bearing system from DePuy.
- LCS Complete Knee System— Another product that can either be mobile bearing or fixed. A cementless option is also available. This is a flexible option that was developed from an original, 1977 DePuy implant design.
- AOX Antioxidant Polyethylene— Similar to Biomet Zimmer’s antioxidant polyethylene option, this product is made from “next generation” materials.
Smith & Nephew offers a wide range of replacement variations that fall under their “Legion” family. This includes replacements made from oxidized zirconium, a nickel-free metal that’s suitable for patients with metal allergies.
- Legion TKS— This replacement is available for many variations of replacements, including revisions and hinge knee replacements. The replacement can also be made with oxidized zirconium.
When it comes to selecting a knee replacement material, style, type and manufacturer, you don’t have to settle for less. In saying this, it’s most important to find an orthopedic surgeon that you trust and follow their lead when it comes to the best recommendation for knee implants. Although the style, brand and material do matter, it’s your trusted care team that will make the biggest impact on the success of your surgery.
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