Types of Knee Implants and Testing Methods

As the population ages, joints require mending and repair to ensure their continued proper functioning. In many cases, complete joint replacement is required to restore full functionality. One of the most common joint replacements today are knee replacements, which involves replacing the organic knee joint with an artificial one. In order to ensure that these artificial knee joints function correctly, extensive testing is required by highly trained experts in the joint replacement field.

Knight Mechanical Testing is a globally accredited medical device testing lab that specializes in testing orthopedic devices for FDA and regulatory approval. We have years of experience in testing various types of knee implants. As a result, our experts possess a deep level of understanding when it comes to each device, the materials, and necessary testing protocols based on the device’s individual profile.

This article serves as an educational guide about knee replacement devices as well as a guide for those in the orthopedic industry looking for information on validating a knee implant for regulatory approval.

Major Knee Implant Brands

There are several popular brands of knee implants on the market today, offering consumers several different styles and types depending on their preference and needs. The most popular brands are ZimmerBiomet, Stryker, DePuy and Smith & Nephew. These brands have the largest market share worldwide and offer fixed and mobile-bearing designs (also known as rotating platforms).

Zimmer Biomet – Formed from a merger between Zimmer and Biomet, this is by far the most popular brand on the market – accounting for over 25% of knee replacements worldwide.

Stryker – One of the most popular makers of knee replacements in the US, Stryker offers partial, total and revision knee replacements for any patient needs. These implants are approved for use in over 100 countries worldwide.

DePuy – DePuy Synthes is an orthopedic company that’s a subsidiary of Johnson & Johnson. DePuy produces the most popular brand for mobile-bearing knee replacements, they produced their original mobile implant in 1977. In addition to their mobile-bearing systems, they also offer fixed versions.

Smith & Nephew – Founded in 1856, they offer a wide range of knee replacement variations. Some of these replacements include those made from nickel-free oxidized zirconium which is ideal for patients with metal allergies.

The engineers at our orthopedic labs have consulted with some of the largest knee implant manufacturers regarding the testing and approval process on various knee replacement devices. Our experts understand what’s involved in testing each component of the device based on guidelines and requirements for various regulatory bodies across the world.

Knee Implant Components

Knee implants are generally made up of three separate components. The first is the femoral component, which is attached to the resurfaced end of the femur. This component is the largest and is subject to the highest amount of wear. The second component is the tibial component, which attaches to the top of the resurfaced tibia – also known as the shin bone. The third component is the plastic spacer, which fills the space between the femoral and tibial components. These spacers provide a smooth surface for the knee joint to flex and bend.

There is also another component sometimes used in knee replacement which is referred to as the patellar component. This is a dome shaped part that replaces the original kneecap that rubs against the thighbone. This is only used when required under certain circumstances during knee replacement surgery.

Femoral Component Testing – As the highest wear item in a knee replacement, the femoral component undergoes extensive testing to ensure its durability and performance meets the application requirements. This test is performed under ASTM F2345, which specifically focuses on ceramic femoral components. During this test, the fatigue strength of the ceramic modular femoral heads is evaluated.

Tibial Component Testing – This test examines the durability of the tibial tray component that holds the plastic spacer insert. This test is performed under ASTM F1800, which specifically focuses on the tibial components. As a part of this test, the fatigue strength of the tibial tray and any additional components is evaluated to ensure that it meets durability and lifecycle requirements.

Plastic Spacer Testing – The plastic spacer in a knee replacement is under a tremendous amount of pressure and strain due to being sandwiched between the femoral and tibial components. The spacer is made of polyethylene and is tested extensively under ASTM F2777. This test is performed to ensure that it meets endurance and deformation standards required for it to properly function in a knee replacement.

Properly testing knee implant components is vital in order to prevent issues with the device such as tibial plate loosening. Failures associated with knee replacement implants may cause serious health complications.

Materials

Knee replacements are usually made of several different materials. The most common of these are cobalt-chromium alloys which are used due to their durability and biocompatibility. The most common application for these is the femoral implant which sees a lot of traction against the plastic spacer during movement.

Titanium cobalt alloys are also used in implants, sometimes in place of cobalt-chromium for those who have nickel allergies. This alloy is also very well suited for the tibial section due to its elasticity and its ability to better form to the surrounding bone. Like cobalt-chromium alloys, it is durable and biocompatible.

The other main type of material for implants is a plastic known as polyethylene. This is a very strong polymer used in the spacer section of the knee replacement and allows the joint to move smoothly. It is also used in the patellar (kneecap) component when that is required to be replaced.

Types of Knee Replacement Devices

Generally speaking, surgeons use and specialize in only one type of knee replacement. The most common of these is referred to as the fixed bearing single radius knee, which offers the greatest level of extension and flexion for the majority of patients regardless of their particular situation or requirements.

The other type of knee implant used is the mobile-bearing knee replacement, which allows for the tibial component to rotate inside the tibial tray mounted on the tibia bone. This is normally only used in less than 10% of applications and requires the user to have strong ligaments to keep it stabilized.