Guide to Knee Replacement Implants and Their Manufacturers

Understanding FDA Oversight

The Food and Drug Administration (FDA) must approve all medical devices used in the U.S. The only exception is if you enroll in a clinical study. In a clinical study, the device used has been clinically proven and the manufacturer is testing it fully to meet FDA requirements. These tests include biocompatibility and verification that the device is able to withstand years of use while retaining its strength and shape during daily use.

Know that with any device—no matter how well designed and how successful it has been—there’s a risk of a defect or recall. Today’s prostheses are remarkably complex and problems occur in a small percentage of cases. The most common problems revolve around knee replacements that fail to bond correctly to the bone. This can lead to pain and additional surgery. However, even the slightest defect in manufacturing or tiniest error on the part of a surgeon can lead to severe problems. Problems are rare; overall, 90 percent of all patients who undergo total knee replacement experience minimal or no complications and have significantly improved knee function. The remaining 10 percent often require follow up surgery within 10 years.

Recalls are extremely rare. Past recalls have centered on devices that do not bond with the bone correctly or were manufactured differently than required specifications. If you’re unfortunate enough to encounter a recall you will require additional consultation with your surgeon and you may be required to undergo additional corrective surgery. It’s wise to check the FDA website periodically. It offers a search tool that lets you enter your specific device and find out if a recall has taken place.

Choosing a Device

There are a number of important factors that differentiate knee implants. A conversation with your surgeon will help you understand which device is right for you.

Implant Materials

Manufacturers offer products with different designs made from a variety of materials. Most devices consist of multiple components made from different medical-grade materials. Metal components usually consist of titanium or cobalt-chromium based alloys. These materials are known for their durability, and have stable (inert) chemical properties such that they do not interact with the body. However, in some rare occasions, patients may have allergies to certain metals that may be a determining factor in which implant is used.

The most common material used to produce plastic components is ultra-high molecular weight polyethylene. Polyethylene is common in many implantable devices because of its ability to glide smoothly within the mechanical joint and closely mimic the way a physical knee moves.

Biocompatibility is crucial. It’s essential that your body doesn’t reject any material used in an implant. It’s also critical that the material doesn’t leach into your body and that the device can withstand the same forces a human knee handles, including weight bearing loads and flexion, which is defined as your ability to bend the joint inward.

Composition/Type

Fixed-Bearing Prosthesis: Surgeons use fixed-bearing systems for the majority of patients. It is the Fixed-Bearing Prosthesis traditional method for implanting an artificial knee and it is often used for older patients, who are less active and unlikely to require a revision. The polyethylene from the tibial component of the device is attached to a metal component underneath. A femoral component rolls on the cushioned surface. The primary goal of any knee replacement is to reduce pain and improve flexibility. One study found that a 20 to 50 percent improvement in knee motion range is common using this approach.

Mobile-Bearing Prosthesis: These systems are designed to provide a few degrees of additional rotation for the knee, compared to a fixed-bearing device. Mobile-bearing prostheses allow the polyethylene insert to rotate short distances inside the metal tibial tray. Mobile-Bearing Prosthesis (femoral component not shown)This prosthesis requires greater support from surrounding soft tissues, including the ligaments. The additional support reduces the risk of dislocation. Studies show that results are on par with the use of a fixed-bearing prosthesis. However, doctors often prefer this approach for younger patients because mobile-bearing devices are designed to accomodate a more active lifestyle and to last longer. The procedure requires a more experienced surgeon.

One 2011 study of age 70 and older patients found that there was no significant difference in knee scores between the two types of implants. Both were equally effective. The mobile-bearing group had greater knee flexion at three and six months, but by two years both groups were similar.

Implant Variations

PCL-Related Features

Your surgeon will examine your posterior cruciate ligament (PCL)—the large ligament that runs along the back of your knee and provides flexion. Depending on its condition, your surgeon may recommend preserving or removing it during a TKR. Different types of implants exist to address either option.

Cruciate-Retaining

As the name implies, this implant enables the surgeon to preserve the ligament. It has a small groove that helps the ligament continue to provide flexion.

Posterior Stabilized

These implants employ a “cam and post” system that substitutes for the posterior ligament. It provides support on the posterior (back) of your knee.

Fixation Method

Surgeons use two types of methods to hold the prosthesis in place and bond the implant to your bone, allowing your leg and the implant to move as a single unit. Some surgeons use a combination of both methods. The most common approach uses a bone cement to hold components in place.

Cemented: The device provides a surface that is cemented to the bone using special cement.

Cementless: Instead of cement, these devices have a special surface that encourages bone to grow into the implant for fixation. Sometimes this method is used in conjunction with a bone graft (allograft).

Instrumentation & Personalization

As implants have improved, so has instrumentation. Knee implant systems come with special instruments called cutting guides that help the surgeon cut your bone precisely. This makes it possible to accurately fit the implant with the bone. In conjunction with computer-assisted surgery, surgeons are able to precisely align and fit knee implants inter-operatively.

Personalized Cutting Guides and Surgical Plans

With the aid of MRIs, surgeons and device companies have begun to produce personalized cutting guides that are ordered prior to surgery. This is useful in difficult cases. The system uses an MRI or CT scan as well as 3D imaging software to view a computer simulation of the knee and create a unique surgical plan. Nearly all manufacturers, including Stryker and Biomet, now provide custom cutting guides. Cutting guides are designed to work with various sizes of implants; custom made or “personalized” guides are not always necessary. Whether or not a medical team uses a personalized guide, surgical planning is important and you should not hesitate to speak to your surgeon about it prior to surgery.

Gender Specific Devices

Traditionally, manufacturers have designed knee implants based on “average” size data. This means a choice of only two or three different implants that vary in size. However, some manufacturers, such as Zimmer and Biomet, market artificial knees as gender-specific, or different for women versus men. Manufacturers claim that these prostheses take into account and adjust for anatomical differences between the male and female.

There’s no conclusive evidence that gender-specific artificial knees are more effective, and some argue that the devices are more about marketing than clinical results. The highest numbers of knee replacement surgeries are women, and a growing array of “women-specific” implants now exist. Although men’s and women’s knees are different, it’s important to note that orthopedic surgeons perform each surgery based on the patient’s unique anatomy and needs regardless of whether an implant is labeled “gender specific” or not. While the term “standard” implant has now become more synonymous with “male” and “narrow” implants have now become more synonymous with “female,” all devices come in different sizes and shapes to match the patient’s anatomy. In the end, surgeons ensure that the bone and device fit together properly.

***

Devices Features and Manufacturers

When researching implants for knee replacement, it is important to realize that while devices used in total knee replacement (TKR) surgeries are the most common, there are also devices designed for partial knee replacement surgery. Of these two types of procedures, partial knee replacement is less common, and addresses less than 10 percent of knee replacement patients. Unlike a TKR, a partial replacement requires removal of only part of the bone in the knee joint. Manufacturers produce variations of devices to address both surgical options, and your surgeon will let you know whether a total or partial replacement is appropriate.

Here are some of the device features that may guide you and your surgeon to the right selection:

• range of motion (maximum flexion and extension angles)
• customizability: size and shape (gender differences)
• stability
• your activity level

Speak to your surgeon about features that are important for you, and know that each device is designed for different types of patient needs, level of joint disease, and anatomy.

The history of a device and its success rate can prove comforting. Some manufacturers share this information. Your surgeon may be able to attest to past results or guide you to useful information online.

Total Knee Replacement

Here are some of the leading TKR devices. We’ve included summaries of product information from device manufacturers. It’s best to visit manufactures’ sites to understand device features and benefits in more detail.

Mobile Bearing

Zimmer: The NexGen® LPS-Flex Mobile and LPS-Mobile Bearing Knees—Both of these systems are designed to bend and rotate freely. The system uses a metal component that is attached at the end of the thigh bone (femur) as well as a metal base plate that replaces the top of the shin bone (tibia). It also includes a plastic articular surface that is loosely attached to the base plate and serves as artificial cartilage. The artificial cartilage is designed to provide 155 degrees of active flexion and 25 degrees of unimpeded internal/external rotation. A surgeon uses bone cement to fix the device to your bones.

Zimmer: The Zimmer Gender Solutions® Patello-Femoral Joint—This device is designed to serve active individuals and it is marketed as an early intervention option. It preserves more bone and requires a smaller incision. The use of this implant could require a surgeon to use special instruments in order to use a minimally invasive approach. The prosthesis claims to “more accurately” accommodate gender differences with its unique and specially shaped components that mirror the anatomy of men and women. It offers 35 different sizing options.

DePuy: DePuy Sigma® Rotating Platform Knees—As the name implies, this mobile-bearing system relies on a rotating platform to mimic natural knee movement. The manufacturer claims that the design reduces internal stress and wear. As a result, it is better equipped to tolerate regular impact and activities—and last longer. DePuy boasts that surgeons have implanted more than 1 million of these devices worldwide. Studies show that about 97 percent of these implants remain in use after 20 years.

Fixed Bearing

DuPuy: DePuy Sigma® Fixed-Bearing Knees—The Sigma prosthetic knee uses low-wear polyethylene and advanced metal components to produce a device that has been shown to have a 99.6 percent success rate after five years. This fixed-bearing knee is designed to maximize stability.

Smith & Nephew: Smith & Nephew OXINIUM®—The system is built to provide a high level of wear resistance. It is built with oxidized zirconium metal, making it more resistant to abrasion and scratching. The manufacturer claims that knee simulator wear testing of the “Oxinium” material using a knee simulator showed an 85 percent decrease in polyethylene wear compared to cobalt-chrome material that is commonly used in other implants.

Stryker Triathlon® Total Knee Replacement SystemStryker: Stryker Triathlon® Total Knee Replacement System—This fixed-bearing system is designed to work with the body to promote easier motion. Research has shown that it can lead to a more rapid return to functional activities after surgery. This is due to the single-radius design of the knee implant. The design allows for the implant to flex, extend and rotate in a circular motion, much like your natural knee.

Biomet: Biomet Vanguard® Complete Knee System—This device is designed to accommodate an individual’s specific anatomy and it is used for both men and women. It offers a high degree of flexion—typically about 145 degrees—and is available in 90 different size combinations and 10 femoral size options to increase overall comfort.

Wright: Wright EVOLUTION® Medial Pivot Knee—Wright’s EVOLUTION Medial-Pivot Knee System has been in use since 1998 and is still widely used today. This prosthesis uses a ball-in-socket design to create a high level of stability and simulate the movement and rotation of a natural knee. It provides essential motion for kneeling, descending stairs, and golfing. The manufacturer claims that the system provides 162 degrees of posterior-stabilized flexion, 152 degrees of cruciate-retailing flexion and 143 degrees of cruciate-substituting flexion.

Partial Knee Replacement (PKR)

Here are a few of the leading partial knee replacement systems:

BioMet: BioMet Oxford® Knee—This PKR device is designed for patients who suffer from limited knee arthritis, known as medial compartment arthritis. The prosthesis—which uses moveable plastic bearings—in implanted on only one side, the medial compartment, of the knee joint. BioMet reports that the procedure removes 75 percent less bone and cartilage. As a result, it is less painful and more likely to speed recovery. According to the manufacturer, it is also more conducive to a natural motion following recovery. Clinical tests on the device show that it has a 98 percent success rate after 10 years and 95 percent at 15 years.

DePuy: DuPuy Sigma® High Performance Partial Knee—The Sigma HP Partial Knee is designed for active patients who require a high degree of flexion but aren’t yet candidates for a TKR. It accommodates deep knee flexion and thus makes it easier to kneel, squat or sit cross-legged. A surgeon can use any of it’s components separately to replace any of the three compartments of the knee. This reduces bone loss and leaves as much of the native knee intact as possible.

Zimmer: Zimmer Unicompartmental (Uni Knee) High Flex Knee®—This PKR device safely provides high flexion and is designed to feel more natural throughout the range of motion. The Zimmer Uni High Flex Knee is designed to safely accommodate up to 155 degrees of flexion. Clinical studies demonstrate a 98 percent success rate after 10 years.

Personalized Systems

Here are two examples of the personalized knee replacement systems available:

Stryker: Stryker Custom Fit® Knee with ShapeMatch® Technology—ShapeMatch technology allows surgeons to perform knee procedures with customized guides designed to provide a fit more closely matching patients’

unique anatomical requirements. ShapeMatch Cutting Guides are developed for each patient based on the patient’s MRI or CT scans. ShapeMatch Technology is available for use only with Stryker’s Triathlon Knee System.

Biomet: Biomet Signature™ Knee—The Biomet Signature Personalized Patient Care system utilizes MRI or CT imaging to generate 3-D visualization options and fine-tune implant size and position to match the precise needs of the patient. The system is used in conjunction with Biomet’s Vanguard® Complete Knee System to create an ideal match.

Final Words

In the end, it’s essential to spend adequate time working with your surgeon to select the right device for you. There’s no simple path to choosing the right procedure and knee replacement device. Rely on your surgeon to discuss the options and evaluate your specific requirements and advice you on the artificial knee that fits you best.