In Emory Orthopaedics, we perform hundreds of hip and knee replacement operations every year. Most of these operations are highly successful, but occasionally, a reoperation is necessary. In these cases, it’s usually because the materials originally used in the knee or hip have worn down over time.
At Emory, we’ve been testing new and improved biomaterials for use in hip and knee replacements for more than a decade, and we’ve found that a new generation of biomaterials is making a significant difference in the longevity of these replacements. This means fewer patients will need reoperations down the road.
The failure rate in a knee replacement operation is directly related to how fast the knee wears, which is affected by how well the knee was put in, the patient’s activity level, the patient’s weight, and the wear resistant properties of the materials used. Think of the tires on a car. The stronger the tire material, the longer it lasts. What if, instead of getting 30,000 miles on a set of tires, you could get 100,000 miles? In essence, that order of magnitude difference is similar to the improved wear resistance of new biomaterials used in hip and knee replacement.
I’ve been using alternative bearing surface materials in hip replacements for about 12 years, starting with metal on metal, then ceramic on ceramic, and now highly cross-linked polyethylene. All three materials have dramatically improved wear resistance and have worked very well for several thousand patients, with no measurable wear on any patient visible through x-rays. But while metal on metal and ceramic on ceramic are useful in hip replacements, they aren’t an option in knee replacements. Until recently, this was also true of cross-linked polyethylene. However, the more recent second generation techniques for cross-linking now make this a viable option for knee replacement also.
Polyethylene, simply put, is a plastic formed from long molecular chains made of carbon and hydrogen atoms linked together. Prior to cross-linking manufacturing techniques, these molecular chains consisted of carbon atoms linked to other carbon atoms in single long chains with the remainder of the molecular bonding sites filled with hydrogen atoms. On a molecular level, cross-linking simply means that the single chains now are cross bonded together to, in essence, create a woven structure. This results in a material that looks identical but is actually a more wear-resistant form of plastic.
Over the past three to five years, we’ve performed approximately 1,000 knee replacement surgeries using cross-linked polyethylene. While all three materials—metal, ceramic, and polyethylene—appear to perform fairly evenly in hip replacement surgery, cross-linked polyethylene is less expensive than ceramic on ceramic. Although the individual patient does not experience a cost difference, this is a benefit to the industry as a whole. Our goal is to develop improved materials that will result in better outcomes and be cost-effective.
If you’re having knee or hip surgery, you can trust your doctor to choose the most effective material for you. Regardless of whether it’s metal on metal, ceramic on ceramic, or cross-linked polyethylene, with all of these new biomaterials, we are cautiously optimistic that wear may no longer be a problem.
Have you had or are you going to have hip or knee replacement surgery? Have you had experience with any of the new biomaterials? We’d like to hear from you. Please take a moment to give us feedback in the comments section below.
About James R. Roberson, MD:
James R. Roberson, MD, chairman of the Department of Orthopaedics and professor of Orthopaedic Surgery, specializes in treating hip and knee arthritis and has performed more than 10,000 hip and knee replacements over the course of his career. Dr. Roberson has practiced at Emory since 1982.