We as orthopedic surgeons have 15+ year follow-up of fixed-bearing knees with excellent results in 90% to 95% of patients. The good survivorship out to 15 years is reassuring but when we look closer at the data, we must realize that many of these patients included in these studies are generally older and less active then some of the patient’s in whom we are currently placing total knees. As baby boomers age and years of activity take their toll, we are seeing more and more patients in their late 40s, 50s, and early 60s who require knee replacement. To obtain 15+ year survivorship in these patients, we may need different implants and/or materials if the results that have been presented in 70+ year-olds are to be equal. There are several different ways of attempting to improve longevity and they include improved bearing surfaces, more durable polyethylene, and mobile-bearing type knees.
Traditionally knee replacements are termed fixed-bearing knees because the polyethylene insert is fixed to the tibial tray. The surface geometry of the implant allows flexion, extension, and some degree of rotation. This results in complex motion patterns between the metal of the femoral component and the polyethylene of the tibial component. Multi-directional motion, as it is termed, tends to lead to faster wear of the plastic.
Mobile-bearing knees (those that allow flexion and extension between the metal femoral component and the top surface of the tibial polyethylene and rotation between the bottom surface of the polyethylene and the tibial tray, convert this complex multidirectional motion into two separate unidirectional motion patterns). Stated more simply, mobile-bearing knees seem to break down complex motion at one surface and create less complex motion at two surfaces. In laboratory testing, this seems to extend the life of the polyethylene and hopefully it will lead to better survivorship of total knee replacements. Most of the major implant manufacturers in the United States have a mobile-bearing knee. Due to FDA classification, only one company has a mobile-bearing knee for common use in the United States. Most of the other companies will see the down-classification of mobile-bearing knees within the next two years.
There are several factors that influence longevity in knee replacement. These include the age and activity of the patient, the surgical technique, and the implant. At least one positive contribution may be the use of mobile-bearing knees in younger active individuals.
