A knee for the long haul

Memphis, TN -Biomedical engineers once had feet of clay when it came to ceramic knees. Ceramics, with their low coefficients of friction and exceptional hardness, could certainly reduce the wear that shortens the life of artificial knees, but these brittle materials just couldn't handle the contact stresses found in even the best implant designs. Or could they? Smith & Nephew has now developed the first knee implants made with a ceramic surfacing technology that creates a zirconia surface over a zirconium substrate. This hybrid material, called "oxidized zirconium," pairs the mechanical properties of a metal with the wear-fighting capabilities of a ceramic.

And fighting wear has become more important than ever. Today's weekend warriors may be young at heart, but they aren't necessarily young of joint. "I used to perform knee replacements almost entirely on patients in their 70s," says Dr. Richard Lasken, co-chief of knee surgery at the Hospital for Special Surgery in New York. "But I'm now seeing a growing number of people who have bad osteoarthritis in their 50s." Earlier knee replacements, coupled with our tendency to live longer, have intensified the search for implant materials that won't wear out before we do. "A 15 to 20 year life span for a knee implant isn't good enough anymore," Lasken says. "We need them to last 30 or more years."

Built to last. Neither a coating nor a through-and-through ceramic, oxidized zirconium comes from a high-temperature oxidation process that changes the surface of wrought zirconium parts into zirconia. The ceramic zone extends about five microns below the surface. For the next few microns, the process leaves a gradient of oxygen-enriched metal, which ultimately gives way to unadulterated zirconium alloy.

Smith & Nephew's ceramics technology addresses the growing need for longevity by addressing the wear that typically occurs as metal femoral components slide on a tibial bearing surface made from ultra-high-molecular-weight-high-density polyethylene (UHMWPE). Over time, metal alloys such as cobalt chrome develop tiny scratches from abrasive and oxidative wear, roughening their surface just enough to eat away the polyethylene bearing. "A single scratch 2 mum deep, with 1 mum adjacent peak height, on a metal counterface can cause a dramatic increase in the wear rate of UHMWPE," explains Dr. Gordon Hunter, a materials engineer who manages research projects for Smith & Nephew.

Hunter cites three ways in which oxidized zirconium's ceramic surface targets wear. For one, the ceramic surface slides with less resistance. Its coefficient of friction on polyethylene is less than half that of cobalt chrome, he reports. For another, the material resists abrasion. In pin abrasion tests against acrylic bone cement, which can produce debris in the joint, the oxidized zirconium exhibited 4,900-times the abrasion resistance of cobalt chrome. Finally, it's more than twice as hard as cobalt chrome, giving it greater immunity to scratches.

The properties seem to have paid off. In a simulator that subjected oxidized zirconium implants to six million physiological loading cycles, the new material reduced the abrasive and adhesive wear of polyethylene by 85% compared to cobalt chrome, according to Hunter. Longer simulator runs have shown that implants made with the material will last as long as 30 years, adds Lasken, who has already installed almost 200 of the ceramic knees during the past 6 years of trials.

What lies beneath. The zirconium alloy, meanwhile, contains niobium to help it achieve the mechanical and physical properties most important in a knee application. In a fatigue test that simulates full-flexion loading, a femoral component made from the oxidized zirconium supported 4.4 kN over 10 million cycles. "That represents a device strength equal to cobalt chrome," Hunter says, explaining that the oxidation process closes a fatigue-strength gap between untreated zirconium and normally stronger cobalt chrome. "Fatigue strength is dominated by surface factors like hardness," he explains. The zirconium also has a lower modulus of elasticity at 100 GPa than cobalt chrome at roughly 250 GPa. Being less stiff makes a small contribution to decreasing contact stresses in the joint and heading off bone re-absorption that occurs when too stiff an implant shields the bone from a healthy amount of loading.

Smith & Nephew will initially use oxidized zirconium in two knee implants first designed for cobalt chrome. "The designs are identical other than the material," says Hunter. This design consistency matters because it allows surgeons to use familiar procedures and tools. It also means that the implants make use of proven design principles that keep contact stresses low enough to minimize polyethylene creep and fatigue wear. Best of all, the material innovation may open up a whole new way of tailoring implants to patient needs, Hunter argues. Older, less active patients may stick with cobalt chrome, while the young and active might go with more oxidized zirconium implants that cost more up front. "In the past, surgeons and their patients had alternatives in design only," Hunter says. "Now they have alternatives in materials too."