New bearing surfaces, methods of crosslinking may result in less wear

Although bearing surfaces have evolved leading to highly wear resistant crosslinked polyethylenes, William J. Maloney, MD, noted that these materials have presented some complications leading researchers to continue to work on improving polyethylene options.

“When you look at what the future holds, it is probably going to come from minor changes of what we are currently doing and probably not revolutionary changes,” Maloney, said during a presentation on the future of bearing couples at the SICOT XXV Triennial World Congress 2011.

While the survivorship of highly crosslinked polyethylene is good at 10 years, in vivo oxidation and reduced mechanical strength remain potential problems leaving room for improvement, Maloney told Orthopedics Today. To address these issues, researchers such as McKellop et al and Muratoglu et al are working on several strategies to improve strength while maintaining wear resistance. One such strategy is surface crosslinking of the polyethylene, where the bearing surface is crosslinked to improve wear resistance, but the rest of the polyethylene is not crosslinked.

William J. Maloney

“When you look at pin on disc wear testing and hip simulator studies, it is comparable to highly crosslinked polyethylene. But when you look at the mechanical strength, you can see it is superior to the highly crosslinked material,” Maloney said.

Another limitation of some highly crosslinked polyethylenes is the non-uniformity in both macro-structure and micro-structure, Maloney said. One solution is thermal crosslinking developed by DC Sun, MD, and coworkers — a method of crosslinking where the material is heated from above the melting point for a long enough time period to rupture carbon-carbon bonds and form crosslinks.

“Thermal crosslinking has high wear resistance in the lab,” Maloney said. “It remains relatively uniform throughout the material, which may be an advantage over time.”

Another technique is mechanical deformation, as described by Muratoglu et al. The polyethylene is irradiated to crosslink the material. It is then strained to quench the free radicals and annealed to remove the residual stress.

Nitride and diamond-like coatings

Researchers in England are looking at nitride coatings for cobalt chromium alloys, which have low wear rates compared to metal-on-metal (MoM) components as described in a study by Fisher et al. Metal-on-ceramic is another option because of its low wear rate compared to MoM configurations. Fisher et al have demonstrated that in the lab, ceramic-on-metal has lower wear than MoM.

“In the laboratory, tests are usually done with optimal implant position. In real life, less than optimal cup positioning is common. I am concerned about suboptimal implant position and edge loading, metallic ion production, metal transfer and implant breakage; essentially the worst of both worlds,” Maloney said.

Ceramic-on-ceramic options will evolve into ceramic monoblock cups that would give the option of large heads that MoM bearings now allow, Maloney said.

Diamond-like carbon coatings are a plasma arc deposited, hard surfaces that have shown low wear rates in simulators. Work by Jeff Taylor et al has demonstrated that these coatings have low coefficients of friction “making them attractive as bearing surfaces” and show a high bond strength between the diamond-like carbon coating and underlying metallic material, Maloney said.

Carbon-fiber reinforced PEEK

Researchers from England, such as Richard Field from London and Professor Neil Rushton from Cambridge, are using a new material consisting of carbon-fiber reinforced polyetheretherketone (PEEK). Maloney noted that laboratory studies on wear and debris show carbon-fiber reinforced PEEK particles are not cytotoxic and are no more biologically active than other biomaterial particulates as demostrated by Ingham and coworkers.

An ongoing surveillance study of the material showed a small number of revisions for non-bearing failure, and generally the explanted implant bearing surfaces look good, Maloney noted. The second generation PEEK acetabular component has been developed by Stryker and is called the physiologic chondosteal replacement — a monoblock socket that addresses stress shielding and flexes with the natural acetabulum.

Maloney noted that Gradion Total Cartilage Replacement (Biomimedica Inc.; Berkeley, Calif.), as studied by Dave Myung, MD, PhD, and co-workers, uses a new synthetic polymer.

“It is an interesting material that is a differentially hydrated and the initial applications are likely to be resurfacing arthroplasty,” Maloney said.

The material’s mechanical properties are similar to cartilage. On the bone side, it is not hydrated and relatively stiff; but on the articular side, it is very hydrated leading to a very low coefficient of friction. Preliminary wear studies on this material at 5 million cycles demonstrate encouraging results. Clinical studies have yet to be done using this material, according to Maloney. – by Renee Blisard

Reference:

• Maloney WJ. International Hip Secrets Symposium: Current use of couples – The future. Presented at the SICOT XXV Triennial World Congress 2011. Sept. 6-9. Prague.

• William J. Maloney, MD, can be reached at Stanford University School of Medicine, 450 Broadway St., Pavilion A, Redwood City, CA 94063; 650-498-7555; email: wmaloney@stanford.edu.
• Disclosure: Maloney receives royalties from Zimmer for hip products (not bearings) and from Wright Medical Technology Inc. for knee products.