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.
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
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
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
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,
The materials 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
- 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: email@example.com.
- Disclosure: Maloney receives royalties from Zimmer for hip
products (not bearings) and from Wright Medical Technology Inc. for knee