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Oxidized zirconium and highly crosslinked polyethylene a good bearing surface for THA
Solution must be found for abrasive third-body particles in THA.
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The best bearing surface for total hip arthroplasty must address
both sides of the articulation, according to Robert L. Barrack, MD.
“Bearings must provide a solution for the presence of abrasive third-body particles that are frequently present in modern-day total hip replacements,” said Barrack, clinical professor of orthopedics at Tulane University, who discussed the topic at the Current Concepts in Joint Replacement — Winter 2003 meeting.
While highly crosslinked polyethylene presents a dramatic improvement in wear resistance of the articulation in total hip arthroplasty (THA), the femoral side “cannot be ignored,” he said.
Oxidized zirconium has an extremely low wear rate, and therefore a combination of oxidized zirconium and highly crosslinked polyethylene appears to be “an attractive solution that meets stringent requirements,” Barrack said.
Short-term clinical studies and laboratory tests have shown that highly crosslinked polyethylene is a promising compound for reducing bearing wear. “The lowest reported wear rated in the orthopedic literature at beyond 10 years were obtained when highly crosslinked polyethylene was combined with a ceramic head,” Barrack explained in his paper. There are some disadvantages, however, including susceptibility to surface damage and the development of cracks. In fact, some highly crosslinked polyethylene materials have a problem with increased wear when they are combined with a roughened counterface.
Several investigators have demonstrated a significant deleterious effect when combining standard polyethylene and highly crosslinked polyethylene with a roughened metal femoral head. Support is building for a combination of highly crosslinked polyethylene and a smooth, scratch-resistant femoral counterface. An obvious choice for this is a fully ceramic head, Barrack said.
Because of the disadvantages associated with ceramic heads — risk of fracture and decreased surgical options — oxidized zirconium was developed. “The base metal is an alloy of zirconium and niobium, and the surface has been transformed to zirconia. This represents a surface modification and not a coating,” he said.
Oxidized zirconium
Oxidized zirconium is abrasion resistant, biocompatible and strong; it is about 5 µm thick and has the strength of metal with no potential to break, he said. Another advantage to oxidized zirconium is its extreme damage tolerance even in the worst circumstances.
Barrack said that heads made of oxidized zirconium wear at least as good as new, undamaged cobalt-chrome femoral heads. When subjected to hip simulator studies under smooth conditions, cobalt-chrome and oxidized zirconium heads paired with highly crosslinked polyethylene reveal virtually no wear.
“But when both heads are exposed to a roughening protocol, the wear rate of cobalt-chrome against highly crosslinked polyethylene rises dramatically to a level higher than the wear rates seen with standard polyethylene against a smooth cobalt-chrome head,” Barrack said. “The oxidized zirconium retains a very low, almost immeasurable wear rate.”
For more information:
- Barrack RL. Femoral surface alteration: one side is not enough. #16. Presented at the Current Concepts in Joint Replacement — Winter 2003. Dec. 10-13, 2003. Orlando, U.S.A.