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Erythrocyte, urine metal ion levels remain elevated in metal-on-metal patients
Investigators find 19-times greater urine chromium levels compared to metal-on-polyethylene patients.
SAN FRANCISCO — The results of a prospective, randomized control trial indicate significantly higher metal ion levels in patients who undergo metal-on-metal total hip arthroplasty compared to those who have polyethylene liners.
Richard W. McCalden, MD, and colleagues studied the erythrocyte and urine cobalt, chromium and titanium levels of 41 patients who underwent total hip arthroplasty (THA) using a metal head with either a metal or polyethylene liner at an average follow-up of 8.1 years.
The investigators discovered a 19-times greater urine chromium level, a 16-times greater urine cobalt level and a 9-times greater level of erythrocyte cobalt in the metal-on-metal patients compared to the polyethylene group at the latest follow-up.
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Continual elevation
“The median erythrocyte cobalt levels [in the metal-on-metal group] remained elevated even after 7 years, and we are finding that this is really not a trend that changes,” McCalden said during a presentation at the American Academy of Orthopaedic Surgeons 75th Annual Meeting.
The investigators found that 42% of these patients showed increasing metal ion levels, they wrote in their abstract. “Basically, we expected to enroll 100 patients in the study,” McCalden said. “We actually stopped at 41 patients, the reason being that the metal levels were so clearly higher in the metal cohort that it was not necessary to continue the study.”
The study included 23 patients in the metal-on-metal group and 18 patients in the polyethylene cohort. All of the patients received the same stem and had 28-mm low-carbon heads. Surgeons used a high-carbon insert in patients in the metal-on-metal group. During the study, one patient died.
“Of note, this particular bearing is an older-generation metal-on-metal bearing, no longer manufactured, with less-than-ideal design characteristics based on our current knowledge,” McCalden told Orthopaedics Today International.
The investigators found variability in the entire study population regarding metal ion levels. However, the investigators discovered significantly elevated cobalt and chromium levels in the urine and greater cobalt erythrocyte levels in the metal-on-metal group overall compared to controls for up to 7 years.
Cobalt ion production
The study also revealed no significant differences between the groups regarding erythrocyte chromium levels. “Both the urine cobalt and chromium continue to be excreted at very large amounts relative to the control group, suggesting that there is also cobalt ion production, but it’s being extracted,” McCalden said.
They also discovered no significant differences between the groups regarding SF-36, WOMAC or Harris Hip scores.
For more information:
- Richard W. McCalden, MD, can be reached at London Health Sciences Centre, University Campus, 339 Windermere Road, London, Ontario, Canada N6A 5A5; e-mail: Richard.mccalden@lhsc.on.ca. The study was funded by Biomet. He receives research support from Smith & Nephew and DePuy and is a consultant for Smith & Nephew and Stryker.
Reference:
- MacDonald SJ, McCalden RW, Bourne RB, et al. Do metal ion levels in metal-on-metal THAs change over time: Midterm results of a prospective RCT. Paper #018. Presented at the American Academy of Orthopaedic Surgeons 75th Annual Meeting. March 5-9, 2008. San Francisco.