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Metal-on-metal bearings in old implants may cause chromosomal aberrations
Study results present more questions than answers about long-term exposure to ion debris.
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Chronic exposure to metal ions from early metal-on-metal hip bearings may result in chromosomal aberrations, according to recent findings, but the consequences of these aberrations remain unknown.
“Are these findings of clinical significance? We have absolutely no idea,” said Edward Dunstan, FRCS, of Stanmore, England. “Our patients are otherwise well.”
Dunstan presented results of a long-term study of the effects of metal ion debris in patients with metal-on-metal hip bearings at the British Orthopaedic Association Annual Congress in Glasgow.
Metal-on-metal bearings create more particulate debris than polyethylene, and the particulate is significantly smaller. Dunstan said that electron microscopy has shown metal debris inside the nuclei of certain cells; polyethylene debris “ends up in the cytoplasm.”
Long follow-up
Dunstan and colleagues studied 25 patients who received metal-on-metal hip bearings between 1965 and 1979.
“The average age at time of surgery was incredibly young — only 33 years of age — and the average follow-up is now 35 years” for the survivors, Dunstan said. He noted that while the acetabular cups evolved from uncemented to cemented design during the period of implantation, the bearing articular surfaces and the neck, shaft and stem did not change in terms of materials.
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Image: Dunstan E |
Dunstan told Orthopaedics Today, “The bearing articular surface was always a cobalt, chromium, molybdenum alloy. The neck shaft and stem of the prosthesis was always a titanium/titanium alloy.” The Stanmore Metal-on-Metal Proximal Femoral replacement implant was used throughout the study.
The study included three groups: a control group matched for age and sex, patients who retained their original metal-on-metal bearings and patients who originally had metal-on-metal bearings, but received a revision to metal-on-polyethylene. Researchers used the 24-color fluorescent in situ hybridization (FISH) chromosome painting technique on peripheral leucocytes to map changes and aberrations in chromosomes. They examined results from 20 different cells per individual.
The FISH technique allowed researchers to view gain or loss of chromosomes and the more significant structural aberrations, Dunstan said.
“There was a significant difference between the number of structural aberrations between the control group and metal-on-metal group. There was also a significant difference in the number of structural aberrations between the metal-on-metal group and the revised metal-on-metal group. There was no difference between the control group and the revised group.”
Reversible changes
Interestingly, Dunstan said that the chromosomal changes appear to be reversible. The revisions from metal-on-metal to metal-on-poly occurred at an average of 26 years postoperatively, or approximately 10 years from the end of the study. From the time of revision to the present, those patients’ chromosomal aberrations declined.
“It may well be that the normal genetic repair mechanisms are overwhelmed or are themselves damaged by the metal ion load,” Dunstan said.
Because this study focused only on peripheral leucocytes, researchers could not determine the overall effects of metal ion wear.
“Are these changes replicated in other tissues in the body? There is evidence from retrieval analysis that they do occur in bone marrow,” Dunstan said. “What about reproductive cells? We just don’t know. But what is absolutely sure is that we must monitor the long-term biological effects of metal wear debris.”
For more information:
- Dunstan E, Ladon D, Whittingham-Jones P, et al. Chromosomal aberrations in the peripheral blood of patients with metal-on-metal hip bearings. Presented at the British Orthopaedic Association Annual Congress. Sept. 27-29, 2006. Glasgow.