Loose THR components linked to higher metal ion levels

Elevated ion levels may be related to implant corrosion, suggests a study of patients with hip implants in situ for 30 years.

A study of total hip replacement patients with implants in situ for more than 30 years found significantly higher metal ion levels in whole blood and urine for all arthroplasty patients compared to controls.

Patients with metal-on-metal articulations and radiologically loose acetabular components had significantly higher levels of several metal ions, including chromium and cobalt, compared to the rest of the arthroplasty patients, however.

“The greatly elevated levels of [cobalt] in the whole blood and urine may well be helpful in identifying those patients in whom implants are loose and generating a higher volume of debris, and may therefore assist in timing of revision surgery,” the authors of the study said.

Tim W. Briggs, FRCS, and colleagues at the Royal National Orthopaedic Hospital Trust in Essex, England, measured concentrations of various metal ions in 10 surviving THR patients who had undergone surgery between 1965 and 1979. These patients received prostheses with cobalt-chrome-molybdenum femoral components fitted to cemented intramedullary titanium shafts, according to the study.

The researchers compared ion levels in samples of whole blood and urine between these patients, four age- and gender-matched control patients and two arthroplasty patients with metal-on-polyethylene components in situ for a mean of 27 years.

“Most other blood studies have been performed on serum for analysis, but we believe that certain metal elements such as [chromium] have an affinity for red blood cells and, therefore, discarding this constituent of blood will adversely affect the true reading,” the authors said.

The researchers found that all arthroplasty patients had significantly elevated cobalt levels. Compared to control patients, arthroplasty patients had blood levels of cobalt 50 times higher and urine levels more than 300 times higher, according to the study.

Two patients with radiologically loose metal-on-metal articulations had considerably higher mean blood levels of chromium compared to the other arthroplasty patients, but this did not reach statistical significance. However, these two patients had significantly higher urine levels of chromium (P=.01), the authors noted.

“This suggests that metal-on-metal articulations do give significant levels of [chromium] wear debris, particularly when loose, but that this is then excreted in the urine rather than accumulating over time in the blood,” they said.

Compared to control patients, all THR patients had significantly higher blood levels of titanium (P<.01) but no significant difference in urinary levels. This suggests that the elevated titanium levels were mainly due to implant corrosion and abrasive wear of soft tissues, they said.

All THR patients also had significantly higher urinary levels of vanadium (P=.006); their vanadium blood levels were also two to three times higher (P<.01). However, mean blood levels reached 0.86 ng/g for the two patients with loose metal-on-metal articulations — significantly higher than the other arthroplasty patients, the authors noted.

For more information:
Dunstan E, Sanghrajka AP, Tilley S, et. al. Metal ion levels after metal-on-metal proximal femoral replacements. J Bone Joint Surg Br. 2005;87-B:628-631.