Abstract
Purpose
Metal-on-metal (MoM) articulations in total hip replacement (THR) have become an attractive option for young, active patients. Short-term reports have demonstrated elevated systemic metal ion levels in the blood and urine. Disseminated concentrations of cobalt and chromium have raised concern regarding cellular toxicity, chromosomal damage and adverse local soft tissue reactions.
Long-term studies are required to support the increased use of MoM bearings in younger patients given their potential deleterious effects. The purpose of the current study was to report the seven to 13 year clinical, radiographic, and metal ion results in patients following MoM THR.
Method
We prospectively followed 165 patients (196 hips) after second-generation MoM THR between July 1997 and November 2003. Functional outcome was measured using the Harris Hip Score (HHS) and the University of California Los Angeles (UCLA) Activity Score. Radiographic analysis was performed using Einzel-Bild-Roentgen-Analyse (EBRA) by two of the authors blinded to the study. Cobalt and chromium metal ions were measured from whole blood and analyzed using inductively coupled plasma-mass spectrometry as previously described.
Results
163 prostheses were analyzed. The mean age at surgery was 50.8 years (17 to 66). There were 80 females and 83 males. The mean follow-up was 8.87 years (range, seven to 13 years). Four hips (2.5%) were revised: two for infection at 0.2 and seven years; one for a loose stem at 1.3 years; and one for a loose cup at nine years. One patient received wound debridement for a superficial infection and did not have any components revised.
The mean HHS and UCLA scores at the last follow-up were 91 and 6.8, respectively. The mean acetabular inclination and anteversion was 40 degrees (range, 24 to 57), and 19 degrees (range, three to 39), respectively.
Median cobalt levels peaked at a value of 2.87 g/L at four years (p<0.0001 vs. pre-operative) and subsequently decreased to 2.0 g/L after nine years (p=0.002 vs. four years). Median chromium levels maximally increased up to 0.75 g/L after five years (p<0.0001 vs. pre-operative) and tended to decrease thereafter to values of 0.56 g/L after seven years.
The Kaplan-Meier survivorship was 91.3% for revision for all causes, and 97.5% when excluding the hips revised for a manufacturers defect at a mean of 8.87 years (range, seven to13 years).
Conclusion
The present seven to 13 year follow-up study of MoM THRs indicates that the clinical and radiological results are satisfactory with low revision rates. Furthermore, our study demonstrates the trend of metal ion levels in whole blood over a long-term. Both cobalt and chromium ion levels peaked at four and five years, respectively, and gradually decreased thereafter.
