Plasma levels of cobalt and chromium ions and
Metal Artefact Reduction Sequence (MARS)-MRI scans were performed
on patients with 209 consecutive, unilateral, symptomatic metal-on-metal
(MoM) hip arthroplasties. There was wide variation in plasma cobalt
and chromium levels, and MARS-MRI scans were positive for adverse reaction
to metal debris (ARMD) in 84 hips (40%). There was a significant
difference in the median plasma cobalt and chromium levels between
those with positive and negative MARS-MRI scans (p <
0.001).
Compared with MARS-MRI as the potential reference standard for the
diagnosis of ARMD, the sensitivity of metal ion analysis for cobalt
or chromium with a cut-off of >
7 µg/l was 57%. The specificity was
65%, positive predictive value was 52% and the negative predictive
value was 69% in symptomatic patients. A lowered threshold of >
3.5 µg/l for cobalt and chromium ion levels improved the sensitivity
and negative predictive value to 86% and 74% but at the expense
of specificity (27%) and positive predictive value (44%). Metal ion analysis is not recommended as a sole indirect screening
test in the surveillance of symptomatic patients with a MoM arthroplasty.
The investigating clinicians should have a low threshold for obtaining
cross-sectional imaging in these patients, even in the presence
of low plasma metal ion levels.
We have evaluated retrospectively the relationship of bony injuries seen on 106 consecutive MR scans in elderly patients of a mean age of 81.4 years (67 to 101) who were unable to bear weight after a low-energy injury. There were no visible fractures on plain radiographs of the hip but eight patients (7.5%) had fractures of the pubic ramus. In 43 patients (40.5%) MRI revealed a fracture of the femoral neck and in 26 (24.5%) there was a fracture of a pubic ramus. In 17 patients (16%) MRI showed an occult sacral fracture and all of these had a fracture of the pubic ramus. No patient with a fracture of the femoral neck had an associated fracture of the pelvic ring or
