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Metal Ion
Study 2002
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NOTE: This paper was presented at
the New Trends of Joint Reconstructive Surgery 2002 Meeting, comparing serum
levels in a control group, conventional THR, McKee Farrar, MOM Hip Resurfacing,
and MOM THR (Sulzer), which concluded that metal ion levels are higher at 1 year
with a small diameter MOM THR (Sulzer) than with resurfacing. As the Sulzer
device has been in use for over 15 years with no associated problems reported to
date, this is further evidence that this is currently not an issue.
V Reddy MD† J Dorairajan MD† J Nevelos PhD* S
Krikler PhD FRCS(Orth)†.
† University Hospitals Coven try & Warwickshire NHS Trust * Corin Medical†
Introduction
The conservative principles of hip resurfacing combined with very low wear
metal-metal bearing technology may provide a very attractive solution for the
younger arthritic hip. The long-term effects of the inevitable metal ion release
however, remain a concern. Serum ion levels from metal-metal resurfacing hip
patients were measured pre-operatively and then every year for up to five years.
Materials and Methods
All patients had had either a McMinn (1995-1996) or Cormet® (1997-) hip
resurfacing (both manufactured by Corin Medical, UK) implanted by a single
surgeon. Blood was collected using a standardised technique to ensure no
metallic contamination. Chromium analysis was by graphite furnace atomic
absorption (ETA-AAS) using a 4100ZL or A800 Perkin Elmer instrument. Cobalt was
analysed by inductively coupled plasma mass spectrometry (ICP-MS). Measurements
were taken from 39 patients with 7 of those having bilateral resurfacings.
Pre-operative levels were only taken from 15 patients, and these were
universally low (below 20nmol/l).
Results – General Population
In all patients the serum levels of cobalt and chromium increased following
resurfacing although the extent of the increase varied greatly between patients.
There was a general decrease in ion levels from ~2 years. The data shows a
definite trend of decreasing ion levels after 3 years. This may be consistent
with running-in wear and healing of the peri-prosthetic tissues providing a
smooth, stable joint. The decline in serum level seems to start slightly earlier
in younger patients (<40) than older patients (>60).
Discussion – General Population
- Ion levels increase over the first two years before dropping to a
generally low level.
- Younger patients’ ion levels peak earlier than older patients – probably
related to activity level.
- There was no difference in ion levels between the McMinn and Cormet
devices.
- Bilateral resurfacings did not appear to elevate the ion levels
significantly.
- Insufficient sample numbers for statistical analysis.
Results – Outliers
The vast majority of patients studied had similar ion level release which
remained below 250nl/l. There were, however three individual patients who had
much higher ion levels which may cast light on possible causes of increased
wear.
Case 1. Patient AP. 22 year old female at primary operation – Steep
Acetabular Cup
Case 2. Patient LB. 42 year old female at primary operation Post
operative Dislocation and Steep Cup
Case 3. Patient SM. 35 year old male at primary operation.
Discussion – Outliers
These cases were performed before the start of this study and so ion levels were
not taken pre-operatively. Their levels were one or two orders of magnitude
higher than the general population. (Note scale on the respective graphs). Also
the ion levels show no sign of decreasing with time unlike the general
population. Steep acetabular cup angles have been shown to be related to
increased wear in hard-hard hip bearings (16) . Steep cups will also inevitably
lead to an increased chance of dislocation (as in case 2). The large diameter
heads used for hip resurfacing are naturally resistant to dislocation and Case 2
of this series remains the only dislocation to date in the senior author’s
personal series of over 250 operations.
Conclusions
In the vast majority of patients with metal-metal hip resurfacing there is a
small but early rise in serum metal ions over the first 2-3 years but the levels
then gradually diminish over time .
Steep cups seem to be associated with increased levels, which could be due to
wear and therefore increased ion release into the body.
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