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**A. J. Hart, FRCSG(Orth), Clinical
Senior Lecturer & Honorary Consultant Orthopaedic Surgeon1; S. Sabah,
BSc, Medical Student1; J. Henckel, MRCS, Clinical Research Fellow &
Specialist Registrar in Orthopaedics1; A. Lewis, FRCS(Orth),
Consultant Orthopaedic Surgeon1; J. Cobb, FRCS, Professor of
Orthopaedic Surgery1; B. Sampson, MRSC, CChem, Director of
Supraregional Trace Element Laboratory1; A. Mitchell, FRCR,
Consultant Musculoskeletal Radiologist1; and J. A. Skinner, FRCS(Orth),
Consultant Orthopaedic Surgeon2
1 Department of Radiology
Imperial College, Charing Cross Hospital, Fulham Palace
Road, London W6 8RF, UK.
2 Department of Orthopaedics Royal National
Orthopaedic Hospital, Brockley Hill, Stanmore,
Middlesex, HA7 4LP, UK.
Abstract:
We carried out metal
artefact-reduction MRI, three-dimensional CT measurement
of the position of the component and inductively-coupled
plasma mass spectrometry analysis of cobalt and chromium
levels in whole blood on 26 patients with unexplained
pain following metal-on-metal resurfacing arthroplasty.
MRI showed periprosthetic lesions around 16 hips, with
14 collections of fluid and two soft-tissue masses. The
lesions were seen in both men and women and in
symptomatic and asymptomatic hips. Using
three-dimensional CT, the median inclination of the
acetabular component was found to be 55° and its
positioning was outside the Lewinnek safe zone in 13 of
16 cases. Using inductively-coupled plasma mass
spectrometry, the levels of blood metal ions tended to
be higher in painful compared with well-functioning
metal-on-metal hips.
These three clinically useful investigations can help to
determine the cause of failure of the implant, predict
the need for future revision and aid the choice of
revision prostheses.
…We included
the first 26 consecutive, consenting patients (9
men with a mean age of 52.3 years (33 to 63), 17
women with a mean age of 52 years (38 to 70)
at primary operation; 18 unilateral, 8
bilateral)) who presented with unexplained, painful MOM
resurfacings from February 2008. Ethical
approval had been granted by the appropriate
committee. All patients had previously undergone
a traditional hip assessment, comprising a
clinical history and examination, blood
infection screen and examination of serial
pelvic radiographs…
Results
…All
the angles of the acetabular component were expressed as
the anatomical angle. There was a median
inclination of 55° (39° to 78°) and version
of 31° (48° to 47°). These values were within
the safe zone of Lewinnek et al18
in only three of 16 patients. All the
acetabular components were anteverted except
one who was retroverted 47°. He described
pain on hip flexion which was restricted to 60°. At
operation the hip was found to be subluxing
during flexion due to anterior impingement.
Ten patients had inclination of the component
greater than 50°, the threshold for a clinically
detectable increase in whole-blood metal ion
levels…
Discussion
…We found three
clinically relevant results. First, 13 of the 16 painful
MOM hip resurfacings that had undergone CT scanning were
associated with positions of the component outside the
safe zone of Lewinnek as measured by 3D CT.18 Secondly,
the patients had higher blood metal levels than those
with well-functioning prostheses.14–17 Thirdly, metal
artefact reduction sequences MRI demonstrated the
presence of periprosthetic masses in 14 of the 26
painful MOM hips…
…Currently, most
patients with a painful MOM hip are assessed by the same
diagnostic algorithms as used for the painful non-MOM
hip. These include the taking of a clinical history and
examination followed by a review of serial plain
radiographs, blood inflammatory markers and hip
aspirates. Isotope bone scanning to determine loosening
and infection10 and CT to determine the position of the
component, particularly in a case of dislocation, are
occasionally used. However, hip resurfacing appears to
have specific modes of failure which are not detectable
using these protocols. These may be attributable to two
main causes, namely high rates of wear with high local
levels of Co and Cr and an adverse response to the metal
wear debris…
…The position of
the component may influence the risk of failure of a MOM
hip because suboptimal acetabular version may cause
impingement, and inclination angles greater than 50° may
cause high rates of wear. The risk of impingement is
greater for a hip resurfacing than a THR because the
head-neck ratio is reduced. High inclination angles in
MOM hips can increase the blood levels of metal wear
debris up to 150 times greater than the median blood
levels in patients with well-functioning MOM hips.
Unfortunately, assessment of the position of the
component of large-diameter MOM hips is difficult in
practice because the large metal head obscures edges of
the acetabular component on plain radiography and axial
CT. Appropriate CT protocols and validated 3D
measurement software are needed to overcome these
difficulties and to minimise the radiation dose. We have
demonstrated these in our protocol…
..The use of blood
metal levels as biomarkers of wear rate is an exciting
possibility for monitoring a MOM hip. There is debate on
the relationship between the wear rate and blood metal
levels.20 Recent reports have shown that patients with
bilateral MOM hips have 1.5 times the level of Co and Cr
in blood than those with unilateral MOM hips21 and that
removal of a MOM hip results in a rapid fall in blood
metal levels.22 In our patients the levels of Co were
significantly greater than in those with
well-functioning MOM hips and this suggests that painful
hip resurfacings have greater wear rates. However, the
levels of Cr were barely raised in patients with
unilateral MOM hips and therefore analysis of blood
metal ions as a screening test for painful MOM hips is
likely to be sensitive but not specific. As seen from
our ranges, for 100% sensitivity the threshold for blood
metal levels would need to be set at around 0.5 ppb for
unilateral and 2.0 ppb for bilateral resurfacings.
However, within our control population we noted that
some patients had extremely high blood metal levels and
therefore, even at 50 ppb, the test was not 100%
specific for painful MOM hips. Our control data suggest
that, with a threshold of 15 ppb, the test would have a
specificity of around 98%. We have begun prospective
metal ion and clinical monitoring of all patients with
high levels to verify the value of this test…
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