Notas / Notes - Active Congress.......

MARTES / TUESDAY
18
METAL-METAL COUPLE – SCIENTIFIC
BASES
Eric Robinson, Tajeshwar Singh Aulakh, Munir Khan,
Jan-Herman Kuiper, James Richardson
The Robert Jones & Agnes Hunt Orthopeadic Hospital.
Oswestry Outcome Centre, Oswestry (Great Britain)
Science is based on observation. Any hypothesis should
then fit those observations and be open to testing. Metal on
metal hip replacement was very successful with a design by
Ring some 30 years ago. There were difficulties however
with initial fixation to bone. I will show a case where a loose
Ring stem was simply revised by re-cementing the original
stem, and 8 years later the patient is functioning well! This
observation helped to question the teaching I had received
against metal-metal bearings.
The metal on polythene hip has stood the test of time but
John Charnley himself insisted it only be used for the older
patient of 65 years or more. Some excellent long term results
have been obtained in patients who had some other limiting
factor from a young age that prevented them attaining a high
mileage each year. We all know the problems of osteolysis
related to the polythene debris from a metal on polythene
joint. It is for these reasons that there is again a logical argument
for using a metal on metal articulation in the younger
and active patient.
The Outcome Centre at Oswestry has been following 5000
hip resurfacing (BHR) now out to 10 years. This is an independent
review where information is gathered directly from patients.
This has found a very high level of function and activity
in patients with metal on metal resurfacing hip replacement.
There is good survival out to 10 years with 95% survival. This
group of patients is from a combined series of several surgeons
across the world who were new to using this technique.
Similarly, I have excellent results now out to 10 years with
the Metasul articulation on a Thrust Plate. This is a short uncemented
stem with strong initial stability and rigidity. These
patients likewise have a high level of activity and the 8 year
follow up was a 98% survival.
What of the metal ions absorbed? Certainly in a minority of
patients these have a hypersensitivity response which causes
pain and cyst formation. All patients absorb some metal in
the form of chromium and cobalt. These levels are highest in
the joint and adjacent to the metal implants. Alterations in
circulating cells have been noted and this causes concern.
However, the observation of a local sarcoma adjacent to a
metal implant is very rare indeed considering the numbers
implanted worldwide and this includes a whole range of metal
implants. The observation therefore is that in practice and
indeed over many years it is unlikely that the risk of a local
cancer is significant. Further observations and studies are
needed to identify and quantify risks of leukaemia.
The ‘bottom line’ is whether patients with hip resurfacing are
dying at a faster rate than expected. Tajeshwar Aulakh has
studied the mortality rates of the Oswestry 5,000 and finds
the rate almost half as low as the expected.
Munir Khan has studied the release and distribution of metal
wear debris from metal on metal prostheses. Average size
of the debris particles is 120 times smaller than a red blood
cell. We found that this debris material is mostly concentrated
in the extra-cellular portion of blood and that chromium has
special affinity for plasma proteins. Therefore, measurements
of plasma metal ion levels will provide more accurate results
to compare release of wear debris in different hip prostheses.
Moreover, our group was the first to show that metal ion levels
increase following an hour of physiological exercise.
The rise is of a few nanomoles that can only be picked if
detection limits of the instrument are less than 1nmol/L. We
found that determining the exercise-related cobalt rise is an
accurate technique of determining in-vivo wear of metal on
metal bearings, compared to resting plasma levels.
We also studied the effects of metal ions on DNA. Using comet
assay we found that cobalt and chromium ions caused
DNA damage, which disappeared in the presence of apoptotic
inhibitor. This apoptotic damage was misinterpreted as mutagenic
in previous studies, but generally it is a safer thing for
an agent to increase apoptosis.
Our studies therefore echo the findings of epidemiologic studies
in which researchers did not find a higher risk of cancer
in patients with metal on metal bearing hip replacement at
up to 30 years follow-up.
METAL/METAL BEARING SURFACES:
THE WAY OF THE FUTURE?
Cecil H. Rorabeck, MD
Health Sciencies Centre.
London-Ontario (Canada)
OBJECTIVE
Polyethylene wear continues to be the most significant issue
following total hip arthroplasty (THA) leading to the current
increase in use of alternative bearing surfaces. We performed
a prospective, randomized, blinded clinical trial comparing
metal versus polyethylene bearing surfaces in patients receiving
THA.