CONSERVE® PLUS Metal-on-Metal Frequently Asked Questions
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1.
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How long has metal-on-metal articulation been in use? |
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George McKee of Norwich, England, was the first to use metal-on-metal
articulation with modified Thompson stems and a one-piece cobalt chrome
socket combination in THR in 1953. The design was primitive, but many
lasted for more than 14 years. In 1986, August et al. reviewed 657
total hip arthroplasties performed by McKee and Farrar. The
combined survival rate of the hip and stem at fourteen years was 84.3%.
Loosening was cited as the main cause of failure, and bone cement was
cited as the main culprit. The authors never implicated
metal-on-metal articulation as a reason for complications in their
paper.5 Although metal wear was detected in devices
that were revised, McKee did not observe any undesirable effects of that
debris on the soft tissues or the bone.6 The early
history of metal-on-metal devices, including Dr. Amstutz ' experience
with the McKee device in New York, has been previously published.7-9 |
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2.
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What is the optimum material for metal-on-metal articulation? |
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Metal-on-metal articulation is typically associated with the cobalt
chromium molybdenum alloy. These alloys are divided into two
categories: high carbon, where the carbon content is above 0.20%; and
low carbon, where the carbon content is less than 0.05%. Several studies
comparing both groups have been conducted. Earlier studies
presented inconclusive results.10 By comparison, later
studies isolated the contribution of factors such as surface finish,
clearance, sphericity and carbon content. There is now general consensus
in the industry that the high carbon alloy has much better wear
resistance than the low carbon type.11
In addition, there are two types of processes used in manufacturing
the cobalt chrome molybdenum components. One method is casting the
components (used by Wright for the CONSERVE® PLUS and CONSERVE® Total
implants). The other is forging the material. Although the chemical
composition can be exactly the same between the two materials, there is
a structural difference. The grain size of the forged alloy is
typically less than 10 microns, whereas the grain size for the cast
material ranges from 30 to 1000 microns. There is also a marked
difference in the appearance of the carbides, in that the carbide
regions tend to be smaller in the forged material. Metal liners
and femoral heads have been produced at Wright with both types of
material. A limited number of couples were tested in a hip wear
simulator. The test showed less wear with cast high carbon alloy
than the forged alloy. Due to the limited number of samples, the
difference had low statistical reliability.11 This
study was the basis for Wright’s decision to use cast cobalt chrome
alloy as the material of choice for their metal-on-metal components.
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| 3. |
Does the clearance between articulating components play a role in
wear debris generation? |
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Absolutely! This is probably the most influential factor in wear
behavior. The proper clearance is essential for entrapping the
synovial fluid between the articulating surfaces. This fluid is
largely responsible for separating the surfaces while the joint is in
motion, thereby reducing wear. If the gap between components is too
small or too large, there is a sharp increase in wear rates.12
A study conducted by Isaac, Dowson and others (DePuy International,
Leeds, UK) compared wrought and as-cast components with various
clearances between those two groups. The results of the hip
simulator study strongly indicated that clearance plays a major role in
wear rates and that "wear appears to be relatively insensitive to
changes in materials that have similar chemical compositions but
different microstructures."20 |
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| 4. |
Does the CONSERVE® PLUS acetabular shell with the big femoral head
use the same clearance for all sizes? |
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No. The clearance between components is size-dependent. The
larger the diameter, the larger the gap between the components. The
range for the entire family of sizes is from 90 to 200 microns of
diametral clearance, each bearing size having an optimized gap for
maximum fluid film thickness. |
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5.
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I've heard a lot about heat-treated cobalt chrome components versus
"as cast" components. What are they talking about and is
there a difference? |
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Cobalt Chrome Molybdenum components that are cast usually go through
the hot isostatic pressing (HIP) and solution annealing processes to
remove microporosities often found in castings and to improve the
ductility and homogeneity of the material. The microstructure of
this type of heat-treated material looks different from that of the
original casting. It is important to note that even though heat-treated
material looks different, it doesn’t affect wear.
Two global metal-on-metal resurfacing manufacturers use the heat-treated
process for the castings (Corin, LTD. and Wright). Midland
Medical, the producer of the Birmingham Hip Resurfacing (BHR) implant,
leaves the castings untreated. The BHR product champion, Derek
McMinn, MD, claims that heat treatment can lead to carbide depletion,
which can adversely affect wear rates. One "pin on disk"
type test suggests that "as cast" material wears slightly less
than "HIP" cast material; however, the data shows so much
scatter that the results are inconclusive.15 In
addition, the linear tracking motion of the type of
"pin-on-disk" used in that study is very different from the
actual hip motion. A linear tracking pin-on-disk test is conducted
by sliding the cylinder on the flat surface back and forth along one
axis. The actual movement of the femoral head inside the socket
produces crossing path motion. Studies in hip simulators are more
relevant since they more closely resemble the actual hip function by
reproducing this crossing path motion. It has been shown that a linear
tracking pin-on-disk test under-estimates UHMWPE wear rates by 10 to 100
times, and overestimates metal-on-metal wear rates as compared to hip
simulators and retrieval studies.17 Midland Medical has
not published any data from a hip simulator to support their claim.
Also, zero clinical studies have been conducted which suggest BHR
components create less wear than heat treated components.
Bowsher et al. conducted a hip simulator wear study in which 40mm
diameter metal-on-metal bearings, either "as cast" or heat
treated, were compared side-by-side.13 Wear rates were
compared for the running-in state (first 1 million cycles), steady
state, and also fast jogging. In all three conditions, there was
no difference between wear rates of the two forms of the alloy.
The authors concluded that HIPing and solution annealing do not
adversely affect the wear rates of large diameter metal-on-metal
articulations. Furthermore, one additional study was presented at
the recent June 2003 Conference on Metal-on-Metal Devices in Montreal
that corroborate the Bowsher study.20
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| 6. |
What is the "steady-state" wear? |
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Typically, metal-on-metal couples in the hip simulators go through the
"run-in" or "wear-in" period where the weight loss
due to wear increases linearly. At some point, usually between 500,000
and 1 million cycles, the wear increase drops dramatically or stops
altogether. It is then said that the metal-on-metal couple reached
the "steady-state" of wear. |
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| 7. |
Does the surface finish affect wear rates? |
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Surface finish has a definite effect on wear rates. The rougher the
surface finish, the higher the peaks of material that eventually will be
removed. Typical surface finish for the CONSERVE® resurfacing
components is 0.008 microns (micrometers). This is an order-of-magnitude
smoother than the finish on typical metal femoral heads articulating
with polyethylene inserts used for THR. |
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8.
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Do larger heads wear less than smaller heads? |
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Theoretically, if the metal couple is dry, larger heads should wear
more than smaller heads due to their longer sliding distance per step.
However, in the presence of the fluid the opposite is true: larger
diameter heads should wear less because of their greater sliding
velocity. Calculations show that larger diameter wear couples can
form a thicker synovial fluid film between components.12
Hmin = 1.64D(_Ú/ED)0.65(W/ED2)-0.21
Where:
Hmin is the minimum film thickness
D is the head diameter
Ú is the entraining velocity
According to the formula above, the larger the articulating diameter,
the larger the Hmin value. A thicker fluid film means
less contact between hard surfaces during motion and, presumably, less
wear. Does this theory prove itself? The study cited above
compared 22mm, 26mm and 35mm diameter metal-on-metal articulations and
found no difference between the three. Isaac compared 16mm, 22mm, 28mm,
36mm and 54.5mm diameter couples and, for diameters 28mm and larger, it
was determined that wear decreases with increasing head diameter.20
In the study of the 54mm articulating couple conducted at Wright, the
wear rates were found to be very similar to the wear rates for the 44mm
CONSERVE® PLUS articulating couple performed at another institution.
These numbers are in agreement with other experimental data obtained
with hip simulators.
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| 9. |
What do they mean when they say that cobalt chrome is
"self-healing"? |
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A cobalt chrome articulation has the ability to polish out the
scratches from abrasive damage such as third-body wear. In
retrieval studies, the deep scratches have often been partially or
entirely polished out of the main contact zones. |
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| 10. |
What is the average particle size for metal wear debris? |
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In one study, the cobalt chrome particles from a McKee-Farrar
metal-on-metal articulation were in the range of 6 to 744 nm
(nanometers), with an average size of 42 nm.16 By
comparison, polyethylene particles range from 0.05 to 5 micrometers (50
to 5000 nm). |
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| 11. |
Can a metal-on-metal articulation prevent osteolysis? |
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Since a metal-on-metal articulation does not eliminate wear entirely,
there is always the potential for an osteolytic reaction. There
are reports of isolated cases of osteolysis with metal-on-metal joints.17However,
these are mostly limited to the first-generation metal-on-metal
components. Those were implanted with acrylic cement, which can
fragment and generate third-body abrasive particles. It is
believed that the metal debris is too small, in comparison to the
polyethylene particles, to initiate an osteolytic reaction. A
study of several metal-on-metal components (Metasul ™ total hip
replacements and McMinn surface replacements) investigated the bone and
tissue reactions to the metal debris.18 It was noted that
metallosis (a grey-black appearance of the soft tissue) was present with
the surface replacements and the total hip replacements.
Macrophages filled with metallic particles were found in all tissues,
but in larger amounts in those with metallosis. Giant cells and
small areas of histiocytic granulomas were also present. The authors
noted that there were fewer macrophages and giant cells than typically
seen in tissues around metal-polyethylene joints, and although an
inflammatory response to the metal particles was present, this was not
as severe as the response to the cement particles. The authors
concluded that the long-term response to these very small CoCr particles
should be monitored. There has been no observed occurrence of
metallosis in connection with CONSERVE® PLUS or CONSERVE® Total
implants. |
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CONCLUSIONS:
Many factors affect metal-on-metal wear behavior. Some of them are more
significant than others. Surface finish, appropriate radial clearance and
high carbon content have been shown to play the greatest role in reducing wear
rates.
The microstructure of the alloy does not play a key role in wear behavior.
"As cast" and heat treated alloys were directly compared in hip
simulators by the scientists from Corin, DePuy, Centerpulse, and in some
independent laboratories, all showing similar wear rates for both types of
alloy. Proponents of "no heat treatment" regimes have not
provided us with laboratory or clinical data to date. McMinn’s claim of
better metallurgy with the "as cast" components is based primarily on
"pin-on-disk" type testing. The "pin-on-plate" or
"pin-on-disk" type experiment can compare the wear of different
materials as a flat surface, but the mechanism of these tests has nothing in
common with the motion of the hip joint.
Hip simulators offer the most reliable way to assess wear in the laboratory,
but keep in mind that the outcome greatly depends on the method, testing
equipment, and measuring equipment. Since we are dealing with tiny amounts
of debris, test results may vary greatly from one hip simulator study to
another. Take that into account when comparing data between two tests
conducted by different people and with different equipment.
Finally, the best proof of a good design is in the clinical outcome.
The CONSERVE® PLUS metal-on-metal articulation has a good clinical history with
over 6 years and over 1300 patients. The paper presenting the clinical
results of the first 400 CONSERVE® PLUS hip resurfacing cases performed at the
JRI has been accepted for publication by the Journal of Bone and Joint
Surgery.
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