Coatings and metallurgy - the facts

[Guest guest]

Having just joined the Group, I am amazed at the depth of technical

information being exchanged, and wonder if I can contribute?

HEAT TREATMENT

There is presently a fundamental disagreement between some

manufacturers about the effect of heat treatment on these devices.

The original metal-on-metal hip resurfacing device was developed by

Corin Medical in the UK in the late 1980's in cooperation with Mr

McMinn (Birmingham UK)and was known as the Corin McMinn Hip

Resurfacing System. As with all the current devices, it was

manufactured from high carbon Cobalt Chrome.

When Mr Mcminn split up from Corin and set up his own company with Mr

Treacy in 1996 (MMT, who manufacture the BHR), they changed the basic

metallurgy of their new device, and manufacture it in its " as cast "

form, ie without any further heat treating.

Mr McMinn claims that this improves the wear characteristics of the

BHR, and that the way that Corin heat treat their current device

(Cormet) means it will not perform as well.

THIS IS PATENTLY UNTRUE.

a) The results Mr McMinn publishes for the original Corin McMinn

device are excellent (98% survivorship at 6-7 years, which compares

well with the gold standard in hip replacement), yet almost all of

these original devices were heat treated!!

" as Cast " components (such as the BHR) have a coarse carbide

structure, with some carbides at the grain boundaries, and a carbide

fraction of about 5%

c) heat treated components (such as the Corin Cormet)have evenly

distributed fine carbides, with occasional carbides at the grain

boundaries, and a carbide fraction of approximately 2.3%.

d) the original Corin McMinn device was heat treated, with fine

carbides and a carbide fraction of 2.3%. This is the device for which

Mr McMin repoerts excellent meduium-term follow-up

e) Corin has continued to heat treat the current Corin Cormet device,

just as the original for which the long term results are available

f) the BHR is not heat treated, and there is no long term data for

this device

Independent wear test data published by Corin proves that there is no

statistical difference in wear between " as cast " and heat treated,

although the results may be marginally in favour of heat treated.

MMT have not published any independent scientific data which supports

claims that wear is significantly different between " as cast " and

heat treated components.

Heat treating is a standard, recognised and well proven method of

producing orthopedic implants. It removes microporosity in the metal

and gives a fine, homogeneous carbide structure (ie the structure is

the same at every point in the component), which is why Corin (and

others) have stuck with this specification.

The blades in a jet engine turbine are heat treated for exactly the

same reasons, and are considrered less likely to suffer catastrophic

failure as a result. I would not wish to fly in an airoplane which

had turbines which were not heat treated, any more than have a hip

joint made from un-treated material.

At the end of the day, it is HIGHLY UNLIKELY that " as cast " or heat

treated components will show any difference in long term

survivorship. This is purely a marketing argument, put forward by one

manufacturer only to try and scare potential pateints away from their

competitors!

What is much more important are the manufacturing tolerances these

devices are made to - the surface finish and diametrical mismatch

contribute FAR more to potential wear than metallurgy! Forget the

irrelevent arguments about metallurgy, and look for devices

manufactured to high standards by experienced orthopedic

manufacturers with a credible track record.

SURGACE TREATMENT

PubMed is an excellent source of material on this subject, and

hundreds of papers publishing decades of results will will tell you

exactly the same thing: that porous coating and HA coating work on

orthopedic implants. Which is why almost all uncemented implants

(whether hip stems, resurfacing cups or knee components) are porous

coated and HA coated.

Porous coating provides a macrosurface for bone to integrate into the

component over a period of months, and provide long-term fixation -

the implant literally becomes attached to the bone itself. Numerous

orthopedic devices use this coating, and have done for many years,

with excellent results.

HA (hydroxyapatite) merely encourages the bone to grow onto the

macrosurface. Since it is absorbed as this happens, it does not last

long, and is not intended to be a permament fixture.

The Corin Cormet uses a proven Bi-coat (both porous coating and HA)

similar to that used by many other companies on many other

orthopaedic devices.

The BHR uses an unproven " cast-in " macro surface, for which there is

no published long-term follow-up data.

SUMMARY

The information above is undisputable fact - these issues are nothing

new and all have been widely reported, written up, validated and

opinions formed. These technical issues are great fun for our

engineers, but are now being unfairly used by certain manufacturers

to confuse and influence patient decisions as to which implant they

should have.

At the end of the day, and in my humble, personal opinion, there are

two IMPORTANT factors when considering your options for any kind of

medical treatment:

a) is the treatment (device) a proven one?

is the practitioner (surgeon) experienced with the

treatment/technique?

If the answer to both is yes, you will almost certainly not go far

wrong.

We have been manufacturing Resurfacing implants since 1989, and I

persoanlly have attended more than 150 Resurfacing operations. If

anyone has any technical questions they would like answering from the

manufacturers point of view, don't hesitate to post them - I will try

to get on the Group every couple of days.

Iain Dunbar

Corin Group

Cirencester

UK