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Artificial hip joints and other surgical
implants make severe demands on the materials from which they are
made. Once inside the human body even stainless steel and titanium
alloys can corrode, become scratched and sometimes break. This wear
and tear means that artificial joints have to be replaced every ten
years or less.
Using advanced surface treatment techniques such as glow-discharge
ion implantation, the researchers have managed to improve the strength,
hardness and corrosion resistance of metal implants.
In spite of the success of surgical implants
such as artificial hip joints, the materials used to make them are
not always quite up to the job. Even stainless steel and titanium
alloys can break under the enormous stresses on load-bearing joints,
and in the salty environment of the body they can also corrode.
Deposits of inorganic salts can scratch bearing surfaces, making
joints stiff and awkward. As a result, the lifetime of an implant
is at most ten years, and often less.
Metallurgists and engineers often treat the surfaces of metal parts
to make them stronger, harder and more resistant to corrosion. This
project has adapted innovative surface treatment techniques to the
alloys commonly used in surgery, and has shown that the resulting
implants have better properties than those treated using conventional
techniques. Despite the medical benefits, however, the response
from implant manufacturers and the medical profession has been disappointing
perhaps for purely cosmetic reasons.
Getting the best of both worlds
Metal parts subject to wear need to be hard, yet hard metals can
be brittle and difficult to work with. One way to overcome this
is to make the part from an alloy that is relatively soft and tough,
and then to modify its surface using heat or chemicals. This can
add hardness, corrosion resistance and other desirable properties
without spoiling the toughness of the original metal. Carbon and
nitrogen are the two most popular elements added during surface
treatment.
Most metal surgical implants are made from one of three groups of
alloys: titanium-aluminium-vanadium (TiAlV), titanium-aluminium-iron
(TiAlFe) or stainless steel (316L). Titanium alloy implants are
currently treated using a process known as chemical vapour deposition
to add nitrogen to the metal surface. This gives a handsome gold-coloured
finish but the resulting layer of titanium nitride is thick (1-2
micrometres) and brittle. As the temperature changes the nitride
layer expands at a different rate from the underlying metal, and
this can cause it to crack or peel off.
Once the metal has lost its protective coating, ions such as chloride
in the body can corrode the surface and start cracks which can cause
the insert to break under repeated loading. There is also the problem
of biocompatibility: the body tolerates pure titanium, but damage
to the metal surface of stainless steel or cobalt-based alloys currently
used can release impurities such as nickel and cobalt, both of which
can provoke an immune response. Corrosion also releases dissolved
metal into the bloodstream, which is undesirable.
Researchers at the Université Louis Pasteur in France decided
that newer forms of surface treatment could increase both the corrosion
resistance and the biocompatibility of metal implants. In particular,
they decided to try surface treatment techniques that add carbon
and nitrogen in their elemental forms (as atoms dissolved in the
metal matrix) instead of chemically combined with the metal (as
carbides and nitrides). By keeping the treated layer thin and avoiding
a sharp junction with the underlying metal, the researchers hoped
to come up with a durable surface that would not crack or peel.
Using new techniques
The Université Louis Pasteur researchers started BE-4375
in October 1991 with the help of a large consortium of other specialists.
The Institut National Polytechnique de Lorraine in France provided
expertise in plasma deposition for treating stainless steel, whilst
Eurorad, a commercial arm of the French government-funded PHASE
laboratory, contributed its knowledge of ion implantation.
Also involved were two French universities, Ecole Nationale Superieure
des Arts et Industries de Strasbourg and the University of Nancy;
the University of Porto in Portugal, which ran the corrosion tests;
the Technical University of Braunschweig, Germany, which specialises
in chemical vapour deposition and has worked with car manufacturer
Volkswagen on nitriding gearbox parts; and two German firms: titanium
supplier Deutsche Titan and medical device manufacturer Peter Brehm.
The researchers used three groups of techniques. The first of these,
ion implantation by glow discharge, is a medium-energy (40-60 keV)
process that can penetrate deep into the material. Because it does
not require the beam of ions to be separated into different atomic
masses, it costs only 20 to 50 per cent as much as conventional
ion implantation, yet gives many of the same benefits. Both titanium
alloys and stainless steel were treated with nitrogen using this
method.
Processes in the second group, also used to add nitrogen, are based
on the use of plasma. Plasma diffusion (plasma nitriding, PDT) is
a low-energy (0.5-2 keV) process whose effects are intermediate
between chemical vapour deposition (CVD) - the conventional nitriding
process - and ion implantation. It was used to treat both titanium
alloys and, in a low-temperature variant, for stainless steel. Plasma-assisted
chemical vapour deposition (PACVD) is a development of CVD in which
plasma is used to give results that approach those of ion implantation.
For titanium alloys the researchers developed a two-stage process
in which PACVD followed PDT.
The third group consists of a single technique, a physical deposition
process known as reactive magnetron sputtering. This was used to
add a layer of carbon-doped stainless steel to the surface of stainless
steel components.
New procedures are promising...
The treated inserts were analysed using metallurgical laboratory
techniques and subjected to a series of functional tests. These
included assessments of the fatigue resistance of the complete inserts,
and the coatings' adhesion, biocompatibility and ability to withstand
corrosion and wear.
Ion implantation by glow discharge gave good results for all three
alloys. Fatigue strength increased by up to 20 percent, biocompatibility
was excellent, and the new treatment improved the corrosion resistance
of stainless steel. This treatment, say the researchers, is very
promising for surgical applications.
PDT and PDT followed by PACVD worked well for titanium alloys. If
the treatment temperature is above 800°C the metal loses fatigue
strength, but at lower temperatures wear resistance increases without
affecting fatigue strength. Plasma-nitrided stainless steel performed
poorly in biocompatibility tests and was rejected for surgical uses.
It should be useful, however, for other engineering applications.
Sputtered carbon coatings on stainless steel give excellent resistance
to wear and corrosion. Because this technique, like ion implantations,
is straightforward and gives reproducible results, the researchers
predict they are well placed for early commercial application.
...but selling it is harder
Despite the fact that the new treatments could be used to make
implants that last for 20 years, commercial interest in the project
has been muted. Eurorad has tried to market both services and equipment
for the new techniques, but implant manufacturers and hospitals
have shown little interest. One of the researchers even suggests
that this is for purely cosmetic reasons: the new techniques lack
the attractive gold finish produced by conventional nitriding, even
though this has been shown to perform less well.
The partners have patented their new techniques, however, and hope
that eventually they will see commercial applications. As well as
surgical implants these could include dental drills and other tools,
and as a tougher replacement for PTFE non-stick coatings on cooking
pans.
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