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Ceramic
material, with its biocompatibility and resistance to wear, is ideally
suited for a wide variety of medical implant applications, from
artificial joints to implantable electronic sensors, stimulators and
drug delivery devices. For well over a decade, alumina,
zirconia
and other ceramics have successfully proven their ability to withstand
the harsh environment of the human body.
Innovative Techniques Extend Areas of Applications of
Ceramics in Medicine
Now,
driven by the industry’s need for longer-lasting and ever smaller – yet
more complex - components, materials scientists are extending the
benefits of ceramics for new medical implant applications with
innovative techniques, including injection molding, engineered coatings
and ceramic-metal assemblies. This article discusses how these
developments in ceramic material and processing are contributing to the
evolution of medical implant applications and the key role that Morgan
Technical Ceramics (MTC) is playing in this industry. Morgan
Technical Ceramics, comprised of Morgan
Advanced Ceramics (MAC) and Morgan
Electro Ceramics (MEC), is a leading manufacturer of innovative
ceramic, glass, precious metal, piezoelectric and dielectric materials.
Ceramics for Artificial Joints
Advances
in the use of ceramics for artificial joints have received a great deal
of attention, especially since golf legend Jack Nicklaus received a
ceramic-on-ceramic total hip replacement in 1999 in an experimental
procedure at New England Baptist Hospital. Ceramic-on-ceramic hip
joints received FDA approval in 2003.
Introduction of Ceramic Materials for
Artificial Joints
Ceramic
materials have been used for artificial joints since the 1970s when the
first generation of alumina products demonstrated superior resistance
to wear, compared to the traditional metal and polyethylene materials.
Advances in material quality and processing techniques and a better
understanding of ceramic design led to the introduction of second
generation alumina components in the 1980s that offered even better
wear performance.
Advantages of Using Ceramics Compared
to More Traditional Materials
Traditional
metal – polyethylene hip system wear generates polyethylene particulate
debris, inducing osteolysis, weakening of surrounding bone and results
in loosening of the implant, a primary cause of costly revision
operations. Ceramic materials generate significantly less polyethylene
debris when used in conjunction with polyethylene acetabular components
in bearing couples. Indeed, state-of-the-art ceramic-on-ceramic
technology, where an alumina femoral head is mated with an alumina
acetabular cup, totally eliminates polyethylene debris and reduces wear
significantly. A study of MAC’s
HIP
Vitox® ceramic-on-ceramic hip joints demonstrated a wear rate
of just 0.032mm3/million
cycles. In addition to resolving the problems caused by polyethylene
debris, the use of ceramic-on -ceramic hip systems alleviates any
concerns over metal ion release into the body if a metal on metal hip
system were used.
This superior wear
performance extends the life of artificial joints, giving
ceramic-on-ceramic joints a predicted life of well over 20 years.
Serving the needs of the increasing numbers of younger patients for
whom such surgery is now a viable operation, these ceramic-on-ceramic
joints allow them to continue leading active lifestyles.
Ceramics for Implantable
Electronic Devices
New
developments in ceramic technology are playing an equally important
role in the evolution of implantable electronic devices. In the
forty-five years since the first cardiac pacemaker was successfully
implanted in the U.S., researchers and doctors have created a wide
array of implantable electronic devices, including pacemakers, defibrillators,
cochlear implants, hearing devices, drug delivery and
neurostimulators.
Examples of Implantable
Electronic Devices
For
example, medical device companies are testing neurostimulators that
pulse various nerves to treat particular medical conditions: the
hypoglossal nerve [in the neck] to treat sleep apnea; the sacral nerve
to treat bowel disorders; the stomach to treat obesity, the thalamus to
treat epilepsy, the vagus-nerve to treat chronic depression, and other
regions of the deep brain to treat migraines and obsessive-compulsive
disorder.
Ceramic Feed-Thrus in Implantable Electronic Devices
These
devices increasingly rely on ceramic components, such as the feed-thrus
that provide the functional interface between the device and body
tissue. A feed-thru is a ceramic to metal seal assembly that contains
metal pins or small tubes that pass through a ceramic component. These
pins allow electricity to pass in or out of the implanted device in
order to sense what is going on in the body and/or to administer an
electrical charge when needed. A feed-thru can also be used to
administer drugs to the patient. The ceramic substrate of the feed-thru
acts as an electrical insulator, isolating the pins from each other. MTC
can also make ceramic housing assemblies to enclose the electronics for
the device, which can attach to a feed-thru.
Properties of Ceramic Feed-Thrus
Feed-thrus
for implanted devices must be hermetic, with a leak tight seal around
each pin. This ensures’ that bodily fluids do not work their way into
the device and destroy the internal electronics and that chemicals do
not inadvertently escape from drug delivery devices. A braze material,
typically 99.99% gold, is used to join each metal pin to the ceramic
insulator. To ensure the braze adheres securely, MAC
has developed a proprietary process, in which the surface of the
ceramic is prepared for brazing by the application of a thin film of
biocompatible metal such as platinum, niobium, titanium via physical
vapor deposition (PVD).
Demands for Ceramic Feed-Thrus
Developers
of new and improved implantable medical devices continually demand
smaller and more complex components. For example, MTC
has created a one-inch diameter ceramic feed-thru for drug delivery
applications that houses 104 separate pins. Voltage passes through each
pin activating different combinations of switches allowing a greater
number, or more complex combinations, of drugs administered at any
given time.
Powder Injection Molding Aids
Component Miniaturization
The
application of powder injection molding (PIM) has furthered the pursuit
of component miniaturization. This method enables the production of
intricate features and unusual geometries, most notably for
hearing-assist devices, bone screws and implantable heart pumps.
Testing of ceramic injection molded objects has shown that net-shape
as-molded parts exhibit significantly less variation in flexural
strength than green machined parts of the same formulation. The
narrower Modulus of Rupture distribution of the PIM parts can be
attributed to lower variability in surface finish than that which
occurs with a comparable machined surface.
Metal Injection Molding As An Economical
Alternative to Traditional Processing
MAC
also offers Metal Injection Molding (MIM) technology, which provides a
low-cost alternative to machining, investment casting, and stamping. A
MIM machine can typically mold parts in about 10 seconds compared to
minutes or even hours through conventional techniques. MIM applications
are ideally suited for high-volume production of intricate components,
ranging from laparoscopic instruments to biopsy jaws and dental
brackets.
Ceramic
Coatings for Implants
An
additional area of ceramic technical development important to medical
implant applications is ceramic-based coatings, such as diamond-like
carbon (DLC),
that provide a biocompatible, sterilization-compatible, non-leaching,
and wear resistant surface for key pivot points and wear surfaces. Such
coatings are used to reduce friction, increase surface hardness and
prevent ion transfer from metal implant components.
Summary
Driven
by the rapidly expanding and evolving market for medical implants,
material scientists and ceramic component manufacturers will continue
to develop new materials and new processes for the smaller, more
sophisticated, and longer-lasting implant applications of the future.
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