Ceramic materials are good candidates for medical implants thanks to stellar biocompatibility and resistance to wear.
Alberox Products Div.
Morgan Advanced Ceramics
New Bedford, Mass.
Alumina, zirconia, and other ceramics have successfully withstood the harsh environment of the human body. A need for better and ever-smaller yet more complex biocompatible components has led scientists to develop innovative techniques that bring ceramics to medical implants. These techniques are in the areas of injection molding, engineered coatings, and ceramicmetal assemblies.
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 during an experimental procedure. Ceramicon-ceramic hip joints received FDA approval in 2003.
Ceramic materials have gone into artificial joints since the 1970s when first-generation alumina products demonstrated wear resistance superior to that of traditional metal and polyethylene. Refinements in material quality and processing techniques, as well as a better understanding of ceramic design, led to the introduction in the 1980s of second-generation alumina components with even better wear performance.
Ceramic materials mated with polyethylene acetabular components in bearing couples generate significantly less polyethylene debris than traditional metal/ polyethylene hip systems.
Polyethylene particulate debris induces osteolysis (a weakening of surrounding bone tissue) and makes the implant loosen. It is a primary cause of costly revision operations.
State-of-the-art ceramic-on-ceramic technology, where an alumina femoral head mates with an alumina acetabular cup, eliminates polyethylene debris and reduces wear significantly. A study of HIP Vitox ceramic-on-ceramic hip joints demonstrated wear rates of just 0.032 mm3/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 as may be the case with traditional metal-onmetal hip systems.
This superior wear performance extends the life of artificial joints, giving ceramicon-ceramic joints a predicted life of well over 20 years. The longer life is a boon for the increasing number of younger patients for whom such surgery is now a viable operation: Ceramic-on-ceramic joints let them continue leading more active lifestyles.
IMPLANTABLE ELECTRONIC DEVICES
New developments in ceramic technology are playing an equally important role in the evolution of implantable electronics. In the 45 years since the first cardiac pacemaker was successfully implanted in the U.S., researchers and doctors have created an array of implantable electronics that include pacemakers, defibrillators, cochlear implants, hearing devices, drug-delivery systems, and neurostimulators.
Medical-device companies are, for example, testing neurostimulators that pulse various nerves to treat particular medical conditions. Neurostimulators implanted into the hypoglossal nerve (in the neck) treat sleep apnea. In the sacral nerve they treat bowel disorders, and in the stomach they treat obesity. Ceramic devices implanted in the thalamus also treat epilepsy, while those in the vagus nerve can treat chronic depression. Other regions of the deep brain can also receive ceramic implants to help treat migraines and obsessivecompulsive disorder.
These implants increasingly rely on ceramic components for such features as feed-thrus that serve as the functional interface between the device and body tissue. A feed-thru is a ceramic-tometal seal assembly that contains metal pins or small tubes that pass through a ceramic component. The pins let electricity pass in or out of the implanted device for power or connections to sensors.
Feed-thrus can also administer drugs to patients. The feedthru's ceramic substrate acts as an electrical insulator, isolating the pins from each other. Ceramic housing assemblies attached to a feed-thru also serve as electronic enclosures for the device.
Feed-thrus for implanted devices must be hermetic, with a leak-tight seal around each pin. The seal ensures bodily fluids won't work their way into the device and destroy the internal electronics. It also ensures that chemicals in drug-delivery devices won't inadvertently escape. A braze material, typically 99.99% gold, joins each metal pin to the ceramic insulator.
A proprietary process ensures the braze adheres securely. Here the ceramic surface is prepared for brazing by physical vapor deposition (PVD) of a thin film made from a biocompatible metal such as platinum, niobium, or titanium.
Developers of implantable medical devices continually demand smaller and more complex components. For example, there is now a 1-in.-diameter ceramic feed-thru available for drug-delivery applications that houses 104 separate pins. Electrical current passes through each pin to activate different combinations of switches. This action lets the device administer a greater number (or more complex combinations) of drugs at any given time.
The application of powder-injection molding (PIM) has furthered component miniaturization. This method is key to production of intricate features and unusual geometries, most notably for hearing-assist devices, bone screws, and implantable heart pumps. Testing of ceramic injectionmolded objects has shown that flexural strength of net-shape asmolded parts varies significantly less than that of green machined parts having the same formulation. The fact that PIM parts have a narrower modulus of rupture distribution comes from a lower variability in surface finish than that of a comparable machined surface.
Metal-injection molding or MIM is yet another low-cost alternative to machining, investment casting, and stamping. A MIM machine can typically mold parts in about 10 sec compared to minutes or even hours through conventional techniques. MIM suits high-volume production of intricate components ranging from laparoscopic instruments to biopsy jaws and dental brackets.
An additional important area of ceramic technology to medical implant development is in ceramicbased coatings such as diamondlike carbon (DLC). These coatings provide a biocompatible, sterilization-compatible, nonleaching, and wear-resistant surface for key pivot points and wear surfaces. DLC coatings are used to reduce friction, make surfaces harder, and prevent ion transfer from metal implant components.
There's a rapidly expanding and evolving market for medical implants. So material scientists and ceramic component manufacturers will continue to develop new materials and new processes for smaller, more sophisticated, and longer-lasting implants of the future.
Alberox Products Div., Morgan Advanced Ceramics Inc., (508) 995-1725, alberox.com