Ceramic Technology Contributes to Advances in Medical Implants

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.

Driven by the industry's need for longer-lasting and ever smaller, more complex components and 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.

Ceramics for Artificial Joints

Advanced ceramics that are used 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.

Ceramic materials have been used for artificial joints since the 1970s, when the first generation of alumina products demonstrated superior resistance to wear. Advances in material quality and processing techniques, along with a better understanding of ceramic design, led to the introduction of second generation alumina components in the 1980s, which offered even better wear performance.

The traditional metal-polyethylene hip system wear generates polyethylene particulate debris, inducing osteolysis – the weakening of surrounding bone. This results in the 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. 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 Fairfield, NJ-based Morgan Advanced Ceramics' (MAC) HIP Vitox ceramic-on-ceramic hip joints demonstrated a wear rate of just 0.032mm3/million cycles. Using ceramic-on-ceramic hip systems also alleviates any concerns about metal ions being released into the body if a metal-on-metal hip system is used.

The superior wear performance extends the life of artificial joints, giving ceramic-on-ceramic joints a predicted life of more than 20 years. This serves the needs of the increasing number of younger patients, making such surgeries a viable option while allowing them to continue leading active lifestyles.

Implantable Electronic Devices

New developments in ceramic technology are playing an equally important role in the evolution of implantable electronic devices. In the 45 years since the first cardiac pacemaker was successfully implanted in the U.S., researchers and doctors have created a broad array of implantable electronic devices, including pacemakers, defibrillators, cochlear implants, hearing devices, drug delivery, and neurostimulators.

Ceramic technology plays an important role in implantable electronic devices.

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; thethalamus to treat epilepsy; the vagus-nerve to treat chronic depression; and other regions of the deep brain to treat migraines and obsessive-compulsive disorder.

These devices increasingly rely on ceramic components, such as the feed-thrus that provide the functional interface between the device and body tissue. A feedthru 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 feedthru can also be used to administer drugs to patients. The ceramic substrate of the feed-thru acts as an electrical insulator, isolating the pins from each other.

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.

Developers of new and improved implantable medical devices continually demand smaller and more complex components. For example, Moran Technical Cermiacs (MTC) has created a 1"-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.

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-assistance devices, bone screws, and implantable heart pumps. Testing of ceramic injection-molded objects has shown that net-shape, asmolded parts exhibit significantly less variation in flexural strength than greenmachined 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 comparably machined surfaces.

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 with 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-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 are also important to medical implant applications. These coatings increase surface hardness and prevent ion transfer from metal implant components.

Material scientists and ceramic component manufacturers continue to develop new materials and new processes for the smaller, more sophisticated, and longer-lasting implant applications of the future.