Advanced Ceramics in Medical Applications
Developments in ceramic materials and product design are offering surgeons and patients new options for joint replacement surgery. Improvements in ceramic production will bring the proven biocompatibility and long-term durability benefits of the material to an increasingly wide range of medical applications. Driven by the medical industry’s need for ever smaller yet more complex components, materials scientists today are making use of innovative processing techniques, including injection moulding, engineered coatings and ceramic-metal assemblies. The results include hand tools, valves, and implantable devices.
Ceramics in Joint Replacements
Most artificial joints used today incorporate a metal or ceramic head against an ultra-high molecular weight polyethylene (UHMWPE) cup. The use of ceramic components in joint replacement surgery was initiated in the 1970s with the introduction of first generation alumina products, when ceramic’s superior resistance to wear in comparison to more traditional metal and polyethylene materials became apparent. Advances in 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 better performance than earlier systems.
Figure 1. Examples of HIP Vitox alumina and Zyranox zirconia ceramic orthopaedic implants and DLC coating technology for femoral head implants
Advantages of Using Ceramics in Joint Replacement Applications
Low system wear rate leads to long life-times, typically over 20 years for ceramic. It also generates less polyethylene particulate debris, a known cause of osteolysis and a major source of revision operations that are both costly and increase the trauma of patients. Some types of ceramic hip joints have a wear rate of just 0.032mm3/million cycles. Ceramic systems also alleviate concerns over metal ion release from the use of metal implants in the body and their possible side effects.
Ceramic-on-Ceramic Artificial Joint Systems
‘Ceramic-on-ceramic’ systems have been developed to eliminate the problems associated with polyethylene and metal altogether. Such systems have by far the lowest wear rate of all bearing couple technologies available today.
These implants have grown in popularity in Europe over the past five years or so and, in 2003, the Federal Drugs Agency (FDA) gave their clearance for use in the USA. The properties of ceramic-on-ceramic make them ideally suited for younger and more active patients. The international golfer Jack Nicklaus has a replacement ceramic hip.
Complex Ceramic Implantable Components
Building on the known benefits of ceramic as a material for bio-implants, manufacturers are now starting to use injection moulding techniques to produce much smaller, more complex parts, most notably hearing-assist devices, bone screws and implantable heart pumps.
Figure 2. Ceramic life science instrumentation, surgical tools and implantable devices
Increasing Rates of Minimally Invasive Surgery
At the same time, surgical procedures have become more intricate, creating a need for smaller and precise instrumentation. This is borne out by the increase in minimally invasive surgery (MIS) in areas including hernia, ulcer repairs and even complete hip replacement.
A survey by the University of Arkansas School for Medical Sciences and Arkansas Children’s Hospital, Little Rock, USA, reported that 82% of paediatric surgeons now perform MIS procedures. This means patients recover quicker, which is good for the patient and more cost-effective for the hospital.
Intricate Implants Made by Injection Moulding of Ceramics
Injection moulding of ceramic allows production of small, high-precision instruments. These strong and complex shapes, such as the hinge joints on powered hand tools, allow for intricate designs. Traditional machining of ceramics would be more time consuming and expensive, and may not enable all the same features as ceramic injection moulding. Internal testing at Morgan Advanced Ceramics (MAC) in Rugby, UK, on injection moulding versus ‘green’ machining showed more consistent strength on the injection moulded parts.
Ceramics in Implantable Electronic Devices
Ceramic technology is also playing a role in the evolution of implantable electronic devices. Medical device companies are testing neurostimulators that pulse various nerves to treat particular medical conditions.
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. The pins allow electricity to pass in or out of the implanted device and sense what is going on in the body. The ceramic substrate of the feed-thru acts as an electrical insulator, isolating the pins from each other.
Increasing Demands on Ceramic Components from Implant Device Developers
The developers of implantable medical devices continually demand smaller and more complex components. The application of powder injection molding (PIM) has furthered the pursuit of component miniaturisation. 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 moulded objects has shown that net-shape as-moulded parts exhibit significantly less variation in flexural strength than green machined parts of the same formulation.
Metal Injection Molding (MIM) technology can be used as a low-cost alternative to machining, investment casting, and stamping. MIM applications are ideally suited for high-volume production of intricate components, such as laparoscopic instruments, biopsy jaws and dental brackets.
Total Hip Replacements
The first recorded attempts at hip replacement were carried out in Germany and used ivory to replace the femoral head. By the 1960s, the modern artificial joint had been developed thanks primarily to the work of Sir John Charnley at Wrightington Hospital, Wigan, UK. He used tribological techniques to produce a design known as the low friction arthroplasty that combined a small diameter metal head articulating against a polymer – initially PTFE but, due to early complications, changed to high density polyethylene, fixed into the femur using bone cement. The replacement joint was lubricated with synovial fluid.
Wear and Polyethylene in Total Hip Replacements
Gradually, evidence began to implicate polyethylene particulate wear debris from the metal-PE bearing couple as the cause of aseptic loosening of total hip replacements, limiting their survival and necessitating surgical revision of the hip system. Such complications led to the need for alternative bearing surfaces in total hip replacement.
The Evolution of Ceramics in Total Hip Replacements
The use of alumina ceramics in total hip arthroplasty began in Europe. Professor Pierre Boutin pioneered the use of ceramics in France in 1970, replacing the traditional metal femoral heads with alumina. Further developments led to the introduction of ceramic acetabular (cup) components to bear against the ceramic femoral heads that reduced wear rates and patient complications associated with polyethylene wear debris. Early ceramic component failures led to worries over its use, however, improvements were made in material quality, manufacturing processes and design requirements to alleviate such concerns. Nowadays, many hip implant components are made of a ceramic material rather than a metal or polyethylene.
Alumina and Zirconia in Orthopaedic Implants
In 1985, Morgan Advanced Ceramics launched Vitox alumina for use in joint replacement, following on in 1990 with their Zyranox zirconia orthopaedic implants. Zirconia ceramic is one of the highest strength ceramics suitable for medical use. Research is continuing on new materials to improve the performance of replacement joints. Through work on ceramic matrix composites, the company hope to introduce a material that is extremely hard and low wearing as well as having excellent mechanical properties for the next generation of hip replacements.
Other advances in medical device technologies are the result of combining existing methods in new ways. In the USA, Morgan Advanced Ceramics is investigating ceramic-metal joining solutions for medical applications using braze alloy systems. In another project the company is using diamond-like carbon (DLC) coating technology for medical applications to offer extremely hard, low friction wear surfaces.
Diamond-Like Carbon Coatings
DLC coating, first developed for automotive and commercial applications, is finding use in coating valves, wear plates and fluid delivery devices. This will make the devices inert, biocompatible, extremely hard (up to 3300kg/mm2) and lubricious (coefficient of friction = 0.1) so that they can handle extreme pressures, while still allowing the instruments to be employed for analysis of small samples.
Over the last 20 years, ceramic materials have been refined and there is now a range of solutions optimised for use in the medical field, from surgical tooling to implants.