MARKET WATCH
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Ceramic components from CeramTec AG (Plochingen, Germany) are processed at temperatures in excess of 1400°C, yielding a material with a hardness surpassed only by diamond.
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Although ceramics have recently found their way into such applications as bone screws, spinal spacing systems, and synthetic bone graft material, joint replacement is still the most common use of ceramics in orthopaedics. One factor contributing to the popularity of ceramic joint implants is the high wear resistance of alumina- and zirconia-based materials. Modern manufacturing procedures such as hot isostatic pressing provide these ceramic implants with a highly uniform molecular structure. As a result, they are much less likely to fracture than the first generation of orthopaedic ceramics, which was introduced in the 1970s.
As demand for joint replacement procedures increases among an aging global population, ceramics appear to be taking a growing share of the market. Ceramic-on-ceramic hip implants have gained popularity in Europe during the last five years, and this segment is expected to continue to grow. According to Frost & Sullivan analysts, ceramic-on-ceramic hip bearings are estimated to capture 26% of the market by 2012—up from 9% in 2004.
Ceramics’ low wear rate makes them good candidates for young, active, or overweight patients in need of joint replacements. Conventional plastic and metal hip implants typically have a life span ranging from 10 to 15 years. By contrast, modern ceramic joint implants could last more than 30 years. Besides lower wear levels than either metal or plastic, ceramics are also better tolerated by the body, and, unlike metals, ceramics do not face corrosion problems or generate metal ions. As they wear, plastic and metal implants generate debris that can lead to loosening of the implants and ultimately the need for revision of the joint. “Using artifical joints made of ceramics has been demonstrated to significantly reduce this problem,” explains CeramTec AG spokesperson Jörg Kochendörfer. In addition, ceramics have been shown to produce minimal wear particles, and the body has been found to tolerate ceramic particles well.
“I cannot think of a better material for joint replacement than alumina- and zirconia-based ceramics,” says Ozan Akkus, associate professor of biomedical engineering at Purdue University (West Lafayette, IN, USA). “[Such joints] are very strong, produce very low friction, and are extremely inert in terms of their biocompatibility.”
A Mixed Reception
Despite ceramics’ wear resistance, the materials are faced with an image problem—particularly in America. “When the first ceramic implants were introduced in the United States, there was a problem with them breaking in the body,” explains John Szivek, professor of orthopaedic surgery at the University of Arizona (Tucson, AZ, USA).
The first generation of ceramics used in joint replacement procedures sometimes failed as a result of manufacturing defects, causing the implants to crack and eventually shatter. Furthermore, the grain size was up to 40× larger than in modern ceramics, which have individual grains as small as 1 µm. Additionally, ceramic joint implants have benefitted from improved quality control and design, and the introduction of ISO 6474, which specifies testing methods and manufacturing criteria for alumina-based ceramic joint implants.
Studies have found fractures of ceramic implants to be quite rare: a French study following 5500 ceramic hip implants from 1977 to 2001 listed only 13 fractures of the alumina component, putting the failure rate at 0.002%. Another study, published in 2000 by CeramTec AG (Plochingen, Germany), analyzed the durability of 2.5 million alumina femoral balls. The fracture rate of the firm’s ceramic femoral heads was 0.026% for first-generation alumina, 0.014% for second-generation alumina, and 0.004% for femoral heads manufactured after 1994.
“In Europe, orthopaedic surgeons were largely happy with ceramics when they were introduced and have continued to use them,” says Szivek. The market for ceramic implants continues to grow more quickly in Europe than in the United States. But the US market for ceramics may catch up, says Steve Hughes, materials technologist at Morgan Advanced Ceramics (Rugby, Warks, UK). “At present, around 5% of hip implants in the United States use ceramics, and the market is growing rapidly since the approval of ceramic-on-ceramic implants in 2002,” Hughes notes. “I suspect that eventually this trend will catch up with some European countries where more than 50% of implants are ceramic.”
A Promising Future?
Although the risk of fracture of ceramic joints has been virtually eliminated as a result of improved manufacturing capability, there have not been many studies that investigate the long-term efficacy of ceramic implants. “Joint replacements can last more than 30 years, and it may require an even longer amount of time for data collection,” states Jonathan Cluett, MD, an orthopaedic surgeon based in North Adams, MA, USA, who hosts http://orthopedics.about.com. “No one knows how ceramics will do in the long run.”
Nevertheless, orthopaedic ceramics have performed well in the laboratory and in short-term studies in patients. And ceramics currently are undergoing extensive research to further improve their material characteristics and bioactivity, according to Feza Korkusuz of the Department of Metallurgical and Material Engineering at Middle East Technical University (Ankara, Turkey). “Adding silica into their formulation significantly improves their load bearing and torque capacities. Such ceramics are called bioglass, and they have been found to be very biocompatible.”
Not surprisingly, one of the most extensive areas of research involves ceramic hip implants. “There is a lot of activity at present: new material developments offer increased strength and greater design flexibility in ceramic hip implants,” says Hughes. “Such developments would also open the way for new applications or improvements on existing devices for spinal disc replacement and total knee arthroplasty.”
The applications of ceramics are expanding as researchers continue to investigate synthetic ceramic bone. Hydroxyapatite, a porous calcium phosphate ceramic, has been used extensively in this research because it mimics the mineral composition of bone. Unlike inert ceramics such as alumina, the material is able to bond with bone. Used as a bone filler, it can form a scaffold that is eventually converted into natural bone by the body. But because it is weaker than natural bone, hydroxyapatite is generally not used for load-bearing applications.
Though hydroxyapatite is currently the most commonly used biomimetic ceramic, research may lead to the development of ceramics in other areas. “With the developing concept of tissue engineering I assume ceramics will eventually be able to mimic bone and cartilage when combined with polymers,” says Korkusuz. Porous hydroxyapatite ceramics could be used in conjunction with polymers that mimic the type-1 collagen component of bone. “When combined with cells and the appropriate type and dose of modulators, such systems could replace bone and cartilage in the near future,” Korkusuz adds.
For more information on CeramTec and Morgan Advanced Ceramics, as well as a range of companies offering products and services relevant to manufacturing orthopaedic products, turn to the company profiles in the following pages of this section.
Laser Processing Yields Precise Orthopaedic Components
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Laser processing services available from a contract manufacturer include radius cutting as small as 0.002 in. diam in tubing ranging from 0.020 to 0.230 in. wide. In addition, Vitaldyne (Waverly, MN, USA) can produce holes as small as 0.0005 in. diam and grooves 0.0008 in. wide. Tubular parts can be fabricated from nitinol, stainless steel, silicone, and medical-grade polymers including PVC, acrylics, and PTFE. Prototype to large-quantity production requirements can be accommodated.
A range of services are offered to meet the needs of medical device designers, including ribbon-wire cutting, profile cutting, slotting, and piercing holes through a single tubing wall. Additional laser-based services include ablation, thin-wall-tube welding, and micromarking, large-area Nd:YAG marking, and batch coding. The firm also provides adhesive and solvent bonding, soldering, pad printing, and packaging of medical components and assemblies.
The company offers laser systems with accurate part-control tooling and real-time laser parameter control for repeatable part accuracy and quality.
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Tubes and Rods Supplied for Orthopaedic Procedures
Custom-manufactured surgical tubes and rods with burr-free slots, notches, cross-holes, multiple angles, and other complex features are available from a contract manufacturer. Using high-precision wire EDM processes, Marshall Manufacturing (Minneapolis, MN, USA) creates surgical tool components that are suited for orthopaedic applications.
Achieving dimensional accuracy to ±0.0005 in., the firm’s CNC wire EDM processes provide flexibility in creating part features to tight tolerances. Typical parts include 0.187-in.-diam stainless-steel rods and 0.125-in.-diam tubes with a swaged tip, cross-holes as small as 0.015 in., and connecting slots and notched areas with specially textured surfaces. Custom points and tips can also be fabricated. Induction brazing, grooving, contouring, threading, forming, and 3-D bending are also available. Additional manufacturing processes include grinding, knurling, milling, stamping, broaching, burnishing, and CNC Swiss machining.
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Alumina Components Resist Wear
A medical-grade alumina ceramic surpasses the requirements of ISO 6474. Available from CeramTec AG (Plochingen, Germany), Biolox Forte has a low wear rate, does not release metal ions, and is not known to cause adverse tissue reactions or allergic reactions. The high-purity alumina ceramic contains a small amount of magnesium to prevent grain growth and to allow the alumina to be sintered to near-theoretical density. The components undergo hot isostatic pressing at a temperature in excess of 1400°C and at a pressure greater than 1000 bar.
Laboratory tests using wear simulators have shown that the Biolox Forte material’s wear rate is equivalent to 0.001 mm per year. By contrast, metal-on-polyethylene systems have a 200× greater wear rate, according to the firm. The material has a Young’s modulus of 380 GPa. The inert material is biocompatible and stable in the body.
Thermoplastic Sheet Material Provides Tensile Strength
High-performance thermoplastic sheet suitable for orthotic and prosthetic applications can be thermoformed and membrane pressed. Offered by Kleerdex (Bloomsburg, PA, USA), the Kydex material can be used to eliminate seams and corners in products that pose cleaning problems. Products laminated with the material avoid the cracking and chipping associated with high-pressure laminates.
The material resists exposure to lipids present in body fats and to gamma radiation, making it suitable for a range of applications. The material also can be engineered to eliminate pressure points typically found in orthotic devices. The thermoplastic’s melting point range is from 320° to 390°F.
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Ceramic Implant Devices Are Suited for a Variety of Applications
A firm designs and manufactures ceramic implant devices for a range of orthopaedic, reconstructive, and surgical applications. Morgan Advanced Ceramics Bioceramics (Rugby, Warks, UK) offers ceramic femoral heads and cup inserts for ceramic-on-polyethylene and ceramic-on-ceramic hip replacement bearings, spinal fusion and artificial-disc replacement devices, and custom orthopaedic products. In addition, the company manufactures ceramic feed-throughs that are designed to enhance the reliability of internal bone growth stimulators.
The company offers flexible manufacturing processes to satisfy specific customer needs, backed by an extensive design and technical support team to assist in all stages of product development. It is certified to ISO 13485.
Firm Machines and Assembles Orthopaedic Components
Precision machining and assembly of a range of orthopaedic and spinal reconstruction components are available from Marox (Holyoke, MA, USA). The firm employs CNC milling, turning, Swiss screw machining, and wire electrical-discharge machining to produce a range of orthopaedic implants from titanium, stainless steel, PEEK, and other materials. Applications include spinal, hand, hip, and knee implants, as well as bone plates and screws. Other services available include prototyping, product launches, and project management. The company is certified to ISO 13485 and ISO 9001:2000 and is registered with US FDA.
