A ground bar in a bushing demonstrating the importance of straightness and roundness of the bar so as not to cause chatter. Photo courtesy of Boston Centerless.

Living in a Material World: The “Raw” Outlook

Continuing advances  can mean more choices for manufacturers—though sometimes at higher cost

Ursula Jones, Contributing Writer

While the orthopedic industry has enjoyed double-digit growth over the past few years, the future is expected to be even brighter. As the Baby Boomer generation approaches its sixties, the need for implants and other orthopedic products will accelerate. In fact, the number of hip and knee replacements performed annually in the United States will nearly double by 2030, according to the American Academy of Orthopaedic Surgeons.

“Factors affecting the growth of the market include an aging population, further penetrations into geographical regions where implants have been less common and an increase in obesity, which can have a significant impact on knees and hips,” said Harvey Stein, GUR global product specialist for Ticona, a Florence, KY-based manufacturer of engineering polymers. “We’re also seeing more joint replacements being performed in younger populations and especially in women who have had sports injuries, for example.”

Another potential development on the horizon will be the growing demand for medical and orthopedic products in China. In the past, the Chinese government has not provided much medical care outside its major cities. Now its rural communities are demanding improved medical care. “The Chinese government has said that, over the next five years, it will begin providing health care to those citizens. This will have a huge impact on the industry,” said Jeff Wise, vice president of sales and marketing for Titanium Industries in Rockaway, NJ.

With this anticipated growth in the orthopedic industry, the demand for raw materials will also increase. Proven materials such as titanium, cobalt-chrome and UHMW-PE will continue to be used, while ceramics will likely gain a growing share of the market. In addition, newer materials such as Nitinol, PEEK and bioabsorbable polymers will give orthopedic manufacturers a wider range of choices going forward.

Titanium

In addition to its strength, titanium is also extremely biocompatible, making it an ideal orthopedic implant material. “Titanium allows bone growth on the implant very readily,” said Wise. “It has a strength-to-weight ratio that is similar to the human body.” It’s also about half the weight of nickel-alloy materials, an important consideration for implants, especially hips and knees.
 

Miniature cable assemblies made from implantable materials. Photo courtesy of Fort Wayne Metals.

According to Wise, the cost of titanium had been comparable to that of nickel alloys. In the past few years, however, high demand for titanium by the aerospace industry has driven up the price by a magnitude of four.

Steven Tamasi, CEO of Boston Centerless in Woburn, MA, estimated that titanium is costing his company approximately 10 times the cost of certain grades of stainless steels. “The aerospace industry drives the availability of titanium—and therefore the price,” he explained.

According to Bob Myers, executive vice president of Fort Wayne Metals in Indiana, some mills will give priority to their large customers—such as aerospace companies—that require more tonnage. “One of the key concerns we have is that the world demand for raw materials used in this industry is at an all-time high,” he said. “As a result of that, some of the raw materials we need are getting more difficult to obtain. That will continue to be a challenge for the next few years.”

However, because the amount of titanium typically required by an orthopedic component is quite small, the cost is not a major factor. “The cost of the part is not a function of the raw material cost; it’s everything subsequent to that,” Wise explained. “Machining, part traceability and quality requirements play a more dominant role in the cost of the part.”

Other Metals and Alloys

Feed-thrus allow electricity to pass in and out of the implanted device to administer an electrical charge. Photo courtesy of Morgan Advanced Ceramics.

In addition to titanium, a variety of other metals can be used in orthopedic applications, including cobalt-chrome and newer materials such as Nitinol. Cobalt-chrome is very strong and has extremely high corrosion and wear resistance. “The alloy is also fairly straightforward to cast using the investment casting process,” said Jim Kiely, sales manager for Muskegon, MI-based Cannon-Muskegon, a manufacturer of premium-grade alloys.

One primary benefit cobalt-chrome has over titanium is that it has the reputation of being somewhat easier to work with. “Cobalt-chrome is similar to titanium in terms of the raw material cost, but it’s cheaper to cast and machine,” Kiely noted.

Selection of the right metal is not always a cut-and-dried decision. “Each design requires specific mechanical properties, which different materials help to achieve,” noted Myers. “Once we determine the designer’s needs, we can help [him or her] select the right material.” Myers additionally noted that his customers are always interested in finding materials that offer good processing capabilities. “We’re developing some processes that will enable the material to come to the customer directly off the spool without having to be straightened,” he explained.

Fort Wayne Metals also can make machining-grade stock out of 35N LT, a proprietary superalloy modification of an aerospace material that has lower titanium, which eliminates inclusions that can lead to fatigue failures. Myers said the material was originally created for the cardiac-rhythm management market, in which smaller-diameter wires are being used in leads for implantable defibrillators and pacemakers. Orthopedic applications include cables, pins and bone screws.

Myers, like many other suppliers, has seen an increased interest in Nitinol by the orthopedic industry due to its unique shape-memory properties and super elasticity. He said that the use of Nitinol from his company’s cardiovascular customers has increased by roughly 40% in recent years, and he expects that trend to carry over into the orthopedic industry as well.

Polyethylene

The most widely used implantable polymer today is ultra-high molecular weight polyethylene (UHMW-PE), due primarily to its high impact strength and biocompatibility. According to Mark Evans, worldwide business manager for MediTECH Medical Polymers in Fort Wayne, IN, many materials have tried to replace UHMW-PE in orthopedic applications but have been unsuccessful. “In many cases, these materials may have looked good in the lab, but they don’t perform as well in actual use,” Evans explained.

Flexible cable assemblies for orthopedic instrumentation. Photo courtesy of Fort Wayne Metals.

That doesn’t mean UHMW-PE can’t be improved. Evans said his company has been working with implant manufacturers and industry experts over the past several years to further reduce polyethylene’s wear.

“While UHMW will continue to be a mainstay within orthopedics, there will also be room for alternate materials, such as metal-metal and ceramic-ceramic implants, for younger patients,” Evans said.

Ticona’s Stein agrees. “We know that many device manufacturers have a complete product line where they will offer ceramics as well as metal-on-metal and UHMW-on-metal,” he explained. Despite the increased competition from other materials, polyethylene will always have its place in the market.

According to Stein, the price of UHMW-PE has been stable for the past several years. “There are long-term contracts between the manufacturers and their customers as well as the converters and the device manufacturers. We don’t want to upset those contracts, but price adjustments may be necessary due to cost and raw material increases. Even then, [Ticona would remain] committed to a long-term pricing strategy,” he said.

PEEK, Other Polymers

One polymer taking off in the orthopedic industry is PEEK, particularly for use in implants. The advantages to this material is that it is both lightweight and strong; the disadvantage, however, is that its expensive, according to Tamasi of Boston Centerless.

According to Michael Callahan, president of Invibio in Greenville, SC, PEEK has gained wide acceptance over the last decade and has a proven history of safe long-term implantation. Invibio sells a proprietary formulation called PEEK-OPTIMA, which is naturally radiolucent, allowing clinicians to assess healing and bone formation around an implant without the interference of X-ray scatter or generation of MRI artifacts.

The PEEK-OPTIMA polymer can be formulated with variable amounts of barium sulphate to tailor the radiopacity of the implant, allowing the desired degree of visibility for confirmation of implant positioning in patients. Additionally, the polymer’s mechanical properties can be altered to meet specific requirements. For instance, carbon fiber additives can be used to match PEEK-OPTIMA polymer’s modulus to that of bone, thereby reducing stress-shielding and related complications.

Combined with the material’s mechanical properties, this opens up new areas for orthopedic design and technologies, whether used alone or in combination with existing materials. “Applications utilizing PEEK-OPTIMA continue to expand because of its imaging compatibility, proven biocompatibility, flexible properties and versatility in processing,” Callahan explained.

Bioabsorbable polymers such as polylactic acid (PLA) and polyglycolic acid (PGA) are also finding new applications in the orthopedic industry. Since these products are designed to dissolve inside the body, they reduce the need for follow-up surgeries to remove metal components. Current applications include sutures, bone pins and screws.

Ceramics

While ceramics have been used in orthopedic applications for decades, they have not yet caught up with their metal and polymer counterparts. Ceramics are more widely used in Europe and Japan than in the United States, but that is expected to change in the near future. According to Steve Hughes, medical materials technologist for Morgan Advanced Ceramics in Fairfield, NJ, ceramic-ceramic systems make up roughly 70%-75% of implants in Europe, and they’re starting to take off in the United States. “A lot of implant companies’ growth will be coming from this area,” he predicted.

The two most significant benefits of ceramic are its extreme hardness and its excellent biocompatibility. “Ceramics offer much longer wearing times
than cobalt-chrome or polyethylene,” Hughes said. “You can also get very good wettability on ceramics, very good lubrication … and extremely low wear in comparison to standard metals and polymers.”

Because of its higher cost, ceramic is unlikely to replace all other implant materials. But for now, the market is growing. Orthopedic manufacturers are adding ceramic-on-polyethylene and ceramic-on-ceramic implants to their product lines to meet the needs of the marketplace. “These products are mostly being marketed to the younger generations because of ceramic’s low wear properties,” said Hughes. “Ceramic systems have been shown to last for 20 to 25 years and longer. And when they do need to be replaced, it’s not a case of the bearings wearing out. Rather, it’s due to a failure of other mechanisms,” he said.

Ceramic’s low wear properties also mean less wear debris. Furthermore, any debris that is generated by a ceramic component won’t interact with the human body. “The wear debris from ceramics is bioinert, unlike the wear debris from polyethylene and even metal, which can cause health problems in some implant patients—particularly over time,” Hughes noted.

Another potential application for ceramics is in hip resurfacing systems, which typically use a cobalt-chrome cap and cup to replace the joint. One concern regarding these metal-on-metal systems is the increase in metal ions in the body resulting from wear. According to Hughes, ceramics could be a solution to the metal ion problem associated with current hip resurfacing systems. “One possible trend you might see in these systems is ceramic on metal or ceramic on ceramic to minimize the wear debris,” he explained.

Also under development are bio-ceramic materials using synthetic Hydroxyapatite, the natural mineral component of bone, which can be used in such applications as dental implants and synthetic bones. The resulting implants bond readily to bone and other tissues without rejection or inflammatory reactions.

In the meantime, ceramics are still classified as a somewhat specialized field. “A lot of orthopedic companies don’t have much experience in-house from an R&D perspective,” explained Hughes. “It’s coming of age slowly but surely.”

What the Future Holds

There’s no doubt that materials will continue to improve, making even greater orthopedic advancements possible. One possible area of focus could be the development of bio-active materials—ie, materials that promote the healing process. Another possibility is the development of biomolecular materials. “These materials could ultimately be used to develop self-repair mechanisms that can respond to damage, or self assembling polymers that mimic tissue life cycles and have regenerative characteristics,” explained Invibio’s Callahan.

As materials research continues, more options will become available for orthopedic manufacturers. These new materials will not only help improve products already on the market, but also will enable the creation of products previously thought impossible. The sky is truly the limit.

Ursula Jones is a freelance writer specializing in the medical device, pharmaceutical and packaging industries.