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Choosing Materials Wisely

As orthopedic designs change rapidly, so must orthopedic materials. Close collaboration between OEMs and suppliers is the best way to keep pace.

Orthopedic manufacturers are always looking to improve designs of implants and instruments. The pace of development is fast and the desire for innovation is strong. To accommodate this kind of growth and change, orthopedic OEMs are always on the lookout for materials with superior properties.
Sometimes that means working with materials suppliers to come up with new materials or new combinations or mixtures of existing ones. Sometimes it means performing new tests on existing materials to see if they can be adapted to new applications.

There are many different ways to go about determining which materials are best for particular orthopedic applications. The common thread is that the most successful projects result from OEMs that have a keen understanding of their suppliers’ products and capabilities, and from suppliers that are attuned to what OEMs are looking for, and are striving to improve their products by anticipating future needs.

“We make sure we keep up with what the markets are looking for,” says Jon Schaeffer, medical commercial manager for Carpenter Technology Corp., a Reading, Pa.-based maker of stainless steel and specialty alloys.” OEMs are always looking to improve properties of materials so they can design better products, and we have made modifications in accordance with their wishes. We are always working on new stuff.”

Some of the Options

There are a number of materials to choose from for orthopedic applications, from metals to thermoplastics to ceramics. Which route to go depends on priorities, including strength, wear, flexibility, and design for manufacturability.

A highly magnified view (15,000 times) of the surface of a textured silicon nitride implant. The proprietary ceramic, manufactured by Amedica Corp., has a crystalline structure that is a matrix with crevices piled on top of each other, which accounts for its superior fracture toughness, according to the company. Photo courtesy of Amedica. Read more about it in the sidebar below.

On the thermoplastics side, polyetheretherketone (PEEK) has become an increasingly popular choice, though “any thermoplastic with great strength characteristics that can meet ISO 10993 requirements and is allowed to be implanted is seeing growth,” said Mark D. Schaefer, corporate vice president of business development for Spectrum Plastics Group, a plastics supplier based in Minneapolis, Minn.

“PEEK is preferred by many implant designers as the mechanical properties mimic those of bone,” explained Ed Hurst, business development executive for Spectrum’s medical device business. “PEEK implants won’t carry all the load (like metal implants will), so this requires the surrounding human tissue to continue to function and support load, at least that’s one of the theories out there. PEEK can be molded very accurately if tooled properly. PEEK is extremely stable and many orthopedic customers have combined PEEK with other implantable substances to further promote bone growth. It is extremely versatile.”

Indeed, there is much innovation going on with thermoplastics right now, according to Schaeffer.

“The universe of implantable thermoplastics with DMFs(device master files) in place has expanded dramatically in last few years,” he said. “It is an area that OEMs should be investigating, and even if not already registered for implant use, there are ways to develop a useable and competitive advantage in a device from thermoplastics for the right applications.”

In ceramics, a major development has been the introduction of silicon nitride, which has shown the ability to promote bone growth and inhibit infection. (For more on this, see sidebar.)

On the metals side, titanium continues to be popular. Craig Schank, vice president at Supra Alloys Inc., a Camarillo, Calif.-based raw material supplier, says there is a lot of demand for two titanium alloys, 6AL-4V and 6AL-4V ELI. Todd Hall, medical products manager for Ulbrich Stainless Steels and Special Metals Inc. and Ulbrich Specialty Wire Products, a supplier based in North Haven, Conn., said his firm is seeing more requests for titanium 3-2.5 in strip (Grade 9) and L605 (or other cobalt-chromium alloys) in both strip and wire form.

Also, said John Schaeffer, metals that show promise with elongation now are getting a closer look by orthopedics manufacturers.

“One property enhancement OEMs are looking for is elongation. A strong material still has to have good elongation in regards to fatigue cycles,” he said. “The better an alloy wears, the better its fatigue lifecycle, the longer a life for the alloy and the part. The goal is not to do revisions on implants. That’s a big driver in the industry.”

For instruments, a material that’s been making inroads is nitinol, a roughly equiatomic nickel and titanium, which offers shape memory, said John F.X. Morley, medical product manager for Johnson Matthey Plc, a London, United Kingdom-based specialty chemicals firm with a ceramics division in West Chester, Pa.

“Under certain conditions the metal can accommodatemassive strain (up to about 8 percent or so) and recover its original shape,” he said. “This property, which is facilitated via a stress-induced phase change (from martinsite to austinite), is commonly referred to as being ‘super-elastic’ because the material acts like a metal rubber band. The same material, again under specific conditions, can recover a pre-trained shape through the addition of heat. This property can allow for a shape change to occur due to heating—from room temperature to body temperature, for instance.”

While mostly known for use in cardiovascular applications like guide wires and stents, nitinol has properties that are appealing to the orthopedics industry as well, he adds.

“Applications in ortho are starting to be the next big thing. The most common applications in orthopedic products for these nitinol materials tend to be in tools which are needed to access difficult-to-reach parts of the anatomy,” he explained. “Imagine a drill bit that can curve to accommodate the particular shape of the femoral stem as a hip surgery is taking place, or a flexible instrument to facilitate a minimally invasive surgery, which would otherwise require a large incision. Or a plate which may be manipulated outside of the body, but when inserted and warmed to body temperature, `remembers’ a specific pre-programmedcontour.”

The Vetting Process

The U.S. Food and Drug Administration (FDA) has gotten more stringent about making medical OEMs prove that their materials are sound and properly suited for the application. That means the vetting process has gotten more extensive. In many cases, the OEMs and suppliers do separate testing; the suppliers run a series of tests to ensure that the materials meet their customers’ specifications, and the OEMs run a more complicated series of tests to ensure that the materials meet FDA requirements.

“It is really theortho device manufacturers who will have the burden of determining if these materials are appropriate for their applications,” said Morley. “This is really something specified by directive of FDA. Material suppliers can provide mechanical properties like tensile, yield, elongation, hardness, corrosion resistance, etc., but the real expertise must be developed (and often resides) with the device manufacturers because the performance of the materials must always be evaluated in the system into which it is to be designed.”

Jon Schaeffer agreed.

“All OEMs have their own quality processes, driven by regulatory requirements. They generally give us guidelines on what parameters to meet,” he said.” We batch test, but they have their own quality procedures to go through. Our own tests are done to make sure we meet the requirements of the customer. We gain information about our processes, and how changes to them affect properties.”

The process can be expensive, especially if there is no DMF on file for FDA to peruse, said Mark Schaefer.

“If the material supplier has not developed and filed a DMF for the material, then the device owner is going to be responsible to do all the vetting required via ISO 10993 and FDA,” he said. “Typically they would do that on a specific lot of material which then means they must procure that entire lot up front and hold in inventory which just adds additional cost. This takes time as well as dollars.”

The process can get even more complicated if additives have to be vetted as well, he noted.

Hurst explained that this means “OEMs and processors alike must have good relationships with the resin suppliers to facilitate selection of approved materials up front and early to avoid costly delays and redesigns later.”

Product Development

Where closer collaboration comes into place is in the early stages of development, when OEMs have identified what properties they need from their materials but may not have figured out which materials are the best fits. This is especially the case when the materials the OEM uses on existing products aren’t proving to be the best fit on prototypes for a new product.

“Often materials suppliers do work with device companies to pioneer new materials which can be incorporated into medical devices,” said Morley. “What usually occurs is that a device manufacturer has reached the limits of what is capable from a given material that is the standard but is stilllooking to reach a new population. In these instances, collaboration will typically come in the form of a joint development project where the material supplier is given a problem to solve, such as ‘I need a higher tensile, but more elongation’or ‘I need this, but need to see it better under X-ray.’We then go back to the lab and look to engineer a solution.”

The process tends to start with OEMs telling suppliers what they want to achieve and what class of material may help them achieve it, according to Hall.

“By suggesting new or different alloys to us and also what their end dimensions need to be, we can work with customers to provide the next step in the development timeline for them,” he said. “Our customers may dictate that a wider form of Titanium 6-4 may be needed or someone may want to try niobium or tantalum. That will drive our research and manufacturing departments to provide the new dimensions and product forms whenever possible. We constantly strive to provide higher quality materials thru process improvements that include both manufacturing and quality control inspections.”

However, he added, OEMs need to give suppliers enough information to hone in on the best choices, but enough flexibility to experiment.

“Whenever possible, all material requirements should be discussed as early as possible,” Hall said.
“Dimensional and tensile tolerances are only the beginning to selecting a correct material. Consider cast, camber, straightness, flatness, surface finish and any other finish requirements that are needed to make the final part successful. Either over or under specifying a material’s properties will lead to a longer development and order cycle.”

Jon Schaeffer agreed. “They may have concerns about machinability. They may have concerns about our ability to hold certain tolerances and diameters. These are very important for them from a processing point of view,” he said. “It is a big cost driver if you can get materials to machine better, achieve tighter tolerances, and use more precise machines to make your products. Cost is always a factor, but it is not the number one priority. That’s to get the best materials for the best designs. Cost has become a bigger part of the overall scheme of things because of the healthcare law, which has some proposed taxes that could affect the industry.”

Schank says uncertainty about the new law also is impacting material development.

“With the new healthcare law, the uncertainty has everyone slowing down. The question is whether companies will continue to focus on developing high-tech products, or if the healthcare law will standardize products, making them become generic in order to keep costs lower,” he said. “But we’re looking forward to a robust year in sales. As we slowly come out of the recession, there are some good signs out there for the metals industry as a whole.”

Design for manufacturability issues also must be ironed out as soon as can be, Hurst noted.

“Just because it meets clinical needs doesn’t mean it will be manufacturable, so the OEM must work closely with resin suppliers and contract manufacturers,” he said. “The little things matter. You need to consider how a material will flow through a mold. You need to consider issues like gate vestige and tool venting. A good molder needs to ask about these things.”

The reward for getting materials issues sorted out early in the development process is quality products and robust growth.

“Biocomposites and other thermoplastic combinations have great potential in the marketplace. The U.S. market is continuing to grow and the global market will explode in the upcoming decade,” says Hurst.” Future patients and customers are demanding and will want better performing products that are implanted less invasively and will last far longer before revisions.”

Spirit of Collaboration

As orthopedic designs get more complicated, so must the materials that are required to make them. It is more important than ever that orthopedic OEMs and their materials suppliers rely on each other during the development process. Generally, it is no longer acceptable during the development of a new product to assume an existing material is going to fit the bill, make specifications based on it, and wait for the supplier to deliver the order without further input. Rather, the OEM’s design expertise and the suppliers’ material expertise must both be brought to bear in a collaborative environment featuring constant communication.

“Suppliers are always looking to help their customers find the answers to their problems and needs,” said Hall. “If we cannot produce it, we’ll tell you, but until we know there may be a need for it, we are less likely to make it. Don’t be afraid to ask.”

Erik Swain is a freelance writer based in Phillipsburg, N.J. He hascovered the medical device industry for 14 years.

Silicon Nitride: A New Solution

It is not common for totally new materials to be introduced into the orthopedic market. Usually, innovation occurs by developing new grades of existing materials, or by coming up with new combinations of different materials. But Amedica Corp., a fledgling orthopedic OEM based in Salt Lake City, Utah, is hoping to make a big splash with silicon nitride, a ceramic that had not been used for orthopedic applications until the company’s founder decided to try it.

Amedica’s Valeo VBR small implant for corpectomy procedures. Photo courtesy of Amedica Corp.

Silicon nitride had been popular with commercial manufacturers, especially in the aerospace industry. Aaron Hoffman, M.D., a world-renowned total-jointsurgeon, thought it might translate well into orthopedics. His research led to the founding of Amedica.

“Dr. Hoffman invented medical-grade silicon nitride in trying to determine how to extend the life of the total-joint-bearing surface, the ultimate goal being a lifetime total joint,” said Eric Olson, Amedica’s president and CEO. “We were already using ceramics in medical devices, and since silicon nitride shares some of the same properties as those ceramics, he thought maybe it would be a good opportunity for medical devices.”

Research has shown silicon nitride to be tougher and to have better wear properties than other ceramics, Olson added.

“One of silicon nitride’s most essential qualities is fracture toughness, which is much higher than any other ceramicmaterial, as much as two times higher,” he explained. “It is not susceptible tohydrothermal degradation, which is an issue with other ceramics, even zirconia-based ones. Because of that, silicon nitride will be the ceramic of choice for femoral head replacement.”

Another advantage is that siliconnitride is radiolucent, and can be seen clearly in MRI and CT scans withoutproducing any artifacts.

Research taking place now is showing that silicon nitride appears to be superior to polyetheretherketone (PEEK) andtitanium in terms of promoting bone growth and preventing infection. This could have a major impact on thelandscape of orthopedic materials.

“Today, if you have an implant and get an infection, it is treated with antibiotics, and if those don’t work, the implant must come out,” explained Thomas Webster, Ph.D., associate professor of engineering in orthopedics at Brown University in Providence, R.I., the study’s lead researcher. “This has become more common with the emergence of bacteria that have developed resistance to current antibiotics. We need to come up with new ways to reduce infection. Silicon nitride appears to be a way to stop bacteria from growing while still providing bone growth.”

In one study, Webster’s team put PEEK, titanium and silicon nitride implants into rat skulls. Then they injected them with bacteria. The results after 14 days were pretty conclusive.

“One great indicator is push-out force. When you remove the implant and surrounding tissue, and put force on the implant, you can see how much force it takes to push out tissue. On bone, the force is pretty high, but if bacteria are infectingthe surrounding tissue, it’s easier,” he

explained. “What we saw was that after 14 days, the PEEK implant required no force to push the implant out of the tissue. With silicon nitride, it took nine newtons of force after 14 days. That means it inhibited growth of bacteria and promoted growth of bone. Titanium was closer to PEEK,taking two newtons of force, which still paled in comparison to silicon nitride, which took four times greater force in the experiment and had four times greater bone growth. And that was happening without the use of antibiotics.”

Webster attributes part of the success to the ionic bonding of the material.

“If you have ionic bonding, that means you have high surface energy, which is what we think repels bacteria,” he said. “Metals have metallic bonding, so the same thing is not observed.”

What may set it apart from otherceramics, Olson says, is its crystalline structure, which is a matrix with crevices piled on top of each other. That, he says, accounts for its superior fracture toughness. He is very positive about the market implications of silicon nitride. A spineapplication hit the market in 2008 and has been a success. A total-joint application is still under review by the U.S. Food and Drug Administration.

“The debate in spine is what to do now that you can no longer use BMPs (bone morphogenetic proteins) in combination with PEEK and titanium,” he says. “We are uniquely suited to go to market and have an impact on the $1.8 billion market forinterbody spacers, thanks to biocompatibility, superior imaging and other properties.”

One of the next steps is to get broad-based claims about the material verified.

“The next step is that as we complete more studies, we will submit data to regulatory bodies and get specific biomaterial claims that silicon nitride has properties such as antimicrobial, promotion of bone growth, and so on,” Olson says. “Taking those claims to the marketplace will produce extremely disruptive technologies. In spine, it is already having a major impact.”

In particular, Olson believes it will be well-suited to dental applications because of its strength and antimicrobial nature, and to femoral heads.

“There have been issues of hydrothermal degradation with zirconia-based heads, and we have the solution to that, we believe,” he says. “A lot of surgeons no longer use metal-on-metal. The appeal of our solution is that it could provide more stability but also increase range of motion. Now, they are using a poly cup articulated on a metal acetabular shell. We are working on silicon nitride acetabular cups that can articulate directly on the poly.”

Amedica does all its manufacturing in-house and is selling directly to physicians. In 2010, it acquired US Spine to give it a more extensive product line. —E.S.

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