To Protect and Serve

Orthopedic coatings improve performance while benefiting surgeons and patients.

Stacey L. Bell
Editor at Large

Harsh environments: Your car has seen them all. From summer’s hot, beating rays of sunshine to the ice and snow buildup of winter, your vehicle’s paint is exposed to damaging elements that could cause rust, chipping or worse. Therefore, car owners who would like to extend the life of their automobile’s exterior will add a coat of wax and polish after regular washings.

Within the orthopedic sector, a protective coating can work wonders as well. Surgical instruments cutting through bone and tissue can lose their edge quickly. Implants are placed within a foreign environment that must be encouraged to allow bone to attach while inhibiting the growth of infection. Coatings can help improve performance in both of these areas, as well as in other situations.

Orthopedic coatings have a wide array of functions: They can facilitate device biocompatibility, lubricity, hardness and durability; offer aesthetic touches; and even stave off infection. Photo courtesy of IonBond LLC.

Consider titanium nitride (TiN). While currently used mainly for aesthetics and identification purposes, TiN also can be very functional, providing depth marks on an instrument so surgeons will have a visual indicator of where to stop drilling to avoid harming a patient. The coating also can extend instrument life, according to Larry Williams, an independent sales representative for Eifeler-Lafer, Inc. in St. Charles, IL.

“A past study found that when an intra-medullary reamer is coated with TiN, the life of the cutting edges is tripled; lubricity (slipperiness) is greatly improved, helping to move the flow of bone chips out of the wound area; and the coated reamer will cut twice as fast as an uncoated reamer,” Williams said. Further, the study showed that the coated reamer, due to its sharper cut and added lubricity, decreased the heat at the point of the cut by nearly 50 degrees. This lower heat lends itself to decreased bone necrosis, thereby improving the chances of newly formed bone attaching to the implant more quickly.

“When thin-film coatings were first introduced to the medical community in the early 1980s, there were some concerns with how well the new technology worked,” Williams said. “The newer generations of coatings, and the types of coating vessels that now apply them, are more technically advanced and promote much better adhesion to the substrates.”

He added that not only do coatings improve the life of surgical instruments and implants, they also play a major role in improving the life of the cutting tools used to produce these products. One study showed that an uncoated three-quarter-inch end-mill used to cut titanium hip implants originally was producing an average of eight pieces per grind. The tool normally could be reground five times before having to be replaced. When a composite coating of titanium aluminum nitrade (TiAIN) was applied to the end-mill, it yielded 43 pieces per grind while retaining the same five regrinds schedule. Further, with the addition of the coating, the CNC machine was able to run 30% faster, thus greatly increasing productivity. “This type of increase in yield and productivity translates into major annual cost savings for an orthopedic manufacturer,” Williams noted.

In fact, perhaps the biggest misconception about orthopedic coatings is that they’re simply an additional cost. “Yes, there is an additional cost up front, but you save much more money later because coatings make products last so much longer. Coatings can expand a product’s life five- to tenfold,” said Ray Harris, director of sales for Cleveland’s Electrolizing Corporation of Ohio (ECO).

Coating Implants: The Next Frontier

Coatings and surfaces resulting from anodizing, oxidizing or other finishing techniques traditionally have been used for improving orthopedic device biocompatibility, lubricity, hardness and durability, as well as for aesthetic or marking purposes. Today, coatings for implants are gaining in popularity as companies find that coatings can be used to promote healing and stave off infection as well.

“More and more, we’re seeing the use of composite materials designed to offer combined benefits and produce better patient outcomes,” reported Maureen Reitman, ScD, principal engineer for Exponent, Inc., a scientific and engineering consulting firm in Bowie, MD. Hydroxyapatite (a complex calcium phosphate) has been used for years to promote bone growth. This bioactive material is of increased interest when combined with silver for its antimicrobial effect or with diamond-like carbon for greater durability.

Shown is a porous coating comprised of asymmetric titanium particles. The scratch-fit and porosity are greater than that of spherical bead coatings, allowing for better implant stability and bone ingrowth into the coating. Photo courtesy of Smith & Nephew.

In other developments, companies are refining coatings to allow for better bone growth. “One drawback to the current generation of coatings has been that there is so much metal rather than open space. This can cause stress shielding, limiting bone ingrowth,” said Jon Moseley, PhD, technical director of implant technology for Wright Medical Technology, Inc., which is headquartered in Arlington, TN. That is, after the bone grows into the implant a little bit, it no longer gets load from the body, so it stops developing. Bone needs stress to grow. “Today’s newer coatings have lower stiffness and more open space to grow stronger, healthier bone,” Moseley said.

Michael Carroll, senior director of implant technology at Wright Medical, added, “New materials used for porous coatings can be used not just as coatings, but also as actual standalone implants to fill bone voids and defects,” replacing bone cement in many applications.

In a related area of research, Wright Medical recently developed a biologic coating that uses a bone void filler, a calcium sulfate material, to coat the roughened surface of hip implants. “The material stimulates new bone growth and fills the space between the coating and the bone,” Moseley said.

Companies also are seeking to improve what’s commonly known as “scratch fit,” the friction between the implant and bone that holds the implant in place. A higher friction coupled with a more stable interface will reduce motion between the bone and implant, allowing for better long-term outcomes.

“New metallic porous coatings are now coming on line. Some of the newer coatings are composed of asymmetrically shaped particles with sharp edges so they have better bite into bone, increasing frictional properties against the bone and increasing porosity as well,” noted Marcus Scott, senior project manager, research for Smith & Nephew, Inc. in Memphis, TN. Previously, coatings used spherical beads of titanium or cobalt alloy that had lower scratch fit and porosity, or used plasma spraying methods that resulted in dense coatings. “By optimizing the porosity and pore size, we can facilitate blood vessel incursion and promote good bone ingrowth into the coatings,” Scott said.

In addition to improving the integration of bone with an implant, coatings also will increasingly be used to reduce the potential problems of infection, Reitman said. Silver has long been known for its antimicrobial properties, and several companies, including Spire Corp. and AcryMed, offer coatings of nanosized silver to reduce infection resulting from implant surgery.

“Silver compounds have been shown to inhibit the adhesion of bacteria to implant surfaces. Since implants can protect bacteria from the body’s natural immune system, this is a hot area of research,” Carroll concurred.

“We’re also seeing a trend toward incorporating more drugs and biologics into the implant to allow for better growth and ossification,” Reitman said.

While the early results of using new biomaterials appear promising, it will be years before clinical experience can prove how they work in real life. “All the animal studies in the world won’t give the real clinical results we need. Did a particular new development let us move from a 15-year survival to a 20-year survival? Only time will tell,” Moseley explained.

“In the future, I expect to see more coatings acting as carriers for drugs and antimicrobials as well as proteins or peptides to promote and maintain osseointegration around the implant,” Scott said. “Numerous issues still need to be addressed, though, including what is the optimal molecule to use, and what is the optimal dosage and release.” Researchers also are studying how growth factors, such as bone morphogenetic proteins, might be incorporated into coatings.

Tackling Today’s Challenges

With the many advances occurring in orthopedic coatings, experts recommend that OEMs consider a number of factors when choosing a coating for a new implant or instrument.

1. Use the right coating. “Too often, companies choose the incorrect coating for a particular product. Proper training in coating properties and their functionality is extremely important. Orthopedic companies can not only greatly increase production throughput, but also realize hundreds of thousands of dollars in savings annually by making educated choices,” Williams said.

Product design details also should figure into any coating decision. “We come across this problem a lot—a design that can’t be coated,” said Harris. “It’s important for coatings professionals to be involved from the ground level.”

An OEM should consult with a coating company before finalizing a product’s design to ensure the most appropriate coating and process can be selected. In addition, Reitman recommended that product designers consider the intended product function and the challenges of this use. Demands on products for the spine, shoulder or hip can vary, but where a device is used ultimately may not be the primary consideration.

“Ask the medical community how current offerings fall short. What is the real need? Prioritize your characteristics based on those findings,” she advised. For instance, is bone growth or cell attachment the chief concern? If so, you might choose a coating with hydroxyapatite or tricalcium phosphate. If preventing infection is a greater priority, silver-based coatings would be more appropriate.

“Don’t get caught up in the trends,” Reitman said. Just because nanotechnology or convergence is hot doesn’t mean coatings in those areas will be right for your project. There is no substitute for the careful application of engineering principles.

“The underlying questions on the implant side, which is a new area for coatings, are: What is the test protocol the customer will subject the product to? What environment will the coating be subject to?” said Ray Fontana, business development director for Rockaway, NJ’s Medthin Division of IonBond. “We offer several alternatives, and we’ll set up a design of experiments, providing a variety of coatings initially. We may find conventional films do the best job, or go with a more exotic option. By allowing us to be involved early in the design phase, we can save customers a lot of time and effort.”

Scott added that any coatings decision comes down to answering several questions: How well will the coating bond to the implant? (Obviously, you don’t want any coating flaking off into the surgeon’s hand or the patient’s body.) What is the effect of applying a coating to the implant on the implant itself? (How will performance and longevity be impacted?) How will the coating affect the patient’s bone around the implant?

2. Hire the right company. “Coatings vary greatly between companies, as do their application processes. Even though coatings may be called by the same names, their performance can differ dramatically depending upon how they were applied to the tool,” Williams explained. “It’s similar to different chefs preparing a meal. They may all use similar ingredients, but some produce better results than others.”

Choose a coatings provider that is experienced in the orthopedics industry and understands the stringent requirements associated with coating medical implants and instruments.

Fontana added that the capability to test for biocompatibility is another key factor in the working relationship. IonBond is in the process of developing FDA master files on the coatings it offers to make their use easier for the industry. Further, while some larger companies tend to go to universities to conduct research for new coatings, Fontana questions whether academia is the best setting for commercializing results. “We do this every day,” he said.

Process control is critical as well. “Five to 10 years ago, validations weren’t necessarily required on a lot of our processes,” Harris said. “As time has passed, and coatings have been proven reliable and that they solve problems, a tremendous amount of attention has now been shifted to performing biocompatibility testing and focusing on validation.”

3. Consider the impact of masking requirements. As the designs of both implants and instruments become more intricate and complex, OEMs are asking for complicated, sometimes near-impossible, mask lines. In many cases, their requirements call for certain areas of the part to be left uncoated. A lack of understanding of coating techniques can result in delayed product launches or worse.

“Uneven surfaces, tapers or recessed areas can be very challenging,” Williams said. “If design engineers would be proactive in discussing new product needs with a qualified coating specialist prior to final design approval, and very early in the design process, many potential problems could be identified and creatively addressed.”

4. Keep everyone aware of the project’s parameters. Another common problem is that design engineers don’t specify on their prints the type of material being used in the product and its tempering temperatures so that the coater can use the proper process.

“Most instrumentation is made out of 17-4 PH stainless steel, which should not be coated above 700 degrees Fahrenheit. Many coating processes offered today need to operate at temperatures that are much higher than this, so they should not be considered,” Williams said. “Arc evaporation equipment, which can comfortably coat at temperatures below 500 degrees Fahrenheit, should be the only process considered. Other types of processes will anneal the substrate and harm the integrity and function of the tool. It would be highly recommended that OEMs research each potential coater before placing their very expensive products in their hands.”

Fueling the Market

Coating companies noted that this sector has been growing by 10% to 20% in recent years, with similar expansion expected for the foreseeable future. Certainly, the orthopedic market is expected to see greater demand in the years to come. In a recent study, Exponent determined that the need for primary total hip arthroplasties may grow by as much as 174% from 2005 to 2030, to 572,000 procedures performed annually. Primary total knee arthroplasties are expected to increase by 673% to 3.48 million. Total hip and total knee revisions are projected to skyrocket by 137% and 601%, respectively, during the same period.

As a result of the projected tremendous demand in these areas, more orthopedic surgeons, efficient operating techniques and resources such as implants and instruments will be needed. “Because coatings improve the integration of bone with an implant and they can be used to reduce the potential problems of infection, in that sense, orthopedic coatings are a significant driver for growth in the orthopedic market,” Reitman said.

In addition, more OEMs are using coatings to provide a competitive advantage or differentiator for their products. “We get a lot of requests for custom coatings,” Harris said. He noted that ECO has expanded its capacity for every medical coating it offers—sometimes twice and, in some cases, by 100%—over the past few years to meet increased demand.

Fontana noted that IonBond customers also are seeking custom coatings. “We’re seeing some interest in oxy-nitrides and oxides. Customers want new coatings with different characteristics that some conventional coatings may not be able to deliver,” Fontana explained. New technology is allowing the company to deposit pure titanium metal, allowing for textured surfaces on cobalt chrome porous bead materials, potentially enhancing cell attachment. “The titanium-coated porous bead surface provides a more biocompatible surface for cell attachment than cobalt chrome does,” Fontana said. “So you have the benefits of cobalt chrome on the articulation side but with better potential for adhesion on the bone side.”

He added that OEMs are expressing interest in developing different colors of coatings as well—as opposed to the traditional gold, bronze, silver and gray typically used.

Eifeler-Lafer recently introduced a newly engineered coating, titanium aluminum silicon nitride (TiAISN), which has a very high resistance to heat and offers a distinctive appearance with its iridescent blue color.

“The industry is excited about the developments occurring in coatings today,” Reitman concluded. “Coatings are opening up new opportunities.”

Stacey L. Bell is a Tampa, FL-based freelance writer who specializes in business and marketing issues.