The Quest for Longevity

Surface modification adds the final touch to myriad orthopedic products.

Jennifer Whitney
Editor

Anyone who knows the name Roy Hobbs surely recalls the most pivotal scene in the movie “The Natural.” The invincible baseball player had an equally invincible tool, “Wonderboy”—the wooden bat Hobbs hand carved as a child from the tree struck by lightning at his father’s gravesite. In the movie’s towering shot, silence was interrupted by the umpire’s scream, “Foul ball,” and Hobbs reached for his bat….
 
In the end, even “Wonderboy” succumbed to the great equalizer to all of man’s innovations: deterioration.   

Quality is the key to any operation involving surface modification. Shown above is a worker performing a visual quality inspection of anodized  products. Photo courtesy of DOT GmbH.

Similar issues affect the orthopedic industry. In the quest to keep implants, instruments and even delivery systems as durable and wear resistant as possible—not to mention allow for more efficient surgical procedures—surface modification technologies have become a mainstay in the design and manufacture of orthopedic products today.

“Surface techniques contribute to longevity, durability and, therefore, to cost effectiveness of implants [as an example], which is important in times of increased price pressure from the health systems. Moreover, some techniques shorten the healing time and, thus, support the early rehabilitation of patients,” said Dieter Pfliegensdörfer, public and customer relations manager for German-based DOT GmbH, a coatings provider.

For all of the industry’s innovation, many service providers have found that the orthopedic community remains conservative in its adoption of new approaches to old problems. Of course, one can hardly blame OEMs—especially those who already have products that can last for decades—to trust newer technology over the tried-and-proven. That said, many specialists in surface modification are working to continuously improve existing technology and even devise new applications while keeping up with their projects utilizing present techniques.

“Surface enhancement treatments offer a viable alternative to providing improved wear and abrasion, reduction of wear debris at [an implant] coupling and reduction of metal ion release. Time and education will ultimately lead the industry here,” said Ray Fontana, business development director for IonBond, a management owned company with global headquarters in Switzerland and US headquarters in Madison Heights, MI. “We are continually evaluating the direction that the orthopedic industry is moving, relative to their need for new and innovative products. We then assess our capabilities and refocus ourselves to satisfy those requirements.”

Depending on the desired outcome, a variety of coatings or other surface treatments are used to protect or enhance orthopedic products.

Ion Technology Is a Mainstay

Processes using ion technology are one example of popular surface treatments used today. For example, Spire Biomedical Inc. specializes in ion implantation, a treatment that doesn’t add material to a part but, instead, modifies the surface. Originally developed for semiconductor applications, ion implantation remains a mainstay in that industry, said Eric Tobin, vice president and chief operating officer of Spire Biomedical, based in Bedford, MA. 

In basic terms, this approach bombards an implant surface with gaseous or metallic ions in an effort to harden the surface and make it more conducive for fluid retention, enabling more reliable joint performance. Although this method is used to treat metal, the goal is to reduce wear on polyethylene components in metal-on-plastic implants. “You want to ensure the metallic surface stays smooth over time,” Tobin explained. 

The advantage with ion implantation, he added, is that it can be performed at a low temperature (thereby having no adverse effect on the material being treated). And since the process isn’t a coating, there’s no risk of delamination.

Plasma-assisted chemical vapor deposition coating equipment  enables extremely smooth surfaces. Photo courtesy of IonBond.

Ion technology also is being used in coating applications. Physical vapor deposition (PVD), which has been used in orthopedic applications for more than 15 years, represents a class of coating processes that involve the evaporation of metal within a vacuum environment to generate ions that produce thin, metallic coatings. Since the typical compositions used by the orthopedic industry (TiN, AlTiN, TiAlN, DLC, etc.) are inert, there is no risk of the coating reacting poorly with the human body. Even better, PVD offers parts a strong, durable surface with an extremely high micro-hardness.

“For example, in a general manufacturing application, a typical drill will perform much more efficiently after it has been PVD coated—the higher micro-hardness and lowered coefficient of friction of the PVD thin-film layer improve abrasion resistance and chip evacuation,” said Matthew Thompson, sales manager for East Petersburg, PA-based Richter Precision Inc.

Aside of performance utility, PVD also is used in orthopedics as a means of differentiating instruments and components visually. Different coatings can be used to produce various colors, which, for example, can aid a surgeon looking for a specific screw during a procedure utilizing several screws in different sizes.

Various deposition technologies are used to generate PVD coatings: hollow cathode reactive plating, magnetron sputtering and cathodic arc deposition. The technology of choice for the orthopedic industry, and stipulated in most OEM coating specifications, is the cathodic arc deposition method.

Thompson cited the cathodic arc technique as one that has seen vast improvement over the years. “We have moved to a filtered arc technology, which is designed to filter out macro-particles,” he reported. “In the past, if you were using cathodic arc to evaporate titanium to generate titanium nitride (TiN), macro-particles embedded in the thin-film layer would give the coating a hazy appearance. Now there are methods to remove these macro-particles; therefore, we create a more stable, homogenous and aesthetically pleasing layer.”

A relative of PVD is chemical vapor deposition (CVD), a gaseous process that diffuses coatings into a substrate. Although it’s similar to PVD, this technology uses high temperatures, which limits its utility in medical devices, experts said. However, IonBond is working with OEM customers on proprietary processes that could make this an emerging technology for implants, helping to protect their metallic content from releasing ions into the body while providing a superior wear surface and interface with ultra-high molecular weight polyethylene.

“The bond with CVD is outstanding and you can develop coatings with some unique properties, such as multilayer coatings,” said Gene Elwood, senior medical accounts manager North America for IonBond. He also noted that the diffusion process promotes better adhesion.

Along these lines, plasma-assisted chemical vapor deposition (PaCVD), a subset of CVD technology, has the benefit of diffusion at very low temperatures and additionally creates “nice amorphous diamond films,” Elwood said.

One area that many providers of PVD and other plasma technologies are investigating is nano-layered PVD thin-films. The next stage in nano-layer development involves super lattices coatings. This technology involves the deposition of very thin, alternating nanometer layers of materials that each are oriented differently within the coated structure.        

“Instead of depositing one layer 3 to 5 microns thick, we may deposit 50 to 60 layers 10 to 100 nanometers thick,” said Thompson. By way of explaining how the technology would work, he explained, “Let’s say a coated component is in use and high loads or stresses cause micro-fracturing in the top layers. With super lattice coatings, propagation of micro-fracturing from layer to layer would be impeded—therefore, providing better overall coating performance and life.”

Richter Precision plans to introduce this technology to the orthopedic market later this year.
   

Some Coatings Aid Patient Health

Along with ion technology, providers of surface modification technologies have plenty of other offerings that not only ensure a part’s longevity, but also help benefit patient health.

For example, DOT GmbH’s ceramic coatings (TiN and TiNbN) help prevent implant materials from causing allergic reactions in the human body as well as reduce wear. “A fully biocompatible titanium nitride or titanium niobium nitride ceramic surface coating on metallic implant components has a beneficial effect on reducing both allergic reactions and wear,” Pfliegensdörfer noted. 

Since the orthopedic community is devoted to bone science, many companies have focused on ways to promote bone regeneration. For example, DOT’s BONIT, an electrochemically deposited calcium phosphate (CaP) coating, helps accelerate healing. According to Pfliegensdörfer, this class of coatings has come a long way.

“The first CaP coatings that came on the market consisted of relatively thick (>50 Ìm) and compact hydroxyapatite layers that were applied to the implant surface using the plasma spray technique,” he explained. “Unfortunately, this process induces thermal degradation that may have an adverse effect on coating quality. In addition, the so called ‘line of sight’ spraying process is unsuitable for porous surfaces and complex implant geometries, as only the external surfaces directly facing the spray gun are coated. These factors, in addition to the limited bonding capacity and inhomogeneous solubility of these coatings, led implant developers to the realization that for CaP coatings to provide long-term stability, plasma sprayed coatings may not be necessary or even desirable. It has since been discovered that bioactive coatings on implant surfaces only need to last as long as it takes for implant osseointegration to be completed. Once this has occurred, the coating has fulfilled its purpose and should biodegrade to make way for new bone.”

Today, he noted, thin (approximately 15 Ìm), fully biodegradable and electrodeposited CaP coatings help eliminate the long-term risks that have been associated with CaP  coatings and provide complete coverage on both structured surfaces and complex implant geometries. In addition, the electrodeposition process enables excellent solubility and resorption—all while being manufactured at room temperature.

Spire Biomedical also has been offering hydroxyapatite coatings in the form of its product IonTite. Acknowledging that conventional CaP coatings can crack and delaminate, Tobin said the company has been working on research that hopefully will produce a new generation of coatings that will help eliminate some of these problems and possibly even integrate some biologic agents into the coatings.
Along these lines, Spire Biomedical is investigating the use of integrating biologics such as bone morphogenic proteins into coatings to encourage bone ingrowth. “I think the industry in general is going more and more toward biologics,” Tobin said. “The more biologically oriented solutions are the ones that will prevail in the long term. By introducing these materials, you can stimulate bone to grow even faster.”

Anodization, Lasers Treat Surfaces

Along with processes that coat surfaces of orthopedic products, a variety of etching and chemical treatments are employed to modify existing surfaces without adding material.

“From a finishing standpoint, you have mechanical ways of finishing, and you have chemical ways of finishing. Certain components require a smooth finish [while] others require a rough, porous surface. [In addition,] certain components require a bright, shiny finish; others require a dull, non-reflective finish,” explained Ray Harris, director of sales for Electrolizing Corp. of OHIO in Cleveland. “There are a lot of differences from the past and how things are done. First of all, there are much more stringent guidelines from the FDA, EPA, etc, on what we can and can’t do in our processes. Furthermore, as the orthopedic components become more and more complex, we are required to find newer, more advanced ways to process them.”

Duralectra, a Natick, MA-based provider of aluminium finishes, specializes in anodization, an electro-chemical process that strengthens surfaces and offers abrasion and corrosion resistance to instruments and the cases and trays that house them.

Laser welding is new to orthopedics, but it’s proving useful for motion-preservation technology. Above, a technician inspects a laser-welded part. Photo courtesy of Mound Laser  and Photonics Center.

Aluminum, a popular material for these products because it is lightweight, plentiful, cost effective and easy to machine, particularly benefits from anodization because the chemical bath produces a layer of oxide that strengthens the material without adding any new layers to the surface.

According to Bob Mills, director of sales and marketing for Duralectra, a major advantage of anodization, which has been widely used by the orthopedic industry for decades, is that it’s inexpensive to achieve while offering impressive performance. “The key is that these items don’t get beat up as easily and will resist the sterilization processes they undergo, allowing them to increase the number of cycles they can go through,” he explained.


This finish especially benefits any equipment being used in Europe, where high-pH chemicals are used to clean instruments and delivery systems, he added. Noting this is a concern to any manufacturer sending products there, Duralectra continues to work on improving the process to further increase a product’s longevity. In the next year, Mills said, the company will introduce a “substantially different” new, highly corrosion-resistant process that will offer even better protection for products undergoing high-pH cleansing.

For now, though, the company has used some of its newer, improved proprietary processes to benefit orthopedic applications. For example, the Sanford Quantum process, which has been in use for the past few years, yields a clear hard coat that helps seal in the bright colors being used today for brand recognition by many OEMs.    

“Typically, when you hard coat, it comes out as a natural color, such as a dark green or grey,” Mills said. “We have found a way to yield a clear coating.”

He added that since the goal is to keep colors as vivid as possible over repeated sterilization cycles, anodization specialists also have been focusing on chemistry to ensure that colors don’t fade and also that they don’t change their shade over multiple batch jobs.

But the use of color is a moot point if it’s just going to chip off over time. Therefore, Duralectra launched its SanfordPrint process a year ago as a solution to the tendency for silkscreened images to chip off over time, Mills said. The process involves building a porous hard coat on the surface of a delivery system to embed dyes and seal them. This way, he explained, a layer of sealant protects the surface and graphic, which means the graphic won’t wear off.

Along with anodization, laser etching long has been used to achieve both decorative and utilitarian finishes on parts. However, laser specialists are finding that some of their other techniques are gaining ground in orthopedics.

Miamisburg, OH-based Mound Laser and Photonics Center (MLPC) has been placing more focus on laser welding for implant components. Although use of this process is fairly new for the orthopedic sector, it’s finding a niche in smaller components for motion-preservation technology, said Kevin Hartke, MLPC’s manager of sales and marketing.

For example, he said, “If you need to put a pin in place or bring two small pieces together, you have limited ways to do it. We can seal things with laser welding and keep them hermetic—keeping biologic fluids out—and it offers permanence.”

Since about half of MLPC’s business is rooted in its research and development activities, the company also has been working universities and OEM customers to explore the use of direct laser machining to selectively remove material from ceramic and metal implants, examining factors such as the texture’s effect on an implant’s compatibility and its propensity for reducing wear or friction.

“Instead of using a machine tool to surface modify, we’re using focused light. The advantage is we can create features that are much smaller,” Hartke said, adding that this technology enables modifications that are as minute as 1 micron (by way of comparison, a human hair is about 80 microns in diameter).

Quality Counts Most for OEM Customers

 
Given the processes and technology used to achieve some of the more complex surface modifications today, providers of these types of services believe there won’t be any shortage of work for them in the near future.

“Although some industries, such as automotive and aerospace, have brought coatings in-house, many medical device manufacturers have not bothered to do so given that it’s not their core competency and the technology continues to make advancements,” Elwood said.

Since these types of finishing services should continue to remain outsourced for the foreseeable future, OEMs will continue to care about one thing above all other factors: quality.

“More recently, we see this market paying more attention to the details that are required to ensure quality and consistency in products and services,” Fontana reported.

Of course, it’s not easy to maintain top quality when it seems every customer wants its product turned around overnight, if possible. Tightening lead times haven’t strangled service providers yet—in fact, they believe the time crunch is serving to make them constantly strive for better ways to perform everyday processes.

“There’s a lot of growth in this industry right now and we don’’ want to be responsible for holding up that growth. We want to ensure we can keep up and maintain the good delivery times and ensure quality at the same time,” Thompson concluded.

Harris also said that the more stringent quality requirements from regulatory bodies will serve to strengthen the industry as a whole. “Everything from FDA guidelines to EPA guidelines, to more stringent quality programs [are] required on our processes. Some view that as being a negative, but in many ways, I see that as a positive,” he said. “It drives our organization to become better. In the long run, that is what our customers want.”