Scratching the Surface: A Roundtable on Coatings and Treatments

Going for the gold can be quite a daunting task.

Consider, for example, the hardships and hazards that fronted the fortune-seeking Forty-Niners in California’s Sierra Nevada foothills; the tremendous pressures and grueling training regimens bedeviling Olympic athletes; the education, discipline, and foresighted thinking required of Nobel Prize laureates; or the effort to unseat cobalt chromium as the metal of choice in orthopedics.

Granted, the latter illustration is (generally) not as physically or emotionally taxing as the others but it nevertheless is challenging. Cobalt chromium is considered the gold standard in joint replacements for its in-body longevity (about 15-20 years), but that durability can be a detriment, too—the metal has been linked to cobaltism (a.k.a., cobalt poisoning).

Cobalt chromium molybdenum alloys are the strongest, hardest, and most fatigue resistant of the metals used for joint replacements. Those properties have bolstered its appeal among knee and hip implant manufacturers in recent decades as the industry strives to improve artificial joint perpetuity and performance.

Yet those same properties have proven detrimental. A research letter that appeared last summer in the Journal of the American Medical Association (JAMA) warned of the extent of potential cobaltism victims. “Only two models of joint replacements have been recalled in the U.S. for cobalt-chrome metallosis complications: one extreme-risk hip with a metal-on-metal articulation (Johnson & Johnson) and one high-risk hip with a modular cobalt-chrome neck (Stryker),” the JAMA letter stated.
“Millions of residents of North America implanted with non-recalled extreme-risk or high-risk implantations are likely not monitored and are likely experiencing cobalturia.”

To reduce the likelihood of future cobalturia cases, Salt Lake City-based Total Joint Orthopedics Inc. (TJO) has developed an artificial knee that is 50 percent lighter than cobalt chromium and rivals the metal in both strength and hardness. The company’s Klassic Knee System—unveiled earlier this spring at the American Academy of Orthopaedic Surgeons 2022 Annual Meeting—is machined from bar stock titanium and coated with titanium nitride (TiN).

“Our team has been working diligently for several years to find a material that can rival cobalt chrome in strength and hardness,” TJO CEO Erin Hoffman said in debuting the Klassic Knee, “and we are thrilled to introduce the first cobalt chrome alternative to our portfolio.”

And that alternative is gold—literally. TJO’s Aurum technology (aurum is the Latin word for gold) uses a patented ion beam enhanced deposition (IBED) process to create a five-micron thick ceramicized surface layer that is interdigitated with the substrate while preserving the implant’s material and geometric properties. Since the Aurum IBED process is physical rather than chemical or thermal, the ion beam enhanced deposition application uses kinetic energy, enabling the coating environment to maintain a much lower temperature (below 400° F), thereby preserving the integrity of TiN’s strength and hardness.

Aurum technology is the latest attempt by orthopedic implant developers to improve joint replacement performance through surface modifications and coatings. Companies have enhanced wear resistance and osseointegration with plasma-sprayed titanium surfaces, non-spherical bead, or calcium phosphate/hydroxyapatite-based coatings, and warded off bacterial infection via silver, copper, magnesium, copper-titanium dioxide, and titanium oxide-based PVD coatings.

Canadian researchers, however, are working on an all-inclusive coating that would prevent bacteria growth, facilitate cell growth, and foster bone reconstruction. The coating under development at Institut national de la recherche scientifique (Quebec) is comprised of three bioactive materials: chitosan, found in shrimp shells (it has antimicrobial properties); collagen (the organic component of bone, facilitates cell migration and growth); and copper-doped phosphate glass, which stimulates blood vessel formation and bone reconstruction.

“The ability to deposit such coatings allows for the potential to make implants with tailored biological properties” Ph.D. student Imran Deen told Science Daily late last year. “It holds promise for biomedical applications, as these coatings can provide better implant-host interactions.”

For insight on the current implant-host interactions provided by existing coatings technologies and surface modifications, Orthopedic Design & Technology spoke to various industry experts over the last several weeks. They included:

• Ulf Brogren, chief commercial officer at Promimic AB, a global developer of nano surface modification technology. In the last three years, the company’s HAnano Surface has been approved and introduced to the U.S. dental and orthopedic implant market on 13 different devices and used in more than 600,000 implantations.
• David Detwiter, director of research; Brian More, CEO; and Jeff Shepherd, chief commercial officer at Nanovis, a Columbia City, Ind.-based firm that develops and commercializes nanotechnology platforms for improving bone growth and fighting infection.
• Keith Edwards, executive vice president, Commercial, at Precision Coating, a Hudson, Mass.-based medical coatings service provider to interventional, orthopedic, and advanced surgical markets.
• Deborah Robbins, market development specialist at Bodycote, a global thermal processing services provider.
• Tim Zentz, general manager at Danco Medical, a Warsaw, Ind.-headquartered provider of metal finishing, anodizing, passivation, and chrome coating for medical device manufacturers.

Michael Barbella: What trends and/or market forces are currently driving innovation in orthopedic surface modifications and coatings? Have these trends changed in recent years or have they remained fairly stable?

Ulf Brogren: Cementless implants are currently a huge trend in arthroplasty procedures. This is being driven by younger patients in need of implant therapies with a longer life span, patients with compromised healing conditions, and the shift toward treatment at ambulatory surgical centers (ASCs). This trend is increasing the need for implants with improved osseointegration properties. A new market segment of thin coatings and surface modifications is currently growing fast to meet these new demands.

For spinal implants, improved osseointegration for better and faster fusion has been a strong trend over the last five years. Spinal fusion with titanium and 3D-printed implants has gained market share compared to PEEK implants. But over the next couple of years, I think the market share of PEEK cages with surface treatments and modifications for better osseointegration properties will grow, since it is easier to assess fusion with PEEK implants compared to titanium.

David Detwiler, Brian More, Jeff Shepherd: The concept of hierarchical structures of macro-micro-nano began to enter the academic sphere in the 2000s with industry adoption in the 2010s. Technology applications start with what industry has available and the level of control that can be exerted over material. The macro level is the easiest to access and came in the form of adding porosity to devices to encourage bone in-growth. The 3D printing revolution also took hold in this era leading to a wave of new devices and design possibilities. Along with macro structures, micron-level structures and roughness were also being incorporated into devices. Some of this was as simple as shot peening or bead blasting surface. Actual coatings of hydroxyapatite, calcium phosphates, and titanium were also adopted in this era with some success and some failures.

The understanding of nanosurfaces has evolved from the “more is better” stage to the engineering of desired biological outcomes associated with the surface topography and chemistry. The nano realm was and is the least well understood and took the FDA a long time to understand the safety and capabilities of nanotechnology. The first few products with FDA Nanotechnology Designations have entered the market in recent years. Early successes of nanotechnology such as the Titan NanoLock surface on interbodies, Nano FortiCore interbodies, and Nano FortiFix pedicle screw product lines have raised the awareness of the industry and raised the bar for the next nanosurface technology. The “me too” era of nanosurfaces is upon us, diluting the differentiation of generic nanosurfaces without associated biological data. This is particularly true of acid etched surfaces because they are easy to create and most processes are off-patent, falling under manufacturing trade secrets. The market now requires biological data, in-vitro and in-vivo, to complement the nanosurface to achieve market differentiation.

The trend of adoption has been in order of manufacturing capability, macro structures first, microstructures second, nanostructures and surface chemistry third. We are now entering the nanostructure and surface chemistry era and it appears most of this innovation starts with smaller enterprises focused on innovation. This trend will continue with a proliferation of nanosurface and surface chemistry offerings, followed by market adoption by larger companies aiming to grow or maintain market share. This will continue until all of the major market players have a macro-micro-nano strategy in place on most of their product lines. After that, who knows, that is the domain of the innovator.

Keith Edwards: The goal remains the same in orthopedic coatings: provide an implant surface that promotes osseointegration and is supported by strong preclinical and histologic evidence. Cost to the HCP institution is a key element as well so newer surface modifications must demonstrate superior healing characteristics to gain acceptance for higher costs. I believe surface coatings will become a standard and expected of all implants ahead. One long-sought wish but nascent offerings are antimicrobial coatings. Regulatory hurdles are immense, which will slow rollout and availability

Deborah Robbins: Austenitic stainless steels offer a variety of advantages in orthopedic devices ranging from corrosion resistance to biocompatibility. With the use of Bodycote’s Kolsterising® process, austenitic stainless steels are further enhanced with improved wear performance and the elimination of galling. Now devices that were often constructed of bi-metals to avoid galling can be made of one single austenitic alloy. This reduces manufacturing costs and additional supply chain challenges.

Tim Zentz: Improved patient outcomes continue to be a strong driver of new, innovative surface modifications. Through Danco Medical’s license agreement with Promimic, the HAnano surface modification is becoming more widely used in this space on FDA-cleared products.

Barbella: What specific surface modifications and coatings are employed to treat orthopedic implants?

Brogren: Historically, it has been plasma sprayed hydroxyapatite and titanium coatings that have been used in the industry. During the last couple of years, thinner coatings (like our HAnano Surface) and subtractive surface modifications (such as acid etching of titanium) have been introduced to the market.

Detwiler, More, Shepherd: Orthopedic implants are treated with a range of surface coating technologies. Plasma spray titanium and hydroxyapatite are common with benefits and drawbacks to both in different applications. Surface modifications such as acid etching and anodization don’t produce a coating but modify the texture of the surface to improve protein and cell interactions. Processes to modify surface chemistry to create superhydrophilic surfaces are generally intended to improve blood and protein contact, which can improve cell attachment and bone contact.

Edwards: Key types include titanium oxides and combination of titanium and hydroxyapatite. Methods of application are heavy on capital investment and hence only a few firms can compete with viable options. No exceptional cost advantage exists to a particular treatment.

Robbins: Kolsterising® is a low-temperature diffusion treatment for stainless steel that can expand the design window of these alloys. This diffusion process results in a significant increase in surface hardness while maintaining the ductility and corrosion resistance of the substrate. It is a proprietary, thermochemical diffusion process that forms a carbon S-phase at the surface while avoiding carbide precipitation, which causes sensitization.

Zentz: Danco Medical’s Type 2 Titanium Anodize provides increased fatigue strength and added lubricity on titanium implantable devices.

Barbella: What challenges do 3D-printed orthopedic products present to surface modification and/or coatings providers? How does the 3D printing process specifically impact the type of surface modification or coating used on an implant?

Brogren: Using 3D printing as a manufacturing process has given the implant industry the ability to produce implants with good micro-porous ingrowth structures. However, traditional implant coatings are not applicable to this type of porous geometry, since they require line-of-site application methods and their relative thickness will clog the porous structure. Also, many of these 3D printing methods result in a smoother implant nano-surface which decreases the surface energy. Using a wet chemical-based technology like our HAnano Surface allows for modification throughout these highly complex porous structures while adding bone chemistry, super hydrophilicity, and nano topography that will improve that implant’s osseointegration.

Detwiler, More, Shepherd: One of the biggest issues with coatings and surface modifications is how they are applied to surfaces. Spray-on processes such as plasma spray hydroxyapatite and titanium or blasting based techniques require line-of-site application. Deep, narrow porous structures are difficult to apply uniform surface modifications. Liquid-based processes such as dipping, acid etching, and anodization can reach deep areas within porous materials, provided the pores are not dead-ended but part of an open interconnected porosity. 3D printing has provided the open porous structures for bone in-growth but limits the types of surfaces that can be applied uniformly. Open porous surfaces generated by 3D printing can provide the right framework for implant fixation. However, manufacturers should consider nanoscale surface modifications that will increase the implant’s ongoing biologic fixation.

Edwards: The promise of additive manufacturing is to eliminate selectively the need for separate implant machining and coating. Add in logistics costs between providers and the opportunity for savings is considerable. The primary hurdle to 3D adoption is generation of extensive clinical equivalence.

Robbins: As the development of 3D-printed medical devices continues to evolve, Bodycote’s Kolsterising® process paired with Hot Isostatic Pressing (HIP) can further improve the hardness of the component’s surface. With the HIP process—densifying the material and reducing or even eliminating any material inclusions—this combination of processes can improve the orthopedic device’s performance and reliability.

Zentz: 3D-printed devices can pose challenges to traditional coating providers, where potential entrapment can occur. Working with your coating supplier before, during, and after processing of product is critical when these devices are being designed. Additionally, HAnano solves many of these challenges by providing a surface modification that does not clog traditional porous structures and 3D-printed devices. 

Barbella: What factors must be considered when choosing a surface modification or coating technique for an orthopedic implant?

Detwiler, More, Shepherd: Biological intent of the surface needs to match the function of the device. Resistance to microbial attachment does not guarantee good tissue integration. There are often tradeoffs for a particular feature. Duration of coating lifetime needs to match the implant lifetime. Do you want the host tissue to form a bond with the coating or with the base implant material? Orthopedic devices see a lot of forces pass through them. Incompatible mechanical properties can cause delamination or breakdown of coatings that can diminish the effectiveness of the orthopedic device, such as microparticle wear debris or delamination of HA coatings.

Edwards: Preclinical and histologic evidence are required. The presence of predicate devices with equal coatings simplifies the submission process. A detailed Master File at the FDA is also a requirement. Lastly, in an ideal world, the surface treatment provider has already completed a battery of ISO 10993 biocompatibility testing.

Robbins: Kolsterising® is often used to replace traditional coatings in medical devices because this diffusion process does not flake or delaminate. Flaking and delamination can create foreign residuals in a variety of orthopedic procedures. Over the next several years as orthopedic devices continue to evolve, traditional additive coatings will continue to challenge the device manufacturers in eliminating the opportunity for foreign material to enter either the surgical zone or the body.

Zentz: A coating’s impact on a patient’s outcome is critical, but even more so is the ability of the coating supplier to provide a service in a consistent and timely manner under strict quality guidelines.

Barbella: What improvements can be made to the surface modifications and/or coatings process (or technology) to achieve better results from orthopedic implants?

Brogren: Differing indications and biomechanical requirements means there will aways be a need for multiple implant material types and geometrical structures. In terms of osseointegration, the perfect coating or surface modification is agnostic to implant material and geometry. HAnano Surface is such a surface modification.

When it comes to osseointegration, it is hard to find a specific implant material that by itself has good properties for osseointegration. From literature, we know the oxide layer of titanium is an interface the bone can grow onto. That makes titanium a good biomaterial to start with. By roughening the surface through blasting or etching, the roughness can be improved to provide better integration properties with bone. Some of these processes create a nanostructured surface which has shown to improve the integrative properties of the titanium even further. If you combine the roughened titanium with a super thin bioactive coating/surface modification, you have a perfect interface for bone to grow onto.

Detwiler, More, Shepherd: The first thing is to match the surface with the intent of the implant. If the coating interferes with the function of the implant or shortens the lifetime of the implant, it is not worth adding it for marketing purposes. Surfaces that are resorbable can provide engineered outcomes after initial healing will be paired with surfaces that are permanent and provide biological cues for fixation in the event of micromotion or mild device trauma.

Surface modifications often provide a second, more biocompatible surface that the tissue will interact with instead of the base implant material. Translation of mechanical forces from host tissue through the surface to the implant, and then on to another point of contact, is often a weak point. The surface material is often inherently different from the base implant material and has different mechanical properties. With enough loading cycles, the interface between the surface and the implant can break down, causing aseptic loosening. Having very thin (i.e., nanometer thin) treatments and coatings allows closer contact between tissue and base implant material.

As surfacing technology becomes more refined, we will see more bioengineered, outcome-specific surfaces. Successful surface technologies will be tunable, meaning it is modified for a specific tissue interaction. This will encourage differentiation or vascular recruitment, which will define next-generation technologies.

As the FDA catches up with academic innovation and industry applications, antimicrobial surfaces will provide solutions for high-risk patients and revision surgeries with critical fixation needs.

Edwards: The nature of the process usually involves high vacuum, vapor depositions that are costly and time-consuming. Changing these processes will prove difficult.

Robbins: Due to the addition of large concentrations of carbon atoms via Bodycote’s Kolsterising® process, high compressive stresses are formed at the surface. These compressive stresses along with occupation of the interstitial sites by carbon atoms cause an increase in the surface hardness of the material and improved mechanical properties without altering part dimensions.

Zentz: Contacting Danco Medical during the design process to review the potential for special racking, masking requirements, or critical tolerance features is an important step to the coating process. Additionally, Danco offers orthopedic customers lunch-and-learn presentations where applications and the applicable processes can be reviewed in real time, while also providing educational material for future development projects.

Barbella: How do you see the surface modification and coatings market evolving over the next five years?

Brogren: Currently a lot of companies are researching the area of antimicrobial coatings and surface modifications. I think we will see some new technologies in these areas over the next couple of years. The trend of improved osseointegration through new implant surface modifications will continue to grow over the next couple of years, since the demographic and health issues of the western world will increase the demand for implants with improved osseointegration.

Detwiler, More, Shepherd: Major players will absorb innovative surface companies or specific technologies and bring them to the mass market. Marketing campaigns will educate the market about new potential of surface technologies, raising the bar for everyone in the market to compete and gain market share. The FDA will allow initial antimicrobial surfaces into select markets. Science driven technologies with substantial in-vitro and in-vivo data will eventually win against “me-too” surfaces with large marketing campaigns because the science drives results.

Edwards: Implants with coatings will dominate the offerings ahead. Due to the considerable product lifecycles and inability to change coatings based on regulatory hurdles, the market is likely to remain stable and growing. A coating improvement may come once a poorly performing product is discontinued and next-generation products follow on.

Zentz: A continued focus on antimicrobial surface modifications for infection reduction and quicker osseointegration for healing response and patient recovery time.