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Orchid Blog

New Trends in Orthopedic Implant Coatings

Posted: Monday, August 17, 2015

By Parimal Bapat, Research Engineer

Most orthopedic implant manufacturers still rely heavily on traditional coatings for their implants, such as sintered bead and plasma-sprayed metallic and hydroxyapatite (HA) coatings. These technologies are well-established, very familiar to both surgeons and the FDA, and have accumulated large bodies of supporting clinical and patient data for validation. However, there are some challenges to traditional coatings—for example, sintered bead coatings do not work on substrates of a different composition, such as a titanium coating adhering to a cobalt-chrome surface. Plasma spray coatings require surface preparation before coating, which can reduce fatigue strength of the implant, especially in titanium implants. Both sintered bead and plasma sprayed coatings are high-temperature processes that require more time and greater control, respectively.

Even though these well-established methods and materials have performed well over the years, new technologies are being developed that can vastly improve the functionality and performance of today’s orthopedic implants—and OEMs (original equipment manufacturers) want them. For example, customers are showing interest in coatings that have specifically engineered surface properties, such as high surface roughness and improved interconnected porosity that can potentially facilitate osteointegration. There is keen attention throughout the industry on the 3D printing of implants, a process that allows engineers to design and build an entire implant, including the roughened surface for osseointegration, in a single process.

Some companies are interested in ultra-thin HA coatings that can be applied to variety of materials and surfaces, including the inside of the porous structures—something that may encourage bone growth and is not possible with today’s current plasma-sprayed HA coatings. Thin HA coating can be applied to a variety of materials, including metals, plastics, and polycarbons—thus opening new avenues for designing implants with different materials which traditionally have not been used in the industry. There is growing interest in antimicrobial coatings, which are designed to help fight infection. These types of coatings work by making the surface of the coating incompatible with harmful microorganisms that cause disease.

OEMs are eager for new and improved technologies and materials that will make their orthopedic implants last longer, perform better, and improve patient outcomes, and are better positioned to offer solutions that balance cost and capability to their present and future coatings and add more value to their customers.

This is especially important for healthcare systems—orthopedic implants that enhance bone growth, last longer, and have fewer problems will reduce the rates of re-hospitalization and corrective surgeries, as well as enhance quality of life for the patient. Below are three areas of technological innovation on the forefront of orthopedic research and development that will improve functionality, performance, and longevity of implants.

Porous Rough Titanium Coatings

A porous rough titanium coating is basically a titanium plasma- sprayed (TPS) coating that is engineered to have a greater surface roughness and larger pore size. Both these enhanced characteristics are believed to promote bone growth into and around the implant. With a pore size greater than 100 microns, a porous rough TPS coating can deliver higher surface roughness and also comply with the FDA’s requirements for its porous coating category. Porous rough TPS coatings still have all the qualities of a standard plasma sprayed titanium coating, such as bio-inertness, bio-compatibility, and the ability to coat dissimilar substrates. For example, a titanium coating can be applied to a cobalt-chrome surface, something that sintered bead coatings cannot do. (See Figure 1)

A TPS coating for polyether ether ketone (PEEK) substrate material is also being developed. PEEK is a bio-inert, radio translucent material that is widely used in the spine industry. Because PEEK is bio-inert, it does not have natural properties that enhance bone growth. A porous coating on PEEK implants would promote osteointegration. Orchid is developing a porous metallic coating for PEEK implants that is similar to a TPS coating with a typical TPS microstructure. An added benefit is that it can be applied with the conventional plasma spray process. Because PEEK has a lower melting point than metals, one of the greatest challenges will be controlling the surface temperature of PEEK during the higher-temperature plasma-spray process.

Additive Manufacturing Technologies

Additive manufacturing (AM) is a rapidly evolving technology that is being used in the medical device, aerospace, and automotive and industries to build complex parts. Not only does AM save considerable time and money compared to standard machining, it also gives design engineers greater freedom for designing innovative parts with unique or challenging geometries.

AM is a good choice for implants with low surface to volume ratios and implants that have complex geometries which are difficult to achieve with traditional forging, casting, and machining techniques. In addition, since a variety of materials such as titanium and stainless steel, can be used in the AM process, functional prototypes can be made out of the same material as production components. This enables more rigorous testing of prototypes while decreasing the development time for new products.

The highly popular 3D printing process is being used to build complete orthopedic implants, including the roughened surface, in one single process. The process is fast, ideal for low runs, and can create nearly production-ready implants. 3D printing utilizes a CAD program to build an implant, one layer at time. Layer thickness can be controlled down to a few microns. Using the CAD software, engineers determine the desired coating specifications and build the entire implant in a single run, with minimum post processing. 3D printing machines use either a laser or an electron beam to fuse the materials and build the parts.

EOS (laser) and Arcam (electron beam) are at the forefront in this market. Both companies have validated processes that orthopedic companies have used to build implants that have received 510(k) approval from the FDA. Ultimately 3D printing saves time and money by completing parts in a matter of hours instead of days, eliminating secondary steps, reducing material waste, and speeding time to market.

Thin HA Coatings

Plasma sprayed HA coatings have seen a great amount of success in the orthopedic industry since the time they were introduced into the market. However, plasma spraying is a high-temperature process that only coats the visible area of the implant. A thin HA coating will not only overcome these drawbacks, but also improve the functionality of the implant. Less than five microns in thickness, the coating covers the entire surface of the implant, including the insides of the pores, which helps osteointegration. As the bone grows into the pores, the HA coating inside the pores continues to promote growth. Applying the thin coatings is a low-temperature process that is relatively fast process, which shortens processing time delivering an improved product.

Thin HA coatings improve functionality compared to traditional coatings because the thin HA coats the inside of the porous surface. This is especially true for geometrically complex implants and devices. Because there are no thin HA coatings yet on the market, one of the greatest challenges in developing thin coatings is regulatory. The FDA has guidelines for plasma HA coating, but not for thin HA coatings. Companies are now conducting animal studies to test biocompatibility and to see if these coatings are beneficial for osteointegration. Coating characterization data will also be prepared and submitted.

Future Developments

Research and development is being conducted for all these technology advancements. The next step is receiving FDA clearance, which has established guidelines for porous metallic and HA coatings. It can take about 6 to 10 months to create a master file with the FDA for each of these coatings.

As these technologies become commercialized and enter the marketplace, OEMs can expect increased functionality and performance from their products. The payback comes in faster time to market, longer life cycle, improved end-user satisfaction, and better patient outcomes. Technology and material discoveries in the orthopedic market will likely have applications in other advanced manufacturing industries as well, such as biotechnology and aerospace.