In Orthopedic Applications, the Coat Makes the Implant

Author: 
Bob Michaels
Orthopedic implants rely on a variety of coatings to achieve such crucial properties as osseointegration, lubricity, and fatigue strength

To ensure the long-lasting success of artificial hips, knees, and other orthopedic implants, designers and manufacturers must be aware of the environmental conditions that their products will confront during surgery and once they are firmly anchored in the body. Success depends on the ability of implants to coexist with the body’s physiology and fuse with native tissue to form a seamless bond. Furthermore, the implant must also be designed to prevent material from shedding particulates into surrounding body tissue while exhibiting sufficient strength to withstand the rigors of weight and motion.

In an effort to satisfy these demands, medical device suppliers and manufacturers have developed a range of materials and deposition techniques to produce coatings for orthopedic implants featuring a diverse array of material properties. Based on these properties, orthopedic implant coatings play a critical role in facilitating osseointegration, minimizing friction between implantable components, and bolstering fatigue resistance. Hence, they are often just as indispensable to the well being of the patient as the implants themselves.

Promoting Osseointegration
Depending on the specific application, coatings can be applied to orthopedic implants either to encourage or to impede bioactivity. For example, one of the prime reasons for applying coatings to orthopedic implants is to promote osseointegration—the process by which bone and other tissues fuse with the implant to ensure proper implant fixation and longevity. While such coatings encourage bioactivity and are of paramount importance for addressing the implant’s viability, other coatings are designed to actually prevent such forms of bioactivity as infections.

Catering to the need to provide a bioactive environment for orthopedic implants, Orchid Orthopedic Solutions (Southfield, MI) specializes in titanium, cobalt-chromium, and hydroxyapatite (HA) orthopedic coating technologies for promoting osseointegration of joint, spine, and other orthopedic devices. While the company’s titanium orthopedic coatings are used to create roughened surfaces comparable to a 100-mesh Al203 finish or to prevent implants from cracking, its spherical-bead, asymmetrical-powder, and titanium plasma spray technologies provide porous surfaces to facilitate bone in-growth.

Applied to titanium or chromium-cobalt substrates, the company’s plasma-spray technology, for example, uses either a titanium or hydroxyapatite plasma spray to coat the implant surface. This method, according to Tony Crivella, Orchid’s senior manager—implant sales, coating and plastics, involves the use of inert gases, electricity to create an arc, and titanium. By running the powder through the arc, particles are created that attain a velocity high enough to blast the coating onto the substrate.

In contrast, the company’s titanium and cobalt-chromium spherical-bead coatings and its Asymmatrix titanium and cobalt-chromium asymmetric-particle coating are applied to orthopedic implants using high-temperature sintering, Crivella explains. “In these techniques, we use binders and apply specific bead sizes or types of beads—meaning spherical or nonspherical—to the bone side of the implant. Then, we use vacuum technology to heat the part to the point at which all of the binders burn off and all of the particles stay in place.” Relying on the use of powders, this method sinters, or spot-welds, the particles, enabling them to bond to the surface of the implant and create particular porosities and pore sizes. “This process, Crivella adds, “gives the bone and tissue an area to grow into.”

The company’s HA coating, on the other hand, differs from its titanium and cobalt-chromium coating technologies in that this very thin bonelike material attracts native bone, according to Crivella. However, while HA coatings allow for better implant fixation than no coatings at all, they lack the porosity of the company’s titanium or cobalt-chromium coatings that is crucial for promoting bone and tissue integration into and around the implant. “Given the limitations of HA coatings, what we’re seeing more and more is that HA is being used on top of either titanium or cobalt-chromium, providing a combination of bone-attracting material and an area for the bone to grow into,” Crivella comments. “However, although this capability is growing in popularity, the downside is that it’s expensive.”

Coatings with Good Vibrations

The ultrasonic vibrations of Sono-Tek’s nozzle deagglomerates nanoparticles to keep them evenly dispersed.

Coatings are often only as effective as the systems that are used to deliver them. To apply coatings to orthopedic implants, coating equipment providers offer a range of systems based on different technologies, including dip coating, sintering, and plasma spray etch. In contrast to these methods, Sono-Tek Corp. (Milton, NY) specializes in ultrasonic technologies, which are used to deposit coatings on orthopedic implants to either promote or inhibit bioactivity.

Encouraging or impeding bioactivity depends on the agents that are incorporated into the liquid being applied to the implant surface using ultrasonic technology, remarks Steve Harshbarger, executive vice president and division director of Sono-Tek. “For example, our coating technology can be employed for applying HA thin films to promote bioactivityosseointegration—or it can deposit antibiotic and antiinflammatory thin films to inhibit bioactivity.

Ultrasonic coating technology uses high-frequency sound vibrations to atomize liquids, creating very small, uniform drops while simultaneously deagglomerating nanoparticles. The technology’s soft, low-velocity atomized spray can be aimed at the desired area of the implantable device to form a thin, durable layer. “Unlike dip-coating methods, the application of coatings using ultrasonic spray technology allows for complete flexibility to adjust coating thickness,” Harshbarger says.

Often composed of liquids containing nanosuspensions, the uniform films applied using the company’s ultrasonic technology can be as thin as 50 to 100 nm, according to Harshbarger. “The ultrasonic vibrations of the coater’s nozzle continuously deagglomerate the nanoparticles in these suspensions, keeping them evenly dispersed,” he adds. “This is important, especially because achieving uniform particle dispersal is a common difficulty associated with depositing nanocoatings such as carbon nanotubes, a recent and exciting ultrasonic implant coating that is used as a scaffold material to promote bioactivity.”

With an eye toward the future, Harshbarger notes that the development of nanosuspensions—on the rise in the past several years—will continue. “These nanomaterials allow for thinner and stronger coatings than were possible in the past. And while these liquid suspensions are often very costly—particularly at the R&D level—ultrasonic spray technology enables manufacturers to minimize the use of materials while still obtaining a very uniform coating.”

Orthopedic Lube Job

Orthopedic screws feature
a titanium anodized
coating from Electrolizing Corp. of Ohio.

While many orthopedic coatings are designed to promote fixation and osseointegration, other coating applications increase surface lubricity—an important property for preventing wear on the surfaces of metal implants and implant components. For example, in orthopedic implant applications, achieving surface lubricity is crucial for preventing galling, remarks John Kalinowski, account manager—technical sales at the Electrolizing Corp. of Ohio (Cleveland). “A form of surface damage that occurs when metals slide against each other, galling results when small titanium bone screws turn against titanium implants and plates,” according to Kalinowski. “It causes microscopic roughening and creates protrusions above the original surface. This property can also damage surgical instruments that incorporate moving parts.”

To address this issue, the company offers Ti-MED II, a coating that is applied to the surfaces of spinal and other orthopedic implants by means of titanium anodizing. “Originally developed for use in the aerospace industry, this anodizing method is particularly beneficial in the medical industry because it increases lubricity and has excellent antigalling properties,” Kalinowski says. “These characteristics lead to significant improvements in implant performance and durability.” Type II titanium anodizing also penetrates the titanium instead of accumulating on the surface, impeding dimensional changes after processing, he adds.

In addition to providing a lubricious surface, coatings applied using the company’s titanium anodizing technique enable orthopedic implants to endure fatigue stresses, Kalinowski comments. Greater fatigue strength is achieved because the anodizing process creates a uniform surface, producing high strength-to-weight surfaces that enable the implant to withstand the natural forces associated with body movements.