Choosing a Coating

The right surface modification technique brings benefits to patients with implants.

Erik Swain

The importance of selecting the right coating or surface modification technique for an orthopedic implant cannot be understated. Metals and ceramics that orthopedic implants are made from are not inherently compatible with the body’s tissues and organs and are prone to wear. The more wear an implant has, the shorter its lifespan in a human body will be.

Selecting the right coating or surface modification technique can help prevent these problems. If processed properly, these methods can reduce the chances of the body having an adverse reaction to the implant and help the body accept the implant, even spurring bone to grow around it.

“The primary objective is to improve the long-term wear performance of the implant,” said Gene Elwood, North America senior medical accounts manager for Ionbond LLC. IonBond de-posits

Choosing the right coating or surface modification technique is key for orthopedic implant firms. IonBond’s PVD coating machine is used to deposit titanium nitride on implants for patients with alloy sensitivity issues. Photo courtesy of IonBond.

BioCeramic coatings through WW Medical Coating Competence Centers. IonBond’s global headquarters is in Olten, Switzerland, and its North American Medical Coating Competence Center is in Rockaway, N.J. “A parallel objective is to provide a protective barrier for alloy-sensitive patients. While today no implant device lasts forever, a longer-lasting implant benefits the patient and helps to reduce healthcare costs. IonBond works closely with the device OEM to improve implant performance while satisfying each OEM’s specific test and evaluation protocols,” Elwood added.

As such, it is not surprising that “there’s a lot of talk about reducing wear, which increases the life of an implant,” said Marie Vennstrom, deputy R&D manager for Sandvik Medtech, a coatings testing company based in Sandviken, Sweden. “If you improve surface conditions, there will be less wear debris in the body. People are sensitive to metals, so the less iron released into the body, the better. And reduced wear allows for implants that stay in the body longer, which is always better for patients.”

Titanium, HA Coatings

There are a number of ways these goals can be accomplished. To aid bone growth onto implants, manufacturers typically choose titanium or hydroxylapatite (HA) coatings, said Colin McCracken, Ph.D., development manager of powder products for Reading Alloys, a Robesonia, Pa.-based division of Ametek Corp., which supplies titanium-based powders used by manufacturers to coat hip, knee and dental implants. “One of the main differences between titanium-based coatings and HA coatings is that titanium-based coatings do not require any fixing agent, while HA requires a fixing agent or cement,” he said. “In the short term, the recovery time is likely longer than it would be with a cemented implant. However, the non-cemented fixation normally lasts longer because it relies on bone ingrowth. HA relies on the strength of the cement to keep the implant in place.”

If an implant manufacturer opts for a titanium-based coating, it can choose between one that is pure titanium and an alloy that contains 6 percent aluminum and 4 percent vanadium, called Ti-6AI-4V, McCracken said.

“Both are engineered to spur bone ingrowth,” he said. “The Ti-6AI-4V has higher strength than pure titanium but does cost more. The market is about 50-50. Both go back a long way. Which gets used often depends on which material the orthopedics company started with, which was grandfathered in. Com-panies generally don’t change those kinds of preferences.”

New technologies are being developed that will do an even better job at promoting bone ingrowth, McCracken added. This in turn is prompting firms such as Reading Alloys to develop new powders that are more compatible with these processes.

“Several medical companies are developing new porous coatings for implants that promote and increase bone ingrowth and reduce bone shielding effects by the use of titanium-based foams or scaffolds,” he explained. “To-day’s technology works by plasma-spraying the powder onto the implant. New technologies will not require plasma spraying. They will result in a higher level of porosity. And that will make the implants much closer to the strength of the bone and reduce the amount of bone shielding that occurs. If you can reduce bone shielding, the life of the implant increases. The scaffolding technology requires a finer particle size distinction. So we are developing new powders to aid those developments.” Also, he noted, “metal injection molding is being used for very small dental implants. Putting Sintering titanium hydride powder in another form through the sintering process also allows the implant to achieve higher density, which leads to im-proved strength.”

Similarly, the device industry is looking into biologics to help promote bone ingrowth, said Elwood. “The device industry is evaluating biologic growth surfaces to enhance cell attachment and promote bone ingrowth. IonBond’s patented TST [titanium surface technology] is at the forefront of enhanced bone cell attachment,”he said.

Elwood also sees two other developments for metal- and ceramic-based coatings coming to the forefront in the near future.

“Primary deposition technologies are PVD, PaCVD and CVD with PVD being currently used to deposit TiN (titanium nitride) on implants for patients with alloy sensitivity issues, currently used widely in Europe. A more recent introduction is a device coated with a multilayer coating, top layer being ZrN (zirconium nitride); addresses both wear and alloy sensitivity,” he said.

Advances in BioCeramic coatings for spine implant applications also will have a major impact to improve wear and eliminate current issues for MRI imaging that are produced by alloys such as CoCr, he said. “Ti (titanium is an excellent alternative biomaterial, but its wear properties are poor; hence, the need for a BioCeramic coating. The unique properties of IonBond’s exclusive Medthin-Diamond (ADLC) has demonstrated positive performance results with cervical discs, for example.”

Polymer Coatings

Another option is polymer coatings. These are sought by implant manufacturers who want to attain certain properties that metals don’t have, said Donald Garcia, director of R&D, Boyd Coatings Research Co. Inc., a Hudson, Mass.-based supplier of polymer coatings.

“They are inert, biocompatible, non-stick, fission-reducing, lubricious, and wear-resistant. Lubricity aids in range of motion,” he said. “They are fairly common and have been used for a long time. They are plastics, as opposed to metals or ceramics. Their inertness is a key element. They are also nonthrombogenic, biocompatible and have a low coefficient of friction. With metal finishes, you have to add something to get those properties.”

Available polymer coatings include:

• PTFE (polytetrafluoroethylene)

• PFA (perfluoroalkoxy)

• FEP (fluorinated ethylene propylene)

• PVDF (polyvinylidene fluoride)

• ETFE (ethylenetetrafluoroethylene)

• PPS (polyphenylenesulfide)

• PAI (polyamideimide)

• PEEK (polyaryletheretherketone)

• MOS (molybdenumdisulfide)

• Nylon (polyamide)

“The key is developing coatings that don’t react and don’t cause any reaction in the patient,” he said. “Inert and biocompatible coatings can act as a barrier coating to fend off offending materials.”

Just as others foresee nanotechnology playing a role in the development of metal coatings, Garcia sees it playing a role in the development of polymer coatings as well.

“In the future, we might see more use of nanotechnology that can allow the molecules in coatings to perform in a different way,” he said. “For example, you might see something that is elastic at one point but stiff at another. As R&D continues, nanotech will probably be the next big step. There is not much in the way of new innovations for polymers.”

Surface Modifications

Another way to help an implant become accepted by the body is to finish it in a way that gives it a surface texture that is similar to that of the bone it will have to become compatible with.

There are many techniques that can accomplish this goal. One is automated blasting, such as that provided by Guyson Corp., based in Skipton, N. Yorkshire, England, and Saratoga Springs, N.Y.

“You normally use blasting when you have a specific surface texture or roughness requirement,” sayid John Carson, Guyson’s marketing manager and group leader of its application team. “Preparation for a coating and mechanical bonding are both improved by attaining a specific degree of roughness. We call it technical surface preparation. With orthopedic implants, you often require all areas of the component to have identical roughness. That is literally impossible to ensure when the components are processed by manual techniques. There is too much variation in hand-blasting. Thus, you need automated blasting. It is often a robotic process. The robot holds the blast nozzle, or manipulates the gun, or is used as a machine loader and unloader. Auto-mated blasting comes in to the picture as a means of eliminating the variability in quality that arises when you have manual blasting procedures. Those are difficult to control and duplicate.”

The technique also comes in handy for surgical instruments, he added.

“We find that a tremendous number of implants and instruments are bead-blasted, as opposed to grit-blasted. For surface prep work, you would grit blast, but for cosmetic finishing, you would blast with a bead of something like stainless steel or glass. The material is sometimes dictated by the surgical community, especially for instruments. Highly polished instruments can reflect light too harshly. They need to be made less reflective.”

For the next wave of robotic blasting to catch on, manufacturers will need to appreciate what advanced programming techniques can accomplish, Carson said.

“With advanced programming tools, preparing a new program for processing a particular type of component can be done repeatedly,” he said. “With a solid 3D-computer model and advanced programming software, you can develop a very consistent and high-quality manufacturing process and consistently attain the desired angles and distances, which is important because some components have very complicated slopes.”

This allows for the most accurate form of blast processing, and the flexibility is compatible with the styles of manufacturing often used in the medical industry today, Carson said.

Storing a program eliminates change-over and the preparation work that would need to be completed with a more archaicstyle of automation. A program can be set up with a bar code scan or the entering of a part number. It can recall and repeat a program as flawlessly as the last time, he added. It opens up tremendous savings in labor and time, but it also eliminates opportunities for error such as overblasting, underblasting and insufficient buffing.

Choosing Properties

In order for any of these solutions to work, however, manufacturers must have a keen awareness of what properties their implants must have and how they are expected to behave in the body.

It is no surprise, then, that many OEMs have a systematic process by which they collaborate with coatings and surface modification providers to pick the best possible solution, or develop a new one.

“Coating selection is based on the implant device and the objectives of the specific OEM,” said Elwood. “Where warranted, IonBond will custom-engineer a coating to meet the OEM’s specific performance objectives.”

Garcia of Boyd said it’s all about what the coatings customers want in the end.

“We look mostly at use conditions and properties that the client needs to impart. We look at how the device is going to be used and what conditions it will be subjected to in the manufacturing process, the surgical process and inside the body. We look at all that and determine which of our offerings can bring value, or we can engineer a coating to suit a specific application. A lot is being done in processing to reduce costs. Lean manufacturing is one part. Use of computer systems to strengthen our operations is another. We are always looking at continuous process im-provements. These types of advances can lower costs. We also look at how we can keep our material costs in check with our vendors.”

An understanding of the chemistry involved also is crucial, said Venn-strom of Sandvik: “Is there a corrosion situation? Are there local conditions? We try to find test methods to try the material against. The aim of our work is to improve the life of an implant, so it can stay in the body as long as possible. We have knowledge about how they are made, what can happen and what is the worst-case scenario and we simulate that. We need to find methods that can tell us in the laboratory what will happen once an implant is put into patients. One goal is to make coatings that can be produced densely at low temperatures. If your surface is dense, you may be able to have a surface that doesn’t have any faults in it. You can make sure it is homogenous and doesn’t have pinholes and cracks. Otherwise you will have problems with corrosion. If you have low temperatures, that won’t affect the properties of the bulk material. High temperatures will. A major concern is to improve the life of implants. That’s an interesting challenge from a materials perspective.”

The physics of an implant come into play too, she said. “You have to determine what impact the load distribution will have on each person. You have to weigh the pros and cons of polymers versus metals. That will affect how you design your implant.”

Processing challenges also must be addressed up front, said Carson of Guyson.

“Each different manufacturer has a different set of steps in their process,” he said. “We work with virtually all different types—tibial trays, femoral parts, hip cups, all kinds of other hardware. It seems like each project has a slightly different focus or emphasis. Sometimes our machinery is used late in the process, but in other cases it is used very early. It often depends on what technologies they’re using up-stream in the process.”