One of the foremost issues on the minds of orthopedic implant makers lately has been the controversy over metal-on-metal (MoM) hip implants. Back in 2008, orthopedic device company Zimmer Holdings Inc. voluntarily recalled its Durom acetabular component; in 2010, DePuy Orthopaedics Inc., now DePuy Synthes, recalled its ASR XL acetabular hip system and ASR hip resurfacing system due to concerns over metal deposits in the surrounding tissue of the implanted devices; and in 2012, Smith & Nephew plc recalled metal liners of the R3 acetabular system due to a higher than expected number of revision surgeries associated with the use of the device in total hip replacements outside the United States.

Like polyethylene, which was the material commonly used in hip implants before MoM became more widespread, metal surfaces give off small particles of debris. In addition, metal surfaces can corrode, giving off metal ions. Metal debris (ions and particles) can enter the space around the implant, as well as enter the bloodstream. This can cause a reaction in some patients, such as pain or swelling around the hip, osteolysis, and very rarely, symptoms in other parts of the body.

Though the failure rate of MoM hips have been shown by some studies to be higher than other types of hip implants, MoM hips are certainly not the only types of implants that show wear when implanted. As a 2006 article published in the Journal of Biomedical Materials Research Part B: Applied Biomaterials showed, significant wear appears in the types of implants that preceded the popularity of MoM—metal-on ultra-high molecular weight polyethylene—as well as ceramic-on-ceramic and ceramic-on-metal hips.¹

Ultimately, the site at which friction occurs is of course the surfaces of the implant. Or drill. Or saw. Or any other implant or tool that interacts with the body during surgery. Coatings, polishing and other surface treatments are devices’ defense against becoming dangerous to the patient, and for this reason they are an important consideration in medical manufacturing.

Coatings: Color, Hardness and Beyond
Coatings as a surface treatment do not usually apply to orthopedic implants, i.e., devices that are placed inside the body. Coatings serve to make surgical tools last longer, give them color (for instance for devices that are color coded for ease of use).

Aluminum anodic coatings are not used for orthopedic implants or parts of an instrument that enter the body. Anodic coatings serve to make surgical tools and equipment last longer and give them color if required (e.g. for devices that are color coded for ease of use). Tim Cabot, president of DCHN LLC, a metal finishing and anodizing company based in Woonsocket, R.I. that primarily focuses on aluminum tools, explained how his company approaches coatings.

“Aluminum anodizing is a 70 or 80 year old technology,” Cabot told ODT. “Although there are some variants, most anodizing uses sulfuric acid as the electrolyte. This type of anodizing can be either decorative to be used on items like pet dog tags for instance; or conventional hard coat such as that used in Calphalon cookware.”

There are three basic types of anodic coating processes. Type I produces a relatively thin coat using a chromic acid bath, type II uses a sulfuric acid bath and type III produces a hard coat of usually higher thickness.

“There are some pretty strong performance limitations on these standard anodic coatings,” Cabot said. “On decorative or type II coatings, you can get bright and shiny parts but they will not be hard and scratch resistant. And you can get hard and scratch resistant parts with type III, but you can’t get clear or bright and shiny. They’re in two separate worlds. Where DCHN plays is to offer customers the best of both. We have different process technology that allows us to offer very hard but highly decorative anodic coatings. The reason is that we address the fundamental coating process differently using Sanford Process technologies.”

DCHN’s special process is called MICRALOX, a mnemonic for microcrystalline aluminum oxide. It provides a hard coat where the anodic coating molecules are phase changed into partially crystalline structures for a reported 10 times the chemical and corrosion resistance of conventional hard coat, while maintaining the same physical properties. This process was developed for devices that need high chemical resistance, such as reusable medical instruments that require constant cleaning and sterilization. MICRALOX coatings are designed to withstand hundreds of Steris, Sterrad and autoclave cycles. Also, MICRALOX is proclaimed to maintain its integrity at pH 13 for two hours or pH 0.9 for 48 hours. The coating is tested to withstand up to 15,000 hours of salt water spray, so it is highly corrosion resistant as well.

“Many medical instruments are reusable, because they’re expensive and not everything can be disposable,” Cabot said. “In order to prepare the instrument for the next procedure, you have to go through a cleaning cycle. There are a couple of steps in a cleaning cycle. One is you’re trying to remove organics from the surface of the instrument, and the next is you’re trying to make sure it’s a sterile surface. The way you remove organics typically is you use soaps and detergents. The more effective soaps and detergents include oxidizers. They’ll be sodium hydroxide or other materials like that. Those detergents strip the anodic finish off the instruments. When you go buy an expensive drill or saw or other device and then run it through your washing and disinfecting process, the anodic coating can start to strip off. Conventional anodic coatings are less resistant to strong chemistry, and the cleaning is the reverse process of how the coating is formed. And obviously the stronger the chemistry or the longer the immersion times or the more frequent the repetition, the faster your coating can dissolve. So people invest a lot of money in very high quality tools and equipment, and then a couple weeks or months later it looks terrible because it’s been going through these cleaning cycles. That’s one issue. The second issue is you have to sterilize. There are two primary sterilization for aluminum equipment: autoclave or Sterrad. Autoclave is super heated steam—which can kill bacteria and viruses and things like that, but it also has the propensity to initiate a chemical reaction with the aluminum and anodic coating. It converts the aluminum oxide to aluminum hydroxide, and that may cause a smutty surface. You get iridescence, mottling, dye leaching, color fading, and other things like that, as well as corrosion. On some instruments one might use Sterrad. But the issue of using Sterrad is it uses hydrogen peroxide which is a very, very strong oxidizer and the process can attack the colors, causing fading and other problems.”

Because DCHN primarily works with aluminum, the company is not providing surface treatments for devices that go into the body. However, when colors fade and corrosion occurs on the housing for surgical tools, this can pose a safety risk to patients. As Cabot explained, if devices are color coded or if they have instructions embedded into the coating, fading and leaching can significantly compromise the surgical procedure and increase the risk for surgical error.

The fear of surgical error due to compromised instruments is not an empty one: The American Academy of Orthopaedic Surgeons (AAOS) has been at the forefront of patient safety since it first launched an initiative in 1997. This year, the organization launched the “Sign Your Site” program, based on a similar program established by the Canadian Orthopaedic Association. The initiative highlighted three actions for patients: a review of the operative procedure with the patient and operating room personnel prior to surgery; a review of the patient’s chart in the operating room prior to surgery; and the patient writing their initials at the operative site. Hence, “sign your site.”

“In medical school, instruction is largely focused on technical skills,” William Robb, M.D., an orthopedic surgeon at the Northshore University Health System in Chicago, Ill., told ODT at AAOS’ 2014 Annual Meeting held in New Orleans, La. in March. “But to give an egregious example of what could go wrong, you could do a perfect operation on the wrong patient, which would be disastrous. We must create highly reliable, systematic, standardized methods of care that mirror processes incorporated in other complex industries and apply them to our own, and learn which ones work and which don’t.”

The extreme example of surgery being performed on the wrong patient—or the wrong limb of a patient—is certainly worse than confusion on how a tool should be used, but misuse of a tool can be disastrous.

“Coating failures can be dangerous in a couple of ways,” Cabot said. “One is that typically in a medical procedure you want to make sure that you’re not only using the right tools but you’re using them in the way they were intended to be used. For instance if the coating is removed, it is likely that the instructions and markings on the surface will also be removed along with the coating. Maybe you have a marking about to set the drill for going in vs. pulling out. Or maybe certain parts are color-coded. You use the green one for this situation but the red one for another situation. If you remove the color coding, suddenly now you increase the risk of the wrong tool being used in a surgical situation. And that’s besides the fact that no one wants to have shoddy-looking equipment when you’re in an operation or not be able to quickly tell whether the blemish on the part is corrosion vs. contamination.”

As well as sterilization and cleaning processes, another limitation conventional anodic coating poses is its use of electricity. Such coatings use the sulfuric bath as a conductive material, and the anodic process slowly builds a coating out of the substrate material, typically aluminum. As more voltage is delivered, heat builds, which imposes limitations on how thick the coating can become because aluminum does have a burning point. Cabot explained that electrolytes can be chilled to remove heat from the surface, but that process causes the coating to darken—which limits the ability for color to be added to the coating.

“A number of years ago we approached this problem a different way,” Cabot said. “Instead of using voltage to drive the coating formation, we used both a DC and AC current as part of a proprietary and patented process. It’s a little technical as to how this works, but the basic idea is that it’s fundamentally different compared to conventional high voltage processes. By delivering the AC current over the DC current, it acts like a windshield wiper on your window wiping away water as you drive down the road. In our case, it removes a major cause of electrical resistance so we don’t have to drive voltage so high, and because we don’t have so much resistance, we don’t need so much voltage, resulting in much less heat. This means we can stop the coating from getting dark. That was one of our first major innovations – to be able to produce a clear hard coat that had all of the functional properties of type III conventional hard coat but with many of the decorative properties of type II or decorative anodizing.”

Venturing Inside the Body
If coatings on devices that never even enter the body prove themselves so important to patient safety, surface treatments on implanted devices (such as the infamous MoM hips) play with much higher stakes.

Danco Anodizing, a Warsaw, Ind.-based anodizing and metal finishing company, offers an exclusive Integrated Color Control (ICC) system for color anodizing titanium implants to its customers. Color anodizing is a process by which color is created by treating the surface to reflect light a certain way, creating a “perceived” color to the human eye. Since the entire spectrum of colors lie within five millionths of an inch of coating thickness, variation within any single color is very difficult to control. Slight variations in processing controls traditionally have caused color differences perceptible to the human eye. Danco’s ICC system purportedly delivers consistent colors in day to day and lot to lot production cycles based on light refraction rather than the human eye’s ability to see.

“When you do titanium colors, you’re doing it by the thickness of the coating and as the light reflects off the coating it refracts the prism colors, so for titanium anodizing you’re limited by the colors in the rainbow,” said Dean Zentz, vice president of operations at Danco Anodizing. “Certain colors like red and forest green are not obtainable.”

Danco’s document on orthopedic implants explains that within the orthopedic industry, products for which titanium color anodizing is often applied include bone plates and screws, intramedullary nails and rods, spine cages, and other hardware commonly associated with trauma or spinal surgery.
Products are color anodized primarily for cosmetic and/or identification purposes. In the operating room, for example, clinical staff might desire to have magenta, blue, gold, green and bronze “colors” that can help visibly differentiate various lengths of screws that possess the same diameter.

Danco also offers a process called low friction chrome for instruments such as cutting blades, needles, drills, cannulas, laparoscopic and endoscopic instruments and clamps.

“The low friction chrome process conforms to AMS 2460 [the current chromium plating specification standard],” Zentz said. “This is used on stainless steel surgical instruments to improve and increase the life of the instrument through steam autoclave. It will also increase the wear and hardness.”

The process, which imparts a smooth finish to the surface of such products, is designed to reduce friction on mating parts, reduce heat buildup during cutting procedures, improves corrosion resistance, is autoclavable many times over, removes the problem of nickel sensitivity in patients because of the barrier formed between possible nickel bearing instruments, and can be used on all grades of stainless steel.

“Smooth” is the name of the game when it comes to certain implants and most surgical tools. Able Electropolishing Company Inc., a surface finishing company that calls Chicago, Ill., home, has developed its own proprietary electropolishing method it calls Brite passivation, a process it has offered since 1954. Passivation is a chemical treatment that slightly alters the very top surface chemistry of metal, making it “passive” and less susceptible to corrosion. In most cases, you cannot tell by visual inspection whether a part has been passivated. Brite passivation is designed to carefully dissolve a small amount of surface metal, and removes discoloration, oxides and imbedded contamination. The metal part is cleaner and brighter as a result, hence the term Brite passivation.
Brite passivation essentially is a light electropolishing process that provides the multiple benefits received with electropolishing that extend beyond just corrosion resistance,” said Able Electropolishing President Tom Glass. “Standard passivation provides a level of corrosion resistance by removing free iron from the surface of the part, whereas Brite passivation is actually removing a small amount of material, removing the embedded impurities, and provides for a level of corrosion resistance that’s far superior than standard passivation.

Often, however, for fusion implants and other such devices that require bone ingrowth, the level of roughness is key and rides a delicate balance between bone formation and resorption of bone remodeling at the interface of the bone implants.

Titanium porous plasma spray being applied to an implantable cup. Image courtesy of MedicalGroup Corp.

MedicalGroup Corporation, a medical manufacturing services provider based in France, specializes in coatings to promote biological fixation of orthopedic implants. The company provides what is called projection plasma spraying, an automated thermal spraying technique. The spray can be in the form of implantable metals such as titanium and cobalt chrome, ceramics such as alumina and hydroxyapatite; or a multi-layer combination of both. The coatings can be applied to the full range of orthopedic prosthetic joints including hip, knee, spine, shoulder, ankle, elbow, hand, foot or tooth. Due to their roughness and porosity, metallic coatings facilitate primary fixation and anchoring of the implant by increasing the surface in contact with the bone. Hydroxyapatite can facilitate the osteointegration of the prosthesis.

“On orthopedic implants, bone attachment to the implant is a key issue that is addressed by creating porosity and faster bone growth on the implant,” Richard Vandevelde, president and founder of MedicalGroup, told ODT. “Therefore a porous coating is the most appropriate solution, the vast majority of the time, being by plasma spray process. Ceramic/calcium phosphate coatings started in the 1980s in Europe on cementless hip stems and cups in order to reduce the mortality rate caused by cement. Hydroxyapatite (HA) coating is deposited by plasma spray and HA may dissolve within a few years. At some point, the question of HA coating dissolution on implants arose. In 1989, MedicalGroup developed a new concept of porous coating on hip implants for the French market. After three years of research, titanium (Ti) porous plasma spray coating, in addition to HA, was applied on hip stems and cups. Gradually, HA coating was replaced by a dual coating. On the 180,000 plasma spray coatings made by MedicalGroup Corp. every year, 75 percent are now dual layer (Ti/HA) mainly for the European and Asian markets. The main substrates to be coated are titanium and stainless steel. However, we see a lot of cobalt chrome and now PEEK (polyetheretherketone). Since the start and wide use of plasma spray coating, the main requests have been for higher roughness. We therefore had to develop coatings with a higher level of roughness.”

3-D Printed Devices: A New Market?
The March/April issue of ODT featured the 3-D printing manufacturing process and how it is becoming more widespread now with patents on the technology expiring as we speak. Direct metal laser sintering (DMLS) is one process of 3-D printing, by which layers of material are sintered by heat lasers to create the final shape of the device.

“DMLS is a little bit rough,” Drew Roberts, engineering manager for Deland, Fla.-based 3D Material Technologies LLC (3DMT), told ODT. “The process inherits itself to have a little bit of a grainy surface finish, but the material is finishable, polishable, and chromable so you can actually design into the part maybe a quarter percent shrinkage in the larger area so you can actually polish it down to a mirror-like finish.”

Since 3-D printing (also known as additive manufacturing, rapid prototyping, and other monikers) is as yet not being used for large lot or large run device manufacturing, surface treatment service providers have not seen an uptick in 3-D printed devices requiring their services.

“New surface typologies will be developed with the increased use of 3-D printing,” MedicalGroup’s Vandevelde said. “Products made with additive manufacturing are still not widely used at this point. Several features and functionalities still have to be improved. Cleaning is still tricky. Thermic and physical treatments are usually necessary and impacting manufacturing costs. Machining is still a necessary process. HA plasma coating is not applicable on very porous surfaces such as additive manufacturing. Therefore, we have developed a new treatment surface of calcium phosphate electroposition at low temperature. This process allows an even coating all the way through the deepest pores.”

As 3-D printing grows, it will likely feed the coating, polishing and overall surface treatment industry. But that time has not come just yet.


Reference

1. “Friction of Total Hip Replacements With Different Bearings and Loading Conditions.” 2005. Claire Brockett, Sophie Williams, Zhongmin Jin, Graham Isaac, John Fisher. Journal of Biomedical Materials Research Part B: Applied Biomaterials.