05.29.08
The Fault Line of Success and Failure
Coatings and surface treatments are seen as the keys to implant longevity. But can they deliver?
Frank Celia
Total joint replacement (TJR) is one of the biggest success stories in recent medical history. A brief generation ago, patients with severe joint arthritis simply had to accept a future of increasing inflammation and dwindling mobility. Now, thanks to artificial joints, millions oTotal joint replacement (TJR) is one of the biggest success stories in recent medical history. A brief generation ago, patients with severe joint arthritis simply had to accept a future of increasing inflammation and dwindling mobility. Now, thanks to artificial joints, millions of patients throughout the world live happier, more active lives, free from debilitating pain. And the industries and medical personnel who provide these devices are enjoying well-deserved success.
Yet beneath all the congratulatory back slapping and marveling at last quarter’s profits, a gnawing truth persists. For all their success, total hip and knee replacements—by far the most common—do not last long enough. Their known lifespan is about 15 to 25 years, while life expectancy in the United States now hovers at 78 (and for those already 65, that figure jumps to 83), with certain demographics really off the charts. A Caucasian woman of better-than-average means, for example, can expect to live to 90. Which is to say nothing of future gains.
If revision surgeries posed less risk, this discrepancy would not be so troubling. But replacing TJRs is far more complicated and dangerous than the initial procedure—itself no walk in the park. The patient is older, the surgery longer, blood loss greater, infection more frequent, etc. Doctors put off revision surgeries as long as possible for these reasons.
With patients in their 40s and 50s undergoing TJRs in greater numbers, researchers are scrambling for ways to increase implant longevity. High-tech coatings and surface treatments that promote osteogenesis and implant fixation have been the primary means of achieving this goal so far. But some in the field are working on more radical solutions, trying to replace metal components with more organic, bone-like substances grown/fabricated in laboratory settings.
Metallic Pros and Cons
Forty years ago, when implants were being designed, metal load-bearing components must have seemed a natural material choice. Metals—strong, hard and durable—in many ways surely appeared an improvement on bone.
Photo courtesy of Boyd Research Coatings Co. |
The more short-term and well-understood flaw of even the most inert metals is, of course, poor to nonexistent interfacial bonding between metallic surfaces and surrounding bone. Because in-vitro tests are limited and in-vivo tests all but impossible (except on animal models), determining exactly how implants malfunction remains problematic. By the time the patient presents a failing implant, the damage already has occurred and pathology is challenging to track. But general scientific consensus holds that when implants do fail, the problem most often originates at the interface between the implant and tissue. Researchers believe a non-adherent fiberous tissue layer forms, creating small gaps between bone and the implant, again permitting movement that leads to implant failure. (It bears mention that some doubt this theory, arguing instead the articulating parts and the tiny particles of non-organic matter they produce signify the true fault line of most implant failures.)
Researchers have targeted this implant-tissue interface in attempts to lengthen implant function. A popular and well-tested method is coating the metal with a layer of calcium phosphate (CaP), a type of bioceramic, usually the mineral hydroxyapatite (HA). Extensive research has shown the close resemblance of HA and the mineral components of tooth bone enhances the biocompatibility of the implant.
Application Arguments
There is broad-based agreement that bioactive CaP coatings are beneficial. However, where opinion differs fiercely is in how, where and under what circumstances these coatings should be applied.
An enormous variety of application methods exist. Many vapor deposition techniques are available, including physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD variants include hollow cathode reactive plating, magnetron sputtering and cathodic arc deposition, just to name a few.
Then there are solution-based application methods, in which the component is dipped into a vat of coating mixture, sometimes at room temperature.
Probably the most widely used and well-understood method for applying CaP coatings is plasma spraying. This process involves spraying molten HA powder via a hose-like nozzle device on the component surface, where the liquid material sticks and freezes. Short-term studies on animal models show plasma-sprayed HA-coated implants promote faster bone growth and better stability, but longer-term studies have proven less encouraging. Evidence indicates such coatings may be subject to degradation, delamination, osteolysis and third-body wear over prolonged periods.
Nobody can say for sure why plasma-sprayed HA-coated implants are failing to live up to expectations. Some blame the extremely high temperatures used during the spraying process (up to 12,000 degrees Celsius). Others think the coatings are too thick and thus unresorbable, leading to early loosening of the implant caused by soft tissue reactions and osteolysis. Still others are quick to point out that a too-thin coating can result in resorbtion occurring too rapidly, which also is not desirable. The coating should be thin enough to resorb, but not before doing its job.
“Worldwide there is an increasing awareness of the importance of thin CaP coatings that are produced in a low-temperature process, and these are generally called biomimetic coatings,” said Peter Zeggel, general manager at DOT GmbH, based in Rostock, Germany. The company has responded to this awareness by introducing its electrochemically deposited BONIT coating process. DOT sees BONIT as maintaining the best qualities of plasma-sprayed HA coatings, while circumventing their drawbacks. “Due to the low temperature process there is no thermal degradation, with its adverse effects on the coating quality,” Zeggel explained. “BONIT has no monolithic layer, and therefore, there is no delamination of the coating. And a further very important reason for its success lies in the fact that it is resorbed by the body in a few weeks, after having fulfilled its function, which is to induce bone growth to the implant surface.” To date, the company has applied this coating to 670,000 implants, and interest in it continues to grow, he added.
Pre-treatments
No matter what surface treatment or application an OEM selects, the substrate will require some kind of pre-treatment before a coating is applied. At the very least, a thorough and intensively effective cleaning is called for, a task often accomplished by ultrasonic technology. (See “A Greener Cleaner” on page 58).
Blasting, another common pre-treatment surface process, involves some type of media—whether it be granular mineral material, such as aluminum oxide or silicon carbide, or a glass bead or stainless steel shot—being shot at high velocity onto the surface of a component, creating a pattern of tiny angular dents or rounded dimples. Blasting may be employed for cosmetic purposes, to obscure tool scars or other superficial marks created during machining processes. But its goal in terms of an impending coating procedure is to render a substrate more conducive to adhesion.
“The texture helps the coating stick to it,” explained John C. Carson, group leader of the Applications Team at the Saratoga Springs, NY-based Guyson Corp., a designer of cleaning, finishing and surface preparation machinery. “It enlarges the surface area by creating tiny little peaks and valleys, if you will. And it also serves as a cleaning process, removing surface oxidation and contaminants that might interfere with a strong bond.”
Titanium plasma-spray coating units. Photo courtesy of DOT GmbH. |
It may be simple, but a slipshod execution could have devastating consequences. “In thermal spraying processing [plasma spraying], you are relying largely on mechanical bonds, so that surface preparation becomes a critical step,” he said. “If you have a variation in the substrate condition, then no matter how good your coating operation is, you will have places where the coating is weak.”
The importance of consistency in quality and repeatability has led many manufacturers to switch to automated blasting units. Carson estimated that 90% of his new blasting customers are shops looking to upgrade from manual to automated tooling.
Other Surface Processes
Not all coating and surface treatments are geared toward enhancing implant longevity. Many companies provide ways to add color to an instrument or component, for aesthetic value or to help surgeons and support staff maintain organization in the operating room. In other cases, a surface coating may enhance instrument performance or allow component parts to function more harmoniously.
Such is the niche of companies including Hudson, MA-based Boyd Coatings Research, which specializes in fluoropolymer coatings (probably the most well known of which is Teflon). These non-stick, non-thrombogenic, electrically insulated coatings have many orthopedic applications.
As minimally invasive surgeries become more popular, instrument components become smaller and more tightly packed together, noted Donald M. Garcia, president of Boyd Coatings. “If you have an instrument designed with intricate moving parts and tight tolerances, you are going to need some kind of lubrication, and it has to be a dry film because you’re not going to put oil on an item designed to be inserted in a patient,” he said. When coated with the right fluoropolymer, “the moving components interact with one another without binding up or just not functioning,” he added. Also, the non-thrombogenic properties of the coatings make them ideal for tubes designed to snake through veins and arteries.
Additionally, fluoropolymers are integral in keeping electronic components functioning properly. “Scopes and things of that nature are becoming smaller, and that means smaller electronics. So the properties of the coatings are more important, because whether it is a conductor or an insulator, these tightly packed electrical components have got to remain separated from each another.”
Another common fluoropolymer application is colorizing instruments. Color-coded instruments help surgeons keep track of items while in use and where they should be placed afterward. In the past, such colorization often was achieved with inks and dyes, but as hospitals continue to seek efficiencies in the autoclaving processes, cleaning solutions have grown more aggressive. Hence, surgeons are finding that higher-quality coatings last longer. “Our product does not chip, and it stands up to these autoclaving and new sterilizing solutions much more effectively,” Garcia said.
Other coatings aim to alleviate more short-term problems, such as allergic reactions. For example, knee implants made from cobalt-chrome contain traces of nickel. Compared to other implants, the articulating surfaces of knees are very large, which means greater risk of metal ion release into surrounding tissue. If the patient is nickel sensitive, an allergic reaction could result. DOT offers a ceramic PVD coating of Titanium Niobium Nitride, which seals the whole surface of cobalt-chrome, preventing the release of nickel ions, according to Dieter Pfliegensdorfer, public and customer relations manager at DOT GmbH. “And, at the same time, it reduces wear.”
There is growing concern in the orthopedic field regarding patient sensitivity to metals and the components of bone cement. Patch testing indicates hypersensitivity to nickel might be as high as 13% in the general population and possibly as high as 20% among women. However, it is unclear how patch testing results correlate with in-vivo reactions to cobalt-chrome implants and nickel ions.
What Works?
This article only scratches at the surface of all the questions, options, concerns, debates and controversies surrounding coatings and surface treatment in TJRs. Volumes could be written on plasma spraying alone. Ongoing areas of research include surface texturing achieved by etching and anodization, both of which aim for better bone growth and adhesion; various surface drugs, such as antimicrobials; and bone growth factors. There is even renewed debate about whether the current industry standard, cementless implants, are all they are cracked up to be—a matter thought to have been settled 20 years ago.
Ultimately, the timelines involved preclude certain knowledge of the how well current coating treatments will perform. Validating improvement in a technology already known to last a quarter century requires an enormous dedication to follow-up study, a task perhaps only achievable by future generations.
But one certainty stands out: The human body does not welcome long-term hosting of metal joints. Metal and metal alloys rust, corrode, produce dangerous particles and are too stiff, heavy and dense. This was the fundamental genesis for inventing coatings in the first place.
Thus, to discuss coatings and surface treatments is, in essence, to discuss materials. For example, porous titanium structures—not porous on the surface, but porous throughout the item’s interior, like coral—rate high among current research trends. This configuration offers lighter weight, increased flexibility and an internal framework closer to that of natural bone. “Materials that replicate the natural anatomy are going to have advantages,” observed Joseph Benevenia, MD, a professor of orthopedics at the New Jersey Medical School in Newark.
Some believe eschewing the high-tech in favor of more natural processes may be the key to solving the longevity problem. “Generally, we see a tendency to substitute cost-intensive technologies, such as plasma spray technologies, with more nature-related technologies—eg, the biomimetic BONIT coating—that are cost saving,” noted Pfliegensdorfer, “which is especially important for marketing products in the emerging economies.”
Better still, might we slice the Gordian Knot and forgo metal altogether? Research teams are attempting to create artificial bone-like materials by combining organic and inorganic elements in a laboratory. Natural bone is composed of the organic (collagen) and the inorganic (dahilite appetite). Investigators have used organic scaffolds of hydrogel polymers to template the mineralization of HA in a way similar to natural bone. The opposite approach, fabricating porous ceramic scaffolds as the base for growing organic composites, also has been studied.1
Both have been slow to achieve much practical success. Duplicating millions of years of evolution in a Petri dish is no simple affair. Nevertheless, payoffs could be substantial. “I welcome these things,” noted Dr. Benevenia. “These are wonderful things. But where we are most likely to see a breakthrough in this technology is probably in applications that cannot be achieved with current implants, such as growing cartilage cells, for example.”
With so much at stake, it is difficult to conceive of a future wherein the TJR longevity conundrum goes unsolved—assuming it has not been solved already. As always in science, today’s misstep provides the knowledge for tomorrow’s success. “The development of coatings and materials is intimately related, and the lessons learned in one field can be applied in the other,” maintains one study, urging an interdisciplinary approach.1 “The end result will not only be better biomaterials, but a better understanding of the natural tissues, and lessons that can be applied to the fabrication of materials for many other applications.”
SIDEBAR:
A Greener Cleaner: The Value of Modern-Day Ultrasonic Processes
Back in the 1980s, when opinion among Western democracies on environmental issues was less divided, an international treaty called the Montreal Protocol was put into effect. Designed to protect the ozone layer, the protocol severely regulated chlorinated solvents, making cleaners such as Freon no longer as viable in a manufacturing setting.
These regulations essentially gave birth to the modern-day ultrasonics industry, which stepped in to replace chlorinated solvents in industries where scouring must get down to a molecular level—industries including medical device manufacturing.
“You might say what we do is a greener process,” said Steve H. Myers, sales manager at Freeport, IL-based Ultrasonic Power Corp., which provides ultrasonic cleaning tooling for orthopedic manufacturers, “because we use water based and biodegradable detergents.”
In addition to being more environmentally friendly, ultrasonics—when applied correctly— arguably are more effective than previous cleaning methods.
The mechanism of action in ultrasonic cleaning is called cavitation. This occurs when a component is submerged in tank full of liquid cleaning solution and is bombarded with ultrasonic sound waves produced by what is known as a transducer. The sound waves hit the object in alternating patterns, which create tiny microjet implosions that penetrate to areas unreachable by other methods. Different frequencies produce varying degrees of cleaning intensity.
“The thing that is really great about this technology is that it is so versatile and flexible,” noted Cheryl Larkin, director of marketing at Miraclean, based in Ashville, NY, explaining why ultrasonics are so popular among orthopedic manufacturers. “You can usually improve or expedite a cleaning or passivating process by adding ultrasonic tanks, for example. There may be further gains achieved by putting different frequencies in different tanks or by optimizing the fixturing of the parts. There are a variety of ways to produce very positive outcomes."
Frank Celia is a freelance healthcare writer based in the Philadelphia area.
Reference:1. Tomsia A, et al. Biomimetic Boneline Composites and Novel Bioactive Coatings. Advanced Engineering Materials. 2005, 7, No.11:999-1004.