While efforts are afoot at the University of Alabama Birmingham (UAB) to develop nanodiamond coatings for use on hip and knee implants, Vista Engineering at Birmingham’s Innovation Depot technology incubator is close to completing studies to employ the coating on implants for treating temporomandibular joint disease (TMJD). Resulting in chronic pain and disability, TMJD is the second-most common musculoskeletal condition in the United States. It affects up to 37,000,000 people, approximately 50 to 67% of whom seek treatment and approximately 15% of whom develop chronic TMJD, according to the national Institute of Dental and Craniofacial Research of the National Institutes of Health.
In an effort to improve the treatment of TMJD, Vista Engineering, with the assistance of Yogesh Vohra’s team at the UAB Center for Nanoscale Materials and Biointegration, are developing and testing a prototype total replacement prosthesis featuring a diamond-on-diamond wear surface. The company’s goal is to replace current-generation metal-on-metal implants and implants combining both metal and ultra-high-molecular-weight polyethylene with diamond-coated technologies. To that end, it relies on the use of a microwave plasma reactor for depositing the nanodiamond coating on metal substrates.
Birmingham is the site of the two largest microwave plasma reactors in the United States: a 6-kW system in Vohra’s laboratory and a 30-kW system installed at Vista Engineering. “Whatever small-scale laboratory ideas or innovative chemistries we come up with for use in the reactors can be transferred to manufacturing,” Vohra comments. “Thus, we develop chemistries at UAB and transfer them to Vista.”
In the following Q&A, Raymond G. Thompson, manager and engineering expert at Vista Engineering, explains how the nanodiamond coating is created and how development efforts to produce diamond-coated TMJD implants could contribute to the development of future orthopedic implants. —Bob Michaels
MPMN: Why diamonds and not some other material?
Thompson: Our technology is going in the direction of developing diamond-on-diamond surfaces. By putting a diamond coating on materials used for orthopedic and dental implants, we are able to eliminate what a lot of other people must do to achieve the required wear resistance. Common materials used today for this purpose include cobalt-chromium-molybdenum and steel. As implant materials, these alloys have many good properties, including high wear resistance. In hip and knee implant applications, they rub up against other materials, such as high-density polyethylene.
In contrast, we are working on implants to treat TMJD. However, TMJD implants have had a troubled history of not working properly. The first big problem is that implants made by Vitek used Teflon pads, which caused a severe reaction with bone tissues, resulting in extreme pain and thinning of the bone leading to the brain.
The diamond-on-diamond concept originated with Patrick Louis, professor and director of the Oral and Maxillofacial Residency Training Program at UAB, and Jack Lemons, professor of dentistry in the department of prosthodontics at the UAB School of Dentistry. Unlike the problems associated with previous-generation TMJD implants, their diamond-on-diamond coating technology enables the fabrication of a very compact implant. As a result, the incision can be half the size of incisions required using older technologies, reducing the probability of running into a nerve and causing facial-muscle problems. Louis and Lemons developed the shape of the implant and the diamond coating. This coating technology enabled the development of the new implant design, and the new design will enable doctors to perform new clinical procedures.
MPMN: What role do gases play in creating diamond coatings on metal substrates?
Thompson: When I was in school, we were told that you can’t make diamonds from gas, from the vapor phase. We make diamonds from methane gas. By the time I was grown up and even when I was a member of the faculty, people thought that it was impossible to make diamonds from gas; you always needed to have the crucible, the earth, the temperature, the pressure. That’s how artificial diamonds are manufactured. It wasn’t until the 1980s that General Electric came up with the clever discovery that you can actually take carbon and build it into a diamond structure under the gas phase. It’s so much easier to build diamonds that way.
It turns out that this gas method is a pretty slick process. Our growth rates are about a millionth of a meter an hour. In contrast, three- to five-carat diamonds, which are pretty large, are grown at a rate of a millionth of an inch an hour.
MPMN: Where does nitrogen come in?
Thompson: The technology that Yogesh Vohra developed was a variation on what General Electronics had done. Vohra’s technology involves adding an impurity—nitrogen—to the methane gas to achieve a smooth diamond surface. The short part of the story is that the addition of nitrogen causes a defect center that makes the diamond smooth. The GE diamonds are very angular and bold—similar to gemstone diamonds. Because our diamonds do not have the angularity associated with previous diamond-manufacturing techniques, they are good for specific applications. For example, they are useful in applications in which surfaces rub up against each other. Because of their smooth, hard surface, diamonds, when rubbed against other diamonds, actually get smoother. They polish themselves, and the longer they do this, the better they get. No other material that we know of can do this.
MPMN: How do you use the microwave plasma reactor to deposit the diamond coating?
Thompson: The diamond coating is produced in the microwave plasma reactor at 1400°F. Because this is a red-hot environment, the coating cannot be applied to plastics. Thus, we apply it to metals and ceramics.
First, we establish a vacuum environment in the reactor chamber, and then we flush in hydrogen. At that point, we start an arc using a high-frequency current. Formed just like a neon light, this arc causes a plasma to form, which lights up like a light bulb. We use hydrogen because when you place hydrogen on metal, it keeps the metal clean. If there is any residual oxygen in the chamber, the hydrogen keeps the oxygen off the metal.
Next, we bring in the methane gas. Because methane gas consists of hydrogen and carbon, its use enables us to bring carbon into the system. When carbon gets into the plasma, the methane gas breaks down from a hydrocarbon into hydrogen and carbon atoms. Like dew on leaves, the carbon atoms condense on the metal, making it cold so that the hot gases will want to condense on it. Carbon can produce graphite, as in a lead pencil; it can make soot, as in your fireplace; or it can make diamonds. But to make diamonds, we have to convince the carbon atoms when they sit down that they want to sit down precisely in the diamond structure of the material.
There are certain metals that allow that process to take place. One of them is titanium, which is also a good implant material. When carbon atoms react with titanium, they form a metallurgical bond. The carbon atoms react with the titanium to form titanium carbide and actually become part of the surface structure. The titanium carbide reacts with the carbon to form a carbon structure, forming a diamond surface on top of the titanium carbide. The resulting product is not just something deposited on a surface without being connected to the base metal. Thus, this technology is not a deposition process, but a growth process. You can’t peel this coating off and find the separation point.
In the TMJD implant, the diamond coating is applied onto a metal substrate such as titanium that is polished prior to the deposition process. The advantage of our technology is that by growing the diamond coating, we eliminate metal-on-metal wear, eliminating the metalosis problem because it prevents metal ions from entering the body. Based on our wear tests, our expectation is that the diamond-on-diamond coating will last most of a human lifetime without getting close to wearing out. If the coating were to release any material, it would be carbon—not a problem for the body since the body is a carbon machine.
MPMN: What types of tests do you perform to determine the viability of the diamond coating?
Thompson: Our test procedures rely on the use of a wear simulator, which can be employed to test an array of different artificial joints, including hips and knees. At present, we are calibrating it specifically to test TMJD implants. The machine provides many different degrees of freedom of motion, including rotation and cantellation. We are now working out the forces and displacements that we need to simulate the TMJ joint. Once our simulator is set up, we will compare implants coated with our diamond coating with other types of implants to determine the coating’s wear properties. Our goal is to simulate 10 years of use from the implant. Although diamond-coated implants haven’t been implanted in humans yet, they have done very well in animal studies.
After we have completed our wear simulations, we will be working with a team at the University of Alabama that will perform finite element modeling in which we will be able to model the stress at every point on the diamond surface. We already know where the stress points and locations of highest stress will be and have produced prototypes based on this modeling. But after our wear tests, finite element modeling will resume to determine if the stress points have changed.
Any foreign particles, including those from diamonds, are going to cause some sort of tissue response. Our goal is to produce much less debris than implants currently in use. Thus, our hope in performing wear simulations is to show that the diamond coating will produce much less wear than other technologies.