Nanodiamond coatings for artificial joints

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2012

A new study evaluates the response of macrophages to nanodiamond particles of various sizes.

KEY CONCEPTS
• A need exists for better artificial joints to minimize friction, wear and the formation of wear-debris particles.
• A nanodiamond coating based on the alloy Ti6Al4V shows comparable performance in a standard wear test and generates fewer particles with smaller particle sizes than the materials commonly used in artificial joints.
• No effect was observed when nanodiamond particles at the concentrations expected to be produced in the human body were exposed to macrophages.

THE FRICTION AND WEAR THAT DEVELOPS BETWEEN JOINTS IN THE HUMAN BODY IS becoming increasingly important as populations age. At some point, joints such as knees and hips become so worn that individuals need to have them replaced with prosthetics.

Most of us who are involved with STLE have friends who have undergone procedures to replace these joints. In fact, more than 418,000 knee replacements and 328,000 hip replacements are performed in the U.S. each year, according to the American Academy of Orthopedic Sciences (see Figure 1).


Figure 1. The increasing number of knee replacements has created a need for improved artificial joints that will generate lower levels of wear-debris particles and, as a result, minimize the amount of pain. Nanodiamond coatings show great promise as a potential coating for a titanium alloy to provide better performance with less discomfort. (Courtesy of the University of Alabama at Birmingham)

The human body has developed several means for providing lubricity between joints. In a previous TLT article, research was discussed that indicates a glycoprotein known as lubricin acts as a lubricant to reduce the friction and wear (1). It appears that lubricin is able to function between hydrophobic and hydrophilic interfaces. 

One of the concerns with using artificial joints is premature wear. Yogesh Vohra, professor of physics at the University of Alabama at Birmingham in Birmingham, Ala., says, “The wear is caused by the interaction of the two most common materials used in hip and knee replacements, which are cobalt- chromium-molybdenum (CoCr- Mo) alloy and polyethylene.”

One added problem is the generation of wear-debris particles that can become situated in fibrous tissue will activate the human body’s immune system to try and eliminate them. Vohra says, “Macrophages are human cells present in tissues that destroy large debris particles with diameters between 1 micron and 3 microns and smaller particles with diameters between 40 and 50 nanometers. But the problem arises that proteins called cytokines may be released during this process which will cause inflammation, leading to pain.”

Vohra indicates that the source of the larger particles is polyethylene while most of the smaller particles originate from the CoCrMo alloy. He adds, “Predicting the degree of wear particles generated for any specific artificial joint is complicated. It depends upon the surface finish and the load. Wear rates ranging from a few microns per year to 100 microns per year can occur.”

The use of nanodiamond coatings has been found to demonstrate less wear and be more suitable for use on artificial joints. Vohra says, “Published research has shown that the wear rate of nanodiamond coatings versus polyethylene dropped by a factor of 2 to 3 as compared to CoCrMo.”

In a past study by the researchers, nanodiamond coated on the alloy Ti6Al4V was evaluated against polyethylene versus CoCrMo through the use of the pin-on-disk wear test (2). Testing was done by applying sterile-filtered defined bovine calf serum at human body temperature to the two surfaces.

The procedure used was ASTM F732, which is a wear test that evaluates polymeric materials used in artificial joints. Each test was conducted for 2 million cycles or an approximate sliding distance of 80 kilometers. The results show comparable performance for both metal alloys.

Nanodiamond coatings, ranging from a thickness of 4 microns up to 30 microns, have shown the potential for the formation of fewer wear particles with smaller particle sizes than for CoCrMo or polyethylene. But there is still concern that the particles formed may lead to similar problems that are seen with the currently used artificial joint materials.

A new study has now been done to evaluate the response of macrophages to nanodiamond particles of various sizes.

IN VITRO STUDY
Vohra and his fellow researchers exposed macrophages to nanodiamond particles ranging in size from 6 to 500 nanometers at concentrations between 0 and 200 micrograms per milliliter in an in vitro study. Dr. Vinoy Thomas, the study’s first author says, “We made nanoparticle solutions through sonication in a phosphate-buffered saline medium. After incubation with 10% human serum at 4 C, the nanodiamonds were added to a solution of the macrophages. Further incubation was maintained for 24 or 48 hours followed by an apoptosis and quantitative gene expression analysis.”

The researchers found that at concentrations where nanodiamond particles are expected to be present (below 50 micrograms per milliliter), there was no effect on the macrophages. This means that the nanodiamond particles are probably not toxic to macrophages.

Gene-expression studies suggest that exposure to nanodiamonds leads to the release of fewer inflammatory chemicals by the macrophages, causing less discomfort. Thomas says, “While we saw some negative effect from nanodiamond particles at a concentration of 200 micrograms per milliliter, we believe that nanodiamond coating can be used in artificial joints because no effect is seen below 50 micrograms per milliliter, the particle concentrations at which they are typically detected as debris in the human body.”

Future work will involve evaluating the interaction of two surfaces, each containing nanodiamond coatings and the interaction of a nanodiamond coated surface with polyethylene. Vohra and his team are hopeful that nanodiamond coatings can be used as an alternative to the commonly used materials in artificial joints. He says, “We hope the reduced wear volume and particle size expected for diamond articulation will represent a major advance over conventional orthopedic bearings.”

Additional information on this work can be found in a recent article (3) or by contacting Vohra at ykvohra@uab.edu.

REFERENCES
1. Canter, N. (2008), “Glycoprotein: The Key to Lubricating Joints,” TLT, 64 (2), pp 12-13.
2. Hill, M., Catledge, S., Konovalov, V., Clem, W., Chowdhury, S., Etheridge, B., Stanishevsky, A., Lemons, J., Vohra, Y. and Eberhardt, A., “Preliminary Tribological Evaluation of Nanostructured Diamond Coatings Against Ultra-High Molecular Weight Polyethylene,” Journal of Biomedical Materials Research Part B: Applied Biomaterials, 85B (1), pp 140-148.
3. Thomas, V., Halloran, B., Ambalavanan, N., Catledge, S. and Vohra, Y. (2012), “In vitro Studies on the Effect of Particle Size on Macrophage Responses to Nanodiamond Wear Debris, Acta Biomaterialia, 7 (5), pp. 1939-1947.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.