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Better Lubricant Leads to Longer-Lasting Bone Implants

One of the greatest problems with prosthetic parts, is that artificial parts don't self-repair, at least at our current level of technology. This means that in the case of non-essential prosthetics such as bones, a patient usually outlives their prosthesis. Worse, in the case of an essential prosthesis such as a heart or liver, the life of the prosthesis is the limiting factor on the life of the patient If an artificial heart is only going to last eight or nine years, then the patient is likely to only live eight or nine years.

We need to find ways to extend the life of the prosthesis, from ten years to twenty, fifty, or ideally a hundred years – longer than the patient themselves. In that way, the length of the prosthetics' lifespan is not the limiting factor on the lifespan of the patient. That way, replacements only occur
when the original is damaged, as opposed to a finite lifespan dictating replacement – if the patient or health service can afford another new one every decade.

We don't have methods to increase the lifespan of all types of prosthetic yet, but we are making progress on several types. In a decisive leap forwards, bone implants – such as artificial hips, knees, wrists or elbows – have become much more sturdy and long-lasting, by identifying what causes wear and tear on titanium joints, and identifying new materials to greatly reduce that wear and tear. Less wear and tear means a far longer operational life.

Researchers from Northwestern University, Rush University Medical Center, Chicago, and the University of Duisburg-Essen Germany have discovered that graphitic carbon is a key element in a lubricating layer that forms on metal-on-metal hip implants. The lubricant is bears a closer resemblance to the lubrication of a combustion engine than that of a natural joint.

However, this makes sense, since we are not talking about natural joints here, but artificial ones, that perhaps have to be lubricated in ways similar to other mechanical joints rather than organic ones as was previously attempted.

“Metal-on-metal implants can vastly improve people’s lives, but it’s an imperfect technology,” said Laurence D. Marks, a professor of materials science and engineering at the McCormick School of Engineering and Applied Science, and a co-author on the paper who led the experimental effort at Northwestern University. “Now that we are starting to understand how lubrication of these implants works in the body, we have a target for how to make the devices better.”

“Hip replacement surgery is the greatest advancement in the treatment of end-stage arthritis in the last century,” said co-author and principal investigator Dr. Joshua J. Jacobs, the William A. Hark, M.D./Susanne G. Swift Professor of Orthopedic Surgery and professor and chair of the department of orthopedic surgery at Rush. “By the time patients get to me, most of them are disabled. Life is unpleasant. They have trouble working, playing with their grandchildren or walking down the street. Our findings will help push the field forward by providing a target to improve the performance of hip replacements. That’s very exciting to me.”

Earlier research by team members Alfons Fischer at the University of Duisburg-Essen and Markus Wimmer at Rush University Medical Center discovered that a lubricating layer forms on metallic joints as a result of friction. Once formed, the layer reduces friction as well as wear and corrosion. This layer is called a tribological layer and is where the sliding takes place, much like how an ice skate slides not on the ice but on a thin layer of water.

But, until now, researchers did not know what the layer was. (It forms on the surfaces of both the ball and the socket.) It had been assumed that the layer was made of proteins or something similar in the body that got into the joint and adhered to the implant’s surfaces.

The interdisciplinary team studied seven implants that were retrieved from patients for a variety of reasons. The researchers used a number of analytical tools, including electron and optical microscopies, to study the tribological layer that formed on the metal parts.

The electron-energy loss spectra, a method of examining how the atoms are bonded, showed a well-known fingerprint of graphitic carbon. This, together with other evidence, led the researchers to conclude that the layer actually consists primarily of graphitic carbon, a well-established solid lubricant, not the proteins of natural joints.

“This was quite a surprise,” Marks said, “but the moment we realized what we had, all of a sudden many things started to make sense.” 

Metal-on-metal implants have advantages over other types of implants, Jacobs said. They are a lower wear alternative to metal-on-polymer devices, and they allow for larger femoral heads, which can reduce the risk of hip dislocation (one of the more common reasons for additional surgery). Metal-on-metal also is the only current option for a hip resurfacing procedure, a bone-conserving surgical alternative to total hip replacement.

“Knowing that the structure is graphitic carbon really opens up the possibility that we may be able to manipulate the system in a way to produce graphitic surfaces,” Fischer said. “We now have a target for how we can improve the performance of these devices.”


Hips that Function Better and Last Longer

"Graphitic Tribological Layers in Metal-on-Metal Hip Replacements", Science 23rd December 2011, pp 1687 - 1690

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