Hip forever? Graphitic lubricant for metal-on-metal implants

Materials World magazine
5 Feb 2012
hip implant

Hip implants that last a lifetime could soon be within reach after researchers found a lubricating layer that forms on metal-on-metal implants.

Due to the friction involved in joint movement, scientists say that a reaction occurs between implants made from cobaltchrome-molybdenum alloy and the body’s biological tissue, or more specifically, the protein in the surrounding fluid. This results in a layer of graphitic carbon that acts as a lubricant.

Professor Laurence Marks, from Northwestern University, based just outside Chicago, USA, says: ‘There is a graphitic carbon layer that forms inside the human body, and this plays a major role in the lubrication of the implants in vivo and reduces the corrosion.’

The research is significant, says Marks, because it fills a crucial knowledge gap in the understanding of how hip implants work. ‘This is the first step. Now, there are lots of different ways we can think about making the graphitic material adhere better to the metal.’

The layer itself is a byproduct of the body and the implant reacting together. ‘The carbon comes from the body, but it is what is taking place at the surface of the metal that produces the graphitic material. Graphite is one of the best known solid lubricants in existence, and it has been manufactured commercially for that purpose for more than 100 years. It’s a classic lubricant that works even in water.’

The team happened upon the discovery when investigating whether there was carbon at the grain boundaries of the surface of implant samples received from patients. ‘We thought there might be carbon getting involved in the grain boundaries and playing a role in the wear of these materials. When we saw evidence for graphitic carbon, it was a surprise, and we had to do a lot more work to make sure it was real.’

To pin down what was causing the graphitic layer to occur, the team ran through a process of elimination, using electron and optical microscopes, and a number of other analytical tools to confirm their findings. ‘If you take electrons and shine them through a sample and measure how much energy they have lost there is a characteristic spectroscopic peak that is associated with graphitic carbon. It actually shows that you have pi-bonding.’

Marks also had to make sure the layer wasn’t being produced by the method used to make the sample. ‘We looked at a model and did similar spectroscopic analysis, and you don’t see anything graphitic. We did tests to make sure the electron beam was not changing things and used Raman spectroscopy as an independent tool.’ He cites the Edisonian approach, which he points out was much more than just trial and error. ‘People think that Edison just tried lots of things, when in fact he didn’t – he used every piece of information he could possibly get to make devices better. He wasn’t just blindly trying different things.’

Although hip implants can be the ‘difference between a wheelchair and being able to walk’, they are still an imperfect technology, according to Marks.

Many implants can last longer than their prescribed 10-year lifespan, but some can fail earlier, particularly those in younger, more active patients.With better understanding of how the implants function, Marks claims the path is open for the development of implants with much longer lifespans. ‘We want these devices to last 30 years. And if you can improve the lubrication and reduce the wear you can only make them better.’

Although prosthetic joints can be made from a number of different materials, including polymers and ceramics, Marks added that the finding only applies to metal-on-metal implants. He adds that more research will be needed to ascertain what effect, if any, the graphitic carbon has on the body.  


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