Case study: Extending the life of hip implants

Orthopaedic hip implant

Orthopaedic hip implant
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By studying stress, researchers from Heriot Watt University and ISIS could extend the life of orthopaedic implants.

  • Orthopaedic implants are often made using titanium coated in a thin layer of hydroxyapatite. This coating process causes stress within the implant, which could cause the implant to fail.
  • By using neutrons to look at these implants, researchers were able to collect experimental data that will be used to develop a computer model.
  • This model can be used by the implant industry to monitor the quality of hydroxyapatite coatings.

Over 50,000 hip replacements are performed in Britain each year, and the number is suspected to increase to over 96,000 by 2030.

Implants are commonly made using titanium coated in a thin layer of hydroxyapatite, which is the main constituent of human bone. The coating encourages bone growth into the implant without breaking down, and bonds the bone to the implant holding it in place.

Thermal spraying is the dominant commercial technique used to coat the implants. The hydroxyapatite coating is typically 220 micrometres thick. Due to differences in the thermal and mechanical properties of titanium and hydroxyapatite at the interface between the two materials, residual stresses form in the coating after spraying. 

Dr Rehan Ahmed and his team from Heriot-Watt University in Edinburgh are using neutron beams to investigate hip implant coatings, collecting vital stress measurements to determine how this relates to implant failure.

Research into the lifespan of medical components is becoming increasingly important in the UK and worldwide, as average life expectancy continues to rise.

“We are developing a stress profile model that can be used by the orthopaedic implant industry to monitor the quality of the hydroxyapatite coating,” explained Dr Ahmed. “The experimental data we collected at ISIS is vital for validation of our model.”

 “We could have used X-rays which penetrate only the near surface coating layer,” said Dr Ahmed. “However, then we would have had to use a layer-removal technique electro-polishing or grinding off material in order to be able to penetrate deeper into the sample. The problem is, every time you remove material, you change the stress profile,  which is difficult to compensate during stress measurement.”

In contrast, neutron beams are highly penetrating and allow researchers to study both the hydroxyapatite coating and the titanium implant substrate simultaneously without the need to remove material from the implant.

At ISIS, a specialised instrument called Engin-X is available to engineers for accurate study of microscopic stresses in complex engineering components. The instrument supports a wide range of sample environments allowing researchers to study the effect of treatment techniques on the stress profile of their sample.

Dr Ahmed and his team were able to compare the change in residual stress profile in the implant coating before and after heat treatment.  

 “Up to 18% of implants fail, for a number of reasons, including stress” explained Dr Ahmed. “If we are going to be able to provide long-lasting implants, then it is critical to have experimental data to back up our computer models,” said Dr Ahmed. “Engin-X is able to provide us with the unique high-resolution information we need.”

Beth Penrose, Rehan Ahmed, Martyn Bull

Research date: January 2010

Further Information

Contact: Dr R Ahmed, R.Ahmed@hw.ac.uk

Further reading: R Ahmed et al., Mater Sci Forum 652 (2010) 309

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