Making a new hip last longer

A Cork engineering student is helping surgeons improve the success of artificial hip joint operations, writes Dick Ahlstrom

Her novel research into a key cause of joint failure has also won her a trip to London next month, and the right to represent Ireland in the annual Institution of Mechanical Engineers' Competition.

Niamh Thompson, a final year degree student in Cork Institute of Technology's Department of Mechanical and Manufacturing Engineering, has already claimed the national engineering award for the best mechanical engineering degree project. The competition involves all third-level institutions with mechanical engineering degree programmes in the Republic. Thompson now goes forward to the international final, competing against the best from Northern Ireland, Wales, England and Scotland.

Her focus is the cement used to lock replacement hip joints into position at the top of the leg. "This is the first time this particular research has been conducted," she says.

Hip joint replacement has become commonplace, with up to a million procedures a year taking place in hospitals around the world. A number of these will fail after a time, forcing more surgery to carry out repairs. Up to 80 per cent of these failures are caused by loosening, either through infection or the failure of the bone cement, says Thompson.

Surgeons from Cork University Hospital and St Mary's Orthopaedic Hospital approached CIT, looking for research into one possible cause for hip replacement failure.

"They had a concern that hydrogen peroxide used during surgery was having an effect on the hip joint cement," says Thompson.

Surgeons flush the top of the leg bone, the femur, with hydrogen peroxide solution before fixing the metal hip joint replacement in place using bone cement. The assumption was that the solution reacted either with the bone or the cement to increase the risk of later joint failure.

Thompson was looking for a fourth-year research project and had developed an interest in biomedical engineering after a stint working with local artificial knee and hip-joint manufacturer, Stryker Howmedica Osteonics. She readily volunteered to pursue this research challenge.

"The objective of this project is to see how the solution is affecting the bone cement," she explains.

She had to devise ways to test the hydrogen peroxide theory, not just looking at physical changes but also at how the solution might speed up failure due to cracking and fatigue under pressure.

She created moulds to form segments of hardened bone cement. The moulds were flushed with the solution before being filled with cement. This simulates what actually happens with the femur before the bone cement is applied.

"It is actually only the outside of the cement that gets contaminated."

These cement segments are now being tested under mechanical load after exposure to varying concentrations of hydrogen peroxide solution. They are tested to failure to establish the influence of the solution on cement breakdown.

The segments also undergo microscopic analysis, looking for "crack growth propagation" that works down into the cement from the surface. "The hydrogen peroxide we believe is causing pores, increasing the porosity of the cement. The more pores, the greater the risk of failure," Thompson says.

She has a good working hypothesis of why these pores form. The solution produces an amount of frothing in the bone as the chemical reacts with proteins in the blood. "It is this air in the socket which is causing inclusions in the bone cement. These air gaps cause the bone cement to fail."

Much remains to be done. "A lot of the work has been in designing the moulds and in testing the segments correctly," she says. "The testing is still very much in progress. Hopefully I will know by the time I finish up in March if the solution is affecting the cement."

Thompson has always been interested in mechanical engineering. "When everyone else was playing with Barbies I was playing with Meccano," she says. "I have always loved hands-on stuff. That was always the area I was most interested in."