New plastic gives patients leg up

By LOUISE THOMAS and KATHERINE HOBY

Two New Zealanders are helping introduce a new type of hard-wearing plastic which is being tested in hip-replacement surgery throughout the world.

The technology could eventually help New Zealanders who have hip replacements predict when their new hip will wear out and how it will fail.

Assessment of the new plastic will use a monitoring technique developed in New Zealand by Peter Devane and Professor Geoffrey Horne at Otago University's Wellington School of Medicine.

The technique allows orthopaedic surgeons to use a computer to measure wear with an accuracy of 0.13mm or better.

It means surgeons can take measurements over the first three to four years to predict how long the patient's hip will take to wear out, when and how it will fail and allow them to reoperate at the best time.

The three-dimensional monitoring technique, known as PolyWare, involves measuring the wear of the plastic in the artificial joint through x-rays.

The x-rays are monitored by a computer program, which accurately measures changes.

The program takes into account patient variables such as obesity and leg position, image variables such as film, cassette type and sharpness, and errors inherent in the PolyWare analytical process.

The technique has been evaluated in Canada and the United States, and Mr Devane says PolyWare is accepted as being the most accurate measurement of plastic wear available.

In the United States, the Food and Drug Administration has a condition that any new material used in total hip replacements must have ongoing monitoring through the method that Mr Devane and Professor Horne have developed.

PolyWare is currently used by about 35 major university centres in the US, Canada, Britain and Europe.

Mr Devane said about 4500 hip replacements were done in New Zealand annually, and up to a million worldwide.

Total hip-replacement surgery involves sticking a metal pin with a ball attached into the middle of the thigh bone. The metal head then fits into a plastic socket attached to the pelvis.

The plastic socket wears away over time.

"Every time you walk on it you grind up little wee microscopic piece off the plastic," said Mr Devane.

"Over 10 years or so, these little wee pieces can cause the prosthesis to fail."

In the past, polyethylene wear of a total hip replacement was measured on x-rays using crude instruments such as callipers, compasses and rulers.

The average wear being measured was in the order of 0.15mm a year, but the accuracy of the technique was plus or minus 0.5mm.

Mr Devane said the new type of polyethylene had performed well in laboratory tests, where sockets from conventional and new plastic had been worn down mechanically and the results compared.

He said the trials were promising and the new plastic was being used in patients with total hip replacements.

It was hoped to establish within four years whether improved wear properties in the laboratory also applied in the patient.

"From the surgeon's point of view, the plastic is exactly the same looking, feeling and touching as the conventional type of plastic, so there are no big changes in terms of doing the operation," he said.

"But the difference will come in how the replacement will perform in 10 years versus the conventional type of plastics."

The PolyWare system meant countries taking part in the international trials could accurately monitor the new replacements rather than just waiting for them to fail, he said

The New Zealand Orthopaedics Association has been compiling a record of every joint replacement in the country for the last three years. Mr Devane hopes it will help monitor how well the new technology works.

The new plastic is also being used for knee-joint replacements.

Funding for the development of PolyWare has come from the Lottery Grants Board and the New Zealand Orthopaedics Association Wishbone Trust.