Spring remedy for knee troubles

Australian medics and engineers have devised an ingenious cruciate ligament replacement. Julia Hinde reports.

A revolutionary design by Melbourne University engineers and doctors will take the pain and long-term trauma out of one of the most common knee operations, and could see sportsmen and women back on the field in days, rather than months.

Thousands of cruciate ligament replacement operations are carried out each year but the replacements - which require a lengthy operation and weeks of physiotherapy to recover - rarely perform as well as the original cruciate and many do not last.

The engineers and medics have devised a replacement that not only better mimics the cruciate ligament and will therefore last longer, but can also be replaced time and again with little trauma.

Currently, torn cruciate ligaments, which act to keep the knee in place, are replaced either using a piece of plastic nailed to the upper and lower leg bones, or with a piece of ligament, normally taken from the Achilles tendon. Neither mirrors the workings of the cruciate effectively and both tend to waste away or break quickly.

To improve the operation, two Melbourne orthopaedic surgeons thought of incorporating a spring in the replacement ligament and took their idea to Andrew Samuel, reader in the university's department of engineering.

Dr Samuel said: "They envisaged a system that was stiff in the section in the bone and soft, and hence bendy, in the middle. They wanted a replacement that would last a while but then, if it went, the patient could come in for day surgery and have it replaced again."

A decade later, the team has designed a new component, including a spring section, that enables considerable extension at low loading, and a plastic component that is stiffer, giving the replacement strength, but also some ability to bend at high loading.

Dr Samuel said: "We have tested the plastic and spring for one million knee cycles under heavy loading in the lab. This now needs to be tested in animals."

The new design involves a permanent cone drilled into the upper and lower leg bones, with the plastic and spring components strung between them. The original insertions of the cones would involve major surgery, but then the moving components - the spring and plastic - could be replaced whenever they failed.

"No one before has come up with a variable stiffness system for an artificial cruciate ligament," said Dr Samuel. "We estimate the structure should last four or five times as long as current replacements, but the beauty is that we have something that can then be replaced with day surgery. It makes it a much more attractive option than what is now available."

Dr Samuel estimated the design could be in use for humans in five years. He said the work has been driven by "Mr and Mrs Average", not by the elite sports market. "Any person with a damaged cruciate has a strong likelihood of developing early osteoarthritis, with the eventual need for a full knee operation. An effective cruciate ligament replacement may improve their chances of avoiding this."