Knees-up for plastic fantastic

By MONIQUE DEVEREUX

A New Zealand surgeon is leading the field in implanting a new artificial knee.

Dr Mark Clatworthy, specialist knee surgeon at Middlemore Hospital, yesterday performed his eighth operation using the artificial joint since it was released worldwide a month ago.

The surgery was relayed to the hospital's academic lecture theatre, where it was watched by 52 orthopaedic surgeons from 10 Asian countries.

DePuy, the Johnson & Johnson subsidiary that developed the knee, brought the specialists to New Zealand.

Because only three sets of the surgical equipment needed to implant the artificial knee are available across Asia, it was easier to bring the surgeons to New Zealand to watch the procedure.

The difference with the new knee is a rotational plastic bearing that sits between three metal components that are screwed into the top of the shin bone, the bottom of the thigh bone and the back of the kneebone.

The bearing slots into a flat metal base that is attached to the shin bone.

It is attached by a "peg" rather than fixed to the flat plate, and can therefore rotate.

The curved metal section that is attached to the thigh bone sits comfortably in the plastic bearing, so when the leg moves, the "knee" turns and pivots much like a natural joint.

A fixed bearing allows for only a more rigid up-down movement.

Little is known about the new knee in comparison with the older - and regularly used - fixed-bearing joint.

Dr Clatworthy is about to start a study that will compare the two. About 200 patients receiving new knees over the next two years will be asked to take part.

The patient who received her new knee yesterday will be a perfect example - she already has a fixed joint in her other leg.

"It will certainly be interesting to watch the differences," said Dr Clatworthy.

"We know the concept sounds a lot better and in theory should offer better, more natural movement as well as last longer, but only time will be able to tell us that for certain," he said.

The patient is placed under general anaesthetic to implant the joint.

Once the knee is propped up at a 90-degree angle, a long cut is made to open the skin.

The skin and muscle are peeled back until the bone is cleared.

Other tissue and sinew around the bone are sliced or burned off to give the surgeon clear access.

The kneecap, usually attached to the thigh and the shin by muscles, is pushed to one side.

Once the knee joint has been properly exposed, the lower end of the thigh bone and the top of the shin are sawn off, using a flat drill.

Holes are drilled into each section and metal pins inserted for attaching the artificial sections.

The back of the kneebone is also cut away to leave a smooth surface.

Holes are drilled in the kneebone to enable another part of the artificial joint to be attached.

Three of the joint's four sections are cemented into position.

The cement, which contains antibiotics, takes 10 to 12 minutes to set, depending on the temperature in the operating theatre.

Once the metal components are set in place the plastic bearing is slotted into the flat shin-bone plate.

The knee is then pulled straight, bent back up and twisted gently to the side to test the manoeuvrability.

The whole procedure takes just over an hour.

While this particular mobile joint is in the early stages of use across the world, the concept has been around for almost 20 years.

There are 1453 people waiting for orthopaedic surgery at Middlemore Hospital.

Dr Clatworthy said he expected just under half of those to be knee replacements.

The rest would be hip replacements.