A hip replacement

High Wycombe-based Calverline, a specialist in orthopaedic support, has developed a better hip hinge.

Time spent in hospital beds is premium and in North American private healthcare it drives innovation. Since hip replacement operations began in the 1960s, doctors have narrowed the time patients spend in bed from weeks to days.

Traditionally the hip hinge has helped increase the chances of a successful recovery. The device is strapped over the hip at the waist and leg to limit movement.

Ken Davis, managing director of Calverline in High Wycombe, is a specialist in orthopaedic support and has taken the hip hinge a stage further: ‘A hip hinge has two simple adjustment devices. One that limits extension and inflection (lifting the leg or bending), and the other limits abduction (leg splay). What this means is you can fit someone up within 24 hours of the operation and literally get them on their feet.’

It is the movement of inward abduction, particularly when sitting down or crossing your legs that can cause a replacement hip to ‘pop’ out again. So the leg is mechanically held out.

‘Now that’s fine when you’re sitting down or getting up which is where you’re prone to pop,’ says Davis. ‘So you set them at 30° abduction. But you and try and walk like that!’

These have been the parameters of hip hinges until now. But a competitor produced a hinge based on two polypropylene wedges rotating together — as you sit down the leg is moved out to mimic the safe and natural movement of the hip. The problem with this is when, under load, there is a lot of friction, which the outpatient must overcome. This also reduces the lifetime of the hinge.

‘So we had to do better than this,’ says Davis, ‘and created what you might call an automatic abduction device.’

Terry Gander, an industrial designer from Datchet-based IDC who worked with Davis on the project says: ‘We looked at all sorts of ways of doing this and in the end we came up with a tie rod device with infinite positions you can adjust. When you sit down your leg moves out at an angle — it is impossible to close your leg, so preventing that crossed-knee injury potential of the joint popping through the muscle.’

What this allows is a near normal gait. A normal walking mode takes you through about 20° of inflection which will correspond to no more than 10° abduction. But as the inflection angle decreases (during bending or sitting), the tie rod tries to shorten the effective radius, pulling the abduction hinge outwards.

‘It’s a bit like a track rod end on a steering system,’ says Terry. ‘As it swings round it has to miss the rest of the mechanism. The breakthrough came when we found this method and it was made possible using our own ball joint design.’

The product will be launched in North America this month.

Calverline can be reached at +44 (0)1494 614 605.