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_____________________[PRO] computer-aided design gets hip

Computer-aided design gets hip

Computer design systems, digitised models of human joints and 'fuzzy logic' are all being pressed into service to build a better hip joint. A Dublin City University team is using the latest technology in an important new research area, 'medical engineering'.

Researchers at Dublin City University are trying to develop a better artificial hip joint, one that lasts longer and causes fewer problems, writes Dick Ahlstrom

Irish Times; Oct 31, 2002

Computer design systems, digitised models of human joints and 'fuzzy logic' are all being pressed into service to build a better hip joint. A Dublin City University team is using the latest technology in an important new research area, 'medical engineering'.

'Research in computer aided design has gone beyond the building of solid models, it now goes on to model how the component will perform when in service,' explains Dr Bryan MacDonald, a lecturer in DCU's School of Mechanical and Manufacturing Engineering. He heads a research group which uses advanced modelling to assess the performance of replacement joints and other engineering components before they are actually put into service.

'We have a research team concerned with computer aided design analysis and we look at engineering problems including medical engineering problems,' he says. 'I am a lecturer in computer aided design so my research tends to focus on that, basically design and analysis of engineering components. In recent years, we have tended to concentrate on the medical design area.'

The activity fits well with developments at DCU. It introduced a new bachelor of engineering degree in medical mechanical engineering.

Computer modelling is a powerful tool when considering medical engineering components such as replacement hip or knee joints. 'The problem with hip replacements is when you put them in you can't take any measurements from them,' he says. 'You only know something has gone wrong when you have to replace it.'

The computer models allow the researchers to put a joint through its paces, to put it under stress and discover what its weak points are. This allows designers to come up with better, longer-lasting designs but also to innovate.

The work is based on 'finite element modelling', which involves building up a composite impression of joint performance by measuring each individual element. In this case, the elements include bone, the titanium replacement, the cement used to hold the two together and any associated tissues. 'In that way, we can see what effect the prosthesis has on the surrounding tissues,' says MacDonald.

To this can be added 'fuzzy logic', algorithms which can be used to help come up with unusual designs, he explains. Elements of a design, for example the shape or size of a joint, are fixed but all other elements are open for modification by the fuzzy logic application.

This can lead to novel changes in original designs. In aircraft design, it is used to devise fresh versions of a component: for example, asking the system to reduce the amount of material in a component to save weight, while fixing all the original, load-bearing aspects.

Replacement joints can loosen over time because the joint takes on more of the load, causing the under-worked bone to waste away. 'One thing we are examining is if we can change the prosthesis to a synthetic material to extend the life of the implant,' he says.

The model allows them to try out polymers, fibre-reinforced polymers and metal-matrix composites instead of the traditional steel and titanium. They can also trial-run experimental designs that could not otherwise be tested on a real patient.

'These are all fairly radical things that a surgeon wouldn't like to try on a patient but we can model on our finite element system,' explains MacDonald.

'We are limited in the materials we can use because they must be biocompatible,' he adds, but this does not prevent innovation. One area under study is the use of special 'diamond-like coatings' to reduce metal on metal wear.

An artificial hip joint duplicates the original joint with its ball and socket, but the metal-against-metal surfaces are prone to wear. 'The diamond-like coatings are put on the inside of the socket and top of the ball joint and can reduce wear, last longer and eliminate this problem,' he says.

A wholly new material can also be tested in the model so long as its characteristics are known. A recent example is the use of silicon inclusions sprinkled through the original titanium design. The silicon seems to act like the reinforcing rods used to strengthen concrete, leaving a joint that is much tougher than titanium alone.


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