Disc with a difference

in the early 90s, when latex allergies were causing medical concern, UK chemist Geoffrey Andrews had the idea that polyurethane would be the ideal replacement material. As well as using it for obvious applications like doctor's gloves, he felt that the versatile polymer could be used in a wide range of products from urethral catheters to angioplasty balloons.

Andrews had experience of making polyurethane, and he called on an old colleague, chemist Robert Snell, to help him develop a method for designing and manufacturing safe medical products.

Their company, Cambridge-based

, has been running profitably since 1995 producing a range of medical products such as the urethral catheters and angioplasty balloons.

Recently, however, the company has turned its entire focus on a spinal disc replacement called Compliant Artificial spinal disc, or CAdisc. Although still under development, the company hopes that in a couple of years, after clinical trials, the device will be a more comfortable and durable alternative to traditional replacements.

Like all the medical products from Ranier, the CAdisc is based on polyurethane and hopes to rival discs such as ProDisc and Charité, which have been available for several years.

'Both of these are based on old hip replacement technology, which is a metal on polyethylene-type articulating surface, that gives rise to different problems that our product would address,' said Snell, who is Ranier's technical director.

'Some of those problems arise from the articulating surfaces which have a tendency to wear, especially if the discs are not placed correctly,' he explained. 'Second, and probably more important, the articulating devices work like a ball and socket so you have a fixed centre of rotation, and although Charité has a slightly floating centre of rotation, it's quite fixed.'

The CAdisc is completely elastomeric and has a dynamic centre of rotation that will translate and bend according to the way a person's anatomy prefers to bend rather than how the device wants it to. CAdisc has one region comprising a harder higher modulus polyurethane and a second comprising a softer version of the material. The two regions are connected by a third, where the hardness varies continuously from the soft to the hard area.

The manufacturing of the devices will rely on a Ranier patented technology called Precision Polyurethane Manufacture (PPM), which the company used to fabricate other polyurethane medical devices. PPM integrates computer-controlled polymerisation of polycarbonate polyurethanes with the CAdisc moulding process to create the disc's unique graduated modulus design. The process allows the device to be chemically bonded throughout and is designed to have no regions of high stress concentration that could lead to device failure under repeated load.

The architecture of the human spine is complex, and fitting it with foreign objects is no easy task. Artificial discs are designed to replace the cushion between two large parts of the vertebrae. A vertebra is one of 33 irregular bones that make up the spinal column.

One of the reasons fixed rotation artificial discs cause pain, said Snell, is they put unnatural stresses on the spine's posterior joints, or facets, that are designed to prevent excessive twisting and allow a small amount of lateral bending and flexing and extension.

The CAdisc's compliance provides a shock-absorbing function, like a natural disc, that prevents stress on the facets and adjacent vertebral discs. Snell said this is something that even 'gold-standard' procedures such as spinal fusion sometimes fail to do.

'There has been a reasonable amount of evidence which shows that by fusing two bones together, you transfer stresses to the adjacent level, which accelerates a degeneration in the adjacent disc,' he said. 'So although you're treating one level, you're just transferring the problem up or down.'

Currently, the CAdisc is designed for the lower back area where most pain and disc problems are experienced. However, Ranier is developing one for the cervical spine, the upper spine, or the neck.

Snell said that working with poly- urethanes has presented some interesting challenges over the years. 'Depending on how you put the material together, you can get completely different results — from something that is a soft elastomeric material like a chewing gum to something which is a hard engineering-type plastic,' he said. 'And depending on the chemistry you use, you could have something that is extremely bio-stable or toxic and particularly nasty to the body.'

Snell said polyurethanes have a bad reputation in the medical field because previous users hadn't selected the ones most suited for medical applications — especially implants.

'The body's defences are quite aggressive to any foreign body within it, so it uses a huge arsenal of different ways of trying to break something down,' he said. 'Some polyurethanes can degrade and with this in mind, the user acceptance of the regulatory authorities of any new material is quite reluctant. You have to provide a huge body of evidence to support the long-term stability and the biological safety of the device.'

One of the most challenging parts of the CAdisc development process was creating a 3D computer model of a device with viscoelastic material properties. 'We have overcome that now,' said Snell. 'We've found some very good partners that can help us work with software to do it.'

If all goes according to plan, when the CAdiscs are manufactured and commercially available in the next couple of years in Europe, they will be available in a range of 24 sizes that will be prescribed to a patient depending on the footprint of the surface area of their vertebral body, the curvature of their spine and the disc height.

'With those three factors, we should be able to provide coverage of 95 per cent of the population,' said Snell.