A team at University College London (UCL) is developing a spinal canal implant that could improve the quality of life and life expectancy for people with serious spinal cord injury.
Previous research has restored function to this patient group by stimulating muscles through the skin using surface electrodes or implanting electrodes in the muscles or between the spinal cord and the muscles.
UCL has for some time been investigating a different approach by putting the electrodes on the nerve roots, which is where the nerves emerge from the spinal cord but remain within the spinal canal.
‘These are relatively fine and fragile fibres within the spinal canal,’ said Prof Nick Donaldson of UCL’s neuroprosthesis engineering department. ‘The complication is that there are many fibres very close together which emerge from the spinal canal and form the major nerves that run down into the legs and also control the bowel and bladder.
‘The advantage from a surgical point of view is that they’re all available in one location, so you can, in a single procedure, field and place the electrodes together rather than having to fit electrodes and route cables over the legs of the patient.’
FineTech Medical makes an implant called the sacral anterior root stimulator for this site in the body which is just used for neurological functions — primarily emptying the bladder and bowel.
‘That has been very successful and made a big difference to patients who’ve had it fitted,’ said Donaldson, ‘but it doesn’t do anything for the legs. I ran a research project in the 1990s where we stimulated the roots a bit higher up — the lumbar roots — and showed that we could get useful leg function, allowing a paraplegic to propel a recumbent cycle.
‘We would like to expand the existing device by giving it more channels so we can add leg function to the existing neurological functions of the implants.’
The surgeon inserting the implant has to connect very small electrodes to individual nerve roots in such a way that the currents which flow between the electrodes just stimulate the target nerve roots, not neighbouring ones. This is achieved using a structure called an ‘active electronic book,’ because the surgeon can place the roots between the ‘pages’ of the device, separating them.
‘The project, which is mainly technological, addresses how we can increase the number of stimulation channels without having many cables going into the spinal canal, said Donaldson. ‘At the moment we have really been working at the limit of what the surgeons think is practical, with 12 channels, each corresponding to a nerve stimulated. The number one might want to stimulate is in the region of 20 to 30, so if we could double or treble the number of channels, we could do more for patients.
‘That requires us having some way of putting the electronics right down near the electrodes. So the idea of the active book is that it has semiconductor switches and perhaps amplifiers within the electrode structure, we call the book and relatively few wires going through the dura (the outer membrane of the spinal cord) into the canal.’
This has the advantage of reducing the risk of infection and cerebro-spinal fluid (CSF) leak.
By the end of the project, the team hopes to show the technology can run in saline for long periods. It aims to demonstrate a method of sealing the electronics so the implant will be reliable for years, and carry out mechanical tests to prove its robustness.
The EPSRC-funded project runs from 2008 until 2010 during which time UCL will carry out the design of the electronics. When complete, approval will be sought from the Medicines and Healthcare products Regulatory Agency (MHRA) to undertake trials, which could take up to 10 years
UCL’s project partners are the Tyndall Institute, which will develop the integrated circuit sealing, and Freiberg University, which has special knowledge of laser cutting tiny electrodes.
A team at University College London is developing a spinal canal implant that could improve the quality of life and life expectancy for people with serious spinal cord injury.