Three FES applications
The Healthy Aims project, funded by the European Union, started in December 2003 to develop a range of implants and diagnostic equipment that integrate micro-, nano- and biotechnology with wireless communications2. The diagnostic equipment and biomaterials under development have been presented in recent issues of Medical Device Technology3,4. Work on new functional electrical stimulation (FES) equipment is described in this article.
The four year project has involved 26 partners from 10 European Union countries, including seven small- and medium-sized companies, six clinical partners, five large enterprises and seven academics and research groups. One FES system for hand and wrist control, the STIMuGRIP, has been taken from specification, through design, manufacture and laboratory trials and has just entered pilot clinical trials. This is described in detail below. Two other FES applications for bladder and bowel control were introduced into the project at the start of year four. For the bladder control system, a demonstrator is being produced and tested in the laboratory. For the bowel control system, acute animal trials have already been successfully completed and chronic animal trials are underway. Pilot clinical trials for the bladder and bowel FES systems will be undertaken by the clinical teams now that the Healthy Aims project is completed.
Disabilities caused by a stroke
In the United Kingdom (UK) approximately 150,000 people suffer a stroke per annum1. Taking into account the population of Europe, this can be extrapolated to be more than 750,000 strokes in Europe per annum. The group most likely to be affected are people aged over 65, although approximately one third of stroke patients are below this age and this group includes children and even babies.
A stroke is the third most common cause of death in Europe after heart disease and cancer and it is the single most common cause of severe disability. Finetech Medical and its partner Salisbury District Hospital (Salisbury, UK) have been working together as part of the Healthy Aims project to develop and trial a new implanted neurostimulator to improve the impaired upper limb function found in this type of disability.
Of all acute stroke patients starting rehabilitation, approximately half will have a marked impairment of function of one arm and approximately one sixth of these will regain useful function. This impairment is mainly the result of a reduction in the ability to selectively activate the finger, thumb and wrist extensor muscles. The practical outcome of this means that if a hand grasps an object, it is not easy to release the object; in addition, the lack of wrist extension during grip leads to a weak grasp and a less functional position of the hand.
Hand control implant
The implanted neurostimulator being developed assists patients who have an upper motor neuron lesion to extend the wrist and open the hand. The device is intended to be a long term implanted system and is comprised of two main parts: a two channel implanted stimulator and an external control box (see Figure 1). The two channels of the implant may be controlled independently of each other to maintain wrist extension while the fingers of the hand are allowed to close and grip an object using the natural spasticity of the affected limb.
The implanted stimulator contains a tuned inductively coupled two channel receiver that powers the stimulating electrodes. The implant becomes active when coupled to the external transmitter and receives the stimulation pattern for each channel independently through the tuned 1 MHz and 2 MHz circuits. The implant is placed under the skin on the centre and outward facing part of the forearm. One channel is used to open the hand by stimulating the posterior interosseous nerve as it passes over the supinator muscle. This is just distal to the branching point of the extensor carpi radialis brevis and supinator muscles. The second channel is used to stimulate the motor point of the extensor carpi radialis brevis muscle to produce the extension to the wrist. This stimulation needs to be maintained to provide support to the wrist while making a grasp.
The implanted device has two pairs of 4 mm platinum epimysial electrodes connected by spiral wound cables to the implant receiver body (see Figure 2); the body contains the tuned receiver circuits that are mounted on ceramic substrates. The device is encapsulated entirely in silicone elastomer except for the exposed electrode faces. The electrodes are used as pairs in a bipolar configuration, which provides an active and indifferent electrode for each channel. The active electrodes are placed over the posterior interosseous nerve and the motor point of the extensor carpi radialis brevis muscle and the indifferent electrodes are placed a few centimetres distally.
The external control unit incorporates a three-axis accelerometer to detect deliberate movement by the patient. An example of the type of movement that starts the stimulation cycle is raising the forearm from the side when the hand is pointing to the ground and taking it to a horizontal position whereby the hand is pointing forward. The accelerometer signals are used as inputs to a state machine algorithm. This determines the onset and termination of stimulation, where the applications predominately focus on wrist extension and opening of the fingers. Further control is added by programmed timeout instructions that can be used, for example, to turn off stimulation to the fingers if no movement is detected by the accelerometer after three seconds. The patient wears the controller unit on the affected limb, directly over the implant site, where it is secured in place by an adjustable elasticised strap. The implant receives its power from the external controller, which has been designed to receive pulses of stimulation energy by means of inductive coupling. The hardware has been designed with four modes: an exercise mode and three functional application modes.
The system, comprising implant and controller unit, has been approved by the Medicines and Healthcare Products Regulatory Agency, the UK’s Competent Authority (www.mhra.gov.uk), for a clinical investigation at Salisbury District Hospital where it is being evaluated.
Prior to receiving the implant, subjects were assessed with an external surface stimulator to condition the muscles, reduce the spastic tone of the antagonist muscles and practice control movements. Following the initial selection and training programme, further selection criteria were performed. These covered:
- demonstration of an improvement in the Action Research Arm Function Test score when using the external device
- demonstration of an improvement in at least one Canadian Outcome Performance Measure task with a change of score of two or more in performance or satisfaction
- demonstration to the satisfaction of the clinical team that the subjects understand the use of the surface stimulation device and that they display the motivation required to use the device.
The device was implanted under general anaesthetic in a day surgery. An incision approximately 12 cm long was made on the dorsal aspect of the forearm, adjacent to the pre-determined position for the implanted receiver stimulator. The posterior interosseous nerve and its branch to the extensor carpi radialis brevis were identified using a surgical test stimulator. The test stimulator uses the same electrodes and stimulation parameters as those of the implant, however the electrodes are mounted on probe bodies to aid position identification during surgery. Using the two stimulation probes, active and indifferent, the best positions of the electrodes were chosen to allow good extension of the fingers, thumb and wrist without significant ulna or radial deviation of the wrist. Care was taken to avoid overflow to neighbouring muscles.
Once the position was determined, the location was marked using surgical dye to ensure the location was not lost while the electrodes are being sutured in position. The implanted receiver was placed in position under the location marks made prior to the operation and sutured. The wound was then closed and a final check of the function of the device made with the external controller in place over the arm.
Two research volunteers have received the implant. Subject 1 is female, aged 58 years, and is 46 years post stroke with right-sided hemiplegia. Subject 2 is also female, aged 37 years, and is 24 years post stroke with left sided hemiplegia. At the time of writing, both subjects are in the early stages of muscle training using cyclic exercise stimulation from the implant following the surgery and two-week healing period. Consequently, full results are not available, however, the following observations have been made.
- Independent control of wrist extension and hand opening has been demonstrated. No cross talk between channels has been observed.
- The feasibility of a bipolar epimysial electrode system has been demonstrated. A stimulation effect was obtained from active and indifferent electrodes. The overall stimulation effect could be modified by choice of position of each electrode. In Subject 2 the wrist extension was provided by a combination of extensor carpi radialis brevis and extensor carpi radialis longus by placing one electrode on each muscle.
- The sensation of the stimulation is negligible, the primary sensation being the feeling of the muscles pulling.
The recent start of the clinical trial of this two channel implanted stimulator for hand opening and wrist support following stroke suggests that the approach is feasible. Although the initial results of the trial are promising, it will be the outcome of the longer-term performance that will need to be closely reviewed. In particular, finding the best standard deliberate movement by the user to trigger the device may need an element of personalisation; however it may be more realistic to expect the user to adapt to standard triggering settings. Furthermore, at this early stage of the trial there is still the need to review the optimisation of the electrode positions and how they perform over the long term. It is planned to commence triggered stimulation in the coming weeks.
The future of FES in Europe
The results described in this article show that it is feasible for new FES systems to be developed within Europe in realistic timescales. However, despite these advances, the product lifecycle in this industry considerably lags behind the available technology because of the lengthy nature of the clinical testing and approval process for new medical devices. This can mean that a company’s technology/device becomes obsolete even before it reaches the market because a competitor introduces a more advanced system. The work performed in the Healthy Aims project combines new technologies with those proven from existing products. Keeping the features and capabilities focused on patient needs and the design simple, and avoiding over ambitious solutions has enabled a relatively rapid and cost-effective device to be developed and trialled within the project period.
The authors would like to acknowledge the support, information and surgery provided by the clinical team at Salisbury District Hosiptial, Salisbury, UK.
1. www.stroke.org.uk. European stroke number estimated using the population of the UK as 60 million and the population of Europe as 370 million.
2. The Healthy Aims project is funded by the European Information Societies Technology Programme (IST-2002-1-001837) under FP6, www.healthyaims.org
3. D. Hodgins et al., “Developments in Sensor Systems,” Medical Device Technology, 18, 5, 32–35 (2007).
4. D. Hodgins et al., “Biocompatible Materials: Developments For New Medical Implants,” Medical Device Technology, 18, 6, 30–35 (2007).
Diana Hodgins, MBE DSc (Honorary) is Project Co-ordinator of Healthy Aims and Managing Director of European Technology for Business Ltd, Codicote, UK, tel. +44 1438 822 822, www.etb.co.uk.
John Spensley is Managing Director of Finetech Medical Ltd, 13 Tewin Court, Welwyn Garden City AL7 1AU, UK, tel. +44 1707 330 942, e-mail: firstname.lastname@example.org, www.finetech-medical.co.uk.