Implanted Dropped Foot Stimulator
The STIMuSTEPTM (formally known as the Finetech Implanted Dropped Foot Stimulator) has been developed by the University of Twente and Roessingh Research. & Development in Holland in collaboration with the UK based company, Finetech-Medical Ltd. The purpose of the device is to make walking easier, safer and faster for people who have a dropped foot as a result of an incomplete spinal cord injury. This is the inability to lift the foot as the foot is brought forward, often causing it to drag or catch. By applying electrical pulses (Functional Electrical Stimulation, FES) to the appropriate leg muscles, the dropped foot can be corrected during walking.
Implantation of the device requires an operation that takes about 1 hour and is performed using a spinal anaesthetic. A single incision of approximately 5cm on the outside of the lower leg just below the knee is required to site the device.
How does the STIMuSTEPTM work?
The nerve that controls the lifting of the foot in walking is called the common peroneal nerve. At a point, just below the knee, this nerve splits into two branches, the deep branch and the superficial branch. The deep branch goes to the muscles that lift (dorsiflex) and turn inward (inversion) the foot while the superficial branch supplies the muscles that turn the foot outwards (eversion). In normal walking, a combination of these movements is required. Therefore an electrode will be surgically inserted in both nerves enabling the movements to be controlled separately. Figure 1 shows the electrodes, which are attached to the implanted receiver / stimulator using flexible silicon leads. To stimulate the nerve very short pulses of electricity are passed through each electrode, times a second.
Figure 1. Implanted receiver stimulator Figure 2. The transmitter
Figure 3 schematic diagram of the STIMuSTEPTM
This causes nerve impulses to travel down the nerve to the muscle in the same way as a naturally occurring nerve impulses. Power to run the stimulator is passed through the skin using radio waves from a small control box strapped to the outside of the leg (figure 2). Figure 3 is a schematic diagram of system.
To stimulate the muscles at the correct time a foot switch (a small pressure pad placed in the shoe) is use to detect when the foot is lifted from the ground. Stimulation begins when the foot is lifted and ends when the heel is returned to the ground. Sensation from the electrical stimulation should be very slight and it is expected that users will quickly become accustomed to it. Use of the system will be automatic; the user only has to set the stimulation intensity using two control buttons. Once healing has occurred the operation site scar should be negligible. It may be possible to palpate the implant under the skin but it is not expected to be noticeable to the eye.
The Department of Medical Physics and Biomedical Engineering together with the Wessex centre for Plastic Surgery, both at Salisbury District Hospital are currently planning the development of a clinical service to implement the STIMuSTEPTM. Initially the service will be restricted to people who have a dropped foot following stroke. It is envisaged that the device will be suitable for people who have used the external Dropped foot system (ODFSIII) for some time and have demonstrated that they can benefit from long term use. It is expected that the device would be particularly useful where there are difficulties with using the external system due to problems reliably locating the electrode positions, the sensation of stimulation or problems with skin irritation.
The precise procedure is yet to be worked out but it is expected that implantation can be an outpatient procedure. The follow up required will be similar to that required for the ODFSIII.
At the present time there is no NHS funding allocated to this service. We will be investigating how this can be obtained. Self-funding may also be possible.
When will the service start?
We hope to start the service in 2005.
How can I obtain the STIMuSTEP?
A medical referral to Prof. Ian Swain at the Department of Medical Physics and Biomedical Engineering, Salisbury District Hospital is required.
STIMuSTEP Implanted Dropped Foot Stimulator Patient Information Sheet