Pilot study to evaluate the safety and efficacy of an implanted dropped foot stimulator (IMPULSE)

Paul Taylor, Geraldine Mann, Duncan Wood

Dept. Medical Physics and Biomedical Engineering, Salisbury District Hospital, Salisbury, Wiltshire SP2 8BJ


John Hobby

Plastic Surgery, Salisbury District Hospital, Salisbury, Wiltshire, SP2 8BJ


The Finetech Implanted Dropped Foot Stimulator excites the deep and superficial branches of the common peroneal nerve to allow independent control of dorsiflexion and eversion to correct dropped foot. In this pilot study 5 subjects increased their walking speed by 24% when the device was used and could walk 37% further in a six minute walk. Dorsiflexion and eversion was increased by 15 and 9 degrees respectively and hip abduction reduced. No evidence of damage to the nerves was found. The device was well accepted by its users who felt that their independence was increased by its use.

1. Introduction

Dropped foot following stroke can be successfully alleviated using surface stimulation devices such as the Odstock Dropped Foot Stimulator (ODFS). These devices stimulate the common peroneal nerve as it passes over the head of the fibula bone, causing the foot to dorsiflex. When timed to the gait cycle using a pressure sensitive switch placed under the heel, the device significantly aids the swing through phase of gait, reducing the effort of walking and increasing walking speed. Despite a good orthotic response and high compliance with these devices, users report difficulty with using them, in particular, correctly replacing electrodes. This problem could be alleviated by the use of implanted electrodes, avoiding the day to day variation in the user placement of electrodes. Additionally the sensation from the stimulation would be reduced and the risk of skin allergy eliminated. The University of Twente, Finetech Medical Ltd and European Technology for Business (ETB) have developed an implantable device1. Two channels of stimulation are used to stimulate the 2 branches of the common peroneal nerve. The deep branch produces dorsiflexion and inversion while the superficial branch produces eversion and plantaflexion of the foot. By adjusting the relative proportions of stimulation to both nerves, the exact movement of the foot can be controlled. This paper reports the findings of a pilot study of the device.

2. Methods

2.1 The ODFS Questionnaire

A questionnaire was sent out to 140 experienced (1 year +) users of the ODFS. They were asked to identify what problems they had experienced and rate the seriousness of those problems on a visual analogue scale. They were also asked if they would consider an implanted device.

2.2 Clinical trial: Selection criteria

  1. First stroke of at least 12 months duration with a stable neurology.
  2. Dropped foot identified by an inability to achieve a normal heel strike during walking.
  3. Able to walk at least 100m with aids.
  4. Must be able to understand the use of the equipment and purpose of the trial.
  5. Able to give informed consent
  6. Aged between 18 and 65

Exclusion criteria

  1. Evidence of inversion contractures;
  2. Serious medical conditions
  3. Regular surface peroneal stimulator users

2.3 Clinical trial: Procedure

Subjects were first assessed through a 1 month base line period to demonstrate stability. After receiving the implant, three weeks were allowed for healing of the operation site. 2 months were then allowed for training with the device and assessments were then repeated in the following three months, once a new gait pattern had become established.

2.4 Clinical trial: Assessments

Safety was assessed by nerve conduction studies. Spontaneous discharges at EMG needle insertion were also examined for evidence of denervation.

Walking speed and physiological cost index (PCI) was measured over 10m. PCI is equal to the difference between resting heart rate and the heart rate at the end of 10m divided by the

Table 1. Walking speed, PCI and Endurance, pre op and post op both with and without

the implant (n=5). The results here are the mean of 4 measurements made in the final 3 months period.


Walking speed m/s

No stim

PCI Bt/m

No stim

Walking speed m/s

With stim

PCI Bt/m

With stim



Pre op mean





Post op mean






(with stim)

Mean % change

pre / post op











Mean % change stim / no stim







Table 2. Hip, knee and ankle angle while walking with and without the implant (n=4), at end month 5.

knee flexion

(max swing)

Hip Flexion

(max swing)

Hip abduction

(max swing)

Dorsiflexion (pre loading response)


(pre loading response)

Mean Degrees

No stim






Mean Degrees

With stim






Difference Degrees






Wilcoxon p






walking speed in m/min. This gives an indication of the effort expended in walking. Three runs were recorded both with and without the stimulator and the order randomised to compensate for fatigue2. Additionally the distance walked in 6 min was recorded by walking repeated lengths of a 13.5m corridor.

The implant's effect on kinematics parameters was recorded using a Biotech Datalink system. Penny and Giles goniometers were placed over the ankle, knee and hip joints to measure dorsiflexion, eversion, knee flexion and hip flexion / extension and adduction abduction.

User opinion of the device was collected using a purpose written questionnaire3.

3. Results

ODFS user survey: 98 questionnaires were returned from 140 sent to current ODFS users. On average they had used the ODFS for 3 years (SD 2.1years). While 52% were well satisfied or 41% moderately satisfied with the ODFS, problems were reported with skin reaction (28% - serious 6%), electrode placement (72% - serious 17%), donning and doffing (58% - serious 15%), coping with wires (58% - serious 19%) and sensation (36% - serious 3%). 67% would consider an implant.

Five people received the implant. 4 were right sided hemiplegics and 1 was left sided following stroke.

No change in nerve conduction velocity was reported following the implant procedure. Nor was there any evidence of denervation observed on EMG needle insertion.

Mean walking speed with the device was 24% faster with the implant compared to pre op without and 14% faster than without the device post op. No significant changes were seen in PCI. Mean distance walked in 6 minutes increased by 37% pre to post op. (table 1)

Dorsiflexion and Eversion were compared walking with and without the device at the point just before heel strike. This is before any loading response has occurred. Use of the implant resulted in a 15 and 9 degree increase in dorsiflexion and eversion. There was also a reduction in hip abduction of 2 degrees that may be due to reduced circumduction in the swing phase, which is a compensatory response to aid ground clearance. A trend to reduced knee flexion when the implant was used was also seen. This could also be less compensatory activity but may indicate knee flexion is reduced by stimulation (table 2).

User questionnaire: 3 users used the device every day while one used it 4 to 6 days a week and one 2 to 3 days a week. On days that the device was used, one user used it all day, one 9 to 12 hours, two 6 to 9 hours and one less than 3 hours a day. The device was used for every type of activity by 3 users while one used it outdoors only and one for longer walks only. Two users regularly walked between 10 and 100 yards, one between 100 and 500 yards, one between 500 yards and 1 mile and one walks more than one mile.

From a list of 12 possible replies, the users were asked to select any reasons that were relevant to them and indicate the most important reason for using the device. All users said they were more independent when using the device and 4 said they were more confident and could walk on uneven ground. 3 users stated they could walk further, walk faster and walk with less effort when using the implant and 2 users used it because they could discard a splint or walking stick. 2 users felt using the device helped to keep them fit while only one user used it because it prevented them from tripping or falling. Each user chose a different main reason for using the device. The reasons were: I can walk faster, I can walk with less effort, I am less likely to trip or fall, I am more independent and The exercise keeps me fit.

Two users required help putting on the device while 3 were independent and the perceived average time to do this was 4.8 minutes (median 4 minutes). Three users believed their spasticity had reduced since using the device, one believed it was the same and one believed it had increased although in the last case this was at odds with the opinion of the physiotherapist. The device had an effect on the use of aids. Two people stopped using walking sticks while walking and one reduced their use of a walking stick. One user reduced their use of a wheelchair, one reduced their use of an AFO and one stopped using an AFO.

Three users stated that the device worked correctly all of the time while 2 stated it worked correctly most of the time while walking. While sitting, 4 users said the device never gave false outputs while 1 reported this happened rarely. 2 users stated they always adjusted the controls themselves while 2 always had an assistant do this for them. Three users stated that adjustments were only occasionally needed, one only when it was put on and one user never adjusted the device. Two felt these adjustments were very easy to make, 1 easy and 2 fairly easy. Three users stated they only adjusted the position of the box when it was put on while the other users adjusted it occasionally or every few hours.

The users were asked if they agreed with the following statements:


I am glad that I have the IMPULSE


All 5 strongly agree


I would recommend IMPULSE to another person


4 strongly agree 1 disagree


IMPULSE allows me more independence


3 strongly agree, 1 agrees and 1 indifferent


I feel more confident when I use IMPULSE


3 strongly agree, 1 agree, 1 indifferent


I am more independent since I received the implant


2 strongly agree, 2 agree, 1 no response


IMPULSE has improved my quality of life


3 strongly agree, 1 agree, 1 strongly disagree


IMPULSE has a good cosmetic appearance when worn


2 agree, 2 indifferent, 1 strongly disagree


The sensation from the electrodes is comfortable


1 strongly agree, 4 agree

3 users stated that the box never got knocked in daily use while one said it occurred occasionally and one said it occurred frequently. The latter user said the control knobs on the box pressed into the thigh when the knee was at 90 degrees and also the box got knocked getting in and out of a car.

Users were asked to rate the sensation from using the device on a 1 to 10 analogue scale where 1 was no sensation 5, was a mild, comfortable sensation and 10 was a severe sensation. Two rated the sensation as 1; two rated it as 5 and one as 6.

4. Discussion and Conclusions

The Finetech implanted dropped foot stimulator was found to perform in a similar manner to surface devices but with the most of the advantages predicted. However, users still experienced some problems with its use. The response from the device is sensitive to small changes in the position of the transmitter, requiring some care in its placement. Also, three of the users were aware of the sensation from the device although this may have been from the strong muscle action required to overcome calf tone. The trend to reduced knee flexion is a possible concern. However, recently, a new version of the device that allows the addition of a rising ramp to the output, has been tried with one subject who reported that knee flexion was made easier. This may be due to reduced spastic tone, which may, in the previous version, have been by induced by the rapid movement causing stretch reflexes.

Overall the users were enthusiastic about the device. A larger trial is now required to fully demonstrate its efficacy.


  1. Kenney L, Bultstra G, et al. An implantable two channel drop foot stimulator: initial clinical results. Artif.Organs 2002;26(3):267-70.
  2. Taylor PN, Burridge JH, et al. Clinical Use of the Odstock Dropped Foot Stimulator. Its Effect on the Speed and Effort of Walking. Arch Phys Med Rehab. 1999; 80 1577-1583
  3. Taylor PN, Burridge JH, et al. Patient's Perceptions of the Odstock Dropped Foot Stimulator (ODFS). Clin. Rehab. 1999; 13: 333-340


We wish to acknowledge the assistance of Laurence Kenny and Rik Buschman of RDD, Gerrit Bultstra of the University of Twente, David Francis and David Keeling of Finetech Medical, Diana Hodgins of ETB and Jonathan Cole of Salisbury District Hospital. This work was funded by Medlink.

FESnet 2002 Final Paper Submission

Isometric joint moments from an implanted drop foot stimulator

Wood DE 1, 2

1 Dept. of Medical Physics and Biomedical Engineering, Salisbury District Hospital Salisbury, Wiltshire, SP2 8BJ, UK


Taylor PN 1, 2, Mann GE 1, Hobby JE 3

2 Academic Biomedical Eng. Research Group, Bournemouth University

3 Department of Plastic Surgery SalisburyDistrict Hospital


Responses to implanted stimulation of both branches of the common peroneal nerve were evaluated in five chronic stroke subjects. Special equipment was used to measure isometric joint moments in three dimensions generated at the ankle while seated and also at the knee and hip with the legs extended. Tests have demonstrated that selectivity in ankle moments, especially dorsiflexion and eversion, can be attained by optimising these two channels of stimulation. Response to stimulation by implant is similar to that by surface stimulation, including the sensitivity to transmitter or, in the case of surface stimulation, electrode position.


1. Introduction

Functional electrical stimulation (FES) using surface electrodes can be used to improve walking for patients following a stroke [1]. Typically, electrodes are placed to stimulate the common peroneal nerve for dorsiflexion and eversion during the swing phase of the gait cycle. Lack of movement and/or control at the hip and knee may also be improved through the same electrodes, by eliciting the flexor withdrawal reflex.

To provide optimal balance of movements electrode placement is critical. Though instruction and training in finding these positions can be provided to the patient, a significant number (44%) say they experience problems and 34% of those who stopped using the device cite it as one reason for stopping [2]. In addition, some patients suffer from allergic reactions to the electrodes and some experience pain during stimulation. These issues may be addressed using implanted stimulation.

Previous work by other groups has used single channel implanted devices, but these have a lack of selectivity in appropriate ankle movements for gait assist. A two-channel implanted stimulator is therefore being piloted [3].

This paper is one of a pair presented at this conference that describes the results from the UK centre on this pilot study. Its companion paper [4] demonstrates the safety and efficacy of the device, whilst this paper reports on isometric joint moments measured while using the implanted stimulator and compares the results to that when using surface stimulation.

2. Methods


The implanted stimulator is a two-channel device, developed at the University of Twente, using sub epineural electrodes to simulate both branches of the common peroneal nerve. Pilot work demonstrated selectivity in ankle movements through stimulation of the deep branch, innervating the dorsiflexor and inverter muscles, and the superficial branch, innervating the everter muscles [3].

Surface stimulation is delivered using the Odstock Dropped Foot Stimulator (ODFS) [1]. When used in these tests, the electrodes are positioned according to standard practice, with the active electrode close to where the common peroneal nerve branches at the head of the fibula and the indifferent electrode over tibialis anterior muscle.

Subject group

Test protocols

Isometric joint moments were measured using the multi-moment chair [5] at least 5 months post operation. At this time, from experience with surface stimulation [6], it is expected that improvements in walking would have stabilised.

Responses to implanted stimulation were first recorded in the seated position, with the knee and ankle at 90 degrees as defined in the study protocol [3], using the transmitter positioned as by the subject. If appropriate, a new position was located by one of the research team. Stimulation intensities were as set by the clinician at previous appointments to assist in walking. Each subject was also asked to produce a maximum voluntary movement into dorsiflexion.

Selectivity and sensitivity tests were conducted in this same position. Selectivity in ankle movements was evaluated by measuring recruitment curves for each of the ankle moments, while adjusting each channel individually. Intensity was measured on a CRO (1MΩ) using a double wire loop held to the body of the transmitter. During these tests the other channel remained at its minimum setting. Sensitivity was defined as change in dorsiflexion moment with proximal/distal and medial/lateral displacement of the transmitter. In 3 subjects, sensitivity was also measured while stimulating by surface electrodes by altering the position of the active electrode.

The ‘chair’ was then adjusted to accommodate the hips and knees in extension to investigate hip, knee and ankle moments arising from either direct stimulation or by eliciting the withdrawal reflex. These tests were conducted both by implanted and surface stimulation for comparison.

3. Results

  1. Stimulation responses
  2. Using the set-up parameters, moments were measured from each subject that were appropriate to assist in walking, i.e. dorsiflexion, eversion and ankle abduction. A typical response is shown in figure 1. The moment is observed to increase rapidly, be maintained at a stable level, before decreasing as stimulation is terminated.

    Fig. 1. Isometric moments from stimulating both nerve branches (s4)

    It is interesting to note that each subject could elicit voluntarily a similar level of dorsiflexion moment to that attained by stimulation, but the responses were not as stable and were often associated with increased inversion.

  3. Selectivity
  4. A typical example of the results from measuring the muscle recruitment curve is shown in figure 2. It illustrates the selectivity in dorsiflexion and eversion moments by adjusting each channel individually. Both channels are observed to cause ankle abduction. Each muscle recruitment curve for each subject indicated that after threshold stimulation intensity there was an almost linear response in moment with increasing stimulation intensity.

    Fig. 2. Recruitment curve (s5)

    For an individual subject and stimulation channel, threshold intensities for each moment in the three dimensions were recorded to be the same. These are shown in table 1.

    Table 1. Channel threshold intensities (in mV)



















  5. Spatial sensitivity
  6. Spatial sensitivity tests indicated that a 10 mm displacement in any direction from the ‘optimal’ position resulted in approximately 50% reduction in dorsiflexion moment. Interestingly, similar sensitivity in altering the position of the active electrode existed for the surface stimulator, figure 3.

    Fig. 3. Effect from transmitter/electrode displacement subject 4; (solid ‘surface’, dotted ‘implant’)

    On presenting for these tests, 2 of the 5 subjects had their transmitter positioned 10+ mm from its optimum position.

  7. Extension tests
  8. Joint moment responses at the ankle, measured from the subject while the legs were extended, were similar to those from the ‘seated’ tests.

    Moments at the knee and hip were more complicated. Though the flexor withdrawal reflex can be elicited using surface electrodes, a strong response is not always observed. These results support that finding, with only one subject (s3) demonstrating the hip and knee flexion. For this subject, there was no observed difference in reflex response in using the implanted or surface stimulator.

    4. Discussion

    Comparing responses from implanted to surface stimulation, both methods, when set up correctly, can generate moments appropriate for gait assist. In regard to hip and knee movements, neither system was superior in eliciting components of the withdrawal reflex. Perhaps the main question is therefore whether implanted stimulation provides improved selectivity in movements and is easier for the patient to use.

    It is our opinion that adjustment of surface electrodes can provide selective movements, but skill is required. Training to the patient supports this, but a high proportion experienced problems [2], complicated by reduced function in the hemiplegic arm. In some cases correct positioning of electrodes by an experienced clinician is difficult. These results for the implanted stimulator demonstrate selectivity in the primary movements (dorsiflexion and eversion), the responses often being linear providing ease in adjustment. This supports the work from the collaborating centre on this pilot trial in the Netherlands [7].

    Perhaps the most disappointing result is the spatial sensitivity while using the implant. Kenney et al. [3] suggested that experience with surface stimulation demonstrated increased sensitivity to electrode position, as compared to this implant. This is not supported by these results. It may be argued that a transmitter is easier to position, but the reader should also be aware that 2 of the 5 subjects presented with their transmitter not in an optimum position. Therefore a level of training to the patient is still required.

    These results are being used to inform the pilot trial of behaviour and selectivity of responses while stimulating by implant.


    [1] Burridge et al. The effect of common peroneal nerve stimulation on the effort and speed of walking – a randomised controlled trial with chronic hemiplegic patients. Clin Rehabil, 11: 201-210, 1997.

    [2] Taylor et al. Patient perceptions of the Odstock Drop Foot Stimulator. Clin Rehabil, 13: 333-340, 1999.

    [3] Kenney et al. Initial results from two trials of an implantable two-channel drop foot stimulator. 7th Int Wshop FES, Vienna Austria, pp.192-195, 2001.

    [4] Taylor et al. Pilot study to evaluate the safety and efficacy of an implanted dropped foot stimulator (IMPULSE). This proceedings, 2002.

    [5] Wood et al. Apparatus to measure simultaneously 14 isometric leg joint moments, Part 2 Multimoment chair system. Med Biol Eng Comp, 37(2): 148-154, 1999.

    [6] Taylor. The use of electrical stimulation for correction of dropped foot in subjects with upper motor neuron lesions. Adv Clin Neuroscience Rehabil, 2(1): 16-18, 2002.

    [7] Hutten et al. The sensitivity and selectivity of an implantable 2-channel peroneal nerve stimulator system for restoration of dropped foot. 7th Annual Conference of IFESS, Ljubljana, Slovenia, pp. 183-185, 2002.



    The work is funded by the UK Medlink programme (IMPULSE project) and Salisbury Health Care NHS Trust.