Long-term results following electrical stimulation of the peroneal nerve using the ActiGait® system in 33 patients with central drop foot

Patients and methods

Gait tests

Gait speed was assessed using the 20 m gait test. Patients walked 20 m at normal and maximum speed, while the time was taken. It was performed pre-operatively without walking aids, and repeated once by using an orthosis and another time by use of a surface stimulation. The same gait test setup was repeated after the implantation of the ActiGait® system after 6 weeks, and after 6, 12 and 36 months.

The endurance of the gait was assessed by the six-min Test. The distance was measured while the patients were walking for 6 min before and after the implantation with and without walking aids (orthosis, surface stimulation, ActiGait® system). Patient’s risk to stumble and fall depend on the reliability and reaction time of the system. Therefore another test “TUG” [6] was performed pre- and post-implantation. Patients were sitting on a chair and were asked to rise, walk 3 m, return and sit down. The recorded time reflects the reaction time of the system.

Changes in the quality of life

The influence of the gait improvement on patients’ quality of life was evaluated using a questionnaire filled out pre-operatively, 6 weeks, 6, 12 and 36 months following implantation.

The ActiGait® system

The peroneal nerve stimulator ActiGait® is a partly implantable system consisting of external components and an implantable stimulator (Figure 1). The latter, comprising the cuff electrode connected to the stimulator body through an electrode cable is implanted in the thigh of the patient. The cuff electrode has four stimulation channels positioned at 90° to each other allowing a 360° spatial control over nerve stimulation. This allows the optimal selection of stimulation channels during the system activation. The external control unit delivers energy and control signals over the external antenna to the implant. Stimulation is synchronized with the gait pattern being triggered by an external heel switch placed in the shoe. The therapist can select the stimulation channels to obtain a balanced foot lift and can adjust stimulation intensity and frequency using the clinical interface and computer software.

Figure 1:

Schematic illustration of the implantable and non-implantable components of the ActiGait®-drop foot stimulator. The system consists of an external control unit (1) with a transmission coil (antenna) (2), which receives signals from the external footswitch (5) via a wireless radiofrequency signal. It enables precise activation depending on the gait cycle and allows for the adjustment of the stimulation parameters. The subcutaneously implanted stimulator (3) is connected to the 4-channel cuff (4), which is placed around the peroneal nerve. The stimulator communicates via a wireless radio frequency signal with the antenna and transmits the stimulation signals.

Surgical procedure

The MRI performed prior to the implantation shows the peroneal nerve path and the level of bifurcation of the sensitive branches in the fossa poplitea region, where the cuff electrode should be placed around the nerve. The surgery is performed under general anesthesia with patients placed in a side position. Two incisions are performed to place the cuff electrode and stimulator body. The common peroneal nerve is dissected over a length of 4 cm above the tibial plateau. The stimulator body is sutured on the muscle fascia in the thigh region and the connecting electrode cable is tunneled under the skin over the biceps tendon between the two incisions. After placing and closing the cuff electrode around the motor branch of the peroneal nerve, the system is functionally tested, and finally the wound is closed. A detailed description of the procedure can be found elsewhere [9].

Postoperative care

The system was activated four weeks after implantation, while incisions have been healed and cuff electrodes are fully encapsulated in scar tissue to avoid sliding and rotation around the peroneal nerve, and allow a stable and reliable stimulation. During activation all individual channels are tested in order to determine the foot movement and finally ensure an optimal foot lift (dorsalflexion) and balance (eversion or inversion). In addition, stimulation intensity (pulse width), ramp-up and frequency were optimized to give a balanced and sufficient foot lift. Follow-ups to optimally re-adjust stimulation were performed every 3 month during the first year and later on annually.

Statistical analysis

Measured values are indicated with mean and standard deviation. Statistical significance was tested using the Wilcoxon rank-sum method using SPSS statistical software (ver.22 IBM). Significance value was set to p<0.05.

Gait endurance (The six-min walking test)

The 33 patients covered in average a distance of 202 ± 41 m during the 6 min walk without walking aids. With the aid of the ActiGait® system, the covered distance increased to 380 ± 30 m (p=0.038). There was no difference in the distance by comparing measurements after 6, 12 and 36 months (p>0.05).

The 20 m gait test

For all 33 patients, the average time required to cover 20 m in a normal gait without walking aids decreased significantly from 31.8 ± 10.2 to 18.5 ± 4.6 s after the implantation (p=0.039) and was stable over 6, 12, and 36 months.

The results of this test by using an orthosis (27.3 ± 8.3 s) or surface stimulation (23.9 ± 6.5 s) suggest similarly a positive trend with increased speed. However, this was not as pronounced as observed after the ActiGait® implantation.

The timed up and go test

In the 36 months post-implantation follow-up, the test required 18.6 ± 4.8 s after switching off the system, which means a functionally relevant mobility impairment. Upon switching on the ActiAGait® system, the time was reduced to 11.8 ± 3.1 s (p=0.041). Notably, a similar trend was observed after 12 months post-implantation [7], [8], which underlines the stability of the system.

Complications

Overall, we observed a few complications during our survey. In two of the 33 patients, postoperative peroneal nerve lesions were noticed, which completely recovered within 24 months following implantation. In both cases immediate re-surgery was performed to exclude a cuff electrode dislocation. One patient suffered from an electrode cable breakage. The problem was solved by replacing the implant. There occurred one case of hematoma around the cuff electrode, three cases of wound healing problems and one case of edema in the thigh region. No irreversible peroneal nerve injuries were observed.

Patient satisfaction survey and subjective quality of life

To assess subjective changes in daily living after implantation of the ActiGait® stimulator, patients were asked to answer four questions related to QoL:

1. Did you notice any changes in mobility in the daily living? (Figure 2a),

2. Did you notice any changes in interpersonal contacts and social participation? (Figure 2b)

3. Did you notice any changes in your quality of life? (Figure 2c)

4. Would you recommend this treatment for other patients? (Figure 2d)

Figure 2:

(a) Quality of life questionnaire. (b) Quality of life questionnaire. (c) Quality of life questionnaire. (d) Quality of life questionnaire.

Thirty-one of the 33 patients reported remarkable improvements of their quality of life following direct electrical nerve stimulation (Figure 2).