Patient Characteristics and Screening

Twenty-seven patients (14 men, 13 women) with a stroke-related spastic hemiparesis, including a drop foot of at least 6-months duration, were treated with the implantable 4-channel drop foot stimulator, ActiGait (for patient characteristics see Table 1). The mean age of the patients was 51.0 years (range 24–66 years). The mean duration of stroke-related drop foot was 63.0 months (range 6–244 months). The drop foot was right-sided in 15 and left-sided in 12 patients. The cause of the central palsy was ischemic in 21 patients and hemorrhagic in 6 patients.

The drop foot was diagnosed according to the criteria of Perry et al. 1995,¹⁴ with a dragging foot during the swing phase. Prior to surgery, the possible benefit of the implantable ActiGait stimulator was tested by external functional surface stimulation of the peroneal nerve (CE-FAR Step II, output 60 mA, frequency 20–100 Hz; Cefar-Compex Scandinavia AB). If a strong dorsiflexion of the ankle could be evoked, the patient was considered to be a candidate for stimulator implantation. In addition, MRI of the leg was performed prior to surgery to exclude any peripheral injury of the peroneal nerve and to determine the anatomical localization of the bifurcation of the sensible branch of the common peroneal nerve for a proper electrode positioning.

Gait Tests and Quality of Life Survey

To evaluate whether patients with an ActiGait stimulator show changes in walking speed, they were asked to walk 20 meters, and the required time for this action was noted. This test was repeated 3 times without walking aids, with an ankle orthosis, with a surface stimulator preoperatively, and 6 weeks after implantation of the ActiGait system. In each case, the task was repeated with normal and maximal gait speed. Further, patients were asked to walk 6 minutes continuously pre- and postoperatively (walk endurance). The distance covered was measured in meters. Finally, the time needed to stand up from a chair, walk 3 meters, and return to sit down again was measured pre- and postoperatively to determine the needed activation time and to assess the risk of falls (timed up-and-go test).¹⁵ Furthermore, the patients were asked in a subjective quality of life survey about their satisfaction with the implanted device (Fig. 1). All tests were performed by a neurosurgeon (K.D.M.).

Quality of life and patient satisfaction survey of the patients who received the ActiGait drop foot stimulator.

Actigait Drop Foot Stimulation System

The ActiGait stimulation system consists of an implantable 4-channel nerve stimulator with a 12-contact electrode cuff, an external control unit, and a heel switch (Fig. 2). The heel switch is worn in a sock and triggers the initiation and termination of each stimulation sequence by a radiofrequency wireless signal to the external control unit when the foot is lifted from the ground. The external control unit is fixed to a belt and can be linked to a personal computer by the physician to set up the individual stimulation parameters. The external control unit enables powering and controlling of the implanted nerve stimulator by an externally wired antenna positioned on the skin over the subcutaneously implanted nerve stimulator body. The nerve stimulator contains a receiver for power and control, and transmits the stimulation to the 12-electrode cuff through a subcutaneous wire.

Illustration of the ActiGait drop foot stimulation system. The hybrid system consists of an implanted nerve stimulator connected to the 12-electrode cuff around the peroneal nerve and an external control unit that contains the power supply, which is triggered by the external heel switch. Copyright Klaus Daniel Martin. Published with permission. Figure is available in color online only.

Surgical Procedure

Surgery was performed under general anesthesia as previously described by Burridge et al.² In brief, the common peroneal nerve was exposed above the knee for 4 cm using a longitudinal incision along the tendon of the biceps femoris muscle. For the implantation of the stimulator body, another curved incision at the anterolateral aspect of the upper leg was performed. Ankle dorsiflexion was tested using a nerve stimulator (GN 015; maximum 6 V, 10 mA, 2.5 Hz; Aesculap). Afterward, the electrode cuff was closed around the nerve and the stimulator body was fixed to the lateral femoral fascia. Before the closure of the incisions, the correct position of the cuff and the stimulator was controlled visually in maximal flexion and extension of the knee, ensuring a free gliding of the cuff.

Postoperative Care and Programming of the Stimulator

During the first 3 to 5 weeks, the patients had to restrict the movement of his or her operated leg to a maximal knee flexion of 30°. During this period the Achilles tendon was passively stretched to avoid contractures. Afterward, the programming of the system was started. Programming parameters were frequency, stimulation intensity, ramping, and timing of the stimulation channels. Every channel was tested for the best dorsiflexion and the stimulator was activated with 1 mA, 20–30 Hz, and an optimal impulse duration of 70 μsec (Table 1). Patients were able to readjust the impulse intensity with a patient programmer. Finally, gait evaluation was performed after 4 weeks, measuring gait speed, walk endurance, and the timed up-and-go test. In addition, the subjective quality of life survey was administered.

Statistical Analysis

The values of the gait tests are expressed as means ± standard deviations. The significance level was calculated with a Wilcoxon rank-sum test using SPSS software (version 22, IBM). A p value < 0.05 was considered statistically significant.

Surgery and Postoperative Care

The implantation of the ActiGait system in all patients was easy and without any technical complications. Postoperatively, the implantable device did not appear bulky, but was palpable in all patients. Patients 1 and 10 suffered from a nerve injury but recovered completely after 18 months. In both patients, a reoperation was completed. In patient 1, no organic correlation was found, whereas in patient 10 the nerve injury appeared to be due to a nerve pull by the cuff electrode, which was corrected intraoperatively. In patient 8, a wound healing disorder was found that was then treated locally with a wound examination; no further complications were found. Patient 27 suffered from neurodermatitis and developed an infection of the implant, which ultimately had to be removed. Bleeding was not observed, although 21 patients with an ischemic stroke were treated with acetylsalicylic acid 100 mg per day continuously.

In all patients, the programming of the ActiGait stimulator was uncomplicated, as described above. Usually, channel 1 and/or 4 were activated to recruit the motor fibers located anteriorly in the nerve bundle. The handling of the ActiGait system was described by all patients as being uncomplicated and easy in daily living. There were no technical defects of the implanted components. However, in 80% of the patients, the external heel switch had to be replaced because of a defect. Depending on the activity level in their daily life, the patients had to charge the ActiGait stimulator every 3 to 7 days for approximately 1 hour.

Walking Speed (20-m Gait Test)

Results from the pre- and postoperative gait tests of the patients who received the ActiGait drop foot stimulator. A: Maximal and pleasant walking speed without walking aids, with ankle foot orthesis, and with a surface stimulator (all preoperatively) compared with postoperative walking speed with the ActiGait system. B: Pre- and postoperative walking endurance measured by the distance that was covered within 6 minutes. C: Pre- and postoperative timed up-and-go test.

Six-Minute Walking Test

Gait endurance was registered by measuring the covered distance of patients once they had been walking for 6 minutes. Without any walking aids, patients covered on average 196 ± 40 meters in 6 minutes. With ActiGait stimulation, the mean distance increased significantly (p < 0.0001) up to 401 ± 32 meters (Fig. 3B).

Timed Up-and-Go test

To compare the activation time and the risk of falls in patients without any walking aids and with the implanted ActiGait stimulator, the timed up-and-go test was performed. Without walking aids, the needed time was 20.5 ± 5.3 seconds, which means a functionally relevant mobility impairment (> 20 seconds needed). With ActiGait stimulation, the time was reduced significantly (p < 0.0001) to 10.6 ±2.7 seconds, which can be described as mild mobility impairment (11–19 seconds needed; Fig. 3C).

Subjective Quality of Life and Patient Satisfaction Survey

For subjective changes in daily living after the implantation of the ActiGait stimulator, patients were asked to answer a short quality of life survey (Fig. 1). Twenty-one patients (78%) noticed a complete return to normal mobility and 5 patients (18%) stated a pronounced improvement of mobility. Only 1 patient (4%) observed no changes. Social participation improved for 2 patients (7%) marginally, 6 patients (22%) reported a marked improvement, and 18 patients (67%) noticed a complete return to normal. One patient (4%) reported no changes in social participation. The quality of life improved markedly for 6 patients (22%) and returned to completely normal for 20 patients (74%). One patient noticed no differences in quality of life. Twenty-three patients (85%) stated they would recommend the operation, while 3 patients (11%) were unsure and 1 patient (4%) would advise against implantation. This patient, who also reported no changes in quality of life, mobility, and social participation, suffered from the nerve injury mentioned above.