A fairly common walking disability is known as dropped-foot syndrome, a chronic condition characterised by the inability to hold the foot raised during the swing phase of walking. Stroke victims are most likely to suffer from dropped- foot syndrome, but also people with incomplete spinal cord injuries and MS patients often have this condition. A sufferer from dropped-foot syndrome has to develop a peculiar and difficult gait in order to be able to make progress by walking, and activities such as ascending and descending stairs can be extremely difficult. As such, there have been considerable efforts at relieving the symptoms of dropped-foot syndrome.

Various mechanical devices have been developed in order to compensate for dropped-foot syndrome. For example, the foot can be held against dropping by way of a moulded ankle-foot orthosis, usually in the form of a lightweight plastic device that fits behind the leg and under the foot. Also known are spring-assisted ankle-foot orthoses, which permit selection of the amount of ankle motion, with selected degrees of dorsiflexion assistance.

Unfortunately, the known ankle-foot orthoses have major drawbacks, including high maintenance, noise and wear and tear on clothing, shoes and furniture.

In view of the generally less than totally satisfactory nature of various designs of ankle-foot orthoses, there have been proposals for directly stimulating the peroneal nerves in an affected leg, to cause appropriate operation of the muscles in the ankle-foot region over the required period of a walking cycle, such that the foot does not drop during the swing phase of the gait. In one known system, a small switch is located between the heel of the patient and a shoe worn on the affected foot, which switch is connected to a control unit by wires leading to a control unit either strapped to the patient's leg or carried elsewhere about the patient's body. That control unit is connected to electrodes which are coupled to the deep and superficial peroneal nerves; the control unit supplies to those electrodes stimulating signals following recognition of the heel being lifted off a surface, at the commencement of the

swing phase. Typically, after a fixed delay following switch operation on the lifting of a heel, a pulsed current for stimulating the peroneal nerves is ramped up to preset values and continues at an appropriate frequency during the swing phase. Following detection of the heel being placed on a surface at the end of the swing phase, the stimulation current continues for a fixed time before being ramped down to zero.

A system such as that described above is capable of greatly improving the gait of a dropped-foot syndrome patient, who becomes able to adopt a near-normal walking gait. However, the provision of a switch in a shoe, below the heel of the patient, leads to various complications including the need always to wear a shoe to ensure the switch is properly located below the heel and is operated on lifting the leg. Further, experience has shown that the switches are less than wholly reliable and need frequent replacement for continued operation of the device. There are also difficulties in ensuring the wires between the switch and the control unit remain connected and are not damaged or otherwise disturbed by the shoe, clothing and general wear and tear.

The present invention aims at improving the known form of apparatus which directly stimulates the nerves in the affected leg of a patient, to mitigate the effects of dropped-foot syndrome, as well as to improve on the known form of system where a heel switch is employed to detect the swing phase of a walking gait.

Accordingly, one aspect of this invention provides apparatus to stimulate peroneal nerves in a leg of a patient to mitigate dropped-foot syndrome, which apparatus comprises a sensing unit adapted to be mounted on the affected leg of a patient, a control unit, and a driving unit having electrodes for connection to the appropriate nerves in the leg of a patient, the sensing unit including sensing means to determine acceleration in two distinct directions and at least one angular rate sensor, the sensing means and the angular rate sensor together being arranged so that when in use the sensing unit detects acceleration in a generally vertical plane and the angular rate sensor detects acceleration about a generally horizontal axis, the outputs of the sensing unit being supplied to the control unit which processes said outputs and supplies a drive signal to the

driving unit to cause triggering of the nerves in the leg of a patient to control foot movement.

It will be appreciated that with the apparatus of this invention, no reliance is placed on the use of a mechanical switch to detect heel lifting and heel striking, at the two ends of the swing phase of a walking cycle. Rather, a determination is made of when to stimulate the peroneal nerves in the affected leg from the dynamics of the walking gait, as detected by a sensing unit adapted to be mounted on the affected leg of a patient and including appropriate sensing devices for leg movement.

It has been established that it is possible to determine the gait phase by analysing inertia movement data from the lower leg. Inertia motion sensors need no external reference for accurate measurement of motion quantities and recent advances in solid state miniaturisation of motion sensors have made them ideal for employment in the apparatus of this invention. The signals from the motion sensors may be analysed in real-time, so that the swing phase of the gait cycle can be determined, on the basis of the output from the sensors.

Micro-system technologies (MST) may be employed in the fabrication of suitable motion sensors, typically using piezo-electric devices to detect both linear acceleration and angular rate. Such sensors may be made by known MST/MEMS (micro-electro-mechanical systems) processes. Since the production of such devices is known in the art and forms no part of this invention, they will not be described in any detail herein.

Preferably, the sensing means comprises two linear accelerometers, the axes of operation (detection) of which are substantially orthogonal. In this case, the axes of operation preferably lie in a common plane, which should be disposed substantially vertically when the sensing unit is in use. Though not essential, it would be possible to use a third linear accelerometer arranged with its sensing axis in a third distinct direction, which preferably is substantially orthogonal to the two distinct directions of the first-mentioned linear accelerometers. This could be used to give better control of foot movement, which may be desirable for some patients. In addition to the accelerometers, the angular rate sensor preferably also is an MST device, in the form of a solid-

state gyroscope. Preferably, the axis of sensing of that device is normal to the plane containing the axes of operation of the two accelerometers of the sensing means.

The driving unit preferably is self-contained and apart from the electrodes which connect to the peroneal nerves, should be encapsulated in a bio-stable material whereby the driving unit may be implanted in an affected leg of a patient. The implantation site must be selected by the surgeon depending upon the patient's anatomy but should be on the outside of the affected lower leg, just below the knee and typically no more than 200mm from the knee joint.

The driving unit need not contain any power source and may derive its power from the control unit, the control and driving units being inductively coupled whereby signals are inductively transmitted by the control unit to the driving unit. Those signals preferably are in two separate channels to permit the transfer of the required control signals for the electrodes connected to the superficial and deep peroneal nerves, respectively, as well as the transfer of power to the driving unit.

In order to minimise the size of both the control unit and the driving unit, it is preferred for the inductive coupling between the two units to be by way of flat-wound coils which are closely juxtaposed but with body tissue therebetween, so minimising both the internal and external intrusiveness of the two units.

In a preferred form of this invention, a power feed-back loop is provided from the driving unit to the control unit, whereby the control unit may adjust the power level supplied to the driving unit so as to lie within a preset range. This can be important to ensure proper stimulation of the peroneal nerves and by monitoring the power transferred from the control unit to the driving unit, it becomes possible to compensate for possible misalignment between the control unit and the driving unit.

The superficial branch of the peroneal nerve supplies the peroneus longus/brevis and sometimes the extensor digitorum brevis. The deep branch supplies the tibialis anterior, extensor hallucis longus, extensor digitorum longus and peroneus tertius muscles. During the swing phase in normal walking ; the

activity of tibialis anterior combines with two other dorsiflexors, extensor digitorum longus and extensor hallucis longus, to dorsiflex the foot, whilst maintaining adequate balance between the everting action of the extensor digitorum longus and the inverting action of the tibialis anterior. By appropriate stimulation of the two branches of the peroneal nerve, it is possible to control dorsiflexion/plantarflexion inversion and eversion of the leg/foot joint but the required balance is extremely difficult to achieve and the result is often excessive eversion resulting from the action of the two peroneus muscles.

Therefore, control over the relative stimulation levels to the superficial and deep branch is necessary to provide the necessary moment to balance inversion and eversion.

The control system of this invention, especially when used in combination with power feedback, permits excellent control of the relative levels of stimulation for the two peroneal nerve branches, so permitting good relief from dropped foot syndrome within a wide range of operating parameters.

By way of example only, one specific embodiment of apparatus of this invention will now be described in detail, reference being made to the accompanying drawings, in which:- Figure 1 diagrammatically shows the system mounting and configuration; Figure 2 is a simplified transmitter diagram; Figure 3 is a block diagram of the operation of the system ; and Figure 4 is a simplified flow chart of the system.

The embodiment of apparatus of this invention is intended to control foot movement, for use by a patient suffering from dropped-foot syndrome. The apparatus comprises a main unit 10 intended to be strapped to the upper part of the lower leg, typically not more than 200mm below the knee joint. The main unit 10 may take the form of a relatively flat essentially rectangular box within which the required electronics and sensors are mounted, the box being retained in position by one or two elastic straps which extend around the calf. When positioned as shown, the sensing unit is above the region of greatest calf muscle girth, such that there will not be any significant tendency for the control unit to descend the calf, when in use.

Contained within the main unit is a battery for powering the apparatus, which battery preferably is rechargeable. Recharging may be achieved inductively or by the physical connection thereto of a suitable charger, typically overnight when the main unit has been removed from the leg of a patient.

In addition to the power source, the main unit encloses a sensing unit which includes two linear accelerometers arranged with their sensing axes orthogonal to each other and lying in a common plane. In use, with the unit strapped-to a leg, the common plane is intended to be essentially vertical, with the accelerometer axes extending generally along the length of a leg and in the general direction of walking, as shown by the directions X and Y on Figure 1.

The sensing unit also includes an angular rate sensor in the form of a solid- state gyroscopic device, operating about an axis of rotation Z as shown in Figure 1, orthogonal to axes X and Y.

Further contained within the main unit 10 are the required control electronics for processing the outputs of the two accelerometers and the rate sensor, and providing an RF signal to a coil within the main unit, for inductively coupling to an implanted driving unit (not shown, to be described below). The control electronics permit the adjustment of the signals generated for peroneal nerve stimulation in order to achieve the required correction characteristics for the dropped-foot syndrome. In particular, allowance may be made for adjustment of the power supplied to the nerves, the duration of the supply, the rise and fall times of the signals and the frequency of the pulses supplied.

Other parameters may be made adjustable as required.

Typically, the control of the supply of the signals may be by means of a processing unit which may be programmed by connecting to the main unit a suitable controller, preferably by way of an RF link to the main unit. The controller may then be operated by a clinician or physician on setting up the system and when completed, the patient will have no ability to adjust the system. Thus, the clinician or physician may adjust the system to suit a patient, the system then operating without further adjustment for an extended period of time. As such, the only control required by the patient is an on/off switch, on the main unit.

The driving unit is implanted subcutaneously, immediately below the normal position of the control unit when secured to the lower leg by the elastic straps, the driving unit having a pair of electrodes which are connected to the superficial and deep peroneal nerves within the affected leg. The process of connecting to those nerves is known and understood by those skilled in the art and will not be described in detail here.

The driving unit takes the form of a two-channel receiver or two parallel receivers operating at different frequencies and connected to a common planar coil all sealed within the driving unit body which typically will be of a button- shape, with a diameter of less than 35mm and a thickness of less than 6mm.

The driving unit should be capable of remaining implanted on a long-term basis, without causing expulsion, erosion or a related pathological event. The driving unit should have suitable suture sites or otherwise provide a clinically- acceptable method of fixation to the surrounding tissue.

Figure 2 diagrammatically illustrates a simplified form of the transmitter system. The transmitter (which is external to the leg and contained within the main unit 10) includes a planar coil LTx the frequency of which can be switched by means of switch S1, to operate at typically 1MHz or at 2MHz. Planar coils give the advantage of increased reliability, a lower profile and can be made to tight tolerances. Within the driving unit, implanted below the skin shown in Figure 2, there are two separate planar coils, one for each of the two frequencies of the transmitter. The resonant frequency of LRx1 and its capacitor C1 is 1 MHz and the resonant frequency of LRx2 and its capacitor C2 is 2MHz, such that discrimination between the two channels can be achieved.

As the driving unit is implanted only just subcutaneously and the transmitter within the main unit is intended immediately to overlie the driving unit, only relatively low powers are required with a range of typically less than 25mm, at the operating frequencies.

Figures 3 and 4 show the operation of the system. The outputs of the X-axis accelerometer, the Y-axis accelerometer and the Z-axis rate sensor are supplied to an inertia system analyser the output of which is supplied to a signal processor within which are set the required trigger levels and timings. A

controller, display and waveform setting module is connected both to the signal processor and to the transmitter, for the production of the required trigger signals at the appropriate power levels, to give the necessary degree of stimulation to the peroneal nerves. An operator interaction device is able to talk to the waveform setting module to permit the setting of the required power levels, pulse frequencies and so on to give proper operation, but that interaction device will normally be used only by a clinician or physician.

The waveform setting module communicates with the transmitter section, for the supply of the RF signals to the driving unit implanted in a leg. Power for all of these described components is derived from a rechargeable battery also located within the main unit and having a voltage regulator as well as a charging unit.

The passive receiver is shown in Figure 3, as receiving signals from the transmitter. In broken lines, there is shown a power feed-back loop giving power control. By having the transmitter monitoring the feed-back loop from the receiver, the power supplied to the transmitter can be controlled to ensure that the required power level is received by the driving unit for proper driving of the peroneal nerves, despite possible misalignment between the transmitter and the receivers of the driving unit.

Also shown in Figure 3 is a biometric feedback path. The receiver stimulates the nerves in a leg which causes a foot to rotate and that rotation will be detected by the sensors of the sensing unit. The response may be compared by the signal processing unit with that which had been demanded by the system and the continuing stimulation of the peroneal nerves may be modified to take into account that response.

Figure 4 shows the processing steps described above, from the swing phase of leg movement being detected to nerve energisation to achieve rotation of the foot, thus minimising the effect of dropped-foot syndrome. The removal of the supply of the signals to the peroneal nerves may be performed in much the same way, by detecting the termination of the swing phase and then shutting down the supply of power to the receivers, so that the energisation of the nerves ceases.

If power control is to be provided, then a feed-back loop is provided from the"receive and decode signal"step of Figure 4, to be transmitted back to a receiver within the control unit, which receiver then causes the waveform setting module suitably to adjust the power supplied to the transmitter of the control unit. Further, there will be biometric feedback in that the inertia sensors will sense the rotation of the foot that takes place, which can then be compared to the demanded rotation by the system.