GB2393659A

GB2393659A - Nerve routing device for bridging non-functional parts of a spinal cord

Info

Publication number: GB2393659A
Application number: GB0322558A
Authority: GB
Inventors: Martin Stuart Jones
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-09-27
Filing date: 2003-09-26
Publication date: 2004-04-07
Also published as: GB0322558D0; GB0222452D0

Abstract

The present invention relates to a prosthetic device for use in at least partially restoring the function in a spinal cord in which said function has been lost. The prosthetic device comprises first and second implants 11, 12 positionable at first and seconds ends respectively of a non-functioning section 17 of the spinal cord so as to contact functioning nerves within respective adjacent first and second functioning sections 21, 22 of the spinal cord. The implants are reactive to and stimulatory for said functioning nerves. A routing device 14 connected to the first and second implants maps and relays signals between said first and second implants, whereby an impulse from a specific functioning nerve within a functioning section of the spinal cord detected by the adjacent implant is mapped by the routing device and relayed to the other implant to effect stimulation of the corresponding specific functioning nerve in the other functioning section of the spinal cord.

Description

1 2393659

! PROSTHETIC DEVICE

The present invention relates to a prosthetic device. More particularly, the present invention relates to a prosthetic device operative to provide at least partial restoration of spinal cord function in circumstances where said function has been lost.

5 An injury to a person's spinal cord can cause severe disability, including paraplegia and quadriplegic. Such injury may occur, for example, in road traffic accidents, falls, etc. The spinal cord may remain intact after an injury, but the spinal cord function may be lost.

The spinal cord is normally protected from damage by the vertebrae. Inside the 10 spinal cord are two distinct areas: grey matter, which is responsible for reflex actions, and the white matter, which is responsible for carrying signals along the length of the spinal cord (see Figure 1). The cord varies in thickness from approximately 70mm to 1 30mm Nerve impulses are transmitted by the axons of nerve cells (see Figure 2).

15 Axons run along the spinal cord and form the white matter. At rest, an axon maintains a potential difference between the axoplasm and the extracellular fluid, these two fluids being separated by the axon membrane. It is this membrane that is responsible for ensuring that a potential difference of approximately -70mV exists between the axoplasm and the extracellular fluid. Nerve impulses travel along the length of an axon 20 as a fluctuation in this potential difference. In order to establish such an impulse, the potential difference must be altered beyond a certain threshold voltage, such as -60mV.

Once this voltage has been surpassed, an 'action potential' is achieved and the nerve is stimulated. Impulses typically only travel in one direction because they are unable to

( travel back to an area of the axon membrane until that area has restored itself to the resting state, an operation that takes time.

Axons in the white matter sections are encased in myelin. This is a fatty, insulating, substance that increases the speed of transmission of impulses. It is also S what gives the white matter its colour. The casing of myehn Is not continuous, but has minute interruptions known as nodes of Ranvier situated along the length of the axon.

These serve as points along which the impulseumps rapidly.

A number of techniques are currently used to attempt to restore spinal cord function. The most commonly cited works Involve nerve regeneration through the use 10 of drugs, stem cells, or electrical stimulation. However, none of these techniques have yet proved to be sufficient or satisfactory. Although it is possible to use an implant to stimulate nerve signals, it is extremely difficult to stimulate or react to individual nerves within a bundle of the type found within the spinal cord.

It is an object of the present invention to provide a means for at least partially 15 restoring spinal cord function.

According to a first aspect of the present invention therefore there is provided a prosthetic device for use in at least partially restoring the function in a spinal cord in which said function has been lost, the prosthetic device comprising first and second implants positionable at first and seconds ends respectively of a non-functiomng section 20 of the spinal cord so as to contact functioning nerves within respective adjacent first and second functioning sections of the spinal cord, the implants being reactive to and stimulatory for said functioning nerves, and a routing device connected to the first and second implants and operative to map and relay signals between said first and second implants, whereby an impulse from a specific functioning nerve within a functioning

( section of the spinal cord detected by the adjacent implant is mapped by the routing device and relayed to the other implant to effect stimulation of the corresponding specific functioning nerve in the other functioning section of the spinal cord.

The prosthetic device of the present invention thus provides a means for 5 circumvention of the non-functioning section of the spinal cord, thereby allowing transmission of specific nerve impulses.

With regard to the first and second implants, these may take any suitable form.

The implants may be substantially the same or different from one another. The implants may take the form of flat plates.

10 Each Implant has a conductive surface capable both of supplying and detecting electricity at specific sites on the conductive surface, said surface comprising a plurality of specific contact points. The contact points have very small surface areas relative to the conductive surface as a whole, so that a high density of contact points is provided on the conductive surface of the implant.

15 Each implant also has a reverse surface that Is non-conductve. In use, the implant is positioned such that the conductive surface of the implant contacts functiomng nerves within the adjacent functioning section of spinal cord, whereas the non-conductive surface faces the non-functioning section of the spinal cord.

The contact points on the conductive surface of each implant are sized and 20 arranged such that each individual functioning nerve fibre within the functioning section of the spmal cord can be addressed individually. A stimulus applied at a specific contact point on the conductive surface of the implant allows innervation of a specific functioning nerve within the functioning section of the spinal cord. Further, the ability

( to detect a potential difference at a specific contact point allows a nerve impulse from a specific individual functioning nerve to be received.

In order to facilitate insertion of the implants, said implants may be provided with at least one cuttmg edge.

5 Once in position, the implants may be fixed so that they move in concert with the spinal cord. Ideally, the positions of the first and second Implants relative to their respective adjacent sections of spinal cord should remain substantially constant.

Fixation of the implants following positioning may be achieved, for example, by attachment of each implant to surrounding vertebrae. Alternatively, or additionally, the 10 implant may be provided with a hole or holes through which tissue will grow to anchor the implant in position following implantation.

The first and second implants are preferably connected to the routing device by cables or the like, which convey signals between said implants and the routing device.

Signals may travel in either direction between the implants and the routing device.

15 With regard to the routing device, this may take any suitable form. For example, the routing device may comprise a small, mobile computer. The routing device may be positioned within the recipient. However, in order to provide the required computational power and energy requirements, the routing device may need to be external to the patient, possibly worn using a belt or sling arrangement. The routing 20 device maps an impulse received from a specific contact point on one of the implants to an outgoing stimulus on a specific contact point on the other implant. In order for the routing device to perform this function effectively, a period of 'learning' is required following implantation, since the relationship between specific contact points on the first and second implants will not initially be known. The implants may not always he

( inserted in exactly the same position in each patient, and the anatomy of each patient's spinal cord will vary. Thus, the present invention is not necessarily intended to align specific contact points on each implant with specific functioning nerves, but rather to establish which contact points correspond to specific nerves after implantation.

5 During the reaming period, the device reams which specific contact points on the conducting surface of one implant correspond to specific contact points on the conducting surface of the other implant so that an in-coming nerve impulse detected by one implant results in specific stimulation of the corresponding nerve by the other implant. In the case where the routing device comprises a computer, appropriate 10 software may be utilised in order to facilitate the reaming. For example, the software may make use of one of the many machine learning techniques, such as rule induction, genetic algorithms, neural networks, etc. The learning process may rely upon the application of a particular specific stimulus to a patient, which results in the generation of a specific nerve impulse. This 15 impulse is detected by a specific contact pomt on an implant. A signal carried to the routing device allows said routing device to map the in-coming impulse on said specific contact point to an out-going stimulation in turn of corresponding specific contact point(s) on the other implant. Through trial and error process, relying to a large extent upon the patient's own assessment, it is possible to identify which specific contact point 20 on the second implant must be stimulated in order to achieve the appropriate reaction in the patient. It will be appreciated that the learning period may therefore comprise a lengthy period of physiotherapy, each stimulus-reaction being matched in tum.

However, importantly, the map contained withm one routing device may serve as the basis for a map established within another routing device. Thus, the reaming

( period may be reduced over time by partially re-using the mforrnaton gathered during previous learning periods with other patients. There will, however, still need to be a period of adjustment for each individual patient.

The implants are preferably provided with more contact points than there are 5 adjacent functioning nerves. Thus, certain contact points will not be useful because they do not align with any functioning nerve fibres. The implants of the present invention could be described as self-aligning. The learning period allows the routing device to determine which specific contact points on the implants correspond to specific individual functioning nerves.

10 The routing device may correct for some movement of the Implants relative to the spinal cord.

The present invention may be used in providing a method of surgical treatment for at least partially restoring the function in a spinal cord in which said function has been lost, the method comprising inserting a prosthetic device as hereinbefore described 15 into the spinal cord of a patient requiring such treatment. It is likely that the insertion of the implants will cause an inflammatory response at the insertion sites. Hence, a steroid, or other drug such as methylprednisolone, may need to be administered in order to protect the surrounding spinal cord from the host inflammatory response.

Current work on spinal cord restoration looks at implanting Schwann cells into 20 the spinal cord in order to allow the reformation of myelin. Implantation of such Schwann cells and other cells believed to facilitate restoration of the body's natural structures may therefore also be performed. After a suitable period of recovery, the process of creating the map contained within the routing device begins.

The present invention will now be described further by way of example only and with reference to the accompanying diagrams in which: Figure I is a diagrammatic cross-sectional diagram of a spinal cord; Figure 2 is a diagrammatic longitudinal cross-section of an axon as found 5 in the white matter region of the spinal cord; Figure 3 is a diagrammatic perspective view showing a prosthetic device of the present invention in use; Figure 4 is a diagrammatic longitudinal cross-section of an axon with an implant in place; 10 Figure 5 is a diagrammatic plan view of one possible arrangement of the surface of an implant according to the present invention; Figure 6 is a diagrammatic plan view of one possible arrangement of the surface of the implant in relationship to an axon; Figure 7 is a diagrammatic plan view showing one possible way that 15 information flows onto and from an implant.

Referring to Figure 1, a spinal cord I comprises grey matter 2 and white matter 3.

Figure 2 provides a cross-sectional view of an axon 4, which has a membrane 5 surrounding the axoplasm 6 and separating it from the extracellular fluid 7. A myehn 20 sheath 8 insulates the axon, the sheath bemg interrupted at nodes of Ranvier 9.

Referring to Figure 3, a prosthetic device 10 comprises first 11 and second 12 implants inserted into the spinal cord 13 of a patient and a routing device 14 connected to the implants I 1, 12 by cables 16. The Implants I 1, 12 are positioned at either end of a non-functioning section 17 of the spinal cord 13. Each implant 11, 12 has a

conductive surface 18 and a non-conductive surface 19. The conductive surface 18 of each implant 11, 12 contacts a functioning section (21, 22 respectively) of the spinal cord 13. The non-conductive surface 19 of each Implant 11, 12 faces the non-

functioning section 17 of the spmal cord 13. Suitable materials for the implants include 5 those used for integrated circuits. Obviously, these materials support the electrical needs of such implants and recent research has developed a silicon 'patch clamp' replacement - a traditional device used in biology to determine voltages withm nerve fibres. Figure 4 shows a single implant 11 in position. The counterpart implant 12 is 10 not shown, its arrangement being almost identical to that of 11. The implant 11 is inserted into the spinal cord 21 so that it Cuts through' the axons. The conductive surface 18 of the implant 11 faces towards a functioning section of the spinal cord 21.

The non-conductive surface of the implant faces the non-functioning section 17 of the cord 21. Each axon consists of a central region, the axoplasm 6, surrounded by an axon 15 membrane 5. Surrounding the axon membrane 5 are myelin sheathes 8. Outside of the myelin sheathes 8 and in occasional contact with the axon membrane 5 at nodes of Ranvier is the extracellular fluid 7. Upon the conductive surface 18 of the implant 1 1 are a plurality of contact points 32, 33, three of which are shown in Figure 4. The contact points are surrounded by an insulating material 31. The contact points are at a 20 small enough scale that no single contact point 32, 33 is large enough to straddle the myelin sheath 8 to make contact with both the axoplasm 6 and the extracellular fluid 7.

In the case that the implant 11 is inserted into location at a node of Randier where there is no myelin sheath 8, then provided the contact point does not straddle the axon membrane 5, the device will continue to work correctly. However, there may be

( situations in which this Is not the case, and that axon might not be adressable by the implant 11. Provided that at least one of the contact points 32 contacts the axoplasm 6 and at least one contact point 33 contacts the extracellular fluid 7 close to the axon in question, then it is possible both to innervate and receive signals from the nerve. In S order for an axon to be stimulated, a potential difference above the action potential must exist across the axon membrane 5. This can be achieved by applying a potential difference between contact point 32 and contact points 33. This potential difference, through conduction in the fluids 6, 7, causes a potential dit'ference at the next node of Ranvier, and so triggers an impulse. An impulse is generated when the potential 10 difference across the axon membrane 5 differs by a given voltage, V. In order to avoid false stimulation of related axons, the voltage at contact point 32 contacting the axoplasm should be raised by V/2 volts, and the voltage at the contact point 33 contacting the extracellular fluid 7 should be lowered by V/2 volts. This means that the potential difference across the axon membrane S of the axon intended for stimulation 15 will equate to the action potential. However, for other axons, the potential dif't'erence will be below the action potential, and so they will not be stimulated. Additionally, this technique allows the implants to work with axons with differing action potentials.

In order to detect an impulse, no potential difference is applied, instead the difference Is measured between the contact points 32 and 33.

20 Figure 5 provides a simphfied representation of one possible configuration of the conductive surface 18 of the implants 11, 12. Contact points on the conductive surface 18 are arranged into a grid 23, controllers 24 connected to the routing device 14 by the cables 16 being operable to control the addressing; of individual sites on the grid.

Of course, it will be appreciated that topologies other than grids may be employed.

( Additionally, the arrangement of sites may be designed to match the areas of white matter 3 within the spinal cord.

Figure 6 shows an Implant I 1, 12 postioncd adjacent a specific individual nerve fibre 26 with axoplasm 6, axon membrane 5 and myelin sheath 8. It can be seen that the 5 conductive surface 18 of the implant I 1, 12 consists of a plurality of contact points 29, surrounded by an insulating material 31. The nerve fibre contacts the conductive surface 18. Suitable contact points 29 must be chosen to generate the required potential difference across the axon membrane 5. Certain contact points 32 only make contact with the axoplasm 6 and the myelin sheath 8. These are the internal contact points.

10 Further contact points 33 are in close proximity to the axoplasm 6 and are in contact with the extracellular fluid 7. These are the external contact points. It should be noted that for clarity not all of the suitable external contact points 33 are indicated. It is between these two sets of contact points 32, 33 that the potential dit'f'crence must be altered in order to generate the action potential. There may be a number of suitable 15 contact points, increasing the chances of any given nerve fibre being addressable.

As hereinbefore described, a learning period is used in order to 'teach' the routing device 14 which specific contact points can be used to innervate individual nerves. Many of the contact points will be unused and it may be desirable to abrogate the conductivity of such unused points, t'or example, contact points that straddle the 20 myelin sheath 8 or are positioned at a node of Ranvier 9. This might be achieved, for example, by 'short-crcuiting' such points.

In order to receive in-coming nerve signals, the potential dit't'erence at known specific contact points is determined. This can be done simply by measuring the potential difference between contact points 32 and 33. A capacitor-type device may be

used to store the potential difference that accumulates over time. Periodically, this may be read to determine whether the stored charge is sufficient to mdieate that a nerve signal has been detected. It may be necessary to assign one of three roles to contact points: points that stimulate nerve signals, receive nerve signals, or those that do not 5 perform a useful function. It may be possible to assign roles to contact points dynamically and after insertion of the Implants I 1, 12. However, this does not preclude static role assignment.

The functioning of implants 11, 12 relies on their ability to detect or apply voltages, under the control of the routing device 14, at one specific contact point 10 independently of all other eontaet points. There is likely to be a large number of eontaet points that must be addressed. Indeed, it may not be practical to permit individual concurrent addressing of the contact points owing to the resulting control electronics being too complicated.

Advantageously, therefore, rather than addressing individual points concurrently 15 the eontaet points may be chained together but capable of operating individually. This may be achieved by providing eontaet points capable of adopting one of two possible states: a passive state and an active state. While the eontaet point is in the passive state, it is accumulatively readmg the voltage at its surface but is not applying a voltage and so does not effect a stimulation of the adjacent nerve. Potentially, all contact points 20 would be in the passive state simultaneously. Although the state is tended passive this is only to indicate that the nerves are not bemg stimulated. The Implant is still doing useful work whilst the eontaet points are in this state. Some of the eontaet points 32, 33 are used in determining potential differences across the axon membrane 5. Possibly all of the contact points will be involved in conveying information that is used when the

( contact points move from the passive to the active state. The information conveyed Is data items representing voltages that should be applied when the change in state occurs.

One possible path that the information may take is shown in Figure 7. The contact pomts 29 are addressed m 'chains' 46 rather than individually. Figure 7 is a 5 simplification of what may occur, the chain shown having only forty contact points. In a reality there may be a different number of contact pomts m each chain, and each implant 11, 12 would have many such chains. The example in Figure 7 would allow the number of contact points 29 with which the routing device 14 must communicate to be reduced from forty to two, and so reduce complexity. While data flows onto the start of 10 the chain 46, data also flows off the end of the chain 46. While the contact points 29 are in the passive state, an input stream of data items 41 of the same length as the chain 46 is fed along the chain. While data Is being fed along the chain 46, a single item at a time, data is also being taken from the end 42 of the chain 46 (it should be understood that the data 41 and 42 do not represent physical objects). The items in the data chain 15 46 represent voltages that are neutral 43, slightly above the extracellular potential 45, or slightly below the extracellular voltage 44. Once all the data items have been fed along the chain 46, the contact points 29 momentarily change from the passive state to the active state. When the contact points 29 change state, they apply the voltage that has been transmitted to them through the chain 46 and simultaneously swap the data that 20 was on the chain 46 with any potential dit't'erences that they have been reading whilst in the passive state. After a short period, the contact points 29 return to the passive state and once more start reading and storing any potential differences that the nerves themselves are supplying. The data that flows off the data cham 46 is used by the routing device 14 to determine which nerves are carrying signals to the Implant 11, 12.

If the implant moves relative to the nerve, then it is possible to correct for such offsets using the routmg device 14.

It is of course to be understood that the present invention Is not intended to be restricted to the details of the above embodiment, which are described by way of 5 example only.

Claims

( CLAIMS

1. A prosthetic device for use in at least partially restoring the function in a spinal cord in which said function has been lost the prosthetic device comprising first and second implants positionable at first and seconds ends respectively of a non 5 tunctiomng section of the spinal cord so as to contact functioning nerves within respective adjacent first and second functioning sections of the spinal cord the implants bemg reactive to and stimulatory for said functioning nerves and a routing device connected to the first and second implants and operative to map and relay signals between said first and second implants whereby an impulse 10 from a specific functioning nerve within a functioning section of the spinal cord detected by the adjacent implant is mapped by the routing device and relayed to the other implant to effect stimulation of the corresponding specific functioning nerve in the other functioning section of the spinal cord.

2. A prosthetic device according to claim 1 wherein the first and second implants 15 are substantially identical.

3. A prosthetic device according to either of claims I or 2 wherein at least the first implant comprises a flat plate.

4. A prosthetic device according to any of claims I to 3 wherein at least the first implant is provided with at least one cutting edge.

20 5. A prosthetic device according to any of claims 1 to 4 wherein the Implants are adapted to be fixed m position to move with the spinal cord.

6. A prosthetic device according to claim 5 wherem the Implants arc adapted to be fixed to vertebrae.

( 7. A prosthetic device according to either of claims S or 6, wherein the implants are provided with at least one hole through which tissue may grow to anchor the implants in position following implantation.

8. A prosthetic device according to any of claims 1 to 7, wherein the implants are 5 connected to the routing device by cables.

9. A prosthetic device according to any of claims I to 8, wherein the routing device is positionable within a recipient.

10. A prosthetic device according to any of claims I to 8, wherein the routing device is positionable externally to a recipient.

10 11. A prosthetic device according to any of claims I to 10, wherem the routing device utilises machine learning software.

12. A prosthetic device according to claim I 1, wherein the machine learning software is selected from software based upon rule induction, genetic algorithms or neural networks.

15 13. A prosthetic device according to any of claims I to 12, wherein the routing device is pre-programmed with data from at least one previous use thereof.

14. A prosthetic device according to any of claims I to 13, wherein at least one implant is provided with a plurahty of contact points being greater in number than the adjacent functioning nerves.

20 15. A prosthetic device according to any of claims I to 14, wherein the routing device functions to correct any movement of the Implants relative to the spinal cord. 16. A prosthetic device according to any of claims I to 15, wherein at least one implant is provided with a plurality of contact points each being susccptblc to

( short circuitry whereby the conductivity of any missed contact pomts may be abrogated following implantation of the implant.

17. A prosthetic device according to any preceding claim, wherein at least one | implant is provided with a plurality of contact points and a capacitor Is provided 5 that is operable following implantation of the implants to store a potential difference between a first contact point In contact with the axoplasm and/or myelin sheath of an adjacent nerve fibre and a second contact point in contact with adjacent extracellular fluid, whereby nerve sgnalling may be detected.

18. A prosthetic device according to any of claims I to 17, wherein at least one 10 implant is provided with a plurality of contact points and said contact pomts each have a function selected from the following: stimulating nerve signals, receiving nerve signals and no useful function.

19. A prosthetic device according to claim 18, wherein the function of each contact point is assignable dynamically following implantation of the implant.

15 20. A prosthetic device according to claim 18, wherein the function of each contact point is assignable statically.

21. A prosthetic device according to any of claims I to 20, wherein at least one implant is provided with a plurality of contact points, said contact pomts being addressable in chains.

20 22. A prosthetic device according to claim 21, wherein each contact point Is capable of adopting either a passive or an active state.

23. A prosthetic device substantially as hereinbefore described with reference to and as Illustrated in the accompanying drawings.