Field of invention

The present invention relates to an implantable electrode array for intracochlear implantation. Background Art

Known cochlear implant devices require several improvements in order to overcome certain deficiencies of a mechanical or structural nature.

One difficulty relates to insertion. Ideally, and in the best surgical results, the electrode array is fully inserted to a depth of 1 1/2 turns of the scala tympani. The average insertion depth is, however, rather less than one full turn. This inhibits the ability of the array to stimulate those parts of the auditory neurons responsible for lower frequency sounds, and as a result induced hearing sensations may be limited to the higher frequency regions only.

A further problem with known intracochlear electrode arrays is that they often do not locate in the optimal position for stimulation. During the surgical insertion procedure a straight electrode array :ends to engage the outer wall of the scala tympani. However, optimal stimulation is obtained from locating the electrodes adjacent to the inner wall, near the modiolus.

Another consideration is the minimisation of trauma to the cochlea. Straight electrode arrays tend to distort the spiral ligament, basilar membrane or other structures during insertion, possibly causing trauma and creating insertion difficulties.

A spiral-shaped intracochlear electrode array has been developed by the University of California at San Francisco (UCSF). This device, however, is relatively thick, occupying almost all the scala tympani, thereby limiting the depth of insertion. Further, it is a "sided" device, whereby the electrodes are presented only on the inner and upper surfaces of the array and left and right electrode arrays are distinct. The limited surface area of the electrodes may result in higher charge densities, and contribute to tissue damage. It is an object of the present invention to provide an electrode array which at least ameliorates the disadvantages of the prior art.

Summary of the Invention

The present invention provides an electrode array having a terminal curved portion, and a straight portion, the curved portion having substantially the shape of a exponential curve with between 1 and 2 turns. Preferably, the device tapers towards the tip. Preferably, the tip of the array is somewhat less curved to facilitate easier insertion. Preferably the array is formed so as to be resilient and have memory, so that once straightened and released it assumes its predetermined shape. Preferably, the curved part of the array describes about one and one quarter turns with its axis corresponding to the function R (θ) = AeB( ^β + c ), wherein A is in the range of about 120 μm to about 144 μm, and B is about 0.2, and C is 7π/6.

Preferably, the array is formed from a silicone polymer such as Silastic MDX 4-4210, and the platinum lead wires have a diameter of about 25 μm and are formed from a platinum alloy having about 20% to about 30% iridium. Preferably the electrodes are formed from platinum bands, selected such that the bands will fit freely within the scala tympani, and the surface area of the bands is large enough to prevent the production of high charge densities in use. Brief Description of Drawings

One embodiment of the invention will be described with reference to the drawings, in which:

Figure 1 illustrates one form of the inventive electrode array; and - Figure 2 illustrates an insertion tool adapted for use with the inventive array.

Detailed Description A preferred embodiment of the invention will now be described. It is emphasised that this is illustrative and not intended to be limitative.

The embodiment of Figure 1 relates to one form of the present invention using band electrodes and being generally similar in construction to the

Cochlear Fry. Ltd. 22 channel device which is commercially available. The present invention relates solely to the construction, dimensions and mechanical properties exhibited by intra-cochlear electrode arrays.

In order to produce an array which was shaped so as to properly engage the desired portions of the scala tympani while maximising insertion depth and minimising trauma, the inventors have more accurately defined the shape of the inner wall of the scala tympani. The inventors made a number of moulds of human cochlear scala tympani from cadavers, characterised the shape relative to the central axis, and derived a mathematical expression to provide a best fit for the experimental data. The object was to provide a curved array which would be suitable for most patients in terms of minimising trauma, maximising insertion depth, and locating the array adjacent to the modiolus.

The following expression for the optimal shape of the longitudinal axis of the implant was obtained:

- R(θ) = AeB( ^β + c ), where R(μm = radial distance of the inner wall from the central axis of the scala tympani, θ = angle about this axis( 0< θ< 5π/2 ) and A, B and C are values obtained from experimental data. It has been observed that A has a mean value of 132 with a standard deviation of 8, B is consistently 0.2, and C is set as 7π/6 . It is accordingly preferred that B is about 0.2 and A is in the range of 116 to 148, preferably about 132.

Referring to Figure 1 , the preferred shape is illustrated. Array 10 comprises an optional angled portion 21 , a straight portion 16 including electrodes 12, and a curved portion 18 also including electrodes 12 which in general forms a spiral about axis 14.

It is noted that the tip region 20, containing about the last 2 bands before the tip, does not conform to the exponential curve, as this would be a very tight curve for the electrode array to pass the basal turn and difficult to insert. It is instead only slightly curved.

The illustrated array is tapered towards the tip. Preferred dimensions are about 0.6 mm at the outer end of straight portion16 and about 0.4 mm at the tip 20. This facilitates deep insertion while minimising trauma to the various structures.

It is important that the array have a structure which is flexible, to minimise insertion trauma, yet resilient so that the correct shape is resumed after the

insertion procedure. There is very little advantage in carefully characterising the shape of the array unless this is maintained after insertion.

One way to achieve this is to use the wires which are provided to electrically connect the electrodes. It is preferred according to the present invention that these be formed from a resilient material rather than 90% platinum

10% iridium alloy wire. One suitable material is an alloy of platinum with 20 to

30% iridium. The wires may be annealed.

A further aspect of the present invention relates to the method of insertion.

It will be appreciated that the inventive array may be inserted using any suitable technique.

One such method proposed according to the present invention involves the use of a suitable insertion tool. It will be understood that the diagram is somewhat exaggerated laterally so as to more clearly illustrate the operation of the tool. Referring to figure 2 .such a tool may comprise a teflon tube 32 with a slit

33 provided longitudinally, and a rod 31 within the tube 32 enabled to be moved longitudinally by depressing plunger 30.

In use, the curved 18 and straight 16 portions of the array 10 are inserted into tube 32 such that the curve of the array 10 is directed away from the slit 33. Array 10 includes flange 22, which in use is engaged by the rod 31 so as to move the electrode array 10 out from the end of the insertion tool. It will be understood that the angled portion 21 of array 1O protrudes from slit 33 in the tube 32.

Hn use, the array 10 is placed within tube 32, and the tube 32 inserted into the straight portion of the basal turn of the scala tympani - typically about 8 mm from the round window. The rod is then moved forward, the array is thus inserted directly into the curved portion of the scala tympani in its predetermined curved shape, until the rod is nearly at the end of the tube and the array is freed.

The tool may be a suitably modified syringe and catheter. A flange or mark may be provided so as to indicate to a surgeon the appropriate insertion depth for the tool.

It will be noted that the present invention, although curved, is not sided: i.e., one device may be manufactured and inserted into either the left or right cochlea.

It will be understood that modifications and variations are possible within the general scope of the present invention.