Patent Publication Number: US-9839778-B2

Title: Impact protection for implantable electric lead

Description:
This application is a divisional of U.S. patent application Ser. No. 14/816,118, filed Aug. 3, 2015, now issued as U.S. Pat. No. 9,433,777, which in turn is a divisional of U.S. patent application Ser. No. 14/163,002, filed Jan. 24, 2014, now issued as U.S. Pat. No. 9,174,039, which in turn claims priority from U.S. Provisional Patent Application 61/756,502, filed Jan. 25, 2013, all of which are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to medical implants, and more specifically to an implantable electrode arrangement used in medical implant systems such as middle ear implants (MEI), cochlear implants (CI) and vestibular implants (VI). 
     BACKGROUND ART 
     A normal ear transmits sounds as shown in  FIG. 1  through the outer ear  101  to the tympanic membrane (eardrum)  102 , which moves the bones of the middle ear  103  (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea  104 . The cochlea  104  is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea  104  forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve  113  reside. In response to received sounds transmitted by the middle ear  103 , the fluid-filled cochlea  104  functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve  113 , and ultimately to the brain. 
     Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea  104 . To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear  103 , a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea  104 , a cochlear implant with an implanted electrode contact can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode. 
       FIG. 1  also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor  111  where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant  108 . Besides receiving the processed audio information, the implant  108  also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead  109  to an implanted electrode array  110 . Typically, this electrode array  110  includes multiple stimulation contacts  112  on its surface that provide selective stimulation of the cochlea  104 . 
     The electrode array  110  contains multiple lead wires embedded in a soft silicone body referred to as the electrode carrier. The electrode array  110  needs to be mechanically robust, and yet flexible and of small size to be inserted into the cochlea  104 . The material of the electrode array  110  needs to be soft and flexible in order to minimize trauma to neural structures of the cochlea  104 . But an electrode array  110  that is too floppy tends to buckle too easily so that the electrode array  110  cannot be inserted into the cochlea  104  up to the desired insertion depth. 
     U.S. Patent Publication 2010/0305676 (“Dadd,” incorporated herein by reference) describes winding the lead wires in the extra-cochlear segment of the electrode lead in a helical shape to make that portion of the electrode lead stronger. Dadd is quite clear that such a helical portion does not extend into the intra-cochlear electrode array which needs to be much more flexible than the extra-cochlear lead in order to minimize trauma to the cochlear tissues when the array is inserted. 
     U.S. Patent Publication 2010/0204768 (“Jolly,” incorporated herein by reference) describes winding the individual lead wires in the intra-cochlear electrode array in an elongated helical shape where each wire is separate and independent. 
     Electrode leads of active implantable medical devices including Middle Ear Implants (MEI&#39;s), Cochlear Implants (CI&#39;s), Brainstem Implants (BI&#39;s) and Vestibular Implants (VI&#39;s) need to be small in diameter but also they carry multiple lead wires. Electrode leads also need to be robust against external mechanical impacts, especially in locations where the electrode lead is placed on top of the skull bone only covered by the skin. In case of a mechanical impact on an unprotected electrode lead, the elastic silicone electrode carrier material is compressed and the electrode lead becomes temporarily locally thinner and elongated. Lead wires at the affected location experience local tensile forces and can even break. This is also the case for helically formed wires within a silicone electrode carrier since they are forced to expand nearly the same amount as the carrier material itself. 
     To deal with this problem, some implant designs arrange for the electrode lead to exits the implantable processor housing so that the electrode lead never lies superficially on top of bone. One disadvantage of such designs in the case of cochlear implants is that the implant housing must be placed in a very exactly defined position relative to the ear. For implant designs where the electrode lead emerges from the side of the implant housing, the surgeon is recommended to drill an electrode channel into the bone, which is time consuming so that not every surgeon follows the recommendations. Some electrode lead design include a rigid impact protector that surrounds the electrode lead, but that approach reduces the flexibility of the electrode lead which in turn makes the surgical implantation procedure more difficult. And in case of a mechanical impact in the area of the electrode lead, a rigid impact protector may protect the electrode from damage but also may cause trauma in the surrounding tissue when it is pressed against the protector. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are directed to an implantable electric lead arrangement for a medical implant system that includes an implantable electric lead with parallel lead wires wound in an elongated helix about a central longitudinal axis. At least one support wire is parallel to the lead wires and has a higher strain energy absorption capacity than the lead wires to mechanically strengthen the lead wires against external impact force. 
     The at least one support wire may be multiple support wires; for example, there may be support wires along outer sides of the helical ribbon of lead wires. The at least one support wire and the lead wires of the helical ribbon may form a single integrated structural element, or they may be structurally separate elements wound together in a helical ribbon. And the electric lead may specifically a cochlear implant electrode lead connected at one end to an implantable cochlear implant processor and connected at another end to an intracochlear electrode array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows anatomical structures in a human ear having a cochlear implant system. 
         FIG. 2  shows the local effect of a mechanical impact force on a portion of an implantable electric lead. 
         FIG. 3  shows an embodiment of the present invention having a conical lead core. 
         FIG. 4  shows an embodiment of the present invention having a rod-shaped lead core with support ribs. 
         FIG. 5  shows an embodiment of the present invention with support wires along each outer edge of the helical wire ribbon. 
         FIG. 6  shows an embodiment of the present invention with a support substrate beneath the helical wire ribbon. 
         FIG. 7  shows an embodiment of the present invention having a hollow lead core. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
       FIG. 2  shows how longitudinal deformation of a cochlear implant electrode lead  109  occurs in response to radial and/or axial deformation caused by an external impact or exerted pressure onto the electrode lead  109  which thins and elongates the resilient material of the lead carrier  201  and creates a local tensile force on the lead wires  202 . Such longitudinal deformation should be prevented or strongly suppressed. Reduced longitudinal deformation as a response to radially induced deformation is especially a challenge in electric leads  109  having helically wound lead wires  202  as in  FIG. 2 . Embodiments of the present invention are directed to an implantable electric lead, for example, a cochlear implant electric lead  109 , which is more robust against radial and/or axial deformation to avoid wire breakage within the electric lead  109 . This most often occurs relatively close to the stimulator housing where after implantation the electric lead  109  runs on top of the skull bone. 
       FIG. 3  shows one embodiment where an electric lead  300  which has an lead carrier material  304  that contains parallel embedded lead wires  301 . As shown in  FIG. 3 , the lead wires  301  are wound in an elongated helix about a central longitudinal axis of the electrode lead  300 . The embedded lead wires  301  may also be arranged in other suitable forms than in an elongated helix. For example, the lead wires  301  may be shaped in a substantially sinusoidal form about a central longitudinal axis of the electric lead  300 . 
     A conical lead core  302  is fixed and enclosed in the lead carrier  304  within the wire helix for providing impact strain relief to the lead wires  301  by resisting radial and/or axial deformation from external impact force. The lead core  302  may be made of a flexible polymer material that may have an elastic module value and/or a shore-A hardness value greater than some given threshold value. For example, the lead carrier  304  material may be silicone of medium shore-A hardness (e.g. NUSIL MED-4244 or Applied Silicone LSR40), while the lead core  302  material may have a higher shore-A hardness (e.g. NUSIL MED-4770). 
     In other specific embodiments, the lead core  302  may be made of a flexible metallic material, for example, a shape memory alloy (SMA) such as Nitinol. The flexible metallic material may be a single wire having a thickness e.g. between 0.4 mm and 0.1 mm, or more preferably between 0.3 mm and 0.2 mm. Or the lead core  304  may be formed from bundled fibers or wires which may run in parallel or be braided. Lead core fibers may be made of inorganic materials such as carbon basalt (e.g. CBF—continuous basalt fibers) or glass, or from organic materials such as polypropylene, polyethylene, polyamide, aramide, spun liquid crystal polymer (e.g. Vectran) or other materials from these groups. Alternatively, they may be made from shape memory alloy such as Nitinol, e.g. having a bundle thickness between 0.4 mm and 0.1 mm, or more preferably between 0.3 mm and 0.2 mm. The material of the lead core  302  resists elongation of the electric lead  300  in case of a mechanical impact and also restricts diametric compression of the electric lead  300 . 
     Whatever the specific choice of the material of the lead core  302 , the flexibility of the electric lead  300  should be preserved. It is important that the surgeon can be able to bend the electric lead  300  to properly implant it, for example, to insert the electric lead  300  through the holed drilled into the skull bone, preferably as easily as without this core element  302  being present. 
     To satisfy the competing requirements of mechanical strength and impact resistance versus high flexibility suggests that it is important to choose an intelligent set of ratios between the radii of the various elements of the electric lead  300 . For example, the ratio between the lead core  302  radius (r C ) and the electric lead  300  radius (r L ) may be selected to be greater than 0.1 and less than 0.7: 0.1&lt;r C /r L &lt;0.7. In a preferred embodiment this ratio maybe between 0.33&lt;r C /r L &lt;0.66. In addition, the ratio between the radius of the helical shape of the lead wires  301  (r H ) and the lead core  302  radius (r C ) should be selected to be between 1+x and 1.5, where x is the ratio between the radius of the lead wires  301  themselves (including isolation) and the lead core  302  (r C ). (A ratio between the radii of the wire helix (r H ) and the core (r C ) represents the value of 1+x when the lead wires  301  are directly wound around the lead core  302 ). In a preferred embodiment, the ratio r H /r C  may be between 1+x and 1.3, or even more preferably between 1+x and 1.25. 
     The conical end  303  of the lead core  302  ensures that there is not an abrupt transition of mechanical lead properties between the impact-protected part of the electric lead  300  and the unprotected part. Where the lead core  302  is made of individual wires or fibers, each of these may extend by different amounts towards the conical end  303  to provide a smooth transition. 
       FIG. 4  shows structural elements of an embodiment of a rod shaped lead core  302  with support ribs  305  that are perpendicular to the lead core  302  and distributed along the length of the lead core  302 . The support ribs  305  help anchor the lead core  302  within the lead carrier  304  material and also help resist radial and/or axial deformation from external mechanical impacts on the electric lead  300 . 
     In contrast to some previous schemes for temporarily using a stiffener element to assist with surgical insertion of the electrode, which is then removed, the lead core  302  element is securely fixed within the lead carrier  304  and remains as a structural element of the electric lead  300  after surgery to provide lasting post-surgical protection from external impacts. Moreover, proper design of the lead core  302  and the electric lead  300  should maintain full flexibility of the electric lead  300  rather than making it stiffer for surgical handling as with the prior art schemes. In addition, the prior art intra-surgical stiffener element is designed to be placed at the cochleostomy opening (or other location where the lead may be buckled during insertion), whereas the lead core  302  is placed close to the basal end of the electric lead  300  near the implant housing where it runs relatively unprotected after surgery on top of skull bone. It is also worth noting that the prior art surgical stiffener element does not describe how to deal with lead wires  301  that wound in an elongated helical shape embedded within an lead carrier  304  as here. 
     Embodiments of the present invention also include other specific approaches for mechanically protecting the lead wires.  FIG. 5  show an implantable electric lead arrangement for a medical implant system where an electric lead  500  has a lead carrier  503  which contains parallel embedded lead wires  501 . As shown in  FIG. 5 , the lead wires  501  are wound in an elongated helix about a central longitudinal axis of the electric lead  500 . The embedded lead wires  501  may also be arranged in other suitable forms than in an elongated helix, for example, the lead wires  501  may be shaped in a substantially sinusoidal form about a central longitudinal axis of the electric lead  500 . 
     At least one support wire  502  is parallel to the lead wires  501  and has a higher strain energy absorption capacity than the lead wires  501  to mechanically strengthen the lead wires  501  against external impact force. In the specific embodiment shown in  FIG. 5 , there are multiple support wires  502 , one along each outer side of the helical ribbon of lead wires  501 . The at least one support wire  502  and the lead wires  501  of the helical ribbon may form a single integrated structural element, or they may be structurally separate elements wound together in a helical ribbon. In order to absorb the tensile forces acting on the lead wires  501  in the case of a mechanical impact onto the electric lead  500 , the support wires  502  may be thicker and/or may be made of a different material (e.g. different metallic alloy, manufactured fiber or polymer) than the lead wires  501 . 
     Embodiments of the present invention also include an implantable electric lead arrangement for a medical implant system such as the one shown in  FIG. 6 . An electric lead  600  has a lead carrier  603  that contains parallel embedded lead wires  601 . As shown in  FIG. 6 , the lead wires  601  are wound in an elongated helix about a central longitudinal axis of the electric lead  600 . The embedded lead wires  601  may also be arranged in other suitable forms than in an elongated helix, for example, the lead wires  601  may be shaped in a substantially sinusoidal form about a central longitudinal axis of the electric lead  600 . 
     A impact protection ribbon  602  lies in a plane beneath the lead wires  601  acting as a wire support substrate that mechanically supports the lead wires  601  and protects the lead wires  601  from external impact force. The lead wires  601  and the impact protection ribbon  602  are wound together in an elongated helical ribbon about a central longitudinal axis of the electric lead  600 . The impact protection ribbon  602  may be made of thermoplastic material, fluorinated ethylene propylene (FEP), polyethylene or poly-etheretherketone (PEEK) material, which may be molded around the lead wires  601  or glued to the lead wires  601  to form a wire support substrate. 
     In some embodiments, the impact protection ribbon  602  may not be fixed connected to the lead wires and thus does not act as a wire support substrate. Rather, the impact protection ribbon  602  may be wound coaxially but separately with the lead wires  601  in an elongated helical ribbon about a central longitudinal axis of the electric lead  600  and mechanically protects the lead wires  601  from external impact force. In such embodiments, the impact protection ribbon  602  may be coaxially outside or coaxially inside the lead wires  601  and may be made of fluorinated ethylene propylene (FEP), polyethylene, poly-etheretherketone (PEEK) material or superelastic Nitinol material. 
       FIG. 7  shows an embodiment similar to that of  FIG. 3  but without a hollow core element  700 . The lead wires  701  are wound in an elongated helix about a central longitudinal axis of the electric lead and embedded in a hollow cylinder of core material  702 . The embedded lead wires  701  may also be arranged in other suitable forms than in an elongated helix, for example, the lead wires  701  may be embedded in core material  702  and shaped in a substantially sinusoidal form about a central longitudinal axis of the electric lead. A hollow lead core  700  reduces the overall volume of silicone in the electric lead and so reduces the amount of silicone displaced in a longitudinal direction during an impact. As a result the longitudinal pulling forces on the lead wires  701  during an impact are reduced, leading to better impact resistance.  FIG. 7  shows the approximate relative proportions of a hollow core for an implant electrode. The ratio of hollow core to electrode lead diameter should be about the same as proposed above. 
     Embodiments of the present invention such as those described above provide protection of an implantable electric lead against mechanical impact with lower risk of lead wire breakage in case of such mechanical impacts. And despite the increased robustness of the electric lead against a mechanical impact, the elasticity and flexibility of the electric lead are less electric lead. Depending on the specific core material used, the electric lead can be elastic (to flip back after being bent), malleable (to retain the new shape when bent), or floppy. Although additional components and manufacturing steps are needed in comparison to an unprotected electric lead, the still uncomplicated electrode structures allow for easy manufacturing processes. 
     Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention.