Impact protection for implantable electric lead

Abstract

An implantable electric lead arrangement for a medical implant system 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.

Claims

1. An implantable electric lead arrangement for a medical implant system, the arrangement comprising: an implantable electric lead having a lead radius r.sub.L and containing a plurality of parallel lead wires wound in an elongated helical ribbon about a central longitudinal axis with a helical radius r.sub.H; a conical shape lead core fixed and enclosed within the helical ribbon and having a lead core radius r.sub.C, the lead core being configured for providing impact strain relief to the electric lead by resisting radial and/or axial deformation from external impact force; and at least one support wire parallel to the lead wires and having a higher strain energy absorption capacity than the lead wires to mechanically strengthen the lead wires against external impact force; wherein a radius ratio between the lead core radius r.sub.C and the lead radius r.sub.L is greater than 0.1 and less than 0.7.

2. The implantable electric lead arrangement according to claim 1, wherein the at least one support wire includes a plurality of support wires.

3. The implantable electric lead arrangement according to claim 2, wherein there are support wires along outer sides of the helical ribbon of lead wires.

4. The implantable electric lead arrangement according to claim 1, wherein the at least one support wire and the lead wires of the helical ribbon form a single integrated structural element.

5. The implantable electric lead arrangement according to claim 1, wherein the at least one support wire and the lead wires are structurally separate elements wound together in a helical ribbon.

6. The implantable electric lead arrangement according to claim 1, wherein the electric lead is 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.

7. The implantable electric lead arrangement according to claim 1, wherein the radius ratio between the lead core radius r.sub.C and the lead radius r.sub.L is greater than 0.33 and less than 0.66.

8. The implantable electric lead arrangement according to claim 1, wherein a radius ratio between the helical radius r.sub.H and the lead core radius r.sub.C is between 1+x and 1.5, where x is a ratio between radius of the lead wires and the lead core radius r.sub.C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows anatomical structures in a human ear having a cochlear implant system.

(2) FIG. 2 shows the local effect of a mechanical impact force on a portion of an implantable electric lead.

(3) FIG. 3 shows an embodiment of the present invention having a conical lead core.

(4) FIG. 4 shows an embodiment of the present invention having a rod-shaped lead core with support ribs.

(5) FIG. 5 shows an embodiment of the present invention with support wires along each outer edge of the helical wire ribbon.

(6) FIG. 6 shows an embodiment of the present invention with a support substrate beneath the helical wire ribbon.

(7) FIG. 7 shows an embodiment of the present invention having a hollow lead core.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(8) 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.

(9) 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.

(10) 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).

(11) 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.

(12) 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.

(13) 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.sub.C) and the electric lead 300 radius (r.sub.L) may be selected to be greater than 0.1 and less than 0.7: 0.1<r.sub.C/r.sub.L<0.7. In a preferred embodiment this ratio maybe between 0.33<r.sub.C/r.sub.L<0.66. In addition, the ratio between the radius of the helical shape of the lead wires 301 (r.sub.H) and the lead core 302 radius (r.sub.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.sub.C). (A ratio between the radii of the wire helix (r.sub.H) and the core (r.sub.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.sub.H/r.sub.C may be between 1+x and 1.3, or even more preferably between 1+x and 1.25.

(14) 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.

(15) 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.

(16) 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.

(17) 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.

(18) 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.

(19) 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.

(20) 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.

(21) 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.

(22) 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.

(23) 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.

(24) 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.