Anchoring structure for implantable electrodes

In one embodiment, the structure comprises, in addition to a body which is usually cylindrical or tubular, a plurality of tines which extend radially from the body. The tines, which are usually flat in shape and tapered, are positioned, relative to the body, with a keying angle other than zero and hence in a generally helical arrangement. The principal axes of the tines in question are preferably also inclined to the longitudinal axis of the body, giving rise to a generally anchor-like shape.

FIELD OF THE INVENTION
 The present invention relates to structures for anchoring implantable
 electrodes. An implantable electrode is any device which can cooperate, in
 an electrically conductive relationship, with human or animal tissue in
 which it has been implanted. The invention has been developed with
 particular attention to its possible application to heart-stimulation
 electro-catheters with passive fixing.
 BACKGROUND OF THE INVENTION
 Structures related to the present invention are described in WO 98/20933
 and the following U.S. Pat. Nos. 5,300,107; 4,796,643; 5,179,962;
 4,402,329; 4,432,377; 4,269,198; 4,479,500; 4,945,922; 4,716,888;
 4,437,475; 4,917,106; 5,336,253; 5,303,740; 5,074,313; 5,090,422;
 5,423,881; 4,721,118; 4,662,382; 4,585,013; 4,582,069; 4,506,679;
 4,497,326; 4,467,817; 4,301,815; 4,444,206; 4,409,994; 4,258,724;
 3,943,936; 3,971,364; 3,902,501; 5,439,485; 4,488,561; 4,360,031;
 4,443,289; 4,988,347; 4,454,888; 4,643,201; 5,016,646; 5,044,375;
 5,231,996; 5,405,374; 4,393,883; 4,332,259; 4,402,328; 4,156,429;
 4,590,950; 4,458,677; 4,236,529; 4,913,164; 4,841,971; 4,722,353;
 4,289,144; 5,476,499; 5,476,500; 5,476,502; 5,425,756; 5,324,327;
 5,261,418; 5,257,634; 5,238,007; 5,738,220; 5,713,945; 5,683,447;
 5,578,068; 5,571,157;5,545,206; 5,562,723; 5,423,881; and 5,645,580. The
 contents of each of these U.S. patents is hereby incorporated by reference
 into this application.
 Practically all of the solutions described in the documents cited above
 provide for the anchoring structure to be produced in the form of a body
 from which one or more projecting anchoring elements usually called
 "barbs" (or "tines" in current terminology) extend in a configuration
 generally comparable to that of an anchor. With a certain degree of
 simplification, but without departing very much from reality, the
 configurations of the tines in the above-mentioned documents can be
 divided substantially into two basic types: (1) the type which provides
 for the tines to be produced in the form of small bars which are generally
 cylindrical throughout their length between the proximal region connected
 to the body of the structure and the distal end (see, for example, U.S.
 Pat. No. 4,269,198); and (2) the type in which the tines have a generally
 flattened configuration, possibly with dimensions which decrease gradually
 from the proximal region (of substantially elongate shape) connected to
 the body of the structure, towards the distal end. An example of this
 second type of configuration is described in U.S. Pat. No. 4,945,922. This
 configuration provides for the use of tines of flattened shape which have
 a slightly arcuate profile in a generally semi-cylindrical configuration
 so that the tines can fit better against the wall, which is usually
 cylindrical, of the body of the anchoring structure when they are folded
 to the position for the insertion of the electrode towards the
 implantation site.
 In the configuration described in U.S. Pat. No. 4,945,922, the proximal
 regions of the tines extend along a path substantially aligned with the
 direction of the planes transverse the principal axis of the body of the
 anchoring structure. In contrast, in the configurations described in U.S.
 Pat. Nos. 4,721,118, 4,585,013, and 4,467,817, this proximal region
 extends along a path substantially aligned with one of the generatrices of
 the cylindrical body of the structure and hence in a direction parallel to
 the principal longitudinal axis of the body. This configuration (see, for
 example, FIG. 4 of U.S. Pat. No. 4,721,118) enables the tines to be
 brought to a position in which they are wrapped around and close to the
 body of the anchoring structure when it is confined inside a sheath used
 for positioning it at the implantation site by catheterization. To adopt
 the terminology which is conventional in the field of the propellers
 (helices) to which reference will be made below, the configuration
 described in U.S. Pat. No. 4,945,922 may be seen as a configuration in
 which the tines of flattened profile have a keying angle of 0.degree.. On
 the other hand, in the configuration described in U.S. Pat. No. 4,721,118,
 the tines in question have a keying angle of 90.degree..
 Tines of the types referred to above have some intrinsic disadvantages,
 even when they are used in combination. For example, tines with a
 bar-like, typically circular profile tend to be too inflexible in the
 proximal region connected to the body of the anchoring structure.
 Moreover, when they are folded close to the body of the anchoring
 structure in the insertion position, these tines tend to project quite
 significantly relative to the outline of the body of the restraining
 structure.
 Flattened tines with "zero" keying angles can be made to fit quite closely
 against the body of the restraining structure at the insertion stage.
 However, their small cross-section in the proximal region means that the
 tines often have inadequate behavior during the resilient opening-out
 stage after positioning at the implantation site. Moreover, the low
 resistance of the proximal region to bending exposes the tines to the risk
 that even a slight stress applied to the electrode in the direction away
 from the implantation site causes the tines to turn over from the
 generally anchor or arrow-like (harpoon-like) configuration which can
 ensure firm anchorage of the electrode at the implantation site.
 Tines with "90.degree." keying angles have the undoubted advantage of
 rendering independent the flexural characteristics of the proximal regions
 of the tines which come into play, respectively, when the tines are
 wrapped around the body of the anchoring structure, and when they are
 unfolded from the body in question, projecting radially like fins relative
 to the anchoring structure. In the first situation, the proximal regions
 of the tines are in fact subjected to bending stress relative to their
 smallest dimension, thus showing a high degree of flexibility. In the
 second situation, the bending stress acts in the direction in which the
 extent of the proximal regions of the tines is greatest so that they show
 much greater strength and stiffness.
 However, even this latter solution is not free of problems. It in fact
 imposes limitations due to the number and radial extent of the tines which
 can be arranged on the body of the anchoring structure in the same region
 of its axial extent. This is because it is necessary to prevent the tines
 from coming close together and interfering with one another while they are
 being wrapped around and close to the body of the anchoring structure.
 This is disadvantageous both because of a possible increase in the radial
 dimensions of the unit due to the superimposition of the tines, and
 because of possible problems of interference during the unfolding stage.
 In this connection, it should be noted that the unit formed by the tines
 and by the body of the anchoring structure is usually a one-piece
 elastomeric component which has the appearance of a bush from which the
 tines extend.
 There is, moreover, a tendency to reject solutions which provide for the
 use of tines which are offset relative to one another along the axis of
 the body of the anchoring structure, since it is usually preferred to be
 able to fit at least four tines uniformly distributed angularly on the
 same axial portion of the body.
 The object of the present invention is to provide an anchoring structure of
 the type specified above in which the above-mentioned problems are finally
 overcome. According to the present invention, this object is achieved by
 means of an anchoring structure having the specific characteristics
 described in this specification and recited in the claims.
 SUMMARY OF THE INVENTION
 In particular, the solution according to the invention provides tines which
 are particularly thin but also stiff to ensure anchorage of the electrode.
 During insertion, the tine is bent along its natural bending plane by a
 twisting movement on the body of the anchoring structure, the slight
 thickness of the tine enabling it faithfully to reproduce the profile of
 the body. During use, however, a high degree of stiffness with respect to
 forces directed along the longitudinal axis of the structure is ensured
 since this is a direction other than the natural direction of bending of
 the tine. These forces are those which are exchanged between the tines and
 the heart trabeculae so that, in the solution according to the invention,
 the tines ensure more effective anchoring than any tine of equal size, for
 example, of the triangular type with a "zero" keying angle. Again in
 comparison with known structures of this type, the solution according to
 the invention enables the width of the tine to be increased, for given
 frontal dimensions. This helps further to stiffen the tine, for a given
 overall size.
 Moreover, during implantation, the tines can be fitted in the insertion
 device (which is usually constituted by a tubular body) by first engaging
 the farthest forward portion of the proximal region of each tine, and then
 proceeding along the remainder of the bodies of the tines. This ensures,
 for the doctor performing the implantation, a smoother feel of the
 catheter of which the anchoring structure constitutes the head portion
 inside the insertion device.
 According to an embodiment of the invention which has been found
 particularly advantageous, a keying angle of about 30.degree. is selected,
 the tines also having a substantially triangular, tapered shape with an
 angle of about 14.degree. at the tip of the triangular profile. With
 regard to the general angle of inclination of the tines (defined as the
 angle formed between the principal longitudinal axis of the tine and the
 longitudinal axis of the body of the supporting structure, this latter
 axis in practice identifying the direction of advance of the anchoring
 structure during implantation), the selection of a value in the region of
 about 45.degree. has been found particularly advantageous.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In the drawings, an anchoring structure for implantable electrodes is
 generally indicated 1. For the meaning of the term "implantable
 electrode", reference should be made to the introductory portion of the
 present description. Also refer to the introductory portion for a
 description of the general criteria for the production and use of the
 structure 1.
 As already indicated above, the invention relates primarily to the tines of
 the structure 1. As for the rest, the characteristics of the similar
 structures described in the documents mentioned in the introductory
 portion of the present description may therefore be reproduced, in
 general, for the structure 1. It will suffice herein to note that the
 structure 1 is constituted in general (including the tines) by a single
 shaped body of generally flexible material compatible with the requirement
 to be implanted in a human or animal body. For example, this may be a
 silicone elastomer material.
 The structure 1 is usually composed of a generally tubular body 2. This
 shape enables the electrode, which is not shown since it is of known type,
 to pass through and/or to be positioned in the body 2. A tip T of the
 electrode (shown in broken outline in FIGS. 1 and 2) is in fact intended
 to project beyond the distal end 3 of the body 2, which extends along a
 principal axis indicated X2. A plurality of tines, generally indicated 4,
 extend from the body 2 (usually in the vicinity of the distal end 3) in a
 geometrical arrangement which, for the reasons explained further below,
 can be defined both as anchor-like and as propeller-like (helix-like).
 Thus, as can best be seen in the detailed view of FIG. 4 (in which one of
 the tines 4 is shown individually), each of the tines 4 preferably has a
 smooth and flattened shape with an approximately rectangular profile with
 rounded side and end edges. Moreover, it should be noted that, in the
 currently preferred embodiment of the invention, the tines 4 are formed
 integrally with the body 2 of the structure 1. The projecting portion of
 the structure of each tine 4 preferably has an isosceles triangular
 profile with an apex angle of the order of about 14.degree..
 An observation of FIGS. 1 and 2 in combination shows that, in contrast with
 the tines with "zero" and "90.degree." keying referred to in the
 introductory portion of this description, the tines 4 according to the
 invention are formed so as to have a keying angle (.alpha.) typically of
 about 30.degree.. This is why they are likened above to a propeller
 structure. In more strictly geometrical terms, the arrangement of the
 tines 4 according to the invention may be described with reference to the
 proximal regions 5 of the tines 4 (see FIG. 4) which have a generally
 elongate shape extending along a helical path. This path is centered about
 the principal axis X2 of the structure 2 and has an angle of twist
 corresponding to the above-mentioned keying angle (preferably about
 30.degree.).
 Another possible way of describing the geometry of the configuration in
 question (with reference, for example, to bodies 2 which do not
 necessarily have a cylindrical shape but, for example, have a prismatic
 shape) is to say that the aforementioned proximal regions 5 extend along
 generally slanting or oblique paths both relative to the direction defined
 by planes perpendicular to the longitudinal axis X2 and relative to the
 axis X2 itself (this being intended to indicate briefly that the aforesaid
 paths are slanting or oblique relative to any of the planes included
 within the family of planes passing through the axis X2). It will also be
 appreciated from an observation of FIGS. 1 and 2, particularly FIG. 2,
 that (according to a known solution) the principal axes of the tines 4 are
 also generally inclined relative to the axis X2. The respective angle of
 inclination (.beta.) selected, indicated in FIG. 1, is preferably about
 45.degree.. This is why they are likened above to an anchor structure.
 When compared with solutions with "zero" keying, in the first place, the
 solution according to the invention benefits from the high degree of
 flexibility of each tine 4 when it is folded (in practice wrapped) close
 to the body 2 by bending of its proximal region 5 transversely where it is
 thinnest, and hence in the direction in which it presents least resistance
 to bending and also takes up the least space once folded against the body
 2. This is combined with the high degree of strength shown by the tines in
 the typical condition of use after unfolding with respect to stresses in
 the direction identified substantially by the longitudinal axis X2 of the
 structure 1 and hence acting on the proximal region 5 to a large extent
 "lengthwise".
 In comparison with solutions with "90.degree." keying, the solution
 according to the invention has two basic advantages. In the first place,
 precisely because the tines 4 arc not folded close to the body 4 in a
 completely circular arrangement but in a helical arrangement, it is
 possible (particularly with the use of tines 4 which are tapered gradually
 towards the distal end) to fit three, four, or possibly even more tines 4
 on the same longitudinal portion of the body 2. This is achieved whilst
 preventing the tines in question from covering and interfering with one
 another whilst they are being folded close to the body 2. If other
 parameters remain the same, this permits the production of tines 4 of
 greater radial length if desired and, in particular, tines which, when
 folded close to the body 2, have a circumferential extent greater than the
 circumferential separation between the proximal regions 5 of adjacent
 tines 4. Naturally, the term "circumferential" relates to the body 2. The
 other advantage lies in the fact that, when fitted in the respective
 insertion device (which is usually constituted by a tubular sheath), the
 structure I does not oppose the insertion movement with tines 4 extending
 completely (particularly with regard to their proximal regions) along the
 axis X2 which is also the insertion axis. Rather, the fact that the
 proximal regions 5 are in a slanting or oblique arrangement relative to
 this axis means that the structure according to the invention is very
 compliant and easy to insert.
 In particular, by adjusting parameters such as the number of tines 4 and
 their keying angle (it should be remembered that the solution described
 herein, which provides for the presence of four tines 4 with a keying
 angle of about 30.degree., is given purely by way of example) it is
 possible to produce anchoring structures 1 with characteristics which are
 differentiated according to specific requirements of use. For example, a
 reduction in the keying angle will produce, in general, a structure 1
 which is more compliant upon being fitted in the insertion device but
 which at the same time is less resistant to stresses along the principal
 axis X2. An increase in the keying angle, on the other hand, will
 generally produce a structure which is less compliant upon being fitted in
 the insertion device but which is intrinsically more resistant to stresses
 at the implantation site.
 It is stressed that the remarks just made also apply to the other
 parameters considered. Clearly, in fact, by adjusting these other
 parameters (the number and thickness of the tines 4, their geometrical
 arrangement, angle of inclination, etc.), as well as the keying angle, it
 is possible to achieve even greater flexibility in the definition of the
 functional parameters.
 Naturally, the principle of the invention remaining the same, the details
 of construction and forms of embodiment may be varied widely with respect
 to those described and illustrated, without thereby departing from the
 scope of the invention as defined in the appended claims.