Fixation for implantable medical devices

A tine portion of an implantable medical device includes a hook segment and a distal segment terminated by a tissue-piercing tip, wherein the distal segment extends from a distal end of the hook segment to the tip. The hook segment, which is elastically deformable from a pre-set curvature, for example, defined by a single radius, preferably tapers from a first width thereof, in proximity to a proximal end thereof, to a smaller, second width thereof, in proximity to the distal end thereof, wherein the tip has a width that is greater than the second width of the hook segment. Alternately, the tine portion may include a hook segment that is defined by two radii and a straight section extending therebetween.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to the commonly-assigned U.S. patent application Ser. No. 13/955,127, now U.S. Pat. No. 9,119,959 and U.S. application Ser. No. 13/955,674, now U.S. Pat. No. 9,155,882, which were filed concurrently herewith and incorporated by reference, in their entirety.

TECHNICAL FIELD

The present invention pertains to implantable medical devices, and, more specifically, to tissue-penetrating fixation components thereof.

BACKGROUND

An implantable medical device, for the delivery of stimulation therapy and/or for diagnostic sensing, may include at least one tissue-penetrating fixation component configured to hold the device at an implant location.FIG. 1is a schematic diagram that shows potential cardiac implant sites for such a device, for example, within an appendage102of a right atrium RA, within a coronary vein CV (via a coronary sinus ostium CSOS), or in proximity to an apex103of a right ventricle RV.FIG. 2is a plan view of an exemplary implantable medical device200, which includes a tissue-penetrating fixation component formed by a plurality of tine portions230.FIG. 2further illustrates device200including a hermetically sealed housing220that contains control electronics201and a power source203shown in broken outlines, and which defines a longitudinal axis2of the device. Housing220may be formed of a medical grade stainless steel or titanium alloy and have an insulative layer formed thereover, for example, parylene, polyimide, or urethane. With further reference toFIG. 2, device200includes a pair of electrodes,261,262, which may form a bipolar pair for cardiac pacing and sensing; tine portions230surround electrode261and are configured to penetrate tissue in order to hold electrode261in intimate contact with tissue, for example, at one of the aforementioned implantation sites, while securing, or fixating device200for chronic implantation at the site. Further description of a suitable construction for device200may be found in the co-pending and commonly assigned United States Patent Application having the pre-grant publication number 2012/0172690 A1.

With reference toFIG. 3A, device200may be delivered to an implant location via a delivery catheter300. For example, with reference toFIG. 1, if the target implant site is located in the right atrium RA, coronary vein CV, or right ventricle RV, a distal end310of catheter300may be maneuvered into the heart through a superior vena cava SVC or an inferior vena cava IVC, according to a transvenous delivery method known in the art.FIG. 3Ashows a partial cross-section of distal end310of catheter300, which is formed like a cup to hold and contain device200for delivery to the implant site.FIG. 3Aillustrates device200having been loaded into distal end310so that a hook segment231of each tine portion230is elastically deformed, from a pre-set curvature thereof, to an open position, at which a distal segment232of each tine portion230extends distally toward an opening313of catheter distal end310. Each tine portion230is preferably formed from a superelastic material, such as Nitinol.FIG. 3Afurther illustrates a deployment element320abutting a proximal end of device200and extending proximally therefrom, through a lumen of catheter300, and out from a proximal opening301thereof. Element320may be moved, per arrow M, by an operator to push device200, per arrow P, out from opening313of distal end310, for example, when opening313has been located by the operator in close proximity to tissue at the target implant site.

FIG. 3B, is an enlarged view of distal segment232of one of tine portions230, wherein a tissue-piercing tip322, which terminates distal segment232, has just been pushed out through opening313of distal end310of catheter300and into contact with tissue T.FIG. 3Billustrates distal segment232supported by the surrounding wall of distal end310, in proximity to opening313, so that the push force of deployment element320is effectively transferred through tip322to first compress the tissue T, as shown, and then to pierce the tissue T for penetration therein, which is shown inFIGS. 3C-D.FIGS. 3C-Dillustrate partial tine penetration and full tine penetration, respectively, as deployment element320continues to push device200out opening313. It can be seen that the elastic nature of each tine portion230, once the constraint of the distal end310is withdrawn, allows the corresponding hook segment231to relax back toward the pre-set curvature thereof within the tissue. The full penetration of tine portions230, shownFIG. 3D, is representative of acute fixation of device200at the implant site, for example, for the evaluation of device performance (e.g., pacing and sensing via electrodes261,262). It should be noted that, at some implant sites, tine portions230may, at full penetration, extend back out from tissue T, for example, generally toward distal end310of catheter300.

With further reference toFIG. 3D, a tether350is shown looping through an eye feature205formed at the proximal end of device200; tether350extends proximally through a lumen of deployment element320to a proximal end351thereof, outside a proximal end of deployment element320, which may be seen inFIG. 3A. Thus, if the performance of acutely fixated device200is unsatisfactory, the operator may use tether350to pull device200back into distal end310, thereby withdrawing tine portions230from the tissue, so that device may be moved by delivery catheter300to another potential implant site. Alternately, if the acutely fixated device200performs satisfactorily, proximal end351of tether350may be severed to pull tether350out from eye feature205of device200, and the fully penetrated tine portions230continue to fixate device200for chronic implant.

The aforementioned co-pending and commonly assigned U.S. Patent Application '690 discloses suitable embodiments of a fixation component having tine portions similar to tine portions230, wherein the tine portions exhibit a suitable baseline performance, for example, in terms of a deployment force, an acute retraction force (for repositioning), atraumatic retraction, and acute and chronic fixation forces. Yet, there is still a need for new configurations of tine portions for implantable devices, like device200, that may further enhance fixation.

SUMMARY

Some embodiments of the present invention encompass implantable medical devices (e.g., cardiac pacemakers) and tissue-penetrating fixation components thereof, which include one or more tine portions configured for increased strain relief during the flexing thereof, either at initial implant (particularly in cases where the retraction of penetrated tines is necessary for repositioning the device), or when subject to cyclic loading during a chronic implant of the fixated device, for example, within a beating heart. These tine portions are, preferably, also configured to reduce the risk of tissue trauma during the retraction thereof from the tissue, for example, for repositioning. In certain embodiments, a tissue-penetrating fixation component for an implantable medical device includes a tine portion configured to mitigate the risk of compressing, for example, to the point of occlusion, blood vessels in proximity to the implant site, without sacrificing chronic fixation performance, and while maintaining adequate strain relief.

According to some embodiments, a tine portion of a tissue-penetrating component of an implantable medical device includes a hook segment and a distal segment terminated by a tissue-piercing tip. The hook segment, which is pre-set to extend along a curvature that encloses an angle of between 135 degrees and 270 degrees, from a proximal end thereof, in proximity to the base portion, to a distal end thereof, and which is elastically deformable from the pre-set curvature to an open position, tapers from a first width thereof, in proximity to the proximal end thereof, to a second width thereof, in proximity to the distal end thereof, the second width being less than the first width. The distal segment, which is pre-set to extend along a relatively straight line, approximately tangent to the distal end of the hook segment, from the distal end of the hook segment, is terminated by a tissue-piercing tip that, preferably, has a width that is greater than the second width of the hook segment. The first width of the hook segment may be approximately two to five times greater than the second width thereof, and the width of the tissue-piercing tip may be two to three times greater than the second width.

According to some embodiments, in which a length of the distal segment of the tine portion is relatively short, to mitigate the risk of vessel compression, the distal segment either extends approximately parallel to a longitudinal axis of the component/device, or away from the longitudinal axis, when the hook segment conforms to the pre-set curvature.

According to some embodiments, in which the tissue-penetrating component further includes a base portion, for example, in the form of a ring, that defines the aforementioned longitudinal axis and is configured to be fixedly attached to the implantable medical device, the tine portion further includes a proximal segment that extends between the hook segment and the base portion, wherein the proximal segment may extend from the base portion toward the longitudinal axis.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides practical examples, and those skilled in the art will recognize that some of the examples may have suitable alternatives.

FIG. 4Ais a schematic representation of one of tine portions230isolated from the above-described implantable medical device200, wherein an exemplary flexing, per arrow F, of tine portion230is illustrated. Such flexing may be encountered by tine portion230, once tine portion230has penetrated tissue to fix device200at a chronic implant site for cardiac monitoring and/or therapy, for example, as illustrated inFIG. 3D. Thus, fatigue life is a consideration influencing the configuration of tine portions for those implantable medical devices that may be subjected to cyclic loading caused by hundreds of millions of heart beats, over the life of their implant. InFIG. 4A, a zone of stress concentration SC, for example, in response to the flexing per arrow F, is circled; zone SC is located in proximity to a proximal end31of hook segment231of tine portion230, where hook segment231joins with a base portion203. Base portion203and tine portion230may be integrally formed, wherein base portion203is configured to be fixedly attached to device200. Stress concentration in zone SC may also result from deformation of hook segment231into the open position (FIG. 3A), for example, upon initial loading of device200and retraction of device200back into distal end310of catheter for repositioning, which, in combination with the repeated force of deployment, can potentially push tine portion230toward an elastic limit and may make tine portion230subsequently more vulnerable to fatigue under the aforementioned cyclic loading. Although rounded edges of tine portions230effectively reduce the concentration of stress, as previously described in the aforementioned commonly-assigned U.S. Patent Application '690, some embodiments of the present invention incorporate tine portions that have tapered hook segments to further address the stress concentration, for example, as illustrated inFIG. 4B.

FIG. 4Bis a perspective view of a tine portion430, according to some embodiments, one or more of which may be integrated into device200, as substitute for tine portions230. A base portion403is shown integrally formed with tine portion430, according to some preferred embodiments, wherein base portion403is configured for attachment to a medical device, such as device200.FIG. 4Billustrates a hook segment431of tine portion430extending from a first end41thereof, in proximity to base portion403, to a second end42thereof, in proximity to a distal segment432of tine portion430, wherein hook segment431tapers from a first width W1, in proximity to proximal end41, to a smaller, second width W2, in proximity to a distal end42of hook segment431. The tapering of hook segment431provides strain relief during the aforementioned deformation/flexing, to alleviate the aforementioned stress concentration.FIG. 4Bfurther illustrates an optional slot48(dashed lines), which may be formed through a thickness t of tine portion430, and extend between first width W1 and second width W2. The inclusion of slot48provides an additional means for providing strain relief, for example, when a limit on how narrow second width W2 may be, for example, no smaller than approximately 0.020-0.025 inch, so that distal segment432does not tear tissue upon retraction therefrom. According to some embodiments, optional slot48may include internal shear tabs (not shown) to help distribute out of plane loads, for example, orthogonal to the illustrated direction of flexing, per arrow F ofFIG. 4A.

With further reference toFIGS. 4A-B, distal segment432of tine portion430is shown having a shorter length than distal segment232of tine portion230, for example, to provide more flexibility in selecting a suitable implant site without risking undue trauma to tissue, upon penetration of tine portion430at the selected site. The shorter length can help to prevent perforation through the wall of a structure, for example, the heart, at some implant locations, and can reduce a probability for penetrated tine portions430to interfere with blood vessels, which interference, for example, may compromise coronary blood supply, as will be described below in conjunction withFIG. 5.

FIG. 5is an estimated tissue penetration path and an ‘as set’ relaxation plot for tine portion230of device200(FIG. 2), wherein tine portion230is formed from approximately 0.005 inch thick Nitinol.FIG. 5includes a solid line, which represents the profile of tine portion230when device200is loaded in distal end310of catheter300(FIG. 3A) with hook segment231deformed to the open position. With reference back toFIGS. 3A-D, the origin, or zero coordinate, along the ordinate axis generally corresponds to the constraining wall of distal segment310of delivery catheter300. The plot ofFIG. 5is made up of a segmented line connecting circles, which corresponds to the estimated penetration path of tine portion230, for example, when device200is pushed out from distal end310and into tissue T (FIGS. 3B-D), and a dashed line, which represents the profile of tine portion230, according to the pre-set curvature, toward which the penetrated tine portion230relaxes over time. The volume of tissue between the segmented line and the dashed line approaches that which is squeezed or compressed by the penetrated tine portion230as it relaxes over time; the greater this volume, the greater the probability for the penetrated tine to compress or pinch one or more blood vessels that perfuse the tissue. For example, the dotted line inFIG. 5represents a potential coronary artery that may be compressed or pinched by tine portion230. As alluded to above, the length of distal segment232is a factor contributing to the volume that is squeezed by penetrated tine portion230, so that reducing the length of distal segment232may be desired. However, if the length of distal segment232is reduced, without modifying other aspects of tine portion230, an orientation of tine portion230relative to tissue T, when hook segment231is in the open position, will be impacted such that tine portion230may be less likely to effectively penetrate into tissue T, for example, upon exiting through opening313of distal end310of catheter300(FIGS. 3A-B). Therefore, with reference toFIG. 4B, the tapering of hook segment431of tine portion430not only relieves strain but also allows for a more favorable orientation of the shorter distal segment432for tissue penetration (e.g., being directed along a line that is closer to normal to the tissue surface), when hook segment431is in the open position.

Various embodiments of tine portions for fixation of an implantable medical device, for example, as described below, incorporate a tapered hook segment and/or a shorter distal segment, to address the above described cyclic loading and/or potential tissue trauma. The following embodiments have been configured with reference to prior art tine portions of tissue-penetrating fixation components for medical devices, such as those described in the aforementioned commonly assigned U.S. Patent Application '690 (generally corresponding to tine portion230), in order to allow a similar fit of devices, like device200, within a delivery catheter, for example, having the tine portions deformed into the open position within distal portion310of catheter300, and to maintain suitable baseline performance, for example, in terms of a deployment force (e.g., no greater than approximately 1-2 Newtons for a fixation component having four tine portions), an acute retraction force, for repositioning (e.g., between approximately 3-4 Newtons for a fixation component having four tine portions), atraumatic retraction, and an adequate acute fixation force (e.g., greater than approximately 2.5 Newtons for a fixation component having four tine portions).

FIG. 6Ais a plan view of a medical device600, according to some embodiments of the present invention.FIG. 6Aillustrates device600including a hermitically sealed housing620and a pair of electrodes661,662; housing620, like housing220of device200, contains control electronics and a power source (not shown), which, for example, together with electrodes661,662, are adapted for cardiac pacing and sensing.FIG. 6Afurther illustrates device600including tine portions630, which are adapted to penetrate tissue in order to secure device600at an implant site, for example, a cardiac site in the right atrium RA or the right ventricle RV (FIG. 1), having been deployed from distal end310of delivery catheter300(FIGS. 3A-D). According to some embodiments, tine portions630are included in a tissue-penetrating fixation component63, which is shown, separate from device600, inFIG. 6B.

FIG. 6Billustrates component63also including a base portion603, from which tine portions630extend, preferably being integrally formed therewith, as described below. According to the illustrated embodiment, base portion603of fixation component63defines a longitudinal axis6of component63and is configured for attachment to device600so that axis6is approximately aligned with a longitudinal axis20of device600. Component63may be part of a subassembly that forms a distal end of device600, and which also includes electrode661; such a subassembly is described in the aforementioned commonly-assigned U.S. Patent Application '690, in conjunction withFIGS. 3A-4Bthereof, the description of which is hereby incorporated by reference.FIG. 6Bfurther illustrates each tine portion630of tissue-penetrating component63including a hook segment631and a distal segment632.

With reference toFIG. 6C, which is an elevation view of component63, each hook segment631extends along a pre-set curvature that encloses an angle θ, from a proximal end61thereof to a distal end62thereof.FIG. 6Cillustrates each distal segment632extending along a relatively straight line that is approximately tangent to distal end62of hook segment631. According to the illustrated embodiment, angle θ is less than 180 degrees, such that distal segment632extends away from axis6.FIG. 6Cfurther illustrates the preset curvature of hook segment631being defined by a single radius R. According to an exemplary embodiment, radius R is approximately 0.085 inch, an angle R, at which distal segment extends relative to axis6, is approximately 20 degrees, and a length LD of distal segment632is between approximately 0.05 inch and approximately 0.1 inch.

According to some preferred embodiments, component63is manufactured by, first, laser cutting base portion603and tine portions630, together, from a tube of superelastic and biocompatible metal (e.g., Nitinol), and then wrapping and holding each tine portion630about a mandrel for a heat setting process that pre-sets the illustrated curvature of each hook segment631. Manufacturing methods such as these are known to those skilled in the art of forming Nitinol components. AlthoughFIG. 6Bshows base portion603of component63formed as a ring, wherein tine portions630are integrally formed therewith and spaced apart from one another about a perimeter of the ring, in alternate embodiments of tissue penetrating fixation components, one or more tine portions may be formed individually and then attached to a base portion that is configured in any suitable fashion for attachment to device600.

FIG. 6Dis a plan view of one of tine portions630, prior to forming the pre-set curvature thereof, in which the above-described tapering for strain relief along hook segment631, from first width W1 to smaller, second width W2 may be seen. When, for example, in the aforementioned exemplary embodiment, component63is manufactured from Nitinol tubing that has a thickness of approximately 0.005 inch, and hook segment631thereof has a length LH of approximately 0.23 inch, first width W1 may be between approximately two to five times greater than second width W2 to provide strain relief for improved fatigue life. Yet, if the smaller, second width W2, for example, being approximately 0.010 inch, were to define an entirety of distal segment632, distal segment632may tear tissue upon retraction therefrom, for example, when repositioning device600. So, with further reference toFIG. 6D, distal segment632of tine portion630is terminated by a tissue-piercing tip622that has a width W3, which is greater than second width W2, for example, approximately two to three times greater, in order to be atraumatic to tissue. In the aforementioned exemplary embodiment, first width W1 is between approximately 0.034 inch and approximately 0.05 inch, second width W2 is approximately 0.010 inch, and third width W3 is approximately 0.02 inch.

FIG. 7Ais an elevation view of a tissue-penetrating fixation component73, according to some alternate embodiments of the present invention, which may be incorporated in device600as an alternative to component63, such that a longitudinal axis7of component73is approximately aligned with longitudinal axis20of device600.FIG. 7Aillustrates component73including a base portion703, similar to base portion603of component63, and a plurality of tine portions730, each of which includes a hook segment731and a distal segment732. Tine portions730and base portion703are preferably integrally formed according to the method described above for component63. Furthermore, each tine portion730, prior to the pre-setting of a curvature of hook segment731, may be configured like tine portion630as described above in conjunction withFIG. 6D, wherein the aforementioned exemplary values for widths W1, W2, W3, thickness t and lengths LD, LH are suitable. However, with further reference toFIG. 7A, the pre-set curvature along which hook segment731extends, from a first end71thereof and a second end72thereof, encloses an angle φ, which is 180 degrees, so that distal segment732extends, between a tissue-piercing tip722thereof and second end72of hook segment731, along a line that is approximately parallel to axis7. The pre-set curvature of hook segment731, like hook segment631, is defined by a single radius R, which may be approximately 0.085 inch.

FIG. 7Bis an estimated penetration path and an ‘as set’ relaxation plot for tine portion730of component73, which may be compared to that of tine portion230(FIG. 5).FIG. 7Billustrates, with a solid line, tine portion730having been elastically deformed into the open position, for example, as would be the case when device600includes component73and is loaded within a delivery catheter, for example, distal end310of delivery catheter300(FIG. 3A). In comparing the solid lines ofFIGS. 5 and 7B, it may be appreciated how the strain relief of tapering flattens the deformed profile of tine portion730relative to that of tine portion230, and that the open position of tine portion730orients distal segment732of tine portion730along a line that is nearly normal to the ordinate axis, which generally corresponds to the above-described tissue surface, for effective tissue penetration. Furthermore, in comparing the estimated tissue penetration path of tine portions230and730(segmented lines connecting the circles), relative to the corresponding relaxed profiles (dashed lines), it can be seen that, due to the shorter length and more open pre-set curvature, tine portion730does not encompass as large a volume of tissue, relative to the pre-set curvature, toward which the penetrated tine portion730relaxes over time, upon full penetration, so that the above described risk of perforation and/or pinching of blood vessels is reduced.

FIGS. 8A-Bare plan views of tine portions830A,830B, prior to pre-setting a curvature thereof, according to some alternate embodiments, either of which may be formed in component63,73in lieu of tine portions630,730, for example, to increase the ease of tolerance control and inspection.FIGS. 8A-Billustrate hook segments831A,831B of tine portions830A,830B having a single-sided, or asymmetric taper. According to the illustrated embodiments, widths W1, W2, and W3 are designated at generally the same locations along hook segments831A,831B and distal segments832A,832B, as previously described for tine portions630and730. Furthermore, it should be understood that, according to some preferred embodiments, a thickness of each tine portion830A,830B (into the page), for example, approximately 0.005 inch, is approximately constant along an entire length of each tine portion830A,830B, since components that would include tine portions830A,830B are preferably formed from a Nitinol tube according to the method described above.FIG. 8Afurther illustrates distal segment832A of tine portion830A being terminated in a tissue-piercing tip822, at which width W3 has a center line that is offset from a center line of second width W2; whileFIG. 8Billustrates a tissue-piercing tip822B of distal segment832B, at which width W3 has a center line approximately aligned with that of second width W2. According to some exemplary embodiments, first width W1 is between approximately 0.034 inch and approximately 0.05 inch, second width W2 is approximately 0.010 inch, and third width W3 is approximately 0.02 inch.

FIGS. 9A-Dare profiles and corresponding estimated penetration path and ‘as set’ relaxation plots of various tine portions930A,930B,930C,930D, according to yet further embodiments of the present invention, wherein the profiles, per the pre-set curvatures of hook segments931A-D, accommodate for a relatively shorter length of distal segments932A-D thereof, for example, compared to that of tine portion230(FIG. 5).FIGS. 9A-Dillustrate the pre-set curvature of each hook segment931A-D being defined by two radii, R1 and R2, wherein R2 is greater than R1. According to exemplary embodiments of tine portions930A,930B, radius R1 is approximately 1.04 mm and radius R2 is approximately 1.65 mm, while in an exemplary embodiment of tine portion930C, radius R1 is approximately 0.5 mm and radius R2 is approximately 1.65 mm, and, in an exemplary embodiment of tine portion930D, radius R1 is 0.25 mm and radius R2 is approximately 2.4 mm. It should be noted that none of tine portions930A-D, as depicted in the corresponding plots, include tapering along the corresponding hook segments931A-D thereof. Yet, it is contemplated that a tapering of hook segments931A-D, for example, similar to that described above, will provide strain relief for improved fatigue life and allow for shorter tine portions930A-D without compromising the orientation of distal segments932A-D, when hook segments931A-D are deformed into the open position.

Each of tine portions930A-D may be one of a plurality, which are included in a tissue-penetrating component, and that extend from a base portion903of the component, wherein base portion903defines an axis9of the component, and may be similar to the above described base portions603,703of components63,73.FIGS. 9A-Dfurther illustrate each of tine portions930A-D including a proximal segment933A-D that extends between base portion903and the corresponding hook portion931A-D. Each of proximal segments933A,933B is shown extending approximately parallel to axis9, while each of proximal segments933C,933D is shown extending from base portion903toward axis9, for example, to increase an overall arc length of each of tine portions930C,930D for added flexibility during retraction into catheter distal end310(FIGS. 3A-C), when the corresponding hook segment931C,931D is being elastically deformed to the open position (solid line of plots). Furthermore, although the orientation of distal segments932C,932D, when tine portions930C,930D are in the open position, is less favorable for ease of tissue penetration that that of other embodiments, the extension of proximal segments933C,933D toward axis9can contribute to a reduction in compressed tissue volume without a tapering of hook segments931C,931D.

With further reference toFIGS. 9B-C, each hook portion931B,931C is also defined by a straight section S that extends between radii R1, R2. With further reference to the solid lines in the plots ofFIGS. 9A-D, it may be seen how straight sections S can somewhat flatten the opened profile of tine portions930B,930C. Finally, in comparing the segmented lines of theFIG. 9A-Dplots, which correspond to the estimated tissue penetration path of each of tine portions930A-D, to that in theFIG. 5plot for tine portion230, it can be appreciated that the relatively shorter lengths of distal segments932A-D, in combination with the corresponding profiles of tine portions930A-D, lead to a reduction in tissue volume that is potentially compressed by each of the penetrated tine portions930A-D during subsequent relaxation toward the pre-set curvature (dashed lines).

Because a reduction in the length, and/or tapering for strain relief of tine portions, can, in some instances, hinder initial tine penetration upon deployment (e.g., according to the method described above in conjunction withFIGS. 3B-C), additional embodiments of the present invention, which are described below in conjunction withFIGS. 10A-CandFIGS. 11A-B, include tissue-piercing distal tips that are configured to enhance initial tine penetration. With reference toFIGS. 3A-B, the initial penetration of tine portions230rely upon a stiffness of tine portions230being greater than that of tissue T, and upon an orientation of tissue-piercing tip322relative to tissue T, when device200is loaded in catheter distal end310, with hook segments31elastically deformed into the open position.

FIG. 10Ais a plan view of an implantable medical device500, according to some embodiments of the present invention.FIG. 10Aillustrates device500including a hermitically sealed housing520and a pair of electrodes561,562; housing520, like housing220of device200, contains control electronics and a power source (not shown), which, for example, together with electrodes561,562, are adapted for cardiac pacing and sensing.FIG. 10Afurther illustrates device500including tine portions530, which are adapted to penetrate tissue in order to secure device500at an implant site, for example, a cardiac site in the right atrium RA or the right ventricle RV (FIG. 1).

FIG. 10Bis a perspective view of a tissue-penetrating fixation component53, according to some embodiments of the present invention, which is shown separated from device500, and which includes tine portions530.FIG. 10Billustrates component53also including a base portion503, from which tine portions530extend. According to the illustrated embodiment, base portion503of fixation component53defines a longitudinal axis5of component53and is configured for attachment to device500so that axis5is approximately aligned with a longitudinal axis25of device500. Component53may be part of a subassembly that forms a distal end of device500, and which also includes electrode561, for example, like the aforementioned subassembly that is disclosed in the above referenced and incorporated by reference passages of the detailed description of commonly-assigned U.S. Patent Application '690.

FIG. 10Bfurther illustrates each tine portion530of tissue-penetrating fixation component53including a hook segment531and a distal segment532. Each hook segment531is shown extending along a curvature that encloses an angle ψ, from a proximal end51thereof to a distal end52thereof; and each distal segment532is shown extending along a relatively straight line that is approximately tangent to distal end52of hook segment531. Each distal segment532is shown extending toward axis5, and, according to an exemplary embodiment, angle ψ is approximately 200 degrees. According to some preferred embodiments, component53is manufactured by, first, laser cutting base portion503and tine portions530, together, from a tube of superelastic and biocompatible metal (e.g., Nitinol), and then wrapping and holding each tine portion530about a mandrel for a heat setting process that pre-sets the illustrated curvature of each hook segment531. As mentioned above, manufacturing methods such as these are known to those skilled in the art of forming Nitinol components. AlthoughFIG. 10Bshows base portion503of component53formed as a ring, wherein tine portions530are integrally formed therewith, and spaced apart from one another about a perimeter of the ring, in alternate embodiments of tissue penetrating fixation components, one or more tine portions may be formed individually and then attached to a base portion that is configured in any suitable fashion for attachment to device500.

In order to provide more flexibility in selecting a suitable implant location for device500, a length of distal segment632of each tine portion630is relatively short compared to that of distal segment232of tine portion230, for example, between approximately 0.05 inch and approximately 0.1 inch. The shorter length can help to prevent perforation through the wall of a structure, for example, the heart, at some implant locations, and can reduce a probability for penetrated tine portions530to interfere with blood vessels, which interference, for example, may compromise coronary blood supply, as described above. However, with reference back toFIGS. 3A-C, after device500is loaded in distal end310of catheter300, and opening313of distal end310is positioned in proximity to tissue at a potential implant site, the reduced length of tine portions530may hinder initial tine penetration. A sharper terminal end of distal segment532can solve this problem but may lead to tissue tearing, upon insertion and/or retraction; thus a relatively blunt terminal end of distal segment532is preferred. So, with further reference toFIG. 10B, each distal segment532includes a tooth520and a relatively blunt end540, which surrounds tooth520.

FIG. 10Billustrates end540including a pair of legs541and a distal arch542that extends between legs541, distal to tip522of tooth520, for example, being spaced apart therefrom by approximately 0.005 inch. Each tooth520has a length, which is defined from a foot521thereof to a tissue-piercing tip522thereof, for example, being between approximately 0.025 inch and approximately 0.045 inch, and legs541extend along the length of tooth520, on opposing sides thereof. Each tooth520and corresponding end540may be laser cut at the same time that tine portions530and base portion503are cut from the aforementioned tube.

According to the illustrated embodiment, legs541of end540are configured to bend in elastic deformation when distal arch542is pushed against tissue at a potential implant site, for example, as illustrated inFIG. 10C, so that tip522of tooth520, which is configured to resist bending, is exposed to pierce the tissue.FIG. 10Cis an enlarged detail view of distal segment532as tine portion530is pushed into contact with tissue T at the implant site. With reference back toFIGS. 3A-B, it should be understood that pushing distal arch542against the tissue T may be accomplished, as described above for device200, after device500is loaded into distal end310of catheter300so that hook segments531of tine portions530are elastically deformed into the open position, at which distal segments532are directed distally toward opening313of distal end310. After tip522of each tooth520has pierced the tissue, in response to the relatively high push force for initial deployment, legs541of end540can relax back into line with tooth520so that distal arch542, upon subsequent penetration/insertion of tine portions530into tissue, and upon retraction thereof from the tissue, if necessary, prevents tip522from tearing the tissue. With reference back toFIG. 10B, according to an exemplary embodiment, a thickness t of each tine portion530, which is relatively constant along the entire length thereof, is approximately 0.005 inch, a width wf of foot521of tooth520is between approximately 0.010 inch and approximately 0.015 inch, a width wt of tip522of tooth520is approximately 0.003 inch, and a width we of legs541and distal arch542is approximately 0.005 inch.

FIG. 11Ais an elevation view of a tissue-penetrating fixation component83, according to some alternate embodiments of the present invention, which may also be incorporated in the exemplary device ofFIG. 10A.FIG. 11Aillustrates component83including a base portion803and a plurality of tine portions830extending therefrom, similar to component53, wherein each tine portion830includes a hook segment831and a distal segment832that are configured to address both of the aforementioned issues related to tissue penetration and fatigue life. Component83may be cut and formed from a Nitinol tube in a manner similar to that described above for component53.FIG. 10Afurther illustrates each hook segment831being pre-set to extend along a curvature that encloses angle φ, from a proximal end81thereof to a distal end82thereof; and each distal segment832is shown extending along a relatively straight line that is approximately tangent to distal end82of hook segment831. According to the illustrated embodiment, angle φ is approximately 180 degrees, so that each distal segment832extends approximately parallel to a longitudinal axis8of component83. The pre-set curvature of hook segment831is defined by a single radius R, which may be approximately 0.085 inch.

FIG. 11Bis a plan view of tine portion830, prior to forming the pre-set curvature thereof.FIGS. 11A-Billustrate each tine portion830including a tapered hook portion831, similar to hook portions631,731of tine portions630,730, described above, wherein second width W2, in proximity to distal end82of hook segment831, is less than first width W1, in proximity to a proximal end81of hook segment831.FIGS. 11A-Bfurther illustrate distal segment832having a width W3 that is greater than the second width W2. Distal segment832, like distal segment532of component53, includes tooth520and end540to facilitate tissue piercing without tearing, as described above. Like component53, a thickness t of each tine portion830, which is relatively constant along the entire length thereof, may be approximately 0.005 inch, and distal segment832thereof may conform to the aforementioned exemplary dimensions of tooth520and end540.

In the foregoing detailed description, the invention has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims.