Patent Publication Number: US-2023158315-A1

Title: Biostimulator having lockable fixation element

Description:
This application is a continuation of co-pending U.S. patent application Ser. No. 16/785,172, filed Feb. 7, 2020, which claims the benefit of priority of U.S. Provisional Patent Application No. 62/803,973, filed Feb. 11, 2019, entitled “Biostimulator Having Lockable Fixation Element,” and these patent applications are incorporated herein by reference in their entirety to provide continuity of disclosure. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates to biostimulators. More specifically, the present disclosure relates to leadless biostimulators having tissue anchors. 
     Background Information 
     Cardiac pacing by an artificial pacemaker provides electrical stimulation of the heart when its own natural pacemaker and/or conduction system fails to provide synchronized atrial and ventricular contractions at rates and intervals sufficient for a patient&#39;s health. Such antibradycardial pacing provides relief from symptoms and even life support for hundreds of thousands of patients. Cardiac pacing may also provide electrical overdrive stimulation to suppress or convert tachyarrhythmias, again supplying relief from symptoms and preventing or terminating arrhythmias that could lead to sudden cardiac death. 
     Cardiac pacing by conventional pacemakers is usually performed by a pulse generator implanted subcutaneously or sub-muscularly in or near a patient&#39;s pectoral region. Well known difficulties exist for conventional pacemakers such as complex connectors and/or risks of mechanical failure. As a result, leadless cardiac pacemakers have been developed. Leadless cardiac pacemakers are self-contained and self-sustainable biostimulators that can be attached to tissue within a dynamic environment, e.g., within a chamber of a beating heart. Leadless cardiac pacemakers can deliver pacing pulses directly to a target tissue. Leadless cardiac pacemakers can include tissue anchors that project from a distal end of the pacemaker to engage the target tissue and hold the pacemaker against the tissue after implantation. 
     SUMMARY 
     Tissue anchors of leadless pacemakers ensure that a sensing and/or pacing electrode of the leadless pacemaker maintains good electrical contact with the target tissue. For example, a tissue anchor can be a helical metal spring having a sharpened end. The sharpened end can engage the target tissue. A proximal portion of the helical metal spring can be mounted on a body of the pacemaker and provide a spring force to pull the pacing electrode toward and into contact with the target tissue when the helical metal spring is screwed into the target tissue. Such a tissue anchor may not, however, be securely mounted on the body of the leadless pacemaker. For example, the helical metal spring may inadvertently rotate relative to the pacemaker body during tissue engagement or long-term use. When the tissue anchor moves relative to the body, the tissue anchor can disengage from the target tissue and/or the electrode of the leadless pacemaker can lose contact with the target tissue. Existing manners of securing the tissue anchor to the body to prevent such dislodgement, such as adhesives, can degrade over time. Accordingly, leadless cardiac pacemakers can benefit from improvements in securing the tissue anchor to the body of the leadless pacemaker. 
     A biostimulator, e.g., a leadless cardiac pacemaker, having a fixation element that is securely mounted on a body of the biostimulator is described below. A leadless pacemaker system including the biostimulator and a method of manufacturing the biostimulator are also described. In an embodiment, a biostimulator, such as a leadless cardiac pacemaker, includes a helix mount mounted on a housing (a body of the biostimulator), and a fixation element mounted on the helix mount. The helix mount therefore couples the fixation element to the housing. The helix mount has a mount flange, which provides a helical channel to receive and hold the fixation element. The mount flange includes a keeper to receive a fastener of the fixation element. More particularly, the fastener can be a portion of the fixation element disposed in the keeper. 
     In an embodiment, the fixation element includes a helix having one or more turns, and the fastener extends between a first turn end of a helix turn and a second turn end of the helix turn. The fastener can extend orthogonal to the helix at the first turn end and the second turn end. More particularly, the fastener can be coupled to the helix at one or more of the first turn end or the second turn end, and can extend through the keeper channel from a portion of the helical channel on a first side of the mount flange to a portion of the helical channel on a second side of the mount flange. The fastener can be a bent portion of the helix, or a stop element that is coupled to the helix by a joint, e.g., a weld. In any case, the fastener can engage the keeper and be rigidly connected to a helical portion of the fixation element such that the keeper interferes with the fastener to resist movement of the fastener and in turn movement of the fixation element relative to the helix mount. Accordingly, the keeper-to-fastener locking mechanism can secure the fixation element to the body of the biostimulator. 
     The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
         FIG.  1    is a perspective view of a leadless pacemaker system, in accordance with the present disclosure. 
         FIG.  2    is a side view of a biostimulator, in accordance with the present disclosure. 
         FIG.  3    is a perspective view of a distal portion of a biostimulator, in accordance with the present disclosure. 
         FIG.  4    is a perspective view of a fixation element of a biostimulator, in accordance with the present disclosure. 
         FIG.  5    is a perspective view of a helix mount, in accordance with the present disclosure. 
         FIG.  6    is a side view of a fastener of a fixation element engaged with a keeper of a helix mount, in accordance with the present disclosure. 
         FIG.  7    is a side view of a fastener of a fixation element engaged with a keeper of a helix mount, in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe a biostimulator, e.g., a leadless cardiac pacemaker, having a fixation element that is mounted on a housing and includes a helix extending to a leading point for piercing tissue. The biostimulator may be used to pace cardiac tissue. The biostimulator may also be used in other applications, such as deep brain stimulation, and thus, reference to the biostimulator as being a cardiac pacemaker is not limiting. 
     In various embodiments, description is made with reference to the figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions, and processes, in order to provide a thorough understanding of the embodiments. In other instances, well-known processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the description. Reference throughout this specification to “one embodiment,” “an embodiment,” or the like, means that a particular feature, structure, configuration, or characteristic described is included in at least one embodiment. Thus, the appearance of the phrase “one embodiment,” “an embodiment,” or the like, in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The use of relative terms throughout the description may denote a relative position or direction. For example, “distal” may indicate a first direction along a longitudinal axis of a biostimulator housing. Similarly, “proximal” may indicate a second direction opposite to the first direction. Such terms are provided to establish relative frames of reference, however, and are not intended to limit the use or orientation of a biostimulator to a specific configuration described in the various embodiments below. 
     In an aspect, a biostimulator includes a fixation element that locks into a helix mount to reduce a likelihood that the fixation element will move relative to the helix mount during use. For example, when a helix of the fixation element is engaged with tissue to fix the biostimulator to a target site, torque applied to the fixation element will not cause the helix to rotate and back away from the tissue into the helix mount. Securing the fixation element to the helix mount can allow for more reliable tissue fixation by reducing a likelihood that the fixation element will rotate relative to the helix mount and/or translate relative to helix mount along a longitudinal axis of the biostimulator. More reliable fixation can reduce variability in tissue scarring around the fixation element because there will be less chance of variable motion of the fixation element. Reduced scarring can promote more consistency in pacing thresholds of the biostimulator. The fixation element described below can also be smaller than other designs, because fewer turns of the helix are required to achieve a secure attachment between the fixation element and the helix mount. In addition to reducing space requirement of the fixation element, assembly of the fixation element to the helix mount can be simplified, since the fastener can securely engage the keeper without using adhesives to bond the fixation element to the helix mount. Accordingly, the lockable fixation element described below provides a compact, easily assembled, and reliable securement between the fixation element and a body of the biostimulator. 
     Referring to  FIG.  1   , a perspective view of a leadless pacemaker system is shown in accordance with the present disclosure. A leadless pacemaker system  100  may be used for delivery and/or retrieval of a biostimulator  102 , e.g., a leadless cardiac pacemaker, into or from a patient. The leadless pacemaker system  100  can include an elongated catheter  104  extending distally from a handle  106  to a distal end  108 . The elongated catheter  104  can be a deflectable catheter, and an operator can use the handle  106  to steer the distal end  108  in the patient. In an embodiment, the leadless pacemaker system  100  includes a guide catheter  110  mounted on the elongated catheter  104 . The guide catheter  110  can be slidably disposed on the elongated catheter  104  such that a distal portion of the guide catheter  110  can slide distally over the distal end  108  of the elongated catheter  104  and/or the biostimulator  102 . Similarly, the leadless pacemaker system  100  can include an introducer hub assembly  112  mounted on the guide catheter  110 . The introducer hub assembly  112  can be slidably disposed on the guide catheter  110  such that a distal portion of the introducer hub assembly  112  can slide distally over the distal end  108  of the elongated catheter  104 . More particularly, the introducer hub assembly  112  can be inserted into an access sheath to gain access to the patient vasculature, and after access is established, the distal portion of the guide catheter  110  and/or the distal end  108  of the elongated catheter  104  can be advanced through the access sheath into the patient. 
     The distal end  108  of the elongated catheter  104  may be selectively connectable to the biostimulator  102 . More particularly, the biostimulator  102  can be mounted on the distal end  108  of the elongated catheter  104 . The biostimulator  102  can be protected by a protective pacemaker sheath of the distal portion of the guide catheter  110  during delivery and/or retrieval of the biostimulator  102  from the patient. Accordingly, the biostimulator  102  can be advanced into the patient along with the distal end  108 . 
     The leadless pacemaker system  100  can be used to implant one or more biostimulators  102  within an atrium and/or a ventricle of a heart of the patient. Implantation of each biostimulator  102  may be achieved, in part, by endocardial insertion of the biostimulators  102 . For example, the elongated catheter  104  of the leadless pacemaker system  100  can be torqueable and can be used to rotate the biostimulator  102 . Rotation of the biostimulator  102  when a fixation element (described below) is in contact with the heart tissue can cause the fixation element to screw into the heart tissue and affix the biostimulator  102  to the heart tissue. Similarly, removal and retrieval of the biostimulator(s)  102  may be accomplished endocardially. For example, the torqueable elongated catheter  104  can be rotated to disengage the biostimulator  102  from the heart tissue. Accordingly, delivery and retrieval systems having a structure similar to that shown in  FIG.  1    may be used to deliver and/or retrieve biostimulator  102  from a target anatomy. 
     Referring to  FIG.  2   , a side view of a biostimulator is shown in accordance with the present disclosure. The biostimulator  102  can include a housing  202 . The housing  202  can be a hermetically-sealed housing, and can include an electronics compartment  203 . The electronics compartment  203  may be located within the housing  202 , and can contain the electronic components necessary for operation, e.g., sensing and/or pacing, of the biostimulator  102 . The hermetic housing  202  can be adapted to be implanted on or in a human heart, and can be cylindrically shaped, rectangular, spherical, or any other appropriate shape. The housing  202  can include a conductive, biocompatible, inert, and anodically safe material such as titanium, 316 L stainless steel, or other similar materials. 
     The biostimulator  102  can include an insulator  204  disposed on the conductive material of the housing  202  to separate a proximal electrode  206  from a distal electrode  208 . The electrodes can include pace/sense electrodes, or return electrodes. A low-polarization coating can be applied to the electrodes, such as platinum, platinum-iridium, iridium, iridium-oxide, titanium-nitride, carbon, or other materials commonly used to reduce polarization effects, for example. In an embodiment, the distal electrode  208  is a pace/sense electrode and the proximal electrode  206  is a return electrode. 
     In an embodiment, the proximal electrode  206  is integral to the housing  202 . Alternatively, the proximal electrode  206  can be connected to the housing  202  at a maximum distance of two centimeters from the housing  202 . The distal electrode  208  can also be referred to as a tip electrode, and can be used to sense and/or pace the target tissue when the biostimulator  102  is implanted in the patient. By co-locating the stimulation electrode and the pacing generator of the electronics compartment  203  on the housing  202 , and by reducing the pulse generator size to fit within the heart, the biostimulator  102  can be leadless. 
     The insulator  204  can be an insulating coating on a portion of the housing  202  between the electrodes, and can include materials such as silicone, polyurethane, parylene, or another biocompatible electrical insulator  204  commonly used for implantable medical devices. In the embodiment of  FIG.  2   , a single insulator  204  is disposed along the portion of the housing  202  between the electrodes. In some embodiments, the housing  202  itself can include an insulator  204  instead of a conductor, such as an alumina ceramic or other similar materials, and the electrodes can be disposed upon the housing  202 . 
     The housing  202  can optionally contain an energy source (not shown) to provide power to the distal electrode  208 . The energy source can be a battery, such as a lithium carbon monofluoride (CFx) cell, or a hybrid battery, such as a combined CFx and silver vanadium oxide (SVO/CFx) mixed-chemistry cell. Similarly, the energy source can be an ultracapacitor. In an embodiment, the energy source can be an energy harvesting device, such as a piezoelectric device that converts mechanical strain into electrical current or voltage. In certain embodiments, the energy source can be located outside of the housing  202 . For example, the energy needed to power the electrical circuits could come from an ultrasound transducer and receiver, which receive ultrasound energy from an ultrasound transmitter located outside of the housing  202 . 
     In an embodiment, the distal electrode  208  is radially inward of a fixation element  210 . The fixation element  210  can be a fixation helix or other flexible or rigid structure suitable for attaching the housing  202  to tissue, such as heart tissue. 
     Referring to  FIG.  3   , a perspective view of a distal portion of a biostimulator is shown in accordance with the present disclosure. The biostimulator  102  can include a header assembly  302 . The header assembly  302  can be mounted on the housing  202  in several ways, including, without limitation, connecting the header assembly  302  to the housing  202  using one or more of a biocompatible adhesive, a threaded connection, or ultrasonic welding. 
     The header assembly  302  generally includes the fixation element  210  and one or more backstop elements  304 . There may be several backstop elements, including forward facing and side facing or laterally extending backstop elements, which provide anti-unscrewing features. More specifically, the fixation element  210  can include a primary helix pointing in a first direction and the backstop elements can include side-facing anti-unscrewing features. The side-facing anti-unscrewing features can include several side-facing sutures extending from an outer surface of the biostimulator  102  in a second direction opposite the first direction. The fixation element  210  may be substantially formed of any suitable biocompatible material including, without limitation, one or more of stainless steel, nickel-titanium alloys (such as Nitinol), nickel-chromium alloys (such as Incoloy®), titanium, and multiphase nickel alloys (such as MP35N® or 35N LT®). The backstop elements may be, by contrast, formed of various flexible biocompatible materials including, without limitation, one or more of polypropylene, polyethylene, polyester, nylon, polyurethane, silicone, poly(lactic acid) (PLA), poly(glycolic acid) (PGA), polyimide, polyether ether ketone (PEEK), and polycarbonate. 
     The header assembly  302  may include a helix mount  306 . The helix mount  306  can couple to and retain the fixation element  210 . More particularly, the fixation element  210  can be mounted on the helix mount  306 . Similarly, the helix mount  306  can be mounted on the housing  202 . Accordingly, the fixation element  210  may be connected to the housing  202  via the helix mount  306 . 
     Referring to  FIG.  4   , a perspective view of a fixation element of a biostimulator is shown in accordance with the present disclosure. The fixation element  210  can include a helix  402  extending distally from a proximal end to a distal piercing tip  404  over one or more turns  406 . Certain dimensions of the fixation element  210  are provided here by way of example and not limitation. The helix  402  can be formed from a wire having wire diameter from and including 0.003 inches to and including 0.03 inches. The wire can be coiled or otherwise formed into a helix corkscrewing about a longitudinal axis. The helix  402  can have a diameter (measured transverse to the longitudinal axis) from and including 0.06 inches to and including 0.3 inches. The helix  402  can have a pitch (measured parallel to the longitudinal axis) from and including 0.01 inches to and including 0.05 inches. 
     Each turn  406  of the helix  402  can spiral about the longitudinal axis of the fixation element  210  between respective turn ends. For example, a turn  406  of the helix  402  can extend distally from a first turn end  408  to a second turn end  410  over a single revolution of the helix  402 . The fixation element  210  can, however, extend over any number of turns  406  from the proximal end to the distal piercing tip  404 . For example, in the embodiment illustrated in  FIG.  4   , the helix  402  continues to extend distally from the second turn end  410  to the distal piercing tip  404  over several, e.g., 2.5, turns or revolutions of the helix  402 . 
     In an embodiment, the fixation element  210  includes a fastener  412  configured to engage a corresponding feature of the helix mount  306 . The fastener  412  can be coupled to the fixation element  210  at any location along the fixation element  210 . For example, the fastener  412  can be coupled to the helix  402  at the first turn end  408 . As described below, the fastener  412  can be integral to the helix  402 , e.g., may be a bent segment of the fixation element  210 , or may be an additional component, e.g., may be a wire segment, bonded to the helix  402 . 
     Referring to  FIG.  5   , a perspective view of a helix mount is shown in accordance with the present disclosure. The helix mount  306  can include a mount body  502 , which can be a central portion of the helix mount  306  that mounts on the housing  202  of the biostimulator  102 . The helix mount  306  can have a cylindrical outer surface facing radially outward from the longitudinal axis, and an interior cavity having a cylindrical inner surface facing radially inward from the longitudinal axis. Accordingly, the helix mount  306  can have an annular wall extending along the longitudinal axis. In an embodiment, the mount body  502  can have an internal thread formed in the inner surface within the interior cavity. The internal thread can engage an external thread on the housing  202  such that the helix mount  306  can be screwed onto a distal end of the housing  202 . 
     The mount body  502  can have a semi-closed end. For example, the mount body  502  can include a distal cap at a distal end of the helix mount  306 . The distal cap can span in a transverse direction across the longitudinal axis to enclose a distal end of the interior cavity of the mount body  502 . Thus, a distal surface of the distal cap can face substantially longitudinally, and a proximal surface of the distal cap can face the interior cavity. The distal cap can be a bulbous portion at a distal end of the helix mount  306  that has a smooth and curved, atraumatic, outer surface. The distal cap can be a semi-closed end of the mount body  502  because a central hole may be formed through the distal cap along the longitudinal axis. The central hole can receive the electrode tip (as shown in  FIG.  3   ). The electrode tip can project distally from the mount body  502  to engage tissue when the fixation element  210  is screwed into the target site. 
     In an embodiment, the helix mount  306  includes a mount flange  504 . The mount flange  504  can extend radially outward from the mount body  502 . For example, the mount flange  504  can spiral around an outer surface of the mount body  502  in a threaded fashion from a distal end of the mount flange  504  to a proximal end of the mount flange  504 . The threaded mount flange  504  spirals around the mount body  502  to form a helical channel  506  between the flange walls. The helical channel  506  can be a helical groove shaped to receive the helix  402  of the fixation element  210 . For example, the helix  402  can be screwed onto the mount flange  504  to secure the fixation element  210  to the helix mount  306  and the housing  202 . Accordingly, the mount flange  504  provides a holding thread to receive and hold the fixation element  210 . 
     The helix mount  306  can have a keeper  508  in the flange walls. The keeper  508  can be configured to receive the fastener  412  of the fixation element  210  to lock the fixation element  210  to the helix mount  306 . More particularly, the fastener  412  of the fixation element  210  can be inserted into the keeper  508  of the helix mount  306  to lock the components together, and the fastener  412  can be removed from the keeper  508  to unlock the components. 
     Referring to  FIG.  6   , a side view of a fastener of a fixation element engaged with a keeper of a helix mount is shown in accordance with the present disclosure. The keeper  508  in the mount flange  504  can include an axial channel  602  extending through the mount flange  504 . For example, the axial channel  602  can be a slot, a cut-out, a pocket, a recess, or another void formed in the threaded body of the mount flange  504  such that an opening is created in the threaded body between a first side  604  of the mount flange  504  and a second side  606  of the mount flange  504 . Whereas the axial channel  602  can extend axially through the mount flange  504 , the helical channel  506  extends on both the first side  604  and the second side  606  of the threaded body of the mount flange  504 . Accordingly, the axial channel  602  intersects the helical channel  506  on the first side  604  and the second side  606  of the mount flange  504 . 
     The axial channel  602  of the keeper  508  connects a portion of the helical channel  506  on the first side  604  of the mount flange  504  to a portion of the helical channel  506  on the second side  606  of the mount flange  504 . The axial channel  602  can be substantially axial, such that a point within the portion of the helical channel  506  on the first side  604  of the mount flange  504  is longitudinally separated from a point within the portion of the helical channel  506  on the second side  606  of the mount flange  504 . The points in the helical channel  506  coincide with the turn ends  408 ,  410  of the helix  402  when the fixation element  210  is located within the helical channel  506 . The points represent ends of the turn  406  of the helical channel  506 , and coincide with the first turn end  408  and the second turn end  410  of the helix  402 . Accordingly, the keeper  508  extends through the mount flange  504  between turn ends  408 ,  410  of the helical channel  506 . 
     In an embodiment, when the fixation element  210  is threaded onto the mount flange  504  of the helix mount  306 , the fastener  412  is disposed in the keeper  508 . The fastener  412  can extend into the keeper  508  from the helix  402 . The fastener  412  can extend substantially axially between the points on either side of the mount flange  504 . Accordingly, the fastener  412  is disposed in the keeper  508  between the first turn end  408  and the second turn end  410  of the helix  402 . 
     Given that the fastener  412  can extend through the keeper  508  in a substantially axial direction, and the helix  402  extends on either side of the mount flange  504  in a spiral or transverse direction, the fastener  412  can be orthogonal to the helix  402  at one or more of the first turn end  408  or the second turn end  410 . For example, the fastener  412  can extend orthogonal to the helix  402  at both the first turn end  408  and the second turn end  410 . Orthogonality between the fastener  412  and the helix  402  can create a stop feature that engages the keeper  508  when torque is applied to the helix  402 . For example, a leftward torque applied to the fixation element  210  of  FIG.  6    may cause the fastener  412  to press against a left face of the keeper  508 . By contrast, a rightward torque applied to the fixation element  210  of  FIG.  6    may cause the fastener  412  to press against a right face of the keeper  508 . In both cases, the keeper  508  can interfere with the fastener  412  and resist motion of the fastener  412 , and thus, can prevent rotation of the fixation element  210  relative to the helix mount  306 . In other words, the pocket in the mount flange  504  acts as a stop in either rotational direction. Since the helix  402  cannot rotate when the fastener  412  is engaged with the keeper  508 , a likelihood that the fixation element  210  can move translationally (forward or backward relative to the housing  202  or the target tissue along a longitudinal axis of the biostimulator  102 ) is reduced. 
     In an embodiment, the fastener  412  is integral to the helix  402 . The fastener  412  can be a segment  608  of the fixation element  210 , which extends from the helix  402 . The segment  608  can extend from a proximal end of the helix  402 . For example, the first turn end  408  may be a proximal end of the helix  402 , and the fixation element  210  may be bent at the first turn end  408 . Accordingly, the segment  608  can be the bent portion of the fixation element  210  that extends from a bend  610  at the first turn end  408 . The bend  610  can be a 90° bend of the fixation element  210  wire at the first turn end  408 . The segment  608  can extend into the keeper  508  from the bend  610  toward the helix  402  on an opposite side of the mount flange  504 . More particularly, the segment  608  can extend from the bend  610  to a segment end  612  distal from the first turn end  408 . 
     The segment end  612  of the fastener  412  can be a free end, or the segment end  612  can be fastened to one or more of the mount flange  504  or the helix  402 . The fastener  412  can extend toward the helix  402  on an opposite side of the mount flange  504  such that the fixation element  210  self-intersects across the keeper  508 . 
     In an embodiment, the segment end  612  is coupled to the helix  402  at a location distal from the keeper  508 . For example, the segment end  612  can be coupled to the helix  402  at the second turn end  410 , which is longitudinally separated from the bend  610 . The fastener  412  can be coupled to the helix  402  by a joint  614 . The joint  614  can be, for example, a thermal or adhesive bond. In an embodiment, the joint  614  is a weld that affixes the segment end  612  to the helix  402  at the second turn end  410 . 
     Referring to  FIG.  7   , a side view of a fastener of a fixation element engaged with a keeper of a helix mount is shown in accordance with the present disclosure. In an embodiment, the fastener  412  is a stop element  702 , which is a separate component than the helix  402 . The fastener  412  can be coupled to the helix  402  at one or more of the first turn end  408  and/or the second turn end  410 . For example, as described above, an integral fastener  412  can be coupled to the helix  402  at the first turn end  408  by the bend  610 , and the fastener  412  can be coupled to the helix  402  at a second turn end  410  by the joint  614 . Similarly, the stop element  702  can be coupled to the helix  402  at the first turn end  408  by a first joint  704 , and the fastener  412  can be coupled to the helix  402  at the second turn end  410  by a second joint  706 . The joints  704 ,  706  can be, for example, thermal or adhesive bonds. In an embodiment, the joints  614   704 ,  706  are welds that affix the stop element  702  to the helix  402  at the turn ends  408 ,  410 . 
     One or more of the first joint  704  or the second joint  706  are optional joints. More particularly, the stop element  702 , which may be a short segment of wire or another piece of material that fits within the keeper  508 , can be joined to the helix  402  on only one side of the mount flange  504 . In an embodiment, the stop element  702  is joined to the helix  402  at the first turn end  408  and extends distally into the keeper  508  to a free end near the second turn end  410 . In another embodiment, the stop element  702  is joined to the helix  402  at the second turn end  410  and extends proximally into the keeper  508  to a free end near the first turn end  408 . Optionally, the stop element  702  is joined to the helix  402  at both turn ends  408 ,  410 . As described above, the keeper  508  can resist movement of the stop element  702  when torque is applied to the fixation element  210 . Accordingly, when the fastener  412  is engaged with the keeper  508 , a likelihood of relative movement between the fixation element  210  and the helix mount  306  is reduced. 
     The fastener  412  can extend from the helix  402  at a location that is at or distal to a proximal end  708  of the fixation element  210 . As shown in  FIG.  7   , the joint  614  between the stop element  702  and the helix  402  is at the first turn end  408  distal to (in a helical direction along the helix  402 ) the proximal end  708  of the helix  402 . By contrast, as shown in  FIG.  6   , the bend  610  between the segment end  612  and the helix  402  is at the first turn end  408 , which is coincident with the proximal end  708  of the fixation element  210 . One skilled in the art can contemplate other configurations, however, in which the fastener  412  is integral to the helix  402  and is distal from the proximal end  708  of the fixation element  210 . For example, the fastener  412  can be a U-shaped bend that extends distally into the keeper  508  from the bend  610  at the first turn end  408  and then extends proximally out of the keeper  508  to a second bend (not shown). The fastener  412  can transition into the helix  402  at the second bend to run proximally around the mount body  502 . Such a configuration can allow for the fastener  412  to be positioned at any turn of the mount flange  504 . For example, although the fixation element  210  has been represented with only a single fastener  412  near the proximal end  708  of the helix  402  in the illustrations, the fixation element  210  may include several fasteners  412  extending through respective keepers  508  at different locations along the mount flange  504 . Accordingly, the biostimulator  102  can have several fastener-keeper locking pairs to secure the fixation element  210  and reduce a likelihood of relative movement between the fixation element  210  and the helix mount  306 . 
     Having described the structure of the fixation element  210  and the helix mount  306  of the biostimulator  102 , it shall be apparent that a method of assembly may be used to lock the fixation element  210  to the helix mount  306 . In an embodiment, the helix mount  306  can be mounted on the housing  202 . For example, an internal thread of the mount body  502  can be engaged with an external thread of the housing  202 , and the helix mount  306  can be screwed onto the housing  202 . At an operation, the fixation element  210  can be mounted on the helix mount  306 . The helix  402  of the fixation element  210  can be screwed into the helical channel  506  defined by the mount flange  504 . While threading the fixation element  210  onto the mount flange  504 , the fastener  412  can ride over a radially outward surface of the mount flange  504 . More particularly, the mount flange  504  can resiliently deflect a portion of the fixation element  210  (having the fastener  412 ) radially outward while the fastener  412  slides over the mount flange  504  toward the keeper  508 . At an operation, the fastener  412  can be engaged into the keeper  508 . When the fastener  412  slides over the mount flange  504  to the location of the keeper  508 , a spring force of the deflected fixation element  210  can resiliently deflect the fastener  412  radially inward into the keeper  508 . The fastener  412  can therefore be locked into the keeper  508  between the first turn end  408  and the second turn end  410  of the helix  402 , and further rotation of the fixation element  210  can be resisted by contact between an inner wall of the keeper  508  and an outer surface of the fastener  412 . 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.