Patent Publication Number: US-2020297518-A1

Title: Method and device for determining a sufficient stent removal force

Description:
The present invention relates to a method and a device for determining a sufficient stent removal force or stent retention force, also referred to hereinafter as an implant removal force. The invention will also be described hereinafter on the basis of the example of a balloon-expandable stent and an associated balloon catheter, but is not limited in principle to this application and is suitable for determining the removal force of an implant in general. 
     Stents or vascular supports are used in the field of blood vessel stenosis in order to hold open the constricted vessel in question or to expand the flow cross-section. The stent in question is for this purpose crimped for example onto a balloon, which is inflatable via a shaft connected to the balloon, such that the stent arranged in the constricted vessel region is expandable to an extended diameter so as to hold open the constricted point of the vessel. 
     An important variable in respect of a stent of this kind is what is known as the stent retention force (SRF), which will also be referred to herein as the stent removal force. This stent removal force refers to a force that has to be exerted on the stent in order to move it axially on the balloon or to remove it from the balloon. In order to determine this force, the stent can be held for example, wherein a tensile force is exerted on the balloon in the axial direction. This tensile force can be exerted on the balloon for example via the shaft extended in the axial direction. 
     Current methods for determining a sufficient stent removal force determine this force for example as follows. In accordance with a first variant the stent-balloon combination is fixed in the centre of a U-shaped holder by means of two adhesive strips. 
     The holder is pulled by means of a certain force, whilst the shaft (balloon) is held at the other end. If the holder is pulled by the desired stent removal force, the stent must remain on the balloon or must not move axially thereon. Otherwise, the test is not deemed to be passed. 
     Alternatively, in accordance with a further test method, the balloon-stent combination is for this purpose left in the crimping device and the shaft is pulled on by a predefined force. 
     There is also the possibility to act on the stent struts using a hook-shaped tool in order to remove the stent from the balloon. 
     A disadvantage of the aforementioned methods is in particular the fact that the fixing of the stent-balloon combination in the aforementioned holder has to be performed manually and is therefore costly. Furthermore, said adhesive strips can not only act on the stent, but also on the balloon arranged therebeneath, which increases the susceptibility to errors of the method accordingly. 
     Proceeding from this basis, the problem addressed by the present invention is that of providing an improved method and an improved device for determining a sufficient stent removal force. 
     This problem is solved by a device having the features of claim  8  and by a method having the features of claim  1 . Advantageous embodiments of the respective aspects of the invention are stated in the corresponding dependent claims and will be described hereinafter. 
     In accordance with claim  1 , a method for determining a sufficient removal force is disclosed, comprising the following steps:
         providing an implant ( 100 ) securely clamped to a catheter part, wherein the catheter part and implant extend along an axial direction (z), such that a removal force (F A ) on the catheter part in the axial direction (z) has to be exceeded in order to move the catheter part relative to the implant in the axial direction (z),   clamping the implant securely clamped on the catheter part between a first and a second holding jaw ( 10 ,  20 ) with a predefined contact pressure (F B ) perpendicularly to the axial direction (z),   exerting a predefined, desired removal force (F A ) on the catheter part in the axial direction (z), at which force the catheter part should not move relative to the implant in the axial direction (z), wherein, if the catheter part moves relative to the implant in the axial direction (z), the removal force (F A ) of the implant is considered to be too low, and wherein if the position of the implant relative to the catheter part is constant, the removal force (F A ) of the implant ( 100 ) is considered to be sufficient.       

     The invention will be described hereinafter on the basis of the example of a method for determining the stent removal force for a stent which is securely clamped on a balloon which is fastened to a catheter shaft. Accordingly, everything explained hereinafter for a balloon fastened to a catheter shaft and a stent securely clamped thereon is to be understood synonymously for a catheter part and an implant securely clamped thereon. However, the invention is suitable in particular for determining the stent removal force for a stent securely clamped on a balloon. 
     Here, movement of the balloon relative to the stent when said predefined, desired stent removal force is exerted is also considered to be a complete removal of the balloon from the stent. 
     In accordance with one embodiment of the method according to the invention, it is also provided that the holding jaws merely contact or hold the stent. In this regard it is provided in particular that the two holding jaws contact or hold merely an outer side of the stent, which runs around the balloon in a peripheral direction running perpendicular to the axial direction. The holding jaws therefore in particular do not directly hold the balloon or the shaft extending from the balloon. 
     In accordance with one embodiment of the invention it is also provided that the two holding jaws are each pressed with a surface against the stent with the contact pressure, wherein said surface is formed from one of the following materials: silicone, polyurethane or other elastomers. 
     In accordance with a further embodiment of the method according to the invention it is provided that the first holding jaw is pressed against the stent by means of a pretensioned spring in order to exert the contact pressure, such that the first holding jaw presses the stent against the second holding jaw and the balloon surrounded by the stent is thus clamped between the two holding jaws by the contact pressure. 
     It is furthermore provided in the method according to one embodiment that the axial direction runs vertically, wherein in particular the stent removal force exerted on the balloon points downwardly in the vertical direction. 
     A further aspect of the present invention relates to the provision of a measurement standard device which defines a measurement standard. This device can be used to exert a precisely predefinable tensile force on the balloon. 
     This measurement standard device can be inserted between a free end of the shaft and a fastening device, wherein a tensile force is exerted on the measurement standard device via the fastening device in the axial direction. This tensile force is also introduced into the free end of the shaft and thus into the balloon via the measurement standard device. 
     The measurement standard device in accordance with one embodiment comprises a first portion and a second portion connected thereto, wherein the two portions detach from one another when they are pulled apart from one another with a predefined force. A predefined tensile force can thus be exerted on the shaft/balloon in the axial direction, since the two portions separate from one another when this force is reached. 
     In accordance with one embodiment the two portions can each be formed by a magnet, wherein the two magnets are attracted to one another by a predefinable force. The force necessary to separate the two portions/magnets can be adjusted for example by a corresponding choice of the magnets. The measurement standard device can also comprise more than two magnets. 
     With use of the measurement standard device, it is possible to exert a tensile force on the measurement standard device in the axial direction in any way (i.e. manually or by machine). The tensile force is exerted or increased here such that the measurement standard device opens, i.e. the two portions of the measurement standard device are detached from one another. Should the force necessary to open the measurement standard device correspond to the stent removal force that is to be tested or that is desired, it can be determined whether the stent has a sufficient stent removal force. Should the stent move axially or be removed from the balloon before the measurement standard device opens (i.e. the two portions of the measurement standard device are detached from one another), the stent removal force is not sufficient, and the balloon-stent combination is discarded as a reject. 
     A further aspect of the present invention relates to a device for determining a sufficient stent removal force. This device can be used in particular to carry out the method according to the invention. 
     The device according to the invention comprises at least:
         a clamping device, comprising a first and a second holding jaw, wherein the two holding jaws are opposite one another and are configured to hold a stent surrounding a balloon with a predefinable contact pressure, and   an actuator for exerting a predefinable tensile force on a shaft fastened to the balloon, wherein the actuator comprises a fastening device for fastening the shaft relative to the actuator, such that the tensile force can be exerted on the shaft and thus the balloon via the fastening device.       

     In accordance with one embodiment of the device according to the invention, it is provided that the first holding jaw comprises a first material region and the second holding jaw comprises a second material region, wherein the two material regions each comprise a surface for contacting and holding the stent, wherein the two surfaces run parallel to one another and perpendicularly to the contact pressure. 
     The two material regions can be formed for example as a hexahedral pad, although other forms are also conceivable. 
     It is furthermore provided in accordance with a preferred embodiment of the invention that the two material regions are each formed from one of the following materials: a silicone (i.e. a material from the group of poly(organo)siloxanes) or PUR. 
     It is furthermore provided in accordance with one embodiment of the device that the first holding jaw comprises a first carrier and that the second holding jaw comprises a second carrier, wherein the two carriers are opposite one another, and wherein the first material region is fastened to the first carrier and the second material region is fastened to the second carrier. 
     In accordance with a further embodiment of the invention it is provided that the first carrier is fastened to a first arm of the clamping device and that the second carrier is fastened to a second arm of the clamping device. 
     Here, it is preferably provided in accordance with one embodiment that the first carrier in order to generate the contact pressure can be pretensioned in the direction of the second carrier by means of a spring. 
     It is furthermore provided in accordance with one embodiment that the spring for adjustment of the contact pressure is supported on a screw that engages via an external thread in an internal thread of the first arm. The spring is preferably arranged here between the first carrier and the screw. By screwing the screw increasingly into the internal thread, the pretension of the spring or the contact pressure is adjustable. Here, however, alternative possibilities for adjusting the force, such as a pneumatic device, are also possible. 
    
    
     
       Further features and embodiments of the invention will be explained hereinafter with reference to the drawings, in which: 
         FIG. 1  shows a sectional illustration of a clamping device of a device according to the invention for holding the stent surrounding the balloon; and 
         FIG. 2  shows a sectional illustration of a fastening device for introducing a tensile force into the shaft connected to the balloon. 
     
    
    
       FIG. 1  in conjunction with  FIG. 2  shows a device  1  according to the invention for testing the stent removal force of a balloon-stent combination  101 ,  100 . Here, the stent  100  is securely clamped on a balloon  101 , such that the stent  100  surrounds the balloon  101  in a peripheral direction U (that is to say transversely to the axial direction z of the stent  100  or of the balloon  101 ). A shaft  102  extends from the balloon  101  in the axial direction z and can be used to inflate the balloon  101 .  FIG. 1  in the present case shows an upper part of the device  1 , specifically a clamping device  2  for the stent  100 . A lower part of the device  1 , which is configured to exert a tensile force F A  on the balloon  101 , which force points downwardly in the axial or vertical direction z, is shown in  FIG. 2 . 
     In order to remove the stent  100  from the balloon  101  or conversely the balloon  101  from the stent  100 , or in order to move the two components  100 ,  101  axially relative to one another, what is known as the stent removal force F A  has to be applied in the axial direction z, which force must be great enough to ensure the functionality of the balloon catheter (stent-balloon combination  100 ,  101 ). 
     Whether a sufficiently great stent removal force F A  is present can be tested in accordance with the invention as follows: Firstly, the stent  100  surrounding the balloon  101  is clamped between a first and a second holding jaw  10 ,  20  with a predefined contact pressure F B  (for example between 1 and 10 N) perpendicularly to the axial direction z. Here, it is preferably provided that the two holding jaws  10 ,  20  in each case by merely a surface  12   a,    22   a  contact or hold merely an outer side  100   a  of the stent  100 , which surrounds the balloon  101  in the peripheral direction U running perpendicularly to the axial direction z and faces away from the balloon  101 . The surfaces  12   a,    22   a  are preferably formed from silicone or PUR or a comparable material. 
     A free end  102   a  of the shaft  102  is clamped in accordance with  FIG. 2  preferably in a fastening device  30  of an actuator  3 . A predefined, desired stent removal force F A  (for example between 1 and 15 N, in particular between 10 and 15 N) is then applied to the balloon  101  via the shaft  102  in the axial direction z via the fastening device  30  by means of the actuator  3 . If the balloon  101  then moves relative to the stent  100  in the axial direction z, the stent removal force F A  of the stent  100  is considered to be too low. Otherwise, the test is deemed to be passed. 
     According to  FIG. 1 , it is preferably provided in respect of the clamping device  2  that the first holding jaw  10  comprises a first material region  12  and the second holding jaw  20  comprises a second material region  22 , wherein the two material regions  12 ,  22  are preferably formed as pads and said surfaces  12   a,    22   a  are designed to hold the stent  100 . The two surfaces  12   a,    22   a  run parallel to one another and perpendicularly to the contact pressure F B . Said material regions/pads  12 ,  22  are preferably manufactured from a silicone or from PUR, as already mentioned above. 
     In order to hold the two material regions  12 ,  22 , the two holding jaws  10 ,  20  preferably each comprise a carrier  13 ,  23 , wherein the two carriers  13 ,  23  are opposite one another, and wherein the first material region  12  is fastened to the first carrier  13  and the second material region  22  is fastened to the second carrier  23 . Here, the first carrier  13  is fastened to a first arm  2   a  of the clamping device, wherein in particular the first carrier  13  engages by means of a protrusion  13   a  in a recess  13   b  in the first arm  2   a.  It is also provided that the first carrier  13  in order to generate the contact pressure F B  can be pretensioned by means of a spring  11  in the direction of the second carrier  23 , wherein the spring  11  in order to adjust the contact pressure F B  is supported on a screw  14  which engages by means of an external thread  14   a  in an internal thread  14   b  of the first arm  2   a.  The spring  11  is arranged here between the first carrier  13  and the screw  14 . Furthermore, the second carrier  23  can also be fastened to the second arm  2   b  of the clamping device  2  in that the second carrier  23  engages by means of a protrusion  23   a  in a recess  23   b  in the second arm  2   b.    
     According to  FIG. 2 , the fastening device  30  already mentioned above can also comprise two clamping jaws  30   a,    30   b  for securely clamping a free end  102   a  of the shaft  2 , such that said tensile force or predefined, desired stent removal force F A  can be exerted onto the balloon  101  via the fastening device  30 . The tensile force F A  can be exerted for example by means of an actuator  3  onto the shaft  102  in the axial direction z via the fastening device  30 . 
       FIG. 2  also shows a measurement standard device  5 , which can be used to exert a precisely predefinable tensile force on the balloon  101 , in particular for the case in which the tensile force F A  is not precisely controllable/measurable, for example in the case of a manual exertion of the tensile force F A . 
     The measurement standard device  5  in accordance with  FIG. 2  can be inserted optionally between a free end  102   a  of the shaft  102   a  and the fastening device  30 , wherein it is possible to exert a tensile force on the measurement standard device  5  in the axial direction via the fastening device  30 . This tensile force F A  is also introduced into the free end  102   a  of the shaft  102  and thus into the balloon  101  via the measurement standard device  5 . 
     The measurement standard device  5  can comprise for example a first portion  51  and a second portion  52  connected thereto, wherein the two portions  51 ,  52  detach from one another when they are pulled apart from one another at a predefined force. In this way, a predefined tensile force F A  can be exerted onto the shaft  102  and balloon  101  in the axial direction z, since the two portions  51 ,  52  separate from one another when this force F A  is reached and precisely delimit the exerted force F A . The two portions  51 ,  52  can be formed for example in each case by a magnet  51 ,  52 , wherein the two magnets  51 ,  52  are attracted by a predefinable force F A , which is adjustable for example by the choice of the magnets.