Abstract:
An introducer assembly provided with a sheath formed of a plurality of elongate sections which are coupled to one another along their longitudinal sides by a dovetail coupling. The sheath sections are slidable longitudinally relative to one another, enabling the sheath to be retracted in stages. This reduces the amount of friction which must be overcome in the use of the sheath and in particular during the deployment of a medical device carried within the sheath. The structure can also substantially increase the precision of deployment of the medical device, as well as decreasing the force required during the deployment operation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Great Britain patent application No. 1402470.7 filed on Feb. 12, 2014 entitled “SHEATH OR CATHETER FOR MEDICAL INTRODUCER ASSEMBLY” the entire contents of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a sheath or catheter for a medical introducer assembly, and to an introducer assembly including such a sheath or catheter. 
       BACKGROUND ART 
       [0003]    Medical introducer assemblies are in common use for the deployment of medical equipment or devices in a patient, and in particular for vascular implantation of medical devices such as stents, stent grafts, vena cava filters, occlusion devices, prosthetic valves and so on. Generally, such assemblies are fed endoluminally through the patient&#39;s vasculature to the treatment site, often over significant distances. For instance, aortic devices may be introduced via the femoral artery and fed all the way up to the aortic arch and even the ascending aorta to the heart. Such introducer assemblies can have lengths of well over one metre, in some cases up to two metres or more. Even assemblies intended to be fed into the patient at a distance closer to the treatment site will generally have significant lengths. 
         [0004]    Introducer assemblies of this nature include an outer sheath which houses the majority of the elements of the assembly, as well as the medical device, treatment or diagnostic tool. The distal end of the sheath normally has a dilator tip extending therefrom, for assisting in guiding the sheath through the patient&#39;s vasculature. The proximal end of the sheath typically remains outside the patient and is connected to a haemostatic valve assembly and deployment triggers. The sheath is generally a tight fit to the components it houses in order to ensure that these are securely held during the deployment process, as well as to ensure that the sheath has the smallest practical outer diameter. The sheath needs to be sufficiently flexible yet sufficiently strong to have good trackability through often tortuous vasculature, as well as good pushability to get its distal end reliably at the treatment site. In most cases the introducer assembly and thus the sheath will be caused to curve, often several times along its length, as a result of curvatures and bifurcations in the vasculature. These characteristics of the sheath and its use create friction within the sheath between the inner wall of the sheath and the components it houses, most typically with the medical device or tool or the elements of the assembly which hold the latter in place at the distal end of the sheath. This friction must be overcome in order to pull back the sheath so as to expose the medical device or tool, which often requires a significant force by the clinician. The application of such a force, however, makes it difficult to hold the distal end of the introducer assembly steady, particularly given that the only part of the assembly which can be held is its proximal end which remains outside the patient. As a result, the distal end can often jump out of position, leading to the risk of misplacement of the medical device or tool. This problem is often exacerbated with medical devices which self-expand and which will thus be pressing against the inner wall of the sheath when held they are held inside it. Further difficulties can arise with medical devices and tools which are orientation dependant, such as fenestrated or branched medical devices, and which must not only be disposed in the correct longitudinal position in a vessel but also in the correct orientation. 
         [0005]    Attempts have been made in the art to alleviate such drawbacks, for instance with splittable or expandable sheaths, but such solutions are not always appropriate and do not necessarily alleviate the problems mentioned above. 
         [0006]    Another issue with such assemblies lies with the provisions necessary to accommodate the guide wire which is often used for placing the introducer assembly in the patient. This is normally fed into the patient first, with the introducer assembly then fed over the guide wire, which acts to train the distal end of the assembly around the patient&#39;s vasculature between the percutaneous entry point and the treatment site. The guide wire will generally be kept in place in the patient for the duration of the medical procedure, particularly important when the procedure involves multiple steps and thus the use of multiple introducer assemblies or components. In order to be able to hold the guide wire in position, this will often have to be of significant length, typically greater than two times the length of the sheath. This can often lead to the guide wire having very significant lengths. 
         [0007]    Known designs of sheath can be found, for example, in U.S. Pat. No. 8,088,154 and US-2010/0049305. 
       DISCLOSURE OF THE INVENTION 
       [0008]    The present invention seeks to provide an improved sheath or catheter for an introducer assembly and to provide an improved introducer assembly. 
         [0009]    According to an aspect of the present invention, there is provided a sheath or catheter for a medical introducer assembly including a plurality of elongate sections, each having first and second longitudinal sides and couplings at said first and second longitudinal sides; said elongate sections being connected together by said couplings to form the sheath or catheter; said couplings allowing longitudinal movement of elongate sections coupled thereby, such that the elongate sections are movable relative to one another. 
         [0010]    In the claims the device is referred to as a catheter but it is to be understood that use of the term catheter is intended to encompass also a sheath, for instance of the type used in an introducer assembly. 
         [0011]    Such a structure enables the sheath to be retracted in stages, such that only a portion of the circumferential extent of the sheath is moved at a time, and thus only a portion of the internal surface of the sheath slides along the internal components of the sheath. The reduced surface area of sheath surface which moves at any one time will present significantly reduced friction to movement and will thus require a significantly reduced deployment force. Moreover, the ability to move elongate sections independently of one another enables the clinician to hold steady the position of the remaining sheath sections, and thus the distal end of the introducer assembly, while retracting one section. The assembly can thus be supported so as to improve positioning accuracy. 
         [0012]    Advantageously, the elongate sections are transversally curved, for instance part-rounded. Thus, the sheath or catheter may be round in transverse cross section, as with a conventional sheath, although it may be non-round, for instance oval. It is to be appreciated that the teachings herein are not limited to a sheath of any specific cross-sectional shape. 
         [0013]    The sheath or catheter may be formed of two elongate sections, although in the preferred embodiment has three sections. Other embodiments may have four or more elongate sections. 
         [0014]    The elongate sections may have equivalent transverse dimensions, so as to be of the same circumferential size, but in other embodiments they may have different transverse sizes. For instance, there may be provided one or more elongate section which is/are narrower than the others. 
         [0015]    The couplings preferably extend for a least a part of said length and most preferably extend for the whole of the length. The couplings advantageously lock the elongate sections together in the transverse direction, thus to prevent radial splitting or separation of the elongate sections from one another. 
         [0016]    Preferably, the couplings include a first coupling type providing a channel with a narrowed opening and a second coupling type providing a rib or series of protrusions shaped to fit into the channel. The couplings are most preferably dovetail couplings. 
         [0017]    The elongate sections preferably include proximal and distal ends and are provided with gripping elements at their proximal ends. These may be tabs or the like or form part of a mechanism such as a handle for gripping and then manipulating the elongate elements. 
         [0018]    Advantageously, the elongate sections are provided with strengthening elements. 
         [0019]    The sheath or catheter may be rigid, so as not to be curved during use, as it may be flexible so as to be able to pass through a patient&#39;s vasculature in an endoluminal deployment procedure. 
         [0020]    According to another aspect of the present invention, there is provided an introducer assembly for the deployment of medical devices or tools, the assembly including a sheath or catheter, the sheath or catheter including a plurality of elongate sections each having first and second longitudinal sides and couplings at said first and second longitudinal sides, said elongate sections being connected together by said couplings to form the sheath or catheter, said couplings allowing longitudinal movement of elongate sections coupled thereby, such that the elongate sections are movable relative to one another. 
         [0021]    It is not necessary with such a catheter or sheath to provide a pusher rod for holding the medical device in position while retracting the cheat or catheter. 
         [0022]    According to another aspect of the present invention, there is provided a method of deploying an implantable medical device or a treatment or diagnostic tool by means of an introducer assembly which includes a sheath or catheter, the sheath or catheter having a proximal end and a distal end and including a plurality of elongate sections each having first and second longitudinal sides and couplings at said first and second longitudinal sides, said elongate sections being connected together by said couplings to form the sheath or catheter, said couplings allowing longitudinal movement of elongate sections coupled thereby, such that the elongate sections are movable relative to one another; the method including the steps of: 
         [0023]    disposing the distal end of the sheath or catheter in a patient; 
         [0024]    retracting one of said elongate sections while holding the other elongate section or sections in position, so as to expose a part of the medical device or tool; and 
         [0025]    retracting the other elongate section or sections while holding said one elongate section in position, thereby to expose the remainder of the medical device or tool. 
         [0026]    It will be appreciated that the sheath or catheter disclosed herein is particularly suitable as the outermost sheath of an introducer assembly. 
         [0027]    Other features and advantages of the teachings herein will become apparent from a consideration of the specific description which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: 
           [0029]      FIG. 1  is a schematic view of an embodiment of introducer assembly; 
           [0030]      FIG. 2  is a perspective view of the distal end of a preferred embodiment of sheath; 
           [0031]      FIG. 3  is a transverse cross-sectional view of one section of the sheath of  FIG. 2 ; 
           [0032]      FIG. 4  is a perspective view of a part of one section of the sheath of  FIG. 2 ; 
           [0033]      FIG. 5  is a transverse cross-sectional view of another embodiment of coupling arrangement for the sheath sections; 
           [0034]      FIG. 6  is a perspective exploded view of another embodiment of coupling arrangement for the sheath sections; 
           [0035]      FIG. 7  is a transverse cross-sectional view of another embodiment of coupling arrangement for the sheath sections; 
           [0036]      FIG. 8  is a transverse cross-sectional and perspective view of the components of another embodiment of coupling arrangement for the sheath sections; 
           [0037]      FIG. 9  is a schematic view of an embodiment of the proximal end of the sheath of  FIG. 2 ; 
           [0038]      FIGS. 10 to 11B  are schematic views of various other embodiments of the proximal end of the sheath; and 
           [0039]      FIG. 12  is a schematic view of the sheath of  FIG. 2  shown partially retracted. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    Referring to  FIG. 1 , this shows in schematic form an embodiment of introducer assembly  10  according to the teachings herein. The introducer assembly may be used for implanting a medical device into a patient, for example a stent, stent graft, vena cava filter, occluder, embolization device, prosthetic valve and so on. It may also be used for deploying diagnostic or other medical treatment equipment. Introducer assemblies suitable for such medical purposes are generally known in the art. 
         [0041]    The assembly  10  includes a proximal end  12  provided with a manipulation unit  14 , which typically includes one or more haemostatic valves, couplings for the administration of flushing fluid, control elements for controlling the operation of the assembly  10 , such as for effecting the deployment of the device carried in the assembly, and so on. Such elements are known in the art and are therefore not described in further detail herein. 
         [0042]    The assembly also includes a sheath  16 , described in detail below, which extends from the manipulation unit  14  to a distal end  18  of the assembly  10 . The sheath may have a length from several tens of centimetres to well over one metre, even up to two metres or more, in dependence upon the intended location of percutaneous insertion of the distal end of the assembly  10  into the patient, which is often far from the treatment site. The majority of the sheath  16  will be fed into the patient during the procedure and it thus needs to be flexible in order to be able to curve around the patient&#39;s vasculature (that is have good trackability), yet sufficiently strong to be able to transmit force applied thereto from the manipulation unit  14  to its distal end  18  so as to push through the vasculature up to the treatment site (that is to have good pushability). 
         [0043]    The distal end  18  of the assembly  10  typically has a dilator tip  20 , although this is not always necessary. 
         [0044]    The sheath  16  provides an internal lumen  15  for housing the components of the assembly and in particular the device to be deployed (not shown) and the elements required to effect such deployment. The device is typically held at or close to the distal end  18  of the assembly  10  and the deployment elements, which may include a device carrier and a device pusher element, are generally located at or proximal of the medical device. It is important that the sheath  16  is a close fit, often a tight fit, over the components it houses for a number of reasons. First, the sheath  16  should have as small an outer diameter as possible, which optimises its trackability. Secondly, the sheath needs to hold the components it houses securely therewithin, leaving little or no room for these components which might enable them to become dislodged during the medical procedure and other handling of the assembly. This is particularly important in the case of orientationally dependent medical devices, for example, branched or fenestrated medical devices. Third, the sheath often plays a part in radially constraining the medical device, in particular devices which are self-expanding. 
         [0045]    This close or tight fit, added to the fact that the sheath is not normally straight at the moment of deployment of the medical device, contributes to significant friction between the internal wall of the sheath and the components it houses. A large force is required to overcome this friction, particularly at the start of the deployment procedure, which can lead to a number of complications, including jolting of the distal end  18  of the assembly  10  and thus loss of the positioning of the medical device, deformation of the medical device, and so on. 
         [0046]    The sheath  16  of this embodiment of assembly  10  has a segmented structure, described in detail below, which enables the sheath  16  to be withdrawn in sections rather than as a whole as with prior art sheaths. 
         [0047]    A better view of the preferred embodiment of sheath  16  can be seen in  FIG. 2 , which shows the distal end  18  of the sheath  16 , in a condition in which two sections thereof have been partially retracted. 
         [0048]    The sheath  16 , in this embodiment, has a conventional round shape in axial cross-section, although in other embodiments the sheath  16  could have other transverse cross-sectional shapes, such as oval. The shape of the sheath  16  in transverse cross-section is not material to the teachings herein. 
         [0049]    The sheath  16  of this embodiment is formed of three elongate sections  22 ,  24 ,  26  which each, in this example, rounded in transverse cross-section, and in particular being third-circular. Each section  22 - 26  includes a first longitudinal side  28  and a second longitudinal side  30 . In the embodiment shown in  FIG. 1 , the sections  22 - 26  are the same as one another. Each longitudinal side  28  is provided, in this embodiment, with a channel  32  extending along the length of its respective section  22 - 26 , while the each longitudinal side  30  is provided with an elongate rib  34 , equally extending along the length of its respective elongate section  22 - 26 . The rib  34  is shaped and sized to fit within the elongate channel  32  of the adjacent section  22 - 26  and in the preferred embodiment in such a way that the rib  34  interlocks with the walls  36 ,  38  of the side  28  which provide the channel  32 , as explained in further detail below. The structure is such that the ribs  34  are able to slide in their respective channels  32 , and thereby such that the sections  22 - 26  are able to slide relative to one another.  FIG. 2  shows the sections  22  and  24  partially retracted relative to the section  26 . 
         [0050]    Referring now to  FIGS. 3 and 4 , these show in better detail the structure of the elongate sections  22 - 26  of the sheath  10 , with preferred dimensions for a sheath having an inner diameter of 10 mm and an outer diameter of 12 mm. As will be apparent, the ribs  34  have a dovetail shape, with the channels  32  being similarly shaped to receive the dovetail ribs  34 . This shape, as will be appreciated can ensure that the ribs  34  cannot be pulled laterally out of the channels  32 , thereby ensuring that the sheath  16  remains intact during its use. In this example, the tape of the dovetail ribs  34  and equivalent dovetail recess  32  is of around 33% reduction. It has been found that this provides good radial strength preventing radial separation of the sections  22 - 24 , yet good sliding qualities. Other tapers are envisaged, particularly of greater than 33% reduction. 
         [0051]    It will be appreciated that the shape of the ribs  34  and the corresponding shape of the recesses  32  can be other than dovetailed as shown. In particular, they could be any shape which enables the sections  22 - 26  to slide longitudinally relative to one another and yet which prevents uncoupling of the sections  22 - 26  from one another in the circumferential/radial direction (such as by being pulled apart). In another embodiment, the ribs  34  may have an enlarged head or beading with a narrower neck fixed to the side  30  and correspondingly shaped channels. In another example, the ribs  34  could have a laterally extending flange, and the channels  32  provided with corresponding laterally extending side channels, again to provide the circumferential/radial locking function. 
         [0052]    The sheath  10  and in particular the sections  22 - 26  are made of a flexible material, so as to give the sheath  16  good trackability and yet of strength sufficient to give the sheath the necessary pushability. Preferred materials include polyamide, polyether block amide such as Pebax™, polyethylene terephthalate (PET), high-density polyethylene (PEHD), polyurethane, polyimide (PI) or polytetrafluoroethylene (PTFE). 
         [0053]    The sections  22 - 26  of the sheath  16  are preferably made of a single layer of material but in other embodiments may have incorporated therewithin strengthening elements, such as braiding, curved strips of metal and so on. These would typically be embedded within the wall of the sections  22 - 26 . 
         [0054]    The connection elements  32 ,  34  may be made of a polymer extruded to a PTFE sublayer, in practice of any suitable material which provides radial strength to the couplings and thus to the assembly could be used. Various examples are given below. 
         [0055]    The sheath  16  may be coated on its internal and/or its external surfaces with PTFE (Teflon). The coupling elements may similarly be coated. 
         [0056]      FIGS. 5 to 8  show different embodiments of connection element structures for the sheath  16  taught herein. Referring first to  FIG. 5 , the sections  22 - 26  are provided with extruded connector profiles  70 ,  72 , with the profile  70  providing a substantially flat strip coupled to the section wall by a narrower neck  72 , whereas the female connection element  74  provides a correspondingly-shaped channel therein. 
         [0057]    The example of  FIG. 6  has connection elements of similar shape to the example of  FIG. 5  but which is provided with strengthened inserts  102  and  104 , made for example of metal or other substantially rigid material, and which have ribbed flanges  106 ,  108  which engage into corresponding channels in the sheath sections  22 - 26  and are able to embed into the material of those channels. The inserts  102  and  104  may be made of any material harder than the material of the walls of the sheath sections  22 - 26 . It will be appreciated that in circumstances where the inserts  102 ,  104  are made of a rigid material, the sheath  16  will be substantially straight and can thus be used, for example, in open surgery procedures where it is not necessary to cause the sheath  16  to curve around the patient&#39;s vasculature or organs. It is not excluded, though, that the inserts  102 ,  104  could be made of a flexible material. 
         [0058]    With reference to  FIG. 7 , this embodiment provides a single insert  110  having a ribbed flange  112  the equivalent to the flanges  106  and  108  of the embodiment of  FIG. 6  and a hooked flap  114  which extends at an angle of approximately 90° to the rib. The fixed flap  114  is able to slide within a correspondingly-shaped channel  116  in the edge of the adjacent sheath section  22 - 26 . 
         [0059]    Another advantage of the inserts is that it is difficult to manufacture parts by extrusion with very tight tolerances. Inserts can avoid this problem given that they can be accurately manufactured or machined. 
         [0060]    With reference to  FIG. 8 , this embodiment provides within the channel  32  of each sheath section  22 - 26  an insert  122 , which may be made of metal or other rigid other substantially rigid material, which is shaped to have a narrower opening at its base. The rib  34 ′ of each sheath section  22 - 26  includes a reinforcement element such as a wire extending through the rib close to its extremity. The reinforcement wire preferably has a diameter similar to or larger than the opening of the insert  122  so as to ensure that the coupling elements of the adjacent sheath sections  22 - 26  remain firmly connected together during use. The reinforcement wire  120  can also cause the outer walls of the rib  34 ′ to bulge slightly, which can have the effect of reducing the surface contact between the rib  34  and the insert  122 , thereby to reduce friction. It will be appreciated that the reinforcement wire  120  and/or insert  122  can readily be made of a material which is simply harder than the material forming the walls of the sheath sections  22 - 26 . 
         [0061]    Referring now to  FIG. 9 , there is shown an example of structure for the proximal end of the sheath  16 . The elongate sections  22 - 26  are coupled, in this example, to respective flanges  40 - 44  which extend radially outwardly from their respective sections  22 . These flanges are fixed to their respective sections and act as gripping elements or handles used in sliding the sections  22 - 26  relative to one another during the deployment operation. The actual structure of the elements  40 - 44  is not material, it being simply important to be able to apply force to the sheath sections  22 - 26  in order to move these relative to one another or, as explained below, to hold one or more of the sections  22 - 26  in position. The elements  40 - 44  could be directly used by a clinician or could be incorporated within the manipulation assembly  14  shown in  FIG. 1  and actuated via appropriate actuators of the manipulation unit  14 , in the manner akin to the actuators used in such introducer assemblies for, for instance, operating trigger wires and other medical device restraining elements. 
         [0062]    Referring now to  FIG. 10 , there are shown various different examples of gripping elements provided at the proximal end of the sheath  16  for use in gripping and moving the sheath sections  22 - 26 . One example includes a plain grip  130 , as in the embodiment of  FIG. 9 , although the flanges of the gripping element  130  may be curved as shown, for user comfort. The example of gripping element  132  provides a plurality of ribs on each flange for reducing any possible risk of sliding of a user&#39;s finger during the manipulation of the gripping elements. The example of the gripping element  134  provides a hole or aperture in each flange of a size which can be gripped by the tip of a user&#39;s finger. The embodiment of grip element  140  provides a radially outwardly extending tapering tab terminating in a ball or other enlarged head able to be readily gripped by a user. 
         [0063]      FIGS. 11A and 11B  show another embodiment of the proximal end of the sheath  16 , in which each gripper element  150  is in the shape of a radially outwardly extending T-bar. This is preferably of a size large enough that each T bar can be gripped between a user&#39;s fingers. 
         [0064]    The skilled person will appreciate that other designs or structures of gripping element can be used in addition with or instead of the examples shown herein. 
         [0065]    Referring now to  FIG. 12 , there is shown in schematic form the major portion of the introducer assembly  10  of  FIG. 1 , showing in particular the elongate section  22  retracted relative to the sections  24  and  26 , achieved, as will be appreciated, by pulling on the flange or other device  40 . This retraction of the sheath section  22  partially exposes the elements held within the sheath  16 , particularly at its distal end and at the device holding zone  50 . In the case where an implantable medical device is located at the distal end of the assembly  10 , and thus at the distal end of the sheath  16 , the retraction of one of the sheath elements  22  will expose a part of that medical device. 
         [0066]    Considering first the issue of friction between the internal wall of the sheath  16  and the devices housed within the sheath  16 , only a fraction of that friction needs to be overcome in retracting, in this example, the section  22  and specifically only a third of that friction given that only a third of the internal area of the sheath is moved. Thus, much less force is required to move the section  22  of the sheath  16 . Similarly, once one of the sections  22  has been partially retracted, the retraction of any of the other sections  24 ,  26  will equally require less force as it will be necessary to overcome only a part of the total friction. In practice, once a first section  22  has been retracted, even partially, there is generally likely to be a reduction in the remaining level of friction between the internal walls of the sheath  16  and the components housed therewithin. This would be in part as a result of the fact that the remaining portions  24 ,  26  of the sheath  16  would be able to expand radially outwardly to a certain extent. In some instances, the components held within the sheath  16  may also be able partly to expand at the zone  50  once the first section  22  has been retracted, leading to further reduction in friction between the sheath  16  and those components. 
         [0067]    It is to be appreciated also that in addition to a reduction in the friction which has to be overcome in order to commence the deployment operation, additional support to the sheath  16  is provided by being able to hold the other sheath sections, in this example the sections  24  and  26 , in position by applying a holding pressure to the proximal ends of the sheath portions  24 ,  26 . In other words, the sheath portions  24 ,  26  can be held and supported in position while a pulling force is applied to the flange  40  to retract the sheath section  22 . This is in addition to being able to apply a similar holding force to the internal components within the assembly  10  and in particular a carrier catheter, pusher element or the like. Being able to hold the sheath  16  in position provides significant advantages in terms of retaining the distal end  18  of the introducer assembly  10  at the correct location and orientation during the commencement of the deployment operation. 
         [0068]    Once one of the sheath sections  22  has been partially retracted, the other sheath sections  24 - 26  can also be retracted, in which case a holding force is applied to the flange  40  of the section  22  and the other sheath section, for example  26 , while pulling back on the flange  42  (or other device) to retract the sheath section  24 . Thus, each of the sheath sections  22 - 26  can be retracted in turn so as to expose gradually the entirety of the medical device held at the distal end  18  of the introducer assembly  10 . This does, of course, also provide for staged deployment of the medical device held at distal end  18  of the introducer assembly  10  and thus a more gradual and precise deployment and positioning of this within the patient&#39;s vasculature. 
         [0069]    It will be appreciated also that such sequential retraction of sheath sections  22 - 26  can be particularly advantageous in the deployment of orientation specific medical devices or tools, such as branched or fenestrated stent grafts and so on, where, for example, a branch could be exposed by the retraction of a first sheath section  22 , positioned as required, before retraction of the remaining sheath sections to deploy the main body of the stent graft or other device. This can provide significant advantages in terms of ease and accuracy of deployment of orientation specific medical devices and tools. 
         [0070]    In addition to the above-described advantages, the provision of a sheath  16  made of elongate sections which are able to slide relative to one another provides better flexibility of the sheath  16  and as a result better trackability within a patient&#39;s vasculature, in that the flexibility of the sheath may not be dependent solely upon the flexibility of the material and structure of the walls of the sheath, but is also improved by the fact that the sheath sections  22 - 26  are able to slide relative to one another as the sheath  16  curves, in particular, to reduce the resistance to curvature of the material. This can also assist in reducing the risk of kinking of the sheath when it is curved to tight angles. 
         [0071]    It is not necessary with introducer assembly  10  taught herein to have a pusher rod of the type conventional in the art, since those sections of sheath  16  which are held in place while one of the sections is retracted, will act also to hold the implantable medical device in position as the result of the higher friction between those stationary sheath sections  22 - 26  and the medical device, relative to the friction between the one sheath section  22 - 26  which is moved and the medical device. This leads to a simpler introducer assembly and also a simpler deployment procedure. It is to be understood, though, that a pusher element of conventional type could be used with this introducer assembly  10 . 
         [0072]    The sheath sections  22 - 26  could be provided with different markings and/or colours, useful in denoting the orientation of the medical device or tool held within the sheath  16 , and also for indicating the order of retraction of the individual sheath sections  22 - 26 , for example to expose first a fenestration or side branch to a medical device held within the sheath. In this regard, the partial retraction of the sheath  16  can assist in the correct orientation of an orientation dependent medical device, such as a fenestrated or branched stent graft, in that a first sheath section  22 - 26  can be pulled back (typically the section overlying the fenestration or side branch), allowing fine positioning of the medical device and in particular the fenestration or side branch given that the major portion of the medical device will be still held in a radially contracted configuration, with that major portion being released from the sheath only once the fenestration or side branch has been correctly positioned in the patient. In this regard, it would also be possible to pass the guidewire through the fenestration or side branch once it has been partially deployed, with the major portion of the medical device still held constrained within the sheath sections which have not yet been retracted. 
         [0073]    A further advantage of this structure of sheath lies in the handling of a guide wire used during the deployment operation, as is often used in the art. As explained above, where such guide wire is used and in circumstances where the medical procedure may involve a plurality of phases and, it is often necessary to have a very long guide wire in order to be able to keep this in place during the withdrawal of the sheath  16  from the patient. However, with a sheath  16  having sections  22 - 26  which can be independently removed, the sheath  16  could be removed from a guide wire by removal of one of the sheath sections  22  and then effectively “peeling off” the remainder the sheath from the guide wire, which can thus be substantially shorter than with conventional single-piece sheaths. An analogy could be made with a rapid exchange introducer arrangement. 
         [0074]    It is to be understood that the number of elongate sections  22 - 26  of the sheath  16  may not be just three as shown. Other embodiments may have a greater or even a lesser number of sheath sections. For example, a sheath could be made of just two sections, whereas it could be made of four sections or even more. Moreover, even though the embodiment described above has sections which are each of the same width, in the example shown third-circular, they need not be the same width. For instance, there may be a narrower section with one or more wider sections, or one wider section with one or more narrower sections. The relative widths of the elongate sections  22 - 26  will be dependent primarily on the desired characteristics of the sheath  16  and of the device housed within the sheath  16  and to be deployed thereby. For instance, one of the sheath sections  22  may be sized in accordance with the size of a side branch or fenestration. 
         [0075]    The sheath or catheter taught herein may be made flexible, in which case its various components are made from flexible material. As mentioned above, the sheath or catheter may also be substantially rigid, in which case one or more of its components may be made of substantially rigid material. 
         [0076]    In all of the above described embodiments, there may be provided between adjacent sheath sections  22 - 26  a breakable connection element, such as a tab, or small protrusion and corresponding recess in abutting wall portions of adjacent sheath sections, for holding the sheath sections  22 - 26  in position relative to one another until a deliberate pulling force is applied to the sheath sections  22 - 26 , thereby to prevent premature sliding movement between the sheath sections. 
         [0077]    It will be appreciated that the sheath or catheter disclosed herein is particularly suitable as the outermost sheath of an introducer assembly. 
         [0078]    It is to be understood that the above-described embodiments are examples only of the invention taught herein and that the invention could be embodied in different forms. For example, the teachings herein could also be used in other catheters and cannulas used for medical applications, including rigid cannulas. 
         [0079]    It is to be understood that all optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.