GB2219213A - A method of making a catheter - Google Patents

Abstract

In the manufacture of a laser catheter which selectively removes biological material in an obstructed lumen a rod 42 of light transparent material is polished, the polished end is inserted into a tube 51, the rod and tube are fused together at the juncture of each, and the portion of the tube and the rod at the exposed end is formed at 44, 55 into an exit lens surface. The surface is operable to emit a divergent beam having a working region of limited axial entent in which the density of energy is sufficient for removal of biological material. (Fig 3A, not shown). <IMAGE>

Description

A METHOD OF MAKING A HOUSING FOR THE DISTAL END OF A CATHETER This invention relates to a method of making a housing for the distal end of a catheter.

In our co-pending Application No. 8605020 (published No. 2171913A) from which this application is divided, we describe and claim a catheter for recanalizing an obstructed lumen by selectively removing sequential layers of biological obstructing material by radiant energy comprising: a) an elongate catheter body containing a flexible optical conductor; b) the proximal end of the catheter having means to enable said radiant energy to enter the flexible optical conductor; c) the distal end of the catheter having an emission aperture from which a beam of said radiant energy may be emitted, said emission aperture having a cross-sectional dimension which substantially corresponds to that of the distal portion of the catheter body; d) said catheter and emission aperture being constructed and arranged to shape the radiant energy beam emitted from the emission aperture to define an unfocused beam having a working region in which the density of energy is sufficient to cause said removal and so that the portion of the beam extending distal to the working region has insufficient energy density to cause said removal; and e) the cross-sectional dimensions of the beam in the working region being no smaller than about the diameter of the distal portion of the catheter body thereby to enable the catheter body to be passed through a recanalized hole formed by said working region, the axial depth of the working region being not substantially greater than the cross-sectional dimension of the distal portion of the catheter.

The present invention relates to a method of making a catheter for recanalizing an obstructed lumen by selectively removing sequential layers of biological material by radiant energy, the catheter having a distal end which includes a housing for pieces of light transparent material which form an optical system from which radiant energy fed therethrough may be emitted, the method comprising the steps of: (a) polishing one end of a rod of said light transparent material; (b) inserting said polished end into a tube having an inner diameter about equal to the outer diameter of the rod, said tube providing said housing for the optical system; (c) fusing the rod and tube at the juncture of each to join the two together; and (d) forming the portion of the rod and tube at the exposed end into an exit lens surface operable to shape the radiant energy emitted therefrom to define a diverging beam having a working region in which the density of energy is sufficient to cause said removal of biological material, the portion of the beam distal to said working region having insufficient energy density to cause said removal whereby the cross-sectional dimensions of the beam in the working region are no smaller than about the diameter of the distal portion of the catheter housing thereby to enable the catheter to be passed through a recanalised hole formed by said working region, the axial depth of the working region being not substantially greater than the cross-sectional dimensions of the distal portion of the housing.

Preferably a heat source is used to bond the pieces together.

The preferred method of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which Figure 1 illustrates a laser catheter; Figure 2 is a section taken on line 2-2 in Figure 1; Figure 3 is a diagrammatic illustration of the distal tip of the catheter showing the divergent beam pattern emitted from the optical housing; Figure 3A schematically illustrates the thermal profile of a heat pattern created in an absorbing medium in response to the combined exponentially decaying energy and geometrically expanding beam pattern which is provided by the catheter; Figure 3B is a graphic representation comparing energy distribution of the catheter of Figure 1 with a Gaussian energy distribution; Figure 4 is an optical-schematic view, greatly enlarged, of an optical system of the catheter and its relation to the distal end of the optical fibre;; Figure 5 is an optical-schematic view similar to-that of Figure 3 illustrating another embodiment of the optical system; FIGS. 6A and 6B are energy distribution plots illustrating substantially uniform energy distribution in the working portion of the energy beam for the system illustrated in FIG. 4; FIGS. 7A and 7B are energy distribution plots illustrating substantially uniform energy distribution at the working portion of the energy beam for the optical system illustrated in FIG. 5; FIG. 8 is a greatly enlarged sectional side view of the distal end of the catheter including an optical system assembly, FIG. 9 illustrates in further detail, the fiber holder and distal tip of the fiber shown in the assembly of FIG. 8; FIG. 10 illustrates dimensional details of the fiber optics conductor;; FIG. 11 is a diagrammatic illustration of the distal end of the catheter in a partially stenosed blood vessel; FIG. 12 is another diagrammatic illustration of the distal end of the catheter in abutment with the stenosis in a fully obstructed blood vessel; and FIG. 13 is an axial-sectional view of another embodiment of an optical system.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS As is shown generally in FIGS. 1 and 2, the catheter is formed from an elongate flexible body 10 and, for example, may be extruded from an appropriate plastic material such as polytetrafluoroethylene. The body 10 has a lumen 12 for enclosing a fiber optic light conductor 14. The distal end of the catheter is provided with an optical housing indicated generally at 16 which contains a net-negative optical lens system. The optical system in the housing receives radiant energy from the distal tip of the fiber optic light conductor 14. The radiant energy is emitted from the optical system in a controlled predetermined pattern from an emission aperture 18.

The proximal end of the catheter includes a molded fitting 20 which is secured to the catheter body 10. Projecting from the proximal end of the fitting 20 are a pair of flexible tubes 22, 24. The tube 22 is adapted to receive the fiber optic light conductor 14, which extends through the fitting 20.

The proximal end of the tube 22 is provided with a connector 26 which is connected to the proximal end of the fiber optic light conductor 14. Connector 26 is adapted to be mounted with respect to the source of radiant energy, such as a laser (illustrated diagrammatically at 27) so that the proximal end of the light conductor 14 may receive the radiant energy and conduct it along its length to the optical system 16. The other tube 24 communicates through the fitting 20 with the lumen 12 of the catheter body 10 and preferably is provided with a conventional luer connector 28.

The catheter body is provided with a plurality of fluid flow apertures 30 near the distal end. The pathway defined between-the luer connector 28, tube 24, main catheter body 10 and apertures 30 provide for communication with the distal region of the patient's blood vessel where the distal end of the catheter is located. It provides a passageway for fluids or gases to flow both to and from the distal region of the patient's blood vessel and also provides a means for making pressure measurements.

In accordance with the invention the optical system forms the beam of radiation so that the beam will be unfocused and will expand geometrically for example, at an angle of about 200 to the optical beam axis 0-0, in saline solution as it leaves the emission aperture 18. FIG. 3 illustrates diagrammatically at 32 the peripheral rays of the beam when the beam is emitted into a saline solution, while FIG. 3A illustrates the response of the material to the energy pattern of the beam with respect to propagation distance from the emission aperture 18. From FIG. 3 it will be appreciated that, owing to the geometrical expansion of the beam along the beam axis, the energy density of the emitted beam decreases in a distal direction along the beam axis 0-0, while the cross-sectional area of the beam increases with propagation along the axis.

This decrease in energy density is in addition to the exponential decay in energy level that is due directly to increasing propagation distance.

In accordance with the present invention, the relatively small diameter region adjacent the emission aperture 18, indicated at W in FIG. 3 and FIG. 3A, is considered to be the working region in which the energy density is sufficient to remove obstructing biological material. From FIG. 3 it will be appreciated that the working region W is comparatively short when the radiation beam is emitted into a low refraction medium such as clear saline solution (not shown). When the beam is emitted into such a medium, the optical system causes the beam to diverge at the aforesaid angle (e.g.: 200) which assures that its effective working power density preferably will not extend more than a millimeter or two beyond the emission aperture 18.

When the emission aperture is brought close enough to biological material (e.g., thrombus, plaque, blood) so that the latter is in the working region W, the beam will operate on (i.e.: remove by thermal, ablative, or other action) the biological material that is in the working region.

From the thermal profile shown in FIG. 3A, it will be appreciated that the invention combines an exponentially decaying energy profile with a geometrically expanding beam pattern, which assures a larger decrease in energy density along the optical axis 0-0 than would be available from a converging or a collimated beam pattern. The thermal profile in an absorbing medium is represented in FIG. 3A by isothermal lines 33, 34 and 35, respectively. The shaded region within the first isothermal line 33 is the thermal response within the working region W. Within that region the energy density, in Joules per cubic centimeter of spatial volume, preferably should exceed 3000 J/cm3, so that the biological material in the working region will be removed (as by ablation, erosion, etc.).Between the first and second isothermal lines 33 and 34, the energy density falls off to a range between 3000 J/cm3 and 272 J/cm3, in which the temperature of the biological material will be about 1000 C. Outside the third isothermal line, the temperature of biological material will be less than 500 C. A temperature of 500 C or above will cause irreversible protein denaturization.

When the temperature is below 500 C, cell trauma typically is insignificant and self reversing.

Within the working region W, the output beam has a substantially uniform energy distribution with respect to displacement from the beam axis 0-0. By way of example, the beam has more than 50% irradiance for radial distances of up to about 70% of the l/e2 beam radius, as is represented in curve A in FIG. 3B. Examples of such an irradiance profile are illustrated in FIGS. 6 and 7. By contrast if the energy distribution of the beam were non-uniform, such as Gaussian, its 50% irradiance 2 point would be located at 58% of the 1/e2 radius as is represented in Curve B in FIG. 3B.

As has been mentioned above, the axial length of the working region W may vary somewhat, depending on the index of refraction of the medium in the working region (e.g.: saline solution) into which it irradiates before being brought to the biological material. An outer limit for the length W is selected to be at a predetermined value which will reduce the chance of projecting the energy beam in a manner which might risk serious damage to biological material located beyond the target, i.e.: more than about one to two millimeters away, so as to minimize the chance of damaging an artery, or other blood vessel. By way of example, for a 1.0 to 1.5 millimeter diameter catheter intended to be used in small bore arteries such as coronary arteries, a maximum length for the working region W of the order of 1.5 millimeters appears desirable.

Further in accordance with the invention, it will be understood that in an imaginary plane transverse to the optical axis 0-0 located about 1.5 mm. in front of the emission aperture 18, the energy density of the emitted beam is substantially uniformly distributed throughout an imaginary circle which is diametrically larger than the catheter 10.

The energy density proximal of and within that circle is adequate to remove biological material so as to form a hole through which the catheter can be advanced. For example, pulses from an argon source, delivered at 25 watt/sec., 25% duty cycle, in a beam lmm. in diameter, through a saline solution, will remove about 0.25mm depth of non-calcific plaque per pulse, across the beam diameter. When the beam perforates or otherwise passes distally beyond the obstruction and the fluid beyond the hole is transparent (e.g.: saline solution), the density of energy which propagates more than about 1.5 mm.

beyond the obstruction will be too low to vaporize other more distant material, such as the wall of a blood vessel.

FIGS. 11 and 12 illustrate somewhat diagrammatically the manner in which the catheter is applied to vascular obstructions. As shown in FIG.

11 the blood vessel V has a lumen L which is partially obstructed by a stenosis S. The catheter is advanced through the patient's vascular system to bring the distal tip of the optical system 16 directly against the stenosis S. FIG. 12 illustrates an enlarged detail of the distal tip of the optical system 16 as it is brought to bear against a totally-blocking stenosis S within the lumen L of a blood vessel. In accordance with the invention, radiant energy emitted from the emission aperture 18 at the distal tip of the optical system 16 will ablate or otherwise remove the stenotic material S. As the catheter is advanced through the blood vessel V and as the radiant energy is applied, preferably in pulses of suitable peak power, discrete layers of the stenotic material will be removed so as to ultimately form a tunnel through the stenotic material S. The recanalized lumen formed by the tunnel is suggested diagrammatically in phantom at L' in FIG. 12. The recanalized tunnel L' thus formed is, as mentioned, slightly greater in diameter than the diameter of the catheter LA to facilitate advancement of the catheter through the blood vessel.

The condition illustrated in FIG. 11 in which the stenosis does not block completely the lumen L of the blood vessel V, may permit some of the radiant energy to pass through the opening in the stenosis so as to be directed toward a distal portion of the inner surface of the blood vessel wall. While that would not be likely to occur if the region distal of the catheter tip is filled with relatively opaque, radiant energy absorbing fluid, it is contemplated that the system may be used with a saline flushing technique and some of the region of the lumen distal of the catheter tip might be filled with a more clear saline liquid, allowing transmission of the radiant energy.Thus, in some circumstances such as where a distal portion of the blood vessel V is curved, as illustrated in FIG. 11, the present invention minimizes the risk that radiant energy which might impinge on a distal portion of the blood vessel wall will not perforate that wall.

FIGS. 4 and 5 illustrate two embodiments of the optical system 16. As shown in FIG. 4 the light-output end 36 of the fiber optics conductor 14 is coupled to a spherical lens 38, a first plano-concave lens 40 and a second plano-concave lens 42, in succession. The intercomponent spacings and component thicknesses along the optical axis 0-0 of the system are indicated on the figure as dl to d6, respectively. Representative design parameters for the optical system of FIG. 4 are stated in Table I, following: Table I.Design Parameters for Optical System Fig. 4 Optical Fiber Component Spacings & BR< Thicknesses dl 0.3574mm Numerical Aperture = 0.3 d2 = 1.00mm Exit Diameter = 0.1 mm d3 = 1.00mm S.S. (Substantially Uniform (Distribution Characteristic d4 = 1.00mm d5 = 1.00mm d6 = 1.00mm Total Length = 5.3574mm Radius Lens Type Material n(530nm) of Curvature 38 Sphere BK-7 1.5200 rl = 0.5 mm 40 Plano-concave BK-7 1.5200 r2 = 1.156 mm 42 Plano-concave Corning 7740 1.477 r3 = 0.867 mm Lens Type Thickness 38 Sphere d2 40 Plano-concave d4 42 Plano-concave d6 The operative portion of the radiation from the system shown in FIG. 4 is located within the region W extending about 1.5 mm from the concave surface 44 of the exit lens 42, which for purposes of illustration is shown bounded by a transverse plane indicated by line 46.Shown also in FIG. 4 are ray tracings 50 from the lower half (below the optical axis 0-0 as seen in the figure) of the light output end 36 of fiber optics conductor 14 to the boundary plane 46, for a wavelength of 530 nm. In order to see the entire ray distribution at the boundary plane one can superimpose a mirror image of the traced rays with respect to the optical axis. The aperture stop is fixed at the back surface of the spherical lens 38, for ray-tracing purposes.

The ray-tracing method used in development of FIG. 4 was consistent with the assumption that the optical fiber 14 behaves like a uniform energy distribution source, to find out the approximate energy distribution at the boundary plane 46. The upper half of the optical fiber tip 36 (0.05mm in extent) was first divided into 200 point sources.

Five rays from each point source (1,000 total) spanning the numerical aperture of 0.3 were traced through the optical system 20 to the boundary plane 46. The distance between the optical axis 0-0 and the outermost dimension (0.75mm from the optical axis) of the fiber optics conductor-lens system at the boundary plane was divided into twelve equal compartments to collect the traced rays. The number of rays which landed in each of these twelve compartments, indicative of beam intensity, are plotted as histograms in FIGS. 6A and 6B, for the wavelengths 530mm and 330nm, respectively. Assuming that each ray carries the same amount of energy, the histograms in FIGS. 6A and 6B approximate the energy distribution at the boundary plane 46 for the optical system shown in FIG. 4.It can be seen from FIG. 6 that this system creates an approximately 1.5mum diameter spot of substantially uniform energy distribution in cross-section, e.g., at the boundary plane 46, 1.5 mm from the concave surface 44 of the exit lens 42. FIG. 6A is an energy distribution plot at the boundary plane for light of wavelength equal to 530 nm. The same plot for 330 nm radiant energy is shown in FIG. 6B.

FIG. 5 illustrates another embodiment of the optical system 16, in which the spherical lens 38 is followed by a single bi-concave lens 48. Otherwise the system of FIG. 5 is similar to the system of FIG. 4. Design parameters for the system in FIG. 5 are stated in Table II following: Table II. Design Parameters for Optical System Fig. 5 Optical Fiber Lens Spacings & BR< Thicknesses Numerical Aperture = 0.3 dl = 0.31mm Exit Diameter = 0.1 mm d2 = 1.00mm Substantially Uniform Energy d3 = 3.19mm Distribution Characteristic d4 = 1.00mum Total Length = 5.50 mm Radius of Lens Type Material n (530 nm) Curvature 38 Sphere BK-7 1.5200 r1 = 0.5 mm 48 Bi-Concave Corning 7740 1.477 r21 = 1.092 mm r31 = 1.158 mm Lens Type Thickness 38 Sphere d2 48 Bi-Concave d4 Energy distribution in the boundary plane 46, for the embodiment of FIG. 5 is shown in FIGS. 7A and 7B for wavelengths 530 nm and 330 nm, respectively.The designs of the systems shown FIGS. 4 and 5 will work particularly well for wavelengths of light in the range from 330 nm to 530nm, but are not limited to that range.

As can be seen from the dimensions in Tables I and II, the optical system 16 is miniature. The system of FIG. 4 has a total length of 5.36 mm; that of FIG. 5 is 5.50 mm long. Each system including the housing for the lenses is only 1.5 mm in diameter.

FIGS. 8 to 10 inclusive, show an optical assembly 16 which facilitates assembly of the lens components 38, 40 and 42 with the required spatial and positioning precision. A glass tube 51 snugly encloses the optical elements, which are spaced apart in the tube with tubular spacers 52, 54 and 56. A holder 58 for the fiber optics conductor 14 is fitted into one end of the tube 51, followed by the first spacer 52 which holds the spherical lens 28 the required distance from the aperture surface 36 of the fiber optics light conductor 14. The next spacer 54 establishes the spacing between the spherical lens and the intermediate plano-convex lens 40. The last spacer 56 establishes the spacing between the intermediate lens and the exit lens 42.

To assure that the distal end of the fiber optics conductor 14 is spaced and oriented in a precise position with respect to the optical system 16, its coupling to the optical system 16 includes a high precision holder 58. The fiber optics conductor holder 58 may be made of glass, ceramic or other material capable of being formed to a high degree of precision tolerance. The fiber optics light conductor 14 is prepared as shown in FIG. 9, with the distal part of its buffer sheath 61 removed. The holder 58 has a precision formed axial bore made up of two sections including an enlarged diameter proximal segment 60 and a narrow diameter distal segment 63. The bore 60, 63 receives the clad fiber of the light conductor 14.To prepare the optical fiber for attachment to the holder 58, the plastic buffer sheath 61 which typically surrounds and protects the optical fiber is removed to an extent such that the projecting portion 65 (see FIG. 9) of the fiber conductor can be extended through the distal small diameter bore 63 in the holder. Care is taken when stripping the buffer sheath 60 so as not to damage the layer of reflective cladding 67 about the core of the conductive fiber 14. The stripped end of the fiber assembly thus is inserted into the holder so that the stripped protruding portion 65 of the fiber extends into the small diameter bore 63 while the proximal portion containing the buffer sheath 61 is contained within the larger diameter portion 60 of the axial bore in the holder 58.The end of the optical fiber which protrudes beyond surface 62 of holder 58 may be finished flush with surface 62 of the holder 58. The foregoing arrangement serves to hold the aperture end 36 of the fiber flush with the distal end surface 62 of holder 58, against which the first tubular spacer 52 abuts. This arrangement establishes precisely the spacing between the aperture end 36 of the light conductor 14 and the spherical lens 28. The rigidity and precision with which the holder 58 can be made also assures precise alignment and positioning of the fiber along the optical axis of the system. The fiber optics light conductor 14 may be held in the holder 58 with an epoxy cement.

The spacers may be made of a thin-wall tubing (e.g: thin-wall tubing having outer diameter 0.040 inch and wall thickness 0.005 inch) which will not cause vignetting. For optimum radiopacity performance a radiopaque material such as tantalum is preferred as a spacer material.

The catheter body 10 is fitted over the narrower back end 64 of the holder 58 spaced a short distance from the shoulder 68 between the two parts of the holder. The glass tube 51 is bent over the shoulder 68, as by fusing the end 65 of the glass around the shoulder. A filler 66, which may be made of a plastic, such as polytetrafluoroethylene fills the annular space between the catheter body 10 and confronting end 65 of the glass tube 51. The outer diameter of the entire assembly, from the catheter body 10 to the glass tube 51, is substantially the same, providing a smooth uniform surface the entire length of the catheter, as is indicated in Figure 1.

The concave surface 44 of the exit lens component 42 is formed after the assembly of the holder 64, lens components 38, 40, 42 and spacers 52, 54, 56 into the glass tube 51- has been completed. Pyrex brand glass No. 7740 is chosen as the material for the exit lens 42 and the glass tube 51. The exit lens 42 begins as a glass rod 1.5 mm long and 1.0 mm outer diameter with the end which will form the interior after assembly polished flat. When assembled into the glass tube 51, the exit lens 42 is fused to the glass tube, Pyrex brand glass being preferred because it has a lower softening temperature than other suitable optical glass materials. Such other materials can be used for the inner lens components 28 and 30. After fusing, the concave exit lens surface 44 is formed, and the exit end edge 55 of the glass tube is rounded to mate smoothly with the periphery of the concave surface.

In FIG. 13, the optical system illustrated comprises a single net-negative lens element 142 at the exit end 55 of the glass tube 51, separated precisely from the nearer transverse surface 62 of the light conductor holder 58 by a radiopaque spacer 154. Preferably the lens expands the light beam 14' exiting from the light conductor 14 to a beam 14" exiting from the lens at an angle of about 200 to the optical axis 0-0. The beam power parameters are adjusted so that in the working region W between the concave exit surface 144 and the nearby transverse plane 46 the radiant energy has the required density, substantially uniformly distributed to perform tissue removal acccording to the invention.

The aperture of the lens opening (44, 144) in the present invention is very close to the full outer diameter of the supporting envelope, namely, the tube 51, so as to provide an expanding beam that is just under the housing diameter close-in to the housing 51, for enabling the housing to be advanced into the hole that is being formed, as well as to maximize the energy that can be delivered through the miniature optical system 16.

From the foregoing it will be appreciated that the invention provides a catheter adapted to transmit and deliver radiant energy of a character adapted to etch or erode biological material, such as a vascular obstruction. The invention may be used with radiant energy in the visible, infra-red, ultra-violet and far-ultraviolet (200nm) ranges. The invention embodies an arrangement for delivering the radiant energy in a manner which avoids the risk of perforating the wall of the vessel. It should be understood, however, that the foregoing description of the invention is intended merely to be illustrative thereof and that other modifications and embodiments will be apparent to those skilled in the art without departing from its spirit.

Having thus described the invention what we desire to claim and secure by letters patent is:

Claims (3)

1. CLAIMS 1. A method of making a catheter for recanalizing an obstructed lumen by selectively removing sequential layers of biological material by radiant energy, the catheter having a distal end which includes a housing for pieces of light transparent material which form an optical system from which radiant energy fed therethrough may be emitted, the method comprising the steps of: (a) polishing one end of a rod of said light transparent material; (b) inserting said polished end into a tube having an inner diameter about equal to the outer diameter of the rod, said tube providing said housing for the optical system; (c) fusing the rod and tube at the juncture of each to join the two together; and (d) forming the portion of the rod and tube at the exposed end into an exit lens surface operable to shape the radiant energy emitted therefrom to define a diverging beam having a working region in which the density of energy is sufficient to cause said removal of biological material, the portion of the beam distal to said working region having insufficient energy density to cause said removal whereby the cross-sectional dimensions of the beam in the working region are no smaller than about the diameter of the distal portion of the catheter housing thereby to enable the catheter to be passed through a recanalised hole formed by said working region, the axial depth of the working region being not substantially greater than the cross-sectional dimensions of the distal portion of the housing.
2. 2. A method as claimed in claim 1 wherein a heat source is used to bond the pieces together.
3. 3. A method of making a catheter substantially as herein described with reference to the accompanying drawings.

   3. A method of making a catheter substantially as herein described with reference to the accompanying drawings.

   Amendments to the claims have been filed as follows CLAIMS 1. A method of making a catheter for recanalizing an obstructed lumen by selectively removing sequential layers of biological material by radiant energy, the catheter having a distal end which includes a housing for pieces of light transparent material which form an optical system from which radiant energy fed therethrough may be emitted, the method comprising the steps of: (a) polishing one end of a rod of said light transparent material; (b) inserting said polished end into a tube having an inner diameter about equal to the outer diameter of the rod, said tube providing said housing for the optical system; (c) fusing the rod and tube at the juncture of each to join the two together; and (d) forming the portion of the rod and tube at the exposed end into an exit lens surface operable to shape the radiant energy emitted therefrom to define a diverging beam having a working region in which the density of energy is sufficient to cause said removal of biological material, the portion of the beam distal to said working region having insufficient energy density to cause said removal, the cross-sectional dimensions of the beam in the working region being no smaller than about the diameter of the distal portion of the catheter housing thereby to enable the catheter to be passed through a recanalised hole formed by said working region, the axial depth of the working region being not substantially greater than the cross-sectional dimensions of the distal portion of the housing.

   2. A method as claimed in claim 1 wherein a heat source is used to bond the pieces together.