Abstract:
A steerable catheter having a segmented tip formed from materials of differing hardnesses. A tip segment formed of softer material provides improved steerability, while an adjacent harder segment provides improved resistance against steering wire pull-through. Wear-resistant sleeves and a coined loop portion of steering wire provide additional resistance against steering wire pull-through. A tip fabrication method is also disclosed. The catheter includes a one-piece inlet housing with integral mounting fins for attachment to the catheter housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part and claims the benefit under 35 U.S.C. §120 of U.S. patent application Ser. No. 09/126,863, filed Jul. 31, 1998; which in turn was a continuation-in-part of U.S. patent application Ser. No. 08/777,548, filed Dec. 30, 1996, now U.S. Pat. No. 6,030,360, issued Feb. 29, 2000. The content of U.S. patent application Ser. Nos. 09/126,863 and 08/777,548 are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to medical devices, and more particularly to a steerable catheter having a segmented tip configured for improved steerability, a tip assembly providing improved resistance to steering wire pull-through, and an improved one-piece inlet housing. 
     2. Description of Related Art 
     Medical practitioners frequently gain access to internal regions of a patient&#39;s body through the use of medical catheters in a variety of medical procedures, in order to reduce or eliminate the need for more invasive procedures. For example, medical catheters may be used to access internal body regions with a fiberoptic scope, light bundles, and/or other surgical instruments or devices, for a variety of diagnosis, treatment and/or material delivery purposes. 
     Steerable catheters have been developed to provide improved access to internal tissue. These catheters typically include a flexible catheter shaft and steering wires for controlling the flexure of the catheter shaft. A problem common to many previously existing steerable catheters is the retention of the steering wire or wires within the flexible material of the catheter shaft. The steering wire or wires typically comprise a small diameter length of high tensile strength material, whereas the flexible catheter shaft material typically must be relatively soft in order to provide sufficient flexibility. Thus, in many instances, the steering wire will cut through the flexible catheter shaft material, or will otherwise become disattached from the catheter shaft, rendering the catheter inoperable or compromising its utility. This problem is especially troublesome with small diameter catheters having one or more instrument access lumens therethrough, due to the reduced material thickness of the catheter. 
     Various approaches have been proposed for addressing the problem of steering wire detachment, a number of which are described in applicant&#39;s previous U.S. patent application Ser. Nos. 08/777,548 and 09/126,863, the content of which are incorporated herein by reference. Previously developed devices and methods have met with varying degrees of success, but none have proven fully successful for all applications. 
     An additional problem that has been found problematic in some previously known catheter designs results from the use of standard Touhy-Borst assemblies  10 , as shown in FIG. 1, as the catheter inlet. The standard Touhy-Borst assembly  10  typically comprises a two-piece housing, having a first housing component  12  coupled to a second housing component  14  by means of a threaded luer  16  or other coupling. The use of this type of two-piece housing results in increased assembly time and expense, and presents a risk of detachment during use. Additionally, the coupling  16  permits relative rotational movement between the first housing component  12  and the second housing component  14 . This is disadvantageous as it is has been found desirable to maintain the flush port  18  in a fixed position relative to the catheter housing. The standard Touhy-Borst assembly  10  is typically affixed to the catheter housing by means of mounting wings  20  provided on the second housing component  14 . Even if the wings  20  are rigidly attached to second housing component  14 , the rotational movement permitted at coupling  16  allows movement of the first housing component  12  and the flush port  18  thereof. In addition, the standard Touhy-Borst assembly  10  presents several steps or discontinuities  22   a ,  22   b ,  22   c  within its internal passage. These discontinuities present obstructions to instrument passage, and can result in abrasive wear and tear on sensitive instruments. The standard Touhy-Borst assembly  10  is also less than fully satisfactory for use as a catheter inlet housing, as it typically includes only two mounting wings  20 . It has been found desirable to provide additional mounting wings angularly spaced about the circumference of the inlet housing for more accurate positioning. It has also been found desirable to increase the thickness and contact area of the mounting wings to provide more secure attachment to the catheter body housing. 
     Thus, it has been found that a need exists for an improved steerable catheter device, and for an improved catheter shaft, tip assembly, and inlet housing for catheters. It is to these and other needs that the present invention is primarily directed. 
     SUMMARY OF THE INVENTION 
     Briefly described, in preferred form, one aspect of the present invention provides a tip assembly for a steerable catheter, which assembly includes a catheter shaft having first and second steering wire lumens extending lengthwise therethrough. One or more access lumens may optionally be provided through the length of the catheter shaft, for permitting passage of a fiberoptic endoscope and other instruments, infused fluids, aspirated materials, and/or otherwise accessing internal regions. The tip assembly further includes a continuous length of steering wire having a first leg extending through the first steering wire lumen, a second leg extending through the second steering wire lumen, and a looped segment connecting the first and second legs. The looped segment of steering wire may optionally include a coined or otherwise formed expanded outer dimension. A first wear-resistant sleeve is preferably provided within the first steering wire lumen adjacent the looped segment of steering wire, and a second wear-resistant sleeve is preferably provided within the second steering wire lumen adjacent the looped segment of steering wire. The wear-resistant sleeves and coined portion of the steering wire provide improved resistance against steering wire pull-through or detachment from the soft material of the catheter shaft. 
     In another aspect, the present invention preferably comprises a segmented catheter shaft for a steerable catheter. The segmented catheter shaft preferably includes a first shaft segment having a rear distal end for connection to a catheter body housing, and a front distal end opposite the rear distal end. The segmented catheter shaft preferably further includes a tip segment having a first end fused to the front distal end of the first shaft segment, and a second end opposite the first end. The first shaft segment is preferably relatively stiff to prevent buckling of the catheter shaft, whereas the tip segment is relatively flexible, as compared to the first shaft segment, for improved steerability. The segmented catheter shaft preferably further includes an end segment having a first end fused to the second end of the tip segment, and a second end opposite the first end. In preferred form, the segmented catheter shaft also includes first and second steering wire lumens extending lengthwise through the first shaft segment, the tip segment, and the end segment; and optionally includes one or more access lumens extending throughout the length of the catheter shaft. A continuous length of steering wire is preferably provided, having a first leg extending through the first steering wire lumen and a second leg extending through the second steering wire lumen. A looped segment of the length of steering wire connects the first and second legs, extending across the second end of the end segment of the catheter shaft. A cover segment is preferably also provided, overlying at least a portion of the second end of the end segment, and encapsulating the looped segment of steering wire between the second end of the end segment and the cover segment. The end segment and cover segment are preferably formed of relatively stiff materials of construction, as compared to the tip segment, to resist steering wire pull-through. Wear-resistant sleeves can be provided in the steering wire lumens of the end segment to provide improved resistance to steering wire pull-through. 
     Another aspect of the present invention provides a method of forming a segmented catheter shaft. The method preferably comprises providing a first shaft segment, a tip segment, and an end segment, each having first and second steering wire lumens extending lengthwise therethrough. One or more access lumens can optionally also be provided through the first shaft segment, the tip segment, and the end segment. Mandrels are inserted through the steering wire lumens and, if provided, the access lumens. The tip segment is arranged on the mandrels between the first shaft segment and the end segment. The first shaft segment, tip segment and end segment are then bonded end-to-end, to form a shaft assembly having a connecting end comprising a portion of the first shaft segment and a free end comprising a portion of the end segment. The mandrels are removed, and a continuous length of steering wire is inserted through the shaft assembly, the steering wire having a first leg extending through the first steering wire lumen, a second leg extending through the second steering wire lumen, and a looped segment connecting the first and second legs adjacent the tip end of the shaft assembly. The looped segment of steering wire is then encapsulated between the free end of the shaft assembly and a cover segment applied to overlie at least a portion of the free end of the shaft assembly. In a further preferred embodiment, the looped segment of the steering wire is provided with an expanded outer dimension larger than an inner dimension of the first and second steering wire lumens, and/or wear-resistant sleeves can be installed in the steering wire lumens of the end segment prior to installing the steering wire. The first shaft segment and the end segment are preferably formed from a material having a first stiffness, and the tip segment from a material having a second stiffness less than the first stiffness. In a further preferred embodiment, a manifold is formed at the connecting end of the shaft assembly by inserting core pins into the steering wire lumens and, if provided, into the access lumen(s), and injection molding the manifold around the core pins. The core pins are then removed from the lumens upon demolding of the manifold. 
     Another aspect of the present invention provides an inlet housing for a catheter. The inlet housing preferably includes a unitary body portion having an instrument inlet, an outlet, an internal passage extending between the instrument inlet and the outlet, and an outer mounting surface. A sealing element is preferably provided adjacent the instrument inlet. In a further preferred embodiment, the inlet housing includes a flush port in fluid communication with the internal passage. One or more mounting flanges can be provided, extending outwardly from the unitary body portion. In a preferred embodiment, four mounting flanges are provided, spaced circumferentially about the unitary body portion at 90° intervals, with one of the mounting flanges generally aligned with the flush port. The internal passage of the inlet housing preferably provides a smooth transition between the instrument inlet and the outlet, whereby tools, instruments or other materials inserted therethrough will not meet with substantial obstruction. 
     A further aspect of the present invention provides a steerable catheter incorporating one or more of the above-described features. In a preferred form, the steerable catheter includes a catheter body having a steering actuator for steering an attached catheter shaft. The steerable catheter shaft preferably is formed from segments of different stiffnesses as described above, and includes first and second steering wire lumens and at least one access lumen extending lengthwise therethrough. The steerable catheter preferably includes a tip assembly comprising a continuous length of steering wire having a first leg extending through the first steering wire lumen, a second leg extending through the second steering wire lumen, and a looped segment connecting the first and second legs of the steering wire. The ends of the first and second legs opposite the looped segment are coupled to the steering actuator. Wear-resistant sleeves are preferably provided within the first and second steering wire lumens adjacent the looped segment of the steering wire. One or more inlet housings are preferably mounted to the catheter body housing, one inlet housing corresponding to each of the access lumens provided through the segmented catheter shaft. One or more access conduits are preferably also provided, coupling each inlet housing to its corresponding access lumen. 
     These and other features and advantages of preferred forms of the present invention are described herein with reference to the drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 shows a side view, in partial cross-section of a prior art Touhy-Borst assembly. 
     FIG. 2 shows a top view of a steerable catheter according to a preferred form of the present invention. 
     FIG. 3 shows a side view of the steerable catheter shown in FIG.  2 . 
     FIG. 4 shows a top view of a portion of the steerable catheter shown in FIG. 2, with the top cover removed to show internal components. 
     FIG. 5 shows a detailed view of certain internal components of the steerable catheter shown in FIG. 2, with the housing removed for clarity and shown generally in phantom lines. 
     FIG. 6 shows a plan view of a tip assembly of a steerable catheter, in partial cut-away view, according to one form of the present invention. 
     FIG. 7 shows a cross-sectional view, taken at line  7 — 7  of FIG. 6, of the tip assembly shown in FIG.  6 . 
     FIG. 8 shows a plan view of a steering wire portion of the tip assembly shown in FIG.  6 . 
     FIG. 9 shows an end view, from line  9 — 9  in FIG. 8, of the steering wire. 
     FIG. 10 shows a plan view of a tip assembly of a steerable catheter, in partial cut-away view, according to another form of the present invention. 
     FIG. 11 shows a cross-sectional view, taken at line  11 — 11  of FIG. 10, of the tip assembly shown in FIG.  10 . 
     FIG. 12 shows a plan view of a steering wire portion of the tip assembly shown in FIG.  10 . 
     FIG. 13 shows a side view, in partial cross-section, of an inlet housing according to a preferred form of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawing figures, wherein like reference numerals represent like parts throughout, preferred forms of the present invention will now be described. As seen best with reference to FIGS. 2-5, the present invention generally comprises a steerable catheter  40 , having a catheter body  42 , a segmented catheter shaft  44 , and at least one inlet housing  46 . In a preferred form, the catheter body  42  comprises an upper housing shell  50  and a lower housing shell  52 . A steering actuator such as, for example, a rotatably mounted dial  54  is preferably mounted in or on the catheter body  42 . In the depicted embodiment, the dial  54  is rotatable about an axis  56  formed by cooperating projections and recesses on and in the dial  54  and one or both of the housing shells  50 ,  52 . The dial  54  is preferably retained in place between the housing shells  50 ,  52 , which are attached to one another in their assembled configuration by adhesive, thermal welding, and/or one or more couplings  58 , such as cooperating pins and holes, resilient couplings, screws, rivets or other fasteners. The catheter body  42  generally comprises a forward end  60 , a rear end  62 , and first and second sides  64 ,  66 , and is generally sized and shaped to be comfortably manipulated by a practitioner&#39;s hand. The catheter body is preferably fabricated from plastic or another substantially rigid material of construction. One or more cutout sections  68  can be provided in one or both of the housing shells  50 ,  52  to provide clearance for the steering actuator. The housing shells  50 ,  52  can also be provided with engagement features and openings, as required for mounting of the inlet housings  46  and the catheter shaft  44 , as is more fully discussed below. The steering actuator can additionally comprise directional indicating means  70  for visually indicating the deflection of the steering actuator, and thereby the expected degree of displacement of the steerable tip of the catheter shaft  44 . 
     The segmented catheter shaft  44  of the present invention will be described with particular reference to FIGS. 2-10. In a preferred embodiment depicted in the figures, the catheter shaft  44  generally comprises a first shaft segment  80 , a tip segment  82 , an end segment  84 , and a cover segment  86 . The first shaft segment  80  comprises a rear distal end  90  for connection to the forward end  60  of the catheter body  42 , and a front distal end  92  generally opposite said rear distal end  90 . The tip segment  82  preferably comprises a first end  94  fused or otherwise attached to the front distal end  92  of the first shaft segment  80 , and a second end  96  generally opposite the first end  94 . The end segment  84  preferably comprises a first end  98  fused or otherwise attached to the second end  96  of the tip segment  82  and a second end  100  generally opposite the first end  98 . The cover segment  86  preferably overlies at least a portion of the second end  100  of the end segment  84 . 
     In preferred form, the first shaft segment  80  is formed from a material having a first stiffness and the tip segment  82  is formed from a material having a second stiffness less than the first stiffness. In this manner, the first shaft segment  80  resists buckling along its length, and the relatively softer, more flexible tip segment  82  permits improved steerability. In a presently preferred example embodiment, the first shaft segment  80  is formed from a 7233 durometer PeBax (plastic) extrusion having a diameter of approximately 0.118 inch, and the tip segment  82  is formed from a 4033 durometer PeBax extrusion of substantially identical cross-section and diameter. The end segment  84  is preferably formed from a material having a third stiffness greater than the second stiffness, for example, a 7233 durometer PeBax extrusion substantially identical to that of the first shaft segment  80 . The provision of the end segment  84  having substantially greater hardness than the tip segment  82  provides increased resistance to steering wire detachment during operation. The cover segment  86  is preferably formed from a relatively hard material such as 7233 durometer PeBax, but can alternatively be formed from a softer material such as 4033 durometer PeBax. The lengths of the segments of the catheter shaft  44  will vary depending upon the intended application. In a presently preferred example embodiment, the first shaft segment  80  is between 10″ to 14″, and most preferably approximately 11″ in length; the tip segment  82  is approximately 1.5″ in length; the end segment  84  is approximately 0.200″ to 0.500″ in length; and the cover sement  86  is approximately 0.200″ or less in length. The segments of the catheter shaft  44  are thermal welded or otherwise fused to one another end-to-end, with any internal lumens aligned between the segments, to form a shaft assembly  104  having a connecting end  106  comprising the rear distal end  90  of the first shaft segment  80 , and a free end  108  comprising the end segment  84  and cover segment  86 . 
     As seen best with reference to FIGS. 6-12, the catheter shaft  44  preferably comprises first and second steering wire lumens  112 ,  114  extending lengthwise through the first shaft segment  80 , the tip segment  82 , and the end segment  84 . The diameter of the steering wire lumens  112 ,  114  may vary depending upon the intended application. In a presently preferred embodiment, the diameter of the steering wire lumens  112 ,  114  is approximately 0.014 inch. The catheter shaft  44  preferably has a generally round cross-section, as seen best with reference to FIGS. 7 and 11. The steering wire lumens  112 ,  114  are arranged generally diametrically opposite one another along a first diameter of the cross-section. In a further preferred embodiment, the catheter shaft  44  optionally further comprises one or more (two are shown) access lumens  116 ,  118  for allowing passage of instruments such as a fiberoptic endoscope or surgical implements, and/or for infusion and aspiration of fluids or other materials. The diameter of the access lumens  116 ,  118  may vary depending upon the intended application. In a presently preferred embodiment, the diameter of the access lumens  116 ,  118  is approximately 0.051 inch. The access lumens  116 ,  118  are preferably arranged generally diametrically opposite one another along a second diameter generally perpendicular to the first diameter. 
     The catheter shaft  44  preferably further comprises a tip assembly  120 , which will be described with particular reference to FIGS. 6-12. The tip assembly  120  preferably comprises a first wear resistant sleeve  122  disposed at least in part within the first steering wire lumen  112  of the end segment  84 , and a second wear resistant sleeve  124  disposed at least in part within the second steering wire lumen  114  of the end segment  84 . The wear resistant sleeves are preferably formed from stainless steel, other metals, ceramics or other materials having a high hardness and resistance to wear. A continuous length of steering wire  126  is provided, having a first leg  128  extending through the first steering wire lumen  112 , and a second leg  130  extending through the second steering wire lumen  114 . In preferred form, the steering wire  126  comprises a 0.010 diameter wire. A looped segment  132  of the length of steering wire  126  extends between the first leg  128  and the second leg  130 . The first leg  128  of the steering wire  126  extends through the first wear resistant sleeve  122 , and the second leg  130  extends through the second wear resistant sleeve  124 . In this manner, as tension is applied to the steering wire  126  during operation, the wear-resistant sleeves  122 ,  124  prevent the wire  126  from cutting or otherwise damaging the softer plastic material of the catheter shaft  44 . In addition, the relatively harder material of construction of the end segment  84  provides improved holding of the sleeves  122 ,  124 , and improved resistance to damage from the steering wire  126  than would be provided by the relatively softer material of the tip segment  82 . 
     In the embodiment of the present invention depicted in FIGS. 6-9, at least a portion of the looped segment  132  of the steering wire  126  is provided with an expanded outer dimension  134 . The expanded portion  134  is larger in at least one dimension than the inside diameter of the opening through the wear-resistant sleeves  122 ,  124 , so that the sleeves  122 ,  124  and the expanded portion  134  of the steering wire  126  cooperate to resist steering wire pull-through. In a preferred embodiment, the expanded portion  134  of the steering wire  126  is a coined portion formed by crimping to deform the wire  126 . The expanded outer dimension  134  is preferably in the direction of the plane of the steering wire  126 , as seen best with reference to FIG.  8 . In this manner, the thickness of the expanded portion, when viewed end-on as shown in FIG. 9, is reduced, thereby reducing or eliminating any potential interference by the steering wire  126  with the openings of the access lumens  116 ,  118 . In a preferred embodiment, the entire looped segment  132  of the steering wire  126 , between the first wear-resistant sleeve  122  and the second wear-resistant sleeve  124 , is coined to have an expanded outer dimension in the plane of the steering wire  126 . In an alternate embodiment shown in FIGS. 10-12, the looped segment  132  of the steering wire  126  is not provided with an expanded outer dimension. Encapsulation of the looped segment  132  of the steering wire  126  between the end segment  84  and the cover segment  86  further affixes the steering wire  126  in place, providing additional resistance to steering wire pull-through. 
     In preferred form, the catheter shaft  44  further comprises a manifold  140 , as shown in FIGS. 4 and 5. The manifold  140  is coupled to the rear distal end  90  of the first shaft segment  80 . Mounting means, such as a radially projecting flange  142 , are preferably provided on the outer surface of the manifold  140  for attaching the catheter shaft  44  to the catheter body housing  42 . For example, in the depicted embodiment, the flange  142  is adhesively secured within cooperating channel elements  144  provided within the upper and lower housing shells  50 ,  52  adjacent the forward end  60 . The manifold  140  preferably further comprises one or more steering wire passages  144  extending therethrough, and communicating with the steering wire lumens  112 ,  114  of the shaft assembly  104 . The manifold  140  preferably further comprises one or more access passages  146  extending therethrough, and communicating with the access lumens  116 ,  118  of the shaft assembly  104 . 
     As seen best with reference to FIG. 4, a first free end  150  of the first leg  128  of the steering wire  126  is attached to a first connection point of the steering actuator, and a second free end  152  of the second leg  130  of the steering wire  126  is attached to a second connection point of steering actuator. For example, in the depicted embodiment, the first and second connection points of the steering actuator comprise slotted posts  156 ,  158  projecting from the steering dial  54 . The free ends  150 ,  152  are inserted into the slots, and secured therein with hot melt or other adhesive. The slotted posts  156 ,  158  are preferably mounted generally diametrically opposite one another on the dial  54 . The steering wire  126  thus extends continuously from the first free end  150  attached to the dial  54 , through a first steering wire passage  144   a  of the manifold  140 , through the first steering wire lumen  112  of the shaft assembly  104 , through the first wear-resistant sleeve  122 , to the looped segment  132  adjacent the free end  108 , and loops back through the second wear-resistant sleeve  124 , through the second steering wire lumen  114  of the shaft assembly  104 , through a second steering wire passage  144   b  of the manifold  140 , to the second free end  152  attached to the dial  54  generally opposite the first free end  150 . So arranged, rotation of the steering dial  54  about axis  56  in a first rotational direction causes displacement of the free end  108  of the shaft assembly in a first direction, and rotation of the steering dial  54  about axis  56  in a second rotational direction opposite the first direction causes displacement of the free end  108  of the shaft assembly in a second direction opposite the first direction. 
     The present invention preferably further provides an improved inlet housing  46 . As seen best with reference to FIG. 13, the inlet housing  46  preferably comprises a unitary body portion  172  having an instrument inlet  174 , an outlet  176 , an internal passage  178  extending between the instrument inlet  174  and the outlet  176 , and an outer mounting surface  180 . The unitary body portion  172  preferably comprises a flush port  182  in fluid communication with the internal passage  178 , for allowing passage of fluids such as, for example, saline fluid, pharmaceuticals, anesthetics, biologically active materials, markers, or other materials. As depicted, the flush port  182  extends outwardly from the exterior surface of the inlet housing  46 , generally perpendicular to the direction of the internal passage  178 , to form a generally T-shaped component. The flush port  182  can alternatively extend outwardly at an angle to form a generally Y-shaped component. A check valve, flow restricting orifice, and/or other flow control devices (unshown) can optionally be provided in the flush port  182 . 
     The outer mounting surface  180  of the inlet housing  46  preferably comprises one or more mounting flanges  184  extending outwardly from the unitary body portion, for attaching the inlet housing  46  to an external structure or device such as, for example, the catheter body  42  described above. In the embodiment depicted in the figures, a circumferential mounting flange  184   c , and a plurality of axial mounting flanges  184   a , are provided. In preferred form, four axial mounting flanges  184   a  are provided, spaced circumferentially about the unitary body portion at approximately 90° intervals. One of the axial mounting flanges  184   a  is preferably generally aligned with the flush port  182 , thereby enabling attachment of the inlet housing  46  to an external structure with the flush port  182  aligned generally parallel to or generally perpendicular to a mounting component of the external structure. Because the unitary body portion  172  of the inlet housing  46  of the present invention comprises a single component formed of a substantially rigid material, the flush port  182  and the mounting flanges  184  are substantially fixed in position relative to one another. In this manner, the inlet housing  46  of the present invention eliminates certain disadvantages found to result from the rotational movement permitted between the housing components of prior art Touhy-Borst fittings joined by threaded connections. 
     The inlet housing  46  preferably further comprises a sealing element within the internal passage, for example, adjacent the instrument inlet  174 . In preferred form, the sealing element comprises a check valve such as an elastomeric duck-bill valve  186 . The duck-bill valve  186  can be retained in place within the internal passage by means of a washer  188 , a bushing  190 , and a retaining cap  192  capable of attachment to the unitary body portion  172  such as by a threaded coupling. 
     The internal passage  178  of the inlet housing  46  preferably comprises a generally smooth transition throughout its entire length, from the instrument inlet  174  to the outlet  176 . As used herein, “generally smooth transition” is intended to mean that the passage  178  presents no reductions in internal dimension, in the direction A of instrument insertion, at an angle greater than approximately 30° measured relative to an immediately adjacent wall surface of the internal passage. 
     The steerable catheter  40  of the present invention preferably comprises at least one inlet housing  46 . For example, and with reference to FIGS. 2-5, two inlet housings  46  are preferably mounted within the catheter body  42  adjacent the rear end  62 , with their instrument inlets  174  and flush ports  182  externally accessible. One inlet housing  46  is preferably provided for each access lumen  116 ,  118  in the shaft assembly  104 . Engaging recesses  200  are preferably provided in the upper and lower housing shells  50 ,  52  to receive mounting flanges  184 . The cooperating flanges  184  are attached within the recesses  200 , preferably by adhesive, compression fit, thermal welding or other attachment means, thereby rigidly fixing the position of the inlet housings  46  and, if provided, the flush ports  182  thereof, relative to the catheter body  42 . An access conduit such as a proximal extension  202  extends between each inlet housing and a corresponding access passage  146  in the manifold  140 , providing communication and passage of instruments, fluids and other objects and materials between the internal passage  178  of each inlet housing  46  and the corresponding access lumen  116 ,  118  of the shaft assembly  104 . In addition, a side extension  204  can be provided extending from each flush port  182 , for connection to an external fluid source. In preferred form, the proximal extensions  202  and the side extensions  204  are preferably formed from a smooth-walled, flexible plastic tubing. 
     The present invention further comprises a method of forming a segmented catheter shaft, the shaft being substantially similar to the segmented catheter shaft  44  described above. A first shaft segment, a tip segment, and an end segment are provided, each having first and second steering wire lumens extending lengthwise therethrough. The first shaft segment, the tip segment, and the end segment preferably further comprise at least one access lumen extending lengthwise therethrough. The first shaft segment is preferably formed from a material having a first stiffness. In a presently preferred embodiment, the first shaft segment is formed from a 7233 durometer PeBax extrusion. The tip segment is preferably formed from a material having a second stiffness less than said first stiffness. In the presently preferred embodiment, the tip segment is formed from a 4033 durometer PeBax extrusion. The end segment is preferably formed from a material having a stiffness greater than that of the tip segment. In the presently preferred embodiment, the first shaft segment is formed from a 7233 durometer PeBax extrusion. 
     The first shaft segment, the tip segment, and the end segment are then bonded end-to-end, with the tip segment arranged between the first shaft segment and the end segment, to form a shaft assembly having a connecting end comprising a portion of the first shaft segment and a free end comprising a portion of the end segment. In order to prevent the internal lumens of the shaft segments from collapsing during the bonding process, mandrels are preferably inserted through the first and second steering wire lumens, and if present the access lumens, of the first shaft segment, the tip segment, and the end segment. The mandrels can comprise, for example, stainless steel rods and/or wires approximately matching the internal diameters of the lumens. According to the preferred method, the several segments of the shaft assembly are then bonded by thermal welding. The mandrels are then removed. 
     Optionally, a manifold can then be formed at the connecting end of the shaft assembly. The manifold is preferably formed by inserting core pins into the steering wire lumens and the access lumens at the connecting end of the shaft assembly, and injection molding plastic into a mold around the core pins to form the manifold. The core pins are then removed upon formation of the manifold. The openings remaining in the manifold after removal of the core pins form the steering wire passages and access passages through the manifold. 
     A continuous length of steering wire is then inserted through the steering wire lumens. Optionally, a first wear-resistant sleeve can be installed within the first steering wire lumen of the end segment, and a second wear-resistant sleeve installed within the second steering wire lumen of the end segment prior to insertion of the steering wire into the steering wire lumens. A first leg of the steering wire is inserted to extend through the first steering wire lumen, and a second leg of the steering wire is inserted to extend through the second steering wire lumen. A looped segment connects the first and second legs adjacent the tip end of the shaft assembly. The method of the present invention can optionally further comprise providing a portion of the looped segment of the steering wire with an expanded outer dimension larger than an inner dimension of the steering wire lumens, for example, by coining. 
     According to the preferred method, the looped segment of the steering wire is preferably encapsulated within a cover segment overlying at least a portion of the tip end of the shaft assembly. Mandrels or pins are inserted into the tip ends of the access lumens to prevent the formation of obstructions therein during the encapsulation process. A short cover segment of plastic, such as a 4033 or 7233 durometer PeBax extrusion is mounted over the pins or mandrels, and thermal welded to the tip end of the shaft assembly. The thermal welding process encapsulates the looped segment of steering wire between the end segment and the cover segment. The pins or mandrels are then removed. 
     While the invention has been described in its preferred forms, it will be readily apparent to those of ordinary skill in the art that many additions, modifications and deletions can be made thereto without departing from the spirit and scope of the invention.