GB2117246A - Vascular prosthesis - Google Patents

Abstract

An artificial vascular prosthesis is made by winding a thread around a tube (3) of warp knitted textile thermoplastics material held on a rigid former (1), removing the thread, then immediately applying an axial thrust to the tube to reduce the length of the tube by formation of corrugations (8) and then heat setting the material of the tube in its corrugated condition. Removal of the thread before compressing axially the tube material allows the tube material compressed by the encircling thread to relax while immediate application of the axial compressive thrust causes the tube to corrugate before the memory of the pressure applied by the thread has had time to dissipate. The finished prosthesis thus has corrugations without any reduction in pore size. <IMAGE>

Description

SPECIFICATION Vascular prosthesis The subject of this invention is vascular prosthesis. More particularly the invention relates to a vascular graft comprising an artificial flexible tube of bio-compatible material intended to replace a portion of a blood vessel in a human or animal body. The prosthesis of the invention also includes bifurcated tubes intended to be used for a section of blood vessel including a bifurcation.

Such artificial grafts are already known. The known tubes are normally made by a knitting process providing a seamless porous tube and to provide the required degree of flexibility combined with resistance to collapsing the tubes are customarily formed with corrugations. The known method for corrugating a tube intended as an artificial graft has been to wind a thread around a plain knitted tube held on a straight former, the thread forming convolutions of wide pitch. Axial pressure is then applied to the tube from both ends causing it to collapse lengthwise, the portion of the tube between adjacent convolutions bulging outwardly to form a continuous helical ridge extending from one end of the tube to the other.

The material of the tube which is knitted using a thermoplastic synthetic fibre such as a polyester is then heat-treated to cause corrugations to become set whereupon the thread is removed. For the prosthesis to be effective the tube has to be porous and this porosity is provided by the knitted structure employed. However, the size of the pores must be within a certain range to be effective. If the pores are too wide haemorrhaging takes place whereas if the pores are too narrow there is insufficient seepage of blood through the tube to promote the ingrowth of fibrous tissue.

The grafts previously known have been reasonably effective but tend to suffer from unevenness of build-up of fibrous tissue. It was not readily apparent why this occurred because the effect was not regular throughout any specific graft, the matter being further confused because in use the build-up of fibrous tissue does not proceed at the same rate over the entire graft. In the course of experiments leading to the present invention it was discovered that one cause of uneven build-up of fibrous tissue is unevenness in the fabric structure. Attempts were made to improve the evenness of the fabric structure but despite refinements in the knitting operation which ultimately produced a fabric structure in which the variation in mesh size was no greater than 0.25% the problem of irregular build-up of fibrous tissue was not solved showing that some other factor was responsible.Further experiment showed that the thread wound around the tube caused the mesh of the fabric structure to be compressed at the points where the thread contacted the tube i.e.

in the positions of the troughs of the future corrugations and during the heat treatment this reduction in pore size at these points was set in the structure and became a permanent feature of the structure.

Thus what is desirable is a vascular graft formed in such a way that the pore size remains substantially constant throughout the area of surface of the graft and it is an object of the present invention to provide such a graft and a method of making the graft, the graft produced by the method having a pore size which remains nearly constant within close limits throughout the entire graft.

A method of making an artificial vascular prosthesis according to the invention comprises placing a warp knitted tube formed of a synthetic thermoplastic yarn over a rigid former which fits the tube closely, winding a thread helically around the exterior of the tube with a predetermined pitch and a predetermined tension, removing the thread, then immediately applying an axial thrust against both ends of the tube whereby to reduce the axial length of the tube resulting in its assuming a corrugated form and subjecting the corrugated tube to a heat treatment sufficient to set the material of the tube whereby the corrugated form of the tube becomes permanent.

Bifurcated grafts are made by following the same method the basic feature of which is that the thread defining the positions of the corrugations is removed before the tube is axially compressed.

Apparatus for performing the process is illustrated in the accompanying drawings in which Fig. 1 illustrates diagrammatically apparatus for winding the thread on to a tube and Fig. 2 illustrates a graft produced by the method.

In the drawings 1 denote a bar rotatable by a driving mechanism 2 and readily removable from the driving mechanism, 3 denotes a knitted tube fitted over the bar, 4 denotes a lead screw rotatable by the driving mechanism 2, the pitch of the lead screw being related to the speed of rotation of the lead screw and the speed of rotation of the bar 1 in such a way that a thread 5 led through the traveller 6 on the lead screw 4 is laid on the surface of the tube 3 with a predetermined pitch while a tensioning mechanism 7 carried by the traveller 6 determines the tension with which the thread 5 is laid on the tube 3. In Fig. 2 the corrugations formed are indicated at 8.

In operation of the device of Fig. 1 the bar 1 is rotated causing the tube 3 to rotate. At the same time the lead screw 4 in rotating causes the traveller 6 to move along the length of the bar parallel to the bar. The thread 5 led through the tensioning device 7 on the traveller 6 forms convolutions on the tube 1 which are widely spaced. Once the desired number of convolutions has been laid on the tube 3 the thread 5 is severed and the bar 1 is removed from the mechanism.

The thread 5 is then unwound from the tube 3.

The bar with the tube 3 is placed into a heattreating and compressing mechanism in which end thrust is applied to both ends of the tube 3 while it is still on the bar 1. It had been found during the experiments referred to that the yarn forming the warp knitted tube retains the memory of the pressure applied by the thread 5 for several minutes which is long enough to allow a regular corrugation to appear by application of an end thrust and the heat treatment then immediately applied sets the tube material but since the setting takes place on a tube unrestricted by an encircling thread the fibres forming the tube are free to relax so that the reduction in pore size caused by the pressure of the thread is eliminated. The resultant graft illustrated in Fig. 2 has a much more nearly constant pore size than previously known grafts and is found to promote an acceptably even build up of fibrous tissue when it is put into use.

Claims (2)

1. A method of making an artificial vascular prosthesis which comprises placing a warp knitted tube formed of a synthetic thermoplastics yarn over a rigid former which fits the tube closely, winding a thread helically around the exterior of the tube with a predetermined pitch and a predetermined tension, removing the thread, then applying an axial thrust against both ends of the tube immediately after removal of the thread whereby to reduce the axial length of the tube resulting in its assuming a corrugated form and subjecting the corrugated tube to a heat treatment sufficient to set the material of the tube whereby the corrugated form of the tube becomes permanent.

2. An artificial vascular prosthesis formed by the method claimed in claim 1.