SURGICAL PROSTHESES

Abstract: A biomaterial is provided which is suitable for use in surgery in a human patient. It comprises a coherent layer of non-human collagenous tissue which has been subjected to glutaraldehyde tanning so as to comprise cross-linked collagen fibrils, and a reinforcement of synthetic material embedded within the coherent layer. The synthetic material has structure features for promoting said embedding, the average density of said features being in situ greater than 50 per square centimetre. Improvements are also provided in a method of producing biomaterials by allowing collagenous tissue growth on mesh structures covering support surfaces implanted into host animals. In one aspect a tubular synthetic fibre mesh structure fits loosely over a support rod or tube, and in the other aspect a sheet support is used and the tissue growing around the sheet support is adapted to form a pocket, pouch or envelope of collagenous material.

Description: SURGICAL PROSTHESES This invention relates to the field of surgery and more particularly to prosthetic grafts for vascular and non- vascular applications. US Patent 4,319,363 discloses a prosthesis for revascularisation made from a biomaterial and it describes a method for making the prosthesis in which a mesh covered silicon rod (mandril) is inserted into a living host animal, preferably a sheep, collagenous tissue is allowed to grow around the mandril for about twelve to fourteen weeks after which the implant is removed and subjected to glutaraldehyde tanning to form a prosthesis for revascularisation. The current invention is based on the surprising discovery that certain variations in the structure, geometry and quantity of the synthetic material or substrate on which it is supported promote improved tissue growth and/or allow the creation of new biological composite materials ("biomaterials") suitable for both vascular and non- vascular surgical application. In accordance with a first broad aspect of the invention there is provided a biomaterial suitable for use in surgery in a human patient, comprising: a coherent layer of non-human collagenous tissue which has been subjected to glutaraldehyde tanning so as to comprise cross-linked collagen fibrils, and a reinforcement of synthetic material embedded within the coherent layer, said synthetic material having structure features for promoting said embedding the average density of said features being in situ greater than 50 per square centimetre. Preferably, the density of said features is greater than 100 per square centimetre. Preferably also, the synthetic material is a fibre mesh and the features for promoting said embedding are the reticulations of the mesh. The fibre mesh may be constructed from polyester yarn. The polyester yarn may also be augmented with polyurethane, either in the form of strands of polyester dipped in polyurethane or strands of polyurethane woven around strands of polyester. Alternatively, the synthetic material may be particulate in nature, in which case said features may be constituted by individual particles of that material. Preferably further, the biomaterial is in the shape of a tube. Alternatively, the biomaterial is in the form of sheet. Preferably also, the biomaterial is smooth on one side to inhibit attachment to surfaces in the patient proximate said one side and rough on the other side to encourage said attachment. In the case where the synthetic material is a mesh, the mesh may be embedded in the coherent layer such that the mesh structure is in a loose unstretched state. In accordance with a second broad aspect of the invention there is provided a method of producing a biomaterial, comprising the steps of: positioning a tubular synthetic fibre mesh structure about a support rod or tube; implan ing the mesh covered support rod or tube in the body of a living, non-human, host animal at such location as to cause growth of collagenous tissue thereon,- allowing said collagenous tissue to grow on the implant until there is formed a coherent wall of said tissue encompassing the rod or tube and having the mesh structure embedded therein; removing the implant and said coherent wall of collagenous tissue from the body of the host animal; subjecting said coherent wall of collagenous tissue to glutaraldehyde tanning in order to produce cross¬ linking of collagen fibrils therein so as to increase the strength of the wall and also to impart immunological inertness and sterility thereto; and removing the rod or tube from within the coherent wall of collagenous tissue at any time subsequent to removal of the rod or tube and coherent wall of collagenous tissue from the body of the host animal; wherein the tubular synthetic fibre mesh structure fits loosely over the support rod. Optionally, the tubular mesh may be substantially larger in a longitudinal direction than the support rod or tube. The tubular biomaterial thereby produced may if desired be cut length-wise to produce a sheet. In accordance with a third broad aspect of the invention there is provided a method of producing a biomaterial, comprising the steps of: implanting a sheet support in the body of a living, non-human, host animal at such a location as to cause growth of collagenous tissue thereon; allowing said collagenous tissue to grow on the implant until there is formed a coherent layer of said tissue on both sides of the sheet support; removing the implant and said coherent layer of collagenous tissue from the body of the host animal; subjecting said coherent layer of collagenous tissue to glutaraldehyde tanning in order to produce cross¬ linking of collagen fibrils therein so as to increase the strength of the layer and also to impart immunological inertness and sterility thereto; and separating the sheet support from the coherent layer of collagenous tissue at any time subsequent to removal of the implant from the body of the host animal to form a pocket, pouch or envelope of collagenous material. Preferably, a synthetic material having structure features for promoting embedding of the synthetic material within the collagenous tissue is positioned on the support sheet so as to encompass both sides of the support sheet. Preferably also, the synthetic material is a mesh structure. The synthetic material may have the features required for the first broad aspect of the current invention. The positioning of the synthetic material may be in accordance with the second broad aspect of the invention. Preferably, for all first, second and third aspects of the invention, the host animal is a sheep. Preferably too, the implant is made beneath the cutaneous muscle of the lateral thoracic wall of the host animal. Preferably further, the implant is allowed in the host animal for at least ten weeks. Preferably also, the tanning step is carried out by immersing the implant and wall of tissue in a bath of buffered glutaraldehyde after removal of the body of the host animal and before removal of the support or tube. Preferably further, the biomaterial is rehydrated for use using heparin. In order that the invention may be more clearly understood preferred embodiments of the current invention will be described with reference to the accompanying tables and figures, where: Figure 1 shows typical tissue growth over the implanted rod or tube overtime for one embodiment of the invention. Figure 2 illustrates four variations in the structure of polyester mesh used in variations II, III and IV of the invention described hereunder. Variation I shown in Figure 2 is the mesh structure used in US Patent 4,319,363. Figure 2a shos a knotting configuration for Variations I, II ,111 ,IV. Figure 3 is a scanning electron microscope (x 52) magnification of the fibre structure in variation III of Figure 2. Figure 4 is a reproduction photomicrograph (H & E x 40) showing a section through the pre-flow of variation I, as used in US Patent 4,319,363. Figure 5 is a similar diagram to Figure 4 (H & E x 40) for variation II of the current invention. Figure 6 is similar to Figure 4 for variation III of the current invention. Figure 7 is similar to Figure 4 for variation IV of the invention described hereunder. Figure 8 is similar to Figure 4 for variation V of the invention described hereunder. Figure 9 is similar to Figure 4 for variation VI of the invention described hereunder. Figure 10 is an angiogram of a variation III prosthesis in the below knee femoropopliteal position in the human passing across the bent knee. Figure 11a is an explanted variation II prosthesis after seven months in the aorta-iliac position in a canine patient. Figure lib is a reproduction photomicrograph of a section through the prosthesis of Figure 11a, SR X 10 Figure 12a is a explanted variation II prosthesis similar to Figure 11a after four years in a canine host. Figure 12b is a reproduction photomicrograph of a section through the prosthesis of Figure 12a, SR X 10. Figure 13 is a reproduction photomicrograph of a section through a variation III prosthesis after six months in the aorta-iliac in a canine host, H & E x 40. Figure 14a shows the cumulative patency of 73 variation I prosthesis in the femoropopliteal position. Figure 14b shows the primary and secondary cumulative patency of 66 variation II prosthesis evaluated by the same surgical unit as shown in Figure 14a. Figure 14c shows the primary and secondary patency in the study undertaken by the same surgical unit as in Figure 14a on 79 variation III prosthesis. Figure 15a shows a wide diameter prosthesis suitable for production of a flat sheet after removal from the host animal. Figure 15b shows the prosthesis of Figure 15a cut longitudinally and laid flat ready for processing. Figure 16 shows various shapes of prosthesis which can be produced for different applications such as ligaments. Figure 17 shows a "bladder" prosthesis suitable for lining an artificial heart, produced in accordance with an embodiment of the invention. Figure 18 shows an oval shaped patch on its rough side, suitable for body wall patching. Figure 19 is a scanning electron micrograph at 400 times magnification demonstrating cracking of tissue in variations I and II. The best method of implementing the improvements the subject of the current invention is to implant the prosthesis in sheep of the following characteristics: 1. Wethers of Border Leicester First Cross, Corriedale, Merino or Polywarth type or any cross breeds of these breeds. 2. Age not less than 3 years and not more than 6 years. 3. Crown to rump length not less than 1 metre. 4. At implant weight not less than 45 kgs. 5. At explant a weight gain of 3 to 5 kgs. The biomaterial is explanted between 12 and 14 weeks. With reference to Figure 1, it can be seen that the maximum tissue coverage occurs at this time. In the above conditions, sheep provide sterile, self- regulating culture conditions suitable for the reliable and reproducible production of the biomaterial. Variation I shown in Figure 2 is the prior art polyester mesh of US Patent 4,319,363. The different meshes of variations I, II, III and IV were knitted on a Raschel Warp knitting machine with a 2-needle bed and 4-bar structure. The knitted loop structure for each variation was designated as shown in Figure 2a. The yarn in each case comprised bundles of approximately 50 polyester strands, each strand being composed of two 44 decitex filaments. The resultant yarn density was 0.6 to 0.8 grammes per metre. In variations V and VI (not demonstrated) woven polyester mesh was dipped in polyurethane in V and polyurethane strands were wound around polyester strands in VI. The mesh weave of variation III is illustrated in Figure 3. Modifications in the mandril-mesh assembly influences the eventual tissue incorporation and form. For instance in U.S. 4,319,363, mandril diameter and tubular mesh diameter were identical and the polyester mesh was stretched over the mandril. Illustrating the second aspect of the invention, in variation III an 8 mm diameter tubular mesh was used on a 6 mm diameter mandril. Tubular mesh 106 cm in length was used on a mandril 75 cm in length. This resulted in a thicker and more even cover of tissue over the flow surface without exposed mesh bundles which probably caused less than optimal results in variation I. In Figures 4, 5, 6, 7, 8 and 9, reproduction photomicrographs of the histology sections of mesh variations I, II, III, IV, V and VI demonstrate the changes in tissue configuration and thickness which occurs with the mesh and mesh/mandril modifications. The tissue cover on the flow surface covering the mesh has increased with each variation and the collagenous tissue has become more compact. The prior art variation I shown in Figure 4 shows that polyester mesh strands are in bundles (M) supporting a delicate collagen tissue membrane (C) . There is a smooth lining to the flow surface (FS) with a thin tissue cover over the mesh bundles. By contrast, the section through the pre-flow variation II shown in Figure 5 shows polyester mesh bundles (M) closer together due to the increased reticulation density and collagen tissue more compact (C) . There is smooth lining to the flow surface (FS) and the tissue cover over the mesh bundles remains thin, as in variation I. In variation III shown in Figure 6, the polyester mesh bundles (M) are completely incorporated within the collagenous tissue (C) which is very compact. The mesh bundles are slightly closer again, and the flow surface (FS) remains smooth. The tissue cover over the flow surface is thick. This is due to the looseness of the fitting of the mesh over the mandril which allows more tissue to invade the space between the mesh and the mandril compared to the stretched mesh configuration of variation I. The section through the pre-flow variation IV as shown in Figure 7, shows that the polyester and polyurethane mesh bundles (M) are well incorporated into the dense collagen tissue (C) . The mesh bundles are very closely aligned. The strength imparted by the thickness of the tissue cover and closely aligned mesh bundles indicate a non-vascular application for this variation would be appropriate. With reference to Figure 8 where variation V is shown, the polyester mesh bundles are dipped in polyester (M) and are closely aligned and well incorporated into the collagen tissue (C) . With reference to Figure 9 where variation VI is demonstrated, the polyurethane strands wound around the polyester mesh (M) result in bundles which are well incorporated into the dense collagen tissue (C) . The physiochemical characteristics obtained with variation I have been retained in variations II and III with some notable differences listed in Table 1 below. Table 1. In vitro studies which demonstrate the improved characteristics of variation III. Study Variation II Variation III Haemocompatibility 16.44 +/- 7.3 10.36 +/- 7 (Platelet consumption - The lower the number the more haemocompatible) . Extension 2.77 +/= .68 5.71 +/- 2.32 Kink radius 15 - 19 9 - 15 (The lower the number the greater resistance to kinking) . Instron test 85.25 +/- 43 125.2 +/- 47 The haemocompatibility as determined by the platelet consumption study using a closed loop system has been enhanced in variation III. The lower the reading the more haemocompatible the surface. Instron testing exposes the material to stretching forces. Variation III has greater strength, desirable in some non-vascular applications. Variation III demonstrates increased longitudinal stretch or elasticity (extension) and greater kink resistance required in a vascular prosthesis as it allows for better placement around the knee joint or other areas where curving is desirable as demonstrated in the human angiogram in Figure 10. Animal studies undertaken with variation I, II and III in the aorto-iliac position in dogs to determine patency and long term performance have demonstrated excellent results (Table 2 below) . Table 2 In vivo studies comparing Variation I, II and III in the aorto-iliac position in the canine model. Prosthesis type no.of dogs days patent %patenσy Variation 1 10 308 - 420 100% 42 1 - 730 80% Variation II 63 63 - >1460 87% 47 365 - >1095 80.8% Variation III 10 28 - 195 90% 12 28 - 373 100% Variation III demonstrates a thicker tissue cover on the flow surface and over the mesh bundles compared to variation II as Figures 11a and lib, 12a and 12b and 13 show, indicating an improved flow surface and reducing the risk of prosthesis failure. Variation I, II and III have been evaluated in human clinical studies for peripheral revascularisation in one surgical centre and the results are shown in Figures 14a, 14b and 14c. The results obtained at four years are superior in Variation III with fewer occlusions occurring in the early time frame, thus documenting demonstrable enhancement in performance as a direct consequence of the modifications incorporated. With reference to Figure 11, an explanted variation II prosthesis after seven months in the aorta-iliac position in the canine model is shown. Blood staining has occurred at th anastomoses (A) due to the thin tissue cover over the mesh on the flow surface (FS) . The prosthesis was patent at explant. With reference also to Figure lib, the flow surface (FS) is smooth and thrombus free however the tissue cover over the mesh (M) is thin. Figure 12a and 12b shows that the same characteristics are present at seven months and four years in the canine model for variation II. A variation III prosthesis in the canine model is shown after six months in the aorta-iliac position in Figure 13. The flow surface (FS) is thrombus free and the tissue cover over the mesh (M) is thick preventing the occurrence of blood staining. Variations I, II and III have been evaluated in human clinical studies for peripheral revascularisation in one surgical centre and the results are shown in Figures 14a, 14b and 14c. The patency at 48 months for variation I is 32%. A large number of failures occurred in the first and second six month periods indicating a less than optimal flow surface to the prosthesis. Figure 14b for variation II prosthesis shows little change from variation I in the primary patency at the six and 48 month time period, though the secondary patency at the six month period is satisfactory. The primary and secondary patency of variation III prosthesis shown in Figure 14c indicates that the improvements in the technology for variation III have transferred to markedly improved clinical performance. The results obtained at four years are superior in variation III with fewer occlusions occurring in the early time frame, thus documenting demonstrable enhancement in performance as a direct consequences of the modifications incorporated in variation III. The improvements in variation II are less dramatic. The sheet form biomaterial is shown in Figures 15a and 15b, where a large-diameter variation III prosthesis was manufactured. The flat material was produced by cutting the tubular prosthesis along its length. Such a prosthesis can alternatively be manufactured in accordance with the third aspect of the invention by implanting in the host animal a sheet support and covering the sheet on both sides by the synthetic material, either in mesh form or alternatively in a painted particulate form. Such prostheses produced from flat sheets by either method would be useful in non-vascular applications such as ligament replacement where strength is a critical consideration. Variations in shape and configurations are shown in Figure 16. Flat rectangular or oval shaped silicone, nylon, acrylic, polyethylene, teflon or polyurethane support sheets in isolation or in combination covered with synthetic mesh results in a bladder, pouch or pocket suitable for many applications (Figure 17, 18) . The most important of them would be an application as a lining for artificial heart chambers. Unique features of an internal, smooth, haemocompatible surface shown in Figure 17 and external non-smooth surface shown in Figure 18 makes this aspect of the invention extremely useful in applications where attachment to external surfaces and non- attachment of the internal surface is required, as for example in hernia repair (Figure 18) . Increased tissue cover obtained has transformed some disadvantages encountered in the original version into additional desirable features. In US Patent 4,319,363, the prosthesis was stored in 90% absolute alcohol. This caused dehydration of the tissue component and necessitated rehydration prior to surgical implantation. In addition, in the original version, because of the thin tissue cover, with dehydration "cracking" occurred, often exposing the polyester through the tissue covering the flow surface, resulting in poor performance (Figure 19) . The current version with increased tissue cover does not exhibit polyester at the flow surface. In addition, during rehydration, instead of physiological saline, heparin is preferably used in th current invention and remains bioactive in the collagen/glutaraldehyde complex of the prosthesis enhancing performance. Tables 3 and 4 below show the results of heparin uptake studies. Heparin uptake studies. Heparin uptake and retention directly and following initial protamine sulphate treatment were studied. Heparin retention was assayed after 120 hours (5 days) . Heparin uptake and retention were superior with PrSo4 treated grafts but it caused stiffness that made it less satisfactory. The direct method which consisted of partial dehydration resulted in a satisfactory prosthesis. Variation III that was tested, as claimed, can retain heparin in effective amounts opening the drug delivery potential in a controlled manner. Table 3 3 H-heparin (cpm/m) in the lumen of graft segments Η-HEPARIN IN LUMEN DIFFERENCE (cpm mD x 10 * IN 'H-HEPARIN CONCENTRATl ON AFTER FIVE DAYS (cpm/ml ) x 10 6 GRAFT BINDING INITIALLY AFTER FIVE DAYS PROCEDURE AVERAGE AVERAGE Var. Ill Direct 3.04 3.17 3.10 1.49 1.52 1.51 1.59 Var. Ill Protamine Sulphate 3.04 3.17 3.10 0.97 1.01 0.99 2.11 Table 4 3 H-heparin (cpm/cm 2 ) bound to graft segments. GRAFT BINDING PROCEDURE 'H-HEPARIN BOUND TO GRAFT (cpm/cm 2 ) x 10 6 Average Var. Ill Direct 0.29 0.24 0.27 Var. HI Protamine Sulphate 0.26 0.25 0.26 The aldehyde and amino groups in the collagen/glutaraldehyde complex can not only retain heparin but other pharmacological agents like antibiotics eg tetracycline. The increased tissue cover combined with alcohol dehydration will enable the prosthesis to be moisture packed rather than fluid packed for end use. The tanning procedure of the current invention is identical to that described in US Patent 4,319,363. Variations may be made to current invention as would be apparent to a person skilled in the art of biomaterial design similar to that described in US Patent 4,319,363. These and other modifications may be made without departing from the ambient of the invention, the nature of which is to be ascertained from the foregoing description, figures and tables and the claims.

Claims: CLAIMS : 1. A biomaterial suitable for use in surgery in a human patient, comprising: a coherent layer of non-human collagenous tissue which has been subjected to glutaraldehyde tanning so as to comprise cross-linked collagen fibrils, and a reinforcement of synthetic material embedded within the coherent layer, said synthetic material having structure features for promoting said embedding the average density of said features being in situ greater than 50 per square centimetre. 2. A biomaterial as claimed in claim 1 wherein the density of said features is greater than 100 per square centimetre. 3. A biomaterial as claimed in claim 1 or claim 2 wherein the synthetic material is a fibre mesh and the features for promoting said embedding are the reticulations of the mesh. 4. A biomaterial as claimed in claim 3 wherein the mesh is embedded in the coherent layer such that the mesh structure is in a loose unstretched state. 5. A biomaterial as claimed in claim 3 wherein the fibre mesh is constructed from polyester yarn. 6. A biomaterial as claimed in claim 5 wherein the polyester yarn is augmented with polyurethane. 7. A biomaterial as claimed in claim 6 wherein the polyurethane is in the form of strands of the polyester dipped in polyurethane. 8. A biomaterial as claimed in claim 6 wherein the polyurethane is in the form of strands of polyurethane woven around strands of the polyester. 9. A biomaterial as claimed in claim 1 or claim 2 wherein the synthetic material is particulate in nature. 10. A biomaterial as claimed in claim 9 wherein said features are constituted by individual particles of that material. 11. A biomaterial as claimed in any one of claims 1 to 10 wherein the biomaterial is formed in the shape of a tube. 12. A biomaterial as claimed in any one of claims 1 to 10 wherein the biomaterial is in the form of a sheet. 13. A biomaterial as claimed in any one of the preceding claims wherein the biomaterial is smooth on one side to inhibit attachment to surfaces in the patient proximate said one side and rough on the other side to encourage said attachment. 14. A method of producing a biomaterial, comprising the steps of: positioning a tubular synthetic fibre mesh structure about a support rod or tube; implanting the mesh covered support rod or tube in the body of a living, non-human, host animal at such location as to cause growth of collagenous tissue thereon; allowing said collagenous tissue to grow on the implant until there is formed a coherent wall of said tissue encompassing the rod or tube and having the mesh structure embedded therein; removing the implant and said coherent wall of collagenous tissue from the body of the host animal; subjecting said coherent wall of collagenous tissue to glutaraldehyde tanning in order to produce cross- linking of collagen fibrils therein so as to increase the strength of the wall and also to impart immunological inertness and sterility thereto; and removing the rod or tube from within the coherent wall of collagenous tissue at any time subsequent to removal of the rod or tube and coherent wall of collagenous tissue from the body of the host animal; wherein the tubular synthetic fibre mesh structure fits loosely over the support rod or tube. 15. A method as claimed in claim 14 wherein the tubular mesh is substantially larger in a longitudinal direction than the support rod or tube. 16. A method of producing a biomaterial, comprising the steps of: implanting a sheet support in the body of a living, non-human, host animal at such a location as to cause growth of collagenous tissue thereon; allowing said collagenous tissue to grow on the implant until there is formed a coherent layer of said tissue on both sides of the sheet support; removing the implant and said coherent layer of collagenous tissue from the body of the host animal; subjecting said coherent layer of collagenous tissue to glutaraldehyde tanning in order to produce cross¬ linking of collagen fibrils therein so as to increase the strength of the layer and also to impart immunological inertness and sterility thereto; and separating the sheet support from the coherent layer of collagenous tissue at any time subsequent to removal of the implant from the body of the host animal to form a pocket, pouch or envelope of collagenous material. 17. A method as claimed in claim 16 wherein a synthetic material having structure features for promoting embedding of the synthetic material within the collagenous tissue is positioned on the support sheet so as to encompass both sides of the support sheet. 18. A method as claimed in claim 17 wherein the average density of said features in situ is greater than 50 per square centimetre. 19. A method as claimed in claim 18 wherein density of said features is greater than 100 per square centimetre. 20. A method as claimed in any one of claims 17 to 19 wherein the synthetic material is a fibre mesh and the features for promoting said embedding are the reticulations of the mesh. 21. A method as claimed in claim 20 wherein the mesh is embedded in the coherent layer such that the mesh structure is in a loose unstretched state. 22. A method as claimed in claim 20 wherein the fibre mesh is constructed from polyester yarn. 23. A method as claimed in any one of claims 17 to 22 wherein the biomaterial is smooth on one side to inhibit attachment to surfaces in the patient proximate said one side and rough on the other side to encourage said attachment. 24. A method a claimed in any one of claims 14 to 23 wherein the implant is made beneath the cutaneous muscle of the lateral thoracic wall of the host animal. 25. A method as claimed in any one of claims 14 to 23 wherein the host animal is a sheep. 26. A method as claimed in any one of claims 14 to 23 wherein the implant is allowed in the host animal for at least ten weeks. 27. A method as claimed in any one of claims 14 to 23 wherein the tanning step is carried out by immersing the imp1-ant and wall of tissue in a bath of buffered glutaraldehyde after removal of the body of the host animal and before removal of the support or tube. 28. A method as claimed in any one of claims 14 to 23 wherein the biomaterial is rehydrated for use using heparin.

Inventors: KETHARANATHAN VETTIVETPILLAI, KETHARANATHAN, VETTIVETPILLAI
Applicants: KETHARANATHAN VETTIVETPILLAI, KETHARANATHAN, VETTIVETPILLAI
Application year: 1996
Main classification: A61L27
IPC codes: A61L27