FI80207C - Method of making a tensile load bearing prosthesis - Google Patents

Description

80207

Method for making a tensile load-bearing prosthesis According to the method presented here, a synthetic prosthesis can be prepared for the replacement and repair of ligaments and tendons.

A generally accepted method of repairing ligaments and tendons is to move tissue to a defective site from elsewhere in the body. This repair method 10 often fails due to many factors, such as insufficient strength to the transplanted tissue, the dependence of the transplanted tissue on re-vascular viability, and insufficient strength of the attachment of the transplanted tissue.

15 The need for prosthetic replacements is a major damage to replace ligaments and tendons, and several attempts have been made to provide such devices. However, there is currently no prosthesis that is widely accepted. The reasons for the lack of Pro-20 prosthesis devices are insufficient tensile strength, lack of adequate attachment, breakage of the device due to mechanical stresses, and deterioration of the prosthesis / tissue interface.

According to the present invention, there is provided a tensile load-bearing prosthesis comprising, in the part requiring attachment to the traction tissue, a plurality of longitudinally adjacent parallel microporous polytetrafluoroethylene (PTFE) filaments having a characteristic interstitial dimension and having a thickness thick enough to allow across the thickness of the thread.

Most preferably, the material is expanded PTFE having a microstructure of node- and fibril-restricted pore-35 spaces and having a characteristic intermediate dimension greater than about 7 microns and a basic tensile strength greater than about 138 MPa and a porosity greater than 3055.

It is further preferred that the parallel strands be formed of a plurality of concentric loops of a continuous fiber of material elongate in the tensile load direction, wherein the ends of the plurality of elongate loops are assembled and bonded together to form at least one loop for attachment to the traction bone tissue.

It is further preferred that the prosthesis includes a means for distributing the tensile load in the individual strands.

The preferred embodiments of this invention allow for the maximization of tissue adhesion to a prosthesis without compromising the strength of the prosthesis and, when used as a prosthetic dressing, provides an initial attachment means for the device that allows for almost immediate patient movement. This shorter period of immobility 20 greatly reduces the rehabilitation time required for a functioning joint.

The invention will be explained in more detail by way of example only with reference to the accompanying drawings, in which Figure 1 is a photomicrograph of the PTFE material used in the structure of Example B prosthesis 25, Figure 2 is a photomicrograph of PTFE material used in another prosthesis structure, Figure 3 is a photomicrograph of laterally stretched characteristic Fig. 4 shows a schematic perspective view of a prosthesis formed from a prosthesis formed in accordance with the present invention, Fig. 5 schematically shows the implantation of a prosthetic prosthesis-35 prosthesis not made in accordance with the present invention, Fig. 6 shows schematically a step in a method of forming a prosthesis according to the present invention, Figure 7 shows a schematic perspective view of another prosthesis formed according to the present invention. Fig. 8a, B and C show the implantation of the prosthesis of Fig. 7 in the knee joint with the anterior cruciate ligament prosthesis.

The inventive method disclosed herein provides a synthetic prosthesis for the replacement and repair of ligaments and tendons. The prosthesis is formed of several filaments made of porous PTFE. The porosity of the strands 25 is characterized by connecting pore spaces.

The dimensions of the strands are small enough to allow tissue to grow into and through the entire strand. The percentage pore space or porosity is greater than 30¾, which allows mechanical attachment to the tissue in the intermediate spaces of the prosthesis to provide sufficient adhesive strength. This degree of porosity is a requirement only for those parts of the device that are intended to be anchored by tissue attachment. Porosity, as used herein, is defined as:

Po 25 ¾ porosity = - ^ - / 100 p! where: p £ = density of the porous material = density of the solid PTFE that forms the solids content of the porous material.

30 For PTFE that has never been sintered, p1 = 2.3 g / cm 2 and for materials that have been sintered, p1 is 2.2 g / cm 2, although this value may in fact vary slightly depending on the sintered and cooling conditions.

Immediate postoperative attachment of the device is accomplished by loops that are attached directly to the bone tissue. This initial attachment expands and eventually becomes unnecessary as the tissue grows into the porous filament material, providing permanent attachment of the prosthesis to the tissue. The tissue can easily grow between the strands and in the 5 strands because they are not attached to each other and are not tightly together. However, the depth to which the tissue can grow in each strand is controlled by the dimensions of the combined pore space passages passing through the porous microstructure. The interconnected pore space is formed by a solid PTFE matrix. In some cases, the matrix is formed of large solid nodes joined by long, flexible, relatively inelastic fibrils. Although the nodes may form rigid inelastic structures in the growing tissue, the protruding tissues may bend and push the fibrils aside. Other microstructures of this invention have much smaller nodes that appear merely as fibril junctions. In both cases, the tensile strength of fibril-20 is very high, and although they can be bent by the tissue, they cannot be stretched significantly. The microstructures of this invention can be characterized by an average interstitial dimension that can be used to predict the depth of tissue ingrowth. Short lengths of fibril-25 prevent tissue proliferation. Thus, in the case of porous filaments of short fibril lengths, the overall dimensions of the filaments themselves must be small enough that ingrowth and attachment occur throughout the filament.

The methods used herein to characterize the fibril length of a particular microstructure are based on a visual inspection of that microstructure. Appropriately magnified photographs can be produced with a scanning electron microscope or, in some cases, with a light microscope. The microporous PTFE materials of this invention can vary sufficiently in microstructure, so that a variety of methods must be used to measure the characteristic interstitial dimension. The fibrous fibers, for example those prepared by the method described in Example B, have a microstructure which can clearly be characterized by fibril-joined nodes. The characteristic intermediate space dimension of these types of materials can be determined by directly measuring the space between nodes. This measurement takes place along a line located in the direction of the strength direction 10 (Fig. 1). Sufficient measurements must be made so that the node spacing is adequately characterized. The average node distance thus obtained is used to characterize the intermediate space and then to predict the ingrowth into this microstructure.

In a filament material made, for example, by the stretching process disclosed in U.S. Patent 3,962,153, or in the products of U.S. Patent 4,187,390, the nodes of PTFE may be smaller and much less regular. In highly stretched products made in accordance with these patents 20, the internode becomes very large and the fibrils are packed together. In the manufacture of these products, the sintering step causes the fibril bundles to coalesce and form secondary attachment points. For this reason, the microstructure of such materials is not very clear under magnification. In determining the characteristic interstitial dimension of these materials, it is necessary to measure the distance of the fibril attachment sites rather than to measure the length of the fibril (i.e., node distance). The intermediate-30 dimensions of these materials can be observed if the samples are prepared for microscopy by stretching the material at slightly right angles to its direction of stretching. When the sample is stretched 10% in the lateral direction by preventing the sample from shrinking in the longitudinal direction at the points where the fibrils are attached, it should become apparent in a microscopic examination. The distance between the fibril junctions is then at all apparent intervals formed between the fibril bundles. This measurement is made in the direction of the stretching direction. As with the node spacing method described above, the number of fibril-5 attachment distance measurements must be sufficient to characterize the intermediate dimensions of the microstructure.

Figure 2 shows how this type of material occurs without lateral stretching compared to Figure 10, which is a photomicrograph of the same material stretched laterally by 10%. This lateral stretch, which is used only to characterize the microstructure of the material, shows a temporary structural re- leensuuntausta. The force applied to the material in the longitudinal direction restores the restoration of the original lateral dimension and the original microstructure. As discussed above, it is believed that the ingrown tissue pushes the fibers that make up this microstructure aside. A method for measuring the characteristic intermediate dimension of materials of this type is shown in Figure 3. · Once the characteristic intermediate dimension has been determined by the methods shown, suitable thread dimensions can be determined.

The maximum thickness that allows tissue to penetrate through its entire strand is nearly twice the depth of tissue penetration. As used herein, thickness means a suitable small cross-sectional dimension of a thread, e.g., the diameter of a thread having a circular cross-section, the thickness of a thread having a rectangular cross-section. In general, the combinations of characteristic intermediate dimension and filament thickness that are considered best because they allow complete tissue penetration across the filament cross-section in a shorter period of time can be determined from the following ratios: 7 80207 ln (filament diameter) ^ 2.28 x 10 ~ 2 (CID) -4 , 36 when CID> 7 microns, {_ 120 microns ln (thread diameter) ^ 6.98 x 10 ~ 2 (CID) -9.94 when CID ^ 120 microns 5 where: CID = characteristic intermediate dimension (microns)

In = natural logarithm The diameter of the thread is shown in inches.

The depth of tissue penetration into the microporous structure decreases radically when the characteristic interstitial dimension falls below 10 microns. This reduction is due to the fact that in structures with this characteristic and less than this distance, only a small number of intermediate passages are large enough to allow a single cell of the desired type to enter.

In characteristic interstitial dimensions of 15,120 microns and above, substantial vascularity follows tissue ingrowth and allows for a much greater depth of penetration. We believe that this results in an increase in the relationship between the intermediate dimension and the depth of tissue penetration.

The greatest requirement for a successful ligament or tendon prosthesis is adequate strength. In many cases, prosthetic materials used to replace these natural structures are subjected to very high tensile loads. In some cases, the strength of the prosthesis must be many times the peak load it is subjected to to compensate for the time-dependent mechanical properties of the prosthesis.

In terms of mechanical strength, the prior art Teva would state that the number of individual strands required for a particular application depends on several factors. These include: the cross-sectional area of the individual strand, the tensile strength of the individual strand, and the need for traction for this particular application, including the safety factors of the creep-35 strength constraints. The individual strands shown in this embodiment 880207 of the invention can be prepared using the methods set forth in U.S. Patent 3,953,566, U.S. Patent 3,962,153, and Example B below. It is desirable to use a material with a high basic tensile strength to minimize all physical dimensions of the device and thereby minimize the size of the boreholes placed in the bone for installation of the device. The basic tensile strength relates to the strength of the polymer used in the porous sample and is used as disclosed in U.S. Patent 10 3,953,566.

In a preferred embodiment of the present invention: The filamentary material is a porous PTFE having a basic tensile strength greater than 138 MPa, a porosity greater than 30 # and a microstructure characterized by interconnected passages formed between nodes and fibrils.

- The fiber dimensions of the microstructure and the characteristic intermediate space dimensions are chosen so that the ingrowth of the tissue 20 through the fiber takes place rapidly. .

- Each thread and finished structure has sufficient strength to meet the mechanical requirements of a particular application.

- The parallel strands consist of multiple loops formed of a continuous fiber of filament material.

- The ends of the multiple loops are assembled together and formed into at least one loop to attach the product to the bone tissue.

30 - The uniformity of the tensile load of the prosthesis under tensile force is improved by: 1. minimizing the differences in loop length used to form parallel strands, 2. compressing the loop strands in loop segments at an elevated temperature sufficiently to allow the matte 9 80207 to coalesce and secure.

- The prosthesis may also include means for distributing the tensile load 5 between the strands as the prosthesis passes over an angle: 1. in a thread bundle around its longitudinal axis, 2. in a loose strand braid.

10 - Although an embodiment of a ligament prosthesis preferably has a pair of opposing loops formed from elongate loops, the present invention also includes a single loop 324 formed from loops assembled for attachment to a single bone. The loops at the other end 316 remain unassembled or chamfered to provide attachment to soft tissue, such as muscle tissue, e.g., by means of a suture (see Figure M).

20 In the latter case, the closed ends of the loop provide additional resistance against the possible slipping of the strands past the sutures. This single loop embodiment of the prosthesis 310 could be used to repair or replace tendons.

Example A

This example shows a prosthesis that did not reach -50 satisfactory system strength because the thickness of the strand was too large for the interstitial dimension that characterizes its microstructure. The thickness (diameter) of the filament was 0.66 cm, the porosity of the filament was about 8θί, and the characteristic intermediate dimension was about 78 microns. The intermediate state-35 was determined as shown in Figure 1. The prosthesis was used to replace the canine anterior cruciate ligament by guiding the material through the boreholes to the tibia and femur. Four holes were drilled in the tibia 2 and femur 4 so that the prosthesis thread 6 formed a material loop with two strands at the original ligament (Figure 5). · The initial attachment was made by tying the ends of the thread together into a knot 8 to form a continuous loop. In the space between the microporous material of tissue ingrowth and formation, it was expected to increase the strength of the initial attachment and distribute the stress on the surrounding tissue. The tensile strength of the thread passing through each knee joint was about 250 kg. The combined strength of the two strands was thus 499 kg. 260 days after implantation, the knee-15 joint was removed.

Boreholes were placed in the tibia and femur for installation on tensile test fixtures. After removal of all supporting parallel structures, the femur around the knee was pulled from the tibia along the axis of the 20 prosthetic ligaments at a constant rate of 500 mm / min until collapse. The length that connected the internal space between the joints between the bone ducts formed the part of the prosthesis that was under tensile load in this test due to the tissue attachment to the prosthesis in the bone ducts. The collapse form of the system-25 was a fracture of the prosthetic device at the exit level from the bone ducts. Surprisingly, this fracture occurred at a value of only 91 kg. Using histological examination, we found that this decrease in strength was proportional to the limit of bone ingrowth, which was generally less than 1 mm in the prosthesis. In the case of a thread with this diameter and a characteristic intermediate dimension, the attachment takes place only in the circumferential ring of the material on the circumference of the device. This reduced area then becomes the only load-bearing prosthetic material when the attraction is initially applied to it.

11 80207

The collapse occurs in this circumferential ring of material first and then it progresses through the center of the prosthesis.

Example B

5 Experience from Example A led to the conclusion that tissue ingrowth must penetrate through the cross-section of the strand to provide a sufficiently long-term system strength. Thus, the member was formed using a thread of similar porosity and characteristic interstitial dimension but much smaller in diameter. The filament material used to form the anterior cruciate ligament prosthesis of this example was prepared as follows: PTFE dispersion powder ("Fluon CD-123" resin, manufactured by ICI America) was mixed with 130 cm 3 of a free "ISOPAR K" solvent (manufactured by Exxon Corporation) PTFE: n per 0.45 kg was compressed into a pellet and extruded into a rod of 0.274 cm in diameter in a piston extruder with a reduction ratio of 96: 1 in cross section from a pellet to an extruded rod.

The extruded bar, which still contained Isopar K solvent, was immersed in an Isopar K tank at 60 ° C and stretched 8.7 times its original length between traction rollers with a production speed of about 26.3 m / min. The diameter of these traction rollers was 7.1 cm and the distance from the center to the center was about 11.4 cm. The diameter of the rod was reduced from about 0.274 cm to about 0.119 cm by this stretch - Isopar K was then removed from the stretched material.

The stretched rod was drawn through a round sealing nozzle heated to 300 ° C. The orifice of the nozzle narrowed at an angle of 10 ° from 0.127 sows to 0.064 cm and then was constant 12 80207 at a length of about 0.064 cm. The production speed of the material leaving the die was 2.19 m / min.

The stretched rod was then heated to 300 ° C by contact with the heated drive 5 'draw rolls and stretched 4 times / 2 times ($ 350) at a production rate of 1.98 m / min. The diameter of these traction rollers was 7 cm and the distance from the center to the center was 11.4 cm.

Finally, the rod was prevented from shrinking and placed in a drying oven at about 367 ° C for 30 seconds.

In finished form, the fiber treated by this method had the following properties: diameter = 0.066 cm basic tensile strength = 511 MPa porosity = 80.8% characteristic intermediate dimension = 74 microns. As shown in Figure 6, the prosthesis 10 was fabricated over two steel coils 42, 44 mounted on a rack (not shown). The spools were supported by tu-20 at 46.48, which were 14 cm from the centerline to the centerline. These steel coils were threaded to allow disassembly of one flange. The strand of PTFE material was transported around the two spools 80 times, so that a total of 160 strands were joined by the two spools. The two free ends of the fiber were tied together by multiple square knots. One spool was disassembled from the support, rotated 180 °, and reassembled on top of the support, creating a half-turn about the longitudinal axis of the structure.

The structure was then wrapped with a thin film of PTFE for 25 turns at three points in each case. This film was prepared according to the method of U.S. Patent 3> 962,153 and had the following properties: width = 0.95 cm 35 thickness = 0.00064 cm longitudinal basic tensile strength 483 MPa porosity = 84% 13 80207 The bundle of strands was coated with this thin film at two points 28, 30 adjacent the sides 42, 44, forming loops 24, 26 at the ends of the structure (Fig. 7). The central portion 38 was also coated with a film. The Ke-5 lat was then mailed from the supports and placed on a rack made of thin metal wire and arranged to prevent rotation and longitudinal shrinkage. This rack was then placed in a 375 ° C drying oven for six minutes. After cooling, coils 10 were removed from the ends of the structure. The space occupied by the halves formed loops by which this ligament structure can be secured to the bone with screws or other suitable fastening means. All areas coated with the film were compressed during the heat treatment due to shrinkage of the film to provide cohesiveness from fiber to fiber. During the heating period described above, there were also a few fiber-to-fiber attachments in the uncoated areas. These fibers were then separated individually using metal chopping. The structure then comprised a 1β0 microporous PTPE strand that attached loops of slightly denser material. The prosthesis included a 180 ° thread along the direction of the tensile load to better distribute the tensile load between the strands. The PTFE tape cover 38 surrounding the strands 20, located nearly midway between the ends 14, 16 of the prosthesis 10, served to maintain the thread by ensuring that the strands 20 were released during implantation. As with the PTFE coatings 28, 30, the coating 38 30 is intended to be placed outside the contact area of the bone so that it does not prevent tissue from growing into the strands 20.

An organ just prepared as shown was implanted in the knee of a sheep to replace the anterior cruciate ligament removed (see Figures 8A, B and C). This implantation was performed by placing a 0.64 cm borehole in both the femur and tibia. The hole was placed in the tibia along the axis of the naturally removed natural anterior cruciate and came out at the insertion site. The placement of the tibial borehole began at the lateral distal surface of the tibia near the lateral spine of the tibia. The channel was 5 mm long so that the outlet hole was formed just near the lateral tibial joint pin on the surface of the tibial bend of the tibia. The prosthesis 10 was passed from the tibia outlet to the space between the articular pins, across the intra-articular space, and through the femoral canal. The loop-10 cat 24, 26 and the coated segments 28, 30 at the ends of the structure were placed outside the drilled bone channels. The placement of the coated segment 32, 34 and 38 in the central region of the structure was the internal space of the joints. The prosthesis 10 was then anchored to the bone with self-tapping Orthopedic-15 screws placed through loops 24, 26. The knee joint was found to be stable immediately after surgery.

After a three-month implantation period, the knee was removed from the animal and boreholes were made in the femoral and tibial-20 bones, where fasteners were placed to perform tensile testing along the axis of the ligament structure. After removal of the muscle tissue and separation of all the supporting parallel structures around the knee, the tibia was pulled apart from the femur at a constant rate of 500 mm / min until collapse occurred. The collapse of the system occurred at 291 kgrs. The collapse occurred in a ligament prosthesis in a loop attached to the tibia. The fracture occurred when the load exceeded the fixation provided by the weather growth of the tissue si-30 into the intraosseous segments and was transferred to the fixation screw. Organ collapse was associated with the discharge of filament material through the loop segments after several filaments had collapsed. Histological examination of this sample showed 35 ingrowth of tissue into and into the filaments. Tissue

II

15 80207 growth had progressed perfectly through the diameter of a few strands. We believe that at longer planting times, most of the filaments would show complete and thorough ingrowth.

Claims (8)

16 80207 A method of making a tensile load-bearing prosthesis to replace or replace ligaments or tendons, the prosthesis comprising a plurality of parallel, longitudinally adjacent, non-adherent, microporous polytetrafluoroethylene or other suitable material strands attached to at least one loop of the prosthesis. for initial attachment to a traction tissue, characterized in that a) a plurality of pins are arranged and spaced apart to define an elongate area according to the desired size of the prosthesis, b) a plurality of elongate loops are formed around the pairs of pins from a continuous fiber fabric until the desired number of parallel strands, and c) assembling the ends of the loops together at least at one end of the elongate region to form a loop and ensuring that the assembled ends of the loops unload against him. A method according to claim 1, characterized in that the loop strands are further wound around the longitudinal axis of the prosthesis to provide a more even load distribution when the prosthesis is arranged to connect the joint. A method according to claim 2, characterized in that the loop threads are twisted 180 °. A method according to claim 1, characterized in that the loop strands are loosely braided to provide a more even load distribution when the prosthesis 35 is intended to connect the joint. A method according to claim 1, characterized in that the securing against unraveling further comprises the step of compressing and heating the assembled ends at a sufficient temperature and for a sufficient time to join the loops together. 5 A method according to claim 1 or 5, characterized in that the securing against unwinding further comprises a pre-step in which the strands of high-strength material are wound around the assembled ends 10 tangentially to the axis direction of the prosthesis at a point adjacent to the loop. The method of claim 6, characterized in that the high strength material is expanded PFTE having a matrix tensile strength of about 483 MPa. A method according to claim 5, characterized in that the assembled ends are compressed and heated in a die having the desired loop shape 20 and dimensions. 25 30 35 18 80207