FIELD OF THE INVENTION

The present invention relates to a hydrogel based device for use as tissue repair.

BACKGROUND OF THE INVENTION

Hundreds of thousands of anterior cruciate ligaments (ACL) are torn every year and this trend has been increasing with the rise of participation in sports in the general population and in particular in females and older participants. For young and/or athletic active individuals the standard care is based on ligament reconstruction. Several replacement tissues can be envisaged using either grafts (auto, alio and xeno) or artificial materials. Xenografts, ligaments from other animals, and allografts from cadaveric human tissue are possibilities that overcome the need to autologous tissues and avoid the risk of donor-site morbidity. However, their use poses several issues including risks of disease transmission, graft rejection and inflammation. Moreover, in the case of allografts, the supply is so small that the market demand can never be met from this source. Autograft tissues extracted from the patellar tendon, quadriceps tendon-patellar bone or the hamstring tendons are currently the most common sources of grafts for ACL reconstruction. Yet this therapy relies on the extraction of healthy tissue which implies risks of donor-site morbidity, an initial low strength, a high probability of rupture at the initial stages and a long and painful recovery period. The use of artificial prosthetic ligaments as an alternative to autografts could bring substantial improvements in the existing reconstruction therapies.

Several prosthetic devices for ACL replacement have been made over the past thirty years using a wide range of materials. The materials which have been considered for these devices including polyester (Stryker Dacron® ligament prosthesis, Leeds-Keio), polytetrafluoroethylene and fluoropolymers (Gore-Tex®), carbon fibers, polyethylene, nylon and polystyrene. However none of these artificial ligaments have demonstrated positive long term results in vivo. Failures of previous devices mostly originate from mechanical failures or from a lack of biocompatibility. Mechanical failures include i) rupture caused by wear, fatigue or severe loading in the knee and ii) laxity in the joint after creep of the prosthetic ligament or loosening of the fixation element in the bone. Biocompatibility issues primarily manifest as immunogenic particulation leading to chronic synovitis. Due to high incidence of such problems, most if not all the previous artificial ligaments have been withdrawn from the commercial market. For example, no such devices are currently approved for clinical use by the Food and Drug Administration of the United States of America (FDA).

Over the last decades, statistics have been collected that confirm the poor long term efficacy of existing artificial ligaments. A study of 855 artificial cruciate ligaments over a 15- year period found that there was 40-78% failure rate (Fu F. H. et al. American Journal of Sports Medicine, 2000, 28(1), 124-130). Another report found that around 80% of knees which had been reconstructed using Dacron prosthetic ligaments had developed significant osteo arthritic symptoms at a 9 year follow up (Fu F. H. cited above). Similarly, a study of 268 patients revealed that Gore-Tex® anterior cruciate ligament prosthesis yielded a failure rate of 42%o with case of effusions, rupture and strong loosening (George M.S. et al, American Journal of Sports Medicine, 2006, 34(12), 2026-2037). Overall, complication rates for artificial ligament operations are of the order of 40-50%, which is much higher than the rate with autologous and allogenic ligaments.

As an alternative to non-biodegradable artificial ligaments, research efforts are being carried out to develop tissue engineered (TE) ligaments for which a biodegradable scaffold first replaces the native ligament and is progressively replaced by a new reconstructed living tissue. Several systems have recently been designed using silk, collagen or polylactic acid biodegradable fibers (Freeman J. W. et al. Journal of Biomechanics, 2010, doi: 10.1016/j.biomech. 2010.10.043). Nevertheless, the management of cell sourcing as well as the control of scaffold degradation while ensuring proper mechanical properties remain unsolved issues that still need to be addressed before clinical use.

Most of the patents deal with the methods of attachment or fixation design to attach an artificial ligament to the bones of a joint. Recent examples are given below.

FR 2 700 111 relates to an artificial ligament consisting of a fixed section and a moving sleeve which form two separate ligaments of the same or different lengths, joined together so that they can slide relative to one another. The two ligaments can be made from plaited, woven or knitted fibres of the same or different materials, with their ends joined together by a thermo-shrink material or a supple adhesive. The outer ligament can be made with sections of reduced resistance which allow its length to be varied. The ligaments can be made from Dacron® (RTM) or other synthetic fibres, or from natural cellulose fibres which are treated to make them biocompatible.

US 2003/114929 discloses a prosthetic ligament including a cord of thermotropic liquid crystal filaments. Preferably the cord is a string or thin rope made by several strands braided, twisted, or woven together. Strands are, preferably, made of a multi- filament thread. US 5800543 relates to an artificial ligament device comprising a plurality of tows of biocompatible material (for example polyester) secured side-by-side in a flat elongate array by braiding, the tows being looped back at one end of the device to form an eye, the flat lengths adjoining the eye being secured to each other side-by-side by stitching, the tows around the eye being grouped together and whipped, and lashing being applied around a base of the eye.

WO 2009/047767 provides a ligament prosthesis having a first end and a second end, comprising a first load bearing element and a second load bearing element, the first and second load bearing elements differing in one or more mechanical properties and being arranged in the prosthesis in series. The load bearing elements may be made from an alloy and, in order to protect nearby organs and tissues from abrasion from the prosthesis and vice versa, may be contained in a sleeve made from biodegradable polymers such as poliglecaprone, polyglycolic acid, polylactic acid, polydioxanone, or co-polymers of the aforementioned polymers.

WO 2009/113076 provides a ligament prosthesis having an undeployed configuration and a deployed configuration. The prosthesis has a resistance to tension in the undeployed configuration that is less than its resistance to tension in the deployed configuration. In the deployed configuration, the prosthesis is capable of twisting and bending. In one embodiment, the prosthesis has a meshwork of filaments woven, knitted or braided into a slender cylinder. The prosthesis may be used to replace an anterior or posterior cruciate ligament.

WO 98/22046 discloses a "free strand" ligament that is naturally self-convoluted between the two ends of the intra-articular median part.

With the foregoing disadvantages of the prior art in mind, it appears that there is a need for a device which i) is biocompatible on the short and long term, meaning after years of implantation in vivo, ii) reproduces closely the non-linear elastic mechanical behaviour of native ligaments and tendons including the stiffness and the toe-region, iii) ensures ultimate tensile stress (strength) and ultimate tensile strain that are safe with respect to the patient's activity. Polymer hydrogels constitute relevant materials in that respect.

Hydrogels, also called aquagels, are hydrophilic polymer networks that can absorb water and swell without dissolving at least temporarily. Depending on the physico-chemical properties of these networks, levels of water absorption can vary greatly from about 10% to thousand times their dry weight. An important characteristic of hydrogels is that they can possess a water content and a molecular structure very similar to those of living tissues. These features confer them biocompatibility, lubricity, rubbery elasticity and possibly biodegradability, which are of interest for biomedical applications and more particularly tissue replacement. Examples of hydrogel forming polymers that are relevant for biomedical applications are polyvinyl alcohol, poly ethylene-glycol, polysaccharides, polylactic acids and their copolymers

US 5981826 provides a poly( vinyl alcohol) hydrogel construct having a wide range of mechanical strengths for use as a human tissue replacement. It may be especially useful in surgical and other medical applications as an artificial material for replacing and reconstructing soft tissues in humans and other mammals. Soft tissue body parts which can be replaced or reconstructed by the hydrogel include vascular grafts, heart valves, esophageal tissue, skin, corneal tissue, cartilage, meniscus, and tendon. However, the reported tensile modulus of elasticity for the so-prepared material is less than IMPa, which is too low as compared to the ultimate tensile stress of ligaments and tendons, which is greater than lOOMPa.

WO/2006/102756 relates to a hydrogel exhibiting anisotropic properties which is poly( vinyl alcohol) produced by preparing a solution of poly(vinyl alcohol) with a preselected concentration, thermally cycling the solution by freezing and thawing, stretching the hydrogel and thermally cycling the hydrogel at least one more time. Said anisotropic hydrogel is used for soft tissue replacement selected from vascular vessels, coronary arteries, heart valve leaflets, heart valve stent, cartilage, ligaments and skin. However, the ultimate tensile stress for the so-prepared materials do not exceed 0.4MPa, which is too low as compared to the ultimate tensile stress of ligaments and tendons, estimated in the range 30-50MPa.

WO/2001/017574 discloses a hydrogel intended for orthopedic applications wherein the tissue is selected from the group consisting of bone, cartilage, meniscus, bursa, synovial membranes, tendons, ligaments, muscle and vertebral disks. Like for WO/2006/102756, the ultimate tensile stress for the preferred material is about 8MPa, which is too low for the replacement of most ligaments and tendons.

A non-biodegradable device that can be installed in-vivo to repair a ligament or tendon, that is biocompatible and that reproduces closely the mechanical properties of native ligaments and tendons as well as ensures the appropriate values of tensile strength and ultimate tensile strain required for ligament or tendon replacement is needed.

DESCRIPTION OF THE INVENTION

The inventors propose a novel type of device, in particular an artificial ligament or tendon which can be placed and fixed in a patient at the location of respectively a ligament or a tendon and which is made of biocompatible hydrogel and reproduces closely the tensile mechanical response of native ligaments or tendon with values of ultimate tensile stress and ultimate tensile strain that are appropriate for ligament or tendon repair. It can be placed in the patient using minimally invasive techniques. Unlike autograft techniques, this device does not necessitate the sacrifice of healthy tendons or ligaments from the patient.

Thus an object of the present invention is a biocompatible device in the form of an elongated body comprising at least a flexible median part between two end parts, said body having a fibrous structure formed from biocompatible hydrogel fibers, said hydrogel fibers being assembled to form the body of the device.

According to the invention, the word "biocompatible" means that the hydrogel and the device elicit little or no immune response in a given organism, or is able to integrate with the tissue.

According to the invention the flexible median part reproduces the behavior of the natural ligament or tendon whereas at least one end part is designed to be fixed on a bone.

According to the invention, the body may be formed by any techniques known from the one skilled in the art, for example by simple assembly of the fibers keeping them individual, by assembly of the fibers into threads or strands, by braiding, by knitting or by weaving.

According to the invention, the device is advantageously an artificial ligament used for repairing any ligament in animals, in particular non human mammals or humans. Ligaments which may be repaired may be selected from the folio wings: head and neck ligaments (cricothyroid ligament, periodontal ligament, suspensory ligament of the lens), wrist ligaments (palmar radiocarpal ligament , dorsal radiocarpal ligament, ulnar collateral ligament, radial collateral ligament), shoulder ligament (rotator cuff), knee ligament (anterior cruciate ligament (ACL), lateral collateral ligament (LCL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), cranial cruciate ligament (CrCL) - quadruped equivalent of ACL, caudal cruciate ligament (CaCL) - quadruped equivalent of PCL, patellar ligament).

The device according to the invention shows a tensile strength between 10 and 200MPa, advantageously between 15 and 50 MPa (corresponding to an ultimate load between 1100 and 4000N for a ligament with a diameter of 1cm) and an ultimate tensile strain between 10 and 100%, advantageously 15-80%.

In an advantageous embodiment, the tensile strength and the ultimate tensile strain of the device defined above are measured after equilibration in water or bodily liquids. In an advantageous embodiment, the device according to the invention shows an ultimate tensile load between 30 and 60000N, advantageously comprised between 300 and 4000N and an ultimate tensile strain comprised between 10 and 100%, advantageously between 15 and 80%.

In an advantageous embodiment according to the invention, the fibrous structure may also contain other type of biocompatible fibers assembled with the hydrogel fibers, for example carbon fibers or polymeric fibers selected from polyethylene, polyethylene terephthalate, polypropylene, polytetrafluoroethylene, polyurethane, polyurethane urea, polysaccharides, elastin, collagen.

According to the invention, the diameter of the device is similar to the one of the natural part to be repaired; for example in the case of an artificial ligament for repairing a human knee ligament the diameter of the device is comprised between 2 mm and 2 cm, advantageously comprised between 5 and 10 mm. The length of the body is also similar to the one of the natural ligament. For the case of the human knee ligament, it is comprised between 0.5 and 5 cm and the length of the whole ligament comprising the median part and the end parts is comprised between 5 and 25 cm, advantageously between 10 and 20 cm, more advantageously is equal to about 15 cm. The choice of the diameter and of the length of the device is well in the hand of the one skilled in the art who may adapt said parameters to the type of ligament or tendon he wishes to repair.

According to the invention any biocompatible hydrogel known from the one skilled in the art may be used if the moisture content is comprised between 10 and 80%. A suitable hydrogel is, for example, the PVA hydrogels disclosed in US 5 981 826, or sold by Salumedica LLC. Commercial hydrogel fibers can be used like PVA fibers Solvron® sold by Nitivy Company Ltd.

According to the invention, the device may further contain anchoring systems at least at one end part which may be any system known from the one skilled in the art, like for example a hook, a screw, a buckle, a bone anchor, an interference screw, a cross pin or a suture button. It may also be an eye spliced at said end of the device.

In an advantageous embodiment according to the invention, either the fibers, or the assembly of fibers (threads, strands, braid, knit fabric or woven fabric) or the whole body or both of them or the three are entrapped in a hydrogel matrix or coated by a hydrogel, said hydrogel being the same or being different from the hydrogel used for the fibers. Said coating or matrix may contain mineral fillers imparting better anchoring on the bone like calcium phosphates (hydroxyapatite and the like) or may be swollen by water in situ in order to have better anchoring on the bone.

Then in an advantageous embodiment of the invention, the anchoring system may be the hydrogel coated or embedded end parts of the device themselves.

Where coating by a hydrogel or embedding in a hydrogel matrix is performed on the fibers or strands, then it is performed prior to assembly of the body. Said matrix or coating plays several roles:

i) It better redeploys the efforts between fibers

ii) It lubricates the contact between the device and surrounding tissues thus diminishing the risks of inflammation and of failure by wear, iii) It provides additional stiffness to the end parts in order to reduce deformation in the bone tunnels and favor adhesion with bone.

iv) According to the properties of the hydrogel, it is used either to prevent cellular adhesion on the median part of the body or to reinforce the bone-device interface and accelerates the osseous integration of the end parts.

The device may have any form suitable for repairing the corresponding natural part. It is generally elongated in shape and the anchoring system at at least one end of the device is adapted for attachment to a musculo-skeletal tissue such as bone.

In an advantageous embodiment of the invention, the device has three parts: a central part and two extreme parts. In the case of a cruciate ligament the extreme parts are called respectively tibial and femoral part and the central part is called ligamentous part.

In an advantageous embodiment according to the invention, the fibers may be assembled to threads, and the strands are strands of threads. The strands may be twisted in the opposite direction of the twist of the threads to gain a greater stiffness and cohesion of the structure.

In another advantageous embodiment, the strands may surround a hollow core. The obtained structure has several roles:

i) The hollow core provides a lower resistance to bending and thus a greater flexibility to the central part.

ii) The hollow core can be filled by a substance like bone cement or bone mulch or an appropriate fixation device like interference screws to better fix one or both of the extreme parts, tibial and femoral parts in the case of knee ligaments. In another advantageous embodiment of the invention, the fibers are oriented in a direction of loading of the prosthetic ligament.

In another advantageous embodiment of the invention, the fibers may be assembled to threads, and the threads are oriented in a direction of loading of the prosthetic ligament.

In another advantageous embodiment the assembly pattern can be different for each part. For example, where the fibers are assembled into strands, strands can be twisted in the central part and braided in the end-parts.

The fibers themselves or the coating or both may contain at least one compound selected from the group consisting of growth factors, drugs like anti-inflammatory drugs or a mineral filler like calcium phosphates (hydroxyapatite and the like).

In an advantageous embodiment, the stiffness of the device of the invention is comprised from about 10 to 500 N/%, preferably from about 20 to about 300 N/%.

The device according to the invention may be prepared by any process known from the one skilled in the art. The process shall permit the modification of each part of the body separately.

Consequently another object of the invention is a process for preparing a device according to the instant invention with steps comprising:

a) assembly of fibers to form the body of the device,

b) immersion of the fibers, the assembly of fibers or whole body of the device in an aqueous solution of hydrogel to coat said fibers, or said threads, or said strands, or said body or to embed them in a hydrogel matrix,

c) cross-linking the hydrogel coating, or the hydrogel fibers or the hydrogel matrix.

In the process, the order of steps a), b) and c) may be modified or one of step b) and c) may be deleted. For example where fibers or threads or strands are coated or embedded in hydrogel, step b) is performed before step a). Step c) may be provided directly on the hydrogel fibers before step a) or step b) may be performed before step a).

Cross-linking in step c) may be performed by any techniques known in the art. Suitable techniques include application of heat and/or irradiation like UV or gamma rays and/or chemicals like dialdehydes in the case of PVA hydrogels and/or freezing/thawing or drying/rehydrating cycles.

In a further embodiment of the process according to the invention, the assembly of fibers of step a) is performed around a rod to form a hollow. In a third embodiment of the process according to the invention, it further comprises an annealing step above the glass transition temperature of the hydrogel forming polymer and below the dissociation temperature of the hydrogel network. In the case of polyvinyl alcohol, the temperature of the said step is preferably in the range between 100°C to 200°C. Said annealing step may be performed directly on the fibers prior to step a) or before or after step c). In a preferred embodiment of the process, the device is maintained under tension during annealing.

According to the invention, it is possible, in the process, to treat differently and separately the end-parts and median parts in order to have condign properties for each part.

The device according to the invention may be used as ligament or tendon including elbow, shoulder, ankle, knee in dog, horse or human, in particular a cruciate ligament, more particularly an anterior cruciate ligament and may be configured therefore.

Consequently another object of the instant invention is a method for inserting a device comprising the following steps:

a) providing a device according to the instant invention,

b) attaching a first end of the device to a first attachment site,

c) attaching the second end of the device to a second attachment site

The first and the second attachment sites may be any musculo-skeletal tissue to which the natural ligament or tendon is attached in a human or non-human animal. In case of the anterior cruciate ligament if the first attachment site is the femur then the second attachment site is the tibia. If the first attachment site is the tibia then the second attachment site is the femur.

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described by way of examples and drawings.

Figure 1 is a schematic drawing of one embodiment according to the invention. About

15 hydrogel threads (1) constituted from about 12 hydrogel continuous fibers are assembled to form a strand (2). Several strands, which number is adjusted to impart the good mechanical properties, are assembled to form the body (3), about 9 to 25 strands can be used for replacement of a human ACL. The fiber assembly may be designed to have a hollow core (7). Strands may be embedded in a hydrogel matrix which is of the same nature as the fibers. The tibial part (5) and the femoral part (6) or both may be coated by a hydrogel matrix (8) differently from the ligamentous part (median part) (4). On this particular design, the anchoring system (9) is a loop spliced at one end of the device. Figure 2 illustrates the load versus strain response during a tensile test for several types of device obtained following the invention. These results are compared to the tensile behavior of real ligaments and tendons as reported in the literature (Woo S. L. et al. American Journal of Sports Medicine, 1991, 19, p. 217; Kim D. H. et al. American Journal of Sports Medicine, 2003, 31, p. 861.).

Figure 3 shows the effect of annealing on the tensile behavior of one type of a device according to the invention.

Figure 4 shows the result for one type of device with and without coating by a hydrogel.

EXAMPLE 1: DEVICE DESIGN

Several devices have been prepared from the assembly of PVA fibers only, or PVA fibers and ultra-high molecular weight polyethylene (UHMWPE) fibers.

PVA fibers (suitably with a dissolution temperature greater than 50°C and 95+% hydro lyzed) were obtained as threads of more than 600 dtex composed of several continuous fibers like the Solvron® MH675 from Nitivy Company Ltd. PVA strands were prepared by winding 15 PVA threads together. UHMWPE fibers were obtained as threads of more than 60dtex like Dyneema® from DSM.

Ten types of assembly were created with various structures, PVA/UHMWPE compositions and treatments. Fiber assemblies were approximately 10mm in diameter, which is approximately the same diameter as the native ligament of the knee and currently used grafts. Geometries, compositions and treatments are summarized in Table 1.

Table 1 :

Sample Components Description

Type 1 16 PVA strands 16 PVA strands are assembled parallel to the direction of loading.

Type 2 16 PVA strands 4 twisted ropes each consisting of 4 PVA strands twisted together.

Type 3 22 PVA strands Central core of 4 braided PVA strands surrounded by 6 twisted ropes each consisting of 3 PVA strands twisted together.

Type 4 21 PVA strands Same as Type 3 but with 27 UHMWPE threads together

27 UHMWPE threads as a unit substituted for one of the PVA strands in the braided central core.

Type 5 16 PVA strands Same as Type 1 but with annealing of the structure at

130°C for 1 hour.

Type 6 16 PVA strands Same as Type 1 but with annealing of the structure at

160°C for 1 hour. Type 7 16 PVA strands Same as Type 1 but with annealing of the structure at

190°C for 1 hour.

Type 8 16 PVA strands Same as Type 1 but with annealing of the structure at

160°C for lO min.

Type 9 16 PVA strands Same as Type 1 but with annealing of the structure at

190°C for lO min.

Type 10 16 PVA strands Same as Type 1 but with coating by PVA hydrogel using the immersion method and applying one freezing-thawing cycle.

To illustrate the effect of annealing, Type 1 assemblies were annealed in an oven at several temperatures and during different annealing times (see Types 5-9) in Table 1. In a preferred method, fiber assemblies are maintained under tension during annealing by clamping them on a metal frame.

To embed or coat the device, a PVA solution with the desired concentration (suitably in the range 5-20wt%) was prepared either by dissolving the PVA fibers or by dissolving another PVA like PVA sold by Sigma Aldrich (suitably with MW of 80,000 to 186,000, 99+% hydrolyzed). These PVA solutions were obtained in distilled water by mixing and heating for about 1 hour around 90°C.

In one method, fiber assemblies were mounted on a metallic frame, immersed for one day at 20°C in the solution and removed from the solution. In another method, they were encased in cylindrical moulds and the PVA solution was poured in these same moulds. To cross-link the hydrogel solution, the devices obtained by these two methods were thermally cycled by freezing at about -18°C for 12 hours, and then thawing in air or distilled water by heating back to 20°C for 12 hours. This process represents one cycle by freezing-thawing. Up to 10 cycles have been applied. Cross-linking was also performed by drying the devices at 20°C in ethanol for one day and then in vacuum for one day and by rehydrating them by immersion in distilled water for one day. This process represents one cycle by drying- rehydrating.

The end parts and median part of the device could be treated separately by these two methods. In the method by immersion, only the end-parts could be embedded in the PVA matrix by partially immersing the sample. In the method with moulds, separate moulds could be used for each end- and median parts. In a first step, PVA embedding was applied to the end-parts and followed by cross-linking and possibly annealing. A second PVA embedding was then applied to the median part only or to the entire device and followed by another cross-linking step. Different stiffnesses and properties for the median and end-parts were produced by varying the degree of cross-linking achieved during the first and second steps. In one method, a solution of PVA with a dispersion of hydroxyapatite was used to coat the end- parts only.

EXAMPLE 2: PROPERTIES OF THE DEVICE

These results correspond to the fiber assembly types presented in Table 1.

1.1. Biocompatibility testing

1H and 13C NMR analysis showed that there is no noticeable distinction between the PVA used for the hydrogel fibers and a biocompatible PVA already used for cartilage replacement like the one described in US 5981826. In particular, there are no detectable traces of other organic compounds and PVA is hydro lyzed over 98%. For this latter PVA, biocompatibility was qualified by testing for ability to produce cytotoxicity, intracutaneous irritation, sensitization by Kligman maximization, Ames mutagenicity, chromosomal aberration, and chronic toxicity. Chronic toxicity was assessed in combination with long-term subcutaneous implantation in a 13-week rat animal model. The material met the acceptance criteria for all testing conducted.

1.2 Dimensions

The diameter of the samples presented in Table 1 was measured and found in the range 10-1 lmm.

1.3. Tensile properties

1.3.1. Measurement

Hydrogel swelling was achieved by immersing fiber assemblies in distilled water at 23°C for 24h prior to testing. Tensile testing was performed on an Instron 5966 apparatus using capstan fixations. Samples were tested less than 5min after removal of water and pulled at a strain rate of about 10-2s-l (1.5mm.s-l) which was fast enough to avoid any significant drying during testing. Testing was performed at 23°C. Instantaneous strain was measured by following ink marks with a video extenso meter. Three samples were tested for each type of design as described previously in Table 1.

1.3.2. Results

Results are given in figure 2, 3 and 4. Figure 2 compares the results for type 1, 2, 3, 4, 5 and 7 with literature data showing the tensile behavior of native ACL and hamstring tendon grafts. The behavior of native ACL was reproduced from the results of Woo et al. (Woo S. L. et al. American Journal of Sports Medicine, 1991, 19, p. 217) taking a ligament length of 20mm to estimate strain from displacement. The behaviour of hamstring tendon grafts was reproduced from the results of Kim D. H. et al. (Kim D. H. et al. American Journal of Sports Medicine, 2003, 31, p. 861.) taking a tendon length of 30mm to estimate strain from displacement.

All types of device exhibited a non-linear elasticity with a toe region as well as an ultimate strain greater than 15% suitable for low tension and wear during the swing phase of walking. The stiffness of the devices is given by the slope of a linear fit of the load versus strain curves in the 0-10% range. All designs exhibited an acceptable value for stiffness within the range of the currently available ACL replacements. The Type 1 and Type 2 devices had the approximate stiffness of allograft replacements or the native ligament of a middle aged person. The Type 3 device had a stiffness value very close to that of a young person's native ACL. Finally, stiffness could be largely increased with the PVA/UHMWPE structure (Type 4) or in the annealed device (Type 7). They had a stiffness up to about three times greater than that of native ligaments and similar to autografts harvested from the patellar tendon or hamstrings. In the case of the PVA/UHMWPE structure shown here, the high stiffness is obtained after the toe-region (in the 30-40% strain range).

Figure 3 shows how annealing can be used to tune the tensile behaviour. These results are obtained on a Type 1 structure but similar results are obtained on the other structures. For a given annealing time (1 hour), an increase in the annealing temperature from 130°C to 190°C increases the stiffness of the device up to five times (Types 5 to 7). A similar effect is obtained for shorter annealing times as shown with Type 9 (lOmin). However, a temperature that is too low and a time that is too short can have limited effect (Type 8).

Figure 4 shows the tensile behaviour for a fiber assembly coated by a PVA hydrogel and cross-linked with one freezing-thawing cycle (Type 10). In this case, the mechanical response is preserved after coating with a slight increase in stiffness at large deformation.

Tensile strength was measured by dividing the load at break by the initial section of the sample. Here the section is approximated to a disk of diameter 10mm for Types 1, 2, 5 to 10 and 11mm for Types 3 and 4. Ultimate tensile strain corresponds to the strain at break. All devices exhibited values of ultimate tensile load (load at break) and ultimate tensile strain that are appropriate for the replacement of ligaments or tendons. Table 2 below compares these mechanical properties derived from figures 2, 3 and 4 with preferred values for the replacement of ACL ligaments as estimated from the literature.

Table 2:

Ultimate Tensile strength, Ultimate Stiffness, N/% tensile load, N MPa tensile strain

Preferred values for 20 (native ACL)

1100-4000 15-50 >15%

ACL replacement 300 (tendon)

* Stiffness measured in the 30-40% strain range.

The ligaments obtained according to the invention exhibit the unique combination of a tensile behaviour close to that of native ligaments and tendons possibly including a very similar toe-region, a tensile strength greater than 20MPa corresponding to ultimate tensile loads greater than 1500N and an ultimate tensile strain greater than 15% as well as the biocompatibility of the selected materials (PVA hydrogel and UHMWPE).