WO1993006790A2

WO1993006790A2 - Tissue augmentation device and method

Info

Publication number: WO1993006790A2
Application number: PCT/US1992/006971
Authority: WO
Inventors: Craig L. Van Kampen
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1991-10-04
Filing date: 1992-08-24
Publication date: 1993-04-15
Also published as: AU2510192A; WO1993006790A3

Abstract

A bioabsorbable tissue augmentation device (10) designed to be used in parallel with biological connective tissue, such as a ligament or tendon, in the repair or reconstruction of biological connective tissue. The device (10) comprises a strap-like element (10) formed of a biocompatible material initially having sufficient mechanical properties at the time of implantation to support at least the working loads normally supported by the biological connective tissue being repaired or reconstructed. The strap-like element (10) comprises a braided structure (10) formed by braiding a plurality of yarns (12, 14), each of which has been formed by twisting together a plurality of fibers (16) to form the yarn (12, 14) with a twist rate of between about 20 and 150 turns per meter. The fibers (16) are formed of polylactic acid, preferably substantially enantiomerically pure poly-L-lactic acid. The strap-like element (10) is adapted for fixation at the ends thereof to biological connective tissue and/or the anatomical structures connected by the biological connective tissue, and the strap-like element (10) is adapted to gradually lose its load-bearing capability after implantation thereby gradually transferring increased loads to biological connective tissue.

Description

TISSUE AUGMENTATION DEVICE AND METHOD

Background of the Invention

The invention relates generally to tissue augmentation devices, and more particularly to a tissue augmentation device that is formed of generally biodegradable material.

The concept of tissue augmentation has been developed in order to protect transferred tissues used in ligament reconstruction by a load-sharing mechanism. Load-sharing is achieved by suturing an implant in parallel with the transferred tissue. An example of a tissue augmentation device is sold under the trade designation "3M KENNEDY LAD Ligament Augmentation Device" by the Minnesota Mining and Manufacturing Company, St.

Paul, Minnesota. That device is described in McPherson e al., "Experimental Mechanical and Histological Evaluation of the Kennedy Ligament Augmentation Device", Clinical Orthopedics and Related Research, Volume 196, pages 186-195 (June 1985) , which is incorporated herein by reference. As used herein, the term "LAD" means ligament augmentation device, and "TAD" means tissue augmentation device.

The "3M KENNEDY LAD" brand device comprises a generally flat diamond braided structure formed of untwisted polypropylene yarns that are braided at approximately ten picks per inch (394 picks/meter) . Each yarn in that LAD is formed of a plurality of untwisted filaments of approximately 13 denier per filament, and standard industrial techniques are used to control static. The polypropylene yarn of the "3M KENNEDY LAD" brand device has high tenacity (7.5 grams per denier), and the strength after braiding is maintained well (6.7 grams per denier) . Polypropylene is considered to be biocompatible, but not biodegradable or bioabsorbable.

While the "3M KENNEDY LAD" brand ligament augmentation device has met a need for a device to augmen the strength of repaired or reconstructed connective ^•tissue, there has been a desire for a similar type of device that would be "biodegradable" or preferably "bioabsorbable". One feature of a bioabsorbable device would be that the material of the device would absorb into the patient's body, leaving no implant material after an extended period of time. Another feature of both biodegradable and bioabsorbable materials is that they degrade over time while the biological connective tissue is healing, with the potential result bring that the strength of ligament augmentation device decreases in some intended relationship to the expected increase in strength of the biological connective tissue during healing.

Co-assigned U.S. Patent Nos. 4,759,765 and 4,834,752 (Van Kampen) , which are incorporated herein by reference, describe a tissue augmentation device for the repair or reconstruction of tendons and ligaments. That device includes one or more flat braided strap-like elements of stable biocompatible material connected in series with biodegradable elements. For example, the stable strap-like element may be formed of polypropylene, and the biodegradable element may be formed of polylactic acid. The biodegradable element of that device is designed to gradually lose its load-bearing capability after implantation in order to gradually transfer increased loads to the repaired or reconstructed tissue.

The strap-like elements of that construction may be formed by braiding nine or thirteen tows or bundles each containing 180 extruded polypropylene filaments. In many respects the braided strap-like elements described in that patent are similar to the braided structure of the "3M

KENNEDY LAD" brand ligament augmentation device described above.

European Patent Publication No. 0 241 252 (Benicewicz et al.) describes an artificial, absorbable ligament prosthesis formed of a polymer of L(-)lactide which may contain up to 10% glycolide as a co-monomer. The preferred structure of that prosthesis is described in the European publication as being an open structure which allows tissue ingrowth formed from "lightly-twisted" tows of "filaments having a 1 to 15 and preferably 2 to 8 twists per inch" (39-590 twists/meter and preferably 79-315 twists/meter) . The idea advanced in the Benicewicz et al. publication is that the prosthesis "will provide a scaffold to induce natural tissue to form along the filaments of the prosthesis. After the natural tissue has grown along the filaments and formed a new ligament or tendon, the filaments will be absorbed, and within some time period will no longer be present at the implant site or anywhere in the body."

U.S. Patent No. 4,792,336 (Hlavacek et al.) describes a flat braided ligament or tendon implant device having texturized yarns, and including an absorbable component comprising a glycolic or lactic acid ester linkage. The structure of that device includes a flat braid having primarily axial (quoit) yarns of an absorbable polymer, sleeve or carrier yarns consisting completely of absorbable material that are turned at about 1.4 turns per inch (55 turns/meter) , and possibly a non-absorbable component blended into the axial yarns.

U.S. Patent No. 4,795,466 (Stuhmer et al.) describes an artificial crucial ligament for a knee joint. That artificial ligament comprises a plurality of tubes, which may be of braided construction, disposed in concentric relation to define a stem, with alternating tubes extending from the stem to form to branches. The tubes of that arrangement are formed by a plurality of polyester threads, which in turn are formed either by twisting a large number (e.g., 60) of monofilaments to form multifilament threads in the case of the outermost layers, or by a single monofilament in^'the case of the innermost tubes.

U.S. Patent No. 4,932,972 (Dunn et al.) discusses a prosthetic ligament comprising a plurality of ultra high molecular weight polyethylene yarns. Each yarn is formed by parallel winding of at least fifty fibers (e.g., 120 fibers). The yarns of that device are formed into an 8-strand, 6-parallel wound plain braid ranging from 5-7 picks/inch (197-276 picks/meter) to provide an average gage section diameter of about 4-6mm.

U.S. Patent No. 4,773,910 (Chen et al.) describes a permanent ligament prosthesis intended to be used as a replacement of either the anterior or posterior cruciate ligament in a human knee. That prosthesis comprises two separately adjustable strands each comprising two tows twisted together to form the strand. The tows are formed by combining a number of individual yarns, which in turn are formed by a number (e.g., 118) of small individual filaments. The total number of filaments in each strand of that prosthesis is between 8,000 and 14,000, and the filaments are formed of polyolefins such as high molecular weight polyethylene and ultra high molecular weight polypropylene. The Chen et al. patent describes the yarns making up the strand as having "very little twist, i.e., a twist of less than 3 twist per inch and preferably is 0 twist per inch" (less than 118 twists/meter and preferably zero twists/meter) .

Summary of the Invention

The invention provides a biodegradable tissue augmentation device designed to be used in parallel with biological connective tissue, such as a ligament or tendon, in the repair or reconstruction of biological connective tissue. More specifically, the tissue augmentation device is designed for fixation to biological connective tissue and/or anatomical structures to augment the strength of the repaired or reconstructed connective tissue connecting the anatomical structures. The device is designed to gradually lose its load-bearing capability after implantation thereby gradually transferring increased loads to the biological connective tissue. The tissue augmentation device is preferably designed to be bioabsorbable so that the material of the device is substantially absorbed by a patient's body over a period of time. Generally, the tissue augmentation device of the invention comprises a strap-like element formed of a biocompatible material initially having sufficient mechanical properties at the time of implantation to support at least the working loads normally supported by the biological connective tissue being repaired or reconstructed. The strap-like element comprises a braided structure formed by braiding a plurality of yarns. Each yarn is formed by twisting a plurality of fibers together to form the yarn with a twist rate of between about 20 and 150 turns per meter. The fibers are formed of crystalline polylactic acid. The strap-like element is adapted for fixation at the ends thereof to the biological connective tissue and/or the anatomical structures connected by the biological connective tissue, and the strap-like element is adapted to gradually lose its load-bearing capability after implantation thereby gradually transferring increased loads to the biological connective tissue.

The method of making the tissue augmentation device generally comprises forming a plurality of fibers of crystalline polylactic acid. A plurality of the fibers are twisted together at a twist rate of between about 20 and 150 turns per meter to form each single yarn. The yarns are then braided to form a braided structure, and the braided structure is bonded and cut into discrete braided strap-like elements that are adapted for fixation at the ends thereof to the biological connective tissue and/or the anatomical structures connected by the biological connective tissue. Other features will be in part apparent and in part pointed out hereinafter.

Brief Description of the Drawing

The invention will be further described with reference to the drawing wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawing, and wherein: Figure 1 is a partial view of a knee joint illustrating a tissue augmentation device of the invention as implanted in a patient;

Figure 2 is a frontal view of a tissue augmentation device of the invention;

Figure 3 is an enlarged fragmentary frontal view of a portion 3-3 of the tissue augmentation view of figure

2;

Figure 4 is an enlarged view of a plied yarn of the type that would be braided together with other plied yarns to form the tissue augmentation device of figures 1 and 2;

Figure 5 is an enlarged view of a single yarn of the type that would be twisted together with other single yarns to form the plied yarn of figure 4;

Figures 6 and 7 are schematic illustrations of the "S" and "Z" directions, respectively, of twist; and

Figure 8 is a flow chart illustrating a method of making the tissue augmentation device of figures 1-3.

Detailed Description of a Preferred Embodiment

As shown in figure 1, a biodegradable tissue augmentation device 10 of the invention is designed to be used in parallel with biological connective tissue (not shown) , such as a ligament, tendon or a graft of a ligament or tendon, in the repair or reconstruction of biological connective tissue. (As used herein, "biological connective tissue" is intended to cover connective load-carrying tissue, and in particular includes ligaments and tendons. A "ligament" is connective tissue that connects two bones together to stabilize a joint, and a "tendon" is connective tissue that connects a muscle to a bone. The term "parallel" is not used in the strict geometric sense, but rather in the biomechanical sense of members sharing a common load.) The tissue augmentation device 10 of this invention is similar in some respects to the device described in co-assigned U.S. Patent Nos. 4,759,765 and 4,834,752 (Van Kampen) .

In use, the augmentation device 10 and the tissue are adjacent one another along their lengths, and there may be inter-twining between the two. Preferably, the tissue augmentation device 10 is sutured or otherwise secured to the biological connective tissue along its length.

The biological connective tissue used in conjunction with the augmentation device may be autogenic tissue from the patient or allogenic tissue from a donor or cadaver. In the most common ligament reconstruction practiced today, i.e., the reconstruction of the anterior cruciate ligament of the knee, the tissue used in the reconstruction is typically a portion of the patellar tendon. Other tissues such as the semitendinosus tendon, the rectus femoris tendon and fascia lata may also be used. In the case of ligament or tendon repair, the biological tissue used in conjunction with the device 10 of the invention is the damaged ligament or tendon itself which has been reapproximated by standard surgical techniques.

The term "reconstruction" refers to the reconstruction of the function of the damaged tissue by replacing it with other tissue, e.g., as discussed above with respect to the anterior cruciate ligament. The term "repair" refers to the repair of the actual damaged tissue.

The device 10 comprises a strap-like element (also 10) formed of a biocompatible material including polylactic acid, preferably consisting of essentially pure poly-L-lactic acid substantially without co-monomers such as glycolic acid and substantially without the D enantiomer. As used herein, the term "strap-like" is used broadly to connote flexibility, and although a preferred embodiment is flat in cross section, any cross-sectional geometry may be used. The strap-like element 10 initially has sufficient mechanical properties at the time of implantation to support at least the working loads normally supported by the biological connective tissue being repaired or reconstructed.

The strap-like element 10 comprises a plurality of yarns 12 braided together to form a flat diamond braid structure. Preferably, each yarn 12 constitutes a "plied" yarn 12 (figure 4) comprising a plurality of single yarns 14 (figure 5) that are twisted together in one of the S and Z directions to form the plied yarn 12. Each single yarn 14 may be formed by twisting together a plurality of fibers 16 at a twist rate of between 20 and 150 turns per meter in the other of the S and Z directions. In other words, each single yarn 14 is formed by twisting fibers 16 together in one direction (e.g., the S direction in figure 5) , and each plied yarn 12 is formed by twisting single yarns 14 together in the direction opposite to the twist of the fibers 16 (e.g., the Z direction in figure 4). The fibers 16 may be formed by extruding one or more filaments (not shown) of poly-L-lactic acid ("PLA") . As used herein, the "S" direction refers to a left hand direction of twist as illustrated in figure 6, and the "Z" direction of twist refers to a right hand direction of twist as illustrated in figure 7. A yarn has S-twist if, when held in a vertical orientation, the spirals conform to the direction of the slope of the central portion of the letter "S". A yarn has a Z-twist if, when held in a vertical orientation, the spirals conform to the direction of the slope of the central portion of the letter "Z". See, e.g., N. Hollen and J. Stadler, Textiles, particularly at page 99 (The Macmillian Company, New York, 3d ed. , 1968).

The term "diamond braid" refers to a braid in which the directions of the yarns 12 being braided forms a generally diamond shaped pattern where the yarns 12 pass over other yarns 12. See, e.g., the diamond-shaped pattern designated by the reference numeral 18 in figure 3. Preferably, the twist rate of each single yarn 14 is between about 40 and 125 turns per meter in one direction, and the twist rate of each plied yarn 12 is between about 20 and 100 turns per meter in the other direction. Most preferably, the single yarns 14 are twisted together at a twist rate between about 40 and 75 turns per meter to form each plied yarn 12.

Preferably, the braided structure has a pick rate between about 6 and 10 picks per inch (240-400 picks/meter) , most preferably between about 7 and 9 picks per inch (280-350 picks/meter) . As used herein, "pick" refers to crossing of one yarn over another in a braided structure. Picks are counted along the longitudinal direction of the braid. A braided structure is considered to be "tighter" the greater the number of picks per unit length.

As discussed above, the filaments are preferably extruded of material consisting essentially of poly-L- lactic acid, without co-monomers such as glycolic acid or the D enantiomer. Such enantiomerically pure poly-L-lactic acid is preferred for a number of reasons including the generally crystalline nature of the L enantiomer with the resulting strength of the fibers 16 under tension, and the greater resistance of the crystalline poly-L-lactic acid to hydrolytic degradation compared to the less crystalline DL form. As used herein, "crystalline" is intended to be a relative term indicating a material having much greater crystallinity than the polypropylene employed in the "3M KENNEDY LAD" brand device or the less crystalline poly-DL-lactic acid.

The relative purity of the L enantiomer of poly-L-lactic acid is indicated by itsOptical rotation, which is preferably approximately -156 degrees (at 25 degrees Celsius in chloroform, with the wavelength of the light being used in the measurement being 589 nanometers) . High purity of the L enantiomer is important in obtaining a sufficiently crystalline poly-L-lactic acid filament. The intrinsic viscosity of the preferred material is approxi ately 2.5 deciliters per gram at 25 degrees Celsius at a concentration of 0.5 milligrams per milliliter poly-L-lactic acid in chloroform. See also, e.g., F. Chabot et al., "Configurational Structures of Lactic Acid Stereocopolymers as determined by ¹³C{¹H} n.m.r.". Polymer, 24:53-59 (January 1983). Unless indicated otherwise, "pure" or "purity" mean chemically and enantiomerically pure, without heavy metal impurities, other impurities or co-monomers. While poly-L-lactic acid is preferred, it is contemplated that substantially enantiomerically pure poly-D-lactic acid could be used to form the fibers 16 used in the ligament augmentation device of this invention. Poly-D-lactic acid is also crystalline so long as it is kept enantiomerically pure. In any event, the process of die extrusion of the filaments results in molecular alignment of the crystalline, enantiomerically pure poly-L-lactic acid and of the crystalline enantiomerically pure poly-D-lactic acid. U.S. Patent No. 3,636,956 and T.H. Barrows, "Degradable Implant Materials: A Review of Synthetic Absorbable Polymers and Their Applications", Clinical Materials, 1:233-257 (1986) are incorporated herein by reference.

Suitable polylactic acid filaments have been produced by Gunze Limited, Kyoto, Japan. In this regard, European Patent Publication No. 0 431 188 (Suzuki et al.) is also incorporated herein by reference.

The strength retention of PLA fibers 16 to be employed in the invention may be tested by placing the fibers in a buffered saline solution at 37 degrees

Celsius, and comparing their tensile strength at selected intervals over time. Fibers 16 that can retain at least fifty percent of their original strength after six months in the saline solution are considered preferred for the ligament augmentation device 10.

As illustrated in figure 1, the strap-like element 10 is adapted for fixation at the ends 20 and 22 thereof to the biological connective tissue and/or the anatomical structures connected by the biological connective tissue. Preferably, in the case of a device 10 being used in the reconstruction of the anterior cruciate ligament, end 20 of the device is mounted along the anteromedial tibia 24, and the other end 22 of the device is mounted on the lateral femur 26, with the device 10 passing through a tunnel 28 drilled through the tibia 24. U.S. Patent Nos. 4,759,765 and 4,834,752, which are incorporated herein by reference, describe various methods for attachment of a ligament augmentation device that may be used in implanting the device 10 of this invention.

The method of making the bioabsorbable tissue augmentation device 10 of the type described above is shown generally in the flow chart of figure 8. First, as represented in step 100 of figure 8, a plurality of filaments are extruded preferably of material consisting essentially of substantially pure poly-L-lactic acid. A plurality (e.g., 12) of the extruded filaments are then brought together in step 102 to form each fiber 16. The filaments are preferably not twisted in forming the fibers 16. It is contemplated that a very long fiber (multiple thousands of meters) could be formed, and then cut to form a plurality (e.g., 24) of shorter but still long fibers. A number (e.g., 24) of the fibers 16 formed in step 102 are then twisted together in either the S or Z directions at a twist rate of between about 20 and 150 turns per meter to form a single yarn 14 in step 104. The single yarn 14 would conveniently be long enough to provide sufficient material such that when cut a plurality (e.g., 4) of single yarns 14 are formed. The length of each single yarn 14 formed in step 104 could be, for example, 1000 meters:

A plurality (e.g., 4) of single yarns 14 are then twisted together in the other of the S and Z directions (the opposite direction of twist to that used in forming the single yarns 14) to form each plied yarn 12 as indicated in step 106. If the length of the single yarns 14 is 1000 meters, the length of the plied yarn 12 formed in step 106 would also be approximately 1000 meters before the plied yarn 12 is cut to form a suitable plurality (e.g., 13) of plied yarns 12.

Step 108 indicates that the plurality of the plied yarns 12 are braided according to standard braiding techniques to form a braided structure. Steps 110 and 112 indicate bonding and cutting, respectively, of the braided structure into discrete braided strap-like elements 10 that are adapted for fixation at the ends 22 and 24 thereof to biological connective tissue and/or anatomical structures connected by biological connective tissue.

Example l PLA fiber was produced of extruded poly-L-lactic acid to form 12 filaments of 4.4 denier per filament.

Single yarns were prepared from 24 fibers with a twist of 100 turns per meter in the S direction. Four single yarns were then plied together to form a plied yarn with a twist of 40 turns per meter in the Z direction. The yarn helix angle of the single yarns was calculated to be approximately 8.8 degrees, and the helix angle of the plied yarn was approximately 5.9 degrees. Yarn balance was achieved with a 2.9 degree offset in the S direction. The tenacity of the plied yarn was 4.2 grams/denier.

Example 2 Thirteen plied yarns prepared according to Example 1 were loosely braided in a flat diamond construction with 8.5 picks per inch (335 picks/meter) . The tenacity of the braided construction was 2.9 grams/denier. The strength loss due to braiding was measured to be 30%.

Dynamic tensile testing was performed to l million cycles by alternately loading and unloading the braid between load levels of 50 Newtons and 500 Newtons at a rate of one cycle per second. This testing showed virtually no loss of strength due to fatigue, and only a limited amount of creep of 0.04% per decade. Bending strength was tested to 1 million cycles by flexing the braid between an angle of 15 degrees and 45 degrees across a blunt edge while under spring tension. As a consequence of this motion and action, the tension on the braid cycled between 150 and 300 Newtons while flexing at a rate of 4 cycles per second. The bending strength was lowered by fatigue in the amount of 8% after 1 million cycles.

Example 3 PLA fiber was produced with 12 filaments of 4.6 denier per filament. Single yarns were prepared from 24 fibers with a twist of 100 turns per meter in the S direction. Four single yarns were then plied together to form a plied yarn with twist of 75 turns per meter in the z direction. The yarn helix angle of the single yarns was calculated to be approximately 8.8 degrees, and the helix angle of the plied yarn was approximately 11.1 degrees. Yarn balance was achieved with a 2.3 degree offset in the S direction. Tenacity of the plied yarn was 4.4 grams/denier.

Example 4

Nine plied yarns prepared according to Example 3 were loosely braided in a flat diamond braid construction with 8.5 picks per inch (335 picks/meter). The tenacity of the braided construction was 3.4 grams/denier. The strength loss due to braiding was measured to be 23%. Several such braided devices were implanted to augment anterior cruciate ligament reconstructions in goats and sheep, and then compared to non-absorbable "3M KENNEDY LAD" brand ligament augmentation devices commercially available from Minnesota Mining and Manufacturing Company, St. Paul, Minnesota (Catalog No. 11340) . Analysis of the implanted materials after 3 and 6 months indicated generally equivalent mechanical performance between the invention and the control materials. No adverse tissue reaction was observed with the materials of this example. Example 5 PLA fiber was produced with 12 filaments of 4.7 denier per filament. Single yarns were prepared from 30 fibers with a twist of 125 turns per meter in the S direction. Three single yarns were then plied together to form a plied yarn with a twist of 75 turns per meter in the Z direction. The single yarn helix angle was calculated to be approximately 11.9 degrees, and the helix angle of the plied yarn was approximately 9.9 degrees. Yarn balance was achieved with a 2.0 degree offset in the S direction. Tenacity of the plied yarn was 4.0 grams/denier.

As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing be interpreted as illustrative and not in a limiting sense.

Claims

CLAIMS :

1. A biodegradable tissue augmentation device (10) designed to be used in parallel with biological connective tissue, such as a ligament or tendon, in the repair or reconstruction of biological connective tissue, the device (10) comprising a strap-like element (10) formed of a biocompatible material initially having sufficient mechanical properties at the time of implantation to support at least the working loads normally supported by the biological connective tissue being repaired or reconstructed, the strap-like element (10) comprising a braided structure (10) formed by braiding a plurality of yarns (12) , each yarn (12) being formed by twisting a plurality of fibers (16) together to form the yarn (12) with a twist rate of between about 20 and 150 turns per meter, the fibers (16) being formed of polylactic acid, the strap-like element (10) being adapted for fixation at the ends thereof to the biological connective tissue and/or the anatomical structures connected by the biological connective tissue, the strap-like element (10) being adapted to gradually lose its load-bearing capability after implantation thereby gradually transferring increased loads to the biological connective tissue.

2. A device (10) according to claim 1 wherein the fibers (16) are twisted together at a twist rate between about 40 and 125 turns per meter to form the yarn.

3. A device (10) according to claim 2 wherein the braided structure (10) has a pick rate between about 240 and 390 picks per meter.

4. A device (10) according to claim 3 wherein the pick rate for the braided structure (10) is between about 280 and 350 picks per meter.

5. A device (10) according to claim 1 wherein each yarn (12) constitutes a plied yarn (12) formed by twisting together a plurality of single yarns (14), the single yarns (14) being formed by twisting together the plurality of fibers (16) .

6. A device (10) according to claim 5 wherein the fibers (16) are twisted together in one of the S and Z directions to form each single yarn (14) , and the plurality of single yarns (14) are twisted together in the other of the S and Z directions to form a plied yarn (12) .

7. A device (10) according to claim 6 wherein the single yarns (14) are twisted together at a twist rate between about 20 and 100 turns per meter to form a plied yarn (12) .

8. A device (10) according to claim 7 wherein the single yarns (14) are twisted together at a twist rate between about 40 and 75 turns per meter to form a plied yarn (12) .

9. A device (10) according to claim 8 wherein each fiber (16) comprises at least one filament, each filament consisting essentially of polylactic acid selected from the group comprising substantially enantiomerically pure poly-L-lactic acid and substantially enantiomerically pure poly-D-lactic acid.

10. A device (10) according to claim 9 wherein the braided structure (10) comprises a flat diamond braided structure (10), the fibers (16) ^'being formed by bioabsorbable filaments consisting essentially of crystalline poly-L-lactic acid having an intrinsic viscosity of approximately 2.5dl/g measured in 0.5mg/ml concentration solution of poly-L-lactic acid in chloroform at 25 degrees Celsius, and an optical rotation of approximately -156 degrees measured in chloroform at 25 degrees Celsius with a light wavelength of approximately 589 nanometers, the filaments having a tenacity of approximately 4.5 grams per denier and at least 50% strength retention after six months exposure to physiological conditions.

11. A method of making a biodegradable tissue augmentation device (10) of the type that is designed to be used in parallel with biological connective tissue, such as a ligament or tendon, in the repair or reconstruction of biological connective tissue; the method comprising:

(a) providing a plurality of fibers (16) of polylactic acid; (b) twisting a plurality of the fibers (16) together at a twist rate of between about 20 and 150 turns per meter to form a yarn (12, 14);

(c) providing a plurality of yarns (12, 14) formed in step (b) ; (d) braiding the plurality of the yarns (12,

14) to form a braided structure (10) ; and

(e) bonding and cutting the braided structure (10) into discrete braided strap-like elements (10) that are adapted for fixation at the ends thereof to the biological connective tissue and/or anatomical structures connected by the biological connective tissue; the braided strap-like element (10) having sufficient mechanical properties at the time of implantation to support at least the working loads normally supported by the biological connective tissue being repaired or reconstructed, the strap-like element (10) being adapted to gradually lose its^' load-bearing capability after implantation thereby gradually transferring increased loads to the biological connective tissue.

12. A method according to claim 11 wherein the step (b) of twisting a plurality of fibers (16) together comprises twisting the plurality of fibers (16) together at a twist rate of between about 40 and 125 turns per meter.

13. A method according to claim 12 wherein the step (d) of braiding a plurality of the yarns (12, 14) comprises braiding the yarns (12, 14) such that the braided structure (10) has a pick rate between about 240 and 390 picks per meter.

14. A method according to claim 13 wherein the step (d) of braiding a plurality of the yarns (12, 14) comprises braiding the yarns (12, 14) such that the pick rate for the braided structure (10) is between about 280 and 350 picks per meter.

15. A method according to claim 11 further comprising the step of twisting a plurality of the yarns (14) formed in steps (b) and (c) , which yarns constitute single yarns (14) , to form a plied yarn (12) ; and providing a plurality of plied yarns (12) formed in the immediately preceding step; the step (d) of braiding a plurality of the yarns (12) to form a braided structure (10) comprising braiding a plurality of the plied yarns (12) to form the braided structure (10) .

16. A method according to claim 15 wherein the step (b) of twisting the plurality of fibers « (16) together to form a single yarn (14) comprises twisting the fibers (16) together in one of the S and Z directions to form each single yarn (14) , and step of twisting a plurality of single yarns (14) together to form a plied yarn (12) comprises twisting the single yarns (14) together in the other of the S and Z directions to form a plied yarn (12) .

17. A method according to claim 16 wherein the step of twisting a plurality of single yarns (14) together to form a plied yarn (12) comprises twisting the single yarns (14) together at a twist rate between about 20 and 100 turns per meter to form a plied yarn (12) .

18. A method according to claim 17 wherein the step of twisting a plurality of single yarns (14) together to form a plied yarn (12) comprises twisting the single yarns (14) together at a twist rate between about 40 and 75 turns per meter to form a plied yarn (12) .

19. A method according to claim 18 wherein the step (d) of braiding the plied yarns (12) to form a braided structure (10) comprises braiding the plied yarns (12) to form a flat diamond braided structure (10) .

20. A method according to claim 11 wherein the step (a) of providing a plurality of fibers (16) of polylactic acid comprises extruding a plurality of filaments consisting essentially of crystalline polylactic acid selected from the group comprising substantially enantiomerically pure poly-L-lactic acid and substantially enantiomerically pure poly-D-lactic acid, and bringing the filaments together to form each fiber (16) .

21. A method according to claim 20 wherein the step of extruding a plurality of filaments consisting essentially of crystalline polylactic acid selected from the group comprising substantially enantiomerically pure poly-L-lactic acid and substantially enantiomerically pure poly-D-lactic acid comprises the step of extruding the filaments through a die of substantially enantiomerically pure poly-L-lactic acid having an intrinsic viscosity of approximately 2.5dl/g measured in 0.5mg/ml concentration solution of poly-L-lactic acid in chloroform at 25 degrees Celsius, and an optical rotation of approximately -156 degrees measured in chloroform at 25 degrees Celsius with a light wavelength of approximately 589 nanometers.