CA2060635A1 - Bioabsorbable medical implants - Google Patents

Abstract

ABSTRACT A completely bioabsorbable reinforced composite material for use in medical implants and a method for making same. The composite material comprises a hybrid yarn of intimately co-mingled reinforcement fibers of a crystalline polymer and matrix fibers of a polymer having a glass transition temperature below the melting point of the crystalline polymer. The hybrid yarn is heated under pressure to a processing temperature between the glass transition temperature of the matrix fibers and the melting point of the crystalline polymer to form a continuous matrix with reinforcing fibers of crystalline polymer. The composite material may be formed by a two step consolidation process.

Description

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ll44 . _ BTOABSORBABLF, MEDICAL IMPLANTS

BACKGROUND OF T~IE TNVENTION
1. F;eld Of The Invention This invention relates to bioabsorbable composite materials for use in medical implants and a method for making same.

2 Background Of The Art It is a common surgical operation to attach a reinforcing rod or plate to a fractured bone so that the brolcen ends may be stabilized to promote healing. Metal implants have often been used because of their strength. However, it is necessary to perform another operation to remove metal implants after the bone has healed. The metal, being stiffer than the bone, becomes the primary load bearing member thereby protecting the bone from stress. Moderate stress is beneficial to bone tissue, though, and if the rnetal implant is not removed, the extended stress protection will cause the bone to atrophy through decalcification or osteoporosis. This eventually results in weakening of the bone.
One way to eliminate the necessity of a second operation and the harm of stress protection is to use a bioabsorbable polymer implant. As the bone heals over a period of time, the implant is gradually absorbed by the body and the mechanical stress of daily activity and exercise is gradually reapplied to the bone.
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~: Bioabsorbable materials are also usefill for other surgical applications, as for example, in making pins, rods, staples an~l fasteners for a variety of medical and orthopedic applications. One problem associated with bioabsorbable surgical devices, however, is that the material of construction is not nearly as strong as metal.
Various methods of reinforcement have been evaluated in an attempt to increase the strength of such bioabsorbable irnplants. One method has been to incorporate a strong non-absorbable fiber or filler, such as carbon fiber, as a rein~orcing agent in a bioabsorbable polymer matrix. The disadvantage of such a system is that the non-absorbable fiber remains in the body tissue and may cause histological reacLion or other undesirable effects in the long term.
One approach has been to employ a bioabsorbable polymeric material reinforced with a di~ferent polymeric material. For example, U.S. Patent No. 4,279,249 discloses a matrix of polylactic acid or a ~-copolymer thereof reinforced by fibers, threads or other reinforcement elements of polyglyolic acid or a copolymer thereof. The implant is made by providing alternating layers of the polymer constituting the matrix and the reinforcing elements, compressing the layers under pressure at a suitable temperature and then rapidly cooling the composite. Another approach has been to employ self-reinforced bioabsorbable polymeric materials, as described, for example, U.S. Patent No. 4,743,257. More particularly, V.S. Patent No. 4,743,257 discloses rnethods whereby a finely milled polymeric powder is mixed with ~ibers, threads or cor-esponding reinforcement units of the same polymeric material or an isomer thereof. In one method, the mixture is heated such that the ~ ~
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finely milled particles are softened or melted, but the reinforcement material is not significantly softened or melted. The reinforcement material thus forms a matrix phase which binds the reinforcement uni~s to form a self-reinforced composite., In alternative methods, fibers, threads or corresponding reinforcement units are combined with a polymeric melt of like material, or absorbable fibers, threads or corresponding structures may be heated in a pressurized mold such that the outer surface softens or melts, thereby acting as a matrix.
Despite these known methods~ however, a method for producing a bioabsorbable material having suitable in ViYo strength and absorption properties in an efficient and controlled manner is needed.

SUMMARY O~ THE INVENTION
Provided herein is a method for making a bioabsorbable composite material for use in medical implants. The method comprises placing a plurality of reinforcement fibers and matrix fibers together in close contigui~r. The reinforcement fibers comprise a substantially crystalline bioabsorbable polymer characterized by a melting point, and the matrix fibers comprise a bioabsorbable polymer charac~erized by a -glass transition temperature which is lower than the melting point of the reinforcing fiber. The closely placed f;bers are then heated under an applied processing pressure to a processing temperature below the melting ~ ~ -point of the reinforcement fiber, such that the matrix fibers soften and ~ ;
flow so as to form a matrix around the reinforcement fibers. After being held at the processing temperature and pressure for a period of time sufficient for the matrix fibers to flo~ around the reinforcement ,...... .. ..... ...... .. .. . . . . . ... .. ..

., , fibers, the resultant reinforcement fiber-containing matrix is permitted to cool. The composite material formed by this process is bioabsorbable and exhibits excellent in vitro strength.
The present invention may further involve formation of the composite material by a two step process wherein the reinforcement and matrix fibers are partially consolidated in a first processing step and then further consolidated in a second processing step.

BRIEF DESCR~PI'ION OF THE DRAWINGS
Fig. 1 is a perspective cross-sectional view of the bioabsorbable composite material before being processed;
Fig. 2 is a perspective cross-sectional view of the bioabsorbable composite material after being processed;
Figs. 3, 4a and 4b are photomicrographs illustrating cross sections of the hybri~ yarn under various degrees of consolidation; and, Figs. 5 to 20 graphically represent data obtained in the examples below.

DETAIL~D DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention described herein is a method for making a completely bioabsorbable reinforced composite material. l'he material can be used in highly loaded applications and does not suffer from the bio-incompatibility of carbon fiber reinforced materials.
Fig. 1 illustrates a bund~e of fibers or yarn prior to being processed into a bioabsorbable composite material. Fig. 2 illustrates the bioabsorbabie composite material prepared in accordance with the .. : . .

- ~r~ s method of the present invention. Reinforcing fibers 2 comprise a bioabsorbable polymeric resin such as, for example, a copolymer of polylactic acid (PLA) an~l polyglycolic acid (PGA). The relative proportion of the components may be chosen to suit the surgical application. For example~ under identical processing conditions, polyglycolic acid is typiccllly the stronger of the two components and more crystalline. However, polyglycolic acid is more rapidly absorbed by body tissue. Hence for surgical applications where it is desired to maintain the implant strength over a longer period of time, the fiber will typically contain more polylactic acid. The fibers can be fibers of the type used in manufactllring suture material.
This invention contemplates that the reinforcing fibers 2 be substantially crystalline. Therefore, the reinforcing material is first spun into fiber form, and then processed to increase its crystallinity, as for example by drawing the fibers as is known in the art to orient the fiber, thereby increasing the crystallinity and strength of the fibers.
As used herein, substantially crystalline fibers are characterized by a crystallinity of at least IO~o by weight. Being a crystalline fiber, it is thermally stable until the melt temperature of the crystals is achieved.
The matrix fibers 3 are fabricated from a bioabsorbable polymer, copolymer, or blend of polymers and/or copolymers such as polyglycolic acid/polylactic acid copolymer blended with polycaprolactone.
The composition of the matrix ~Ibers 3 includes a bioabsorbable ;
component which has a melting point or glass transition temperature below the processing temperature ~i.e. below the melting point of-the ;`
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-reinforcing fibers). Suitable relatively low melting point components include polycaprolactone, polytrimethylene carbonate, polydioxanone, polyorthoesters, copolymers of polycaprolactone and lactic acid and/or glycolic acid, copolymers of polytrimethylene carbonate and lactic acid and/or glycolic acid, and copolymers of polydioxanone and lactic acid and/or glycolic acid. The addition of these materials also has been found to improve the impact resistance of articles fabricated therefrom.
A preferred blending component is polycaprolactone, which has amelting point of about 60~C. Thus, for example, at typical process;ng temperatures for reinforcing and matrix fibers of PGA/PLA copolymers, a low melting point component in the matrix fiber such as polycaprolactone is in a molten state and thereby facilitates flow of the matrix material. Another preferred flow agent is polytrimethylene carbonate.
The matrix fibers may be substantially amorphous and left in the as-spun condition rather than drawn as were the reinforcing fibers.
Amorphous polymers are typically characterized by a glass transition temperature rather than a melting point. Above the glass transition temperature the amorphous polymer acts as a highly viscous fluid: it can be made to flow under high pressure. Thus the glass transition ternperature of the matrix fibers will be below the melting point o the reinforcing fibers.
The matrix fibers 3 and reinforcing ~Ibers 2 are combined to form a hybrid yarn 1 in which the matrix and reinforcing fibers 2 are intimately co-mingled and in close contiguity. For example, one or more matrix fibers 3 and one or more reinforcing fibers 2 may be pl;ed - - - . ~ . . - . , ~ .

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. together. Close physical approximation of the matrix fibers 3 to the reinforcing fibers 2 enables the matrix to flow around and substantial]y encompass the reinforcement fibers 2 with minimal flow distance. The matrix material is thereby converted from the form of individual fibers 3 to a single mass 3a.
The hybrid yarn can be made into unidirectional tapes, woven broad goods, and roving suitable for filàment winding, pultrusion, laminating, braiding, etc. l'referably, the reinforcement material constitutes 50-65% volume ~o of the hybrid yarn 1. The deniers of the matrix and reinforcing fibers are not critical. Typical matrix fiber deniers range from 60 to 300, and typical reinforcement fiber deniers range from 60 to 30û. The relationship of the deniers of the respective fibers is typically from about 1:2 to 2:1 reinforcement fiber 2 to matrix fiber 3, and preferably about 1:1 to 1.5:1.
In a preferred embodiment of the present invention, the hybrid yarn 1 is treated to remove monomer and other impurities prior to processing. This treatment is typically accomplished by heating the hybrid yarn 1 under vacuum until monomer and/or impurities have been removed.
Typical processing conditions for monomer and impurity removal include processing temperatures of from about 75' to about 1200C and vacuum conditions such that vapori2ed monomers and/or impurities are exhausted from the system, e.g., pressures of from about 50 to about lS0 Torr, optionally with an inert gas purge, e.g., dry nitrogen. The processing time will vary depending on the initial purlty of the material - , ~

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being treated and the quantity of such material, but treatment times on the order of approximately 48 hours are typically employecl.
After the matrix fibers 3 and reinforcing fibers 2 are cornbined to form the hybrid yarn 1 and optionally treated to remove monomer impurities, the hybrid yarn 1 is heated to a processing temperature which is above the glass transition temperature of the matrix fibers 3 and below the melting point of the reinforcing fibers 2. Pressure is applied to induce flow of the matrix material such that it surrounds the reinforcing fibers 2, thereby transforrning the hybrid yarn 1 into a unitary composite mass.
The processing conditions for the reinforcing and matrix materials described above will depend on the polymeric composition of the individual fibers, and particularly the matrix fibers 3. Thus, for es~ample, if the matrix fibers 3 and reinforcing fibers 2 are 18~o PGA, 82%
PLA, then the glass transition temperature is approximately 600C and the melt temperature is about 1600C. For a hybrid yarn 1 which comprises the aforesaid matrix and reinforcing fibers 2, typical processing conditions include a temperature above about 600C and below about 1600C, and typically a temperature of from about 700C to about 1400C~ Typical processing pressures are from about 0 psig to about 600 psig, and preferably about 300 to about 500 psig and more preferably about 400 psig.
As can readily be seen, the method of the present invention allows processing temperatures far below the melting point of the reinforcing fibers 2. Consequently, this reduces danger of weakening the ~-reinforcing fibers 2 by melting.

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~, After the hybrid yarn 1 has been processed for a sufficient period of time, it is typically cooled to ambient temperature while at the processing pressure. The processing pressure is maintained during the cooling step because as the hybrid yarn 1 is cooled, the fibers have a tendency to shrink. By maintaining pressure, this tendency is inhibited.
As the hybrid yarn 1 cools, the matrix material 3a solidifies.
There is no need for a rapid cooling because the procçssing temperature employed need not approach the melt temperature of the reinforcing fibers 2. Typically, the hybrid yarn 1 of the present invention is force cooled, e.g., by passing air and/or a cooling fluid through the platens of the processing equipment.
The processing time for tlle hybrid yarn 1 depends on several factors which include the number, mass and geometry of the hybrid yarns 1 being processed, the processing temperature and pressure employed, the compositions of the matrix fibers 3 and reinforcing fibers 2, and the degree to which the hybrid yarn 1 is to be consolidated, i.e., the degree -to which void spaces in the hybrid yarn 1 are to be eliminated.
Typically, the processing equipment includes a vacuum source to withdraw vapors from the hybrid yarns 1 and to thereby prevent degradation (e.g., due to the presence of impurities or moisture) and to assist in the consolidation thereof.
Although it is possible to produce a finished medical irnplant in a single processing step according to the present invention, in an alternate process to produce medical implants, and particularly medical implants having non-rectilinear or curved geometrics, a two step process - can be used. This two step method involves placing a plurality of matrix ~ 1 -..
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fibers 3 and reinforcing fibers 2 (hybrid yarn) in close contiguity, processing the hybrid yarn 1 at a processing temperature and press~lre, and under vacuum, for a period of time to partially consolidate the hybrid yarn 1, e.g., for from 3 to 10 minutes, cooling the hybrid yarn 1 to ambient temperature, and removing the pressure therefrom. Processing conditions for partial consolidation of the hybrid yarn 1 can include a temperature of from about 600C to about 1600C and an applied mechanical pressure of from about 0 to about 600 psig, preferably about 300 to about 500 psig, and more preferably about 400 psig. The partially consolidated hybrid yarn 1 is then ~urther consolidated in a second processing step.
Prior to entering the second processing step, the hybrid yarn 1 may be cut into multiple pieces such that the hybrid yarn 1 may be efficiently placed in a compression mold, e.g., a mold for a pin, rod, or staple, or such that pieces of the yarn may be stacked to form a multiple layer product. The two step process is thus particularly advantageous where the desired medical implant is to be produced in a compression mold. The fact that the hybrid yarn 1 is only partially consolidated upon entering the compression mold allows freer movement of the matrix material relative to the reinforcing fibers 2 than would be possible had the hybrid yarn 1 been fully consolidated in the initial step.
The processing conditions for the second processing step are typically the same as are employed in the initial processing step, although as will be apparent to one of ordinary skill in the art, pressure applied to a part in a compression mold is difficult, if not irnpossible, to quanti~y. Total applied mechanical force may be as high as 30,000 pounds.

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,, , ;. : , -: It is also diff;cult to quantify tbe degree to which a hybrid yarn 1 has been consolidated at any given time according to the present invention. Cross-sectional SEM photograp}ls give some indication of the degree to which void spaces exist in a hybrid yarn 1. As used herein, the term "partially consolidated hybrid yarn" means a hybrid yarn that has had a substantial percentage of its void spaces removed, e.g., at least 10% of the original void space eliminated. Typically, the initial processing step is believed to remove from about 30% to about 70% of the void space originally present in the hybrid yarn 1, but as noted above, the percentage is difficult to measure. The second processing step typically removes at least about 10~o of the remaining voids in the partially consolidated hybrid yarn.
Fig. 3 is a photomicrograph showing a cross section of an unconsolidated hybrid yarn at 200x magnificatio~
Fig. 4a is a photomicrograph showing a cross section of a partially consolidated hybrid yarn at 200x magnification.
Fig. 4b is a photomicrograph showing a cross section of a pin at 50x magnification after two consolidations.
As can be seen from the above photomicrographs the partial consolidation removes at least some of the void spaces. After two consolidations, void spaces have been removed to a degree such that individual fibers cannot be distinguished.
The description thus far h~s been directed to the processing of a single hybrid yarn 1 for purposes of clarity. However, the method of the present invention is preferably employed to simultaneously process multiple hybrid yarns 1. Thus, individual hybrid yarns 1 may be oriented 11 i c~

: unidirectionally, as for example, by winding around an aluminum paddleand by pultrusion or they may be oriented in various directions as for exarnple, by weaving into broad goods, by filament wind;ng, by laminating biased ply lay ups, by braiding, etc. It is also within the scope of the present invention to orient the fibers unidirectionally and transversely at various cross-over angles in one or more succeeding layers.
Any bioabsorbable polymeric material or combination of polymeric materials may be employed as matrix fibers 3 and rein~orcement fibers 2 provided a differential exists between the glass transition temperature of the matrix fibers 3 and the melting point of the reinforcement fibers 2. Thus, for example, polymers and copolymers of such synthetic bioabsorbable materials as polylactic acid, polyglycolic acid, polydioxanone, polytrimethylene carbonate, poly(ethylene carbonate), poly~iminocarbonates), polycaprolactone, polyhydroxybutyrate, polyalkylene oxalates, polyalkylene succinates, polytmaleic acid), poly(1,3-propylene malonate), poly(ethylene terephthalate), or poly(amino acids) rnay be employed. Selection of a suitable absorbable material will depend on such factors as the desired in vivo strength properties and absorption rate for the medical implant being processed.
A further embodiment of the present invention involves the use of matrix fibers of differing composition, e.g., a percentage of individllal fibers having a high glycolide content and a percentage of fibers of low glycolide content, or the use o~ reinforcing fibers of differing composition, or both. The selection of individual matrix and/or reinforcing fiber compositions may be made in such case to achieve 1~

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~r~ s the desired absorption rate and tensile strength in the composite rnaterial.
The implant material produced according to the present invention is strong, yet sompletely bioabsorbable. The use of fibers for both the matrix and the reinforcing elements of the hybrid yarn 1 ensures that the matrix and reinforcing elements of the yarn are in close contiguity, and permits efficient and controllable processing of the hybrid yarns 1 into high strength bioabsorbable medical implants. Further details may be derived frorn the examples below.

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~ EXAMPLE 1 An absorbable composite was mamlfachlred by combinin~ a reinforcement fiber made from a copolymer of 18% glycolic acid and 82%
lactic acid with a matrLx material consisting of a blend of ~he above copolymer with 20% polycaprolactone (PCI,). The as-spun matrix yarn was 221 denier and the four reinforcement yarns were 87.7 denier. The reinforcement fiber was drawn so as to increase its tenacity and modulus, while the matrix material was utilized in as-spun fiber form. The two yarns were ~hen co-wound onto a flat aluminum paddle. The reinforcement yarn represented 61.1% of the material by volume. The paddle was then compression molded in a vacuum press for about 5 minutes at 110oC and approximately 400 psi pressure to facilitate flow of the matrix material about the reinforcement fibers. The fiber was then cooled to ambient temperature. The temperature of the processing is below the melt temperature of the reinforcement fibers. The resultant material was a unidirectional reinforced thermoplastic material suitable for the manufacture of a variety of medical products.

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_XAMPLE 2 An absorbable composite was manufachlred by combining a drawn reinforcement fiber multifilament yarn made from a copolymer of 18%
glycolic acid and 82% lactic acid and an as spun fiber of the same material. Five spools of drawn reinforcement fibers having an average denier of 85.4 each were combined with one as-spun yarn of 338.8 denier.
The reinforcement fiber constituted 56.6% of the material by volume. The material was wrapped onto a flat alum;num paddle and consolidated at llOoC and approximately 400 psi pressure. The result was a unidirectional reinforced thermoplastic material suitable for laminating.

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t ~2~'~J .~'"~5 ' EXAMPLE 3 Two larninae of Example 1 are stacked one upon the other and placed in a compression mold with a cylindrical cavity of 0.051" (liameter and 2.0" length. The mold was placed intu a vacuum press moidetl at l10~C
for five minutes under 20,000 Ibs. of f'orce, and cooled to 300C while under pressure. The cylindrical pins were tested in flexure and shear with the following results: Young's Modulus - 769.5 Ksi, Flexural Strength'- 28.0 Ksi, Shear Strength - 15.4 Ksi. ~or comparison purposes an injection molded pin of a copolysner consisting of 18% glycolic acid and ~2% lactic acid was similarly tested with the following results:
Young's Modulus - 466.2 Ksi, Flexural Strength - 19.1 Ksi, Shear Strength 7.6 Ksi. Several pins were molded at 110nC then placed in buffered solution for in v Q testing. The results can be seen in Figs. 5, 6 and 7.
All flexural tests for this and succeeding examp}es were accornplished using four point bend configuration with a 16:1 span to depth ratio, a 3:1 span to load nose ratio, and a cross head rate of 0.50"/min. All shear testing was accomplished using a double shear fixture which sheared the pin across its cross section, at 0.20 inches per minute.

. I!, ~r~ s ' EXAMPLE 4 Two laminae of Example 2 were stacked one upon the othel and placed in a compression mold with a cylindrical cavity of 0.051" di~meter and 2.0" lengtb. The cylindrical pins were tested in flexure and shear with the following results: Young's Modulus - 928.1 Ksi, Flexural Strength - 34.1 Ksi, Shear Strength - 18.1 Ksi. For comparison purposes an injection molded pin of a copolymer consisting of 18% glycolic acid and 82% lactic acid was similarly tested with the following results:
Young's Modulus - 466.2 Ksi, Flexural Strength - 19.1 Ksi, Shear Strength 7.6 Ksi. Several pins were molded at 110oC then placed in buffered solution for in vitro testing. The results can be seen in Figs. 8! 9 and 10. '' , `''' ~.

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Two larninae of Example 2 were placed one upon the other in a compression mold with a rectangular cavity with complicated end geometries. The material was molded at 110~C with pressure in order to demonstrate molding characteristics. The part was later tested in flexure. The Young's Modulus was determined to be 1.191 Msi and the specimen withstood 18.5 Ksi flexural stress without yielding.

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2~ 'r?,5 ~, EXAMPLE 6 An absorbable composite was manufactured by combining a drawn reinforcement fiber multifilament yarn made from a copolymer of 18%
glycolic acid and 82% lactic acid and an as spun fiber multifilament yarn manufactured ~rom a blend consisting of 75% of the above copolymer and 25% polycaprolactone. The reinforcement fiber constituted 53.6% by weight of ~he total composite yarn. ~e composite yarn was then wrapped onto an aluminum plate, partially consolidated at 500 psi and 110oC and later laminated and compression molded into small pins measuring 0.051"
in diameter and 2.0" in length.
A second batch of the mat`erial was manufactured as described above, with the exception that the wound aluminum paddle was subjected to a monomer/impurity removal treatment cycle ("post treatment") of 110~C
and partial vacuursl of 80 Torr, with a 6 cfm flow of dry nitrogen for a period of 48 hours in order to remove residual monomers from the material. After post ~reatment, the paddle was partially consolidated and srnall pins were compression molded as described aboYe.
Both sets of pins were then placed in buffered solution for in Vitro testing. The results can be seen in Figs. Il, 12 and 13.

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- ; _XAMPLE 7 An absorbable composite was manufactured by combining a drawn rein~orcement fiber multifilament yarn made from a copolymer of 18%
glycolic acid and 82% lactic acid and an as spun fiber multifilament yarn manufactured from the same copolymer. The reinforcement fiber constituted 54.8~o by weight of the total composite yarn. The composite yarn was then wrapped onto an aluminum plate, partially consolidated at S00 psi and 110oC and later laminated and compression molded into srnall pins measuring 0.051" in diameter and 2.0" in length.
A second batch of the material was manufactured as described above, with the exception that the wound aluminum paddle was subjected to a monomer/impurity removal treatment cycle ("post treatmen~") of 110~C
and partial vacuum of 80 Torr with 6 cfm flow of dry nitrogen for a period of 48 hours in order to remove residual monomers from the materiaL After post treatment, the paddle was partially consolidated and small pins were compression molded as described above.
Both sets of pins were then placed in buffered solution for in Vitro testing. The results can be seen in Figs. 14, lS and 16.

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, : ,: . : ,. ,.. -- I EXAMPI,E 8 An absorbable composite was manufactured as follows: The matrix material was made of an as spun yarn consisting of a blend of 80U/o trimethylene carbonate and 20% lactic acid copolymer introduced into a copolymer of 18~o glycolic acid and 82~o lactic acid in a ratio of 25% to 75~o respectively. This as spun multifilament yarn was combined with four yarns of drawn reinforcement material consisting of a copolymer of 18%
glycolic acid and 82% lactic acid and then wound around an alumimlm paddle. l~e reinforcement material constituted 56.0~o weight of the total hybrid yarn. The wound paddle was post treated to remove monomers and vacuum compression molded at 110'C into partially consolidated flat stock. Two pieces oE the resulting material were placed in each cylindrically shaped mold cavity and the entire mold was likewise vacuum compression molded at 110'C to form pins measuring 0.052 in diameter and 3.5" in length.
The pins were then cut to a length of approximately 1.5".
Several pins were ETO (ethylene oYide) sterilized and then placed in Sorenson's Buffer solution at 37'C while another set of pins were placed , -directly in Sorenson's Buffer solution at 375C without sterilization.
The results can be seen in Figures 17, 18, 19 and 20 showing the sterilized and non-sterilized pins.
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Claims (23)

1. A method for making a bioabsorbable composite material for surgical implants, said method comprising:
a) providing a reinforcing fiber comprising a substantially crystalline bioabsorbable polymer characterized by a melting point, and a matrix fiber comprising a polymer characterized by a glass transition temperature which is lower than the melting point of the reinforcing fiber, b) placing said reinforcing and matrix fibers together in close contiguity to form a hybrid yarn, and c) heating said hybrid yarn under an applied processing pressure to a processing temperature below said melting point of the reinforcement fiber and above said glass transition temperature of the matrix fiber to consolidate said hybrid yarn to form a bioabsorbable composite material.

2. The method of Claim 1 wherein each of said reinforcing fiber and said matrix fiber individually is spun from a polymer selected from the group consisting of polylactic acid, polyglycolic acid, polydioxanone, polytrimethylene carbonate, poly(ethylene carbonate), poly(iminocarbonates), polycaprolactone, polyhydroxybutyrate, polyalkylene oxalates, polyalkylene succinates, poly(maleic acid), poly(1,3-propylene malonate), poly(ethylene terephthalate), poly(amino acids), and copolymers thereof.

3 The method of claim 2, wherein said reinforcing fiber is processed to increase its crystallinity after being spun into fiber form.

4 The method of claim 2, wherein the matrix fiber is substantially amorphous.

5. The method of claim 1, wherein the matrix fiber further comprises a bioabsorbable component having a melting point or glass transition temperature below said processing temperature.

6. The method of claim 5, wherein said bioabsorbable component is selected from the group consisting of polycaprolactone, polytrimethylene carbonate, polydioxanone, polyorthoesters, copolymers of polycaprolactone and lactic acid and/or glycolic acid, copolymers of polytrimethylene carbonate and lactic acid and/or glycolic acid, and copolymers of polydioxanone and lactic acid and/or glycolic acid.

7. The method of claim 1, wherein the reinforcing fibers constitutes about 50 to 65 volume per cent of the hybrid yarn.

8. The method of claim 1, wherein said processing temperature is from about 60°C to about 160°C, and said processing pressure is from about 300 psig to about 500 psig.

9 The method of claim 1, further comprising the step of cooling said bioabsorbable composite material.

10. The method of claim 1, wherein said hybrid yarn is treated to remove monomer and/or impurities prior to being heated under an applied pressure, said treatment comprising heating said hybrid yarn to a temperature of from 75°C to 120°C under at least a partial vacuum for a period of time sufficient to remove a substantial amoumt of the monomer and/or impurities.

11. The method of claim 1 wherein said hybrid yarn when viewed in cross section comprises a plurality of void spaces and a plurality of layers, each layer having unidirectionally oriented fibers, the fibers of some layers being in angular disposition to the fibers in other layers.

12. A method for making a bioabsorbable composite material for surgical implants comprising:
a) placing a matrix fiber in close approximation with a crystalline reinforcing fiber to form a hybrid yarn;
b) treating the hybrid yarn in a first processing step to cause the matrix material to flow around the reinforcing material to achieve a partial consolidation of the yarn; and c) treating the partially consolidated yarn in a second processing step to achieve a further consolidation of the yarn.

13. The method of claim 12, wherein said hybrid yarn is partially consolidated by treating said hybrid yarn at a partial consolidation temperature and a partial consolidation applied mechanical pressure for a period of time sufficient to eliminate at least 10%, but not all of the void spaces.

14. The method of claim 12, wherein said matrix fiber is substantially amorphous.

15. The method of claim 13, wherein said partial consolidation temperature is from about 60°C to 160°C, said partial consolidation applied mechanical pressure is from about 300 to 500 psig.

16. The method of claim 12, wherein said partially consolidated hybrid yarn comprises a layer having unidirectionally oriented fibers.

17. The method of claim 16, wherein said hybrid yarn comprises a plurality of layers, each layer having unidirectionally oriented fibers which are unidirectionally aligned with the fibers in the other layers.

18. The method of claim 13, wherein the partially consolidated hybrid yarn is cut into multiple pieces prior to said second consolidation step to improve placement in a compression mold.

19. A bioabsorbable composite material for surgical implants, said material comprising a hybrid yarn of intimately co-mingled first fibers of a bioabsorbable crystalline polymer characterized by a melting point and second fibers of a bioabsorbable polymer characterized by a glass transition temperature below the melting point of the crystalline polymer, said hybrid yarn being heated under pressure to a processing temperature above the glass transition temperature of the second fibers and below the melting point of the crystalline polymer.

20. The bioabsorbable composite material of claim 19, wherein each of said bioabsorbable crystalline polymer and said bioabsorbable polymer of said second fiber individually is spun from a polymer selected from the group consisting of polylactic acid, polyglycolic acid, polydioxanone, polytrimethylene carbonate, poly(ethylene carbonate), poly(iminocarbonates), polycaprolactone, polyhydroxybutyrate, polyalkylene oxalates, polyalkylene succinates, poly(maleic acid),poly(1,3-propylene malonate), poly(ethylene terephthalate), poly(amino acids),and copolymers thereof

21. The bioabsorbable composite material of claim 19, wherein the bioabsorbable polymer of said second fibers are substantially amorphous and contain a bioabsorbabie component having a melting point below said processing temperature.

22. The bioabsorbable composite material of claim 20, wherein said component is selected from the group consisting of polycaprolactone, polytrimethylene carbonate, polydioxanone, polyorthoesters, copolymers of polycaprolactone and lactic acid and/or glycolic acid, copolymers of polytrimethylene carbonate and lactic acid and/or glycolic acid, and copolymers of polydioxanone and lactic acid and/or glycolic acid.

23. The bioabsorbable composite material of claim 20, wherein said hybrid yarn is treated to remove monomers and/or impurities before said hybrid yarn is heated to the processing temperature, said treatment comprising heating said hybrid yarn to a temperature from about 75°C to 120°C under at least a partial vacuum for a period of time sufficient to remove a substantial amount of the monomers and/or impurities.