MOLDABLE, HAND-SHAPABLE BIODEGRADABLE IMPLANT MATERIAL

Abstract: This invention provides molded, biodegradable porous polymeric implant materials having a pore size distribution throughout the material which is substantially uniform. These materials can be molded into implants of any desired size and shape without loss of uniformity of pore size distribution. The implants are useful as biodegradable scaffolds for cell growth in healing of tissue defects. When manufactured to have an aspect ratio grater than about 3, the implants can be further hand-shaped to fit the defect into which they are placed and the desired shape for the regrown tissue.

Description: MOLDABLE, HAND-SHAPABLE BIODEGRADABLE IMPLANT MATERIAL Cross-Reference to Related Applications This application is a continuation-in-part of co-pending U.S. Serial No. 08/540,788 filed October 11, 1995, fully incorporated herein by reference. Field of the Invention This invention is in the field of biodegradable polymeric implant materials, specifically such materials which are moldable into a wide variety of sizes and shapes and which can be hand-shaped at body temperature, while maintaining structural integrity. Background of the Invention Biodegradable polymers useful for implantation into tissue defects and providing scaffolding for tissue ingrowth have been described, for example, in PCT publication WO 9315694, incorporated herein by reference. Such polymers may be manufactured to have mechanical properties matching those of the tissue into which they are to be implanted. Implants formed of biodegradable polymeric materials may be preseeded with cells of the desired tissue type, for example as described in U.S. Patents 4,963,489, 5,032,508, 5,160,490 and 5,041,138. Implant materials as described above generally contain pores and/or channels into which the tissue grows as the biodegradable material erodes, thus providing new tissue growth of roughly the same size and shape as the implant. A shapable implant material useful for a dental implant is described in U.S. Patent No. 3,919,773 to Freeman (Sybron Corp.) issued November 18, 1975 for "Direct Moldable Implant Material," however this implant material is not biodegradable. PCT publication WO 92/15340 dated 17 September 1992 of Lundgren et al. (Guidor AB) discloses a malleable bioresorbable material used to repair periodontal defects around the tooth. Polylactic acid (PLA) and copolymers of polylactic acid and polyglycolic acid (PGA) are disclosed as usually being very brittle in their pure state. The reference discloses modifying the polymers with plasticizing agents to make them more malleable. The plasticizing agents tend to cause undesirable swelling of the polymers in vivo and to decrease structural stability; however, through careful selection of the plasticizer and polymer and through the use of lOμm perforations through the material, swelling can be minimized. Such perforations can be created by a laser process as described in PCT Publication WO 92/22336 dated 23 December 1992 to Mathiesen, et al. (Guidor, AB) . U.S. Patents 4,844,854 issued July 4, 1989 for "Process for Making a Surgical Device Using Two-phase Compositions," 4,744,365 issued May 17, 1988 and 5,124,103 issued June 23, 1992 for "Two-Phase Compositions for Absorbable Surgical Devices," and 4,839,130 issued June 13, 1989 for "Process of Making an Absorbable Surgical Device," to Kaplan et al. (United States Surgical Corporation) disclose that biodegradable surgical devices of PLA/PGA can be made less brittle by using a two-phase polymer having a continuous lactide-rich phase interpolymerized with a continuous glycolide-rich phase or a continuous lactide-rich phase having dispersed throughout it discrete particles of a glycolide-rich phase. The material is then annealed to raise the temperature at which the material can be distorted. Malleability is thus disclosed as an undesirable property. As discussed above, it is desirable that a biodegradable implant designed for tissue ingrowth have mechanical properties similar to those of the tissue into which it is placed. Typically accepted values for the elasticity (Young's modulus) of cartilage are less than 1 MPa (Brown and Singerman (1986), J.Biomech. 19 (8) :597-605) . Poisson's ratio (measuring the tendency of the material to distort sideways when pressed upon from the top) values for cartilage are low, no more than about 0.3 (Frank Linde (April, 1994) Danish Med. Bull. Volume 4, No. 2). It is thus apparent that providing a biodegradable polymeric material with a low Poisson's ratio, which is also hand-shapable at body temperature for use as an implant for promotion of cartilage, bone and other tissue ingrowth is not a trivial problem. A simple malleable substance having a high Poisson's ratio would not be suitable. Porous biodegradable polymeric implant materials for use as scaffolds for cell ingrowth have previously been limited by the difficulty in achieving uniform porosity in implants of a size larger than a few millimeters in any dimension. Molding such materials in closed molds in vacuum ovens has resulted in the formation of scattered large bubbles and thin spots. Cell ingrowth iε best encouraged in a material of uniform porosity. Thus, there is a need for a method of making a biodegradable implant material larger than a few millimeters in cross section with pores of uniform size and distribution. In traditional bone graft procedures, autogenous cancellous bone from a source such as the iliac crest is often used for filling bone defects. Allogenic bank bone has been advocated as an alternative to autogenous bone, but the effectiveness of the graft is often compromised by nonunions, fatigue fractures, and both clinical and histological evidence of resorption of the graft. Further, allogenic bank bone is often in short supply and may carry disease factors. A major disadvantage in using autogenous bone from the patient's iliac crest is that taking this material is an extremely painful procedure (patients undergoing spine fusions tend to complain more about iliac crest pain than spine pain) . Sterile materials useful as substitutes for autogenous and allogenic bone are therefore desirable, especially sterile biodegradable materials serving as scaffolds for in-growing bone. Biodegradable implant materials for use in healing bone defects include particulate materials such as those described in Ashman, et al. U.S. Patents No. 4,535,485 issued August 20, 1985 for "Polymeric Acrylic Prosthesis" and 4,547,390 for "Process of Making Implantable Prosthesis Material of Modified Polymeric Acrylic (PMMA) Beads Coated with PHEMA and Barium Sulfate." These materials bond together inside the defect to form a porous mass. This implant material is not disclosed to have the mechanical properties of bone. It is desirable in promoting tissue ingrowth to provide an implant material with properties similar to those of the tissue in question insofar as possible to provide a microenvironment such that cells growing into the implant will find conditions as close as possible to the natural conditions for which they were designed. This invention provides such particulate materials. Summary of the Invention One aspect of this invention provides a molded, biodegradable, porous polymeric implant material having at room temperature (20 to 25°C) , a Poisson's ratio less than about 0.3, preferably less than about 0.25, and more preferably less than about 0.1, and a porosity between about 60 volume percent and about 90 volume percent, preferably between about 60 and about 80 volume percent, and more preferably between about 65 and about 75 volume percent, wherein the pore size distribution throughout the material is substantially uniform, and having an aspect ratio of about 3 or more, wherein said molded implant material is hand-shapable at or slightly above body temperature without loss of structural integrity. It is critical that the porosity not be greater than about 90%. Implants with too great a porosity cannot provide the stiffness required to promote cell differentiation which requires a stiffness comparable to the tissue type, e.g., cartilage or bone, ingrowing into the implant. The term "substantially uniform" in reference to pore size distribution throughout the material means that the size distribution of pores as measured in every portion of the material is the same. In the preferred embodiment, target or average pore size is about 100 μm to 200 μm diameter, and this average pore size is found in all portions of the material. A range of pore sizes is present above and below this average, e.g., about 5 μm to about 400 μm, and this range is substantially the same in all portions of the material. The pores are substantially evenly distributed throughout the implant material so that the density of the material at different points does not vary. The term "aspect ratio" as used herein refers to the ratio of the longest dimension of the implant to the shortest dimension of the implant. For example, a molded wafer of this invention having the dimensions 20x40x3mm would have an aspect ratio of 40/3, or 13.3. The term "structural integrity" as used herein means that the porosity and distribution of the pores does not significantly change, and the material does not crack or break when it is hand-shaped as described herein. As the implant materials of this invention are biodegradable, they maintain structural integrity for a period of time sufficient to effect tissue ingrowth and healing of the defect into which they are placed and subsequently biodegrade. The term "hand-shapable" as used herein means that the material may be distorted by hand or by hand tools, so as to be shaped to fit a defect, or twisted and bent to provide desired support for tissue regrowth. The term does not refer to shaping by means of cutting tools. The hand-shapability of the implant material is correlated with its mechanical properties, e.g. elasticity (also referred to as "stiffness" herein) and compressibility. Young's modulus defines the elasticity of a material, measured as the stress divided by the strain. Young's modulus is a measure of how much force must be applied to the material per unit area to deform it a given amount per unit length. Poisson's ratio is a measure of how much the material will contract in the directions at right angles (sideways) to the direction of stretching (as when a rubber band becomes thinner when stretched) , or how much it will expand sideways when compressed (as when a ball of clay expands sideways when compressed from the top) . At body temperatures, and slightly above, the elasticity of the materials of this invention increases (Young's modulus decreases) so that the materials are generally more easily hand-shaped than at room temperature. The materials of this invention may be hand-shaped at body temperatures; however, it is preferred they be warmed to at least about 45θC and more preferably to at least about 50oc, such as by warming on a hot plate prior to shaping. They should not be warmed about a temperature so high that they are uncomfortable to the touch. Polymeric implant materials of this invention can be molded during manufacture into a wide variety of shapes and sizes to address many different tissue defect situations. The molded materials can be further hand-shaped by bending or can be carved to suit each individual implant application. It is desirable in the implant materials of this invention to have structural properties, including mechanical properties such as elasticity, similar to those of the tissue into which the implant is to be placed. It is desirable that the implant materials be porous to facilitate tissue ingrowth. In general, the implant materials of this invention preferably have a Young's modulus between about 0.1 and about 200 MPa at room temperature, more preferably between about 0.1 and about 50 Mpa, and most preferably between about 0.1 and about 5 MPa or 10 MPa. Cartilage has a Young's modulus of less than about 1.0 MPa, and thus implant materials of this invention designed to be placed next to cartilage tissue preferably have a Young's modulus less than about 1.0 MPa. Bone has a Young's modulus from about 1.0 up to about 1700 MPa. However, for hand-shapability the Young's modulus at shaping temperature should not be greater than about 10 MPa. A preferred embodiment has a glass transition temperature (Tg) between about 38 and about 50. This invention also provides molded, porous, biodegradable implant materials having a porosity between about 60 and 90 volume percent wherein the pore size distribution throughout the material is substantially uniform, and wherein said molded materials have at least one dimension greater than about 7 mm. Prior attempts to make such implants have failed because of the inability of prior workers to achieve uniform porosity when the dimensions of the implant exceeded 7 mm. These implant materials having uniform porosity may have Young's moduli at room temperature up to or greater than that of bone, i.e. up to about 1700 MPa or greater. Preferably they have elasticities, Poisson's ratios and porosities as described above. The implant materials may be used to make multi-phase implants such as the two-phase implants as described in PCT publication WO 9315694, incorporated herein by reference. These two-phase implants preferably have an upper cartilage phase and a lower bone phase and are inserted into a defect extending from cartilage into bone with the appropriate phases adjacent the same tissues. The term "cartilage phase" as used herein means that the material has mechanical properties, e.g. stiffness or elasticity, substantially the same as cartilage. Similarly the term "bone phase" means that the material has mechanical properties substantially the same as bone. In a preferred embodiment of the two-phase implant, each phase has a different color to aid in distinguishing the phases so as to correctly place each phase adjacent the appropriate tissue. The implant materials of this invention preferably have an average pore size of between about 5μm and about 400μm, more preferably between about lOOμm and about 200μm. It iε preferred that the pores be interconnected to allow free flow of fluid therethrough. It is also preferred for cartilage phase implants that the implant not contain large channels having a diameter greater than about 1 mm, as this may make it difficult for ingrowing cells to bridge the gap without formation of fibrous tissue. Larger channels may be present in bone phase implants, as these can be packed with blood and marrow cells, when the implant is placed in vivo to encourage rapid cell growth. High porosities are preferred in the implants of this invention, consistent with maintaining structural integrity, as these provide most useful scaffolds for tissue ingrowth. The polymeric implant materials of thiε invention may also comprise cells compatible with the host for which they are intended, for example as described in the above-referenced patents directed to polymeric scaffold materials seeded with cells. Implant materials of this invention, with and without channels, may be infiltrated with nutrient and/or cellular material such as blood and narrow cells, cartilage cells, perichondrial cells, periosteal cells, and other cells of mesenchymal origin, e.g., osteoblasts, chondrocytes, and their progenitors, adipocytes, muscle cells, tendon cells, ligament cells, dermal cells and fibroblasts, to facilitate tissue growth. A number of suitable biodegradable polymers for use in making the materials of this invention are known to the art, including polyanhydrides and aliphatic polyesters, preferably polylactic acid (PLA) , polyglycolic acid (PGA) and mixtures and copolymers thereof, more preferably 50:50 to 80:20 copolymers of PLA/PGA, most preferably 70:30 to 80:20 PLA/PGA copolymers. Single enantiomers of PLA may also be used, preferably L-PLA, either alone or in combination with PGA. Polycarbonates, polyfumarates, caprolactones, trimethylene carbonate, polydioxanone and polyhydroxybutyrate may also be used to make the implants of this invention. The polymer preferably has an average molecular weight between about 30,000 daltons and about 150,000 daltons prior to use, more preferably between about 60,000 daltons and about 120,000 daltons. The polymers are designed to biodegrade in vivo over a period of weeks. Preferably the materials maintain their structural integrity so as to serve as supportive scaffolds for ingrowing tissue for a period of at least about two weeks, and preferably about four to ten weeks for cartilage repair, and thereafter biodegrade. For bone repair, the materials maintain their structural integrity for at least about two weeks and preferably about three to eight weeks, and thereafter degrade. The implant materials of this invention may be formed into convenient sizes and shapeε during manufacturing, εuch aε waferε, tubes, cubes, balls, cylinders and the like up to about 100 mm thick, including irregular shapes. For hand- shapable use, preferred forms are wafers having an aspect ratio greater than about 3. The die-punched wafers produce cylinders, chunklets and resultant honeycomb lattices as hereinafter described. The implant materialε may be reduced in εize to particles of preferably about 50 μm to about 1 cm in diameter, e.g., by shredding as hereinafter described. The implant materials tend to retain their shape at room temperature for ease in shipping and εtoring. However, in one embodiment of thiε invention, when warmed, e.g., in the physician'ε handε or by heating means εuch as a hot plate, to about 450C to about 50θc, and preferably not above a comfortable handling temperature, e.g., about 50θc, and/or placed within a tissue defect in a patient's body, they may be readily hand-shaped to conform to the shape of the defect and the desired shape for the regrown tissue. The implant materials of this invention may be shaped to (1) repair facial bone defects and certain defects in the mandible and maxilla; (2) dental defectε εuch as alveolar ridge defects; and (3) other bone defects such as defectε in the εpine, cranium, tibia, radiuε, glenoid fossa, etc. New bone growth, taking the shape of the implant, can thus be directed to match the existing defect structures. The materials of this invention can also be used in other bone repairs such as spinal fusionε, or uεed in cartilage repair such as reεurfacing, e.g. femoral condyle resurfacing, or repair of cartilaginous tissue such as in the nose and ears, ribs, and chin. The implant materials of this invention are also readily carvable, and excesε material can be cut from the implant in the process of sizing and shaping the material to fit the defect. After hand-εhaping to fit the defect, the implant material may be sutured, glued or otherwise secured into place if desired. Techniques for suturing cartilage are known to the art, e.g. as described in Brittberg, M. , et al. (1994), "Treatment of Deep cartilage Defectε in the Knee with Autologous Chondrocyte Transplantation," New Engl. J. Med. 331:889-895. Glues known to the art such as fibrin glue, cyanoacrylate, or others, may be used to εecure the implant in place. Other securing means are known to the art including bioresorbable screws, staples or other fixation devices. At body temperature, the tendency of the implant to return to its pre-shaped form is reduced from this tendency at room temperature, and suturing to maintain the desired εhape of the implant may not be neceεsary since the implant may be held in place with surrounding soft tissue. The implant materials of this invention may also incorporate bioactive agentε εuch aε enzymes, growth factors, degradation agents, antibiotics and the like, designed for release over time, as described in U.S. Patent Application 08/196,970 incorporated herein by reference. For best biocompatibility it is preferred that the implant material be subεtantially free of solvent. It is recognized that some residual solvent will be left in the polymer, but preferably lesε than about 100 ppm. Solventε known to the art may be uεed, e.g., acetone, methylene chloride or chloroform. The materialε of this invention are readily sterilizable, have been found safe and effective in pre-clinical animal studies, and are safe for use in the human body. This invention also comprises a method for making a molded biodegradable, porous polymeric implant material of a desired geometry, including a material as described above, wherein the pore size distribution throughout the material is substantially uniform, comprising: a) preparing a precipitated gel-like polymer mass; b) extracting liquid from the polymer mass; c) kneading the polymer masε to form an extensible composition; d) placing the extensible composition into a mold of the desired geometry having small spaced vent holes for solvent removal; e) treating the extensible composition in the mold in a vacuum oven at a temperature and time sufficient to cure the composition. The preparation of precipitated polymers is well-known to the art. In general, the process comprises mixing a dried polymer mix with a solvent, preferably acetone, precipitating the polymer mass from solution with a non-solvent, e.g. ethanol, methanol, ether or water, extracting solvent and precipitating agent from the mass until it is a coherent mass which can be rolled or pressed into a mold, or extruded into a mold, and curing the composition to the desired shape and stiffness. In order to produce an implant having optimal uniform porosity and the desired elasticity coupled with the ability to maintain structural integrity when hand-shaped, kneading of the polymer mass prior to molding is necesεary. The kneading may be done by hand or machine and εhould be continued until the mass becomes extenεible, e.g. can be pulled like taffy. Prior to reaching thiε εtage, the polymer maεs will snap apart or break when pulled. Curing the polymer in a mold to form an implant having an aspect ratio of three or greater is also important in achieving the desired elasticity. The temperature and time of molding may be selected, depending on the polymer being used, to achieve the desired elasticity and other properties, as more fully described below. The mold used for curing the polymer should be perforated with spaced holes or vents, preferably evenly spaced, and preferably having a diameter small enough that the polymeric material does not flow out of the holes, e.g. , between about 0.3 mm and about 0.7 mm, and preferably spaced evenly and about 3 mm to about 20 mm apart. This allows εolvent and air to escape from the material uniformly as it is cured, so that uniform porosity is achieved. The amount of material used can be varied to vary the percent porosity and pore size of the material, as will be apparent to those skilled in the art. The molded implant material made by the above method, which may be large blocks having dimensions up to about 30 cm x 30 cm x 10 cm may be reduced to particles such aε by grinding, pulverizing, milling, cruεhing or εhredding, e.g., in a knife mill, to particleε having an average diameter between about 50 μm and about 1 cm, preferably between about 250 μm and about 2000 μm. Size fractionε having an average particle diameter between about 250 μm and about 850 μm are preferred for dental uses. Size fractions between about 850 μm and about 2000 μm are preferred for other orthopaedic use. Such particulate material may be mixed with a patient's marrow and blood cells (or other cells) and/or cancellous or other bone graft material to provide an autologous bone graft augmentation or subεtitute material which can be packed into a wound to promote healing. When this material is made with the stiffness properties of bone, it provides a microenvironment for ingrowing mesenchymal cells closely approximating bone and therefore providing a conducive environment for proliferation of bone progenitor cells and differentiation of bone and marrow. Osteoconductive proteinε known to the art may also be included in the material. (See, e.g., Brekke, J.H. (1996) , "A Rationale for Delivery of Osteoconductive Proteins," Tisεue Engineering 2:97-114.) A method for implanting the material of thiε invention iε also provided for allowing selected tisεue ingrowth into a defect in a patient'ε body compriεing: a) εelecting a biodegradable polymeric implant having an aεpect ratio of at least about 3, composed of a material having a Poisson'ε ratio less than about 0.3; b) hand-shaping said material to fit said defect; c) prior to, or after, hand-shaping said material, placing said material into said defect. Preferably, the implant selected has a stiffneεs substantially εimilar to that of the εelected tissue. It is preferred that prior to placing the implant into the defect, it be wetted by soaking, preferably under vacuum, with a biocompatible liquid such as sterile saline, plasma or blood, bone marrow, or other cell suεpenεionε. The hand-shaped implant may be sutured into place if desired, and/or may be exposed to crosε-linking activatorε whereby its elasticity (tendency to return to its original shape) is decreaεed, or it may εimply be held in place by surrounding εoft tissues. A method is also provided for resurfacing a femoral condyle comprising: a) hand-εhaping into place over at leaεt a portion of the surface of said femoral condyle a sheet of a biodegradable, porous polymeric implant material having a Young's modulus between about 0.1 and about 200 MPa, a Poisεon'ε ratio leεs than about 0.3, a poroεity between about 60 volume percent and about 90 volume percent, wherein the pore εize diεtribution throughout the material iε εubεtantially uniform, and having an average pore size of between about 5 μm and about 400μm, and having an aspect ratio of at least about 3 ; b) securing said material to existing cartilage on said femoral condyle; c) allowing cartilage tissue to ingrow into the pores of said material and subεequently allowing said material to biodegrade, whereby the surface of said femoral condyle is covered with a uniform layer of cartilage. The uniformity of the layer of cartilage is critical, as lumps and bumps tend to abrade the opposing tissue when the joint is in use, causing damage and pain. The biodegradable implant materials of this invention can also be used for repair of tiεεueε other than cartilage and bone, including tendonε, ligamentε and organε εuch as liver, pancreas, and other organs. Detailed Description of the Preferred Embodimentε The preferred biodegradable implant materials of this invention are designed to have mechanical properties (e.g. elaεticity (Young's modulus) and compresεibility (Poisson's ratio) similar to the tissueε into which they are designed to be placed. Preferred tissueε for placement of the implants of this invention are cancellouε bone and articular cartilage. Cartilage phaεe implant materials with mechanical properties similar to cartilage are also considered to be suitable for use with bone, especially when highly curved regions are to be treated. Cartilage phase implants have a Young's modulus of less than or equal to 1.0 MPa, and bone phase implants have a Young's modulus of greater than or equal to 1.0 MPa. The materials of this invention have a Poisson's ratio lesε than about 0.3 and preferably leεε than about 0.1. The materialε are εomewhat flexible at room temperature (20 to 25°C) . The cartilage phaεe material iε hand-εhapable at normal body temperature e.g. when warmed in the handε or placed into a body cavity, and iε more preferably heated to about 45-50OC for εhaping. The bone phaεe may be manufactured at varying stiffnesses. The smaller the Young's modulus, the more readily hand-shapable the material will be. Using the methods of this invention, one skilled in the art can εhape the implant to fit the tiεεue defect into which it is to be placed. Polymers known to the art for producing biodegradable implant materials may be used in this invention. Examples of such polymers are polyglycolide (PGA) , copolymers of glycolide such as glycolide/L-lactide copolymers (PGA/PLLA) , glycolide/trimethylene carbonate copolymers (PGA/TMC) ; polylactideε (PLA) , stereocopolymers ot PLA such as poly-L- lactide (PLLA) , Poly-DL-lactide (PDLLA) , L-lactide/DL-lactide copolymers; copolymers of PLA such as lactide/tetramethylglycolide copolymers, lactide/trimethylene carbonate copolymers, lactide/δ-valerolactone copolymerε, lactide e-caprolactone copolymerε, polydepsipeptides, PLA/polyethylene oxide copolymers, unsymmetrically 3,6- substituted poly-1,4-dioxane-2, 5-dioneε; poly-β- hydroxybutyrate (PHBA) , PHBA/β-hydroxyvalerate copolymerε (PHBA/HVA) , poly-β-hydroxypropionate (PHPA) , poly-p-dioxanone (PDS) , poly-δ-valerolatone, poly-e-caprolactone, methylmethacrylate-N-vinyl pyrrolidone copolymerε, polyesteramides, polyesters of oxalic acid, polydihydropyrans, polyalkyl-2-cyanoacrylates, polyurethanes (PU) , polyvinyl alcohol (PVA) , polypeptides, poly-β-maleic acid (PMLA) , and poly-β-alkanoic acids. Young's modulus E, a measure of elasticity also referred to as "elastic modulus" is the ratio of stress to strain: E - °- where streεε σ is the normal force per unit area, and strain e is the elongation per unit length. Young's modulus may be measured for the relatively rigid materialε of this invention by meanε known to the art εuch aε the three-point bending test. In this teεt, a εpecimen in the form of a wafer is placed horizontally over two "sawhorse"-shaped wedgeε at both endε. Another wedge, pointed edge down and parallel to the support wedges, is pressed down onto the top of the center of the wafer. Force or load P applied against the top wedge and deflection distance δ are determined. The Young'ε modulus is calculated: 48δJ where P is the applied load, L is the distance between end supports, δ is the deflection distance produced by the load P and I is the moment of inertia with respect to the centroidal axis computed from the geometry of the wafer as: where b is the width of the wafer and h is the thickness. Poisson's ratio v may be determined by an indentation test such as that deεcribed in allowed U.S. patent application no. 08/231,612 incorporated herein by reference. In an indentation teεt, a rigid, cylindrical indenter is pressed into the εurface of a εpecimen εuch aε a wafer of the implant material of thiε invention. The force P with which the cylinder is pressed into the specimen and the vertical distance the material is indented δ are measured. If the Young'ε moduluε E of the material is known, and if the distance the material is indented δ is very small compared to the height of the material, and the radiuε r of the cylindrical indenter iε very εmall compared to the height of the material, then Poiεεon'ε ratio v can be calculated aε: v- ■ fi, 1 2Erϊ> When the Poiεεon's ratio v is small, i.e. lesε than about 0.3, such that the material has little tendency to expand sideways, the Young's modulus E (elaεticity) iε almost the same as the aggregate modulus H» of the material. An important aspect of achieving the desired elastic properties and uniform porosity is kneading the polymer mass during preparation until it becomes extensible. A further important expedient is providing small spaced vent holes in the mold and subjecting the polymer to vacuum extraction of solvent prior to curing, all as described in the Examples hereof. The temperature at which the polymer is cured is also important in determining the elasticity. For achieving the elasticity of cartilage using a 75:25 PLA/PGA polymer having a molecular weight of approximately 70 kD, a temperature of about 50-60°C over a curing time of about 24-48 hours may be used. To achieve the elasticity of bone, a temperature of about 37-42°C may be used over a curing time of about 24 hourε, followed by a temperature of about 47°C another 6-24 hourε. The mechanical properties of bone, cartilage and other tissueε are known to the art, e.g. aε described in U.S. patent application 08/196,970 incorporated herein by reference, or may be readily determined by the testing methods described above, or other testing methods known to the art. As some polymers tend to be lesε elastic than others, the ratio of monomers in the polymer may be adjuεted, aε will be evident to thoεe of ordinary εkill in the art, to achieve the deεired propertieε in the final product. Similarly, the time and temperature of curing may be varied without undue experimentation by thoεe of ordinary skill in the art to achieve the desired properties using the testing methods described above to optimize the process. In addition, cross- linking agents and enhancers, and plasticizers, as known to the art, may be used to modify the desired properties. The target porosity of the materials of this invention is achieved by adding more or less polymer to the mold. For example, in preparing N number of wafers to have a selected target porosity Q, in a mold having a length a (mm) , a width b (mm) and a depth c (mm) , using a polymer of density p (g/cm 31 , the masε of polymer M to be used is calculated by: M _ _l___Zp { _ -Q) N . 1000^ As is known to the art, the lifetime of the material in vivo may be increased by increasing the amount of L-PLA content, molecular weight and degree of crystallinity, or decreaεed by decreasing the same factors. The lifetime of the material may be varied independently of the stiffness as will be apparent to those skilled in the art, for example by increasing the PLA content and at the same time decreasing molecular weight to achieve a longer lifetime without increasing stiffnesε. The implant material may incorporate cellε, bioactive agentε aε is known to the art, pH-adjusting agents, for example as described in U.S. patent application 08/361,332 incorporated herein by reference, including Bioglasε®, of U.S. Biomaterialε, which iε also uεeful for binding growth factors, and other additives known to the art such as matrix vesicles or matrix vesicle extracts as described in U.S. patent application 08/250,695, incorporated herein by reference, and other bioceramics. Waferε formed of the materialε of thiε invention may be readily die-punched to uniformly produce implants of various sizes and shapes. Multi-phase wafers prepared as described in the Examples hereof may alεo be punched to produce multi-phaεe implants of different sizeε and εhapes, preferably to produce cylindrical two-phase implants. A preferred honeycomb lattice material of this invention is prepared by die-punching holes of a diameter of about 1 mm to about 4 mm, preferably about 2 mm, in a wafer having a thickness of about 1 mm to about 4 mm, preferably about 3 mm. The holes are spaced approximately 2 mm to about 4 mm apart. The wafer after punching out the holes is referred to herein as a "honeycomb lattice" material. Thiε honeycomb lattice material is more flexible than the unpunched wafer and is preferred, using bone-phase material, for bone repair such aε bone graft onlay for variouε bone repairs including spinal fusion, that require shaping of the implant. In this process, the honeycomb lattice material is allowed to soak in a mixture of blood and marrow cells prior to affixing into the bone defect. The small cylindrical chunklets punched out of the honeycomb lattice material preferably have a volume between about 1 m ' and about 16 mm J per chunklet. They are uεeful aε a εterile εubstitute for allogenic and autologous bone filler materials in bone graft procedures. These chunklets may be packed into the defect and used aε bone graft filler when other types of bone graft material are at a minimum. It is preferred that bone-phase chunklets be used for this purpose. These chunklets, having mechanical properties (e.g. elasticity) and physical properties (e.g. poroεity) similar to those of bone, provide the ingrowing cells with an environment as close as possible to their natural environment, thus fostering and encouraging growth of the cells. The polymeric implant materials of this invention may be shaped to any desired geometry up to cubic volumes of about 9000 cc. Shredded or carved pieces of the implant material are especially useful as substitutes or extenders for bone graft material. Examples Example 1. Method of making polymeric wafers, cylindrical implants, honeycomb lattices and chunkletε. Five grams of PLA/PGA (75:25) polymer, molecular weight 80,000 D, intrinsic viscosity about 0.6 to about 0.75 in chloroform, were weighed into a Teflon beaker. A 3/4" Teflon- coated magnetic stirring bar was placed in the beaker and the beaker placed on a magnetic stirplate. 30 ml acetone was added and the mixture stirred (at setting 8 on the stirplate) for about 20 minutes until the polymer was completely disεolved. Polymer waε precipitated by adding 30 ml ethanol and stirring for about 20 seconds (at setting 3 on the stirplate) to agglomerate the polymer gel masε. The εupernatant liquid waε then decanted and the gel mass turned onto a Teflon plate to be used as a work surface. The stirbar waε separated from the masε by uεing a Teflon policeman, recovering as much polymer as possible. Excess liquid was blotted away using care not to touch the polymer with the Ki wipe blotter. The polymer masε waε then rolled and flattened to a thin sheet (l±O.l mm thick) using a bar of round Teflon stock about 3/4" in diameter. The Teflon plate with the polymer was then placed in a vacuum desiccator, and vacuum was applied for several minuteε (2 to 4.5 min) uεing a KNF reciprocating diaphragm vacuum pump until the polymer mass became blistered and bubbly as the solvent was removed. The vacuum was released and the Teflon plate with the polymer was removed from the desiccator. Using rubber gloves, the polymer gel was hand-rolled into a ball and kneaded using thumbs and forefingers until the material became soft and extensible. During this procesε a εmall amount of residual solvent was released and the polymer felt slightly wet. Kneading was continued until no more liquid was evident. The gel was then rolled out into a thin sheet using the Teflon bar and being careful not to allow the polymer to wrap around the bar, as the polymer at this point was quite sticky and readily adhered to itself upon contact. The polymer was then again placed in the desiccator and vacuum was applied for several more (2 to 4.5) minutes until the gel expanded and appeared "foamy," having many fine bubbles distributed throughout the matrix. The polymer was removed from vacuum and again kneaded as before until it was soft and extensible and took on the lustre of spun sugar and a "satiny" appearance. The mass of the polymer gel at this point was recorded. The polymer gel was then divided into five equal pieces, and the pieces were εhaped to fit the well of a mold. The mold waε wafer-εhaped, approximately 20 mm x 40 mm x 3 mm, and perforated with holeε having a 0.7 mm diameter spaced approximately 3 mm to 10 mm apart. Care was taken to shape each piece to fit the well of the mold, making sure that the surface was uniform and even with no thin spots and that the material filled the mold edge to edge. The molds (without top) were then placed into the desiccator and vacuum was applied for two minutes. The molds were then removed from the desiccator and the tops of the wafers flattened without completely compressing the expanded polymer. The top plates of the molds were then affixed using appropriate nutε and boltε. The moldε were then placed in a vacuum oven at 60-65°C under vacuum of leεs than 50 mTorr for 24-48 hourε. For cartilage phase materials, i.e. wafers having mechanical properties of cartilage, the treatment vacuum oven was continued at the same temperature for an additional 24 hours. After curing, the polymer was substantially free of solvent. The resulting polymeric cartilage phase wafers were uniform in porosity, having an average pore εize of about lOOμm and a percent poroεity of about 65 volume percent. They were flexible and, when slightly warmed in the hand to about body temperature, were easily hand-shapable. The resulting polymeric bone phase waferε were alεo uniform in poroεity, having an average pore size of about 150μm and a percent porosity of about 70 volume percent. Although they were not as flexible as the cartilage phase wafers at room temperature, they could be hand-shaped at body temperature. A polymeric gel mass as described above was placed into a spherical mold perforated as described above, approximately 1 inch in diameter, and treated in the vacuum oven for 24 hours at 42°C, then at 47°C for about 24 hours. The resultant polymeric sphere had a porosity of 70 volume percent and an average pore size of about 150μm. Smaller and larger sphereε can be molded uεing thiε technique. Two-phase cylindrical implants having a cartilage phase atop a bone phase as deεcribed in PCT Publication WO 9315694 were made by layering the prepared cartilage phaεe over a pre¬ formed bone phaεe. The bone phaεe waε prepared as described above, then polymer gel to form the cartilage phaεe was overlaid on the bone phase and the two-phase wafer was vacuum treated for 48 hours at 50 mTorr at 42°C. Two-phase cylindrical implants were then die-punched uεing the appropriate size cutter, e.g. for osteochondral defects, a 1.0 mm thick cartilage phase was layered over a 2 mm thick bone phase, and a 2-7 mm diameter cylinder was cut from the resulting two-phase wafer. Uniform cylindrical chunklets suitable for use in filling bone defects as an alternative bone graft material were punched out of bone-phase wafers formed by the procesε described above, having a thickness of about 2 mm, with a 2 mm punch, leaving evenly spaced holes in the wafer approximately 3 mm apart. The wafers from which the implants and chunkletε were removed had a honeycomb lattice structure and were suitable for use as implants for promoting healing of bone defects, including spinal fusionε. Example 2 ♦ Uεe of Hand-Shapable Cartilage Phaεe Material for Femoral Condyle Reεurfacing. A portion of the cartilage covering the femoral condyle surface of a patient, in which a defect has been noted, is removed to form a regular-shaped recess. A cartilage phase water of Example 1 having a thickness the same as that of the recess to be filled is cut to form an implant sized to fit the recess then pressed and hand-shaped into the recess so as to exactly fit with no portion raiεed above the cartilage εurface. The implant is then sutured into place with 9.0 Vicryl sutures, and the εite closed. The implant material maintains its structural integrity for a period of 4-10 weeks while new tisεue grows into the defect. The new cartilage forms a smooth, uniform surface over the femoral condyle within a period of about six weeks, and the implant degrades and disappears within a period of about 8-16 weeks. Example 3. Use of Honeycomb Lattice Material for Bone Graft Onlay. Bone phase honeycomb lattice material as described in Example 1 is used to foεter bone regrowth. Thiε implant can be uεed to εtrengthen, via onlay, weakened bone, or it can be used to shape and conform to spinal pedicles for spinal fusion. The honeycomb lattice material is saturated with blood and marrow by soaking and may be sutured into place using standard suturing techniques. Over the course of approximately two to six weeks, new bone growε into the implant to εtrengthen or fuse a defect area, and the implant subsequently degrades and disappears. Example 4. Preparation of Bone Graft Substitute/Extender Material in Chunklet and Shredded Form The implantε uεed for this study were made from a 75:25 DL PLA/PGA copolymer with intrinsic viεcoεity (I.V.) = 0.83 (Birmingham Poly erε Inc. Lot # 101-100-1) in a clean room environment without terminal sterilization. The I.V. was determined in HFIP at 30OC. Two different geometries of the materials were prepared. Cylindrical bone graft extenders were made by punching cylinders (1.5 mm diameter x 1.5 mm high) from wafers (60 x 60 x 1.5 mm) made by the method of Example 1. Shredded extenders were made by εhredding waferε in a Leibinger Teεεier osseous microtome (Leibinger & Fischer LP, Irving, TX) . The shredded particles were then placed into a sieve and shaken. The fraction between 10 and 20 mesh (850-2000 μm) waε collected for uεe in theεe studieε. The shredded fragments were of approximately the same dimensionε as the chunklets and were irregularly shaped. All materials were packaged in sterile Eppenderoff tubes and placed in sterile autoclave bags. Example 5. Use of Bone Graft Substitute Material as Bone Graft Extenders. The shredded material and chunklets of Example 4 were used to study wound healing in 30 6-month old male adult New Zealand White (NZW) rabbits. Ostecto ieε 1 cm in length were created in the diaphyεiε of the right ulna of each rabbit: 6 rabbits received autologous cancellous bone chips (CAN) ; 6 rabbits received CAN and 20 chunklet particles (50:50 v/v); 6 rabbits received 40 chunklet particles; 6 rabbits received an equivalent weight of εhredded particles; and 6 rabbitε were untreated (EMP) . Perioεteu overlying the defect site was retained whenever possible and no internal fixation was used. The defects were examined radiographically at 6 and 12 weeks and histologically at 12 weeks. At 6 weeks there was radiographic evidence of healing in all groups at the osteotomy lines; healing in the treatment groups was enhanced with no obvious differences as a function of implant type. At 12 weekε, the untreated controlε were mechanically unεtable to manual palpation. In contrast, the majority of the treatment specimens were mechanically stable. Radiographically, there were no significant qualitative differences among the treatment groups; all were better than the untreated control. Statistical analysis of semi-quantitative measures showed significant differences between the chunklet containing groups and the EMP group. In general, histological results supported the radiographic evaluations. When autologous bone graft (100 or 50%) was used, cortical plates were restored. While this was delayed in the 100% chunklet and shredded particle animals, bony union was generally complete and only small amounts of residual scaffold remained. In contrast, the entire width and length of the defect was not bridged in any of the EMP specimens. There was mild to moderate focal inflammation in all treatment groups associated with the implants. The inflammation did not appear to impair the wound healing response, and repair mechanisms in the treatment were normal. At 12 weeks, healing was not complete in any of the animals; however, the post¬ mortem gross observationε, radiographic, and hiεtological evaluations indicate that normal healing iε occurring in the chunklet and εhredded material treatment groups and that these materials may be effectively utilized as a bone graft extenderε or bone graft substitutes.

Claims: Claims : 1. A molded biodegradable, porous polymeric implant material having a Poisson's ratio at room temperature less than about 0.3, a porosity between about 60 volume percent and about 90 volume percent, wherein the pore size distribution throughout the material is substantially uniform, and having an aspect ratio of about 3 or more, wherein said molded implant material is hand-shapable at body temperature without loss of structural integrity. 2. The implant material of claim 1 made of a copolymer of PLA and PGA. 3. The implant material of claim 1 shaped to fit a bone or cartilage defect. 4. The implant material of claim 1 having a Young's modulus between about 0.1 and about 200 MPa at room temperature. 5. The implant material of claim 1 having a Young'ε modulus between about 0.1 and about 10 MPa at room temperature. 6. The implant material of claim 1 having a Young's modulus between about 0.1 and about 10 MPa at shaping temperature. 7. The implant material of claim 1 having a porosity of between about 60 volume percent and about 80 volume percent. 8. The implant material of claim 1 fabricated in sheets having a thickness between about .4 mm and about 100 mm. 9. The implant material of claim 7 wherein said sheetε are perforated with spaced holes having a diameter between about 1 mm and about 4 mm and spaced about 2 mm to about 4 mm apart. 10. The implant material of claim 1 shaped to repair a fractured orbital bone. 11. The implant material of claim 1 shaped to correct a chin defect. 12. The implant material of claim 1 shaped to correct an alveolar ridge defect. 13. The implant material of claim 1 comprising bioactive agents and capable of releasing said bioactive agents in use over time. 14. The implant material of claim 1 comprising Bioglasε. 15. The implant material of claim 1 which is substantially free of residual solvent. 16. The implant material of claim 1 capable of maintaining structural integrity in vivo for at least about two weeks. 17. A molded, porous, biodegradable implant material having a porosity between about 60 and 90 volume percent wherein the pore size diεtribution throughout the material iε εubεtantially uniform, and wherein said material has at least one dimension greater than about 7 mm. 18. The implant material of claim 17 having a porosity between about 60 and about 80 volume percent. 19. The implant material of claim 17 which is hand-shapable at body temperature without loεε of εtructural integrity. 20. The implant material of claim 17 having a Young's modulus at room temperature of about 0.1 to about 200 MPa. 21. The implant material of claim 17 having a Young's modulus at room temperature of about 1.0 to about 1700 MPa. 22. A method for making a molded biodegradable, porous polymeric implant material of a desired geometry, wherein the pore size distribution throughout the material is subεtantially uniform, comprising: a) preparing a precipitated gel-like polymer mass; b) extracting liquid from the polymer mass; c) kneading the polymer masε to form an extensible composition; d) placing the extensible composition into a mold of the desired geometry having spaced perforations to allow escape of gas; and e) treating the extensible composition in the mold in a vacuum oven at a temperature and time sufficient to cure the composition. 23. The method of claim 22 wherein the polymeric composition is cured to a selected stiffness corresponding to the stiffnesε of bone or the stiffness of cartilage. 24. The method of claim 22 wherein the time and temperature of step e) are selected so as to produce a molded implant material having a Young's modulus selected from a value between about 0.1 and about 200 MPa at room temperature. 25. A molded, porous, biodegradable material made by the method of claim 22. 26. A sterilizable chunklet having a largest cross-section diameter between about 1 mm and about 4 mm made from the material of claim 25. 27. A chunklet of claim 26 having a Young's modulus greater than about l.0 MPa. 28. A multi-phase implant formed of implant materials of claim 22. 29. A two-phase implant formed of implant materials of claim 28 having an upper cartilage phase affixed to a lower bone phase. 30. A two-phase implant of claim 29 wherein the two phases are of different colors. 31. A particulate biodegradable implant material made by forming particles from the material of claim 25. 32. The particulate implant material of claim 31 made by shredding the material of claim 25 to an average particle size between about 250 and about 850 μm in diameter. 33. An autologous bone graft substitute material for a patient comprising the particulate implant material of claim 32 mixed with marrow and blood cells of said patient. 34. An autologous bone graft substitute material for a patient comprising chunklets of claim 26 mixed with marrow and blood cells of said patient. 35. A honeycomb material made by punching holes in a sheet of material of claim 28. 36. A method of implanting a biodegradable polymeric material having uniform pores for selected tissue ingrowth into a defect in a patient's body comprising: a) selecting a biodegradable polymeric implant composed of a material having a Poisεon'ε ratio leεε than about 0.3; b) hand-shaping said material to fit said defect; c) prior to, or after, hand-shaping said material, placing said material into said defect. 37. The method of claim 36 wherein said molded material of step b) is sutured into place. 38. A method of resurfacing a femoral condyle comprising: a) hand-shaping into place over at least a portion of the surface of said femoral condyle a sheet of a biodegradable, porous polymeric implant material having a Young's modulus between about 0.3 and about 1.0 MPa, a Poisson's ratio lesε than about 0.3, and a porosity between about 60 volume percent and about 90 volume percent, wherein the pore size distribution throughout the material is substantially uniform, and having an average pore size of between about 25 μm and about 400μm; b) securing said material to existing cartilage on said femoral condyle; and c) allowing cartilage tissue to ingrow into the pores of said material and subεequently allowing said material to biodegrade, whereby the surface of said femoral condyle is covered with a uniform layer of cartilage.

Inventors: LEATHERBURY NEIL C, NIEDERAUER MARK O, WALTER MARY ANN, LEATHERBURY, NEIL, C., NIEDERAUER, MARK, O., WALTER, MARY, ANN
Applicants: OSTEOBIOLOGICS INC, OSTEOBIOLOGICS, INC.
Application year: 1996
Main classification: A61L27
IPC codes: A61F2, A61L27, A61L31