Bone fixation materials and methods for preparation

Abstract: The present invention relates to a bone fixation material comprising a polymer which is biodegradable and bioabsorbable into the living body organism, a hydroxyapatite and optionally an alkaline inorganic compound, having an initial bending strength at break of more than 250 MPa, maintaining its strength more than 3 months in a living body organism and being biodegradable and bioabsorbable into and eliminatable from an organism within a range of 6 months to 3 years and a process of preparing a bone fixation material comprising: (A) melting a polymer which is biodegradable and bioabsorbable into and eliminatable from an organism, a hydroxyapatite and optionally an alkaline inorganic compound, (B) applying molecularly orientated means selected from the group consisting of a uniaxial extension, an extruding extension, a milling and compression molding, to the melt of (A) at an extendible and moldable temperature, (C) extending and orienting the chain molecules of the polymer parallel to the major axis or plural axes.

Description: The present invention relates to bone fixation materials easily biodegrading and bioabsorbing into the living body organism, and methods for preparing the materials. More particularly, the present invention relates to bone fixation materials for bone medical treatment use in plastic surgery, oral surgery, thoracic surgery and in addition animal bone treatment, and methods for preparing these materials. Artificial medical devices having a high bending strength are employed in order to redress bone fractures or support rib for fractured bone treatment in plastic surgery, oral surgery, and thoracic surgery. It is necessary that the bone fixation materials such as plates and screws are applied in the period the bone is redressed and are removed as soon as possible after healing in order to prevent the bone from weakening. Nowadays, although the bone fixation materials for clinical use are almost all composed of metals or ceramics, there are problems that materials cause the bone to degrade, because they have too high level modulus, or happen to biodegrade or metal ions are possibly dissolved into the bone. If materials having similar modulus to the bone, and which are biodegraded and bioabsorbed into a living organism, are employed in order to redress the bone fracture, various evil effects on the redressing of the bone will disappear because no foreign substances are located in the living body for a long time. According to the above approach lactic acid polymers which have the characteristic of being biodegraded and bioabsorbed into the living body organism are presently employed as bone fixation materials in clinical application use. It has been well-known for many years that the lactic acid polymers could be employed as bone fixation materials. For example, Japanese applied patent 59-97654 discloses a method to synthesize the lactic acid polymer, but fails to disclose a lactic acid polymer composite having high level modulus and bending strength. Japanese patent 3-63901 discloses molding conditions, in which a lactic acid polymer composite containing hydroxyapatite and having the characteristic of being biodegraded and bioabsorbed into the living body organism is improved by uniaxial extension, mechanical properties such as modulus and bending strength useful of bone fixation materials. PCT patent WO/05312 discloses other molding conditions similar to said patent, in which a lactic acid polymer rod was uniaxially extended to a relatively high draw ratio (7 to 12 times) at a relatively high temperature and attained a high level bending strength and modulus by fibrillation. Japanese applied patent 1-198553 discloses bone cementing materials, the lactic acid polymer of which was composed of a viscosity-average molecular weight of more than 200,000 after molding and uniaxial extension. Furthermore, Japanese applied patent 9-135892 also discloses molding conditions, in which molecules were oriented parallel to not the main-, but to the plural axes during molding in order to reduce anisotropy of the bending strengths in a composite. European Patent application EP0 795 336 A discloses a material for osteosynthesis comprising polylactic acid or a polylactic/polyglycolic acid copolymer with hydroxyapotite dispersed therein. The material in axially oriented by compresion moulding. However, the patents mentioned above only concern initial bending strength and the hydrolyzing durability of bone fixation materials and fail to disclose its bending strength in a living body organism and its durability. There is not enough description concerning biodegradation and bioabsorption into the living body organism after redressing, and especially no description of the method to promote biodegradation and the bioabsorption after the bending strength has left. It takes only about 6 to 8 months to perfectly redress a bone fracture. However, the bending strength of the bone connecting materials which are biodegradable and bioabsorbable, gradually decreases after implanting into the body in accordance with the decrease of the molecular weight, and comes to 0 within 4 months. The molecular weight of the materials reduces from about 200,000 at first to several thousands, to several tens of thousands, while a lot of granular crystallite still remains for 3 to 5 years (J.E. Bergsma et al., Biomaterials, 16(4), 267-274, 1995). As a result, when the bone connecting materials mentioned above are employed for a material to redress bone fractures, it takes, for example, from 3 to 5 years for the fibrous tissues to perfectly retrieve bone tissue, because the velocity of biodegradation and bioabsorption is too slow. In addition, recreated bones cannot invade into an implanting with parts of the bone connecting materials even after healing and, therefore, ulcers in the bone remain almost forever. It causes a reduction in mechanical tenacity which bears a load at first. Added to the above problem, many serious late complications can occur after healing (J.E. Bergsma et al., Biomaterials, 16(1), 25-32, 1995). The present invention relates to a bone fixation material comprising a polymer which is biodegradable and bioabsorbable into the living body organism, a hydroxyapatite and an alkaline inorganic compound, having an initial bending strength at break of more than 250 MPa, maintaining its strength more than 3 months in a living body organism and being biodegradable and bioabsorbable into and eliminatable from an organism within a range of 6 months to 3 years. It is preferred that the chain molecules of the polymer are extended and oriented parallel to the major axis or plural axes. Preferably the polymer is a lactic acid polymer composed of an L type and D,L type mixture of 70 to 100% in a mole percentage of the L type, or a copolymer of a lactide and a glycolide. It is also preferred that the composite ratio of the polymer and the hydroxyapatite is in a weight ratio range of 99:1 to 60:40, and the composite ratio of the hydroxyapatite and the alkaline inorganic compound is in a weight percentage range of 99.9:0.1 to 80:20. Furthermore, the present invention relates to a process of preparing a bone fixation material comprising (A) melting a polymer which is biodegradable and bioabsorbable into and eliminatable from an organism, a hydroxyapatite and an alkaline inorganic compound, (B) applying molecularly orientated means selected from the group consisting of a uniaxial extension, an extruding extension, a milling and compression molding, to the melt of (A) at an extendible and moldable temperature, (C) extending and orienting the chain molecules of the polymer parallel to the major axis or plural axes. It is preferred that the extension is a ram extrusion at high pressure or a hydrostatic extrusion in a temperature range of 80 to 160°C. The bone fixation material of the present invention is composed of materials selected from the group consisting of a polymer with the characteristic of being biodegradable and bioabsorbable into the living body organism, a hydroxyapatite and alkaline inorganic compound. The polymer is preferably a lactic acid polymer, more preferably an L type lactic acid polymer having more than 95% optical purity. However, it is highly preferable that it is in an optical purity range of 80 to 95%, to prevent the polymer from having too high crystallinity. Of course, the crystallinity relates to the initial modulus and bending strength at break and a bone fixation material having high tenacity and modulus is easily composed from a high crystallinity polymer. However, as objective tenacity and modulus are possibly derived from the orientation of the molecular chain in a crystalline state during molding, the crystallinity is preferably lowered in order to be better bioabsorbable into and eliminatable from the organism as soon as possible after biodegradation. It is highly preferred in the present invention that the polymer, as a raw material, is a lactic acid polymer composed of an L type and D,L type mixture of 85 to 95% in a mole percentage of the L type, or a copolymer of a lactide and a glycolide. The polymer is preferably composed of viscosity-average molecular weight of more than 80,000, and more preferably in a range of 100,000 to 500,000. The hydroxyapatite, such as a cow bone powder or its mixture instead of a pure hydroxyapatite, is preferably employed in the present invention. Because the cow bone powder also is alkaline, it is preferred that the hydroxyapatite is alkalized by an alkaline inorganic compound beforehand. Furthermore, the hydroxyapatite can possibly be fabricated by the method, in which a ceramic is heated and mixed by a supersonic or hyper-sermere and decomposed by heating. The alkaline inorganic compound includes, for example, a hydroxide group containing molecule consisting of potassium hydroxide, calcium hydroxide and magnesium hydroxide, a chloride group containing molecule consisting of potassium chloride, calcium chloride, magnesium chloride and zinc chloride, further alkali metal salts, and basic organic compounds such as amine, and is cohered on a hydroxyapatite surface. A chloride compound such as calcium chloride can possibly be employed in spite of its original neutrality because the chloride compounds sold in the market contain several percentage points of calcium hydroxides and their chloride solutions show strong alkalinity. The composite ratio of the polymer and the hydroxyapatite is preferably in a weight ratio range of 99:1 to 60:40, and more preferably 80:20 to 70:30. The composite ratio of the hydroxyapatite and the alkaline inorganic compound is preferably in a weight percentage range of 99.9:0.1 to 80:20, and more preferably 95:5 to 90:10. It is preferred that the alkali inorganic compound is mixed with the hyroxyapatite, or that the hydroxyapatite is dipped in the alkali inorganic compound solution in order to coat the hydroxyapatite surface. A composite of a lactic acid polymer with the characteristic of being biodegradable and bioabsorbable into the living body organism or a copolymer of a lactide and a glycolide having more than 100,000 molecular weight, and a hydroxyapatite having an excellent bone affinity, is preferably employed as a bone fixation material and is uniaxially extended in order to maintain bending strength at break of more than 250 MPa for a period of more than 3 months until the bone fracture is redressed in the organism. However, the bone fixation material mentioned above certainly improves the initial bending strength and hydrolyzing durability, but it does not improve the maintenance of the bending strength in a living body organism and its durability while implanting into the body. Therefore, the materials cannot completely disappear from the organism and the bending strength cannot come to 0 within 4 months. According to the present invention the material is rendered biodegradable and bioabsorbable into the living body organism as soon as possible after the bending strength comes to 0. It is based on the finding that the lactic acid polymer with the characteristic of being biodegradable and bioabsorbable into the organism is a kind of fatty acid ester, and is hydrolyzed by an enzymatic reaction in the living organism and this hydrolyzing velocity is low in an acid state and high in an alkali state (Flyon et al., Degradation of high molecular weight poly L-lactide in alkaline medium, Biomaterials, 16, 833-843, 1995). Although the lactic acid polymer composite shows almost no molecular weight decrease caused by the hydrolysis in the organism within 4 months, then water molecules deeply invade into the organism and the composite shows a large molecular decrease as a whole owing to hydrolysis. In an ordinary case, the hydrolysis in the organism takes place as follows: 1. When a partial pH changes to acid state in the accordance with hydrolysis, 2. The rate of molecular weight decrease changes slowly, 3. And prior hydrolysis occurs in an amorphous area at the same time, 4. Crystallite, having a relatively low molecular weight (1,000 to 10,000), remains in the bone organism and, 5. The hydrolyzing velocity further slows. According to the present invention, it is possible that when water molecules invade into the bone organism after about 4 months, hydrolyzing velocity becomes higher and the material is then biodegraded and disappears from the organism owing to the increase in pH in the bone part to an alkali state (pH of more than 7). The method of making a bone fixation material in the present invention is as follows: (A) a melt characteristic to be biodegradable and bioabsorbable into the living body organism is fabricated from a polymer with the characteristic of being biodegradable and bioabsorbable into the living body organism, a hydroxyapatite and an alkaline inorganic compound. (B) The melt is applied in a desirable shape, such as rod or sheet, by molecularly orientated means selected from the group consisting of a uniaxial extension, an extension extrusion, a milling and compression molding, at an extendible and moldable temperature. (C) The melt composite is extended and oriented parallel to major axis or plural axes. Molecular orientation denotes an orientation in polymer molecule which is occurred in crystallitic, pseudo-crystallitic or amorphous regions by means of a uniaxial extension, an extruding extension, a milling and a compression molding. A ram extrusion at a high pressure or a hydrostatic extrusion are preferably employed as the extension means in order to create a material having an initial bending strength of more than 250 MPa. The temperature, as stated above, is preferably in a range of a glass transition temperature (about 60°C) to a melting temperature (about 180°C) of the lactic acid polymer, more preferably in a range of 80 to 140°C. In conclusion, the bone fixation material of the present invention which has the characteristic of being biodegradable and bioabsorbable into the living body organism is a material having both initial high bending strength and a promotion effect on biodegradation and bioabsorption after 6 months, and therefore, it is nearly an ideal one, and it is impossible to be gained from a usual bone fixation material which is composed of a usual lactic acid polymer. The bone fixation materials of the present invention are useful for bone medical treatment use in plastic surgery, oral surgery, thoracic surgery and animal bone treatment involving shaped objects, including plates and screws. It works in the period it takes for the bone to redress and is removed as soon as possible after healing in order to prevent the bone from weakening. EXAMPLES These examples give a detailed explanation, but do not restrict the scope of the present invention. The compressed bending strengths and modulus mentioned in the examples are estimated in accordance with the three-point bending test described in JIS-K. Examples 1 to 6 An L lactic acid polymer, and a copolymer of L lactide and D,L lactide were synthesized from respective raw materials sold in the market as a polymerization initiator of tin octylate by ring opening polymerization under reduced pressure. The copolymer composite ratios of L lactide and D,L lactide were 100:0 and 85:15. Viscosity average molecular weights of said polymer and copolymer were about 360,000, respectively. The alkalized hydroxyapatite was fabricated by dipping hydroxyapatite (HA) powder (calcined at 1200°C to 5.5 µm average particle diameter) in a 30% calcium hydroxide solution and drying. Molded rods of about 10 mm were fabricated by the method, in which a mixture of the above 2 polymers and the alkalized hydroxyapatite powder at a mixture ratio of 70:30 by weight percentage was dried a day and a night at a relatively high temperature (120 to 130°C) under reduced pressure and extruded at about 200°C. The rods were extended 3 times at 120°C in liquid paraffin and deformed to about one-fifth of the original by hot pressing at 140°C. Furthermore, the rods were extended to 3 x extended magnification in a glycerin bath by a hydrostatic extrusion. Bending strengths of the molded bars are shown in Table 1. Polymer (%) Alkalized Hydroxyapatite (wt. %) Initial Bending Strength (MPa) PLLA 100% P(L-LA-co-DL-LA) 85/15% Example 1 Uniaxial Extension(x3) 100 0 30 280 Example 2 Compression Molding(x1/5) 100 0 30 310 Example 3 Hydrostatic Extrusion(x3) 100 0 30 370 Example 4 Uniaxial Extension(x3) 0 100 30 250 Example 5 Compression Molding(x1/5) 0 100 30 255 Example 6 Hydrostatic Extrusion(x3) 0 100 30 330 Comparative Examples 1 to 6 A copolymer of L lactide and D,L lactide was synthesized as 85:15 of a copolymer composite ratio of L lactide and D,L lactide similar to example 1. Three kinds of molded rods were fabricated without the alkalized hydroxyapatite, similarly to the method mentioned in example 1. Bending strengths of the molded rods are shown in Table 2. Polymer (%) Alkalized Hydroxyapatite (wt. %) Initial Bending Strength (MPa) PLLA 100% P(L-LA-co-DL-LA) 85/15% Comparative Example 1 Uniaxial Extension(x3) 100 0 0 220 Comparative Example 2 CompressionMolding(x1/5) 100 0 0 240 Comparative Example 3 Hydrostatic Extrusion(x3) 100 0 0 280 Comparative Example 4 Uniaxial Extension(x3) 0 100 0 210 Comparative Example 5 CompressionMolding(x1/5) 0 100 0 220 Comparative Example 6 Hydrostatic Extrusion(x3) 0 100 0 250 Examples 7 to 12 The molded rods fabricated in the above examples 1 to 6 were dipped in a 10 ml physiological solution and weight variations of the molded rods were observed after hydrolyzing a long time (about 1 to 2 years) at 37°C. Polymer (%) Alkalized Hydroxyapatite (wt. %) Residual Material(%) PLLA 100% P(L-LA-co-DL-LA) 85/15% After 1 Year After 2 Years Example 7 Uniaxial Extension(x3) 100 0 30 23 0 Example 8 CompressionMolding(x1/5) 100 0 30 27 0 Example 9 Hydrostatic Extrusion(x3) 100 0 30 31 0 Example 10 Uniaxial Extension(x3) 0 100 30 0 0 Example 11 CompressionMolding(x1/5) 0 100 30 0 0 Example 12 Hydrostatic Extrusion(x3) 0 100 30 0 0 Comparative Examples 7 to 12 The molded rods fabricated in the above comparative examples 1 to 6 were dipped in a 10 ml physiological solution and weight variations of the molded rods were observed after hydrolyzing a long time (about 1 to 2 years) at 37°C similarly to examples 7 to 12. Polymer (%) Alkalized Hydroxyapatite(wt. %) Residual Material(%) PLLA 100% P(L-LA-co-DL-LA) 85/15% After 1 Year After 2 Years Comparative Example 7 Uniaxial Extension(x3) 100 0 30 62 32 Comparative Example 8 Compression Molding(x1/5) 100 0 30 65 37 Comparative Example 9 Hydrostatic Extrusion(x3) 100 0 30 73 45 Comparative Example 10 Uniaxial Extension(x3) 0 100 30 48 30 Comparative Example 11 Compression Molding(x1/5) 0 100 30 57 30 Comparative Example 12 Hydrostatic Extrusion(x3) 0 100 30 59 31 As shown in table 1, the bone fixation materials with the characteristic of being biodegradable and bioabsorbable into a living body organism, mentioned in the examples 1 to 6 of the present invention, have bending strengths of more than 250 MPa after applying molecularly orientated means selected from the group consisting of a uniaxial extension, an extension extrusion, a milling and compression molding to a mixture composed of an alkalized hydroxyapatite and a polymer with the characteristic of being biodegradable and bioabsorbable into the organism. In addition, as shown in table 3 of examples 7 to 12, residual material has completely disappeared from the organism and the weight of the material is reduced to 0 within 2 years from beginning the hydrolyzing experiment. As a result, the molded composite is shown to be easily decomposed by the effect of the alkalized hydroxyapatite. On the other hand, as shown in table 4 of comparative examples 7 to 12, if a molded bar contains no alkalized hydroxyapatite, residual material of more than 30% remains in the organism even after 2 years of beginning the hydrolyzing experiment, and the biodegradation velocity looked very slow. The difference is clearly seen between both these examples and the comparative examples.

Claims: A bone fixation material comprising a polymer which is biodegradable and bioabsorbable into the living body organism, and a hydroxyapatite, characterized in that it further comprises an alkaline inorganic compound. The material according to claim 1, wherein the chain molecules of the polymer are extended and oriented parallel to the major axis or to plural axes. The material according to one of claims 1 or 2, wherein the polymer is a lactic acid polymer composed of an L type and D,L type mixture of 70 to 100% in a mole percentage of L type. The material according to one of claims 1 or 2, wherein the polymer is a copolymer of a lactide and a glycolide. The material according to one of claims 1 to 4, wherein the composite ratio of the polymer and the hydroxyapatite is in a weight ratio range of 99:1 to 60:40. The material according to one of claims 1 to 5, wherein the composite ratio of the hydroxyapatite and the alkaline inorganic compound is in a weight percentage range of 99.9:0.1 to 80:20. A process of preparing a bone fixation material comprising: (A) melting a polymer which is biodegradable and bioabsorbable into and eliminatable from an organism, a hydroxyapatite and an alkaline inorganic compound, (B) applying molecularly orientated means selected from the group consisting of a uniaxial extension, an extruding extension, a milling and compression molding, to the melt of (A) at an extendible and moldable temperature, (C) extending and orienting the chain molecules of the polymer parallel to the major axis or plural axes. The process according to claim 7, wherein the extension is a ram extrusion at high pressure or a hydrostatic extrusion. The process according to one of claims 7 or 8, wherein the temperature is in a range of 80 to 160°C. The process according to one of claims 7 to 9, wherein the polymer is a lactic acid polymer composed of an L type and D,L type mixture of 70 to 100% in a mole percentage of L type. The process according to one of claims 7 to 9, wherein the polymer is a copolymer of a lactide and a glycolide. The process according to one of claims 7 to 11, wherein the composite ratio of the polymer and the hydroxyapatite is in a weight ratio range of 99:1 to 60:40. The process according to one of claims 7 to 12, wherein the composite ratio of the hydroxyapatite and the alkaline inorganic compound is in a weight percentage range of 99.9:0.1 to 80:20.

Inventors: GEN SHOKYU, GEN, SHOKYU
Applicants: BMG INC, BMG INCORPORATED
Application year: 1998
Main classification: A61L31
IPC codes: A61L27, A61L31