LACTIDE-CONTAINING BONE FIXATION DEVICE

Abstract: A medical material which contains a lactide-containing polymer having weight-average molecular weight lower than 400000 as determined by GPC and has a lactide content of 4000 ppm or lower.

Description: The present invention relates to a bone fixation device and a process for producing the same. A medical material comprising polylactide which is used in operation has the advantage that it does not need to be taken out of the body after an operation since it is hydrolyzed and absorbed in vivo, while such medical material has a drawback that it is low in retention of strength and deteriorated in strength in vivo in a relatively short period of time. Molecular weight of polylactide has heretofore been increased to improve the strength thereof; however, polylactide tends to be deteriorated in thermal processability if the molecular weight is too high. For example, when polylactide is heated to exceed its melting point for molding, it is largely decreased in the molecular weight due to thermal decomposition. Thus, there is a limit for retaining the strength against heat with maintaining the same molecular weight. An object of the present invention is to provide a bone fixation device which is made of a material excellent in thermal processability and which maintains high strength for a long period of time, and a process for producing the same. Fig. 1 shows relationships among molecular weight, residual amount of lactide and retention of pull-out strength of each of the PLLA samples obtained in Preparation Examples 1-8. Fig. 2 shows measurement results of retention of flexural strength. Fig. 3 shows measurement results of retention of pull-out strength. Fig. 4 shows results obtained by testing the acetone-washed PLLA sample. The inventors conducted an extensive research considering the above drawbacks and found that poly-L-lactide (PLLA) having low molecular weight and high retention of strength can be obtained by lowering a content of low molecular lactic acid components, especially of lactide, to be contained therein. The present invention provides the following bone fixation device and process for producing the same. Item 1. A bone fixation device comprising a poly-L-lactide having a weight-average molecular weight of less than 400,000 measured by GPC, and having a lactide content of not more than 4,000 ppm, said bone fixation device being a pin, a screw, a nail, an intramedullary nail, a plate, a washer, a staple, an anchor or a wire. Item 2. A process for producing the above-defined bone fixation device, characterized by subjecting a poly-L-lactide (PLLA) to at least one step selected from the group consisting of (1) a step of vacuum thermodrying, (2) a step of washing with a solvent and (3) a step of reducing a pressure during polymerization to reduce the residual lactide content to not more than 4,000 ppm. s Item 3. A method for adjusting retention of strength of a bone fixation device comprising a poly-L-lactide (PLLA), the method comprising the step of adjusting the initial flexural strength to be not less than 100 MPa and the residual lactide content to be not more than 4,000 ppm by subjecting the lactide-containing polymer to a pressure-reducing step during polymerization. A content of lactide (L) in the backbone of the lactide-containing polymer is not less than 30 mol%, preferably not less than 50 mol%, more preferably not less than 70 mol%, yet more preferably not less than 80 mol%, particularly preferably not less than 90 mol%. Weight-average molecular weight of the PLLA measured by GPC is less than 400,000, preferably 50,000-350,000, more preferably 100,000-250,000, particularly preferably 130,000-210,000. Initial flexural strength of the bone fixation device of the present invention is about 100-300 MPa, preferably about 180-250 MPa. Retention of flexural strength of the bone fixation device of the present invention after being immersed in phosphate buffered saline (PBS) at 37°C for 90 days is not less than 80 %, preferably not less than 85 %, more preferably not less than 90 %. The low molecular weight lactic acid components encompass, in addition to lactide which is a starting material for polymerization, lactic acid, lactic acid dimer, lactic acid trimer and so forth which are formed by thermal decomposition in the course of melt molding. A content of such lactide is not more than 4,000 ppm, preferably not more than 3,500 ppm, more preferably not more than 3,000 ppm, yet more preferably not more than 2,000 ppm, particularly preferably not more than 1,000 ppm. Lactide can be reduced to be lower than a predetermined concentration by at least one step selected from the group consisting of (1) a step of vacuum thermodrying, (2) a step of washing with a solvent and (3) a step of reducing a pressure during polymerization. The step of vacuum thermodrying can be carried out at a pressure of about 1-10 mmHg, for about 10-20 hours and at a temperature of about 120° C. The step of washing with a solvent can be carried out by immersing the PLLA in an organic solvent such as acetone, ethyl acetate, butanol or the like, subjecting the PLLA to Soxhlet extraction for 5-12 hours, and then volatizing the organic solvent. The step of reducing a pressure during polymerization can be carried out by reducing a pressure by several tens of mmHg for several hours when the polymerization has proceeded to a certain stage. The bone fixation device of the present invention is obtainable by melt-molding or extruding the PLLA having a lactide content of not more than 4,000 ppm, drawing the molded article and pressing or cutting the drawn article. According to the present invention, even if the PLLA used as a starting material for producing the bone fixation device contains more than 4,000 ppm of lactide, a lactide content of the resulting bone fixation device comprising PLLA can be reduced to not more than 4,000 ppm by carrying out a step of vacuum thermodrying or a step of washing with a solvent (i) when the polymer is in the form of polymer chips, (ii) after the melt molding or extrusion, (iii) after the drawing or (iv) after processing as a product by pressing or cutting. The drawing step can be carried out by an arbitrary method such as a uniaxial, biaxial or multiaxial tension-drawing, solid phase extrusion, hydrostatic extrusion or the like. A draw ratio may typically be about not more than 10. The PLLA to be used for the bone fixation device of the present invention may optionally be undrawn. The bone fixation device of the present invention is obtainable by, for example, pelletizing the PLLA having a lactide content of not more than 4,000 ppm using a pelletizer; extruding the pellets by injection molding to obtain a molded article; drawing the molded article by hydrostatic extrusion at a temperature not lower than the glass transition temperature and not higher than the melting point; and cutting using a lathe, fraise and the like or pressing using a molding press. Examples of materials to be added to the PLLA of the bone fixation device of the present invention include bio-ceramics such as hydroxyapatite, A-W crystallized glass, alumina, zirconia, tricalcium phosphate (TCP), carbon, bio-glass, calcium phosphate and bone growth factors such as BMP (bone morphogenetic protein), TGF-β, IGF-I and IGF-II (insulin-like growth factors I and II). The bio-ceramics each has a particle size of about 1-300 µm, and the bone fixation device may comprise such bio-ceramics in an amount of about 1-90 %. These materials may be added to the PLLA as being kneaded therewith or as being applied as a coating on the surface of the bone fixation device. As to a process for producing the bone fixation device, it is possible to optimize the retention of flexural strength for each of the use applications by adjusting the amount of lactide to be comprised in the bone fixation device. A bone fixation device comprising PLLA having a lactide content of 100-4,000 ppm is suitably used for parts which are cured within 8-16 weeks such as bones. A bone fixation device comprising PLLA having a lactide content of 1-100 ppm is suitably used as a bone fixation device, which is required to retain strength for over 16 weeks after an operation (ex. leg extension operation). A medical material having a lactide content of 5,000-8,000 ppm can be used for parts which are cured within 1-2 weeks such as the muscle and skin. A medical material having a lactide content of 4,000-5,000 ppm can be used for parts which are cured within 2-8 weeks such as the lung and liver. The bone fixation device of the present invention can be remarkably improved in retention of various kinds of strength such as pull-out strength, tensile strength and so on in addition to flexural strength even if the bone fixation device is a low molecular PLLA. The present invention is illustrated based on, but is not limited to, the following examples. Lactide contents are measured by HPLC under the following conditions. Detector: RID-6A (product of Shimadzu Corporation); Pump: LC-9A (product of Shimadzu Corporation); Column Oven: CTO-6A (product of Shimadzu Corporation); Column: Serially-arranged Shim-pack GPC-801C, -804C, 806C, -8025C (products of Shimadzu Corporation); Analysis Conditions: Solvent: chloroform; Flow Rate: 1 ml/min; Amount of Sample: 200 µl (0.5 W/W% of a sample was dissolved in chloroform); Column Temperature: 40° C. The weight-average molecular weights are measured by GPC under the following conditions. Detector: RID-6A (product of Shimadzu Corporation); Pump: LC-6A (product of Shimadzu Corporation); Column Oven: CTO-6A (product of Shimadzu Corporation); Column: Serially-arranged shodex K-802, K-804 and K-806 (products of Showa Denko K. K.); Analysis Conditions: Solvent: Chloroform; Flow Rate: 1 ml/min; Amount of Sample: 100 µl (0.5 W/W% of a sample was dissolved in chloroform); Column Temperature: 40°C. Preparation Examples 1-8: Preparations of Poly-L-lactide PLLA polymers (Lot Nos. 1-8) were synthesized by conventional bulk ring opening polymerization of L-lactide. Each of the obtained PLLA materials (granular) had weight-average molecular weight of 200,000-220,000. The granular PLLAs were subjected to vacuum thermodrying (under a reduced pressure of 1-10 mmHg, at 120°C, for 10 hours) to give granular PLLA polymers that were reduced in lactide contents. Each of the granular PLLA polymers was directly molded by an extruder without being pelletized, at a molding temperature of 200-210°C to give a PLLA bullet in the form of a cylinder having a diameter of 16 mm. The PLLA bullets were subjected to vacuum thermodrying (under a reduced pressure of 1-10 mmHg, at 120°C, for 10 hours) to give PLLA bullets that were reduced in lactide contents. Further, each of the PLLA bullets was drawn at a draw ratio of 2.5 by using a hydrostatic extruder at a mold temperature of 120-140°C to prepare a PLLA rod of 10 mm. Each of the PLLA drawn rods was cut to prepare a PLLA screw sample having a crest diameter of 4.5 mm and a overall length of 30 mm (samples No. 1-No. 8). Weight-average molecular weights measured by GPC and residual lactide contents of the samples measured by HPLC are shown in Table 1. Test Example 1: Measurements of retention of pull-out strength After immersing the screw samples Nos. 1-8 obtained by Preparation Examples 1-8 in PBS (pH 7.4) at 37°C for 3 months, retention of pull-out strength of each of the samples was measured. A pull-out test was carried out by using AUTOGRAPH AG-5000B, a product of Shimadzu Corporation, at a test speed of 1 mm/min. By way of a degradability test, measured were physical properties of the samples which had been immersed in a phosphate buffered saline (PBS) and taken out after a predetermined period of time. Results are shown in Table 1 and Fig. 1. Sample Molecular Weight Residual Lactide (ppm) Retention of Pull-out Strength (%) No. 1 189,000 1319 93 No. 2 176,000 1442 90 No. 3 149,000 1874 89 No. 4 187,000 3298 87 No. 5 181,000 3959 82 No. 6 179,000 4236 70 No. 7 174,000 4569 45 No. 8 169,000 4638 26 From the results shown in Table 1 and Fig. 1, it was confirmed that the retention of pull-out strength changes sharply at around 4,000 ppm of the residual lactide. Test Example 2: Measurement of retention of flexural strength Granular PLLA polymers each having a lactide content of 100 ppm, 2,500 ppm and 5,000 ppm were synthesized by conventional bulk ring opening polymerization of L-lactide. Obtained granular PLLA polymers were subjected to vacuum thermodrying (at a reduced pressure of 1-10 mmHg, at 120°C, for 10 hours) to give granular PLLA polymers reduced in lactide contents. Each of the granular PLLA polymers was directly molded by an extruder without being pelletized at a molding temperature of 200-210°C to give a PLLA bullet in the form of a cylinder having a diameter of 16 mm. The PLLA bullets were subjected to vacuum thermodrying (under a reduced pressure of 1-10 mmHg, at 120°C, for 10 hours) to give PLLA bullets reduced in lactide contents. Further, each of the PLLA bullets was drawn at a draw ratio of 2.5 by using a hydrostatic extruder at a molding temperature of 120-140°C to prepare a PLLA rod of 10 mm. The PLLA drawn rods were cut to give pins (3.5 mm) as samples (each having a lactide content of 80 ppm (weight-average molecular weight measured by GPC of 130,000), 1,400 ppm (weight-average molecular weight measured by GPC of 156,000) and 4,800 ppm (weight-average molecular weight measured by GPC of 131,000). Retention of flexural strength of each of the samples was measured after being immersed in PBS (pH 7.4) at 37°C for 10 months. In addition, the flexural strength was measured in accordance with JISK7203. The retention of flexural strength (%) was calculated from an expression of (flexural strength after being immersed in PBS for a predetermined period of time)/(initial flexural strength) x 100. Results are shown in Fig. 2. The symbols in Fig. 2 denote as follows: ◆ (lactide content of 80 ppm),  (lactide content of 1,400 ppm) and ▴ (lactide content of 4,800 ppm). Retention of flexural strength after 3 months of immersion of the sample containing 4,800 ppm of lactide is 50 %, that of the sample containing 1,400 ppm of lactide is 85 %, and that of the sample containing 80 ppm of lactide is 95 %. Test Example 3: As to retention of pull-out strength The screw samples each containing 80 ppm, 1,400 ppm and 4,800 ppm of lactide, which were obtained by cutting PLLA drawn rods used in Test Example 2, were measured for retention of pull-out strength in the same manner as in Test Example 1. Results are shown in Fig 3. The symbols in Fig. 3 denote as follows: ◆ (lactide content of 80 ppm), (lactide content of 1,400 ppm) and ▴ (lactide content of 4,800 ppm). Retention of flexural strength after 3 months of immersion of the sample containing 80 ppm of lactide is 98 %, that of the sample containing 1,400 ppm of lactide is 90 %, and that of the sample containing 4,800 ppm of lactide is 15 %. Test Example 4: Washing with acetone Granular PLLA polymer having weight-average molecular weight of 1273,400 (measured by GPC) was subjected to vacuum thermodrying (under a reduced pressure of 1-10 mmHg, at 120°C, for 10 hours) to give granular PLLA polymer reduced in a lactide content. The granular PLLA polymer was pelletized, and a part the PLLA pellets was washed with acetone for 10 hours using a Soxhlet extractor. The washed PLLA pellets were placed in a vacuum thermodryer to remove acetone under the conditions at 120°C, for 10 hours under 1-10 mmHg. Each of the acetone-washed PLLA pellets and acetone-unwashed PLLA pellets were subjected to bullet molding by using an injection molding machine, drawing using a hydrostatic extruder (at a draw ratio of 2.5) and a cutting process in the same manner as in Preparation Examples 1-8, thereby preparing PLLA screw samples each having a crest diameter of 4.5 mm and an overall length of 30 mm. The PLLA screw sample which was not washed with acetone had a lactide content of 4,200 ppm and weight-average molecular weight of 250,000. In turn, the screw sample which was washed with acetone had a lactide content of 1,000 ppm and weight-average molecular weight of 310,000. Results are shown in Table 2, Table 3 and Fig. 4. In Fig. 4,  denotes the acetone-washed PLLA sample and ▪ denotes the acetone-unwashed PLLA sample. Molecular Weight, Retention of Flexural Strength and Retention of Pull-Out Strength of Acetone-Washed PLLA Sample Time of Immersion Molecular Weight Retention of Flexural Strength (%) Retention of Pull-out Strength Initial Value 310,000 100 100 4 weeks 280,000 102 91 8 weeks 230,000 102 95 12 weeks 180,000 108 96 16 weeks 170,000 98 - Molecular Weight, Retention of Flexural Strength and Retention of Pull-Out Strength of Acetone-Unwashed PLLA Sample Time of Immersion Molecular Weight Retention of Flexural Strength (%) Retention of Pull-out Strength Initial Value 250,000 100 100 4 weeks 140,000 92 53 8 weeks 70,000 47 3 12 weeks 30,000 17 3 16 weeks 30,000 7 -

Claims: A bone fixation device comprising a poly-L-lactide (PLLA) having a weight-average molecular weight of less than 400,000 measured by GPC, and having a lactide content of not more than 4,000 ppm, said bone fixation device being a pin, a screw, a nail, an intrame-dullard nail, a plate, a washer, a staple, an anchor or a wire. A process for producing a bone fixation device comprising a poly-L-lactide (PLLA) having a weight-average molecular weight of less than 400,000 measured by GPC, and having a lactide content of not more than 4,000 ppm, said bone fixation device being a pin, a screw, a nail, an intramedullary nail, a plate, a washer, a staple, an anchor or a wire, characterized by subjecting a poly-L-lactide (PLLA) to at least one step selected from the group consisting of (1) a step of vacuum thermodrying, (2) a step of washing with a solvent and (3) a step of reducing the pressure during polymerization to reduce the residual lactide content to not more than 4,000 ppm. A method for adjusting the retention of strength of a bone fixation device comprising a poly-L-lactide (PLLA), the method comprising the step of adjusting the initial flexural strength to be not less than 100 MPa and the residual lactide content to be not more than 4,000 ppm by subjecting the lactide-containing polymer to a pressure-reducing step during polymerization.

Inventors: KAGAWA HIDEJI, SHIMAMOTO TAKESHI, KAGAWA, HIDEJI, SHIMAMOTO, TAKESHI, KAGAWA, Hideji Medical Material Ctr. of Gunze Ltd., SHIMAMOTO, Takeshi Medical Material Ctr. Gunze Ltd
Applicants: GUNZE KK, GUNZE LIMITED
Application year: 1999
Main classification: A61L27
IPC codes: A61L31, C08G63, A61L17, A61L27, A61L29