JP2005211353A - Artificial vertebral pulp disk and method for improving its wear resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To raise load supporting property in an artificial vertebral pulp disk made of a PVA hydrogel. <P>SOLUTION: The artificial vertebral pulp disk made of the polyvinyl alcohol hydrogel is for performing prosthesis of the vertebral pulp of the intervertebral disk, and has two upper and lower sliding faces arranged mutually nearly back to back which slide with the vertebral body cartilage and side faces arranged between these sliding faces. The moisture content at the portion near the sliding faces is 20-50%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an artificial nucleus pulposus disk used in fields such as orthopedics and neurosurgery, in particular, an intervertebral disc that has been damaged due to a disease or accident such as an intervertebral disc herniation, and only the nucleus pulposus that is the main component of the intervertebral disc. The present invention relates to an artificial nucleus pulposus disk made of polyvinyl alcohol (PVA) hydrogel for prosthesis and a method for producing the same.

  The intervertebral disc is a joint having a very complicated function and structure, both functionally and anatomically. These consist of functional structures of the annulus fibrosus, the vertebral endplate, and the nucleus pulposus.

  In the intervertebral disc, degeneration or damage may occur due to trauma, disease, or aging. In this case, the nucleus pulposus forms a hernia and protrudes into the intervertebral foramina. The protruding nucleus pulposus compresses the spinal nerve and causes pain and paralysis of the surrounding tissues, or pain and paralysis of the lower limbs.

  For patients with intervertebral disc herniation and other diseases that currently play an important role in vivo, intervertebral bodies using autograft bone, bone cement, or artificial vertebral spacers after removal of the affected disc Fixation is being performed.

  However, these treatments are aimed at fixing the vertebral body and inherently ignore the mechanical properties of the intervertebral disc such as flexibility and load buffering action. This adversely affects the adjacent upper and lower vertebral bodies, causing secondary disc damage.

  On the other hand, intervertebral disc herniation is known to be caused by aging of the nucleus pulposus. For relatively mild cases, treatment that replaces only the aging nucleus pulposus is preferable to vertebral body fusion. In other words, partial and total replacement treatment is performed with an artificial prosthesis that has the function of maintaining normal intervertebral body space and supplementing the sufficient mobility of the intervertebral disc while imitating the natural physiological function of the intervertebral disc. It is desirable.

  Therefore, Bao Qi-Bin et al. Have attempted a prosthesis of living nucleus pulposus with hydrogel (see Patent Documents 1, 2, and 3). Charles D. Ray and others have also produced a prosthesis for the purpose of replacing the nucleus pulposus in combination with polyethylene and hydrogel (see Patent Documents 4 and 5).

  The PVA hydrogel is suitable as an artificial nucleus pulposus disk material because it has a repeated load resistance and a shock relaxation property due to a pump action such as discharge and suction of structural water in the hydrogel.

  These artificial nucleus pulposus discs are inserted into a region surrounded by vertebral body cartilage called an annulus fibrosus and a vertebral body end plate in a diseased intervertebral disc region. The inserted artificial nucleus pulposus disc vigorously rubs the adjacent vertebral body cartilage while receiving a load several times the body weight in vivo.

The surface roughness of the sliding surface with the vertebral body cartilage in the artificial nucleus pulposus disc shown in Patent Documents 1, 2, and 3 is about 2 to 5 μm, and the moisture content in the vicinity of the sliding surface is about 70 to 99%. there were. On the other hand, the sliding surface with the vertebral body cartilage in the artificial nucleus pulposus disc shown in Patent Documents 4 and 5 is embodied by a polymer fabric jacket such as polyethylene, and the surface roughness is about 0.5 mm, Furthermore, the water content in the vicinity cannot be expected.
US Patent Publication No. 5047055 US Patent Publication No. 5192326 US Patent Publication No. 5976186 US Patent Publication No. 5824903 US Patent Publication No. 6132465

  However, since the artificial nucleus pulposus disk described above had a moisture content of about 70 to 99%, the load supporting ability was low, and there was a fear that the hernia state could be reproduced due to deformation caused by a high load load. The reconstruction of the tissue relied on the condition of the surrounding fiber ring. That is, when the state of the annulus is incomplete, the function as an intervertebral disc is not achieved, and there is a risk of failure due to dropout. In addition, if the load supportability is insufficient, an instrument for vertebral body fixation is required to make up for it, which may increase the burden on the patient.

  In view of the problems of the prior art, the present invention increases the load support in an artificial nucleus pulposus disk made of PVA hydrogel, and thereby has a sufficient intervertebral disc load support function without depending on the state of the surrounding annulus. An object of the present invention is to provide an artificial nucleus pulposus disk having the same.

  In order to solve the above problems, the artificial nucleus pulposus disk of the present invention is an artificial nucleus pulposus disk made of polyvinyl alcohol hydrogel for prosthetic nucleus pulposus damaged by intervertebral disc disease, and slides back to back with vertebral body cartilage. The upper and lower two sliding surfaces and the side surface arranged between these sliding surfaces are disposed, and the water content in the vicinity of the sliding surface is 20 to 50%.

  According to the configuration of the present invention, sufficient load supporting ability can be exhibited with a moisture content of 20 to 50%. By exhibiting sufficient load supportability, the artificial nucleus pulposus disk alone can support a load several times the weight generated when a human lives. Therefore, it is possible to avoid the risk of breakage, excessive deformation, dropout and the like during load application. In addition, there is no need for a vertebral body assisting instrument and the burden on the patient can be reduced.

  Moreover, according to the moisture content in the above range, it is possible to positively influence the crosslinking efficiency by gamma rays irradiated to improve the wear characteristics. The water content of the PVA hydrogel is more preferably 25 to 35%.

  When the moisture content is less than 20%, a sufficient amount of water cannot be obtained at the interface with the vertebral body endplate cartilage that is in contact, and a lubricated wear state cannot be created. In addition, it is extremely difficult to produce a homogeneous hydrogel having a saturated water content of 20% or less, which is industrially disadvantageous.

  On the other hand, when the water content exceeds 50%, it is difficult to satisfy the mechanical properties such as compression stiffness, axial Young's modulus, torsional stiffness, and torsional Young's modulus of human intervertebral disc, and fulfill the original function of the intervertebral disc. Becomes difficult.

In this invention, the measuring method of the moisture content of PVA hydrogel is performed as follows. The PVA hydrogel is sufficiently dried at 40 to 60 ° C. in a vacuum, and the weight (W ₁ ) at the time of drying is measured. After dry weight measurement, immerse in warm water at 40 ° C for 48 hours or more. After the water content of the hydrogel reaches saturation, it is quickly transferred to a weighing bottle, capped, and the weight (W ₂ ) at the time of water content is measured and calculated according to the following formula.

Moisture content (%) = (W ₁ -W ₂ ) x 100 / W ₂

Next, the method for improving the wear resistance of an artificial nucleus pulposus disk according to the present invention includes a step of crosslinking the upper and lower sliding surfaces by irradiation with 50 to 100 kGy of gamma rays.

  And by irradiating gamma rays with a moderate irradiation dose of 50-100 kGy, it causes cross-linking of PVA molecular chains without sacrificing the original mechanical properties, Young's modulus, and moisture content of PVA hydrogel, and wear resistance. Is to improve.

  This gamma ray irradiation step is preferably performed in low oxygen concentration water substituted with nitrogen. This is because when dissolved oxygen is present in water, these oxygens first cause molecular chain scission, so that the PVA hydrogel tends to oxidatively deteriorate and wear resistance and other properties tend to deteriorate.

  When the gamma ray irradiation dose is less than 50 kGy, the main chain is mainly broken, the main chain is not recombined and cross-linked, and the contribution to the improvement of the wear resistance is low.

  On the other hand, when the gamma ray irradiation dose exceeds 100 kGy, the wear resistance function of the PVA hydrogel can be sufficiently exerted, but due to its high rigidity, the upper and lower vertebral body endplate cartilages may be damaged. .

  In the present invention, the gamma ray dose is measured using a film dosimeter, polymethyl methacrylate dosimeter, and calorimeter. When performing gamma sterilization, the PVA hydrogel is packaged in a sealed container with a saline solution substituted with nitrogen. The above-mentioned dosimeter is installed in the vicinity of the outside of the sealed container to perform measurement.

  Furthermore, in the present invention, mucopolysaccharides such as sodium hyaluronate may be added to the above-described PVA hydrogel artificial nucleus pulposus disk on the upper and lower sliding surfaces. These serve as cartilage sliding aids, and exhibit good sliding characteristics by creating an elastohydrodynamic lubrication state and a boundary lubrication state depending on the concentration.

  The artificial nucleus pulposus disc of the present invention has an upper and lower sliding surface that slides on the vertebral body cartilage, and a center line surface roughness Ra of the upper and lower sliding surface is 0.01 μm to 0.15 μm. preferable. Such an artificial nucleus pulposus disk can be produced by a mold having a molding surface with a centerline surface roughness Ra of 0.01 μm to 0.5 μm. Specifically, as shown in the production schematic diagram of FIG. 1, the PVA solution is poured into a mold that has been polished to an arbitrary surface roughness in advance, and gelled by rapid cooling. Thereby, the hydrogel with the surface property comparable to a metal or ceramics can be produced.

  In general, metals and ceramics of artificial joint materials have a smooth sliding surface by surface polishing to realize excellent wear resistance. On the other hand, hydrogel cannot be polished due to its soft and weak physical properties. Therefore, as described above, a metal or ceramic mold surface is polished in advance, a PVA solution is poured into the gel, and gelled, whereby a PVA hydrogel to which the properties of the metal or ceramic mold surface are transferred is obtained. Can be produced. As described above, excellent wear resistance is realized by the smooth sliding surface.

  As the metal or ceramic mold, a metal material such as titanium or a titanium alloy or a cobalt chromium alloy, a ceramic material such as alumina, hydroxyapatite, or quartz glass can be used.

  In addition, even if the case where the centerline surface roughness Ra is smaller than 0.01 μm is reproduced, in the hydrogel state, a substantial effect is not achieved due to the elastic deformation. In addition, the processing of the molds prepared for producing these is very difficult and industrially disadvantageous.

  Further, when the center line surface roughness Ra is larger than 0.15 μm, when the adjacent vertebral cartilage is rubbed vigorously while receiving a load several times the weight in vivo, Risk of significant damage.

  The center line surface roughness Ra is measured as follows. Using a scanning confocal laser microscope manufactured by Olympus Optical Co., Ltd., an average value of roughness curve elevation absolute values was obtained as a centerline surface roughness Ra by roughness analysis.

  According to the present invention, an artificial body made of PVA hydrogel has a vertical sliding surface that slides on the vertebral body cartilage and a moisture content in the vicinity of the vertical sliding surface is 20 to 50%. Increases the load supportability of the nucleus pulposus disc, thereby avoiding the risk of breakage, excessive deformation, dropout, etc. during load application. Can be reduced.

  According to the present invention, the vertical sliding surface of the artificial nucleus pulposus disk made of PVA hydrogel having a moisture content of 20 to 50% in the vicinity of the vertical sliding surface is irradiated with 50 to 100 kGy of gamma rays. Wear resistance can be improved by crosslinking.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to the drawings, but the present invention is not limited to these examples.

Example 1
PVA having a polymerization degree of 8800 was dissolved in a mixed solution prepared at a weight ratio of dimethyl sulfoxide (DMSO): water = 80: 20 to obtain a PVA solution. After this PVA solution was melted at 120 to 140 ° C., it was poured into an alumina mold and quenched at −15 ° C. Thereafter, DMSO and water were washed with ethanol, and vacuum heat treatment was performed at 60 ° C. for 12 hours and at 140 ° C. for 3 hours. PVA was hydrogelated by immersing in nitrogen-substituted water for 48 hours or more. In this state, 50 kGy of gamma irradiation and sterilization were performed. The water content of this PVA hydrogel was 35%.

  The obtained PVA hydrogel was adjusted to be a square plate type test piece of (L = 30) × (W = 40) × (B = 6) mm. The compressive strength of the test piece was measured using a universal testing machine (INSTRON 1123) manufactured by INSTRON. Measurement was performed using a 25 kN load cell and applying a compressive load up to 2300 kgf at a load speed of 1.0 mm / min. The test piece was taken out from 37 ° C. physiological saline immediately before the test, and measured in the atmosphere (room temperature) in a wet state. The breaking stress, the apparent elastic modulus, and the yield load were calculated from the obtained stress-strain curve.

  2 to 3 show a stress-strain curve and a stress-modulus diagram of the test body. The stress-strain curve had a very gentle S-shape. This indicates that the apparent elastic modulus changes as the stress changes in the stress-modulus diagram of FIG. In the stress-modulus diagram, the apparent elastic modulus was very low in the low load region, and the rigidity gradually increased as the load was increased. After that, it showed an almost constant value, and thereafter the apparent elastic modulus tended to decrease. After a load of 1200 kgf, the apparent modulus increased. Further, the test piece did not break even after a load of 2300 kgf. That is, these mechanical characteristics suggest that PVA hydrogel exhibits sufficient load supportability as an artificial nucleus pulposus disk.

Example 2
PVA was dissolved in a mixed solution prepared at a weight ratio of dimethyl sulfoxide (DMSO): water = 80: 20 to obtain a PVA solution. The PVA solution was melted at 120 to 140 ° C and then rapidly cooled at -15 ° C. Thereafter, DMSO and water were washed with ethanol, and vacuum heat treatment was performed at 60 ° C to 140 ° C. PVA was hydrogelated by immersing in nitrogen-substituted water for 48 hours or more. By these methods, PVA hydrogels having a moisture content of 25%, 30%, 35%, and 60% were prepared. Although an attempt was made to produce a PVA hydrogel having a water content of 15%, it was difficult to produce a sufficient test piece. The obtained PVA hydrogel was processed into a cylindrical shape having a diameter of 20 mm and a thickness of 3 mm. These PVA hydrogels were loaded with a load speed of 0.5 mm / min using an Aiko compression tester. Table 1 shows the evaluation of compression characteristics observed by the compression test together with the PVA hydrogel preparation conditions. As a comparative reference, biological specimens having a major axis diameter of 24 mm, a minor axis diameter of 16 mm, and a thickness of 3 mm were collected from the dog lumbar vertebrae and evaluated together.

The PVA hydrogel having a moisture content of 25 to 35% exhibited high compression rigidity and compression Young's modulus compared to a biological specimen having the same size. That is, a PVA hydrogel that achieves sufficient load supportability even in vivo is obtained.

Example 3
PVA was dissolved in a mixed solution prepared at a weight ratio of dimethyl sulfoxide (DMSO): water = 80: 20 to obtain a PVA solution. The PVA solution was melted at 120 to 140 ° C and then rapidly cooled at -15 ° C. Thereafter, DMSO and water were washed with ethanol, and vacuum heat treatment was performed at 60 ° C to 140 ° C. PVA was hydrogelated by immersing in nitrogen-substituted water for 48 hours or more. By these methods, a PVA hydrogel having a water content of 35% was prepared. The obtained PVA hydrogel was irradiated with gamma rays of 25 kGy, 50 kGy, 75 kGy, 100 kGy, and 125 kGy in low oxygen concentration water substituted with nitrogen. The gel fraction of the obtained PVA hydrogel was measured, and the crosslinking effect by gamma ray irradiation was evaluated. The gel fraction was measured as follows. Vacuo each fabricated PVA hydrogel, dried thoroughly at 40 to 60 ° C., measured weight at the drying (W _a). After the weight measurement, the PVA gel was dissolved in boiling water. After boiling, the remaining PVA gel fraction was sufficiently dried at 40 to 60 ° C. in vacuum, the weight (W _b ) at the time of drying was measured, and calculated by the following formula.

Gel fraction (%) = W _b x 100 / W _a
Table 2 shows the relationship between the gamma ray irradiation dose and the gel fraction of the PVA hydrogel.

As the amount of gamma irradiation increased, the gel fraction increased and showed a tendency to saturate at about 90% at 50 kGy or more. That is, it can be said that highly cross-linked PVA hydrogel is produced by gamma ray irradiation of 50 kGy or more.

Example 4
PVA was dissolved in a mixed solution prepared at a weight ratio of dimethyl sulfoxide (DMSO): water = 80: 20 to obtain a PVA solution. The PVA solution was melted at 120 to 140 ° C and then rapidly cooled at -15 ° C. Thereafter, DMSO and water were washed with ethanol, and vacuum heat treatment was performed at 60 ° C to 140 ° C. PVA was hydrogelated by immersing in nitrogen-substituted water for 48 hours or more. The obtained PVA hydrogel was irradiated with 75 kGy of gamma rays in low oxygen concentration water substituted with nitrogen. This PVA hydrogel was processed into a cylindrical shape having a diameter of 20 mm and a thickness of 3 mm. These PVA hydrogels were loaded with a load speed of 0.5 mm / min using an Aiko compression tester. FIG. 4 shows the compression characteristic evaluation observed in the compression test. In all the water content PVA hydrogels evaluated, an increase in compressive Young's modulus was observed upon irradiation with gamma rays. In the cross-linking by gamma irradiation, further load support was realized.

Example 5
PVA having a polymerization degree of 8800 was dissolved in a mixed solution prepared in a weight ratio of dimethyl sulfoxide (DMSO): water = 80: 20 to obtain a PVA solution. The PVA solution was melted at 120 to 140 ° C., poured into an alumina mold, and rapidly cooled at −15 ° C. Thereafter, DMSO and water were washed with ethanol, and vacuum heat treatment was performed at 60 to 140 ° C. PVA was hydrogelated by immersing in nitrogen-substituted water for 48 hours or more. In this state, 100 kGy of gamma irradiation and sterilization were performed. By these methods, PVA hydrogels having a moisture content of 25%, 45% and 60% were prepared. Although an attempt was made to produce a PVA hydrogel having a water content of 15%, it was difficult to produce a sufficient test piece. In addition, a living cartilage sample was cut out from the patella of an edible pig having a weight of 100 kg at 6 months of age and processed into a cylindrical shape having a diameter of 10 mm and a height of 10 mm.

  A unidirectional end face type friction tester shown in FIG. 6 was used to perform the test under severe lubrication conditions. A living cartilage sample was placed on the upper side of the testing machine, and a PVA hydrogel was placed on the lower side. A friction / wear test was conducted using the above-mentioned testing machine, using a 37 ° C. lubricating liquid prepared from bovine serum and a phosphate buffer solution with a surface pressure of 1.0 MPa and a sliding speed of 20 mm / sec.

  FIG. 7 shows a comparison of the friction coefficients of 25% and 45% water content PVA hydrogel immediately after the start of the friction test. The coefficient of friction at start-up showed a lower value for the 25% water content PVA hydrogel.

The friction and wear evaluations observed in these friction tests are shown in Table 3 together with the PVA hydrogel preparation conditions.

  FIG. 8 shows a PVA hydrogel surface after a typical friction test. In the PVA hydrogel irradiated with gamma rays and having a moisture content of 45%, some wear marks were observed on the surface in the same manner as the hydrogel not irradiated with gamma rays. On the other hand, in the 25% water content PVA hydrogel, it was observed that wear was suppressed by gamma ray irradiation. Many wear marks were observed on the surface of PVA hydrogel having a relatively high 60% water content.

  In conditions 5-8 of Table 3, it is a PVA hydrogel prepared by the same production method as in Example 6 shown below, and has a very high affinity with a living cartilage sample in contact with its surface properties. Not only the hydrogel but also the surface condition of the paired biological cartilage sample was good.

Example 6
The mold was made of alumina ceramic and the surface was polished. The center line surface roughness Ra of the molding surface was 0.020 mm.

  PVA having a polymerization degree of 8800 was dissolved in a mixed solution prepared at a weight ratio of dimethyl sulfoxide (DMSO): water = 80: 20 to obtain a PVA solution.

  The PVA solution was melted at 120 to 140 ° C., then poured into an alumina ceramic mold, and rapidly cooled at −15 ° C. Thereafter, DMSO and water were washed with ethanol, and vacuum heat treatment was performed at 60 ° C. for 12 hours and at 140 ° C. for 3 hours. PVA was hydrogelated by immersing in nitrogen-substituted water for 48 hours or more. In this state, 50 kGy of gamma irradiation and sterilization were performed. The water content of this PVA hydrogel was 35.4%.

  The surface properties of the obtained hydrogel were observed using a confocal laser microscope. FIG. 2 shows a confocal laser microscope image of an alumina ceramic surface transfer PVA hydrogel. The surface roughness (Ra) of the alumina ceramic surface transfer PVA hydrogel was 0.045 mm.

Example 7
Similar to Example 1, this is an example of a PVA hydrogel obtained by transferring the surface properties of a mold and subjected to gamma ray crosslinking. In this example, the mold was made of a cobalt chromium alloy. The center line surface roughness Ra of the molding surface was 0.026 mm. The water content of this PVA hydrogel was 34.8%. In the same manner as in Example 1, the surface properties of the PVA hydrogel were evaluated with a confocal laser microscope. FIG. 3 shows a confocal laser microscope image of the cobalt chrome alloy surface transfer PVA hydrogel. The surface roughness (Ra) of the cobalt chrome alloy surface transfer PVA hydrogel was 0.071 mm.

Example 8
Similar to Example 1, this is an example of a PVA hydrogel obtained by transferring the template surface properties and subjected to gamma-ray crosslinking. In this example, the template was made of quartz glass. The center line surface roughness Ra of the molding surface was 0.020 mm. The water content of this PVA hydrogel was 35.1%. In the same manner as in Example 1, the surface properties of the PVA hydrogel were evaluated with a confocal laser microscope. The surface roughness (Ra) of the quartz glass surface transfer PVA hydrogel was 0.021 mm.

Experimental Example A unidirectional end face type friction tester shown in FIG. 5 was used to perform a test under severe lubrication conditions. A living cartilage sample was placed on the upper side of the testing machine, and a PVA hydrogel was placed on the lower side. A friction / wear test was performed using the above-described testing machine, using a 37 ° C. lubricating liquid prepared from bovine serum and a phosphate buffer solution with a surface pressure of 1.0 MPa and a sliding speed of 20 mm / sec. Abrasion wounds were observed for both PVA hydrogel and the living biological cartilage.

  As a result, in Examples 6 to 8, both the PVA hydrogel and the biological cartilage to be paired were in a good sliding state with no trace of wear. On the other hand, in the comparative example, both the hydrogel and the paired living cartilage remained worn. From this result, it can be seen that the present invention has a great advantage over the comparative example.

It is the outline | summary of the preparation methods of the PVA hydrogel artificial nucleus pulposus disk by this invention. 2 is a stress-strain curve diagram of the PVA hydrogel of Example 1. FIG. 2 is a stress-modulus diagram of the PVA hydrogel of Example 1. FIG. 6 is a graph showing changes in compressive Young's modulus depending on the amount of gamma ray irradiation of the PVA hydrogel of Example 4. FIG. 6 is a schematic diagram of a unidirectional end face type friction tester used in Example 5. FIG. It is a figure which shows the friction coefficient immediately after the test start of the PVA hydrogel of Example 5. FIG. 2 is a surface micrograph of a PVA hydrogel before and after a friction test in Example 5. (a) is a photomicrograph of the surface of the PVA hydrogel before the friction test, (b) is a microphotograph of the surface of the PVA hydrogel not irradiated with gamma rays, 25% after the friction test, and (c) is 45% after the friction test. Micrograph of PVA hydrogel surface without moisture content and gamma irradiation, (d) 25% moisture content after friction test, micrograph of 100 kGy gamma irradiation PVA hydrogel surface, (e) 45% after friction test It is a microscope picture of the surface of PVA hydrogel of moisture content and 100 kGy gamma ray irradiation. It is a confocal laser microscope image of the surface of the PVA hydrogel artificial nucleus pulposus disk produced by the alumina casting_mold | template shown in Example 6. FIG. It is a confocal laser microscope image of the surface of the PVA hydrogel artificial nucleus pulposus disk produced by the cobalt chromium alloy casting_mold | template shown in Example 7. FIG.

Explanation of symbols

1. Test load direction 2. Load cell 3. Upper test piece, living cartilage sample 4. Lubricating fluid 5. Lower test piece, PVA hydro Gel sample 6 ... Rotating motion of test

Claims (3)

An artificial nucleus pulposus disk made of polyvinyl alcohol hydrogel for prosthesis of the nucleus pulposus of the intervertebral disc, which is arranged between the two upper and lower sliding surfaces that are arranged back-to-back relative to the vertebral body cartilage. And an artificial nucleus pulposus disk characterized by having a moisture content in the vicinity of the sliding surface of 20 to 50%. The artificial nucleus pulposus disk according to claim 1, wherein the moisture content is 25 to 35%. The method for improving the wear resistance of an artificial nucleus pulposus disk according to claim 1 or 2, comprising a step of irradiating the vertical sliding surface with gamma rays of 50 to 100 kGy.

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