High-strength crystallized glass ceramic comprising wollastonite, hydroxyapatite and akermanite

Abstract

The present invention relates to a crystallized glass ceramic comprising 30 wt % to 40 wt % of each of CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, and Ca.sub.2Mg(Si.sub.2O.sub.7); a crystallized glass ceramic composition comprising CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, and Ca.sub.2Mg(Si.sub.2O.sub.7) in a predetermined weight ratio; a bone graft material comprising the glass ceramic; and an intervertebral spacer or medical device for replacement of bone tissue, which is manufactured using the bone graft material.

Claims

1. A crystallized glass ceramic comprising CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, and Ca.sub.2Mg(Si.sub.2O.sub.7) at a weight ratio of 30 to 40:30 to 40:30 to 40.

2. The crystallized glass ceramic of claim 1, wherein CaSiO3 comprises a wollastonite, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 comprises a hydroxyapatite (HA), and Ca.sub.2Mg(Si.sub.2O.sub.7) comprises an akermanite.

3. The crystallized glass ceramic of claim 1, wherein the crystallized glass ceramic has increased compressive strength or flexural strength compared to glass ceramics comprising CaSiO.sub.3 and Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 due to Ca.sub.2Mg(Si.sub.2O.sub.7).

4. The crystallized glass ceramic of claim 1, wherein the crystallized glass ceramic is formed by sintering at a temperature of about 850 C. to about 1,100 C.

5. The crystallized glass ceramic of claim 1, which is manufactured by sintering a glass ceramic composition, wherein the glass ceramic composition is manufactured by a process comprising boiling a raw composition comprising SiO.sub.2, hydroxyapatite, Ca(OH).sub.2, MgO, CaF.sub.2, and B.sub.2O.sub.3, wherein the raw composition comprises SiO.sub.2 hydroxyapatite, Ca(OH).sub.2, MgO, CaF.sub.2 at a weight ratio of 25 to 35:25 to 35:18 to 22:4 to 6:4 to 5.

6. A bone graft material comprising the glass ceramic according to claim 1.

7. An intervertebral spacer or medical device for replacement of bone tissue: manufactured using the bone graft material of claim 6; or, comprising the bone graft material of claim 6.

8. The intervertebral spacer or medical device of claim 7, wherein the intervertebral spacer or medical device has compressive strength in a range of about 3,000 N to about 35,000 N or the torsional strength in a range of about 0.6 Nm to about 1.5 Nm.

9. The intervertebral spacer or medical device of claim 7, wherein the intervertebral spacer or medical device is manufactured using a bone graft material, wherein the bone graft material comprises a dense molded product having a value of relative density corresponding to about 95% or higher relative to that of theoretical density.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph showing the analysis results of crystal components of the composition according to the present invention.

(2) FIG. 2 is a graph showing the strength of glass ceramics according to sintering temperature. FIGS. 2(A) and 2(B) show images of glass ceramics manufactured by sintering at 800 C. and 900 C., respectively.

(3) FIG. 3 is a graph showing the XRD analysis results for confirming the crystallization of the glass ceramic according to the sintering temperature of the glass ceramic of the present invention.

(4) FIG. 4 is a graph showing the volume, relative density, and compressive strength of the glass ceramic according to the sintering temperature of the glass ceramic of the present invention.

(5) FIG. 5 is a view showing the shapes of sintered bodies according to the sintering temperature of the glass ceramic of the present invention.

DETAILED DESCRIPTION

(6) A first aspect of the present invention provides a crystallized glass ceramic comprising 30 wt % to 40 wt % of each of CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, and Ca.sub.2Mg(Si.sub.2O.sub.7).

(7) A second aspect of the present invention provides a crystallized glass ceramic composition comprising CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, and Ca.sub.2Mg(Si.sub.2O.sub.7) at a weight ratio of 30 to 40:30 to 40:30 to 40.

(8) A third aspect of the present invention provides a bone graft material comprising the above glass ceramic.

(9) A fourth aspect of the present invention provides an intervertebral spacer or medical device for replacement of bone tissue manufactured using the above bone graft material.

(10) Hereinafter, the present invention will be described in detail.

(11) The present invention, which is related to the discovery of a novel bone graft composition, is based on the first confirmation of the optimal ratio that can provide the required strength when processed into medical devices such as an intervertebral spacer, etc., by controlling the mixing ratio of CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, Ca.sub.2Mg(Si.sub.2O.sub.7).

(12) The present invention relates to a crystallized glass ceramic comprising 30 wt % to 40 wt % of each of CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, and Ca.sub.2Mg(Si.sub.2O.sub.7). Preferably, CaSiO.sub.3 may be wollastonite; Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 may be hydroxyapatite (HA); and Ca.sub.2Mg(Si.sub.2O.sub.7) may be Akermanite.

(13) As used herein, the term wollastonite is a calcium inosilicate mineral represented by the chemical formula of CaSiO.sub.3, which may contain small amounts of iron, magnesium, and manganese instead of calcium. Naturally, wollastonite may be formed when limestones or dolostones with impurities are subjected to high-temperature and high-pressure conditions in the presence of silica-bearing fluids, as in the cases of skarns or contact metamorphic rocks. The relevant minerals may include garnets, vesuvianite, diopside, tremolite, epidote, plagioclase feldspar, pyroxene, and calcite. For example, wollastonite may be produced by reacting silica with calcite, which releases carbon dioxide:
CaCO.sub.3+SiO.sub.2.fwdarw.CaSiO.sub.3+CO.sub.2

(14) Wollastonite can be used in ceramics, friction products such as brakes and clutches, metalmaking, paint fillers, and plastics. The main countries of wollastonite production are China, India, USA, Mexico, Finland, etc.

(15) As used herein, the term hydroxyapatite (hereinafter, referred to as HA) of the present invention is a naturally-occurring mineral form of calcium apatite, which has the chemical formula of Ca.sub.5(PO.sub.4).sub.3(OH), but it can normally be expressed as Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 because the crystal unit cell contains two independent bodies. Hydroxyapatite refers to a single hydroxy component of a complex apatite group and OH-ion may be substituted with fluoride, chloride, carbonate, etc., to form fluorapatite, chlorapatite, etc. Pure hydroxyapatite powder may be white but natural apatite can be brown, yellow, or green. Hydroxyapatite may be formed naturally or by wet chemical deposition, biomimetic deposition, a sol-gel process which is also referred to as wet chemical precipitation, or electrodeposition. Hydroxyapatite may be present in teeth and bone tissue in the human body. Accordingly, hydroxyapatite may be used as a filler replacing a cut bone tissue or as a coating agent to promote the in-growth of bone tissue into a prosthetic implant.

(16) As used herein, the term Akermanite, expressed as Ca.sub.2Mg[Si.sub.2O.sub.7], refers to a melilite mineral of the sorosilicate group containing calcium, magnesium, silicon, and oxygen. Akermanite may be formed by contact metamorphism of siliceous limestones and dolostones, and rocks of sanidinite facies. Akermanite ranks a 5 or 6 on the Mohs scale of mineral hardness and may be found gray, green, brown, or colorless. Additionally, Akermanite may have a white streak and a vitreous or resinous luster.

(17) The present invention is characterized in that it provides a material with significantly enhanced strength compared to glass ceramics which contain CaSiO.sub.3 and Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, by further comprising Ca.sub.2Mg(Si.sub.2O.sub.7).

(18) Preferably, the crystallized glass ceramic of the present invention may be formed by sintering at a temperature of 850 C. to 1,100 C. When the sintering temperature is less than or equal to 800 C., the crystallized glass ceramic may be damaged due to rapid crystallization, and thus may be impossible to be used as a product. Meanwhile, the sintering temperature over 1,100 C. is undesirable because energy waste due to unnecessary heating is not only accompanied but also mechanical properties of the manufactured ceramic may be deteriorated due to excessive crystallization of the glass component.

(19) For providing glass ceramics with enhanced strength, the present invention provides a crystallized glass ceramic composition comprising CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, and Ca.sub.2Mg(Si.sub.2O.sub.7) at a weight ratio of 30 to 40:30 to 40:30 to 40.

(20) A bone graft material of the present invention may comprise the above glass ceramic. As described above, for enhancement of strength, the glass ceramic may be manufactured by sintering a crystallized glass ceramic composition, which comprises CaSiO.sub.3, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, and Ca.sub.2Mg(Si.sub.2O.sub.7) at a weight ratio of 30 to 40:30 to 40:30 to 40, at a high temperature. The preferred sintering temperature is the same as described above.

(21) The bone graft material with enhanced strength may be used for manufacturing an intervertebral spacer or medical device for replacement of bone tissue.

(22) In particular, the intervertebral spacer or medical device for replacement of bone tissue is characterized in that it includes the crystallized glass ceramic according to the present invention in an area directly bonded to the surrounding bones. In a specific example of the present invention, it was confirmed that the spacer showed a bonding in a significantly increased area when the spacer made of the glass ceramic material according to the present invention was used, as compared to when the autograft bone was transplanted in a titanium cage (Table 2). Accordingly, in manufacturing the intervertebral spacer or medical device for replacement bone tissue, it is preferred that the glass ceramic according to the present invention, which has excellent compatibility with the surrounding bone tissue after in vivo transplantation, be included in a region in contact with the surrounding bone tissue.

(23) Examples of the international test standards related to the physical/mechanical evaluation of the intervertebral spacer may include ASTM F2077, ASTM F2267, etc. Among them, the former specifies the experimental environment including the jig for the static compression and torsion test and the dynamic fatigue test, and provides related test protocols.

(24) Preferably, the intervertebral spacer or medical device, manufactured using the bone graft material of the present invention, has compressive strength in a range of 3,000 N to 35,000 N or the torsional strength in a range of 0.6 N.Math.m to 1.5 N.Math.m. Additionally, fatigue strength higher than or equal to the maximum compressive strength, which is not broken even after repeating 5,000,000 cycles at a repetition rate of 5 Hz and a stress ratio of 10, may be obtained. Therefore, the intervertebral spacer made of the bone graft of the present invention or medical device for replacement of bone tissue may be used as an intervertebral spacer both for a cervical spine and for a lumbar vertebra requiring higher strength.

(25) As used herein, the term compressive strength refers to the maximum stress of a material which can withstand a compressive load. The compressive strength of materials that are broken into pieces upon compression may be defined as an independent nature in a narrow sense, but the compressive strength of materials that are not broken into pieces upon compression may be defined by the amount of stress required to deform any material with a random quantity. The measurement may be made by plotting the force applied to a test device against the deformation. In compression tests, compressive strength may be calculated by dividing the maximum load by the initial cross-sectional area of the specimen.

(26) As used herein, the term torsional strength or torsion refers to the degree of capability of a material to withstand a torsional load, in which the torsional strength is the maximum strength of the material subjected to the torsional load, and may be the maximum torsional stress that can maintain the material before fracture, and also called modulus of fracture or shear strength. As the measurement unit, Newton meter (N.Math.m) or feet pound force (ft.Math.lbf) may be used.

(27) As used herein, the term fatigue strength refers to the amount of fluctuating stress required for the fracture of a fatigue test specimen by applying a predetermined number of repeated loads, in which the number of repetitions is called fatigue life. Fatigue strength can generally be measured directly from the SN curve, but is not limited thereto. ASTM defines fatigue strength (S.sub.Nf) as the stress value at which the fracture of the number of N.sub.f cycles occurs.

(28) For example, the intervertebral spacer or medical device for replacement bone tissue made of the bone graft material according to the present invention may be one manufactured using a bone graft material, which is a dense molded product having a value of relative density corresponding to 95% or higher relative to that of theoretical density, but not limited thereto. When products manufactured using the bone graft material, which is a dense molded product, can provide enhanced strength to be advantageous for withstanding loads, these products may be effectively used as intervertebral spacers or medical devices for replacement of bone tissue.

(29) Hereinafter, the present invention will be described in more detail with reference to the following Examples. However, these Examples are for illustrative purposes only and the invention is not intended to be limited by these Examples.

Example 1: Manufacture and Composition Analysis of High-Strength Crystallized Glass Ceramics

(30) SiO.sub.2, hydroxyapatite (HA), Ca(OH).sub.2, MgO, B.sub.2O.sub.3, CaF.sub.2, etc., in the form of powders were boiled at a high temperature of 1,400 C. or higher for 2 hours or more, and then rapidly cooled in water to manufacture raw glass powder. Each raw material was mixed in a ratio of 25 wt % to 35 wt % of SiO.sub.2, 25 wt % to 35 wt % of hydroxyapatite (HA), 18 wt % to 22 wt % of Ca(OH).sub.2, 4 wt % to 6 wt % of MgO, 4 wt % to 5 wt % of B.sub.2O.sub.3, and 4 wt % to 5 wt % of CaF.sub.2. The thus-manufactured glass powder was molded in the same manner as the general method for manufacturing ceramic molded bodies known in the art and then sintered at a high temperature to be crystallized. The crystalline phases obtained by the final crystallization were shown to be mixed with wollastonite, hydroxyapatite and Akermanite in similar proportions. This was analyzed by an X-ray diffraction pattern and the results are shown in FIG. 1. Specifically, 2, which is the main diffraction line of each material, was in the range of 29.5 to 30.5 for wollastonite, 31.5 to 32.5 for hydroxyapatite, and 30.5 to 31.5 for Akermanite, and the strength ratio was 365%, 335%, and 315%, respectively.

Example 2: Evaluation of Sintering Characteristics According to Crystallization Temperature

(31) To determine the optimum sintering temperature for crystallization of wollastonite, hydroxyapatite, and Akermanite, glass powders of the same composition were sintered at different temperatures to manufacture glass ceramics, and the strength of thus-manufactured glass ceramics was confirmed. Reviewing the sintering tendency of the glass powder according to temperature, the sintering was performed at about 700 C., and when the sintering was performed at this temperature for 2 hours, about 5% of shrinkage occurred. When the sintering was performed at a temperature of 750 C. to 800 C., the glass powder was rapidly crystallized and the linear shrinkage rate reached 18% to 21%. However, when sintering was performed at 750 C., the sintering was performed well, but crystallization of wollastonite did not occur. Meanwhile, it was confirmed that when the sintering was performed at 800, there was damage on the product due to discontinuation of crystallization (FIG. 2). Therefore, this indicates that the above temperature range is unsuitable for sintering to produce products. When the sintering was performed by further increasing the temperature to 900 C. or higher, it was confirmed that wollastonite was crystallized and the sintering was performed stably without damage on the product. Then, the temperature was increased further and the same crystal phase was maintained up to 1,100 C., indicating that the temperatures up to the above range were suitable for sintering. However, considering the efficiency of the process, the optimum temperature for sintering may be 1,000 C. at which the half-value width of the XRD diffraction peak caused by crystals was significantly reduced (FIG. 3).

(32) Specifically, as shown in FIG. 3, the crystallized glass ceramics according to the present invention, which were manufactured by sintering at 900 C. to 1,100 C., showed three distinct peaks corresponding to each of wollastonite, hydroxyapatite, and Akermanite, which were formed at a similar rate regardless of the temperature, which indicates that the crystal phase was well formed in the above temperature range. When sintered at a temperature of 1,000 C. or higher, the half-width was significantly reduced indicating that an enhanced level of crystallization occurred.

(33) The glass ceramics were manufactured by the method according to the present invention under the same conditions except that they were sintered at a temperature of 850 C. to 1,100 C. in increments of 50 C. within the above temperature range, respectively. The volumes, relative density, and compressive strength of the glass ceramics were measured and shown in Table 1 below, plotted together in the graph, and shown in FIG. 4.

(34) TABLE-US-00001 TABLE 1 Volume Relative Density Compressive Strength (mL) (%) (N) 850 C. 2.8820 99.4733 16157.95 900 C. 2.8603 100.3449 21963.62 950 C. 2.8502 100.6912 28983.78 1,000 C. 2.8631 99.8791 28940.22 1,050 C. 3.1315 91.4724 33016.09 1,100 C. 3.8299 74.9538 21218.79

(35) As shown in FIG. 4, as the sintering temperature increased to 1,000 C. or higher, the relative density began to decrease along with the increase in volume. The compressive strength showed the maximum value at 1,050 C. and gradually decreased at or below 1,050 C. However, all of the compressive strength exceeded 16,000 N in the sintering temperature range of 850 C. to 1,100 C.

(36) Additionally, the final shape of the hexahedral sintered bodies of glass ceramics manufactured by sintering at each of the above temperatures were photographed and shown in FIG. 5. As shown in FIG. 5, it was confirmed that the glass ceramics manufactured by sintering at a temperature of 1,050 C. or higher had a somewhat convex external shape at the middle portion of the surface. From these results, it is possible to find sintering conditions which provide a combination of volume, relative density, and compressive strength suitable for the use of the glass ceramics. For example, for use of the glass ceramics as a bone graft material, the sintering temperature of 1,000 C., at which compressive strength was high and no external change (i.e., no volume expansion) occurred, was selected.

Example 3: Measurement of Strength of Bone Graft Material According to the Present Invention

(37) The strength of the bone graft material, which contains a high-strength crystalline glass ceramic comprisings wollastonite, hydroxyapatite, and Akermanite at a predetermined ratio according to the present invention, was measured and compared to the values of the wollastonite/HA composite (i.e., an existing glass ceramic material) and HA (i.e., a bioceramic sintered body).

(38) The wollastonite/HA composite glass ceramic, which was manufactured by molding and sintering at 1,000 C. of the glass powders manufactured using SiO.sub.2, hydroxyapatite, and Ca(OH).sub.2 as raw materials, and a HA sintered body, which was manufactured by sintering 100% hydroxyapatite at 1,200 C., were used as Comparative Examples. The strength of the crystalline glass ceramics of the present invention manufactured according to Example 1 and the two kinds of Comparative Examples was measured, and the results of comparison and analysis are shown in Table 2 below. The final sintered body was manufactured as a 1 cm-long cube and the faces were homogenized by polishing to minimize errors in the measurement of strength.

(39) TABLE-US-00002 TABLE 2 Compressive Flexural Fracture Strength Strength Toughness Classification (MPa) (MPa) (MPa .Math. m.sup.1/2) Wollastonite/HA Composite 1103 94 180 10 1.54 0.07 Glass Ceramic HA Sintered Body 832 35 53 1 1.51 0.03 Crystalline Glass Ceramic 1321 40 253 13 3.0 0.17 of Example 1

(40) As shown in Table 2 above, the bone graft material according to the present invention, which contains a high-strength crystalline glass ceramic comprising wollastonite, hydroxyapatite, and Akermanite, has increased compressive strength by about 20% and 60%, and increased flexural strength by about 40% and 375%, compared to those of the wollastonite/HA composite (i.e., an existing glass ceramic material) and HA (i.e., a bioceramic sintered body), respectively. Additionally, the bone graft material according to the present invention showed significantly increased fracture toughness (i.e., about a 2-fold increase) compared to those of the wollastonite/HA composite and HA, respectively.

Example 4: Manufacture of Intervertebral Spacer Using Crystalline Glass Ceramic According to the Present Invention and Analysis of Characteristics Thereof

(41) An intervertebral spacer for a cervical spine can be manufactured by processing the high-strength crystallized glass ceramic material according to the present invention to have compressive strength of 3,000 N or higher, whereas an intervertebral spacer for a lumbar vertebra can be manufactured by processing the high-strength crystallized glass ceramic material according to the present invention to have compressive strength of 8,000 N or higher. That is, as confirmed in Example 2 above, the high-strength crystallized glass ceramic material according to the present invention exhibited compressive strength of about 1,321 MPa. Therefore, it was confirmed that the high-strength crystallized glass ceramic material can theoretically meet the requirements as an intervertebral spacer for a cervical spine and for a lumbar vertebra when manufactured to a size of 2.27 mm.sup.2 and 6.06 mm.sup.2, respectively. Additionally, it was confirmed that considering that general vertebral spacers have a length, width, and/or depth in the range of several mm to several cm, the crystallized glass ceramic material of the present invention can meet the required strength as an intervertebral spacer for a cervical spine and for a lumbar vertebra suggested above.

(42) Additionally, it was confirmed that the spinal spacer manufactured using the crystallized glass ceramic material according to the present invention had fatigue strength without damage even when the spacer was over 5 million cycles at a repetition rate of 5 Hz and a stress ratio of 10 and when subjected to an additional load with the maximum compressive strength. Furthermore, the torsional strength of the spacer was measured and confirmed to have a value of 0.6 N.Math.m or higher.

(43) The results of clinical tests performed in the human body using the spinal spacer manufactured from the crystallized glass ceramic material according to the present invention showed that the spinal spacer had a binding force with the neighboring bones similar to that of a case where an autograft bone was transplanted to a titanium cage (control group), which is a general surgery method. Among the 39 subjects in whom the corresponding spacer was transplanted to the lumbar region, 35 subjects (89.7%) had been showing excellent clinical results for 12 months and the transplanted spacer directly bonded to the neighboring vertebral body. In particular, as a result of calculating the bonded area between the vertebral body and the spacer, as shown in Table 2 below, the bonded area of the crystallized glass ceramic material according to the present invention to the spacer was statistically significantly higher than that of the autograft bone filled in the titanium cage (p<0.001). The calculated area of the spacer or autograft bone associated with the calculated vertebral endplates was compared as shown in Table 3.

(44) TABLE-US-00003 TABLE 3 Classification Upper Vertebral Endplate Lower Vertebral Endplate Example 1 86.0 48.0 mm.sup.2 81.4 48.6 mm.sup.2 Control Group 36.4 16.1 mm.sup.2 39.3 14.7 mm.sup.2