Bone graft system

Abstract

The present invention relates generally to the field of bone graft substitutes and methods for making the same, particularly the invention relates to bone graft substitutes for use in dental or orthopaedic implants. The bone graft substitutes described herein comprise a silicate based material. The silicate based material is a silicate network with a porous structure. The silicate network has one or more metal cations incorporated therein. Preferably a phosphate is also incorporated into the silicate network. The bone graft substitute may have a low density, preferably a density of less than 1.1 g/cm.sup.3. The bone graft substitute may be an aerogel or a cryogel.

Claims

1. A bioactive bone graft substitute comprising a silicate based material which is a silicate network having a porous structure wherein one or more metal cations are incorporated into the silicate network and wherein the density of the bone graft substitute is less than 0.7 g/cm.sup.3 and the average pore diameter of the bone graft substitute is from about 1 to about 99 nm, wherein the metal cations are ionically bound to the silicate network, and wherein the metal cations comprise calcium.

2. The bone graft substitute of claim 1 wherein the silicate network comprises between 0.01 and 70 mol % of metal cation.

3. The bone graft substitute of claim 1, wherein a phosphate is also incorporated into the silicate network.

4. The bone graft substitute of claim 3 wherein the silicate network comprises between 1 and 20 mol % of phosphate.

5. The bone graft substitute of claim 1, wherein the metal cations are ionically bound to the silicate network via an oxygen anion.

6. The bones graft substitute of claim 3, wherein the phosphate is ionically bound to the silicate network via one or more of the metal cations.

7. The bone graft substitute of claim 3, wherein the phosphate is a PO.sub.4.sup.3 anion.

8. The bone graft substitute of claim 3, wherein the phosphate is covalently bound to the silicate network via a SiOP covalent bond.

9. The bone graft substitute of claim 1, wherein the bone graft substitute is a porous solid gel, wherein the porous solid gel has a gas as the dispersed phase, an aerogel, or a cryogel.

10. The bone graft substitute of claim 1, wherein the metal cation is calcium and optionally one or more of strontium, sodium, zinc, magnesium, potassium, titanium, cobalt, aluminum, silver.

11. The bone graft substitute of claim 1, wherein the silicate to metal cation ratio is between 0.3 and 2.

12. The bone graft substitute of claim 1, wherein the bone graft substitute contains between 10 and 70 mol % of metal cation.

13. The bone graft substitute of claim 1, wherein the metal cation is derived in whole or in part from any one of hydroxyapatite (Ca.sub.10)(PO.sub.4).sub.6(OH).sub.2), hydroxycarbonatedapatite octacalcium phosphate (Ca.sub.8H.sub.2(PO.sub.4).sub.6.Math.5H.sub.2), brushite (CaHPO.sub.4.Math.2H.sub.2O), monetite (CaHPO.sub.4), fluorapatite (Ca.sub.10)(PO.sub.4).sub.6F.sub.2), chlorapatite (Ca.sub.10)(PO.sub.4).sub.6Cl.sub.2), fluorohydroxyapatite (Ca.sub.10(PO.sub.4).sub.6(OH).sub.2-xF.sub.x), tetracalcium phosphate (Ca.sub.4(PO.sub.4).sub.2O), tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2) or any combination thereof.

14. The bone graft substitute of claim 3, wherein the phosphate is P.sub.2O.sub.5.

15. The bone graft substitute of claim 3, wherein the metal cation to phosphate ratio is between 0.2 and 20.

16. The bone graft substitute of claim 3 wherein the bone graft substitute contains between 1 and 20 mol % of phosphate.

17. The bones graft substitute of claim 3, wherein the phosphate is derived in whole or in part from any one of hydroxyapatite (Ca.sub.10)(PO.sub.4).sub.6(OH).sub.2), hydroxycarbonatedapatite octacalcium phosphate (Ca.sub.8H.sub.2(PO.sub.4).sub.6.Math.5H.sub.2O), brushite (CaHPO.sub.4.Math.2H.sub.2O), monetite (CaHPO.sub.4), fluorapatite (Ca.sub.10)(PO.sub.4).sub.6F.sub.2), chlorapatite (Ca.sub.10)(PO.sub.4).sub.6Cl.sub.2), fluorohydroxyapatite (Ca.sub.10(PO.sub.4).sub.6(OH).sub.2-xF.sub.x), tetracalcium phosphate (Ca.sub.4(PO.sub.4).sub.2O), tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2) or any combination thereof.

18. The bone graft substitute of claim 1, wherein the silicate is a silicon oxide or a silicon fluoride.

19. The bone graft substitute of claim 18 wherein the silicate is a silicon oxide.

20. The bone graft substitute of claim 19 wherein the silicon oxide is SiO.sub.2.

21. The bone graft substitute of claim 1, wherein the bone graft substitute contains between 20 and 80 mol % of silicate.

22. The bone graft substitute of claim 1, wherein the bone graft substitute further comprises hydroxyapatite (Ca.sub.10)(PO.sub.4).sub.6(OH).sub.2), hydroxycarbonatedapatite octacalcium phosphate (Ca.sub.8H.sub.2(PO.sub.4).sub.6.Math.5H.sub.2O), brushite (CaHPO.sub.4.Math.2H.sub.2O), monetite (CaHPO.sub.4), fluorapatite (Ca.sub.10)(PO.sub.4).sub.6F.sub.2), chlorapatite (Ca.sub.10)(PO.sub.4).sub.6Cl.sub.2), fluorohydroxyapatite (Ca.sub.10(PO.sub.4).sub.6(OH).sub.2-xF.sub.x), tetracalcium phosphate (Ca.sub.4(PO.sub.4).sub.2O), tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2) or any combination thereof.

23. The bone graft substitute of claim 1, having a density of less than 0.3 g/cm.sup.3.

24. The bone graft substitute of claim 1, having a density of greater than 0.001 g/cm.sup.3.

25. The bone graft substitute of claim 24 having an average pore diameter from about 10 to about 25 nm.

26. The bone graft substitute of claim 1, having a pore volume from about 1 to about 20 cm.sup.3/g.

27. The bone graft substitute of claim 26 having a pore volume from about 4 to about 8 cm.sup.3/g.

28. The bone graft substitute of claim 1, having a surface area greater than 400 m.sup.2/g.

29. The bone graft substitute of claim 28 having a surface area greater than 850 m.sup.2/g.

30. A process of making the bioactive bone graft substitute of claim 1 comprising: (i) a gel formation stage; (ii) a liquid phase replacement stage; (iii) a gel drying stage; and (iv) a calcination stage; wherein the gel formation stage comprises the steps of dissolving a metal cation in a first solvent, adding a silicate to the solvent and gelling of the resultant mixture; and wherein the liquid phase replacement stage comprises the step of soaking the gel in a second solvent; and wherein the gel drying stage is carried out by freeze-drying or supercritical drying; and wherein the calcination stage comprises the step of heating the dried gel.

31. The process of claim 30 wherein the gel formation step further comprises the step of dissolving a phosphate in the first solvent before adding the silicate.

32. The process of claim 30, wherein the gel formation step further comprises the step of adding any one of hydroxyapatite (Ca.sub.10)(PO.sub.4).sub.6(OH).sub.2), hydroxycarbonatedapatite octacalcium phosphate (Ca.sub.8H.sub.2(PO.sub.4).sub.6.Math.5H.sub.2O), brushite (CaHPO.sub.4.Math.2H.sub.2O), monetite (CaHPO.sub.4), fluorapatite (Ca.sub.10)(PO.sub.4).sub.6F.sub.2), chlorapatite (Ca.sub.10)(PO.sub.4).sub.6Cl.sub.2), fluorohydroxyapatite (Ca.sub.10(PO.sub.4).sub.6(OH).sub.2-xF.sub.x), tetracalcium phosphate (Ca.sub.4(PO.sub.4).sub.2O), tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2) or any combination thereof to the first solvent.

33. The process of claim 30 wherein the second solvent is an organic solvent.

34. The process of claim 33 wherein the second solvent is ethanol.

35. The process of claim 30, wherein the calcination stage is carried out at a temperature of at least 400 K.

36. The process of claim 30, wherein the calcination stage may be carried out for at least 1 hour.

37. The process of claim 30, wherein the metal cation is provided by a metal salt.

38. The process of claim 37 wherein the metal cation is provided by a calcium salt.

39. The process of claim 38 wherein the calcium salt is selected from calcium nitrate tetrahydrate, calcium acetate and calcium nitrate.

40. The process of claim 30, wherein the metal cation is provided in whole or in part by any one of hydroxyapatite (Ca.sub.10)(PO.sub.4).sub.6(OH).sub.2), hydroxycarbonatedapatite octacalcium phosphate (Ca.sub.8H.sub.2(PO.sub.4).sub.6.Math.5H.sub.2O), brushite (CaHPO.sub.4.Math.2H.sub.2O), monetite (CaHPO.sub.4), fluorapatite (Ca.sub.10)(PO.sub.4).sub.6F.sub.2), chlorapatite (Ca.sub.10)(PO.sub.4).sub.6Cl.sub.2), fluorohydroxyapatite (Ca.sub.10(PO.sub.4).sub.6(OH).sub.2-xF.sub.x), tetracalcium phosphate (Ca.sub.4(PO.sub.4).sub.2O), tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2) or any combination thereof.

41. The process of 36, wherein the phosphate is provided by triethylphosphate.

42. The process of claim 31, wherein the phosphate is provided in whole or in part by any one of hydroxyapatite (Ca.sub.10)(PO.sub.4).sub.6(OH).sub.2), hydroxycarbonatedapatite octacalcium phosphate (Ca.sub.8H.sub.2(PO.sub.4).sub.6.Math.5H.sub.2O), brushite (CaHPO.sub.4.Math.2H.sub.2O), monetite (CaHPO.sub.4), fluorapatite (Ca.sub.10)(PO.sub.4).sub.6F.sub.2), chlorapatite (Ca.sub.10)(PO.sub.4).sub.6Cl.sub.2), fluorohydroxyapatite (Ca.sub.10(PO.sub.4).sub.6(OH).sub.2-xF.sub.x), tetracalcium phosphate (Ca.sub.4(PO.sub.4).sub.2O), tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2) or any combination thereof.

43. A composition comprising the bone graft substitute of claim 1 and at least one other biomaterial.

44. The composition of claim 43 wherein the at least one biomaterial comprises hydroxyapatite (Ca.sub.10)(PO.sub.4).sub.6(OH).sub.2), hydroxycarbonatedapatite octacalcium phosphate (Ca.sub.8H.sub.2(PO.sub.4).sub.6.Math.5H.sub.2O), brushite (CaHPO.sub.4.Math.2H.sub.2O), monetite (CaHPO.sub.4), fluorapatite (Ca.sub.10)(PO.sub.4).sub.6F.sub.2), chlorapatite (Ca.sub.10)(PO.sub.4).sub.6Cl.sub.2), fluorohydroxyapatite (Ca.sub.10(PO.sub.4).sub.6(OH).sub.2-xF.sub.x), tetracalcium phosphate (Ca.sub.4(PO.sub.4).sub.2O), tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2) or any combination thereof.

45. A method of promoting bone growth in a void, cavity, or fracture site in a subject, comprising: introducing a bone graft substitute of claim 1 into said void, cavity, or fracture site.

46. The bone graft substitute of claim 1, further comprising fluoride.

47. A toothpaste comprising the bone graft substitute of claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1: Shows a graph of the N.sub.2 adsorption and description isotherm of Sample 2.

(2) FIG. 2: Shows the FTIR analysis of Sample 2 when treated with simulated body fluids.

(3) FIG. 3: Shows a scanning electron micrograph of Sample 2 after calcination.

(4) FIG. 4: Shows a zoomed in scanning electron micrograph of Sample 2 after calcination.

(5) FIG. 5: Shows scheme 1.

(6) FIG. 6: Shows structure 1.

(7) FIG. 7: Shows the .sup.31P NMR spectra of Sample 1 after 3 hours, 6 hours, 1 day, 3 days immersion in SBF solution.

(8) FIG. 8: Shows the .sup.31P NMR spectra of Sample 2 after 3 hours, 6 hours, 1 day, 3 days immersion in SBF solution.

(9) FIG. 9: Shows the .sup.31P NMR spectra of Sample 4 after 3 hours, 6 hours, 1 day, 3 days immersion in SBF solution.

(10) FIG. 10: Shows the .sup.31P NMR spectra of Sample 5 after 3 hours, 6 hours, 1 day, 3 days immersion in SBF solution.

(11) FIG. 11: Shows the .sup.31P NMR spectra of Sample 6 after 3 hours, 6 hours, 1 day, 3 days immersion in SBF solution.

EMBODIMENTS OF THE INVENTION AND EXPERIMENTAL DATA

(12) The present invention is now illustrated with reference to the following non-limiting examples and accompanying figures.

Example 1

(13) Example 1 describes a method used to produce bone graft substitutes of the present invention.

(14) Reagents

(15) 0.14M NaF solution Absolute (100%) ethanol tetraethyl orthosilicate (TEOS, (Si(OC.sub.2H.sub.5).sub.4)) Calcium nitrate tetrahydrate (Ca(NO.sub.3).sub.2.Math.4H.sub.2O) Triethyl phosphate ((C.sub.2H.sub.5).sub.3PO.sub.4)

(16) Calcium nitrate tetrahydrate was dissolved in 0.14 M solution of NaF, after which ethanol was added. This mixture was then stirred for 5 minutes and then triethyl phosphate ((C.sub.2H.sub.5).sub.3PO.sub.4) added.

(17) Finally the TEOS was combined slowly with this solution and allowed to stir for thirty seconds.

(18) 4 ml of the solution was cast into cylindrical moulds (11 mm50 mm height, via syringe). Each mould was then covered with film and placed into glass container.

(19) Each sample was then gelled for 48 hours at 60 C.

(20) Each sample was then placed into 60% ethanol. After 24 hours the solution was changed for 80% ethanol. After another 24 hours it was changed once again for 95% ethanol. Finally the solution was replaced with 100% ethanol.

(21) Each sample was dried using the CPD method using a Tousimis 931 critical point drier. Each sample was run through three stasis cycles of eight hours each.

(22) After critical drying each sample was then calcined at 700 C. for three hours.

Example 2

(23) Six samples were prepared using the method of example 1. Table 1 shows the chemical composition and density of samples 1-6.

(24) All samples were produced with the following molar ratios of H.sub.2O, ethanol and TEOS of 17.26:16.71:1.00.

(25) TABLE-US-00001 TABLE 1 Chemical compositions SiO.sub.2 P.sub.2O.sub.5 CaO Density Composition (mol %) (mol %) (mol %) (g cm.sup.3) Sample 1 38.00 6.00 56.00 0.126 Sample 2 40.00 6.00 54.00 0.190 Sample 3 42.25 6.00 51.75 Not determined Sample 4 44.69 6.00 49.31 0.122 Sample 5 47.50 6.00 46.50 0.248 Sample 6 50.80 6.00 43.20 0.132

(26) The data presented in table 1 shows both the compositions of and densities of five compositions of bioactive aerogels.

(27) This data demonstrates the ability to produce a range of compositions. It is also clear that very low densities can be achieved for the bone graft substitutes of the present invention.

(28) Density is an important property for bone graft substitutes. The ability to produce bone graft substitutes having low densities represents one of the key advantages of the present invention.

(29) Bone graft substitutes with low densities may provide rapid remodelling times and significantly reduce any potential pH rise associated with the ion exchange processes.

Example 3

(30) Further analysis of sample 2 was conducted, and key properties measured. These are set out in Table 2 below. The average pore diameter and pore volume were obtained from N.sub.2 adsorption using the NLDFT method. The surface area was obtained from N.sub.2 adsorption using the BET method. The density was measure by calculating the volume of the sample and weighing the sample.

(31) Sample 2 was produced according to the method outlined in example 1.

(32) TABLE-US-00002 TABLE 2 Key properties of Sample 2 Property Value Density 0.19 g cm.sup.3 Average Pore Diameter 16.0 nm Surface Area 892 m.sup.2 g.sup.1 Pore Volume 4.92 cm.sup.3 g.sup.1

(33) Table 2 shows a number of properties of sample 2 measured using N.sub.2 adsorption analysis. This data supports the previous data showing low densities and confirms pores in the nm scale and a high pore volume.

(34) Current sol-gel glasses used as bone graft materials have a maximum surface area of around 450 m.sup.2 g.sup.1. The data for sample 2 show that bone graft substitutes of the present invention can have a surface area of nearly double the maximum of currently used bone graft substitutes.

(35) The surface area data show another advantage of the bone graft substitutes of the present invention. It has been shown that the high surface areas of silicate based bone graft substitutes lead to superior in-vivo performance.

(36) Without wishing to be bound by theory it is proposed that the higher surface area allows for improved adhesion of proteins and cells that are involved with osseointegration and remodelling.

Example 4

(37) FIG. 1 shows the N.sub.2 adsorption isotherm of sample 2.

(38) Bioactivity testing (ability to precipitate hydroxyapatite) was carried out on sample 2 using a simulated body fluid test.

(39) FIG. 2 shows the absorbance spectra after 3 hours of immersion in simulated body fluid. The precipitation of hydroxyapatite is confirmed by the presence of two bands at 560 and 600 cm.sup.1.

(40) This is an industry standard test to demonstrate that a material is bioactive. This test is widely accepted to demonstrate that a material which is bioactive in simulated body fluid would, once in the body, be able to form bone on its surface. This is an essential property for bone substitute materials.

(41) FIGS. 3 and 4 show a scanning electron micrograph of sample 2 after calcination.

(42) The structure of the unreacted sample 2 shows silica spheres forming a bioactive aerogel structure.

(43) This data demonstrates that the bone graft substitutes of the present invention are bioactive and exhibit low densities and high surface areas, compared to typically used bones graft substitutes.

Example 5

(44) .sup.31P magic angle spinning nuclear magnetic resonance (MAS-NMR) was performed on a Bruker 600 MHz spectrometer at the 242.9 MHz resonance frequency. The powder samples were packed into a 4 mm rotor and spun at 12 kHz. The measurements were done using 60 s recycle delay and 85% H.sub.3PO.sub.4 was used to reference the chemical shift scale.

(45) FIGS. 7-11 show the .sup.31P NMR spectra of samples 1, 2, 4, 5 and 6 (from table 1) respectively. Measurements were taken after 3 hours, 6 hours, 1 day and 3 days immersion in SBF solution.

(46) A typical .sup.31P shift at 2.9 ppm is beginning to appear after 3 hours with all samples and is clearly visible after 6 hours. This demonstrates that the material has the ability to form hydroxyapatite (or similar) structures on its surface. This is a standard test used to demonstrate the bioactivity of a material.

Example 7

(47) Example 7 provides a method which can be used for producing bone graft substitutes of the present invention.

(48) Reagents

(49) 0.14M NaF solution Absolute (100%) ethanol tetraethyl orthosilicate (TEOS, (Si(OC.sub.2H.sub.5).sub.4)) Brushite (CaHPO.sub.4.Math.2H.sub.2O) Brushite is dissolved in 0.14 M solution of NaF, after which ethanol is added. This mixture is then stirred for 5 minutes.

(50) Finally the TEOS is combined slowly with the solution and is allowed to stir for thirty seconds.

(51) 4 ml of the solution is cast into cylindrical moulds (11 mm50 mm height, via syringe). Each mould is then covered with film and placed into glass container.

(52) Each sample is then gelled for 48 hours at 60 C.

(53) Each sample is then placed into 60% ethanol. After 24 hours the solution is changed for 80% ethanol. After another 24 hours it is changed once again for 95% ethanol. Finally the solution is replaced with 100% ethanol.

(54) Each sample is dried using the CPD method using a Tousimis 931 critical point drier. Each sample is run through three stasis cycles of eight hours each.

(55) After critical drying each sample is then calcined at 700 C. for three hours.

Example 8

(56) The compositions outlined in table 3 may be produced by the method of example 6.

(57) TABLE-US-00003 TABLE 3 Chemical compositions SiO.sub.2 P.sub.2O.sub.5 CaO Composition (mol %) (mol %) (mol %) Sample 7 60.00 13.33 26.67 Sample 8 65.00 11.66 23.34 Sample 9 70.00 10.00 20.00

Example 9

(58) Example 9 provides a method which can be used for producing bone graft substitutes of the present invention.

(59) Reagents

(60) 0.14M NaF solution Absolute (100%) ethanol tetraethyl orthosilicate (TEOS, (Si(OC.sub.2H.sub.5).sub.4)) Brushite (CaHPO.sub.4.Math.2H.sub.2O) Calcium nitrate tetrahydrate (Ca(NO.sub.3).sub.2.Math.4H.sub.2O)

(61) Brushite is dissolved in 0.14 M solution of NaF, after which ethanol is added. The mixture is then stirred for 5 minutes, calcium nitrate tetrahydrate is then added to the mixture and left to dissolve for 5 minutes.

(62) Finally the TEOS is combined slowly with the solution and allowed to stir for thirty seconds.

(63) 4 ml of the solution is cast into cylindrical moulds (11 mm50 mm height, via syringe). Each mould is then covered with film and placed into glass container.

(64) Each sample is then gelled for 48 hours at 60 C.

(65) Each sample is then placed into 60% ethanol. After 24 hours the solution is changed for 80% ethanol. After another 24 hours it is changed once again for 95% ethanol. Finally the solution is replaced with 100% ethanol.

(66) Each sample is dried using the CPD method using a Tousimis 931 critical point drier. Each sample is run through three stasis cycles of eight hours each.

(67) After critical drying each sample is then calcined at 700 C. for three hours.