Patent Grant
7122139
1. Field of the Invention
The present invention is related to a prosthetic bone implant made of a hardened calcium phosphate cement having an apatitic phase as a major phase, and in particular to a prosthetic bone implant comprising a dense cortical portion bearing the majority of load and a porous cancellous portion allowing a rapid blood/body fluid penetration and tissue ingrowth.
2. Description of the Related Art
It is advantageous if a prosthetic bone implant is bioresorbable and is supportive at the same time. Accordingly, an article made of calcium phosphate will be preferable than that made of a metal, if the former has strength which is comparable to a human cortical bone. One way of making such a bone implant is by sintering a calcium phosphate powder, particularly a hydroxyapatite (HA) powder, into a block material at a temperature generally greater than 1000° C. Despite the fact that the high temperature-sintered HA block material has an enhanced strength, the bioresorbability of the material is largely sacrificed, if not totally destroyed, due to the elimination of the micro- and nano-sized porosity during the sintering process.
The conventional spinal fusing device is composed of a metallic cage and a bioresorbable material disposed in the metal cage, for example the one disclosed in U.S. Pat. No. 5,645,598. An inevitable disadvantage of this fusion device is the sinking of the metallic cage sitting between two vertebrae to replace or repair a defect spinal disk, because the hardness and the relatively small size of the cage wear out or break the bone tissue, and in particular the endplate of the vertebra.
A primary objective of the invention is to provide a prosthetic bone implant free of the drawbacks of the prior art.
The prosthetic bone implant constructed according to the present invention is made of a hardened calcium phosphate cement having an apatitic phase as a major phase, which comprises a dense cortical portion bearing the majority of load and a porous cancellous portion allowing a rapid blood/body fluid penetration and tissue ingrowth.
The prosthetic bone implant of the present invention is made by a novel technique, which involves immersing an article molded from two different pastes of calcium phosphate cement (CPC), one of them having an additional pore-forming powder, in a liquid for a period of time, so that the compressive strength of the molded CPC article is significantly improved after removing from the liquid while the pore-forming powder is dissolved in the liquid, creating pores in a desired zone or zones of the molded article.
Features and advantages of the present invention are as follows:
a to 1d show schematic cross sectional views of four different designs of prosthetic bone implants constructed according to the present invention.
a to 2f are schematic cross sectional views showing steps of a method for preparing a prosthetic bone implant according to one embodiment of the present invention.
a and 3b are schematic vertical and horizontal cross sectional views of a prosthetic bone implant prepared according to another embodiment of the present invention, respectively.
Preferred embodiments of the present invention includes (but not limited to) the following:
Four different designs of prosthetic bone implants constructed according to the present invention are shown in
A suitable method for preparing the prosthetic bone implant of the present invention includes placing a tubular partition wall 10 in a hollow cylindrical mold 20, as shown in
An alternative method for preparing the prosthetic bone implant of the present invention from the same raw materials includes pouring the second paste in a first mold, pressing the second paste to remove a portion of the setting liquid from the second paste before the hardening reaction is completed, so that the liquid/powder ratio in the second paste decreases, and allowing the hardening reaction undergo in the mold for a period of time, e.g. 15 minutes starting from the mixing of the CPC powder, the pore-forming powder and the setting liquid, to obtain a cylindrical block having a diameter of 7 mm. Then, the cylindrical block is removed from the first mold, and placed in the center of a second mold having a diameter of 10 mm. The first paste is poured into the annular space in the second mold, and a press having a dimension corresponding to the annular shape is used to pressure the first paste to remove a portion of the setting liquid from the first paste before the hardening reaction is completed, so that the liquid/powder ratio in the first paste decreases. Again, the first paste will undergo a hardening reaction to convert into apatitic phase. The hardened cylinder having a diameter of 10 mm is removed from the second mold, followed by immersing in an immersing liquid, where the pore-forming powder contained in the second paste is dissolved in the immersing liquid while the hardened CPC thereof gaining compressive strength, to obtain the prosthetic bone implant of the present invention, as shown in
The following examples are intended to demonstrate the invention more fully without acting as a limitation upon its scope, since numerous modifications and variations will be apparent to those skilled in this art.
A Ca4(PO4)2O (TTCP) powder was prepared by mixing Ca2P2O7 powder with CaCO3 powder uniformly in ethanol for 24 hours followed by heating to dry. The mixing ratio of Ca2P2O7 powder to CaCO3 powder was 1:1.27 (weight ratio) and the powder mixture was heated to 1400° C. to allow two powders to react to form TTCP.
The resulting TTCP powder from PREPARATIVE EXAMPLE 1 was sieved and blended with dried CaHPO4 (DCPA) powder in a ball mill for 12 hours. The blending ratio of the TTCP powder to the DCPA powder was 1:1 (molar ratio) to obtain the conventional CPC powder. Particles of this C-CPC powder have no whisker on the surfaces thereof.
The TTCP powder prepared according to the method of PREPARATIVE EXAMPLE 1 was sieved and blended with dried CaHPO4 (DCPA) powder in a ball mill for 12 hours. The blending ratio of the TTCP powder to the DCPA powder was 1:1 (molar ratio). The resultant powder mixture was added to a 25 mM diluted solution of phosphate to obtain a powder/solution mixture having a concentration of 3 g powder mixture per 1 ml solution while stirring. The resulting powder/solution mixture was formed into pellets, and the pellets were heated in an oven at 50° C. for 10 minutes. The pellets were then uniformly ground in a mechanical mill for 20 minutes to obtain the non-dispersive TTCP/DCPA-based CPC powder (ND-CPC). The particles of this ND-CPC powder have whisker on the surfaces thereof.
Dense Blocks
To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE 3 was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. The resulting paste was filled into a cylindrical steel mold having a length of 12 mm and a diameter of 6 mm, and was compressed with a gradually increased pressure until a maximum pressure was reached. The maximum pressure was maintained for one minute, and then the compressed CPC block was removed from the mold. At the 15th minute following the mixing of the liquid and powder, the compressed CPC block was immersed in a Hanks' solution for 1 day, 4 days, and 16 days. Each test group of the three different periods of immersion time has five specimens, the compressive strength of which was measured by using a AGS-500D mechanical tester (Shimadzu Co., Ltd., Kyoto, Japan) immediately following the removal thereof from the Hanks' solution without drying. The CPC paste in the mold was compressed with a maximum pressure of 166.6 MPa, and in the course of the compression the compression speeds were about 5 mm/min during 0˜104.1 MPa; 3 mm/min during 104.1˜138.8 MPa; 1 mm/min during 138.8˜159.6 MPa: and 0.5 mm/min during 159.6˜166.6 MPa. The measured wet specimen compressive strength is listed Table 1.
It can seen from Table 1 that the compressive strength of the compressed CPC blocks is increased remarkably after one-day immersion in comparison with the non-immersed block, and declines a little for a longer immersion time.
The procedures of EXAMPLE 1 were repeated by using the C-CPC powder prepared in PREPARATIVE EXAMPLE 2 and the ND-CPC powder prepared in PREPARATIVE EXAMPLE 3. The maximum pressure used to compress the CPC paste in the mold in this example was 156.2 MPa. The results for one-day immersion time are listed in Table 2.
It can be seen from Table 2 that the compressive strength, 62.3 MPa, of the immersed compressed CPC block prepared from the conventional CPC powder (no whisker) is about 1.7 times of that (37.3 MPa) of the non-immersed compressed CPC block in Table 1, and the compressive strength, 138.0 MPa, of the immersed compressed CPC block prepared from the non-dispersive CPC powder (with whisker) is about 3.7 times of that of the non-immersed compressed CPC block in Table 1
Ca4(PO4)2O (TTCP) powder as synthesized in PREPARATIVE EXAMPLE 1 was sieved with a #325 mesh. The sieved powder has an average particle size of about 10 μm. To the TTCP powder HCl aqueous solution (pH=0.8) was added according to the ratio of 1 g TTCP/13 ml solution. The TTCP powder was immersed in the HCl aqueous solution for 12 hours, filtered rapidly and washed with deionized water, and filtered rapidly with a vacuum pump again. The resulting powder cake was dried in an oven at 50° C. The dried powder was divided into halves, ground for 20 minutes and 120 minutes separately, and combined to obtain the non-dispersive TTCP-based CPC powder, the particles of which have whisker on the surfaces thereof. A setting solution of diammonium hydrogen phosphate was prepared by dissolving 20 g of diammonium hydrogen phosphate, (NH4)2HPO4, in 40 ml deionized water. The procedures in EXAMPLE 1 were used to obtain the wet specimen compressive strength for one-day immersion time, wherein the maximum pressure to compress the CPC paste in the mold was 156.2 MPa. The results are shown in Table 3.
The trend same as the TTCP/DCPA-based CPC powder in Table 2 of EXAMPLE 2 can be observed in Table 3.
The procedures of EXAMPLE 1 were repeated except that the maximum pressure used to compress the CPC paste in the mold was changed from 166.6 MPa to the values listed in Table 4. The period of immersion was one day. The results are listed in Table 4.
The data in Table 4 indicate that the compressive strength of the CPC block increases as the pressure used to compress the CPC paste in the mold increases.
The procedures of EXAMPLE 1 were repeated except that the maximum pressure used to compress the CPC paste in the mold was changed from 166.6 MPa to the values listed in Table 5. The liquid leaked from the mold during compression was measured, and the liquid/powder ratio was re-calculated as shown in Table 5. The period of immersion was one day. The results are listed in Table 5.
The data in Table 5 show that the compressive strength of the CPC block increases as the liquid/powder ratio decreases during molding.
The procedures of EXAMPLE 1 were repeated. The period of immersion was one day. The CPC blocks after removing from the Hanks' solution were subjected to post-heat treatments: 1) 50° C. for one day; and 2) 400° C. for two hours with a heating rate of 10° C. per minute. The results are listed in Table 6.
It can be seen from Table 6 that the post-heat treatment enhances the compressive strength of the CPC block.
Porous Blocks
To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE 3 was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. KCl powder in a predetermined amount was mixed to the resulting mixture by stirring intensively. The resulting paste was filled into a cylindrical steel mold having a length of 12 mm and a diameter of 6 mm, and was compressed with a gradually increased pressure until a maximum pressure of 3.5 MPa was reached. The maximum pressure was maintained for one minute, and then the compressed CPC block was removed from the mold. At the 15th minute following the mixing of the liquid and powders, the compressed CPC block was immersed in a deionized water at 37° C. for 4 days, 8 days, and 16 days. The compressive strength of the specimens of the three different periods of immersion time was measured by using a AGS-500D mechanical tester (Shimadzu Co., Ltd., Kyoto, Japan) after the specimens were dry. The measured dry specimen compressive strength is listed Table 7.
It can seen from Table 7 that the dry compressive strength of the porous CPC blocks decreases as the KCl/CPC ratio by weight increases.
The procedures of EXAMPLE 7 were repeated by using sugar, KI, C17H33COONa and C13H27COOH instead of KCl. The immersion time was 14 days in deionized water. In the cases where the C17H33COONa and C13H27COOH were used, the CPC blocks were further immersed in ethanol for additional four days. The conditions and the results are listed in Table 8.
a)S = Pore-forming powder/CPC by volume.
b)C.S. = dry compressive strength (hereinafter abbreviated as C.S.).
c)Porosity: Porosity (vol %) was measured by Archimedes' method, and calculated as in ASTM C830.
It can be seen from Table 8 that various powders which are soluble in water can be used in the preparation of a porous CPC block according to the method of the present invention.
Dual-Functional Block
To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE 3 was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. KCl powder in a ratio of KCl powder/CPC by volume of 2 was mixed to the resulting mixture by stirring intensively. The resulting paste was filled into a cylindrical steel mold having a length of 12 mm and a diameter of 7 mm, and was compressed with a gradually increased pressure until a maximum pressure of 3.5 MPa was reached. The maximum pressure was maintained for one minute, and then the compressed CPC block was removed from the mold at the 15th minute following the mixing of the liquid and powders.
The resulting cylinder having a diameter of 7 mm was placed in another cylindrical steel mold having a diameter of 10 mm. To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE 3 was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. The resulting paste was filled into the gap between said cylinder and said another mold, and was compressed with a gradually increased pressure until a maximum pressure of 50 MPa was reached. The maximum pressure was maintained for one minute. At the 15th minute following the mixing of the liquid and ND-CPC powder, the CPC/KCl composite block was immersed in a deionized water at 37° C. for 4 days. KCl powder was dissolved in the deionized water, and a dual-functional CPC block having a porous CPC cylinder surround by a dense CPC annular block was obtained.
The compressive strength of the specimen was measured by using a AGS-500D mechanical tester (Shimadzu Co., Ltd., Kyoto, Japan) after the specimens were dry. The measured dry specimen compressive strength is 68.8 MPa.
The porosities of the porous CPC cylinder and the dense CPC annular block were measured by Archimedes' method, and calculated as in ASTM C830, after the dual-functional CPC block was broken intentionally, and the results are 74% and 30%, respectively.
X-ray diffraction pattern of the powder obtained by grinding the dual-functional CPC block shows broadened characteristic X-ray diffraction peaks of apatite at 2θ=25–27° and 2θ=20–35° with a scanning range of 2θ of 20–40° and a scanning rate of 1°/min. results indicate that the powder is a mixture of apatite and TTCP with apatite as a major portion.
This application is a continuation of and claims priority to Non-Provisional Patent Application Ser. No. 10/852,167 entitled “DUAL FUNCTION PROSTHETIC BONE IMPLANT AND METHOD FOR PREPARING THE SAME” filed on May 25, 2004, now U.S. Pat. No. 6,994,726.
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