Patent Application
20070283849
Calcium-strontium-hydroxyphosphate cement (strontium-apatite) preparations are described herein. In one embodiment, the cement comprises a powder mixture (i.e., the dry element), which contains components sodium (Na), potassium (K), calcium (Ca), strontium (Sr), phosphates (PO4) and/or carbonates (CO3), together with an alkali salt or an ammonium salt of phosphoric acid or some other anion, and with water and/or an aqueous solution (i.e., the wet element). In one variation, the powder mixture contains, as a Na-component, NaHCO3, as K-components KHCO3 and/or K2HPO4, as a Ca-component, Ca3(PO4)2 (i.e., TCP), and as Sr-components SrHPO4, and/or strontium carbonate (SrCO3) and/or Sr3(PO4)2. Moreover, the powder component optionally further contains hydroxyapatite (Ca5(PO4)3(OH)). As the aqueous mixing solution for the formation of the strontium-apatite cement, an aqueous solution of an alkali salt or an ammonium salt of the phosphoric acid is suitable.
Calcium phosphates enhance the accretion of bone to a non-biological surface, and are able to fill voids in bone created after surgery or disease. When used as a bone void filler, calcium phosphates are replaced by living bone through the bone cascade and remodeling processes. The rate of replacement or resorption is a function both of the crystallinity of the material and its porosity.
Thus, the present invention relates to bone cement compositions or formulations as well as methods of preparing self-setting calcium phosphate materials which contain macroscopic pores of varying size and density, thereby creating a foamed structure with a highly increased surface area which provides substantially more regions for bone turnover than the non-foamed analogue. In a variation of one embodiment, pore sizes may typically be in the range of 4 to 200 microns in diameter. In another variation, pore sizes may be in the range of 10 to 100 microns in diameter. It should be understood that other pore sizes are contemplated. In an embodiment, there is a distribution of varying pore sizes in the above mentioned ranges.
A typical composition of the dry element of the present invention includes one or more of the following components: tricalcium phosphate (TCP), strontium hydrophosphate (SrHPO4), strontium phosphate (Sr3(PO4)2), strontium carbonate (SrCO3), precipitated hydroxyapatite (pHA), sodium phosphate dibasic (Na2HPO4), K2HPO4, and/or a cohesion promoter. The sodium phosphate dibasic and potassium phosphate dibasic may act as a setting accelerator when present in the dry element. Alternatively, these dibasic salts may be to keep a solution acidic, for example, when a buffering capacity is overwhelmed.
The cohesion promoters can be added to either of the wet element or the dry element of the cement prior to mixing. Typical cohesion promoters include soluble starch and/or hyaluronic acid.
Additional components that can be added include (NH4)2CO3, (NH4)HCO3, and/or M2-yHyCO3 (wherein M is a metal in a+1 oxidation state and y is 0, 1, or 2). Metals that can be added include lithium, sodium, potassium, rubidium, cesium, and/or francium.
Typically, the wet element of the present invention will contain a buffer solution that when mixed with the dry element is able to maintain the mixed components at an acidic pH. A buffer solution that has been found to be suitable is a phosphate buffer, such as dipotassium phosphate dibasic and potassium phosphate monobasic (which has a pKa2 of 7.2). Phosphate buffer is a polyprotic acid that has one of its pKa's close to the physiological pH of a neutral solution (i.e., a pH of 7) so it can be readily made so that the buffer solution is acidic. Phosphate buffer is also a suitable buffer because some of the dry components that are being used in the cement contain phosphates in them. If the ratio of molarities of dipotassium phosphate dibasic to potassium phosphate monobasic (i.e., K2HPO4/KH2PO4) is greater than about 5/3, then the buffer will have a pH that is acidic. Maintaining an acidic pH will allow foaming to occur. While it may be desirable to have an acidic pH for the buffer, in certain application, it may also be desirable that the pH be 5 or higher.
In an embodiment, sufficient buffer should be used so that the dry components do not overwhelm the buffering capacity. In an alternative embodiment, the powder can overwhelm the buffering capacity of the solution as long as there are components in the powder element that allows the solution to stay acidic. As an example, by having the acidic calcium phosphate salt, or the appropriate potassium phosphate salt or an organic acid in the powder, one can maintain the acidic nature of the solution even though the buffering capacity of the buffer would ordinarily be overwhelmed. Using this alternative embodiment should aid one in avoiding the use of too much buffer so that one obtains the requisite bone filling formulation consistency that allows for proper filling of bone.
Although phosphate buffer is suitable for the instant invention, it is contemplated, and therefore within the scope of the invention, that other buffers with pKa's that are somewhat acidic or close to a neutral solution are suitable for the present invention. Examples of these buffers include but are not limited to acetate, propionate, malate (pK2), pyridine, piperazine (pK1), cacodylate, succinate (pK2), MES (2-Morpholinoethanesulfonic acid), citrate (pK3), maleate (pK2), histidine, bis-tris, ethanolamine, ADA (N-[carbamoylmethyl]iminodiacetic acid), carbonate (pK1), ACES (N-(2-acetamido)-2-aminoethanesulfonic Acid), PIPES (piperazine-1,4-bis(2-ethanesulfonic acid)), MOPSO (3-(N-morpholino)-2-hydroxypropanesulfonic acid), imidazole, BIS-TRIS propane, BES (N,N-Bis(2-hydroxyethyl)-2-aminoethanesulfonic acid), MOPS (3-(N-morpholino)-propanesulfonic acid), HEPES (N-Cyclohexyl-2-aminoethanesulfonic acid), TES (N-Tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid), MOBS (4-(N-morpholino)butanesulfonic acid), DIPSO (3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxy-1-propanesulfonic acid), TAPSO (3-[N-Tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid), triethanolamine (TEA), pyrophosphate, HEPPSO (4-(2-Hydroxyethyl)piperazine-1-(2-hydroxypropane sulfonic acid)), tricine, hydrazine, glycylglycine (pK2), and Trizma (tris) buffers.
If a phosphate buffer is used as the wet element, an embodiment of the dry element includes components in the following amounts: 30-70 g tricalcium phosphate (TCP), 10-30 g strontium hydrophosphate (SrHPO4), 3-15 g strontium carbonate (SrCO3), 1-5 g precipitated hydroxyapatite (pHA), 0.5-5 g of sodium phosphate dibasic (Na2HPO4) 10-30 g K2HPO4, and/or 0.5 to 5% w/w of a cohesion promoter. If the dry mixture is mixed with a 1 to 1 solution of potassium phosphate buffer containing 4M dipotassium phosphate dibasic and 1.5M potassium phosphate monobasic (K2HPO4 (4M) KH2PO4 (1.5M)), the pH will be approximately 6.8. Because of the relatively high molarity of both dipotassium phosphate dibasic and potassium phosphate monobasic, the buffer solution will have good buffering capacity and the amount of dry ingredients shown above will allow the buffer to stay acidic.
In an embodiment, when the above dry components are in an acidic solution, carbon dioxide is generated. The underlying reaction to generate carbon dioxide is CO2+H2OH2CO3. When the above system is acidic the reaction is driven to the left, CO2 is generated and the CO2 escapes into the cement. Because the buffer keeps the pH at a relatively constant level (unless or until the buffer capacity is exceeded), CO2 will continue to be generated until the solution becomes more alkaline (causing the reaction to equilibrate or to move to the right). Likewise, if the buffer capacity is exceeded, the presence of organic acids and/or salts that maintain an acidic environment will allow the formation of CO2 bubbles.
The formation of CO2 in the cement will generate bubbles. Some of the bubbles will escape the cement and some of the bubbles will not escape. The formation of CO2 bubbles will cause the cement to foam, leading to a plurality of different size macroscopic pores of varying sizes and densities. These pores will provide substantial regions for bone turnover leading to better accretion.
An example of mixing dry components with a wet element are described in, for example, U.S. Pat. No. 6,497,901 to Royer, which is incorporated herein by reference in its entirety for all purposes. Other mixing of dry components with a wet element are described in U.S. Pat. No. 5,462,356 to Murray, which is also incorporated herein by reference in its entirety for all purposes.
It is contemplated and therefore within the scope of the present invention that the bone cement may contain additional components. These additional components may include one or more antibiotics such as gentamicin, gentamicin sulfate, erythromycin, tobramycin, vancomycin, cefazolin, oxacillin, cefotaxime, colistin, clindamycin, and/or fusidic acid. In one variation, when the cement of the present invention foams, antibiotic that is added to the mixture will have the tendency to spread uniformly throughout the cement. This can lead to a more uniform release of the antibiotics when it is applied to bone. For example, gentamicin sulfate may be a particularly suitable antibiotic for incorporation into the foaming bone cement for certain applications, since it is wide spectrum antibiotic that can be used to attack a large variety of bacteria.
Alternatively, and/or additionally, additional components that can be added to the bone cement of the present invention include one or more radiopacifier compounds such as barium sulfate, 2-[2′,3′,5′-triiodobenzoyl]ethyl methacrylate (TIBMA), 3,5-diiodine salicylic methacrylate (DISMA), and/or zirconium(IV) oxide. It is contemplated that other compounds that can be seen under fluoroscopic guidance can be used as radiopacifier compounds.
Furthermore, anticancer agents can be added to the bone cement including, but not limited to, 6-mercaptopurine, methotrexate and/or cisplatin.
Other components that can be added include re-enforcing materials such as additional hydroxyapatite (HA) powder, K2O—Na2—CaO—MgO—SiO2—P2O5 crystallized glass powder, other bioactive glasses, calcium phosphate, carbon, graphite, aramid, bone particles, bone chips, polyethylene, titanium, other metals, ultra high weight polyethylene, polymethylmethacrylate fibers in a cement matrix, tricalcium phosphate, and hydroxycarbonate apatite, and the like.
A common means of evaluating strength of bone filling cements is to use X-Ray Diffraction Spectroscopy (XRD). Alternatively, FTIR (Fourier Transform Infrared Spectroscopy) can be used. Measuring samples using XRD and FTIR are well known to one of ordinary skill in the art. For an example of XRD and FTIR measurements in bone cement, please see U.S. Pat. No. 5,914,356, which is herein incorporated by reference in its entirety for all purposes. A commonly used method in both XRD and FTIR involves taking spectroscopic samples of explanted bone material for observation.
Typical strengths of the cements of the present invention were found to be between about 0.5 and about 2.5 MPa. The strengths of the cements can be increased by adding the above mentioned re-enforcing materials. The amount that is added is dependent on where the bone cement formulation is to be used and whether or not those bones are load bearing bones. For example, if it is to be used on vertebral bones, one might add sufficient re-enforcing materials to attain a strength of 2-12 MPa.
It is contemplated and therefore within the scope of the invention that the bone cement can be used in conjunction with one or more bone surgical screws, metal rods or plates (such as titanium rods or plates), NITINOL alloy structural devices, and/or other mechanical structural devices that add structural strength to the bone. When these structural devices are used, the cement may be used with or without one or more of the above-identified re-enforcing agents. When the bone cement formulation is used without one or more re-enforcing agents, the bone cement serves, in essence, as a lattice that allows accretion of bone into the macroscopic voids. The accretion of bone into the lattice (in combination with the bone cement formulation) may lead to bone that has greater structural strength.
Mixing antibiotics or other biologically useful compounds into the cement will prevent the development of infections or serve some other useful biological purpose when the bone undergoes accretion into the macroscopic voids.
A dry mixture of 60 g tricalcium phosphate (TCP), 20 g strontium hydrophosphate (SrHPO4), 10 g strontium carbonate (SrCO3), 3 g precipitated hydroxyapatite (pHA), and 0.93 to 4.65 g of sodium phosphate dibasic (Na2HPO4), is combined. 0.5 to 5% w/w (of dry constituents) of a cohesion promoter is optionally added to the dry mixture. The dry mixture is mixed with a 1 to 1 solution of potassium phosphate buffer containing 4M dipotassium phosphate dibasic and 1.5M potassium phosphate monobasic (K2HPO4 (4M) and KH2PO4 (1.5M)) pH≈6.8). The liquid to powder ratio is about 0.33 to 0.35 ml. liquid to 1.0 gram powder. The outgassing of CO2 in the slightly acidic mixture from the carbonate generates internal bubbles in the mixture, yielding a foamed structure upon curing of the cement. This foamed structure is then ready for use as bone filler.
In an alternative embodiment, the following components are added in the dry element to generate the bone cement:
added to a 1:1 mixture of K2HPO4 (3M) KH2PO4 (1M)) pH≈6.7.
In another embodiment, the following components are added in the dry element to generate the bone cement:
added to a 1:1 mixture of K2HPO4 (3M) KH2PO4 (1M)) pH≈6.7. The L/P (liquid to powder) ratio is 0.22.
In another embodiment, the following components are added in the dry element to generate the bone cement:
added to a 1:1 mixture of K2HPO4 (3.5M) KH2PO4 (1M)) pH≈6.7 at a liquid to powder ratio. The L/P (liquid to powder) ratio is 0.35.
In another embodiment, the following components are added in the dry element to generate the bone cement:
added to a 1:1 mixture of K2HPO4 (3.5M) KH2PO4 (1M)) pH≈6.7 containing monocalcium phosphate monohydrate (MCPM). A 4% solution of Na2HPO4 and 5% solution of NaHCO3 is used.
Accordingly, in an embodiment, the present invention relates to a bioactive agent or cement composition comprising a dry element and a wet element; wherein said dry element contains tricalcium phosphate, hydroxyapatite and one or more members selected from the group consisting of strontium carbonate and sodium bicarbonate; and said wet element contains a buffer sufficient to keep the bioactive agent or cement composition acidic while curing. The cement in one embodiment is a bone cement.
The composition may contain either of or both of strontium carbonate and sodium bicarbonate. The composition may further strontium hydrophosphate.
In an embodiment, the composition contains 30-70 g tricalcium phosphate (TCP), 10-30 g strontium hydrophosphate (SrHPO4), 3-15 g strontium carbonate (SrCO3), 1-5 g precipitated hydroxyapatite (pHA), 0.5-5 g of sodium phosphate dibasic (Na2HPO4) 10-30 g K2HPO4. Further, the composition may further comprise 0.5 to 5% w/w of a cohesion promoter.
In an embodiment, the wet element is a buffer that is a phosphate buffer. The pH of the buffer is between 6.5 and 6.9 or alternatively, the buffer has a pH between 6.6 and 6.9, or alternatively, the buffer has a pH that is between 6.6 and 6.8.
The present invention also relates to methods of making a foamed self-curing cement comprising:
mixing together a dry element and a wet element;
wherein the dry element contains tricalcium phosphate, hydroxyapatite and one or more members selected from the group consisting of strontium carbonate and sodium bicarbonate;
and wherein said wet element contains a buffer sufficient to keep the cement composition acidic while curing to generate the foamed self-curing cement.
The dry element contains either or both of strontium carbonate and sodium bicarbonate, and optionally further comprises strontium hydrophosphate.
In an embodiment, the method of present invention has a dry element that contains 30-70 g tricalcium phosphate (TCP), 10-30 g strontium hydrophosphate (SrHPO4), 3-15 g strontium carbonate (SrCO3), 1-5 g precipitated hydroxyapatite (pHA), 0.5-5 g of sodium phosphate dibasic (Na2HPO4) 10-30 g K2HPO4. The method optionally employs 0.5 to 5% w/w of a cohesion promoter.
In an embodiment, the method employs a buffer that is a phosphate buffer wherein the pH of the buffer is between 5.0 and 6.9 or alternatively, between 6.0 and 6.9 or alternatively, between 6.5 and 6.9 or alternatively, the buffer has a pH between 6.6 and 6.9, or alternatively, the buffer has a pH that is between 6.6 and 6.8.
In an embodiment, the present invention is also directed to kits containing the above mentioned compositions/bone cements.
An alternative embodiment is directed to a bone piece containing voids wherein the voids contain the above mentioned compositions/bone cements.
The present invention has been described above with regards to a plurality of different embodiments. It is contemplated and therefore within the scope of the present invention that any one or more of the elements discussed above can be combined with any other one or more elements discussed above. It should also be understood that minor modifications can be made to the invention without departing from the scope and spirit of the invention. In addition, where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modification are in accordance with the variations of the invention. Furthermore, certain steps may be performed concurrently in a parallel process when possible, as well as performed sequentially as describe above. Moreover, when a range is disclosed, it is contemplated and therefore within the scope of the present invention that any real number that falls within the scope of this range is a contemplated endpoint. For example, if a range of 6.5 to 6.9 is given, it is contemplated that an endpoint for the range can be 6.726 even though this real number is not explicitly mentioned. In any event, the present invention is not to be limited by the foregoing written description but is rather to be defined by the following claims.
