GB2142915A - Inorganic implant material - Google Patents

Abstract

An inorganic implant material for filling in a defect or hollow portion of bone comprises an inorganic material having a negative zeta potential. The implant material accelerates the formation of new bone. Specified materials are baked calcium phosphate materials, titanium oxide and cattle bone.

Description

SPECIFICATION Inorganic implant material Field of Invention: The present invention relates generally to an inorganic material to be implanted in a living body, and more particularly to an inorganic implant material which is compatible with the living body without causing appreciable foreign body reaction and which is capable of promoting new bone formation.

In surgical and orthopedic treatments, prosthesis operations are often required for filling in defects or hollow portions of bone resulting from the fracture of bone or the surgical removal of a bone tumour. To cope with such a case, it has been a common practice to resect ilium from the patient to fill in the defect or hollow portion of bone, thereby to promote early repair of the bone tissue. However, by means of such an operation, normal bone tissue must be removed from an unspoilt portion, and this causes additional pain to the patient in addition to complicating the operation. Moreover, when the volume of the defect or void in the patient's bone is large, the amount of bone obtainable from his own body is not always sufficient for fully filling in the defect or void.In such a case, it is inevitable to use a substitute for the patient's own bone tissue, the substitute material being selected from the same or different sorts of bone tissue. As to the implantation of the same kind of bone, investigations have been made into the use of bones stored in a frozen state, and into the use of decalcified bones. However, such kinds of bone have not yet been deemed as practically utilizable implant materials. On the other hand, a so-called keel bone, i.e. a material prepared by removing proteins from cattle bones, has been proposed as a different sort of implant bone in some cases. However, whether the substitute material is the same or a different sort of bone tissue, it can still cause foreign body rejection reaction or other problems, and it shows only slight encouragement for the formation of new bone.For these reasons, the post-operation recovery is not always satisfactory.

Another measure adopted conventionally with the aim of decreasing the time required for recovery is the internal-fixation method wherein the fractured portion is directly fixed by means of a metal plate, pin or screw. However in many cases a long time, possibly six months to one year or even longer, is needed for complete recovery even if such an internal-fixation method is adopted. Additionally, the material used for the internal fixation should be removed from the patient's body after the complete recovery of the fractured bone, and this involves unnecessary pain, stress and expense.

A variety of metallic and plastic materials have hitherto been used as substitutes for the hard tissues in the living body. However, it has been recognized that these materials may have a tendency to dissolve or otherwise to deteriorate in the environment of living tissue, or may be toxic to the living body, or may cause a so-called foreign body reaction.

Ceramic materials have attracted public attention, since they have good compatibility with living tissues. Recent proposals have included an artificial bone, a joint or a dental root made of monocrystalline or polycrystalline alumina (Al203) and an artificial dental root made of a sintered product of tricalcium phosphate Ca3(PO4)2 or hydroxyapatite Ca5(PO4)30H. It has been reported that these materials have a good compatibility with the living body. For instance, a sintered product of hydroxyapatite implanted in living bone tissue does not cause the formation of a foreign matter membrane, which shows that the implanted sintered product has been directly combined with the bone tissue.However, it has not yet been clarified what types of ceramic implant materials should be implanted to gain the optimum results of little foreign body reaction, good compatibility with the living body and early formation of new bone.

The Invention: The invention provides an inorganic material for filling in defects or hollow portions of bone, which material has a negative zeta potential. It has been found that such materials have a good compatibility with the living body, cause a ie foreign body reaction and stimulate the formation of new bone tissue within a short period of time.

The term "zeta potential", as used throughout this Specification, is determined by the streaming potential determination method. In detail, the sample to be determined is finely pulverized and placed in a test cell to form a diaphragm through which a liquid is forcibly passed using an inert gas, such as nitrogen gas, as the pressure source to detect the potential difference between the end faces of the diaphragm.The zeta potential is calculated by substituting the applied pressure and the detected potential difference for P and E in the following equation (Helmholtz-Smoluchowski's equation): Zeta Potential= 4s E e P wherein 71 is the coefficient of viscosity (poise) of the liquid, A is the specific conductivity (-1cm-1) of the liquid, e is the dielectric constant (-) of the liquid in air, E is the detected potential difference (mV) and P is the applied gas pressure (cm H20).

It is preferred, for best compatibility with the living body, that the inorganic implant material of the invention should be one containing calcium phosphate or a calcium phosphate compound as the main ingredient, although the present invention is not limited only to the use of such calcium phosphate compounds provided that the inorganic material has a negative zeta potential, and is not toxic to the living body.

Examples of the calcium phosphate compounds that may be used in the invention, after suitable adjustment of their zeta potentials, include tricalcium phosphate, tetracalcium phosphate, hydroxyapatite, fluoroapatite, animal bones and mixtures thereof. These materials are preferred since they are the same as or closely resemble the inorganic composition in the hard tissue of the living body, and exhibit a good compatibility with the living body. Amongst them, hydroxyapatite is the most preferable material, since it is the same as the hard tissue composition of the living body.

The calcium phosphate compound is preferably in the form of a glass, advantageously one containing calcium phosphate as a main ingredient and having an atomic ratio of Ca2+:P043- in the range of 0.2:1 to 3.0:1. The total calcium phosphate content, calculated as CaO plus P20s, is preferably not less than 1 5 wt%.

Any other inorganic materials each having a negative zeta potential may be used in the present invention, irrespective of whether they are synthesized through the known methods or present as natural resources. Examples include animal bones, phosphor minerals and fluoroapatite.

In order to obtain an inorganic material having a negative zeta potential according to the present invention, if the inorganic material is a calcium phosphate compound, the raw material calcium phosphate compound should be baked at 500"C or higher and the total content of inorganic metal ions in the inorganic material such as Mg2+, A13+ or Si4+ should be 10 wt% or less, preferably 5wt% or less as calculated in the form of MgO.When a glass mainly composed of calcium phosphate is used, the melting temperature during the preparation thereof should be from 800"C to 1 700"C and the total content of inorganic oxides other than CaO and P20s, such as Na2O, MgO, Awl203, SiO2, Fe203 and TiO2, should be less than 85 wt%.

Amongst the aforementioned inorganic materials those each having a zeta potential in the range of minus 0.05 mV to minus 20.0 mV, particularly minus 0.2 mV to minus 10.0 mV, are preferable since they have superior compatibility with the living body and result in earlier formation of new bone.

The inorganic implant material of the invention may be used in the form of a powder, granules, a porous body, a plate, a cylindrical or rectangular column, a frustum shape (including conical, triangular and polyhedral frusta), or fibres.

When the inorganic implant material of the invention is used in the form of powders, an inorganic raw material prepared by the above process may be pulverized and used directly as a filler or may be filled in the form of a slurry in an isotonic sodium chloride solution or in blood.

It may be also used in the form of granules, where the inorganic material is first pulverized thoroughly into a fine powder which is then formed into granules by the pan-granulation method. The granules may be used directly as a filler as in the case of powders, or may first be wetted by the addition of an isotonic sodium chloride solution or blood prior to filling.

When the material of the invention is used in the form of a porous body, it may be prepared by baking a spongy bone taken from a living body or by a process including the steps of allowing a slurry of the powdered inorganic material to adhere to an organic porous body followed by removing the organic material by burning. The porous body prepared by the lattermentioned process is preferable to a dense implant material prepared by sintering. When the porous body is used as a filler for filling in a defect or hollow portion of bone, new bone is grown at portions deeper in the thus implanted porous body to promote earlier integration of the implant material with the growing living tissue.

When used in the form of a flat plate, the ircrcanic material of the invention in powder form is first moulded and then sintered to obtain a plate ,ihich may be used for an application where particularly high strength is necessary, as in the case of a plate for fixing bones.

Cylindrical, rectangular column-shaped, and circular or squarish frustum-shaped implant materials may be prepared through a process similar to that utilized for the production of a plate, and these products of such configuration may be used as, for example, nails or screws for fixing bones.

The inorganic implant material according to the present invention may be used not only in the art of orthopedic treatment, but also in the art of dental treatment. For example, it may be used in the form of powders, granules or porous bodies to cure pyorrhea alveolaris or periostitis alveolaris, and may be used in the form of a cylinder, square or rectangular column shape of frustum (including circular, triangle or squarish or polyhedral frusta) as a dental point for filling in a dental root canal.

EXAMPLES: Example 1 Using distilled water as the liquid passing through test samples in a test cell, zeta potential was measured by using a streaming potential determination device (Model ZP-10B, produced by Shimazu Seisakusho Limited) to find the zeta potential of each of the following samples: a baked powder of a cattle bone consisting of inorganic components only (organic ingredients being baked out by baking at 900"C); a baked powder of tricalcium phosphate (baked at 1000"C; content of inorganic metal ions: 0.2 wt% calculated as MgO); a baked powder of hydroxyapatite (baked at 900"C; content of inorganic metal ions: 0.3 wt% calculated as MgO); tetracalcium phosphate (baked at 1 350 C; content of inorganic metal ions: 0.4 wt% calculated as MgO); and a baked powder of titanium oxide (content of inorganic metal ions other than titanium: 0.1 wt% calculated as MgO). The-results are shown in Table 1.

Table 1 Material Zeta Potential (mV) Baked Catle Bone - 0.6 Tricalcium Phosphate - 0.4 Hydroxyapatite - Q.6 Tetracalcium Phosphate - 8.3 Titanium Oxide - 0.3 Each of the aforementioned powders was filled in a defect artificially formed in a femur of a rabbit, the defect having the dimensions of 3 mmf X 4mmL. Formation of new bone was observed at the filled portion after one month from the implantation in the cases implanted with the aforementioned materials. The defect has been completely remedied with a large volume of newly grown bone tissue in each of the cases where the samples produced by baking the cattle bone and hydroxyapatite were used. The volumes of newly grown bones in the defects reached the level next to the cases of cattle bone and hydroxyapatite, when tricalcium phosphate or tetracalcium phosphate was used.The repair degree obtainable by the use of titanium oxide was lower than with the other cases although some newly grown bone was observed.

Example 2 Hydroxyapatite samples were synthesized through the wet process under the conditions set forth in the following Table 2, and the zeta potentials of the resultant samples were measured similarly to Example 1. The representation in Table 2 identified by MgO means the content of inorganic metal ions, calculated as the equivalent molar amound of MgO.

Table 2 Sample Identification Baking Zeta No. of Sample Temp ( C) Potential (mV) No. 1 Hydroxyapatite MgO,0.3wt%) 300 +0.0 2 1 " 500 -0.05 3 " " 700 -0.2 4 " It 900 -0.6 5 5 1 I1 1250 -10.0 6 II " 1350 -20.0 7 7 Hydroxyapatite MgO,3.0wt% 300 +0.2 8 N " 500 -0.01 9 9 SI 700 -0.1 10 1 " 900 -0.4 11 " " 1250 -6.0 12 1 " 1350 +0.1 Each of the sample powders of hydroxyapatite, as set forth in Table 2, was filled in a defect artificially formed in a femur of a rabbit (3 mmf X 4 mmL) in the form of slurry, and the postoperation course was observed after one month to learn the new bone formation and the compatibility with the living body.

As to the compatibility with the living body, favorable results were found except in Samples Nos. 1, 7 and 1 2. As to the formation of new bone, particularly preferred results were found when the powders of Samples Nos. 3, 4, 5, 10 and 11 were used, wherein new bone tissues had been grown almost to fill up the centre portions of the defects.

Example 3 A dog was subjected to an operation to drill a hole having a diameter of 0.8 mm and a depth of 3 mm at its dental root canal in which was filled a cylinder or pillar having the dimensions of 0.9 mmf x 10 mmL and composed by hydroxyapatite (baked at 1 250 C, Content of inorganic metal ions: 0.1 wt% as calculated as MgO) having a zeta potential of minus 3.4 mV when measured by using distilled water. At the same time, a piece of the neck bone of the same dog was resected, the resected piece having the dimensions of 5 mm square and a height of 3 mm, and the resected portion was filled using an implant material in the form of a square frustum having the dimensions of 2 mm square and a height of 5 mm and made of the same hydroxyapatite.

After one month from the implantation operation, the regions implanted with the hydroxyapatite implant materials of the invention were picked out and the tissue contiguous with the implanted materials was observed. The results were that the interfaces and gaps between the implant materials and the living tissues of both operated regions were completely filed up by newly grown bone tissues without appreciable foreign body reaction to reveal acceptable and greatly improved compatibility with the living body.

Example 4 Glasses each containing calcium phosphate as a main ingredient and including other components as set forth in the following Table 3 were prepared by melting them at temperatures shown in the Table followed by rapid cooling to prepare products of granular form.

The granules were pulverized into powders which were tested as in Example 1 to determine the zeta potentials thereof.

Table 3 Sample Ca2+/Po43 CaO + P205 Components Melting Zeta No. (Atanic Ratio) (wt%) other than Temp potential CaO + P205 ( C) (mV) 1 0.1 5 M2OwSiO2 750 +8.0 2 15 2 770 +3.2 3 " 80 ' 850 +0.0 4 0.2 5 Na2O,SiO2 780 +6.5 5 BI 15 ZI 880 -10.6 6 " 80 l 950 -14.5 7 2.0 5 A12 3tSi 2 1710 +2.0 8 @ ; 15 II 1600 -0.05 9 II 80 II 1350 -3.1 Each of the glass powders, as set forth in Table 3, was sieved and the 1.0 to 0.5 mm mesh fraction was filled in a defect of bone (3 mmf x 4 mmL) which had been artificially formed in a femur of a dog. The post-operation course of each of the dogs after 4 weeks was observed.

The results were that no appreciable new bone formation was observed between the glass particles in those cases where Samples Nos. 1 to 4 and 7 were used, whereas networks of newly grown bone tissues were observed between the glass particles in those cases where the samples other than Sample Nos. 1 to 4 and 7 were implanted. The network of newly grown bone tissue obtained by the use of Sample No. 9 was the most significant in that it was thick and tight.

Claims (9)

1. An inorganic material for filling in defects or hollow portions of bone which material has a negative zeta potential.

2. A material according to claim 1, having a zeta potential of minus 0.05 mV to minus 20.0 mV.

3. A material according to claim 2, having a zeta potential of minus 0.2 mV to minus 10.0 mV.

4. A material according to any preceding claim, being a material selected from the group consisting of calcium phosphate compounds and glasses each containing calcium phosphate as a main ingredient.

5. A material according to claim 4, being tricalcium phosphate, tetracalcium phosphate, hydroxyapatite, animal bones or a mixture thereof.

6. A material according to claim 5, which has been prepared by baking the calcium phosphate compound at a temperature of not lower than 500"C to prepare a baked product containing 10 wt% or less of inorganic metal ions other than the ions orginating from the calcium phosphate compound, when calculating the other inorganic metal ions as MgO.

7. A material according to claim 4, being glass with an atomic ratio of Ca2+:P043- of from 0.2:1 to 3.0:1 and a total content of calcium phosphate, calculated as CaO plus P205 of not less than 15 wt%.

8. A material according to claim 7, wherein the glass has been prepared by melting the raw material at a temperature of 800"C to 1 700'C, and wherein the content of inorganic oxides other than CaO and P2O5 contained in the glass is less than 85 wt%.

9. An inorganic material for filling in defects or hollow portions of bone, being a material having a negative zeta potential as disclosed in any of the Examples herein.