The present invention relates to a composite ceramic mate¬ rial having a high strength combined with bioactive proper¬ ties when the material is used as a dental or orthopedic implant. The invention relates also to a method to manufac¬ ture the composite ceramic material.

BACKGROUND ART

Ceramic materials and particularly structural ceramic mate- rials generally have a high resistance to corrosion and ero¬ sion. This is true of e.g. several oxides, nitrides, carbid and borides. Also, said materials have no toxic properties. When used as implant materials said materials are complete¬ ly inactive, i.e. neither positive nor negative reactions with surrounding tissues take place, and consequently it is possible to attain a biological integration to bone tissue without any intermediate connective tissue. Such materials are termed inert when used as implant materials. These pro¬ perties make several oxides, nitrides, carbides and borides potentially very valuable as inert dental and orthopedic implant materials.

However, it is desirable that materials having a favorable it biocompatibility are not only inert, i.e. able to fasten, mechanically to a bone tissue, but also bioactive, i.e. ιe implant can be bonded chemically to a bone tissue. Oxides, nitrides, carbides and borides do not have this property. On the other hand it is known that phosphate-based materi¬ als, having a chemical composition similar to the "inorga- nic" or "ceramic" matter in bone tissue, can display bio¬ active properties. Such a phosphate-based material is e.g. hydroxylapatite, Ca(P0 ₄ ), , However, a synthetic hydroxyl¬ apatite has a low tensile toughness and hence a low strengt and also a tendency to gradually develop a continuous crack growth. Another example of a bioactive material having a calcium phosphate-base is tricalcium phosphate Ca ₃ (PO.) - , but this compound has an unsatisfactory strength. Also, it

has a not negligible water solubility and consequently may be dissolved before the bond to the bone tissue has deve¬ loped. Thus, in this respect, hydroxylapatite is preferred as compared to tricalcium phosphate.

DISCLOSURE OF THE INVENTION

The object of the invention is to suggest a composite cera¬ mic material having a so called duo-quality , i.e. a high strength combined with a bioactivity. This and other object can be attained by using a material, which comprises, when it is used as an implant material, an inert matrix having a high strength and in the matrix evenly distributed 5-35 per cent by volume of at least one second phase, which mainly comprises at least one material having a calcium phosphate- base. This calcium phosphate-base can e.g. be hydroxylapa¬ tite and/or tricalcium phosphate, while the matrix can comp rise mainly one or several oxides and/or nitrides. The mat¬ rix comprises preferably mainly one or several oxides be¬ longing to the group which comprises titanium dioxide, zirconium oxide and aluminum oxide.

The used hydroxylapatite can be entirely synthetic or con¬ sist of a bone ash, which also contains other compounds than hydroxylapatite in small contents.

The material is produced by preparing a powder mixture, which mainly consists of partly a first powder, which in the condition the powder material is in during the admix¬ ture or it will obtain after a subsequent chemical reaction can form a biologically inert matrix in the finished materi al when it is to be used as an implant material, and partly a second powder, mainly consisting of a material having a calcium phosphate-base. A raw compact is made of this pow¬ der mixture and densified.

According to one aspect of the invention said raw compact is densified by a hot isostatic pressing at a pressure of

higher than 50 MPa, preferably at a pressure of at least 150 MPa and preferably not more than 250 MPa. The raw com¬ pact is in this case suitably produced by a cold isostatic compacting, which precedes the hot isostatic compacting, which is facilitated by the admixture of the calcium phos¬ phate powder into the oxide powder. The hot isostatic com¬ pacting preferably is carried out at a temperature of 900- 1300°C. Despite the comparatively low temperature during the hot isostatic compacting it is possible to attain a density of at least 97 %, if the matrix consists of oxides. The comparatively low sintering temperature is advantageous e.g. for the following reasons:

- The grain growth is limited, which favors a high strengt

- A decomposition of the calcium phosphate material (hydro ylapatite or the like) is avoided or limited to an accep¬ table extent; and

- Undesirable reactions between oxides, e.g. titanium diox ide, and the calcium phosphates are prevented.

When the powder material is consolidated, no chemical reac tions take place. The powder mixture suitably is composed in such a manner, that the calcium phosphate-phase will appear as small islands, i.e. as discrete particles, in the matrix, which according to the first preferred embodiment of 'the invention consists of one or several oxides belong¬ ing to the group, which comprises titanium dioxide, zirco¬ nium oxide and aluminum oxide. One might expect that 5-35 percent by volume of calcium phosphate material in the in¬ ert matrix is not sufficient to ensure the desired bioacti vity but at the same time will result in a risk of a sub¬ stantial deterioration of the strength properties. However we have found that these fears are groundless. Thus, clini cal experiments, performed on living animals, have shown, that the material according to the invention has a bioacti vity, which is entirely comparable to the bioactivity of pure hydroxylapatite. Thus, it is not necessary to coat the inert matrix with pure hydroxylapatite in order to ob-

tain the desirable bioactivity, which otherwise is customa¬ ry according to Jnown practice. However, the admixture of calcium phosphate material into the matrix probably must be very finely dispersed and even in order to obtain a very large number of islands per exposed surface unit, the dis¬ tance between adjacent islands of calcium phosphate at the same time being very small. These conditions probably will facilitate the addition of a new bone tissue, but the cau¬ sal relations have not been completely explained. The very finely dispersed and even nature of the distribution of the calciumphosphate fraction in the matrix can also explain the retained very high strength. Thus, provided one regard the calcium phosphate particles as defects of the matrix, it is possible to calculate the largest possible size of the calcium phosphate islands in different oxide materials in order to obtain a certain so called critical intensity fac¬ tor (critical toughness) , known for the matrix. Thus, if the matrix is tougher, the islands can be larger than in a brittle matrix and vice versa. These theoretical considera- tions and practical results lead to the following recommen¬ dations for a composite consolidated material consisting of a matrix, which comprises metal oxides, and a calcium phos¬ phate material dispersed in the matrix, preferably hydroxyl-

» 1 apatite.

Matrix Maximum size of Mean distance between the calcium phos¬ the calcium phosphate phate particles particles τio ₂ 10 um Max. 5 um, pref. max.2 um 1 ₂ 0 ₃ 15 um

Zr0 ₂ 30 um

Thus, as regards the strength, zirconium oxide is preferred to aluminum oxide, which in its turn is better than titanium oxide. Also, the oxides differ as to their chemical resis- tance. Thus, whereas the temperature during the hot isosta¬ tic pressing - should not be higher than 1000°C, when the

matrix is composed of titanium dioxide, it can be as high as 1300°C and preferably 1100-1250°C, when a matrix of a. ^" minum oxide and/or zirconium oxide is used.

Whereas according to known practice one believes that it i necessary to provide an implant with a coat, which entirel consists of a bioactive material, which means that in cer¬ tain cases the implant has been provided with an outer coat of a calcium phosphate material, according to the present invention it is possible to use the duo-properties of the composite material according to the invention, an extra ou ter coat of hydroxylapatite or the like then not being ne¬ cessary. However, it is also, within the scope of the pre¬ sent invention, possible per se to cover a "duo-material", completely or partially, with layers of materials having another composition or to produce implants, in which diffe rent parts have different compositions, including at least one part consisting of a composite material according to the invention having duo-properties and at least one part consisting of a homogenous material, e.g. a ceramic materi without any admixture of a calcium phosphate material or a entirely metallic material. These and other aspects of the invention are also set forth in the patent claims and will be further elucidated in the following description of a few preferred embodiments.

According to an alternative process of producing a compo¬ site material according to the invention a powder mixture is prepared, in which the first powder mainly consists of elementary silicon, whereas the second powder in this case mainly is tricalcium phosphate having a particle size of not more than 5 um. A raw compact is made from the powder, composed of 5-15 percent by volume tricalcium phosphate an the remainder silicon (and pores) , and nitridized by means of nitrogen gas. As an alternative principally also ammoni and other nitridization agents can be used. Through the ni ridization the silicon is combined with nitrogen to form

silicon nitride Si N, and due to the mass build-up the raw compact is condensed to a density of between 65 and 85 % of the theoretical maximum density. Thus, this composite ceramic material is considerably more porous than that ma- terial, which is densified through a hot isotatic pressing, e.g. according to the embodiment described above, which of course unfavorably influences the strength. However, this can to a high degree be made up for due to the fact that silicon nitride is a very strong material. As far as the bioactivity is concerned, the porosity has the valuable effect, that in the pores minor surface-located areas of tricalcium phosphate Ca ₃ (P0 ₄ ) ₂ are exposed. Thus, when a bone growth inwards into the matrix, i.e. into the pore system of the silicon nitride, takes place, Ca-ions as well as phosphate groups are available, which are the necessary building units for a bone growth.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be explained in more detail bymeans of a few illustrative examples and experiments carried out, reference being made to the accompanying drawings, in whic

Fig. 1 shows a microstructure of a material according to a first preferred embodiment according to the invention;

Fig. 2 is a drawn picture, based on an X-ray photograph, which shows how an implant according to the invention has adhered to a natural bone material;

Fig, 3 shows a longitudinal section of a product according to a possible embodiment of the invention;

Fig. 4 shows a longitudinal section of a product according to another possible embodiment of the invention; and

Fig. 5 shows a longitudinal section of a product according to an additional possible embodiment of the invention.

DESCRIPTION OF EXPERIMENTS CARRIED OUT AND EMBODIMENTS Raw materials used in the trials are listed in Table 1.

Monolithic materials (titanium dioxide and hydroxylapatite respectively) as well as composite materials (combinations

of oxides and calcium phosphate materials) , Table 2, were produced from powders of these raw materials. The powder mixtures and a silicon nitride grinding agent in petroleum ether were admixed in a ball mill and were ground for 20 h. Subsequent to an evaporation in an evaporator raw compacts were produced from the powder mixtures by a cold isostatic compacting (CIP) at a pressure of 300 MPa. The raw compacts thus obtained were encased in glass and densified by a hot isostatic pressing (HIP) at a pressure of 200 MPa for 1 h at a maximum temperature of 925°C for the TiO _p -based mate¬ rials and at 1225°C for the other materials. Subsequent to the densification the materials displayed a density of more than 99 % of the theoretical maximum density. The HlP-tempe- ratures and the obtained densities are also shown in Table 2,

Table 1 - Raw materials Designation Description

BA Hydroxylapatite of bone ash

HA Hydroxylapatite, grade Merck TCP -tricalcium phosphate, grade Merch

A °< -aluminum oxide, AKP-30, Sumitomo

R Titanium dioxide, Tioxide Ltd

Z Zirconium dioxide (including 3 mol-%

Table '2 - Powder mictures which have been compacted by a hot isostatic pressing; densities of HIP-produced specimens

Specimen Powder, HIP- Density m 3 g/<c

No. % by volume temp. Recordecl Theoretical

°C

1 70HA/30A 1225 3.39 3.40

2 25HA/75A 1225 3.75 3.77

3 15BA/85A 1225 3.85 3.85

4 15HA/85R 925 4.02 4.09

5 7.5TCP/7. 5HA/85A 1225 3.77 3.85

6 7.5TCP/7. 5HA/85R 925 4.04 4.09

7 15HA/85Z 1225 5.64 5.65

Specimen Tensile Weibull- Toughness Hardness no. strength odule (MPa ² ) (5N)

(MPa) (m) (GPa)

HA 110 18 1.1 + 0. .1 3.9_ 0.3

1 250 n.d.* 2.0 7.1

2 535 n.d.* 4.0 20.2

3 601 19 3.5 18.9

4 252 9 2.9 8.4

5 446 10 3.4 17.8

6 397 n.d.* 2.6 10.9

7 820 n.d.* >7 13

A 400-560 - 4 22

R 405 10 - 12

Z 980 n.d.* >7 14

* not measured

Test bars were made with the size 3x3x30 mm. The test bars were examined in a three-point test to measure the compres- sive strength in bending. The Weibull-modules were measured The toughness was measured using Vicker"s indentation depth method, as well as the hardness at a load of 10 N och 5N respectively. Some specimens were etched for 20 seconds in a 0.1 % HF-solution in order to study the microstructure in SEM.

In order to study the bioactive properties of the materials cylinders were made with a diameter of 3.1 mm and a length of 7 mm.Identical specimens of pure aluminum oxide (negative control)^and pure hydroxylapatite (positive control) were also made to be used as reference materials. The implants were inserted by operation in a large hole (3.2 mm diameter in lateral cortex in rabbits (femora-rabbit franNew Zeeland After a healing period of three months the animals were put to death and the implants were examined by X-ray radiogra¬ phy , subsequent to the removal of surrounding soft tissue.

Fig. 1 shows the microstructure of a material according to

the invention, being composed of 15 percent by volume hyd¬ roxylapatite, the remainder being aluminum oxide (specimen 3) . The hydroxylapatite-phase is evenly distributed through out the aluminum oxide-matrix, in which the hydroxylapatite forms particles (grains) or islands (insulets) having a max imum length of < 6 um . The specimen is somewhat overetched in Fig. 1 in order to be able to identify the microstructur more easily. Some of the smallest grains can be pores. An. X-ray diffraction analysis showed that no phase alterations had taken place during the HIP-treatment.

The mechanical properties are shown in Table 3. As is expec ted, the aluminum oxide-based duo-ceramic materials, speci¬ mens 1, 2, 3 and 5, are stronger than the titanium dioxide- based materials, specimens 4 and 6. The strength level of the aluminum oxide-based duo-ceramic materials is comparabl to the strength level of commercial dental implants made of polycryεtalline aluminum oxide, which is 400-560 MPa. The tensile strength is also comparable. The zirconium oxide- based duro-ceramic materials have the highest strength and tensile strength. The results show, that the duo-ceramic ma terials according to the present invention can be used for dental implants, at least as regards the mechanical proper- duo-ceramic ate- rials according to the invention, which are based on alumi num oxide and zirconium oxide, but also the titanium-based duo-ceramic materials in all likelihood can be used as im¬ plants, at least in those instances when the mechanical properties are a critical factor.

Fig. 2 shows in a drawing, based on an X-ray radiograph a ceramic cylinder, made of specimen no. 3 in Table 3 and in serted by operation. A new cortical bone material has grown towards the implant (at a) as well as along the surface of the implant (at b) . The pattern of bone growth for the duo ceramic material according to the invention is mainly iden tical with that for pure hydroxylapatite, as was shown in

a comparison test. A similar pattern was also obtained with specimens no.4 and no. 7, which had a matrix of titanium dioxide and zirconium oxide respectively.

These trials show, that bioactive ceramic materials having a high strength can be produced by means of a hot isostatic pressing of a raw compact, which is composed of at least two powder fractions, a bioactive phase being obtained, whic consists of hydroxylapatite and is evenly distributed in an oxidic matrix, which gives the material the required strengt The bioactive phase appears as distinct points, the size of which can be allowed to vary depending on the strength of the matrix, but the mean distance between adjacent points must be smaller than 5 um , preferably smaller than 2 um .

In the description above for the invention it has been men¬ tioned that it, within the scope of the inventive concept, also is possible to produce compacts (bodies), in which dif¬ ferent parts have different compositions. This will now be illustrated by means of a few possible examples. According to a first embodiment of this aspect of the invention 15 percent by volume hydroxylapatite powder and 85 percent by volume zirconium oxide powder are mixed. The powder is pre- pared in the same manner as is explained in the description abo e of experiments carried out. The powder mixture is* ^' * poured into a polymer can to fill the can up to half its height. Pure aluminum oxide without any admixture of any substance is then added to fill the can completely. The can is closed and the powder is isostaticly compressed in a cold condition at 300 MPa. The compact specimen is then iso¬ staticly compressed in a hot condition at 1225° for 1 h and at a pressure of 160 MPa. The compact specimen is cut into test bars, in which the center line roughly corresponds to the boundary between pure aluminum oxide and the aluminum oxide/hydroxylapatite-mixture. The test bars (7 of them) with the dimensions 35x3x3 mm are examined using a three- point-bend'.-testing method. A mean value of the tensile strength was measured to 720 MPa (the lowest value was 540

MPa and the highest 810 MPa) .

It is also possible to produce a material similar to the previous one by producing two separate raw compacts, one of them consisting of a powder mixture of a calcium phos¬ phate powder and an oxidic powder and the other one solely consisting of an oxidic powder, and combining the raw com¬ pacts through a common isostatic compacting in a hot con¬ dition. Fig. 3 shows schematicly an example of a work piece produced in this way, in which the joint-ball can consist of a ceramic substance made of pure oxide and the stem can consist of a duo-ceramic substance according to the inven¬ tion.

Fig. 4 shows schematicly another example. In this instance a pure oxidic powder 1 is coated with a duo-ceramic powder 2, powder 2 being partially covering powder 1, subsequent to which the combined powder is isostaticly compressed in a cold condition and after that is encased and isostaticly compressed in a hot condition.

Fig. 5 shows the opposite instance, namely that a raw com¬ pact 3 of a duo-ceramic powder according to the invention

» I partially is coated with an oxidic powder 4, before the com pact is consolidated through a combined isostatic pressi'hg in a cold condition followed by an isostatic pressing in a hot condition.

A compact of a duo-ceramic material according to the inven- tion can also be treated in order to remove the bioactive phase in the surface area of a section of the component. This can be done through a chemical dissolving of the cal¬ cium phosphate phase in the surface layer or through blas¬ ting. In either case small cavities are obtained in those areas where the phosphate material previously was present and these cavities can function as liquid reservoirs, in case implants for joints are to be produced. Thus, a cer-

tain amount of liquid can be kept and in this way the fric¬ tion can be reduced within those areas, where a sliding is to take place in the joint. Of course, the remaining parts of the duo-ceramic work-piece are to be left intact in order to be able to utilize the bioactive properties of the duo- ceramic work-piece, where this property is desirable.