COMPOSITION, MANUFACTURING, AND USE OF SILICON NITRIDE AS A BIOMATERIAL FOR MEDICAL PURPOSE

Abstract: The invention refers to the composition, manufacturing and usage of silicon nitride as a biomaterial for medical usage as a surface carrying, orthopaedic biomaterial for wearing surfaces in artificial joints, for example the hip and the knee joint. The manufacturing is performed by preparing and mixing a mixture of powder containing 70-100 volume % silicon nitride with the addition of 0-30 volume % of one or a combination of the following materials: zirconium dioxide, yttrium dioxide, titanium dioxide, calcium oxide, silicon carbide and mullite. From the mixture a body is formed which is subjected to a high pressure greater than 50 MPa and temperatures between 500-3000 DEG C under sintering (HIP). The material can be used for one, both or all the wearing surfaces in an artificial joint.

Description: Composition, manufacturing, and use of silicon nitride as a biomaterial for medical purpose, TECHNICAL AREA The present invention refers to composition, manufacturing, utilisation and function of silicon nitride for medical purposes, for example as a biomaterial for wearing surfaces in artificial joints. Patients with joints, which are destroyed from fractures or degenerative joint disease, suffer from pain from the involved joint, e.g. the hip joint. In order to relieve the pain it is necessary, in addition to using pain relievers, to surgically change the involved joint. Another alternative is to replace the involved joint with an artificial joint, where the joint surfaces are manufactured from a biomaterial. The biomaterial must be durable against wear, since a joint is heavily loaded. It is necessary to choose a biomaterial, which fulfils these demands, so that a lifetime of the prosthesis can exceed the life expectancy of the patient. Today about 10% of hip prosthesis must be replaced within ten years of the implantation due to wear and loosening caused by particles which originate from the joint surfaces. It is difficult to find new biomaterials with the necessary characteristics for implantation in humans. Ceramic materials or substances, and in particular structural ceramic materials, are generally considered to be highly resistant against corrosion and erosion. This is true for many oxides, nitrides, carbides and borides. The materials are completely inactive, that is they do not cause a reaction either indirectly or directly from — 2 — the surrounding tissue, which makes them suitable as orthopaedic implants. Silicon nitride is such a ceramic substance. Silicon nitride and mixtures in the form of powder can be pressed under high temperature and high pressure, so called isostatic pressing (HIP), to a preshaped porous body consisting of the material of the powder. Silicon nitride is also a ceramic material which we have shown has extraordinary characteristics as a biological material and is extremely well tolerated of the biological organism. In addition the material has a high coefficient of wear, which means that it has a high wear resistance. Compared to other ceramics (see the table below) silicon nitride has excellent characteristics as a prosthetic material for implantation. The invention is defined as a mode of usage of such a ceramic substance or it's mixtures as a biomaterial in a biological environment. More specifically the invention can be defined as a way to use such a ceramic substance in the form of silicon nitride as a basis, and it's mixtures, as a supporting material/wear surface e.g. in prostheses to be used for biological implantation in humans and/or animals. Biomaterial means in this context a material or a substance, which can be sustained by biological tissues without causing a pronounced rejection. A supporting material means a material, which functions as a joint surface in an artificial joint for example in the hip joint or the knee joint, see figure 1 and 2 below. Biological usage means that the prosthesis is surgically implanted in or carried by a biological organism. THE PRESENT STATE OF THE ART When using silicon nitride as a biomaterial in the tissues of biological organisms is rejection of a minor degree without importance, that is the biological organism sustains the foreign material well and the material can therefore be used as an implant. Silicon nitride is shaped by isostatic pressing under high temperature and high pressure to a joint surface, e.g, the joint surfaces of the hip and the knee. In order to acquire a sufficient quality of the wearing surface it is necessary that certain biological demands are met. Through the Swedish patent application 90.04134-4 (KN 7334 SE) is it known how to perform isostatic pressing of ceramic materials to the highest degree of pore quality. We refer to the following publications: T. Mukarami and N. Ohtsuki, Bioceramics, Ishiyaku Euroamerica, 224-46, 1989; P. Kumar et al . , Adv. In Bio.Eng. 9, 373-8, 1990; H. Oonishi et al . , Bioceramics, Ishiyaku Euroamerica, 272-7, 1989; H. McKellop et al . , JBMR 15, 619-53, 1981; B. Weightman and D. Light, Biomaterials 7, 20-9, 1986; L. Zichner, Trans. Soc . Biomaterials Imp. Retrieval Symp ., p.22, 1988. DESCRIPTION OF THE INVENTION The purpose of the invention is to solve such problems, which have been described above, involving prosthesis loosening in biological organisms, including the human, as a result of increased wear at the joint surfaces of the mechanical prosthetic joint. According to the invention silicon nitride is used which, compared to other ceramics, is wear resistant and has excellent characteristics for implantation in the form of prosthesis, as demonstrated in the following table: —4- TABLE Characteristics Alumina Zirconia Silicon nitride Bending strength 600 1000 900 (MPa) Fracture toughness 3-4 >8 4,5 (MPa/m 2 ) Hardness (GPa) 22 12 19 The joint surfaces are manufactured from silicon nitride and/or it's mixtures by isostatic pressing under high temperatures, high pressures and sintering, whereby: 1. Silicon nitride as a base and it's mixtures (see below) creates the prerequisites for a wear resistant material which is suitable for joint bearing components in prostheses for biological organisms, including man, 2. The powder material from which the object is formed/manufactured consists of a ceramic material, silicon nitride, but it can also be composed of intermetallic compounds or composites consisting of mixtures of metallic and ceramic materials. 3. One, both or all joint surfaces in the same artificial joint are manufactured of the mixture according to the example below, 4. The minimal porosity of the joint surface by using HIP for it's manufacturing, limits the wear, 5. The polished surface contributes to decreased wear, 6. The specific microstructure of the joint surface increases the liquid film between the joint surfaces in order to limit wear by increased lubrication from the body fluid. 7. The processing of the joint surface leads to a decreased risk for the formation of cavities or crevices. 8. The angular movement of the joint influences the wear, and manufacturing of small components e.g. a hip with a small prosthetic head (ball) which can be 22 mm or less in diameter from silicon nitride and/or mixtures of the biomaterials mentioned below, 9. Special methods of desinfection which avoids a risk of infection for the biological organism. The invention is of particular value for the manufacturing of surfaces in artificial joints, for which good surface characteristics, such as hardness and wear resistance, is a necessary requirement, and where surface defects such as irregularities, crevices and cracks can not be tolerated. Examples of products with such requirements are joint surfaces, i.e. both the prosthetic head (ball) and the so called cup (socket), in hip prostheses, joint surfaces, i.e. both the thigh bone ( emoral) and the shin bone (tibial) component in knee prosthesis and joint surfaces, e.g. in the shoulder, the ankle and so forth, where the characteristics of the joint surface is decisive, and where silicon nitride and/or mixtures of the biomaterials mentioned below make up the prosthetic body. FIGURE TEXT One presently suggestive variant of the invention will be described below with reference to drawings included where figure 1 shows the wearing surfaces in an artificial hip joint , figure 2 shows the wearing surfaces in an artificial knee joint , figure 3 shows the wearing surfaces in an artificial shoulder joint. figure 4 shows the components of an artificial hip joint, and figure 5 shows the components in an artificial knee joint . DETAILED MANUFACTURING DESCRIPTION Below are a couple of examples of suitable material compositions and material characteristics. Example 1 shows silicon nitride mixed with 2 volume % yttrium dioxide which results in the following data: Example 1 Characteristics Value Bending strength (3 pt . ) 1000 MPa Weibull modulus (m) 22 Fracture toughness 4 , 5 MPa/m 2 Stress exponent >100 Hardness 19 GPa • 7 — Example 2 shows silicon nitride mixed with 10 volume % of aluminium oxide in the form of short fibres, which results in the following data: Example 2 Characteristics Value Bending strength (3 pt . ) 930 Mpa Weibull-modulus (m) 23 Fracture toughness 6 , 5 MPa/m 2 Stress exponent >100 Hardness 20 GPa Manufacturing of the material is performed by preparing a powder mixture according to one of the examples above with silicon nitride as a base or the addition of other biomaterials, which have the desired characteristics. The mixture is formed to a body and sintered under high pressure (greater than 50 MPa, in particular pressures higher than 150 MPa, but not higher than 350 MPa) and high temperature (500-3000 °C) according to the HIP- method. The HIP-method is the suitable method, since the porosity is a decisive factor for wear e.g. of the joint surfaces. After cooling the surfaces are polished, cleaned and disinfected before medical use. When the material has been consolidated, there is no chemical reaction . For the wear at the joint surfaces of the mechanical joint to be used for implantation to be minimised a special method of polish of medical materials which fulfils class III is necessary. In connection with the figures below, some examples of the usage of the biomaterial in connection with different artificial joints are shown. Figure 1 demonstrates an example of a hip prosthetic head and it's socket, where the joint surfaces are — 8 — manufactured from silicon nitride or it's mixtures according to one of the examples above. The ball has a diameter of 22 mm, but can be larger or smaller. The ball is spherical with a conical fitting to be attached to the neck of the prosthetic body. The construction is suitable for artificial hip joints in general. Figure 2 demonstrates a knee prosthesis consisting of a thigh bone (femur) and shin bone (tibia) component which are manufactured of silicon nitride or it's mixtures and which is attached to the thigh bone and the shin bone respectively. The sizes vary according to the patient's weight and size. The construction is well suited for artificial knee prosthesis in general. Figure 3 shows an example of a shoulder prosthesis in the form of a prosthetic ball and it's socket, which are manufactured by silicon nitride or it's mixtures. Figure 4 shows the assembly of the components in an artificial hip joint consisting of a shaft, ball and a socket, whereby the joint surface carrying components are composed of silicon nitride or it's mixtures. A corresponding overview, from the side and form the front, of an artificial knee joint is shown in figure 5. Even in this case the joint surface carrying components, that is the thigh bone and the shin bone components, are composed of silicon nitride or it's mixtures, The invention is not limited to the designs described above which can be varied within the framework of the following patent description, Thus one, both or all wearing surfaces in an artificial joint can be composed from one or many materials according to the invention. The invention is neither limited to the hip, the knee or the shoulder, but can also be used for other types of joints such as elbow, ankle, finger- and other possible joints .

Claims: PATENT CLAIMS l.A method for manufacturing of a ceramic material with a high stress tolerance and tissue tolerance for medical usage as a orthopaedic biomaterial for implantation, c h a r a c t e r i s e d b y 1.1. The preparation/mixture of a powder composition containing silicon nitride as a base consisting of 70- 100 volume % silicon nitride with addition to 0-30 volume % of one or the combination of the following materials: zirconium dioxide, yttrium dioxide, titanium dioxide, calcium oxide, silicon carbide and/or 0-20 volume % of short fibres of aluminium oxide, silicon carbide and mullite, or independent mixtures of those, 1.2, To create a body of the mixture, 1.3. To subject the body to high pressures exceeding 50 MPa and temperatures between 500-3000 °C by isostatic pressing and sintering (HIP) in order to acquire desirable porosity of the body. 2. Method according to patent claim 1 c h a r a c t e r i s e d b y the densi fication of the body under a pressure between 50 MPa and 350 MPa. 3. Method according to the patient claim 1 c h a r a c t e r i s e d b y the sintering of a mixture of silicon nitride and/or a described added material under a temperature of between 500-3000 °C. 4. Method according to patent claim 1 c h a r a c t e r i s e d by the shaping of the silicon nitride and it's mixtures to a body of different sizes and forms in prosthesis, e.g. a hip ball, a hip socket, a knee joint wearing surface, etc. 5. Method according to patent claim 4 c h a r a c t e r i s e d by that the biomaterial is surface carrying and is used as a wear surface in an orthopaedic implant. — 10 — 6. Method according to patent claim 5 c h a r a c t e r i s e d b y the usage of the biomaterial on one, both or all wear surfaces in an artifical joint, for example prosthesis or orthopaedic implant. 7. A ceramic material with a high stress tolerance and tissue tolerance for medical usage as a surface carrying, orthopaedic biomaterial for implantation, c h a r a c t e r i s e d by the composition of the material, a powder containing silicon nitride as a base, consisting of 70-100 volume % of silicon nitride with an addition of 0-30 volume % of one or a combination of the following materials: zirconium dioxide, yttrium dioxide, titanium dioxide, calcium oxide, silicon carbide and/or 0-20 volume % of short fibres of aluminium oxide, silicon carbide and mullite, or independent mixtures of these, whereupon the material mixture is formed to a body which is subjected to high pressure greater than 50 MPa and temperatures of between 500-3000 °C by isostatic pressing and sintering (HIP) for a desired porosity of the body and a polished surface of the body. 8, Ceramic material according to patent claim 4 i s c h a r a c t e r i s e d b y the densi fication of the body under a pressure of between 50 MPa and 350 MPa. 9. Ceramic material according to patent claim 5 i s c h a r a c t e r i s e d b y the sintering of the body under a temperature of between 500-3000 °C. 10. Ceramic material according to patent claim 9 c h a r a c t e r i s e d b y being surface carrying and the usage of it as a wear surface of an orthopaedic implant, whereby the body is modelled, for example as a hip joint head, a hip socket or knee joint wearing surfaces in a prosthesis. AMENDED CLAIMS [received by the International Bureau on 18 August 1999 (18.08.99); original claims 1-10 replaced by amended claims 1-9 (2 pages)] l.A method for the manu acturing of a ceramic material with a high stress tolerance and tissue tolerance for medical usage as a surface carrying, orthopaedic biomaterial for wear surfaces in artificial joints, for example the hip or the knee, wh i c h i s c h a r a c t e r i s e d by 1.1. The preparation/mixture of a powder composition containing silicon nitride as a base consisting of 70- 100 volume % silicon nitride with addition to 0-30 volume % of one or the combination of the following materials: zirconium dioxide, yttrium dioxide, titanium dioxide, calcium oxide, silicon carbide and/or 0-20 volume % of short fibres of aluminium oxide, silicon carbide and mullite, or independent mixtures of those, 1.2. To create a body of the mixture, 1.3. To subject the body to high pressures exceeding 50 MPa and temperatures between 500-3000 °C by isostatic pressing and sintering (HIP) in order to acquire desirable porosity of the body, whereupon 1.4. The body is cooled and the surfaces are polished and decontaminated clinically before medical usage. 2.Method according to patent claim 1 ch ar a c t e r i s e d by the densification of the body under a pressure between 50 MPa and 350 MPa. 3.Method according to the patient claim 1 ch ar a c t e r i s e d by the sintering of a mixture of silicon nitride and/or a described added material under a temperature of between 500-3000 °C. 4.Method according to patent claim 1 ch ara c t e r i s e d by the shaping of the silicon nitride and it's mixtures to a body of different sizes and forms in prosthesis, e.g. a hip ball, a hip socket, a knee joint wearing surface, etc. -12- 5. Method according to patent claim 4 c h a r a c t e r i s e d by the usage of the biomaterial on one, both or all wearing surfaces in an artificial joint, a so called prosthetic joint or an orthopaedic implant, 6. A ceramic material with a high stress tolerance and tissue tolerance for medical usage as a surface carrying, orthopaedic biomaterial for wearing surfaces in artificial joints, e.g. the hip or the knee, c h a r a c t e r i s e d by the composition of the material, a powder containing silicon nitride as a base, consisting of 70-100 volume % of silicon nitride with an addition of 0-30 volume % of one or a combination of the following materials: zirconium dioxide, yttrium dioxide, titanium dioxide, calcium oxide, silicon carbide and/or 0-20 volume % of short fibres of aluminium oxide, silicon carbide and mullite, or independent mixtures of these, whereupon the material mixture is formed to a body which is subjected to high pressure greater than 50 MPa and temperatures of between 500-3000 °C by isostatic pressing and sintering (HIP) for a desired porosity of the body and a polished surface of the body. 7. Ceramic material according to patent claim 6 i s c h a r a c t e r i s e d by the densification of the body under a pressure of between 50 MPa and 350 MPa. 8. Ceramic material according to patent claim 7 i s c h a r a c t e r i s e d by the sintering of the body under a temperature of between 500-3000 °C. 9. Ceramic material according to patent claim 8 i s c h a r a c t e r i s e d by the modelling, for example as a hip joint head, a hip socket or knee joint wearing surfaces in a prosthesis.

Inventors: JIANGUO LI, LINDGREN URBAN, OLSSON KENT, JIANGUO, LI, LINDGREN, URBAN, OLSSON,KENT
Applicants: BIOMAT SYSTEM HB, JIANGUO LI, LINDGREN URBAN, OLSSON KENT, JIANGUO, LI, LINDGREN, URBAN, OLSSON,KENT
Application year: 1999
Main classification: C04B35
IPC codes: A61F2, A61L27, C04B35