DE2621124A1 - Prosthesis for joint replacement - comprising carbon reinforced with carbon fibres and coated with alumina in friction zones - Google Patents

Abstract

Prosthesis for replacing a joint in a living body contains a carrier, having shape similar to that of joint. Carrier consists of C fibre-reinforced C, coated with an Al oxide layer in friction surface zone. In multi-axially stresses prosthesis, e.g. for hip joints, C fibres are pref. arranged as parallel, reciprocally inclined strips, to form structures similar to natural bone. Prostheses are pref. provided with a threaded shank, to facilitate anchoring in bone tissue. Implants have long service life. They can be anchored without bone cement and have improved impact strength. Elasticity modulus of C fibre-reinforced C prosthesis corresponds to that of bone modulus, averaging 10-20kN/mm2. Al oxide layer has low frictional coefft. and abrasion.

Description

Joint prosthesis

The invention relates to a prosthesis for replacing a joint with a support whose shape is similar to the joint to be replaced.

The replacement of contaminated and unloaded by degenerative inflammatory, traumatic or other causes significantly impaired in their functionality Joints caused by artificial joint parts is known. For the satisfactory operation of the implanted joint replacement, it is important that the implant is facing each other is inert to human body tissue, i.e. good tissue compatibility having. Other prerequisites are resistance to the diverse mechanical forces acting on the joint and the uniform dissipation of the Forces on the body's own bone tissue. Finest For example, particles not only roughen the sliding surfaces and thus increase them the coefficient of friction, but trigger harmful connective tissue reactions. Instabilities, i.e. loosening of the implant is caused by chemically and mechanically induced Tissue changes and resorptions, whereby according to Wolff's law of transformation the material composition and the structure of the bone correspond to the respective stress stimuli changed accordingly.

Metals such as noble are particularly suitable as materials for joint prostheses r steels and iron-free alloys, synthetic resins such as polytetrafluoroethylene and polyethylene and combinations of these substances have been proposed. For stressed joints like the hip joint, is because of the favorable abrasion and sliding properties and the adequate mechanical strength the combination of metal / plastic is widely used. The prosthetic Supply by cementing the metal parts, e.g. the prosthesis stem, could however, cannot be solved satisfactorily because the metallic prosthetic stems Due to their great rigidity, loosening and degradation phenomena in the bones can induce.

Other possible causes of loosening are those related to the attachment of the stem used bone cement itself and foreign body reactions that lead to a Lead connective tissue encapsulation of the plastic material. All factors cause a limitation of the functional time of the joint replacement, so that, for example, for Hip joint prostheses the half-life is only about 5 years. As in increasing To the extent that younger patients are also treated with joint prostheses, the proportion is growing of patients constantly undergoing surgery several times have to. The complication rate and the chances of success increase with each new intervention are significantly lower.

According to German Offenlegungsschriften 1 902 700 and 2 138 146 is it was finally known to be artificial limbs and similar parts for the human body made of a carbon fiber reinforced synthetic resin and strength and power flow in the prosthesis through a special orientation of the carbon fibers to adapt to the respective geometric conditions. According to the German Offenlegungsschrift 2 142 820 is a prosthetic limb for the replacement of a joint with a carbon fiber containing pyrocarbon-coated carrier become known. The suggestions however, are due to the different elastic behavior of bone and prosthesis not suitable to solve the problem of loosening prosthetic joints.

It is an object of the invention to loosen joint prostheses by resorption of the bone or connective tissue changes and to prevent the Increase survival rate of artificial joints.

It is another object of the invention to provide joint replacement without the Use a bone cement to anchor in undamaged bone.

According to the invention, the object is achieved with a prosthesis of the type mentioned at the beginning Kind of dissolved, the carrier of which is made of a carbon reinforced with carbon fibers consists, which is provided with a layer of aluminum oxide in the area of the sliding surface is.

The invention is based on the knowledge that the main disadvantage of the physiologically inert and tissue-compatible carbon, namely the brittleness against shock loads by reinforcing the carbon with carbon fibers completely repealed and a tissue-compatible one suitable for implants and at the same time mechanically strong material is obtained. The main advantage over known materials used for joint prostheses, their compatibility and mechanical stability is also more or less satisfied, now consists in that the modulus of elasticity that consists of carbon fiber reinforced carbon Prosthesis corresponds to the module of bone, which on average is about 10-20 kN < amounts to. The elasticity equality causes uniform excessive loading of the bone reducing force transfer from the bone to the prosthesis and vice versa and thus eliminates disruptive stimuli that could cause the prosthesis to grow together permanently prevent in bone tissue. The prosthesis wearer eventually contains in the area the sliding surface a layer of aluminum oxide firmly bonded to the carrier, which has small coefficients of friction and wear rates. The thickness of the wear-resistant The layer is dimensioned in such a way that the elastic behavior of the prosthesis does not is affected.

For multi-axis loaded prostheses it is advantageous to use the reinforcing To arrange carbon fibers in several mutually inclined piles, with the Longitudinal extension of the fibers within a group with a main load direction coincides. Such structures, which are similar to natural bones, make it possible a targeted introduction and discharge of forces. For anchoring in the bone tissue has the prosthesis preferably has a stem-shaped part, which is in a corresponding Engages recess of the undamaged bone. The stem-shaped part and the Are recess appropriately provided with a thread that a exact fixation of the prosthesis facilitated.

In the context of the invention, carbon is exclusively or to understand moldings consisting almost exclusively of the element carbon, thus also moldings made of graphite. Carbon and graphite are physiologically inert Materials, which inter alia due to their porous structure with the bone tissue can enter into a permanent connection. Carbon bodies are particularly favorable for this purpose with a mean pore diameter> approx. 0.4 mm. The use of a special Bone cement to fix the prosthesis is therefore not required. For the production Rather, it is a permanent connection with great mechanical strength Sufficient, parts of the prosthesis that are, for example, stem-shaped or peg-shaped, to let into appropriate recesses of the bone and the joint for the duration to calm the overgrowth. Screw connections behave particularly well, wherein the threads are cut into the pin and into the recess. By Large areas of thread can increase the area of adhesion and thus the pressure be reduced accordingly in the critical interface. Tensions and relative movements between implant and bone are because of the mismatched or almost mismatched elastic behavior of implant and bone not or only in one for the Resorption of bone substance and thus the loosening of the prosthesis harmless Extent possible.

For the production of carbon fibers, it is known, threads from a organic material, such as rayon or polyacrylonitrile, usually after a previous one thermal or catalytic stabilization by heating to about 1000 0C carbonize. It is through the choice of raw materials and manufacturing parameters possible elasticity and strength of carbon fibers in a wide range to vary. Fibers suitable for reinforcement purposes are in the form of continuous filaments, Ropes, staple fibers or woven fabrics available.

A method of making carbon fiber reinforced carbon consists in impregnating fabric layers with pyrocarbon, including one Temperature above about 1000 0C hydrocarbon-containing gases on the fiber surfaces be decomposed.

A second suitable manufacturing process is impregnation of laid-out fiber layers, of fabrics or of staple fibers with a synthetic resin, which is first hardened and then carbonized by further heating. The at Excess porosity formed by carbonization can be increased by one or more times Impregnating the body with a synthetic resin or even a pitch can be reduced. Each impregnation step is followed by a carbonization step, to achieved the desired mechanical properties and the preferred pore structure are. The breaking strength of the carbon body is about 50 - 100 N / mm2, the E-module preferably 10 - 40 kN / mm2.

For certain prostheses, such as a hip joint prosthesis, it is due to the different directions of loading beneficial to the fibers parallel to the respective direction of force application. Fabrics or fibers are for this purpose arranged inclined to one another, one above the other and next to one another.

Carbon, especially in the form of graphite, has a relatively high small coefficient of friction even under dry running conditions.

However, the wear resistance is not always sufficient for an implant requirements to be made.

The prostheses according to the invention are therefore in the area of the sliding surface with a layer of aluminum oxide, in particular of densely sintered aluminum oxide provided, which has a similarly favorable frictional behavior and practically none Subject to wear and tear. The oxide layer is made by known methods, e.g. Plasma or flame spray, applied to the sliding surface or after another Process are thin slices or plates of aluminum oxide in the recesses The prosthesis is inserted and glued to it.

To attach the joint prosthesis, it is useful to use the bone thread itself or the medullary canal. For hip joint prostheses is for example after resection of the femoral head and the femoral neck in the Thigh shaft cut a thread into the compact bone from the medullary canal and the stem-like threaded portion of the prosthesis into that thread screwed in. The pan is anchored, for example, by itself in recesses cones extending to the iliac, ischium and pubic bone.

The invention is illustrated below with the aid of examples.

Example 1 Carbon yarn was used to produce a hip joint prosthesis with the properties -tear strength - 1.6 kN / mm2 modulus of elasticity - 0.16 MN / mm2 drawn continuously through a 10% solution of phenol-formaldehyde resin in ethanol and the solvent by storing the yarn in the temperature range between 20 and 30 0C removed for the most part.

The thread was cut into sections of equal length, which were then cut into a semi-cylindrical, stem-like tapering mold are inserted, the Threads near the wall of the mold followed their contours and with increasing distance from the wall were increasingly oriented parallel to the median plane of the mold.

The assembly then became a body under a pressure of 0.5 bar compacted with a fiber content of approx. 50%.

The composite body 30 was used to cure the phenol-formaldehyde resin min to 100 ° C. and then a further 30 min to 150 ° C. and for carbonizing the resin in a nitrogen atmosphere with a temperature gradient of approx. 20 ° C / h 900 0C heated. The composite body was then coated with a coal tar pitch, whose softening point was 75 0C, impregnated and used in an autoclave for this purpose, its printing first reduced to about 0.3 bar and after addition the tar pitch was increased to about 2 bar. The total residence time was approx. 30 min, the temperature 150 ° C. The impregnated body was then as described above heated to 900 0c.

The prosthesis with a detached ball head and a stem-shaped extension with an incised thread and on the surface of the spherical head was with a direct plasma torch an approximately 30 µm thick layer of aluminum oxide is applied to the surface with Diamond paste has been carefully polished.

The properties of the joint prosthesis were -2 flexural strength - approx. 80 N / mm modulus of elasticity - approx. 20 kN / mm2 bulk density - approx. 0.8 g / cm3 The fracture and especially the elastic behavior corresponds to the average behavior human bone.

Example 2 For a knee joint prosthesis, they were stacked one on top of the other Sections of a carbon fabric in satin weave in a reaction chamber Heated 1150 0C and propane under a partial pressure of about 0.4 bar through the chamber directed. After a reaction time of 24 hours were the spaces between the individual threads were filled with pyrocarbon and the compact body became the prosthesis is carved out with a convex sliding surface. For better anchoring in the thigh bone, the back was provided with several grooves and on the The convex sliding surface was a 2 mm thick plate of densely sintered alumina glued, the convex surface of which was very finely polished.

The properties of the knee joint prosthesis were - flexural strength - approx. 100 Nimm2 modulus of elasticity - approx. 22 kN / mm2 bulk density - 1.3 g / cm3 5 claims

Claims (3)

Claims #. Prosthesis for the replacement of a joint in one living body, with a support whose shape resembles the joint to be replaced is indicated by the fact that the one with carbon fibers reinforced carbon existing carrier in the area of the sliding surface with a Layer of aluminum oxide is provided. 2. Prosthesis according to claim 1, characterized in that it is e k e n n -z e i c h n e t that the modulus of elasticity of the support corresponds to the average modulus of bone is equivalent to. 3. Prosthesis according to claim 1 and 2, characterized g e -k e n n z e i c h n e t that the mean pore diameter of the carbon support is at least 0.4 mm. 4o prosthesis according to claim 1 and 2, characterized in that it is not n e t that several mutually inclined groups of parallel carbon fibers are arranged in the carrier. 50 Prosthesis according to Claims 1 to 3, characterized in that it is not valid n e t that the prosthesis has a stem-shaped threaded portion.