WO1989005161A1

WO1989005161A1 - Sliding layer for joint endoprostheses and process for producing it

Info

Publication number: WO1989005161A1
Application number: PCT/EP1987/000768
Authority: WO
Inventors: Heiko Gruner
Original assignee: Plasmainvent Ag
Priority date: 1987-12-09
Filing date: 1987-12-09
Publication date: 1989-06-15
Also published as: EP0348389A1

Abstract

To ensure good adhesion, perfect biocompatibility and corrosion resistance of a sliding layer (31, 32) applied to the sliding surface (2) of a joint endoprosthesis (1), a biocompatible Ti layer (31) is sprayed on to the sliding region of the metal endoprosthesis (1), followed immediately by a bio-inert TiO2 layer (32) with a non-porous, particularly compact, pronounced lamellar structure. In special cases, the adhesive Ti layer (31) can be dispensed with. The sliding layer (31, 32) is produced by a thermal spraying process, preferably by vacuum plasma discharge.

Description

SLIDING LAYER FOR JOINT OPROSTHESES AND METHOD FOR THE PRODUCTION THEREOF

The invention relates to a sliding layer made of TiO ₂ applied directly to the sliding surface of a joint endoprosthesis or via an intermediate layer, and to a method for the production thereof.

Due to the required physical tolerance, only certain materials are approved for endoprostheses. A distinction is made between biotolerant, bioinert and bioactive substances. The first group includes e.g. Bone cement with which an endoprosthesis is anchored in the bone tissue immediately during the operation. The metallic endoprostheses themselves can also be assigned to the first group. Body compatibility in connection with the required mechanical stability limit the metals suitable for endoprostheses to essentially stainless steel, CoCr alloys, titanium and titanium alloys.

With the help of coatings, attempts are made to impart bio-tolerant or even bioactive properties to the surface of biotolerant endoprostheses. The aim of such coatings is to use a special surface structure to have a favorable effect on the growth of the bone. From DE-A-2 313 678, for example, a coating of titanium, TiO ₂ and Al ₂ O _{3 is} known, with a porous structure produced by flame spraying. WO 86/06617 also describes a very rough and open-pore layer made of titanium and bioactive materials, whereby as outermost surface is also expressly proposed as TiO ₂ .

With joint endoprostheses, e.g. artificial knee or hip joints, in addition to mechanical stability and special surface design, good sliding properties in the joint area are required, while maintaining body tolerance. Unfortunately, the metals and suitable for endoprostheses

Metal alloys not satisfactory sliding properties. Especially the titanium and titanium alloys, which are preferred for reasons of weight and because of their good mechanical properties, are not sufficiently resistant to abrasion under the specific joint load. The corrosive conditions in the human body intensify this effect, whereby metallic abrasion and corrosion products arise in the artificial joint capsule. The toughness of the titanium alloys, their tendency to weld with the machining tool and their poor thermal conductivity also make it more difficult to produce suitable smooth sliding surfaces.

The combination of ceramic and plastic has proven to be an almost ideal joint design. The state of the art is, for example, to provide a hip joint prosthesis made of Ti alloy with a joint head made of Al ₂ O ₃ ceramic and to combine this with a joint socket made of polyethylene as a joint partner, which in turn is inserted into a metallic joint socket. Unfortunately, knee joints, for example, cannot be realized in this embodiment. Instead of the ceramic surface, the metallic femur slides in one in an artificial knee joint plastic tibia attached to the tibia. For this reason, the much heavier CoCr alloy is preferred for the production of the metallic femur, which has relatively better, but poorer sliding properties compared to ceramic. But artificial shoulder or hip joints also require metallic joint balls in certain cases.

For this reason, attempts were made early on to improve the lubricity of the metallic supports by means of coatings. For example, known to coat wear-stressed components made of titanium materials with very hard titanium nitride or titanium carbide, which have excellent sliding properties. Unfortunately, such layers deposited from the chemical or physical gas phase with layer thicknesses in the μm range are not suitable for smoothing mechanically insufficiently prepared sliding surfaces. Despite their stability, they are too thin to withstand the specific load in the joint area for a long time.

Due to the good sliding properties of ceramics, attempts were made to coat the joint surface directly with an Al ₂ O ₃ spray layer, using flame and plasma spraying to produce the layer. Such Al ₂ O ₃ layers can be mechanically reworked. Sufficiently smooth surfaces can even be produced in a grinding and subsequent polishing process. Unfortunately, in contrast to the full ceramic ball, an Al ₂ O ₃ spray layer always consists of a mixture of α and Phase phases. Since the ɣ phase dissolves in the body, the life of the injected is Al ₂ O ₃ - sliding surface limited. It is not possible to convert the ɣ phase into the α phase by means of an annealing process after spraying, since the mechanical properties of the metallic carriers change unfavorably at the required temperatures.

TiO ₂ has also already been produced on joint balls, firstly galvanically (Biomaterials 1981, pages 221 to 224) and secondly by anodic oxidation. Both types of layers are not dense enough or tend to flake off.

The invention is therefore based on the object of providing a sliding layer of the type described at the outset, which adheres very well to the metallic materials for endoprostheses, is absolutely tolerable to the body and corrosion-resistant, in particular does not dissolve in the body and has internal structural stability in order to achieve the necessary mechanical processing allow for the production of smooth sliding surfaces, which the

Ceramic should have sliding properties combined with its abrasion resistance.

This object is achieved according to the invention in that the sliding layer sprayed onto the sliding area of the metallic joint endoprosthesis is a bioinert TiO ₂ layer with a pore-free, particularly compact and pronounced lamellar structure, which is, however, only visible in the metallographic micrograph by etching.

A biotolerant Ti layer as an intermediate layer and is advantageous on the joint endoprosthesis Retainer and then immediately sprayed onto the bioinert TiO ₂ layer.

A method according to the invention for producing such sliding layers is characterized in that, after a method of thermal spraying which enables very high particle speeds, TiO ₂ powder particles are melted and used as

Liquid droplets with high kinetic energy are deposited on the surface of the joint endoprosthesis. They burst and form flat spray cakes, which build up the compact and pore-free spray layer in a lamella-like manner.

Before melting and separating the TiO ₂ powder particles, it is advisable to first melt the Ti powder particles and separate them as liquid droplets with high kinetic energy.

The sliding layer is advantageous by

Vacuum plasma spraying (VPS) manufactured, taking advantage of the known advantages of this

Coating technology, e.g. are described in DE-A-34 22 718. This coating technique is also preferred for reasons of hygiene.

This process allows the TiO ₂ ceramic to be injected so that it is structurally stable and at the same time so thick that on the one hand the required surface smoothness is achieved by mechanical finishing, on the other hand enough layer as a grinding reserve is present, for example to minimize joint roundness values.

Particularly advantageous and expedient developments of the sliding layer according to the invention and the method according to the invention result from claims 3 to 8 and 12 to 16.

In particular, the layer porosity is well below 1%, which is very important for the surface quality that can be achieved.

The surface of the joint endoprosthesis to be coated is expediently degreased in the area of the sliding surface by sandblasting with pure cC-Al-O-. roughened, freed of oxide skin immediately before coating with a transferred arc and at the same time slightly warmed and degassed.

It is recommended to first spray on a Ti layer as an adhesion promoter, whereby the layer thickness should be less than 50 μm. The Ti wettable powder grain size must be limited to 40μ m so that a relatively smooth boundary with the subsequent TiO ₂ layer is created.

It has proven to be particularly advantageous to largely process the sliding surface to a fit before coating, so that the sliding layer according to the invention only has to be polished.

The TiO ₂ sprayed layer has a layer thickness of at least 50 μm, is preferably 0.1 mm thick and only exceeds 0.2 mm in special cases. In a preferred embodiment, the TiO ₂ layer is produced using wettable powder with a grain size of 5 to 25 μm.

Up to six joint endoprostheses are advantageously moved in the spray jet with the aid of appropriately trained substrate holders in such a way that the sliding surfaces are coated simultaneously.

The coating is preferably carried out at a chamber pressure of 100 mbar for the Ti layer and the TiO ₂ layer as the actual sliding layer.

In a preferred embodiment, the Ti layer of the sliding layer according to the invention is sprayed on with a non-reducing plasma flame and the TiO ₂ layer is sprayed on with a reducing plasma flame.

In special cases, the Ti layer as an adhesive layer can be dispensed with, in particular if the TiO ₂ layer does not exceed 100 μm.

The installation of an intermediate layer composed of Ti and TiO ₂ , sprayed on immediately after the Ti adhesive layer and immediately before the pure TiO ₂ cover layer, is deliberately avoided.

The invention is explained below with reference to the drawing and an exemplary embodiment.

Fig. 1 shows a section parallel to the femur axis in a schematic representation of a femoral prosthesis for an artificial knee joint. Fig. 2 shows an enlarged view of the

Sliding surface sprayed on sliding layer in detail A from FIG. 1.

1 shows a joint endoprosthesis 1. In the example shown, a femoral prosthesis was deliberately chosen, the uncoated, original sliding surface 2 of which was roughened by sandblasting with α-Al ₂ O- ₃ particles to an average roughness of approximately 30 μm, with the aid of a transferred electric arc immediately before the natural coating

Surface oxidation layer was freed and dried, degassed and heated to at least 150 ° C. This preparation secures the adhesive force of a sliding layer 3 according to the invention even if, in a special embodiment, a TiO ₂ layer 32 without a Ti layer 31 is sprayed on as a lower, intermediate or adhesive layer. At the same time, this pretreatment ensures safe sterilization of the surface to be coated.

The surface 2 to be coated is cleaned with the sliding layer at a chamber pressure of 100 mbar using a non-reducing plasma flame, preferably with an Ar / He gas mixture in the absence of H ₂ . This is particularly important for endoprostheses made of Ti or Ti alloys in order to avoid hydrogen embrittlement of the surface, which can have a dangerous effect on the mechanical properties. Immediately after cleaning, a Ti layer 31 is first sprayed on tightly and firmly, the flame energy thus is set that all powder particles are melted. The layer thickness of this first layer of the sliding layer 3 is less than 50 ^ -m, its surface 33 has an average roughness due to the selected Ti powder grain size less than 20 μm. The chamber pressure corresponds to the pressure for surface cleaning. The plasma gas composition and the flame energy are adjusted so that the injected powder particles melt completely and form a thick layer composite.

The TiO ₂ layer is sprayed on as the actual sliding layer without interruption, with the chamber vacuum remaining constant, but with adaptation of the flame energy by regulating the arc current and the flame enthalpy by changing the Ar / He mixing ratio and adding H ₂ . Their structure is extra low in pores and compact. Due to the kinetic impact energy of the molten TiO ₂ particles, the desired lamellar layer structure 35 is formed, which, however, is not visible in the pure micrograph. Due to the particle speed in the ArHeH ₂ -

Vacuum plasma flame the residence time of the molten TiO ₂ droplets above the process temperature critical for a loss of O _{2 is} much shorter than in comparable processes of thermal spraying. Despite the reducing plasma flame, the TiO ₂ layer 32 produced according to the invention is harder and more stable and does not yet show the otherwise typical lamellar structure in the pure micrograph, which is caused by O ₂ losses. These properties enable the mechanical finishing of the sliding layer surface 34 and the production of the surface 36 with medium roughness well below 0.1μm.

On the basis of the explanation given for the specific properties of the TiO ₂ sliding layer, it makes sense to use other methods of thermal spray technology for their production, which likewise generate a high gas and therefore particle speed, for example coating using the detonation process or using flame spraying Acceleration of the gas flame. The reasons of layer hygiene, the advantages of surface cleaning and the possibility of being able to apply the Ti layer 31 as an adhesive layer without an O ₂ reaction make the VPS technology the preferred and most suitable manufacturing method for the sliding layer according to the invention.

The layer thickness of the TiO ₂ layer 32 is preferably 100 μm and exceeds 200 μm only in special cases. For layer thicknesses below 100 μm, the Ti layer 31 as an adhesive layer can be dispensed with. The prerequisite for this is the mechanical preparation of the joint surface practically to the final dimension. The sliding layer only has to be sprayed on so thick that only the TiO ₂ layer 32 is always processed during the production of the surface quality, ie the Ti layer 31 or the carrier material is not worked out. In almost all cases, 100 μm TiO ₂ thickness is sufficient for this.

TiO ₂ powder with a grain size of 5 to 25 μm is preferably used for the TiO ₂ layer 32. On the one hand, the sliding layer surface 34 already has an average roughness of less than 10 μm, which means that the mechanical Postprocessing reduced. On the other hand, the structural stability is improved by the finer spray cake 35, which favors the mechanical finishing.

Fine granularity of the spray powder in connection with the high flame temperature and particle speed extend the flight distance within which the injected particles have melted. Therefore, several joint endoprostheses are advantageously moved in the spray jet with special substrate holders so that their sliding surfaces are coated simultaneously and immediately one after the other.

The adhesive force of the sliding layer according to the invention made of TiO ₂ , its spray layer density and internal structural stability and the surface quality thereby achieved in mechanical post-processing recommend its usability as a sliding surface in technical applications, for example for protecting a highly stressed propeller shaft.

The reference symbols in the claims are not intended to limit the scope of protection.

Claims

PATENT CLAIMS

1. Sliding layer (3) made of TiO ₂ applied directly or via an intermediate layer to the sliding surface (2) of a joint endoprosthesis (1), characterized in that the sliding layer of the metal is applied

Joint endoprosthesis (1) sprayed-on sliding layer (3) is a bioinert TiO ₂ layer (32) with a pore-free, particularly compact and pronounced lamellar structure.

2. Sliding layer according to claim 1, characterized in that a biotolerant Ti layer (31) is sprayed onto the joint endoprosthesis as an intermediate layer and adhesion promoter and immediately afterwards the bioinert TiO ₂ layer (32).

3. Sliding layer according to claim 1 or 2, characterized in that the porosity of the TiO ₂ layer (32) is significantly less than 1%.

4. Sliding layer according to one of claims 1 to 3, characterized in that the TiO ₂ layer (32) has a layer thickness of at least 50 μm and preferably 0.1 mm.

Sliding layer according to claim 4, characterized in that the TiO ₂ layer (32) has a layer thickness of over 0.2 mm.

6. Sliding layer according to one of the preceding claims, characterized in that the TiO ₂ layer (32) is produced using spray powder with a grain size of 5 to 25 μ m.

1. Sliding layer according to one of claims 2 to 6, characterized in that the Ti layer (31) has a layer thickness of less than 50 μm.

8. Sliding layer according to one of claims 2 to 7, characterized in that the Ti layer (31) is produced using spray powder with a grain size of at most 40 μm.

9. A method for producing a sliding layer (3) made of TiO ₂ applied to the sliding surface (2) of a joint endoprosthesis (1), characterized in that TiO ₂ powder particles are melted using a thermal spraying process and deposited as liquid droplets with high kinetic energy on the Surface of the joint endoprosthesis (1) are deposited.

10. The method according to claim 9, characterized in that before the TiO ₂ powder particles are melted and deposited, Ti powder particles are first melted and deposited as liquid droplets with high kinetic energy.

11. The method according to claim 9 or 10, characterized in that the sliding layer (3) is produced by vacuum plasma spraying (VPS).

12. The method according to claim 11, characterized in that the surface of the joint endoprosthesis (1) to be coated is degreased in the area of the sliding surface (2), roughened by sandblasting with pure α-Al ₂ O ₃ , with a transferred arc immediately before the coating of oxide membranes is freed and at the same time heated and degassed.

13. The method according to one of claims 9 to 12, characterized in that the sliding surface (2) is largely machined to fit before coating.

14. The method according to one of claims 9 to 13, characterized in that up to six joint endoprostheses (1) are moved in the spray jet with the aid of appropriately designed substrate holders in such a way that the sliding surfaces (2) are coated simultaneously.

15. The method according to one of claims 9 to 14, characterized in that the sliding layer (3) is sprayed on at a chamber pressure of 100 mbar.

16. The method according to one of claims 10 to 15, characterized in that a Ti layer (31) as an adhesive layer of the sliding layer (3) with a non-reducing plasma flame and a TiO ₂ layer (32) as a cover layer of the sliding layer (3 ) is sprayed on with a reducing plasma flame.