WO2009000774A1

WO2009000774A1 - Method for producing surfaces for metal articles

Info

Publication number: WO2009000774A1
Application number: PCT/EP2008/057877
Authority: WO
Inventors: Paolo Celeghini; Francesco Giovagnoni; Matteo Orlandi; Enrico Sandrini
Original assignee: Societa' Azionaria Materiale Ospedaliero - S.A.M.O. S.P.A.
Priority date: 2007-06-22
Filing date: 2008-06-20
Publication date: 2008-12-31
Also published as: ITBO20070436A1

Abstract

A method for producing, on the surface of manufactured articles made of metal, such as for example dental and orthopedic prostheses, a macrostructure and a microstructure, wherein the macrostructure is capable of facilitating the mechanical coupling of the implant and the regrowth of mature trabecular bone structures and the microstructure is capable of facilitating the engagement, at the chemical and physical level, of the surface of the implant by stimulating the osteoinductive/osteoconductive processes of bone tissue. The method comprises the steps of: applying a mask to a metal article; selectively removing part of the mask by means of a computerized system and a laser; applying an electrochemical cycle to remove the exposed material of the article. The electrochemical cycle provides for the application of an electrochemical scanning cycle, which allows to identify the correct working potential and then the application of an electrochemical cycle at constant potential in the work area identified by means of the preceding cycle; removing the remaining mask; generating chemically or electrochemically a micro-rough and/or microporous surface.

Description

METHOD FOR PRODUCING SURFACES OF METAL ARTICLES

Technical Field

The invention relates to a method for providing a macro- and microstructured surface on a metal article, such as for example a dental or orthopedic prosthesis. The surface thus provided is capable of facilitating the regrowth of mature bone trabeculae and of stimulating osteoinductive processes, avoiding the formation of fibrous tissue at the bone-implant interface. This method allows to obtain surfaces which facilitate short- and long-term mechanical stability while ensuring bone-implant adhesion at the mechanical and biological level by means of a high chemical and physical compatibility. Background Art

It is well-known that the use of surfaces with a very specific shaping can facilitate, at the mechanical level and at the purely biological level, the integration of a given prosthetic article (for example an acetabular cup for hip prostheses, a femoral stem for hip prostheses, a tibial plate and a femoral component in the application of a prosthesis to the knee joint, transosteal surface of a fixture for dental implants) with the surrounding bone tissue. This texturizing can relate to different levels of scale and can affect different integration mechanisms on different levels of scale. It is well-known that mechanical stability must be sought by means of structures whose characteristic dimension is compatible with the characteristic dimension of bone trabeculae (one is referring, therefore, to dimensions in the range of 400-1000 microns), while integration, understood as addition of mature bone tissue which adheres closely to the surface of the article without the interposition of fibrous tissue, must be found by stimulating the cellular component, whose response can be modulated by means of a surface pattern which has a characteristic dimension which oscillates between 0.5 and 20 microns. A further level of integration can be sought by stimulating chemical and physical mechanisms, i.e., by providing the surface with particular charges (such as for example those linked to the presence of OH, COOH, H₃O groups, et cetera) which can interact with the protein component on a nanometer scale.

Such surfaces for facilitating integration, understood in its most generic meaning, have now been sought for more than half a century, with more or less adequate methods. This stability as a primary need was expressed in US 5,298,1 15 of 29 March 1994 in the name of Ian Leonard, in US 5,456,723 of 10 October 1995 in the name of S. G. Steinemann, in US 5,603,338 of 18 February 1997 in the name of K. D. Beaty, in US 5,853,561 of 29 December 1998 in the name of B. A. Banks, in US 5,965,006 of 12 October 1999, in the name of R. Baege.

Specific methods are well-known for producing these surface shapings and have been used extensively through the years. One of these methods consists in coating the surface with titanium microspheres which are bonded to the surface by means of a superficial melting process (sintering) (US 4,834,756; US 4,644,942). This method allows an assured improvement in terms of integration with respect to simple metal, but there are doubts as to the actual mechanical stability of this coating. Further doubt arises from the critical size of the voids left between the microspheres, which are too small to allow accurate trabecular regrowth. Other questions arise from the negative effect that this solution can have on the mechanical strength of the article itself, without neglecting the difficulties that can arise in operations for washing and for removing machining oils.

Other widely used methods provide for the use of pure titanium or titanium and hydroxyapatite coatings by means of plasma spray technology in vacuum and in air. Finally, a very common practice is to coat such articles with metallic microfilaments, generally made of titanium, which have the same advantages and disadvantages as coatings performed with titanium microspheres. The use instead of so-called photochemical etching techniques has been disclosed in specific patents, such as: US 3,359,192, US 5,606,589, US 5,814,235, US 5,258,098, US 5,507,815, US 6,193,762. All these methods do not provide for repeatability of the result, since none of these methods offers control of the process in all of its steps. Further, these methods provide only and exclusively the generation of surface patterns, which ensure a mechanical coupling, without ever considering the actual nature of osteointegration, a process which is substantially of a chemical-physical type, with the addition of bone directly in contact with the metal substrate. All these methods completely neglect the fundamental role of a microscopic scale (0.5-20 microns) and do not ensure repeatability of the macrostructuring processes (500-1000 microns). A further limit of these processes is the use of technologies which are purely of the chemical type to remove the excess material, ignoring all the risks linked to metal corrosion processes and due to the generation of gas on the surface. This gas can be oxygen, but also hydrogen, whose role as an embrittlement agent of the structure of metal is well-known.

Accordingly, there is the need to provide a method which instead allows absolute control over the texturization processes, control of removal processes and provision of shaping at the micro level, which facilitates osteointegration processes and not only mechanical locking processes. Disclosure of the Invention

The aim of the present invention is to provide a method for providing a macro- and microstructured shaping on the surface of a metal article, such as a dental or orthopedic prosthesis, which ensures high mechanical stability with an adjacent body and facilitates the chemical-physical bonding between the surface of the metal being considered and the adjacent tissue.

Within this aim, an object of the present invention is to provide a method for providing a surface shaping which ensures mechanical stability by means of areas in which trabecular bone structures can find shelter and regrow, forming mature bone tissue. Another object of the invention is to provide a method for preparing said surface on two scales of levels: the first one with dimensions of 500-

1000 microns, which facilitates the mechanical bond, and the second one with dimensions of 0.5-20 microns, which facilitates cell activity. Another object is to provide a method which allows to control all the parameters of the process in its different steps so as to ensure consistency of results and repeatability of the process.

Another object is to provide a method which allows to produce controllable and repeatable shapings both during the provision of the pattern of the shaping and during the step for removal of the excess material.

Another object is to provide a method which ensures respect for the mechanical strength of the article, i.e., ensures conditions of electrochemical removal of the excess material in a range of potential which does not entail the risk of hydrogen-related embrittlement. Another object is to provide a substrate which is suitable for bone regrowth.

Another object is to provide a substrate which is absolutely decontaminated from any kind of pollutant originating from mechanical processes. Another object is to provide a method for ensuring controlled removal of material in the three directions of space.

Another object is to provide a method which allows to provide said surface shaping without undercuts, which can constitute a trigger point of cracks. Another object is to provide a method for avoiding the formation of undercuts, which act abrasively on tissues.

Another object is to provide a method for generating a surface shaping with hollows for regrowth of bone trabeculae and a surface shaping for optimum cell adhesion, without sharp edges, so as to preventing these edges from causing abrasion and inflammation of bone tissue. Another object is to provide a method for preparing implant surfaces having the characteristics cited above, made of implantable materials, such as for example titanium and titanium alloys, zirconium and zirconium alloys, tantalum and tantalum alloys, stainless steel and chromium-cobalt base superalloys.

Another object is to provide a substrate which is suitable for the subsequent deposition of osteoinductive coatings, such as calcium phosphates deposited by chemical or electrochemical processes.

Another object is to provide a method for improving the adhesion of bone tissue and the integration of the implant surface by reducing micromovement at the interface, which is acknowledged as the main cause of the formation of fibrous tissue.

This aim and these and other objects which will become better apparent hereinafter, are achieved by a method for providing a macro- and microstructured surface on a manufactured article made of metal, comprising the steps of: a) depositing a mask of protective film, constituted by a masking agent which is resistant to corrosion in acid and/or basic solutions, on the surface of the article; b) stabilizing the mask of protective film; c) exposing part of the masked surface of the article by selective and controlled removal of the mask according to a previously generated pattern; d) electrochemically etching the material that constitutes the article on the surface exposed in step c) to generate a macrostructure, in conditions of protection against embrittling chemical agents; e) removing the remaining part of the mask by means of a mechanical and chemical treatment; f) applying a microstructure by means of a surface patterning method on a micrometer scale. Brief description of the Drawings

Further characteristics and advantages of the invention will become better apparent from the following detailed description of a non-exclusive embodiment thereof, illustrated by way on non-limiting example in the accompanying drawings, wherein:

Figure Ia is a view of the pattern generated for removal of the mask on an implantable article;

Figure Ib is a view of a detail of Figure Ia;

Figure 2 plots the amperage as a function of voltage during the application of a potentiodynamic cycle;

Figure 3 plots time as a function of amperage;

Figure 4 plots time as a function of voltage;

Figures 5a and 5b are views of an acetabular cup;

Figure 6 is a view of a spongiosa screw; Figures 7a and 7b show a femoral prosthetic stem, viewed from two angles;

Figures 8a and 8b are views of the proximal components of a prosthetic stem;

Figures 9a and 9b are views of the distal components of a prosthetic stem;

Figure 10 is a perspective view of a femoral shield;

Figure 11 is a perspective view of a tibial plate;

Figure 12 is a perspective view of a dental implant. Ways of carrying out the Invention The invention consists of a method for providing a macro- and microstructured surface on a manufactured article made of metal, which comprises the steps of: a) depositing a mask of protective film, constituted by a masking agent which is resistant to corrosion in acid and/or basic solutions, on the surface of the article; b) stabilizing the mask of protective film; c) exposing part of the masked surface of the article by selective and controlled removal of the mask according to a previously generated pattern; d) electrochemically etching the material that constitutes the article on the surface exposed in step c) to generate a macrostructure, in conditions of protection against embrittling chemical agents; e) removing the remaining part of the mask by means of a mechanical and chemical treatment; f) applying a microstructure by means of a surface patterning method on a micrometer scale.

The method as a whole leads to the provision of two distinct shapings on the surface of the article. The first shaping, with macroscopic dimensions, provides the structure with high mechanical stability with an adjacent body; further, the macrostructures offer shelter to growing bone trabecular structures, allowing them to regrow, forming mature bone tissue. The second shaping, characterized by microscopic dimensions, instead facilitates the cellular activity of the bone tissues which will integrate with the article, modulating the response of the cellular component and mediating bone addition phenomena directly in contact with the article.

Typically, the article can be made of implantable materials. Preferably, the article is made of a metal which is selected from the group that comprises titanium, zirconium, niobium, tantalum, aluminum, and their alloys, alloys of stainless steel and cobalt/chromium and nickel/titanium base alloys. Such article can be selected from the group that comprises a dental or orthopedic prosthesis and a generic surgical instrument. The prostheses can be for example a component for hip prostheses, such as acetabular cups 1, spongiosa screws 2, femoral prosthetic stems 3 and proximal 3a, 3b or distal components 3c, 3d of prosthetic stems 3, an implantable component of knee prostheses, such as femoral shields 4 and tibial plates 5, a spinal prosthesis, a component for traumatology and a dental implant 6 in the area in contact with bone tissue.

During a first step of the method, a mask of protective film constituted by a masking agent is deposited on the article. The mask can be deposited totally or partially on the surface of the metal article. The masking agent must be resistant to corrosion in acid and/or basic solutions. Further, the masking agent can be in a form selected from the group that comprises a liquid, a gel and a sol-gel (paste).

The masking agent can be applied by means of a method selected from the group that comprises spraying of the masking agent onto the article, immersion of the article in the masking agent and sprinkling of the masking agent onto the surface of the article. The homogeneity of the resulting film depends on the rate of application, on the geometry of the article and on the viscosity of the liquid agent. The choice of the adequate masking agent depends on the applications; in particular, it depends on the type of chemical solution adopted to remove the metal, on the definition of the pattern to be provided and on the metallurgical nature of the article that is intended to treat. Generally, the choice falls on an agent selected from the group that comprises epoxies, acrylates and polyesters. Further, in order to increase the mechanical strength of the mask, it is possible to use filler-type agents of various kinds, both inorganic and organic.

The step that follows the masking of the article provides for stabilizing the deposited film mask. This operation can occur by treatment with a form of energy such as heat, white light, light at a particular wavelength (for example in the infrared or ultraviolet range) and gamma rays. The form of energy that can be used varies as a function of the properties of the masking used, of the type of masking agent chosen and of the metallurgical nature of the article being processed. In one embodiment of the invention, stabilization can occur by drying, for example by treatment in an aspirated oven at temperatures between 100 and 300 ⁰C. At the end of this process, the article is coated by a coherent layer which is mechanically stable and resistant to chemical agents.

Once stabilized, the mask is removed in the regions where one wishes the subsequent electrochemical etching to remove the material of the article, so as to create in low relief the macrostructure that facilitates the regrowth of trabecular bone tissue. The pattern along which the mask is removed is generally drawn beforehand, typically by means of a CAD system. The removal of the mask, with the consequent exposure of part of the surface of the article, then occurs typically by using a laser. For example, the pattern of the shaping of the macroscopic structure is drawn initially by means of a CAD system and subsequently imported on an Nd-YAG laser marking/cutting machine, whose energy allows the mechanical and thermal degradation of the mask and therefore the exposure of some areas of the metal that constitutes the article. The laser marking parameters, such as current intensity, voltage and frequency, as well as the type of optical system to be used, depend on the type of masking agent chosen and on the geometry and metallurgical nature of the article being worked, and are in any case generally within the grasp of the person skilled in the art. The use of a CAD system and of the laser allows to produce controllable and repeatable shapings both during the step for production of the shaping and during the step for removing the material of the mask.

Once part of the mask has been removed from the surface of the article, one proceeds with the provision of the macrostructure at the exposed surface by means of an electrochemical etching, during which the metallic material that constitutes the article is removed. During this removal step, some parameters must be defined accurately by the person skilled in the art, the technology being potentially applicable to all implantable metals and partly also to non-implantable metals, such as those used for surgical instruments and specific tools. To ensure optimum removal of material, a value of electrochemical etching is preset; this presetting can occur by scanning the electrochemical behavior of the article, applying different potentials at short time intervals, by scanning the behavior of the article at cathode and anode potentials, as shown in Figure 2. From this viewpoint, the application of a potentiodynamic cycle from -5 V to +5 V is particularly recommended in order to be able to determine the point of electrochemical etching of the surface of the article. In this manner, it is possible to determine the best attack conditions, which correspond to an anodic behavior of the metal which protects against embrittlement linked to the development of hydrogen and most of all allows to find the most favorable etching area. In this manner it is possible to achieve quick removals of material with high definition. For electrochemical etching, for example combinations of reducing agents with oxidizers are used. Metals such as titanium, tantalum, zirconium, aluminum, niobium and alloys based on these metals, in addition to the well-known stainless steels and cobalt/chromium base alloys, in fact owe their stainless nature to an adhering and coherent layer of oxide which covers their surface; these oxides must be removed by means of reducing agents, such as fluorine-, chlorine-, sulfur-base acids. The addition of an acid oxidizer such as oxalic acid, acetic acid, nitric acid allows to ensure constant removal of the metal underlying the oxide film, reducing the risks of an excessive generation of gaseous and atomic hydrogen, which might penetrate the metallic structure, causing its embrittlement.

After determining the most favorable electrochemical etching area, static conditions are applied in the neighborhood of the etching point, so as to keep the metal that constitutes the article electrochemically susceptible to the surrounding chemical agents, thus prolonging the removal of material for the desired time as a function of the quantity of material that is intended to remove. This quantity depends on the flowing charge according to a theoretical rule whose equation is as follows Δr[μm/h] = |(e_qchim/F)|-i-(3600s/p) where:

- Δr[μm/h] is the thickness of metal removed, in microns per hour

- e_qchim is the chemical equivalent of the removed metal - F is Faraday's constant (96,500 coulomb)

- i is the current density expressed in mA/cm²

- p is the density of the removed metal.

The flowing charge differs from reality according to simple efficiency parameters, which can be determined by the person skilled in the art during the practical step. The operating conditions during the chemical etching step are shown in Figures 3 and 4, showing that the condition of removal of the metallic material is constant in terms of current and voltage during the process, after an initial period during which the working point is reached starting from nil initial values. It is noted that the operation for removing the metallic material consists in maintaining, for a sufficiently long time, the value of electrochemical potential of the article in the neighborhood of the value predefined by means of the operation for scanning the electrochemical behavior, described earlier. Figure 4 shows the behavior of the current over time, the measurement of which allows to calculate, minus a parametric efficiency error, the depth of the removal and the quantity of dissolved metal in solution.

The electrochemical etching generates a macrostructured shaping which can have dimensions of at least 10 microns. Preferably, this shaping has dimensions from 500 to 1000 microns, the same characteristic dimensions as bone trabeculae.

Once the electrochemical etching step has ended, it is necessary to subject the article to a step in which the remainder of the mask is removed.

This treatment can consist in immersing the article in an acid or basic solution, depending on the nature of the masking agent first used, for a period long enough to allow the separation of what is left of the mask from the surface of the article. For example, a basic solution can be a solution of a base selected from the group that comprises sodium hydroxide, calcium hydroxide, potassium hydroxide and hydrogen peroxide. The immersion time can be for example 10 minutes to 48 hours, depending on the composition, temperature and concentration of the solution.

During the last step of the method, a microstructure is applied by means of a known method for surface patterning on a micrometer scale. This method can be selected from the group that comprises chemical or electrochemical etching, chemical or electrochemical deposition of calcium phosphates and anodizing at high voltages (Anodic Spark Deposition), only if the material that constitutes the article is a metal selected from the group that comprises titanium, zirconium, niobium, tantalum, aluminum and alloys thereof. The microstructure thus applied preferably has dimensions between 0.5 and 20 microns. A microstructure with these dimensions, by cooperating with the macro structure generated previously on the surface of the article, allows to modulate the response of the cellular component and the mechanical and biological coupling of the bone tissues that surround the article; further, it allows to mediate bone addition phenomena directly in contact with the metal, facilitating close integration and long-term stability without any interposition of fibrous tissue.

Further, the invention provides metallic articles with a macro- and microstructured surface provided by means of the described method. These articles can be dental or orthopedic prostheses or generic surgical instruments, which thanks to the macro- and microstructured surface are characterized by higher stability of the instrument in surgeons' hands.

Moreover, the invention also provides a use of a dental or orthopedic prosthesis with a macro- and microstructured surface, provided with the method described here, for bone regrowth.

The disclosures in Italian Patent Application No. BO2007A000436 from which this application claims priority are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

1. A method for providing a macro- and microstructured surface on a manufactured article made of metal, comprising the steps of: a) depositing a mask of protective film, constituted by a masking agent which is resistant to corrosion in acid and/or basic solutions, on the surface of the article; b) stabilizing the mask of protective film; c) exposing part of the masked surface of the article by selective and controlled removal of the mask according to a previously generated pattern; d) electrochemically etching the material that constitutes the article on the surface exposed in step c) to generate a macrostructure, in conditions of protection against embrittling chemical agents; e) removing the remaining part of the mask by means of a mechanical and chemical treatment; f) applying a microstructure by means of a surface patterning method on a micrometer scale.

2. The method according to claim 1, wherein the article is constituted by a metal selected from the group comprising titanium, zirconium, niobium, tantalum, aluminum, and alloys thereof, stainless steel alloys and cobalt/chromium and nickel/titanium alloys.

3. The method according to claim 1 or 2, wherein the article is selected from the group comprising a dental or orthopedic prosthesis and a generic surgical instrument.

4. The method according to claim 3, wherein the dental or orthopedic prosthesis is selected from the group comprising a component for hip prostheses, an implantable knee prosthesis component, a spinal prosthesis, a traumatology component and a dental implant in the area in contact with bone tissue.

5. The method according to claim 4, wherein the component for hip prostheses is selected from the group comprising acetabular cups, spongiosa screws, femoral prosthesis stems and proximal or distal components of prosthesis stems.

6. The method according to claim 4, wherein the implantable knee prosthesis component is selected from the group that comprises femoral shields and tibial plates.

7. The method according to claim 1 , wherein the mask is deposited totally or partially on the surface of the article.

8. The method according to claim 1 , wherein the masking agent is in a form selected from the group comprising a liquid, a gel and a sol-gel.

9. The method according to claim 1, wherein the masking agent is applied by means of a technology selected from the group comprising spraying of the agent on the article, immersion of the article in the agent and sprinkling of the agent onto the article.

10. The method according to claim 1, wherein the agent is selected from the group that comprises epoxies, acrylates and polyesters.

1 1. The method according to claim 1 , wherein the mask is deposited with the further application of an inorganic or organic filler.

12. The method according to claim 1, wherein the mask is stabilized by treatment with a suitable form of energy.

13. The method according to claim 12, wherein the form of energy is selected from the group comprising heat, white light, light at a particular wavelength, and gamma rays.

14. The method according to claim 13, wherein the light at a particular wavelength is selected from the group comprising infrared light and ultraviolet light.

15. The method according to claim 12, wherein the treatment is a drying process.

16. The method according to claim 15, wherein drying occurs in an aspirated oven at a temperature between 100 and 300⁰C.

17. The method according to claim 1, wherein the part of the surface of the article is exposed by means of a laser beam.

18. The method according to claim I ₅ wherein the pattern of the macrostructure is generated beforehand by means of a CAD system.

19. The method according to claim 1 , wherein an electrochemical etching value is determined before step d).

20. The method according to claim 19, wherein the electrochemical etching value is determined by applying a potentiodynamic cycle to the article.

21. The method according to claim 20, wherein the potentiodynamic cycle is applied between -5 V and +5 V.

22. The method according to claim 19, wherein the electrochemical etching in step d) occurs by applying an alternating potentiostatic cycle in the neighborhood of the set etching value, for a time which corresponds to the quantity of material that is intended to remove.

23. The method according to claim 1 , wherein the electrochemical etching occurs in the simultaneous presence of a reducing agent and of an oxidizing agent.

24. The method according to claim 23, wherein the reducing agent is a fluorine-, chlorine- or sulfur-base acid.

25. The method according to claim 23, wherein the oxidizing agent is selected from the group comprising oxalic acid, acetic acid and nitric acid.

26. The method according to claim 1, wherein the generated macrostructure has dimensions of at least 10 microns.

27. The method according to claim 26, wherein the macrostructure has dimensions from 500 to 1000 microns.

28. The method according to claim 1, wherein the mechanical and chemical treatment in step f) consists in immersing the article in a basic or acid solution.

29. The method according to claim 28, wherein the immersion is performed for 10 minutes to 48 hours.

30. The method according to claim 28, wherein the basic solution is a solution of a base selected from the group comprising sodium hydroxide, calcium hydroxide, potassium hydroxide and hydrogen peroxide.

31. The method according to claim 1 , wherein the method for surface patterning on a micrometer scale is selected from the group comprising chemical or electrochemical etching, chemical or electrochemical deposition of calcium phosphates and anodization at high voltages when the material that constitutes the article is a metal selected from the group comprising titanium, zirconium, niobium, tantalum, aluminum and alloys thereof.

32. The method according to claim 31, wherein the generated microstructure has dimensions between 0.5 and 20 microns.

33. A metal article with a macro- and microstructured surface, provided with the method according to one or more of claims 1 to 32.

34. The article according to claim 33, characterized in that it is a dental or orthopedic prosthesis.

35. The article according to claim 33, characterized in that it is a generic surgical instrument.

36. Use of the dental or orthopedic prosthesis according to claim 34 as a substrate for bone regrowth.