Coating processes – Medical or dental purpose product; parts; subcombinations;... – Implantable permanent prosthesis
Reexamination Certificate
1999-06-28
2001-08-28
Parker, Fred J. (Department: 1762)
Coating processes
Medical or dental purpose product; parts; subcombinations;...
Implantable permanent prosthesis
C427S002260
Reexamination Certificate
active
06280789
ABSTRACT:
DESCRIPTION
The present invention relates to a process for coating a substrate with a coating of carbonated hydroxyapatite, to a structure comprising a substrate coated with a coating obtainable by said process, as well as an orthopaedic or dental prosthesis or implant comprising said structure.
It is known that mineral components of animal bones and teeth consist essentially of calcium phosphates and in particular of phosphates belonging to the class of hydroxyapatite, which is represented by the formula Ca
10
(PO
4
)
6
(OH)
2
, wherein the molar ratio calciun ions to phosphate ions is 1.67.
It is also known that hydroxyapatites forming animal bones and teeth (also called “biological hydroxyapatites”) are non-stoichiometric hydroxyapatites, which contain extraneous ions, such as carbonate, sodium, potassium, magnesium and fluorine, and are characterized by a molar ratio calcium ions to phosphate ions lower than 1.67.
Synthetic hydroxyapatites can be used for preparing orthopaedic and dental prostheses and implants. In order to be suitable for said use, synthetic hydroxyapatites must have structure and composition similar to those of biological hydroxyapatites, must be non-toxic with respect to the animal body, must be biocompatible (i.e. compatible with the animal body without giving place to adverse reactions with respect to the body functions), osteoconductive (i.e. capable of strongly adhering to the animal bone and promoting its healing and growth), bioresorbable or biodegradable (i.e. capable of being dissolved in the animal body) and bioactive (i.e. capable of facilitating cellular activity and repair of living tissues).
In the preparation of prostheses and implants, synthetic hydroxyapatites are used for example in the form of coatings which are deposited on metallic or ceramic substrates. Such coatings are obtained by dry processes at high temperature, such as plasma-spraying, which however cause decomposition of the material, formation of other phases and presence of impurities in the coatings; moreover, the thus obtained coatings are made of crystals having rather great sizes, and have rather small specific surface area and surface reactivity; consequently, protein adsorption, which is necessary to promote attachment of osteoblast cells and their proliferation, is insufficient. Hence, long term stability, osteoconductivity, bioresorbability and bioactivity of said coatings are unsatisfactory.
Processes for obtaining hydroxyapatite coatings by deposition from solutions containing calcium and phosphate ions have been proposed. However, such processes are slow, complicated and difficult to be controlled. Moreover, they provide coatings which have scarce adhesion to the substrate, are unsatisfactory in terms of composition and crystalline structure and, therefore, do not comply with the requirements of osteoconductivity, bioresorbability and bioactivity. An object of the present invention is therefore that of providing a process which is easy to be carried out and controlled, and allows the formation of hydroxyapatite coatings having excellent adhesion to the substrate, scarce crystallinity (i.e. being made of crystals of rather small sizes), high specific surface area and surface reactivity, and complying in an excellent way with the requirements of biocompatibility, osteoconductivity, bioresorbability and bioactivity.
This and other objects are achieved by the present invention which provides a process for coating a substrate with a coating consisting essentially of carbonated hydroxyapatite.
By the term “carbonated hydroxyapatite” a hydroxyapatite is meant, wherein phosphate and/or hydroxy ions are partially replaced by HCO
3
bicarbonate ions.
The process of the present invention comprises:
(a) immersing the substrate to be coated in an aqueous solution containing calcium ions in a concentration from 1 to less 3.8 mmoles/liter, phosphate ions in a concentration from 1 to less 3.8 mmoles/liter and bicarbonate ions in a concentration from 0.08 to 0.25 moles/liter, with a molar ratio calcium ions to phosphate ions from 0.8 to 2.0, a pH from 6.8 to 8.0 and a temperature lower than 50° C.;
(b) heating at least the portion of the solution in contact with the substrate to a temperature from 50 to 80° C. until having a pH greater than 8;
(c) maintaining the substrate in contact with the heated supersaturated alkali solution obtained in step (b), thus causing formation of a carbonated hydroxyapatite coating to take place spontaneously directly at the contact with the surface of the substrate; and
(d) taking the substrate off the solution, after the requested thickness of coating has been obtained, and subjecting the coating to drying.
Another object of the present invention is a structure comprising a substrate consisting of metallic or ceramic material coated with a coating obtainable by the above mentioned process.
A further object of the present invention is an orthopaedic or dental prosthesis or implant comprising said structure.
Substrates, to which the process of the present invention can be applied, can be made of metallic or ceramic material.
Preferably the substrate surface must have a certain porosity and/or roughness. If necessary, porosity and/or roughness of the surface can be increased by means of appropriate treatments, such as laser treatment, sanding or plasma-spraying treatment.
By way of example, the substrates can be made of a metal selected from titanium, titanium alloys, zirconium, zirconium alloys, vanadium and vanadium alloys, or a material coated with a coating of said metals or metal alloys.
Other examples are substrates made of a ceramic material selected from hydroxyapatite, fluoridated hydroxyapatite and other calcium phosphates, or substrates consisting of a metallic material coated with a coating of said ceramic materials, said coating having been obtained by a dry process at high temperature, e.g. by plasma-spraying.
By the term “fluoridated hydroxyapatite” a hydroxyapatite is meant, wherein hydroxy ions have been partially replaced by fluorine ions in order to reduce the dissolution and degradation rate of the hydroxyapatite at the contact with other substances which are present in the animal body.
The Applicants have found that the process of the present invention is particularly suitable and advantageous for coating substrates, which have been previously coated with hydroxyapatite by plasma-spraying.
In particular the Applicants have found that multilayer structures comprising a metal substrate, coated with a first coating of metallic material, preferably the same metallic material as that of the substrate, with a second coating of hydroxyapatite, optionally fluoridated, and with a third coating of carbonated hydroxyapatite, the latter having been obtained by the process of the present invention, are characterized by excellent mechanical properties and other properties, such as biocompatibility, osteoconductivity, bioresorbability and bioactivity. Consequently, said multilayer structures can be advantageously used for making orthopaedic and dental prostheses and implants.
Particularly advantageous are multilayer structures wherein both said first metallic coating (with a thickness of preferably 30-100 &mgr;m) and said second hydroxyapatite coating are obtained by plasma-spraying under vacuum or in inert atmosphere. Preferably, before depositing said second coating, porosity and/or roughness of the metal surface can be increased by appropriate treatments of the above mentioned types.
The aqueous solution of step (a) is a solution which is stable and can be easily prepared at ambient temperature and handled without a particular care.
In step (a) the solution is utilized preferably at ambient temperature and can be prepared by dissolving any compounds of the requested ions in deionized water. Preferably calcium ions are introduced in the form of calcium nitrate Ca(NO
3
)
2
, phosphate ions in the form of ammonium hydrogenphosphate (NH
4
)
2
HPO
4
and bicarbonate ions in the form of sodium bicarbonate NaHCO
3
.
Other ions, such as for example
Ranz Xavier
Rey Christian
Biocoatings S.r.l.
Darby & Darby
Kolb Jennifer
Parker Fred J.
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