WO2000074637A1

WO2000074637A1 - Dental implants and method for their coating

Info

Publication number: WO2000074637A1
Application number: PCT/FI2000/000488
Authority: WO
Inventors: Kari Luotio
Original assignee: Osfix International Ltd Oy
Priority date: 1999-06-02
Filing date: 2000-05-31
Publication date: 2000-12-14
Also published as: AU4762100A; FI991264A0

Abstract

A method for coating of dental implant, implant supported structure and other dental laboratory metallic work or framework made from cast CoCr -alloy or other metal or metal alloy and a dental implant, implant supported structure or other dental laboratory metallic work or framework. The coating is a DLC film manufactured with e.g. a PVD deposition method. The coating prevents Co and Cr or other ions from dissolving under the circumstances of the mouth.

Description

Dental implants and method for their coating

Field of the invention

The present invention relates to dental implants, implant supported structures and other dental laboratory metallic works and to a method for the coating of the same.

Technical background

Dental implant frameworks (i.e. mesio- and super-structures) are made mostly of gold alloys. However, mainly out of economical reasons, cobalt chrome alloy is a material for the same purpose. Cobalt chrome alloy is commonly used in prosthodontics and it is a well known material by both dentists and dental technicians. One of the disadvantages of cobalt chrome alloy is an eventual allergic reaction to cobalt, even though allergic reactions are not very common. Nevertheless, all structures must be designed to enable the removal of cobalt chrome alloy components from implant and dental structures. It is to be mentioned that, even though in the following specification cobalt chrome is taken as an example, also other metal alloys are useful and may be used according to the present invention. Such metals or metal alloys are for instance titanium or alloy of gold and palladium. Other alloys may be used as well as the present invention will bring the solution for certain problems which are common when using non-noble metals.

According to studies known from literature, all cobalt chrome alloy components may also dissolve in oral conditions. Galvanic corrosion has been claimed to cause loss of bone around the implant. It is assumed that a coating on cast CoCr-alloy might be a more practical and economical method to achieve more corrosion resistant implant frameworks. The other choice would be the use of a titanium prosthesis. However, casting of titanium requires considerably more complex technology, and a protective gas atmosphere and is thus more complex to use and also the material itself is quite expensive.

Brief description of the invention The aim of the present invention is thus to achieve a dental implant, implant supported structure or other dental laboratory metallic works or framework in which the solubility of cobalt chrome components is decreased thus avoiding allergic reactions and also other adverse reactions caused by the material used for dental implants and framework inside the bone tissue or soft tissue or outside the same. Further aim of the invention is to achieve an adequate method for coating dental implant frameworks.

The above aim of the present invention are achieved by a dental implant or framework and the method for producing of the same, the characteristic features of which have been given in the accompanying claims.

The implant or framework and the method according to the present invention basically uses thin film technology for achieving the desired advantageous result.

Detailed description of the invention

The present invention is described in more detail in the following by describing the procedure, steps and materials required for the present invention. The method and the product will become clear from the description.

The present invention is based on a thin film which has been made from DLC (Diamond Like Carbon) film. In a conventional way the base material is first covered by a very thin film of titanium or other suitable material to avoid difficulties in layering the DLC over the base material. The method of making the said thin layer of DLC on the base material is, for instance, a pulsed carbon plasma arc PVD method. The deposition procedure is carried out in a vacuum in room temperature.

However, the present invention is by no means restricted to the used method as it is evident to a person skilled in the art that also other methods are applicable when DLC is layered over the base structure. For instance; low pressure chemical vapour deposition (CVD), various plasma evaporation techniques, various ionization processes or chemical gaseous phase deposition are techniques that can be used. The base material was cast CoCrMo alloy (Wironit extra hard, Bego, Germany), with nominal composition of Co 63; Cr 30; Mo 5; Si 1.1 ; Mn 0.5; C 0.4. The test plates were custom made and hand polished using standard methods of dental laboratories. The size of the test plates was 4 cm² and thickness 0.5 mm.

Numerous test were made, after the production of the test plates, to find out the physical/physicochemical properties of the thus formed product.

The microstructural studies of the films were carried out with a Philips XL30 scanning electron microscope (SEM). The coating thickness was determined from the SEM micrographs.

The circumstances and methods of the Corrosion tests

The corrosion tests were all made using a 0.9 % physiological saline solution (Medipolar) as a corrosion medium. The pH-value was set to 4.7 with HCI. The duration of the immersion test was 2 months and the weight loss was measured with the accuracy of 0.1 mg. The electrochemical polarization measurements were done with EG & G Pare computer controlled potentiostat/galvanostat Model 273A. Reference electrode was Ag/AgCI and counter electrode platinum. A potentiodynamic measuring technique was used and the potential was changed at a speed of 0.5 mV/min. Dissolution tests were done by polarizing the samples electrochemically near corrosion potential.

The immersion time before the first measurement was 24 hours after which the electrolyte was changed to a new one. After that the immersion time was 48 hours; so the total testing time was 72 hours. The amount of ions dissolved were determined from the solution by ICP analysis.

Thickness and structure of the films

As the production of the film was made as an uninterrupted procedure, composition changes gradually from pure titanium to diamond like carbon structure. When the film thickness during deposition reaches the value of 400...500 nm, the morphology of the film changes, and this can be seen in the SEM-micrograph, seen in Figure 1. The total thickness of the film is approximately 1 μm. Some porosity can be seen on the surface of the film as well as some macro particles. These are formed in the upper half of the film, when the amount of the DLC is more than 50 % and they are macro- droplets from the process.

The pores are mainly situated near the agglomerates and so they are also present only within the upper half of the film. The average pore size is approx. 200 nm.

Corrosion tests

Immersion tests

After two months immersion time in the 0.9 % physiological saline solution only very small weight changes were measured. For comparison, also uncoated plate was tested. The backside of the testplates was covered with with Araldit® glue during test. After immersion the plates were dried in 80^βC for 30 min. No actual weight loss was detected, only a very slight weight increase due the corrosion products. The weight loss was approximately of the order of 0,1 %.

After testing the samples were studied with SEM to find out if corrosion products or other damage were present. No differences were detected in the coated materials when compared to the as-deposited films. Instead, two different phase areas could be seen in the pure base material, CoCr-alloy after test. This is evidently due to the etching effect of the saline solution. Corrosion phenomena were observable on the uncoated plate after 1440 hours in physiological saline solution at room temperature but not on the coated plates.

Electrochemical polarisation tests

In the electrochemical potentiodynamic polarization tests the potential was scanned from -600 mV to 1200 mV. The corrosion current density was about 0.02 μA/cm² for DLC-films and base material. All tested examples had some open porosity, which affected both the corrosion current density and the corrosion potential. The corrosion potential was -380 mV vs. Ag/AgCI for CoCr-plate and around -200 mV for the coated plates. No clear passivation effect was detected in this saline solution. Based on these results, the chloride content in physiological saline solution is high enough clearly to disturb the formation of the protective passive layer on CoCr-alloy.

Measurements of dissolved ions

In order to have more information on the protective behaviour of the coatings, the dissolved metal ions in the corrosive media were measured. Diluted into the liquid, cobalt was found to dissolve at much higher rate (nearly 3 times more) than chromium from the base material. No other ions from the cast alloy were detected. With DLC-film the amount of Co was reduced to 30 % of the uncoated alloy.

Medical point of view

The examined materials have been accepted as highly biocompatible materials in medical literature. From a clinical point of view the surface hardness of the exposed metallic framework is important because calculus removing is a procedure which is highly wearing and damage causing. Thus the DLC is a very interesting material. Moreover, it is in common use in heart valve prosthesis and under studies for hip prosthesis. The biocompatibility of DLC is well documented using both cell culture and animal tests.

In dental use, if not implanted, the only theoretical channel for systemic effects is the digestive canal. The question is only academic, not practical, because great amounts of diamond is swallowed for decades after every dental operation as a result of the wearing of diamond drills. Moreover, diamond is carbon which has been a medicine for centuries.

When coatings are used in dental prostheses, we have to accept that these frameworks are not serial products but custom made. Thus the structures always contain areas which are not perfectly polished and the behaviour of the coating material in such surfaces needs further studies.

The studied films contained small amount of porosity due to coating process. The surface scratches were found to be the preferred sites for pore formation. The DLC film contained some small agglomerates within the upper half of the film. The coatings were able to reduce the amount of dissolved ions from base material. The reduction of dissolved Co-ions was remarkable.

DLC film is a promising coating to prevent CoCr ion release and the hardness of the surface of DLC is also excellent for the purposes of this use.

make it most interesting. The problem of DLC is that the gradient process with titanium is not in common use and the commonly used process with wolfram does not produce stabile coatings for biocompatible use.

Claims

1. A method for reducing dissolution of metal, for example cobalt and chromium or other metal or metal alloy ions from a base material of dental implants, implant supported structures and other dental laboratory metallic works, comprising coating the surface of the framework with a thin layer of diamond like carbon (DLC).

2. A method according to claim 1 , wherein the base material mainly consists of non- noble metal or of an alloy in which at least one of the components is a non-noble metal.

3. A method according to claim 1 , comprising a step of first covering the surface of the framework with a very thin layer of titanium or other priming material.

4. A method according to claim 1 , wherein the covering with DLC is effected by pulsed carbon plasma arc PVD method or any other suitable method.

5. A dental implant, implant supported structure or other dental laboratory metallic work or framework consisting e.g. of CoCr-alloy or other metal or metal alloy and having on its surface a film (layer) of diamond like carbon (DLC).

6. A dental implant or framework according to claim 5, and having on its surface, under the layer of DLC a thin film (layer) of titanium or other priming material.