GB2550146A

GB2550146A - Metal alloy for dental Prosthesis

Info

Publication number: GB2550146A
Application number: GB1608145.7A
Authority: GB
Inventors: Cascone Paul
Original assignee: Argen Corp
Current assignee: Argen Corp
Priority date: 2016-05-10
Filing date: 2016-05-10
Publication date: 2017-11-15
Also published as: GB201608145D0

Abstract

A metal alloy powder comprising: 40-95wt% palladium; 5-35wt% chromium; 0-8wt% gold; 0-10wt% silver; 0-10wt% platinum; and 0-3wt% of ruthenium, rhenium or iridium; where the total proportion of palladium, gold, silver and platinum is 65-95wt%; and where the total proportion of palladium, chromium, gold, silver and platinum is 95-100wt%. Preferably, the alloy is made up of palladium. Preferably, the alloy powder is 85% palladium and 15% chromium. Preferably, the powder has average diameter 20-40µm and has thermal expansion coefficient of 13-15x10-6 °C-1 at 500°C. The powder alloy is made into a metal frame for a dental prosthesis using selective layer sintering, preferably in an inert atmosphere, and comprises a porcelain coating attached to the frame.

Description

METAL ALLOY FOR DENTAL PROSTHESIS

The present invention relates to a metal alloy for use in a dental prosthesis, a dental metal frame for porcelain bonding, a dental prosthesis and a method for manufacturing a dental metal frame for porcelain bonding.

Dental prostheses designed to replace whole teeth or parts of teeth typically consist of a porcelain coating bonded to a metal frame. It is desirable for the metal frame to be made from an alloy containing a high proportion of a noble metal as this leads to a more predictable bond with the porcelain coating and improves the corrosion resistance of the prosthesis.

Conventionally, the metal frame has been manufactured by casting a metal alloy, which requires a skilled dental technician to ensure that the cast forms the alloy into the required shape. As a result, the process of manufacturing the metal frame for a dental prosthesis tends to be relatively slow and expensive. Various alloys have been developed that can be used to manufacture dental metal frames in this way.

The present inventors have realised that metal frames for dental prostheses can be manufactured more efficiently by using an automated additive manufacturing technique. However, many existing metal alloys for use in dental prostheses are unsuitable for use in such techniques due to their inability to absorb laser light and other physical properties. In particular, it has proved difficult to find a dental alloy suitable for additive manufacturing that has a sufficient noble metal content to meet accepted standards for dental prostheses, including UK National Health Service (NHS) standards.

After extensive experimentation, the inventors have discovered an alternative alloy for use in a dental prosthesis that is particularly suited to being used in an additive manufacturing process and also contains a sufficiently high noble metal content to meet NHS standards. The resulting prosthesis exhibits a reliable metal frame-porcelain bond and is highly corrosion resistant.

According to a first aspect of the present invention, there is provided a metal alloy powder comprising palladium in a proportion of 40% to 95% by mass with respect to the total mass of the metal alloy powder and chromium in a proportion of 5% to 35% by mass with respect to the total mass of the metal alloy powder. The metal alloy powder contains 0% to 8% by mass of gold with respect to the total mass of the metal alloy powder, 0% to 10% by mass of silver with respect to the total mass of the metal alloy powder, 0% to 10% by mass of platinum with respect to the total mass of the metal alloy powder, and 0% to 3% by mass of one or more elements selected from the group consisting of ruthenium, rhenium and iridium with respect to the total mass of the metal alloy powder. The total proportion of palladium, gold if present, silver if present and platinum if present in the metal alloy powder is between 65% and 95%, and the total proportion of palladium, chromium, gold if present, silver if present and platinum if present in the metal alloy powder is between 95% and 100%.

Preferably, the balance of the metal alloy powder is made up of palladium.

Preferably, the proportion of chromium in the metal alloy powder is between 10% and 20% by mass. Preferably, the proportion of palladium in the metal alloy powder is between 70% and 90% by mass.

More preferably, the proportion of chromium in the metal alloy powder is substantially 15% by mass. Preferably, the proportion of palladium in the metal alloy powder is substantially 85% by mass.

Preferably, the mass ratio of palladium to chromium in the metal alloy powder is between 3.5 and 7. Suitably, the metal alloy powder consists substantially entirely of palladium and chromium.

In one embodiment, the one or more elements selected from the group consisting of ruthenium, rhenium and iridium are present in a combined proportion of between 0.5% and 1.5% by mass with respect to the total mass of the metal alloy powder.

Suitably, the metal alloy powder is substantially silver-free. Alternatively, the metal alloy powder contains 1% to 10% by mass of silver with respect to the total mass of the metal alloy powder.

Preferably, the average particle diameter of the powder is between 20 μη and 40 μη, the average particle diameter being defined as the particle diameter in the particle size distribution of the powder obtained by laser diffractometry at which the cumulative mass of powder particles reaches 50%. Preferably, the metal alloy powder has a thermal expansion coefficient in the range 13 to 15 x 10 6 / °C at 500 °C.

According to a second aspect of the present invention, there is provided a metal frame for a dental prosthesis, the metal frame comprising palladium in a proportion of 40% to 95% by mass with respect to the total mass of the metal frame and chromium in a proportion of 5% to 35% by mass with respect to the total mass of the metal frame. The metal frame contains 0% to 8% by mass of gold, 0% to 10% by mass of silver and 0% to 10% by mass of platinum with respect to the total mass of the metal frame. The metal frame contains 0% to 3% by mass of one or more elements selected from the group consisting of ruthenium, rhenium and iridium with respect to the total mass of the metal frame. The total proportion of palladium, gold if present, silver if present and platinum if present in the metal frame is between 65% and 95%, and the total proportion of palladium, chromium, gold if present, silver if present and platinum if present in the metal frame is between 95% and 100%.

Preferably, the balance of the metal frame is made up of palladium. Preferably, the metal frame has a thermal expansion coefficient in the range 13 to 15 x 10'6 / °C at 500 °C.

According to a third aspect of the present invention, there is provided a dental prosthesis comprising the metal frame described above and a porcelain coating attached to the metal frame.

In one embodiment, the dental prosthesis is a short span bridge.

According to a fourth aspect of the present invention, there is provided a metal alloy consisting of 5% to 35% by mass of chromium, 1% to 10% by mass of silver, 0% to 8% by mass of gold, 0% to 10% by mass of platinum, 0% to 3% by mass of one or more elements selected from the group consisting of ruthenium, rhenium and iridium, and palladium making up the balance of the metal alloy. The total proportion of palladium, silver, gold if present and platinum if present in the metal alloy is between 65% and 95%.

Preferably, the alloy contains 10% to 20% by mass of chromium with respect to the total mass of the metal alloy. Preferably, the one or more elements selected from the group consisting of ruthenium, rhenium and iridium are present in a combined proportion of between 0.5% and 1.5% by mass with respect to the total mass of the metal alloy.

According to a fifth aspect of the present invention, there is provided a method of use of the metal alloy powder described above to manufacture a metal frame for a dental prosthesis.

According to a sixth aspect of the present invention, there is provided a method for manufacturing a metal frame for a dental prosthesis, the method comprising providing a layer of the metal alloy powder described above in a build space of a selective laser melting apparatus. The method includes selectively melting portions of the layer by applying laser light to the portions of the layer using a laser light source of the selective laser melting apparatus, so that the melted portions of the layer form parts of the metal frame, providing another layer of the metal alloy powder on top of the previously provided layer in the build space, selectively melting portions of the another layer by applying laser light to the portions of the another layer using the laser light source, and repeating the steps of providing another layer of the metal alloy powder and selectively melting portions of the another layer until the metal frame is formed.

Preferably, the method is performed in an inert atmosphere.

Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram of an atomization process used in manufacturing a metal alloy powder of the invention;

Figs. 2A and 2B show scanning electron microscope (SEM) pictures of a metal alloy powder according to the invention;

Fig. 3 is a 500X magnified image showing the internal structure of an atomized metal alloy powder according to the invention;

Fig. 4 is a chart showing a typical particle size distribution of an atomized metal alloy powder according to the invention;

Fig. 5 is a schematic diagram of a selective laser melting (SLM) apparatus used in an embodiment of the present invention;

Fig. 6 shows a cross section of a dental metal frame according to an embodiment of the invention; and

Fig. 7 shows a cross section of a dental prosthesis according to an embodiment of the invention.

An embodiment of the invention is a metal alloy powder to be used in manufacturing a dental prosthesis. The alloy powder is formed from a metal alloy including as its main components chromium (Cr) and palladium (Pd). The characteristics of the alloy are described below.

The majority of the alloy is made up of Pd and it is preferred that the proportion of Pd in the alloy be 65% by mass or greater with respect to the total mass of the metal alloy. It is more preferred that the Pd proportion be 70% by mass or greater. Still more preferably, the Pd proportion is 80% by mass or greater.

Since Pd is a noble metal, is desirable to have a high proportion of Pd in the metal alloy in order to improve the corrosion resistance of a dental metal frame made from the alloy.

It is also preferred for the Pd content of the metal alloy to be 95% by mass or lower with respect to the total mass of the metal alloy. It is more preferred for the Pd content of the metal alloy to be 90% by mass or lower. The reason for limiting the Pd content is that the thermal expansion coefficient of a dental metal frame made from the alloy must be compatible with that of the porcelain material used to coat the metal frame to form a dental prosthesis. If the thermal expansion coefficient of the alloy used to make the metal frame differs too much from that of the porcelain material then the porcelain will crack when it is baked onto the metal frame. A Cr-Pd alloy having too high a proportion of Pd will have a thermal expansion coefficient that is too low to be compatible with many commercially available dental porcelains.

For the above reason it is also preferred for the metal alloy to have a thermal expansion coefficient in the range 12 to 15 x 10 6 / °C at 500 °C. More preferably, the alloy has a thermal expansion coefficient of at least 13 x 10 6 / °C at 500 °C. The most favourable range for the thermal expansion coefficient of the alloy is 13.8 to 15.0 x 10 6 / °C at 500 °C.

As for the Cr content of the alloy, it is preferred that the proportion of Cr be 25% by mass or lower with respect to the total mass of the metal alloy. It is more preferred that the proportion of Cr be 20% by mass or lower. The inventors have discovered that Cr proportions higher than 20% by mass tend to make the resulting alloy brittle. This is thought to be because the solubility of Cr in Pd decreases at higher proportions of Cr. This leads to inter-metallic compounds being formed in the alloy at high Cr contents, which is thought to result in brittleness.

In one particularly preferred embodiment, the metal alloy includes 13-17% by mass of Cr and 87-83% by mass of Pd. The inventors have found that alloys with these proportions of Cr and Pd exhibit good compatibility with commercially available dental porcelains and are particularly suited for use in additive manufacturing techniques such as selective laser melting (SLM), as well as having a high noble metal content. Of these metal alloys, an alloy containing approximately 15% by mass of Cr and 85% by mass of Pd is especially preferred.

It is preferred that the metal alloy has a ratio of Cr to Pd by mass between 0.1 and 0.33. It is more preferred that the mass ratio of Cr to Pd in the powder be between 0.12 and 0.25. It is particularly preferred that the mass ratio of Cr to Pd in the powder be between 0.15 and 0.2.

The Pd content in the metal alloy may be partially substituted by platinum (Pt), silver (Ag) and/or gold (Au) while preserving the desirable properties of the alloy. Ag may be present in the metal alloy up to a proportion of 10% by mass, while Au may be present in the alloy up to a proportion of 5% by mass. It is more preferred that the metal alloy be silver-free and/or gold-free.

Adding Ag to the metal alloy has the effect of reducing the melting point of the alloy. This is beneficial in some embodiments as it makes it easier to melt the alloy before atomising it in order to form the metal alloy powder. Adding Ag to the alloy also reduces its cost. When silver is added to the alloy for these reasons, it is particularly preferred that the metal alloy contain between 1% and 10% by mass of Ag. Due to the immiscibility of silver and chromium, proportions of silver above 10% in the Pd-Cr metal alloy tend to result in a weakened two-phase crystal structure in the alloy, which is undesirable.

When gold is added to the alloy, it is particularly preferred that the metal alloy contain between 1% and 5% by mass of Au.

Pd may be partially substituted by Pt in the metal alloy as Cr also exhibits good solubility in Pt. However, increasing the proportion of Pt in the alloy will increase the cost of the dental alloy. Increasing the proportion of Pt in the alloy will also reduce the thermal expansion coefficient of the alloy, which makes it more difficult to ensure compatibility with available dental porcelains as discussed above. It is therefore preferred that the ratio of Pd to Pt by mass in the alloy be 2 or greater. More preferably, the ratio of Pd to Pt by mass in the alloy is 3 or more. It is also preferred that the proportion of Pt in the metal alloy be 5% or less by mass with respect to the total mass of the metal alloy.

When platinum is added to the alloy, it is particularly preferred that the metal alloy contain between 1% and 5% by mass of Pt.

The metal alloy may also include other elements in small quantities. These elements include ruthenium (Ru), rhenium (Re) and iridium (Ir), which may be included in order to reduce the grain size of the alloy. In order to have the effect of reducing grain size, it is preferred that the combined content of Ru, Re and Ir in the alloy be 0.1% by mass or more. The combined content of Ru, Re and Ir in the alloy is preferably 2% by mass or less with respect to the total mass of the metal alloy. More preferably, the combined content of Ru, Re and Ir in the alloy is 1% by mass or less.

The alloy described above is the result of an extensive research programme by the inventors to develop a dental alloy that can be used in a SLM manufacturing process having a high proportion of Pd. The inventors realised that using a high proportion of Pd in a dental alloy is desirable for the reasons given above, i.e. the noble metal properties of Pd including corrosion resistance, the compatibility of the thermal expansion coefficient of Pd with existing dental porcelains and the relatively low cost of Pd compared to Pt. However, many attempts to produce a workable dental alloy for use in a SLM process having a high proportion of Pd (e.g. around 70% by mass) failed.

Alloying conventional dental alloy elements such as Ag, indium (In), tin (Sn) and gallium (Ga) with palladium failed because the metal frames produced by the SLM process using these alloys were found to release gas when the porcelain coating (also known as a ceramic veneer) was baked onto the frame to produce a dental prosthesis. This degassing of the alloy forming the metal frame prevented an even porcelain coating from being securely bonded to the metal frame in the baking process, resulting in degradation of the quality of the resulting dental prosthesis. Surprisingly, the inventors found that the Cr-Pd alloy described above does not suffer from this problem when used to manufacture a metal frame in a SLM process. The inventors also discovered that the mechanical properties, such as tensile strength, of the Cr-Pd alloy above are sufficient to be used to manufacture metal frames for both single unit dental prostheses and bridges.

Physical properties of an example of an alloy containing 85% palladium and 15% chromium by weight are described in the table below.

Table 1

Metal alloys as described above can be formed into metal alloy powders for use in a SLM process for manufacturing a dental metal frame. Various methods for forming powders from metal alloys are known. In one preferred example, the metal alloy is melted and the molten alloy is atomised. The atomised molten alloy then forms a metal alloy powder as it cools. A specific example of a process for producing the metal alloy powder is described below.

Elemental palladium and chromium are weighed to have the correct proportions by weight, for example 85% palladium and 15% chromium. The elements are placed in a vacuum furnace for melting and casting. The vacuum furnace is evacuated and back filled with argon. The materials are then heated to approximately 1600 °C until totally molten and then poured into a rectangular mold contained within the vacuum furnace. The melting temperature of the palladium/chromium alloy is about 1400 °C. Once cooled, the material forms an alloy block shaped by the mold.

The alloy block is then rolled until the material begins to break up. The pieces are then atomized. This process ensures that there is no segregation in the melted material and that the atomized particles will be of the same homogenous alloy.

The atomization process is as follows, and is performed using the apparatus 10 illustrated in Fig. 1. The alloy pieces are placed in a crucible 2 within a furnace with a protective atmosphere of argon and melted. The crucible 2 has an orifice at the bottom that is plugged. Once the alloy is fully molten the plug is released allowing the liquid alloy to form a thin stream 6. At the same time, an inert gas 4 under high pressure is allowed to impinge upon the liquid alloy stream 6. The pressure forces the alloy stream 6 to break up into spheres 8 which cool rapidly and are collected at the bottom of the atomizer 10.

The above process produces homogenous alloy powder having a uniform spherical shape as shown in the SEM pictures of Figs. 2A and 2B. A chemical analysis of two examples of the resulting atomized powder in weight percent is shown in the table below.

Table 2

In the example metal alloy powders of Table 2, the small amounts of aluminium (Al), zirconium (Zr), carbon (C), sulphur (S), oxygen (0) and nitrogen (N) present are impurities. The definitions of alloys in this specification and in the appended claims should be read as encompassing the presence of such impurities, which are unavoidable.

Fig. 3 shows the internal structure of the atomized powder demonstrating a uniform equi-axed grain size. The image of Fig. 3 has a magnification of 500X.

After atomization the powder is screened and air classified in order to produce a particle size distribution that will function well in a selective laser melting machine. A typical particle size distribution is shown in Fig. 4. It can be seen that the peak of the distribution occurs at a particle size of between 20 and 30 μηι, and around 90% of the particles fall with a particle size range of 10 to 60 μηι.

Commercially available equipment can be used to perform this atomisation powder manufacturing process.

The metal alloy powder preferably has an average particle diameter between 10 μηι and 50 μηι. This particle size range allows the powder to be used in SLM manufacturing processes using current commercially available equipment. More preferably, the average particle diameter of the powder is between 20 μηι and 40 μιτι.

The average particle diameter is defined as the particle diameter in a particle size distribution of the powder obtained by laser diffractometry at which the cumulative mass of powder particles reaches 50%.

Once the metal alloy powder has been formed, it is used to manufacture a dental metal frame using a SLM process.

To begin the process of manufacturing a dental prosthesis, a dental laboratory technician starts with the traditional plaster model made from the patient's impression sent by the dentist. Instead of manually applying wax to create the design of the prosthetic device, the model is scanned with a dental scanner (e.g. the BShape 850 (RTM) manufactured by 3Shape A/S). The scanner collects information about the surface of the model and, in particular, the contours of the prepared surface upon which the prosthetic device will be cemented. This information is then displayed on a computer screen and the technician designs a virtual crown.

Specialized software can be used for the dental design process, for example 3shape Dental Designer (RTM) available from 3Shape A/S. For example, when a crown for a molar is being designed a coping that will be built by the SLM machine is designed by the technician within the software. This virtual crown is output as an electronic file in the STL extension format for example. This file type is normally associated with stereo-lithography manufacturing processes.

The electronic file is then sent to a facility that has a SLM machine. In order to prepare the file for printing, supports are added to the design and the combined files are sent to the machine.

The molar unit is then manufactured by the SLM machine with the supports attached, using a process as described below for example.

An example of an apparatus 100 used to perform the SLM process is illustrated in Fig. 5. The apparatus 100 includes a laser light source 102 and an inclined focusing mirror 104 for deflecting and focusing the laser light onto an object 106 in a build space 120. The build space 120 is an enclosure housing a movable platform 110, which can be moved towards and away from the mirror 104 in the direction of the deflected laser light.

The apparatus 100 also includes a powder delivery system 130, which consists of an enclosure 132 containing the metal alloy powder to be used in the SLM process and a movable platform 134 that can be moved towards the opening of the enclosure 132 to push powder out of the enclosure 132. The powder delivery system 130 also features a blade 136, which can be moved across the opening of the enclosure 132 to carry powder across a horizontal surface 150 of the apparatus 100 from the powder delivery system 130 to the build space 120.

Both the enclosure 132 of the powder delivery system 130 and the build space 120 are recessed into the horizontal surface 150 of the apparatus. When the blade reaches the opening of the build space it continues to move horizontally across the opening, thereby spreading the powder evenly across the opening of the build space to form a new layer of powder on the surface of the object 106.

During the SLM process, each new layer of powder is subjected to selective melting by the laser light being applied to the surface of the powder. The point of contact between the laser light from the light source 102 and the surface of the powder is the burning point 140, at which melting of the powder takes place. The burning point is moved across the surface of the powder according to a predefined pattern to form a two dimensional pattern of molten alloy powder in the top layer of powder. The burning point may be moved relative to the surface of the powder by tilting the mirror 104 to adjust the direction of the deflected laser light beam and/or by moving the platform 110 of the build space 120.

The alloy powder that is melted by the laser light coheres to form solid sections of metal alloy within the remaining un-melted powder 108. Once melting of one layer of powder has been completed, the SLM process continues by retracting the platform 110 downwards away from the surface 150 by the thickness of one layer of powder. A fresh layer of powder is then spread over the top of the previous layer by the blade 136 from the powder delivery system 130 and the fresh layer is then melted by the laser light in turn. This process continues until the entire dental metal frame 106 has been built up layer by layer from the sections of melted alloy powder. The frame 106 can then be removed from the remaining un-melted powder 108.

Each layer of powder used in the SLM process has a thickness of between 20 μηι and 35 μιτι. This provides sufficient resolution in the manufacturing process to form the dental metal frame 106 at the required level of accuracy for a dental prosthesis.

Once the molar unit has been manufactured by the SLM machine, the supports are removed and the surfaces of the crown are ground with a bur to allow for porcelain application.

An example of a dental metal frame 200 for a molar unit manufactured by the process described above is illustrated in cross section in Fig. 6. The frame is formed of metal alloy material 202 as described above.

To complete a dental prosthesis, a porcelain coating is applied to the metal frame and the metal frame and the porcelain coating are then baked to harden the porcelain and bond the porcelain to the metal frame. A conventional method of porcelain coating for dental prostheses can be used with the metal frame of the present invention. A commercially available dental porcelain material such as VITA VMK Master (RTM) produced by VITA Zahnfabrik H. Rauter GmbH & Co. KG can be used to coat the metal frame.

An example of a dental prosthesis 300 manufactured by the process described above is illustrated in cross section in Fig. 7, including the metal frame 302 and the porcelain coating 304.

When silver is present in the metal alloy material of the dental metal frame 302, it is preferred that a porcelain material that is resistant to discoloration in the presence of silver be used for the porcelain coating 304. An example of such a porcelain material is Noritake EX-3 (RTM) produced by Kuraray Noritake Dental Inc.

The foregoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention as defined by the claims.

Claims

1. A metal alloy powder comprising: palladium in a proportion of 40% to 95% by mass with respect to the total mass of the metal alloy powder; and chromium in a proportion of 5% to 35% by mass with respect to the total mass of the metal alloy powder, wherein: the metal alloy powder contains 0% to 8% by mass of gold with respect to the total mass of the metal alloy powder, the metal alloy powder contains 0% to 10% by mass of silver with respect to the total mass of the metal alloy powder, the metal alloy powder contains 0% to 10% by mass of platinum with respect to the total mass of the metal alloy powder, the metal alloy powder contains 0% to 3% by mass of one or more elements selected from the group consisting of ruthenium, rhenium and iridium with respect to the total mass of the metal alloy powder, the total proportion of palladium, gold if present, silver if present and platinum if present in the metal alloy powder is between 65% and 95%, and the total proportion of palladium, chromium, gold if present, silver if present and platinum if present in the metal alloy powder is between 95% and 100%.

2. The metal alloy powder of claim 1, wherein the balance of the metal alloy powder is made up of palladium.

3. The metal alloy powder of claim 1 or claim 2, wherein the proportion of chromium in the metal alloy powder is between 10% and 20% by mass.

4. The metal alloy powder of any of claims 1 to 3, wherein the proportion of palladium in the metal alloy powder is between 70% and 90% by mass.

5. The metal alloy powder of any of claims 1 to 4, wherein the proportion of chromium in the metal alloy powder is substantially 15% by mass.

6. The metal alloy powder of any of claims 1 to 5, wherein the proportion of palladium in the metal alloy powder is substantially 85% by mass.

7. The metal alloy powder of any of claims 1 to 6, wherein the mass ratio of palladium to chromium in the metal alloy powder is between 3.5 and 7.

8. The metal alloy powder of any of claims 1 to 7, wherein the metal alloy powder consists substantially entirely of palladium and chromium.

9. The metal alloy powder of any of claims 1 to 7, wherein the one or more elements selected from the group consisting of ruthenium, rhenium and iridium are present in a combined proportion of between 0.5% and 1.5% by mass with respect to the total mass of the metal alloy powder.

10. The metal alloy powder of any of claims 1 to 9, wherein the metal alloy powder is substantially silver-free.

11. The metal alloy powder of any of claims 1 to 7 and 9, wherein the metal alloy powder contains 1% to 10% by mass of silver with respect to the total mass of the metal alloy powder.

12. The metal alloy powder of any of claims 1 to 11, wherein the average particle diameter of the powder is between 20 μιτι and 40 μιτι, the average particle diameter being defined as the particle diameter in the particle size distribution of the powder obtained by laser diffractometry at which the cumulative mass of powder particles reaches 50%. IB. The metal alloy powder of any of claims 1 to 12, wherein the metal alloy powder has a thermal expansion coefficient in the range

13 to 15 x 10 6 / °C at 500 °C.

14. A metal frame for a dental prosthesis, the metal frame comprising: palladium in a proportion of 40% to 95% by mass with respect to the total mass of the metal frame; and chromium in a proportion of 5% to 35% by mass with respect to the total mass of the metal frame, wherein: the metal frame contains 0% to 8% by mass of gold with respect to the total mass of the metal frame, the metal frame contains 0% to 10% by mass of silver with respect to the total mass of the metal frame, the metal frame contains 0% to 10% by mass of platinum with respect to the total mass of the metal frame, the metal frame contains 0% to 3% by mass of one or more elements selected from the group consisting of ruthenium, rhenium and iridium with respect to the total mass of the metal frame, the total proportion of palladium, gold if present, silver if present and platinum if present in the metal frame is between 65% and 95%, and the total proportion of palladium, chromium, gold if present, silver if present and platinum if present in the metal frame is between 95% and 100%.

15. The metal frame for a dental prosthesis of claim 14, wherein the balance of the metal frame is made up of palladium.

16. The metal frame for a dental prosthesis of claim 14 or claim 15, wherein the metal frame has a thermal expansion coefficient in the range 13 to 15 x 10~6 / °C at 500 °C.

17. A dental prosthesis comprising: the metal frame of any of claims 14 to 16; and a porcelain coating attached to the metal frame.

18. The dental prosthesis of claim 17, wherein the dental prosthesis is a short span bridge.

19. A metal alloy consisting of: 5% to 35% by mass of chromium with respect to the total mass of the metal alloy; 1% to 10% by mass of silver with respect to the total mass of the metal alloy; 0% to 8% by mass of gold with respect to the total mass of the metal alloy; 0% to 10% by mass of platinum with respect to the total mass of the metal alloy; 0% to 3% by mass of one or more elements selected from the group consisting of ruthenium, rhenium and iridium with respect to the total mass of the metal alloy; and palladium making up the balance of the metal alloy, wherein the total proportion of palladium, silver, gold if present and platinum if present in the metal alloy is between 65% and 95%.

20. The metal alloy of claim 19, wherein the alloy contains 10% to 20% by mass of chromium with respect to the total mass of the metal alloy.

21. The metal alloy of claim 19 or claim 20, wherein the one or more elements selected from the group consisting of ruthenium, rhenium and iridium are present in a combined proportion of between 0.5% and 1.5% by mass with respect to the total mass of the metal alloy.

22. A method of use of the metal alloy powder of any of claims 1 to 13 to manufacture a metal frame for a dental prosthesis.

23. A method for manufacturing a metal frame for a dental prosthesis, the method comprising: providing a layer of the metal alloy powder of any of claims 1 to 13 in a build space of a selective laser melting apparatus; selectively melting portions of the layer by applying laser light to the portions of the layer using a laser light source of the selective laser melting apparatus, so that the melted portions of the layer form parts of the metal frame; providing another layer of the metal alloy powder on top of the previously provided layer in the build space; selectively melting portions of the another layer by applying laser light to the portions of the another layer using the laser light source; and repeating the steps of providing another layer of the metal alloy powder and selectively melting portions of the another layer until the metal frame is formed.

24. The method of claim 23, wherein the method is performed in an inert atmosphere.

25. A metal alloy powder substantially as hereinbefore described with reference to any of Figs. 2A, 2B, 3 and 4.

26. A metal frame for a dental prosthesis substantially as hereinbefore described with reference to Fig. 6.

27. A dental prosthesis substantially as hereinbefore described with reference to Fig. 7.

28. A method for manufacturing a metal frame for a dental prosthesis substantially as hereinbefore described with reference to Fig. 5.