RELATED ART The so called Lost Wax technique is a standard method within dental laboratories, to manufacture parts of, or complete restorations in various materials, e. g. gold al- loys, titanium, ceramics, etc. The Lost Wax technique includes producing a replica of the prepared tooth, often in a plaster material, with the aid of a bite impression.

With the plaster die as base, a dental technician sculptures and designs a restoration in a wax or similar material. The wax-restoration is embedded in a material that will form a mould with the shape of the wax-restoration. The wax is removed from the mould by placing it in an oven, whereby the wax evaporates and disappears. The restoration is cast by letting a metal or some other material in a liquid, or semi- liquid, form enter into the mould through a special channel. The technique used for making the liquid metal reach all parts of the mould is, for example, by rotation us- ing centrifugal force, or by using vacuum/pressure.

A major problem with this traditional casting method is that it is labour intensive, especially concerning the wax model design part. For each restoration, a trained dental technician must design the restoration by hand in order to achieve a wax model of the restoration for use in subsequent casting steps. The actual casting step may however be done in a quite efficient manner.

CAD/CAM based systems from the manufacturing of dental restorations are known in the art, for example: Duret :"Vers unit prothese informatisee"Tonus Dentaire No 73,1985pp. 55-57,

Duret et al:"CAD-CAM in dentistry", JADA, Vol. 117, November 1988, pp.

715-720, Williams:"Dentistry and CAD/CAM: Another French Revolution", Journal of Dental Practice Administration, January/March 1987, 'solin, Sundh, Bergman:"The Decim System for Production of Dental restora- tions", International Journal of computerised Dentistry 1999: 3. [1].

Thus, the first CAD/CAM systems for manufacturing of dental restorations ap- peared in the 1980's. In a dental CAD/CAM system a method is used which in- cludes measuring the surface of the plaster model, produced in the same manner as in the Lost Wax technique, described above. Data about the topology of the pre- pared tooth and possibly also the surrounding teeth are stored in a computer. An op- erator interacts with the CAD part of the system in order to design the restoration on the scanned surface and to create a computer model of the desired restoration. The computer model is transferred to the CAM part of the system, where tool paths are generated for machining of the restoration out of a given blank, made out of, e. g. ceramics or metal. In the last step the tool paths are transferred to a CNC-machine for machining.

The CAD/CAM systems use the possibilities that computers and software give in order to design the restoration more or less automaticly in a cost effective way.

However, in the manufacturing step, there are some costs that are difficult to avoid: 1. When machining in ceramics or in metal, there will always be a significant cost for tool wear.

2. When a CNC-machine is used for manufacturing, there will always be a signifi- cant capital cost.

3. When machining in expensive material such as gold alloys or similar, there is a need for a cost-effective re-cycling system for the cutting chips and excess mate- rial.

4. When expensive material such as gold alloys or similar is to be machined out of a solid block, instead of being cast, a lot more of said material need to be pres- ent, say ten times more. Even if most of it could be re-cycled at low cost, just the presence of an expensive material may cause problems and costs related to secu- rity.

SE 470 346 B discloses a method for producing artificial ceramic dental restora- tions. The method comprises the steps of -registering the surface of a prepared tooth using a three-dimensional optical method on a plaster model of the preparation, -reproducing the registered surface (of the preparation) on a first body, using a computer controlled mill, -forming a second body by expanding the registered surface and milling an outer surface corresponding to the expanded surface, or by applying wax onto the first body, -generating an imprint of the second body in an elastic material, -using the first body and the imprint for a pressing tool, and -forming a coping using the pressing tool in a press casting process.

The method in SE 470 346 B is adapted to an industrial process, involving heavy and expensive equipment, not easily made available in a dental technician labora- tory. Further, the method in SE 470 346 B cannot be easily used in the existing casting method presently used in laboratories.

OBJECT OF THE INVENTION The object of the present invention is to provide a method for producing a dental restoration, which method reduces or avoids the problems mentioned above.

It is a further object of the invention to provide a method for producing a dental restoration, that is more time effective than traditional casting methods, and which

can be easily implemented in facilities, where traditional casting methods are em- ployed.

SUMMARY OF THE INVENTION The object of the invention is obtained by a method according to claim 1.

By creating a computer model for the design of the restoration, instead of, as is the case in the Lost Wax technique, sculpting by hand in wax material, the labour cost is reduced. The tool cost for machining in wax will be significantly lower compared to machining in ceramics or metals. As the machining time per restoration will be lower when machining in wax compared to other materials, the capital cost per res- toration will be lower. Since the method according to the invention does not give rise to the need of a re-cycling system for any material presently used for restora- tions, there is also no need for an expensive filtering system for the capturing of contamination in the re-cycled restoration materials. There is no need for logistics and a security system for handling of large amount of gold.

DESCRIPTION OF THE FIGURES The invention will now be described closer with reference to the accompanying drawings, in which figures 1-4 are schematic perspective views of items used in stages in a method described here below. Fig. 5 is a schematic perspective view of an item used in a method according to an alternative embodiment of the present in- vention.

DETAILED DESCRIPTION In the following description the term preparation includes a prepared tooth, a pre- pared plurality of teeth or a prepared part of a tooth. To produce a restoration for a preparation, the first step is to create a replica of the preparation, here called a

preparation replica. This is done, in a known manner, by creating a bite impression of the preparation, using an appropriate material, usually silicone. The preparation replica, often made in plaster material, is formed by engaging the silicone with plaster material.

Fig. 1 illustrates a step in the method according to the present invention, whereby the surface of the plaster model, i. e. the preparation replica 1 is measured, using a measuring device 2. Data about the topology of the prepared tooth are stored in a data storage means 3, for example a computer 3. The measuring could also include surrounding teeth, whereby the data of the topology of these would be stored as well.

Referring to fig. 2, using a suitable software, such as a CAD/CAM system, a com- puter model 4 of the desired restoration is created, using data acquired in the step depicted in fig. 1. An operator may interact with the design part, e. g. the CAD part of the system in order to design the restoration on the scanned surface, whereby various types of software tools may be present in order to make the design job easier and quicker.

The computer model 4 is transferred to the manufacturing part of the software, e. g. the CAM part, where tool paths for the control of a machining equipment, such as a CNC-machine, are generated.

With reference to fig. 3, the tool paths are transferred to a CNC-machine 5 for ma- chining of a restoration replica 6 of the dental restoration. The CNC-machine could be of the type commonly used in dental laboratories for machining of a restoration out of ceramics or metal. Preferably, a material with a relatively low evaporation temperature is used for the restoration replica 6. The material could be wax of the type commonly used in dental laboratories, for the Lost Wax technique, known to the person skilled in the art. Alternatively plastic or any other suitable material could be used for the restoration replica 6.

With reference to fig. 4, the wax-restoration, i. e. the restoration replica 6 is embed- ded in a material that will form a mould 7, that will obtain an inner surface, essen- tially complementary to the outer surface of the restoration replica 6. The material for the mould could be one commonly used in dental laboratories.

Preferably, by elevating the temperature of the mould, the restoration replica mate- rial is removed by letting it evaporate. For this, an oven equipment commonly used for the Lost Wax technique can be utilised. Alternatively, the restoration replica material 6 can be removed by letting it melt.

Subsequently, the restoration is cast by letting a metal or other material in a liquid, or semi-liquid, form enter into the mould through a special channel 8. A technique that could be used during casting, in order to make the liquid or semi-liquid material reach all parts of the mould, is either rotation, generating centrifugal force, or the use of vacuum/pressure, in the same manner as is done in the Lost Wax technique.

Fig. 5 shows an alternative machining step in the method according to the invention.

A restoration replica blank, preferably in wax material, used to machine the restora- tion replica 6, can be provided with a protrusion 9. The protrusion can serve as a fastening device in the CNC-machine, whereby it is engaged with a holding device in the machine, to fix the blank during machining. Subsequently, during mould preparation, the protrusion 9 can be used as a channel plug 9. When the restoration replica material is removed, e. g. by evaporation, as described above, the channel plug 9, being made out of the same material as the rest of the restoration replica 6, will be removed too. Thereby a channel is provided in the mould, which can be used for entering the restoration material into the mould during casting.

In the method described, a CAD/CAM system is used in order to let the computers do the time consuming design work, while an efficient cast process is used in order to avoid the drawbacks with machining the restoration directly.