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Title:
A METHOD OF AND AN ARRANGEMENT FOR A DENTAL RESTORATION
Document Type and Number:
WIPO Patent Application WO/2002/076327
Kind Code:
A1
Abstract:
The present invention relates to a method of and an arrangement for generating a three-dimensional computer model of a tooth restoration (50) or a part thereof where the computer model is intended to be used in a manufacturing process of said restoration (50). The method comprises the steps of: storing information representing the external shape of a preparation (2), generating a computer model of the preparation (2), registering and/or storing at least one value of at least one restoration parameter (h1, h2, h3, h4, h5, g1, g2) defining the location of the restoration (50) in relation to the location of the preparation (2) in a configuration consisting of the restoration (50) and the preparation (2), where said preparation (2) comprises a margin (4), and using the at least one registered and/or stored value of the at least one restoration parameter (h1, h2, h3, h4, h5, g1, g2) for the generation of the computer model of the restoration (50).

Inventors:
Grebacken, Pontus (Lövstigen 33 Skellefteå, S-931 64, SE)
Sjölin, Roger (Tuvagårdsvägen 62 Skellefteå, S-931 49, SE)
Application Number:
PCT/SE2002/000520
Publication Date:
October 03, 2002
Filing Date:
March 19, 2002
Export Citation:
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Assignee:
DECIM AB (P.O. Box 753 Skellefteå, S-931 27, SE)
Grebacken, Pontus (Lövstigen 33 Skellefteå, S-931 64, SE)
Sjölin, Roger (Tuvagårdsvägen 62 Skellefteå, S-931 49, SE)
International Classes:
A61C13/00; G05B19/4099; A61C9/00; (IPC1-7): A61C13/00; G05B19/42
Domestic Patent References:
WO1996010371A11996-04-11
WO1998027890A11998-07-02
Attorney, Agent or Firm:
ALBIHNS STOCKHOLM AB (P.O. Box 5581 Stockholm, S-114 85, SE)
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Claims:
CLAIMS
1. A method of generating a threedimensional computer model of a tooth restora tion (50) or a part thereof where the computer model is intended to be used in a manufacturing process of said restoration (50), comprising the steps of : storing information representing the external shape of a preparation (2), generating a computer model of the preparation (2), said method further char acterised by the steps of : registering and/or storing at least one value of at least one restoration parameter (hl, h2, h3, h4, h5, gl, g2) defining the location of the restoration in relation to the location of the preparation in a configuration consisting of the restoration and the preparation, where said preparation comprises a margin (4), and using the at least one registered and/or stored value of the at least one restoration parameter (hl, h2, h3, h4, h5, gl, g2) for the generation of the computer model of the restoration (50).
2. A method according to claim 1, wherein the restoration parameters comprise pa rameters (hl, h2, h3, h4, h5, gl, g2) defining: the distance (gl) between the external surface of the preparation and the corre sponding surface of the restoration (50) at a first part of the configuration con sisting of the restoration (50) and the preparation (2), and the distance (g2) between the external surface of the preparation (2) and the cor responding surface of the restoration (50), at a second part of the configuration consisting of the restoration (50) and the preparation (2), and further the distance (hl) defining the beginning of the first part of the preparation in re lation to the margin (4) of the preparation (2), as seen from the margin (4), and the distance (h2) defining the end of the second part of the preparation (2) in re lation to the margin (4) of the preparation (2), as seen from the margin (4).
3. A method according to claim 1 or 2, wherein the restoration parameters comprise a parameter defining at least one dimension, e. g. an end radius, of at least one material removal tool (T) to be used in the manufacturing process for removing material, and/or the smallest radius of curvature of a concave surface that could be accomplished during the manufacturing process by a material removal tool (T), the method further comprising the steps of identifying at least one small radius cavity region in the computer model, com prising a cavity, recess or concave part, at which at least one radius of curvature is smaller than the smallest radius of curvature that could be accomplished dur ing the manufacturing process, and adjusting the surface at the at least one small radius cavity region so that the sur face has no radius of curvature being smaller than the smallest radius of curva ture that could be accomplished during the manufacturing process by the mate rial removal tool (T).
4. A method according to claim 3, wherein the step of adjusting the surface at the at least one small radius cavity region comprises moving the surface so that the extension of the computer model, at the at least one small radius cavity region, is not greater than the extension of the computer model, at the at least one small radius cavity region, before the execution the step of adjusting the surface at the at least one small radius cavity region.
5. A method according to any of the preceding claims, wherein the restoration pa rameters (hl, h2, h3, h4, h5, gl, g2) comprise at least one margin parameter (h3), which can assume a value, associated with a near margin design type, selected out of a predefined set of near margin design types, each indicating a separate type of design of the restoration at an end region thereof, by the margin.
6. A method according to any of the preceding claims, wherein the restoration pa rameters (hl, h2, h3, h4, h5, gl, g2) comprise an outer layer thickness parameter (h4), intended to assume a value indicating the desired thickness of an outer layer (70), intended to be placed outside the restoration (50), further comprising the steps of registering and/or storing information about the external shape of at least part of at least one neighbouring tooth (12,14) and/or opposite tooth (16,18,20), generating at least part of the external surface of the outer layer (70), using at least some of the information, registered and/or stored in the step of registering and/or storing information about the external shape of at least part of at least one neighbouring tooth and/or opposite tooth, registering and/or storing a value for the outer layer thickness parameter (h4) and generating at least a part of the external surface of the restoration (50), using the value for the outer layer thickness parameter (h4), and the external surface of the outer layer (70).
7. A method according to any of the preceding claims, wherein the restoration pa rameters (hl, h2, h3, h4, h5, gl, g2) comprise a restoration thickness parameter (h5), intended to assume a value indicating the desired thickness of at least part of the restoration (50).
8. A method according to any of the preceding claims, wherein the restoration (50) is a reduced crown.
9. An arrangement for generating a threedimensional computer model of a tooth restoration (50) or a part thereof where the computer model is intended to be used in a manufacturing process of said restoration (50), comprising: means (42) for storing information representing the external shape of a prepa ration (2), means (44) for generating a computer model of the preparation (2), the arrangement further characterised by: means (42) for registering and/or storing at least one value of at least one resto ration parameter (hl, h2, h3, h4, h5, gl, g2) defining the location of the restora tion (50) in relation to the location of the preparation (2) in a configuration con sisting of the restoration (50) and the preparation (2), where said preparation (2) comprises a margin (4), and means (46) for using the at least one registered and/or stored value of the at least one restoration parameter (hl, h2, h3, h4, h5, gl, g2) for the generation of the computer model of the restoration (50).
10. An arrangement according to claim 9, wherein the restoration parameters com prise parameters (hl, h2, h3, h4, h5, gl, g2) defining: the distance (gl) between the external surface of the preparation (2) and the cor responding surface of the restoration (50) at a first part of the configuration con sisting of the restoration (50) and the preparation (2), and the distance (g2) between the external surface of the preparation (2) and the cor responding surface of the restoration (50), at a second part of the configuration consisting of the restoration (50) and the preparation (2), and further the distance (hl) defining the beginning of the first part of the preparation (2) in relation to the margin (4) of the preparation, as seen from the margin (4), and the distance (h2) defining the end of the second part of the preparation (2) in relation to the margin (4) of the preparation, as seen from the margin (4).
11. An arrangement according to claim 9 or 10, wherein the restoration parameters comprise a parameter defining at least one dimension, e. g. an end radius, of at least one material removal tool (T) to be used in the manufacturing process for removing material, and/or the smallest radius of curvature of a concave surface that could be accomplished during the manufacturing process by a material re moval tool (T), the arrangement further comprising means (44) for identifying at least one small radius cavity region in the computer model, comprising a cavity, recess or concave part, at which at least one radius of curvature is smaller than the smallest radius of curvature that could be accom plished during the manufacturing process, and means (46) for adjusting the surface at the at least one small radius cavity region so that the surface has no radius of curvature being smaller than the smallest ra dius of curvature that could be accomplished during the manufacturing process by the material removal tool (T).
12. An arrangement according to claim 11, wherein the means (46) for adjusting the surface at the at least one small radius cavity region comprises means (46) for moving the surface so that the extension of the computer model, at the at least one small radius cavity region, is not greater than the extension of the computer model, at the at least one small radius cavity region, before the execution the step of adjusting the surface at the at least one small radius cavity region.
13. An arrangement according to any of claims 9 to 12, wherein the restoration pa rameters (hl, h2, h3, h4, h5, gl, g2) comprise at least one margin parameter (h3), which can assume a value, associated with a near margin design type, selected out of a predefined set of near margin design types, each indicating a separate type of design of the restoration at an end region thereof, by the margin.
14. An arrangement to any of claims 9 to 13, wherein the restoration parameters (hl, h2, h3, h4, h5, gl, g2) comprise an outer layer thickness parameter (h4), in tended to assume a value indicating the desired thickness of an outer layer (70), intended to be placed outside the restoration (50), further comprising means (42) for registering and/or storing information about the external shape of at least part of at least one neighbouring tooth (12,14) and/or opposite tooth (16,18,20), means (44) for generating at least part of the external surface of the outer layer (70), using at least some of the information, registered and/or stored in the step of registering and/or storing information about the external shape of at least part of at least one neighbouring tooth and/or opposite tooth, means (42) for registering and/or storing a value for the outer layer thickness pa rameter (h4) and means (44) for generating at least a part of the external surface of the restora tion (50), using the value for the outer layer thickness parameter (h4), and the external surface of the outer layer.
15. An arrangement according to any of claims 9 to 14, wherein the restoration pa rameters (hui, h2, h3, h4, h5, gl, g2) comprise a restoration thickness parameter (h5), intended to assume a value indicating the desired thickness of at least part of the restoration (50).
16. An arrangement according to any of claims 9 to 15, wherein the restoration (50) is a reduced crown.
Description:
A METHOD OF AND AN ARRANGEMENT FOR A DENTAL RESTORATION TECHNICAL FIELD OF THE INVENTION The present invention relates to a method of and an arrangement for generating a three-dimensional computer model of a tooth restoration or a part thereof where the computer model is intended to be used in a manufacturing process of said restora- tion.

DESCRIPTION OF RELATED ART 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 Sjolin, Sundh, Bergman:"The Decim System for Production of Dental restora- tions", International Journal of computerised Dentistry 1999: 3. [1] Methods for creating 3D-models of tooth restorations are also known, for example 'The patent application WO 98/27890 (DDS): Method for producing a dental prosthesis Swedish patent 501 411 (Procera) :"Forfarande och anordning vid tredimension- ell kropp anvandbar i manniskokroppen".

The methods used in several dental CAD/CAM systems available today are based on"reverse engineering"technology, which means that a prepared tooth (or replica of it), possibly together with the dental environment in terms of neighboring and

opposite teeth, is scanned. This information is used as base when a user via interac- tive work at a terminal designs the restoration, and hence creates the mathematical model of the restoration in the computer. An alternative approach to solving the de- sign problem is to sculpture the restoration in wax, clay or similar material, scan both the prepared tooth and the sculptured restoration, correlating the different scans to each other and from this information create a 3D-model of the restoration.

In known technology the internal part of the restoration can be defined by defining the scanned surface of the prepared tooth, or a simple offset of said surface, as the internal surface of the restoration.

There are several disadvantages with the design methods described above: Firstly, both of them require quite a lot of operator time. Secondly, in the wax-up method, there is still dependency upon the traditional labour-intensive craft work, and thirdly in the latter method, this work has been replaced with a new kind of labour intensive work in front of the computer, designing the restoration by click and drag operations with the mouse. Fourthly, the models created in today's methods are not always pos- sible to manufacture in the subsequent step. In most dental CAD/CAM systems, a CNC machine is used for manufacturing. Such a machine can use tools of various types, dimension and geometrical shapes. Typically, tools with bigger diameter are used in the beginning of the machining sequence in order to take away a lot of mate- rial, while tools with smaller diameter are used for the finishing. However, even if very small tools are used, there can always be pockets in the restoration in which the tool can not enter. In these cases material will be left, and more manual after treat- ment will be required in order to make the restoration fit.

SUMMARY OF THE INVENTION The object of the invention is to bring about a method and an arrangement relating to creating a three-dimensional computer model of a tooth restoration or a part

thereof decreasing the number of and intensity of manual work steps needed when creating a tooth restoration.

This is achieved by use of a method of generating a three-dimensional computer model of a tooth restoration or a part thereof where the computer model is intended to be used in a manufacturing process of said restoration, comprising the steps of storing information representing the external shape of a preparation, generating a computer model of the preparation, and registering and/or storing at least one value of at least one restoration parameter defining the location of the restoration in rela- tion to the location of the preparation in a configuration consisting of the restoration and the preparation, where said preparation comprises a margin.

The method and arrangement according to the invention has the following advan- tages: It is possible to register, store and use restoration parameters which reduce the required operator time. Further, these parameters are quickly entered by the opera- tor. Moreover, the steps according to the invention also require less training of a person designing the restoration. Also, the restoration can be designed in such a way that the veneering porcelain of a crown has a more or less uniform thickness, which is advantageous with respect to mechanical strength.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the appended drawings, wherein: - fig. 1 shows a schematic side-view of a plaster model of a prepared tooth, - fig. 2 shows a schematic side-view of a model of a jaw, - fig. 3 shows a schematic side-view of a model of the prepared tooth, - fig. 4 shows a schematic side-view of a model of the prepared tooth, and sche- matically a computer comprising a processor and a memory,

- fig. 5 shows a schematic, partly sectioned side-view of a model of the prepared tooth and a restoration, - fig. 6 shows a perspective view of a model of the prepared tooth, - fig. 7 is an illustration for the explanation of an algorithm according to a pre- ferred embodiment of the present invention, - fig. 8 shows a part of a restoration and a part of a manufacturing tool, and - fig. 9 also shows a part of a restoration and a part of a manufacturing tool.

DETAILED DESCRIPTION OF EMBODIMENTS On the drawings, Fig. 1 shows schematically the occlusal part of a plaster model 2 of a prepared tooth, i. e. a tooth preparation. The plaster model 2 of the prepared tooth can be obtained by making an imprint of the prepared tooth at the dentist, in a manner known in the art. Besides plaster any other suitable material can be used.

The boundary 4 between the prepared part 6 at the top of the tooth and the unpre- pared part 8 is commonly known as the margin 4.

As shown in Fig. 2, it is also possible to use a model of a full jaw 10 comprising a model of the prepared tooth 2 and models of the surrounding teeth 12,14,16,18,20 in the method of generating a three-dimensional computer model of a tooth restora- tion 50 or a part thereof where the computer model is intended to be used in a manu- facturing process of said restoration 50. The models of the different teeth are sepa- rate from each other and releasably fixed to the model of the jaw 10. In this way the model of the prepared tooth 2 and the models of the surrounding teeth 12,14,16, 18, 20 can be removed and reinserted in sockets (not shown) in the model of the full jaw 10 without changing their relative positions in space with reference to the model of the jaw 10 when placed in their respective sockets.

As shown in Fig. 3, the plaster model 2 is trimmed below the margin 4 before the creation of the three-dimensional computer model of the preparation, so that an un-

dercut region 30 is created there. In this way the undercut region 30 is hidden when the plaster model 2 is viewed from above.

Thereafter, to collect data about the topology of the preparation 2, the plaster model 2 is scanned, in a manner known in the art, and information representing the exter- nal shape of a preparation 2 can be stored in a memory 42 in a computer 44 where- upon a computer model 40 of the preparation 2 comprising the part of the prepara- tion 2 on the occlusal side of the margin 4, as shown in fig. 4, can be generated by a processor 46 in the computer 44. The computer model 40 is built up of dental lines (not shown) that can be manipulated individually to create the desired shape in known manner. In the computer model a reference axis R is determined, the direc- tion of which corresponds to the insertion axis, i. e. the insertion direction of the restoration to be manufactured, in relation to the preparation.

As shown in Fig. 5, the internal surface of a restoration 50, preferably intended as a structure for a crown, is generated in the computer 44 by the processor 46 by off- setting the surface of the computer model 40 of the preparation 2, whereby the dis- tance of the off-set is determined by entering values of gap parameters, as will be described further as well as the steps of registering and/or storing in the memory 42 at least one value of at least one restoration parameter hl, h2, h3, h4, h5, gl, g2 de- fining the location of the restoration 50 in relation to the location of the preparation 2 in a configuration consisting of the restoration 50 and the preparation 2, where said preparation 2 comprises a margin 4, and the use of the at least one registered and/or stored value of the at least one restoration parameter hl, h2, h3, h4, h5, gl, g2 for the generation in the processor 46 of the computer model of the restoration 50. The space 60 between the restoration 50 and the preparation 2 is intended to be filled with cement for fixing the restoration 50 to the preparation 2.

A first part PI of the preparation 2 is defined as follows: The jaw end of the outer surface of the first part P 1 is delimited by a first line L1, which is located at a dis-

tance hl from the margin 4. The value of hl is stored in the memory 42 in the com- puter 44, or entered by an operator, hl preferably being around 1200 pm. The occlu- sal end of the first part P1 equals the occlusal end surface of the preparation.

A second part P2 of the preparation is defined as follows: The jaw end of the outer surface of the second part P2 is delimited by the margin 4. The occlusal end of the surface of the second part P2 is delimited by a second line L2, which is located at a distance h2 from the margin 4. The value of h2 is stored in the memory 42 in the computer 44, or entered by an operator, h2 preferably being around 800 pm.

A third part P3 of the preparation is defined as follows: The jaw end of the outer surface of the third part P3 is delimited by the second line L2. The occlusal end of the surface of the third part P3 is delimited by the first line LI.

For the first part P 1 the inner surface 52 of the restoration 50 is generated by the processor 46 by defining a first gap gl between the preparation 2 and the restoration 50, the first gap gl being constant around the first part P1. Preferably the first gap gl is approximately 60 um. Preferably, for the second part P2 the inner surface of the restoration 50 is generated by the processor 46 by making it to correspond to the outer surface of the preparation 2. Alternatively, the same algorithm is used as for the inner restoration surface in the first part PI as described above. When the algo- rithm is used for the second part P2, a value for a second gap g2 is either stored in the memory 42 in the computer 44 or entered by an operator of the computer 44. For the third part P3 the inner surface of the restoration 50 is generated by the processor 46 using the first gap gl at the first line LI and the second gap g2 at the second line L2 so that the distance between the preparation 2 and the restoration 50 decreases linearly from the first line LI to the line second line L2. Alternatively, a non-linear change of said distance can be arranged between the first line LI and the line second line L2. Thus, the restoration parameters comprise parameters hl, h2, gl, g2 defin- ing the distance gl between the external surface of the preparation and the corre-

sponding surface of the restoration at a first part of the configuration consisting of the restoration 50 and the preparation 2, the distance g2 between the external surface of the preparation 2 and the corresponding surface of the restoration 50, at a second part of the configuration consisting of the restoration 50 and the preparation 2, the distance hl defining the beginning of the first part of the preparation 50 in relation to the margin 4 of the preparation, as seen from the margin 4, and the distance h2 defining the end of the second part of the preparation 2 in relation to the margin 4 of the preparation, as seen from the margin 4.

Fig. 6 illustrates a preferred algorithm for generating the first line LI. An arbitrary point PM on the margin 4 is the centre of a sphere S, with the radius hl. The loca- tion of a point PL on the first line LI is determined as the position of the intersec- tion between the sphere S, the surface of the preparation 2 and a plane P, coinciding with the reference axis R and the point PM on the margin 4. For a finite number of points on the margin 4, the location of the same number of points defining the first line LI is determined. Line L2 can be generated in a corresponding way.

Referring to fig. 7, preferably, for an arbitrary point PP on the surface of the prepa- ration 2, an off-set axis O is determined by the processor 46, the direction of which is perpendicular to the surface of the preparation, and which intersects with the point PP. The location of a point PR of the inner surface 52 on the restoration 50 is deter- mined by the processor 46 to be on the off-set axis O, at a distance, corresponding to the first gap gl. The second gap g2 can be generated in a corresponding way.

Fig. 8 illustrates a problem that can arise during manufacturing of the restoration 50.

Tools for manufacturing of the restoration 50 may comprise cylindrical tools with a spherical tool end having a radius equalling half the diameter of the tool, or cylin- drical tools with a tool end having rounded edges with a radius less than half the di- ameter of the tool. Even other types of tools are possible to use. If a cavity 54 in the model of the restoration 50 presents a radius that is smaller than an end radius of a

tool T, used during manufacturing to mill the restoration 50, the tool T will not be able to reach into the cavity and remove a sufficient amount of material to accom- plish the desired surface of the restoration 5. According to the invention as shown in Fig. 5, the cement space is increased in this case in order to make it possible for the smallest tool available to remove the required amount of material. Preferably, a di- mension of the tool T, or the smallest radius of curvature of a concave surface that could be accomplished during the manufacturing process, is registered or stored in the memory 42 in the computer 44. Small radius cavity regions 54 are identified in the computer model of the restoration 50 by the processor 46, the small radius cavity regions 54 comprising a cavity, recess or concave part, at which radiuses of curva- ture are smaller than the smallest radius of curvature that could be accomplished during the manufacturing process. Referring to fig. 8, preferably in the computer model of the restoration 50 the surfaces at the small radius cavity regions 52 are re- generated by the processor 46, so that the surface obtains a radius of curvature of essentially the same as, or slightly bigger than, the tool radius. The surface regener- ated is fully located outside the original surface, which gives a bigger cement space locally. As these changes of the surface most often are made in the top of the resto- rations, this extra space is clinically acceptable. In order to avoid this kind of extra cement space near the margin, a function is provided which makes the processor 46 to check the distance to the margin and prohibits the surface modifications below a distance h3 from the margin. The parameter h3 is configurable. Thus, the restoration parameters comprise a parameter defining at least one dimension, e. g. an end radius, of at least one material removal tool T to be used in the manufacturing process for removing material, and/or the smallest radius of curvature of a concave surface that could be accomplished during the manufacturing process by a material removal tool T.

At least one small radius cavity region in the computer model, comprising a cavity, recess or concave part, is identified by the processor 46 at which region at least one radius of curvature is smaller than the smallest radius of curvature that could be ac-

complished during the manufacturing process, and the surface is adjusted at the at least one small radius cavity region so that the surface has no radius of curvature being smaller than the smallest radius of curvature that could be accomplished dur- ing the manufacturing process by the material removal tool T.

The step of adjusting the surface at the at least one small radius cavity region com- prises moving the surface so that the extension of the computer model, at the at least one small radius cavity region, is not greater than the extension of the computer model, at the at least one small radius cavity region, before the execution the step of adjusting the surface at the at least one small radius cavity region.

A simple coping restoration is characterised by the uniform thickness of the com- plete restoration. This form is relatively simple to accomplish since it is an offset of the preparation surface. The reduction of the crown is uniform and will give the dental technicians an excellent base for the porcelain work. When a tooth is in bad condition, it is sometimes necessary to remove quite a lot of the tooth. When a sim- ple coping is made for this type of preparation, there will be a very thick and uneven layer of porcelain which builds up the crown. As the porcelain is much weaker than the coping, or core, this will affect the strength of the final restoration. It may also be aesthetically unattractive. In some cases when modern leucit porcelain is used it is not even possible to use the simple coping because it locally will require a thick- ness of the porcelain layer outside of the manufacturer specification. It is also time consuming to build a crown which requires a lot of porcelain as the different layers may have to be burned after each other in a furnace.

Thus preferably, the outer surface of the restoration is generated by the processor 46 using the following method of generating a three-dimensional computer model of a tooth restoration 50 or a part thereof where the computer model is intended to be used in a manufacturing process of said restoration 50, comprising the steps of :

Firstly, preferably but optionally, information about the external shape of at least part of at least one neighbouring tooth or teeth 12,14, or part thereof, and opposite tooth or teeth 16,18,20, respectively, is registered and/or stored in a memory 42 in the computer 44.

Preferably but optionally, in a first scan set the complete jaw 10 with the prepared tooth 2 and the neighbouring teeth 12,14,16,18,20 is used. This scan set is used to give the operator information about the size, shape and position of the teeth sur- rounding the prepared tooth. This set can be displayed on the screen together with the preparation scan to give information and then be turned off as it otherwise may interfere with the design of the restoration. The holder is taken out from the scanner and the neighbouring teeth removed from the sockets.

The holder is inserted into the scanner and the prepared tooth is scanned thus storing information in the memory 42 regarding the topology of the prepared tooth. By us- ing an absolute reference system for the holder it can be reinserted without losing orientation.

An interesting area in the surface of the tooth reconstructed for preparation is se- lected by the user and the area is recalculated with higher resolution in order to re- duce the amount of handled data and to improve the speed when manipulating the surface on the screen.

For each tooth a template built up of dental lines (not shown) and resembling a complete crown is stored in the memory 42. A template corresponding to the tooth is loaded and automatically adjusted to attach to he margin line.

The restoration parameters comprise at least one margin parameter h3, which can assume a value, associated with a near margin design type, selected out of a prede- fined set of near margin design types, each indicating a separate type of design of the restoration at an end region thereof, by the margin.

In the following, some pre-defined margin designs are described: In a knife-edge type margin, the coping is very thin near the margin and no coping material will be visible.

In a cut-back type margin, the coping does not extend all the way down to the mar- gin in order to give space for porcelain.

In a collar type margin, a band of the coping material is visible all the way around the crown, and In an extended collar type margin, the band of coping material has a bigger exten- sion.

Often knife-edge or cut-back type margins are used buccally of the tooth to make room for porcelain, and collar or extended collar type margins are used lingually to make the crown stronger.

The template may now be manipulated by the user by using draggers. The shape and size can be adjusted to correspond to the neighbouring teeth and an angle in the horizontal plane can be adjusted to align the tooth with the surrounding teeth.

Secondly, a value h4 indicating the desired thickness of an outer layer, intended to be placed outside the restoration 50, is registered or stored in the memory 42.

Thirdly, at least part of the external surface of the outer layer is generated by the computer operator using at least some of the information, registered and ! or stored in the step of registering and/or storing information about the external shape of at least part of at least one neighbouring tooth and/or opposite tooth. The surface of the crown is calculated using the dental lines and the crown is visualised on the screen.

A value for the outer layer thickness parameter h4 is registered and/or stored in the memory 42.

Fourthly, using the value of the desired thickness, i. e. the outer layer thickness pa- rameter h4 and the external surface of the outer layer, at least part of the external surface of the restoration 50 is generated by the processor 46 with an algorithm cor- responding to the one described above with reference to fig. 7.

The internal parameters are chosen together with a reduction distance. A new exter- nal surface is calculated by the processor 46 using the reduction distance and the crown surface. The reduction has a minimal thickness, e. g. 0,5 mm, which is main- tained even if the reduction of the crown has resulted in a smaller thickness.

The reduced crown will solve the problem with a too thick layer of porcelain as the external shape is a uniform offset of the final crown.

Alternatively, a value of the restoration thickness h5 is stored or registered in the memory 42, and using the information about the internal surface of the restoration 5, the external surface is generated by the processor 46 with an algorithm correspond- ing to the one described above with reference to fig. 6.

The restoration parameters hl, h2, h3, h4, h5, gl, g2 comprise an outer layer thick- ness parameter h4, intended to assume a value indicating the desired thickness of an outer layer, intended to be placed outside the restoration.

The restoration parameters hl, h2, h3, h4, h5, gl, g2 comprise a restoration thick- ness parameter h5, intended to assume a value indicating the desired thickness of at least part of the restoration 50.

What has been said above about the restoration 50 can apply to other types of dental restoration as well, for example crowns, bridges, pillars, veneers, implant compo- nents, etc.