WO1994027523A1

WO1994027523A1 - Procedure and arrangement for dental restoration

Info

Publication number: WO1994027523A1
Application number: PCT/SE1994/000478
Authority: WO
Inventors: Fredrik ÅSTRÖM; Erik ODÉN
Original assignee: Tritech Mekatronik Ab
Priority date: 1993-05-21
Filing date: 1994-05-20
Publication date: 1994-12-08
Also published as: SE9301749D0; SE501820C2; SE9301749L

Abstract

The present invention relates to a method and to an arrangement for preparing a tooth restoration model for the restoration of a full tooth or part of a tooth. The arrangement functions to determine measurement values, by measuring a first surface (31) of an object (2) to be measured. The object (2) may be a tooth, the copy of a tooth, an impression of a tooth, or a prepared part of a tooth. The arrangement also functions to move the object so as to enable the object to be scanned at a plurality of points thereon, for which points a distance is calculated by optical triangulation. By compilation with data relating to movement of the object there are obtained measurement values which constitute a data description of the outer surface (31, 51) of the object or a part of this outer surface. The arrangement includes a computer unit which processes the measurement values and generates the aforesaid model therefrom in the form of a data description of its surfaces.

Description

PROCEDURE AND ARRANGEMENT FOR DENTAL RESTORATION.

The present invention relates to a method of the kind defined in the preamble of Claim 1 and also to an arrangement for putting the method into effect.

In order to be able to create a tooth restoration model, either a full tooth restoration model or a part tooth restora¬ tion model, it is necessary first to scan a tooth surface and then process the values obtained numerically.

Several dental CAD/CAM systems are known to the art, all of which include functional units for scanning and numerical processing. Tooth surfaces can be scanned either with a mechanical scanner or with an optical scanner. The mechanical scanners have limited accuracy and, above all else, it is impossible to determine the correct geometry from the scanned values, since the point of mechanical contact with the tooth surface is indeterminable. According to one known mechanical method, a position sensor includes a tip-mounted ball which is caused to run over the surface of the object to be mea¬ sured. Since the sensor registers the position of the centre of the ball, it is impossible to measure correctly distinct changes in outline, because the radius of the ball is a limiting factor in registering distinct changes in surface topography.

Swedish Patent Application SE-89 027 48-6 describes a method for the three-dimensional measuring of teeth in the oral cavity. The method described requires a number of light paragons to be stored in the measuring arrangement, and that these paragons are projected onto the tooth. The method described requires the resulting light images to be assembled and analyzed.

It is also known to photograph the object to be measured. In order to succeed in this regard, high demands are placed on the geometry of the surface, which must be prepared prior to scanning. According to one known scanning method, it is necessary to prepare the tooth to be scanned. The surfaces of the tooth must be planar and relatively vertical, since parts of the walls may be hidden if the surfaces are not properly prepared. The tooth is then covered with a powder in order to increase the contrast, whereafter a camera is swept over the tooth and the result is presented on a monitor. Since this will only present a two-dimensional image, it is necessary for an operator to interpret the image information. Furthermore, this known method is only able to manage inlays.

One problem with known techniques is that the surface of a tooth or a tooth impression cannot be measured without modifying the original geometry.

Another problem encountered with known techniques is that scanning cannot be effected with sufficient accuracy to enable a geometrical model which after manufacture will fit the original object to be measured, in this case the tooth, on the basis of the values obtained.

A mechanical measuring system which includes a measuring arm equipped with a ball which is intended to run over the surface is sensitive to, e.g., changes in temperature but are liable to affect the mechanical components. It is therefore necessary to calibrate the system repeatedly, resulting in extra work for the operator and the wastage of effective time.

Another serious problem encountered when scanning tooth surfaces is to find the preparation border. When practicing known methods, it is necessary to introduce this border manually after scanning the surface of the object to be measured. OBJECTS OF THE PRESENT INVENTION

The main object of the present invention is to provide a method which will facilitate the production of a tooth restoration.

Another object of the present invention is to provide a method for producing a set of measured values which describe at least one limitation surface for the restoration of a tooth.

Another object of the present invention is to achieve contact- less registration of a surface and with the aid of the registered measured values describe a surface of a tooth or a tooth model.

Yet another object of the present invention is to provide a method for analyzing the measurement values that are read-off and therewith obtain a set of measured values that describe the limitation surfaces of a tooth restoration or the restora- tion of part of a tooth.

A further object of the invention is to provide a method for creating a model of a cap, a facing or a bridge on the basis of the measured values.

Still a further object of the present invention is to provide a method which will enable the position of the preparation border to be established automatically.

Another object of the present invention is to create a three- dimensional model of a geometrical body, such as a tooth restoration or part of a tooth restoration.

Yet another object of the invention is to provide a method for creating a model of such nature that part of the surface of the model will coincide with or fit the scanned surface. Still another object of the present invention is to provide an arrangement by means of which the inventive method can be put into effect.

Still another object of the present invention is to provide a method for measuring automatically the shape of a prepared tooth which is to be provided with a restoration body and a method for determining automatically the shape of the prospec¬ tive restoration body.

These objects and also other objects made evident in the following description are achieved in accordance with the invention with the method defined in the characterizing clause of Claim 1. Further features and further developments of the inventive method and an arrangement for carrying out the method are set forth in the remaining Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate an understanding of the present invention, the invention will now be described in more detail with reference to an exemplifying embodiment thereof and also with reference to the accompanying drawings, in which Fig. 1 illustrates schematically an arrangement for carrying out the inventive method;

Fig. 2 shows the arrangement in Fig. 1 from one side; Fig. 3 illustrates the principle of optical triangulation; Fig. 4 shows a tooth curve seen from an occlusal aspect, i;e; from the chewing surface; Fig. 5A illustrates a base part 28 with a tooth crown 29, wherein the base part 28 may be an object 2 to be measured which the surface 31 is scanned by means of an inventive method, and wherein tooth restoration may consist in a tooth crown, for instance; Fig. 5B is a magnification of an encircled area 510 taken from

Fig. 5A;

Fig. 6A illustrates a facing 38 of the facial surface of an incisive 24;

Fig. 6B is a magnification of an encircled area taken from

Fig. 6A;

Fig. 7 shows a bridge 39 with a hanging joint 40; wherein the bridge abutments 41 and 42 are in this case prepared for a crown; wherein a bridge abutment may equally as well be a facing or an inlay; and wherein a bridge may consist in several abutments and several hanging joints;

Figs. 8.1, 8.2 and 8.3 show a premolar 26 prepared for an inlay; wherein the prepared cavity may be filled with amalgam or dental composite, for instance; and wherein the preparation border 30 may indicate the border between the original tooth surface and the drilled cavity; and

Fig. 9 illustrates an object 2 to be measured having a surface which is to be scanned.

The present invention relates to a method and to an arrange¬ ment for creating a tooth restoration model, this model being created on the basis of a scanned surface of a prepared tooth or a tooth impression.

The invention also relates to an arrangement and to a method for contactless scanning or measuring of the surface geometry of an object to be measured. The object 2 to be measured may be spot-illuminated with a diode laser 7, the reflected light can be collected by a lens 9 and impinge on a light-sensitive element 10. The shape of the measured object can be determined to a high degree of accuracy, when using the principle of optical triangulation.

According to the inventive method, the measured values may be used in a mathematical or numerical analysis in a computer 12. According to the method, there can be generated a set of coordinates which describe limitation surfaces of representa- tives of a tooth restoration or the restoration of part of a tooth, such as a cap or coping, stored in the computer. This representation is also named model. Thus, it is possible to generate a model on the basis of the surface geometry of the scanned object, and this model can be analyzed and changed in accordance with the preferences of the operator, with regard to magnification factors, preparation borders and so on. The resultant model contains information that can be used to manufacture a tooth restoration. For instance, in accordance with the invention, the arrangement may include a preparation tool, e.g. a computer controlled milling device, which manufactures a tooth restoration with the aid of information from the model, by preparing a piece of starting material.

A DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates schematically a system by means of which the inventive method can be carried out. To this end, there may be used a rotational table 1, an object 2 to be measured, a test bench 3 to which equipment is attached, an instrument holder 4 for securing lenses and laser/sensor, holder fixtures 5 having grooves in which instrument holders are secured, a plane convex lens 6 for focusing the laser beam, a diode laser with built-in collimator 7 which emits a laser beam 8, an achromatic lens 9 for collecting reflected light, a light- sensitive sensor 10, driving electronics and electronics for analyzing the measurement signals 11, a computer 12 in which measurement values are stored and in which a numerical analysis is made, together with an aperture 13 which makes the laser beam smaller and round. The angle between the laser beam and the axis through the centre and the achromatic lens is referenced .

Fig. 2 illustrates the system shown in Fig. 1 from one side. A driving system 17 which includes a motor drives a screw 15 on which the rotational table or plate 1 is mounted, this screw causing the table 1 to rotate at a speed commensurate with the speed of the motor. The screw is seated in a nut 16, which is fixed, and the table or plate will rise as the screw turns in the nut. The laser beam 8 defines an angle β with the horizontal plane 50, wherein the horizontal plane is a plane whose normal vector is parallel with the rotational axis of the object 2 to be measured.

The object 2 may be a tooth, a tooth impression or a tooth copy. The tooth copy may be a cast of the tooth and may be produced from a readily moulded material, such as a plastic material for instance. The preparation border 30 marks the border of an area 51 which corresponds to a tooth area that will not be covered by the restoration. It is therefore unnecessary to scan the geometry of the surface 41 in manufac¬ turing a geometrical model of the restoration.

The object 2 is held fixed on the table 1. As mentioned in the aforegoing, the drive system 17 will cause the object to be measured to rotate and also to rise. Since the screw 15 meshes with the threads of the aforesaid nut, there is obtained a distinct relationship between the speed at which the screw rotates and the extent to which the object rises. Alternative- ly, the object to be measured may be fixed and the laser- sensor-assembly caused to rotate around the object. Thus, essentially the whole of the surface of the object 2 can be scanned. There may be mounted adjacent the rotational table an initial sensor Gl which is able to deliver a measurement value that corresponds to the angle of rotation φ around the rotational axis of the object to be measured to a computer 12. Information concerning the surface of the object in an initial dimension is obtained by repeatedly storing measurement values corresponding to the angle of rotation φ (Fig. 1) .

The arrangement may also include a second sensor G2 capable of delivering information from which a second dimension of the measured point on the object to be measured can be calculated. The second sensor G2 may, for instance, include a linear position sensor which measures movement of the screw in a direction parallel with the rotational axis of the table. Alternatively, the measurement values of the second dimension may be calculated by the computer with the aid of such parameters as the rotational angle φ of the object to be measured and the rising or climbing angle of the screw.

According to a further embodiment of the invention, the light source can be moved by changing the angle β , as illustrated in Fig. 2. The angle β can be caused to move within the angular range of 0° to 180° while measuring the measurement points on the object to be measured. The angle β is registered and the measurement value is stored in the computer 12 at the same time. In this way, there is obtained by the scan an outline of the object to be measured. Measurement information which includes information about the three-dimensional structure of the object to be measured is obtained by scanning several outlines in combination with turning the object to be measured.

The light beam 8, SI, may be directed in towards the rotation- al axis of the object to be measured. The object can be caused to rotate and scanning of the object can commence. As the object to be measured rotates and the position coordinates of the measurement points in the first and the second dimension are established in the aforedescribed manner, it is possible to determine the coordinates of the measurement points in a third dimension, by optical triangulation, as described below.

A light source illuminates the object 2 at a point which defines an angle β (Fig. 2) with the horizontal plane 50. This angle β may be an angle between 0° and 180°, but is preferably between 30° and 70°. In the case of the first embodiment, the angle is 45°. The light source may be a laser 7 which delivers collimated light. The light beam, or light ray, 8, has a small diameter and a relatively low divergence so as to contribute to a large depth of field. It is necessary for the light beam to be narrow, because it is important that the light beam will project a small light spot 210 on the object to be measured, since the measurement value is the mean value of the surface illuminated by the light spot. More precise values as to the appearance or configuration of the true surface of the object to be measured are obtained when the light spot can be made small. Optimally, the light beam will have one and the same diameter over the whole of the measuring range and divergence must be kept low in attempting to attain this optimum. The diameter of the light beam can be reduced, by screening-off the light beam with the aid of an aperture 13 and/or focusing the light with the aid of a lens 6. The diameter of the light beam must be so small that the light spot that impinges on the tooth or the tooth impression 2 will also be small. In view of the fact that the surface of the tooth or of the tooth impression is very uneven in proportion to the wavelength of the light, the light will be reflected and diffused. The light reflected can be collected by an achromatic lens 9 at a given angle α (Fig. 1) from the optical axis of the laser light. The angle α may be varied within the angular range of 0-90° for instance, although preferably within the range of 10-40°. According to one embodiment, this angle is 30°. The lens 9 may also be of a kind other than an achromatic lens. The reflected light impinges on a light-sensitive element at a point 14. This element is mounted on a sensor 10 whose outward signal is proportional to where the maximum intensity of the light spot impinges on the element, in other words when the position of the light spot changes, the output signal of the sensor will also change in a corresponding manner. When applying the principle of optical triangulation (Fig. 3) , there is obtained a functional relationship between the position of the measure- ment point 210 in a radial direction of the object and the maximum intensity of the light on the sensor element. The laser light and the sensor may be modulated by one and the same frequency, so as to reduce the influence of ambient light. Optical triangulation will provide information concern- ing deviations of the distance h in the direction of the third dimension. An absolute value of the distance from the rota¬ tional axis of the object to its surface can be determined by calibrating the measuring arrangement, using the values obtained by scanning an object of known surface geometry.

According to another embodiment of the invention, the output signal of the sensor 10 may also indicate the intensity of the reflected light, as described in more detail below.

Fig. 3 illustrates the principle of optical triangulation. A light beam SI from the light source S impinges on the surface B, which is uneven. The illuminated point b is reflected in the point b' in the plane D through the lens L in the angle φ from the optical axis of the light source. Assume that the surface B is moved through the distance or path h. The illuminated point c will now be reflected in C at a distance a from b' in the plane D, said distance a being determined by the invertible function: a = F(h) .

The aforesaid surface B may be a surface of the object 2 and the plane D may be a surface on the sensor 10.

The sensor output signal, which will vary according to the point on which the light spot impinges on the sensor element, is processed and measured in the electronic unit 11 for storage in a storage area in a computer 12. The object 2 is rotated by the motor. Points on the object 2 are measured at a determined frequency. The entire surface of the object can be scanned and determined as the object rotates and rises slightly with each revolution.

Fig. 5A illustrates a base part 28 having a restoration in the form of a tooth crown 29. The base part 28 may be the copy of a prepared tooth whose shape or configuration corresponds to the shape of the base part. Alternatively, the base part 28 may be the copy of an artificial pillar, for instance when the tooth is root-filled. The restoration/the crown 29 may be made completely homogenous in one single material, e.g. gold, or in two parts 33, 34 each consisting of a different materia. In this latter case, a coping, e.g. a titanium coping, is seated next to a prepared tooth surface 31. A dental techni¬ cian is able to build the outer contours on the outside of the coping, for instance in porcelain. The outer part 34 has an outer surface 43. There is a narrow cement slot 36 between the tooth surface 31 and the inner surface 32 of the coping. The border line 30 of the prepared surface is referred to as the preparation border 30.

Fig. 5B is an enlarged view of the encircled part 510 in Fig. 5A.

Fig. 6A illustrates a facing 38 on the facial surface of an incisive 24. A facing may be made of plastic or ceramics, for instance. The facing is affixed to the prepared tooth surface 31 by cement in the slot 36. The inner surface 37 of the facing follows the prepared tooth surface 31. The cement slot 36 may be about 50 μm in size. The numeral 30 identifies the preparation border.

Fig. 6B is an enlarged view of an encircled part in Fig. 6A.

In the illustrations presented in Figs. 5A, 5B, 6A, 6B and in Fig. 9, the surface 31 of the object to be measured is scanned on a first or initial side of the preparation border 30. In order to ensure that all information of interest is obtained, a reading can also be taken on the surface that constitutes the preparation border 30 together with a part of the surface 51 which is situated on the other or second side of the preparation border 30 (see Fig. 9) .

The preparation border 30 can be determined, by marking-out the border before beginning the scan, by marking-out the area 51 on the object 2, i.e. the area on said other side of the preparation border 30. Marking can be effected, for instance, by colouring the surface 51 with a colour whose reflection characteristic differs from the reflection characteristic of the remainder of the object.

According to one embodiment of the invention, the area 51 is marked with a black colour, whereby the reflection of light will be interior to that of the original surface.

According to an alternative embodiment of the invention, solely one line at the preparation border is marked, in stead of marking the complete area 51. When scanning, a much weaker signal, or no signal at all, is detected when scanning or reading the marked surfaces. The location of the preparation border 30 can be determined, by analyzing the measuring data collected in the scan.

According to another embodiment of the invention, the area 51 is covered with a material whose reflection factor differs from the reflection factor of the remainder of the surface of the object. The results of the measuring process can be stored together with synchronization data from the motor drive stage, therewith making it possible to determine where on the surface the respective values derive. The scanned points on the surface are transformed to points in a coordinate system, e.g. cartesian, polar or cylindrical. The scanned points belonging to this group together represent a model of the scanned surface.

The information collected in the aforedescribed manner can then be used to create an extremely accurate model of the complete restoration, as described below.

A solid restoration model contains surfaces. These surfaces can be represented as points, planar polygon surfaces or as parametrically described surfaces. Furthermore, lines, curves and points on the surface may have their own representation, for instance description or marking of the preparation border.

By a surface which is represented in the form of points is meant that a plurality of points create a representation of the surface. The points in the plane are classified in accordance with type and may be edge points, corner points or surface point. The edge points and corner points form respec- tively the edges and corners of the surface.

By a planar polygonal surface is meant a surface where all the corners of the polygon lie in one and the same plane. The edges of the polygonal surface are defined as lines between two corners.

By a parametrically described surface is meant a surface which can be described as a function P of two variables u and v. In a cartesian coordinate system, the positions of the coordi- nates for the surface can be represented by the parametrical vector function:

P(u,v) = (x(u,v), y(u,v), z(u,v))

Parametrically described surfaces may be Bezer surfaces and Spline surfaces.

A line can be represented by its two end points. A curve can be represented in a parametrial form, i.e. by the envariable parametrial vector function P(u) which in a cartesian system of coordinates can be described as:

P(u) = (x(u), y(u), z(u))

For instance, a circle in the xy-plane with a centre in the origin of coordinates can be described in a parametrial form with the following functions:

x(u) = r cos(2τru), y(u) = r sin(2τru) , z(u) = 0

where u can take values between 0 and 1. When using a measuring arrangement according to one embodiment of the present invention, a complete tooth restoration model can be obtained on the basis of one or more scanned surfaces in combination with other numerically obtained surfaces. Surfaces, borders and points on the restoration model can be represented by the aforedescribed representation models, for instance. There is a number of different types of tooth restorations.

According to the present invention, the obtained measurement values may represent geometrical surfaces and all their constituent components, i.e. solids, parts of surfaces, edges and points, and the measurement values can be modified or erased by a CAD system constructing in accordance with the invention. The information collected through the measuring procedure may be used to produce a coping or cap, of which the inner surfaces correspond wholly or partially with the upper surface and side surfaces of the measured tooth (Fig. 1) . However, when manufacturing a coping or cap, it is often beneficial to permit a slot or gap between the outer surface of the tooth and the inner surface of the coping. Consequent¬ ly, there is a need to modify the measurement information. This modification may involve translation, scaling, e.g. enlarging or decreasing, and/or rotation. Modification may also involve changes in the vector function on a surface or curve described parametrically.

Modification may also be a scaling of geometrical objects conditioned by function. For instance, scaling of this kind may include a modification of the measurement values that represent the scanned surface, so that a slot of gap will be formed in relation to the scanned surface when the two surfaces are fitted together.

It is conceivable that the slot at the preparation border 30 shall be equal to zero, and the surface is therefore scaled successively from the preparation border up to a given distance from the preparation border, from which the remaining points on the surface are scaled equally, Figs. 5A, 5B. When the scaled surface is fitted against the original, the result will be a gap or slot 36 which has a uniform width between the main part of the surfaces (Figs. 5B and 6B) . The gap narrows proximal to the preparation border 30 and is zero at the border itself. Another form of function-determined scaling is direction-dependent scaling. A coping for a crown 33 may, for instance, be enlarged by different scale factors both verti- cally and radially.

In the case of a dental CAD system constructed in accordance with the present invention, an operator is able to mark that part of a scanned surface which will fit the restoration. The border of this area is the preparation border 30. The prepara¬ tion border can also be obtained automatically from the data available, provided that the operator has marked the prepara¬ tion border prior to scanning. The position of the light spot and its intensity are both detected. Since this intensity will vary in accordance with the colour and shape of the measured surface, it is possible to detect an area which has previously been marked by the operator.

The remaining surfaces of the restoration can be defined by the operator. These surfaces meet and coincide with the inner surface of the coping at the preparation border 30 marked by the operator on the scanned surface.

The operator is able to scale the scanned measurement values and therefore also produce other positional coordinates which describe an initial model surface 32, so that a slot will be formed for the original surface 31 when the transformed model surface 32 is fitted to the scanned surface 31. By transforma¬ tion, it is possible to generate positional coordinates which describe the surface 32, so that the slot will be formed on one of the two sides of the scanned surface. In the case of a dental crown, the transformed surface will embrace the scanned surface, while in the case of an inlay, the trans¬ formed surface will be embraced by the scanned surface.

When the preparation border is given or worked-out, it is possible to generate a model of certain kinds of restorations, such as a coping for a single crown, directly from the scanned surface, the preparation border and parameters. The function for generating the model of the coping may be parameter- dependent, so that models of copings can be generated in accordance with certain standardized forms, for instance of even thickness.

Similarly, the creation of facing and inlay models may also be parameter-controlled.

A restoration model can be generated by scanning several surfaces. For instance, the positional coordinates describing the outer surfaces 43 of the model (Fig. 5A) can be generated by scanning the surface of a restoration prototype. This restoration prototype may be a coping which has been sculp¬ tured manually to a suitable form, for instance. By scanning with the aid of optical triangulation in the aforedescribed manner, the outer surface 43 of the model can be obtained in a simple manner favourable to the operator.

According to one embodiment of the present invention, a tooth is worked to obtain a prepared surface. A copy of the prepared tooth is made and used as a measurement object, so that the object will have a surface 31 which coincides with the prepared surface of the tooth. Positional coordinates which describe the surface 31 are established by optical triangula¬ tion. A coping, or cap, of mouldable material is placed on the tooth copy, such that the coping will cover essentially the whole of the surface 31. The coping is sculptured to obtain a surface which corresponds to the outer surface 43 of the desired tooth restoration. The sculptured coping now consti¬ tutes a restoration prototype. The sculptured coping or the restoration prototype can now be used as a measurement object and its outer surface 43 can be scanned by means of a method which includes optical triangula¬ tion, as described above.

Computerized processing of the two scanned surfaces 31 and 43 will enable these two surfaces to be positioned one above the other such that the three-dimensional shape and volume of the coping will be defined by the scanned surfaces.

The intersection curve is the preparation border. The resul¬ tant measurement values of the points thus obtained constitute a model of the restoration prototype or the coping.

Accordingly, an inlay can be created by, e.g., scanning a plaster cast of a prepared tooth with an inlay made of wax or similar material. An impression or a plaster cast of the cavity is also scanned. A model of the inlay is created when these two surfaces are fitted together at the preparation border.

A model generated on the basis of several scanned surfaces can then be modified. It may be necessary to change one of these surfaces in order to obtain a slot between the restoration model and the prepared tooth surface. Reasons may be found to change the size of the restoration model.

The finished restoration model can be used to obtain data for manufacturing a true restoration, for instance with the aid of a computer-controlled milling machine.

According to one embodiment of the invention, a restoration is produced in the following manner:

A cast is made of the tooth, in the patient's mouth, which is to be treated. The inner surfaces of the casting will there¬ fore correspond to the outer surfaces of the tooth. The cast can be used to produce a tooth copy 2 (Fig. 1, Fig. 2) . The preparation border 30 is marked on the tooth copy 2 with a pen, when the operator wishes the preparation border to be identified by the inventive arrangement.

The tooth copy 2 is fixated in the measuring system as the object 2 to be measured, as described above, and the outer surfaces 31, 30, 51 of the tooth copy, and therewith also of the tooth, can be registered extremely accurately in three dimensions (Fig. 9, Fig. 5A and Fig. 5B) by means of the aforedescribed optical scanning procedure. The collected measurement information can be transformed to positional coordinates of a large number of measurement points, this number being so large as to enable the surfaces of the tooth to be determined to a high degree of accuracy.

The position coordinates are stored in a memory system in the computer.

The position coordinates are used as initial values for describing a second surface 32 of which the outlines or contours correspond to the outline or contour of the surface 31 of the tooth. Fig. 5B, by mathematical computation.

An operator is able to influence the mathematical computation, for instance so that the second surface 32 of the coping is able to merge with the outer surface of the tooth at the preparation border 30 when the coping is placed over the tooth.

The convex outer surface of the coping can be described by indicating outer coordinates. These coordinates can be calculated automatically and/or are obtained by using a CAD system. All surfaces of the coping have been determined once the outer surface has been described, and a suitable coping can be produced (Fig. 1) by transferring the form data describing the desired coping from the computer 12 to a software-controlled manufacturing device 220. The manufactur¬ ing device 220 may include a computer-controlled milling machine, for instance.

According to another embodiment of the invention, the object 2 to be measured may consist in an impression of two or more teeth. By scanning one impression at a time and by registering the distance between these impressions, it is possible to build a model on the basis of the cast section as a whole.

Fig. 4 shows a tooth curve as seen from an occlusal aspect, i.e. from the chewing surface. The set of teeth is divided into four quartiles. Each quartile includes two incisors 24, one canine 25, two bicuspids or premolars 26 and three molars 27. The surfaces 21 which face towards the tongue are called lingual surfaces. The facial surfaces of the teeth face outwards 18, 19, distal proximal surfaces 22 faced backwards and mesial proximal surfaces face forwards in the mouth. The chewing surfaces 20 are called occlusal surfaces.

Claims

1. A method for generating a model for tooth restoration or for restoration of part of a tooth, wherein the method includes determining measurement values by scanning a surface (31, 51) of an object (2) to be measured, characterized in that the object (2) is a tooth, a tooth copy, a tooth impres¬ sion or a prepared part of a tooth; in that the object to be measured is moved successively in relation to a measurement line (8) , along which the point of intersection (210) of the surface (31, 51) of the object with the measurement line (8, SI) is determined together with data relating to movement of the object; in that the position of the intersection point is determined by means of optical triangulation in at least one positional dimension; in that the resultant measurement values constitute a data description of the outer surface (31, 51) of the object to be measured or a part of this outer surface; and in that the resultant measurement values are processed so as to obtain said model in the form of a data description of its surfaces.

2. A method according to Claim 1, characterized in that the resultant measurement values are processed by means of a computer (12) , therewith generating first position coordi- nates, wherein first position coordinates describe points on the first surface (31, 51) ; in that second position coordi¬ nates are generated in dependence of the first position coordinates, wherein the second position coordinates describe points on a first model surface (32) ; in that third position coordinates are generated, wherein the third position coordi¬ nates describe points on a second model surface (43) ; and in that the first model surface (32) and the second model surface (443) constitute delimiting surfaces for the three-dimensional model.

₃. A method according to Claim 1 or 2, characterised in that the first model surface (32) is caused to follow the first surface (31) and to form with the first surface an overall gap or slot, and so that the first surface will intersect the first model surface at a preparation line (30) at most.

4. A method according to Claim 2 or 3, characterized in that the optical triangulation includes the step of generating a light beam (8, SI) which coincides with the measurement line, said light beam being reflected at a point (210, b, c) on the object to be measured (2, B) and the reflected light beam impinges on a sensor means (10, D) ; in that a first output signal is delivered in accordance with the position (b', c') on the sensor means (10, D) at which the reflected beam impinged on said sensor, said first output signal correspond¬ ing to the change in position (h) of the measurement point along the measurement line (8, SI).

5. A method according to any one of the preceding Claims, characterized by marking selected surfaces on the measurement object (2) and covering said selected surfaces such as to influence the reflectivity of said surfaces; detecting the intensity of the reflected light beam when scanning the object to be measured, and combining the light intensity data with the obtained measurement values to determine the position coordinates of the selected surfaces.

6. A method according to Claim 5, characterized in that the marked surface on the object (2) to be measured defines a preparation border line (30) therewith accomplishing automatic scanning of the preparation border line.

7. A method according to any one of the preceding Claims, characterized in that the third position coordinates are generated by scanning the surface of a restoration prototype, thereby determining the position in at least one position dimension by means of optical triangulation.

8. A method according to any one of Claims 2 to 6, charac¬ terized in that the second position coordinates (32) are generated as a function of the first position coordinates and of the preparation border, by data processing in a computer.

9. A method according to any one of the preceding Claims, characterized in that the third position coordinates (32) are generated as a function of the first position coordinates (31), the second position coordinates (32), and the prepara- tion border (30) , by data processing in a computer.

10. A method according to any one of the preceding Claims, characterized in that the third position coordinates (32) are generated by interactive data processing.

11. The use of a model generated in accordance with the method according to any one of Claims 1-10 in the manufacture of a tooth restoration or in the restoration of part of a tooth.

12. An arrangement for carrying out the method according to any one of Claims 1-10 for creating a model of a tooth restoration or of part of a tooth restoration, wherein the arrangement is constructed to determine measurement values by scanning a first surface (31) of an object (2) to be measured, characterized in that the object (2) is a tooth, a tooth copy, a tooth impression or a prepared part of a tooth; in that the arrangement includes means for moving the object for the purpose of scanning the object at a plurality of points thereon, wherein the distances between said points are measured with optical triangulation and wherein measurement values which constitute a data description of the outer surface (31, 51) of the object or a part of said surface are obtained by combining said distances with data relating to movement of the object; and in that the arrangement also includes a computer which is programmed to process the obtained measurement values and to generate the model therefrom in the form of a data description of its surfaces.

13. An arrangement according to Claim 12, characterized in that the arrangement includes measuring units which function to determine the measurement values of the position of a measurement point in three dimensions; in that the arrangement further includes a light source and a sensing means adapted to determine a measurement value which indicates the position of the measurement point on the first surface (31) of the object to be measured at least in a first position dimension by optical triangulation; in that the measuring units are constructed to deliver the determined measurement values to a computer; in that the computer is programmed to generate first position coordinates in accordance with the obtained measurement values, wherein the first position coordinates describe points on the first surface (31, 51) ; in that the computer is programmed to generate second position coordinates in accordance with the first position coordinates, wherein the second position coordinates describe points on a first model surface (32) ; in that the arrangement includes means for scanning or measuring third position coordinates, which describe points on a second model surface (43) ; and in that the first model surface (32) and the second model surface (43) constitute delimiting surfaces for a three-dimensional model.