WO2002032340A2

WO2002032340A2 - Method for determining a computer model for a dental object, such as a crown

Info

Publication number: WO2002032340A2
Application number: PCT/NL2001/000760
Authority: WO
Inventors: Nicole Leonarda Wilhelmina Eikelenberg; Malte De Moll; German Enrique Knoppers; Merijn Wijnen
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 2000-10-17
Filing date: 2001-10-16
Publication date: 2002-04-25
Also published as: AU2002212831A1; NL1016420C2; WO2002032340A3

Abstract

A model for a dental object, such as a crown, is defined by an upper side and boundary surfaces, determined by scanning, of the jaw where the object is to be placed and between adjacent teeth and/or molars. Provided on the jaw is a filling material, which is machinable at least on the upper side, such that by performing some chewing movements on the filling material an impression of a dynamic chewing surface is formed. The dynamic chewing surface is scanned and recorded in a computer, as well as the scanned boundary surfaces, the computer model being determined on the basis of these data. The dynamic chewing surface can be scanned by means of an optical scanner fixed with respect to a fixed point on the jaw, after which the formed filling material is removed. With the scanner, the form of the part of the jaw and adjacent teeth and/or molars that is complementary to the formed filling material is then scanned.

Description

Title: Method for determining a computer model for a dental object, such as a crown.

This invention relates to a method for determining a computer model for a dental object, such as a crown, which is defined by an upper side and boundary surfaces, which boundary surfaces form the limits between the object, the jaw where the object is to be placed, and adjacent teeth and/or molars, and which boundary surfaces are determined by scanning, and which upper side forms the side of the object facing away from the jaw.

Such a method is known from the American patent US-A 4,575,805. This patent describes a method in which by means of a scanner the surface characteristics of an organ to be restored, such as, for instance, a preparation of a tooth or molar, are determined. On the basis of the recorded surface data, the three-dimensional form of a restoration (crown) is calculated, following which this object is manufactured mechanically. By means of the scanner from this patent, it is possible to obtain detailed information on the preparation and the immediate surroundings of the preparation. This information is necessary to realize a proper fit of the crown on the preparation and between the adjacent teeth and molars.

To realize a proper fit of the crown with respect to the opposing teeth or molars, the antagonists, it is necessary, however, to also analyze the chewing movement. In fact, this chewing movement defines the space in which the crown may be located. The apparatus according to the above- mentioned patent does not provide a solution thereto, because the system can only determine the above-mentioned boundary surfaces of the jaw where the object is to be placed and of adjacent teeth and/or molars.

The invention has for its object to provide a method for determining a computer model for a dental object, with which the upper surface of the object can be shaped in an accurate manner, and with which the above- described drawbacks are avoided. This object is achieved by a method of the type mentioned in the opening paragraph, which method comprises the following steps: providing on the jaw a filling material, which is machinable at least on the upper side, such that by performing some chewing movements on the filling material, an impression of a dynamic chewing surface is formed on the upper side; scanning the thus formed impression; recording in a computer the scanned boundary surfaces and the impression, the computer model being determined on the basis of these data. With a method according to the invention, it is ensured that the form of the dynamic chewing surface can be determined very accurately. Such a dynamic chewing surface defines an upper side, which, besides the boundary surfaces defined by adjacent teeth and/or molars, the preparation and the jaw, defines the space within which a dental object can be shaped. In particular, it is ensured that this form is stamped by the antagonists into the surface of the machinable material by repeatedly performing a chewing movement. Thus, a dental object can be manufactured while preventing the antagonists, on closure of the jaws, from being fixed in one specific position or from unpleasantly connecting together. The occurrence of this phenomenon is rather normal in the present traditional dental practice and is usually solved by using a so-called biting plate with which the relation between the lower and the upper jaw is recorded. With it, a dental technician can in most cases determine, on the basis of his experience and expertise, the fit of a crown or bridge on the opposing molars. However, such a technique is not suitable for determining a computer model.

By using a machinable material, an impression of a dynamic chewing surface is formed, contrary to a chewing surface formed by the use of a plastic material. It is observed that, in the present practice, similar plastic materials are already being used to make an impression of the antagonists. Because here a chewing surface is recorded according to a last chewing movement, the employed materials are unsuitable for forming a dynamic chewing surface, as is realized in a method according to the invention. It is conceivable ^"to scan the surfaces in different manners, with several combinations with known techniques being possible. These combinations comprise: removing from the mouth as one whole the filling material formed into a model, and scanning it outside the mouth, or making a plaster impression of the formed filling material by using a silicone impression of the lower and the upper jaw. However, taking such a silicone impression is rather unpleasant to the patient. Besides, making such an impression takes a rather long time, so that such a technique is not preferred. It is further conceivable that the dynamic chewing surface is separately determined by scanning either in the mouth or outside the mouth the surface formed by the method of the invention, and that the boundary surfaces are determined in the traditional manner, such as by means of the scanning technique of the above-mentioned patent or by means of a plaster impression.

In a preferred embodiment, the model is determined by scanning an impression of the dynamic chewing surface by means of an optical scanner fixed with respect to a fixed point on the jaw; removing the formed filling material; and scanning with the fixed optical scanner the form of the part of the jaw and adjacent teeth and/or molars that is complementary to the formed filling material, so that the boundary surfaces are determined. The thus employed method has the advantage that the spatial coordinates of the model can always be determined from one point. The combination of scanning from one point both the dynamic chewing surface formed in the filling material and the boundary surfaces has the practical advantage that problems of connection between the boundary surfaces and the above- mentioned upper surface can be avoided. According to a further preferred embodiment, scanning is performed with a scanner with a freely movable head, such that a measuring bundle from the scanner can strike at a relatively great angle an upper side or boundary surface of the model to be scanned. With a thus employed scanner, an increased measuring accuracy can be ensured, since according as an angle at which a measuring bundle falls on a surface to be measured ^"~~ is smaller, the distance from the point to the scanner can be determined less accurately. Moreover, the risk of disturbing reflections increases, with the distance from a point on a measuring surface no longer being determinable at all.

In a preferred embodiment, the scanned form parameters of the dynamic chewing surface are combined with general profile data of a tooth or molar. In a reconstruction of a tooth or molar, such general profile data identify a number of fixed profile characteristics in the dynamic chewing surface.

The invention further relates to filling material suitable for determining a model of a dental object, such as a crown, according to at least one of the above-mentioned methods, the filling material being machinable at least on the upper side, so that by performing some chewing movements, an impression of a dynamic chewing surface can be formed. The filling material may comprise a layer of machinable material and a non- machinable bonding layer, the layer of machinable material having a relatively small thickness. What is thus prevented is that through the machinable layer undesirably breaking off an inaccurate model for a dental object is obtained. This effect can further be avoided, if the contact surface between the bonding layer and the machinable layer is enlarged with respect to a flat contact surface. The bonding layer increases the form stability and firmness of the model. Preferably, the non-machinable bonding layer is suitable for receiving the form of the part of the jaw and adjacent teeth and/or molars where the object is to be placed. A suitable material for forming the machinable layer is a silicone gel into which talc is mixed. To form the bonding layer, a thin-liquid silicone gel may be used.

The invention further relates to a holder for placing in the mouth a filling material for forming a model of a dental object as described above, the holder comprising a transverse surface with two flexible, substantially parallel surfaces on both sides thereof, in which transverse surface an opening is provided. The flexible surfaces are suitable for clamping over adjacent teeth or molars, so that a limited volume is formed in which the filling material can be provided. This can be done by injecting a filling material through the opening provided in the transverse surface.

A further aspect of the invention is formed by a scanner for scanning a dynamic chewing surface and a boundary surface to determine the form of a model according to the above-discussed method, which scanner comprises a fastening member, which can be firmly fastened to a part of a tooth, molar or a jaw; an optical sensor movable with respect to the fastening member, suitable for detecting with respect to the sensor the relative position of a point of the boundary surface or of the upper side of the formed filling material; and a connecting means for forming a connection between the optical sensor and the fastening member, so that the sensor always assumes a defined relative position with respect to the fastening member. The scanner may have a freely movable head, such that a measuring bundle of the scanner can strike at a relatively great angle a surface of the model to be scanned.

The invention will be explained in more detail with reference to the drawing, in which:

Fig. 1 is a diagrammatic view of the formation of a dynamic chewing surface by means of a filling material according to the invention; Fig. 2 is a block diagram of the steps to be taken to form a computer model for a dental object according to the method of the invention;

Fig. 3 is a diagrammatic side view of a scanner for determining the form of a computer model for a dental object according to the method of the invention;

Fig. 4 is a diagrammatic top view of the scanner of Fig. 3; and

Fig. 5 is a diagrammatic perspective view of a holder for placing in the mouth a filling material according to the invention.

In the figures, the same or corresponding parts are represented by the same reference numerals.

Between two molars 1 on a jaw 2 a preparation (stump) 3 is prepared so as to be able to receive a dental object, in this case a crown (not shown). Provided on the preparation 3 is a filling material 4, in the example of the drawing consisting of a bonding layer 5 and a layer of machinable material 6. The bonding layer 5 is relatively thick, so that the filling material 4 reaches to a height just somewhat smaller than the height of the surrounding molars 1.

The layer of machinable material 6 is applied on the side 7 facing away from the jaw 2 and consists, for instance, of a relatively thin layer of silicone gel, into which talc is mixed. The layer 6 is relatively thin, which prevents too rapid machining of the layer, so that through the layer undesirably breaking off an inaccurate model of the dental object is obtained. The bonding layer 5 may have a consistency suitable for receiving the form of the part of the jaw and adjacent teeth or molars where the object is to be placed. If the bonding layer 5 has this consistency, for instance, when the bonding layer 5 consists of thin-liquid silicone material, then a model pre-formed in the mouth can be removed from the mouth, and the form thereof can be determined outside the mouth. If the bonding layer does not have such a consistency, it is impossible or less possible to determine the model outside the mouth, and form determination takes place in the mouth, for instance in a manner as will be described further.

In the example of Fig. 1, the contact surface 8 between the bonding layer 5 and the layer of machinable material 6 is shown by a wave line. This indicates that the contact surface between the bonding layer and the machinable layer is relatively larger than a flat contact surface, so that a better bond of the layer of machinable material 6 and the bonding layer 5 is obtained. It is conceivable to further improve this bond by using specific bonding materials, such as glue, or through a filling material 4, which, in the direction of the side 7 facing away from the jaw 2, has a gradual increase in concentration of talcum powder. Of course, such variants are deemed to fall within the scope of protection of the claims.

In Fig. 1, the dynamic chewing surface 9 is shown by the curvatures formed in the layer 6 by machining the side 7 facing away from the jaw 2 by repeatedly performing a chewing movement. This dynamic chewing surface 9 defines, besides the boundary surfaces determined by adjacent teeth and/or molars, the preparation and the jaw, the space within which a dental object can be shaped. Fig. 2 shows a block diagram of the steps to be taken to form a computer model for a dental object according to the method of the invention. In a first step 10, a first bonding layer is applied, such as the layer 5 of Fig. 1. It is assumed that preparing the preparation 3 and cleaning the mouth and adjacent teeth and/or molars 1 has already taken place. The layer is applied by means of a holder, as will be described further.

In a second step 11, the contact surface 8 is enlarged, for instance by treating it with a spatula. This step 11 can be left out, if the materials selected for the bonding layer 5 and the machinable layer 6 in themselves have a sufficient mutual bond. In a subsequent step 12, the machinable layer 6 is applied. The layer 6, too, is applied by the above-mentioned holder, to be described further.

The filling material 4 thus placed in the mouth, consisting of the two layers 5 and 6, must harden for some time, in practice for a few minutes. Then the product is firm enough to allow the performance of a next step 13, the formation through a chewing movement of a dynamic chewing surface in the side 7 facing away from the jaw 2. This chewing movement comprises repeatedly chewing (at least more than once, in practice about 5 times) with the antagonists on the machinable material, so that small pieces are broken off.

By thus treating the filling material 4 placed in the mouth, a model is formed of the boundary surfaces with adjacent teeth and/or molars, the preparation and the jaw, and of the dynamic chewing surface. The forms of this physical model are to be scanned by means of a scanner to form a computer model for the dental object, which can then be manufactured mechanically by means of a computerized treatment.

In the example of the diagram of Fig. 2, scanning takes place as follows: In step 14, an optical scanner is clamped in the mouth, so that it is fixed with respect to a fixed point on the jaw where an object is to be placed. By using such a fixed scanner, the spatial coordinates of the model can always be determined from one point.

With the scanner, the dynamic chewing surface 9 is scanned in step 15, by means of an optical distance determination. The scanner will be described further with reference to Figs. 3 and 4. In a subsequent step 16, the formed filling material 4 is removed from the mouth, and the preparation 3 and adjacent teeth and/or molars 1 are cleaned, so that the contours thereof can be scanned in a step 17 by means of the scanner, which remains fitted in the mouth in the intermediate period. Because also the boundary surfaces of the jaw 2 where the object is to be placed and adjacent teeth and/or molars 1 are scanned from the same fixed point, problems of connection between the boundary surfaces and the dynamic chewing surface 9 are prevented.

In step 18, the data determined by scanning are entered into a computer and stored, after which, through numerical analysis of the scan data, a computer model is calculated. The thus calculated computer model may serve for the manufacture of a dental object in a computerized treatment process (CAD/CAM). As already discussed, this may also involve general profile data of a crown or another dental object to be manufactured. Figs. 3 and 4 are respectively a diagrammatic side view and top view of a scanner 19 for determining the form of a model for a dental object according to the method of the invention. The scanner 19 has a freely movable head 20, such that a measuring bundle 21 of the scanner 19 can strike at a relatively great angle a chewing surface or boundary surface of the model to be scanned. With a thus employed scanner 19, an increased measuring accuracy can be obtained, since according as an angle at which a measuring bundle falls on a surface to be measured is smaller, the distance from the point to the scanner can be determined less accurately. Moreover, the risk of disturbing reflections increases, with the distance from a point on a measuring surface no longer being determinable at all. In Figs. 3 and 4, this movability is diagrammatically shown by the arrows P. The scanner 19 consists of a fastening member 22, which can be firmly fastened to a part of a tooth 23; a head 20 movable with respect to the fastening member 22. In the head, an optical sensor (not shown) is provided, which is suitable for detecting with respect to the sensor the relative position of a point of the boundary surface or of the chewing surface of the formed filling material. A connecting member connects the head 20 and the fastening member 22, so that the head 20, and hence the sensor, always assumes a defined relative position with respect to the fastening member 22. Fig. 5 diagrammatically shows a holder 24 for placing in the mouth a filling material for forming a model of a dental object, as described above. The holder has a transverse surface 25, which, in use, is substantially parallel to the chewing surfaces 26 of adjacent teeth or molars 27. The holder is clamped over the jaw 28 on the teeth or molars 27, so that the two flexible, substantially parallel surfaces 29 form limits, between which the filling material of the invention can be provided. After the holder 24 has been placed, the filling material can be injected into the opening 30.

It will be clear to those skilled in the art that the invention is not limited to the exemplary embodiments described with reference to the drawing, but may comprise all kinds of variations thereof. Thus, in the above description of the drawing, an embodiment of a crown according to the invention has been discussed. The invention, however, may also be used in a bridge or a so-called plate. Such variations are deemed to fall within the scope of protection of the appended claims.

Claims

1. A method for determining a computer model for a dental object, such as a crown, which is defined by an upper side and boundary surfaces, which, boundary surfaces form the limits between the object, the jaw where the object is to be placed, and adjacent teeth and/or molars, and which boundary surfaces are determined by scanning, and which upper side forms the side of the object facing away from the jaw, characterized in that the method comprises the following steps:

— providing on the jaw a filling material, which is machinable at least on the upper side, such that by performing some chewing movements on the filling material an impression of a dynamic chewing surface is formed on the upper side;

— scanning the thus formed impression;

— recording in a computer the scanned boundary surfaces and the impression, the computer model being determined on the basis of these data.

2. A method for determining a computer model for a dental object according to claim 1, characterized in that the model is determined by

— scanning an impression of the dynamic chewing surface by means of an optical scanner fixed with respect to a fixed point on the jaw;

— removing the formed filling material; and

— scanning with the fixed optical scanner the form of the part of the jaw and adjacent teeth and/or molars that is complementary to the formed filling material, so that the boundary surfaces are determined.

3. A method for determining a computer model for a dental object according to claim 2, characterized in that the scanner has a freely movable head, such that a measuring bundle of the scanner can strike at a relatively great angle an upper side or boundary surface of the model to be scanned.

4. A method for determining a computer model for a dental object according to at least one of the preceding claims, characterized in that when determining the computer model the scan data of the dynamic chewing surface are combined with general profile data of a tooth or molar.

5. A filling material suitable for determining a model of a dental object, such as a crown, according to the method of at least one of the preceding claims, characterized in that the filling material is machinable at least on the upper side, so that by performing some chewing movements an impression of a dynamic chewing surface can be formed.

6. A filling material according to claim 5, characterized in that the filling material comprises a layer of machinable material and a non- machinable bonding layer, the layer of machinable material having a relatively small thickness.

7. A filling material according to claim 5 or 6, wherein the contact surface between the bonding layer and the machinable layer is enlarged with respect to a flat contact surface

8. A filling material according to at least one of claims 5 - 7, characterized in that the non- machinable bonding layer is suitable for receiving the form of the part of the jaw and adjacent teeth and/or molars where the object is to be placed.

9. A filling material according to at least one of claims 5 — 8, characterized in that the machinable layer consists of a silicone gel into which talc is mixed.

10. A filling material according to at least one of claims 5 - 9, characterized in that the bonding layer consists of thin-liquid silicone gel.

11. A holder for placing in the mouth a filling material for forming a model of a dental object according to at least one of the preceding claims, characterized in that the holder comprises a transverse surface with two flexible, substantially parallel surfaces on both sides thereof , in which transverse surface an opening is provided.

12. A scanner for scanning a dynamic chewing surface and a boundary surface to determine the form of a model according to any one of the preceding claims, comprising

- a fastening member, which can be firmly secured to a part of a tooth, molar or a jaw;

- an optical sensor movable with respect to the fastening member, suitable for detecting with respect to the sensor the relative position of a point of the boundary surface or of the upper side of the formed filling material; and

- a connecting member for forming a connection between the optical sensor and the fastening member, so that the sensor always assumes a defined relative position with respect to the fastening member.

13. A scanner according to claim 12, characterized in that the scanner has a freely movable head, such that a measuring bundle of the scanner can strike at a relatively great angle a surface of the model to be scanned.