The present invention relates to a system for testing the strength of a tooth, wherein the system comprises a testing apparatus and a test unit. Moreover, the present invention relates to a fixture for use in the system for testing the strength of a tooth. Additionally, the present invention relates to method for testing the strength of a tooth.

BACKGROUND OF THE INVENTION

The strength of dental fillings in teeth is essential. Thus, scientists and manufactures of dental fillings test the strength of teeth by subjecting a tooth replica having a dental filling to a compressive force. This is done in order to determine the maximum force, which the dental filling can withstand. However, the tests are often performed in a non natural situation in that the dental filling and tooth replica are free standing in the test situation. In other words, the tooth replica is not positioned next to one or more neighbouring teeth. However, this situation is far from the real life situation wherein the tooth is positioned next to one or two neighbouring teeth. In fact studies have shown that in real life, the strength of a tooth is dependent on the neighbouring tooth, as neighbouring teeth abut each other. The effect is that a part of the force applied to one tooth is transferred to the neighbouring tooth.

Accordingly, the closer two teeth are to each other - preferably touching each other - the larger the force is, which one of the teeth may withstand without breaking. It is an object of one or more embodiments of the present invention to provide an apparatus and/or a device and/or a method which allows for testing the teeth in a situation which is as close to a real life situation as possible.

DESCRIPTION OF THE INVENTION

In a FIRST aspect the present invention relates to a system for testing the strength of a tooth, the system comprising a testing apparatus and a test unit comprising a fixture which is adapted to accommodate:

- the tooth which defines an upper surface, and one or more lateral surfaces; and

- a first neighbouring element each of which is adapted to imitate a neighbouring tooth; wherein the testing apparatus comprises a force application element which is arranged such that when the test unit is provided in the testing apparatus, a compressive force may be applied to the upper surface of the tooth by means of the force application element, thus causing at least a part of tooth to be forced towards the first neighbouring element. One advantage of providing a neighbouring element during the test situation is that a part of the compressive force applied to the tooth during the test may be transferred to the neighbouring element. This is closer to the real life situation, and thus provides for a more accurate test method.

It will be appreciated that both in the real life situation and in the test situation, the distance between the tooth and the neighbouring tooth is decisive for how much force can be transferred from the tooth to the neighbouring tooth without causing the tooth to break . Thus in a set of teeth, where the teeth generally abut each other, the individual tooth can be subjected to a larger force than in a set of teeth, where the teeth generally do not abut each other, i.e. are provided at a distance from each other. Thus in one embodiment of the invention, the distance between the tooth and the neighbouring element may be varied so as to emulate different spaces between the teeth.

The system according to the present invention may be adapted to test the strength of the tooth by determining the maximum compressive force and/or the maximum shear stress which the tooth can withstand before breaking. The testing apparatus may be adapted to gradually increase the force applied by means of the force application element to a tooth, until the tooth breaks. It will be appreciated that in order to determine this maximum force and/or the maximum shear stress, the testing apparatus will typically comprise a means for determining the force applied. Moreover, the apparatus may comprise a data processing unit, which is adapted to determine the shear stress on the basis of the compressive force applied. In order to do so, the testing apparatus may comprise an input device by means of which, the user enters data about the geometry and the position of the rupture. Alternatively, the testing apparatus may comprise one or more 2 dimensional or 3 dimensional scanners, by means of which, the geometry and the position of the rupture can be determined.

In one embodiment, the force is applied in an altering manner, so as to imitate a chewing motion of the tooth. In one embodiment, the strength of the force applied reciprocates between zero Newton and up to a predetermined maximum force. It will be appreciated that by applying the force in an altering manner, rupture due to fatigue may be determined. During such repetitive application of compression force, the force may remain at the same level during all the applications of the force. Alternatively, the force may be altered over time, e.g. such that the force is gradually increased with each cycle. The test apparatus comprises a fixture which is adapted to accommodate one or more of:

- the tooth which defines an upper surface, and one or more lateral surfaces; and

- the first neighbouring element each of which is adapted to imitate a neighbouring tooth.

In one embodiment, the tooth is a natural tooth, an artificial tooth or a tooth replica. In the present context, the term 'tooth replica' shall be understood as an element which has some of the same properties as a real tooth. Accordingly, the tooth replica is not necessarily shaped exactly like a natural tooth. As an example, the tooth replica may be cylindrical. However in some embodiments, the shape of the tooth is close to the shape of a real tooth.

The tooth defines one or more lateral surfaces. In the context of the present invention the term 'lateral' shall be understood as any side surface i.e. a surface forming an edge with the upper end surface. This edge need not be a sharp edge, but can in some embodiments be rounded edge. The number of lateral surfaces depends on the shape of the end surface. If the end surface defines a circle the tooth defines a cylindrical cylinder, whereby the tooth defines only one single lateral surface namely the curved circumferential surface. If the end surface is quadrangular, the tooth forms a rectangular parallelepiped (i.e. a prism whose faces are all quadrangular), whereby the tooth defines four lateral surfaces. In the latter case, an edge is formed between any two neighbouring lateral surfaces. This edge extends in a direction away from the end surface of the tooth.

The force application element may define an impact surface, which is used to apply the compressive force. The impact surface may be formed by an end part of the force application element. At least a part of the end part may have a shape corresponds to at least a part of a sphere, or a part of a natural tooth, or a part of a cone. In one embodiment, the width of the force application element is smaller than the width of the tooth.

In one embodiment, the tooth comprises a dental filling of a material which is suitable for use as a dental filling, the dental filling being provided in an area facing the first neighbouring element. Examples of dental filling materials are metals such as gold, gold alloys, gold- platina alloys, silver amalgam, or non-metals such as composite resins, or cements or ceramics.

Moreover, the fixture may be adapted to accommodate a first spacing element which abuts one or more of the lateral surfaces of the tooth and the first neighbouring element so as to imitate direct physical contact between the tooth and the first neighbouring element. The first spacing element may in one embodiment abut at least a part of one of the lateral surfaces. In another embodiment, the first spacing element abuts at least a part of some or all the lateral surfaces. In cases where the first spacing element is adapted to be abut all the lateral surfaces of the tooth, the first spacing element may form a passage having a cross-section corresponding to the cross section of the tooth. Accordingly, if the tooth is cylindrical, the first spacing element may be ring-shaped with a circular passage.

The first spacing element may be retained between the tooth and the first neighbouring tooth by means of the pressure caused by the distance between the tooth and the first

neighbouring element being smaller than the thickness of the first spacing element in the same direction. Alternatively or as a supplement, the first spacing element may be adhered to the lateral surface of one to the tooth and the neighbouring element.

In one embodiment, the first spacing element comprises an elastically or plastic deformable material. Examples are composite materials or resilient materials such as natural or synthetic rubbers, or silicones. Other examples are deformable metals, such as lead or zinc. Yet other examples are polybutylene and polyisoprene. In one embodiment, the first spacing element is adapted to change hardness and/or size e.g. by application of a force to the first spacing element or by application of an electrical current to the element. One example of such an electrical element is a piezo electric element.

It will be appreciated that an alternative to providing the first space defining element which imitates direct physical contact between the tooth and the neighbouring element, is that the tooth and the neighbouring element abut each other directly. In such cases the tooth or the neighbouring elements may be shaped such that the area of contact between the tooth and the neighbouring element has a size corresponding to the normal contact area between two teeth.

In one embodiment, the upper surface of the tooth defines a cavity with a bottom and one or more side surfaces. It will be appreciated that such a design causes the upper surface to have a geometry which is closer to or similar to the geometry of the upper surface of a natural tooth. The cavity may define a cone shaped cavity terminating at a bottom (which is the lowest point in the cone shaped cavity). Such a cone shaped cavity defines one big side surface which encirculates the bottom of the cavity. The force application element may be adapted to apply the compressive force to at least one of the side surfaces, when the test unit is accommodated in the testing apparatus and the force application element is forced towards the tooth. In one embodiment, the force application element is adapted to apply the force to only one point of the one or more side surfaces. In another embodiment, the force application element is adapted to apply the force to at least two points of the one or more side surfaces, such as at two points, or three points, or four points.

In one embodiment, the system further comprises a second neighbouring element. As is the case with the tooth and the first neighbouring element, the tooth and the second

neighbouring element may be spaced apart or may abut each other directly.

Moreover, a second space defining element may be provided which abut one or more of the lateral surfaces of the tooth. It will be appreciated that by providing a first and a second neighbouring element, the setup is even more similar to the natural situation, in which any tooth has two neighbouring teeth (except for the wisdom teeth). The second space defining element may be arranged in the same way as the first space defining element. Moreover, the second space defining element may comprise the same materials as the first space defining elements. In other words: the second space defining element may comprise any combination of features and elements of the first space defining element. In fact, the first and the second space defining elements may in one embodiment form one unitary element. This one unitary element may be formed by two elements which are attached to each other e.g. by means of an adhesive or by being welded together. Alternatively, the one unitary element may form a monolithic element i.e. one element without any seams.

It will be appreciated that in a natural set of teeth, each tooth is retained the same position by means of its root. Thus in order to improve the similarity between the natural situation and the test situation, the fixture may retain at least a part of the tooth and/or the first neighbouring element and/or the second neighbouring element in a predetermined position. In one embodiment, the fixture retains a lower part of the tooth and/or the first neighbouring element and/or the second neighbouring element in a predetermined position. In order to retain the tooth, the fixture may comprise retaining means, which typically will abut a part of the tooth. In one embodiment, retaining shall be understood as preventing a lower part of the tooth from moving closer (horizontally) to the first and/or second neighbouring element, while the tooth is not being prevented from pivoting about the lower part. Alternatively or as a supplement, retaining shall mean that the tooth is also prevented from pivoting about a lower part

In a second aspect the present invention relates to a fixture for testing the strength of a tooth, the fixture comprising any combination of features and/or elements of the fixture according to any of the preceding claims. The fixture according to the second aspect may comprise any combination of features and/or elements of the fixture according to the first aspect.

In a third aspect the present invention relates to a method for testing the strength of a tooth by means of a testing apparatus and a test unit according to any of the preceding claims, the method comprising the steps of:

- applying a compressive force to the upper surface of the tooth by means of the force application element, thus causing at least a part of tooth to be forced towards the first neighbouring element.

Moreover, the method may further comprise the step of providing the tooth in the fixture. Additionally, the method may comprise the step of providing the tooth in the fixture, and/or providing the first neighbouring element in the fixture and/or providing the first spacing element in the fixture and/or providing the second neighbouring element in the fixture and/or providing the second spacing element in the fixture.

In one embodiment, the method further comprises the step of applying the compressive force comprises the step of applying the compressive force to the sides of the cavity of the tooth.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described with reference to the figures, in which :

Fig. 1 discloses a top view of a first example of the invention,

Fig. 2 discloses a side elevational view of the first example of the present invention, Fig. 3 discloses rupture of the tooth in a side elevational view of the first example of the present invention,

Fig. 4 discloses a top view of a second example of the invention,

Fig. 5 discloses a side elevational view of the second example of the present invention,

Fig. 6 discloses rupture of the tooth in a side elevational view of the second example of the present invention, Fig. 7 discloses a top view of a third example of the invention, Fig. 8 discloses a side elevational view of the third example of the invention, Fig. 9 discloses a side elevational view of a fourth example of the present invention, Fig. 10 discloses a fourth example of the invention, and Figs 11 and 12 discloses cross-sections of the fourth example of the invention.. DETAILED DESCRIPTION OF THE FIGURES

Figs. 1-3 disclose a first example of a test unit 100 comprising a fixture 102 defining four side surfaces 104 and a bottom 106 (in Figs. 2 and 3, the side surfaces 104 are not shown but are instead illustrated by means of arrow 105). The four side surfaces 104 and the bottom 106 define a cavity 108. Inside the cavity 108 is provided a tooth 110, a first neighbouring element 112 and a second neighbouring element 114. Each of the first neighbouring element 112 and the second neighbouring element 114 is spaced apart from the tooth 110 by means of a first spacing element 116 and a second spacing element 118, respectively. In the embodiment of Fig. 1, the first neighbouring element 112 and the second neighbouring element 114 define circular elements with a flat upper surface 120 and side surfaces 122. Similarly, the tooth 110 is circular and defines a concave upper surface 124 and side surface 126. The concave upper surface 124 defines a bottom point 128. In the embodiment of Figs. 1-3 the first spacing element 116 and the second spacing element 118 are provided in the form of flat elements which do not encirculate the tooth 110. However, it will be appreciated that in other embodiments, the two elements 116, 118 forms one unitary ring-shaped element which encirculating the tooth 110. In the latter case, the ring-shaped element will abut not only the first neighbouring element 112 and the second neighbouring element 114, but also two of the side surfaces 104.

In one embodiment, the height of the side surfaces 104 correspons to at least a fourth of the height of the side surface 126, such as a third of the height of the side surface 126, such as half of the height of the side surface 126, such as two thirds of the height of the side surface 126, such as three quarters of the height of the side surface 126.

In one embodiment, the center of gravity of the first spacing element 116 and/or the second spacing element 118 may be positioned at a position corresponding to at least a fourth of the height of the side surface 126, such as a third of the height of the side surface 126, such as half of the height of the side surface 126, such as two thirds of the height of the side surface 126, such as three quarters of the height of the side surface 126.

In one embodiment, the height of the first spacing element 116 and/or the second spacing element 118 may correspond to at least a fourth of the height of the side surface 126, such as a third of the height of the side surface 126, such as half of the height of the side surface 126, such as two thirds of the height of the side surface 126, such as three quarters of the height of the side surface 126.

In one embodiment, the first neighbouring element 112 and the second neighbouring element 114 have the same height as the tooth 110, whereas in other embodiments they have different heights.

In one embodiment, the first spacing element 116 and/or the second spacing element 118 are compressed in its/their longitudinal direction (i.e. in the up and downwards direction of Fig. 1) so as to change its/their dimension in the direction transverse to the longitudinal direction. In the embodiment of Figs. 1-3, a force application element 130 is positioned as close to the bottom point 128 of the tooth 110 as possible. However, as the application element 130 defines a spherical surface 132 and as the cavity 108 is inwardly cone shaped, the application element 130 does not contact the bottom point 128. Instead the spherical surface 132 contacts the concave upper surface 124 in one ring shaped contact point 134 the geometrical center of which coincides with a vertical line extending through the bottom point 128.

In the test situation, the force application element 130 is moved downwards in Figs. 2 and 3 whereby a compressive force 136 is applied. As the force application element 130 abuts the tooth in one ring shaped contact point 134, the force causes a transverse force 138 to be applied to the concave upper surface 124. Although not shown in the figure, it will be appreciated from simple vector theory that this force 138 has a vertical and a horizontal component. When the compressive force 136 is large enough, the force causes the tooth 110 to rupture along a rupture line 140. However due to the provision of the first neighbouring element 112 and the first spacing element 116, the force 136 which can be applied to the tooth 110 is larger than if these elements 112,116 were not provided. Thus, this test method is closer to the natural use situation than a test method without any neighbouring teeth.

Figs. 4-6 disclose a second example of the invention which is relatively similar to the first example, and thus identical reference numbers refer to identical objects/features. The differences are as follows: In Figs. 4-6, the tooth 110 is prevented from moving to the sides by means of retaining elements 142. Moreover, the first spacing element 116 and the second spacing element 118 are not as tall as in Figs. 1-3.

In Figs. 4-6, the tooth 110 does not define an inwardly concave upper surface 124, but rather two inclined surfaces 144 terminating in a bottom line 146. Moreover in Figs. 4-6, the force application element 130 does not coincide with the longitudinal axis (not shown) of the tooth 110. Instead the force application element 130 is arranged such that the force 136 applied by the force application element 130 is only transferred to one point 134. Accordingly, the resulting force applied to the inclined surface 144 results in a vertical force 138 having a left component 148 and a right component 150.

Fig. 7 and 8 discloses a third example of the invention in which each of the first and the second neighbouring elements 112,114 abut the tooth 110 directly, in an area of contact 152. In order to make the contact between the first and second neighbouring elements 112, 114 and the tooth 110 as close to the real life contact between teeth in a mouth, the first and the second spacing elements 116, 118 are shaped such that the area of contact is limited to the area of contact 152. Accordingly, in the embodiment of Figs. 7 and 8, the first and second neighbouring elements 112,114 are not cylindrical.

It will be appreciated that in another embodiment, the same effect may be achieved by providing cylindrical first and second neighbouring elements 112, 114 together with a tooth 110 forming the same lateral protrusions 154 as the first and second neighbouring elements 112, 114 in Figs. 7 and 8. However, as the first and second neighbouring element 112, 114 normally will be reused and as it is easier and cheaper to manufacture a cylindrical tooth 110, the embodiment in the Figs. 7 and 8 is advantageous from an economic point of view.

In the embodiment of Figs. 7 and 8, the first spacing element 116 and the second spacing element 118 are provided between the side surface 104 and the first spacing element 116 and the second spacing element 118, respectively. By providing said space defining elements 116, 118, it is possible to emulate the real life ability of a tooth to move/flex relative to the jaw.

Figs. 7 and 8 illustrates a fourth example of the invention. Again identical reference numbers refer to identical elements/features. The two main differences relative to the previous figures is that only one retaining element 142 is provided. However this element is ring shaped and thus prevents movement of the tooth 110 in any direction in the plane defined by the bottom The other difference is the provision of pressure adjustment screws 156, which may be used to adjust the pressure applied from the first neighbouring element 112 and the second neighbouring element 114 to the first spacing element 116 and the second spacing element 118, respectively. By increasing the pressure, an increased contact/pressure between the tooth 110 and the two neighbouring elements 114, 116 is emulated. Similarly, by decreasing the pressure, a decreased contact/ pressure between the tooth 110 and the two neighbouring elements 114,116 is emulated.

Figs. 10-12 disclose a fourth example of the invention. Again identical reference numbers refer to identical elements/features. Like in the third example, a pressure adjustment screw 156 is provided which is used to adjust the pressure applied from the first neighbouring element 112 to the tooth 110. In the case of Figs. 10-12, the first neighbouring element 112 is provided in the form of a cylindrical element which is received in a housing 158. A resilient/space defining element 118 may be provided between the pressure adjustment screw 156 and the first neighbouring element 112. By providing said space defining element 118, it is possible to emulate the real life ability of a tooth to move/flex relative to the jaw.

The tooth 110 is circular and defines a concave upper surface 124 and side surface 126. The concave upper surface 124 defines a bottom point 128.

In order to test the tooth, a force application element 130 is provided. The force application element 130 is secured to a force application unit 160 which is movable towards and away from to the housing 158. Thus, when the force application unit 160 is moved down towards the housing 158, the lower surface 162 of the force application element 130 is brought into contact with the concave upper surface 124 of the tooth whereby force is applied to the tooth. Due to the provision of the first neighbouring element 112 apart of the force is applied to said first neighbouring element 112. It will be appreciated that the lower surface 162 of the force application element 130 advantageously may define the inverse geometry of the concave upper surface 124. Accordingly, this surface 162 may define a part of a concave surface. In one embodiment, the concave upper surface defines a part of a cone and thus the lower surface 162 may also define apart of a cone.

The force application unit 160 is free to move towards and away from the housing 158, such that the abovementioned application of force may be applied. However it will be appreciated that lateral movement of the force application unit 160 relative to the housing 158 is undesirable as this will cause the mating surfaces 124,162 not to be aligned. Accordingly, a rod-shaped guide element 164 may be provided which is secured to the housing 158, while allowing the force application unit 160 to move freely relative to the rod-shaped guide element 164, as the element 164 is received in a passage 166 of force application unit 160 but not secured thereto. Moreover, lateral guiding elements 168, is provided which extend down into engagement with lateral surfaces 170 of the housing.