Field

The invention relates to a method of measuring the geometric quality of a manufactured dental component and apparatus for carrying out said method.

Background

A well understood problem in the field of manufacturing individualised components is that of performing quality control on each component.

A component is first designed using a CAD system to have a customised shape. The component is then milled according to the CAD shape and sintered to produced a final hardened dental component. During the process of milling and sintering a component, a number of errors and distortions maybe introduced to the geometric shape of the component. When sintering, the component is predicted to shrink, although the degree and consistency of the shrinkage is hard to control. Consequently, each component must be checked.

As each component will have a different geometric configuration, the quality control process must be arranged so as to verify each individual geometric configuration to a high precision. This process is typically performed manually by an inspector trained to identify mismatches between the manufactured component and a control die. A control die is a fabricated replica of the scanned tooth which is based on the original scanned shape. From an ocular inspection, the inspector can see if the control die fits the component or if it is loose. The inspection may be based on a tentative feel of how the crown fits the control die. Special care is to inspect how the coping fits the preparation line. The preparation line is where the coping crown meets the tooth forming the border between the prepared and unprepared part of the tooth. The inspector then makes a determination of quality and rejects the component if the quality is not high enough. The problem with this process is that a poorly trained or fatigued human inspector may be inconsistent with their quality control standards, allowing otherwise poor quality components to pass.

What is needed is an automated technique which allows accurate and consistent quality control of individualised components.

Summary

The invention is defined in the independent claims. The dependent claims describe preferred embodiments of the invention.

According to a first aspect of the invention, there is provided a method of making a dental component, comprising the steps of:

- providing a dental component geometric data having a plurality of reference markers,

- manufacturing the dental component according to said dental component geometric data, said dental component having physical reference markers corresponding to the reference markers of the dental component geometric data

- obtaining an image of the dental component including said physical reference markers

- determining a geometric relationship between the reference markers of the dental component geometric data and the reference markers of the image of the dental component

- determining the quality of the dental component in dependence on the geometric relationship.

According to a second aspect of the invention, there is provided a method of inspecting a geometric quality of a manufactured dental component:

providing a dental component geometric data having a plurality of reference markers

providing said dental component manufactured in dependence on the dental component geometric data, said dental component having physical reference markers corresponding to the reference markers of the dental component geometric data obtaining an image of the dental component including said physical reference markers

determining a geometric relationship between the reference markers of the dental component geometric data and the reference markers of the image of the dental component

determining the quality of the dental component in dependence on the geometric relationship.

Figures

Aspects of the present invention will now be described by way of example with reference to the accompanying drawing. In the drawings:

Figure 1 shows the process flow for the manufacturing and quality control process. Figure 2 shows the oral situation of the patient before applying a component.

Figure 3 shows the surface data of the surface scan.

Figure 4 shows the virtual component.

Figure 5 shows the addition of the reference markers to the virtual component.

Figure 6 shows an embodiment of a reference marker.

Figure 7 shows the finished component as applied to a tooth stem.

Description of embodiments

Specific embodiments of the invention will now be described with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like numbers refer to like elements.

The following description focuses on an embodiment of the present invention applicable to the quality control of manufacture of individualised dental components. However, it will be appreciated that the invention is not limited to the use of individualised dental components, but may be applied to the dental components in general, or even any manufactured component.

The following is a description of a preferred embodiment of the invention.

Figure 1 shows the process flow for the manufacture and quality control of a dental component.

In step 10, data determining the necessary shape of the dental component is obtained. In the preferred embodiment shown in figure 2, the dental component is a dental crown 210 configured to fit to an existing tooth stem 220 of a patient. Consequently, the surface of the tooth stem 230 will define at least part of the shape of the dental component, the surface to fit against the tooth stem. Therefore, in step 10, the surface of the tooth stem is imaged with a high degree of accuracy (e.g. to a tolerance of 10 μηη) using a 3D surface scanner and the resulting surface data 330 shown in figure 3 is provided to a CAM/CAM system.

In an alternative embodiment, the dental component is a bridge, denture or partial denture. Any customised component that is designed to fit a specific patient may benefit from the present invention.

The surface data could be in the form of a point cloud, triangulated data or any common data format such as IGES or STL data formats.

In step 20, a virtual dental component 410 is designed by a technician using the CAD/CAM system. In the preferred embodiment shown in figure 4, the virtual dental component is designed, using the tooth stem surface data, to have a surface 430 to fit to the tooth stem. The body 420 of the virtual dental component is then designed by the technician to provide a good aesthetic outcome and suitable occlusal surface in order to make a corresponding physical crown comfortable in the patient's mouth.

In step 30, as shown in figure 5, a number of reference markers 540 are added to surface 430 of the virtual dental component 410. This step maybe performed by the technician by selecting suitable sites on the surface 430 of the virtual component and placing them manually using the CAD/CAM software. In the preferred embodiment, the reference markers are placed only on the portion of the surface 430 of the component which is configured to fit against the surface of the tooth stem.

Alternative, the reference markers may be placed automatically. In some embodiments, the position of the reference markers are chosen to match those of ideal support points for providing optimal robustness for the fit of the component to the tooth. In these embodiments, determining the positions of the reference markers may include performing the algorithm of Wang et al., Optimizing Fixture Layout in a Point-Set Domain, IEEE Transactions on Robotics and Automation, Vol. 17, No3, June 2001. Alternatively, a configuration of random seed positions as prospective marker positions may be generated and then the marker positions iteratively moved to find an optimal configuration with the best support robustness. In one embodiment, a total of 6 reference markers are employed.

In the preferred embodiment shown in figure 6, the reference markers are dome shaped and extend 25 pm from the surface of the virtual component. In another embodiment, the reference markers are merely a portion of textured surface, adding no extra volume to the component. In another embodiment, the reference markers are n-gonal pyramid (i.e. Tetrahedron, Square pyramid, Pentagonal pyramid, Hexagonal pyramid). The apex point provides a very precise reference point for high resolution scanners.

Once the design of the virtual component is complete, the virtual component and reference markers are recorded as component geometric data.

In step 40, the dental component is manufactured according to the component geometric data. Preferably, the component is first milled from a pre-sintered pressed powder bar in dependence on the component geometric data. The pre-sintering allows sufficient hardness to allow the accuracy of the mill to mill the reference markers, without hardening the bar too much to mill. The resulting milled component is then sintered to further shrink and harden it, making it suitable for placement in the patient's mouth.

In step 50, the manufactured dental component is imaged. In the preferred embodiment, this is performed using a high resolution 3D surface scanner. The resolution of the scan is sufficient to accurately record the surface of the component and the position of the reference markers on the surface of the component.

Alternatively, the scan may be performed using high resolution CT scan or other scanning technique known in the art.

In step 60, the geometric relationship between the reference points of the imaged dental component and the corresponding reference points of the virtual component added in step 30 is determined. Many automatic matching programs are known, such as Geomagic® and Mathlab®.

In step 70, a determination of the geometric quality of the component is determined in dependence on the geometric relationships of step 60. In one embodiment, the recorded distance between each component and the corresponding virtual component could be represented in a plot or graph and a maximum threshold value could be set for each recorded distance. Where a recorded position for a reference marker is 25 μιη from the intended position defined by the virtual refrence marker, this represents a 25 μητι of the component at least around the vicinity of the marker. In one embodiment, any distortion of greater than 10 m results in the rejection of the component.

In one embodiment, if the measured position of any reference marker deviates from its position by more than the threshold value then it is determined that the final geometric shape of the manufactured component is too distorted from the designed virtual component fulfil the intend function of the component, e.g. fit to the tooth stem of the patient or provide a comfortable occlusal surface. If the measured position of each one of the reference markers deviates from their positions by less than the threshold value, the component is determined to be sufficiently accurate to provide a high quality product for the patient.

In an alternative embodiment, the quality determination is based on a more complex analysis of multiple reference points at a time.

In step 80, the dental component is fitted. In the preferred embodiment shown in figure 7, the dental crown is cemented to the tooth stem of the patient. Typically, the layer of cement between the tooth stem and the crown is below 100 pm. Consequently, as the reference markers extend only 25 pm from the surface of the crown, they do not interfere with the fitting or cementing of the crown to the tooth stem.