Patent Application
20070172787
The invention relates to a method for the fabrication of a dental prosthetic item, for example, frame structures such as dental copings or bridges, on the basis of high strength, brittle materials, especially ceramic materials such as zirconium oxide and aluminum oxide or sintered metals.
Such materials are not imparted with their ultimate strength properties until a second process step is carried out, for example a sintering process, in the case of ceramic material.
In the methods disclosed in the prior art, restorations are produced by process steps carried out as follows.
First of all the blank is manufactured. To this end, the raw materials for a batch are produced, and then the mill blank is manufactured by compressing the raw material. Finally, the sintering shrinkage parameters of typically 25% for the batch are defined. Sintering shrinkage parameters vary from batch to batch typically within a tolerance range of ±2%.
The sintering shrinkage parameters for individual batches can be determined by a laborious procedure with a degree of accuracy typically ranging from 0.1% to 0.2%; the batches can be designated accordingly.
Furthermore, a blank with a connecting geometry for attachment thereof to a holder is known from the manufacture of implants.
Then the restoration is manufactured, which involves making an impression of the situation to be restored in the patient's mouth and subsequently producing a scan model. This is followed by 3D scanning of the model and designing the restoration by CAD/CAM methods. Once the design data are obtained, a suitable blank is selected and the sintering shrinkage parameters of the selected blank are imported. The design data are adjusted with reference to the sintering shrinkage parameters to suit the selected blank prior to 3D machining thereof. Following carving, a shaped part is produced which is further processed, in the case of ceramics by sintering, to achieve its ultimate strength properties. Finally, this shaped part having its ultimate strength properties can be veneered with veneering ceramics.
The disadvantages of this method are that exact process control is required for the manufacture of the blank, a process step is required to determine the shrinkage parameters, and high demands are placed on the sintering process. Systems available on the market show that the sum of all of the errors arising from the process steps can lead to considerable deviations in the dental prosthetic items produced, which prosthetic items are generally in the form of a framework.
According to the invention, the 3D shaping operation is divided into a coarse machining operation on the blank and a precision finishing operation on the shaped part having its ultimate strength properties, for the production of the final shape of the dental prosthetic item.
The advantage thereof is that minimal demands with respect to precision are placed on the first 3D shaping operation and the process used for achieving ultimate strength properties. Only the dimensional deviations need to be corrected during the finishing operation.
In order to be machined, the blank is advantageously attached to a holder having a first connecting geometry, which is designed such that the blank in its uncompacted form can be mounted in a defined position.
Along with the sintering shrinkage parameters, the coarse machining operation can advantageously be such as to provide oversizing in order to cover the entire tolerance range of the fabrication of the dental prosthetic item, in other words the tolerance band of a batch.
In this manner, the process of defining the sintering shrinkage parameters of the batch as well as the importing of the parameters prior to each machining step can be omitted. Due to the low tolerance band of ±2%, with typical restorations of ±20 mm in length there is only an excess of ca. ±400 μm in the direction of the largest dimension. Consequently, this also leads to simplified process control for the manufacture of the raw material for the blank.
Advantageously, the degree of oversizing is collected dependent on to a local position on the shaped part having its ultimate strength properties.
Since the shrinkage caused during sintering is substantially homogeneous, the degree of oversizing can be calculated starting from the center of the shaped part relative to the distance from the center, and the shape of the shaped part can thus be further largely approximated to the shape of the dental prosthetic item.
During coarse machining, an unmachined residual region is advantageously left on the blank in the vicinity of the holder. This achieves stable attachment of the coarsely machined shaped part to the holder during further machining operations.
The shaped part advantageously remains on the residual block during the coarse machining operation in order to provide a positioning aid for repositioning.
In another advantageous embodiment, a precise reference block is formed on the blank or produced on the shaped part during the coarse machining operation.
According to a further development, the machining data for the finishing operation can be acquired by measuring the reference block on the shaped part or the blank. The shaped part and the reference block are sintered during the sintering operation. The position of the reference block relative to the blank is known to the control software. After sintering, the reference block or blocks reproduce the exact shrinkage parameters in all directions in space. The measurement of the reference block imparted with the ultimate strength properties takes place inside the machining unit, preference being given to an optical operation or a modified tool-tactile operation. The machining schedule is generated from the measured shrinkage parameters. Such measurement can take place outside the unit, if desired.
The errors in the entire process chain are thus corrected, ie, the additional adjustment effort required on the part of the dental technician is minimal.
The shaped part imparted with its ultimate strength properties is advantageously attached to a holder with a connecting geometry that takes the shrinkage parameters into account. Secure attachment is thus ensured for the finishing operation and the shaped part imparted with the ultimate strength properties assumes a precisely defined position relative to the machining tools.
According to a further development, the machining data for the finishing operation can also be obtained by scanning the shaped part after it has been imparted with the ultimate strength properties. The machining schedule is generated from the comparison of these data with the original scanned data. The machining schedule can thus be optimized to allow for criteria such as high speed, high precision, or low wear on the tools. Scanning can be achieved by means of a scanning device on the machining unit or outside the machining unit.
In both types of scanning operations, exact repositioning of the pre-fabricated restoration on the block holder is not absolutely necessary.
According to an advantageous development, scanning of the shaped part imparted with the ultimate strength properties is undertaken only in certain regions that require a high degree of precision. Finally, a comparison of these scanning data with the coarse machining data is used to generate the machining schedule for the finishing operation, allowance being made for the shrinkage parameter acquired from the comparison. The advantage in the acquisition of the shrinkage parameter is that shorter scanning times are made possible.
Since the final shaping of the occlusal regions, the wall regions, and the connecting links is carried out during veneering with veneering ceramics when frameworks are used in the manufacture of dental prosthetic items, in some cases the oversizing used in said regions is tolerable and no machining is necessary to achieve the calculated ultimate dimensions.
In such cases it is advantageous for scanning of the shaped part imparted with the ultimate strength properties to take place only in these regions.
Thus to produce a framework during the finishing operation, it is in some cases sufficient to machine only the following regions: the internal mating surface of the dental coping and the mating surface in the region of the preparation border.
The removal of the shaped part imparted with the ultimate strength properties from the residual region advantageously takes place during the finishing operation, so that the shaped part assumes a precisely defined position relative to the machining tools during the entire machining operation.
An advantageous development of the method includes scanning of the shaped part imparted with the ultimate strength properties by means of a scanning device on the machining unit. On the one hand this saves time, as the shaped part does not have to be mounted in some other device and on the other hand it contributes to the precision of the operation in that accidental misalignment of the shaped part on the holder is avoided.
Another aspect of the invention relates to a holder set that comprises at least two holders, of which each holder has a connecting geometry for an item to be held therein, a first holder has a connecting geometry for a blank not imparted with the ultimate strength properties and a second holder has a second connecting geometry for a shaped part imparted with the ultimate strength properties that has been produced from the blank by machining followed by a compacting operation, wherein said first and second connecting geometries differ from each other by the shrinkage parameters of the blank that will be incurred in the final compacting operation.
Furthermore, the invention relates to a holder for a blank not imparted with the ultimate strength properties and for a shaped part imparted with the ultimate strength properties, wherein a first connecting geometry for said blank not imparted with the ultimate strength properties and a second connecting geometry for said shaped part imparted with the ultimate strength properties that has been produced from the blank by machining, are provided, wherein said first and second connecting geometries differ from each other by the amount of shrinkage of the blank that will be incurred in the final compacting operation.
Such a holder set or such a holder presents a very good foundation for the implementation of the method proposed by the invention.
A final aspect of the invention relates to a shaped part for producing a dental prosthetic item, wherein said shaped part consists of a material not yet having its ultimate strength properties and which is machined from a blank so that it approximates the final shape but is oversized to allow for the shrinkage that will be incurred by the final compacting operation and that covers the tolerance range of the 3D shaping of said dental prosthetic item and preferably includes the tolerance band of each batch of blanks.
An advantageous development of the shaped part has a reference block of known position and dimensions.
The reference block can serve for the measurement of the shrinkage parameters of the shaped part prior to and/or after sintering, because, as a first approximation, the sintering operation causes homogeneous shrinkage throughout the material.
A final advantageous development consists of a shaped part that includes a positioning aid on a holder on the unmachined region of said shaped part or on which said positioning aid is produced from the blank during the manufacture of a shaped part by means of a 3D shaping operation. This simplifies the exact positioning of the shaped part in the machining unit.
The method of the invention is explained below with reference to the drawings, in which
a to 5b show a holder set for holding the workpiece prior to and after the compacting operation, and
A shaped part 2 attached to a holder 1 is shown in
Furthermore, a reference block 6 is provided on the shaped part shown, said reference block serving for the exact measurement of the shaped part prior to and after the compacting operation in order to determine the shrinkage parameter Xs in all directions in space.
Parts of the structure represented in
By sintering, the shaped part 2 is converted into the shaped part 2′ (not shown) having its ultimate strength properties, which is still to be finished.
In
In
The shrinkage parameters of the material of the blank fluctuate from batch to batch within a certain range, which is typically 2%.
Furthermore, the manufacturing tolerances resulting from the manufacturing method, which can lie within a range of from 5 to 100 μm, are taken into account.
The sum of all of the parameters yields a rough size Xr resulting from the production of the shaped part 2.
After sintering, the oversize that must still be machined off has decreased by the amount defined by the shrinkage parameter Xs, so that only the residual oversize needs to be removed, at least at the sites that are relevant.
Such sites are identified for a framework in
Depending on the dental prosthetic item, however, it may even be necessary to subject the entire dental prosthetic item during the final carving operation.
Scanning of the shaped part having its ultimate strength properties can be performed either optically in the manner already known from dental grinding units or by a modified tool-tactile operation within the machining unit.
The grinding schedule for the ultimate grinding operation is generated from the measured shrinkage parameters.
Of course, the sintered shaped part can be scanned optically outside the machining unit. Compared with scanning using a point sensor or a tool-tactile operation, such scanning has the advantage in that it is much quicker. An intra-oral scanning camera is particularly well suited for this purpose. In order to take full advantage of the measuring range of such a camera, an appropriate scanning device for the camera and for the shaped part to be scanned can be provided. In this manner the risk of camera shake is avoided, on the one hand, while on the other hand it is possible to discern clearly the region in which scanning can be performed. In restorations of considerable length, a plurality of scanned data sets can be combined with each other in a manner suitable for the generation of an overall data set.
a and 5b show a holder set comprising the holders 51 and 52. The illustrated holders 51 and 52 have connectors 53, 54 that accommodate a workpiece. The connector 53 of the holder 51 is configured so that a blank 55, in which the material has not yet been imparted with the ultimate strength properties, can be fastened thereon. After the shaped part has been coarsely machined from the blank and subsequently sintered, all parts of the material shrink to approximately the same extent. It is therefore necessary, for additional machining, to use a holder 52 with a connector 54, of which the connecting geometry is reduced in size relative to the connector 53 by the shrinking factor. The dental prosthetic item 56 can be held thereon in an exactly defined position.
A holder is shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 020 369.5 | Apr 2004 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/51762 | 4/21/2005 | WO | 10/18/2006 |
