Dentistry – Method or material for testing – treating – restoring – or... – Crown
Reexamination Certificate
1998-10-30
2001-09-11
Lucchesi, Nicholas D. (Department: 3732)
Dentistry
Method or material for testing, treating, restoring, or...
Crown
C433S223000
Reexamination Certificate
active
06287121
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to a material for a dental prosthesis made of plastic reinforced with glass fibers.
BACKGROUND OF THE INVENTION
For a long time, dental prostheses have been produced, depending on their use, as inlays, onlays or bridges and, depending on the expense justified in the individual case, from an amalgam, gold or porcelain. When using an amalgam, shaping is performed in situ, i.e. in the mouth of the patient, in that the moldable amalagam material is placed in the not yet hardened state into the cavities to be filled. Some build-up of material outside the cavity is also possible, wherein the shaping takes place either in the form of molding the pliable amalagam, or by means of grinding tools for the hardened amalagam. When using gold or porcelain, shaping is performed by the dental technician, generally in accordance with a cast, or respectively duplicate, of the dental prosthesis to be produced, which was previously made in the mouth of the patient.
Since amalagams as well as gold or porcelain have not given satisfactory results in every respect, it has been tried for some time to produce dental prostheses from plastic. It has been shown here that the durability of such dental prostheses can be considerably improved, if the plastic used for this is reinforced with glass fibers. However, until now glass fiber-reinforced plastics have been little used in dental medicine. Known dental prostheses are produced from bundles of glass fibers, which are soaked in a liquid plastic material acting as an adhesive. Shaping is performed manually in the manner of molding. Because of the manual process steps, the physical properties of such dental prostheses are not assured, in addition, changes in volume can take place during hardening of the known materials.
SUMMARY OF INVENTION
It is therefore an object of the invention to propose a material, with which dental prostheses can be produced, which preferably can be made by means of an automated process, whose shape and volume is constant and which have material properties which do not change either during processing or in use or by aging.
The novel material is a glass fiber-reinforced plastic, from which bodies, which constitute blanks for the dental prostheses, are produced by means of a special injection molding process. The shaping of the dental prostheses is then performed by machining, either in the form of a type of copy-milling, or by means of electronically controlled tools. By means of the production in accordance with a special casting method it is achieved, that the reinforced glass fibers in the plastic material can be arranged as needed. The final physical properties of the material have been reached after hardening and change neither because of mechanical or chemical stresses, nor during processing, nor by aging.
Partially aromatic polyamide has proven itself to be a particularly suitable plastic material, whose mechanical properties are strengthened by the reinforcing glass fibers. For example, it is possible to achieve a modulus of extension of 22 GPa, and a Brinell hardness of 280 MPa. Without admixtures, the load at rupture is approximately 1,150 N, and with admixtures it can reach more than 1,500 N. At the same time this material is light, its specific mass is approximately 1.7 g per cubic centimeter, so that it is approximately 2.6 times lighter than titanium, which already is considered to be very light. In addition, it is a poor thermal conductor, so that complaints by the patient arising from thermal conduction do practically not arise. In an esthetic way the material is more than satisfactory, it is translucent, i.e. lets light through, and has a color similar to tooth enamel.
Depending on the use of the blank for producing inlays, crown or bridges, the material in its end configuration is subjected to different types of stresses. With stresses which are the same on all sides, a material which is isotropic, or at least quasi-isotropic, is preferably used, which is understood to mean that it acts like an isotropic material. This occurs, because the glass fibers are molded in an unaligned manner into the plastic.
However, if stresses placed on the dental prosthesis are preponderant in defined directions, a material which acts anisotropically is preferably employed. Such can be created in that the glass fibers are molded in an aligned arrangement.
Furthermore, the invention relates to a method and a device for determining the shape of a duplicate to be provided with a dental prosthesis, and to a device for executing this method.
For the production of dental prostheses from blanks by means of a machining process it is necessary to know the shape, or respectively the boundaries of the area of the remaining teeth to be provided with the dental prosthesis material. Often the area of the remaining teeth is a single remaining tooth, which is intended to be complemented by an inlay or a crown. In certain cases, for example if bridges are to be made, the area of the remaining teeth includes several remaining teeth. Customarily an automated determination of these boundaries takes place for this purpose, for which several methods are suitable. For example, actual scanning methods, as well as contactless, so to speak optical methods by means of line, strip or pattern projection, holographic interferometry, grey-scale evaluation, distance measurements by means of various sensors, profile measurements and tomographic X-ray image evaluation are employed.
Since the determination of the shape of remaining tooth areas in situ, i.e. in the mouth of the patient, is unpleasant for the patient and arduous for the dentist, and often cannot be performed with the required accuracy, a duplicate is generally made initially, and then the shape of the duplicate of the remaining tooth area is determined. This method, too, has two important disadvantages. For one, shaded areas, or respectively dead corners or undercuts, cannot be detected, because they are not accessible to scanning instruments, or respectively because with the contactless process the direct line of connection between the radiation source, or respectively the sensor, on the one hand, and the area to be scanned on the other is interrupted, namely generally by another part of the very body to be scanned. For another, in certain cases the result of scanning is not sufficiently accurate, since it greatly varies as a function of the dimensions of the body to be scanned. Customarily the resolution of digital measuring sensors is not expressed in length measurements, such as millimeters or meters, but in image particles, which are called pixels. The quotient formed from the size of the measured field and the number of pixels is defined as the actual resolution. Thus, a sensor with 1000 pixels will detect a length of 10 mm at a resolution of 10 &mgr;m, but a length of 100 mm, possibly consisting of a multitude of partial lengths, only at a resolution of 0.1 mm, which corresponds to one tenth of the first mentioned resolution, wherein this reduced resolution no longer meets the requirements of dental medicine.
It is therefore also the object of the invention to provide a method and a device for executing the method, which makes possible the determination of the shape of duplicates of remaining tooth areas to be provided with dental prostheses, while avoiding the above described disadvantages.
The method invention is based on the following considerations: the resolution with which the individual areas of the boundaries to be scanned are determined must be very high, while the relative position of these areas does not necessarily have to be measured with a resolution of the same strength. Known shaping methods for preparing a duplicate of the region to be scanned meet these requirements for accuracy. In contrast to the original, i.e. an area of the set of teeth, a duplicate can be divided into several duplicate sections, after which the shape of each individual one of these duplicate sections is determined. With a suitable division of the section
Efroni Eran
Guiot Jean-Bernard
Traber Tony
Chambers, Esq. Guy W.
DCS Forschungs & Emtwicklungs AG
Lucchesi Nicholas D.
Townsend and Townsend / and Crew LLP
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