Dental Prosthesis With Cobalt Alloy Frame

The present invention relates to a new and improved dental prosthesis.

In its more particular aspects the present invention specifically relates to a new and improved dental prosthesis which constitutes a ceramically coated dental prosthesis containing a metal frame and dental ceramic material. Such dental prosthesis may constitute a coated crown, a bridge or the like.

It is a basic prerequisite for alloys which are intended for use in the human body that such alloys must be physiologically safe. Besides the fact which is obvious as such, that the alloy must not dissolve in the human body, a number of further requirements exist when using such alloys in the oral cavity. With respect to the very high corrosive stresses, for example, extreme fluctuations of the pH-value in the oral cavity accompanied by simultaneously variable concentrations of corrosion promoting ions, for example, chloride ions, these alloys still additionally must have properties which are of importance in the course of manufacturing dental prostheses, bridges, or the like. For reasons of appearance as well as of durability, dental prostheses are coated with a ceramic material and this ceramic material is applied to the metal surface by means of firing processes. When firing the ceramic material it must be considered that the temperature is raised by approximately 1,000.degree. C. and, therefore, the differences in the thermal coefficients of expansion may not be too great. Furthermore, if such differences do exist, the thermal expansion of the alloy component must be smaller than that of the ceramic component, so that the ceramic portion is not subjected to tensile stress but to compressive stress after cooling of the dental prostheses because ceramic material possesses only low tensile strength, however, high compressive strength.

Further there often exists the desire that the ceramic material should be applied not by means of a single firing process but by means of a number of firing processes and the behavior during these individual firing processes should correspond to the behavior as initially explained.

Therefore, with the foregoing in mind it is a primary object of the present invention to provide a new and improved dental prosthesis containing a metal frame having a linear thermal expansion coefficient which is unaffected by multiple firing processes required for preparing the dental prosthesis.

Now in order to implement this and still further objects of the invention which will become more readily apparent as the description proceeds, the dental prosthesis of the present development is manifested by the features that the metal frame comprises an alloy containing, each in percent by weight: carbon 0 to. 0.4, preferably, 0.05 to 0.3, silicon 0.1 to 5.0, preferably 0.1 to 3.0, manganese 0.01 to 8.0, preferably 0.3 to 3.0, chromium 25 to 35, preferably 27 to 33, molybdenum 1.0 to 8.0, preferably 4 to 7, niobium 0.1 to 5, preferably 0.5 to 3, nickel 0 to 0.3, iron 0 to 1.0 and, if desired, copper 0.1 to 5.0, preferably 0.5 to 2.0, the remainder being cobalt and impurities resulting from manufacturing conditions.

An alloy having the inventive composition possesses the coefficient of expansion desired for dental frames to be coated with a ceramic material. At the same time the required mechanical values, especially with respect to elongation of fracture and hardness are also satisfied. However, it turned out completely surprising that this alloy possesses high dimensional stability even after exposed to a plural number of thermal loads. These high thermal loads not only occur during the plural firings of the ceramic layer but also are caused by the fitting operation of the prosthesis, for instance, to the counter-denture. During such fitting operation the dentist carries out an adaptation to the counter-denture by mechanically removing the ceramic layer, whereby extreme local thermal loads can arise. If then an irreversible dimensional alteration takes place, this can result in premature destruction of the ceramic material, or in an ugly discoloration of the ceramic material, for example, due to the formation of hair cracks and the penetration of coloring matter.

In the following the invention is further explained with reference to Examples.

During the firing of ceramic materials containing, for instance, 80% by weight of feldspar, 18% by weight of quartz and 2% by weight of boron frit, onto the frame to be coated, the sample is heated one or more times to temperatures between 950.degree. C. and 1,000.degree. C. Heating is generally for 10 minutes with intermediate cooling to room temperature in order to apply, for example, the mass to be fired. For simulating a triple firing and at the same time determining dimensional alterations, different test bodies having a-composition in accordance with the following Table and a diameter of 10 mm and a height of 10 mm, were first heated to 1,000.degree. C. in vacuo, maintained at this temperature for 10 minutes and then rapidly cooled to room temperature. This procedure was repeated three times. No alterations in the diameter as well as in the height could be determined after the first, the second as well as the third firing.

As can be seen from the Table, the coefficient of thermal expansion is in the range between 13.7.times.10.sup.-6 .degree. C..sup.-1 and 15.5.times.10.sup.-6 .degree. C..sup.-1, and thus lies within the desired limits for the thermal expansion in order to maintain the ceramic material to be applied under a desired compressive stress and not under a tensile stress. The hardness between 290 and 410 HV10 also corresponds to the requirements and therefore the proof stress and elongation of fracture permit mechanical processing in correspondence therewith.

While there are described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims. ACCORDINGLY,