Method For Making A Dental Appliance

Abstract

An alloy consisting essentially of 8 to 16 weight-percent chromium, 5 to 15 weight-percent nickel, 1 to 10 weight-percent titanium, remainder cobalt, as a material that can be bonded to ceramic masses by burning, whereby an integrally assembled alloy-porcelain dental applicance is formed for making non-removable dental prostheses.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method of using a cobalt-chromium-nickel-titanium alloy for high-temperature bonding to porcelain for dental purposes.

An alloy with 5 to 15 weight-percent chromium, 5 to 15 weight-percent nickel, 4 to 10 weight-percent titanium, contents of carbon, silicon, manganese, aluminum, and iron up to 1 weight-percent, contents of molybdenum up to 3 weight-percent, remainder cobalt, may be used as a material for tooth prostheses and general surgical implants. This alloy is especially suitable for these purposes, because it possesses good resistance to corrosion and excellent receptivity by the human body, in addition to having better mechanical properties (especially elongation and tensile strength) than the alloys previously used for such purposes.

The present invention concerns the new use of a certain alloy as a material to be bonded by heating to porcelain to make dental appliances, such as crowns and bridges. The joining of metal alloys with ceramic masses by high-temperature bonding to make such appliances places a number of special requirements on the metal alloy. Among the alloy properties of importance are the permanence of color, the ability to form a strong bond with a ceramic mass, and a certain and not overly difficult procedure for working the alloy.

The strength of the bond between the alloy and the ceramic mass is determined essentially by the oxides formed on the surface of the alloy. These surface oxides must not react so strongly with the ceramic mass that a discoloration of the ceramic mass occurs.

Generally, noble metal alloys are relatively less desirable for high-temperature bonding to ceramic masses, since as compared with other metals they have relatively high density and smaller values for their mechanical properties. Therefore, nickel-based alloys have been developed to serve as material to which ceramic masses can be bonded, Some of these alloys contain, along with about 60 to 70 weight-percent nickel, about 15 to 20 weight-percent chromium, about 1 to 5 weight-percent of each of the elements silicon, molybdenum, manganese, and aluminum, and about 0.5 to 1 weight-percent beryllium. These alloys require, however, considerable safety measures when being made or worked, because of their beryllium content.

Other alloys which have been used for bonding to ceramics contain about 60 weight-percent nickel, about 15 to 16 weight-percent of each of the elements chromium and molybdenum, and about 0.5 to 5 weight-percent of each of the elements silicon, tungsten, manganese, vanadium, iron, and cobalt.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to add to the group of alloys which can be used for the bonding of porcelain for dental purposes an alloy which has been discovered to be especially suited for such purposes, meeting in excellent manner the requirements as above given and possessing outstanding mechanical properties.

This as well as other objects which will become apparent in the discussion that follows are achieved, according to the present invention, by using an alloy consisting essentially of 8 to 15 weight-percent chromium, 5 to 15 weight-percent nickel, 1 to 10 weight-percent titanium, 0 to 3 weight-percent molybdenum, 0 to 2 weight-percent tungsten, 0 to 1 weight-percent manganese, 0 to 0.1 weight-percent carbon, remainder cobalt, as a cast material which is provided in several operations with ceramic masses which are deposited by burning for bonding to the said metal body and to each of the successive ceramic layers, whereby the last deposited ceramic porcelain mass with glazing is shaped by burning to form artificial teeth so that the metal body in conjunction with the deposited ceramic masses can be used as a non-removable dental prosthesis. During the burning of the first deposited ceramic layer, the so-called basic mass, oxides are formed on the metal body surface which produce an excellent bond between metal and ceramic mass.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is permissible for the alloy used in the present invention to contain the impurities which are commonly present in the raw materials used for its manufacture. Examples of such impurities are silicon and aluminum, each to be present in the alloy preferably at substantially less than 1 weight-percent.

The alloy for use in the present invention surpasses the previously used alloys in terms of mechanical properties and additionally forms bonding surface oxides more easily because of its titanium content.

In preparing the alloy for practice of the present invention, the alloy may be melted in vacuum induction furnaces using either a vacuum of 10.sup.-.sup.3 to 5 mm Hg or a protective gas atmosphere (for example, argon) at a pressure of about 1 to 300 mm Hg. Casting of the alloy is usually done in burned ceramic molds such as used for small, fine-detail castings.

Conventional casting processes can be used with the alloy in the present invention, since the melting point of the alloy is about 1,350.degree. C. It can be formed in ceramic molds using the techniques of centrifuge casting. The alloy may be melted using an oxygen-acetylene flame adjusted neutrally (that is neither oxidizing nor reducing).

Test specimens for determining the mechanical properties of the alloy used in the present invention are produced using a centrifuge casting method. The cast specimens are turned to final size and polished. They have a diameter of 2.1 to 2.6 mm and a gage length of 25 mm. This corresponds closely to the directives of British Standard 3366:1961 entitled "Specification for Dental Cobalt Chromium Casting Alloy." The strain rate used in obtaining 0.2 percent yield strength data is about 10.sup.-.sup.4 sec.sup.-.sup.1.

In order to test the compatibility of the alloy used in the present invention with the bodies of animals and, in particular, humans, corrosion and electrochemical measurements were carried out.

The corrosion measurements were carried out both in artificial saliva and in an aqueous solution containing 1 percent lactic acid and 0.25 percent NaCl. The solution was held at 40.degree. C. The test specimens were left in such solution for more than one year. Even for such a long test time, the alloy gave the extremely small corrosion velocities of 0.01 mm/year or less. Such velocities are below the accuracy of measurement of usual equipment.

The artificial saliva was an aqueous solution containing the following additions per liter;

0.95 grams K.sub.2 HPO.sub.4 0.23 grams NaCl 0.16 grams CaCl.sub.2 0.55 grams KCl 0.22 grams KSCN 0.13 grams (NH.sub.2).sub.2 CO 0.77 grams NaHCO.sub.3

current-time curves were recorded in artificial saliva in the electrochemical measurements. Potentials of +100 and +150 mV as measured against a saturated calomel electrode were used. These potentials can arise should the alloys of the present invention come in contact with gold in the mouth of an individual. In these experiments, the alloys gave current densities of 10.sup.-.sup.4 to 10.sup.-.sup.5 milli-amperes per square centimeter. Such values are hardly measurable using conventional equipment. In no case in these experiments was evidence of corrosion found.

In addition to the above experiments, the alloy of the present invention has been in the mouths of humans now for more than 3 years. Neither evidence of corrosion nor any type of disturbance or effect on the persons involved has been noted.

Further illustrative of the present invention are the following examples:

EXAMPLE I

An alloy usable in the present invention has the following composition: 13.6 weight-percent Cr, 7.9 weight-percent Ni, 0.045 weight-percent C, 8.5 weight-percent Ti, remainder Co. And, it has the following mechanical properties: a 0.2 percent yield strength of 75.7 kiloponds/mm.sup.2, a tensile strength of 116.0 kiloponds/mm.sup.2, and an elongation of 12.4 percent.

The test specimens used for determining the mechanical properties of this alloy were manufactured in the following manner:

At first, cobalt, chromium and nickel mixed in proportions to obtain the desired composition of the alloy were melted in a vacuum induction furnace using a vacuum of 10.sup.-.sup.1 to 10.sup.-.sup.3 mm Hg. After complete degassing of the metal bath, commercially pure titanium melted in a vacuum arc furnace was added to the liquid alloy in the quantity required to obtain the desired composition. After complete solution of the titanium in the metal bath, the liquid alloy was cast at a temperature of about 1,430.degree. C into a burned ceramic mold placed in a vacuum induction furnace. This burned ceramic mold had been manufactured in the conventional manner by the lost wax process. The cast alloy which after solidification was available in cylindrical or rectangular parallelopiped form was blasted with steel shot and subsequently cut up into pieces weighing between 15 and 30 grams. For making the test specimens, these metal pieces were melted using an oxygen-acetylene flame adjusted neutrally, whereupon the liquid metal was cast centrifugally into burned ceramic molds also made by the lost wax process. Before casting, these molds were preheated to about 950.degree. C and the casting temperature was again about 1,430.degree. C. The solidified as-cast specimens had a diameter of 2.8 mm. When turned and polished, the test specimen had a diameter of 2.4 to 2.5 mm and a gage length of 25 mm. The mechanical properties as above given represent the average values obtained from measurements on four different test specimens.

EXAMPLE II

An alloy B usable in the present invention was prepared having the following composition: 11.9 weight-percent Cr, 9.0 weight-percent Ni, 0.53 weight-percent Mo, 0.003 weight-percent C, 8.2 weight-percent Ti, remainder Co.

EXAMPLE III

An alloy C usable in the present invention was prepared having the following composition: 12.1 weight-percent Cr, 9.1 weight-percent Ni, 1.3 weight-percent Mo, 0.008 weight-percent C, 8.2 weight-percent Ti, remainder Co.

EXAMPLE IV

An alloy D usable in the present invention was prepared having the following composition: 9.8 weight-percent Cr, 11.9 weight-percent Ni, 0.040 weight-percent C, 8.1 weight-percent Ti, remainder Co.

EXAMPLE V

An alloy E usable in the present invention was prepared having the following composition: 12.0 weight-percent Cr, 7.8 weight-percent Ni, 0.53 weight-percent Mo, 0.030 weight-percent C, 8.4 weight-percent Ti, remainder Co.

EXAMPLES VI TO X

Alloys F to J usable in the present invention were prepared having the following compositions in weight-percent: ##SPC1##

The alloys of Examples II to X exhibited the following properties:

Alloy 0.2% yield strength tensile strength elongation in kiloponds/mm.sup.2 in kiloponds/mm.sup.2 in % B 77.5 113.5 11.4 C 81.5 112.2 10.0 D 72.0 108.0 10.0 E 70.8 107.0 12.2 F 79.5 108.4 10.3 G 58.5 92.0 13.6 H 61.8 111.8 14.5 I 67.7 92.3 14.0 J 76.7 104.7 10.1

EXAMPLE XI

An alloy K was prepared having the following composition: 12.0 weight-percent chromium, 9.0 weight-percent nickel, 8.1 weight-percent titanium, 1.0 weight-percent molybdenum, 0.011 weight-percent carbon, remainder cobalt. This alloy had the following mechanical properties:

0.2% yield strength 79.5 kiloponds/mm.sup.2 tensile strength 116.4 kiloponds/mm.sup.2 elongation 11.5% Brinell hardness 357 kiloponds/mm.sup.2

This alloy was provided in several operations with ceramic masses which were deposited and bonded to the metal body and to each of the successive ceramic layers by burning, whereby the last deposited ceramic porcelain mass with glazing was shaped by burning to form artificial teeth so that the metal body in conjunction with the deposited ceramic masses could be used as a non-removable dental prostheses. It was found that all requirements as above given were met in outstanding manner. Additionally advantageous is that the average linear coefficient of thermal expansion for this alloy between room temperature and 600.degree. to 900.degree. C amounts to (15.4 to 16.4) .times. 10.sup.-.sup.6 .degree. C.sup.-.sup.1 and consequently adequately matches that of fired porcelain, which lies from about (14 to 15) .times. 10.sup.-.sup.6 .degree. C.sup.-.sup.1.

The alloys for use in the present invention have been found to be suited for making alloy-to-porcelain bonded dental appliances from the ceramic masses presently available on the dental supplies market. The making of the bonded appliances can proceed according to the instructions provided by the manufacturers of the ceramic and porcelain masses.

The mentioned ceramic masses are produced and delivered for example by the firms VITA-Zahnfabrik H. Rauter KG, Saeckingen (BRD) and IVOCLAR AG, Schaan (Liechtenstein). The burning takes place mainly with temperatures between 650.degree. and 1,000.degree. C.

The terms "0.2 percent yield strength," "tensile strength," and "elongation" are used as defined on pages 4 and 5 of "Elements of Materials Science" by Lawrence H. Van Vlack, 2nd edition, 1964, Addison-Wesley Publishing Co.

The Brinell hardness was calculated with the formula as set forth on page 94 of the "Metals Handbook," American Society for Metals, 1948, where the test load was 187.5 kiloponds and the ball has a diameter equal to 2.5 millimeters.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims

We claim:

1. The method of using an alloy consisting essentially of 8 to 16 weight-percent chromium, 5 to 15 weight-percent nickel, 1 to 10 weight-percent titanium, remainder cobalt, comprising placing such an alloy in contact with several layers of ceramic masses as parts of a dental appliance, and heating the contacting alloy and masses for causing their bonding to one another, whereby an integrally assembled alloy-porcelain dental appliance is formed.

2. The method as claimed in claim 1, wherein said alloy contains up to 3 weight-percent molybdenum, up to 2 weight-percent tungsten, up to 1 weight-percent manganese, and up to 0.1 weight-percent carbon.

3. The method of using an alloy consisting essentially of 8 to 16 weight-percent chromium, 5 to 15 weight-percent nickel, 1 to 10 weight-percent titanium, up to 3 weight-percent molybdenum, up to 2 weight-percent tungsten, up to 1 weight-percent manganese, up to 0.1 weight-percent carbon, remainder cobalt, comprising bonding a body of such an alloy to ceramic masses by burning for making non-removable dental prostheses.