Dentistry – Orthodontics – Bracket
Reexamination Certificate
1999-10-18
2001-07-10
Wilson, John J. (Department: 3732)
Dentistry
Orthodontics
Bracket
C433S002000
Reexamination Certificate
active
06257882
ABSTRACT:
TECHNICAL FIELD
The present invention is in the field of orthodontic appliances. More specifically, the present invention is in the field of orthodontic appliances in which at least a portion of the components of the appliances are brazed together using a corrosion resistant, nickel-based alloy.
BACKGROUND OF THE INVENTION
In the assembly of components of an orthodontic appliance, e.g., as part of a system commonly referred t o as “races,” brazing is often used to bond the components together. Brazing is intended to provide bonds having great strength, temperature resistance, and corrosion resistance.
Brazing refers t o a process of joining metal components, often of dissimilar composition, to each other. Typically, a brazing alloy that has a melting point lower than the melting point of the components to be joined is interposed between the components to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing, alloy but below the melting point of the components to be joined. The brazing, alloy melts and wets the joint between the components, often by capillary flow of the melted brazing alloy. In some instance, a certain degree of metallurgical reaction might also occur between the brazing alloy and the base metal of the components. Once the joint is wetted, the assembly is cooled so that the brazing alloy solidifies, thus forming a strong bond between the components.
Brazing alloys used in joining, stainless steel components in the orthodontic and dental fields, categorized by the element of highest weight percentage, have been silver-based, gold-based, or nickel-based. In orthodontic and dental applications, the selection of a brazing alloy class impacts the visual aesthetics (e.g,., the color match between the resulting joint and the components), strength, corrosion resistance, and tarnish resistance of the resulting brazed appliance.
Silver-based brazing, alloys are typically alloyed, individually or in combination, with copper, tin, or zinc in order to lower the melting, point of the brazing, alloy. In brazing, stainless steel, silver-based brazing, alloys have the advantage of providing a low melting temperature and good bond strength. However, the color of the brazed joint resulting from the use of silver-based brazing, alloys does not match stainless steel because the brazed joint tarnishes to a dark color over time. Additionally, data suggest that using silver-based brazing alloys that do not include nickel can result in interface corrosion between the base metal of the components and the braze fillet due to nickel depletion at the braze fillet/base metal interface. See, e.g., I. Kawakatsu, Welding J., June (1973) pp. 233-239.
Gold-based brazing alloys are typically alloyed, individually or in combination, with nickel, copper, palladium, and silver. Gold-based brazing, alloys arc often used with stainless steel because gold-based brazing alloys have good strength, ductility, and tarnish resistance. However, the yellow color of gold-based brazing, alloys contrasts with the stainless steel used in the components to be brazed. Moreover, gold is galvanically noble to the active stainless steel along the braze fillet/base metal interface. The galvanic action results in interface corrosion in laboratory tests in physiological saline (Ringer's solution).
Nickel-based brazing, alloys offer an excellent metallic color match to the stainless steel in orthodontic and dental appliances. Conventional nickel-based brazing, alloys are most commonly alloyed with boron in order to lower the melting, point of the brazing, alloy. Chromium also is often present to increase the ductility of the braze fillet while increasing the resistance to high temperature and corrosion. However, under manufacturing or use conditions, the braze joint formed from conventional nickel-based brazing, alloys is preferentially corroded.
What is needed, therefore, is a brazing alloy that has a color that is compatible with the stainless steel used in orthodontic and dental appliances, can form a strong, bond, and is corrosion and tarnish resistant.
SUMMARY OF THE INVENTION
It has now been discovered that the corrosion problem of orthodontic appliances having components brazed together with nickel-based brazements is substantially due to the presence of boron used in these alloys. In the oral cavity, the boron can react with the chromium of the brazement and/or the joined stainless steel components, leaving the braze interface easily corrodible. The strength of the brazed joint also can be weakened.
Accordingly, the present invention is based, at least in part, upon the concept of using boron-free, nickel-based alloys as a brazement for orthodontic assemblies. In preferred embodiments, the alloy may include an agent other than boron that helps to reduce the liquidus temperature of the alloy to allow brazing to occur at conveniently low temperatures. Brazed orthodontic assemblies formed using the boron free, nickel-based alloy of the present invention are highly corrosion resistant, even in harsh saline environments such as Ringer's Solution. The brazement also has good mechanical strength and provides a good color match with stainless steel for good aesthetics. Preferred embodiments also are produced with smooth fillet surfaces by using high heating rates through the melting range of the brazement.
In one aspect, the present invention relates to a method of brazing a first component of an orthodontic appliance to a second component of the orthodontic appliance, comprising the step of brazing the first component to the second component using a brazing medium comprising a boron-free brazing alloy that includes at least about 50 weight percent nickel.
Another aspect of the present invention relates to an orthodontic appliance comprising a first component and a second component, wherein the first and second components are brazed together by a boron-free brazing alloy of the present invention comprising at least about 50 weight percent nickel.
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I. Kawakatsu, Welding J. June (1973), pp. 233-239.
Composition and in Vitro Corrosion of Orthodontic Appliances, Copyright 1994-97, Optimedia, Ltd.
E. Lugscheider et al., “Development of Nickel-Chromium-Silicon Base Filler Metals”; Welding Journal including Welding Research, vol. 57, No. 10, Oct. 1978, pp. 319-s-323-s.
3M Innovative Properties Company
Christoff James D.
Wilson John J.
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