WO1992006665A1

WO1992006665A1 - Dental clasp

Info

Publication number: WO1992006665A1
Application number: PCT/GB1991/001862
Authority: WO
Inventors: James Spence; Rex Palmer
Original assignee: Trucast Limited
Priority date: 1990-10-23
Filing date: 1991-10-23
Publication date: 1992-04-30
Also published as: GB9023047D0

Abstract

A cast article of a tissue-compatible Cr-Co-Mo alloy containing also C and 1.2 to 2 % w/w N2, is characterised by an elongation after fracture (according to BS 3366) of at least 5 %. Such a composition is particularly well suited to the production of an integrated working dental clasp which has good workability.

Description

DENTAL CLASP

Field of the Invention

This invention relates to dental clasps and to materials suitable for forming such clasps.

Background of the Invention

A full denture may be constructed to a mould of the user's mouth; in such a case, there is no problem in supporting the denture within the mouth. In the case of a partial denture which replaces some of the natural teeth given that other natural teeth are retained in the same arch, it is often necessary to provide wires known as clasps which relate to or hook around the natural teeth and thereby retain and/or support the denture. For the purposes of this specification, the combination of plate, skeletion or denture, with clasp(s) constitutes a "working dental clasp".

A problem associated with this construction of partial denture is that the workability of the wires is limited. In other words, the wires cannot sustain much of the bending or twisting that allows them to be worked and shaped, to engage a natural tooth, without fracturing. Even when the denture is fitted, the low workability may cause fracturing during use or on removal/refitting.

Cast dental clasps of a cobalt-chrome alloy are known. The procedure comprises attaching a piece of softened preformed wax pattern by a suitable fixing system to a mould of the teeth. This wax pattern consists of the denture plate/skeleton and the clasp.

Using a traditional lost wax casting process, the dental alloy is cast to replace the constructed wax pattern. The resulting clasp suffers from the disadvantage of lacking workability, leading to potential fracture of the clasp arm under working or adjustment conditions.

An object behind the present invention is to produce cast clasps which have working properties tending towards those of wrought clasps (which are formed from wire). GB-A-2000188 discloses Cr-Co-Mo alloy articles containing N₂ and C, primarily intended for surgical implants, following hot-forging. The possibility of dental casts is mentioned in passing. Three methods of controlling the nitrogen content in an ingot of the alloy are described: in two, casting is conducted under an argon or nitrogen atmosphere, but in none of the methods are any steps taken to expel or exclude oxygen throughout.

Summary of the Invention

According to the present invention, a working dental clasp is formed of a cast tissue-compatible Co-Cr-Mo alloy having good workability. A novel composition is characterised, alternatively or in addition, by its oxide- free nature.

Description of the Invention

The present invention is based on the discovery that certain alloys have sufficient toughness, i.e. high proof stress coupled with high elongation, that make them particularly suitable for use in dentistry. The preferred composition of such an alloy is as follows (amounts in % w/w):

C 0.04 - 0.08

Si 1.00 max

Mn 1.00 max

Al 0.03 max

Cr 27.0 - 30.0

Fe 0.75 max

Mo 5.0 - 7.0

N₂ 0.16 - 0.20

Ni 0.5 max

Ti 0.05 max

Co balance

The presence of up to 2000 ppm nitrogen promotes an increase in strength through interstitial stiffening of the matrix. This increase in strength enables the reduction of grain boundary carbide through reducing the carbon levels in the alloy. The combination of the high nitrogen and low carbon results in an alloy which is tougher than the standard material, but which is also easier to machine or trim. The low level of nickel caters for the small minority of patients who might be nickel-sensitive. Another advantageous aspect of the alloy, for use in dentistry, is that it is beryllium-free.

An alloy of the type described can be produced by alloying the major elements under vacuum, followed by introducing a high back-pressure of nitrogen (but below normal air pressure), when nitrogen can be added in the form of high nitrogen chrome without problems of fade. The process of melting under vacuum followed by back- pressurisation by inert gas prevents the ingress of oxygen, thereby promoting a clean oxide-free material.

A further advantage of maintaining a high nitrogen back-pressure in the furnace, is that it minimises the problem of the metal "spitting" during refining. In consequence, the alloy does not contain components such as aluminium and titanium which might otherwise be used to improve quiescence.

A particularly preferred composition of an alloy for use in this invention is 0.04% C, 0.6% Si, 0.5% Mn, 28.5% Cr, 6.0% Mo and 1800 ppm N₂, the balance being Co.

The composition given above has been found to have unexpected advantages in the manufacture of dental clasps. An example of a suitable clasp is illustrated in the accompanying drawing which is a schematic view of an arch of teeth including (e.g. acrylic) false teeth 1 mounted on a metal partial dental plate 2 of the alloy composition, having two clasp wires or arms 3 cast integrally. The clasp wires are used to hook around natural teeth 4. The false teeth may be attached to the dental plate by any suitable means.

The high elongation values associated with the cast alloy used in this invention mean that the clasp wires are easily worked, without the usual danger of breakage. Thus the dentist can be confident that a clasp of the invention can be manipulated as much as necessary, when putting it in place, or subsequently removing and refitting it. This minimises the need for duplication of clasps for a given patient, and allows problems associated with the inevitable partial loss of precise matching between cavity and mould, when producing the clasp, to be overcome. This can all be achieved within the context of an integral structure, i.e. an integrated cast working dental clasp.

Another advantage of the invention is that the production technique described above provide good consistency. Moreover, the material is easily trimmed to shape.

A dental clasp or other cast article of the invention may be prepared by the lost wax process. Therefore, the advantages described above are associated with a cast material, i.e. without the expense of shaping under pressure, as in drawing, rolling or forging.

The following Examples illustrate the invention. Here and throughout the specification, elongation values are given for test bars cast by the lost wax method in accordance with British Standard 3366: Part l: 1988: ISO 6871-1987 (incorporated herein by reference).

Examples 1 tt 3

Three alloy compositions were prepared by the method described above. Their compositions and mechanical properties are as follows:

Claims

1. A cast article of a tissue-compatible Cr-Co-Mo alloy containing also C and 0.12 to 0.2% w/w N₂, characterised by an elongation after fracture (according to BS 3366) of at least 5%.

2. A cast article of a tissue-compatible Cr-Co-Mo alloy containing also C and 0.12 to 0.2% w/w N₂, that is substantially oxide-free.

3. An article according to claim 1 or claim 2, wherein the alloy has the following composition (amounts in % w/w):

C 0.08 max

Si 1.00 max

Mn 1.00 max

Al 0.03 max

Cr 27.0 - 30.0

Fe 0.75 max

Mo 5.0 - 7.0

N₂ 0.12 - 0.20

Ni 1.0 max

Ti 0.05 max

O₂ substantially zero

Co balance

4. An article according to any preceding claim, wherein the alloy contains 0.16 to 0.2% w/w N₂.

5. An article according to any preceding claim, wherein the alloy contains 0.02 to 0.08% w/w C.

6. An article according to claim 5, wherein the alloy contains 0.04 to 0.08% w/w C.

7. An article according to any preceding claim, wherein the alloy contains no more than 0.5% Ni.

8. An article according to any preceding claim, wherein the elongation after fracture is at least 9%, e.g. 12 to 30%.

9. An article according to any preceding claim, in the form of an integrated working dental clasp.

10. An article according to any preceding claim, for therapeutic use.

11. Use of an article according to any of claims 1 to 9, for cosmetic dentistry.