Background and Prior Art

This invention relates to the use of lining materials for use with composite restorative materials by the "sandwich techniqu Hitherto, the application of composites alone has produced microleakage at the gingival margin, which is probably caused by polymerisation shrinkage forces of the composite creating a channel between the composite and the gingival margin which allows access to bacteria. Such shrinkage forces are greater than the forces of adhesion between the composite and dentine, necessitating the development of either a new technique or other material to eliminate the channel. Previous workers hav utilised glass ionomer cements as a primary layer at the cavit floor or gingival wall prior to the placement of the composite. Glass ionomers adhere strongly to dentine, and their curing mechanism does not create shrinkage forces as great as those for the setting of composites, so reducing the likelihood of microleakage. Moreover, the use of glass ionomers as linings has the desirable effect of continuous fluoride release into ncl.lnt.pnt. Luotli structure, especially relevant in cases of high caries rate and susceptibility.

Glass Ionomer Liners

Ceramic/acrylic fillings are transparent to X-rays. As a resul it is impossible for the dentist to inspect restorations, and t detect decalcified dentine, caries, overhanging margins, defect and voids when examining radiographs of teet restored with non-metallic fillings.

The first three problems can be overcome by using a glass ionomer/acrylic cement containing a filler to confer radiopacit such as barium glass or barium sulphate. The glass ionomer cem would πiiπiπiluu duntino deculclfication and secondary carles if a high fluoride ion-leachable glass were used, together with its constituent calcium salt content. Although such a radiopaq cement should enable detection of defects and voids, it is HP. ^C HTΉH 1-n ΓPHIIΓP thf'Lr occurrence as far as possible.

Other desirable criteria which this liner should meet include:

(a) translucency;

(b) formulation as a two-paste system,to facilitate dispensation from tubes and syringes for mixing by spatula. This would avoid excessive air entrapment and consequent voids, and when used for cementing, would further reduce the .incidence of voids and defects in the restoration;

(c) long shelf like with no separation;

(d) low solubility in oral fluids;

(e) strong adhesion to tooth;

(f) suitable working and setting times;

(g) adequate strength

The Invention

The glass ionomer cement of this invention comprises an ion- leachable fluoroaluminosilicate glass together with 15 to 30%, preferably about 2096 by weight of barium glass or other suitable radiopaquer agents, to provide radiopacity. Blended with the glass is 0.1 to 10.0%, suitably 3 to 5% by weight of an organic peroxide, preferably benzoyl peroxide _* , Both glasses are in the form of fine powders, having a wet film thickness of less than about 15u when dispersed in water.

The frit of the ion-leachable glass has a ternary oxide composition consisting of chemically combined oxides of calcium aluminium and silicon, wherein:

(a) alumina comprises 15-30% by weight of the total glass composition;

(b) calcium oxide comprises up to 50% by weight of the total glass composition;

(c) silica comprises from 10-65% by weight of the total glass composition;

(d) the weight ratio of calcium oxide to silicon dioxide is greater than 0.92;

(e) the weight ratio of Calcium Oxide: Aluminium ortho phosphat is between 0 and 0.74;

(f) the glass comprises less than 14% by weight of fluorine, introduced by the addition of aluminium fluoride, calcium fluoride and/or fluorine containing minerals such as cryolite.

The composition of the glass helps to control the setting time of the cement, which should be between one and ten minutes, an preferably two to five minutes, to allow the dentist adequate working time to mould the cement into the desired shape.

The mixture of powders is blended with an approximately equal weight of a resinous binder to produce a free flowing white paste. Resins pertinent to this work are those used in restorative compositions, particularly:

(a) the addition product of bisphenol A and glycidyl methacryl usually referred to as BIS-GMA;

(b) the reaction product of an organic di-isocyanate with a hydroxyalkyl methacrylate, termed a ύrethane dimethacrylate.

When this glass/resin paste is thoroughly blended with an approximately equal weight of an aqueous solution of a poly- carboxylic acid, preferably poly (acrylic acid), of concentra¬ tion 30-50% by weight, setting takes place to form a cement. The polycarbo;xylic acid solution should contain a maximum of 5% by weight of an amine activator. Representative amines include N,N-dimethyl-p-toluidine, N,N-diethyl-p-toluidine, N,N-dihydroxyethyl-p-toluidine, N,N-dimethyl-3,5-xylldine and p-(dimethylamino) phenylacetic acid, with N,N-dihydroxyethyl- p-toluidine being the preferred activator _β Setting occurs by two mechanisms, namely:

(a) extraction of cations from the ion-leachable component of the glass powder, and their migration and interaction with ani of the polycarboxylic acid to form a partially cross-linked structure;

(b) polymerisation of the resin by free radicals generated fro the interaction of the peroxide in the powder component with the amine dissolved in the polycarboxylic acid solution.

An alternative embodiment of this invention is the subsitution of the chemical curing ^~ agents with a visible light curing systemo A suitable photosensitising system comprises two components, namely an^-diketone photosensitive species togethe with an amine reducing agent„ Representativeς(-diketones inclu biacetyl, camphoroquinone, 2,2' - furil, p-toluil,c-andtf-napht benzil, phenanthraquinone and naphthoquinone, with camphoro¬ quinone being the preferred photoinitiator.

Representative amines include tripropylamine, tributylamine, N-alkyl dialkanolamines and 2-(dimethylamino) ethyl methacrylat with the latter being the most preferred. Either or both components may be incorporated into either or both pastes, although it is preferred that both pastes contain both components,, For reasons of colour, curing time and stability, the quantity of both ingredients is carefully controlled within the range ^0.01 to 0.2% by weight of the paste, and most prefer ably within the range 0.03 to 0.08% by weight. When irradiated with an intense beam of light of wavelength 420-450nm , hardenin should occur in 5 to 60 seconds, and suitably in 10 to 20 seconds. '

The set glass ionomer cement of this invention is acid-etched with a 30-40% by weight aqueous solution of orthophosphoric acid for 30 to 60 seconds, washed with water and dried. It is then optionally treated with a dentinal bonding agent prior to the application of a composite restoration.

Suitable composites include two-paste chemically cured or single paste visible light cured formulations for the restora¬ tion of either anterior or posterior teeth. In the former case, the two components are thoroughly blended by spatulation for about 30 seconds and immediately placed on the glass ionomer liner, shaped, and allowed to set. With light cured composites, the composite is shaped in the lined cavity, and exposed to an intense beam of light of wavelength 420-450nm for 10 to 60 seconds preferably 10 to 20 seconds, to induce hardening.

The composite adhered fairly strongly to the glass ionomer liner as separation was difficult.