1. Field of the Invention
The present invention relates generally to a dental cement composition, and specifically to a dental glass ionomer cement composition. More specifically, the present invention is directed to a dental glass ionomer cement composition making use of the curing of a polymerizable unsaturated organic compound by a polymerization reaction in combination with the curing of a fluoroaluminosilicate glass powder with an .alpha.-.beta. unsaturated carboxylic acid polymer by a neutralization reaction.
2. Prior Art
Many types of dental cements have been available in various applications. Typical of dental cements used so for in the art are zinc phosphate cement making use of the reaction of zinc oxide with phosphoric acid, polycarboxylate cement making use of the reaction of zinc oxide with a polycarboxylic acid, zinc oxide eugenol cement making use of the reaction of zinc oxide with eugenol, glass ionomer cement making use of fluoroaluminosilicate glass powders with a polycarboxylic acid, and resin cement making use of the polymerization of an acrylic monomer.
These dental cements have both merits and demerits. An ideal cement is not still found. For instance, the zinc phosphate cement shows no adhesion to dentine and has a stimulating action peculiar to phosphoric acid at the initial stage of curing; the polycarboxylate cement provides a cured material the final strength of which is low; the eugenol cement is used only for temporal sealing and attachment due to its low strength and its poor durability in the oral mouth, but it has a stimulating action inherent in eugenol; and the resin cement presents a problem in terms of bioaffinity.
In contrast, the glass ionomer cement is characterized in that it is of a very excellent bioaffinity and shows adhesion to dentine, and yields a cured material that is of semi-transparency and has a good-enough aesthetic appearance as well. In addition, fluorine contained in the glass is expected to make some contribution to dentine reinforcement. By taking advantage of many such characteristics, the glass ionomer cement has wide applications in the dental field, and are now used for the restoration and filling of caries cavities, the attachment of crowns, inlays, bridges and orthodontic bands, the lining of cavities, core building, and preventive filling.
A grave problem with the glass ionomer cement, however, is that its curing reaction is inhibited upon contact with moisture such as saliva at the initial stage of curing, resulting in some considerable degradation of its final physical properties. The reason is that the glass ionomer cement is easily affected by water, since the neutralization reaction between the polycarboxylic acid (an acidic component) and the fluoroaluminosilicate glass (a basic component) occurs in the presence of water. As the glass ionomer cement comes into contact with water at the initial stage of curing, its surface becomes brittle and cloudy, resulting an aesthetic problem. Many attempts have heretofore been made so as to eliminate this problem.
To achieve a sharp curing rate, for instance, Japanese Patent Publication No. 54(1979)-21858 and Japanese Patent Laid-Open No. 57(1982)-2210 teach the addition of a chelating agent and a fluorocomplex salt, respectively. Even according to these teachings, no complete solution can be provided to the above-mentioned problem or the degradation due to moisture of glass ionomer cement at the initial stage of curing.
To solve this problem, we have come up with a dental glass ionomer cement composition which comprises a liquid component containing a polymerizable unsaturated organic compound and a polymerization catalyst in addition to a conventional polyacrylic acid, so that it can be sharply cured by exposure to visible light, as disclosed in Japanese Patent Publication No. 6(1994)-27047.
In this glass ionomer cement, the neutralization reaction of the fluoroaluminosilicate glass powder with polyacrylic acid takes place at the initial stage of curing with the concurrence of the polymerization reaction of the unsaturated organic compound by exposure to light, so that it can cure quickly. This eliminates the problems in association with conventional glass ionomer cement compositions; their embrittlement or disintegration due to contact with moisture at the initial stage of curing. In addition, this cement can be more easily manipulated by curing by light than ever before. In particular, this glass ionomer cement composition are improved in terms of physical properties such as initial hardness, adhesion strength to dentine, bending or flexural strength, and transparency. Even in this cement, however, there are still some problems waiting solutions.
Among the advantages of the above-mentioned light curing type of unsaturated organic component-containing glass ionomer cement, there are improved physical properties such as initial hardness, flexural strength, adhesion strength to dentine and transparency in addition to use of visible light with which it can be more easily manipulated than ever before and resistance to embrittlement due to contact with moisture. However, this cement cannot be used for the purpose of attaching crowns or inlays to dentine at where irradiated light will not reach, because the polymerization of the unsaturated organic compound must be carried out by irradiating it with visible light. Another disadvantage of the light curing type of glass ionomer cement is that some limitation is placed on its curing depth achieved by exposure to visible light; that is, as when it is filled thick in a cavity, some cement layer tends to remain uncured due to incomplete polymerization in a deep portion. Rapid polymerization by exposure to light incurs an unavoidable contraction, which results in a gap or other cavity being formed between the cement and dentine. This, in turn, leads to detachment of the filler, or allows saliva to enter the gap or cavity, inducing secondary caries.
Thus, there has been a strong demand for glass ionomer cement which, while taking advantage of the unsaturated organic compound-containing cement, can be cured without recourse to exposure to visible light.
One possible way for dealing with this is to make use of a redox reaction in which a redox catalyst, typically a peroxide represented by benzoyl peroxide or KPS etc. takes part. However, the peroxide is unstable in glass ionomer cement, and so degrades immediately unless stored always at 4.degree. C. or lower. This cement system is unsuitable for dental therapy because it evolves heat of considerable high temperature upon curing. The peroxide has a stimulating action on, and so harmful to, dentine. This accounts for a strong need for a glass ionomer cement composition containing a polymerizable unsaturated organic compound, which, even without recourse to use of a conventional redox reaction system and to exposure to visible light, can be cured into a material having physical properties good-enough for practical dental therapy, and can be stored over an extended period of time and harmless to dentine.