Elastomeric State Glass Ionomer Cement
Technical Field
The invention relates to an improved, elastomeric state, castable glass ionomer cement, suitable for use as a liner or base in dental restorations, as a replacement for 5 dentine, or for the replacement of enamel.
Background Art
Glass ionomer cements have been available for a number of years, and used for applications such as dentine substitutes, restoration of primary teeth, and luting cements. They are generally supplied in two parts, a powder and a liquid that are mixed together ι o to form a paste that can be applied to surfaces or placed as a restoration. An acid-base reaction occurs that provides a poly(carboxylic) acid component of the liquid with ionic cross-links, and the mixture sets to a hard cement. Traditionally, glass ionomer cements have been slow to cure, lengthening the time required to complete the restoration.
In recent years, light curable glass ionomer cements have been introduced that have ι ε improved the convenience to the dentist by shortening the time required to complete the restoration, but in general they rely heavily upon a free radical polymerisation process to generate a cured cement, and no longer rely as heavily upon the acid base reaction that generates fluoride ions and provides good compatibility with dentine. Light curable glass ionomer cements tend to be hard or even glassy materials after light curing that
20 show a lower level of fluoride ion release, and a higher volume shrinkage than classical glass ionomer cements. The present invention relates to a glass ionomer cement that is elastomeric after light curing, and is very close in character to a classical glass ionomer cement.
It has been found that a liquid precursor of a glass ionomer cement, comprising an
25 aqueous solution of polymerisable acid or non-acidic monomers and a poly (carboxylic) acid component, when mixed with a powdered precursor of a glass ionomer cement and subjected to a free radical cross-linking reaction, or an anionic or cationic polymerisation reaction, forms an elastomeric pre-cured material. The water-swollen polymer network of the pre-cured material then reacts with the glass within the powdered glass ionomer
30 cement precursor via an acid-base reaction, providing ionic cross-links which produces a cured glass ionomer cement.
The elastomeric nature of this cavity liner after curing provides some previously unrealised benefits. These advantages include: stress relaxation after curing and allows
. for good adaptation to the cavity. Furthermore, it reduces the potential for pull-off from
35 the dentine surface when curing the first increment of composite that is placed on the glass ionomer cement. Moreover, it allows for greater ease of handling and use. Further, the glass ionomer cement has the ability to undergo a free radical
polymerisation that is initiated by a catalysed peroxide. This would ensure that the free radical component of the setting process is taken to completion.
Object of the Invention
An object of the invention is to provide a curable glass ionomer cement that is elastomeric after curing, and suitable for use as a liner or base in cavity restoration.
Disclosure of the Invention
According to a first embodiment of the invention, there is provided an elastomeric material obtainable by curing a composition comprising a mixture of a liquid precursor of a glass ionomer cement and a powdered precursor of a glass ionomer cement, said liquid precursor comprising at least one polymerisable monomer present in a range of between 5 to 50% by weight of said liquid precursor of a glass ionomer cement, a carboxylic acid polymer, and an aqueous solvent, wherein said liquid precursor of a glass ionomer cement and said powdered precursor of a glass ionomer cement are in a ratio of between about 2.5: 1 and about 1 : 1 by weight. According to a second embodiment of the invention, there is provided a method of treating dental caries, comprising applying the elastomeric material in accordance with the first embodiment of the invention as a liner or base for dental or cavity restoration, curing the mixture to form an elastomeric-like material, manipulating the resulting elastomeric material to provide adaptation to the cavity, and allowing an acid-base reaction to proceed to produce a cured glass ionomer cement.
According to a third embodiment of the invention, there is provided an elastomeric material obtainable by curing a composition in accordance with the first embodiment of the invention, wherein the material further comprises a casein phosphopeptide, wherein said liquid precursor of a glass ionomer cement, said powdered precursor of a glass ionomer cement and said casein phosphopeptide are present in a ratio typically from between about 1: 1 :8 to about 2.5: 1:0.0001 by weight.
According to a fourth embodiment of the invention, there is provided a method of treating dental caries comprising applying the elastomeric material in accordance with the third embodiment of the invention as a liner or base for dental or cavity restoration, curing the mixture to form an elastomeric-like material, manipulating the resulting elastomeric material to provide adaptation to the cavity, and allowing an acid-base reaction to proceed to produce a cured glass ionomer cement.
Typically, the elastomeric material is obtainable by curing a composition - comprising a mixture of a liquid precursor of a glass ionomer cement, comprising polymerisable acid monomers, a carboxylic acid polymer, a solvent, a free radical initiator and an activator for the free radical initiator, and a powdered precursor of a glass ionomer cement, wherein said liquid precursor of a glass ionomer cement, said powdered precursor of a glass ionomer cement are in a ratio typically from between
about 2.5: 1 to about 1: 1 by weight, wherein said curing is achieved by a free radical polymerisation reaction.
The preferred ranges for the components of the liquid precursor of the composition in accordance with the first embodiment are as follows: 5 The polymerisable acid or non-acidic monomer may be present in an amount up to
90% by weight based on the total of the liquid precursor of the glass ionomer cement. Typically, the polymerisable acid or non-acidic monomer is present in a range of between 2 to 50% by weight, more typically in a range between 2 to 45 % , 2 to 40%, 2 to 35 %, 5 to 35 % , 8 to 35 % , 10 to 35 % or 12 to 35 % by weight, and even more ιo typically in a range between 15 to 35 % by weight.
Typically, the carboxylic acid polymer may be present in an amount up to 90% by weight based on the total of the liquid precursor of the glass ionomer cement. Typically, the carboxy c acid polymer is present in a range of between 5 to 50% by weight, more typically in a range between 5 to 45 % , 5 to 30% , 5 to 25 % , 10 to 45 % , or 10 to 40% by i s weight, even more typically in a range between 15 to 40% by weight.
Similarly, the aqueous solvent may be present in an amount up to 80% by weight based on the total of the liquid precursor of the glass ionomer cement. Typically, the aqueous solvent is present in a range of between 10 to 75% by weight, and more typically in a range between 30 to 50% by weight. o Where the elastomeric material is obtained through a free radical polymerisation curing process, a free radical initiator may be present in an amount up to 5 % by weight based on the total of the liquid precursor of the glass ionomer cement. Typically, the free radical initiator is present m a range of between 0.01 to 2% by weight, and more typically in a range between 0.1 to 0.5 % by weight.
25 An activator for the free radical initiator may be present in an amount up to 5% by weight, based on the total of the liquid precursor of the glass ionomer cement. Typically, the activator is present in a range of between 0.01 to 2% by weight, and more typically in a range between 0.1 to 0.5% by weight.
The casein phosphopeptide withm the composition in accordance with the third 0 embodiment of the invention may be present in an amount up to 80% by weight based on the total of the liquid precursor of the glass ionomer cement. Typically, the casein phosphopeptide is present in a range of between 0.01 to 10% by weight, and more typically in a range between 0.01 to 5% by weight.
Typically, the ratio of the liquid precursor of a glass ionomer cement and the 5 powdered precursor of a glass ionomer cement in accordance with the first embodiment of the invention is about 1.5: 1 by weight.
Typically, the ratio of the liquid precursor of a glass ionomer cement, the powdered precursor of a glass ionomer cement and the casein phosphopeptide in accordance with the third embodiment of the invention is about 1.5: 1 :0.001 by weight.
The liquid precursor of the glass ionomer cement may be comprised of a variety of polymerisable acid or non-acidic monomers, including any acidic or non-acidic monomers that will take part in a free radical polymerisation reaction Acid monomers are those acids that contain carbon-carbon double bonds. These include methacrylic acid, acrylic acid, itaconic acid, maleic acid, and maleic anhydride. The polymerisable non-acidic monomers may include such monomers as. 2-hydroxy ethyl methacrylate, acrylamide, methacrylamide, or tetrahydrofurfuryl methacrylate. Further, these polymerisable monomers may also be combined with acidic or neutral monomers containing more than one carbon-carbon double bond such as l ,5-dιallyl-2,4-benzene dicarboxylic acid, triethylene glycol dimethacrylate or triallyl- 1 ,3, 5-tπazιne- 2,4,6(lH,3H,5H)-tπone.
The liquid precursor of the glass ionomer cement may be comprised of a variety of carboxylic acid polymers including any homopolymers with a single type of unit along their side chain, such as poly(acrylιc acid) poly(methacryhc acid), and (itaconic acid). The carboxylic acid polymers may also include any copolymers, such as poly(vιnyl methyl ether co-maleic acid), poly(methacrylιc acid co-acrylic acid), poly(styrene co- acryhc acid co- methacrylic acid). Furthermore, the polymer may have double bonds along the side chain, making the polymer capable of taking part in a free radical reaction. The liquid precursor of the glass ionomer cement will contain an amount of an aqueous solvent. Suitable aqueous solvents include water, but may aiso include a mixture such as water and a water miscible liquid such as ethanol or isopropanol.
Both the liquid and powdered precursors of the glass ionomer cement may also contain a free radical initiator such as camphorquinone, azobisisobutyromtrile or riboflavin.
The liquid precursor of the glass ionomer cement may also contain a free radical inhibitor such as butylated hydroxy toluene, hydroquinone and methyl ethyl hydroqumone.
Suitable powdered precursors of the glass ionomer cement include any powder containing any amount of divalent or trivalent metal ions. Examples of these include calcium aluminium fluorosihcate glass, phosphates of zinc and calcium, oxides and hydroxides of calcium, zinc, barium, strontium and aluminium.
The powdered precursor may be comprised of a solid that will generate an acid in the presence of water or an acidic solution. Such a solid may be phosphorous pentoxide, disodium tartrate or disodium maleate.
The powdered precursor may also be comprised of a fluoride as a source of fluoride ions. Suitable examples include sodium fluoride, calcium fluoride, strontium fluoride or sodium aluminium hexafluoπde The liquid precursor may also contain these fluoride sources
The powdered precursor may also contain a peroxide initiator so that the powder- liquid mixture will undergo a free radical polymerisation in the absence of light, Suitable examples of such a peroxide initiator include: benzoyl peroxide or methyl ethyl ketone peroxide. When preparing the glass ionomer cement, part, or all of the poly(carboxylic) acid component of the glass ionomer cement may be added to the powdered precursor, so that the dry poly(carboxylic) acid polymer swells or dissolves in the liquid component, when the powdered and liquid precursors of the glass ionomer cement are mixed together.
Typically, the free radical polymerisation reaction is light activated and is brought about by adding to the liquid formulation, a small amount of an initiator such as camphorquinone and an activator such as a tetramethyl amine. A suitable example of such an amine is N,N-3,5-tetramethyl aniline.
Typically, N,N-3,5-tetramethyl aniline may be present in a range of between 0.1 % to 5 % by weight based on the total of the liquid precursor of the glass ionomer cement. More typically, N,N-3,5-tetramethyl aniline is present in an range of between about 0.1 to about 0.7% by weight, and even more typically in a range between about 0.2 to about
0.5% by weight.
One method of obtaining the elastomeric material of the pre-cured glass ionomer cement is via a free radical polymerisation reaction. The free radical polymerisation curing reaction is initiated by exposing the admixed liquid and powdered glass ionomer cement precursors to light that contains a significant amount of light at a wavelength at or close to 470nm. The curing time may vary from about 5 to about 80 seconds, but more preferably from about 10 to about 60 seconds. An example of such a light source is provided by the Optilux 401 curing lamp (Demetron Research Corporation).
Alternative methods of obtaining the elastomeric material of the pre-cured glass ionomer cement are to utilise a cationic or anionic polymerisation process. Combinations of Lewis acids and proton donors are important initiators for cationic polymerisation. A suitable combination includes boron trifluoride and water. Anionic polymerisation can be initiated by anionic species by transferring a negative charge to the vinyl double bond, for example potassium amide or a mixture of sodium and naphthalene.
The acid-base reaction that occurs to form a cured glass ionomer cement involves the neutralising of the acid groups in the polymer network by multivalent metal ions such as calcium ions and aluminium ions as provided by the powdered precursor. The acid- base reaction is slow, and the rate of the reaction is limited by the diffusion of metal ions out of the glass powder into the polymer network, and subsequent ionic crosslinking.
The casein phosphopeptide is as disclosed in United States Patent 5,015,628, the disclosure of which is incorporated herein by reference.
The giass ionomer cement may also contain an amount of a heavy metal that would render the material opaque to X-rays, ie radio-opaque. Examples of such heavy metals include barium, bismuth, gold, silver, tin, lead, cadmium, antimony, palladium, platinum, tungsten or iridium. The heavy metals should be in a form sufficiently bound such that undesirable heavy metals are unable to be leached in vivo.
Generally, the glass ionomer cement exhibits fracture toughness and flexural modulus values similar to those obtained for classical glass ionomer cements, that is, about 0.4MNm and 6 GPa respectively after 72 hours of curing at ambient temperature.
Best Modes and Other Modes of Carrying Out the Invention
Typically, the preferred compositions of the invention fall within the following ranges, wherein all proportions are calculated by weight:
(a) Liquid Precursor:- Poly(carboxylic acid) 10 to 55%
Polymerisable acidic or neutral monomer 5 to 50% Solvent 10 to 75%
Reducing agent 0.1 to 1.0%
Light activated initiator 0.1 to 1.0%
Free radical inhibitor 0. 1 to 1.0%
(b) Powder Precursor:- Oxide of tetravalent non-metal 15 to 40% Oxide of trivalent metal 25 to 40%
Oxide of divalent metal 5 to 15 %
Oxide of monovalent metal 2 to 15%
Phosphorous pentoxide 10 to 25 %
Fluoride ion 8 to 20% Free radical initiator 0.01 to 5%
Powder to provide radio-opacity 0 to 50% eg Barium glass The powdeπliquid ratio is in the range of 1.0: 1 to 2.2: 1. A typical formulation for Barium glass may fall within the following range: Silicone dioxide 5 to 70%
Barium oxide 1 to 50%
Boron oxide 0 to 30%
Aluminium oxide 0 to 25 % Typically, the preferred compositions of the invention, including the casein phosphopeptide, fall within the following ranges:
(a) Liquid Precursor:- Poly (carboxylic acid) 10 to 55 %
Polymerisable acidic or neutral monomer 5 to 50%
Solvent 10 to 75%
Reducing agent 0.1 to 1.0%
Light activated initiator 0.1 to 1.0%
Free radical inhibitor 0.01 to 1.0%
(b) Powder Precursor:- Oxide of tetravalent non-metal 15 to 40%
Oxide of trivalent metal 25 to 40%
5 Oxide of divalent metal 5 to 15 %
Oxide of monovalent metal 2 to 15 %
Phosphorous pentoxide 10 to 25%
Fluoride ion 8 to 20%
Free radical initiator 0.01 to 5 % o Powder to provide radio-opacity eg Barium glass 0 to 50%
(c) Casein phosphopeptide 0.01 to 10%
The powder: liquid: casein phosphopeptide ratio is in the range of 1.0: 1:8 to 2.5: 1:0.0001.
A more preferred composition of the invention falls within the following ranges: 5 (a) Liquid Precursor:- Poly (carboxy lie acid) 15 to 40%
Polymerisable acidic or neutral monomer 5 to 35 %
Solvent 15 to 65 %
Reducing agent 0.2 to 0.5 %
Light activated initiator 0.2 to 0.5 % 0 Free radical inhibitor 0.02 to 0.1 %
(b) Powder Precursor:- Oxide of tetravalent non-metal 22 to 28%
Oxide of trivalent metal 28 to 34%
Oxide of divalent metal 7 to 11 %
Oxide of monovalent metal 5 to 10% 5 Phosphorous pentoxide 1 1 to 17%
Fluoride ion 10 to 16%
Peroxide initiator 0.05 to 3%
Powder to provide radio-opacity 0 to 50% eg Barium glass o The powdeπliquid ratio is in the range of 1.3: 1 to 2.0: 1.
A more preferred formulation for Barium glass may fall within the following ranges:
Silicone dioxide 5 to 55 %
Barium oxide 1 to 35 % 5 Boron oxide 0 to l5%
Aluminium oxide 0 to 15 %
The invention will now be described in greater detail by reference to specific
Examples, which should not be construed as limiting on the scope thereof.
Example 1
A novel light curable glass ionomer cement was prepared according to the 5 following:
(a) Liquid Precursor:- Poly(acrylic acid) 26.0 grams
Methacrylic acid 16.0 grams l ,5-diallyl-2,4-benzene dicarboxylic acid 8.0 grams
Water 50ml ι o N,N-3,5-tetramethyl aniline 0.36 grams
Camphor quinone 0.34 grams
Butylated hydroxy toluene 0.20 grams
(b) Powder Precursor:- Calcium aluminium fluorosilicate glass powder 99.8%
Benzoyl peroxide 0.2 % 15 The powder: liquid ratio is 1.5: 1.
The powdered and liquid precursors of the glass ionomer cement are supplied separately. A paste is produced in small quantities as required by introducing small amounts of the liquid precursor to the powdered precursor on a suitably sized plate or 0 mixing receptacle. The liquid and powder precursors are then mixed.
The resulting composition is then cured to an elastomeric state by a free radical polymerisation process. This process is preferably catalysed by light curing. The composition is mixed with an appropriate photoinitiator, either UV or visible light sensitive and an amine accelerator. A free radical inhibitor is added to improve the shelf
25 life of the liquid precursor. An acid-base reaction occurs between the acid groups of the polymer network and the divalent and trivalent metal ions of the powdered precursor to produce a glass ionomer cement.
Example 2
A novel light curable glass ionomer cement was prepared according to the 30 following:
(a) Liquid Precursor:- Poly(acrylic acid) 18.0 grams
Methacrylic acid 16.0 grams l ,5-diallyl-2,4-benzene dicarboxylic acid 16.0 grams
Water 50ml
35 N,N-3,5-tetramethyl aniline 0.36 grams
Camphorquinone 0.34 grams
Butylated hydroxytoluene 0.02 grams
(b) Powder Precursor:- Calcium aluminium fluorosilicate glass powder 99.8 %
Benzoyl peroxide 0.2%
The powder: liquid ratio is 1.5: 1.
The glass ionomer cement is produced in accordance with the method described in Example 1.
Example 3
A novel light curable glass ionomer cement was prepared according to the following:
(a) Liquid Precursor:- Poly(acrylic acid) 18.0 grams
Methacrylic acid 20.0 grams l ,5-diallyl-2,4-benzene dicarboxylic acid 12.0 grams
Water 50ml
N,N-3,5-tetramethyl aniline 0.36 grams
Camphor quinone 0.34 grams
Butylated hydroxytoluene 0.02 grams (b) Powder Precursor:- Calcium aluminium fluorosilicate glass powder 79.8%
Barium glass 20.0%
Benzoyl peroxide 0.2%
(c) Casein Phosphopeptide 2 % The powder: liquid: casein phosphopeptide ratio is 1.5: 1:0.001. The glass ionomer cement is produced in accordance with the method described in Example 1.
Industrial Applicability
The glass ionomer cement of the invention can be used in place of amalgam in dental restorations.