Method of preparing radioopaque cross-linked poly(carboxylic acid) dental cement

Abstract: Alkaline earth metal aluminofluorosilicate glasses suitable for use as ion-sources in the poly(carboxylic acid) cement compositions contain strontium in order to provide radiopacity. The invention also provides methods for making poly(carboxylate) cement using the glasses of the invention as ion-sources.

Description: The invention relates to a method of preparing radioopaque cross-linked poly(carboxylate) dental cement. Poly(carboxylic acid) cement compositions are well known and established. Such compositions basically comprise (i) a polymer containing free carboxylic acid groups (typically a homo- or co-polymer of acrylic acid) and (ii) an acid leachable source of polyvalent metal ions (e.g. an acid leachable glass such as a calcium aluminofluorosilicate glass). In the presence of water, the polyacid leaches polyvalent metal ions from the source thereof and these serve to cross-link the polymer chains to give a solid material (a cement). Poly(carboxylic acid) compositions are discussed authoritatively in, for example, "Organolithic Macromolecular Materials" Wilson, A.D. Crisp, S.; Applied Science Publishers, 1977; see especially chapter 4. Poly(carboxylic acid) cement compositions have been found to have particular application as dental restorative materials but suffer from one practical disadvantage in that they are radiolucent, that is they are essentially transparent to X-rays. As a result, it is not possible, for example, to examine a dental restoration, carried out using such a cement, using an X-ray technique and, further, it is not possible to locate any portion of a dental restoration which may become dislodged and ingested by a person, using an X-ray technique. WO-A-80/00409 relates to hardenable compositions (dental cements) comprising e.g. (i) a polycarboxylic acid), (ii) a particulate ion-leachable material and (iii) a filler having a high flexural modulus and being anisotropically dimensioned, as a mercury-free substitute for amalgam, however, not mentioning strontium as a glass ingredient. US-A-3 971 754 discloses an enamel-matching X-ray opaque and optically translucent dental filling composition using, however, as a binder olefinically unsaturated polymerizable esters not reacting with ion-leachable glass, hence, not yielding a dental cement with added water. It has now been found that, in accordance with the invention a method of preparing a radioopaque cross-linked poly(carboxylate) dental cement comprising blending a fluorine containing glass powder, prepared using components wherein strontium, represented as SrO, comprises from 5 to 35% by weight of the glass polyacrylic acid having a molecular weight of from 20,000 to 125,000, and water, to provide a powder blend of glass and polyacrylic acid to produce a dental cement having a radioopacity equal to or greater than that of dentine or dental enamel gives a set material which is radioopaque, that is opaque to X-rays, and still has acceptable properties with regard to strength, hardness, translucency etc. The set cement may have a radioopacity equal to or greater than that of natural tooth material. (The radioopacity of a material may be conveniently defined in terms of the equivalent thickness, in mm, of aluminium required to give the same radioopacity as one mm of the material. In these terms, the radioopacity of dental enamel is generally from 1.3 to 2.7 mm aluminium per 1 mm enamel; that of dentine being lower, about 0.6 to 0.9 mm aluminium per 1 mm dentine). In order to achieve acceptable radioopacity in use, the glass contains from 5 to 35% preferably from 5 to 24%, most preferably from 12 to 18% by weight, of strontium (calculated as SrO). In order to achieve this it is most convenient that all the alkaline earth metal in the glass be strontium, i.e. that the glass be a strontium aluminofluorosilicate glass substantially free of other alkaline earth metals such as calcium. The glasses of the invention may be prepared from a mixture of alumina, silica, aluminium fluoride, aluminium phosphate and strontium fluoride (in place of calcium fluoride) in the presence of, for example cryolite (sodium aluminium fluoride) or boric acid as fluxing agents. Thus, preferred glasses of the inventioin comprise: strontium (calculated as SrO), 8 to 32 % by weight; aluminium (calculated as Al₂O₃), 25 to 40 % by weight; silica (calculated as SiO₂), 23 to 33 % by weight ; fluorine (calculated as F₂), 0 to 10% by weight; sodium (calculated as Na₂O), 1 to 7% by weight; and phosphorus (calculated as P₂O₅) 0 to 10% by weight. The polymer containing free carboxyl groups is preferably a homopolymer of acrylic acid. Copolymers of acrylic acid with one or more other ethylenically unsaturated carboxylic acids such as maleic or itaconic acid, may be employed but are less preferable. The acrylic acid polymer or copolymer has a molecular weight of from 20,000 to 125,000, preferably from 35,000 to 70,000, most preferably from 45,000 to 75,000. For the sake of convenience the polymer containing free carboxylic acid groups will hereinafter simply be referred to as a "polyacrylic acid". The glass is in particulate form and suitably has a particle size of from 0.5 to 60µm. The particle size of the glass may vary within these limits depending upon the intended end use of the poly(carboxylic acid) cement composition. Thus, for example, where the resin is to be used as a dental restorative material (i.e. as a filling or stopping material) or as a base under an amalgam or a so-called "composite" dental restorative material (i.e. a mixture of an ethylenically unsaturated resinous material, an inert particulate filler and a curing agent for the resinous material), the particle size is suitably 0.5 to 60 µm; preferably 1 to 40 µm and most preferably from 5 to 25 µm.. When intended for use as a luting agent the particle size is suitably 0.5 to 30 µm, preferably 0.5 to 25 µm and most preferably 1 to 20 µm. The weight ratio of polyacrylic acid to glass is suitably from 0.15 : 1 to 0.5 : 1, preferably 0.2 : 1 to 0.4 : 1; and the weight ratio of water to glass is preferably 0.4 : 1 to 0.1 : 1. The reaction of the polyacrylic acid and glass may be carried out in the presence of other materials serving to alter or modify the working time and/or setting time of the mixture, e.g. a hydroxycarboxylic acid such as tartaric acid serves to increase the rate of set of the composition. Compositions for forming a cement from the glass of the invention and polyacrylic acid may be put up as two-part packs, one part comprising an aqueous solution of the polyacrylic acid (and optionally working/setting time modifiers) and the other part comprising a particulate glass. Alternatively, a dry blend is formed of particulate glass and a powdered polymer for subsequent addition of water to form a cement-forming composition. In this latter case working/setting time modifiers may be present in the dry blend or in the water. The cement is translucent and suitably has a radioopacity of at least 1 mm of aluminium per mm of structure, preferably at least 1.5 mm, and more preferably at least 2.0 mm, of aluminium per mm of strontium. In order that the invention may be well understood the following Examples are given by way of example only. In the Examples all percentages are by weight unless otherwise stated. EXAMPLE 1 A glass was prepared by fusing together 14.8% aluminium oxide, 25.9% silica, 3.3% strontium fluoride, 4.8% aluminium fluoride, 21.3% aluminium phosphate and 20.0% cryolite. The resultant glass (which contained 10.8% of strontium calculated as strontium oxide) was ground to a Sauter Mean Diameter of 2.5 µm and was blended with polyacrylic acid (molecular weight 45,000) and tartaric acid to give a cement-forming powder containing 83.6% of the glass, 15.1% of polyacrylic acid and 1.3% of tartaric acid. The powder was subsequently mixed by hand, using a dental spatula and a glass block, with water (water:powder ratio = 0.15:1) to give a restorative material (cement). This virtually translucent cement typically had a working time of 1 minute and 45 seconds from the start of mixing, a compressive strength, after 24 hours, of 250 MPa and a radioopacity of 1.2 mm aluminium per 1 mm cement. EXAMPLE 2 A glass was prepared by fusing 16.6% of aluminium oxide, 29.0% of silica, 34.3% of strontium fluoride, 5.3% of aluminium fluoride, 9.9% of aluminium phosphate and 5.0% of cryolite. The resultant glass (which contained 29.2% of strontium calculated as strontium oxide) was used to prepared a cement-forming powder by blending it with the same polyacrylic acid as used in Example 1 and tartaric acid in the same proportions as used in Example 1. The powder was evaluated as described in Example 1 and it was found that the cement typically had a working time of 1 minute and 50 seconds from the start mix, a compressive strength after 24 hours of 245 MPa and a radioopacity of 2.8 mm aluminium per mm cement. EXAMPLE 3 A cement-forming powder was prepared from the glass of Example 2, the polyacrylic acid of Example 1 and tartaric acid; the cement containing 74.4% of the glass, 24.8% of the polyacrylic acid and 0.8% of the tartaric acid. The powder (which was intended for use as a luting cement) was evaluated as described in Example 1 (at a powder:water ratio of 3.4:1) and typically had a working time of 3 minutes, a compressive strength of 90 MPa and a radioopacity of 2.2 mm aluminium per 1 mm cement.

Claims: A method of preparing a radioopaque cross-linked poly(carboxylate) dental cement comprising blending a fluorine containing glass powder, prepared using components wherein strontium, represented as SrO, comprises from 5 to 35% by weight of the glass, polyacrylic acid having a molecular weight of from 20,000 to 125,000, and water to provide a powder blend of glass and polyacrylic acid to produce a dental cement having a radioopacity equal to or greater than that of dentine or dental enamel. A method as claimed in claim 1 in which the glass comprises, by weight, in addition to strontium, from 25 to 40%, Al (calculated as Al₂O₃), from 23 to 33%, Si (calculated as SiO₂) up to 10%, F (calculated as F₂) from 1 to 7%, Na (calculated as Na₂O) and from 0 to 10%, P (calculated as P₂O₅). A method as claimed in claim 1 or claim 2 in which the polyacrylic acid is a homopolymer of acrylic acid having a molecular weight of from 35,000 to 70,000. A method as claimed in any one of the preceding claims in which the glass has a particle size of from 0.5 to 60 micrometers. A method as claimed in any one of the preceding claims in which the glass and acrylic acid are blended in a weight ratio of polyacrylic acid to glass of from 0.15:1 to 0.4:1, and the weight ratio of water to glass is from 0.4:1 to 0.1:1. A method as claimed in any one of the preceding claims in which a hydroxycarboxylic acid is introduced in the mixing step, the hydroxycarboxylic acid serving to alter or modify the working time or setting time of the cementing reaction or both. A method as claimed in claim 6 in which tartaric acid is used to decrease the setting time of the reaction. A method as claimed in any one of the preceding claims in which the glass is substantially free of alkaline earth metals other than strontium. A method as claimed in any one of the preceding claims in which strontium fluoride is added to the glass composition in the preparation thereof. A method as claimed in any one of the preceding claims which produces a dental cement having a radioopacity of at least 1.5 mm of aluminium per mm of cement.

Inventors: BILLINGTON RICHARD WILLIAM, WILLIAMS JILL ANN, BILLINGTON, RICHARD WILLIAM, WILLIAMS, JILL ANN
Applicants: DENTSPLY INT INC, DENTSPLY INTERNATIONAL, INC.
Application year: 1987
Main classification: A61K6
IPC codes: C04B28, C03C3, A61K6, C03C4, C04B22