Electric heating – Microwave heating – With diverse device
Reexamination Certificate
1999-09-20
2002-08-27
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
With diverse device
C219S705000, C219S709000, C219S748000, C422S021000, C264S402000, C425S17480E, C522S001000
Reexamination Certificate
active
06441354
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of dentistry. More specifically, the present invention relates to a microwave polymerization system for dentistry that utilizes specifically controlled microwave energy to cure polymer materials so as to produce dental prosthetics and dental composites having improved physical characteristics.
BACKGROUND OF THE INVENTION
The use of polymer materials in the dental arts for the restoration of lost or damaged teeth is well known. Such uses fall into two general categories: (i) the use of polymer materials to produce dental prosthetics, such as dentures, bridges and crowns, that are either permanent or removable articles, and (ii) the use of polymer materials to create dental composites for fillings to repair teeth instead of using conventional amalgam fillings or as veneers to refinish tooth enamel surfaces. The first category of dental articles, dental prosthetics, are created outside of the patient (i.e., extra-oral), typically by making an impression of what the desired article should look like and then molding the article to match the impression. The second category of dental articles, dental composites, are created directly in the patient's mouth (i.e., intra-oral) as fillings or veneers to repair or resurface teeth. Regardless of which category is being considered, dental articles made of polymer materials must have adequate strength, durability, and dimensional stability and must also be biocompatible and chemically inert. It is also important to be able to process each type of dental article rapidly, conveniently, safely and economically.
An example of a dental prosthetic in the first category of dental articles that is created using polymer materials is a removable denture. Most commercial dentures are created using a paste or resin matrix formed of various polymers, co-polymers and monomers (typically methyl methacrylate), as well as certain crosslinking agents, initiators, accelerators and other additives. This resin matrix is formed into a plaster mold and is then hardened or cured by applying energy in the form of heat. Typically, the plaster mold containing the uncured denture is prepared in a dental laboratory based on an impression taken by the dentist. To cure the resin matrix, the plaster mold is placed into a flask that is then put in a thermal water-bath for up to 8 hours. This conventional process of curing a denture takes such a long time because both the plaster molds and the polymers in the resin matrix are relatively poor thermal conductors and are heated only from the outside via the thermal water-bath. The conventional process can also result in large numbers of voids and significant shrinkage during curing due to uneven thermal conduction and non-uniform polymerization of the resin matrix. These problems are discussed in Feilzer A J et al., “Curing contraction of composites and glass-ionomer cements,”
Journal of Prosthetic Dentistry
, Vol. 59, pp. 297-300 (1988); and Ferracane J L et al., “Wear and marginal breakdown of composite with various degrees of cure,”
J Dent. Res
., Vol. 76, No. 8, pp. 1508-16 (1997). The lack of completely uniform polymerization of the denture also leaves residual monomers that are toxic and can act as irritants to oral tissues and compromise the physical characteristics of the denture.
In an effort to overcome the long cure times associated with the conventional thermal water-bath technique, a technique of using commercial microwave ovens to heat and cure polymer resins to form dental prosthetics has been developed. In the conventional thermal curing method for polymer dental articles, a temperature differential is required to force heat by conduction from the surface of the flask and mold to the center of the article. Because the heat penetrates from the outside to the internal portions of the material by thermal conduction, overheating and degrading polymers can occur at the outer surface of the article. When microwave are used to initiate the thermal curing processing, it is possible for the article to be heated uniformly as the electromagnetic radiation instantaneously penetrates deeply and heating occurs throughout all three dimensions of the irradiated article. The main advantages provided by microwave energy include a rapid internal heating, independent of the heat flow through the surface, as well as minimal thermal lag and thermal gradients throughout the interior of the article, which results in a more homogeneous curing of the article with a higher degree of conversion of monomers into polymer chains.
Comparisons of these two techniques can be found in Hayden W J, “Flexure strength of microwave-cured denutre baseplates”,
General Dentistry
, Vol. 343, pp. 367 (1986); Al Doori D et al. “A comparison of denture base acrylic resins polymerised by microwave irradiation and by conventional water bath curing systems,”
Dental Materials
, Vol. 4, pp. 25-32 (1988); and Geerts G et al., “A comparison of the bond strengths of microwave and water bath-cured denture materials,”
The Journal of Prosthetic Dentistry
, Vol. 66, No. 3, pp. 403-07 (Sep. 1991). Various types of flasks and molding equipment that can be used in conjunction with a commercial microwave oven for processing and curing dental articles made of polymers have been developed as described, for example, in U.S. Pat. Nos. 4,971,735, 5,151,279, 5,324,186 and 5,510,411, European Patent No. 0 687 451 A2 and Japanese Patent No. JP7031632A. The repair of dentures and related articles using microwave processing is also described in Turck M D et al, “Microwave processing for dentures, relines, repairs and rebases,”
The Journal of Prosthetic Dentistry
, Vol. 69, No. 3, pp. 340-43 (1993). Generally, dentures cured by commercial microwave ovens have improved mechanical properties, and often have better adaptation than those cured by the water-bath method. The primary advantage of microwave curing, however, is the reduced processing times which can be shortened from 8 hours or more to as little as a few minutes.
While the use of commercial microwave ovens to cure dental prosthetics solves some of the problems of conventional thermal water-bath cured prosthetics, dental prosthetics processed in this manner can be less than satisfactory in terms of their physical and biocompatibility characteristics because varying degrees of cure, micro-shrinkage and porosities are still present. Any large degree of micro-shrinkage or porosities in the polymers of dental prosthetic cured using conventional microwave ovens will lead to fitting inaccuracy and unreliability. These problems are discussed in Wallace P W et al., “Dimensional accuracy of denture resin cured by microwave energy,”
The Journal of Prosthetic Dentistry
, Vol. 68, pp. 634-40 (1992); and Salim S. et al. “The dimensional accuracy of rectangular acrylic resin specimens cured by three denture base processing methods,”
The Journal of Prosthetic Dentistry
, Vol. 67, pp. 879-85 (1992).
It understood in the dental arts that micro-shrinkage is primarily due to the resin matrix. The physical and mechanical properties of a polymer material, such as hardness, stiffness and abrasion resistance and strength, are highly influenced by the arrangement of the resin matrix when the fillers and coupling agents are fixed during the curing process. Micro-shrinkage results from the shorter distance between atoms in the resin matrix after polymerization than before polymerization. The monomers in the resin matrix are located at Van der Waals distance, which change to a covalent bond distance once the resin matrix is polymerized. If all of the monomers in the resin matrix are not converted into polymer chains during the polymerization curing process, then this change in distance can induce mechanical stresses in the form of micro-shrinkage in those areas where there was not complete conversion of the monomers into polymers. Commercial resin matrices are found to undergo volume shrinkage of as much as 7% with most resin matrices shrinking 2-3%. This kind of mi
Durand Jean-Pierre
Seghatol Marc
Leung Philip H.
Patterson Thuente Skaar & Christensen P.A.
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