In general, studies of commercial resin matrixes indicate volumetric shrinkage in the curing process as great as seven percent, with most undergoing shrinkage of two or three percent (Seghatol & Durand, 1999). In dentistry, the comparatively small dimensions of the products composed of polymers and the specific point-load stresses sometimes encountered by dentures magnifies the undesirable effects of even small percentages of incomplete polymerization and volumetric shrinkage. Moreover, dental materials are typically designed to fit with much closer mechanical tolerances than two or three percent if they are expected to perform as designed throughout their intended lifespan (Ashby & Jones, 1996).

Ultimately, even with different compositions of monomers selected for their favorable response to heating, curing dental polymers in commercial microwaves cannot provide a complete solution. Commercial microwaves are universally designed so that their magnetrons cycle on and of instead of emitting a constant flow of microwave energy. For this reason, the most common technique of dental polymer curing through the use of commercial microwave ovens involves a two-step process whereby the microwave energy heats the water bath first (Urban, Machado, et al., 2007).

The water retains a constant thermal energy level irrespective of the fluctuations in microwave energy. Single-step techniques have also been developed, wherein the necessity of a thermal intermediary is eliminated in conjunction with the development of specialized resin compounds designed to absorb heat directly. However, while that technique partially solves the problem associated with curing dental polymers in commercial microwaves, it is less effective at addressing others.

More specifically, that method reduces the thermal differential between the external and internal surfaces thereby solving the problem of degrading the external surface by overheating. However, it...

Specialized Microwave Equipment for Dental Polymerization:
The most comprehensive approach to reducing incomplete polymerization and eliminating residual monomers is the implementation of a specially-designed microwave apparatus intended specifically for the purpose of curing dental polymer products. By designing a microwave device solely intended for dental polymer curing, this approach provides the most comprehensive solution available to all the problems identified in connection with other curing methods. The actual design is relatively simple as represented by the diagram below for a device patented for dental use in 1999. (Seghatol & Durand, 1999)

Essentially, this device employs a mechanical method of precisely positioning a mold of a dental device, such as the crown pictured above, into a secure housing in order to permit the accurate directed flow of microwave energy into a very small area focused by the operator. In principle, the device eliminates incomplete polymerization, microscopic void formation, and residual monomers by allowing precision-controlled, steady (i.e. non-cycling) heating without the need for two-step thermal energy transference such as through a water-based medium.

References

Ashby, M., Jones, D. (1996). Engineering Materials. p. 191-195. Oxford: Butterworth-

Heinermann. Seghatol, M., Durand, J. (1999). "Microwave polymerization system for dentistry." United States Patent # 6737619; Retrieved, November 20, 2008, at http://www.freepatentsonline.com/6737619.html

Urban, V., Machado, a., Oliveira, R., Vergani, C., Pavarina, a., Quezia, B. (2007). "Residual monomer of reline acrylic resins: Effect of water-bath and microwave post-polymerization treatments." Dental Materials, Vol. 23, No. 3; pp. 363-368 (3/07).