Dental sealants and adhesives are widely used in clinical settings. Desirable properties include safety, efficacy, durability, and favorable cosmetic properties. It is preferred that dental compositions be shelf stable, easy to formulate, and that they do not set so rapidly as to make them difficult to apply to a patient.
Dental compositions frequently contain monomers which are polymerized by the dentist or technician (e.g. by light, self-cure, or dual-cure). However, many dental compositions form a problematic “oxygen inhibited layer” (OIL) or “uncured layer” on their surface. This layer's polymerization is inhibited due to the presence of molecular oxygen radicals in ambient air. As a result, incomplete polymerization occurs. Such layer often render the surface sticky or tacky, making the dental composition more difficult to mold or shape. Such incomplete polymerization also tends to lead to lower hardness of the surface and/or no curing if a thin surface is present.
Extoral is a visible-light cured dental resin formulation sold by AFR Imaging Corp. (Portland, Oreg.). Extoral can be used for surface treatment or as a denture resin. Extoral cures rapidly and produces a glossy, hard surface upon irradiation with normal dental light. Extoral is significant in that it does not have an “oxygen inhibited layer”, even when cured with low intensity light. One difficulty with using Extoral is its volatility, leading to a very strong odor. The smell is objectionable to both patients and dentists/technicians, making it difficult to use in a laboratory, and nearly impossible to use in a clinical setting. Another difficulty is the very brittle surface created by Extoral, which may limit its use in certain dental applications such as surfaces subject to compressive forces.
Extoral's odor is suggestive of the presence of methyl methacrylate. Methyl methacrylate is a volatile compound used in several dental products. In addition to its unpleasant smell, exposure to methyl methacrylate has been linked to various health concerns. Numbness, paraesthesia, reduced pulmonary function, and reduced respiratory function have been observed in dental technicians who have been chronically exposed to methyl methacrylate (Sadoh, D. R. et al., British Dental J. 186(8): 380–381, 1999; Nishiwaki, Y. et al., J. Occup. Health 43: 375–378, 2001). The U.S. Environmental Protection Agency describes methyl methacrylate as an irritant of the nose and throat, and mentions that exposure for short periods of time can cause headache and fatigue (EPA 749-F-95-014 Fact Sheet, November 1994).
It would be of great value to develop a dental composition that does not have an “oxygen inhibited layer” and that does not contain methyl methacrylate or other volatile compounds that are irritating and potentially dangerous to dentists, dental technicians, and their patients.