Discoloration of Dental Cements and Composites in a Sulfide Solution

Abstract

In an early study the discoloration of certain hardened silicate cements, after exposure to an atmosphere of hydrogen sulfide (H₂S) for 24 h at room temperature, was ascribed to the formation of dark-colored sulfides of base metal impurities (Paffenbarger et al. JADA 25, 32, 1938). A recent study noted that, in general, silicate and glass ionomer cements were more prone to color shifts than composites after exposure to H₂S for 9 weeks (Sugawara, Ph. D. Thesis, Nihon Univ.). The aim of the present study was to devise a simple, aqueous sulfide exposure test for esthetic restorative materials. The general procedure was to expose specimen disks to a 0.1% (w/v) sodium sulfide solution, adjusted to pH 9, for 1-7 days at 37° or 55°C. The 55°C-Na₂S exposure was designed as an accelerated test. Materials studied included : 1 silicate and 2 silicophosphate cements of known lead content, a glass ionomer cement (FIIF), several commercial composites and an experimental, hydrophilic composite. Known amounts of base metal contaminants in the form of appropriate salt solutions were added to the liquid components of FIIF and the composites. Specimens exposed to distilled water under the same conditions served as controls. Exposure to the aqueous sulfide medium resulted in the following ranking in order of decreasing discoloration : Glass ionomer cement> silicophosphate cement> silicate cement> hydrophilic composite> hydrophobic composite. Generally, the results of the aq. Na₂S test paralleled those obtained with H₂S. The degree of discoloration is dependent on a number of factors : the nature, concentration and leachability of the metal impurities, and the hydrophilicity and permeability to sulfide of the esthetic restoratives.