Modified Appen Model for Developing Restoratives with Enhanced Clinical Aesthetics

Objectives: Acid-degradable and inert glasses containing CaF₂ and SrF₂ are widely used as fillers in aesthetic dental restorative materials. Aesthetically superior polyalkenoate cements and dental composites are produced by matching the refractive index (RI) of the glass to the resin or the polysalt matrix to avoid light scattering at the interfaces. RIs of glasses can be calculated by the means of Appen factors. However, there were no Appen factors for amorphous metal fluorides published to date. Hence, the objective of this study was to empirically derive Appen factors for amorphous Calcium and Strontium fluorides.
Methods: Several series of acid-degradable fluoroaluminosilicate glasses of varying metal fluoride content were selected for this study. The Appen equation was used to estimate RIs of glasses by using previously published RIs for the crystalline metal fluorides, in addition to Appen factors for the glass oxides. RIs of glasses were measured by the Becke line technique. Fragments of coarse glass samples were dispersed in mineral oils of varying RI (1.45-1.55) and observed under a phase contrast microscope until RI match was found. Subsequently, once experimental RIs of glasses became available, Appen factors for amorphous CaF₂ and SrF₂ were derived.
Results: Empirical Appen factors for the amorphous metal fluorides were found to be higher than those for the crystalline metal fluorides. It was also found that the RIs of glasses reduced linearly (R²=0.98) with increasing fluorine content.
Conclusions: Empirical Appen factors have been successfully derived for the amorphous metal fluorides that enable accurate RI estimation of fluoride-containing glasses for the development of dental restoratives with enhanced clinical aesthetics.