Specimen aspect ratio and light transmission in photoactive dental resins

To test the influence of specimen dimensions on light transmission and shrinkage strain properties of curing dental resins.

Photocurable resin specimens (Bis-GMA/TEGDMA) with aspect ratios (AR) of 2 (4mm×2mm); 4 (4mm×1mm and 8mm×2mm); 8 (8mm×1mm); 12 (AR: 12mm×1mm); and 24 (12mm×0.5mm) were light cured. Light transmission and shrinkage-strain data were recorded throughout, and upper and lower surface hardness measurements were performed following cure.

Light transmission was significantly affected by the specimen aspect ratio even at similar thicknesses (p<0.05). By comparing light transmission through a negative control resin without photoinitiator, the lowest AR specimens showed a relative increase in transmission above 100% throughout curing, which was caused by specimen constraint. The extent of lower surface cure (as assessed by increasing hardness) was principally affected by cavity height and decreased for thicker specimens (p<0.05). Only the 2mm thick specimens showed a significantly greater lower to upper hardness ratio with increasing cavity diameter (p<0.05). The highest AR specimen showed the greatest lower to upper hardness percentage (p<0.05), and was expected since this AR was obtained by reducing the sample thickness to 0.5mm. Generally, total shrinkage strain increased and shrinkage strain per unit mass decreased with increasing AR.

Specimen constraint in low AR cavities may compromise light transmission as unexpected light intensity variations may occur for low configuration factors, which ultimately affect polymer conversion of light-cured resin-based restorations through depth.