Elastic Modulus Maturation Effect on Flexure of Restored Primary Molar

Objectives: Residual stresses depend not only on volumetric changes, but also elastic modulus. We measured elastic modulus development over a 4-week period for compomers, composite, and resin-modified glass ionomer (RMGI), and calculated its effect on cuspal flexure using a finite element model of a restored primary molar subjected to polymerization shrinkage.
Methods: Elastic modulus of compomers (Dyract, Dentsply; Compoglass, Ivoclar), composite (EsthetX, Dentsply), and RMGI (Ketac Nano, 3M ESPE) were determined using four-point bending of 2mm×2.5mm×25mm rectangular beams at 10min, and after 24hr, 1wk, 2wk, and 4wk storage in water at 37°C (N=10). Modulus data were analyzed using ANOVA and Student-Newman-Keuls post hoc test (α=0.05). Using the measured elastic modulus at 10min and 4wk along with previously published post-gel polymerization shrinkage data (Suiter et al, JDR 2016) as input, cuspal flexure was calculated in a cross-sectional finite element model of a restored primary molar.
Results: Dyract had the highest elastic modulus followed by Compoglass, EsthetX, and Ketac Nano (Table). Elastic modulus values were lowest at 10 min regardless of material, then rapidly increased in 24hr in compomers and composite while gradually increasing in RMGI. Percentage of modulus increases over 4 weeks was 46%, 49%, 107%, 83% for Dyract, Compoglass, EsthetX, and Ketac Nano, respectively. Cuspal flexure predicted by finite element analysis ranged between 4.2 to 9.1 microns. Percentage cuspal flexure increase during the 4 weeks was 3%, 4%, 9%, 12% for Dyract, Compoglass, EsthetX, and Ketac Nano, respectively.
Conclusions: Elastic modulus development is material dependent – rapidly increasing in composite and compomers, while more gradually increasing in RMGI over 4 weeks. The contribution of the elastic modulus maturation to cuspal flexure, and hence residual stresses in the restored tooth, was not proportional.