Hygroscopic expansion in a Class II restoration

Objectives: Restorative materials exhibit contraction and expansion due to polymerization shrinkage and water sorption. Both processes affect residual stress. These dimensional changes were investigated for a restored tooth.  Methods: Extracted molars were mounted in acrylic resin along with spherical references. Large MOD slots were cut and restored in two horizontal layers with a nano-filled resin-modified glass-ionomer (Ketac Nano, 3M ESPE). The teeth were kept moist at all times. The restored teeth were suspended in a water bath for up to 2 months. The specimens were digitized at various stages to determine the deformation (N=7+2 controls). Free water sorption was measured for discs of the restorative material (27mm diameter, 1.4 and 2.3mm thick), recording weight and dimensions (N=5). Diffusion parameters and hygroscopic expansion were derived from the discs using a finite element model for water sorption. Combined with shrinkage measurements, deformation was calculated for a restored molar. Results: Occlusal restoration surfaces expanded after immersion, while buccal and lingual tooth surfaces tended to move inward during shrinkage and outward after immersion. Tooth movements, however, were close to the capability of the measurement system, owing to low post-gel shrinkage (0.23%lin) and modulus properties. The discs absorbed 71±7 and 52±1 µg/mm³, while volume expanded 1.1±3.2 and 3.1±1.4% over 4 days for the 1.4 and 2.3mm thickness, respectively. A diffusion factor (2E-12) was derived and applied. Correlation between the finite element analysis and experiments was R>0.98 for the weight change and R>0.66 for the dimensional versus weight change. The finite element analysis showed similar occlusal deformation patterns but substantially higher expansion than observed in the experiment. Conclusion: Hygroscopic expansion measured for unrestrained discs did not reflect the restored tooth deformation. The results suggest that the confinement of a cavity alters the amount of hygroscopic expansion. Acknowledgment: Supported by MDRCBB and Non-Tenured Faculty Grant (3M Foundation).