Can hygroscopic expansion compensate polymerization shrinkage? Part I. Deformation of restored teeth.

Objective: Restorative materials exhibit contraction and expansion due to polymerization and water absorption. Each process deforms and stresses a restored tooth structure in opposite direction. This study evaluated the tooth deformations during these competing processes.

Methods: Large MOD cavities were prepared in 10 extracted molars. Five were restored with a hydrophobic resin composite (Filtek Supreme, 3M ESPE) and the others with a hydrophilic resin-modified glass-ionomer (Ketac Nano, 3M ESPE). The restored molars and two unrestored controls were stored in water for 24 weeks. The molars were digitized with an optical scanner at baseline, after preparation, restoration, and at 1, 2, 4, 8, 16, and 24 weeks water immersion. The digitized buccal, lingual, and restoration surfaces were analyzed to determine their deformation patterns. The results were statistically analyzed using ANOVA followed by Student-Newman-Keuls post hoc tests (p=0.05).

Results: The buccal and lingual tooth surfaces moved 13-14 μm inward after restoration. After water immersion, cuspal deformation in the resin composite group gradually decreased, reversing the shrinkage deformation within four weeks. The immersed resin-modified glass ionomer group reversed shrinkage deformation within one week, and continued to expand further to 28 μm after 24 weeks. Cuspal deformations after water immersion were significantly different with the two restoratives. Restoration surfaces also expanded after water immersion, while the control teeth showed no significant deformation.

Significance: Polymerization shrinkage deformation was compensated by hygroscopic expansion within 4 weeks in teeth restored with a hydrophobic resin composite, while a hydrophilic restorative over-compensated polymerization shrinkage within 1 week causing tooth expansion.