Influence of Hygroscopic expansion on cuspal deflection of tooth composite restoration (an in vitro study)

Purpose: This study evaluates the effect of hygroscopic expansion on the cusp deflection of tooth composite restoration.
Materials and Methods: Eighty (80) human premolars extracted for orthodontic reasons stored in normal saline were used. Large Mesiooccluso distal cavity (MOD) cavity was prepared. The specimens were divided into two main groups (40 each). each main group divided into two groupsaccording to bonding used (G-bond & composite consensual adhesive) each group (20 each), subdivided into four equal subgroups as follows: group A: Using low shrinkable resin composite (Filtek™ P90 Silorane shade A2; 3M ESPE,St Paul, MN, USA) with its adhesive system. group B: Using low shrinkable composite (Filtek P90 Silorane shade A2; 3M ESPE) with Gbond (GC, Tokyo, Japan). group C: Using Filtek™ Z350 (3M ESPE) composite with Gbond (GC). group D: Using Filtek Z350 (3M ESPE) composite with Adhe SE (Ivoclar Vivadent , Schaan, Liechtenstein). Specimens were stored in water for four time interval (immediate, 1, 2, 4&12) weeks. Each group was further divided into equal subgroup (5 teeth per each) according to immersion in normal saline. Cuspal deflection was detected by Universal measuring microscope (Carl Zeiss, Jena, Germany) and Universal horizontal metroscope (Universal Langenmesser; Carl Zeiss). The buccal and lingual cusp movements were recorded for 2000 s and the measured value (as a function of time) was stored on a computer through a data acquisition board. The buccal and lingual cusp movements were recorded again after the specimens were immersed in deionized water. The results were statistically analyzed using ANOVA followed by Student–Newman–Keuls post hoc tests.
Results: The cavities which restored with the silorane (P90) resin-based composites recorded less cuspal deflection than the methacrylate-based (Filtek Z350) group the cavities which restored with silorane (P90) resin-based composites and bonded with its consensual adhesive recoreded the least cuspal deformations. cuspal deformation of the restored teeth gradually decreased, reversing the shrinkage deformation. The two hydrophobic resin composite restored teeth showed a gradual decrease of the shrinkage deformation due to hygroscopic expantion.
Conclusion: Polymerization shrinkage deformationwas 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.