Abdusalam, Elhejazi A A.
Hygroscopic-Stress-Development of Resin-Based Luting and Restorative Materials.
J Pak Dent Assoc Jan ;11(2):67-72.

OBJECTIVE: The aim was to determine the magnitude and rate of development of lateral stress arising from hygroscopic expansion over 6 months. A suitable methodology was described by Momoi and McCabe. MATERIALS & METHODS: Nine materials (4 Composites, 1 Compomer, 1 Resin-modified glass-ionomer cements, 1 adhesive and 2 dual-cure luting cements) were selected for this investigation. 693 brass moulds were used. Each brass mould had an outer diameter of 6.0 mm and height of 3mm. These were reamed inside to form a cylindrical hole with smooth inner surface wall. The internal diameter of the inner walls had a taper of 1ø and the cavity was 4.4 mm in diameter at the base. Materials were placed according to manufacturers` instructions and light-cured using a layering technique. Groups (n=11) were prepared for water-storage in situ at 37øC for each time-period (1& 7d; 1,3,6 m). Controls were dry-stored. RESULT: After each storage period, the specimens were expelled from their cavities and the axial extrusionforce was converted into lateral stress via the frictional-coefficient relationship. The mean values of displacement (6) for the control groups were zero or approximately zero for CMF and TRC. After one day of water storage, the displacement stress ((y) increased significantly. The resin-composite materials exhibited a slow increase in the hygroscopic-stress: 24h < 1 week < 1 month < 3 months < 6 months. This result indicated that hygroscopic expansion of biomaterials began to compensate polymerization shrinkage after a short period of water storage. CONCLUSION: The increased water uptake and material expansion towards the walls of the mould enhanced the force required to displace a specmmnn from the mould.

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