Viscoelastic stability of resin-composites under static and dynamic loading.

OBJECTIVES

To compare the viscoelastic behavior (creep) of dental resin-composites under both static and cyclic loading in compression.

METHODS

Ten cylindrical specimens (4 mm × 6 mm), divided into two subgroups (n = 5) were prepared from each of four commercial resin-composites, using a divisible stainless steel mold. They were thoroughly cured from all sides. Groups 1 and 2 were loaded statically and dynamically respectively after 1d of fabrication and dry storage. Group 1 was loaded with a constant static load of 35 MPa and it was applied for 2 h followed by 2 h of strain recovery to obtain the static creep (%) and permanent set (%) respectively. To Group 2 a cyclic load between 1 MPa and 50 MPa was applied at a frequency of 0.25 Hz for 30 min to obtain the "dynamic" creep strain (%). Regression and correlation analysis (α = 0.05) was performed to examine possible correlations between static and "dynamic" creep.

RESULTS

For the resin-composite investigated, a good correlation was found between "dynamic" creep strain (%) and maximum static creep strain (%) (r = 0.92) and a strong correlation was also found between "dynamic" creep strain (%) and permanent set (%) (r = 0.97), SIGNIFICANCE: Maximum static creep was significantly higher than "dynamic" creep. A direct numerical equivalence was not expected between static and "dynamic" creep values, as in the case of "dynamic" creep, loading was cyclic and was applied for a shorter overall period. Nevertheless a strong correlation was found between the static and dynamic creep measurement.