Objectives . To evaluate the effect of instrument compliance on the polymerization shrinkage stress measurements of dental composites.
Materials and methods . The contraction strain and stress of composites during light curing were measured by a custom made stress-strain analyzer, which consisted of a displacement sensor to measure a strain, a cantilever load cell to measure a stress, and a negative feedback mechanism to control instrument compliance. The instrument was able to measure the polymerization shrinkage stress by two modes: (1) High compliance mode (4.7 μm/N) without feedback, or (2) Low compliance mode with a feedback mechanism. The shift between two modes was controlled by an On–Off switch of the negative feedback circuit.
A flowable (Filtek Flow, 3 M ESPE) and two universal hybrid (Z100 and Z250, 3M ESPE) composites were studied. A silane treated metal rod with a diameter of 3.0 mm was fixed at the free end of the load cell, and other metal rod was fixed on the base plate. Composite of 1.0 mm thickness was placed between the two rods and light cured. The axial shrinkage strain and stress of the composite were recorded for 10 min by light polymerization (Elipar FreeLight 2: 700 mW/cm 2 , 3M ESPE) by either of two modes. The measurements were repeated five times for both modes per each material. The tensile modulus of the cured specimens was also determined with the instrument. The statistical analysis was conducted by ANOVA, paired t -test and Tukey’s test ( α < 0.05), and regression analysis.
Results . The measured axial strains (μm), shrinkage stresses (MPa) with high compliance or low compliance mode, and tensile modulus (GPa) for three composites ( n = 5).
| Composite | High compliance (no feedback) | Low compliance (with feedback) | Tensile modulus | |
|---|---|---|---|---|
| Axial strain | Stress | Stress | ||
| Z100 | −10.73 (0.32) b | 2.91 (0.10) b | 17.08 (0.89) a | 2.31 (0.18) a |
| Z250 | −7.53 (0.57) c | 1.94 (0.09) c | 9.46 (1.63) b | 2.05 (0.20) b |
| FF | −11.77 (1.02) a | 3.11 (0.13) a | 10.11 (0.29) b | 1.41 (0.11) c |
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