Purpose: To compare the ultimate tensile strength (UTS) of a compomer (Dyract AP) and a composite (Spectrum TPH) that were stored in an aqueous or a non-aqueous medium for up to 180 days.
Methods: 156 hourglass-shaped specimens of each material with cylindrical cross-sectional areas along the test regions were prepared. For each material, 12 beams were stressed to failure using a microtensile testing method to obtain the baseline UTS data. The rest were divided into two groups. One group was stored in de-ionized water (W), and the other (control) in silicone fluid (O) at 37 degrees C. 12 beams were removed from the respective storage medium after 1, 7, 14, 30, 90, 180 days for evaluation of the NTS. Materials retrieved from the intact central cylindrical portion of the fractured 180-day specimens were prepared for transmission electron microscopy (TEM).
Results: Baseline UTS for Dyract AP (D) and Spectrum TPH (S) were 76 +/- 16 MPa and 92 +/- 13 MPa, respectively. One-way ANOVA on ranks and Dunn's multiple comparison tests showed that UTS of Groups D-O, S-O and S-W remained stable during 180 days and were not significantly different from the baseline results. There was a significant drop in UTS in Group D-W after 90 and 180 days when compared with the 1-day result (P < 0.05). A highly significant nonlinear correlation was also observed in Group D-W between the mean UTS after the first day and the storage time (P < 0.01). No difference could be seen between the filler-matrix interface in S-O and S-W. A very thin glass-ionomer phase (ca. 200 nm thick siliceous hydrogel layer) could be observed in D-W, but not in D-O.