Effect of relining, water storage and cyclic loading on the flexural strength of a denture base acrylic resin

This study investigated the effect of relining, water storage and cyclic loading on the ultimate flexural strength (FS U) and on the flexural strength at the proportional limit (FS Pl) of a denture base acrylic resin (Lucitone 550-L). Rectangular bars of L were made (64 mm × 10 mm × 2 mm) and relined (1.3 mm) with four relining resins (Kooliner-K, Ufi Gel Hard-UGH, Tokuso Rebase Fast-TR and New Truliner-NT). In addition, specimens relined with L and intact L specimens were made (64 mm × 10 mm × 3.3 mm). A three-point flexural test was applied on the specimens ( n = 10) after (1) polymerization; (2) water storage (30 days); (3) cyclic loading (10,000 cycles at 5 Hz) and (4) water storage (30 days) + cyclic loading. Data (MPa) were analyzed with three-way ANOVA and Tukey's HSD tests ( α = 0.05). To test for a possible correlation between FS U and FS Pl, a linear regression coefficient “ r” was calculated. After water storage, L-UGH and L-TR demonstrated an increased FS U (41.49–50.64 MPa and 49.95–57.36 MPa, respectively) ( P < 0.05). Only L-TR demonstrated an increased FS Pl (20.58–24.21 MPa) after water storage ( P < 0.05). L-L had the highest FS U (between 78.57 and 85.09 MPa) and FS Pl (between 31.30 and 34.17 MPa) ( P < 0.05). The cyclic loading decreased the FS U and FS Pl of all materials ( P < 0.05). Regression analysis showed a strong linear correlation between the two variables ( r = 0.941). Water storage improved the FS U of L-UGH and L-TR and the FS Pl of L-TR. L-L produced the highest FS U and FS Pl. The FS U and FS Pl of all materials were detrimentally influenced by cyclic loading.