Effect of polyethylene and glass fiber reinforcement on the radiopacity of flowable resin composites: an in-vitro comparative study

Chairside fiber reinforcement of flowable composites may alter their radiographic behavior, yet different fiber systems have not been directly compared within the same composite matrix. This study compared a polyethylene fiber mesh (Ribbond) and a pre-impregnated glass fiber strip (EverStick) for their effect on the radiopacity of two flowable composites with different filler systems. Clearfil Majesty Flow (CMF; barium glass, 81 wt%) and G-aenial Universal Injectable (GAI; strontium/lanthanoid glass, 69 wt%) served as negative controls and EverX Flow (EXF), a factory short-fiber-reinforced composite, as positive control. Ribbond (polyethylene mesh) and EverStick (pre-impregnated glass fiber strip) were incorporated as single 4 × 2 mm interlayers in disc specimens (10 mm × 2 mm; n = 5 per group). Aluminum-equivalent radiopacity (mmAl) was calculated from digital radiographs using ImageJ. Enamel and dentin sections from one extracted third molar served as descriptive references. Data were analyzed by one-way and two-way ANOVA with Tukey HSD tests (α = 0.05); effect sizes and 95% confidence intervals were reported. Significant differences existed among the measured series [F(8, 36) = 335.03, p < 0.001, ηp² = 0.987]. EverStick reduced radiopacity by 40.3% in CMF (7.66 ± 0.25 to 4.58 ± 0.27 mmAl) and by 32.4% in GAI (3.94 ± 0.24 to 2.66 ± 0.22 mmAl; p < 0.001 for both). Ribbond did not significantly alter radiopacity ( p > 0.05). Two-way ANOVA of the six composite groups confirmed significant main effects and interaction ( p < 0.001). All groups exceeded the ISO 4049 minimum (≥ 2.0 mmAl); GAI + EverStick produced the lowest mean (2.66 mmAl) and was numerically lower than the enamel reference (4.11 ± 0.27 mmAl). Ribbond produced no statistically detectable change in radiopacity in either composite background, whereas EverStick reduced it significantly. The differences were statistically significant, but all constructs still met the ISO 4049 requirement and no diagnostic-accuracy assessment was performed, so a clinical consequence remains to be demonstrated. The difference between the two systems most plausibly reflects their combined properties — fiber composition, bundle dimensions, resin pre-impregnation and adhesive infiltration — rather than fiber type alone. Radiographic behavior may reasonably be considered alongside mechanical performance when a reinforcement system is selected.