Color stability of polyetheretherketone and graphene-reinforced polymethyl methacrylate after in-office bleaching

This in vitro study evaluated the color stability of polyetheretherketone (PEEK) and graphene-reinforced polymethyl methacrylate (G-CAM) following direct exposure to a high-concentration in-office bleaching gel.

Thirty square-shaped specimens (10 × 10 × 3 mm) were prepared: 15 PEEK (BioHPP, Bredent, Germany) in shade A2 and 15 G-CAM (Graphenano Dental, Spain) in shade A2, with baseline color coordinates recorded using a spectrophotometer (VITA Easyshade Advance 4.0, Vita Zahnfabrik, Germany). All specimens were surface-ground, ultrasonically cleaned, and stored in distilled water at 37 °C for 24 h prior to measurements. A 40% hydrogen peroxide bleaching gel (Opalescence Boost, Ultradent Products, South Jordan, UT, USA) was applied for 20 min and repeated after a one-week interval, with color measurements obtained after each session. Color differences were calculated using the CIEDE2000 formula (ΔE₀₀). The observed CIELAB coordinates (L*, a*, b*) were analyzed using a 2 × 3 mixed-design ANOVA (material × bleaching stage), whereas the derived ΔE₀₀ contrasts were evaluated secondarily, with Mann-Whitney U tests used as supplementary between-material analyses (α = 0.05).

A significant material × time interaction was found for all CIELAB coordinates (L*, a*, b*; all p < .001), indicating that the two materials followed distinct temporal patterns of color change. After the first bleaching session, G-CAM exhibited a markedly higher color change (ΔE₀₀ = 7.97 ± 3.12) compared with PEEK (ΔE₀₀ = 0.55 ± 0.23). Between the first and second sessions, G-CAM continued to show a higher incremental color change (ΔE₀₀ = 7.02 ± 2.61) than PEEK (ΔE₀₀ = 3.66 ± 0.26). When total color change from baseline to the second bleaching session was evaluated, G-CAM demonstrated a lower ΔE₀₀ value (1.81 ± 1.04) compared with PEEK (3.86 ± 0.34).

PEEK and G-CAM exhibited markedly different color-stability behaviors under high-concentration in-office bleaching. Under the tested direct-exposure conditions, the mean baseline-referenced ΔE₀₀ of G-CAM exceeded the 1.8 acceptability threshold after the first session and was approximately at the threshold after the second session (1.81 ± 1.04), whereas that of PEEK remained below the threshold after the first session and exceeded it after repeated exposure. Under these in vitro conditions, the two materials behaved differently, but the results require clinical confirmation and should not be interpreted as direct treatment guidance. These findings are specific to the direct-exposure protocol used and should not be extrapolated to lower-concentration home-bleaching systems.