Optical stability and underlying mechanisms of nine brands of ultra-translucent zirconia: a study under in vitro aging simulation

Background Ultra-translucent zirconia is widely used in dental restorations for its excellent optical properties, yet long-term evidence regarding the stability of its esthetic performance in clinical scenarios remains limited. This study aimed to systematically evaluate the optical stability and underlying mechanisms of 9 commonly used ultra-translucent zirconia materials before and after artificial aging. Materials and methods Ninety specimens ( n = 3 per group) were prepared from nine ultra-translucent zirconia brands, namely Wieland (ZNT), 3 M ESPE (LVP), Amann Girrbach (AG), Kuraray Noritake (KAT), Aidite (ADT), Upcera (UPC), Besmile (BSM), Ivoclar Vivadent (ZLT), and Sagemax (NZ), and one lithium-disilicate glass-ceramic control, Ivoclar Vivadent IPS e.max Press (PLT), at thicknesses of 0.5 (for veneers), 1.0 (for veneers or monolithic crowns), and 1.5 mm (for monolithic crowns). Specimens were hydrothermally aged at 134 °C and 0.2 MPa for 4, 8, 12, and 16 h. Translucency parameter (TP) and color difference (the CIEDE2000 ∆E 00 ) were measured before and after each aging duration. Crystalline characteristics and surface morphology were analyzed via X-ray diffraction (XRD) and scanning electron microscopy (SEM). A three-way independent-samples ANOVA with Bonferroni-adjusted pairwise comparisons was applied to TP. ∆E 00 was evaluated descriptively against the acceptability threshold. Results TP was brand- and thickness-specific, with the control group showing the highest values. UPC showed significant TP reduction across all thicknesses, while ZNT (1.5 mm), BSM (1.0 mm), and the PLT control (1.0 mm and 1.5 mm) at selected thicknesses exhibited declines after different aging durations. ∆E 00 exceeded the acceptability threshold of 2.25 in most material–thickness combinations from 12 h of aging onward. LVP was the only material that remained below 2.25 at all three thicknesses across all aging times, whereas all other ultra-translucent zirconia materials and the control (PLT) exceeded the threshold at one or more aging durations. XRD confirmed monoclinic phase formation in all zirconia groups and SEM images revealed surface changes of most groups’ specimens after aging. Conclusions Translucency and color stability of ultra-translucent zirconia are brand- and thickness-dependent under hydrothermal aging conditions, offering preliminary comparative optical data that may inform future comprehensive material evaluations.