Abstract
Purpose: This in vitro study evaluated the effects of thermocycling and chemical ageing on the optical (specifically translucency, colour stability (ΔE₀₀), and masking ability), and physical (surface roughness, and fracture strength) properties of highly translucent polychromatic monolithic zirconia (HTPMZ) crowns subjected to different surface treatments.
Methods: Eighty HTPMZ crowns (IPS e.max ZirCAD Prime Esthetic, Ivoclar) were divided into four surface treatment groups: control (untreated), polished, glazed, and polished + glazed. Half of each group underwent artificial ageing through 20,000 cycles (~ two years) in deionised water at 5°C and 4% acetic acid (pH 2.47) at 55°C, following ISO 6872. Measurements were taken before and after ageing for optical translucency (TP), colour differences (CIEDE 2000 [ΔE₀₀]), surface roughness (Sa, Sq, Sz), and fracture strength. Masking ability was evaluated using visual thresholds (ΔE₀₀: acceptability 1.8, perceptibility 0.8). Fractographic features were examined by SEM. Non-parametric statistics with Bonferroni correction were applied (α = 0.05).
Results: Ageing significantly increased translucency in all groups, with the polished + glazed group showing the greatest change (p < 0.05). ΔE₀₀ ranged from 2.4 to 4.0, with no significant intergroup differences. Polished and polished + glazed crowns had higher proportions of clinically acceptable colour changes, with the polished + glazed group reaching 50%. Surface roughness differed significantly among groups (p < 0.05), lowest in polished and highest in polished + glazed crowns. Ageing did not significantly affect surface roughness or fracture strength. SEM analysis revealed typical fracture patterns originating at crown margins, with manufacturing defects such as milling marks and glaze voids.
Conclusion: Thermocycling and chemical ageing increase translucency of HTPMZ crowns but do not compromise surface topography or fracture resistance. Surface treatments, particularly polished and polished + glazed, enhance optical stability and masking ability, supporting their use in anterior restorations. Appropriate finishing protocols optimise aesthetics without affecting mechanical integrity, highlighting the clinical durability of HTPMZ crowns.