Effect of multiple firing cycles on optical and mechanical properties of lithium disilicate ceramics fabricated via pressable and CAD/CAM technologies: a systematic review and meta-analysis

Multiple firing cycles are commonly used during the fabrication and adjustment of lithium disilicate restorations but may affect their optical and mechanical properties. This systematic review and meta-analysis evaluated the effects of repeated firing on colour change (ΔE/ΔE₀₀), translucency parameter (TP/TP₀₀), and biaxial flexural strength (BFS) of lithium disilicate ceramics, while assessing material-specific responses and clinically relevant firing thresholds. Four materials were included: IPS e.max CAD, IPS e.max Press, Amber Mill, and Amber Press. A systematic review and meta-analysis were conducted in accordance with the PRISMA 2020 guidelines. Comprehensive electronic searches were performed in Scopus, Web of Science, PubMed/MEDLINE, the Cochrane Library, and Google Scholar. Following screening and eligibility assessment, 40 studies were included in the systematic review, and 20 studies met the criteria for quantitative synthesis. Random-effects meta-analyses were performed using Hedges’ g, and statistical heterogeneity was assessed using the I² statistic. Forty studies were included in the systematic review. The meta-analysis comprised 17 translucency datasets, 11 colour-change datasets, and 13 BFS comparison datasets. Translucency showed a small, non-significant secondary pooled effect (Hedges’ g = + 0.20; 95% CI: −0.11 to + 0.50). Formula-specific colour-change analyses showed large standardised effects for both ΔEab (k = 9; g = + 3.58; 95% CI: +3.13 to + 4.03) and ΔE₀₀ (k = 2; g = + 4.08; 95% CI: +3.76 to + 4.41). BFS showed no statistically significant secondary pooled effect (g = + 0.58; 95% CI: −0.28 to + 1.44). Responses varied by material and firing protocol. Repeated firing was associated most consistently with colour change, whereas translucency and biaxial flexural strength varied according to the material and firing protocol. Firings beyond five cycles may increase the risk of clinically unacceptable colour alteration in some materials and settings. IPS e.max CAD appeared comparatively stable across several tested protocols, while the smaller evidence base for Amber ceramics indicated greater optical variability. These findings support minimising avoidable refiring, particularly when aesthetic stability is critical; however, the evidence remains entirely in vitro.