Impact of material selection, processing protocol and aging on the flexural strength of lithium silicate ceramics

To evaluate the effect of thermomechanical aging on the flexural strength of different lithium disilicate-based CAD/CAM ceramics under clinically relevant processing conditions.

A total of 192 bar-shaped specimens (15 mm×4 mm x 1.5 mm) were prepared from three CAD/CAM ceramics: two lithium disilicate (Amber Mill (AM) and IPS e.max CAD (EM)) and one lithium di/alumina-silicate (CEREC Tessera (CT)). Specimens were glazed using IPS e.max CAD Crystall./Glaze Spray or Universal Spray Glaze Fluo and subsequently processed according to material-specific crystallization protocols (conventional crystallization (cc) vs. speed crystallization (sc)), and sequences for material variants with different optical properties (high translucency processing (ht) vs. medium opacity processing (mo)). For each group (n = 32), half of the specimens were tested initially (non-aged), while the other half was subjected to artificial aging in a chewing simulator (1.2 million cycles, 50 N, 5°C/55°C). Flexural strength was determined via three-point bending. Data were analyzed using Kolmogorov-Smirnov, Kruskal-Wallis, and Mann-Whitney U tests (α=0.05). The Kaplan-Meier estimates together with the Log-Rang-test, as well as Weibull moduli, were computed.

Comparing processing protocols within each material, differences in flexural strength were observed only for artificially aged CEREC Tessera specimens (p < 0.05). Artificial aging significantly reduced flexural strength in both AMht, AMmo and CTcc. No differences were observed among non-aged groups. Among thermomechanically aged specimens, EMcc and EMsc, and CTsc demonstrated the highest flexural strength of all groups.

In most cases, the selected processing protocol did not influence the flexural strength, supporting the clinical feasibility of using faster or alternative crystallization programs when endorsed by the manufacturer. Artificial aging revealed material- and processing protocol-specific reductions in flexural strength, highlighting the importance of both material composition and processing conditions in ensuring long-term clinical durability. These findings emphasize that performance differences between materials may only become clinically relevant after exposure to simulated oral aging conditions.