Evaluation of the Residual Glass Phase in Lithium Silicate Glassceramics

Objectives: CAD/CAM processing of lithium silicate glass ceramics (LS) is often performed in a meta-crystallized state followed by a final crystallization step. Incidence of crown fracture was observed upon cooling from crystallization temperature, hypothesizing significant internal stress build-up during crystallization. The aim of this study was to investigate the chemical composition of the residual glass phase, to measure coefficients of thermal expansion (CTE) and to conclude on the internal stress level.
Methods: The chemical compositions of Suprinity PC (SUP, Vita Zahnfabrik) and e.maxCAD (EMC, IvoclarVivadent) were identified using XRF and ICP-OES techniques while the crystalline phases were analyzed using XRD. By subtracting the chemical composition of the crystalline phases, we calculated the composition of the residual glass phases. Glasses were melted accordingly and the CTE of the parent and residual glass was measured in Dilatometry and compared with the single components.
Results: The chemical analysis revealed a fraction for of 9.8 wt% (SUP) / 0.8 wt% (EMC) zirconia in the parent glass, which enriched up to 19.2 wt% (SUP) / 2.2wt% (EMC) in the residual glass, while the crystalline phases covered 49.9 wt% (SUP) / 66.5wt% (EMC) of the material. The residual glasses resulted in a reduced CTE compared to the parent glasses.
Conclusions: A comparison of CTE of the single crystalline phases with the residual glasses led us to the conclusion that a high mismatch was found responsible for build-up of microscopic internal stress influencing the overall stress state and in turn resulting in spontaneous fracture of the crown at certain processing conditions. EMC showed less internal stresses compared to SUP, most likely due to the lack of zirconia in the residual glass and due to less amount of lithium metasilicate. Slow cooling upon crystallization temperature was found to reduce the failure susceptibility.