Influence of Finishing Procedures on Mechanical Properties of Monolithic Ceramics

Objectives: Considering monolithic restorations, polishing, glazing and extrinsic staining can be performed in order to better mimic natural teeth. Nonetheless, additional firing cycles or thicker stain/glaze layer may influence the mechanical properties of the restorations.
This study evaluated the influence of different surface finishing protocols on the mechanical properties of ceramics.


Methods: Discs of zirconia (Z); lithium disilicate (LD) and zirconia-reinforced lithium silicate (ZLS), received six surface finishing protocols: polishing (P); glazing (1 firing cycle) (G); staining + glazing (1 firing cycle) (SG); one staining layer then glazing (2 firing cycles) (S/G); four staining layers then glazing (5 firing cycles) (4S/G); and four staining layers then two glazing layers (6 firing cycles) (4S/2G); (n=150/material). Evaluation of surface roughness using an optical profilometer; hardness and elastic modulus with a nanoindenter; and biaxial flexural strength with a universal testing machine were carried out. Zirconia phase transformation was investigated using x-ray diffraction.
Results: Roughness was influenced by material and finishing procedure (p<0.05). For zirconia, P group was rougher than others (p<0.05). For ZLS and LD, no significant difference was shown between P and G groups; however they differed from the stained groups (p<0.05). Hardness and elastic modulus were also influenced by material and finishing procedure (p<0.05). While zirconia P group evidenced higher hardness and elastic modulus, 4S2G group showed significantly lower values than other zirconia finishing procedure (p<0.05). For LD and ZLS there was no difference among the surface finishing protocols (p>0.05). The flexural strength was different concerning to material and finishing procedure (p<0.05). Additional firing cycles decreased the flexural strength for Z e ZLS (p<0.05); there was no significant difference for LD (p>0.05). Also, additional firing cycles increased monoclinic phase content for zirconia discs (p<0.05).
Conclusions: Additional firing cycles and glaze thickness can affect mechanical properties of Z and ZSL.