Influence of external and internal surface roughness modifications on ceramic flexural strength

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After the occlusal adjustment of a ceramic restoration, polishing is necessary to promote surface smoothness and increase the ceramic flexural strength. However, the internal adjustment procedure may affect ceramic flexural strength. The purpose of this study was to evaluate the effect of external adjustment with or without the polishing procedure and of internal adjustments on the surface roughness and flexural strength of a ceramic. Sixty bar-shaped specimens were fabricated from a reinforced glass ceramic lithium disilicate and divided into 6 groups (n = 10): G, glaze (control); EA, external adjustment; EAP, EA + polishing; IA, internal adjustment; EIA, EA + IA; and EPIA, EA + polishing + IA. The entire external and internal surfaces were ground with an extrafine diamond rotary instrument positioned perpendicularly to the long axis of the specimen. External surface roughness was determined by measuring the Ra of the specimens. All the specimens were submitted to a 3-point bend test. One additional specimen from each group was prepared so that the surface morphology could be observed by scanning electron microscopy (SEM). The data were analyzed with 1-way ANOVA and the Bonferroni and Dunnett tests. Group EAP presented statistically lower surface roughness than G, EA, IA, and EIA but was similar to EPIA (P<.001). No statistically significant differences were found in the flexural strength of the 6 groups (P=.081). External adjustments without the polishing procedure did not affect the surface roughness of the ceramic. However, when followed by polishing, a significant reduction was found in surface roughness. This reduction in surface roughness did not influence the ceramic flexural strength, as well as the association of the others' surface roughness modifications and internal adjustment.