Fractographic analysis of lithium-disilicate crowns fabricated with different processing techniques

Method: Two fractured anterior crowns, made of monolithic and bilayered Li2Di respectively, were recovered. They were examined utilizing first a stereomicroscope and second a scanning electron microscope (SEM). The stereomicroscope analysis was performed at low magnifications (57x) with oblique lighting in order to identify classic fractographic patterns such as arrest lines and hackles. A preliminary fracture surface map of the stereo-observations was drawn and used as a guide for the SEM analysis that followed and interpreted back to the origin of failure. Small fracture features, such as wake hackle within veneering ceramic and the glaze layer were highlighted.

Result: The stereomicroscope analysis provided an overview of the fractured topography. The greater magnifications (SEM analysis) showed the presence of wake hackles, indicators of the crack propagation direction. A general map of the fracure events could be reconstructed starting with the irregularities at the internal aspect of the margins of both crowns. The bilayered Li2Di crown showed a feather-edge-like margins. In the monolithic crown two fracture events could be identified. Hackle and wake hackle of the crack front emanating from this mesial margin arose from hoop stresses and propagated through the full crowns thickness towards the distal axial walls. The main crack seemed to start from the mesial internal margin, the secondary event occurred from the distal axial wall.

Conclusion: The rapid cooling technique and the internal adjustments of the crown, by mean of grinding, performed by the laboratory seemed to play a role in fracture starting and propagation. Fractography could be an useful tool for understanding the failure process in brittle restorative materials, as well as for assessing possible design or firing cycles or technique inadequacies.