Fractographic analyses of three ceramic whole crown restoration failures

Develop fractographic techniques to perform failure analyses of three ceramic whole-crown clinical failures.

Three clinical whole-crown failures of different ceramic material systems were studied. The systems included appropriate veneers applied to core materials of cold isostatically pressed alumina, injection molded alumina/alumina-magnesia spinel, and hot pressed lithium disilicate. The surface topography was examined for classical fractographic features, utilizing both optical and scanning electron microscopes independently as well as group examination by four fractographers.

All three restorations displayed many common fractographic features, the most important of which was wake hackle. Wake hackle occurs when an approaching crack front sweeps by a small bubble or discontinuity in a brittle material, leaving a trail. The resulting trails can be used to trace back crack progression to an initial starting area. Wake hackle was found to be common in the veneer portions of the crowns, and the strategy of creating a wake hackle map enabled crack paths to be traced from a core area outward in all three restorations. The fracture origin areas were located where hoop stresses are high. Another helpful fracture feature was compression curl, an indication of flexural stress, most visible under low incident optical lighting.

This study indicates that fractographic analyses can be productively performed on ceramic whole-crown failures. Optical examination under low incident lighting and wake hackle mapping were developed as promising techniques. Hoop stress was implicated as a concern in the three studied specimens, and should be considered when analyzing other whole-crown failures.