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The Influence of Material Type, Preparation Design, and Tooth Substrate on Fracture Resistance of Molar Onlays


Tooth colored all ceramic restorations have been the treatment preference of many patients for esthetics and biocompatibility. This study aimed to test the fracture resistance of posterior ceramic onlays milled with computer-aided design and computer aided manufacturing (CAD/CAM) machines.

The effects of material type, preparation design, and tooth substrate were evaluated using a full-block design. Ninety teeth were tested. Three different CAD/CAM ceramic onlay material types were included: a lithium disilicate glass-ceramic (IPS-e max CAD), a nano-filled resin-composite (Lava Ultimate), and a feldspathic porcelain (CEREC Blocs C).

Three different preparation designs were included: an anatomical occlusal onlay preparation, a concave occlusal onlay preparation, and a flat occlusal onlay preparation. Two different tooth substrates were used: all in enamel, or in dentin with enamel peripheries. All teeth were restored and artificially aged. A load-to-failure test was used to measure the resistance to catastrophic fracture.

Three-way ANOVA determined that of all the simple main effects, and all of their possible interactions, the only significant effect on failure load was that of material type (p < 0.0001). The glass-ceramic and resin-composite materials were stronger than the feldspathic porcelain. Multiple range analysis determined that restorations fabricated from feldspathic porcelain were significantly weaker than those made of glass-ceramic or resin-composite materials.

The restorations in all 90 specimens fractured catastrophically; but only 26 of the 90 teeth fractured. Material type influenced the incidence of tooth fracture (Chi-square = 12, p < 0.05). Preparation design influenced the incidence of tooth fracture (Chi-square = 7, p < 0.05). Tooth substrate type did not influence the incidence of tooth fracture (Chi-square = 2, p > 0.05). A glass-ceramic restoration, and a concave preparation design were associated with increased incidences of tooth fracture.

Resistance to catastrophic failure is desirable, as is a failure mode that avoids vertical tooth fracture, but these results showed that these goals might be mutually exclusive.

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