Title: 1741 - Fractographic Analysis of Lithiumsilicate Crown Failures During Sintering
Ulrich Lohbauer (Presenter)
University of Erlangen
Michael Wendler, University of Concepción
Anselm Petschelt, University of Erlangen
Renan Belli, University of Erlangen
Objectives: Lithiumsilicate based ceramics exhibit superior strength and toughness than feldspathic or leucite reinforced dental ceramics, being thus preferred materials for the fabrication of single-unit dental prostheses. Subtractive crown manufacturing via CAD/CAM procedure is often performed in a meta-crystallized state followed by a final crystallization step. Depending on the specific heat transfer within the crown during crystallization firing, we observed reproducible crack formation upon cooling lithiumsilicate crowns from the sintering temperature.
Methods: Lithiumsilicate molar crowns (Suprinity, Vita Zahnfabrik) were CAD/CAM manufactured in a CEREC system (inLab MC XL, DentsplySirona) and crystallized on dark ceramic firing tray/pin in a Vacumat 4000 (Vita Zahnfabrik) according to the IFU (840°C, 8min). Three cooling protocols were applied (n=6 crowns per group), defined by the furnace opening temperature: 840°C (fast cooling, FC), 680°C (reference cooling, RC), and 550°C (slow cooling, SC). An additional thermocouple was installed in the crown lumen. Crowns that fractured during the crystallization process were fractographically examined under a Stereomicroscope (Discovery V8, Zeiss) and SEM (ISI 50, Leitz). Additional measurement of the glass transition temperature Tg was performed using thermo mechanical analysis (TMA; Netzsch).
Results: The crowns fired at RC or SC protocols remained intact after crystallization. The FC protocol produced 4 bulk fractures in an identical manner: The fracture started at the contact point of the crown with the firing pin in the crown lumen. A chill-check crack type was identified as fracture origin. First crack formation was observed at 650°C chamber temperature (versus 450°C in the crown lumen). TMA measurement identified the Tg of the fully crystallized material at 640°C.
Conclusions: In light of the results, the manufacturer’s IFU regarding cooling protocol is a prerequisite for material integrity, minimizing internal stress build-up. FC protocols are contraindicated for lithiumsilicate dental ceramics. The use of firing pins could also be questioned due to localized thermal heat transfer.
The submitter must disclose the names of the organizations with which any author have a relationship, the nature of the relationship, and the clinical or research area involved. The following is submitted: NONE