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Title: 1741 - Fractographic Analysis of Lithiumsilicate Crown Failures During Sintering

Authors:

Ulrich Lohbauer (Presenter)
University of Erlangen

Michael Wendler, University of Concepción
Anselm Petschelt, University of Erlangen
Renan Belli, University of Erlangen

Abstract:

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.

Disclosure Statement:
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

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