Impact of Various Cavity-Preparation Designs on Fracture Resistance and Failure Mode of CAD/CAM Fabricated Ceramic Inlays and Onlays  
[PERSON]; [PERSON]; [PERSON]; [PERSON] Impact of Various Cavity-Preparation Designs on Fracture Resistance and Failure Mode of CAD/CAM Fabricated Ceramic Inlays and Onlays. _Appl. Sci._**2024**, _14_, 3816. [[https://doi.org/10.3390/appl409816](https://doi.org/10.3390/appl409816)]([https://doi.org/10.3390/appl409816](https://doi.org/10.3390/appl409816))  
Footnote †: Copyright** © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))).  
Footnote †: Copyright** © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/0/](https://creativecommons.org/licenses/by/4.0/0/)]([https://creativecommons.org/licenses/by/4.0/0/](https://creativecommons.org/licenses/by/4.0/0/))).  
Footnote †: Copyright** © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.  
and dentist [6]. Many materials, including zirconia, can be used to provide indirect restorations for posterior teeth. These materials are biocompatible, long-lasting, aesthetically pleasing, and resistant to fatigue [7].  
For posterior teeth with significant cavities caused by caries, only restorations are a desirable treatment option because they decrease tooth tissue loss and offer preferable aesthetics [8,9]. Onlays also offer beneficial stress distribution by covering multiple tooth cusps, decreasing the chance of tooth and/or restoration fracture [10]. The degree to which indirect ceramic restorations resist fracture is one of the most crucial factors in determining their efficacy [11].  
Clinical fracture or failure of zirconia ceramic restorations is influenced by various parameters: the mechanical characteristics of the restoration, the width of the isthmus, the depth of preparation, the morphology of internal line angles, and other aspects of cavity geometry [12,13].  
If a tooth's fracture resistance is compromised during cavity preparation, it is challenging to restore [14]. Dentists have been advised to use the preparation design that provides the highest resistance to fracture. Fracture-resistance tests have been used to measure the forces that may cause such restorations to fracture. However, research on the effects of preparation design on the stress distribution and fracture resistance of tooth structures restored with partial ceramic crowns has produced confusing and inconsistent results [14].  
Restorations performed with an indirect approach employing a CAD/CAM system may have been selected in cases where there has been a greater loss of tooth structure, according to [PERSON] et al. [15]. Many research and papers have addressed the isthmus width in direct restoration, even though it is crucial for the preservation and integrity of the restoration and teeth [14, 15, 16].  
Since its impact on indirect restoration is still unknown, the purpose of this study is to investigate how isthmus width affects fracture resistance and failure modes in premolar teeth by investigating its effects in various cavity-preparation designs for zirconia ceramic inlays and onlays made with CAD/CAM technology [17]. The null hypothesis stated that the fracture resistance and mode of failure of premolars restored with zirconia inlays and onlays are not significantly impacted by variations in cavity-preparation designs.