# Influence of Cavity Geometry on the Fracture Strength of Dental Restorations: Finite Element Study
## 1 Introduction  
The restorative treatment of teeth with a considerable amount of loss structure remains a great challenge because of their susceptibility to fracturing [1; 2]. The optimum tooth preparation depends on several design variables, which can vary from the materials used to the cavity geometries, passing through marginal or occlusal surface considerations. Although several variables achieve ideal tooth preparation, the objective is always to prevent fracturing and restore good tooth functionality. Nevertheless, the restoration materials and the cavity design geometry should also ensure a balance between preserving the tooth structure and maximizing the strength of the restoration [2]. In this context, ceramic inlays, onlays, veneers and complete-coverage crowns have gained popularity [3]. Several review studies have been developed to identify if different types of indirect restorations used for single teeth had different biological and technical complications and survival rates [4; 5; 6]. Within the nine studies selected by [PERSON] et al., the caries was the main biological complication for all types of restorations, followedby a root or tooth fracture incidence of 11.34%. Ceramic fractures represented the most common technical complication, followed by loss of retention and porcelain chipping. Hence, two different ceramic preparation designs were selected in this study: inlay and onlay. These preparations are often used for indirect restoration when a molar or premolar is too damaged to support a basic filling, but not so severely that it needs a crown [7]. Both structures are prepared outside the patient's mouth and, posteriorly, are cemented or bonded to the tooth. Both restorations were assumed to be CAD/CAM produced using IPS Empress(r) CAD (Ivocalar Vivadent, Schaan, Liechtenstein) dental restorative materials, which has been developing modern composites and ceramic-based materials that are almost indistinguishable from natural dentition. These indirect restorations are specially indicated to posterior teeth treatments, wherein they should be capable of offering superior mechanical properties, improved anatomy, proper contour, interproximal contact, marginal adaptation and surface texture [3; 8]. Nevertheless, due to the lack of marginal ridges and their specific morphology, the prevalence of fractures in the maxillary premolars is higher than in the molars and mandibular premolars [9]. A great number of in vivo [7; 10; 11], in vitro [2; 3; 12] and numerical studies [8; 13; 14; 15] have been developed to evaluate the outcome of several design parameters of the inlay and onlay preparations for the longevity of tooth restoration. In a five-year retrospective study, [PERSON] et al. [16] concluded that when comparing inlay and onlay restorations, there was a tendency toward a higher number of failures in the inlay group. Later, in 2005, [PERSON] et al. [17] reported that teeth with inlays appeared more resistant to fractures than those with onlays. Nevertheless, the results of several finite element studies [18; 19; 20] show that significant defects in molars and premolars should be restored with ceramic onlays and not inlays. However, in 2011, [PERSON] et al. [21], in an experimental study of the fracture resistance of premolars restored with inlay and onlay ceramic restorations, showed that cuspid coverage did not result in improved fracture resistance of the restored teeth. Hence, inconsistent and conflicting reported results might be related to the use of different restoration materials, dimensions and criteria for reducing dental cusps and the use of varying experiment set-ups [16; 18; 19; 20; 21; 22]. Hence, in the present study, preparation of both the onlay and inlay cavities was performed with the same dimensions at the cervical region. The main difference is only the cusps' material i.e., the height and geometry of the dental cusps are the same for both restorations.