When an indirect restoration is selected as the treatment option for posterior teeth, the clinician must determine the configuration of the cavity preparation. Several designs have been proposed for preparing all-ceramic resin-bonded posterior restorations, as guided by the particular mechanical and structural characteristics of ceramic restorative materials. mechanical and structural characteristics of ceramic restorative materials. The primary causes of failure of ceramic onlay restorations are cohesive bulk fractures and marginal deficiencies, which manifest clinically as marginal discoloration and secondary caries. Tooth preparation designs for posterior ceramic restorations have been based on traditional cast metal restoration designs, but with more occlusal tooth reduction and with a slightly increased taper. These preparations may involve the removal of considerable tooth structure. As more structure is removed, higher tooth strain and lower fracture resistance may occur.5 The increased tooth structure loss may increase cuspal flexure, thereby reducing the tooth fracture resistance, or open the restoration-tooth interface .~However, it has been demonstrated that cusp recovery results in fewer failures, likely increasing the longevity of posterior ceramic restorations. Recently, minimally invasive cavity preparations for posterior indirect restorations were demonstrated to present the benefit of conservation of tooth structure, as well as improved stress distribution. However, the performance of posterior restoration is also material dependent. Due to the continuous advancements in dental ceramics and innovative manufacturing techniques.~Maximum preservation of sound tooth structure and maintenance of the vitality of restored teeth is critical for the longevity of teeth as well as restorations, especially in cases, where a large amount of tooth tissue has been lost due to wear and/or trauma. As the demand for conservative tooth treatment increases, so does the need for partial ceramic crowns. In addition to traditional cusp capping, simplified designs have been recommended in certain cases, such as fractured teeth and teeth with large caries.. According to the cusp coverage, the types of restorations can be classified as inlays, which is not covered cusps, onlays, which is covered at least one cusp, or overlays, and which is covered all cusps.~Onlay restorations not only provide superior esthetics but also minimize tooth tissue loss, making them a good treatment choice for posterior teeth with extensive cavities formed due to caries. Moreover, by covering more than one tooth cusp, onlays provide a favorable distribution of stress, reducing the risk of tooth and restoration fracture. preparation design may also play a role in tooth/restoration fracture. Some authors have shown that occlusal reduction reduces the chance of restoration failure. Others have shown that large preparation designs results in a reduced chance of possible fracture occurred in tooth tissue.~While different preparation designs have been described in the literature, the most appropriate design will vary according to the restorative material to be used. Despite the fact that partial ceramic crowns are increasingly advocated as alternative restorations for extensively damaged teeth, the literature includes limited studies examining the use of different preparation designs with this type of restoration. To date, confusing and contradictory results have been obtained regarding the effects of preparation design on the fracture resistance and stress distribution of tooth structure restored with partial ceramic crowns. Therefore, this study investigated the effects of three different preparation designs on the fracture of ceramic onlays. The hypothesis that different designs would affect the fracture resistance of onlay restorations.
Tooth preparation designs for posterior ceramic restorations have been based on traditional cast metal restoration designs, but with more occlusal tooth reduction and with a slightly increased taper. These preparations may involve the removal of considerable tooth structure. As more structure is removed, higher tooth strain and lower fracture resistance may occur.5 The increased tooth structure loss may increase cuspal flexure, thereby reducing the tooth fracture resistance, or open the restoration-tooth interface .~However, it has been demonstrated that cusp recovery results in fewer failures, likely increasing the longevity of posterior ceramic restorations. Recently, minimally invasive cavity preparations for posterior indirect restorations were demonstrated to present the benefit of conservation of tooth structure, as well as improved stress distribution. However, the performance of posterior restoration is also material dependent. Due to the continuous advancements in dental ceramics and innovative manufacturing techniques.~The aim of this study is to evaluate the clinical functional performance of ceramic onlay restorations with butt joint preparation design and compare them to shoulder preparation design.