الفهرس 
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Abstract
Tooth fracture is one of the commonest causes of tooth loss. Many factors contribute to fracture such as tooth anatomy, cavity design and restorative material. Because of their morphology and occlusal forces, maxillary premolars have a higher fracture rate than molars and mandibular premolars. To improve the fracturing strength of the teeth, indirect restorations are recommended. According to cuspal coverage, restorations can be classified as inlays, onlays, or overlays.
Inlays and onlays can be fabricated either in the laboratory or using CAD/CAM technology. Cuspal coverage is thought to be increasing fracture resistance. Moreover, CAD/CAM hybrid materials that have combination of ceramic and polymer phases, are believed to have better flexural strength, elasticity, and hardness similar to natural tooth structure.
The aim of the present study was to evaluate the fracture resistance and the mode of fracture of maxillary premolars restored with polymer infiltrated ceramic and resin nano-ceramic CAD/CAM inlays and onlays.
This in-vitro study included forty-five extracted human maxillary premolars. Nine teeth were left intact as the control group (group I) and the remaining thirty-six specimens were divided into two main groups (n=18) according to the MOD cavity preparation design. Preparations were made as: group II: MOD inlay, group III: MOD onlay with both functional and non-functional cusps reduction. Each group was subdivided into two subgroups (n=9) according to the type of CAD/CAM materials used; VITA Enamic and Lava Ultimate. RelyX™ U200 self -adhesive resin cement was used for cementation. All samples were subjected to thermocycling (5,000 cycles at 37°C, 5°C, 37°C, and 55°C). All specimens were axially compressed in a universal testing machine using a metal sphere of 4 mm diameter at speed of 0.5mm/min. Mode of fracture was examined under stereomicroscope.
Results showed that the highest mean fracture resistance was shown in the intact group (group I), while the lowest fracture resistance was manifested by Lava Ultimate onlays (group IIIb). Lava Ultimate inlays (group IIb) exhibited higher values than Vita Enamic inlays (group IIa) though there was no significant difference between them. However, Vita Enamic onlays (group IIIa) showed higher fracture resistance than Lava Ultimate onlays (group IIIb). There was only significant difference between group I (control group) and group IIIa and group IIIb, while the remaining differences were not statistically significant. Mode of fracture analysis showed the highest rate of pattern III (fracture in both tooth and restoration) was in group IIa (Vita Enamic inlays), while the highest rate of pattern I (fracture in restoration only) was in group IIIa (Vita Enamic onlays) and group IIIb (Lava Ultimate onlays) equally. And pattern IV (fracture in both restoration and tooth structure and through the long axis) was mainly in group IIb (Lava Ultimate inlays).
Based on the results of this study, we recommend choosing the inlay preparation design and conservation of tooth structure, when there is no need for more extension and cuspal coverage, to provide maximum fracture resistance to the restored teeth as the inlays showed higher mean fracture resistance than the onlays. Also, both Vita Enamic and Lava Ultimate CAD/CAM materials showed comparable results, apart from the cavity design, and can be used efficiently and reliably in fabricating inlays and onlays for restoring posterior teeth.