Tehran University of Medical Sciences Frontiers in Dentistry 2676-296X 12 12 2016 08 15 874 881 EN Hamid Jalali Assistant Professor, Dental Research Center, Dentistry Research Institute, Tehran University of Medical Sciences, Tehran, Iran; Department of Prosthodontics, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran Leyla Sadighpour Assoicate Professor, Department of Prosthodontics, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran Ali Miri Faculty of Dentistry, Tehran University of Medical Sciences Ahmad Shamshiri Dentistry Research Institute, Department of Community Oral Health Faculty of Dentistry, Tehran University of Medical Sciences 2016 01 01 2016 03 11 Objectives: The purpose of this in vitro study was to compare the marginal adaptation and fracture resistance of a zirconia-based all-ceramic restoration with two preparation designs. Materials and Methods: Twenty-four mandibular premolars were randomly divided into two groups (n=12); the conventional group received a peripheral shoulder preparation and the modified group received a buccal shoulder and proximal/lingual chamfer preparation. The marginal fit of the zirconia crowns (Cercon) was evaluated using a stereomicroscope. After cementation, load was applied to the crowns. The mean fracture load and the mean marginal gap for each group were analyzed using t-test (P=0.05). Results: The mean marginal gap was 71±16µm in the conventional group and 80±10µm in the modified group, with no significant difference (P=0.161). The mean fracture strength was 830±153N for the conventional group and 775±125N for the modified group, with no significant difference (P=0.396). All but one fracture occurred in the veneering ceramic. Conclusion: Less aggressive preparation of proximal and lingual finish lines for the preservation of tooth structure in all-ceramic restorations does not adversely affect the marginal adaptation or fracture strength of the final restoration. https://fid.tums.ac.ir/index.php/fid/article/view/985 https://fid.tums.ac.ir/index.php/fid/article/download/985/849