Background After root canal treatment, most tooth defects need to be restored. Onlay restoration is widely used to restore dental defects. Endocrown is a new type of onlay; however, dentists have yet to obtain a full understanding of the clinical effects of marginal forms of endocrowns. Here, we present a multicenter protocol to compare the clinical efficacy of two marginal forms (flat and 90-degree shoulder) for tooth restoration. The efficacy will be evaluated by marginal fit, marginal discoloration, and integrity of restoration. Methods The proposed flat and 90-degree shoulder marginal endocrown assessment trial is an open-label, parallel-group, multicenter randomized controlled trial involving two hospitals. A total of 200 patients will be included in this trial, and the following patient inclusion criteria will be applied: good oral hygiene habits, no periodontal diseases, receipt of standard root canal treatment, and need for endocrown restoration. Patients will be enrolled after providing signed informed consent and will be divided into two groups (flat and 90-degree shoulder endocrown) in accordance with a random number table. Treatment allocation will be balanced (1:1). Endocrowns will be cemented by dual-cured luting composite. Clinical evaluations will be performed at baseline and at 24 months after treatment in accordance with modified US Public Health Service criteria by two independent evaluators. The primary outcome will be marginal fit; secondary outcome measures will include debonding, marginal discoloration, and integrity of restoration. All acquired data will be analyzed by an independent statistician. Wilcoxon one-sample tests will be used for intra-group comparisons, and Wilcoxon two-sample tests will be used for inter-group comparisons. The Bonferroni method will be used to correct for multiple comparisons, and hierarchical logistic regression will be applied to determine central effects. Discussion The results of this trial will provide a clinical basis for clinicians to restore teeth by endocrowns and to improve long-term restoration for patients. Trial registration ClinicalTrials.gov identifier: NCT03398395 . Registered on 12 January 2018. Electronic supplementary material The online version of this article (10.1186/s13063-019-3530-1) contains supplementary material, which is available to authorized users.
Background This study aimed to evaluate the stress distributions in endocrown restorations as applied to endodontically treated teeth (ETT), according to the factors of “margin design” (four levels) and “restorative material” (six levels). Methods Four 3D-finite elements models were constructed for endocrown restored molars considering different margin designs. Model A was prepared with a flat butt joint margin and received an endocrown with a 2.0-mm occlusal thickness. Model B was prepared with a 20° bevel margin and received an endocrown with a 2.0-mm occlusal thickness. Model C was prepared with an axial reduction and 1-mm shoulder margin and received an endocrown with a 2.0-mm occlusal thickness. Model D was prepared with an anatomic margin and received an endocrown with a 2.0-mm occlusal thickness. The following endocrown materials were used: In-Ceram Zirconia (Zr), Vita Suprinity (VS), IPS Empress (IE), Grandio blocs (GR), VisCalor bulk (VS), and CopraPeek Light (CP). The Load application (600 N) was performed at the food bolus and tooth surface during the closing phase of the chewing cycle. The results for the endocrown and tooth remnants were determined according to the von Mises stress. The failure risk of the cement layer was also calculated based on the normal stress criterion. Results Model D (with an anatomic margin) showed the greatest stress concentrations, especially in the irregular and sharp angles of the restoration and tooth remnants. The stress concentrated on the dentin was significantly lower in Model B with a 20° bevel margin (20.86 MPa), i.e., 1.3 times lower than the other three margin designs (27.80 MPa). Restorative materials with higher elastic moduli present higher stress concentrations inside the endocrown and transmit less stress to the cement layer, resulting in lower bonding failure risks. In contrast, materials with an elastic modulus similar to that of dentin presented with a more homogeneous stress distribution on the whole structure. Conclusions An endocrown with a 20° bevel margin design could be a favorable preparation option for ETT. Composite resins (GR and VC) exhibit a more even stress distribution, and seem to be more promising materials for endocrown molars.
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