Finite Element Evaluation of Stress Distribution in Mature and Immature Teeth
INTRODUCTION: Traumatic injuries may affect the vitality and development of tooth. So divergence or parallelism of canal walls induces an open apex which is very susceptible to fracture. The aim of this study was to determine the stress distribution pattern in immature teeth and compare it with mature teeth. MATERIALS AND METHODS: In order to analyze stress distribution using finite element method, models were first designed according to actual samples by "ANSYS" software. Two models of the maxillary central incisors were designed, one mature and one with open apex. Mature tooth was designed as having undergone root canal therapy with gutta-percha and the immature one with both MTA plug and gutta-percha. Samples were loaded in seven stages and then, the stress distribution in each model was measured, using the "ANSYS" software. RESULTS: During gutta-percha condensation, the immature tooth transfers the stress directly to the external root surface where force is directly applied, whereas during masticatory occlusal forces, the stress is transferred to middle third of buccal and lingual surfaces and to buccal cervix in lunar shape. During traumatic forces, stress concentration was on the cervical region of buccal surface and middle third of buccal and lingual surfaces. MTA plug prevents stress distribution toward apical region and forces concentrate in dentin at the point where stress terminates. CONCLUSION: Within the limitations of this simulated mechanical analysis, it was confirmed that the pattern of stress distribution in mature and immature teeth is different. Cervical area of the buccal surface in immature teeth is one of the stress concentration areas, which contribute to the high rate of fracture in this area.
Long‐term Success of Nonvital, Immature Permanent Incisors Treated With a Mineral Trioxide Aggregate Plug and Adhesive Restorations: A Case Series from a Private Endodontic Practice
Abstract This case series evaluated the long‐term clinical outcome of nonvital immature teeth treated with mineral trioxide aggregate (MTA) as an apical barrier and an adhesive restoration with or without a fiber post. Eighty‐three teeth in 72 patients were treated by the first author with an apical MTA plug and an adhesive restoration of composite resin and in 45 of the 83 teeth 1 or more fiber posts. All of the patients had been referred to the first author's private endodontic practice with at least 1 immature tooth with signs of pulpal necrosis and subsequent apical periodontitis that had been caused by a variety of traumatic dental injuries. Three teeth presented had dens invaginatus. Of 83 teeth, 69 teeth in 60 patients were available for follow‐up after 5 to 15 years (recall rate = 83%). The mean follow‐up time was 8.29 years. No teeth were lost because of a root fracture. Ninety‐six percent (66/69) of the recalled teeth were characterized as healed. Based on periapical radiographs and clinical examination, 96% of teeth treated with the MTA barrier technique and adhesive restorations were characterized as “healed” and were in function after 5 to 15 years (mean = 8.29 years). These results indicate that this is a viable and predictable treatment approach for the long‐term success of nonvital immature teeth.
EFFECT OF SAMPLE PREPARATION ON WATER TREE RESISTANCE OF XLPE/SEBS BLENDS
The effects of an aromatic polymer, such as poly[styrene-b-(ethylene-co-butylene)b-styrene] (SEBS), and its preparation processes on water tree resistance, morphology, crystallinity, dielectric constant and dielectric loss tangent of crosslinked polyethylene (XLPE)/SEBS and XLPE/SEBS/ethylene vinyl acetate (EVA) blends were investigated. The XLPE/SEBS samples were characterized by means of differential scanning calorimetry, scanning electron microscopy, dielectric spectral analysis and the accelerated water treeing experiment. The results clearly showed that although SEBS was partially compatible with LDPE and it was dispersed evenly in XLPE matrix, the blends demonstrated excellent dielectric performances, such as dielectric constant and dissipation factor. The crystallinity of XLPE/SEBS blends was decreased with increase in SEBS content and it was further decreased with the addition of EVA. SEBS could effectively improve the water-tree resistance of XLPE and both SEBS and EVA were found to improve the water tree resistance of XLPE synergistically as expected. The differences in sample preparation processes could change the phase morphology and dielectric performance of the blends, and could also influence the dispersion of SEBS in XLPE matrix as well as the crystallinity of the blends. Different preparation processes led to different water tree resistance behaviours of the samples. The improvement of SEBS dispersion in XLPE matrix helped to improve the water tree resistance of XLPE.