Mechanical Researches Of Mandibular Posterior Teeth Restored With Endocrown

Endocrown is an effective restorative technique for endodontically treated teeth which relies on the pulp chamber as the retentive resource. This restoration maximally preserves the remaining tooth tissue which benefits the resistance of the restoration and tooth tissue. However, the preparation of retention forms needs to remove a certain amount of tooth structure to meet the requirement of fitting and retaining of the restoration, which might result in weakening of the remaining tooth tissue and the contradiction between retention forms and resistance forms. How to reach a good balance is a great challenge to reconstructive dentists. Endodontically treated teeth are more prone to fracture comparing with vital teeth. Yet there has been little information about the difference of fracture resistance between endocrown and conventional post-core supported crown currently, especially of mandibular posterior teeth. Therefore, the aims of this present study were to evaluate the fracture resistance of endodontically treated mandibular posterior teeth restored by endocrown and conventional post-core supported crown, and to explore the optimal designs for abutment preparing using mechanical methods, thus to provide reference for clinical endocrown restoration of mandibular posterior teeth.In our study, the difference of fracture strength between the endocrown and the conventional crown of mandibular posterior teeth was explored using fracture resistance test. Meanwhile, the three-dimensional finite element method was used to analyze stress values and stress distribution patterns on residual tooth tissues with different marginal designs, different thickness of pulp cavity walls and floors.PART I:A comparison of the fracture resistance of endocrowns and glass fiber post supported conventional crowns for endodontically treated mandibular posterior teethObjective:This study aimed to evaluate the fracture resistance and failure modes of endodontically treated mandibular premolars and molars restored by endocrowns and conventional post-core supported crowns. Methods:Thirty mandibular premolars and thirty mandibular molars were assigned into three groups (n=10):teeth with endocrowns; teeth with conventional post-core supported crowns; intact teeth. Except for intact teeth, all specimens were endodontically treated and accepted abutment preparation, then both endocrowns and conventional crowns were fabricated from lithium-disilicate blocks using a CEREC 3D CAD/CAM unit. All specimens were subjected to thermocycling and then received a compressive load until fracture occurred. The fracture resistances and fracture patterns were recorded and data was analyzed. Results:The mean fracture resistance ± standard deviation of endocrown group, post-core supported group and intact teeth group for mandibular premolar was recorded as follows:479.05+180.56N,510.10 ± 190.96N,997.10 ± 166.25N.There was a significant difference between restorative groups and intact teeth group(P<0.010), while no significant difference was found between the two restorative groups(P=0.702). As to molars, the values of endocrown group, post-core supported group and intact teeth group were 953.30±256.41N,725.15±205.51N,1154.05±262.27N respectively, the post-core supported group had a significant difference both with endocrown(P=0.045) and with intact teeth(P=0.001), but no significant difference was found between endocrown group and intact teeth group(P=0.075). Regarding the failure modes, whether the mandibular premolar or the mandibular molar, most specimens of restorative groups exhibited unfavorable fractures, and the intact teeth were more prone to occur favorable fractures patterns. Conclusion:The endocrown provides a feasible alternative for endodontically treated mandibular molars. However, for mandibular premolars, it shows no advantage in fracture resistance compared with the conventional method.PART II:Influence of different designs of marginal preparation on stress distribution in the mandibular posterior teeth restored with endocrownObjective:To evaluate the effect of different designs of marginal preparation on stress distribution in the mandibular premolar and molar restored with endocrown using three- dimensional finite element method. Methods:Four models with different designs of marginal preparation, including the flat margin.90° shoulder.135° shoulder and chamfer shoulder, were established to imitate mandibular premolar and molar restored with endocrown. A load of 100N was applied to the intersection of the long axis and the occlusal surface, either parallel or with an angle of 45° to the long axis of the tooth. The maximum values of Von Mises stress and the stress distribution around the cervical region of the abutment and the endocrown with different designs of marginal preparation were analyzed. Results: ? The load along to the long axis of the tooth caused obvious stress concentration on the lingual cervical region of both the tooth tissue and the restoration of premolar, while on the mesial and distal portions of the molar. The stress distribution characteristics on the cervical region of the models with a flat margin and a 90° shoulder were more uniform than those in the models with a 135° shoulder and chamfer shoulder.? Loading at 45 ° to the long axis of the tooth caused stress concentration mainly on the buccal portion of the premolar cervical region and the buccal and mesial region of the molar, and the model with a flat margin showed the most favorable stress distribution patterns. Under this circumstance the stress values was greater than that of corresponding models with a parallel loading. ? Irrespective of the loading direction, the stress value was the lowest in the flat margin model. The stress value in the cervical region of the endocrown was greater than that in the counterpart of the tooth tissue.? The stress level on the enamel was higher than that on the dentin nearby in the flat margin model. Conclusion:Endocrowns with flat margin followed by a 90° shoulder are recommended for restoration of mandibular posterior teeth.PART III:Influence of thickness of pulp cavity walls on stress distribution in mandibular posterior teeth restored with endocrownObjective:To evaluate the effect of different thickness of pulp cavity walls on stress distribution in the mandibular premolar and molar restored with endocrown using three-dimensional finite element method. Methods:Four models with different thickness of pulp cavity walls (1.0mm?1.5mm?2.0mm?2.5mm) of mandibular premolar and molar restored with endocrown were established. A load of 100N was applied to the intersection of the long axis and the occlusal surface, either parallel or with an angle of 45° to the long axis of the tooth. The maximum values of Von Mises stress and the stress distribution of the pulp cavity walls were analyzed. Results:? The load parallel to the long axis of the tooth caused obvious stress concentration on the lingual portions of pulp cavity walls in premolar. while in molar, stress concentration was observed on the mesial and distal portions of pulp chamber walls. ? Loading at 45° to the long axis, stress concentration was mainly on the buccal portion of pulp cavity walls of premolar and the buccal and mesial region of molar. ? Whether the load direction was parallel to the long axis of the tooth or at 45 ° to the long axis, the stress at pulp cavity walls reduced with the increase of thickness. Conclusion:Preserving pulp cavity walls tissue leads to favorable stress patterns of endocrown restoration and the stress values of mandibular premolar was significantly higher than that of mandibular molar with the restoration of endocrown.PART IV:Influence of pulp cavity floor thickness on stress distribution in the mandibular molar restored with endocrownObjective:To evaluate the effect of thickness of pulp cavity floor on stress distribution in the mandibular molar restored with endocrown using three-dimensional finite element method. Methods:Three models with different thickness of pulp cavity floor (0.5mm?1.0mm?1.5mm) of mandibular molar restored with endocrown were established. A load of 100N was applied to the intersection of the long axis and the occlusal surface, either parallel or with an angle of 45° to the long axis of the tooth. The maximum values of Von Mises stress and the stress distribution of the pulp cavity floor were analyzed. Results: ? Parallelling load lead to stress concentration on the mesial and distal line angle of the molar pulp cavity floor.? Loading at 45 ° to the long axis of the tooth,the buccal and lingual portion of the pulp cavity floor of molar presented obvious stress concentrations.? No matter what the loading direction was, the stress of the floor of pulp cavity showed no significant changes with the increase of its thickness.Conclusion:The thickness of the pulp cavity floor doesn't affect the stress distribution of mandibular molar restored with endocrown.