Investigation Of Effects Of Pulp Chamber Preparation And Retention Form Of Restoration On Stress Distribution In Tooth After Root Canal Therapy By Three-dimensional Finite Element Analysis

The first permanent mandibular molar erupts early in the oral cavity and takes main effect of mastication, and also tends to suffer from dental caries and other disease caused by stress fatigue, maintenance of which is important to mastication. At the present time, the issue is still open as to the cavity form of pulp chamber preparation during the process of root canal therapy. In this study, we analyzed the effects of pulp chamber preparation and retention form of restoration on stress distribution in tooth after root canal therapy using three-dimensional finite element method and the results would provide useful data for clinical treatment.Objectives and Methods:1.To construct a 3-D finite-element model of the mandibular first molar. Spiral CT scanning was used to obtain the morphological pictures of mandibular first molar. PHOTOSHOP 7.0 and Matlab software ware used to remove the Dead Pixes and improve the definition and contrast of the pictures which was saved as BMP. Mimics software was used to form the primary 3D finite element models. Geomagic Studio 7,Parten,UG software were used to form the contour of different types of tissues. And then import the 3D finite element models in MSC.MARC software to establish models of the tooth after pulp chamber preparation with cavity diameter of 2mm and 4mm in occlusal surface respectively.2. To analyze three kinds of stress of two types of cavity with central Concentrated Loads, and get the trend of change by obtaining the all-sided stress nephogram and Stress in the Cross Section of the tooth crown in different layers. Changes in stress distribution and concentration area in tooth, restorations (composite/ amalgam) and Zinc Phosphate Cement were observed by comparing the stress nephograms and stress curves in different groups.3. To analyze the restoration of large destruction with different retention forms with central Concentrated Loads and get the trend of change by obtaining the all-sided stress nephogram and stress in the Cross Section of the tooth crown and distal root, mesiobuccal root, mesiolingual root in different layers. Changes in stress distribution and concentration area in tooth and restorations (composite/ amalgam) were observed by comparing the stress nephograms and stress curves in different groups.Results:1. Established a direct, complete 3-D finite element model of the first permanent mandibular molar. The model has favorable comparability and was fit for biomechanics analysis.2. Experiment on pulp chamber preparation①High stress concentration was observed in dental cervical area, especially in distal side in both groups.②Stress level of the dental crown was higher in the tooth with cavity diameter of 2mm than in 4mm.③No difference was found in the bottom of the restoration between 2 groups if filled with composite, while higher stress level was showed in the tooth with cavity diameter of 2mm than in 4mm if filled with amalgam.④Only slight difference existed in stress level of the 2 groups.⑤Filling material had influence on stress distribution in the bottom of pulp chamber, stress level in amalgam filled tooth was higher than in composite filled tooth.3. Experiment on retention form①Stresses in dental crown induced by 4 retention forms were different, amalgam post group was the highest, followed by fiber post and direct composite filling, stainless steel post was the least.②stresses in dental roots induced by 4 retention forms were different, higher stress concentration occurred mainly in distal root, stainless steel post group was the highest among the 4 groups, then amalgam post, fiber post and direct composite filling groups didn't show stress concentration in root areas.③whenever composite was used to restore decay, stress along interface between dental substances and filling material would decreased significantly. Conclusions:1. This study established six three-dimensional finite element models of normal mandibular first molar, large defected mandibular first molar with different occlusal cavities and restoration with different retention form using thin CT and soft wares such as MSC and MARC. The model established by this method was of high precision, similarity and comparability.2. Pulp chamber preparation after removing the occlusal tooth tissue and the roof of pulp chamber and exposing the root canal orifice is good for root canal preparation and filling, and stress induced in tooth after the treatment is lower compared with the tooth prepared with pulp chamber not fully exposed. In clinic we used to think that removing the tooth tissue as less as possible is the key point of repairing the tooth with enough strength. But this study showed that not removing the tooth tissue that should be removed in the treatment would have harmful effect on tooth strength.3. The retention form of large defected pulpless tooth should depend on the specific condition of the tooth. Bonding was the best repair way. Fiber post was the best choice for increasing the retention force. Metal post might result in stress concentration in the root and root fracture, so it should be carefully used. Silver amalgam post was not advised to use. Silver amalgam post not only effected root strength, but also cause leakage around the restoration and secondary caries because of the high stress concentration in the interface of the restoration and tooth tissue.