| The residual root is the one of the most common disease in dental defect. Post-and-core crown restoration is the main method to repair residual roots, because if try to save the residual root and use it to complete the restoration, it will avoid stripping the adjacent teeth after the restoration of fixed bridge. But for some reasons, such as lengthened teeth, residual roots emerged to gingival, teeth with severe caries, broken roots and frontal resorption etc, the residual in alveolar bone are shorter than normal. As for the short roots, the stress upon periodontal tissue will increase after post-and-core crown restorations. And the increased stress may do some damage to the teeth and sustentacular tissues. The research uses the photoelastic experimental stress analysis, and chooses different mandibular central incisor roots in length after post-and-core crown restorations to analyze the different stress and the factors of effecting the stress distribution. We can make out the serviceable range suit for the restorations of single-core crown and combined crown, and provide the reference of the restorations of post-and- core crown in clinic.The way apllied in this study is an experimental mechanics method. First, according to principles of physical and chemical similarity , polymethyl methacrylate, epoxy resin and sulphurated siliastic have been used to manufacture the photoelastic model used in this experiment .The models are setting off nine groups that are the models of the residual roots of the first in C zone with normal length, shortening 1/6,2/6 and 3/6 with single-core crown restoration and normal length, shortening 1/6,2/6 and 3/6 with combined crown and the models as control group of the first in C zone which includes healthy teeth with normal length of root, alveolar bone and pericementum membrane. Second, it is to do the experiment of measuring the material streak value for the same photoelastic material. According streak numbers, N, calculates the f value that is the material streak value. Third, finished the photoelastic models simulate healthy periodontal tissue. The residual roots of the first in the C zone with different length restored with single-core crown and combined crown are loaded on cutting edge from vertical direction and inclined direction with an angle of 45°. The streak numbers of photoelastic models can be measured after loaded. According the f value and the formula (δ=Nf/h) the streak numbers convert pressure. At last, it is to make out the serviceable range suit for the restorations of single-core crown and combined crown.Result of experiment1. The ratio during the artificial tooth, alveolar bone and pericementum of the photoelastic model is 1.35∶1∶0.00030, it is similar to the ratio during the tooth, alveolar bone and pericementum in the oral, which is 1.35:1:0.00025.2. It is to measure the material streak value for the photoelastic material of experiment in the experiment of measuring the material streak value. The material streak value is 12.28N/mm(f =12.28N/mm).3. The residual roots of the first in the C zone with different length, under the vertical load the stress is localized in the zone of the root tip, under the inclined load, the stress is localized in the zone of cervix. The stress under the inclined load is higher than the stress under the vertical load significantly, so the stress under the inclined load is more harmful to periodontal tissue than the stress under the vertical load.4. After the restoration of single-core crown, the peak value of the control group on the buccolingual is 2.22±0.150MPa, the peak value on the mesial and distal is 2.23±0.178MPa, the peak value of the residual roots with normal length on the buccolingual is 2.10±0.122MPa , the peak value on the mesial and distal is 2.15±0.164MPa , the peak value of the residual roots with the length of shortening 1/6 on the buccolingual is 3.75±0.210MPa, the peak value on the mesial and distal is 3.80±0.334MPa, the peak value of the residual roots with the length of shortening 2/6 on the buccolingual is 5.17±0.171MPa, the peak value on the mesial and distal is 5.33±0.151MPa, the peak value of the residual roots with the length of shortening 3/6 on the buccolingual is 6.80±0.284MPa, the peak value on the mesial and distal is 6.51±0.352MPa. According to the experimental result, we could find out that the peak value of the residual roots with normal length is lower than the control group a little(p>0.05), the peak value of the residual roots with the length of shortening 1/6 is higher than that of the control group, but no more than twice higher(p<0.05), the peak value of the residual roots with the length of shortening 2/6 is two times higher than the control group a little(p<0.05), the peak value of the residual roots with the length of shortening 3/6 is two times higher than the control group far. According to the principle of the periodontal potential (the occlusalforce of chewing foods in normal masticatory movement is the half of the strength that the periodontium can bear ) we can make out that the stress of abutment is no more than two times higher than nature tooth's, so the residual roots with normal length and the length of shortening 1/6 should be repaired with single-core crown, the residual roots with the length of shortening 2/6 should be repaired with single-core crown cautiously, the residual roots with the length of shortening 3/6 should not be repaired with single-core crown.5. After the restoration combined crown, the peak value of the control group on the buccolingual is 2.22±0.150MPa, the peak value on the mesial and distal is 2.23±0.178MPa, the peak value of the residual roots with normal length on the buccolingual is 2.03±0.171MPa, the peak value on the mesial and distal is 2.08±0.190MPa , the peak value of the residual roots with the length of shortening 1/6 on the buccolingual is 2.93±0.096MPa, the peak value on the mesial and distal is 3.00±0.212MPa, the peak value of the residual roots with the length of shortening 2/6 on the buccolingual is 3.87±0.135MPa, the peak value on the mesial and distal is 3.88±0.155MPa, the peak value of the residual roots with the length of shortening 3/6 on the buccolingual is 5.12±0.255MPa, the peak value on the mesial and distal is 4.93±0.185MPa. According to the experimental result, we could find out that the peak value of the residual roots with normal length is lower than the control group(p>0.05), the peak values of the residual roots with the length of shortening 1/6 and 2/6 are higher than that of the control group, but no more than twice higher(p<0.05), the peak value of the residual roots with the length of shortening 3/6 is two times higher than the control group a little(p<0.05), According to the principle of the periodontal potential we could find that the residual roots with normal length and the length of shortening 1/6 and 2/6 should be repaired with combined crown, the residual roots with the length of shortening 3/6 should be repaired with combined crown cautiously.
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