This article was to study the micro-leakage of 3 different root canal sealers (Endomethasone, AH-Plus and GuttaFlow) by fluid filtration test, and to observe the micro-structure between walls of root canal and the sealers by SEM. The results indicated that the micro-leakage of GuttaFlow was the least and Endomethasone was the most. Statistics difference were found between all the groups. Different root canal sealer got different micro-structure between walls of root canal and sealers. So we consider that the canal sealing ability of GuttaFlow is the best among these sealers.

Objective:To investigate the occurrence rate and distribution pattern of three-rooted mandibular molars in children using CBCT.Methods:CBCT images of 206 children aged 4-11 years were retrospectively analyzed.The root numbers of the bilateral man-dibular first and second deciduous molars,and the permanent mandibular first molars were recorded.A chi-square test was used to de-tect the gender and side difference.Spearman correlation test was used to analyze the relation between the bilateral homologous teeth and the mandibular first and second deciduous molars on each side.Results:The occurrence rate of three-rooted mandibular first and second deciduous molars and mandibular permanent first molars was 10.9％(19/175),25.1％(46/183)and 28.4％(57/201)for individuals,respectively(x2=18.543,P＜0.01),and 8.7％(32/366),19.0％(73/384)and 23.6％(95/402)for teeth(x2=30.692,P＜0.01),re-spectively.Gender difference was not detected for each tooth type(all P＞0.05),while side differences were detected in mandibular sec-ond deciduous molars and the frequency of three-rooted molar on the right and left side was 23.7％and 14.2％(P＜0.05),respectively.The concurrence rate of bilateral three-rooted mandibular deciduous first and second molars and mandibular permanent first molars was 57.9％(11/19),47.8％(22/46)and 66.7％(38/57),respectively,and the rho was 0.710,0.597 and 0.745,respectively(all P＜0.01);between the three-rooted deciduous first and second molars on the left side,rho=0.188(P＜0.05)and on the right side,rho=0.304(P＜0.01).Conclusion:The occurrence rate of three-rooted mandibular molars in children increases in the following sequence:first deciduous molars＜second deciduous molars＜first permanent molars.They frequently occur bilaterally and exhibit a moderate to high degree of correlation.However,the correlation was very weak between the mandibular deciduous first and second molars.

Objective:To explore the method and curative effect evaluation of repairing and rebuilding the long segmental bone defect after resection of long bone malignant tumors in adolescents.Methods:Reviewed our experience in 10 patients. Among them, 4 were male and 6 were female. The age was 10.2 years old (range, 6-16 years old). Including 7 cases of osteosarcoma, 3 cases of Ewing's sarcoma; 2 cases of humerus tumors, 7 cases of femoral tumors, and 1 case of tibial tumors-who presented between April 2015 and October 2019, in which the proportion of bone defect account for 71.4%-89.3% of the total length of diseased bone after tumor resection, with an average value of 80.9%. Considering the epiphyseal growth of young people, on the premise of ensuring enlarged tumor resection, we use traditional fibula autograft and prosthesis combined with steel plate, 3D printing design to retain the super-prosthesis of the epiphysis, and 3D printing bone trabecular was processed for the sake of making it grow successfully into the binding part of prosthesis and bone as we expect, simultaneously, the design of the short-force arm retaining the bilateral epiphysis can assit the steel plate and interlocking screws to enhance the stability.Results:The operation was completed successfully, and there were no complications such as nerve and blood vessel injury during the operation. After follow-up of 4-8 months, with an average follow-up of 22.9 months. One patient's review of X-ray film after 2 years of the operation showed that the transplanted fibula had a sign of bone resorption, and the internal fixation was in it's original position without obvious abnormalities. Two cases' auxiliary steel plates had fractured within one and a half to two years after surgery. With replacing the fractured steel plates, the internal fixation was good until now. According to the evaluation standards of the International Society of Limb Salvation: 2 cases of humerus osteosarcoma were respectively scored 23 points, 26 points, an average of 24.5 points; 4 cases of femoral osteosarcoma and 3 cases of femoral Ewing's sarcoma respectively scored 26 points and 25 points, 19 points, 30 points, 27 points, 24 points, 28 points (average, 25.6 points); 1 case of tibial osteosarcoma scored 26 points; all obtained relative satisfying treatment results.Conclusion:The combination of 3D printing and traditional machining techniques to produce an individualized long segmental prosthesis that preserves the epiphysis which applicated in the limb salvage treatment of bone malignant tumors can achieve excellent therapeutic effect.