[Effect of different root canal file tapers on fracture strength of root canal wall].

PURPOSE: To compare the effects of different nickel-titanium rotary tapers on fracture strength of root canal and the application value of finite element analysis in root canal therapy. METHODS: Twenty four mandibular premolars were selected and all crowns were removed. All samples were randomly divided into 4 groups: A, B, C and D. Group A and C were instrumented with Z-K3 nickel titanium file as 0.04 taper for root canal preparation. Group B and D were instrumented with Z-K3 nickel titanium file as 0.06 taper. Group C and D were sliced into 2 mm thick slices 3 times after root canal preparation. Two teeth were selected from group A and group B and scanned with micro-CT. The model was imported into different softwares to optimize. The assembly model was built and imported into ANSYS for finite element analysis. Universal testing machine and force bar were used to apply 100 N force to samples until fracture occurring at any place of tooth sample. The fracture load was recorded and comparative analysis was conducted. RESULTS: The results of finite element analysis showed that the maximum stress of 0.04, 0.06 taper models was 138.88 MPa and 78.812 MPa, respectively. The average fracture load of group A, B was 490.12 N and 501.83 N, respectively. In group C and D, the maximum average fracture load was 93.61 N and 141.53 N, respectively. From the neck to the middle and then to the tip, the average fracture load of root canal decreased in turn. CONCLUSIONS: For normal form mandibular premolars, the fracture strength of the root instrumented with 0.06 taper nickel titanium rotary file was significantly higher than that instrumented with 0.04 taper. Three dimensional finite element analysis could guide dentists to choose a suitable taper file in root canal treatment to reduce the incidence of root fracture.

Morphology study of freeze-drying mononuclear cells of human cord blood.

The research on haematopoietic stem cells of human cord blood has become more important recently. At present, cord blood is mainly preserved at ultra-low temperatures. In the former study, we compared the effects of preserving mononuclear cells (MNC) and whole human cord blood by freeze-drying. This time, a further study was conducted on freeze-drying mononuclear cells. Samples in the presence of PVP, sucrose, mannitol and FBS were firstly frozen to -38 degrees C. Afterwards, they were vacuum-dried at a selected shelf temperature of -30 degrees C for the main drying stage, and then vacuum-dried at 15 degrees C for the second drying stage. The entire time of freeze-drying process was 41 hours. Samples were stored at room temperature for 7 days prior to evaluation. Subsequently, the dried samples were resuspended in an isotonic phosphate-buffered saline solution. The residual moisture content was 6.5 +/- 0.87%. The recovery of the cells was tested by a haemacytometer, and the numerical cell count recovery of rehydrated MNC increased by 8%. Morphology of the fresh and rehydrated MNC was analyzed respectively using standard light microscopy, scanning electron microscope and transmission electron microscope. The results showed that karyons changed and cytoplasm decreased after rehydration, but it is still unknown that whether these changes will influence the proliferative ability of the stem cells.

Freeze-drying of mononuclear cells and whole blood of human cord blood.

The research on haematopoietic stem cells of human cord blood has become more important recently. People have concentrated on the preservation of cord blood stem cells. At present, cord blood can be preserved at ultra-low temperatures. In this study, we try to preserve cord blood and its constituents by freeze-drying. The experiments on both the mononuclear cell content and the whole blood of human cord blood were carried out respectively. The samples were frozen firstly by different cooling protocols in the presence of PVP, sucrose, and mannitol. Afterwards, they were vacuum-dried at a selected shelf temperature of -30 degree C for the main drying stage, and then vacuum-dried at 15 degree C for the second drying stage. The entire time of the freeze drying was 52 hours. Samples were stored at room temperature for 2 days prior to evaluation. Subsequently, the dried samples were suspended in an isotonic phosphate-buffered saline solution. The recovery of the cells were tested by a haemacytometer, and the highest cell numerical count recovery of MNC was 75.0 percent (SD = 4.1 percent) (P = 0.01), obtained in the protocol of 40 percent PVP + 20 percent sucrose + 10 percent Mannitol. The viability of the nucleated cells measured by PI staining and the ratio of the number of CD34+ to the number of lymphocytes (by the FITC anti-human CD34+ conjugated antibody method) were measured using a flow cytometer (FCM). The protocol of 40 percent PVP + 20 percent sucrose + 10 percent fetal bovine serum had the highest viability of 98.6 percent (SD = 0.7 percent) (P = 0.01). The highest ratio of CD34+ to lymphocytes was 1.2%, and the highest recovery of CD34+ was 68.4 percent (SD = 39.5 percent) (P = 0.05). Comparing the results of the lyophilized MNC subfraction with that of the whole blood, the lyophilization of the isolated MNC was more successful than that of whole blood.