| Objective: 1 To investigate the relationships between the human dental pulpal histological changes and different orthodontic force and the different strength. 2 To investigate the pain regularity aroused by the orthodontic force and the relationships between the pain and the different strength.Methods: 1 Thirty-six patients undergoing extraction of four of the first premolar teeth (144 dens) were randomly divided into three groups: 0g group control group(n=24); 100g group orthodontic force of 100g (n=60); 200g group orthodontic force of 200g (n=60). And each group were then randomly divided into three groups: d 3 group , d 7 group and d 14 group.2 Set up the model: four first molar were separated from others for a week then spliced and banded. Four first premolar teeth were orthodontic bracketed as usual, and the enamel of proximal surface was removed about 0.5mm and then polished. 3 100g group orthodontic force of 100g: one terminal of the 0.012 inch Ti-Ni pull-spring was ligated to the retractor of the band of the first molar teeth by a 0.20mm ligation, and the other terminal was ligated to the bracket of the first premolar teeth by a strength of 100g tested by ergometer. 200g group orthodontic force of 200g: give a 200g orthodontic force in the same way as 100g group.group I control group: just orthodontic bracket and band without orthodontic force. After added the orthodontic force, patients were taught to use the VAS(Visual Analog Scales)to record the pain degree everyday. The results were recorded after evaluating doctor measured the datas by sliding caliper.pulpal histological changes: 3d,7d,14d after added the orthodontic force, the teeth was exelcymosised under 0.5% lidocaine local anesthesia and immediately the teeth was split open and the dental pulp was taken out and fixed to make light and electron microscope slices. The histopathological changes of the dental pulp was observed under light and electron microscope and the teeth showed dental pulp degeneration was counted. Data was statisticaly analyzed by SAS 6.12 software.Results:1 The histopathological changes of the dental pulp1.1 100g group orthodontic force of 100g: at d 3 showed part blood capllary dilated and hyperemiaed, endochylema exudated, dentin cell arrangement disord; and at d 7 individual dental pulp partly showed odontoblastic layer vacuolization and pulp reticular atrophy. And at d 14 the histopathological changes of the dental pulp was similar as d 7.1.2 200g group orthodontic force of 200g: at d 3 showed most blood capillary dilated and hyperemiaed, endochylema exudated, and dispersed inflammatory cells infiltration, dentin cell arrangement disord; and at d 7 most dental pulp showed odontoblastic layer vacuolization and pulp reticular atrophy, dental pulp cells decreased, etc. And at d 14 most teeth showed odontoblastic layer vacuolization and pulp reticular atrophy, dental pulp cells decreased, dental pulp fibrous degeneration.1.3 There was no significant difference in the dental pulp degeneration between 100g group and 200g group at d 3 (P>0.05);but there was significant difference at d 7 and 14 (P<0.05). There was no significant difference between 0g group and 100g group at d 3, d 7 and 14(P>0.05). There was no significant difference between 0g group and 200g group at d 3(P>0.05); but the difference was significant at d 7 and 14(P<0.05).1.4 Under electron microscope the fibroblasts of 100g group and 200g group showed endochylema abundant, rough endoplasmic reticulum, Golgi's complex and mitochondria developed, and secretory granule appeared. But group I showed endochylema abundant deficient, rough endoplasmic reticulum, Golgi's complex and mitochondria undeveloped, dental pulp cell was like fibrocyte.2 Pain intensity (VAS scores) after treatment2.1 The time regularity of pain after treatment: pain arose in 100g group and 200g group within 1 d, reached peak at d 2 or d 3, and at d 5-7 began to descent, and dispeared at d 14. 2.2 Pain of 200g group was significantly stronger than 100g group at d 1, d 2, d 3 and d 4(P<0.05).but there was no significant difference at d 5, d 6, d 7, d 14 between these two groups(P>0.05). Conclusion:1 Orthodontic force of 100g on the first premolar teeth will not cause severe histopathological changes in the initial stage.2 Orthodontic force of 200g on the first premolar teeth may cause severe histopathological changes, and this suggests that the orthodontic force on the first premolar teeth clinically had better less than 200g.3 The strength increase can aggravate the orthodontic pain, this suggests that we had better select less strength clinically.
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