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PURPOSE: To detect the changes of MMP-1 and MMP-9 in gingival crevicular fluid (GCF) of the rats in different age groups with orthodontic appliances. METHODS: 40 female Wistar rats were selected as the animal model, in which there were 20 6-week-old rats and 20 12-week-old rats. The orthodontic appliances were fixed onto the maxilla of the rats after general anesthesia. The maxillary first molar in the experimental side was drawn medially by applying an excess force of 50g, with both of the maxillary central incisors as anchorage, in the control side, the maxillary first molar was winded around the dental cervix with ligature wire. The GCF was collected every week. and MMP-1 and MMP-9 were assayed in quantitation by SDS-PAGE. Western blot and Gel Doc quantitative analysis system, respectively. The data were analyzed for ANOVA using SPSS11.0 software package. RESULTS: No statistical difference of the original concentration of MMP-1 and MMP-9 in GCF was found between adolescent group and adult group (P>0.05), but the adult group's contents were a little more than adolescent group's, and is the same as the 1st week's content. The concentration of MMPs in adult group changed more greatly than that in adolescent group. At the beginning of the experiment, the MMPs' increasing speed of adult group was higher than that of adolescent group. But at the end, the rate of descent of adult group was lower than that of adolescent group. CONCLUSIONS: The expression of MMPs in gingival crevicular fluid of adult rats is higher than that of adolescent rats. It may be more serious of the orthodontic root resorption in adult rats than in adolescent rats. [ABSTRACT FROM AUTHOR]