Abstract: Abstract:Objective To observe the  changes of expression level  of matrix metalloproteinase-8(MMP-8) in periapical tissues of SD rats and to investigate the relationship between MMP-8 and the development of periapical periodontitis  reinfected by Enterococcus faecalis. Methods The maxillary first molar teeth of 42 SD rats  received lipopolysaccharide(LPS) and  were exposed to oral environment to establish primary infected periapical periodontitis  model. The rats were randomly divided into chronic periapical periodontitis group (no extra treatment)，calcium hydroxide disinfection group (disinfection with calcium hydroxide paste after the root canal preparation),formaldehyde cresol (FC) disinfection group (disinfection with FC cotton ball after the root canal preparation),and periapical periodontitis  reinfected by Enterococcus faecalis group (reinfection group,implantion with  Enterococcus faecalis 2 weeks after FC disinfection),respectively. Six rats were respectively sacrificed at the 6th week in chronic periapical periodontitis group and calcium hydroxide disinfection group,the 2nd week  of FC disinfection group and the 1st,2nd,3rd,and 4th week in reinfection  group. The suppermaxilla were obtained and subjected to immunohistochemical staining in order to test the changes of integral absorbance (IA) values  of MMP-8 in periapical tissue in various groups. Results The expression levels of MMP-8 in FC disinfection group(4.49±1.08)and calcium hydroxide disinfection group(14.84±2.60)were both lower than that in chronic periapical periodontitis group(27.76±2.60)(P<0.01),and the expression level of MMP-8 in  FC disinfection group was lower than that in calcium hydroxide disinfection group(P<0.01). The expression level of MMP-8 at the 1st week in reinfection group(42.08±4.19)was higher than that in chronic periapical periodontitis group,and the MMP-8 expression levels were gradually increased at  the  1st week,2nd week (61.32±5.81),3rd week(79.45±7.40),and 4th week (114.67±10.40) in a time-dependent manner,which were higher than those in the other groups,respectively(P<0.05).Conclusion The expression level of MMP-8 is correlated with the progression of tissue destruction of SD rats with periapical  periodontitis reinfected by Enterococcus faecalis,and the higher the expression of MMP-8 is,the worse of the course expansion is.

Key words: matrix metalloproteinase-8, Enterococcus faecalis, periapical periodontitis, immunohistochemistry, rat,SD