Objective:To Study the expressions of Matrix metalloproteinase-9(MMP-9)in human dental pulps in patients under the effects of orthodontic forces.Methods:10 orthodontic patients were selected

the orthodontic teeth to be extracted were decided before orthodontic treatments had started.One side of the orthoodontic tooth underwent routine orthodontic forces.The teeth were extracted within 1 to 7 days after orthodontic forces had been used.There were 20 teeth as under-forces group.The other side of the pre-extracted teeth did not receive any orthodontic forces.There are also 20 teeth as homologized control group.20 third molares clinically diagnosed with endodontitis and without worth of remaining were as inflamed group.Iimmunohistochemical streptavidin peroxidase method(SP method) was used.The expression of MMP-9 was analyzed by analysing images and half-quantitative analysis.Results:There was expression of MMP-9 in control

under-forces and inflamed groups.In under-forces pulpal tissues

MMP-9 was mainly expressed in odontoblasts and fibroblasts.In under-forces pulps

the expression of MMP-9 was manifestly stronger than that of controls

but weaker than that of the inflamed one.The differences among the three groups had statistical significance(P<0.05).Conclusions:MMP-9 is mainly expressed in cytoplasts of odontoblasts and fibroblasts in human dental pulps.In physiological conditions

the expression level of MMP-9 is low.During orthodontic treatments

effected by the orthodontic forces

the dental pulps are in stringent state.The expression of MMP-9 is strengthened which should arouse the attention of the dentists.