Abstract: Objective To investigate the expression and the localization of the matrix metalloproteinase - 1（MMP - 1） and tissue inhibitors of metalloproteinase - 1（TIMP - 1）in root resorption tissue, and to elucidate the molecular changes and mechanism of root resorption induced by orthodontic force in order to establish a foundation for systemic study and prevention of root resorption in clinic. Methods Six experimental Chinese miniature pigs provided by Beijing Agricultural College were selected. Twelve mandibular deciduous lateral incisors of these six pigs were divided into four groups at random, and forces of 0, 0.98, 1.96, 2.94 N were put on each group respectively. The forces were exerted every two weeks. The animals were killed after forty- five days, and the specimens were made which were observed by light microscope. The cells in root resorpion tissue that produced MMP-1 or TIMP- 1 were identified using immunohistochemistry. Results The expression of MMP- 1 in root resorption
tissue in Group 0.98 N, Group 1.96 N and Group 2.94 N increased evidently compared with Group 0 N, and the expression of TIMP- 1 in root resorption tissue in Group 0.98 N and Group 1.96 N increased evidently compared
with Group 0 N. But the expression of TIMP- 1 in root resorption tissue in Group 2.94 N had no difference with Group 0 N. Conclusion MMP- 1 and TIMP- 1 participate in the normal physiological metabolization of extracellular
matrix and the normal remodeling of periodontal tissue during teeth movement on orthodontics treatment MMP-1 and TIMP- 1 play a significant role during root resorption.