Abstract: Abstract: Objective To compare the changes of enamel micro-cracks before and after debonding with different bracket adhesives. Methods Ninety fresh premolar teeth were collected for orthodontic treatment, which were divided into two groups (45 in each group). The first group:obvious microcracks group (visible to the naked eyes under natural light), and the second group: weak microcracks group(invisible in natural light but visible under scanning electron microscope); each group was further divided into three groups(each group of 15): 3M light-cured adhesive group, 3M chemically-cured adhesive group, control group, each group of 15 . Before and after debonding, microcracks length and width changes were measured by the use of scanning electron microscopy. Results: In the obvious microcrack group, the average microcrack width before and afterthe debonding 3M chemical curing adhesive group and 3M light-cured adhesive group had statistically significant difference (P <0.05); while the average length of the microcrack before and afterde-bonding had no significant difference between the two groups (P> 0.05). The differences in width and length before and after treatment were statistically significant (P <0.05). In the weak microcracks group: the average width and length of microcracks were not statistically significant when comparing with 3M light-cured adhesive group and 3M chemically-cured adhesive group (P>0.05) The differences between the width and the length of the chemically-cured adhesive group before and after treatment with 3M light-cured adhesive group were statistically significant (P <0.05). Conclusion The width and length of the enamel microcracks group before and after removing brackets are both increased. Although 3M light-cured adhesive and 3M chemically-cured can both satisfy the requirements of adhesive strength of the enamel, 3M chemical curing group has more damage to the tooth enamel after removing the brackets. Therefore, it is recommended to use 3M light-cured adhesive to meet the clinical requirements which can minimize its damage to the enamel surface.

Key words: brackets adhesives , enamel , microcracks , scanning electron microscopy