Abstract: Objective: To analyze and compare the fitness of material and teeth through observing the bonding interface between nano hydroxyapatite composite resin and enamel. Methods: 30 healthy first molars were prepared and randomly divided into three groups: group A, group B, and group C. Group A was filled with new nano-hydroxyapatite composite resin. Group B was filled with Karisma composite resin, and group C with glass ionomer cement. After the cold and hot cycle aging test, the teeth and material filling position was cut by low speed Emery slice under flowing water, which was then polished into a thin slice. The bonding interface between the filling material and enamel was observed under scanning electron microscope and the gap was measured. Results: The interfacial gap between nano-hydroxyapatite composite resin and enamel was small, the gap between Karisma composite resin and enamel was larger, and the gap between glass ionomer cement and enamel was the largest. There was significant difference among the three groups (P<0.001). Conclusion: The new nano-hydroxyapatite composite resin has good adhesion to enamel.

Key words: Nano-hydroxyapatite, Bonded interface, Enamel, Composite resin, Adaptation