Abstract: Objective: To study the apical surface morphology after root canal preparation with different nickel-titanium instruments horizontal to the anatomic apical. Methods: 40 freshly extracted tooth due to periodontal disease which were randomized to 2 groups were taken images by SEM to record the apical surface morphology. Then they were prepared with K3 or TF respectively under Work Length (horizontal to the anatomical apical foramen). After root preparation, the apical surface morphology of each tooth was observed by SEM, then the apical foramen shift, dentin defects and crackle surrounding the apical foramen was recorded. Results: The apical foramen diversifications (include apical foramen shift, dentin defects and crackle) were observed in both K3 group and TF group. The difference of apical foramen shift and crackle number between two groups didn’t have statistically significant while the numbers of tooth which had dentin defects between two groups have statistically significant(P<0.05). Conclusion: The integrity of the apical foramen would be destroyed after root canal preparation with two kinds of nickel-titanium instruments horizontal to the anatomic apical maybe due to super the apical foramen. The surface morphology of apical foramen would have a lot of diversifications include apical foramen shift, dentin defects and crackle cracks and dentin defects surrounding the apical. Compare with k3 file, TF file group have less apical foramen shift and dentin defects.

Key words: Nickel-titanium instruments , Root canal preparation , Apical morphology, Working length of root canal