Abstract: PURPOSE: To evaluate the effect of one-couple orthodontic NiTi auxiliary arch on the premolar extrusion which had subgingival decayed defect. METHODS: A total of 30 cases with subgingival decayed defects over 2 mm under the proximal gingival margin were chosen including 18 first premolars and 12 second premolars. The first and second molars on the same side were connected steadily and the one-couple orthodontic force system was designed. The NiTi segmental arch was used to extrude the teeth. Student's t test was performed with SPSS 8.0 software package. RESULTS: All the defect margins were 1mm over the gingival margins and the extrusion distance was 2.6~4.1 mm. After extrusion, the crown-root ratio was 1.24±0.30, the depth of the gingival sulcus was 1.87±0.28 mm. There was no significant difference before and after treatment in the angle between the axis of the extruded teeth and that of the first molar (P>0.05). The anchorage teeth kept almost the same relationship with the opposite. CONCLUSION: One-couple orthodontic NiTi auxiliary arch could extrude the premolars with subgingival decayed defects effectively and precisely. The designed force system had little effect on the anchorage teeth.

Key words: Subgingival decayed defect, One-couple, Auxiliary arch, Extrusion