Abstract: PURPOSE: To evaluate the stability of mini-implant by resonance frequency analysis and to determine the optimal loading timing of orthodontic force. METHODS: Fourteen orthodontic patients who had upper first premolars extraction and needed mini-implants to reinforce the anchorage were enrolled in this study. The patients were divided into 2 groups: delay-loading group and selective-loading group. A joined head mini-implant was inserted in the buccal interradicular area on each side of maxilla for each patient and 28 mini-implants in total were used. Osstell resonance frequency device was used to measure the ISQ (implant stability quotients) value. The measurements were performed immediately after the implant insertion and weekly afterwards until week 16. After the ISQ value was stable, orthodontic force of 150g was loaded on the mini-implants in selective-loading group. The same level of force was applied in delay-loading group at week 12. The data was analyzed by one-way ANOVA and t test using SPSS 19.0 software package. RESULTS: Twenty-three mini-implants were stable during the study. Twelve unloading mini-implants in delay-loading group showed maximal ISQ values immediately after insertion and at week 1 (21.48±5.25). The ISQ values decreased gradually from week 2 to week 5 and reached a plateau after week 6 (11.26±3.36). Eleven mini-implants survived in selective-loading group were then loaded at week 6. There were no significant differences of ISQ values in loading and unloading mini-implants after 6 weeks. The remaining 5 mini-implants were loosing during the first 4 weeks, which showed a significant lower initial ISQ values compared to the successful implants. CONCLUSIONS: The stability of mini-implants decreases 1 week after insertion and maintains after 6 weeks. Orthodontic loading after week 6 has no influence on the stability. Lower initial stability is an important factor for the failure of mini-implants.

Key words: Mini-implant, Stability, Loading time, Resonance frequency analysis