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Analysis of time to failure of orthodontic mini-implants after insertion or loading

Objectives This study was performed to evaluate patterns of failure time after insertion, failure rate according to loading time after insertion, and the patterns of failure after loading. Materials and Methods A total of 331 mini-implants were classified into the non-failure group (NFG) and failure group (FG), which was divided into failed group before loading (FGB) and failed group after loading (FGA). Orthodontic force was applied to both the NFG and FGA. Failed mini-implants after insertion, ratio of FGA to NFG according to loading time after insertion, and failed mini-implants according to failed time after loading were analyzed. Results Percentages of failed mini-implants after insertion were 15.79%, 36.84%, 12.28%, and 10.53% at 4, 8, 12, and 16 weeks, respectively. Mini-implant failure demonstrated a peak from 4 to 5 weeks after insertion. The failure rates according to loading time after insertion were 13.56%, 8.97%, 11.32%, and 5.00% at 4, 8, 12, and 16 weeks, respectively. Percentages of failed mini-implants after loading were 13.79%, 24.14%, 20.69%, and 6.9% at 4, 8, 12, and 16 weeks, respectively. Conclusion Mini-implant stability is typically acquired 12 to 16 weeks after insertion, and immediate loading can cause failure of the mini-implant. Failure after loading was observed during the first 12 weeks.

For evaluation of failure rate according to loading time after insertion, the failure rate, calculated as the ratio of FGA to NFG, was analyzed according to loading time (weeks) after insertion. To analyze the patterns of failure after loading, the failed mini-implants were analyzed according to failure time (weeks) after loading. The statistic analysis was done using SPSS 12.0 for Windows (SPSS Inc., Chicago, IL, USA).
This study was approved by the Ethics Committee for Research (Seoul National University Bundang Hospital Ethical Board, B-1011-115-110).

III. Results
In this study, 57 of 331 inserted mini-implants experienced failure. Of these, 29 were in the FGA, and 28 were in the FGB. The mean age of the subjects was 22 Fig. 2) In the failed mini-implant group, 75.44% of failures occurred within 16 weeks after insertion. The failure of mini-implants had a peak from 4 to 5 weeks. (Fig. 2) The mini-implants loaded during the first 4 weeks after insertion showed the highest failure rate. (Table 3, Fig. 3) The failure rates in relation to loading time after insertion were 13.56%, 8.97%, 11.32%, and 5.00% at 4, 8, 12, and 16 weeks, respectively. Mini-implants loaded during the first week after insertion showed a 42.50% failure rate. (Fig. 3) Percentages of failed mini-implants after loading in this group were 13.79%, 24.14%, 20.69%, and 6.90% at 4, 8, 12, and 16 weeks, respectively. (Table 4, Fig. 4) Mini-implant failures after loading in this group occurred mostly during the first 12 weeks after loading. From 4 to 12 weeks after loading, the failure rate of mini-implants in this group was 58.62%. Failures were There was no significant difference in marginal bone loss between immediate loading and delayed loading in such implants 17 .
However, the bone remodeling process involving bone resorption and apposition after insertion of a mini-implant needs time to heal in order to provide stable support of the mini-implant 18 .
Because of the high failure rates of orthodontic miniimplants, loading time is believed to be important. Analysis regarding the failure time of orthodontic mini-implant after insertion and loading could help guide clinical practice when using orthodontic mini-implants. This study was conducted to evaluate the pattern of failure time after insertion, failure rate according to loading time after insertion, and the pattern of failure after loading.

II. Materials and Methods
Subjects included 134 patients (mean age, 20.08±7.52 years) with 331 inserted mini-implants (Miangan; Biomaterials Korea, Seoul, Korea) with a diameter of 1.2 mm and length of 7.0 mm. This study was done from July 2006 to June 2010 in dental clinic of Seoul National University Bundang Hospital (Seongnam, Korea). The surfaces of the miniimplants were machined but not treated. All mini-implants were installed on the buccal alveolar bone between the first premolar and the second molar of the maxilla and mandible.
Mini-implants were inserted by 1 operator using a selfdrilling method after orthodontic leveling and alignment.
The mini-implants were classified into the failure group (FG) and non-failure group (NFG). The FG was divided into the failed group before loading (FGB) and failed group after loading (FGA). Orthodontic force was applied to both the NFG and FGA.
The failure rate was calculated for FG and NFG. To evaluate the pattern of failure time after insertion of miniimplant, the failed mini-implants were analyzed according to the failure time (weeks) after insertion in the FG.

IV. Discussion
The results of this study showed some initial failures after insertion. About 75% of failed mini-implants occurred during the 16 weeks after insertion. (Table 2) Specifically, when the observed intermittently until 44 weeks after loading. (Fig. 4) Tables 2-4 illustrate the data in 4-week intervals. Fig. 2-4 present the data in 1-week intervals to provide greater detail. Values are presented as number (%).

Jong-Wha Jeong et al: Analysis of time to failure of orthodontic mini-implants after insertion or loading. J Korean Assoc Oral Maxillofac Surg 2015
No. of failed mini-implants    layed loading 24 . However, the failure rate in relation to loading time after insertion in the present study was highest during the first week. (Fig. 3) This might indicate that immediate loading after insertion activates bone resorption and results in mini-implant failure 25 . Because mini-implants have a smaller diameter, shorter length, and less surface area contacting the bone compared to conventional implants, mechanical stability in the early stages could be affected and reduced by limited bone resorption around the mini-implant 26 .
In this study, the failure of loaded mini-implants happened in case of the load was applied during the first 12 weeks after insertion except 37 weeks. This suggests that the load until 12 weeks after insertion affects the stability of a mini-implant, and loading is thus recommended at approximately 12 weeks after insertion.
Failure after loading occurred frequently until 13 weeks (Fig. 4), with a high failure rate from 6 to 13 weeks. It is recommended that the clinician should be careful until 3 months after loading to sustain the mini-implant because of failure. Additionally, failure after loading was observed intermittently until 35 weeks. This suggests that the stability of the miniimplant is affected by bone remodeling after tooth movement through the end of treatment. This might be the result of insufficient surface area of the mini-implant in contact with bone in order to obtain sufficient surrounding support 27 .
Failure of the mini-implant results in difficulties and affects the treatment plan. Although there are various causes of miniimplant failure, selection of loading time after insertion is an important factor for avoiding failure. Immediate loading is not recommended, and delayed loading of the mini-implant is suggested until 12 to 16 weeks after insertion.
For more stable use of mini-implants in clinics, further studies regarding surface treatment of mini-implants are loading time after insertion was less than 12 weeks, the failure rates of mini-implant were high. These results signify that the stability of the mini-implant is acquired 12 to 16 weeks after insertion. They indicate that the bone remodeling process after insertion occurs during the first 3 to 4 months after insertion. For implant success, loading time is important. Time for bone healing of tissue damaged through the process of implantation is needed. Therefore, delayed loading has been suggested by other studies focusing on implant stability 19,20 .
Before loading, the failure rate during the first 4 to 5 weeks was the highest. (Fig. 2) It is possible that bone resorption in the bone remodeling process is most active around 4 weeks after insertion in humans, and that bone apposition is achieved 3 to 4 months after insertion.
Some studies have reported that immediate or early loading of mini-implants shows good stability [21][22][23] . A previous animal study has suggested no significant differences in success rate or histological findings between immediate loading and de-