A recent meta-analysis reported that the average overall success rate of mini-implants to be approximately 86 % . This analysis included studies for mini-implants placed in different maxillomandibular locations. However, the vast majority of the studies reporting on mini-implant failure rate have predominantly focused on those placed in interradicular sites [3, 4, 11, 17, 18]. The findings of our study show that IZ mini-implants have slightly lower success rate (78.2 %) than that of the average mini-implant. This is in contrast to Liou et al.’s  findings who reported 100 % success of mini-implants placed in this region.
The reason for the different results in success rates with our study may be attributed to the size of the mini-implants. In their study, the length of the mini-implants was 17 mm. Additionally, the success rate on that study was based on a limited time period of 9 months compared to our study where mini-implants were loaded for an average of 13 months. Furthermore, mini-implant mobility, recorded as displacement, was reported in Liou’s study in 44 % of the patients. Thus, failure could have been evidenced at a later time point for these patients. Finally, although the mini-implants in our study were either placed by an experienced operator, or supervised by an experienced operator who had placed more than 50 mini-implants, our experience in placement of the IZ mini-implants developed through the duration of the study. It is possible that the perfect success rate reported in Liou’s study might be related to experienced operators with more than 50 mini-implants placed in this specific region.
One important variable for the different success rates of mini-implants is skeletal facial pattern. Moon et al.  found similar success rates (77 %) to those of our study for mini-implants placed interdentally in patients with high Frankfurt-mandibular plane angle (FMA). This skeletal type was prevalent in the majority of our patients where the average FMA and mandibular plane angle (MPA) was 31.3° and 39.9°, respectively. This finding is also in agreement with a study by Miyawaki et al.  who also reported that mini-implants placed in patients with high MPA had lower success rates (72.7 %). Indeed, it has been found that patients with an increased vertical skeletal pattern have reduced cortical bone thickness, which may affect primary stability of the mini-implants . However, it is unknown if this reduced cortical bone thickness is also present in the infrazygomatic region.
An evaluation of patient-, mini-implant-, orthodontic-, surgical-, and mini-implant maintenance-related factors that could affect the stability of mini-implant was performed. Among all these factors, none were associated with greater odds of failure. Poor oral hygiene showed a trend to be associated to failure rates. Although this is an expected finding, there is controversy of the role of oral hygiene in mini-implant failure. Sharma et al.  reported that poor oral hygiene and inflammation were associated to mini-implant failure. On the other hand, Park et al.  found that oral hygiene played no role, but local inflammation around the mini-implants did.
Perhaps the type of mucosa surrounding the mini-implant may play a more important role in the inflammatory reaction and thus the success of the mini-implant. It has been reported that nonkeratanized gingiva may be a risk factor for mini-implant failure. Viwattanatipa et al.  found low survival rates of mini-implants placed in the infrazygomatic region or vestibular area (46 % after 1 year). In this study, all the mini-implants were placed on nonkeratanized tissue which could be less resistant to the effects of plaque and thus compromise mini-implant stability. Possibly a longer mini-implant that approximates the attached gingiva may reduce the potential for the development of an inflammatory process.
Although there were some mini-implants that became mobile, some of these did not fail. This is consistent with the findings of Liou et al.  who specifically evaluated IZ mini-implants and found this type of screws have some degree of mobility without failure. However, we observed that mobility appeared to be closely related to failure.
One surprising finding was the fact that operator experience was unrelated to mini-implant failure. Since this type of mini-implant placement has more technical difficulty, it was expected that non-experienced operators would have more failures. This nonsignificant finding in the regression analysis may be related to the fact that these mini-implants, although placed by residents, were still supervised by the experienced operator.
One important factor that could contribute to the failure rate is the angle of placement and the direction of loading force in mini-implants placed in the IZ region. In fact, Perillo et al.  found in a recent study using a finite element analysis that the insertion angle of the mini-implant and the direction of force have a significant influence in the stress on the bone. This parameter was not evaluated in the present study, as it would have needed to be examined in a prospective nature. Moreover, recording the direction of the force vector may be difficult as it may vary as treatment progresses based on the biomechanical needs.
The main limitation of this study is its retrospective nature. Although success rates can be reported when a categorical variable is reported as yes or no, the factors associated to these failures are more difficult to extract from chart notes. In fact, the selected patients from the clinic database may have not accounted patients where the IZ mini-implant was placed and removed immediately due to inadequate primary stability, thus underestimating the true failure rate. Although there is a possibility for this, based on the authors’ experience, inadequate primary stability of the mini-implants in this IZ region has rarely been observed. Regardless of these limitations, this study provided data for expected success rates in mini-implants placed in the IZ region, which from a biomechanical perspective, provide significant versatility for orthodontic tooth movements difficult to achieve from anchorage drawn from mini-implants placed in interradicular sites.
The present study was designed to be a pilot explorative study of a multitude of patient- and provider-related factors associated with failure of IZ mini-implants. A multitude of patient- and provider-related factors could influence outcomes (in this case—failure of IZ mini-implants) and the precise role of each variable on the outcome is difficult to elucidate with a small sample size. This is particularly true when there are variations in the distribution of covariates. The current study was designed to be a pilot project and we identified a mix of patient-related factors that are associated with IZ mini-implant failures. We intend to use results from the present study to design a future prospective study to identify factors associated with failure of IZ mini-implants. Our study included 30 patients (55 mini-implants). These patients were selected based on a chart review over a 6-year time period. Our unit of analysis was each individual mini-implant. In effect, our sample size was 55. This number is still inadequate and the present study may be underpowered considering the number of variables we included in the regression models. It is difficult to increase the sample sizes for such single center studies owing to the fact that very few patients elect to have IZ mini-implants and relatively few number of orthodontists place the IZ mini-implants. The solution will be to increase sample sizes by conducting multi-center studies where we can capture an adequate number of patients that are also heterogeneous in terms of covariate distribution. The present study results will aid in designing better controlled multi-center prospective studies. Therein lies the importance of the present study.