## 4 Discussion  
To the best of our knowledge, this is the first review with meta-analysis that investigates the current literature with the best evidence (RCTs) on the efficacy of orthodontic retainers to maintain the occlusal results after fixed orthodontic treatment. The results of this meta-analysis will provide clinicians a quantitative evaluation of the efficacy of fixed and removable retainers and consequently a quantitative evaluation of orthodontic relapse during the retention period. The pooled data show that the intercaline width does not change significantly during retention with all evaluated retention appliances [37; 38; 39; 40; 41; 42; 43; 44]. The data show that the fixed retainers are also able to avoid significant modifications of LII (\(p=0.07\)). However, this datum should be interpreted with caution because of the fewer number of considered trials. Conversely, the use of removable retainers showed a significant increase of LII. This datum was obtained by pooling the data from five trials for the maxillary arch (Figure 2f) and from seven trials for the mandibular arch (Figure 2e); it can be considered a consistent meta-analytic datum. The amount of variation for LII is 0.48 and 0.72 mm, respectively, for the maxillary and mandibular arch. This result clearly shows that, despite the use of a removable retainer appliance, the teeth of both arches present a small statistically significant (but clinically insignificant) alignment alteration. These results, especially for intercanine width, must be interpreted with caution due to the young age of the sample participants, as this type of alteration can influence the outcome due to residual growth and patient compliance. Interestingly, our data show a significant alteration of LII in the absence of modification of the intercaline width. These results could be explained by interpreting the alteration of LII as a consequence of a single tooth rotational relapse, rather than a transversal inter-canine diameter relapse. The reported modifications of LII could be related to specific factors associated with the use of removable retainers, such as patient compliance and the limitations of using retainers notcurrently. This last hypothesis, however, does not seem to be supported by the evidence. Indeed, one of the considered RCTs compared \begin{table}
\begin{tabular}{c c c c c} \hline \hline  
* Reports of the levels of study quality, consistency of measured outcomes, and strength of recommendations according to the Strength of Recommendation Taxonomy (SORT) system.  
\end{table} 
Table 7: Strength of recommendations for each outcome investigated in the present study. removable retainers used full time and removable retainers used part-time [45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55]. The authors did not find significant differences in terms of relapse after 12 months of retention. Compliance is certainly an issue related to removable retainers. However, the results of this review clearly show that fixed retainers are not free of issues. In fact, the failure rate of fixed retainers (caused by wire breakage or bonding failure) is, on average, 41% and 36% for maxillary and mandibular retainers, respectively (Table 4). These data are in accordance with previously published data [56; 57; 58; 59; 60; 14]. One limitation of this meta-analysis is the small number of included trials; this aspect affected the I2 index, undervaluing the extent of between-study heterogeneity [62; 63]. The I2 values reported a high total variation across the studies, with a mean value and a standard deviation equal to 97.8% and 2.2%, respectively. This variation was assumed to be due to the clinical heterogeneity of the different appliances used in the considered RCTs. Moreover, the source of heterogeneity could be related to other factors such as patient compliance, final occlusal results, orthodontist expertise, type of malocclusion, and type of treatment (with or without extractions) [13; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77].