the effectiveness of RME (Rapid Maxillary Expander) on the sagittal dental or skeletal parameters is still controversial because very little has been written regarding the behavior of antero-posterior mandibular changes in Class II growing subjects who underwent RME as the phase 1 treatment intervention. The reported significant occlusal improvement could be attributed to other reasons, ie, skeletal growth or the use of additional appliances during transition from mixed to permanent dentition. Moreover, the majority of the studies show some limits: they are not randomized, they are not prospective, and they have no control group or they use patients from growth studies as a source for the control group. The primary objective of the present investigation was to conduct a pilot randomized controlled trial (RCT) evaluating the changes in the antero-posterior mandibular position induced by bonded or banded RMEs compared with an untreated Class II control group

Class II malocclusions are commonly observed in orthodontic patients. During treatment planning among the several dento-skeletal pattern combinations of Class II malocclusion, it is important to consider the maxillary transverse deficiency, which is often overlooked.

it has been showed showed an underlying posterior interarch transverse discrepancy of 3 to 5 mm in subjects in early mixed dentition with Class II malocclusions without posterior crossbites in centric occlusion. When these Class II patients are asked to posture their lower jaw forward in a Class I molar relationship, this transverse discrepancy (ie, maxillary constriction) can be observed clinically. It was postulated that in these subjects the mandible is kept in a distal position relative to centric relation because the constricted maxilla is holding it back. The presence of a primitive transverse discrepancy between the dental arches induces a backward position of the mandible, as the occlusal goal is to obtain the highest number of functional contacts.

As reported by several authors, widening the maxilla with rapid maxillary expansion often leads to spontaneous forward posturing of the mandible during the retention period. The orthopedic expansion removes occlusal interferences, allowing the mandible to posture forward, thus improving the sagittal relationships. The mandibular arch acts as a ''foot'' that moves forward after the ''shoe'' is widened.

However, the effectiveness of RME on the sagittal dental or skeletal parameters is still controversial because very little has been written regarding the behavior of antero-posterior mandibular changes in Class II growing subjects who underwent RME as the phase 1 treatment intervention. The reported significant occlusal improvement could be attributed to other reasons, ie, skeletal growth or the use of additional appliances during transition from mixed to permanent dentition. Moreover, the majority of the studies show some limits: they are not randomized, they are not prospective, and they have no control group or they use patients from growth studies as a source for the control group.

Considering that it was not possible to estimate from previous studies the standard deviation to be used for sample size calculation of the main trial with special regards to type of intervention and observation intervals, the primary objective of the present investigation was to conduct a pilot randomized controlled trial (RCT) evaluating the changes in the antero-posterior mandibular position induced by bonded or banded RMEs compared with an untreated Class II control group.

To evaluate the sagittal mandibular response induced by RME therapy in mixed dentition patients with Class II malocclusion, comparing the effects of bonded RME and banded RME with a matched untreated Class II control group

This is a single center, prospective 3-arm parallel group randomized clinical trial with a 1:1:1allocation ratio. 30 (thirty) of subjects are planned. Each subject in the treated groups will be treated with a Rapid Maxillary Expander (RME), bonded or banded respectively. Subjects will be assigned to the groups in random order. Evaluations will be taken at baseline and at the end of the retention period/follow up (for a total of 12 months). Subjects who meet all inclusion criteria and none of the exclusion criteria will be entered into the study.

Allocation of patients to the three groups in a 1:1:1 ratio was determined by a computer-generated randomization list using Rv.0.1 software and by a block size of 4. Then, the allocation information (randomization results) was concealed in opaque and sealed envelopes by the statistician.

The study was blinded in regard of the statistical analysis. Data were recorded and blinded for the statistician: blinding was obtained by eliminating from the elaboration file every reference to patient group assignment

To determine the reliability of the method 15 radiographs chosen at random were traced and digitized by the same investigator on 2 separate occasions at least one month apart. A paired t-test was used to compare the two measurements (systematic error). The magnitude of the random error was calculated by using the method of moment's estimator (MME). Exploratory statistics revealed that not all cephalometric variables were normally distributed (Kolmogorov-Smirnov test) with equality of variances (Levene's test).Kruskal-Wallis test or ANOVA with Tukey's post-hoc tests were used to compare the T2-T1 changes in the three groups. All changes were considered significant at P<0.05. All statistical computations were performed with Statistical Package for the Social Sciences software (Version 12, Chicago, IL)..

10 randomized subjects will be included in the treated group 1, 10 randomized patients will be included in the treated group 2 and 10 randomized patients will be used as untreated control group.

The baseline age was 8.1 ± 0.6 years (range 6.6-9.1 years).

Sample size for this pilot trial was calculated according to the method proposed by Whitehead et al. For a standardized effect size of 1 (a clinically relevant change of 2.0 mm with a combined Standard Deviation of 2.0 mm derived from Guest et al.) for the 8 primary outcome variable Pogonion to Nasion perpendicular, a sample size of 10 subjects per group was required for a Type I error rate of 5% and a power of 80%.

Inclusion Criteria:

• early mixed dentition with first molars fully erupted,
• Class II malocclusion (full-cusp or end-to-end molar relationships),
• negative posterior transverse interarch discrepancy ≥ 4 mm,
• Overjet ≥ 5 mm,
• and prepubertal stage of development (Cervical Stage 1 - Cervical Stage 2 in cervical vertebral maturation)

Exclusion Criteria:

• previous orthodontic treatment,
• extracted or congenitally missing teeth,
• craniofacial syndromes or clefts,
• use of additional orthodontic devices during the observation period

no individual participant data are to be shared