In supplement to extant empirical evidence elucidating the effects of Rapid Maxillary Expansion (RME) on maxillary structures [21], scholarly literature has also posited the short-term impacts of RME on mandibular alterations, such as spontaneous Class II corrections or clockwise mandibular rotations [22, 23]. However, the latter phenomenon, predominantly manifested in dental relationships and compounded by skeletal variations engendered by natural growth, remains speculative in the absence of a comprehensive control group for comparative analysis [9]. Consequently, this retrospective investigation was designed to examine mandibular developments in patients with varying vertical growth patterns induced by RME, juxtaposing these findings against a Control group with more than two years of follow-up data.
The temporal parameters demarcating both the pre-treatment and post-treatment phases revealed no statistically significant disparities between the RME and Control groups (Table 1), thus confirming the homogeneity of the cohorts. Cephalometric measurements at Time 1 (T1) corroborated this observation, the sole exception being the Overjet measurement (Table 2). Notably, the Control group exhibited a reduced initial Overjet value of 3.1 mm, relative to 5.4 mm in the RME group. Although this variance in initial Overjet measurements constrains the scope for comparing dentoalveolar compensations, it does not vitiate the legitimacy of comparisons pertaining to skeletal modifications.
In our research, skeletal changes in the mandible in the sagittal dimensions were analyzed via angular indices, including SNB, ANB, and linear metrics such as Wits, Co-Gn, Go-Me, and Overjet. The ANB angle serves as the predominant cephalometric indicator for describing discrepancies between skeletal bases; however, the method is susceptible to variations linked to the Nasion point's positional changes, particularly due to ongoing growth and development [24]. By incorporating Wits values, which utilize linear measurements to gauge the sagittal relationships of the jaws, we mitigated the influence exerted by cranial base reference points; nonetheless, the parameter remains vulnerable to fluctuations in the occlusal plane [25]. Numerous authors advocate employing both ANB and Wits metrics to diagnose antero-posterior discrepancies in skeletal bases [26]. The metric of Overjet, delineating the horizontal distance between the upper and lower incisor tips (U1 to L1), is significantly affected by dentoalveolar compensations and thus may not serve as a precise indicator of skeletal alterations; it is merely considered a reference for sagittal relationships.
Our findings disclosed that, in comparison to the Control group, the RME cohort manifested a statistically significant decrement in both the ANB angle (-0.9°/-0.1°, P=0.046) and Overjet (-1.0/1.5 mm, P < 0.0001). A diminishing trend was also noted in Wits values, albeit devoid of statistical significance (-0.9/-0.3 mm, P=0.372). As per the SNB angle, mandibular advancement in the RME group was quantified at 1.0° (P=0.047) vis-a-vis the Control group. Measurements of mandibular length, as assessed through Co-Gn and Go-Me metrics, evinced significant growth across both groups; however, the RME cohort did not display an amplified trajectory of mandibular growth relative to the Control group.
Fascinating observations emerge upon juxtaposing facial growth pattern groups in accordance with their mandibular plane angles. In the RME even angle group, significant decrements were noted in ANB (-1.1°, P=0.002), Wits (-1.9 mm, P=0.011), and Overjet (-1.8 mm, P < 0.0001), whereas SNB (2.4°, P=0.053) manifested an increment when compared to the Control even angle group. Conversely, in the RME high angle group, merely Overjet (-0.4 mm, P=0.014) demonstrated a discernible trend towards retraction vis-à-vis its Control counterpart. The RME even angle cohort, when juxtaposed with the RME high angle cohort, evinced more pronounced sagittal transformations, substantiated by a greater trend of decrement in Wits (-1.9/0.0 mm, P=0.016). Subgroup analyses did not yield statistically significant impacts of maxillary expansion interventions on mandibular body growth, corroborating the findings of Susan et al [27], who elucidated that bonded rapid maxillary expanders employed in early mixed dentition Class II Division 1 subjects could mitigate Class II malocclusion as a secondary consequence, both skeletally and dentally.
The promotion of mandibular growth due to RME therapy has been a matter of debate. The prevailing hypothesis postulates that RME therapy alleviates maxillary constriction, thereby facilitating natural three-dimensional mandibular growth [28]. An alternative conjecture implicates functional shifts instigated by occlusal disruptions; the employment of RME could disrupt the existing occlusion, thereby compelling the patient to reposition the mandible anteriorly into a more ergonomically favorable alignment. Subsequent condylar remodeling and stabilization of the mandibular position ensue over time [29, 30]. The absence of augmented mandibular body growth in the RME group relative to the Control group may be attributable to the study's observational timeframe, which was confined to a two-year follow-up period.
For the appraisal of vertical skeletal modifications in the mandible, relative angular measurements such as Frankfort Mandibular Plane Angle (FH-MP), and linear metrics like Nasion to Menton (N-Me), Sella to Gonion (S-Go), Facial Height Index (FHI; N-Me/S-Go), and Articulare to Gonion (Ar-Go) were scrutinized. Some extant short-term studies have asserted that RME can induce buccal inclinations of molars, culminating in an inadvertent elevation of vertical height [23, 31, 32].
Post-treatment mandibular plane angles in this investigation were -0.7°±2.4° and -1.2°±2.0° for the RME and Control groups, respectively, with no statistical significance in their disparities (P=0.403). Explicitly, both the high angle and even angle expansion cohorts recorded post-treatment mandibular plane angles of -1.1°±2.6° and -0.3°±2.0°, which were not statistically divergent from their corresponding Control groups (P > 0.05). Longitudinal observations denoted a trend of decrement in the mandibular plane angle growth, in harmony with the contributions of Garib et al [14] and Chang et al [33], who observed physiological contractions in the mandibular plane angle in their long-term evaluations of normative samples, recording average contractions of -0.7° and -0.9°, respectively.
When scrutinizing longitudinal alterations across other vertical dimensions—N-Me, S-Go, FHI (N-Me/S-Go), and Ar-Go—no statistical variance emerged among any of the cephalometric variables under consideration. Such observations suggest that patients with high mandibular angles who undergo RME treatment manifest comparable likelihoods of experiencing alterations in mandibular plane angles to those individuals possessing even angles.
Consistent with this finding, extant long-term studies collectively assert an absence of substantial modifications in the mandibular plane angle subsequent to rapid maxillary expansion [2, 33, 34].Research conducted by Matteo Rozzi et al [13] discerned that, in the immediate aftermath of RME, subjects characterized by elevated mandibular angles underwent clockwise rotation of the mandible. This may be due to lower muscle strength in high-angle subjects, resulting in lower dental anchorage and greater buccal inclination. Nonetheless, no marked vertical transformations were discernible between the two groups one year post-RME treatment cessation. Corroboratively, Matthew W et al [15], in a long-term surveillance of hyperdivergent subjects undergoing RME, identified no detrimental ramifications upon vertical skeletal relationships, in either the short or long-term continuum.
Although our study did not directly observe clockwise rotation in the vertical direction within the high angle RME group, there was also no significant evidence of mandibular remodeling in the sagittal direction, as seen in the even angle RME group. This suggests that the tendency for clockwise rotation may offset the anterior mandibular positioning remodeling, further studies conducted with larger sample size are necessary to confirm present results.
In the soft tissue comparison, it can be observed that the even angle RME group retruded more compared with the high angle RME group, both groups show statistically significant retruded to their respective control groups. Such observations may be attributable to alterations in maxillary width; namely, the upper lip's thickness can be significantly attenuated due to transverse stretching effects on the lips [35, 36].
The primary limitation of the current study resides in the selective inclusion of cases. Owing to database constraints, it proved challenging to incorporate skeletal Class II cases (ANB > 4°) who had solely undergone long-term RME treatment.
Additionally, the control group's composition, which might include a subset of anterior crossbite cases, it may potentially influence the comparison of dental items to some extent. To mitigate this, we strategically concentrated our analytical efforts on intermaxillary positioning and mandibular growth metrics. Another encumbrance was the unavailability of Cone Beam Computed Tomography (CBCT) examinations, which compromised our capacity to quantitatively evaluate correlations between alterations in maxillary dimensions and mandibular positional shifts. Further studies using three-dimension evaluation might be helpful to provide more information.