Previous CT studies of the effects of RME on palatal and buccal alveoli evaluated only on one axial section [12, 14,15,16], which might not sufficiently delineate the effects of RME on the entire alveolar bone thickness. This present study could be the first to investigate the immediate effects of Alt-RAMEC on the alveolar bone of the same patients by evaluating on four CBCT axial sections. Our results revealed that the Alt-RAMEC reduced the BABT (0.54 ~ 0.70 mm) and thickened the PABT (0.31 ~ 0.41 mm) at the anterior anchor teeth. All the BABT reduction by the Alt-RAMEC was within the scope of the initial BABT, which indicated that the Alt-RAMEC did not cause root or bony dehiscence or compromise the periodontal support of all the anchor teeth.
The first premolars had the greatest BABT reduction by the Alt-RAMEC. This could be due to the double-hinged expander which expands maxilla more at the anterior than posterior [5, 6], and the first premolars were the anterior anchor teeth. The first premolars therefore were expanded in a wider range and heavier force resulting in a more BABT reduction. The first premolars also had the thinnest initial BABT, and a thinner initial BABT has been revealed having more buccal bone reduction and deleterious post-expansion effects [15, 18]. The range of expansion and initial BABT of the anchor teeth of a maxillary expander seems critical to the BABT reduction no matter if it is RME or Alt-RAMEC. On the other hand, the second premolars had the least and negligible BABT reduction because they were not banded or bonded to the expander, and they were the indirect anchor teeth.
Maxillary expansion results in either buccal crown tipping of the anchor teeth of a hyrax expander [21, 22] or bodily movement of the anchor teeth of a Hass type expander with palatal acrylic pads [12, 17, 21, 22]. Interestingly, the double-hinged expander resulted in buccal crown tipping of the posterior anchor teeth but bodily movement of the anterior anchor teeth. Our results revealed that the BABT reduction and PABT thickening were both more at the cervical (AS1 and 2) than at the apical regions (AS3 and 4) of the posterior anchor teeth (fist molars), but they were similar at the cervical and apical regions of the anterior anchor teeth (first premolars).
In comparison to the 1-week RME protocols [15, 16, 18], the protocol of Alt-RAMEC had three more sets of alternating rapid expansions and constrictions of maxilla. And it is reasonable to assume that the Alt-RAMEC has more impacts on the BABT reduction and PABT thickening than RME and has more root and alveolar bone dehiscence. However, the results of this study showed that the BABT reduction and PABT thickening were both within the scope of initial BABT and PABT. The impacts of Alt-RAMEC were less than we expected.
This could be due to the alternating expansion and constriction that is a reciprocal recovery period to each other, which makes the impacts on alveolus less. For a 7-week Alt-RAMEC, the three constrictions reciprocally relieve the expansion force and alternate the buccal alveolus from pressure to tension side and the palatal alveolus from tension to pressure side. Similarly, the four expansions reciprocally relieve the constriction force and alternate the buccal alveolus from tension back to pressure side and the palatal alveolus from pressure back to tension side. At the same time, the expansion and constriction force also reciprocally disarticulates the circumaxillary sutures with less impact on the sutures. It has been suggested that maxillary orthopedic protraction is better before fusion of the circumaxillary sutures [2, 5, 23, 24].
There are certain limitations of this clinical study. Although, the ICC was .992–.996 indicating high reliability, however, the validity (reproducibility) of this study would be more meaningful if the study’s measurement was conducted by at least two individuals with an inter-operator reliability testing. This study was a case-series cohort study rather than a randomized clinical trial. Our findings on BABT reduction and PABT thickening were the immediate effects observed after 7 weeks of the Alt-RAMEC protocol without removal of the expander. The consequence or recovery process of the BABT reduction and PABT thickening by Alt-RAMEC were not studied. It has been revealed that the BABT reduction by RME was reversible and had no evident deleterious effects and would recover after 6 months of retention period [12, 25].The current study did not investigate the impacts of Alt-RAMEC on alveolar bone height of the anchor teeth of expander. It has been revealed that RME seldom causes alveolar bone apical displacement, especially when the process is sterile and noninflammatory . The overall impacts of Alt-RAMEC on the alveolar bone still need further studies.