The present findings would confirm that RME reduces the radicular volume of the maxillary first molars, first and second premolars in the post-retention stage (T0-T1). The first molars showed consistently greater values of volume loss; however, when this parameter was calculated as a percentage of total radicular volume, no significant differences were identified among the investigated teeth. In the tooth-borne RME, this would mean that posterior teeth could be exposed equally to ERR, whether they act as abutment teeth (P1 and M1 in this study) or as un-anchored teeth (P2), that is, even if they received different loads, as also suggested by previous evidence [12]. The values of volumetric loss detected in the TB group were similar to those reported by an earlier study testing conventional maxillary expanders [10]. Considering that the age of the study sample (TB = 13.3 ± 1.02; BB = 14.7 ± 1.15) approximates nearly the final maturational stage of the premolars [17], it could be assumed that the ERR detected may have disrupted the final developmental stage of these teeth.
We also assessed radicular length changes and the shape between the radicular T0 and T1 3D models. All investigated teeth reported a reduction in radicular length in the post-retention stage, with M1p and P1 being the roots mostly involved in the TB group (respectively 0.33 mm and 0.31 mm of length reduction), and with M1p being the root mostly involved in the BB group (0.11 mm of length reduction). In this regard, one of the main concerns of ERR is the harmful consequence of root shortening on the tooth longevity; however, the values registered in this study should be far from threatening the function of the dentition significantly, at least from a quantitative perspective, considering that 2-mm root shortening was found to reduce the total attachment area of 5–10% [18, 19]. In summary, these differences are likely to be considered clinically irrelevant.
The color-coded map obtained from T0 to T1 deviation analysis showed that the reduction in cementum (showed by blue-tone) was localized in the apical, bucco-apical, and bucco-medial radicular areas of both abutment and un-anchored teeth in the TB group. A similar resorption pattern was identified in the BB group, despite the absence of detectable deviation at the apex. This would suggest that modifications in this region were likely irrelevant, as confirmed by the linear measurements of radicular length. These findings corroborate previous evidence from histological materials reporting the generation of radicular resorption on the buccal surface of the roots in the form of small irregularly shaped lacunae [5, 8], and from recent well-conducted micro-CT studies [3]. In this regard, it robustly supports such patterns of ERR: The forces generated by RME are orientated toward the buccal side of the dento-alveolar arch, causing the compression of the periodontal ligament and subsequent hyalinization on the buccal side of the roots, and ERR occurs during the elimination of the hyalinization tissue on the compressed side [20]. Furthermore, the root apex could be a sensitive area since greater force per unit of the surface area generated during RME and because of the presence of a thicker and more rigid bone compared to the trabecular bony architecture of the cervical region [21].
A recent CBCT study [10] showed that maxillary premolars and first molars featured a slight recovery of radicular volume between the active and post-retention phases of RME. This recovery would reflect the process of repair of the damaged cementum. It has been seen that when the orthodontic forces ceased or are below a certain level, the removal of the hyalinized necrotic tissue begins with the subsequent cemental repair [20, 22,23,24]. Thus, it may be possible that the amount of root resorption detected in the present investigation could have been influenced by an active process of cementum repair that might have mitigated the damage that occurred after the active expansion phase.
The null hypothesis of the present study is rejected since the amount of root resorption recorded was significantly greater in TB group compared to the BB group. These findings are explained since no direct forces were applied to the dentition in the BB group, supporting the evidence from a recent split-mouth study [3]. Conversely, our findings disagree with another recent study where the authors have limited the observation to 2D linear measurements of radicular length [25]. However, it must be emphasized that the differences found in the present study between TB and BB expanders are not likely of clinical relevance, and should not influence the clinical decision of the anchorage system to use.
The fact that some amount of volume loss and length reduction was detected in the BB group may be related to the design of skeletal anchorage, which consists of two mini-screws placed on the palatal slope between the second premolar and the first molar area, concentrating the force on the posterior region that may have been somehow transmitted to the dentition due to its potential proximity [26]. In this regard, further studies testing different skeletal anchorage designs and using a consistent methodology for the evaluation of ERR are warmly recommended to provide more conclusive evidence.
