Skeletal and alveolar changes in conventional rapid palatal expansion (RPE) and miniscrew-assisted RPE (MARPE): a prospective randomized clinical trial using low-dose CBCT

Orthodontic management of maxillary transverse deficiency was first reported in 1860 [1] based on the biomechanical principle of orthopedic separation of the two palatal halves by exerting expansion forces at the midpalatal and intermaxillary sutures [2, 3]. Since then, the rapid palatal expander (RPE) has been widely used and proven to be effective for the correction of maxillary posterior crossbites [4], transverse dental arch discrepancies [5], and deficient arch perimeter [6, 7].

In the conventional tooth-borne Hyrax RPE, anchorage is provided mainly by the maxillary first premolars and first molars, concentrating the expansion forces over the dentoalveolar area. Forces applied to these structures, therefore, are related to undesirable side effects, including dentoalveolar tipping; root resorption; periodontal side effects, such as reduction of alveolar bone height, bone dehiscence, and gingival recession [8, 9]; limited basal bone expansion effects; questionable long-term stability [10]; and tissue swelling and ulceration [11]. As overall maxillary expansion is a result of both skeletal and dentoalveolar displacement, the periodontal status after surgical and non-surgical treatments, especially in thin periodontal biotypes, is a major clinical concern [12‐14]. Due to the known limitations, various bone-borne anchorage devices have been introduced and have shown clinical success [15‐17]. Most of the currently available expanders are hybrid in nature and are composed of both miniscrews and tooth-borne parts. However, the role of miniscrews throughout the expansion and consolidation periods have not been well clarified, possibly because of the lack of well-controlled clinical trials.

Apart from the conventional notion that orthopedic midpalatal expansion in individuals over 15 years of age would be very challenging [18, 19], favorable sutural separation in postpubertal adolescents as well as in mature adults has been reported with success [15, 20]. This indicated that human facial sutures are likely to remain patent even in later decades of life, unlike the calvarial sutures that are largely obliterated around the twenties [21‐23]. However, evidence demonstrating that the midpalatal suture changes from a wide and smooth suture to a progressively interdigitated pattern [18, 19] suggests that the same mechanical force may produce different biological and biomechanical effects in immature and mature bone, leading to potentially different side effects [24]. Therefore, the variation of appliance design according to age also needs to be studied.

Three-dimensional radiologic evaluation using currently available computed tomography provides valuable information; however, because of the high radiation dose, a controlled study using serial radiologic evaluation is largely unacceptable. Hence, this study followed a low-dose cone beam computed tomography (CBCT) protocol, which produces only a fraction of conventional CBCT radiation.

One of the purposes of orthopedic maxillary expansion is to secure buccolingual alveolar surroundings along with lateral displacement of the buccal segment for the establishment of proper transverse dimension in the maxilla. In a previous study, tooth-borne expanders have shown unavoidable side effects, such as bony dehiscences and short-term skeletal relapse at the end of the consolidation period in children due to anchor teeth’s buccal displacement [12], which can lead to a reduction in the BBPT. This study implicated that the side effects on the anchor teeth may persist even in successfully separated sutures throughout the expansion and consolidation phase.

On the other hand, purely bone-borne expanders, despite the absence of the side effects in the alveolar bone, exhibit significantly less increase in the intermolar width compared to the conventional tooth-borne expanders in adolescents [31]. Accordingly, a tooth-and-bone-borne maxillary expander, which is a simple combination of a conventional expander and bone-borne anchorage devices, has shown favorable orthopedic and dentoalveolar expansion even in young adults [20, 32]. Nonetheless, the respective roles of the tooth-borne anchorage parts and miniscrews have been unclear. In order to analyze the changes during expansion and consolidation, a comprehensive three-dimensional observation of the maxilla at the end of the expansion and consolidation periods was necessary; therefore, a low-dose CBCT was crucial to evaluate the changes at all three time points.

When compared to adults, children and adolescents are highly sensitive to radiation during their growing stages and therefore they may be affected by the conventional radiation dose. Hence, in this study, an effort was made to reduce the patients’ radiation exposure by obtaining CBCT images with a low-dose protocol. In 2008, Palomo et al. [33] reported that, when the electric current was lowered to 2 mA, the average effective dose delivered to each organ could be lowered about 0.18 times compared to a 15 mA current. In the same year, Ballanti et al. [26] reported their study, which they performed with a low-dose CT protocol for dental care, 80 kV compared with 120 kV resulted in reduced total radiation without compromising the information provided by the diagnostic images. Additionally, a preliminary study conducted with human skulls confirmed that difference observed between low-dose and conventional CBCT images with a voxel size of 0.39 mm was minor and did not significantly affect the measurements of periodontal tissue and dental units [28].

Suture separation was observed in 92.5% of the participants (37/40), which was similar to the results of previous studies [20, 34]. The frequency of midpalatal suture separation was 90% and 95% for the RPE and MARPE groups, respectively.

In the present study, the initial expansion pattern was slightly triangular in the coronal plane as well as in the axial plane. In the coronal plane, expansion of the overall craniofacial structure may be depicted as a triangular pattern with the base at the level of the dental arch. The increase in the nasal width, as a result of the expansion, was lower (ranging from 1.95 to 2.88 mm in the molar region and 2.18 to 2.66 mm in the premolar region) than the maxillary expansion (ranging from approximately 2.59 to 3.31 mm in the molar region and 3.48 to 4.40 mm in the premolar region), which in turn was low when compared to the interdental width of the anchor teeth (from approximately 6.11 to 6.70 mm in the molar region and 6.17 to 6.33 mm in the premolar region. On the basis of these results, an upwardly decreasing expansion gradient was observed, in agreement with the results of previous studies [2, 29] and with a previous systematic review on the topic, in which the MARPE reportedly provided a skeletal component of expansion ranging from 25 to 61% [28, 29, 35]. It is worth mentioning that the skeletal/dentoalveolar expansion pattern may be attributed to individual craniofacial characteristics, such as the degree of midpalatal suture maturation. There is a need to investigate the relationship between the craniofacial characteristics and the alveolar apparatus after use of the MARPE [36, 37]. In the axial plane, the increase in the NPF distance was greater than the increase in the GPF region, indicating a V-shaped expansion of the dental arch, decreasing from anterior to posterior (NPF > GPF). It might be attributed to the anatomic distance between the GPF and the point of expansion force application, as well as to the complex posterior articular surface with the cranial base which limited the effect of the expansion forces.

Carlson et al. [32] reported significant expansion in the zygoma area using MARPE. The relatively small average expansion in the zygoma area observed in the present study may be attributed to individual variations, usually observed in a prospective study design. Even when MARPE was applied, the line of action still passes below the presumed center of resistance of the maxilla, which is grasped by several bony structures on top, including the calvarial bones. Accordingly, a triangular expansion is a common finding regardless of the presence of miniscrews (Table 3) [2, 29]. Moreover, there was no significant difference between the RPE and MARPE groups with regard to expansion at the ZMS, indicating that the zygomatic expansion may be attributed to both the tooth-borne part and the miniscrews. This triangular expansion allows a greater enlargement of the dentoalveolar area, which enables a greater perimeter enlargement compared to the amount of basal bone expansion. Interestingly, all groups exhibited reduction in the basal bone dimension during the consolidation period (T2-T1), supporting the findings by Garib et al. [12] who demonstrated buccal bone dehiscence in younger patients. The significant difference between the M-NW in the RPE and MARPE groups at T1-T0 and T2-T0 indicates that the miniscrews may have played a role in the magnitude of skeletal expansion, and also, maintaining the maxillary bone segments during consolidation.

In our study, the MARPE and RPE groups showed similar dentoalveolar changes except for the maxillary width (PM-MW, M-MW). Intergroup differences in the maxillary width may be because these measurements are a part of the alveolar bone area. Buccal inclination of the anchor teeth resulting from maxillary expansion has been a common finding [9]. We also observed increases in anchor teeth inclinations in both the RPE and MARPE groups. In both type of interventions, the inclination of the first molars significantly increased after expansion (3.4°–3.94°) and decreased during consolidation, confirming the results of previous studies [9]. Therefore, the presence of miniscrews does not appear to guarantee the translation of the anchor teeth to the buccal side.

In both groups, buccal bone thickness of the first molars decreased by approximately 0.4–0.7 mm, whereas palatal bone thickness increased by 0.5–0.9 mm immediately after the RPE and MARPE expansion, indicating buccal displacement of the anchor teeth within the alveolar bone, in accordance with the results of previous studies [9, 12].

Garib et al. [12] reported that rapid maxillary expansion induced buccal bone dehiscences on the anchorage teeth, especially in participants with thinner buccal bone plates. In other studies, the main factors contributing to gingival recession were buccally located or displaced teeth, bone dehiscences, and a thin periodontal biotype [38‐40]. According to the results of the present study, as the decrease of the anchorage teeth’s buccal bone thickness was minimized in the MARPE group, it may be assumed that the chances of bone dehiscences were also reduced.

However, regarding bone thickness changes around the premolar and the first molar, significant differences between RPE and MARPE group were observed in both buccal and palatal bone thicknesses, implying the more stable positioning of the anchor teeth in the MARPE group during the expansion and consolidation periods. Admitting the limitation of the study, it can be claimed that the miniscrews play a role in maintaining the anchor teeth within the maxillary basal bone segment during the consolidation period, enforcing periodontal safety during orthopedic expansion. The elasticity of the basal bone may produce a constant force from the zygomatic buttress against the anchor teeth. Therefore, without miniscrews, at the end of the consolidation period, the actual amount of basal bone may be decimated. According to our study, MARPE played a critical role by eliminating some negative side effects of the maxillary expansion procedure, whereas RPE resulted in buccal alveolar bone thinning of the anchor teeth. A significant positive correlation between the amount of expansion and periodontal variables of the anchor teeth during expansion (T1-T0) in the MARPE groups indicated the consistent relationship between the amount of expansion and the changes in the palatal bone plate thickness of premolar region. In contrast, the lack of correlation in the RPE groups appeared to imply the reduced predictability of the changes associated with the host factor (Table 6).

Taken together, within scope of the present study, it can be summarized that the reinforcement of RPE with miniscrews doesn’t affect the midpalatal suture separation ratio, however it appear to contribute to the maintenance of the basal bone during the consolidation period leading to less periodontal side effects, such as buccal dehiscence. Influence on the greater skeletal expansion was not evident. Future studies with long-term observations would be able to build the clinical significance of MARPE in terms of stability of transverse dimension and safety of the surrounding buccal bone.

Taken together, the reinforcement of RPE with miniscrews appears to contribute to the maintenance of the basal bone during the consolidation period leading to less periodontal side effects, such as buccal dehiscence.