Impact of rapid maxillary expansion on palatal morphology at different dentition stages

One hundred fourteen (67 female, 47 male) out of 167 patients who received a rapid maxillary expansion between 2010 and 2020 with an RME appliance including a Hyrax screw anchored to four teeth were included in the study, using the following inclusion criteria:

Treatment exclusively by the same orthodontist, no prior orthodontic treatment, Caucasian origin, transverse maxillary arch deficiency, uni- or bilateral crossbite, corresponding high-quality dental casts prior to treatment and immediately after RME removal and the number of Hyrax screw activations had to be almost equal in both groups. The application of these strict criteria ensured that therapeutic effects could be evaluated and interpreted without restriction.

The division into two groups was based on the dentition stage of the patient when the appliance was inserted. Patients with an early mixed dentition were assigned to group 1 (early group). Patients with a late mixed dentition, where the first premolars had to be fully erupted, and patients at the beginning of the permanent dentition were in group 2 (late group). The youngest patient was 7.16 years, and the oldest patient was 17.24 years old at treatment onset. The mean age was 11.03 ± 2.59 years (group 1, 9.13 ± 1.33 years; group 2, 12.93 ± 2.09 years). The RME appliance remained inserted for mean 6.15 ± 1.98 months (group 1, 6.20 ± 2.24 months; group 2, 6.11 ± 1.70 months). The exact data on patient age, wear time average number of Hyrax screw activations, percentage of maximum possible turns of the Hyrax screw (in %), location of the crossbite and mandibular deviation can be found in Table 1.

A Hyrax screw RME appliance with solely dental anchorage was used in all patients of this study to ensure comparability of treatment effects. This appliance (palatal screw type S with a lift height of 0.2 mm, Forestadent, Pforzheim, Germany) was fixed with two occlusal rests on the 1st premolars or deciduous molars and with two pre-fabricated bands on the first permanent maxillary molars (Fig. 1 a and b). No premolar bands were used in any of the patients; anterior fixation was gained by bonded occlusal rest on the first premolars or deciduous molars only.

The activation was performed twice daily until the therapeutically desired posterior arch width was reached. Overcorrection was planned with 25% additional widening to the correction of the maxillary deficiency.

The appliance then remained passively in situ for approximately 6 months to stabilise the treatment result (see Table 1).

Two hundred twenty-eight dental casts (114 of group 1 and 114 of group 2) were measured after conversion into 3D models. The original plaster casts were made during treatment at a defined interval:

The impressions were made using alginate from Kaniedenta (Yellow Print Alginate, Kaniedenta, Herford, Germany) and Rim-Lock impression trays. Subsequently, the impressions were cast with plaster (Kanistone Classic, hard plaster type 3, Kaniedenta, Herford, Germany) and trimmed three-dimensionally. The orthoX® scan 3D scanner (Dentaurum, Ispringen, Germany) was used to scan casts and produce their three-dimensional data set (accuracy of < 20 µm with a scan time of 45 s per model).

The obtained 3D data sets were virtually enhanced, trimmed and exported as an STL file through the software OnyxCeph® 3TM (Image Instruments GmbH, Chemnitz, Germany). The subsequent virtual analysis of the digital models was performed with the software 3D-Tool-Free (3D-Tool-GmbH & Co. KG, Weinheim, Germany).

The dental arch width was measured anteriorly at the first premolars or deciduous molars and posteriorly at the first permanent molars (Fig. 2a). The palatine width was measured between the most coronal points of the gingival margin at the first premolars or deciduous molars and at the first permanent molars (gingival-alveolar plane). Starting from these points, the width was determined in 2 mm steps cranially up to a distance of 6 mm (skeletal-basal plane) (Fig. 2 b and c). The anterior/posterior ratio was calculated on three  planes (dental, gingival-alveolar and skeletal-basal) to qualify the expansion as parallel or triangular.

For the determination of the palatine height, the raphe median line was connected perpendicularly to the most coronal point of the gingival margin on the first premolars or deciduous molars and the first permanent molars. The mean value for these measurements was calculated at the respective teeth in both quadrants. The same measurement was performed for the first deciduous molars or premolars 5 mm to the right (1st quadrant) and left (2nd quadrant) of the palatal centre and for the first permanent molars 5 mm and 10 mm to the right and left of the palatal centre (Fig. 3 a and b).

The modified palatal index according to Kim et al. [24] was used to assess palatal shape changes. Starting from the first quadrant, the angles between the horizontal reference line from the gingival margin of opposing first premolars and first permanent molars and the lowest point in the centre of the palatal vault were measured (Fig. 4 a and b). The measurements were conducted using the open-source software GIMP (GNU Image Manipulation Program, The GIMP Team).

The data was collected in a structured manner using spreadsheet software (Excel®, Microsoft Corporation, Redmond, USA) on a computer with the operating system Microsoft® Windows 10 (Microsoft Corporation Redmond, USA). The collected data were subsequently imported into statistical software (SPSS® 23, Armonk, NY, USA) for Windows® (Microsoft Corporation) and analysed. Normal distribution was evaluated visually and with the Shapiro–Wilk test. Treatment-associated changes in variables were analysed using the linked t-test for intra-group comparisons and the independent t-test for inter-group comparisons. Mean and standard deviation were reported for each variable. Statistical significance was assumed at p-values < 0.05. The significance level was defined as follows: p ≥ 0.05 not significant, p < 0.05 significant, p < 0.01 highly significant and p < 0.001 highly significant.

To determine the combined error of the method (MF) according to Dahlberg [25], 25% of the models were randomly selected for this purpose and measured again by the same investigator after a period of 3 months. The error of the method for linear (height, width) and angular measurements was calculated with the formula MF = √(∑d2/2n) to determine the validity of the measurement method, where d is the difference between two measurement results and n is the number of duplicate measurements. The MF in the present study was < 1 for all measurements (height 0.60 mm, width 0.53 mm, angle 0.65°).

In the present clinical study, morphological changes of the palate were evaluated three-dimensionally after rapid maxillary expansion. The treatment effects of rapid maxillary expansion are largely determined by the therapeutic point of force application to the rotational centres of the maxilla. According to various studies, these centres are located dorsally in the area of the median palatine suture and close to the frontomaxillary sutures [26‐31].

Many studies focus on changes within the transverse plane, especially on those of the median palatine suture. Both Bazargani et al. [10] and Liu et al. [32] found primarily no consensus in their systematic reviews as to whether RME treatment leads to triangular, i.e., greater anterior opening, or to a parallel widening of the median palatine suture. In the individual studies examined, however, measurement methods and recording techniques vary greatly. The results of the present clinical study in conjunction with the case studies suggest that the opening of the median palatine suture depends on the patient age at treatment onset: it opens approximately parallel in the early group and triangularly in case of a later therapeutic intervention. A parallel suture opening is also described in the studies by Christie et al. [33] and Podesser et al. [23] for patients with a chronological age of 10 years or less. Habersack et al. [34] also described age-dependent differences for the therapeutic effects of RPE treatment on median palatal sutures using CT data from two comparable cases: a 10-year-old patient with mixed dentition showed a parallel opening, whereas the 16-year-old patient with permanent dentition experienced a greater opening in the anterior region than in the posterior region and thus a triangular expansion.

A possible connection between patient age and opening mode of the median palatine suture seems obvious. Several authors attribute the decreasing expansion in the region of the first permanent molars with advancing age to the onset of ossification of the median palatal suture at this particular level [35‐38]. It is undisputed that the median palatine suture is subject to age-related changes [39]. With increasing age, the degree of ossification increases and progresses from posterior to anterior, whereby the onset and progress of obliteration are subject to strong inter- and intraindividual variations [38, 40]. Wehrbein and Yildizhan [41] as well as Knaup et al. [37] were able to demonstrate in studies on human palates that even in advanced age only minor ossifications of the median palatine suture were present and that the mean sutural width also decreased only slightly compared to younger individuals. They concluded that an increased resistance to transverse expansion in adult patients was probably due to other factors, such as pronounced sutural interdigitations or increased bone rigidity. In a micro-CT analysis of 28 palatal specimens from humans aged 14–71 years, the only age-related factor determined was bone density (BV/TV [%]) in the sagittal plane among the other investigated parameters obliteration index in the frontal plane, suture length, linear sutural distance, interdigitation index in the horizontal plane and bone density. Thus, the morphology of the suture does not seem to have a limiting factor on the mode of opening but rather the bone density of the suturally adjacent maxillary bone [40] and the increasing rigidity of the pterygomaxillary pillars [42].

The triangular pyramidal maxillary expansion in the frontal plane has corresponding effects from the nasal cavity to the alveolar processes [43, 44] and includes orthopaedic and orthodontic components. In the present study, a triangular expansion of the maxilla between the dental plane, the gingival-alveolar plane and the skeletal basal plane is seen in the frontal plane in both the anterior and posterior regions in all patients (see Table 2). This “bending up of alveolar processes” has also been described in CT studies by Podesser et al. [23] and Weissheimer et al. [45] at least for the posterior region.

In the sagittal plane, the late group shows a greater height increase anterior than posterior, while in the early group, it is almost uniform. Although palatal growth is mainly genetically determined, other factors such as adjacent anatomical structures, growth- or therapy-related changes in the position of the teeth or the position of the tongue also play a role [46, 47]. In a Korean longitudinal study on digitised models, Yang et al. [48] documented the growth of the palate in untreated and non-treated subjects between the ages of six and 14. Both palate height—median and right and left paramedian—and width increase more in the posterior region than in the anterior region. Even though ethnic reasons make the comparison with the participants of this study difficult due to the Asian origin of the subjects, and despite differences in measurement points and distances, it is particularly remarkable that the palatal height increase after RPE treatment is different in both treatment groups of the present study. Five millimetres and 10 mm to the right and left of the median raphe, the lateral increase is always greater than at the median raphe itself in absolute terms. In addition, the height increase in the early group is the same anteriorly and posteriorly. In the late group, this is more pronounced in the anterior region than in the posterior region and thus even contrary to the results of the longitudinal growth study by Yang et al. [48].

The modified palatal index according to Kim et al. [24] documents highly significant changes in the shape of the palate in the anterior and posterior regions with early intervention. Patients in the late group show a highly significant flattening of the palatal morphology only anteriorly. These results are consistent with those in the transverse, frontal and sagittal planes.

The therapeutical forces and moments generated through activation of the Hyrax screw act both upon the maxilla and on deeper cranial structures [18], especially the palatine bones and the pterygoid processes of the sphenoid bone. The rising tensions are initially concentrated on the anterior palate and then proceed dorsally along the median palatine suture and via the palatine bone to the sphenoid bone, the zygomatic processes and the medial orbital walls [14]. A possible reason for the triangular expansion found in adolescent patients may be that the anatomical proximity of the maxilla to the pterygoid processes of the sphenoid bone presents a rising resistance to the opening of the suture in the posterior region with increasing age [45, 49].

Holberg [42] used the finite element method (FME) to show that these stresses and deformations are only moderate in the juvenile sphenoid bone, whereas in adults, due to the decreasing elasticity of bony structures, the lateral bending up of both pterygoid processes in the area of the maxillary canal (foramen rotundum ossis sphenoidalis), the inferior maxillary foramen (foramen ovale ossis sphenoidalis) and the superior orbital fissure can cause considerable stresses, which could lead to microfractures with injury to nervous and vascular structures.

The interaction of the various centres of rotation is the cause of the therapeutically induced changes in the height and shape of the palate.

Age-dependent and in favour of the early intervention is the parallel and thus more even opening of the median palatine suture. Post-therapeutic functional stabilisation of the dilation through the establishment of a physiological swallowing pattern and harmonisation of the tongue rest position is thus more likely to be guaranteed with a uniform opening and is essential for long-term stability. In the case of a more V-shaped dilatation after late treatment, an increased caudal tongue rest position and thus an increased risk of recurrence due to a lack of functional stabilisation is to be expected [50].