Study and appliance design
The aim of the present study to provide a higher degree of evidence on the efficacy of of MARPE was supported by a number of methodological choices. In contrast to most previous studies, a prospective study design was applied. The selection bias, one of the main shortcomings of previous reports , was reduced to a minimum through the inclusion of consecutive patients who presented with transverse maxillary discrepancy. Regarding the minimum age for inclusion, a clinical approach was followed, as the age of 16 is the clinically applied cut-off point between RPE and SARPE. Furthermore, a sufficient population size was present to provide the power that was required.
The D-MED, developed in 2018, was the first 3D digitally designed, patient-specific, and 3D-printed MARPE appliance with the aim of delivering more predictable expansion results. The 3D-printed steel structure aimed to provide more rigidity, decreasing the risk of fracture. The accompanying miniscrews (2.0 mm diameter) were thicker than the miniscrews used in other MARPEs (1.5 mm [11, 12] or 1.8 mm diameter [13,14,15,16,17,18]), delivering a stronger anchorage and transmission of expansion forces to the palate. Based on the height of the screw holes, the distance to the palatal mucosa, the thickness of the mucosa, and of the palate, 11 mm or 13 mm miniscrews were applied to ensure bicortical anchorage in all cases, even though for some patients with very thin palatal bone, shorter miniscrews could have been appropriate as well.
Efficacy of MARPE
The mean age (27.0 ± 9.4 years) of the study population was significantly higher than in previous MARPE studies (range: 20.1 ± 2.4–24.9 ± 1.8 years) [11,12,13,14,15,16,17,18,19,20,21,22,23]. Even with the higher mean age, the success rate of MARPE by D-MED is comparable to and even somewhat higher than the mean success rate of MARPE (92.5%) . One of the two patients whose midpalatal suture did not separate was significantly older (56.0 years), which is in line with the findings of Shin et al. and Jeon et al. that there is a significant negative correlation between age and midpalatal suture opening by MARPE [18, 23].
The total transverse expansion (IMW) by D-MED was comparable to the mean by MARPE (6.55 ± 1.05 mm)  and by SARPE (7.0 ± 0.85 mm) . However, the amount of skeletal expansion by D-MED was significantly larger than the mean by MARPE (2.33 ± 0.70 mm)  and larger than that by SARPE (3.3 ± 0.55 mm) . Concordantly, the percentage of skeletal expansion by D-MED (60.4%) was significantly higher than by MARPE (35.6%)  and by RPE and SARPE, ranging from 40 to 55% [25, 26] and 21.5 to 46.3% [27,28,29,30], respectively. Considering the differences in amount of required expansion, the extent of screw activation differed between patients, leading to a greater variance in the absolute measured values. Standard deviations of the contribution of alveolar, skeletal, and dental expansion were calculated to express this variance. In comparison to the M1, the expansion at P1 was almost fully skeletal. This could be attributed to the expansion forces being directed at the first molars via the bands and the four rigid connectors, which led to more dentoalveolar expansion at M1, while this was nearly absent at P1 level. There was also great variance in the age of the participants. A recent study by Jia et al. on the age-dependent effects of MARPE found a significant difference in the amount of skeletal expansion between patients younger and older than the age of 20 . In the current study, patients < 20 years (N = 8) had a skeletal component of 72.1% and patients ≥ 20 years (N = 24) of 56.5% (p = 0.056), demonstrating a similar age-related effect. The expansion was achieved in a short time span of ca. 31 days, comparable to other MARPE studies with a rapid expansion protocol (range: 27–35 days) [11, 15, 17].
There was no statistically nor clinically significant difference in suture width at M1 and P1, showing a parallel anteroposterior midpalatal suture opening pattern, as was also found in previous studies [16, 32]. On the other hand, the expansion at the nasal cavity indicated that MARPE affected not only the maxilla, but also the circummaxillary structures with a more pyramidal or V-shaped expansion pattern in the coronal plane. These findings coincide with previous reports on RPE [33, 34], SARPE [29, 30], and MARPE [17, 31, 32], while a few MARPE studies reported a parallel expansion pattern [6, 16].
Considering that there are no major differences in expansion protocol between the available MARPE appliances, the favorable skeletal expansion results in this study could be attributed to the individualized design and larger diameter of the miniscrews, which led to better fitting and increased anchorage and force transmission with the D-MED. A precise comparison of the MARPE appliances, however, was complicated due to methodological differences of the studies.
Side-effects of MARPE
As described in previous RPE [10, 35, 36], MARPE [14,15,16,17], and SARPE [2, 3, 27] studies, expansion with the D-MED in the present study also had some side-effects. Buccal crown tipping was potentially clinically significant, as the consequential buccal alveolar crest height decrease could lead to gingival recessions . Different factors, such as tipping of the miniscrews, low palatal thickness, or bone density, could have contributed to tipping, as they could have led to increased force delivery on the anchoring molars [18, 37]. However, the mean increase of clinical crown height was not clinically relevant, as well as the decrease of buccal bone thickness, which was similar to that found in earlier MARPE reports [15,16,17], while studies on RPE presented contrasting results, with minimal changes in some , and a significant decrease in others [10, 36]. However, in patients with a weak periodontium, thinning of the buccal bone could be a potentially relevant side-effect, as these patients may be more prone to further periodontal deterioration . This was the case in one out of two failed expansions observed in this study.
Study limitations and future perspectives
Considering study design, ideally, an RCT would provide stronger scientific evidence by comparing the effects of MARPE and SARPE. However, due to the surgical nature of SARPE, random allocation of patients to a surgery and a non-surgery group may face challenging ethical issues. In order to address concerns pertinent to the ALARA principle of radiation exposure, the CBCT at T1 had a smaller field of view. On the other hand, this created a challenge relevant to consistency between the first and the second image acquisition, which, nonetheless, was overcome by the selection of the reference frame.
Despite the positive treatment outcome of the D-MED in the immediate post-expansion period, the observation time was very short and studies regarding the long-term effects and stability following MARPE expansion would be recommended. Due to the limited observation time, it was not possible to evaluate root resorption as a potential side-effect. Furthermore, as there were only two failures, a reliable prediction on which participants might not achieve a successful expansion could not be made. This indicates the need for further research on the prognostic factors, such as palatal morphology in a prospective clinical setting. Nonetheless, it can be expected that the use of MARPE for the correction of transverse maxillary discrepancy will continue to increase.