This study was conducted on two groups of adult patients, in all of which successful maxillary expansion had been performed using the same expander but with different activation protocols. The Slow Expansion Group utilized the FCPC-activation protocol [11] while the Rapid Expansion Group continuously opened the expander two turns per day similar to the MARPE activations protocol [6]. The objective was to assess the outcomes of the two groups in terms of skeletal, sutural and dental changes, as well as to establish an efficiency relation to evaluate the effectiveness of the expansion procedures in both groups.

Recently, the assessment of low-dose CBCT technology (lower kV settings with larger voxel sizes) has proven to be an effective tool for measuring bone and dental landmarks clinically, as well as for evaluating mid-palatal and circummaxillary sutures and their changes in width [3, 16]. However, this recent low-radiation technology could not be applied because this retrospective study, spanning several years, did not allow for its use at that time. The high-resolution CBCT imaging used in the present study is essential for detecting sutural changes, bone remodeling, and potential asymmetries that could influence treatment planning and follow-up. This necessity justifies performing an additional CBCT after 3 months to assess treatment efficacy and analyze cranial changes, providing critical clinical information that other methods could not offer.

In concordance with other MARPE studies [5, 25, 26] the mechanical opening of the maxillary bone with bone borne expanders, had a greater impact on the lower midfacial bone structures (maxilla, palatal bone and sphenoid bone), and also affected distant structures such as the zygomatic bones (Table 2) [25, 27]. This opening in coronal cross-sectional direction has a pyramidal configuration with the base in the palatine bones and the apex in the frontal process of the maxillary bone [12]. This is also observed in a MARPE study where the increase in nasal cavity width and nasal floor width, by 1.61 ± 0.94 mm and 2.20 ± 1.01 mm respectively, significantly improves the upper airway and can be beneficial for OSAS (Obstructive Sleep Apneas Syndrome) patients by enhancing airway passage through the nose [19].

At T2 we observed in our adult patients in MARPE-G an increase in distance in the frontal process of the maxilla with mean 0.32 mm and in the (slow) FCPC-MASPE-G a statistically significant with mean increase inter distance of 0.64 mm (p = 0.000, Table 2). This leads to the conclusion that during rapid expansion skeletal structures have less time to adapt to deformation. During rapid activation (2 × 0.2 mm /d) Ahmida et al. [17] observed in children (12.7 ± 1.74 years) frontonasal process widening values of 1.00 ± 0.54 mm. This observation suggests that the greater sutural widening in this young age group is due to more adaptable sutures. Therefore, the present study demonstrates that the increase in nasal width, which is less than 1 mm, has little clinical significance from an aesthetic perspective. However, this should be considered in patients who do not have OSAS with broad nasal widths, where SARPE with disjunction of the pterygomaxillary suture is more indicated [10].

Between the initial measurement (T1) and the follow-up measurement (T2), there was a significant increase in the distances between the median plates and the lateral plates of the pterygoid processes in both groups. The increase amounted to 0.87–1.35 mm for FCPC-MASPE-G and 2.04–3.04 mm for MARPE-G (Table 2). This indicates that even with a slower activation rate and limited force (FCPC-MASPE-G), deformations in the pterygoid processes still occur, albeit approximately 50–60% less than in the MARPE-G. These pterygoid processes deformation (3 out of 4 measurements) are statistically more effectively detected in the lateral pterygoid plate due to the measurements being taken over a greater caudal distance, as it involves angular deformation (Table 1) [8]. Clinically, this suggests that, with less deformation of the pterygoid processes during the process of maxillary disjunctions, cranial stress and tension are also reduced or better controlled, thereby decreasing the risk of unpredictable cranial complications [8, 9], although not eliminating them [20]. Performing slow disjunctions over months of activation allows for better cranial adaptation to mechanical expansion [11], leading to less deformation of the pterygoid processes. This is because the extended time frame gives cranial sutures a greater capacity to absorb tensile forces and compressions, functioning as areas of increased mechanical resilience [11]. From a clinical perspective, despite the differences observed in the deformations of the pterygoid processes between the two groups, it should be noted that SARPE is the treatment of choice, particularly in cases with high mechanical resistance and in mature adult age groups [10].

Hybrid skeletal expanders resulted in even greater levels of pterygoid process deformation due to the higher cantilever arm of the wire arms placed on the molar bands [28]. This finding is supported by another MARPE study conducted by Cantarella et al., which reported mean pterygoid process deformations of 1.35 mm on the right side and 2.17 mm on the left side of the pterygoid process with 80% statistical power [21].

All of these observations align with other finite element studies that have demonstrated deformations of approximately 2 mm per side in the pterygoid processes following rapid palatal expansion [8, 20, 21]. Given that the pterygoid processes are integral components of the unpaired sphenoid bone, this increase in distance leads to notable deformations and risk of cranial fractures. Formularende.

In this study, the analysis of suture widening within the FCPC-MASPE-G revealed a consistent and significant increase across all examined sutures, except for the pterygopalatine sutures (Table 2, paired t-test). At T2, even in both groups during slow and rapid activation, there was almost no widening of the pterygopalatine sutures, with minimal variability but lacking statistical significance (0.01 mm to 0.30 mm). This could be explained by the pyramidal part of the palatine bone, due to maxillary expansion, acting mechanically like a wedge-shaped structure that slides between the pterygoid process and maxillary tuberosity [9, 21], leading to a compression of the pterygopalatine sutures. This would elucidate why some pterygopalatine suture measurements were even zero millimeters [21]. Variations of the spatial orientation of the pterygoid process against the pyramidal process could explain the statistically significance in the comparison post-treatment groups (T2 FCPC-MASPE-G vs. T2 CG, independent samples Test) of the right pterygopalatine suture (p = 0.027) (Table 1), where significant widths increasing was observed.

It is well known that maxillary disjunction generates strains and stresses in the cranial structures, especially in the zygomatic arch [29], as it represents a buttress of great resistance against maxillary expansion with high tension exerted in the orbit of the unpaired sphenoid bone, orbital fissure, and the pterygoid processes [30]. Headache and diplopia due to increasing intracranial pressure (pseudotumor cerebri syndrome) have been reported [31]. Moreover, in both groups, even with rapid activation, the distance between both optic foramina and both pterygoid canals did not change between T1 and T2 (Table 2).

This is consistent with the unchanging diameter of the optic nerve sheath further supporting the safety of the MARPE technique [32].

But, according to Boryor’s study [33], the upper rim of the zygomatic bone is also subjected to higher compressive stress, where the zygomaticomaxillary and zygomaticotemporal sutures act as mechanically elastic cranial components. Exceeding their elastic limits, especially in mature adult skulls that are less resilient, leads to a decrease in mechanical absorption, and therefore the occurrence of negative consequences [33, 34].

Our findings indicate that at T1, the mean width of the zygomaticomaxillary and zygomaticotemporal sutures in the FCPC-MASPE-G ranged between 0.52 and 0.55 mm. At T2, this group exhibited a significant increase in width in both sutures, approximately one-tenth of a millimeter (p < 0.001 to 0.035, Table 2). In the MARPE-G group at T2, the widening of only the anterior zygomaticomaxillary sutures was significant (p = 0.02), contrary to the insignificant widening of the posterior sutures (Table 2). These results in the MARPE-G sutural behavior are consistent and similar to those found in the study by Ghoneima et al. [22]. Here a rapid maxillary expansion protocol applied to younger patients (mean age, 12.3 ± 1.9 years) showed a statistical absence of zygomaticotemporal suture widening. As no widening of the more posterior sutures (zygomaticotemporal) was observed during rapid activation, slow (and polycyclic) maxillary expansion, allowing for the development of metabolic activity, should be preferred, as observed in the FCPC-MASPE-G at T2. However, it should be noted that the statistical differences observed in the zygomaticotemporal sutures between T1 and T2 in both groups are very small, ranging positively from 0.05 to 0.13 mm (mean values), indicating an adaptive sutural response with minimal clinical relevance. But, due to the larger number of individuals in the FCPC-MASPE-G group, it is easier to detect statistically significant intragroup differences and may explain the absence of significance in the MARPE-G group. Additionally, this small difference between groups is not observed in the anterior zygomaticomaxillary sutures, which demonstrates statistical consistency following the “V” shape maxillary disjunction, despite the difference in the number of individuals between the groups.

However, in the post-treatment comparison of FCPC-MASPE-G and MARPE-G using the independent sample Test, both analyzed zygomatic sutures showed an absence of statistical significance (Table 1). This indicates that the small differences in widening could not be detected statistically or can be explained due to the metabolic activity of the sutures with additional bone apposition at their margins [35], particularly after the 3 months of treatment in the FCPC-MASPE-G. This lack of statistical significance can clearly be observed in Fig. 3a.

Considering that FCPC-MASPE-G enhances sutural response, the success rate of mid-palatal suture opening has been demonstrated to be 100% in adult patients under 30 years of age (n = 21) [11] and has been further supported by a study conducted by Ponna [12], with an average age of 24.1 years, achieving a 100% success rate (n = 17).

In February 2014, while still using the “2 activations/day” MARPE protocol [5, 6], we encountered a cranial complication in a 43-year-old female patient. The rapid widening resulted in an unexpected excessive unilateral opening of the frontomaxillary suture, approximately 6 mm, causing asymmetric widening of the nose and leading to an unaesthetic facial appearance (Fig. 5).

Detachment of the right frontal process of the maxilla during palatal expansion in an adult patient who did not adhere to the prescribed force limitation (1200cN at the activation key)

This experience has prompted us to limit the uncontrolled expansion force to 500 cN for activating the torque wrench and to change the rapid activation rate in adult skeletal expansions to a slow activation rate [11, 12]. The resulting expansive force level is consistent with other clinical studies in late adolescents, where 100 to 120 N have been clinically recorded as effective [36, 37]. Slow expansion and sutural response after midpalatal suture opening were investigated in an experimental animal study. Mature New Zealand rabbits underwent miniscrew-assisted slow maxillary widening with a light expansive force of only 100 g over 3 months, demonstrating that this was sufficient for the midpalatal sutures to open continuously. Consequently, the response in the sutures led to histologically proven formation of new bone after 2 weeks [35](Fig. 3a).

Moreover, the alternate opening- closing of sutures has also been tested in animal studies and proved to be more effective in generating anabolic stimuli with cell proliferation as a sutural response than continuous forces and therefore more suitable to weakening circummaxillary sutures [38].

In the context of circummaxillary sutural strategy, the incorporation of maxillary contraction/expansion activations on a weekly basis along with maxillary protraction strategies in class III treatments, even among late adolescents, has been demonstrated by Liou as Alt-RAMEC [39]. This approach allows for the advancement of the maxilla by 2.5 to 5 mm, indicating improved disarticulation of the pterygopalatine suture of the pterygoid process [39, 40].

In addition to the already implemented Force Control for adult maxillary expansion, the above findings led to a further improvement of the protocol by adding polycyclic closing-opening activations twice a day, resulting in the FCPC protocol.

Analyzing the sutural response between the FCPC-MASPE-G and the MARPE-G, it can be concluded that the FCPC protocol generates a superior bone and sutural response, and this difference is statistically significant in the paired T-test comparison (Table 2). The primary distinction between the two groups was the FCPC protocol characterized by its “slow,” “force-controlled,” and “discontinuous polycyclic” nature, as opposed to the “rapid,” “unlimited in force,” and “continuous” approach during maxillary opening.

This demonstrates that mini-screw-anchored slow palatal expansion (MASPE) in adult patients conducted over a period of 3–4 months is feasible, possible, and more recommendable [11, 12]. Therefore, weakening circummaxillary sutures over this period may reduce the risk of potential cranial base fractures or micro-fractures of the interdigitated osseous bone surfaces [8, 41, 42], particularly in adult patients with stiffer elastic properties [9, 33].

This study comprised two highly similar groups, both utilizing identical expansion devices with the same mini-screw configuration, very similar average ages, and solely successful outcomes. The only distinction lied in the activation protocol. By comparing expander opening to midpalatal suture opening (M.O.R.E.-factor), an efficiency ratio between the two groups was established to determine the expansion procedure with the fewest side effects.

The following formula has been used:

Mean Mid-palatal Opening: Mean Expander opening x 100 = performance %.

Ideally, if the jack screw of the expander were opened by 4 mm, the midpalatal suture should also open by exactly 4 mm. In such a scenario, the M.O.R.E.-factor would be 100%.

Three principal variables strongly influence the M.O.R.E.-factor: Firstly, the prevailing availability and quality of bone and the maturation of sutures, which correlate with age. Secondly the activation protocol, particularly whether it is rapid or slow (in addition eventually being polycyclic). And thirdly the variables of the expansion device, particularly in terms of rigidity and design of the expander [43], including its wire arms, length and diameter of the mini screws used and their stability in the bone [7, 44, 45].

While the FCPC-MASPE-G exhibited a M.O.R.E.-factor of 88.9%, the MARPE-G reached values of 50.2%. This difference suggests that limiting the expansion force and applying a slow and polycyclic protocol reduces the risk of expander complications and deformations during mid-palatal suture opening, with fewer implant displacements [46] and OMI bending [43, 47].Hybrid skeletal expanders (OMI:1.8 mm x 7 mm ) achieved 43,2%, which is half of the aforementioned value [19]. In contrast, tooth-anchored expanders exhibit a M.O.R.E.-factor of approximately 30%, which is only one third (!) of the value mentioned above [48].

This study demonstrates that using the novel, slow contraction-expansion FCPC-protocol for non-surgical skeletal expansion in mature patients is advisable and advantageous.

In summary, while conventional tooth-borne expanders may be inadequate for adult patients, innovative techniques such as MARPE and slow expansion protocols offer promising alternatives. With careful consideration of expansion protocols and advancements in imaging technology, orthodontic treatment can be tailored to optimize outcomes while minimizing risks for adult patients requiring maxillary expansion.

One limitation was the relatively small size of the MARPE group (n = 6) compared to the FCPC-MASPE group (n = 35). Since this was a consecutive retrospective study, the assignment of treatment between the two groups was not random, which may pose a potential risk of selection bias. However, the pretreatment comparison demonstrated no statistically significant or relevant differences between the two groups, indicating intergroup homogeneity. This justifies comparing the two groups, although the rapid group comprised fewer patients.

After the introduction of the polycyclic slow activation protocol more than 10 years ago, which led to improved outcomes and fewer side effects, it was deemed unethical to continue enlarging the rapid group. Additionally, the limited number of CBCT radiographs in the MARPE-G group is less accessible in a retrospective study spanning several years, where this type of pre- and post-radiographs was particularly infrequent. Another limitation was that for the MARPE group, T2 measurements was one month after the end of maxillary expansion, while for the FCPC-MASPE group, it was three months.