Early Treatment with a Slow Maxillary Ni–Ti Leaf Springs Expander

The aim of this brief report is to analyse the available literature on the clinical outcomes of a particular appliance for slow maxillary expansion that consists of one or more nickel–titanium springs. Materials and methods: The main medical databases (Scopus, Web of Sciences, Pubmed and Google Scholar) were scanned up to January 2020 using “slow maxillary expan*”, “slow palatal expan*”, “leaf expander” and “NiTi Palatal Expander” as keywords. Skeletal changes in the maxilla after expansion with the Leaf Expander (L.E.) or similar appliances were taken into consideration while reviewing relevant manuscripts. The review focuses on the comparison between the L.E. and conventional expanders (i.e., Haas and Hyrax) regarding the increase in both the distance between the palatal cusps of the upper first molars and the distance between the palatal cusps of the upper second deciduous molars, as well as the increment of nasal structures and pain connected to expansion procedures. Results: Bibliographic research retrieved 32 articles that were considered eligible for the present study. The limited number of articles currently available in international medical databases is allegedly partly due to the fact that these expanders are currently produced by only one patent holder company, which affects its diffusion. Conclusion: Despite the reduced number of published articles, due to the recent introduction of the L.E. device, most of the authors have found that the effects of the L.E. device are clinically and radiographically comparable to those achievable with the rapid palatal expander.


Introduction
Transverse maxillary hypoplasia is one of the most frequent problems in interceptive orthodontics and one of the most studied topics in orthodontics. This condition is frequently related to as unilateral or bilateral cross-bite and/or antero-superior crowding. Posterior cross-bite is a common malocclusion in children with deciduous or mixed dentition with a prevalence between 8% and 22% [1]. Frequently, transverse deficit is also associated with a lateral shift of the mandible and/or a space deficit involving superior permanent canines, raising the risk of impacted canines. Even if cross-bite occurs in 6-30% of the general population, spontaneous resolution is very low (0-9%) despite the elimination of oral habits and other etiological factors [2][3][4][5]. Maxillary expansion is considered the gold standard to treat this type of malocclusion. It can be achieved using different types of appliances depending on the clinical defects that have to be treated [6][7][8]. The possibility to achieve palatal expansion decreases with age, so this treatment is to be executed as soon as possible, or until the median palatal suture remains fibrous. The suture starts to obliterate at around 14-16 years old, and past 13-14 years in women and 15-16 years in men it is difficult to achieve orthopaedic expansion. If maxillary expansion is executed with proper timing, it results in a fast, predictable and minimally invasive treatment. There are several orthodontic appliances leading to a maxillary expansion, which differentiate 2 of 10 themselves for three main reasons: (a) patient's age (with deciduous, mixed, or permanent dentition); (b) type of applied force (light/orthodontic or heavy/orthopaedic); and (c) time of force applications (continuous/discontinuous). The main difference concerns the type of resulting expansion: Rapid Maxillary Expansion (RME) or Slow Maxillary Expansion (SME) [9][10][11]. RME is the gold standard for the treatment of posterior cross-bite because the opening of the median palatal suture and the maxillary transverse diameter is increased [12][13][14][15]. The main disadvantages of RME are discomfort and pain for the young patient and the need for compliance by parents and patients together for the activation of the appliance. In the literature, among early orthodontics treatments RME is considered one of the most painful therapies in terms of symptoms reported by patients (up to 98%) [16][17][18]. Pain could be related to rapid expansion protocol, because the expressed force could reach up to 10 lbs for each screw activation (0.2 or 0.25 mm) [19].
Since 1970, several authors have suggested that slow expansion is effective in opening the mid-palatal suture in growing patients, thereby reducing pain and discomfort. Some studies, including that of Lanteri et al. [20], showed that slow maxillary expansion through a nickel-titanium expander may also have orthopaedic effects in deciduous or mixed dentition. Studies to date that evaluate nickel-titanium maxillary expanders are few in number [21,22] and the most of them are focused "on the memory screw appliance" [23]. A nickel-titanium expander is able to exert a seamless, light and continuous force, thus producing maxillary expansion while maintaining tissue integrity [24,25] during midpalate suture remodelling [7,26,27].
The characteristics of the reactivable and pre-activated NiTi leaf spring expanders are as follows:

Leaf Expander
The L.E. is an orthodontic appliance first constructed in 2013 by the development of a previous appliance, E.L.A (Espansore Lento Ammortizzato) [20], at the Centre of Bioresearch Leone in Sesto Fiorentino, Florence, Italy. The L.E. has similar features to a conventional rapid palatal expander, but instead of the central screw, it has a double nickel-titanium leaf spring ( Figure 1) which returns to its original shape during deactivation, resulting in calibrated expansion of the upper arch [21]. is executed with proper timing, it results in a fast, predictable and minimally invasive treatment. There are several orthodontic appliances leading to a maxillary expansion, which differentiate themselves for three main reasons: (a) patient's age (with deciduous, mixed, or permanent dentition); (b) type of applied force (light/orthodontic or heavy/orthopaedic); and (c) time of force applications (continuous/discontinuous). The main difference concerns the type of resulting expansion: Rapid Maxillary Expansion (RME) or Slow Maxillary Expansion (SME) [9][10][11]. RME is the gold standard for the treatment of posterior cross-bite because the opening of the median palatal suture and the maxillary transverse diameter is increased [12][13][14][15]. The main disadvantages of RME are discomfort and pain for the young patient and the need for compliance by parents and patients together for the activation of the appliance. In the literature, among early orthodontics treatments RME is considered one of the most painful therapies in terms of symptoms reported by patients (up to 98%) [16][17][18]. Pain could be related to rapid expansion protocol, because the expressed force could reach up to 10 lbs for each screw activation (0.2 or 0.25 mm) [19].
Since 1970, several authors have suggested that slow expansion is effective in opening the mid-palatal suture in growing patients, thereby reducing pain and discomfort. Some studies, including that of Lanteri et al. [20], showed that slow maxillary expansion through a nickel-titanium expander may also have orthopaedic effects in deciduous or mixed dentition. Studies to date that evaluate nickel-titanium maxillary expanders are few in number [21,22] and the most of them are focused "on the memory screw appliance" [23]. A nickel-titanium expander is able to exert a seamless, light and continuous force, thus producing maxillary expansion while maintaining tissue integrity [24,25] during midpalate suture remodelling [7,26,27].
The characteristics of the reactivable and pre-activated NiTi leaf spring expanders are as follows:

Leaf Expander
The L.E. is an orthodontic appliance first constructed in 2013 by the development of a previous appliance, E.L.A (Espansore Lento Ammortizzato) [20], at the Centre of Bioresearch Leone in Sesto Fiorentino, Florence, Italy. The L.E. has similar features to a conventional rapid palatal expander, but instead of the central screw, it has a double nickel-titanium leaf spring ( Figure 1) which returns to its original shape during deactivation, resulting in calibrated expansion of the upper arch [21]. Over the years, different models of the L.E. have been introduced: all appliances lead to a predictable maxillary expansion, with low and constant force and predetermined for intensity, direction, and amount of correction.
The main difference concerns the two basic appliances: (a) Reactivable; (b) Preactivated. Over the years, different models of the L.E. have been introduced: all appliances lead to a predictable maxillary expansion, with low and constant force and predetermined for intensity, direction, and amount of correction.
The main difference concerns the two basic appliances: The reactivable L.E. has a metal structure of chrome-cobalt (Cr-Co) with a central screw whose activation generates the compression of two or more nickel-titanium leaf springs. The screw action does not act directly on cemented teeth, but generates the compression of a nickel-titanium leaf spring. Later, the screw deactivates the spring by recovering its original size, thus producing a balanced expansion. During early mixed dentition, it is preferable to lean on second deciduous molars (E) through two bands, but if needed, first permanent molars are also accepted. To optimise the expansion and stability of the appliance, it can also have two anterior extensions to the canines. Currently, four reactivable L.E. types are available, which differ in the maximum amount generated (in mm) and the entity of force generated (in gr) (1) 6 mm screw-450 gr; (2) 6 mm screw-900 gr; (3) 9 mm screw-450 gr; (4) 9 mm screw-900 gr.
According to the transverse discrepancy calculation on the models (in mm), a 6 mm screw is used when the deficit is less than 5 mm, while, when the discrepancy is greater, a 9 mm screw is used. Generally, in the case of unilateral cross-bite, 6 mm screws are indicated; in the case of bilateral cross-bite, 9 mm screws will be useful. Usually, in deciduous/mixed dentition 450 gr forces are used, while in permanent dentition 900 gr forces are used. A 6 mm screw has two leaf springs and allows for up to 30 activations; the 9 mm one is characterised by three leaf springs and can be activated up to 45 times. In both, each turn/activation produces a 0.1 mm expansion. The number of leaf springs determines the amount of the expansion while their thickness determines the entity of the force, which remains constant regardless of the degree of compression. Regarding appliance cementation, both on bands and on custom made metal structures, it is essential to know whether the teeth surface is undergoing caries prevention procedures or enamel demineralisation treatment. The application of fluoride varnishes before bonding could significantly reduce the SBS value (Shear Bond Strength). In these cases, it is recommended to use prophylactic agents such as CPP-ACP paste, ozone or nano-hydroxyapatite instead of fluoride varnishes [28][29][30][31].
The L.E. has some advantageous features compared with other palatal expander appliances: Reduced need for clinical checks; -Painless; -Controlled, light and graded force production; -No risks of overexpansion; -Easiness of activation; -Vestibular tooth inclination control.
Considering that each screw activation generates a 0.1 mm expansion, 10 activations will be required to obtain a 1 mm expansion. In a 6 mm appliance, the maximum number of activations will be 30; in a 9 mm type, it will be 45, generally divided into three sessions. The lateral arms of the screw should remain offset from the palatal mucosa, approximately 2.5 mm, to avoid the possibility of creating decubitus ulcers. Generally, active expansion takes four to six months. After that, L.E. must be maintained in situ for at least three more months. About one year after cementation, as recommended for other palatal expander types, the appliance can be removed without the need for additional retention ( . The superimposition of the models is made using the palatine rugae as a reference (C).

Leaf Self Expander
The Leaf Self Expander ® (L.S.E.) design is similar to that of an L.E.: the differences are the presence of three double pre-activated nickel-titanium springs and the absence of the reactivation screw [2] (Figure 3). The new technology of the L.S.E. respects the principle of producing light, constant forces of predetermined direction, without any intervention by the patient, parents or orthodontist. To date, four L.S.E. types are available: (1) 6 mm-450 gr; (2) 6 mm-900 gr; (3) 9 mm-450 gr; (4) 9 mm-900 gr.
The L.S.E. is made in a laboratory according to the amount of expansion prescribed by the orthodontist and requires no further modification or reactivation. Leaf springs, compressed during the production of the device, gradually deactivate, recovering their original shape and generating a constant force, predetermined by direction and amount of movement. The L.S.E. is generally supported by E deciduous teeth. The compact size, 11 mm × 12 mm × 4 mm, allows placement even in a very constricted palate. Active expansion requires approximately four to six months, after which the appliance must be maintained in place for at least three months, to stabilise the result.
The aim of this brief report is to analyse the available literature on the clinical outcomes of a particular appliance for slow maxillary expansion that consists of one or more nickel-titanium springs.

Research strategy
The literature review is based on the collection of articles from primary and secondary sources. The main medical databases (Scopus, Web of Sciences, PubMed and  A,B). The superimposition of the models is made using the palatine rugae as a reference (C).

Leaf Self Expander
The Leaf Self Expander ® (L.S.E.) design is similar to that of an L.E.: the differences are the presence of three double pre-activated nickel-titanium springs and the absence of the reactivation screw [2] (Figure 3).  A,B). The superimposition of the models is made using the palatine rugae as a reference (C).
The L.S.E. is made in a laboratory according to the amount of expansion prescribed by the orthodontist and requires no further modification or reactivation. Leaf springs, compressed during the production of the device, gradually deactivate, recovering their original shape and generating a constant force, predetermined by direction and amount of movement. The L.S.E. is generally supported by E deciduous teeth. The compact size, 11 mm × 12 mm × 4 mm, allows placement even in a very constricted palate. Active expansion requires approximately four to six months, after which the appliance must be maintained in place for at least three months, to stabilise the result.
The aim of this brief report is to analyse the available literature on the clinical outcomes of a particular appliance for slow maxillary expansion that consists of one or more nickel-titanium springs.
The L.S.E. is made in a laboratory according to the amount of expansion prescribed by the orthodontist and requires no further modification or reactivation. Leaf springs, compressed during the production of the device, gradually deactivate, recovering their original shape and generating a constant force, predetermined by direction and amount of movement. The L.S.E. is generally supported by E deciduous teeth. The compact size, 11 mm × 12 mm × 4 mm, allows placement even in a very constricted palate. Active expansion requires approximately four to six months, after which the appliance must be maintained in place for at least three months, to stabilise the result.
The aim of this brief report is to analyse the available literature on the clinical outcomes of a particular appliance for slow maxillary expansion that consists of one or more nickeltitanium springs.

Research strategy
The literature review is based on the collection of articles from primary and secondary sources. The main medical databases (Scopus, Web of Sciences, PubMed and Google Scholar) were scanned up to January 2020 using "slow maxillary expan*", "slow palatal expan*", "leaf expander" and "NiTi Palatal Expander" as keywords, and using Boolean operators "OR" and "AND" to combine the search terms. Inclusion and exclusion criteria were predetermined to filter the results.

Results
Bibliographic research retrieved 32 articles that were considered eligible for the present study. After an accurate revision, duplicates were removed, and only clinical studies (prospective, retrospective, case-control) were considered and analysed; seven articles were finally evaluated in the present research [11,21,[31][32][33][34][35]. Table 1 summarises the characteristics of the selected articles. The limited number of articles currently available in international medical databases such as Scopus, Embase and PubMed is allegedly due to the use of two types of nickel-titanium leaf spring expander, the L.E. and L.S.E., which to date are currently produced by only one patent holder company, which affects its diffusion.

Deciduous Upper Intermolar Width (E-E)
As reported by Cossellu and Lanteri V. [11,34], posterior cross-bite was solved in all the treated patients and causes particularly significant deciduous intermolar diameter (E+E) variations, similar to the screw excursion. All the interdental widths of the maxilla increased significantly: second primary molar +5.06 mm (SD 0.98 mm), first primary molar +5.41 mm (SD 1.81 mm), primary canine +5.22 mm (SD 1.91 mm).

Nasal Diameter Width
In the research conducted by Lanteri V. [33,34], after treatment with the slow nickeltitanium spring expander, in all the patients a significant greater increase in both nasal cavity width and maxillary width has been reported, with nasal width having increased by +1.79 mm on average. The results in the L.E. group were comparable to those obtained with RME and SME treatment with no significant differences.

Upper Intermolar Width (6-6)
Lanteri V. [11,34] also reports a significant increase in permanent upper intermolar width (+3.60 mm on average) in patients treated with slow nickel-titanium spring expander positioned on the E-E. Lanteri V. [11,21] therefore asserts that it cannot be just an orthodontic movement, but that morphological remodelling also occurs since the appliance is not bonded to the permanent teeth. Her study also demonstrates an increase in maxillary intercanine diameter almost equal to the screw excursion (+6.07 mm on average), demonstrating a "fan-shaped" maxillary expansion, similar to that occurring with a traditional expansion. Thus, RME and MME could be considered two efficient treatment options to improve transverse width and to obtain space in the upper and lower dental arches [36]. Furthermore, with the slow nickel-titanium spring expander, a self-expansion of the lower arch is obtained, consequent to the upper expansion. In the study of Lanteri V. [31], the intercanine diameter in the mandibular arch results increased by 0.77 mm on average.

Absence of Pain
The recent study by Cossellu G et al. [32] demonstrates that the L.E. activation protocol produces calibrated and painless expansion. In fact, this continuous and slow activation Appl. Sci. 2021, 11, 4541 6 of 10 significantly reduces the mechanical forces transmitted to the bone and sutural complex, thus decreasing the inflammatory reaction related to the palate expansion protocol. Cossellu et al. [32] demonstrated how pain reported during rapid maxillary expansion is influenced by clinical activation protocol and screw size. Patients treated with the slow nickel-titanium spring expander reported significantly lower pain levels in the first seven days of treatment than those treated with RPE.

Upper airways and periodontal effects
With regard to the effects of the NiTi leaf spring expander on the upper airways, the device demonstrated no statistically significant difference compared with RME in the increase in nasal cavity, nasopharynx and maxillary sinus volume [35]. Moreover, it appears that after L.E. therapy there was no difference at the level of alveolar buccal bone plate, showing an efficient and safe treatment in the correction of the maxillary transverse dimension during mixed dentition. Subjects that took ketoprofen lysine salt experienced lower levels of pain during the fourth, fifth and sixth days of screw activation showing to be more effective than paracetamol/acetaminophen, probably due to the anti-inflammatory properties of the analgesic drug. The use of the analgesic during the first three days seems to be even more effective reducing pain until the first day of activation with almost no pain for the whole activation period. The results of this research confirm the effectiveness of SME in treating maxillary hypoplasia in the mixed dentition. This treatment appeared effective in increasing pharyngeal airway and MSV in patients with maxillary hypoplasia. No statistically significant difference was noted when comparing its results to those obtained using a conventional Hyrax-RME.

Discussion
Considering the slow nickel-titanium spring expander, Lanteri V. [20] demonstrates that the effects after expansion are clinically comparable to those achievable with RME by both digital model overlaps and CBCT before and after expansion.
In the meta-analysis conducted by Zhou et al. [9], it is possible to conclude that SME could provide a more effective maxillary expansion than RME in the molar region. Similarly, the values obtained after rapid palatal expansion reported by Christie et al. [37] (deciduous upper intermolar width increase of 3.4 mm on average) are found to be lower in comparison to those of Lanteri V. et al. [20].
This reflects the study conducted by Garrett et al. [38] in which the intermolar width at the level of the second deciduous molars results in a screw expansion of 55% after treatment with RPE, which is therefore lower than that achieved with the slow nickeltitanium spring expander.
In a very similar vein, based on CT observations, Martina R. et al. [39] state that RME is no more effective than SME in posterior cross-bite correction. On the contrary, the review by Lagravère et al. [40] does not find particularly significant data or results concerning SME; in particular, they emphasise the limited bibliography available and the need to conduct further research on this subject.
In another study, Lanteri V et al. [34] compare dental and orthopaedic effects of the slow nickel-titanium spring expander with the effects of the rapid palatal expander, based on posteroanterior cephalometric studies at the beginning of the treatment (T1) and after nine months (T2). Different parameters were analysed among a group of 30 subjects divided into two groups: nasal, maxillary, mandibular and upper intermolar widths. The authors conclude that no statistically significant differences were found between the different groups, confirming the efficacy of SME in the correction of transverse maxillary deficit.
In addition, Lanteri and Beretta et al. [21] analysed a sample of 10 patients, treated with slow a nickel-titanium maxillary expander bonded on deciduous teeth (E). Based on a digital scan of the models at the beginning of the treatment (T1) and at the removal of the palatal expander (T2), they show the complete correction of posterior cross-bite in all patients in four months, with relevant increase in dento-alveolar transverse diameters and anterior mandibular arch (+1 mm).
In fact, many studies illustrate important changes in the transverse diameters of the maxilla at all ages, not only as an orthopaedic effect in younger patients when early intervention is performed, through light forces acting on still active sutures [40][41][42]. Indeed, the results of the slow nickel-titanium expander [35] are similar to those obtained with conventional rapid palatal expander, as we find in the study by Garib et al. [43]. In fact, the increase in nasal width would correspond to 1/3 of the screw opening. Christie et al. [43] also find rapid expansion after a 37% (3 mm) increase in screw activation.
Ugolini et al. [33] underline that the main limitation of their study is that the skeletal effects of the slow nickel-titanium spring expander on the palatine suture have not yet been understood; in fact, they point out the need for further research.
Manzella et al. [44] in their recent study found a suture reaction after treatment with the slow nickel-titanium spring expander. Despite the reduced number of published articles due to the recent introduction of the nickel-titanium slow spring expander, most authors found that the effects of the L.E. are clinically and radiographically comparable to those achieved with the classic rapid palatal expander. As we saw, some studies showed significant changes in maxillary transverse diameters in all age ranges, as well as orthopaedic effects in younger subjects when early treatment was performed on still-active sutures. Thus, clinical results have demonstrated the effectiveness, efficiency and ease of use of these types of devices; therefore, these devices represent a valuable alternative for maxillary transverse deficit treatment in deciduous/mixed dentition. In this context, we expect a significant contribution from the improvement of non-invasive diagnostic tools, deriving from the most advanced technologies [7,45].

Conclusions
In summary, the main advantages of the maxillary expander with nickel-titanium leaf springs are the following: Conversely, the main disadvantages of the rapid palatal expander identified in the literature review are greater pain perception, especially at the beginning of the treatment; the need for compliance by parents and/or young patients; and the need for multiple close clinical checks. Further multicentre studies on larger samples are needed to confirm the promising preliminary data collected so far on more solid scientific bases. Compared to rapid maxillary expansion protocols, the slow expander with nickel-titanium leaf springs can be a pain-free alternative proposal, simplifying clinical procedures and reducing the number of activations and the need for clinical checks. These characteristics are expressed in an increase in the predictability of the results and a decrease in operator-dependent error.