Modified Arnold expander: an alternative for mandibular arch expansion

ABSTRACT Introduction: Due to the anatomical constraints of the mandible, mandibular dental arch usually serves as a guideline to determine the required changes in the maxillary transverse dimension. The Schwarz appliance and the Lip Bumper are the traditional orthodontic appliances for mandibular arch expansion in patients with borderline amounts of crowding, and/or transverse discrepancy. However, they often require patient cooperation, which may be a concern for orthodontists in daily practice. Objectives: This article illustrates a simple fixed orthodontic device as an alternative to achieve mandibular arch expansion in patients with moderate tooth-size/arch-length discrepancy. The four reported cases refer to 8 to 10-year-old patients in the mixed dentition, with an Angle Class I or Class II malocclusion, transverse deficiency in both arches, moderate crowding and/or posterior crossbite, combined with compromised smile aesthetics. The patients were treated with rapid maxillary expansion (RME) using Hass expander appliance and the modified Arnold expander (MAE). Conclusion: This low-cost compliance-free orthodontic appliance provided dentoalveolar decompensation by means of uprighting the posterior teeth, with minimal or no adjustments during treatment. The final results were achieved in only three to four months, and fulfilled all treatment objectives, such as an increase in the arch perimeter and width, and a better teeth alignment.


INTRODUCTION
The transverse dimension and shape of both dental arches varies widely between individuals, according to dental alignment, tooth shape and size, musculature, jaw size and shape, facial and cranial patterns and the dental occlusion. 1 The transverse discrepancy between the maxillary and mandibular arches is one of the most commonly seen malocclusions in the primary and mixed-dentition stages. 2 The prevalence of posterior crossbite is 14% in the primary dentition and 8% in the mixed dentition. 3 These patients may present narrow posterior transarch widths, related crowding, wide buccal corridors, and decreased anterior arch contour. 4 However, although the constriction of the jaw bones is frequently associated to posterior crossbite, this is not a mandatory condition, considering that the maxilla and mandible can be dentoskeletal compensated in order to maintain jaw relationships with function, 5,6 In other words, patients without posterior crossbites can have significant transverse discrepancies that might need treatment.

EFFECTS OF RAPID MAXILLARY EXPANSION (RME)
The transverse malocclusions do not self-correct without treatment, and the expansion of one or both arches is widely recommended, especially during the mixed-dentition period. 7,8 The ideal goal of RME is to achieve minimal dental and maximum orthopedic effect. 9 Different studies have reported that it affects the circummaxillary sutures, specifically the midpalatal Andrade Jr. I, Paschoal MAB, Figueiredo NC Modified Arnold expander: an alternative for mandibular arch expansion 5 one, compresses the periodontal ligament, bends the alveolar processes and induces a buccal tipping of the anchoring teeth, among other skeletal and dental effects. [10][11][12] The transverse expansion will result in varied intra-arch dimensional changes, in addition to potentially altering the occlusal relationships in the three planes of space. It has been shown that RME therapy can increase the maxillary arch perimeter by 0.7 mm for every millimeter of posterior expansion 7,12 . However, it is noteworthy that the amount of expansion created by a given RME protocol is variable and relies on the goals of the orthodontist. As an example, Haas recommends opening the expander to the full extent of the screw (10.0 to 10.5 mm), thereby maximizing the increase in arch width. 7, 9,10 Other study 13 demonstrated that patients who were treated with RME during the mixed-dentition phase followed by fixed appliances had a maxillary arch perim-

MANDIBULAR EXPANSION AND WALA RIDGE
In order to correct these transverse deficiencies and maximize the RME, the mandibular expansion can be a meaningful tool, particularly in cases of mild to moderate discrepancy between tooth size and arch lenght. 7 However, gaining space in the mandibular arch has been considered as a limiting factor, by anatomic reasons and due to the belief that the expansion is not stable. Housley et al. 14 demonstrated that an increase in mandibular arch width of 1.52mm in permanent canines, 2.11mm in first premolars, 2.12mm in second premolars, and 0.92mm in permanent first molars, carried out with an expanding lingual arch appliance, relapsed in 0.8mm, 0.72mm, 0.67mm and 0.15mm, respectively, after a mean postretention period of 6 years and 3 months (± 2 years and 4 months). Nevertheless, the mean pretreatment age in this study was 12 years and 5 months, and most patients were in permanent dentition.
Despite the noted relapse effect, particularly in the anterior arch region, it can be speculated that the transverse expansion performed in the deciduous or early mixed dentition may present a different behavior. Early widening of the dental arches might positively influence the subsequent growth and development of bone jaws, besides a favorable adaption of the muscular environment, which can alter the eruptive paths of the permanent teeth in a buccal direction. 15 Andrade Jr. I, Paschoal MAB, Figueiredo NC Modified Arnold expander: an alternative for mandibular arch expansion Furthermore, it has been reported that the mandibular arch form has a correlation to the shape of the underlying basal bone, which can potentially be used as a reliable diagnostic reference for determining the best position of the mandibular teeth, providing a more stable orthodontic treatment outcome. 1,16 With that purpose, Andrews and Andrews 17 proposed the WALA ridge as an anatomic reference on the mandibular alveolar process that demarcated the soft-tissue band immediately superior to the mucogingival junction, 18 which is located close to the same vertical level as the horizontal center of rotation of each tooth. 19 The WALA ridge is easy to identify and might be clinically useful for individualizing dental arch shape 20 (Fig 1). Typical treatment protocols for patients needing mandibular expansion are the removable mandibular Schwarz appliance 21 and the Lip Bumper. 22 The protocol may begin with maxillary expansion or mandibular decompensation. The Schwarz appliance, which is usually activated once a week for approximately 5 to 6 months, provides a dentoalveolar decompensation of the mandibular arch, establishing a "reference" arch width to which the maxillary arch can be expanded. 23 Later on, the Schwarz appliance should be worn full-time as a passive retainer until the maxillary expander is removed. The primary purpose of the Lip Bumper is to reduce dental arch crowding 24 through an increase in arch width and length, 25,26 by altering the equilibrium between lips, cheeks, and tongue. 27,28 However, as removable appliances, the expansion rate is slow, due to problems with retention and compliance, which might be an important clinical drawback. 29,30

MODIFIED ARNOLD EXPANDER (MAE)
An interesting device to overcome these issues is the Arnold expander, which became popular in the 1970s by Berkowitz 31 as a way to produce slow expansion of the maxillary or mandibular arches, especially in cleft-palate patients, as a non-compliance alternative solution for the correction of tooth size/arch length discrepancy. 32 However, its asymmetric expansion, difficulties of cleaning the exposed open coil, and common tongue injuries have discouraged the use of this appliance. Thus, in the present article, four cases treated in a private office will be presented, in which a modified Arnold expander (MAE) overcame these issues and promoted an increase in the transverse dimension of the mandibular arch in a quick and cheap way. This device has a split lingual frame, a 0.040-in stainless steel tube that was welded to the lingual side of the permanent first molar band and a 0.038-in stainless steel wire welded to the opposite molar band. In both sides, the structure runs lingual to the deciduous molars and canines, and turns at a 90 o angle at the midpoint of the canine. The two parts fit together, with the wire sliding through the tube at the midline, like a telescopic system (Fig 2).  The orthodontic treatment objectives for the first phase were to:     In order to achieve these objectives, a RME was promoted by means of a Hass palatal expander, followed by a MAE in the mandibular arch, in order to coordinate the transverse dimensions of the maxillary and mandibular arches (Fig 6). Furthermore, a cervical pull headgear was proposed to correct the sagittal discrepancy.
As instructed, the parents activated the screw by two turns per day during 10 days, when an excellent orthopedic response was verified (5-mm diastema between central incisors, with an increase of 4 mm in the intermolar width and 4.5 mm in the intercanine width). The MAE was kept in place for four months.
At the end of this period, a significant improvement in the arch perimeter (4 mm on the maxillary arch and 3 mm in the mandibular arch) and width was obtained, as can be observed in the post-treatment photographs (Figs 6G and 6H). After four months of treatment, there was plenty of space for the maxillary canines (Fig 7), the mandibular incisor trauma was eliminated, and the gingival recession was improved (Fig 6E).  correction of the crossbite (Fig 9). After five months of treatment, there was a significant improvement in smile aesthetics due to the increased maxillary incisors display and harmonic buccal corridors (Fig 10C). An Angle Class II molar relationship still persisted in both sides, but a significant improvement  (Figs 10D -10H).

DISCUSSION
The size and shape of the dental arches have an important effect on space available, stability of the dentition, and dental esthetics. 19 Although most orthodontic treatment for transversal discrepancies focuses on the maxilla, it is important to recognize that dental compensations exist for both dental arches. Therefore, the orthodontist must be able to diagnose differentially the cause of any transverse discrepancy, and the presence or absence of posterior crossbite should not be used as a major and unique guide. 33 The correction of crowding in cases with tooth-size/dental arch length discrepancy might be a key factor when deciding between extraction and nonextraction orthodontic treatment.
In order to achieve that, RME is often used during treatment, but mandibular arch widening has primarily been limited to uprighting of posterior teeth, since there is no midline suture, as in the maxilla. 32 been discouraged by some authors due to potentially relapse effect, 21,38 it has been showed that when a crowded mandibular arch is expanded before the eruption of the permanent teeth, the path of eruption of the mandibular permanent canines and premolars might be altered to an increased width. 15,39 Moreover, the greatest growth changes in the dentoalveolar area occur during the eruption of permanent teeth. 40 In this way, it seems reasonable to take advantage of the eruption dynamics to potentially improve the development of the dentoalveolar area.
In the first three reported cases, the patients were first submitted to RME, until the lingual cusps of the maxillary posterior teeth contacted the buccal cusps of the mandibular posterior teeth (Figs 11A and 11B). Later on, the MAE was installed and provided a buccolingual decompensation of the posterior teeth and a proper intercuspation (Fig 11C) . Once the ideal transversal relationship was achieved, and after the stabilization period, these appliances were replaced by a transpalatal arch (TPA) and a lingual arch in the maxillary and mandibular arches, respectively (Fig 11D) . Moreover, with intervention in the early mixed dentition, orthodontists can eliminate potential irregularities and facilitate dental eruption. In permanent dentition, the mandibular expansion is even more controversial. 14 30 produce gingival recession when expressed beyond the alveolar bone. 45 The key for maintaining attachment is to produce movement that results in tooth movement within the alveolar bone, thus preventing any dehiscences.
The WALA ridge can be used as a template for expansion of both arches, which will match the anterior and lateral borders of the WALA ridge. According to Andrews, 17 the shape will be uniquely correct for each patient, regardless of race or sex.
The WALA ridge may resolve orthodontics' long-lasting controversies regarding the anterior and lateral arch-border positions and archwire shapes, as well as whether to extract or expand, or both. It also solves the maxillary arch-border and the maxillary width controversies because a uniquely correct mandibular arch's lateral borders serve as the landmark for the lateral borders for both the upper dental arch and the maxilla. The use of MAE may be a good option in these cases. Regardless, the decision whether to expand or extract requires proper diagnosis, and must be made on a case-by-case basis. In the four cases reported in the present article, the treatment objectives were achieved, with excellent esthetic and functional results.
Last but not least, it has been shown that parents/caregivers have a determining and critical role in cooperation of their children. 46,47 Moreover, patient compliance may decline due to discomfort such as soft tissue irritation, tooth ache, lack of confidence in public, and speech and respiratory disorders.
In the first two cases of this article, the treatment begun with the Schwarz appliance or the Lip Bumper, however the lack of patient-parent cooperation was an issue in the beginning of the treatment. Thus, the MAE was an alternative to removable appliances to correct constricted mandibular arches by posterior teeth uprighting, to relieve crowding, to promote a proper posterior intercuspation and to improve the morphology of the mandibular dental arch without patient cooperation. Furthermore, a fixed mandibular expander offers some advantages, such as a minimal chair-time, no following adjustments, low financial costs, and a symmetrical expansion.

FINAL CONSIDERATIONS
Taking all together, the use of MAE may be clinically advantageous, since it can be effective in decompensating the mandibular posterior teeth buccally, which allows a greater amount of maxillary expansion that, in turn, can be favorable for increasing arch perimeter of both dental arches. Moreover, the MAE is a low-cost device that do not need adjustments (less chair-time) and patient compliance, and do not present complaints about oral hygiene and/or injuries. An adequate housing of the roots in the bony envelope during the mix dentition stage, and a correct intercuspation upon the end of treatment are important to maintain the final results.
Experimental studies are needed to evaluate the effectiveness of the presented protocol, as well as its long-term stability.