Mandibular Behavior after Rapid Maxillary Expansion: A Clinical Study

1. INTRODUCTION

Rapid Maxillary Expansion (RME) is a routinely used orthopedic treatment for many clinical conditions [1-3]. The earliest published paper was written by E.C. Angell (the father of rapid maxillary expansion) [4]. After the reintroduction of this procedure by Haas, RME has been commonly used in the field of orthodontics [5, 6]. Several appliances have been introduced with new modifications in the expander or the appliance design. The bonded acrylic splint expander, Transverse Sagittal Maxillary Expander (TSME) [7], Ragno expander [8], the memory screw [9, 10], and the differential maxillary expander [11], have new mechanisms of action. Some concerns have been raised regarding the oral hygiene and food control in patients wearing orthodontic appliances [12], especially in patients with restorations [13, 14] or those with incorrect behavior [15]. However, some types of expanders are considered more hygienic than others [16].

Many studies investigated different aspects of the RME procedure. Some studies were histological [17]. Others investigated the relapse [18], stability [19] and the side effects related to RME [20].

Increasing the Vertical Dimension (VD) after the maxillary expansion is a primary consideration in treatment planning [21]. Some authors did not find any statistically significant increase [22]. Others claimed that there was a statistically significant increase without any clinical importance [21]. Lineberger stated that the VD increasing had no prejudicial effects on the vertical skeletal relationships. According to long term studies, it was a transient effect [23].

A maxillary displacement after RME has been reported [24]. Mandibular displacement is more controversial, keeping in mind that the mandible is the dynamic part of the dentofacial structures. Some authors reported an anterior mandibular displacement, which improved class II malocclusion, either through dental casts studies [25] or by cephalometric evaluation [20]. The “foot and shoe” theory and the “spontaneous correction of class II malocclusion” phenomenon were introduced [26]. In fact, these had been pointed in the German literature since the early 1900s by korbitz [26, 27]. However, the mandibular backward rotation induced by the maxillary expansion has been described [28].

Despite the large number of clinical studies that investigated the effects of RME, there is a lack of information regarding patients treated by RME as the only treatment intervention. The aim of this study was to investigate the mandibular sagittal and vertical changes subsequent to rapid maxillary expansion.

2. MATERIALS AND METHODS

The lateral cephalograms used in this study were obtained from a previous thesis and no patient was treated, especially for this article. The sample consisted of twenty-eight patients (16 males and 12 females) with indications for transverse expansion aged 4-14 years [9 ± 1.12], and without needing any mandibular intervention during the activation and retention periods. To determine the sample size, G*Power 3.1 (Heinrich-Heine-Universitat, Dusseldorf, Germany) program was used based on the results of a similar previous study [29]. Including a control group was unfeasible due to ethical reasons. It is unethical not to intervene when a diagnosed transverse discrepancy is present [30].

The inclusion criteria were:

• Maxillary constriction.
• Posterior crossbites.
• Class III face mask therapy [phase one].
• Functional therapy of class II [phase one].

Exclusion criteria were:

• Previous orthodontic treatment.
• Syndromes or congenital disorders.
• Class II division two.
• Asymmetries in another plane.
• Need for mandibular orthodontic intervention.

This study was conducted according to the World Medical Association (WMA) guidelines (Fortalenza, Brazil, October 2013). Patient health and safety were the primary considerations [31]. Informed consents were performed. Lateral cephalometric radiographs (Cranex^® Excel Ceph, Soredex, Tuusula, Finland) were taken before treatment (T1). The appliance design was similar to McNamara design (bonded rapid maxillary expander with occlusal acrylic coverage), as shown in Fig. (1). Hyrax® expander (Dentaurum, Pforzheim, Germany) was used. The expander was activated one turn per day until the palatal maxillary cusps reach the buccal mandibular cusps. Overcorrection of 2-3 millimeters was gained. The overcorrection was visually estimated. The appliance was left in place during the retention phase. The average retention period ranged from 164 to 192 days (178 ± 7). After the retention phase, lateral cephalograms were taken (T2).

3. RESULTS

The intraoperator error was limited (average correlation results: r=0.98). As an example, Fig. (4) shows all the repeated measurements of the variable SN. MP is between the upper and lower limit of agreement, which validates the tracing method. The null hypothesis was rejected for three vertical parameters and one sagittal parameter. Descriptive statistics and treatment changes for the cephalometric variables derived before and after orthodontic treatment are shown in Table 2

Table 2.

-		Pretreatment				Posttreatment				-
Vertical	unit	MEANS	SD	MIN	MAX	MEANS	SD	MIN	MAX	DF	P VALUE
SN-MP	deg	38.85	5.09	28.3	44.5	39.51	3.96	32.2	44.8	0.66	*
MMA	deg	29.79	6.33	20.8	39.2	30.69	6.28	21.1	37.8	0.9	**
LAFH/TAFH	%	55.45	1.59	52.3	58	55.58	1.77	52.1	58.2	0.13	NS
S.AR/AR.GO	%	83.46	7.90	73.8	98.3	87.23	13.49	71.7	113	3.77	*
Sagittal	-
SNB	deg	75.33	4.05	70.8	85	75.27	3.20	71.2	81.3	- 0.06	NS
S.N.POG	deg	75.69	4.18	70.7	85.8	75.63	3.26	71.9	81.9	- 0.06	NS
N.A.POG	deg	3.24	6.33	-7.7	13.8	4.4	5.65	-6.7	11.8	1.16	*

SD: standard deviation. N: number. MIN: minimum MAX: maximum. DF: difference. * P value < 0.05, ** P value < 0.01, NS: not significant.

4. DISCUSSION

The aim of the present study was to investigate the mandibular changes in the sagittal and vertical planes after RME as the only treatment intervention. RME was performed using the bonded acrylic splint expander.

One of the limitations of this study was the absence of a control group. The maxillary transverse deficiency is not a self-corrected malocclusion. It is not ethical to leave patients with maxillary deficiency without intervention. Pandis also considered the use of historical control groups problematic [32]. Another limitation was the unbalanced population (16 males versus 12 females), caused by the additional criterion used in the present study (the absence of necessity for mandibular intervention). This criterion decreased the number of patients available for the present study. It was challenging to find patients who need only maxillary orthodontic intervention during the first six months of the treatment period. However, the unbalanced population would not affect the biomechanical behavior of the mandible after maxillary expansion because it is not fully controlled by hormones. For the same reason, it was unfeasible to divide the patients into groups according to each type of malocclusion, or to increase the sample size.

Schudy emphasized that increasing the VD may exacerbate the sagittal discrepancy [33]. Almost every orthodontic malrelationship has a vertical, sagittal and transverse component [21]. The present study results showed a mandibular clockwise rotation and a bite opening effect after RME. As MMA increased significantly (p < 0,05), it could be concluded that the clockwise rotation resulted from the dentoalveolar changes rather than the maxillary vertical displacement, which might affect the ratio of LAFH/TAFH. The LAFH/TAFH showed no significant changes after RME (p > 0,05). The dentoalveolar changes could be like a flexion of the alveolar ridges [34], in addition to the tipping and extrusion of the anchor teeth [35, 36]. These changes could be compensated in the next stage if the patient had to go through a fixed appliance phase of treatment. This could be accomplished through the modification of the maxillary posterior teeth torque. However, the present study results were favorable because if the maxillary vertical displacement was responsible for the clockwise rotation, it should be controlled through extraoral appliances (like vertical pull chin cap), as suggested earlier [37]. Furthermore, this could be the reason for controversy between short term and long term clinical studies. The short term studies recorded VD increases after RME [24]. The long term studies suggested that the VD increase was transient and had no long term deleterious effect [23]. The long term study, because of duration, would include a second phase of fixed appliances therapy with many variables to control; like the torque and prescription of posterior teeth brackets. The short term study may include a phase of fixed appliances therapy and may not.

The significant increase in S.AR/AR.GO (p < 0,05) was caused either by S.AR increase or by AR.GO decrease. AR.GO decrease could be a result of condylar repositioning into a more upper or centric position. This may be produced by the elimination of the deviating contacts after expansion. The increase in S.AR could be induced if the expansion affected the spheno-occipital synchondrosis as suggested by some authors [38]. However, the lateral cephalometric radiographs are insufficient to evaluate the RME effect on the mandibular condyle position in its fossa. Further investigations are needed using other methods like Magnetic Resonance Imaging (MRI) and Cone Beam Computed Tomography (CBCT).

There was no mandibular displacement in the sagittal plane. However, there was a clockwise rotation which could camouflage any forward repositioning. The angle of convexity increased a mean of 1.16 degree (p < 0,05), which could be resulted from the mandible clockwise rotation or the advancement of point A. Haas reported a forward displacement of point A after RME [6]. However, this convexity increase (1.16 degrees) in the growing patients could be compensated by the continuous growth of the mandible. Pancherz reported a late skeletal growth after the age of twenty [39].

CONCLUSION

Within the limitation of this study, the results indicate:

• There was a clockwise rotation of the mandible and a bite opening effect after rapid maxillary expansion.
• The skeletal convexity increased after RME.
• Mandibular clockwise rotation and increased skeletal convexity are not contraindications for rapid maxillary expansion.
• RME may affect the condyle position and the posterior cranial base.

REFERENCES