Dental Arch Changes with Two Different Trans-Palatal Arch

1. INTRODUCTION

Rapid maxillary expansion (RME) is a dentofacial orthopedic treatment often used to treat patients with the constricted palate and poor transverse maxillary growth [1].

RME is used to open the mid-palatal suture and widen the roof of the mouth and the floor of the nose in patients before it is fully fused [2]. This treatment leads to correction of posterior dental crossbites, coordination of the maxillary and mandibular dental arches and gains in arch perimeter in patients with tooth size/arch size discrepancies [2]. However, the long-term stability of these skeletal and dental changes still remains controversial [3, 4]. Previous studies of Krebs [5] and Skieller [6] and the more recent of Akkaya et al. [7] reported that the rapid maxillary expansion appliances show stable results [8]. Conversely, according to Germane et al. [9], the outcomes of a RME treatment are not completely stable due to the increase in cheek or lip pressure in case of a small dental arch. This could potentially bring teeth back to the original positions [10]. Therefore, to stabilize the results obtained with RME and to limit or avoid the relapse, the use of Transpalatal Arch (TPA) is recommended [11]. According to McNamara, TPA is generally used to control the transverse dimension of the maxilla [11, 12]. The conventional TPA, as designed by Goshgarian, is a fixed appliance composed of a round stainless steel wire that goes across the palatal contour with a center U-loop. It is inserted into the lingual sheath of the first permanent molars bands [11-14]. According to recent studies, the use of TPA in mixed dentition followed by fixed appliances was effective in significantly increasing dental arch width and perimeter [10]. The changes remained stable after an average 6.7- years follow-up [10]. Moreover, a modified TPA with palatal arms extended up to canines may be considered as a valid retention tool after RME treatment. Thus, TPA stabilizes the palatal arch posteriorly and transversely and, when needed, helps preserve leeway spaces during the transition of the dentition [15]. Both auxiliary appliances, conventional and modified TPA, when placed passively, are used to maintain the position of maxillary molars in 3 dimensions, keeping the intercuspid widths that mostly tend to relapse [15]. To evaluate differences in dental arches after two different treatments, the adequate choice is to use digital three-dimensional (3D) dental casts [16]. Indeed, plaster models have the disadvantages, such as breakage, loss, and storage requirements [17, 18]. Therefore, the use of digital models makes the measurements more predictable compared with caliper measurements taken from plaster models. Indeed the latter ones have intra-examiner and inter-examiner measurements errors [19-24]. In literature, there is a deficiency of efficiency of two different TPA with the use of digital models. So, the purpose of this study was to evaluate short-term maxillary changes in patients treated with rapid maxillary expansion (RME) followed by TPA with and without palatal arms.

2. MATERIALS AND METHODS

This observational retrospective cohort study was performed in accordance with the Declaration of Helsinki and appropriate ethical approval was granted by the Health Research Ethics Board of University of Campania “Luigi Vanvitelli” (Naples, Italy; approval number 717/18). A signed informed consent from the subjects’ parents was obtained. The sample size was estimated based on preliminary data. A minimum sample of 28 subjects was needed in order to achieve 80% power, with an alpha of 5% to detect a 0.5mm difference (SD 0.5 mm). 30 patients, in particular, 16 females and 14 males, (mean age of 9.7 years ranging between 9 and 12) were collected from the archives of Orthodontic Program of the same university (Table 1).

Inclusion criteria of this retrospective study were:

• Mild to moderate maxillary crowding
• Mixed dentition with fully erupted upper permanent molars

Prepubertal skeletal maturation (CS1 or CS2)

• Treatment with RME followed by conventional TPA (TPAc) or TPA with arms extended to canine (TPAa)
• Presence of dental cast in the three treatment steps.

While Exclusion criteria were:

• Previous orthodontic treatment
• Previous tooth extraction
• Craniofacial anomalies

Recruited subject was divided into two groups based on TPA design.

The samples were treated by expansion with a palatal Hyrax-type expander with rapid maxillary expansion protocol and the operator started activation with two turns of the expansion screw (0.25 mm per turn). Then parents continued the activation two turns per day. Patients were monitored once every 2 weeks until maxillary palatal cusps touched buccal lower cups. Then the appliance was maintained in situ for at least 6 months to preserve the outcomes achieved.

After this period, the Hyrax device was removed and one of the two different types of TPA was used.

The two different TPA were made of 0.036-inch stainless steel wire with a loop directed mesially in the middle: the TPA maintained the original design, while the TPAa presented arms extended to canines (Fig. 1).

For each patient, gypsum cast models for each treatment step were collected: before (T0) and after expansion, including 6 months of retention with the same appliance (T1), and after using the two types of TPA (T2).

The dental cast models were scanned by 3Shape D700 and exported in STL surface mesh, then imported in Viewbox 4.0.1.7 software (dHAL Software, Kifissia, Greece, Athens).

Dental reference points (Table 2) were identified to carry out linear measurements as shown in Table 3 and Figs. (2-4).

3. RESULTS

Each group consisted of 15 patients. In TPA group, there were 11 females and 4 males with a mean age of 9.73 years (age range from 9 years to 11 years), while in TPAa group, there were 5 females and 10 males with a mean age of 9.63 years (age range from 9 years to 12 years).

4. DISCUSSION

The objective of the present retrospective study was to evaluate differences in patients treated with rapid maxillary expansion (RME) followed by TPA with and without palatal arms. Several authors, such as De Luca Canto G and Fleming PS, reported that a retention period of at least 5/6 months is necessary to permit adequate mineralization of the midpalatal suture in order to minimize the relapse tendency after rapid maxillary expansion^20-21 . For this reason, the records at T1 were undertaken after 6 months with RME.

After the 6 months with RME, TPA (conventional and modified) was applied to each patient to maintain but also to improve the transverse diameters during the transition period, before starting the fixed appliance.

In this study, it was important to evaluate the difference between ap (cervical evaluations) and the occ (occlusal evaluations) to understand the changes of the position of the tooth, influenced by its inclination, therefore to differentiate the real movement of the tooth from the tipping. The maintained arch length was useful to correct crowding. The difference in T2 of the arch length between TPA and TPAa was not significant, therefore even if we have had significant reductions in transverse diameters, clinically there would be no difference in the diameters.

In this study at T0-T1 there were no statistical differences at the beginning of treatment between the two groups. All patients, after RME activation, showed significantly increase of all the linear transverse measurements in both groups, so there was no difference.

After the treatment with RME (T1), each group had a relapse during the TPA phase (T2). This decrease in transverse diameters in this phase, in both groups, is attributable to relapse due to expansion with RME. It is therefore clear that everyone has had a significant decrease in all transverse diameters in the retention phase and that is why there lies a big difference between T1 and T2. Certainly, however, between the great relapse of the two groups, the TPAa group maintained more transversal diameters than the TPA group. This is because of the use of the arms, adhering to the palatal part of the teeth up to the canines, in the relapse phase, they have maintained, even if only in the form of torque, the transverse diameters. While, as regards length, there was no significant difference at T2 between the two groups. From the results, it is clear how effectively in both groups there has been an effective expansion of the upper maxillary arch. Results showed that the amount of treatment changes between T1-T0 means that the groups are homogeneous and comparable. The T2-T0 results showed an increase in all transverse diameters indicating that the treatment required for the patients was completed, with an expansion of the maxillary arch. Results showed that using modified TPA the transversal dimension was maintained better and showed less relapse. This means that after RME, the TPA with the arms maintains the inter-molar, inter premolar and inter-canine diameter above all the better than the conventional TPA.

CONCLUSION

- At the end phase of expansion, teeth showed a significant decrease after TPA treatment.

- The use of modified TPA with arms to canines showed higher stability than conventional TPA.

- Thus, results confirm the hypothesis that TPA with arms may allow maintaining dental transverse dimensions better. Statistically significant differences in dental measurements were found between TPA with arms and conventional TPA.

- The change in A-P length and arch length was not significant.

- TPA with arms up to the canines allows us to maintain and also to improve the results obtained during the expansion treatment.

REFERENCES