Dental arch changes after anterior open bite treatment in the mixed dentition produced by miniscrew-supported palatal crib vs conventional fixed palatal crib:A randomized clinical trial

Anterior open bite (AOB) can be defined as the absence of vertical overlap between the maxillary and mandibular anterior teeth when the posterior teeth are in occlusion.¹  It can be divided into two main categories: either dental or skeletal according to the etiology. Dental AOB can occur as a result of environmental causes such as tongue thrust or the tongue posture at rest, whereas the etiology of skeletal AOB is mainly related to genetic factors.² 

Several options have been proposed to treat a dental AOB, mainly aiming at the cessation of the habit such as by the use of spurs,^3–6  quad helix/crib appliance,⁷  or removable or fixed palatal crib.^4,6,8–10  A recent systematic review¹¹  concluded that crib therapy, regardless of the appliance design, was effective in the treatment of AOB. However, there are contradictory data as to whether crib therapy causes mesial movement of the maxillary first molar leading to a Class II relationship and, thus, worsening the malocclusion.^7–9 

Miniscrews have proven to be an effective means of anchorage,¹²  and a recent case report was published in which a miniscrew-supported palatal crib (MSPC) was able to correct AOB in a single patient.¹³  The reported advantage of such a technique was its independence of molar support, so it could be used in cases with bonded molar attachments, when molar movement was planned, or in patients with multiple missing teeth.

No randomized clinical trial has evaluated the effectiveness of the MSPC in the closure of AOB, nor whether a conventional fixed palatal crib (CFPC) produced mesial movement of the maxillary first molar. Therefore, this study aimed to test the effectiveness of both appliances as well as their anchorage load. The null hypothesis was that there would be no difference in the effects between MSPC and the CFPC for treatment of patients with AOB and a tongue-thrust habit in the mixed dentition stage.

The design of this randomized clinical trial was a parallel group, two-arm trial with a 1:1 allocation ratio. The trial was registered on the Pan African Clinical Trial Registry (PACTR201801002981142). No changes to the methods occurred after trial commencement.

Patients were recruited from the Outpatient Clinic at the Department of Orthodontics, Faculty of Dentistry, Cairo University from September 2018 to February 2020. The Research Ethics Committee of the Faculty of Dentistry, Cairo University approved this study. All patients were informed about the study procedures and signed informed consent. The patient eligibility criteria are shown in Table 1. At the start of treatment, a lateral cephalogram was obtained for each patient as well as upper and lower impressions. The impressions were poured, and the stone model was scanned using a 3Shape R500 laser scanner (3Shape A/S, Copenhagen, Denmark).

Sample size was calculated using G*Power (University of Düsseldorf, Düsseldorf, Germany) for the AOB correction outcome. A similar study was used as a reference,³  and the calculation indicated that, for a trial with a power of 80% and an α of 0.05, 10 participants were required per group. To account for patient loss to follow-up, a dropout rate of 25% was accounted for, and a sample size of 26 participants was selected.

Simple randomization was performed by writing numbers from 1 to 26 in the first column and using Kutools for Excel (Microsoft, Redmond, Wash) to randomly sort the numbers. The first 13 numbers were assigned to the MSPC group, whereas the other 13 numbers were assigned to the CFPC group. The randomization numbers were written on opaque white papers that were folded three times to form sealed envelopes and kept inside a box. After acquiring diagnostic records, allocation of each patient to either group was done by selecting one envelope from the box.

In the MSPC group, the insertion site of the miniscrew was planned virtually. The lateral cephalometric radiograph and the digital stereolithography (STL) file of the digital model were superimposed using DDS-Pro software (Uniontech Orthodontic Lab, Parma and Milan, Italy). A 3-dimensional image STL file of a Dentaurum miniscrew (Tomas, Dentaurum, Newtown, PA) was obtained and imported into the software library. The miniscrew was a mushroom-shaped head type and had the same dimensions as those to be placed (1.6 mm in diameter and 10 mm in length). The miniscrew position was determined paramedially on the digital model, and fine adjustments were set on the lateral cephalogram. The identified screw position was then marked on the stone model and sent to the laboratory for construction of the appliance.

A wax template was designed to indicate the vertical and transverse dimensions of the crib.¹⁴  The crib was planned to be located in the region between the two maxillary canines transversely and extending from the hard palate to 0.5–1 mm short of the floor of the mouth. The crib was fabricated from a 0.036-inch stainless steel wire along the custom-designed wax template, and it incorporated 5 to 7 loops depending on the intercanine width. An acrylic button (1.5 mm in thickness) was fabricated covering the base of the crib and two holes, 1.5 mm in diameter each, were drilled in the acrylic button at the planned site of miniscrew insertion.

In the CFPC group, a crib was fabricated using the same technique as the previous group. The crib was adapted and soldered to the palatal surface of the maxillary first molar bands. An acrylic button was fabricated covering only the base of the loops. The patients in both groups were trained to adapt to the new swallowing pattern. The patients were instructed to press their tongue against the acrylic part of the appliance, bring their teeth into centric occlusion, and close their lips to swallow. In each group, the appliance was removed 9 months after the start of treatment, and a posttreatment digital model was obtained.

The primary outcome was to assess the amount of AOB closure, whereas the secondary outcomes were to evaluate the amount of permanent maxillary first molar antero-posterior movement as well as other dental changes. AOB was considered corrected if the overbite was zero (end-to-end vertical incisor relationship) or had a positive value. The treatment effects were assessed using measurements obtained on the pretreatment and posttreatment digital models using DDS-Pro software by a single blinded assessor. All measured variables and their references are defined in Table 2 and Figure 1.

Five pretreatment and posttreatment digital models were analyzed again by the same external assessors to assess intra- and interobserver reliability using Cronbach's α reliability coefficient and intraclass correlation coefficients (ICCs). There were no outcome changes after commencement of the trial.

Blinding was not possible for the principal operator as well as the participants. Only the outcome assessor was blinded and performed measurements independently.

Statistical analysis was performed using SPSS (Version 20.0; IBM, Armonk, N.Y.). Data were explored for normality by checking the data distribution using the Kolmogorov-Smirnov and Shapiro-Wilk tests. All data showed a parametric distribution, and a paired t-test was used to study the changes after treatment within each group, whereas independent t-tests were used to compare pretreatment measurements and changes between the two groups. Qualitative data were presented as frequencies (n) and percentages, and the χ² test was used in comparisons.

Recruitment began in September 2018 and continued until February 2020. A total of 26 patients were recruited and randomly assigned in a 1:1 ratio to either the MSPC group (n = 13) or the CFPC group (n = 13). All treatment follow-ups were completed by June 2021. A Consolidated Standards of Reporting Trials (CONSORT) flow diagram shows the progression of the participants in the clinical trial (Figure 2).

One participant from each group dropped out as they did not come for follow-up visits because of the outbreak of the COVID-19 pandemic. Therefore, the data from 12 of 13 participants for each group were included in the analysis.

The baseline characteristics (Table 3) as well as pretreatment cephalometric variables (Table 4) were similar in the two groups at the start of treatment as there were no statistically significant differences between them.

After a 9-month follow-up period, all participants showed improvement in the amount of AOB closure. However, two patients in the MSPC group and three in the CFPC group still had some AOB. Figures 3 and 4 show photos of corrected AOBs in each group.

In the MSPC group, there was a statistically significant increase in the amount of overbite and overjet by 3.97 ± 1.44 mm and 0.96 ± 0.83 mm, respectively. The maxillary first molar moved mesially by 0.53 ± 0.32 mm; however, the molar relationship almost stayed the same. The maxillary and mandibular incisors were significantly extruded by 1.94 ± 1.66 mm and 1.06 ± 0.71 mm, respectively. Also, there was a significant increase in the clinical height of the maxillary and mandibular central incisors by 0.49 ± 0.63 mm and 0.76 ± 0.32 mm, respectively. Finally, there was a significant increase in Little's Irregularity Index by 1.84 ± 1.85 mm.

In the CFPC group, there was a statistically significant increase in the amount of overbite and overjet by 3.97 ± 0.89 mm and 1.07 ± 1.15 mm, respectively. The maxillary first molar moved mesially by a significant 1.42 ± 0.99 mm, leading to a significant change in the molar relationship by 0.59 ± 0.54 mm. The maxillary and mandibular incisors were significantly extruded by 2.00 ± 1.09 mm and 1.22 ± 1.16 mm, respectively. In addition, there was a significant increase in the clinical height of maxillary and mandibular central incisors by 0.65 ± 0.44 mm and 0.99 ± 0.66 mm, respectively. Finally, there was a significant increase in Little's Irregularity Index by 1.87 ± 2.75 mm.

There was no statistically significant difference between the two groups regarding all measurements except for the change in the antero-posterior position of the maxillary first molar and in the molar relationship. Tables 5 through 7 show the changes after treatment in each group and compared together.

A few harms were observed during the trial, although none led to major damage. Despite continuous instructions and training by the principal operator, it was noticed that a few patients in both groups could not adapt to the new pattern of swallowing. These patients would instead press their lower lip between the mandibular and maxillary anterior teeth to achieve an oral seal.

In addition, of a total of 24 miniscrews inserted, 6 failed. The appliance remained in place; however, the two miniscrews became loose. This was discovered during the follow-up visit in each of the three cases. The miniscrew was removed and reinserted after 2 to 3 weeks of healing. In the CFPC group, two of the solder joints broke and were sent back to the laboratory for repair.

Cronbach's α reliability coefficient and ICC were used for the analysis of method error, and the range of values was greater than 0.7, indicating very good inter- and intraobserver agreement.

Various habit-breaking appliances have been used, with a recent systematic review¹¹  concluding that the use of the palatal crib, regardless of the appliance design, was effective in treating AOB. However, there is contradictory data as to whether those appliances cause mesial movement of the maxillary first permanent molar.^7–9  A recent case report suggested a new design for the crib appliance: anchoring it to the palate using two miniscrews.¹³  However, no study has evaluated its treatment effects on a large sample of patients. Hence, the aim of this study was to evaluate the effectiveness of the MSPC and to assess whether the CFPC produced mesial movement of the maxillary first molars.

The null hypothesis of this trial was rejected. Although the amount of AOB closure was similar in both groups, there was a tendency for greater mesial movement of the maxillary first molar in the CFPC group. The intermittent forces produced during swallowing as well as the considerable continuous force produced by the tongue with its rest position being caged behind the loops were transferred to the maxillary first molars in the CFPC group. These combined forces caused mesial movement of the first molar that led to a statistically significant difference between the two groups regarding the change in molar relationship.

The amount of AOB closure was similar in both groups and was attributed to extrusion of the maxillary and mandibular incisors along with an increase in their clinical crown heights. The crib changed the posture of the tongue as well as prevented it from thrusting forward between the maxillary and mandibular anterior teeth. As a result, the only forces acting on these teeth were produced from the labial side by the action of the upper and lower lips. The amount of AOB closure in this trial was similar to that reported previously in the literature^4,10  and slightly higher than that reported in a recent systematic review (3.1 mm).¹¹  Whereas bonded spurs have been shown to result in an increase in the amount of overbite achieved by 3.09–4.38 mm.^4,5 

A few of the patients could not adapt to the new tongue position during swallowing and instead would press their lower lips between the maxillary and mandibular front teeth to form an oral seal. This led to increased lip pressure being applied to the mandibular incisors, resulting in a significant increase in Little's Irregularity Index and overjet in both groups. Thus, it can be recommended that a lip bumper be placed in conjunction with the palatal crib to remove such pressure.

The arch perimeter and length decreased similarly in both groups, with a greater decrease observed in the mandibular arch compared with the maxillary arch. This may have been attributed to the loss of Leeway space that occurred in most of the patients as a result of exfoliation of the primary molars and canines; the Leeway space is greater in the mandible.¹⁵ 

AOB was considered to be corrected only if the overbite was zero (end-to-end vertical incisor relationship) or had a positive value. Accordingly, the success rate was 83.3% in the MSPC group and 75% in the CFPC group. This may have been attributed to the slightly short follow-up period that, if extended, could have shown better progress. Another reason may have been that the amount of AOB to start with in those patients who did not achieve positive overbite was considerably large.

The failure rate of the paramedian miniscrews in this trial was 25%, which is considerably higher than that reported in the literature (5.5%).¹⁶  This could be attributed to the heavy intermittent forces produced by the tongue during swallowing as well as a continuous force at rest. Another factor to be considered was the participant's age; most of the studies were conducted on adolescents or adults, whereas in this trial the patients were still young, and the palatal bone might not have been dense enough.¹⁷ 

In summary, the results of this clinical trial revealed that both the CFPC and the MSPC appliances resulted in similar improvement in the amount of AOB closure that occurred as a result of the extrusion of the incisors. A few patients developed a lip-trap habit, which led to an increase in the amount of overjet as well as the amount of crowding in the mandibular anterior segment. Finally, the main difference between the two groups was that the maxillary first molar moved mesially more in the CFPC group compared with the MSPC group.

The addition of an untreated control group would have helped to differentiate between the treatment effects of both appliances and normal growth changes. However, this was not done because of the ethical concerns of leaving children without treatment despite their need for immediate intervention. Another limitation of the trial was that it included both sex groups and was not restricted to one specific sex type, with an unequal number of girls and boys.

The generalizability of the results of this clinical trial should be confined to children with AOBs who have similar age and dentoskeletal characteristics. The study was conducted on one race (Caucasians), was limited to only one dental center, and, finally, only one postgraduate student carried out the procedures.

• The MSPC is a viable treatment for the management of AOB attributed to a tongue-thrusting habit, with comparable results with the CFPC.

• Both appliances resulted in similar improvements in the amount of AOB closure, which was attributed to maxillary and mandibular incisor extrusion.

• Both appliances resulted in an increase in the amount of overjet as well as an increase in Little's Irregularity Index in the mandibular arch.

• There was statistically significant mesial movement of the maxillary molars in the CFPC group compared with the MSPC group; however, the clinical significance of this was minor.