Objectives

To compare the buccal and palatal bone changes of maxillary posterior teeth produced by hybrid hyrax (HH) and conventional hyrax (CH) expanders in growing patients.

Material and Methods

A sample of 32 patients with posterior crossbites in the late mixed dentition was recruited and randomly allocated into two groups. Group HH was composed of 18 individuals with a mean age of 10.7 years (six female, 12 male) treated with a hybrid expander with two anterior parasagittal miniscrews. Group CH was composed of 14 individuals with a mean age of 11.4 years (six female, eight male) treated with a conventional Hyrax expander. Cone-beam computed tomography (CBCT) exams were obtained before expansion (T1) and after 11 months when the expander was removed (T2). Buccal and palatal bone plate thickness and height of maxillary posterior teeth were measured. Intergroup comparisons were performed using t or Mann-Whitney tests (P < .05).

Results

The CH group showed greater decreases of the buccal bone plate height (mean change: 1.27 mm) at the maxillary first premolars compared to the HH group (mean change: 0.11 mm, P = .001). No intergroup difference was found for changes in the buccal and palatal bone thickness.

Conclusions

Hybrid expanders showed a tendency to cause less negative impact on the buccal bone plate height of first premolars compared to conventional Hyrax expanders. However, the difference was not clinically significant. Both hybrid and conventional Hyrax expanders are safe for the alveolar bone morphology in the late mixed dentition.

Rapid maxillary expansion (RME) is an orthopedic procedure frequently performed during orthodontic treatment. RME is indicated to treat maxillary constriction in growing patients, improving the transverse interarch relationship.1  The conventional Hyrax expander is a tooth-borne appliance commonly used for RME.2  The Hyrax expander concentrates the forces on the anchoring teeth, causing orthopedic and dentoalveolar effects.2  Buccal tip of maxillary posterior teeth can cause negative periodontal effects including buccal bone dehiscence and a decrease in the buccal bone plate thickness.3,4  Anchorage teeth are more prone to develop gingival recession in the long term.4 

Previous studies compared RME dentoskeletal and periodontal effects between conventional Hyrax and Haas-type expanders using computed tomography images.5,6  Garib et al. demonstrated that both expanders produced similar orthopedic effects.5,6  However, conventional Hyrax expanders caused a greater reduction of the buccal alveolar bone plate height of supporting teeth.5  The maxillary first premolars were affected by more extensive buccal bone dehiscence than maxillary first molars.5  Brunetto et al. associated the Haas-type expanders to loss and reduction of height and thickness of maxillary first molar alveolar bone.7  In contrast, Rinaldi et al. demonstrated that the Hyrax expanders led to dehiscence, fenestration, and exposure of the root at the buccal aspect of maxillary first molars.8 

RME with skeletal anchorage was first described in 2008 with the aim of preventing periodontal side effects to posterior teeth.9  Wilmes et al.10  were the first to use orthodontic miniscrews as RME anchorage in growing patients. The hybrid Hyrax expander had two parasagittal miniscrews in the anterior region of the palate as an anchoring unit. In addition to achieving favorable expansion in the region of premolars and molars, these same teeth showed an increase in their buccal inclinations as an effect of the therapy.10 

A recent randomized clinical trial comparing hybrid and conventional Hyrax expanders in growing patients showed greater increases in the nasal cavity width, maxillary width, and buccal alveolar crest width for the hybrid Hyrax expander.11  A previous study compared the periodontal effects produced by conventional Hyrax and hybrid expanders in growing patients. The hybrid Hyrax expander did not cause changes in the buccal bone plate thickness of the maxillary first premolars.12  However, there was no standardization in the amount of screw activation performed between groups.12 

No previous study has evaluated changes in the bone plate height after expansion with the hybrid Hyrax appliance. There is an assumption that the hybrid expander could prevent buccal bone dehiscence on the anchorage teeth of adolescent patients. This information would better inform clinicians regarding the type of expander to be chosen for adolescents, especially for those patients exhibiting a thin periodontal biotype.

Objective

The aim of this study was to compare buccal and palatal bone changes of maxillary teeth produced by the Hybrid (HH) and Conventional Hyrax (CH) expanders in growing patients. The hypothesis was that both expanders would show similar impact on the alveolar bone morphology after treatment.

Trial Design and Settings

This was a secondary data analysis from a randomized clinical trial (RCT).13  The primary clinical trial was registered under the number NCT03712007 at Clinicaltrials.gov. The study was approved by the Ethics in Research Committee of Bauru Dental School, University of São Paulo, Brazil (CA: 48292721.9.0000.5417).

This study was carried out at the Orthodontic clinic of the Bauru Dental School, University of São Paulo. A sample of 32 patients aged between 9 and 13 years of age with maxillary constriction was recruited and randomly allocated into two study groups with a 1:1 ratio. The inclusion criteria were: (1) patients of both sexes; (2) late mixed or early permanent dentition; (3) Class I and III malocclusion; (4) need for RME either because of posterior crossbite or in association with orthopedic maxillary protraction. The exclusion criteria were: (1) patients with a history of previous orthodontic treatment; (2) patients with systemic or neurological conditions; (3) patients with cleft lip, cleft palate, and/or other craniofacial anomalies.

Interventions

The sample was allocated into two different groups. HH group was composed of 18 patients (12 male, six female) with a mean age of 10.7 years treated with a HH maxillary expander with two anterior miniscrews. CH group was composed of 14 patients (eight male, six female) with a mean age of 11.4 years treated with a CH maxillary expander (tooth-supported anchorage).

In HH group, a premanufactured hybrid expander (PecLab Ltda., Belo Horizonte, MG, Brazil) was anchored on bands cemented to the maxillary first permanent molars. Two miniscrews were placed in the anterior region of the palate at a parasutural position. The miniscrews of 1.8 mm diameter, 7 mm length, and 4 mm transmucosal length were installed in the slots after placement of the expander (Figure 1). In CH group, bands were placed on the maxillary first molars (Figure 1).

Figure 1.

(A) Hybrid Hyrax. (B) Conventional Hyrax.

Figure 1.

(A) Hybrid Hyrax. (B) Conventional Hyrax.

Close modal

In both groups, the expander screw was activated one-quarter turn twice a day for 14 days, achieving 5.6 mm expansion. At the end of the active expansion period, the expander was maintained as a retainer for 11 months until maxillary protraction therapy was performed for a previous study.13  None of the 18 participants in the HH group analyzed in this study demonstrated miniscrew instability. Patients and their parents received guidance on maintaining an appropriate level of oral hygiene throughout the treatment period. Perimplant chlorhexidine gel (2%) was prescribed twice a day during active treatment. One patient from HH group showed periimplantitis during the expansion retention phase and the use of chlorhexidine was increased.

Cone-beam computed tomography (CBCT) was performed with the i-CAT 3D system (Imaging Sciences International, Hatfield, Pa) before (T1) and after 11 months when the expander was removed (T2). The protocol of 120 kVp, 8 mA, 0.25-mm voxel size, scan time of 27 seconds and a field of view of 13 cm in height and 16 cm in depth was used. Patients were positioned in the device with a standardized head position, maintaining the Frankfurt plane parallel to the ground and the sagittal plane perpendicular to the ground.

Outcomes

The primary outcomes were evaluated in a previous study.13  The outcomes of this secondary data analysis from the RCT were the thickness and height of the buccal and palatal alveolar bone plates measured on CBCT scans.

Measurements were performed using NemoScan software by Nemotec (Nemostudio Nx Pro, version 8.8, 2 uv 8, Madrid, Spain). Before measurements were made, the head position was standardized in three planes of space using the multiplanar reconstruction screen. A bispinal line in the axial and sagittal planes was positioned perpendicular and parallel to the horizontal plane, respectively. In the coronal section, the infraorbital line was positioned parallel to the horizontal plane.14 

An axial section passing through the right maxillary first permanent molar trifurcation was used to evaluate the thickness of the buccal and palatal bone plates of maxillary posterior teeth (Figure 2A). Parasagittal sections passing through the center of the roots of posterior teeth in the axial section were used to measure the buccal and palatal bone plate heights of maxillary posterior teeth (Figure 2B). For the first molar, three parasagittal slices were generated passing through the center of the mesiobuccal, distobuccal, and palatal roots. In each parasagittal section, the height from buccal and palatal bone plates to the cusp tip of the corresponding side was measured.

Figure 2.

Variables measured on the CBCT images. (A) Buccal and palatal bone thickness were measured in an axial section passing through the trifurcation of the maxillary right first molar. BBPT indicates buccal bone plate thickness; PBPT, palatal bone plate thickness. (B) Buccal and palatal bone plate heights were measured in a cross-section passing through the center of maxillary premolars and the first molar. BBPH indicates buccal bone plate height; PBPH, palatal bone plate height.

Figure 2.

Variables measured on the CBCT images. (A) Buccal and palatal bone thickness were measured in an axial section passing through the trifurcation of the maxillary right first molar. BBPT indicates buccal bone plate thickness; PBPT, palatal bone plate thickness. (B) Buccal and palatal bone plate heights were measured in a cross-section passing through the center of maxillary premolars and the first molar. BBPH indicates buccal bone plate height; PBPH, palatal bone plate height.

Close modal

Sample Size Calculation

Sample size was calculated to provide a test power of 80% and a significance level of 0.05. For an intergroup difference of 1 mm and a standard deviation of 0.63 mm in first premolar buccal bone plate height,15  a minimum sample of eight patients was required for each group.

Randomization

Randomization was performed electronically using the randomizarion.com website. Random block sizes were used. The allocation was performed using sequentially numbered, opaque, and sealed envelopes. The randomization list generation, allocation concealment, and implementation process were performed by different operators.

Blinding

No blinding was accomplished considering operator and patient were aware of the type of treatment performed. The CBCT scans were de-identified during the assessment to ensure blinding during measurement and data analysis.

Statistical Analysis

After 1 month, 30% of the sample was randomly selected and remeasured by the same examiner. The reliability of repeated measures was assessed by the intraclass correlation coefficient (ICC) and Bland-Altman (B-A) limit of agreement.

Shapiro-Wilk test was used to verify normal distribution. Intergroup comparison of the interphase changes was performed using t or Mann-Whitney U-test, depending on the data normality. The analyses were performed using Jamovi software (The Jamovi project Version 2.3, 2022). A significance level of 5% was regarded.

The final sample of the HH group comprised 18 patients (six female and 12 male) with a mean initial age of 10.8 years (SD = 1.04). The mean treatment time for the HH group was 11.3 months. The sample of the CH group comprised 14 patients (six female and nine male) with a mean initial age of 11.4 years (SD = 3.98). The mean treatment time was 11 months. Group HH was composed of 5% of Class I patients and 95% of Class III patients. Group CH had 14% Class I patients and 86% Class III patients. According to a previous classification by Angelieri et al.,16  61% of patients in the HH group were at palatal suture maturational stage B, 22% were at maturational stage A, and 17% were at maturational stage C. In CH group, 72% were at palatal suture maturational stage B, 14% were at maturational stage A, and 14% were at maturational stage C. Group HH had 55% of patients in the early permanent dentition and 45% in the late mixed dentition. Group CH had 50% of patients in the early permanent dentition and 50% in the late mixed dentition. All patients in the mixed dentition had complete eruption of the maxillary first premolars. Measurements for second premolars that were not erupted or fully erupted (four patients in the HH group and three patients in the CH group) were not considered in the analysis.

Good to excellent reproducibility of repeated measurements was found for all variables, with ICC varying from 0.757 (second premolar bone plate palatal thickness) to 0.964 (first premolar bone plate buccal thickness). The variable with the greatest limits of agreement was the mesiobuccal bone plate height of first molars (−1.46 and 2.09). The variable with the smallest limits of agreement was the buccal bone plate height of first premolars (−0.44 and 0.34).

Similar characteristics regarding sex, initial age, and treatment time were found for both groups (Table 1). Both groups were compatible before treatment (T1) regarding all variables (Table 2). All patients in both groups demonstrated a midpalatal suture split and posterior crossbite correction.11 

Table 1.

Baseline Characteristics of Groups and Treatment Timesa

Baseline Characteristics of Groups and Treatment Timesa
Baseline Characteristics of Groups and Treatment Timesa
Table 2.

Intergroup Comparisons at T1 Before Expansion (t-Test and Mann-Whitney U-Test)a

Intergroup Comparisons at T1 Before Expansion (t-Test and Mann-Whitney U-Test)a
Intergroup Comparisons at T1 Before Expansion (t-Test and Mann-Whitney U-Test)a

The buccal bone plate thickness of posterior teeth decreased after expansion similarly in both groups (Table 3). The palatal bone plate thickness of posterior teeth slightly increased in most regions, with no intergroup differences.

Table 3.

Intergroup Treatment Change Comparisons (t-test and Mann-Whitney U-Test)

Intergroup Treatment Change Comparisons (t-test and Mann-Whitney U-Test)
Intergroup Treatment Change Comparisons (t-test and Mann-Whitney U-Test)

The conventional Hyrax expander produced a greater reduction in the buccal bone plate height of maxillary first premolars compared to hybrid expanders (Table 3). No intergroup difference was found for changes in the buccal bone plate height of maxillary second premolars and first molars (Table 3). The palatal bone plate height remained stable after expansion with no difference between groups (Table 3).

The use of CBCT allowed the visualization and quantification of buccal and lingual bone structures, including the alveolar bone crest level, with high sensitivity.15  The CBCT protocol was chosen based on the lowest possible radiation dose while offering sufficient sharpness for identification of the bone structures to be evaluated, following the ALARA (as low as reasonably achievable) principle. The assessment of the alveolar bone morphology through CBCT scans permitted the analysis of RME consequences on anchoring teeth. Previous studies recommended a smaller voxel size to evaluate delicate structures such as buccal bone plate.17,18  The voxel size in this study was 0.25 mm and the images had high definition and sharpness. When the alveolar bone thickness is thinner than the voxel size, the bone-height measurements are likely to be overestimated.19  For this reason, results should be considered with caution in regions with a very thin alveolar bone plate. Adequate reproducibility was found for all variables in agreement with a previous study that reported excellent intraexaminer reproducibility of the bone morphology assessment in CBCT.19  In this study, CBCT images were clinically used for planning miniscrew installation at T1 and planning comprehensive orthodontic treatment at T2.

The HH used in this research has a design modification compared to that described by Wilmes.10  Palatal extensions and buccal c-clasps were added to all appliances for both groups, making their design compatible and comparable. The palatal extensions were made so that, during the activation phase, the premolars could follow the expansion movement, simplifying the subsequent phase of the orthodontic treatment. The buccal c-clasps were added to increase the resistance of the appliance.

The hybrid expanders share the expansion load between posterior teeth and palatal miniscrews.11  Regarding the skeletal effects, previous prospective and retrospective studies have not found differences between hybrid and conventional expanders.12,20  On the other hand, two randomized clinical trials demonstrated a greater increase of the nasal cavity and maxillary width with hybrid expanders.11,21  Regarding the dental effects, previous studies reported less buccal tipping of first premolars with hybrid expanders.12,21  Considering that the HH expander might produce an increased skeletal effect and more limited dental side effects compared to conventional Hyrax expanders, it is logical to speculate that the impact on the buccal and palatal bone plates would be different between the two expanders.

HH and CH expanders produced a similar reduction in the buccal bone plate thickness of posterior teeth. These findings were in disagreement with previous studies, since a smaller decrease of the buccal bone thickness of maxillary first premolars with the HH expander was found.12,20  The possible explanation are differences in sample age. The current study was performed at an earlier age in the late mixed dentition while Toklu et al. and Schauseil et al. evaluated patients in the permanent dentition. Another difference was related to the amount of screw expansion that was 5.6 mm in the present study and 10 mm in a previous study.12  At the palatal aspect of posterior teeth, the bone plate slightly increased in the HH and CH groups. This increase was related to the buccal movement of posterior teeth considering an extension to first premolars was used in both expander designs (Figure 1). These findings were in agreement with a previous study evaluating hybrid expanders.12 

The impact on the buccal bone plate height of the first premolars significantly differed between groups. A greater decrease of the buccal bone plate height of the first premolars was observed in the CH compared to HH group. The error analysis showed that this measurement was precise and reliable, exhibiting the smallest limits of agreement. No previous study evaluated buccal bone dehiscence formation using hybrid expanders. Since the hybrid expander was anchored on anterior palatal miniscrews, the forces delivered to the first premolars were decreased. Consequently, there was decreased buccal tipping of maxillary first premolars and a smaller buccal bone height change occurred. However, the mean change in buccal bone height was small (1 mm) and not clinically relevant. In other words, both CH and HH expanders can be used in the late mixed dentition. At later ages of adolescence (15 to 18 years), the impact of use of a hybrid expander on buccal bone height of first premolars might be more relevant.

Future studies should evaluate the periodontal outcome of hybrid expander use in late adolescence to understand the ideal type of expander indicated for use during the later stages of the adolescent growth spurt, when the orthopedic effects of expansion appliances may become more limited.

  • Hybrid expanders showed a tendency to produce a lesser decrease of buccal alveolar bone height of first premolars compared to conventional expanders. However, the difference was not clinically significant.

  • Clinically, HH and CH expanders are safe, considering their effects on alveolar bone morphology in the late mixed dentition.

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.

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Author notes

a

PhD Student, Department of Orthodontics, Bauru Dental School, University of São Paulo, Bauru, Brazil.

b

Postdoctoral Fellow, Department of Orthodontics, Bauru Dental School, University of São Paulo, Bauru, Brazil.

c

Maxillofacial Surgeon, Hospital for Rehabilitation of Craniofacial Anomalies, University of São Paulo, Bauru, Brazil.

d

Professor, Department of Orthodontics, Bauru Dental School, Hospital for Rehabilitation of Craniofacial Anomalies, University of São Paulo, Bauru, Brazil.