Dentoskeletal comparison of miniscrew-anchored maxillary protraction with hybrid and conventional hyrax expanders: A randomized clinical trial


This randomized clinical trial aimed to compare the skeletal and dental effects of miniscrew-anchored maxillary protraction (MAMP) using hybrid hyrax (HH) and conventional hyrax (CH) expanders in growing patients with Class III malocclusion.


This was a randomized, parallel, controlled trial. Forty growing patients with Class III malocclusion and maxillary deficiency (Wits appraisal of less than −1 mm) were randomized into 2 groups. Patients were recruited at the Orthodontic Clinic of Bauru Dental School, University of São Paulo, Brazil. The HH group was composed of patients with Class III malocclusions in the late mixed or early permanent dentition treated with a HH expander with 2 miniscrews in the maxilla and 2 miniscrews in the anterior region of the mandible. Class III elastics were used from the maxillary first molars to the mandibular miniscrews placed between permanent canines and first premolars. The CH group was composed of patients treated with a similar protocol except for the use of a CH expander in the maxilla. The primary outcomes included the frequency of overjet correction and sagittal skeletal effects produced with treatment. Allocation was performed with a simple randomization process. Blinding was performed only during assessments. Data were analyzed blindly on an intention-to-treat basis. Intergroup comparison was performed using analysis of covariance. Mean differences (MD) and 95% confidence interval (CI) were obtained for all variables.


The final sample for the HH group was 20 subjects (8 female, 12 male; initial age of 10.7 years), whereas the final sample for the CH group was 15 subjects (6 female, 9 male; initial age of 11.5 years). The frequency of overjet correction observed in the HH and CH groups was 94.4% and 71.4% (risk ratio, 1.32; 95% CI, 0.93-1.88), respectively. Both groups presented similar skeletal sagittal and vertical outcomes after maxillary protraction. The maxillary length (CoA) showed a similar increase in both groups (MD, 1.12 mm; 95% CI, −0.03 to 2.27). The CH group demonstrated a greater mesial displacement of maxillary first molars after treatment than the HH group (MD, 1.22 mm; 95% CI, 0.33-2.11). HH and CH groups produced 2.88 and 1.97 overjet corrections (MD, 0.53 mm; 95% CI, −0.52 to 1.59), respectively.


MAMP using HH and CH expanders produced a frequency of overjet correction of 94.4% and 71.4%, respectively. Similar skeletal effects were observed between MAMP using HH and CH expanders. Greater control of the mesial displacement of maxillary first molar during maxillary protraction using hybrid expanders was observed.


The trial was registered at , under the identifier NCT03712007.


This trial protocol was not published.


This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brasil (CAPES) – Finance Code 001, and by the São Paulo Research Foundation (FAPESP) – Grants nos. 2017/04141-9, 2017/24115-2, and 2019/03175-2.


  • Two miniscrew-anchored maxillary protraction protocols were compared.

  • The hybrid group used a miniscrew-assisted rapid palatal expander.

  • The comparison group used a conventional hyrax expander with elastics.

  • Overjet correction was 94.4% and 71.4% in hybrid and hyrax groups, respectively.

  • Similar skeletal effects were observed in both groups.

Innovations in Class III malocclusion orthopedic interventions were remarkable in the last decade. Facemask therapy constitutes the standard protocol to treat growing Class III malocclusion with maxillary deficiency. Maxillary protraction with facemask therapy produces a combination of skeletal and dental effects. A forward and downward movement of the maxilla, extrusion of the posterior maxillary teeth, and a counterclockwise rotation of the palatal plane can be observed after facemask therapy. As a consequence, the mandible rotates down and backward, increasing the lower anterior facial height. Baccetti et al compared the outcomes of facemask therapy in 2 age groups of growing Class III malocclusion. The early treatment group (mean age, 6.9 years) showed the significant forward movement of the maxillary structures when compared with an untreated Class III malocclusion sample. Conversely, the late treatment group (mean age, 10.3 years) did not produce significant changes in maxillary position after treatment compared to the untreated sample. These findings are in concordance with previous studies that reported more favorable effects of facemask therapy in younger age groups. , ,

A new protocol for treating growing Class III malocclusion using skeletal anchorage was described by De Clerck et al. Bone-anchored maxillary protraction (BAMP) uses Class III elastics attached to titanium miniplates on the infrazygomatic maxillary crests and between the mandibular canines and lateral incisors, bilaterally. BAMP produced favorable skeletal effects in late treatment groups (mean age, 11.10 years). A mean maxillary advancement of 3.5 mm with minimal undesirable dentoalveolar effects were found using BAMP therapy. ,

BAMP-derived therapies were later described for patients with Class III malocclusion in the late mixed or early permanent dentition. Wilmes et al used a hybrid hyrax (HH) expander as anchorage in the maxilla and modified miniplates in the mandible to anchor Class III elastics in young patients (mean age, 10.6 years). Recently, Miranda et al described a miniscrew-anchored maxillary protraction (MAMP) protocol using a HH expander in the maxilla and 2 miniscrews in the mandible. MAMP produced maxillary protraction with an adequate overjet correction.

Specific objectives and hypotheses

This study aimed to compare the dentoskeletal effects of MAMP using HH and conventional hyrax (CH) expanders. The null hypothesis was that maxillary protraction with HH and CH expanders present similar orthopedic and orthodontic changes.


Trial design and any changes after trial commencement

This study is a single-center randomized clinical trial with 2 parallel arms and a 1:1 allocation ratio. Changes in participant numbers were performed after trial commencement and were described in the flow chart ( Fig 1 ). This clinical trial was registered under the number NCT03712007 at

Fig 1
Consolidated Standards of Reporting Trials flow chart.

The study followed the Consolidated Standards of Reporting Trials guidelines. The study was approved by the Ethics in Research Committee of Bauru Dental School, University of São Paulo, Brazil (protocol number 67610717.7.0000.5417). All participants and parents signed the written informed consent before treatment.

Participants, eligibility criteria, and settings

The patients were recruited in the Orthodontic Clinic, Bauru Dental School, University of São Paulo, Brazil, from July of 2017 to March of 2018. The sample consisted of 40 patients with Class III malocclusion aged from 9 to 13 years. The eligibility criteria included: (1) both sexes; (2) late mixed or early permanent dentition; (3) skeletal Class III malocclusion with maxillary deficiency (Wits appraisal of less than −1 mm); (4) anterior crossbite or incisor edge-to-edge relationship. Exclusion criteria included patients with a history of previous orthodontic treatment, nonerupted mandibular permanent canines, special needs, or syndromic patients.


The HH group was composed of growing patients with Class III malocclusion treated with MAMP anchored in a hybrid expander ( Fig 2 ). The therapy consisted of a HH in the maxilla and 2 mandibular miniscrews positioned distally to the permanent canines, bilaterally ( Fig 2 ). A premanufactured hybrid expander (PecLab Ltda., Belo Horizonte, MG, Brazil) was supported by bands in the maxillary first permanent molars and 2 miniscrew placed in the anterior region of the palate in a parasutural position ( Fig 2 , A ). The miniscrews with 1.8 mm diameter, 7 mm length, and 4 mm transmucosal length were installed in the screw slots after placement of the expander ( Fig 2 , A ). In the mandible, 2 miniscrews with 1.6 mm diameter, 6 mm length, and 1 mm transmucosal length were placed on the buccal aspect between the permanent canines and first premolars at the level of the mucogingival junction ( Fig 2 , B ).

Fig 2
MAMP using the HH expander. A , Hybrid expander placed in the maxillla with two miniscrews. B , Class III elastics were used from the maxillary first molar hooks to the mandibular miniscrews.

The CH group was composed of growing patients with Class III malocclusion treated with a similar protocol to the HH group except for the use of CH expander as anchorage for Class III elastics in the maxilla ( Fig 3 ).

Fig 3
MAMP using the CH expander. A , Conventional hyrax expander placed in the maxillla. B , Class III elastics were used from the maxillary first molars hook to the mandibular miniscrews.

The screw activation protocol and Class III elastics were similar for both groups. The expander screw was activated 1/4 turn twice a day for 14 days, achieving 5.6 mm expansion. In the maxillary molar bands, distal hooks of 1 mm-round–stainless steel wire were soldered to accommodate the Class III elastics and provide a more horizontal force. The Class III elastics were used from the maxillary molar distal hooks to the mandibular miniscrews ( Figs 2 and 3 ). Traction started with a load of 150 g of force per side in the first month and 250 g of force per side in the following period. Patients were instructed to wear the elastics full time, changing them every morning and night. Composite build-ups on the occlusal aspect of the mandibular permanent first molars were used to open the bite during maxillary protraction. Treatment was performed by the same operator in both groups (F.M).

Patients and parents were oriented to maintain an adequate level of oral hygiene during treatment. Peri-implant chlorhexidine gel (2%) was prescribed twice a day after oral hygiene during active treatment. Patients were followed monthly. In every appointment, oral hygiene procedures and the importance of compliance with Class III elastics were reinforced for both groups. Maxillary protraction was maintained for a mean of 11.3 and 11.0 months for the HH and CH groups, respectively ( Table I ). After appliance removal, a chincup was recommended for nighttime wear as active retention. Comprehensive orthodontic treatment was started only after the end of this study when necessary.

Table I
Baseline characteristics of the groups and treatment time
Variable HH CH
Sex, n
Male 12 9
Female 8 6
Total, n 20 15
Mean initial age (SD), y 10.7 (0.9) 11.5 (1.2)
Treatment time (SD), m 11.3 (3.9) 11.0 (3.7)

SD , standard deviation.

Cone-beam computed tomography (CBCT) was obtained before (T1) and after therapy (T2) with the i-CAT 3D system (Imaging Sciences International, Hatfield, Pa). The protocol of 120 kVp, 5 mA, 0.25-mm voxel size, scan time of 40 s, and field of view of 13 cm in height × 16 cm in depth was used. All CBCT data were exported in Digital Imaging and Communications in Medicine format to Dolphin 3D Imaging software (version 11.5; Dolphin Imaging and Management Solutions, Chatsworth, Calif). The head orientation was standardized in the right sagittal view positioning the Frankfurt plane parallel to the horizontal plane; in the frontal view, the orbital plane was positioned parallel to the horizontal plane; and in the axial view, and the midsagittal plane passed through the anterior and posterior nasal spines.

Outcomes (primary and secondary) and any changes after trial commencement

The primary outcomes were the overjet correction and the CoA changes. The secondary outcomes included changes in the mandibular plane, incisor inclination, and molar relationship. Reformatted lateral cephalometric images were obtained using Dolphin 3D Imaging software. A cephalometric analysis with 20 linear and angular variables was assessed with Dolphin 3D Imaging software.

Sample size calculation

The sample size was calculated to provide a power of 80%, an alpha error of 5%, and a minimum intergroup difference of 2 mm for maxillary length (CoA) changes, with a standard deviation of 1.4 mm, a sample of 9 patients was required for each group.

Interim analyses and stopping guidelines

Not applicable.

Randomization (random number generation, allocation concealment, and implementation)

The randomization process was performed on the Web site ( ). Allocation concealment corresponded to opaque, sealed, and sequenced numbered envelopes. Each envelope contained the group name according to the randomization sequence. A different operator was responsible for randomization sequence generation, allocation concealment, and processing.

The allocation process started after recruitment for the patients who met the inclusion criteria and signed the informed consent. Before opening the envelope, the patient’s name and allocation date were irreversibly identified on the external surface. Inside each envelope, a card containing the group name was found. During treatment, undesirable dental effects in the maxillary arch were observed in the CH group. For ethical reasons, treatment in the CH group was interrupted after 11 months of trial commencement and before appliance installation in the last 5 volunteers.


No blinding was accomplished considering both operator and patient were aware of the type of treatment performed. CBCT scans were unidentified before assessment. The statistician was also blinded during the analysis (J.R.P.L).

Statistical analysis

After 1 month, 30% of the sample were randomly selected for remeasurement by the same examiner. The reliability of repeated measures was assessed by the intraclass correlation coefficient and Bland-Altman limit of agreement.

The statistical analyses were carried out on an intention-to-treat basis, using multiple imputations to deal with the missing data of the dropouts. Multiple imputations were performed in the SPSS software (version 21.0; IBM, Armonk, NY). Five datasets were generated during the imputation for the missing data; then, the analysis was performed considering the pooled results of the 5 datasets using Rubin’s rule. A complete patient analysis considering only the completed treated patients was also provided.

Intergroup comparisons were performed using analysis of covariance, considering T1 data as a covariate. Intergroup comparison of the overjet correction frequency rate was performed using chi-square tests. Statistical analyses were performed using SPSS software. A significance level of 5% was regarded.


Participant flow

Twenty patients were allocated to the HH group. The final sample of the HH group comprised 20 patients (8 female and 12 male) with a mean initial age of 10.7 years. Two patients had missing data for T2. One patient interrupted treatment, and the other demonstrated a palatal miniscrew instability during the active expansion period ( Fig 1 ). The mean treatment time for the HH group was 11.3 months.

Twenty patients were allocated to the CH group ( Fig 1 ). However, the last 5 participants were not treated because of collateral effects during therapy in 2 out of 15 patients in treatment. These side effects consisted of extreme mesial movement of the posterior teeth and maxillary canine labial displacement. The final sample of group CH comprised 15 patients (6 female and 9 male) with a mean initial age of 11.5 years. The mean treatment time was 11 months. One patient had missing data for T2 because of treatment interruption ( Fig 1 ).

Baseline data

Table I demonstrates the baseline characteristics of the sample. Table II describes the starting forms of all variables for both groups. The frequency of anterior functional shift before treatment was 30% in the HH group and 26.6% in the CH group.

Table II
Starting forms
Variable HH CH
Sagittal skeletal
SNA (°) 84.75 (4.87) 82.06 (3.24)
SNB (°) 83.87 (4.29) 81.99 (3.29)
ANB (°) 0.73 (2.05) −0.14 (1.61)
Wits appraisal (mm) −5.26 (1.91) −5.76 (2.85)
Mx/Md diff (mm) 27.22 (3.65) 28.27 (4.04)
Co-A (mm) 78.59 (4.52) 76.63 (5.47)
Co-Gn (mm) 110.76 (6.01) 110.45 (9.79)
Co-Go (mm) 48.64 (3.79) 50.73 (4.93)
Na-AP (°) 177.21 (6.05) 178.61 (3.47)
Vertical skeletal
Occ plane to FH (°) 6.80 (3.60) 6.96 (3.85)
Palatal plane to FH (°) −1.02 (3.08) −2.62 (3.53)
FMA (MP-FH) (°) 26.33 (4.95) 25.76 (4.24)
Co-Go-Me (°) 125.34 (5.27) 125.41 (5.30)
ANS-Me (mm) 59.71 (5.71) 61.43 (6.17)
U1.palatal plane (°) 119.11 (7.10) 118.94 (6.88)
IMPA (°) 89.70 (5.93) 91.19 (7.43)
U6-A pt 23.56 (1.88) 22.98 (1.50)
Overjet (mm) −1.00 (1.84) −0.52 (2.08)
Overbite (mm) 1.26 (2.33) 0.25 (2.10)
Molar relation (mm) −3.45 (2.47) −2.64 (2.35)
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Dec 24, 2021 | Posted by in Orthodontics | Comments Off on Dentoskeletal comparison of miniscrew-anchored maxillary protraction with hybrid and conventional hyrax expanders: A randomized clinical trial
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