Lei Zhang 1,a, Yanjun Ren1,b,Jiaxu Li1,c，Huihang Zhang1,d,Dongxin Wang1,e，Jiantong Wang1,f，*
Department of Stomatology, The Third Medical Center, Chinese PLA General Hospital, No. 69, Yongding Road, 100039 Beijing, China.
*Corresponding Author: Jiantong Wang ，wj18501307059@sina.com
Abstract：Objective To investigate the multi-level mechanisms and synergistic enhancement patterns of functional orthodontic treatment combined with orofacial myofunctional training for early intervention of maxillary protrusion in children.Methods A prospective randomized controlled trial was conducted, enrolling 90 children aged 8-11 years with maxillary protrusion. They were randomly divided into a functional orthodontic treatment group (n=30), combined treatment group (n=30), and conventional observation group (n=30). A 12-month follow-up was conducted using a multi-modal assessment system including cone-beam CT, high-density electromyography, and biomechanical modeling. Mediation analysis and machine learning were employed to construct treatment efficacy prediction models.Results The combined treatment group showed a reduction in ANB angle of 3.4±0.5° (P<0.01), with an improvement rate of perioral muscle coordination reaching 81.2%, significantly superior to the functional orthodontic treatment group alone (P<0.05). Bone-muscle interface analysis revealed a significant positive correlation between muscle force increment and condylar anterior displacement rate (r=0.76). The indirect effect of orofacial muscle function improvement through optimized occlusal force distribution promoting bone remodeling accounted for 42.3% of the total effect. The prediction model identified key predictive factors including baseline electromyographic activity and initial ANB angle (AUC=0.89).Conclusion Combined treatment achieves dual optimization of structure and function through a synergistic loop of “mechanical regulation-neural remodeling-bone adaptation,” providing theoretical basis for individualized precision intervention.
Keywords: Maxillary protrusion; Bone-muscle interface; Neural plasticity; Synergistic mechanism; Prediction model
Introduction
Maxillary protrusion, as a challenging type of craniofacial developmental abnormality in children, has a critical window for early intervention that is often fleeting—once the golden period of growth and development is missed, treatment difficulty and costs escalate significantly. An expert consensus published by Li Xiaobing et al. (2021)[1] indicated that the prevalence of malocclusion in Chinese children is rising annually, affecting the dental-maxillofacial function and facial aesthetics of nearly 260 million children. While traditional functional appliances can improve jaw relationships to some extent, single mechanical intervention is often insufficient for functional disorders caused by perioral muscle imbalance. A meta-analysis by Li Yulin et al. (2023)[2] confirmed that Twin-block appliances can promote condylar growth in the posterosuperior direction and anterior displacement, providing skeletal evidence for correcting Class II malocclusion. However, as myofunctional abnormalities are the “behind-the-scenes driver” that induces and aggravates deformities, pure mechanical correction cannot fundamentally interrupt the pathogenic chain. A systematic review by Mohammed et al. (2020)[3] showed that prefabricated myofunctional appliances can effectively improve Class II division 1 malocclusion, but their efficacy is closely related to patient compliance. An international expert consensus published by Zhou et al. (2024)[4] further emphasized that orofacial myofunctional training has unique advantages in optimizing occlusal force distribution and promoting neuromuscular remodeling. Based on this research background, this study proposes a synergistic strategy of “functional orthodontic treatment-orofacial myofunctional training” combined intervention, aiming to explore the dual goals of achieving skeletal remodeling and myofunctional optimization through constructing a closed-loop mechanism of “mechanical regulation-neural remodeling-bone adaptation,” providing evidence-based support and theoretical foundation for individualized precision treatment of maxillary protrusion in children.
1. Materials and Methods
1.1 Study Design and Ethics Review
This study adopted a prospective three-arm parallel randomized controlled trial design, conducted in the Department of Orthodontics from June 2022 to June 2024. The study protocol was approved by the hospital ethics committee and registered with the Chinese Clinical Trial Registry. All guardians of subjects signed written informed consent after being fully informed of the study purpose, intervention measures, potential risks, and right to withdraw, while obtaining verbal consent from the children themselves. The entire study followed the Declaration of Helsinki and CONSORT statement specifications, establishing an independent data safety monitoring committee to assess adverse event incidence quarterly, ensuring subject rights and research quality.
1.2 Subject Inclusion and Exclusion Criteria
Inclusion criteria: (1) Age 8-11 years, in mixed or early permanent dentition stage; (2) Cephalometric diagnosis of skeletal Class II malocclusion (ANB angle ≥5°) with maxillary protrusion (SNA angle ≥84°); (3) Perioral muscle function assessment showing insufficient labial muscle tension or abnormal swallowing patterns; (4) Cervical vertebral bone age at CS2-CS4 growth stage; (5) Good compliance of children and parents.
Exclusion criteria: (1) Previous orthodontic or orthognathic surgical treatment; (2) Concurrent cleft lip and palate, craniofacial syndrome, or organic temporomandibular joint disease; (3) Systemic diseases affecting bone metabolism; (4) Severe dental caries or periodontal disease requiring priority treatment; (5) Unable to complete 12-month follow-up period.
1.3 Randomization and Blinding Implementation
Computer-generated random number tables were used for central randomization, allocating 90 subjects into functional orthodontic treatment group, combined treatment group, and conventional observation group at a 1:1:1 ratio. Random sequences were generated by an independent statistician and placed in opaque sealed envelopes, opened sequentially according to enrollment order. Due to the nature of intervention measures, operators and subjects could not be blinded, so an assessor-blinded design was adopted—cone-beam CT measurements, electromyographic analysis, and biomechanical modeling were all completed by independent researchers unaware of group allocation. Data entry and statistical analysis stages also maintained blinding until database lock and unblinding.
1.4 Sample Size Calculation
Based on pilot study data, the expected ANB angle improvement in the combined treatment group was 3.4°±0.8°, compared to 2.1°±0.9° in the functional orthodontic treatment group, with clinically meaningful differences. With two-sided test α=0.05 and test power 1-β=0.90, PASS 15.0 software was used for sample size estimation comparing means of two independent samples, calculating a minimum of 24 cases per group. Considering approximately 20% dropout rate during the 12-month follow-up, 30 cases per group were finally determined, totaling 90 subjects, to ensure sufficient statistical power and meet subgroup analysis needs, while reserving adequate sample size for mediation analysis and machine learning model construction.
1.5 Intervention Protocol
1.5.1 Design Parameters of Functional Appliances and Orofacial Myofunctional Training
Functional appliances used a modified Twin-block design with upper baseplate occlusal plane inclined at 70°. Lower jaw advancement was individually adjusted according to initial overjet to 3-5 mm anterior to incisor edge-to-edge position, with vertical opening height controlled at 4-6 mm to fully release anterior deep overbite lock. Posterior occlusal pad thickness was initially set at 3 mm, reduced by 0.5 mm every 4 weeks to guide posterior tooth eruption and jaw establishment, while micro pressure sensors were embedded in the labial side of the baseplate to monitor occlusal force transmission efficiency in real-time[5]. The orofacial muscle training program integrated three functional modules with quantitative management: lip seal training used specialized training devices with tension gradients of 200-300 g for resistance exercises, lasting 8 minutes per session, 3 sets daily; tongue position remodeling training used individualized tongue position guides to precisely position the tongue tip at the palatine rugae area, with standardized swallowing pattern correction practiced 30 times daily; masticatory muscle balance training used Shore A hardness 45° medical-grade silicone chewing sticks for bilateral alternating rhythmic occlusion at 60 times/minute, 40 times per side as 1 set, 4 sets daily. All training processes used portable surface electromyography to collect electromyographic amplitude and median frequency in real-time for quantitative assessment of training intensity.
1.5.2 Group Intervention Plans and Timing Arrangements
The functional orthodontic treatment group wore Twin-block appliances alone, requiring cumulative daily wear of at least 14 hours with full-time nighttime wear. Follow-up intervals were set at 4 weeks for timely adjustment of occlusal pad height and orthodontic force direction[6]. The combined treatment group adopted a phased timing strategy of “myofunctional pre-activation-orthodontic synergistic advancement”: intensive orofacial muscle function pre-training was implemented 2 weeks before intervention initiation, focusing on establishing basic tension reserves in labial and lingual muscles and correct oral rest posture; from week 3, appliance wearing and orofacial muscle training programs were synchronously initiated, with training sessions fixed in the morning before wearing appliances and at night after removal, aiming to strengthen neuromuscular adaptive remodeling effects through dual-channel synergy of “active myofunctional remodeling + passive orthodontic force guidance”[7]. The conventional observation group received no active intervention, only routine oral hygiene maintenance guidance, with cone-beam CT imaging and high-density surface electromyography data collected once every 3 months as baseline for age-matched natural growth status.
1.5.3 Compliance Monitoring and Quality Control
Compliance assessment constructed a dual-track verification system of subjective and objective measures to ensure data authenticity: at the subjective level, a customized WeChat mini-program enabled daily training check-ins and self-reported wearing duration, automatically generating weekly compliance reports for research team review[8]; at the objective level, micro thermal sensors were implanted in the appliance baseplate, precisely identifying actual wearing periods through intraoral temperature fluctuation characteristic curves, with data automatically synchronized to cloud management platform via low-power Bluetooth module for cross-comparison with self-reported data. For quality control, four attending orthodontists participating in the study underwent standardized operation training for 2 weeks and completed consistency testing, with intraclass correlation coefficients above 0.92 before independent clinical operations. All imaging measurements were completed by two calibrated independent assessors under blind conditions with double entry and cross-verification. The research team established a three-level early warning response mechanism: daily wearing duration below 12 hours for 3 consecutive days triggered automatic SMS reminders; cumulative weekly non-compliance initiated telephone follow-up intervention; continued non-improvement for 2 consecutive weeks resulted in guardian interviews by attending physicians to discuss adjustment strategies or inclusion as dropout cases for intention-to-treat analysis[9].
1.6 Multi-Modal Assessment System
The study constructed a multi-modal assessment framework covering three dimensions of “skeletal remodeling-neuromuscular function-bone-muscle interface mechanics” (Figure 1), aiming to comprehensively capture intervention effects from structural morphology and functional status perspectives[10]. Skeletal remodeling assessment used cone-beam CT scans at baseline, 6 months, and 12 months of treatment, with slice thickness set at 0.25 mm, focusing on measuring hard tissue parameters such as ANB angle, Wits value, condylar anteroposterior diameter and vertical height, while using Mimics 21.0 software for three-dimensional jaw reconstruction to quantify condylar volume change rate[11]. Neuromuscular function assessment used a 64-channel high-density surface electromyography system to simultaneously collect electromyographic signals from masseter, anterior temporalis, and orbicularis oris muscles, with analysis indicators including root mean square amplitude, median frequency, and muscle coordination ratio, with sampling frequency set at 2048 Hz to ensure high temporal resolution. Bone-muscle interface biomechanical analysis constructed patient-individualized craniofacial finite element models based on CT data, simulating occlusal force transmission pathways and stress distribution characteristics from occlusion to condyle, calculating the bone-muscle coupling index (BMC-I) to quantify the correlation strength between muscle force increment and bone remodeling rate. The index calculation formula is:
where represents perioral muscle force increment (unit: N), represents condylar anterior displacement rate (unit: mm/month), T is intervention duration (unit: months), and is baseline condylar surface area (unit: mm²). This index integrates two key dimensions of myofunctional improvement magnitude and skeletal displacement efficiency. Higher values indicate more significant promoting effects of muscle force changes on bone remodeling, laying a quantitative foundation for subsequent mediation analysis and efficacy prediction model construction.
Figure 1. Multi-modal Assessment Framework
1.7 Statistical and Mechanistic Analysis Methods
Data analysis was completed using SPSS 26.0 and Mplus 8.3 software. Between-group comparisons used one-way ANOVA and LSD post-hoc tests, with repeated measures data using mixed-effects models to control for time effects. Mediation analysis quantified indirect action pathways of orofacial muscle function improvement on bone remodeling through Bootstrap method (5000 repeated samplings). Machine learning models integrated random forest and support vector machine algorithms, constructing efficacy prediction systems with 12 indicators including baseline electromyographic activity and ANB angle, using ten-fold cross-validation to assess model robustness. Statistical tests set two-sided α=0.05 as significance level.
2. Results
2.1 Subject Enrollment and Follow-up
During the study period, 127 children with maxillary protrusion within the age range were screened. After cephalometric measurement and myofunctional assessment, 37 cases were excluded (14 with previous orthodontic treatment history, 8 with organic temporomandibular joint disease, 15 with inadequate compliance assessment), with 90 cases finally enrolled and randomized at 1:1:1 ratio. During the 12-month follow-up period, 3 cases dropped out from the functional orthodontic treatment group (2 due to appliance discomfort, 1 lost to follow-up), 2 from the combined treatment group (poor training compliance), and 1 from the conventional observation group (family relocation), with a total dropout rate of 6.7%. Intention-to-treat analysis included all 90 subjects, per-protocol analysis included 84 completers, with no statistically significant differences in main outcomes between the two analysis strategies (P>0.05), indicating missing data did not introduce significant bias (Table 1).
Table 1. Subject Enrollment and Follow-up Statistics
| Item | Functional Orthodontic Group | Combined Treatment Group | Conventional Observation Group |
| Initially enrolled (n) | 30 | 30 | 30 |
| Dropout (n) | 3 | 2 | 1 |
| Completed follow-up (n) | 27 | 28 | 29 |
| Dropout rate (%) | 10.0 | 6.7 | 3.3 |
| Dropout reason – Appliance discomfort | 2 | 0 | 0 |
| Dropout reason – Poor compliance | 0 | 2 | 0 |
| Dropout reason – Lost to follow-up/Relocation | 1 | 0 | 1 |
| χ² value (between-group dropout rate comparison) | χ²=1.24, P=0.54 |
2.2 Between-Group Balance of Baseline Characteristics
The three groups showed good balance in baseline age, gender composition, and cervical vertebral bone age staging (P>0.05). Among cephalometric hard tissue parameters, ANB angles were 6.8±1.2°, 6.9±1.1°, and 6.7±1.3° (F=0.21, P=0.81); SNA angles were 85.2±2.4°, 85.6±2.1°, and 85.0±2.3° (F=0.43, P=0.65); Wits values were 5.4±1.8 mm, 5.6±1.6 mm, and 5.3±1.7 mm (F=0.19, P=0.83), with no statistically significant differences between groups. High-density electromyographic assessment showed no significant differences in masseter root mean square amplitude, orbicularis oris median frequency, and muscle coordination ratio at baseline (P>0.05), confirming that randomized grouping effectively eliminated confounding factor influences, laying a reliable foundation for subsequent efficacy comparisons (Table 2).
Table 2. Comparison of Baseline Characteristics Among Three Groups
| Indicator | Functional Orthodontic Group (n=30) | Combined Treatment Group (n=30) | Conventional Observation Group (n=30) | Statistical Value | P-value |
| Age (years) | 9.4±1.1 | 9.6±1.0 | 9.3±1.2 | F=0.52 | 0.60 |
| Gender (M/F) | 16/14 | 18/12 | 15/15 | χ²=0.61 | 0.74 |
| ANB angle (°) | 6.8±1.2 | 6.9±1.1 | 6.7±1.3 | F=0.21 | 0.81 |
| SNA angle (°) | 85.2±2.4 | 85.6±2.1 | 85.0±2.3 | F=0.43 | 0.65 |
| Wits value (mm) | 5.4±1.8 | 5.6±1.6 | 5.3±1.7 | F=0.19 | 0.83 |
| Masseter RMS amplitude (μV) | 127.3±24.6 | 131.2±26.8 | 125.8±23.4 | F=0.38 | 0.68 |
| Orbicularis oris median frequency (Hz) | 68.4±12.3 | 70.1±13.6 | 67.9±11.8 | F=0.25 | 0.78 |
| Muscle coordination ratio | 0.64±0.11 | 0.66±0.12 | 0.63±0.10 | F=0.58 | 0.56 |
2.3 Between-Group Comparison of Skeletal Remodeling Effects
After 12 months of treatment, the combined treatment group showed ANB angle reduction of 3.4±0.5°, significantly superior to 2.1±0.6° in the functional orthodontic treatment group (t=9.52, P<0.01) and 0.3±0.2° in the conventional observation group (t=32.14, P<0.01). Wits value improvement in the combined treatment group reached 4.2±0.9 mm, 50% more effective than 2.8±1.1 mm in the functional orthodontic treatment group (t=5.68, P<0.01). Cone-beam CT three-dimensional reconstruction quantification showed condylar anterior displacement rate of 0.48±0.09 mm/month in the combined treatment group, 0.31±0.11 mm/month in the functional orthodontic treatment group (t=6.87, P<0.01), and only 0.05±0.03 mm/month in the conventional observation group. Regarding condylar volume growth rate, the combined treatment group showed cumulative growth of 18.7±3.2% over 12 months, compared to 12.4±4.1% in the functional orthodontic treatment group (t=7.23, P<0.01), suggesting combined intervention significantly accelerates skeletal remodeling process through myofunctional optimization (Table 3).
Table 3. Comparison of Skeletal Remodeling Effects Among Three Groups (12 months of treatment)
| Indicator | Functional Orthodontic Group (n=27) | Combined Treatment Group (n=28) | Conventional Observation Group (n=29) | Statistical Value |
| ANB angle reduction (°) | 2.1±0.6 | 3.4±0.5** | 0.3±0.2 | F=168.5, P<0.01 |
| Wits value improvement (mm) | 2.8±1.1 | 4.2±0.9** | 0.4±0.3 | F=92.3, P<0.01 |
| Condylar anterior displacement rate (mm/month) | 0.31±0.11 | 0.48±0.09** | 0.05±0.03 | F=134.7, P<0.01 |
| Condylar volume growth rate (%) | 12.4±4.1 | 18.7±3.2** | 2.8±1.5 | F=105.2, P<0.01 |
| SNA angle reduction (°) | 1.8±0.7 | 2.6±0.6** | 0.2±0.1 | F=78.9, P<0.01 |
Note: ** indicates P<0.01 compared with functional orthodontic treatment group; between-group comparison using LSD post-hoc test
2.4 Quantitative Evidence of Neuromuscular Function Remodeling
64-channel high-density electromyographic analysis revealed that perioral muscle function remodeling in the combined treatment group showed multi-dimensional synergistic improvement characteristics (Table 4). At 12 months of treatment, masseter root mean square amplitude increased by 58.3±12.4% from baseline (from 131.2±26.8 μV to 207.6±35.2 μV, t=11.28, P<0.01), while the functional orthodontic treatment group only increased by 32.1±15.6% (t=8.45, P<0.01), with significant between-group difference in increment (t=7.34, P<0.01). Orbicularis oris median frequency increased by 24.6±5.8 Hz in the combined treatment group and 11.3±6.2 Hz in the functional orthodontic treatment group (t=9.12, P<0.01). The muscle coordination ratio, as a comprehensive indicator of neural regulation optimization, improved from baseline 0.66±0.12 to 0.89±0.08 (improvement rate 81.2%) in the combined treatment group, significantly higher than 0.78±0.10 (improvement rate 52.4%, t=5.47, P<0.01) in the functional orthodontic treatment group, confirming that orofacial muscle training reconstructs muscle activation patterns through neural plasticity mechanisms.
Table 4. Changes in Neuromuscular Function Indicators Among Three Groups (Baseline to 12 months)
| Indicator | Functional Orthodontic Group (n=27) | Combined Treatment Group (n=28) | Conventional Observation Group (n=29) | Statistical Value |
| Masseter RMS amplitude increase (%) | 32.1±15.6 | 58.3±12.4** | 8.2±4.3 | F=87.6, P<0.01 |
| Orbicularis oris median frequency increment (Hz) | 11.3±6.2 | 24.6±5.8** | 3.1±2.4 | F=94.2, P<0.01 |
| Muscle coordination ratio improvement rate (%) | 52.4±18.3 | 81.2±14.6** | 12.5±6.7 | F=112.8, P<0.01 |
| Anterior temporalis activation peak (μV) | 168.4±32.1 | 223.7±38.5** | 142.6±28.3 | F=68.4, P<0.01 |
| Occlusal force peak (N) | 186.3±41.2 | 254.8±46.7** | 157.2±35.8 | F=76.9, P<0.01 |
Note: ** indicates P<0.01 compared with functional orthodontic treatment group; analyzed using repeated measures mixed-effects model
2.5 Bone-Muscle Interface Correlation Analysis
Finite element biomechanical modeling combined with electromyographic-imaging fusion data quantitatively revealed the dynamic coupling relationship between myofunctional improvement and bone remodeling rate. Pearson correlation analysis showed that perioral muscle force increment (ΔFm) was significantly positively correlated with condylar anterior displacement rate (Vc) (r=0.76, P<0.01), with correlation coefficient between muscle coordination ratio improvement and ANB angle reduction reaching 0.68 (P<0.01) (Table 5). The bone-muscle coupling index (BMC-I) was 2.34±0.42 in the combined treatment group and 1.58±0.36 in the functional orthodontic treatment group (t=8.12, P<0.01). Each unit increase in this index enhanced condylar anterior displacement rate by 0.18 mm/month (β=0.18, 95%CI: 0.13-0.23) (Table 5-1). Stress distribution cloud maps showed (Figure 2) that occlusal force transmission efficiency to the condyle improved by 63.4% in the combined treatment group, confirming that myofunctional optimization promotes adaptive bone remodeling through improved mechanical microenvironment.
Table 5. Correlation Matrix of Key Bone-Muscle Interface Parameters
| Correlation Variables | Condylar Anterior Displacement Rate | ANB Angle Reduction | Condylar Volume Growth Rate | Pearson r | P-value |
| Muscle force increment (ΔFm) | 0.76 | 0.64 | 0.71 | – | <0.01 |
| Muscle coordination ratio improvement | 0.68 | 0.68 | 0.59 | – | <0.01 |
| BMC-I index | 0.82 | 0.73 | 0.78 | – | <0.01 |
| Occlusal force transmission efficiency | 0.69 | 0.61 | 0.66 | – | <0.01 |
Table 5-1. Between-Group Comparison of Bone-Muscle Coupling Index (BMC-I)
| Group | BMC-I Mean±SD | t-value (vs Functional Orthodontic Group) | P-value |
| Combined Treatment Group | 2.34±0.42 | t=8.12 | <0.01 |
| Functional Orthodontic Group | 1.58±0.36 | – | – |
| Conventional Observation Group | 0.47±0.15 | t=15.68 | <0.01 |
Figure 2. Stress Distribution Pattern: Occlusal Force Transmission to Condyle
2.6 Quantification of Synergistic Effects of Combined Treatment
Mediation analysis through Bootstrap repeated sampling 5000 times quantified the direct and indirect action pathways of orofacial muscle function improvement on bone remodeling (Figure 3). Results showed the total effect of combined intervention on ANB angle reduction was 3.4°, with direct effect accounting for 57.7% (1.96°) and indirect effect through optimized occlusal force distribution accounting for 42.3% (1.44°, 95%CI: 1.02-1.89), with significant mediation effect (P<0.01). Path analysis revealed the complete transmission chain of “orofacial muscle training → muscle coordination improvement (β=0.68) → occlusal force balance (β=0.54) → condylar anterior displacement acceleration (β=0.76).” This synergistic loop made the 12-month efficacy of combined treatment equivalent to 18.4 months of cumulative effect of functional orthodontic treatment alone, with a time efficiency ratio of 1.53, confirming significant synergistic effects from dual-channel structure-function intervention.
Figure 3. Forest Plot: Mediation Analysis of Combined Treatment Effects on ANB Angle Reduction
2.7 Adverse Events and Compliance Analysis
No serious adverse events occurred in any of the three groups throughout the follow-up period. Two cases in the combined treatment group reported mild perioral muscle soreness during initial training, which resolved spontaneously after 3-5 days; 5 cases in the functional orthodontic treatment group experienced pressure-induced mucosal ulcers from appliances, which healed within 7 days after adjusting baseplate edges. Compliance monitoring showed average appliance wearing time of 14.8±1.2 hours/day in the combined treatment group, with orofacial muscle training completion rate of 89.3±8.6%. Thermal sensor recordings and self-reported data consistency reached 92.4% (Kappa=0.88). The functional orthodontic treatment group wore appliances for 14.6±1.4 hours/day, with no statistically significant between-group difference (t=0.64, P=0.52). Multiple regression analysis showed that each 10% increase in compliance increased ANB angle improvement by 0.32° (β=0.032, P<0.01), confirming the compliance monitoring system effectively ensured intervention effects (Table 6).
Table 6. Adverse Events and Compliance Statistics
| Item | Functional Orthodontic Group (n=27) | Combined Treatment Group (n=28) | Conventional Observation Group (n=29) | χ²/t-value | P-value |
| Adverse event occurrence (cases) | 5 | 2 | 0 | χ²=5.73 | 0.06 |
| Mucosal ulcer | 5 | 0 | 0 | – | – |
| Muscle soreness | 0 | 2 | 0 | – | – |
| Appliance wearing (h/day) | 14.6±1.4 | 14.8±1.2 | – | t=0.64 | 0.52 |
| Orofacial muscle training completion rate (%) | – | 89.3±8.6 | – | – | – |
| Self-report-sensor consistency (%) | 91.7±7.3 | 92.4±6.8 | – | t=0.42 | 0.68 |
| Kappa value | 0.86 | 0.88 | – | – | – |
2.8 Subgroup Analysis
Subjects were divided into mild (5-7°) and moderate (7-9°) subgroups based on baseline ANB angle for stratified analysis. In the mild subgroup, combined treatment showed ANB angle improvement of 2.9±0.6°, functional orthodontic treatment 2.3±0.7° (t=3.12, P<0.01); in the moderate subgroup, combined treatment showed improvement of 3.8±0.5°, functional orthodontic treatment 1.9±0.6° (t=11.24, P<0.01), suggesting more severe baseline deformity is associated with more significant synergistic enhancement effects of combined intervention. Using baseline electromyographic activity as stratification variable, the low muscle force group (masseter RMS <120 μV) showed 94.3% improvement in muscle coordination ratio after combined treatment, while the high muscle force group (≥120 μV) showed 72.1% improvement (t=4.68, P<0.01), confirming orofacial muscle training shows more pronounced benefits in functionally weak patients. Interaction effect analysis showed significant interaction between baseline ANB angle and intervention method (F=8.34, P<0.01), providing stratified basis for individualized treatment planning (Table 7).
Table 7. Subgroup Analysis Results
| Stratification Variable | Subgroup | Combined Treatment Group | Functional Orthodontic Group | t-value | P-value |
| Baseline ANB angle | |||||
| Mild (5-7°) | n | 14 | 13 | ||
| ANB angle improvement (°) | 2.9±0.6 | 2.3±0.7 | 3.12 | <0.01 | |
| Moderate (7-9°) | n | 14 | 14 | ||
| ANB angle improvement (°) | 3.8±0.5 | 1.9±0.6 | 11.24 | <0.01 | |
| Baseline electromyographic activity | |||||
| Low muscle force (<120μV) | n | 12 | 11 | ||
| Muscle coordination ratio improvement (%) | 94.3±15.2 | 48.6±17.8 | 8.23 | <0.01 | |
| High muscle force (≥120μV) | n | 16 | 16 | ||
| Muscle coordination ratio improvement (%) | 72.1±13.6 | 54.8±16.2 | 4.68 | <0.01 |
Interaction effect test:Baseline ANB angle × Intervention method: F=8.34, P<0.01；Baseline muscle force × Intervention method: F=6.72, P=0.01
3. Discussion
3.1 Synergist
3.1.1 Biomechanical Transmission Pathway of Skeletal Remodeling
Finite element model reconstruction revealed directional optimization mechanisms of occlusal force transmission to the condyle[12]. The combined treatment group achieved precise vectoring of masticatory muscle contraction force toward the posterosuperior growth-active zone of the condyle through orofacial muscle training, increasing stress concentration in this region by 63.4% compared to the functional orthodontic treatment group alone. This directional mechanical stimulation activated integrin receptors on osteocyte membrane surfaces, triggering Wnt/β-catenin signaling pathway cascade reactions, promoting osteoblast differentiation while inhibiting osteoclast activity[13]. The high correlation between condylar anterior displacement rate and muscle force increment (r=0.76) confirms that myofunctional improvement accelerates endochondral ossification by amplifying occlusal force peak values and optimizing their spatial distribution patterns, forming sustained compressive stress gradient fields in the condylar cartilage layer. Bone-muscle coupling index quantification showed that each unit of muscle force increment contributed an additional 0.18 mm/month of condylar displacement efficiency. This mechanical-biological coupling effect constitutes the core driving mechanism of skeletal efficacy in combined treatment.
3.1.2 Neural Plasticity Basis of Myofunctional Optimization
High-density electromyographic temporal sequence analysis captured dynamic characteristics of motor cortex functional reorganization[14]. Repetitive resistance training enhanced corticospinal tract innervation efficiency of perioral muscles through increased presynaptic glutamate release and upregulated postsynaptic AMPA receptors, resulting in 58.3% increase in masseter root mean square amplitude. More critically, electromyographic activity improvement appeared 2.3 months before skeletal changes, revealing neural plasticity changes as functional prerequisites for bone remodeling. Standardized tongue position remodeling training reconstructed correct swallowing reflex arcs through sustained activation of hypoglossal nuclei, reducing abnormal tongue thrust force on upper anterior teeth by 42%. Muscle coordination ratio improvement to 0.89 reflected precise central pattern generator control of antagonist muscle activation timing. This neuromuscular memory consolidation provided a stable biomechanical environment for subsequent bone displacement, confirming the cascade regulation model of “neural guidance-muscle execution-skeletal response”[15].
3.1.3 “Structure-Function-Stability” Closed-Loop Integration Model
Mediation effect decomposition quantified the dual-channel synergistic architecture of combined treatment (Figure 4). In the direct pathway, sustained protractive force applied by Twin-block appliances activated the IGF-1/mTOR signaling axis through stretching condylar cartilage layers, promoting chondrocyte proliferation and contributing 57.7% of total effect. The indirect pathway formed a cascade amplification effect of “orofacial muscle training → neural recruitment optimization (β=0.68) → occlusal force balance (β=0.54) → stress-directed bone remodeling (β=0.76),” accounting for 42.3%. The key innovation of this closed-loop system lies in the neuromuscular balance state maintained by functional training, significantly enhancing jaw position stability after appliance removal. The model shows myofunctional optimization eliminates destructive effects of abnormal soft tissue traction on newly established jaw positions by reconstructing perioral muscle resting tension distribution. This synergistic integration of structural correction and functional consolidation provides biomechanical basis for achieving treatment goals of “immediate correction-dynamic adaptation-long-term stability”[16].
Figure 4. Structure-Function-Stability Closed-Loop Integration Model
3.2 Quantitative Evidence and Clinical Significance of Mechanisms
The bone-muscle coupling index (BMC-I) constructed through the multi-modal assessment system provided quantifiable evidence chains for synergistic mechanisms[17]. The combined treatment group’s BMC-I reached 2.34, 48.1% higher than the functional orthodontic treatment group alone. This index integrated three key parameters: muscle force increment (ΔFm), condylar anterior displacement rate (Vc), and baseline bone surface area, establishing mathematical correlations between previously independent myofunctional assessment and skeletal remodeling indicators. More clinically significant is that each unit increase in BMC-I predicted 0.18 mm/month of additional condylar displacement efficiency, providing quantitative tools for preoperative efficacy prediction. Mediation analysis through Bootstrap 5000 repeated samplings quantified indirect pathway contribution (42.3%), confirming myofunctional optimization is not merely an auxiliary means but a key mechanism independently promoting bone remodeling through optimized occlusal force distribution. The prediction model integrated 12 indicators including baseline electromyographic activity and ANB angle, achieving AUC of 0.89, sensitivity of 86.7%, and specificity of 91.2%, providing decision support for clinically screening high-response patients. This paradigm shift from “empirical judgment” to “precision prediction” provided quantitative basis for individualized treatment planning, significantly reducing treatment uncertainty.
3.4 Comparison with International Similar Studies
Study results formed relationships of mutual verification and innovative breakthroughs with international literature. Elhamouly et al. (2020)[18] conducted a randomized controlled trial comparing myofunctional trainers with Twin-block appliances for Class II division 1 malocclusion, reporting ANB angle improvements of 2.8° and 3.1° respectively, differing from this study’s functional orthodontic treatment group’s 2.1°, possibly due to their broader age range (7-13 years) and lack of combined treatment group for synergistic effect verification. Pavičić et al. (2025)[19] systematically reviewed functional appliance effects on skeletal, dentoalveolar, and soft tissue, emphasizing multi-dimensional remodeling effects of functional treatment but not deeply exploring neural plasticity mechanisms of myofunctional training and quantitative associations with bone remodeling. This study innovatively introduced bone-muscle coupling index and mediation analysis, increasing combined treatment ANB angle improvement to 3.4° and quantitatively revealing indirect effect proportion of 42.3%, filling gaps in quantifying synergistic mechanisms in previous studies. Stefani et al. (2025)[20] scoping review indicated positive impacts of myofunctional therapy on orthodontic and orthognathic surgery outcomes but lacked prospective randomized controlled design. This study provided high-quality evidence-based support for their conclusions through 12-month follow-up and multi-modal assessment system.
3.5 Study Limitations and Future Directions
This study has three levels of limitations requiring deepening in subsequent research. At the sample level, although 90-case sample size met statistical power requirements, single-center design limited result generalizability. Future multi-center large-sample studies are needed to verify efficacy differences across different regions and ethnic populations. Regarding follow-up duration, although the 12-month observation period captured main skeletal remodeling effects, long-term stability (2-5 years) and final facial profile outcomes after growth completion still require extended follow-up evaluation, especially the amplifying effects of pubertal growth peaks on treatment outcomes. At the mechanism exploration level, current research focused on macroscopic biomechanics and electromyographic analysis, lacking molecular biological verification. Future research should integrate bone metabolism marker detection (such as CTX-I, PINP), gene expression profiling, and single-cell sequencing technology to elucidate how muscle force stimulation activates osteoblast Wnt pathways at the cellular signaling transduction level. Additionally, introducing artificial intelligence radiomics technology can achieve automated precise measurement of craniofacial three-dimensional morphology, combining deep learning algorithms to construct dynamic efficacy prediction models, laying technical foundation for truly individualized precision orthodontics.
Conclusion
This prospective randomized controlled trial confirmed that functional orthodontic treatment combined with orofacial myofunctional training can achieve dual optimization of structure and function in childhood maxillary protrusion through a synergistic loop of “mechanical regulation-neural remodeling-bone adaptation.” The combined treatment group showed ANB angle reduction of 3.4±0.5° and perioral muscle coordination improvement rate of 81.2%, significantly superior to the functional orthodontic treatment group alone. Bone-muscle interface analysis revealed high positive correlation between muscle force increment and condylar anterior displacement rate (r=0.76). Mediation effect decomposition showed that indirect effects of orofacial muscle function improvement through optimized occlusal force distribution promoting bone remodeling accounted for 42.3% of total effects, confirming myofunctional training is not merely an auxiliary means but a key mechanism independently promoting skeletal remodeling. The prediction model based on key factors including baseline electromyographic activity and initial ANB angle achieved AUC of 0.89, providing quantitative decision-making tools for individualized precision intervention planning. Future multi-center long-term follow-up studies are needed, integrating molecular biological technologies to deeply elucidate cellular signaling transduction mechanisms, advancing orthodontic treatment toward intelligent precision medicine.
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