Following MTBA treatment, mandibular retrognathia was alleviated, upper airway space was widened, and normal movements of the hyoid bone were achieved, thereby improving the breathing status of the children. The results of this study indicate that the MTBA has a good effect on OSA children with mandibular retrognathia. This study provides more options and evidence for treating children with OSA and mandibular retrognathia.We will discuss it from the following aspects.

Airway change and measurement index selection

The airway measurement can be three-dimensional(3-D) or two-dimensional(2-D)0.3-D can accurately measure the airway. But Compared with computed tomography(3-D) [18], X-ray imaging(2-D) is associated with lower radiation and cost; At the same time, 2-D can accurately assess the airway [19], thus, the latter was conducted in this study.As a child grows, the airway will change.Jeans WD [20] et al. measured the area of the nasopharynx and its contents on lateral cephalometric radiographs of 41 normal children, and they found that soft tissue appeared to grow faster than the nasopharynx from 3 to 5 years of age, thus reducing the airway. Subsequently, the soft tissue area remains relatively constant, while the size of the nasopharynx increases, allowing the airway to gradually expand.Arens R [21] et al. studied the changes of the upper respiratory during childhood. They concluded that from year 1 to year 11, the bones of the lower face grew linearly along the sagittal and axial planes, and the soft tissues around the upper respiratory grew in proportion to the bone structure.Therefore, we included normal children with class I without mandibular retrognathia as a control group for the study to reveal changes in airway dimensions due to the effect of treatment.

The landmarks selected in this study represent the morphology of jaw bone, airway and hyoid, which have also been reported in previous literatures [22, 23].All parameters in this study were correlated with the changes of jaw and airway. When an appliance is applied to the jaw, as the jaw shifts forward, airway dimensions changes, sleep indicators change, hypoxia status changes, and inflammatory markers theoretically associated with hypoxia change. At the same time, when the jaw moves, the hyoid bone also changes.

The MTBA selection and treatment

Owing to the complex structure of traditional twin-block appliances, herein, a (MTBA) with higher efficiency with the biomechanical characteristics of tooth movement was designed to treat children with OSA who had mandibular retrognathia.Although MTBA has its advantages, doctors need to pay close attention to monitoring the situation of children wearing the appliance to improve the therapeutic effect of the appliance.

In this study, maxillary expansion was performed to remove the limitation of maxillary dental arch on mandibular dental arch. Fernandez-Barriales M [24] et al. found no statistical difference between RME and watchful waiting. In children with OSA, no convincing evidence was found to prove that RME was superior to watchful waiting. Therefore, changes in AHI and secondary indicators in this study mainly considered changes brought about by mandibular leadership.The standard for the end of orthodontic treatment was improved mandibular retrognathia. Following treatment, the mandible could not revert to its original position. The occlusal relationship between internal and posterior teeth changed from the distant relationship of Angle’s class II to the neutral relationship of Angle’s class I. Coordination between the upper and lower jaws was observed. The overjet of the anterior and posterior teeth was normal.

Analysis of sleep monitoring indicators and airway and maxillofacial changes after treatment

Herein, AHI in children with OSA treated with MTBA decreased from 6.92 ± 4.77 before treatment to 2.75 ± 2.12 after treatment (50% decrease), indicating a successful treatment [25]. The main outcome index, AHI, validated the conclusion of the American Society of Orthodontists’ White Paper [26] that MAA can reduce AHI. Herein, AHI decreased and LSaO2 increased following treatment (all P < 0.05). Furthermore, nighttime sleep improved in children treated with MTBA, indicating that the correct use of MTBA in children with OSA leads to good clinical effects. Similar results were reported by Schütz [27] and Idris [12].

Following MTBA treatment, the SNB significantly increased and ANB significantly decreased (all P < 0.05), which indicated that the mandible moved forward. SPP–SPPW, U-MPW, PAS, and V-LPW also significantly increased (P < 0.05), suggesting that treatment with MTBA can drive the tongue away from the pharyngeal cavity and widen the airway, which positively influences the airway patency of children. A previous study revealed that during the growth of patients with class II malocclusion, the use of functional orthopedic equipment increased the volume of the pharyngeal airway, which is expected to reduce the risk of OSA [28]. Zhao et al. [29] reported that self-adjusting OAs can increase the upper airway volume in children with OSA and mandibular retrognathia. The results of this study were similar to those of previous ones [28, 29].

Analysis of changes in the hyoid bone position following treatment

The hyoid bone is involved in speech, swallowing, and mandibular movements and is associated with airway patency. Savoldi et al. [30] reported that the measurement of the upper airway morphology (defined as intermural space) and hyoid bone position on the lateral cephalometric radiographs of children is a reliable strategy. Soares et al. [31] reported a direct correlation between the severity of OSA and inferior and posterior positions of the hyoid bone in children aged 7–10 years. However, the reason behind the low hyoid bone position in children with OSA remains unclear and is suggested to be due to long-term mouth breathing or mandibular retrognathia and the backward and downward movement of the hyoid bone. In addition, this study revealed that AH-CV3ai and AH-CVP increased following MTBA treatment, indicating that the hyoid bone moves away from the cervical vertebra. Furthermore, a decrease in AH-MP suggested that the hyoid bone moves upward. These comprehensive data revealed that the hyoid bone moved from the posterior to anterior–superior position following MTBA treatment, which improved the position of the posterior inferior hyoid bone. Future studies should explore the correlation between the hyoid bone position and symptoms or incidence of pediatric OSA.

Herein, AH-RGn and AH-FH differed significantly before and after MTBA treatment in the experimental group; no significant difference was observed between the experimental and control groups. These values decreased following treatment in the experimental group; however, the values in the experimental group before and after treatment were larger than those in the control group before and after observation, indicating that the treatment affects these indices. Furthermore, AH-PP significantly differed before and after treatment in the experimental group; however, no significant difference was observed between the experimental and control groups, indicating that the treatment affects these indices.

Serum markers of OSA

Intermittent hypoxia and reoxygenation during sleep are the pathophysiological bases of endothelial dysfunction and inflammation due to OSA [32]. Long-term chronic hypoxia caused by OSA can lead to several physiological and biochemical modifications, such as changes in the NLR, PLR, and levels of C-reactive proteins and markers of serum inflammation [33,34,35,36,37]. A few studies have investigated the association between NLR, PLR, and OSA severity in patients with OSA and mandibular retrognathia. Therefore, considering the key pathogenic role of inflammation in hypoxemia, whether inflammatory marker levels change in children with OSA and mandibular retrognathia following treatment must be determined.

De Luca Canto et al. [38] revealed that plasma IL-6 and hsCRP levels can help distinguish patients with OSA and those without morbidity. Bhattacharjee et al. [39] reported that assessing hsCRP levels following AT may facilitate the prediction of residual OSA. PLR is a new biomarker of inflammation. Tang et al. [35] demonstrated that PLR is closely associated with the severity of OSA and complications of cardiovascular disease. NLR is a novel systemic inflammatory response indicator. Sunbul et al. [40] reported that NLR in the OSA group was significantly higher than that in the control group and was correlated with AHI. Another study reported that NLR was significantly higher in patients with severe OSA than in those with mild-to-moderate OSA and healthy controls [41].

Following the parents’ consent, 18 children were evaluated in this study, and 16 were evaluated for hsCRP levels. However, the differences in PLR, NLR, and hsCRP levels before and after treatment were not significant. This could be because ~ 75% of the patient population had experienced mild-to-moderate OSA. Although clinical symptoms disappeared after treatment, it was insufficient to cause changes in the levels of blood inflammatory indicators. Therefore, future studies should assess children with mild, moderate, and severe OSA separately to obtain more definite results.

Study limitations

(1). As parents demand early treatment for maxillofacial deformity and OSA symptoms, no positive control group of OSA with mandibular retrognathia was considered in this study. Further multicenter, large-scale, randomized double-blind controlled trials should be conducted in the future to evaluate the therapeutic effect of the MTBA in treating children with OSA with mandibular retrognathia.

(2). This study did not pay attention to the changes in the quality of life and sleep structure of children, and it is necessary to continue to study the improvement of the MTBA treatment on other symptoms of OSA children in the future.