Our findings asserted that there were differences in HBP between MB and NB children, in addition antero-posterior skeletal Class (Class I and Class II), and age affected HBP.
Different methodologies had been used to evaluate the HBP; Lateral cephalometric was more predominant for determination of HBP [5, 24, 25]. As reported previously, the CBCT scan obtained before orthodontic diagnosis and treatment planning can help in gaining a clear clinical judgement of hyoid bone position and its surrounding structures . Numerous CBCT studies [17, 18] had evaluated HBP in nasal breathing subjects. The current study used CBCT to evaluate the effect of different breathing patterns on HBP.
All CBCT images were taken when only CBCT was expected to add additional information which would aid in orthodontic diagnosis and treatment planning. The authors’ institution follows the ALARA principle “as low as reasonably achievable”  ensuring not to expose the patients to unnecessary ionizing radiation.
We believe that recognition of mouth breathing patients should be conducted through a multidisciplinary approach by orthodontists and otolaryngologists as recommended by Costa et al.  Previous literature relied on visual and clinical examination only to diagnose mouth breathing, which in fact could lead to improper mouth breathing recognition protocol . This might clarify the different findings and contradictions between various researchers.
Few studies relied on otolaryngologists' diagnosis of mouth breathing [22, 28]. However, previous literature found that orthodontists can accurately diagnose nasal breathing and advised collaboration between orthodontists and otolaryngologists regarding mouth breathing diagnosis . Lymphoid tissue develops fast after birth, reaches its peak size in childhood, begins to regress around the age of 8–10 years, and usually entirely diminishes around 12–14 years . Hence, in our study: the history taking and clinical tests were used as preliminary screening tests for detection of mouth breathing, and the otolaryngologists confirmed the diagnosis, cooperation between the two disciplines led to a better diagnosis and treatment planning .
It has been documented that the HBP could be influenced by the anteroposterior sagittal skeletal patterns , vertical skeletal patterns , and age . In our study, patients with similar age cohorts and skeletal patterns were compared to detect the differences in HBP between MB and NB, given that all of our participants were normal vertical growers.
Chung et al.  compared mouth breathing and nasal breathing children, concluded that mouth breathing patients had elevated HBP compared to the nasal breathing children, although they didn't classify their participants into different antero-posterior sagittal classes, but they found that most of mouth breathing participants had a tendency toward having Class II malocclusion, this concurred with our finding that Class II MB children aged 7–9 years exhibited an upward HBP compared to Class II NB with similar age group. Furthermore, this finding was also corroborated by Chaves et al.  who emphasized that asthmatic patients with mouth breathing had an elevated HBP in relation to the mandible and 3rd cervical vertebrae.
However, Cuccia et al.  and Behlfelt et al.  claimed that mouth breathing children showed extended head posture as well as a lower HBP. Our study found that in 7–9 years group, MB children with skeletal Class I displayed a lower HBP than their matched NB group. A recent study by Vuong and Kang  found a positive correlation between the superior HBP and the constricted airway. Furthermore, skeletal class II pattern and mouth breathing habit have been stated to predispose to a constricted airway [22, 35]. This could explain our finding that MB patients with Class II pattern exhibited more superior HBP than Class I MB patients.
Behlfelt et al.  suspected that the lower HBP in mouth breathers was attributed to lower position of the tongue and to allow for more airway patency as the airway volume might be decreased in the mouth breathers , whereas, Chaves et al.  suggested that the upward HBP is a compensatory mechanism; as mouth breathing is accompanied with clockwise rotation of the mandible, which might release the tension applied by suprahyoid muscle to hyoid bone, thus led to an inferior HBP and constricted pharynx as well, then mouth breathers tended to extend their heads to allow for more airway patency, this posture exaggerated the tension applied by suprahyoid muscle, which consequently pulled hyoid bone to a superior position. Moreover, in our study, we also used Sella point to determine HBP as it is a more stable reference point than cervical vertebrae and mandible .
According to Janicka and Halczy-Kowalik  who assessed the different HBP in mouth and nasal breathers, their sample ranged between 9 and 35 years old; they presumed that mouth breathers had a backward HBP described by the parameter (C3-H). This finding was consistent with our findings in 10–12 years group, in which MB in both skeletal classes demonstrated a backward HBP. However, this finding is contradicted with other findings expressed by Juliano et al. , who assumed an anterior HBP for mouth breathers which is attributed to their head extension to enhance breathing capacity. However, it is worthy to note that previously mentioned studies used varied study designs and methodologies to evaluate HBP.
In the current study, hyoid bone descent in the older age group. Pae et al.  conducted a longitudinal study and described the early descent of hyoid bone as a physiological phenomenon related to speech development, while late descent is associated with increased resistance of airway with aging which usually seen on OSAS patients.
Mouth breathing habit was reported to cause alteration in the normal growth of the craniofacial complex and could be a risk factor for developing OSAS . Our regression model was capable of predicting mouth breathers based on hyoid bone measurements with 76.2% overall accuracy. Moreover, C3-Me and H-EB were found to be significant predictors, for each one-unit increase in the previously mentioned predictors the odds of being mouth breather increase by 2.27 and 1.16, respectively. Increased values of these parameters speculate that MB subjects had extended head posture and hyoid bone modifies its position to enhance breathing capacity .
Previous studies have shown an association between HBP and the severity of OSAS. Chang and Shiao  reported that the distance from the hyoid bone to the mandibular plane (MP-H) was positively correlated to the Apnea Hypopnea Index (AHI), and subjects with longer MP-H distance suffered from more daytime sleepiness. A recent review by Haskell et al.  discussed the favorability of AHI response to oral appliances. Treatment favorability was linked to the anatomical position and angulation of hyoid bone beside maxillofacial and pharyngeal parameters. Moreover, several studies reported hyoid bone positional changes after orthodontic treatment with functional appliances [39, 40]. Such findings highlight the need of gaining a better knowledge of HBP to permit early intervention and a better prognosis.
Our study demonstrated in-depth grouping for age and anteroposterior skeletal classes to detect the differences in HBP between NB and MB, but one limitation is we didn't include Class III patients and we didn’t consider gender differences due to the lack of samples; another limitation is our cross-sectional retrospective study is not capable to assess the causality principle; the differences between NB and MB were detected on one-time point, However, this study adopted a cross-sectional design because of the ethical concerns associated with possibility of increased radiation doses accompanying longitudinal studies; also we didn't include subjects less than 7 years old or older than 12 years old, and this study only included Chinese participants.