The immediate impact of bonded rapid maxillary expansion on the naso-pharyngeal airway patency : a prospective CBCT study

Introduction: Recent studies have utilised cone beam computed tomography (CBCT) for the assessment of the volume of the nasopharyngeal airway space (Guijarro-Martinez and Swennen, 2013, Lenza et al., 2010, Woodside and Linder-Aronson, 1979, Chang et al., 2013). Some of these investigations are based on the analysis of slice data obtained from three dimensional (3D) radiographic images. The usage of a CBCT scan to measure the nasopharyngeal airway volume has drawbacks which include the effect of respiration and tongue position (Abbott et al., 2004), the impact of head posture, lordosis (cranio-cervical inclination) and mandibular morphology on the accuracy of measuring air oro-pharyngeal airways. In addition, published figures to date has not considered the detailed anatomical boundaries of the nasal cavity space, paranasal airway space and other pharyngeal sections collectively (Chang et al., 2013). Aims and objectives: The aim of the study was to assess the validity of the free access software package like ITK Snap in measuring the airways spaces, investigate, using CBCT, the three-dimensional effect of rapid maxillary expansion on the maxillary sinus, the lower part of the nasal cavity, the upper nasopharynx and the upper oropharynx (upper and lower retropalatal spaces), and to correlate the changes in these anatomical spaces with the measured RME appliance split, the dentoalveolar expansion and the gender of the subjects. Materials and methods: This study was carried out on seventeen patients (8 boys, 9 girls; mean age 12.6 + 1.8 years) who required maxillary expansion for the management of narrow upper dental arch. Sample size was calculated using the Researcher’s Toolkit calculator and this indicated that a sample size of 14 patients would produce an Alpha error level or confidence level at 95% and a Beta error level at 20%. Therefore, it was decided to recruit 17 subjects to overcome potential exclusion due to irreproducibility in the head orientation and lordosis. Pretreatment (T1) and immediate post-RME (T2) CBCT images were taken for all the patients. In all CBCT images, head orientation and lordosis were measured using OnDemand 3D software packages. Cases were excluded from the study if the difference in the head orientation and lordosis between the CBCT of T1 and CBCT of T2 was more than 5 degrees. The two scans, T1, T2, were orientated according to a specific protocol and superimposed on the cranial base to standarise the volumteric segemtation and measurements. The impact of RME was assessed by measuring, using ITK snap and OnDemand 3D software packages, the changes in the distance between the intermolar dentoalveolar width at level of molar alveolar crest (IMD), the magnitude of appliance expansion (AE), the volume of respiratory region or the lower part of the nasal cavity (LNC), the volume of the right and left maxillary sinus (RMS and LMS), the volume of the upper nasopharynx (UNP), the subdivisions of the upper oropharynx including the upper and lower retropalatal space (URP and LRP) at T1 and T2. Segmentation of the oro-naso-pharyngeal spaces into multiple segments allows a deailed localisation of the changes and aids in exclusion of any potential masking change of one airway space on adjacent or remote airway space as each segment is associated anatomically and physiologically to different function and/or disorder. The normality of the data was tested using Kolmogorov–Smirnov test. The reproducibility of meaurements was analysed using Paired t-test and interclass correlation coefficient. The volumteric and linear changes was assessed using Student t-test (P<0.05) and Pearson correlation coefficients was used to test the correlation of these changes. Results: Bonded RME has an effective dentoalveolar expansion effect in growing patients (P=0.001) and produced a signiﬁcant increase in UNP (P=0.045). There was a statistically significant reduction on the URP space (P=0.042), especially in males. There was strong correlation between the increase of the volume of the right and left maxillary sinuses (PCC=0.86) and between appliance expansion and dentolavelar expansion (PCC=0.75). Conclusions: ITK-SNAP software is a reliable package and a single threshold value (-450 grey) is an accurate value. Additionally, this software can be used to measure the size of bony defect in patient with cleft palate before secondary alveolar bone grafting. Bonded RME was an effective dentoalveolar expander in growing patients and the immediate expansion of LNC and UNP might be associated with a reduction in nasal resistance, improvement in the nasal breathing and it can be considered as an option for treatment of Paediatric Obstructive Sleep Apnea Syndrome. Findings of this dtudy showed that there is a sexual dysmorphisim secondary to RME but did not reach the statistical significance. Generally, the effect of the RME on the upper naso-oro-pharyngeal airway spaces followed a mushroom like pattern with the upper parts expanded, the middle part was significantly narrowed while the lower part was mildly, but insignificantly statistically, reduced. However, it is essential to consider that regardless of the benefit of the increase nasal patency of this orthopeadic procedure, it should not done merely for the above purposes solely but only when it is linked to a right indication for RME. A future studies could include a colour mapping for detailed assessment of changes in different part of the oro-naso-pharyngeal space, as the shape changes of the airway space is as important as volumetric changes. Finally, a further randomised clinical trial or comparative study with larger sample size and long term follow up would be beneficial in estimating the real impact of the RME on the airway confirm the findings of this study.