The study’s purpose was to investigate whether there is any correlation between the size of the maxillary sinus and skeletal morphological indexes. For the vertical analyses, as far as we know, there was only one related research publications. Okşayan[11] found that the maxillary sinus lengths and widths were shorter in patients with a high angle, but there was no significant correlation between the volume and vertical growth patterns. However, our research found that the maxillary sinus length, width, and volume of high angle patients were smaller than those of low angle patients. Therefore, the conclusions of these two studies are basically the same, and the discrepancy is perhaps due to the age range of the patients included. The average age of patients enrolled by Okşayan was 29.90 ± 10.91, which was much wider. Although, in general, the development of the maxillary sinus has reached the size of an adult at the age of 15[8], many physiological and pathological factors affect the volume of maxillary sinus over time[22, 23]. Considering that the growth of the mandible lasts 18 to 20 years[16], the age of the patients included in this study was 15 to 20 years old, in order to match the longitudinal growth of the mandible and its correlation with the maxillary sinus. Therefore, the ages of included patients probably lead to the support of distinction in the results of these studies.
For the sagittal analyses between the craniofacial bones and maxillary sinus, previous studies were controversial. Oktay[9] found that the maxillary sinuses of females with Class II were relatively larger. Moreover, Weng Jiahua[21] and Dah-Jouonzo[24] also found that the maxillary sinus is larger in patients with skeletal Class II. In contrast, Endo[10] found there was no significant correlation between the volume of the maxillary sinus and the sagittal skeletal malocclusion. In this study, we found no significant difference in the volume of the maxillary sinus in patients with different sagittal skeletal deformities. The reason for this difference might be that the ages of the included patients varied greatly, and the accuracy of the two-dimensional measurement was different from that of the three-dimension measurement. The patients included by Oktay were 6 to 30 years old and were analyzed using panoramic radiographs, while for Weng Jiahua and Endo, the included patients were 12 to 16 years old, the former using CBCT and the latter using the lateral cephalometric radiographs for the study.
For the correlation between the maxillary sinus and mandibular body length, as far as we know, we have only found relevant research based on two-dimensional lateral cephalograms[12]. This two-dimensional study asserted that the length and cross-sectional area of the maxillary sinus were positively correlated with the length of the mandible[12]. However, the three-dimensional measurement of the maxillary sinus has been dramatically improved due to the advancement of digital technology. Traditional two-dimensional planar radiographic pictures lose much information, omitting the perspective from the third dimension[25]. However, three-dimensional digital cone-beam CT (CBCT) has many advantages compared with previous technologies, such as lower radiation, shorter imaging time, low cost[26, 27], and relatively complete maxillary sinus information.
Meanwhile, the prediction of mandibular growth is intractable, but quite a crucial issue. Because in clinical practice, the cause of recurrences in many adolescents with skeletal Class III is the continued growth of the mandible after orthodontic treatment[17]. At present, even the most classic prediction of the mandibular growth using the wrist radiograph is not effective enough[28]. Hand-wrist radiographs can predict the peak of growth and development, but there is no significant correlation with the quantity of mandibular growth[29]. The cervical vertebral maturation (CVM) method could not validly predict the mandibular growth peak[30]. For the first time, this study found that the volume of the maxillary sinus was positively correlated with the length of mandible. Moreover, we did not find a significant correlation between maxillary sinus volume and SN, which means that maxillary sinus has a more intimate relationship with the jaw bone than the whole skull. If the prediction formula established by adding the maxillary sinus as one of the referential indicators can effectively predict the growth and development trend of the mandible, it will be of certain guiding significance for the clinical orthodontic treatment of skeletal Class III patients. This predictive effect is for further research, requiring a larger sample size, and conducting a long-term longitudinal study and statistical analysis.
The results of this study indicate that the maxillary sinus is larger in low angle patients. Therefore, low-angle patients need a more careful evaluation of the shape and size of the maxillary sinus when implanting mini-screws in the maxilla or moving the posterior maxillary teeth to avoid penetrating the maxillary sinus[31]. At the same time, the study results indicate that the volume of the maxillary sinus is positively correlated with the length of mandible. Therefore, for adolescent patients who are skeletal Class III with larger maxillary sinus volume, the possibility of further development of the mandible after orthodontic treatment is higher, and the possibility of recurrence is correspondingly increased. These possibilities mean that orthognathic surgery for this type of skeletal Class III patient might be more appropriate as a treatment selection.
Only limited data of clinical case material could be collected, which resulted in mainly a cross-sectional study with the imaging findings from CBCT. Considering that the maxillary sinus volume has basically completely developed after the age of 15[8], the cross-sectional data of orthodontic patients in this age group were used to analyze the correlation between the volume of the maxillary sinus and the length of the mandible. However, there is no doubt that stronger evidence can be obtained from a longitudinal study, which could more precisely validate the trends of growth and development. Therefore, next, our team will collect more information, especially from the ENT and growth and development disciplines, as well as expanding the sample size and including more measurement indicators, and conducting longitudinal tracking to obtain more convincing evidence.