Our principal finding was that the transverse distance between all three maxillary landmarks studied increased with age. Hence, the null hypothesis that these landmarks do not change with growth is rejected. Although transverse widening of the maxilla during growth has been described previously, and the changes as such are not unexpected, the present study is the first to quantify these changes using 3D imaging data and three sets of easily identifiable skeletal landmarks at various vertical levels of the maxilla. While the average annual change was 0.5 mm for GPFd, 0.3 mm for NCd, and 0.7 mm for IOFd in the overall sample, the annual increases in GPFd and IOFd were significantly greater in males than in females. Although the peak height velocity of females lies within the age range of the patients studied, while that of males lies somewhat outside of that range8,9, this finding is barely surprising. Females are generally smaller in stature and it is logical that their average changes with growth would be proportionally smaller, too. The annual transverse change was not statistically associated with age at T1 for any of the variables measured. This means that there was no clear growth peak for the landmarks studied. This corroborates the findings of other studies, which suggest that chronologic age and other biological indicators do not always correlate with skeletal or facial growth status and that sometimes a real peak may be missing8,9,10,11,12,13. It is also of note that craniofacial structures grow proportionally less than structures further away from the head14. This may, at least in part, explain the small annual increases in the distance between bilateral structures in the facial area found in the present study.
A recent study on a novel predictor of skeletal response to maxillary expansion used the same landmarks as in our study to quantify long-term effects of RME treatment15. In contrast to previous reports of pyramidal expansion with RME16, the authors found a larger proportion of expansion at the level of the infraorbital foramina than at the more caudal levels of greater palatine foramina and nasal cavity15. Our study showed similar changes with natural growth—a larger increase at a more cranial level: the IOFd increased an average of 0.7 mm per year, whereas the GPFd and NCd increased 0.5 and 0.3 mm per year, respectively. This suggests that measurements using the infraorbital foramina as landmarks are significantly influenced by growth, which must be considered when the outcome of expansion treatment is assessed. Prior to this study, little was known about how the foramina of the maxilla change during the prepubertal period.
Some growth studies have demonstrated that the cortical bone lining the inner surface of the nasal cavity undergoes periosteal surface removal of bone as its endosteal side receives simultaneous deposits of new bone, contributing to an increase in nasal width17. The present study sheds more light on the rate of this increase during adolescence.
The greater palatine foramina are located in the hard palate. Our results suggest that they move laterally as the palatine process of the maxilla grows. The transverse growth occurs mainly by midpalatal suture separation, which is greater posteriorly than anteriorly16,18. The classic implant studies by Björk and Skieller suggest that this transverse growth occurs at 0.42 ± 0.12 mm per year18,19. Similar growth rates were reported by Korn and Baumrind with 0.51 ± 0.16 mm per year20. The present results correspond well with the classic implant studies and suggest an annual growth rate of 0.50 ± 0.31 mm for the distance between the greater palatine foramina (GPFd). It was hitherto not known if the results of the classic growth studies could be applied to foramina or other bony landmarks in the maxilla.
It would be logical to assume that the landmarks associated with the maxilla would change concurrently in the transverse dimension. However, the correlations among the annual changes in the distances between the landmarks were relatively low. These findings suggest that the growth of the maxilla cannot be simplified to assume that all landmarks grow simultaneously or by the same amount. This corroborates Enlow’s postulate that growth proceeds in a complex variety of directions in some major regions of the maxilla17.
Our findings have potentially valuable clinical applications. The most common transverse problem in orthodontics is a narrow maxilla, which may cause posterior crossbites. Identifying a crossbite as a transverse problem in patients is not difficult; however, there is little data available to help clinicians quantify the amount of skeletal transverse deficiency of a patient. The data obtained in this study, taken from a large sample of growing dental Class I subjects, may reflect population averages of transverse distances between landmarks. Clinicians may use this data as a diagnostic tool to help them quantify the transverse skeletal problems of their patients.
The treatment of choice for a maxillary transverse skeletal problem is often rapid maxillary expansion (RME) to open the midpalatal suture21,22. When a tooth-borne expander is used, the heavy forces generated by the expander transmit through the teeth into the maxillary bones and separate the hemimaxillae, which leads to subsequent bone deposition at the suture. This skeletal expansion occurs together with dentoalveolar expansion in the form of dental tipping and alveolar bending22,23,24,25,26,27. Quantifying the amount of skeletal expansion has previously been difficult because RME is typically performed on preadolescent patients so long-term effects are a combination of treatment and naturally occurring growth. Up to now, our understanding of maxillary skeletal growth was based mainly on decades-old studies using implants, frontal cephalograms, and dental model measurements with all their limitations18,19,20,28. Confirming that the transverse growth rate at the greater palatine foramina is similar to the maxillary growth reported in the classic implant studies is helpful in allowing clinicians to more confidently estimate the amount of long-term skeletal expansion achieved with RME.
Some limitations of the present study must be considered when interpreting its results. The study population was a convenience sample of patients who had undergone orthodontic treatment. As a result, the CBCT scans were acquired based on clinical need rather than research purposes. For this reason, not all patients were of the same age at the time of the initial CBCT scan or had a standardized time period between T1 and T2. Consequently, we reported any change with growth as average change per year, not as a total amount. In addition, although unlikely in a Class I dento-skeletal sample, the presence of fixed orthodontic appliances might have influenced the development of the maxillary transverse dimension. The ideal study sample would have been an untreated Class I sample with serial CBCT scans and no history of orthodontic treatment. However, such a sample is impossible to find, as it would be unethical to expose individuals to ionizing radiation without diagnostic or therapeutic benefit. After all, the guiding principle of radiation safety is “ALARA,” which stands for “as low as reasonably achievable.” This principle means that even if it is a small dose, if receiving that dose has no direct benefit, you should try to avoid it.
On the other hand, the present study has some substantial strengths. Measurements were performed on a total of 100 subjects, a sample size substantially larger than in most other studies that evaluated the transverse dimension of the maxilla, making its results generalizable to a larger population. In addition, CBCT allowed visualization of the whole maxillofacial complex without any magnification or superimposition of anatomic structures commonly associated with two-dimensional images2,3.
In conclusion, the present results show that the distances between the greater palatine foramina, lateral walls of the nasal cavity, and infraorbital foramina increase significantly during growth suggesting that the positions of these landmarks change in conjunction with the transverse growth of the maxilla. The annual change is generally smallest for the NCd and largest for IOFd with males having greater annual changes than females for GPFd and IOFd, but not NCd. The growth rates found in this study provide normative data on maxillary transverse growth, which can serve as population average and may allow orthodontic clinicians to more confidently estimate the amount of skeletal transverse deficiency or evaluate the long-term effects of skeletal expansion treatment.