The current study derived a formula to objectively determine cervical vertebral bone age (CVBA) in the Asian South Indian population. Similar objective methods of determining CVBA have been utilized by previous researchers, in other ethnic groups. Mito et al. [19] was the first to suggest an objective method for determining CVBA, and their formula was derived for Japanese girls. However, when this formula was applied to Brazilian patients, Caldas et al. [6] noted that it was reliable only for Brazilian girls. Subsequently, they developed different formulae for both genders and validated these for use in the Brazilian population [20]. Kumar et al. also attempted to use Mito’s formula for determining CVBA in the Asian North Indian population [21] and observed that it was reliable only for female patients. They stressed the need for developing a separate formula for males. Varshosaz et al. [22] derived a slightly different formula for the Iranian population.
The slight differences in formulae for the above studies could be explained by the fact that growth patterns tend to vary with race and gender. Zhang et al. [23] showed that ethnic and racial differences can affect growth patterns, and with subjective methods, bone age was overestimated in Asian and Hispanic populations. Therefore, it becomes necessary to identify objective methods of evaluating bone age specific to each ethnic group. Till date, no study exists for objectively evaluating cervical vertebral bone age in the Asian South Indian population. Indians, particularly South Indians, constitute one of the world’s most significant diaspora. Deriving a specific formula for this population would be very relevant in the global scenario.
Although skeletal maturation usually occurs in all seven vertebrae, only the vertebral bodies of C3 and C4 were selected for measurements. The first cervical vertebra is not clearly visible, and the second cervical vertebra shows minimal morphological changes. Cervical vertebrae below C4 cannot be visualized when a thyroid protection collar is worn during radiation exposure [6, 19, 20]. To derive the formula, cervical vertebral ratios that were found to most closely correlate with the patient’s chronological age were used. For both phases of the study, we chose patients who were between 9 and 15 years of age, as this age range corresponds to prepubertal and pubertal growth phase in most patients, and this is when most patients seek orthodontic treatment. The same age range in both phases also ensured reliability of the formulae. We also analyzed male and female patients as separate groups. This was done to account for the differences in the timings of morphological changes in the cervical vertebrae between genders.
In the current study, for both genders, growth acceleration occurred between 13 and 14 years of age. This was reflected through changes in the anterior, middle, and posterior heights, as well as antero-posterior lengths. A distinct and pronounced growth was also noted in the posterior height of C3 in girls and anterior and middle height of C3 in males between 13 and 14 years of age. These findings contrast with those of Caldas et al., who noted that in their study, accelerated growth occurred in anterior, middle, and posterior height of C3 and C4 from 10 to 13 years in females [20]. In males, growth occurred in these regions only in C3, from 12 to 15 years of age, while there was no change in C4. Mito et al., on the other hand, observed that accelerated growth in the anterior, middle, and posterior height occurred between 10 and 13 years of age, in both C3 and C4 of girls [19]. These findings reflect that there is clear ethnic and gender variation in growth of the C3 and C4 cervical vertebrae.
The formula derived by stepwise regression analysis was found to be substantially different from other studies. For Japanese girls, the formula included ratio of anterior height to anteroposterior length and anterior height to posterior height [19]. The study in the Brazilian population focused on ratios of anterior height and middle height to anteroposterior length [20]. In the Iranian population, anterior vertical height alone was found to be a strong predictive factor. In the current formula, however, apart from these parameters, the ratio of the posterior height to anteroposterior length and the ratio of the lower border concavity to the middle height were also taken into account. The depth of concavity was not measured by the previous authors [19, 20, 22]. However, Roman et al. reported that lower border concavity of cervical vertebrae was the best morphological vertebral parameter to estimate skeletal maturation [11]. Therefore, it was measured in the current study, and its ratio with middle vertebral height yielded a significant correlation.
For validating the formula, the Tanner-Whitehouse 3 method was used for evaluation of bone age from hand-wrist radiographs [24]. The Tanner-Whitehouse 3 method is reproducible and reliable and is not as dependent on subjective evaluation as the Greulich and Pyle method, which involves comparison with an atlas [9]. It also allowed for easy comparison of estimated bone age with bone age calculated from cervical vertebrae. The current study showed good correlation between the values derived from both methods. The formulae established in the present study are therefore reliable for objectively determining cervical vertebral bone age and skeletal maturation from lateral cephalograms of Asian South Indian patients of both genders.
Establishing skeletal bone age from cervical vertebrae can in turn predict other variables that may be useful for treatment planning. For instance, Sato et al. found that mandibular growth potential could be accurately assessed based on CVBA, which was useful in planning the timing of treatment, and treatment options in patients with class 3 malocclusions [25]. Studies have also shown that CVBA is correlated with dental eruption [26] and dental maturation of the lower permanent canine and second molar [27]. This can aid in orthodontic treatment planning.
Limitations and future directions
The main limitation of this study is that it was cross-sectional, and measurements for all patients were only taken at one point in time. A longitudinal study would have allowed further validation of the formula at different stages of maturation. However, this would have led to further radiation exposure and may have been prone to attrition bias.
Most formulae generated for calculating cervical vertebral bone age in different ethnic groups are cumbersome and prone to error if worked out manually. The next logical step, as suggested by Caldas et al. [21], would be to develop a software that could automatically calculate CVBA for different ethnic groups based on measurements obtained from cervical vertebrae on lateral cephalograms. Research has already begun in this direction. Kok et al. recently compared the accuracy of different artificial intelligence algorithms for assessing cervical vertebral maturation [28]. However, a common platform that would include formulae for different ethnicities is needed; the world today is a global village and it is not uncommon for orthodontists to encounter young patients from different ethnic groups and populations.
In conclusion, the present study highlights that skeletal maturation indicators have slight variations across diverse ethnic groups, and may require different methods of objective evaluation. The formula derived in the current study has been validated for the Asian South Indian population and may be effectively used to determine cervical vertebral bone age from lateral cephalograms in this population.