Growth modification treatments require information on the growth potential. The orthodontist should be aware whether growth is happening and how much of the remaining incremental growth should be expected [13]. Moreover, evaluation of time of occurrence of this growth is also crucial [8].
As indicated by the results of several studies [3, 4, 7–12], the stages of cervical vertebral maturation are related to mandibular growth changes taking place during puberty. The main goal of the present study was to provide the orthodontist with an easy tool to determine the mandibular GP. This was to be accomplished by analyzing the changes of the cervical vertebrae on the lateral cephalometric radiograph of the patient head, a type of film used routinely in orthodontic diagnosis.
In this study, only girls were examined because of sex-dependent differences with regard to the timing of morphological changes in cervical vertebral bodies [6]. A group of 9- to 11-year-old girls was considered because usually at these ages, children refer to orthodontists. On the other hand, maturation indicators of brief duration are more informative than those of the longer duration [14]. A wide age range of the population may affect the correlation result because of the inability of skeletal maturity methods to detect changes in skeletal maturity precisely when the subjects are either too young or too old, i.e., too far ahead of or too far past the pubertal growth spurt [15]. Ar-Pog was used in this study as mandibular length because it is easily located; [16] the difference between the two stages was used as the MLI.
A stepwise regression analysis was used in this research to define prediction models that could be used to forecast individual future growth changes of the mandible. The stepwise method was used to select the explanatory variables. As the result of the statistical analysis on the present sample, two independent variables (AP3 and PH4) were significantly selected amongst variables studied in order to explain the dependent variable MLI by MLI = 76.21-3.145(AP3) − 2.677(PH4). AP3 and PH4 showed significant statistical correlation (P < 0.001) with MLI. The variability of the dependent variable, which is defined by R2, was 57.7% and adjusted R2 was 54.9%. According to the statistical rule, the number of samples must be at least twice as many as the number of independent variables [17]. The present study consisted of 33 cases which was a satisfactory number to make the regression coefficients, and R2 and adjusted R2 values truly represents the actual population.
Here, adjusted R2 is presented, which is a modification of R2 that adjusts for the number of explanatory terms in a model. Unlike R2, the adjusted R2 increases only if the new term improves the model more than what would be expected by chance. The higher R2 while there were fewer independent variables exhibits an easier method with better prediction accuracy [17].
Chen et al. [12], reported that the combination of AH3, AH4, and AP3 explained the variability of MLI (R2 = 61.35%). The reasons for the differences between their results and the present study can be racial factors in addition to the limited age range in our study. Furthermore, subjects in their study include both Class I and Class II subjects.
The longitudinal nature of this study allows determination of the prediction accuracy of our equation by best means. So, the actual growth can be determined individually in all subjects. As reported in several studies [2, 4, 7, 8, 10, 11], the peak in mandibular growth occurs from CVMS II and CVMS III, and the duration of this peak interval in normal occlusion is 1 year [18]. In our study, all the subjects were in CVMS I or II at the time of first examination. The 24-month interval allowed reaching the CVMS III in nearly all of the cases. Thus, it can be said that the peak has occurred in the majority of subjects.
The formula obtained from this study can be easily adopted in the daily clinical practice as shown in Figures 2, 3, 4, and 5. S.J. is a 10-year and 2-month-old patient who was referred to our private practice in February 2008 with a chief complaint of her lower incisor crowding. Her parents were also concerned about her slightly prominent chin. Our formula can be useful in predicting further mandibular growth and discussing our prediction with the patient/parents. Lateral cephalogram was obtained, and mandibular growth increment was calculated. In March 2010, another cephalograms were obtained after finishing orthodontic treatment. After superimposition on the mandible, it turned out that our prediction was reasonably accurate.
We compared the predictive accuracy of our equation with the method of Mito et al. [11], which is suitable for predicting mandibular growth potential in skeletal Class I patients, and our method showed a better prediction accuracy. The reason for lower prediction accuracy for Mito et al.'s method in our subjects can be explained as follows:
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1.
The formula presented by Mito et al. [11] was designed for Japanese girls. Racial, environmental, and genetic differences between Japanese and Iranian girls can result in differences in developmental biologic clock.
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2.
The subjects consisted of a wider range in Mito et al.'s study [11] (7 to 21 years old) which included individuals before, during, and after growth spurt. However, in our study, the samples were in a more limited age range.
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3.
Our study included only subjects with Class I normal occlusion but Mito et al. [11] used Class II as well as Class I cases.
Our equation predicted the MLI in circumpubertal growth spurt, which is a golden period in regard to treatment efficiency [13].
In a study conducted by Fudalej and Bollen [19], the effectiveness of CVM method in 15 to 27 years old orthodontic patients in postpeak period was evaluated. They found this method as only modestly effective. This may seem controversial with the findings of our study; nevertheless, as they mentioned, there is some weakness in this mixed longitudinal study design, namely:
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a)
The unknown amount of late postadolescent growth
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b)
Although few but there were statistically significant differences between orthodontic patients and those with a normal occlusion (such as our samples) in terms of growth.
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c)
Data of both genders should not be combined when the aim is to present mean ages.
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d)
Furthermore, the samples analyzed by Fudalej and Bollen [19] were in a far later stage of CVM than ours.
These may explain different conclusions
According to the longitudinal survey of Ball et al. [20], the most frequent stages at which mandibular increment occurs is CS4 which is compatible with stage II and III in the method by Baccetti et al. [7]. This is in complete agreement with our study in which most of the cases where in CVMS II through III. On the other hand, Ball et al. [20] concluded that it is not possible to predict peak maturation growth velocities by cervical vertebrae alone, this is a controversial issue. However, any controversies between two conclusions is due to differences in case samplings: our findings are based on 9 to 11 years old girls with Class I occlusion, whilst the study by Ball et al. [20] concentrated on 9 to 18 years old boys without any particular Class of Angle's classification.
In the current study, there are few limitations regarding age and gender of samples used as well as their ethnicity that is limited to Iranian girls. These limitations can be addressed in future studies.