Longitudinal study of mandibular behavior in Class I subjects with vertical and horizontal growth

Objective: To evaluate possible differences in mandibular behavior in Class I individuals with vertical and horizontal growth patterns. Methods: The sample consisted of 20 untreated Class I individuals divided into: Group 1 comprising 10 individuals with vertical growth pattern and Group 2 comprising 10 individuals with horizontal growth pattern, all of them belonging to the Burlington Growth Center files, University of Toronto-Canada, radiographically followed-up at ages 9, 12 and 21. Cephalometric radiographs, determined mean values for a long-term evaluation of mandibular behavior using the following measurements: SNB, Co-GN, SN.GoMe, anterior facial height and posterior facial height. Results: SNB and CoGn values were higher in horizontal growth group at all of the ages studied; SN.GoMe measure was significantly lower in horizontal growth group; anterior facial height (AFH) showed lower values in individuals with horizontal growth pattern; and posterior facial height (PFH) showed lower values in individuals with vertical growth pattern. Conclusion: Long-term comparisons of Class I individuals’ growth tendencies indicate that there are significant differences between both groups. Mandible showed a trend to clockwise rotation in Group 1. Group 2 showed a trend to brachycephalic facial form, due to the deficit in vertical development with regard to anterior facial height.


IntROduCtIOn and lIteRatuRe RevIew
Aiming a safe and correct orientation of the treatment plan for correction of malocclusions, it is necessary to know the pathological and physiological development pattern, on other words, to know the morphological characteristics of Angle's Class I, II and III malocclusions.Sagittal and vertical development of the mandible can be analyzed in longitudinal growth study in samples of untreated Class I individuals.
The craniofacial growth studies of subjects with malocclusion, orthodonticatlly untreated, are important for the assessment of the effects induced by mechanotherapy, since the growth of these individuals is extremely variable, difficult to predict and different when compared to the growth of individuals with normal occlusion, thus complicating the interpretation of the effects of mechanotherapy during growth. 28gan et al 21 conducted a comparative longitudinal study of skeletal changes of subjects with Class I and Class II malocclusion Division 1.It was used a sample of 40 individuals (20 Class I and 20 Class II) ranging from 7 to 14 years.The changes observed in Class II individuals, compared to Class I, are described as follows: The mandible (SNB and SNPog) was significantly more retruded, measures such as (Ar-NG) and (Go-Gn) were found reduced, the Y axis and mandibular plane angle increased, in order to contribute to the retruded position of the mandible in these individuals.
Buschang and Martins 7 observed the anteriorposterior and vertical skeletal development relationship of mandible of Class I and II individuals from 6 to 15 years.The results showed that the vertical and anterior-posterior relationship are not stable during growth and vary according to age, gender and type of malocclusion.
Chung et al 9 analyzed the skeletal and dental morphology of 85 Class II untreated patients, from 9 to 18 years , from the records of Bolton-Brush and Burlington Growth Center.It was investigated the craniofacial growth of those individuals.It was observed a decrease in the mandibular plane angle in all groups and counterclockwise rotation of the mandible, however individuals who had decreased mandibular angle had a greater rotation.
Thilander et al 27 conducted a longitudinal study with subjects from 5 to 31 year-old in Swedish population.The sample was composed of cephalometric radiographs of 469 subjects with normal occlusion orthodontically treated.The results showed that the mandibular length increased in male subjects ; mandibular growth spurt occurred between 13 and 16 years of age.They observed continuous decrease of goniac angle and, consequently, mandibular counterclockwise rotation and the SNB angle increased continuously during the observation period (77 to 81 degrees) in these individuals.
Previous studies shown that sagittal facial growth consists of vertical growth (base) and horizontal growth (front).If the vertical growth of facial sutures and alveolar process is greater than the growth of the condyle, the mandible turns back (clockwise direction), resulting in greater anterior facial height.On the other hand, if vertical growth of the condyle is larger than the sum of the components of vertical growth of facial sutures and alveolar process, the mandible turns to front (counterclockwise).The final growth vector is the result of the competition between horizontal and vertical growth.Longitudinal studies on growth may help answer some clinical questions: What changes might occur in the mandible during growth?Can the clinician detect during growth different directions of mandibular growth in Class I individuals? 5,15,18n the literature, studies concerning the development of Class II malocclusion are more frequent. 1,2,4,6,7,8,11,16,17However, the literature shows that the morphological characteristics of malocclusions are still controversial and debated.Longitudinal studies of growth evaluating the development of Class I, 4,12,27 in horizontal and vertical patterns are scarce.
Based of the reduced number of researches concerning the assessment of differences in vertical and sagittal development of the mandible in untreated Class I individuals at different times of growth, new studies are anticipated with the aim of knowing the characteristics of these individuals, to evaluate facial shape tendency and to investigate longitudinal changes in mandibular growth.The knowledge of such data is critical to the success of orthodontic treatment, allowing the clinician to understand the behavior, of the mandible during growth and possible implications during the orthodontic treatment.

MateRIal and MetHOdS
The sample comprised of 60 radiographs from the Burlington Growth Centre, University of Toronto, Canada, based on longitudinal records of 20 Class I individuals orthodontically untreated followed up at ages 9, 12 and 21 years, being 10 individuals with vertical growth pattern and 10 horizontal .
In this longitudinal study, Group 1 with Class I malocclusion, vertical growth pattern (n = 10; 5 male and 5 female) was compared with Group 2, comprised of Class I malocclusion with horizontal growth pattern (n = 10; 5 male and 5 female).The sample was divided into 2 groups (vertical and horizontal), based on cephalometric measures in T 1 : SNA, SNB, ANB, SN.GoMe, NAP. 25,26Normal values of these measures for initial age of 9 years were described by Riedel, 23 SNA = 80.79, SNB = 78.02,ANB = 2.77, SN.GoMe = 32.27,NAP = 4.22, values above or below the standard deviation of each measure characterize individuals with vertical or sagittal growth tendency, allowing the formation of two groups in this study.
The individuals were included according to the following criteria: 1) present cephalometric radiography from 9 to 21 years of age, 2) Class I malocclusion (ANB ≤ 4° and >0° determined in T 1 at 9 years 23 3) Be healthy and without history of orthodontic treatment.
The sample belonging to the files of University of Toronto, Burlington Growth Centre Department was documented in the period 1952 to 1971.This material is the most extensive longitudinal study of craniofacial growth held up today.All the radiographs were made in the same X-ray machine with magnification of 9.84%, with a constant focus / object distance and fixed in 152.4 cm and the chassis was positioned at a distance of 15 cm in the midsagittal plane of the individuals. 22In this study, 60 cephalometric tracings of the profile were conducted by the same operator, using 0.3 mm mechanical pencil with Ultraphan paper in negatoscope, in a dark room, framed with black cardboard, exposing the corresponding areas to anatomical drawing.
The points were then entered into a Numonics AccuGrid digitizing table and evaluated in computer by means of Dentofacial Planner Plus 2.01 software.Cephalometric analysis was composed by angular and linear measures (Figs 1 and 2), obtained through the program of computerized cephalometrics DFPlus.The mandibular behavior was examined at intervals of ages with reference to the lateral cephalograms.
To differentiate horizontal or vertical mandibular development component, the following linear and angular measures were used: SNB and Co-Gn, for analysis of horizontal position of the mandible (

ReSultS
The comparison of the mean of each of the measures in the two groups in each time was carried out using the Student's t test for equality of two independent populations.Results show that the means of the measures SNB, SN.GoMe and AFH were different in the two groups, in all ages considered.The SN.GoMe value was greater in the group with horizontal growth at all ages.The SN.GoMe means were significantly higher in the group with horizontal growth and the AFH means were significantly higher in the group with vertical growth.
While there were significant differences in the means of Co-Gn and PFH variables, it was observed that the means of the measure Co-Gn and PFH were always higher in the group with horizontal growth pattern (Table 1).
To evaluate possible differences in the growth pattern of the two groups, the change that occurred in each individual, in two distinct ages, (between 9 and 12 years, between 12 and 21 years and between 9 and 21 years) was calculated, for each variable.The results presented in Table 2 show that, except for changes in SN.GoMe variable occurred between 9 and 21 years of age, there is no statistically significant difference in the mean of all other measures in any period considered.Despite the variation among the age groups have undergone changes in mean that can be clinically considered, when statistically measured, only SN.GoMe presented significant changes between 9 and 21 years of age.

dISCuSSIOn
In this longitudinal study, the sagittal and vertical mandible development on sample of Class I individuals with horizontal and vertical pattern were studied longitudinally in a period of clinical growth significance: Between 9, 12 and 21 years of age.

Sagittal development
In respect to the sagittal positioning of mandible among individuals with vertical and horizontal growth pattern a statistically different position between the two groups was observed.The angular (SNB) and linear (Co-Gn) measures indicated a mandibular retrognathism in individuals with Class I with vertical growth showing statistically significant differences between groups in SNB measures (Fig 3; Tables 1 and 2).The value of SNB and Co-Gn was higher in the group with horizontal growth in all ages.Thilander et al 27

vertical development
The vertical position was evaluated on the basis of angular and linear measures, SN.GoMe angle anterior facial height and posterior facial height (Fig 2; Tables 1 and 2).
The measure Sn.GoMe in individuals with horizontal growth pattern was significantly lower than in individuals with vertical growth.This angle shows a tendency to decrease with aging in subjects with hori-  1 and 2).The SN.GoMe and AFH were always lower in the group with horizontal growth, which is in agreement with other authors following a pattern already demonstrated by Iseri et al. 12 The posterior facial height (PFH) presented lower values in all age groups in individuals with vertical growth when compared to individuals with horizontal growth, although not statistically significant.The posterior facial height also influences other measures.Class I individuals with vertical growth presenting increased Sn.GoMe values, for example, show a tendency to a clockwise rotation of the mandible (Fig 7, Tables 1 and 2).

Facial morphology
The anterior facial height showed significant increase in Group 1 (vertical) when compared with Group 2. Changes were noted in this measure among the ages in Group 2, as the increase of anterior facial height is significantly lower in this group.

COnCluSIOnS 1. Sagittal development
In the sample of individuals with vertical growth pattern, a greater retrognathism was noted when compared with the other group.The retrognathic position of the mandible, in the vertical group, was present at 9 years of age, and the growth increment was similar in both groups studied.
In the group with horizontal growth pattern the position of the mandible varied, presenting more orthognathic position.

vertical development
In the group with horizontal growth pattern, there was a deficit in terms of anterior facial height.This deficit was not found in the vertical facial development group.In addition, increases in development of anterior facial height at the time between 9 and 21 years of age were significantly lower in Class I horizontal pattern.

Facial shape
Despite of inter-group differences being statistically significant for some measures, Class I horizontal group showed a more brachycephalic tendency in facial shape.It is suggested that the deviation of the facial morphology could be caused by the deficit in vertical development of anterior facial height.
Fig 1).The vertical position was analyzed by means of the following measures (Fig 2): SN.GoMe angle, N-Me (anterior facial height) and S-Go (posterior facial height).

riod, from 3
to 21 years in Class I individuals.Bishara et al 4 longitudinally assessed Class II and normal individuals.In normal individuals the SNB angle increased during the period, not showing a significant Class II difference.

Table 1 -
observed that the SNB angle increased continuously during the observation pe-Means and standard deviations of the measures by age and group, and results of Student's t test for the equality of means (independent populations).

Table 2 -
Means and standard deviations of the changes in the measures, by group and period, and results of the Student's t test for the equality of averages (independent populations).Figure 4 -Means of SN.GoMe measures in each age and group.zontalgrowth.On the other hand, in individuals with vertical growth, this angle remains virtually the same between 9 and 21 years of age (Figure3, Table1 and 2).
Figure 3 -Means of SNB measures in each age and group.