A correlation between a new angle (S-Gn-Go angle) with the facial height

A correlation between a new angle (S-Gn-Go angle) with the facial height Esraa S. Jassim B.D.S., M.Sc. (1) Marwan S. Al-Daggistany B.D.S., M.Sc. (1) Jinan E. Saloom B.D.S., M.Sc. (1) ABSTRACT Background: Y-axis has been used by many authors as a growth parameter while horizontal growth of mandible is outlined by moving forward along the Go-Gn line. The purposes of this study were to introduce a new angle that determine the vertical growth direction and establish its relation with the facial heights in different skeletal classes. Materials and methods: The sample consisted of 146 digital true lateral cephalometric radiographs collected from the files of the patients attending the Orthodontic clinic in the College of Dentistry, University of Baghdad. According to the ANB angle, the sample was classified into: CL I (17 males and 21 females), CL II (30 males and 40 females) and CL III (20 males and 18 females). 2 linear and 3 angular skeletal variables were measured using AutoCAD computer program 2007. Descriptive statistics applied for all the measurements for both genders and total sample in each class. Independent samples t-test was used to compare the measurements between both genders, while ANOVA test was used to compare the measurements among the skeletal classes, and Pearson correlation coefficient was applied to find the relation between the new angle and other variables. Results: The mean values of S-Gn-Go angle were established and found to be higher in Class I then Class II and III; also they were higher in males than females. This angle showed direct significant correlation with posterior facial height in all of the skeletal classes. Conclusions: Y-axis and S-Gn-Go angle is very important angles in determination the direction of mandibular growth. Key words: Y-axis, S-Gn-Go angle, vertical growth. (J Bagh Coll Dentistry 2010;22(3):96-100). INTRODUCTION The overall picture of head growth has been studied. The reason for this will bear a word of explanation, it should be recognized that the head is a complex of different parts, each one of which serves different functions (1). The mandible is influenced by the posterior brain base since the glenoid fossa, which receives the mandibular condyle, is a part of the temporal bone. This bone is thought to travel with the occipital bone and hence the mandible would be influenced by the behavior of the posterior cranial base (2). Growth of the mandible occurs essentially at the condyle, the growth at the condyle usually does not occur in the direction of ramus, as is commonly imagined, but slightly forward. Individual variation in the direction of growth at the condyles is large and, in the adolescent period, has been found to vary by almost 45 degrees. Growth is not always linear in direction but usually curves slightly forward or occasionally even backward (3). Ricketts and associates (4) described the mandibular growth pattern as racial in nature. (1) Assistant lecturer / Department of Orthodontics, College of Dentistry, University of Baghdad According to their hypothesis, the mandible rotates around a center of rotation which is exterior to its structure and the radius of the arc determines the direction of growth. Facial skeletal development was independent from the dentoalveolar apparatus, which tend to compensate for basal disharmonies. Schudy (5) related dental distoclusion or mesioclusion to mandibular rotation. He suggested that the vertical drift of the maxillary and mandibular molars is responsible for the vertical growth of the jaw. This concept of vertical drift can vary considerably among different individuals having different facial types as well as between the anterior and posterior parts of the dental arch (6). Downs (7) introduced the Y-axis as a line extended from the point Sella to point Gnathion. He measured the angle formed between this line and the Frankfort plane and called it Y-axis angle. Jarabak (8) and Rakosi (9) modified this angle and measured it between the Y-axis and the SN planes. It has a mean value of 66 degree; if it is greater than that, the mandible is in a posterior position, with growth predominantly vertical. If the angle is less than 66 degree, the mandible is in an anterior position relative to the cranial base, and growth predominantly anterior (7). In addition to the Y-axis angle, the results of anterior and posterior facial height help to Orthodontics, Pedodontics and Preventive Dentistry 96

determine the direction of growth of mandible, in vertical growth, the anterior rotation will result in a relative decrease in the anterior face height whereas posterior rotation results in its increase (8). The purposes of this study were to introduce a new angle that determines the vertical growth direction and to establish its relation with the other measurements in different skeletal classes. MATERIALS AND METHODS Sample The sample consisted of 146 digital true lateral cephalometric radiographs collected from the files of the patients attending the Orthodontic clinic in the College of Dentistry, University of Baghdad. The inclusion criteria for sample selection were: the entire sample was of Iraqi origin with an age ranged between 17 30 years. All had complete permanent dentition regardless to the third molars with no history of previous orthodontic treatment, craniofacial disorders, or facial trauma. The sample was classified according to ANB angle (11) into: 1. Skeletal Cl I: 2 ANB 4 (17 males and 21 females). 2. Skeletal Cl II: ANB > 4 (30 males and 40 females). 3. Skeletal Cl III: ANB < 2 (20 males and 18 females). Methods Cephalometric Analysis Every true lateral cephalometric radiograph was analyzed by AutoCAD computer program 2007 to calculate the angular and linear measurements. Once the picture is imported to the AutoCAD program, it will appear in the master sheet on which the points and planes were determined, and then the angular and linear measurements were obtained. The angle was measured directly as it was not affected by magnification while the linear measurements were divided by scale for each picture to overcome the magnification. Cephalometric landmarks, planes and measurements (Figure 1) A. Cephalometric Landmarks 1. Point S (Sella): The midpoint of the hypophysial fossa (9). 2. Point N (Nasion): The most anterior point on the nasofrontal suture in the median plane (9). 3. Point Go (Gonion): A point on the curvature of the angle of the mandible located by bisecting the angle formed by the lines tangent to the posterior ramus and inferior border of the mandible (11). 4. Point Gn (Gnathion): A point located by taking the midpoint between Pogonion and Menton points of the bony chin (11). 5. Point A (Subspinale): The deepest midline point on the premaxilla between the Anterior Nasal Spine and Prosthion (7). 6. Point B (Supramentale): The deepest midline point on the mandible between Infradentale and Pogonion (7). B. Cephalometric planes 1. Mandibular plane (MP): Formed by a line joining Gonion and Gnathion (9). 2. Sella-Nasion (S-N) plane: It is the anteroposterior extent of anterior cranial base (9). 3. Y-axis: A line from sella turcica to Gnathion (7). 4. N- A line: Formed by a line joining Nasion and point A (7, 9). 5. N- B line: Formed by a line joining Nasion and point B (7, 9). C. Cephalometric measurements 1. Total anterior facial height (AFH): It s measured from N to Gn (12). 2. Posterior facial height (PFH): It s measured from S to Go (9). 3. Y-axis angle: It is the angle between S- N and S-Gn planes anteriorly (7). 4. S-Gn-Go angle: It is the angle between S-Gn and Gn- Go planes. 5. ANB angle: The angle between lines N- A and N-B. It is the most commonly used measurement for appraising anteroposterior disharmony of the jaws (10). Statistical analyses All the data of the sample were subjected to computerized statistical analysis using SPSS version 15 (2006) computer program. The statistical analyses included: 1. Descriptive Statistics; include Mean, standard deviation (S.D.) and statistical tables. 2. Inferential Statistics; include Independent-samples t-test: for the comparison of the measurements between genders in each class. ANOVA test: for the comparison of the measurements among the classes in each gender and in the total sample. Orthodontics, Pedodontics and Preventive Dentistry 97

LSD test: used to test the significant P value of ANOVA between every two groups. Pearson correlation coefficient test: to find the relationship between the measured variables. In the statistical evaluation, the following levels of significance were used: Non-significant NS P > 0.05 Significant * 0.05 P > 0.01 Highly significant ** 0.01 P > 0.001 Very highly significant *** P 0.001 Figure 1: Cephalometric bony landmarks planes and measurements used in this study RESULTS 1. Descriptive statistics, genders difference and classes difference Table 1 showed the descriptive statistics, genders difference and classes difference for the measured variables. Anterior facial height In total sample and male group, the mean value of the anterior facial height is larger in Class III, while in female group, the anterior facial height is larger in Class II. ANOVA test reveals non-significant difference among the classes in both genders and in the total sample. value of the anterior facial height is larger in males than females. Student t-test reveals very high significant differences between both genders in all of the skeletal classes. Posterior facial height value of the posterior facial height is larger in Class I, with a non-significant difference among the classes. In total sample and both genders, the mean value of the posterior facial height is larger in males than females. Student t-test reveals very high significant difference between both genders in all of the skeletal classes. Y-axis angle For the total sample and both genders, the mean value of this angle is larger in Class II than Class I and III with a very highly significant difference among the classes in the total sample, highly significant difference in females, and significant difference in males. Using LSD test, there is very highly significant difference between Class I and II and Class II and III in the total sample, while there is high significant difference between Class I and II in male group and between Class II and III in female group. The mean value of this angle is higher in females compare to that in males in Class I and II while it is higher in males in Class III. Student t-test indicated the presence of significant difference between males and females in Class I only. S-Gn-Go angle value of this angle is larger in Class I followed by Class II then III with significant difference among the classes in the total sample and females. LSD test reveals significant difference between Class I and II and between Class I and III in female group, while there is high significant difference between Class I and III in the total sample. In all of the skeletal classes, the mean value of this angle is larger in males than females. Student t-test revealed a significant difference between both genders in Class II only. 2. Correlation Table 2 showed the correlation between the measured variables. Generally, in all skeletal classes, there is direct very high significant correlation between posterior facial height with S-Gn-Go angle and anterior facial height. S-Gn- Go angle shows indirect weak significant correlation with the anterior facial height in Class II and III. On the other hand, Y-axis angle correlated significantly with positive direction with the anterior facial height and negatively with S-Gn-Go angle in Class II and III. ANB angle showed non-significant correlation with all of the measured variables. Orthodontics, Pedodontics and Preventive Dentistry 98

Table 1: Descriptive statistics, genders differences and classes differences Variables Sex Class I Class II Class III ANOVA test LSD test Mean S.D. Mean S.D. Mean S.D. F-test p-value I-II I-III II-III Total 66.34 3.43 68.72 3.29 66.1 3.5 8.42 0.000 *** *** (NS) *** Male 65 3.02 67.91 3.28 66.4 3.62 3.84 0.03 * ** (NS) (NS) Y-axis Female 67.42 3.42 69.25 3.22 65.78 3.49 5.86 0.005 ** (NS) (NS) ** t-test -2.28-1.54 0.46 p-value 0.03 * 0.13 (NS) 0.64 (NS) Total 36.05 2.94 34.72 3.68 33.82 3.28 3.73 0.03 * (NS) ** (NS) Male 36.35 3.08 35.95 3.77 34.8 2.54 0.97 0.39 (NS) S-Gn-Go Female 35.8 2.89 33.91 3.45 32.78 3.74 3.78 0.03 * * * (NS) t-test 0.56 2.13 1.71 p-value 0.58 (NS) 0.04 * 0.1 (NS) Total 113.96 7.31 114.83 6.21 115.43 7.49 0.39 0.67 (NS) Male 119.11 5.86 118.29 5.81 119.85 5.45 0.34 0.71 (NS) AFH Female 109.79 5.54 112.56 5.43 110.69 6.5 1.68 0.19 (NS) (mm) t-test 5.02 3.82 4.12 p-value 0.000 *** 0.000 *** 0.000 *** Total 75.61 6.75 73.29 5.84 74 6.24 1.61 0.2 (NS) Male 80.66 4.84 78.17 5.26 77.93 4.03 1.69 0.19 (NS) PFH Female 71.52 5.14 70.08 3.55 69.8 5.45 0.87 0.43 (NS) (mm) t-test 5.58 6.99 4.58 p-value 0.000 *** 0.000 *** 0.000 *** Total 3.21 0.66 6.27 1.37 0.08 1.24 340.49 0.000 *** *** *** *** Male 3.12 0.78 6 1.26 0.3 1.34 140.32 0.000 *** *** *** *** ANB Female 3.29 0.56 6.48 1.43-0.17 1.09 201.89 0.000 *** *** *** *** t-test -0.77-1.45 1.17 p-value 0.45 (NS) 0.15 (NS) 0.25 (NS) Table 2: Correlation test between different measurements Variables PHF (mm) AFH (mm) S-Gn-Go Y-axis Class I -0.06 (NS) 0.16 (NS) -0.05 (NS) 0.23 (NS) ANB Class II -0.09 (NS) -0.08 (NS) 0.004 (NS) 0.09 (NS) Class III 0.25 (NS) 0.29 (NS) -0.009 (NS) 0.04 (NS) Class I -0.22 (NS) 0.24 (NS) 0.1 (NS) Y-axis Class II -0.35 ** 0.26 * -0.41 *** Class III -0.23 (NS) 0.38 * -0.59 *** Class I 0.61 *** 0.12 (NS) S-Gn-Go Class II 0.38 *** -0.38 *** Class III 0.42 ** -0.34 * Class I 0.68 *** AFH (mm) Class II 0.6 *** Class III 0.67 *** DISCUSSION Anterior and posterior facial heights In total sample and male group, the mean value of both anterior and posterior facial height is larger in Class III due to forward growth rotation of the mandible, while in female group, the anterior facial height is larger in Class II due to backward rotation of the mandible. value of the posterior facial height is larger in class I, with a non-significant difference among the classes, this may indicate that the sample of this study have no vertical discrepancies. In the total sample and both genders, the mean values of both anterior and posterior facial height are larger in males than females; this comes in agreement with the principle that the females are slightly smaller than males in all dimensions Y-axis angle Y-axis angle represents the anteroposterior position of the chin to anterior cranial base which represented by S-N line, so for that reason, the mean value of this angle is larger in Cl II than Cl I and III due to downward Orthodontics, Pedodontics and Preventive Dentistry 99

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