J Bagh College Dentistry Vol. 29(2), June 2017 Assessment of Pedodontics, Orthodontics and Preventive Dentistry 104 Assessment of mandibular third molar position by using computed tomography and reconstructed lateral radiograph Istabraq M. Mohamed, B.D.S. (1) Nidhal H. Ghaib, B.D.S., M.Sc. (2) ABSTRACT Background: Consideration of mandibular third molar is important from orthodontic perspective due to several factors such as, lower anterior arch crowding, relapse in lower anterior region, interference with uprighting of mandibular first and second molars during anchorage preparation and molar distalization. The aims of this study were to assess of gender differences in the mandibular third molar position and compare and evaluate whether there is any differences in the results provided by CT scan and lateral reconstructed radiograph. Materials and Methods: The sample of present study consisted of 39 patients (18 males and 21 females) with age range 11-15 years. CT images for patients who were attending at Al Suwayra General Hospital/the Computerized Tomography department. Computed tomographic images were obtained for The distance from Xi point to distal surface of permanent mandibular second molar was measured in both three dimensional volumetric images and two dimensional CT derived lateral image. The statistical analyses included: means, standard deviations. Paired t-test was used to compare between the two methods and independent t-test was used in verifying the genders difference. Results: The results showed that there was high significant method difference between 3D CT and 2D image and gender differences was observed in values of linear measurements of present study, as males showed higher mean values than females. Conclusion: There is high accuracy of measurement on CT images, so CT scan is advisable during the diagnosis and treatment plan of orthodontic cases. Key words: Mandibular third molar, computed tomography, lateral radiograph. (J Bagh Coll Dentistry 2017; 29(2):104-107) INTRODUCTION The third molar is a tooth that show great difference in its formation, developmental position and size compared to the other teeth in the jaws. This variability in third molars is due to the gradual dimension in the growth of the jaw bone with a consequent reduction in the space available for the teeth (1). Since third molars are the last tooth to form and erupt, they will become liable to be affected by abnormalities like congenital absence, ectopic eruption and impaction (2). This has led to advise removal of the third molar bud at the age of 7-10 years when unsuccessful eruption is predicted (3). The evaluation of mandibular third molar depends on good clinical diagnosis and radiographic aid examination like periapical, panoramic and cephalometric radiograph to analyze the different factors that related to the surrounding structures such as; the amount of space available, angulation of the tooth, its height in the jaw, its relation to the mandibular second molar and to the occlusion (4). The mandibular third molar exhibits the highest rate of impaction. The rates, as reported by Quiros and Palma (5): -------------------------------------------------------- (1) orthodontist wasit ,Ministry of Health. (2) Professor, Depart ment of Orthodontics, College of Dentistry, University of Baghdad.  Hellman 9.5 %  Björk 25 %  Ricketts 50 %  Richardson 35 % The ability for evaluation of the third molars position is important for dentist (6). If they erupt, they have advantage for anchorage, prosthetic abutments, or transplantation (7). If they impacted, they have disadvantage because of adjacent root resorption, inflammatory process, temporomandibular joint problem and late lower anterior crowding (8); thus early removal minimize risk to a patient as extraction later in life (9). Early removal of third molar can minimize the risk of post-operative complications related to surgery on a fully developed third molar such as nerve damage with parasthesia, dry socket, inflammation, bleeding, and pain (10). Thus, assessment of third molar position and its eruption is important for the patient management. MATERIALS AND METHODS The sample of the present study consisted of 39 patients (18 males and 21 females with mean age of 13 years) who were attending at Al- Suwayra General Hospital/ the Computerized Tomography department, who met a special selective criteria were selected. The following criteria were used in the selection of the total sample: J Bagh College Dentistry Vol. 29(2), June 2017 Assessment of Pedodontics, Orthodontics and Preventive Dentistry 105 1- Iraqi Arab subject their age from 11-15 years. 2- Normal general health status, by taking medical history from parents. 3- Skeletal Class I relationship assessed in three planes of space(11). 4- No history of dentofacial deformities, pathologic lesions in the jaws or facial trauma. 5- Full set of teeth with developing mandibular third molar. 6- No congenital missing or supernumerary teeth 7- Normal overbite and over jet (2-4 mm) measured by sliding caliper (Dentarum ® – Germany). 8- No shifting in dental midline. 9- Mild crowding (not more than 2 mm) measured by sliding caliper (Dentarum ® – Germany). 10- Mild spacing (not more than 2 mm) measured by sliding caliper (Dentarum ® – Germany). 11- No previous orthodontic treatment like habits breaker or chin-cap. Methods For every patient in the sample; a clinical examination and computerized tomographic imaging had been done using Brilliance™ 16 CT (Philips C, Netherland), then the CT images were collected from the workstation of the CT unit of and the imaging data were analyzed with the software provided by the manufacturer. Firstly, the mesio-distal crown dimension of mandibular 1st molar was measured clinically using vernier. This is done to compare it with the measurements obtained from the 3D and 2D images. On each image, the distances from "Xi" point to the distal surface of permanent mandibular second molar (12), in both 3D and 2D images was measured. Xi point: A point located at the geometric center of the mandibular ramus. Location of Xi is keyed geometrically to Frankfort Horizantal plane (FH) and perpendicular through Pt (pterygoid vertical [PtV]; a line perpendicular to FH at the posterior margin of the pterygopalatine fossa), in the following steps as show in figure [1](13): 1. Planes perpendicular to FH and PtV are constructed. 2. The constructed planes that tangent to points R1, R2, R3, and R4 on the borders of the ramus. R1-mandible: The deepest point on the curve of the anterior border of the ramus, one half the distance between the inferior and superior curves. R2-mandible: A point located on the posterior border of the ramus of the mandible. R3-mandible: A point located at the center and most inferior aspect of the sigmoid notch of the ramus. R4-mandible: A point on the lower border of the mandible, directly inferior to the center of the sigmoid notch of the ramus. 3. The constructed planes form a rectangle enclosing the ramus. 4. Xi is located in the center of the rectangle at the intersection of the diagonals. (Figure1):Obtaining the location of Xi point Statistical Analysis All the data of the sample was subjected to computerized statistical analysis using SPSS for windows XP. The statistical analysis included: A. Descriptive statistics  Means.  Standard deviations.  Statistical tables. B. Inferential statistics Paired sample t-test: it was used to compare the measurements between the CT and the reconstructed lateral view. Independent sample t- test was used to verify the gender differences. RESULTS Table 1 and 2 showed the descriptive statistics and gender difference of the measured variables in 3D and 2D images. Generally, the mean values was slightly higher in males than females. J Bagh College Dentistry Vol. 29(2), June 2017 Assessment of Pedodontics, Orthodontics and Preventive Dentistry 106 Comparing the two methods of measurements revealed highly significant difference between them in all measurement with 3D measurements slightly larger than 2D (Table 3). Paired sample t-test was done to detect the method difference in the mean values for the permanent mandibular first molars mesio-distal width between the direct clinical measurement and the 3D CT and 2D images. A high significant difference was found between the direct clinical measurements and the 2D image and between the 3D and 2D methods with the same mean value for the clinical and 3D methods as seen in table (4). DISCUSSION It is important to mention that direct comparisons with results from other studies will not be always possible, since this study represents the first approach to compare the 3D CT and the 2D reconstructed lateral view in the assessment of mandibular third molar position. The age of samples ranged between 11-15 years old because development of mandibular third molar was not completed at this age, early removal of third molar at this age is simple and atraumatic (12). About the distance from Xi point to distal surface of permanent mandibular second molar, the result of the present study was agreed with the finding of Forsberg et al. (14) and Venta et al. (15) there was gender difference in the mean value of this measurement, since the mean value of this measurement in males higher than females. In present study, all the measurements on 3D and on 2D images show statistically high significant difference between them. This may be explain by that the two dimensional diagnostic imaging including the reconstructed lateral view have certain analysis limitations such as geometric distortion, superimposition of structures, rotational errors and linear projective transformation. To compare between the clinical and image method of measurement, the mean value of the width of mandibular 1st molar measured clinically and by 3D image is coincide, while it is about 0.8 mm smaller than 2D image. This result gives an impression about the accuracy of 3D image in measurement and diagnosis of orthodontic problems. Although the method difference is statistically significant but clinically is of no value (0.3). Table 1: Descriptive statistics and gender difference for the variables measured in 3D image Measurements Descriptive Statistics Gender Difference (d.f.=37) Total sample (N=39) Males (N=18) Females (N=21) Mean S.D. Mean S.D. Mean S.D. Mean Difference t-test p-value Xi to 7(mm) 19.98 0.77 20.02 0.83 19.96 0.72 0.06 0.24 0.812(NS) Table 2: Descriptive statistics and gender difference for the variables measured in 2D image Measurements Descriptive Statistics Gender Difference (d.f.=37) Total sample (N=39) Males (N=18) Females (N=21) Mean S.D. Mean S.D. Mean S.D. Mean Difference t-test p-value Xi to 7(mm) 18.74 0.81 18.77 0.80 18.71 0.83 0.06 0.20 0.843(NS) Table 3: Descriptive statistics and image difference for the variables measured Measurements Descriptive Statistics Image comparison (d.f.=38) 3D image 2D image Mean S.D. Mean S.D. Mean Difference t-test p-value Xi to 7 19.98 0.77 18.74 0.81 1.25 34.47 0.000 (HS) J Bagh College Dentistry Vol. 29(2), June 2017 Assessment of Pedodontics, Orthodontics and Preventive Dentistry 107 Table 4: Descriptive statistics and measurements difference for the MD width of permanent mandibular first molars MD of 6 measurement Descriptive Statistics Measurements difference (d.f.=38) Mean S.D. Mean difference t-test p-value 3D image 10.28 0.31 0.80 125.73 0.000 (HS) 2D image 9.48 0.30 Clinical 10.28 0.32 -0.001 -0.007 0.994 (NS) 3D image 10.28 0.31 Clinical 10.28 0.32 0.799 11.531 0.000 (HS) 2D 9.48 0.30 REFERENCES 1. Bindayel A. The role of third molar in orthodontic treatment. Pakistan Oral & Dental J 2011; 31(2): 374- 7. 2. Hellman M. Our third molar teeth, eruption, presence and absence. Dent Cosmos 1936; 78: 750-62 3. Ricketts, RM, Roth, RH, Chaconas, SJ, Schulhof, RJ, Engel, GA. Orthodontic diagnosis and planning. in: Rocky Mountain/Orthodontics, Denver, Colorado; 1982:243–260. 4. Laskin DM. Evaluation of third molar problem. JADA 1971; 82: 824-8. 5. Quiros J, Palma A. The mandibular third molar: A method for predicting its eruption. Ortho J 1999; 2(4): 8-15 6. Qamruddin I, Qayyum W, Mahmood S, Wasif S, Rehan F. Differences in various measurements on panoramic radiograph among erupted and impacted lower third molar groups. J Pak Med Assoc 2012; 62(9): 883-7. 7. Hattab FN, Alhaija ES. Radiographic evaluation of mandibular third molar eruption space. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999; 88: 285-91. 8. Daley TD. Third molar prophylactic extraction, a review and analysis of the literature. Gen Dent 1996; 44: 310-2. 9. Venta I, Murtomaa H, Ylipaavalniemi P. A device to predict lower third molar eruption. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997; 84: 598- 603. 10. Niedzielska IA, Drugacz J, Kus N, Kreska J. Panoramic radiographic predictors of mandibular third molar eruption. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006; 102: 154-8. 11. Foster TD. A Textbook of Orthodontics. 3rd ed. Oxford; London: Blackwell Scientific Publications; 1990. p. 83-5. 12. Ricketts RM. Third molar inoculation: Diagnosis and Technique. J Calif Dent Assoc 1976; 4:52-7. 13. Bjork A, Jensen E, Palling M. Mandibular growth and third molar impaction. Acta odonto Scand 1956;14:231-72. 14. Forsberg CM, Vingren B, Wesslen U. Mandibular third molar eruption in relation to available space as assessed on lateral cephalograms. Swed Dent J 1989; 13(1-2): 23-31. 15. Venta I, Murtomaa H, Turtola L, Meurman J, Ylipaavalniemi P. Assessing the eruption of lower third molars on the basis of radiographic features. Br J Oral Maxillofac Surg 1991; 29: 259-62. المستخلص رجوع من الضروري االخذ بنظر االعتبار موقع سن العقل االسفل من الناحیھ التقویمیھ وذلك لعدة عوامل منھا ازدحام االسنان االمامیھ السفلى, وتداخل مع عالج االضراس السفلى. الحالھ في المنطقھ االمامیھ السفلى بعد العالج لعقل األسفل ومالحظھ الفرق بالنتائج بین الجنسین بواسطھ االشعة ثالثیھ االبعاد واالشعة الجانیھ ھذا البحث یھدف الى تحري ومقارنھ موقع سن ا ثنائیھ االبعاد المصنعھ بواسطھ جھاز المفراس الحلزوني. )سنة وقد تم 15- 11لقد تم فحص وتشخیص تسعھ وثالثین مریضا (ثمانیھ عشر من الذكور واحدى وعشرین من االناث) وباعمار تراوحت ما بین ( (تم تقیمھا باستخدام االشعھ المقطعیھ ثالثیھ االبعاد) والطریقھ الثانیھ (تم تقیمھا باستخدام االشعھ الجانبیھ المصنعھ تقیمھم بطریقتین:الطریقھ االولى ثنائیھ االبعاد) وقد تم اخذ التقیم االتي لكل طریقھ: المسافھ من المركز الھندسي لعظم الفك االسفل الى اقصى الضرس الثاني االسفل. المعدل الحسابي بالنسبھ للجنس,حیث تبین ان المعدالت الحسابیھ للقیاسات الماخوذه للذكور اعلى من االناث.لقد ظھر وجود فرق ب ه الدراسة.كذلك لوحظ وجود اختالفات ذوات اھمیھ احصائیھ ما بین االشعھ الثالثیھ االبعاد والثنائیھ االبعاد فیما یتعلق بالقیاسات المتعلقھ بھذ الدراسھ ان االشعھ الحلزونیھ ثالثیھ االبعاد توفر معلومات دقیقھ وقیمھ, لھذا ننصح باستخدام المفراس المقطعي الحلزوني تبین وفقا لنتائج ھذه الثالثي االبعاد خالل التشخیص والتقیم العالجي لحاالت التقویم.