|Year : 2012 | Volume
| Issue : 6 | Page : 795-800
|Relationship of third molar movement during orthodontic treatment and root angulation
Morteza Oshagh1, Nariman Iraji Nasrabadi2
1 Member of Orthodontic Research Center, Associate Professor, Department of Orthodontics, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran
2 Dentist, Student Research Committee, Shiraz Biomaterial Research Center, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran
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|Date of Submission||15-Jul-2010|
|Date of Decision||28-Sep-2011|
|Date of Acceptance||13-Oct-2011|
|Date of Web Publication||3-May-2013|
| Abstract|| |
Background: Curved roots of teeth might cause difficulty in root canal treatment and removal. Beside genetic factors, root morphology of the third molars is affected by environmental factors such as mesial migration. However no studies examine the relation between the amount of third molars' movement and their root angles. The purpose of this study was to evaluate the relation between the amount of third molar movement during orthodontic treatment and the angulation of third molar roots.
Materials and Methods: The records of 39 patients who had apical closure after orthodontic treatment were evaluated. Root curvature was measured on panoramics using the angle between an occlusal reference line and central line of the most apical ninth of the root. The amount of tooth movement of third molars was measured on the pre- and post- treatment lateral cephalometric radiographs by measuring tooth distance from a perpendicular line. Pearson correlation analysis was preformed to obtain relation of the amount of third molar's movement and its root angulation.
Results: In maxillary arch the correlation between tooth movement and root angulation was positive and not significant relation but in mandibular arch this relation was negative and not significant. ( P > 0.05)
Conclusion: The relation between the amount of third molar movement and its root angulation was not statistically significant. Therefore movement of third molars during orthodontic treatment should not be assumed as an etiologic factor of root angulations.
Keywords: Orthodontics, root angulation, third molar movement
|How to cite this article:|
Oshagh M, Nasrabadi NI. Relationship of third molar movement during orthodontic treatment and root angulation. Indian J Dent Res 2012;23:795-800
Human teeth are provided mostly with curved roots , and angled roots might cause difficulty in both root canal treatment and removal.  Angled roots may also contribute to eruption disorder because the resultant force is upward when the root grows down in the bone.  Therefore determination of the cause of angled roots would facilitate prevention and treatment, especially in the case of removal. 
|How to cite this URL:|
Oshagh M, Nasrabadi NI. Relationship of third molar movement during orthodontic treatment and root angulation. Indian J Dent Res [serial online] 2012 [cited 2020 Dec 4];23:795-800. Available from: https://www.ijdr.in/text.asp?2012/23/6/795/111263
Although many studies have described the effect of genetic factors on dental development, ,, it is not clear whether the angulation of the roots is related to genetic disorders or whether it has multiple causes, including environmentally acquired conditions such as infection, trauma, scar formation, inadequate space, tooth migration or movement by orthodontic treatment during tooth formation. ,,,,
Variation in tooth and root shape of the third molars within a population could be the result of environmental factors. , Incomplete eruption of third molars caused by limited space may result in angled roots through induction of root formation.  Yamaoka et al. found that angled roots occur more frequently with incomplete impaction than with full eruption. They also stated that a suitable environment may produce full eruption and a normal shaped root. 
Beside genetic factors, root morphology is also affected by mesial migration.  Marklund and Persson  described that variation in root curvature is associated to the mandibular growth pattern, which in turn is correlated with the magnitude of forward tooth migration. The differences in root forms may be related to variations in the mesial tooth migration or movement of dentition. Bicakci et al. found that mesial tooth migration of mandibular third molars reduces the amount of root curvature developing on this tooth. They studied tooth migration after early loss of mandibular first molar and they only stated that the extraction space was fully or partly closed but they did not measure how much mesial migration had occurred. Also they did not evaluate root morphology in maxillary arch. 
In the history of orthodontics, almost every tooth has been considered for extraction and the extraction therapy is associated with mesial movement of the molars. ,,, This mesial movement of third molars during orthodontic treatment would increase the eruption space and thus compression in bone mass would be decreased.  The presumption is that this suitable environment may produce normally shaped roots. The effects of extraction of premolars and molars on the eruption of the third molars were evaluated by several investigations. Bjork et al.,  Fanning,  Plint,  Richardson et al. and Richardson  reported that the closer the extraction site is to the third molar, the more effect it will have on the development of that tooth. Many researchers have attempted to determine the effect of second molar extraction on the eruption or developmental condition of the third molars. ,,,,,, However no studies were found examining the relation between the amount of third molars' movement and their root angles. The purpose of this study was to evaluate the relation between the amount of third molar movement during orthodontic treatment and the angulation of third molar roots.
| Materials and Methods|| |
The material consisted of pre- and post-treatment panoramic and cephalometic radiographs of 39 white Iranian adolescent patients (30 girls, 9 boys) who had finished orthodontic treatment. Patients' record were selected from the records of individuals treated in orthodontic department of Shiraz University of medical sciences, Shiraz- Iran from 2004 to 2009. All of these patients had orthodontic treatment by pre- adjusted orthodontic appliance (0.22 inch slot dimension, MBT (McLaughlin-Bennett-Trevisi), Dentaurum- Germany) by one expert orthodontist with routine sequence of wires and biomechanics. No third molars were banded or bonded. These patients had been treated non- extraction ( n = 4) or with the extraction ( n = 35).
The pre- treatment subjects' chronological ages range from 13 to 24 years. The mean age was 16.20 ± 2.64 years old. The mean duration of orthodontic treatment was 3.20 ± 0.65 years.
- Free of any known serious illness: The subjects had no craniofacial disorders (e.g., cleft lip and/or palate).
- Normal growth and development and dental conditions.
- Those patients selected that their development stage of third molars before orthodontic treatment was not reached to apical closure stage and after orthodontic treatment had apical closure according to Demirjian et al.'s study.  This was evaluated by assessing pre- treatment panoramic radiographs.
Records of 320 patients were evaluated and 281 patients were excluded for poor radiographic quality, inadequate records and completed apical formation of third molars before treatment or not complete apical formation after treatment. The final sample consisted of 39 patients. Radiographs (panoramic and cephalograms) with good quality images which were taken at the same radiology center were used in this study.
- Image deformity affecting third molars: The patients whose panoramic radiographs did not allow identification of the apices of the third molars were excluded.
- Panoramic showing obvious dental pathology around third molars: The third molars showed no decay extending to the pulp and had no periapical lesions. No tooth which was in doubt due to poor radiographic quality or the stage of development was recorded in this study.
- Patients with tumor, ameloblastoma, follicular dental cyst, odontoma or any conical fused root were not included in this study.
- Patients with third molars which had the apex of the fully formed root on pre- treatment panoramic radiographs were not included in this study. Also those patients who did not have third molars with the fully formed apex root on post- treatment panoramic radiographs were not included in this study.
Measurement of root angle
From the post- treatment panoramic radiographs, the angulation of the apical part of maxillary and mandibular third molars' roots on both sides were evaluated. This evaluation was based on reference lines constructed perpendicular to occlusal plane of third molars (tangent to the mesial and distal surface of crowns). Mesial and distal roots were divided into three equal parts. The most apical third of the roots were divided into thirds again. Apical root angles were measured using the angle between the reference lines and central line of the most apical ninth of the root through the root apex.  [Figure 1]
|Figure 1: Apical root angle (A) was measured using the angle between the reference lines (perpendicular to occlusal plane of third molars (tangent to the mesial and distal surface of crowns)) and central line of the most apical ninth of the root through the root apex|
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Measurement of tooth movement
All cephalographs were taken with the teeth in centric occlusion and the lips in repose. The amount of tooth movement of mandibular and maxillary third molars during orthodontic treatment was measured on the pre- and post- treatment lateral cephalometric radiographs. This was evaluated by measuring tooth distance from a perpendicular line to constructed horizontal line. On these radiographs, first S-N (Sella- Nasion) plane was drawn and then constructed horizontal line was drawn through nasion at an angle of 7 degrees to the S-N line.  From sella a perpendicular line (vertical line) to this constructed horizontal line was drawn. The point of intersection of occlusal plane line of third molar to the reference line perpendicular to occlusal line (tangent to distal surface of third molar crown) was assumed a point of which it's distance was measured from that vertical line. [Figure 2] If images of right and left third molars were not superimposed on cephalograms, an average line between the images of left and right third molars was drawn and the measured position was recorded for both sides. From each measurement obtained from the lateral cephalograms, pre- treatment values were subtracted from post- treatment values to reveal the amount of third molars' movement during orthodontic treatment. Upper and lower third molar movement was measured to the nearest 0.5 mm with a digital caliper (Mitutoyo corp. -Tokyo- Japan- Digimatic caliper, No 50 -652).
|Figure 2: On the cephalograms, constructed horizontal line (constructed FH) was drawn through nasion at an angle of 7 degrees to the S-N line. From sella a perpendicular line to this horizontal line was drawn. The point of intersection of occlusal plane line of third molar to the reference line perpendicular to occlusal line was assumed a point which it's distance (B) was measured from that vertical line|
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All assessments were made by the same examiner in a darkened room with a radiographic illuminator to ensure contrast enhancement of the images.
To test the reproducibility of the assessments, the same investigator reevaluated radiographs of 20 randomly selected subjects 4-6 weeks (mean: 40.4 days) after the first evaluation. The reliability between the two measurements was high according to the results of intra-class correlation coefficient test. ( P = 0.00)
Pearson correlation analysis was performed to obtain relation of the amount of third molar's movement and its root angulation. These relations were analyzed for statistical difference at the P < 0.05 level of significance. Descriptive statistics were obtained by calculating the means and standard deviations of the ages. The statistical package for social science (SPSS Inc., 10.0 Chicago, Illinois, USA) was used for statistical analysis.
| Results|| |
The sample included 9 boys (23.1%) and 30 girls (76.9%). The mean age of the patients was 16.20 ± 2.64 years old. (girls: 16.18 ± 2.78 years and boys: 15.50 ± 1.85 years)
From these patients 66.7% had treated by only fixed orthodontic appliances and 33.3% had been treated by removable appliance for growth modification and then by fixed orthodontic appliances.
These patients had been treated non- extraction ( n = 4) or with the extraction ( n = 35). In 35 extracted patients, 53.8% had extracted four first premolars, 2.6% had extracted four second premolars, 12.8% had extracted two maxillary first and two mandibular second premolars and 30.8% had extracted another combination of teeth.
[Table 1] shows the percentage of mesially and distally curved roots in maxillary and mandibular third molars.
|Table 1: The percentage of mesially and distally curved roots in right and left maxillary and mandibular third molars|
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The mean amount of mesial third molar movement during orthodontic treatment in maxillary arch was 0.69 ± 2.59 millimeters and in mandibular arch was 1.41 ± 3.40 millimeters.
The correlations between the amount of third molar's movement and its root angulation in four quadrants were not statistically significant. ( P > 0.05)
In maxillary arch the Pearson correlation showed positive and not significant relation between tooth movement and root angulation but in mandibular arch this relation was negative and not significant. (P > 0.05) [Table 2] and [Table 3]
|Table 2: The pearson correlation and P value between the mean of maxillary third molar movement during orthodontic treatment and the mean angle of mesial and distal roots of maxillary third molars|
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|Table 3: The pearson correlation and P value between the mean of mandibular third molar movement during orthodontic treatment and the mean angle of mesial and distal roots of mandibular third molars|
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| Discussion|| |
In this study the lower one third of the root was chosen for identification of angled roots. Although roots could be programmed according to molecular regulation by growth factors, the apical portion of the roots is vulnerable to local effects. , Since the purpose of this study was to evaluate the effects of a local factor (orthodontic treatment) on roots, we limited our analysis to the apical one- third of the roots which Yamaoka et al. reported to cause difficulty in root canal treatment and removal of the third molars.
In this study in maxillary arch, more third molar movement during orthodontic treatment caused more angulation in roots but in mandibular arch it caused less angulations. However the relation between the amount of third molar movement and its root angulation was not statistically significant. ( P > 0.05) In Bicakci et al.'s study mesial movement of the second molar caused by early extraction of the first molar had occurred. They found that both mesial and distal roots were 8° more vertical on the extraction sides than on the non- extraction sides.  Yamaoka et al. found that angled roots occur more frequently in teeth with incomplete impaction than in those with full eruption. According to their findings, incomplete impaction may produce large forces or strains, and a change in the regulation of root formation. ,, Mesial movement of developing third molars creates an increase in the eruption space for the third molar, thus relieving the compression exerted on the roots and causes the roots to develop more vertically. ,,,,,,
Although Bicakci et al. found that mesial migration of third molars caused more vertical roots, but Marklund and Persson  reported that apical portion of the roots in mesially- migrating teeth would develop more distally in first molars and first premolars. Marklund and Persson  concluded that migration of teeth with immature roots would probably result in a gradual displacement of already- formed hard tissue in relation to developing soft tissues and that mesially- migrating teeth would thus develop distally curved roots. Both of those studies did not evaluate the effect of tooth movement by orthodontic treatment on root angulation. The different results might be attributed to the fact that different studies evaluate different teeth and most of them did not consider the amount of migration of teeth. , The differences also might be due to sampling variations, racial characteristics, and/or the clinician's own definition of root's angles.
The Sella- Nasion plane is suitable for assessment of changes induced by treatment within the same individual over time. Low variability in identifying sella turcica and nasion is an advantage of using this plane.  Since the purpose of this study was measurement of horizontal movement of third molars, the S-N plane which is not usually a horizontal line was not appropriate for this study. Frankfort horizontal (FH) has also been used extensively in cephalometry but the locating porion reproducibly is difficult.  As an alternative, Legan and Burstone suggested using a constructed horizontal which is a line drawn through nasion at an angle of 7 degrees to the SN line. This constructed horizontal which was used in this study tends to be parallel to true horizontal. 
In this study the patients selected who had started orthodontic treatment before the roots of third molars formed and showed apical closure at the end of treatment. According to these criteria the chronological age of patients at the beginning of treatment was from 13 to 24 years and the mean age was 16.20 ± 2.64 years old. It is in agreement with the results of Kullman et al.'s study  that found mineralization of the third molar root starts at about 15 years age. During development of the third molars, a faint circle may be seen as early as age 8  and as late as 14,  with full formation at about age 20.  Shiller  stated that positional changes in the mandibular third molar are still taking place at age twenty. The roots of third molars are not fully formed until 18 to 25 years of age. ,
Yavuz et al. found that the development of the third molars on the extraction side was significantly accelerated when the first molars were extracted early compared with the contra- lateral teeth. Also it can be said that the eruption of the third molars in the extraction quadrant is accelerated in subjects who lost their permanent first molar at an early age.  Fanning  found that with extractions, emergence of the third molars was accelerated by 1.35 years in the maxillary arch and 1.6 years in the mandibular arch. Moffitt found that extraction of a second molar hastens the eruption of the third molar when compared with the norms.  It is well established that extraction therapy is associated with mesial movement of the molars. ,,, Mesial movement of the molars during closure of the extraction site could have a larger effect on third molar impaction in the mandible than in the maxilla. However Kim et al. suggested that this might not be the case and the difference in the impaction rate of the third molars between extraction and non extraction patients was similar in both arches. The eruption of third molars was not considered in this study.
There are very few studies about possible effects on apical root curvature in the literature. Although a strong genetic component could influence intra- alveolar dental development in the mandible, local factors could be related to the incidence of angled roots unique to mandibular third molars. , The physical and biological dynamics of bones and teeth could lead to changes in the shape of the lower one- third of the root even in genetic factors function of both sides in most cases. 
Amount of third molar movement is dependent on many variables, such as growth, mechanics and patient cooperation during orthodontic treatment. The magnitude of forward tooth migration is correlated to the mandibular growth pattern.  None of these variables were controlled in this study, and it was assumed that patients were treated with similar mechanics. Since all of those patients were treated by one expert orthodontist (associate professor of orthodontic department) this assumption might be reasonable.
As complicated root anatomy could not be observed completely in routine radiographs, the association between orthodontic treatment and shape of roots should be studied three- dimensionally. The difference between populations, the different methodology, and the dissimilarity among observers are other important shortcomings.
| Conclusion|| |
Although our study design precludes conclusions about cause and effect relationships, the results suggest that the relation between the amount of third molar's movement during orthodontic treatment and it ],[ s root angulation was not statistically significant. In maxillary arch the correlation between tooth movement and root angulation was positive and not significant relation but in mandibular arch this relation was negative and not significant. Therefore movement of third molars during orthodontic treatment should not be assumed as an etiologic factor of root angulations.
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Member of Orthodontic Research Center, Associate Professor, Department of Orthodontics, School of Dentistry, Shiraz University of Medical Sciences, Shiraz
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]
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