|Year : 2017 | Volume
| Issue : 3 | Page : 101-107
Evaluation and prediction of impacted mandibular third molars by panoramic radiography: A retrospective study
Anubha Verma, Payal Sharma, Shalaj Bhatnagar
Department of Orthodontics and Dentofacial Orthopedics, I.T.S-C.D.S.R, Ghaziabad, Uttar Pradesh, India
|Date of Web Publication||14-Jun-2017|
D-14, Indian Oil Colony, Sector-O, Aliganj, Lucknow - 226 024, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
Aim: The aim of this study was to evaluate the factors associated with impaction of mandibular third molars and to find a method for predicting eruption of the third molars using panoramic radiographs.
Materials and Methods: Ninety pretreatment panoramic radiographs of patients with a full complement of teeth were selected. The mandibular third molars were divided into two groups depending on their eruption status: Group 1: erupted and Group 2: impacted molars. Various angular and linear measurements were made on the panoramic radiographs.
Results: Shapiro–Wilk test showed that the data were not normally distributed. Mann–Whitney U-test showed that mean rank of angulation between third molar and second molar (M3M2), gonial angle (GoA), and mesiodistal width of third molar (MDW3M) of impacted group was significantly higher than the erupted group. Spearman's correlation coefficient showed positive correlation between angulation of third molar with mandibular plane (M3MP) and retromolar space (RS) with eruption of mandibular third molar. M3M2, GoA, and MDW3M were found to have negative correlation with eruption of mandibular third molar.
Conclusions: Increased RS and M3MP were strongly associated with eruption of the mandibular third molar. An increase in the M3M2 and MDW3M predisposed impaction of third molar. GoA was smaller and mandibular length was larger in the erupted group although both had a weak correlation.
Keywords: Eruption, mandibular third molar, panoramic radiograph, prediction
|How to cite this article:|
Verma A, Sharma P, Bhatnagar S. Evaluation and prediction of impacted mandibular third molars by panoramic radiography: A retrospective study. Int J Orthod Rehabil 2017;8:101-7
|How to cite this URL:|
Verma A, Sharma P, Bhatnagar S. Evaluation and prediction of impacted mandibular third molars by panoramic radiography: A retrospective study. Int J Orthod Rehabil [serial online] 2017 [cited 2018 Jan 19];8:101-7. Available from: http://www.orthodrehab.org/text.asp?2017/8/3/101/208062
| Introduction|| |
Third molars are the last teeth to erupt in the oral cavity. The process of development and eruption of the third molar has been shown to be highly variable and they are the most commonly impacted teeth. Third molars exhibit great variation in size, shape, position, root formation, time of development, and path of eruption. Time of eruption of third molars varies considerably between populations and gender.
In treatment planning, the orthodontist is often challenged with predicting the probability of eruption or impaction of mandibular third molars in young patients. This prediction is commonly based on a radiographic assessment, and it often gives ambiguous answers. Several methods predictive of lower third molar eruption have been presented since 1936. Most of the studies are based on lateral cephalographic measures.,, However, other projections such as periapical views, bitewing radiographs, and anteroposterior views have also been used.,, Since panoramic tomograms are easily available to most practicing dentists, it would be beneficial if they could be used for predicting the development of third molars. The benefit of using panoramic radiographs for predicting lower third molar eruptions is that it is very simple to use and has a high reliability.
No accurate predictive method has been developed because the etiology of mandibular third molar impaction is complex. Previous studies have found several factors related to impaction of third molars such as lack of space in the third molar region, angulation and ectopic position of the third molars, obstruction in the path of eruption, and late third molar mineralization. Sex, racial, and socioeconomic differences and genetic and endocrinologic factors can also influence eruption. An unfavorable path of eruption and excessive initial mesial angulation and minimal up righting during follow-up might increase the likelihood of impaction.
The purpose of this study is to elucidate the factors associated with mandibular third molar impaction and to find a simple, reproducible, and reliable method for predicting eruption of the third molar in the mandible by measurements taken on panoramic radiographs.
The null hypothesis of the study was that there is no difference in the various factors being studied between cases of mandibular third molar impaction and those with erupted mandibular third molars.
| Materials and Methods|| |
The study was conducted in the Department of Orthodontics and Dentofacial Orthopedics. Approval for the study was obtained from the Institutional Ethical Review Committee.
Ninety pretreatment panoramic radiographs of patients aged between 21 and 45 years attending the Department of Orthodontics were selected. Selection criteria were no previous orthodontic treatment, no history of trauma, all permanent teeth including third molars present in the mandible, no missing, carious, or supernumerary teeth, all teeth till the second molar at the occlusal level, and root formation of third molar complete or nearly complete.
The following landmarks and planes were traced on the panoramic radiographs bilaterally on matte acetate paper [Figure 1]:
- Mesial most contact point of the third molar
- Distal most point of the third molar
- Distal contact point of the second molar
- Distal most point on the second molar
- Anterior border of the mandibular ramus: the intersection between a line connecting cusps of the first and second molars and the contour of the anterior mandibular ramus - Ra
- Tangent point to the posterior mandibular corpus - Mpp
- Tangent point to the anterior mandibular corpus - Mpa
- The most dorsal/lateral point on the mandibular condylar process - Cp
- The posterior point on the posterior border of the mandibular ramus - Rp
- The most anterosuperior point on the lower alveolar margin - Id (infradentale)
- Long axis of the second molar - a line drawn through the midpoint of the occlusal surface and the bifurcation of the second molar
- Long axis of the third molar - a line drawn through the midpoint of the occlusal surface and the bifurcation of the third molar
- Mandibular plane - a line connecting Mpp and Mpa
- Ramal line - a line connecting Cp and Rp.
The following parameters were measured bilaterally [Figure 2]:
- Angulation of mandibular third molars to the mandibular plane (angle M3MP) - the angle formed between the long axis of the third molar and the mandibular plane
- Angulation of mandibular third molars to the second molars (angle M3M2) - the angle formed between the long axes of the second and third molars
- Retromolar space (RS) - the distance between the distal contact point of the second molar and the anterior border of the ramus (Ra)
- Mesiodistal width of the third molar (MDWM3) - measured as the greatest distance between mesial and distal surfaces of the crown
- The gonial angle (GoA) - the angle between the mandibular plane and the ramal line
- Length of the mandible (M and L) - the distance from the anterior border of the mandibular ramus (Ra) to infradentale (Id).
For the purpose of statistical analysis, each third molar was considered as a separate sample. The sample was divided into two groups according to eruption status of the mandibular third molars as follows:
- Group 1: Erupted mandibular third molars (n = 90)
- Erupted third molars are defined as teeth that were at the occlusal level with the second and first molar
- Group 2: Impacted mandibular third molars (n = 90)
- Impacted third molars are defined as incomplete eruption with radiographic evidence of apical closure.
The data were subjected to statistical analysis using Statistical Package for Social Sciences version 15.0 (IBM Corporation 1 New Orchard Road, Armonk, New York, United States) statistical analysis software. The descriptive statistics, correlation coefficients, and mean ranks were calculated for each of the experimental groups. The Shapiro–Wilk test was used to determine whether the data used were normally distributed. Spearman's correlation coefficient was used to assess the correlation between all the variables and eruption status of the third molars. Mann–Whitney U-test was applied to compare the measured variables between erupted and impacted groups and a multiple regression model was used to formulate a prediction equation.
| Results|| |
[Table 1] shows the means, standard deviations, and standard error means tabulated for all the angular and linear measurements recorded on panoramic radiographs. The data did not show a normal distribution; hence, nonparametric tests were used.
|Table 1: Descriptive statistics for angular and linear measurements of various groups|
Click here to view
On applying Mann–Whitney U-test [Table 2] and [Graph 1], [Graph 2], it was found that the mean ranks of M3M2, GoA, and mesiodistal width of third molar (MDW3M) of impacted group were significantly higher than the erupted group (P < 0.001). The mean ranks of M3MP, RS, and M and L of impacted group were significantly lower than the erupted group (P < 0.001). The angulation between the mandibular third molar and mandibular plane (M3MP) and RS were found to have a strong positive correlation with eruption of mandibular third molar (0.816, 0.806), while the mandibular length had a weak positive correlation (0.18). Angulation between second and third molar (M2M2), GoA, and MDW3M were found to have negative correlation with eruption of mandibular third molar. The angulation between mandibular second and third molar had a strong negative correlation (−0.861), whereas mesiodistal width and GoA had a weak negative correlation (−0.322, −0.233) [Table 3].
To predict the eruption of mandibular third molars, linear regression was applied in which eruption was taken as dependent variable and M3M2, M3MP, RS as independent variables. The coefficient of determination was found to be 80%.
The regression equation derived was as follows:
Y = 0.982 − 0.015 M3M2 + 0.001 M3MP + 0.061 RS
On solving the equation, a value of 1 denoted an impacted mandibular third molar and a value of 2 showed an erupted mandibular third molar.
To check the accuracy of the regression equation, twenty additional panoramic radiographs were collected and divided into two groups, Group 1 (n = 10) panoramic radiographs showing erupted third molars and Group 2 (n = 10) panoramic radiographs showing impacted third molars. The linear and angular measurements were measured for all the samples. The prediction equation was applied using the measurements of the variables, and it was found to be 90% accurate [Table 4].
| Discussion|| |
The prediction of likelihood of third molar impaction might be of considerable clinical significance. Previous studies have suggested that increased eruption space due to mesial molar movement after premolar extractions can reduce the risk of third molar impactions.,,, In cases presenting with several treatment options, the one which would favor eruption of the third molar may be chosen. However, in borderline extraction cases with a minimal predicted chance of impaction, the nonextraction alternative might be favored. Furthermore, in patients judged to be at increased risk of impaction at debonding, the optimal timing for surgical removal of the third molars can be determined.
The mesiodistal width of the third molar, GoA, and angulation between the second and third molars were significantly less in the erupted group as compared to the impacted group, whereas RS, mandibular length, and the angle between the mandibular third molar and the mandibular plane were larger.
M3M2 showed a strong negative correlation with eruption. This finding was in agreement with Behbehani et al. who suggested that increased mesial angulation of the third molar bud increased the risk of impaction. Ventä et al. stated that teeth inclined mesially more than 45° remain impacted. In this study, the erupted group showed an angulation of 7.56° ± 3.38° and the impacted group 37.27° ± 13.82°. It has been suggested that angulation of the third molar may change during the final stages of root development and less than half of erupted third molars reach an ideal angulation in the dental arch.
The angulation between third molar and mandibular plane showed a strong positive correlation with eruption. Richardson  suggested that a steep angle of the third molar will increase the distance that it will have to travel to erupt, making it less likely to do so, especially if the available space is inadequate. He also demonstrated that the initial angulation of the lower third molar to the mandibular plane could be a factor in predicting impaction.
The GoA was found to have a weak negative correlation with eruption. In contrast, Hattab and Alhaija  reported that a smaller, more acute GoA was more common among in the impacted group. Begtrup et al. found no correlation between jaw angles and erupting third molar. The jaw angle was, therefore, not a relevant measurement when trying to find a parameter for predicting the erupting third molar. This finding also does not correspond with Björk et al. who found that in the anterior rotated growth types with minor resorption of the anterior border of the ramus, the RS was not adequate for erupting third molar. In a similar study, Richardson  found that a small jaw angle measured at the age of 18 years was more often associated with impaction than erupting third molar.
The RS was found to have a strong positive correlation with eruption of third molar, more the RS; higher are the chances of eruption. Earlier techniques for predicting lower third molar eruption were based on evaluation of the space between the center of the ramus (Xi point) and the distal aspect of the lower second molar on lateral radiographs. The critical distance for eruption was found to be 25 mm. Björk et al. reported that the space behind the second molar was reduced in 90% of cases with mandibular third molar impaction. On the other hand, Richardson  found that the presence of space is not an indication whether the third molar will erupt or not. Ganss et al. reported that the ratio of the RS to lower third molar width remained almost constant between the ages of 16 and 20 years in the impacted group, whereas it increased between the ages of 13 and 20 years in the erupted group. In agreement with our study, Hattab and Alhaija  stated that the RS was significantly larger in the erupted group than in the impacted group. Among the variables tested, lack of space was the single most important cause of impacted of third molars. Hattab and Alhaija  found that the chances of third molar eruption would increase if the RS was at least equal to the third molar crown width. Ventä et al. stated that if the RS is at least 16.5 mm, the probability of eruption is 100%. In our study, it was seen that RS was 15.82 ± 1.96 mm for the erupted group and 10.66 ± 2.30 mm for the impacted group.
The mesiodistal width of the third molar was found to have a positive correlation with the eruption of third molar suggesting that a small third molar is less likely to erupt than a larger one. The mesiodistal width of the third molar in the erupted group (12.66 ± 1.16 mm) was significantly smaller than that in the impacted group (13.52 ± 1.32 mm). This is in accordance with the findings of Richardson  and Ventä et al. who also found a tendency for the third molars to be slightly larger in the impacted group although differences in third molar size were small and insignificant.
The mandible was found to be significantly shorter in the impacted group than in the erupted group; however, mandibular length showed a weak positive correlation with erupting mandibular third molars. A short mandibular length predisposed to mandibular third molar impaction. However, Kaplan  and Dierkes  did not find significant differences in mandibular length between subjects with impacted and erupted third molars.
In the present study, the angulation between second molar and third molar (M3M2), angulation between third molar and mandibular plane (M3MP), and RS were found to be highly correlated with eruption of third molar. These variables were used to predict probability of eruption using a logistic regression analysis. The accuracy of the derived equation was tested on an additional validation sample of 20 panoramic radiographs. The prediction was found to be correct in 18 out of 20 cases giving an accuracy of 90%.
The predictive accuracy for third molar eruption may be affected by the normal physiological growth of bone and dentoalveolar structures, which take place at least until mid-adulthood. Changes in the mandible, which favor eruption of third molars, are increase in the length of the mandible, condylar growth in a distal direction, and mesial eruption of the dentition.
The imaging method employed may also reduce the accuracy of prediction. In our study, we have used panoramic radiographs as they provide complete overview of the dentition regarding tooth developmental stage, number of teeth, malformations of teeth, eruption, and resorption processes. Panoramic radiographs are better for measuring the space available for mandibular third molars compared with lateral cephalograms, bitewings, and lateral oblique projections. However, it is difficult to measure skeletal parameters such as size and shape accurately on panoramic radiographs.
In addition, the prediction of the eruption of a tooth is mainly related to the chronological age of the patient and not to individual skeletal maturation. An important consideration in younger age groups is the development of bone in the third molar area. Predictions made before age 20 may be inaccurate because the angulation of a lower third molar changes during development. According to Richardson, accurate prediction of third molar impaction from radiographic measurements is not possible at the age 10–11 years.
This study has not considered sexual dimorphism in eruption of third molars. The other limitation of this study is a small sample size.
| Conclusions|| |
Within the limitations of the study, it can be concluded that:
- An increased RS and angulation between the mandibular third molar and mandibular plane were strongly associated with eruption of the mandibular third molar
- An increase in the angulation between the second and third molar strongly predisposed to the impaction of the mandibular third molar
- There was a tendency for the third molars to be slightly larger in the impacted group
- GoA was smaller and mandibular length was larger in the erupted group although both had a weak correlation.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]