Download Radiographic Assessment for Predicting the Mandibular Third

International Journal of Dental Health Concerns (2015), 1, 1-6
Doi: 10.15713/ins.ijdhc.2
ORIGINAL ARTICLE
Radiographic Assessment for Predicting the Mandibular Third Molar
Eruption after Orthodontic Treatment in First Premolar Extraction
Group and Non-Extraction Group: A-retrospective Study
Arth K. Patel, Sonali V. Deshmukh, C. R. Naik, Sandeep Jethe, Krunal A. Patel
Department of Orthodontics and Dentofacial
Orthopedic, Dr. D. Y. Patil Dental College and
Hospital, Pune, Maharashtra, India
Correspondence: Dr. Arth K. Patel,
A/703 Krishna Tower, Opp. Sachin Tower,
100 Feet Anand Nagar Road, Satellite,
Ahmedabad - 380 015, Gujarat, India.
Phone: +91-9998133564. Email: around_the_
[email protected]
How to Cite:
Patel AK, Deshmukh SV, Naik CR, Jethe S,
Patel KA. Radiographic assessment for
predicting the mandibular third molar eruption
after orthodontic treatment in first premolar
extraction group and non-extraction group:
A-retrospective study. Int J Dent Health
Concern 2015;1:1-6.
Received: 27.10.2014
Accepted: 05.12.2014
ABSTRACT
Introduction: The third molars are the most congenitally missing teeth. If present,
they might follow ectopic eruption path and become impacted. Because it is the last
tooth erupt in the arch, and usually there is no space in the arch. Effects of orthodontic
treatment may also affect the position of third molar after treatment and may changes
the eruption pattern of third molar. Materials and Methods: 30 patient’s pre-treatment
and post-treatment lateral cephalogram and orthopantomogram (OPG) were taken out
of which 15 patients were treated with first pre-molar extraction, and 15 patients were
treated with non-extraction approach. Cephalomatric and OPG tracing were done at
T1 (pre-treatment) and T2 (post-treatment). Distance from Xi-point to distal surface
of second molar, distance from anterior border of ramus to the distal surface of second
molar, angulation of third molar in relation with second molar and mesio-distal width
of third molar were checked. Descriptive analysis was perform for both the groups and
paired t-test and un-paired t-test were performed for first pre-molar extraction and nonextraction group with two different methods such as panoramic radiograph (OPG) and
lateral cephalograms at significant level P < 0.05. Results: In extraction group, there were
more possibility of the eruption, more mesial movement and more angulation changes
noted compare to non-extraction group. Conclusion: Panoramic radiographs were more
accurate when compared to lateral cephalogram while checking minor changes.
Key words: Orthodontic extraction, radiographic assessment, third molar.
INTRODUCTION
eruption of the dentition rather than remodeling at the anterior
border of the ramus will give more space for third molar.[6,7]
Previous third molar studies have concentrated on the
influence that third molars have on the rest of the dentition rather
than on the control that the dentition has on the third molars. The
impact of third molar eruption on mandibular incisor crowding
has been extensively studied, but the results vary.[8-10]
The purpose of this study was to investigate the change in
mandibular third molars angulation, relative to the Xi point and
second molars, in cases treated orthodontically with extraction
of first premolars and to compare these changes with nonextraction cases.
The most commonly congenitally missing teeth are third molars.
If present, they might follow an abortive eruption path and
become impacted.[1] In modern populations; the impaction rate
of third molar is 98% of all impacted teeth.[2] Impaction of the
third molars rate is 73% in young adults of Europe.[3]
Inadequacy of the retromolar space may be one of the reasons
for third molar impaction. Eruption of the mandibular third
molars might be blocked if the remodeling resorption at anterior
aspect of the mandibular ramus is not adequate.[4] Similarly,
eruption of maxillary third molars could be prevented by lack of
compensatory periosteal deposition at the posterior outline of
the maxillary tuberosity.[5]
During the functional phase of the eruption, the space for the
mandibular third molars is also affected by the direction of tooth
movement. The retromolar space will increase if the posterior
teeth erupt more anteriorly. Ricketts stated that mesially-directed
MATERIALS AND METHODS
This observational cross-sectional study was carried out in the
Department of Orthodontics and Dentofacial Orthopedics,
Dr. D. Y. Patil Dental College and Hospital Pune. Ethical
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clearance was obtained from the institutional review board.
The sample size was derived based on the previous literature.
The sample consisted of pre-treatment and post-treatment
lateral cephalograms of 30 patients and pre-treatment and posttreatment panoramic radiographs of 30 patients. Informed
consent was taken from all the patients included in the study.
The eligibility criteria for the study sample were those subjects
who had undergone orthodontic treatment; maxillary second
molars fully erupted, with third molar present either erupted
or unerupted and chronological age of 15 years and above. The
subjects were divided into two groups.
Group 1:Extraction group (patients whose first premolar was
extracted for the orthodontic treatment) (n = 15).
Group 2: Non-extraction group (n = 15).
Each of the above group was further divided into pretreatment and post treatment. Lateral cephalograms and
panoramic radiograph were obtained from the same X-ray
machine (Planmeca Proline XC Dimax3) with the subject in the
natural head position, with teeth in maximum intercuspation and
lips in repose in the Department of Oral Medicine Diagnosis and
Radiology, Dr. D. Y. Patil Dental College and Hospital, Pune.
Tracings were done using a 75 µm lacquered polyester paper
along with a sharp 0.03″ lead pencil. A protractor and a plastic
scale were used to measure the lines and angles. A single operator
performed the tracings in a standardized manner to avoid errors
due to intra-operator variations.
Measurements made on the cephalometric radiographs
[Figure 1] were:
1. The mesio-distal width of the mandibular third molar
2. The distance from the ramus to the distal surface of the
mandibular second molar
3. The distance from “Xi” point, or the geographic center of the
ramus to the distal surface of the mandibular second molar
4. The inclination of the mandibular third molar to its apical
base in relation with mandibular second molar.
Measurements made on the panoramic radiographs [Figure 2]
were:
1. The mesio-distal width of the mandibular third molar
2. The distance from the ramus to the distal surface of the
mandibular second molar
3. The distance from “Xi” point to the distal surface of the
second molar
4. The inclination of the mandibular third molar to its apical
base in relation with mandibular second molar.
All the measurements were analyzed using the Statistical
Package for Social Sciences software (SPSS version 10). Means
and standard deviations were calculated and paired and unpaired t-tests were performed to make comparisons.
Figure 1: Measurements made on the cephalometric radiographs
Figure 2: Measurements made on the panoramic radiographs
RESULTS
Descriptive analysis was performed to calculate means and
standard deviations of the various measurements in both the
groups i.e., First premolar extraction and non-extraction group
with two different methods i.e. with panoramic radiograph
(orthopantomogram [OPG]) and lateral cephalograms. Paired
t-test was used to compare pre-treatment and post treatment
lateral cephalogram and OPG measurements in the extraction
and non-extraction groups. The changes after treatment in
the various measurements between the extraction and nonextraction groups were compared using unpaired t-test.
Table 1 shows lateral cephalogram changes in the position
of developing mandibular third molar in first premolar
extraction and non-extraction group. It was seen that there
was a significant improvement in distance from Xi point-distal
surface of second molar and distance from anterior border
of ramus-distal surface of second molar in extraction group
as compared to non-extraction group. The AB/DC ratio had
also significantly improved in extraction group as compared
to non-extraction group. The change in angulation of third
molar was more in the extraction group when compared to
non-extraction group, however, the difference was statistically
non-significant.
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Table 2 shows the OPG changes in the position of developing
mandibular third molar in first premolar extraction and nonextraction group. It was seen that there was a significant
improvement in distance from Xi point-distal surface of second
molar and distance from anterior border of ramus-distal surface
of second molar in extraction group compared to non-extraction
group. The AB/DC ratio had also significantly improved in
extraction group as compared to non-extraction group. The
change in angulation of third molar was more in the extraction
group as compared to non-extraction group; however, the
difference was statistically non-significant.
attrition. The study of the mandibular third molars have
always aroused greatest interest in clinical practice.[12] Previous
studies[13] regarding third molars have centered on investigating
the effects of third molars on the eruption in dental arches. Many
studies[14,15] have been done to check the changes in position and
inclination of third molar after extraction of first premolar or
second molar for the orthodontic treatment.
Various growth studies have suggested two important
mechanisms for the development of retromolar space in the
mandible: (i) Resorption at the anterior border of the ascending
ramus and (ii) mesial migration of the posterior teeth during the
functional phase of tooth eruption. Both these mechanisms might
depend more on the amount and direction of condylar growth
than on the presence of the third molars.[6,16,17] Thus, in a growing
patient, the effects of all the above mentioned mechanisms
would mask the effect of orthodontic treatment. Problem in the
previous studies was that the subjects included were <18-20 years
old. The mean age of our study sample was 25 years where all
third molar roots showed radiographic evidence of apical closure
at examination which excludes the bone remodeling as well as
mesial migration phenomenon of the third molars. This added
to the strength of the present study where we could determine
exclusively the effects of orthodontic treatment on position of
third molars.
According to Kim et al.,[5] premolar extraction therapy
decreases the rate of third molar impaction because of increased
eruption space concomitant with mesial movement of the molars
during space closure. They had also checked the impaction in
cases where non-extraction orthodontic treatments were done
but they found no significant result. Previously, most of the
studies were done to check the impaction of third molars after
extraction or non-extraction orthodontic treatment. Thus,
there was a need to check the changes in inclination as well as
positional change in third molars after orthodontic treatment to
check impaction rate as well as mesialization of third molar in
extraction as well as in non-extraction orthodontic therapy.
Previous studies were done either on lateral cephalograms or
OPG to check changes in position and angulations of third molar
but until now no studies have used both the tools to check and
compare the change in inclination and position of third molars.
Radiographs are 2-dimensional (2D) images of the 3D structure
so it is required to check the inclination as well as positional
change of third molar in both the radiographs. Thus in this study,
both the radiographs were taken into consideration.
DISCUSSION
The mandibular third molar is the most frequently impacted
tooth. The prevalence of mandibular third molar impaction is
variable in different populations, which is 9.5-39%.[11] This has
been attributed increased the retromolar space due to mesial
drift of the posterior teeth because of excessive interproximal
Table 1: Lateral cephalogram changes in the position of developing
mandibular third molar in first premolar extraction and non-extraction
group
Measurements
Extraction Non‑extraction P value
group`s lateral group’s latral
cephalogram cephalogram
Distance from Xi point‑distal
4.47 mm
1.93 mm
0.015*
surface of second molar
Distance from anterior
4.2 mm
1.8 mm
0.029*
border of ramus‑distal
surface of second
molar (AB)
Angulation of third molar in
4.2°
1.07°
0.240
relation to second molar
AB/DC
0.346
0.113
0.007*
*Significant, Un-paired t-test
Table 2: OPG changes in the position of developing mandibular third
molar in first premolar extraction and non‑extraction group
Measurements
Extraction
Non‑
P value
group’s
extraction
OPG
group’s
OPG
Distance from Xi point‑distal
7.2 mm
3.86 mm
0.069*
surface of second molar
Distance from anterior
6.74 mm
3.33 mm
0.028*
border of ramus‑distal
surface of second
molar (AB)
Angulation of third molar in
5.54°
3.07°
0.113
relation to second molar
AB/DC
0.414
0.105
0.028*
Changes in Eruption Space for Mandibular third Molar
Turley and Schulhof[18] evaluated several methods of measuring
the available space and check for the change in position of third
molars. They concluded that the most useful parameter was
the distance from “Xi” point (center of the ramus) to the distal
surface of the second molar. Bjork[16] conducted a study using
*Significant, un‑paired t‑test, OPG: Orthopantomogram
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lateral cephalograms and assessed the distance from the anterior
edge of the ramus to the distal surface of the second molar. This
helped in evaluation of the positional change in third molars after
orthodontic treatment. Both these parameters were indicative
and confirmative about the mesial movement of the third molar.
In present study, we evaluated both the above mentioned
parameters in extraction and non-extraction group with the help
of both the radiographs to confirm mesial movement of third
molar. Comparisons were made between groups and within
groups as well. In both the groups, there was an increase in
the distance from Xi point to the distal aspect of second molar
as well as from anterior border of ramus to the distal aspect
of second molar post-orthodontic treatment as compared
to pre-treatment. However, the increase in this distance was
significantly higher in the extraction group as compared to nonextraction group. In the non-extraction group, an average mesial
movement of 1 mm was seen on lateral cephalogram, whereas an
average of 3 mm of mesial movement of third molar was seen on
panaromic radiographs. In this group, the mesial movement of
third molars depends on the amount of spacing present before
orthodontic treatment. It also could be dependent on angulation
on first and second molars before treatment. In the extraction
group, lateral cephalograms showed a mean 5 mm of mesial
movement of third molar whereas on panaromic radiographs
a mean 6 mm of mesial movement of third molars was seen.
Thus, it can be concluded that there is a definite mesialization
of third molar in both the groups, the extraction group showing
better results. This finding was in accordance with the findings
of studies conducted by Kim et al.[5] and Poosti et al.,[19] Kim
et al.[5] had found that the variation in mesial movement was
considerable ranging from 1.5 to 8.3 mm in the extraction
group patients, whereas 2.9-3.5 mm in the non-extraction group
patients suggesting that premolar extraction therapy decrease
the frequency of third molar impaction because of increased
eruption space along with mesial movement of the molars
during space closure.
after orthodontic treatment, whereas in non-extraction group,
the ratio was 0.81 on lateral cephalogram and 0.86 in panaromic
radiographs suggesting less possibilities of third molar eruption.
Based on this, it can be concluded that there was a high possibility
of impaction of third molar in orthodontic cases treated by nonextraction approach as compared to first premolar extraction
approach. Dierkes[11] and Faubion[21] had found that only 15%
of mandibular third molars erupted in good position after nonextraction therapy. Richardson and Dent[18] reported that in the
orthodontic patients treated by non-extraction approach, 56%
of the mandibular third molars were either impacted or had
problems that needed surgical removal. Thus, the findings of this
study suggests that the impaction of mandibular third molars
were more likely in patients treated by non-extraction approach
in accordance with the previous studies.[11,18,21,22]
Changes in the Angulation of Mandibular third Molar
Richardson and Dent[18] conducted a study in which he found
that 34% of the orthodontic cases treated with non-extraction
approach presented with impaction, and the impaction decreased
to 28% in cases treated with premolar extractions. He concluded
that most of the impacted third molars have been straightening
out to some extent and that the degrees of their angles have been
augmented. Ahmed et al.[23] conducted a study to determine the
angulation changes of mandibular third molars in cases where first
premolar extraction and non-extraction approach was planned.
He concluded that the differences in angulation were like other
morphological differences but changes in angulation may or may
not be related to extraction or non-extraction treatment approach.
In the present study, we found that in extraction group, lateral
cephalograms showed 4° of mean inclination change whereas
panaromic radiograph showed 5° of mean inclination change
suggesting uprighting of the third molars in relation to the second
molars. In the non-extraction group, lateral cephalograms showed
only 1° of mean inclination change but panaromic radiographs
showed 3° of mean inclination change in third molars with respect
to second molars suggesting uprighting of the third molars.
Dierkes[11] in his study showed that uprighting of the third molars
in first premolar extraction group by 67.5% whereas in second
premolar extraction group, 72.5% uprighting of the third molars
followed by normal eruption of it. Thus, a change in inclination
of third molar was noticed in both the groups. However, this
change was non-significant on lateral cephalogram of nonextraction group and significant on the panoramic radiograph.
This indicates that lateral cephalograms should not be done
alone to check the changes in inclination and positional change
of third molar. On the other hand, this change was significant on
both lateral cephalogram as well as panoramic radiographs. Thus,
to conclude, the results of this study supported the findings of
Richardson and Dent[18] and hence, impaction of third molar is
likely in cases treated with non- extraction approach.
Prediction of Impaction Based on AB/CD Ratio
Henry and Morant,[20] proposed that the impaction of the third
molar on lateral cephalogram could be predicted by establishing
an index of molar space, which could be measured by the
mesio-distal width of the third molar (CD) and space between
the anterior edge of the ramus to the distal surface of second
molar (AB). If CD is same or lesser than the available space, the
eruption possibilities are good. When CD is greater, impaction is
likely. After dividing the value of AB/CD, if the value is ≥1, then
there is a good possibility of eruption. And if the value is <1, then
there is a less possibility of eruption of third molar. The results
of this study showed that in the extraction group, this ratio was
0.92 on lateral cephalogram and 1.06 on panaromic radiograph;
suggesting there are more eruption possibilities of third molars
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Although our study design suggest that the increased potential
for mesial molar movement during extraction site closure, with
increase in retromolar space, might be the major reason for
the intergroup difference in non-extraction and first premolar
extraction group.
Clinically this study can be helpful while taking decision
of extractions of third molars. Studies[16] have suggested that
third molars could cause anterior crowding, so many clinicians
preferred to extract third molars after orthodontic treatment.
Many studies also suggested extraction of third molars in
borderline cases to solve space problem. A frequent argument
for premolar extraction in borderline cases has been that the
procedure might be considered as a substitution for third molar
extraction where moderate anchorage could help in mesialization
of third molar as well as solving the space problems. So this study
confirms the conclusion of previous studies, which suggests that
there is definite mesial movement and inclination change of third
molars after orthodontic treatment irrespective of the approach
used, whether extraction or non-extraction.
This study also helps in predicting the impaction of third
molars after orthodontic treatment. Sometimes before
orthodontic treatment, inclination of third molars indicates
chances of impaction and clinicians advice the removal of third
molars. This could be averted by simply doing the orthodontic
treatment and changing the inclination of the tooth, thus
reducing the likelihood of impaction of third molars.
thereby an increase in the eruption space for the third molars
5. Our findings also suggested that the panoramic radiographs
were more accurate as compare to lateral cephalogram while
checking minor changes. It is difficult to trace the third molar
on to the lateral cephalogram due to the superimposition of
contra-lateral side soft and hard tissue whereas in panoramic
radiograph it is easy to trace the third molar.
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CONCLUSION
Following conclusion can be put forward on the basis of this
retrospective study of third molars position and inclination after
orthodontic treatment.
1. Angulation of third molar in respect with second molar was
significantly increased in extraction group as compare to
non-extraction
2. Eruption possibility of third molars showed significant
increased chances of eruption in extraction group as
compared to non-extraction group after treatment
3. There was an increase in the distance from Xi point to the
distal aspect of second molar as well as from anterior border of
ramus to the distal aspect of second molar post-orthodontic
treatment as compared to pre-treatment suggesting definite
mesial movement of third molars. However, the increase in
this distance was significantly higher in the extraction group
as compared to non-extraction group
4. Our results suggested a clinically significant reduction in the
rate of impaction of mandibular third molars in extraction
groups compared with non-extraction group patients.
This phenomenon might be due to the fact that premolar
extraction approach is associated with an increase in the
amount of mesial movement of the mandibular molars,
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Orthod Dentofacial Orthop 1993;104:566-74.
16. Bergstrom K, Jensen R. Responsibility of the third molar for
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17.Silling G. Development and eruption of the mandibular
third molar and its response to orthodontic therapy. Angle
Orthod 1973;43:271-8.
18. Richardson ME, Dent M. Some aspects of lower third molar
eruption. Angle Orthod 1974;44:141-5.
19. Poosti M, Basafa M, Hosseini M, Parvizi F. Changes in the
position of mandibular third molars following extraction
and non-extraction orthodontic treatments. J Dent Mater
Technol 2012;1:47-52.
20.Ricketts RM, Turley P, Chaconas S, Schulhof RJ. Third
molar enucleation: Diagnosis and technique. J Calif Dent
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