Keywords
arch - crowding - dimensions - third molars
Introduction
Anterior dental crowding is one of the most common problems encountered in orthodontic
practice; the relapse of anterior crowding following the completion of retention period
in orthodontically treated patients has provoked much speculation in the dental literature.[1] Crowding occurs when teeth deviate within the same dental arch. Late incisor crowding
is a multifactorial phenomenon, but there is a controversial opinion in orthodontics
that third molars contribute to the development of malocclusion or relapse after orthodontic
treatment. The arches' size and form also have substantial implications in orthodontic
treatment planning affecting the space available for dentition and the stability of
normally aligned teeth. Leighton and Hunter[2] established a relationship between dental crowding and the morphological characteristics
of the mandible. The anterior component of occlusal force is also essential when it
comes to late lower arch crowding.
According to Richardson,[3] pressure from the back of the arch is an important cause of late mandibular incisor
crowding during the teenage years. Such pressure because of physiologic mesial drift,
the anterior component of the occlusion force on mesially inclined teeth, or the presence
of a developing third molar may cause forward movement of the buccal teeth, with a
shortening of the arch and increased crowding. Lindauer et al stated that American
orthodontists and surgeons believe that mandibular molars can cause dental crowding
and should be removed.[4] In another study conducted by Husain and Rengalakshmi, it is concluded that third
molars may be one of the reasons for mandibular incisor crowding, if not the only
one.[5] However, Tüfekçi et al reported that Swedish and US orthodontists do not believe
that impacted mandibular third molars can cause lower anterior crowding even with
their anterior force.[6] Similarly, Cotrin et al reported that anterior teeth relapse is unrelated to mandibular
third molars.[7] Many researchers have aimed to reveal the influence of potential factors on the
stability of the obtained orthodontic results.[8]
[9]
[10] And yet, we have not been able to single out only one factor responsible for anterior
crowding. However, due to the presence of so much controversy and unfruitful results,
we aimed to assess whether arch dimension and impacted third molars have a relationship
with anterior dental crowding.
Hence, this study aims to determine the extent to which third molars contribute to
lower dental arch crowding by assessing their position and angulation and comparing
the measurements between a crowding and a noncrowding group. The specific objectives
are presented below:
-
• To measure the dimensions of the lower arch, including arch length, arch width,
and arch perimeter.
-
• To measure the retromolar space to lower third molar crown width (Ganss ratio) on
a panoramic radiograph.
-
• To investigate the position and angulation of lower third molars by measuring angle
A and angle B on panoramic radiographs and comparing all measurements between the
crowding and noncrowding groups.
Materials and Methods
A total of 240 subjects were selected for this study. The sample population was chosen
from the patients seeking orthodontic treatment at the Department of Orthodontics.
Only this number of patients consented to participate in the study. This cross-sectional
study was conducted between October 2018 and October 2019. The sample was divided
into two equal groups; 120 participants with Class I normal occlusion and 120 with
Class I crowding. All subjects had healthy lower dental arches and angle Class I molar
relationship without any artificial crowns or anomalies in crown morphology. Periodontal
diseases, decay, and ongoing dental treatments were ruled out.
The authors selected the noncrowding occlusion group from patients who had undergone
orthodontic examinations, with all permanent teeth, including erupted third molars,
and a normal transversal relationship, leading to optimal intercuspation in both jaws.
However, the crowding group had unerupted third molars. The study utilized panoramic
radiographs, alginate impressions, and study models. The Ganss ratio was calculated
to assess the relationship between dental arch dimensions and mandibular anterior
segment crowding. In addition to the Ganss ratio, other measurements were taken, such
as intercanine and intermolar widths, arch perimeter, and arch length.
The mandibular dental arch was examined clinically, and each subject had a panoramic
radiograph taken using a standardized machine with appropriate criteria and specifications.
The overjet and overbite were in normal values (from 2 to 4 mm). The mandibular dental
arch was thoroughly examined using a Dentsply Sirona Orthophos E two-dimensional panoramic
OPG (orthopantomogram) machine. Alginate impressions were taken to allow the casting
of study models, and measurements were made by a single investigator to eliminate
interexaminer variability and were assessed at least twice. The reference lines of
the panoramic radiographs are presented in [Fig. 1].
Fig. 1 Panoramic radiograph of crowding group (unerupted) lower third molars.
Measurements of Ganss Ratio (the Retromolar Space to the Width of the Crown A/B)
-
The retromolar space to the width of the crown of the third molar (known as the “Ganss
ratio”) was measured. First, the width of the third molar crown was measured, and
then the distance from the distal surface of the third molar to the anterior border
of the ramus of the mandible was measured as a reference point for the retromolar
space. The Ganss ratio was then calculated by dividing the retromolar space by the
crown width, following the method described by Ganss et al.[11]
-
The angle A was measured between the long axis of the mandibular third molar and the
long axis of the mandibular second molar. The long axis of each tooth was drawn from
the midpoint of the mesial and distal surfaces to the apex of the tooth, which was
considered to be the reference point for the measurement. The angle was measured on
both the right and left sides of the dental arch. The anatomic landmarks for drawing
the lines were the mesial and distal surfaces of the teeth and the apex of each tooth.
-
The angle B measurement was taken by drawing a line from the distobuccal cusp of the
lower second molar to the midpoint of the line connecting the mesial and distal tips
of the mandibular symphysis. Another line was drawn from the mandibular plane tangent
to the lower border of the mandible. The angle between these two lines was measured
as the angle B. The anatomic landmarks used for this measurement were the distobuccal
cusp of the lower second molar, the midpoint of the line connecting the mesial and
distal tips of the mandibular symphysis, and the mandibular plane. The measurement
of angle B is shown in [Fig. 2].
Fig. 2 Angle B—third molar angulation to the base of the mandible.
Panoramic radiographs, which are routinely taken from orthodontic patients, were selected
according to inclusion criteria, second and third molars were traced on overlying
acetate paper, and analyzed by a single observer. Mesially inclined third molars were
taken into consideration.
Measurements of the Lower Arch Dimensions
The measurements of the lower arch dimensions on study models were made with the following
parameters:
Arch Length
The measurement of arch dimensions was conducted in segments, which were further categorized
into anterior and posterior regions for both the right and left sides. The measurement
technique employed was based on the dimensions proposed by Lavelle and Foster,[12] which included three out of the six dimensions. However, the method used by Niedzielska[13] was adopted for measuring the arch width. The measurement of arch length and width
was carried out using the Korkhaus caliper.
Arch Width
To measure the arch width, the Korkhaus caliper was used to take three measurements:
intercanine width, interpremolar width, and intermolar width. The intercanine width
was measured as the distance between the cusp tips of the canines. The interpremolar
width was measured as the distance between the buccal cusps of the first and second
premolars. The intermolar width was measured as the distance between the mesiobuccal
cusps of the first and second molars.
Arch Perimeter
The arch perimeter was measured by Lundstrom[14] method on the right and left sides, using a vernier caliper with an accuracy of
0.01 mm. The arch perimeter measurements were made from the distal aspect of the permanent
first molar on one side to the distal aspect of the permanent first molar on the other
side.
The measurements of lower arch dimensions are presented in [Fig. 3].
Fig. 3 Measurements of lower arch dimensions (arch width a, b, c; arch length a, b, c, d;
arch perimeter S1-S6) on study models.
Statistical Analysis
Statistical analysis was conducted with IBM Statistics V.26.00. Statistically significant
value was set at p-value less than 0.05. Descriptive statistics: Mean, ± 95% confidence interval, were
done for Ganss ratio, and the distribution of data in series with numerical characters
is tested by the Kolmogorov–Smirnov test, Lilliefors test, and Shapiro–Wilk test.
These tests were chosen because of the normal distribution of data. Differences in
analyzed parameters between two independent samples (right side and left side)/crowding
group and noncrowding occlusion group on the series with numerical characters were
tested by the Mann–Whitney U test (Z/U). Correlation between two analyzed parameters
was tested by Spearman R coefficient in correlation (R) and correlation between one
dependent parameter (S3 & S4) and several independent parameters (Length/a,b,c,d/;
Width/a,b,c/; segment/S1, S2, S5, S6/; Ganss Ratio; Angle A; Angle were examined with
Multiple Regression Model (R).
Results
Among the crowding group, 54 (45%) patients were male, and 66 (55%) were female. In
contrast, the noncrowding group consisted of 70 (58.33%) males and 50 (41.66%) females.
The Pearson chi-squared test was performed, and the p-value was more than 0.05 (0.69), indicating no statistically significant difference
in the gender distribution between the two groups. The descriptive results of the
study are summarized in [Tables 1A] and [1B].
Table 1A
Frequency and percentage distribution of gender
|
Crowding group
|
Noncrowding occlusion group
|
Gender
|
Frequency
|
Percent
|
Frequency
|
Percent
|
Male
|
54
|
45.00
|
70
|
58.30
|
Female
|
66
|
55.00
|
50
|
41.66
|
Total
|
120
|
100.00
|
120
|
100.00
|
Pearson chi-squared test
|
0.12
|
p-Value
|
0.69
|
Table 1B
Age distribution among the crowding and noncrowding groups
|
Age in years
|
Crowding group
|
Noncrowding group
|
Number of subjects
|
120
|
120
|
Mean age
|
18.05
|
18.87
|
Std. deviation
|
1.57
|
1.52
|
Minimum age
|
16
|
16
|
Maximum age
|
21
|
21
|
Pearson chi-squared test
|
2.75
|
p-value
|
0.006
|
The mean age of subjects in the crowding group was 18.05 years, with a standard deviation
of ± 1.57 years. In contrast, the mean age in the noncrowding group was 18.87 years,
with a standard deviation of ± 1.52 years. The difference in mean age between the
two groups was found to be statistically significant (Z = − 2.75, p = 0.006), with subjects in the noncrowding group being significantly older than those
in the crowding group.
[Table 2] displays the results of the retromolar space/Ganss ratio and angle B measurements
on the right and left sides for participants in the crowding and noncrowding groups.
The analysis reveals that the values of the Ganss ratio on the right and left sides
of the noncrowding group were significantly higher than those of the crowding group
(p < 0.01). Similarly, the values of angle B on the right and left sides of the noncrowding
group were significantly greater than those of the crowding group.
Table 2
Differences of the retromolar space/Ganss ratio, angle B on the right and left side
between crowding and noncrowding groups
Ganss ratio
|
Rank sum
crowding
|
Rank sum noncrowding
|
U
|
Z
|
p-Value
|
Ganss ratio/R
|
7284.50
|
21635.0
|
24.50
|
−13.34
|
0.00
|
Ganss ratio/L
|
7300.00
|
21620.0
|
40s.00
|
−13.31
|
0.00
|
Angle B/R
|
10920.0
|
18000.0
|
3660.00
|
−6.58
|
0.000
|
Angle B/L
|
9649.00
|
19271.0
|
2389.00
|
−8.95
|
0.000
|
The Spearman R coefficient analysis revealed a weak and insignificant negative correlation
between the right and left sides of the angle A and Ganss ratio. However, the study
found that an increase in angle A on either side is associated with a reduction in
retromolar space of third molars, as shown in [Fig. 4A and B].
Fig. 4 (A) Examined relation with Spearman R coefficient of angle A/R and Ganss ratio/R, (B) of angle A/L and Ganss ratio/L, (C) of angle B/R and Ganss ratio/R, and (D) of angle B/L and Ganss ratio/L.
On the other hand, a weak negative insignificant correlation was observed between
angle B and Ganss ratio/R using the Spearman R coefficient. Nevertheless, a weak positive
significant correlation was found between angle B and Ganss ratio/R on the left side,
as depicted in [Fig. 4C and D]. Notably, an increase in angle B on both sides corresponded to an increase in the
retromolar space of third molars.
Multiple Regression Analysis
The multiple regression results are shown in [Table 3]. The result reveals that the increase in distance of length b/R, the distance of
segment S2 by 1 mm, will significantly increase the distance of the segment (S3).
The individual parameter changes in length a/R, length d/R, width a, width c, S1,
Ganss ratio/R lead to increase in segment S3. However, the increase is not statistically
significant, with a p-value greater than 0.05.
Table 3
The relationship between the segment of the arch perimeter (S3) as a dependent variable
and Ganss ratio/R, angle A/R, angle B/R, length, width, and segments of the lower
arch perimeter as an independent variable
|
Beta
|
SE. of beta
|
Beta
|
SE of beta
|
t (27)
|
p-Value
|
Intercept
|
|
|
5.96
|
1,17
|
5.11
|
0.00
|
Length a/R
|
0.10
|
0.11
|
0.03
|
0.03
|
0.96
|
0.34
|
Length b/R
|
0.36
|
0.10
|
0.11
|
0.03
|
3.71
|
0.000
|
Length c/R
|
−0.07
|
0.11
|
−0.02
|
0.02
|
−0.68
|
0.50
|
Length d/R
|
0.06
|
0.12
|
0.02
|
0.05
|
0.54
|
0.59
|
Width a
|
0.001
|
0.12
|
0.0003
|
0.04
|
0.01
|
0.99
|
Width b
|
−0.05
|
0.12
|
−0.009
|
0.02
|
−0.37
|
0.71
|
Width c
|
0.15
|
0.11
|
0.03
|
0.02
|
1.31
|
0.19
|
S1
|
0.09
|
0.10
|
0.04
|
0.05
|
0.94
|
0.35
|
S2
|
0.22
|
0.09
|
0.08
|
0.03
|
2.43
|
0.02
|
Ganss ratio/R
|
0.003
|
0.09
|
0.01
|
0.28
|
0.04
|
0.97
|
Angle A/R
|
−0.03
|
0.09
|
−0.002
|
0.01
|
−0.34
|
0.73
|
Angle B/R
|
−0.10
|
0.09
|
−0.004
|
0.004
|
−1.11
|
0.27
|
Abbreviation: SE, standard error.
R = 0.59; F(12.107) = 4,73; p < 0.000.
Furthermore, the multiple regression results of the relationship between the segment
on the left side of the arch perimeter (S4) (dependent variable) and Ganss ratio/L;
angle A/L; angle B/L; length; width, segments of the lower arch perimeter (independent
variable) are shown in [Table 4].
Table 4
The relationship between the segment on the left side of the arch perimeter (S4) as
the dependent variable and Ganss ratio/L; angle A/L; angle B/L; length; width, and
segments of the lower arch perimeter as an independent variable
Parameter
|
Beta
|
SE of beta
|
B
|
SE. of beta
|
t (27)
|
p-Value
|
Intercept
|
|
|
5.03
|
1.23
|
4.08
|
0.00
|
Length a/L
|
0.25
|
0.13
|
0.09
|
0.05
|
1.91
|
0.06
|
Length b/L
|
0.14
|
0.09
|
0.06
|
0.04
|
1.56
|
0.12
|
Length c/L
|
−0.18
|
0.12
|
−0.04
|
0.03
|
−1.47
|
0.14
|
Length d/L
|
0.08
|
0.15
|
0.03
|
0.06
|
0.56
|
0.58
|
Width a
|
−0.01
|
0.11
|
−0.004
|
0.04
|
−0.10
|
0.92
|
Width b
|
0.21
|
0.12
|
0.04
|
0.02
|
1.77
|
0.08
|
Width c
|
−0.11
|
0.12
|
−0.02
|
0.02
|
−0.93
|
0.36
|
S5
|
0.04
|
0.10
|
0.02
|
0.05
|
0.45
|
0.66
|
S6
|
0.28
|
0.11
|
0.16
|
0.06
|
2.56
|
0.01
|
Ganss ratio/L
|
0.02
|
0.09
|
0.05
|
0.28
|
0.19
|
0.85
|
Angle A/L
|
−0.03
|
0.10
|
−0.001
|
0.01
|
−0.27
|
0.79
|
Angle BL
|
−0.02
|
0.10
|
−0.001
|
0.004
|
−0.24
|
0.81
|
Abbreviation: SE, standard error.
From the result, the individual changes of parameters—angle A/L; angle B/L; length
c/L; width a; width c—lead to a reduction in segment S4; however, the reduction is
not statistically significant.
Furthermore, the increase in distance in segment S6 for 1 mm leads to an increase
in the distance of segment S4 by 0.16 mm (B = 0, 16), thus making it statistically
significant with a p-value of 0.01.
Discussion
Dental crowding is a common problem in orthodontic clinics and is defined as a discrepancy
between the space available and the space required in dental arches.[15] The etiology of crowding is multifactorial, and it has been suggested that tooth
size, arch dimension, unerupted molars, and environmental factors play a role in its
development.[16]
[17] Many researchers have investigated the relationship between crowding and these parameters
in detail.[18]
[19]
While crowding can affect the entire arch, it is more commonly localized to the anterior
segment of the lower arch.[20] The cause of this anterior crowding is still under debate; however, it has been
suggested that it may be due to the inadequate growth of the mandible or an imbalance
between tooth size and arch dimension.[21] Whether impacted molars affect dental crowding is still highly controversial.[22] A study conducted in China has revealed that mesiodistally angulated molars impact
anterior teeth crowding.[23]
According to Howe et al,[24] the authors found that arches with crowding were shorter than those without crowding.
This was confirmed in our study, and it was observed that the crowded group's arch
was shorter than the noncrowded group's. This may be attributed to the fact that crowded
teeth rotate and tip, leading to decreased available arch space. This can cause the
arch to become narrower, shorter, and more constricted, which can contribute to crowding.
However, it is essential to note that the difference in arch length between crowded
and noncrowded arches in the study was not significant for all parameters, indicating
that other factors may also be involved in the development of crowding. As a result,
we cannot establish a direct causative relationship between arch length and dental
crowding.
In the works of Waheed-ul-Hamid and Imran,[25] arch length was found to be greater in noncrowded arches as compared to the crowded
group. This is in correlation with the findings of this study because, in the noncrowding
group, the length of the lower arch is significantly greater in relation to the crowding
group. These findings support the importance of considering arch length as a contributing
factor to dental crowding. In addition, a study by Muhammed et al[21] showed that tooth size discrepancies are a significant factor in dental crowding.
This highlights the complex interplay between various dental parameters in the etiology
of dental crowding.
Research has shown that dental crowding is more commonly observed in the anterior
segment of the dental arch due to several factors. The anterior teeth are more susceptible
to tipping and rotation, leading to misalignment and crowding in the anterior segment.[26]
[27] The individual changes in the retromolar region and arch dimensions were crucial
in reducing or increasing the anterior segment of the lower arch.[17] This suggests that variations in arch dimensions may contribute to the development
of dental crowding. Moreover, other studies have highlighted the importance of considering
factors such as tooth size, arch width, and depth in evaluating dental crowding.[28]
[29]
Lateral cephalometric radiographs have been used by several researchers to assess
the position of third molars, but they are not the most accurate radiographs due to
the significant amount of superimposition present.[26]
[29] OPGs are considered more reliable radiographs for assessing the position and angulation
of third molars. Therefore, the authors of this study opted to use panoramic radiographs
to evaluate the angulation and position of third molars and to measure the space/third
molar width ratio.
Hattab and Alihaija[28] found that the space/third molar width ratio was significantly more prominent in
the group of erupted third molars than in the impacted group. This is consistent with
the findings of the current study, where the noncrowding group (erupted third molars)
had significantly larger values of the space/third molar width ratio than the crowded
group (unerupted third molars). Zachrisson[30] noted that mandibular incisor crowding is often caused by a developing mandibular
third molar with insufficient space, and a mesially directed force can contribute
to early mandibular arch crowding. This is consistent with our study, as the authors
found an insufficient retromolar space in the crowding group, where the Ganss ratio
values were smaller than in the noncrowding group.
Niedzielska et al[31] suggested that the ratio of the third molar angle to the base of the mandible and
the third molar angle to the second molar inclination can be used to predict the position
of the lower third molar in the dental arch, facilitating a decision to retain or
remove it. Therefore, early diagnosis and management of developing third molars are
important to prevent crowding and reduce the need for extraction.
This study provides compelling evidence of a significant association between lower
arch crowding and third molars. The results demonstrate that the angle between the
third molars and the mandibular plane is smaller in the crowding group compared to
the noncrowding group. However, we have established that participants who do not have
enough space for the proper alignment of their front teeth do not have enough space
for wisdom teeth either. Furthermore, the angle between the third and second molars
is higher in the crowding group than in the noncrowding group. This suggests that
the mesial inclination of third molars may exert mesial force on lower arch teeth.
This study has certain limitations, such as a small sample size and a cross-sectional
study design. Due to our study design, we could not establish a direct causative relationship
between third molars and mandibular incisor crowding. We suggest extensive cohort
and longitudinal study designs, which also exclude other contributing factors.
Conclusion
According to our study, third molars appear to be a contributing factor to mandibular
anterior segment crowding. There are several potential points to consider regarding
the difficulties in orthodontic treatment planning and making a timely decision about
third molar removal.