Received: 26 December 2019

Revised: 17 May 2020

Accepted: 19 May 2020

DOI: 10.1002/cre2.306

ORIGINAL ARTICLE

Relationship between oral function and mandibular anterior
crowding in early mixed dentition
Yoma Ohno |Yuko Fujita

Division of Developmental Stomatognathic
Function Science, Department of Health
Promotion, Kyushu Dental University,
Kitakyushu, Japan|

Keirato Ohno |Kenshi Maki

Abstract
The position of the dentition is considered to be determined by a combination offorces exerted by the perioral muscles and tongue. We hypothesized that mandibular

Correspondence
Yuko Fujita, Division of Developmental
Stomatognathic Function Science, Department
of Health Promotion, Kyushu Dental
University, 2-6-1 Manazuru, Kokurakita-ku,
Kitakyushu 803-8580, Japan.
Email: y-fujita■kyu-dent.ac.jp

anterior crowding may be related to abnormalities in the development of oral func-

Funding information
Grants-in-Aid from the Ministry of Education,
Culture, Sports, Science and Technology,
Japan, Grant/Award Number: 17K11967

ipants, and their dental casts and lateral cephalograms were evaluated. Little's irregu-

tion. To determine the relationship between oral function and mandibular anterior
crowding in early mixed dentition. A total of 61 children (30 boys, 31 girls) with early
mixed dentition were included in this study. Height, body weight, maximum occlusal
force, lip-closing strength, and maximum tongue pressure were measured in all particlarity index (LII), evaluated by dental casts, was used as an indicator of mandibular
anterior crowding. Maximum occlusal force and lip-closing strength were not significantly correlated with the LII, although they were positively correlated with maximum tongue pressure, mandibular intercanine perimeter distance, and upper central
incisor (U1)-to-NL angle (p < .05). Maximum tongue pressure was negatively correlated with LII (p < .05). Maximum tongue pressure and LII were significantly positively
correlated with the mandibular intercanine perimeter distance and U1/NL angle, andnegatively correlated with the interincisal angle (p < .05 for all). Crowding of the mandibular anterior teeth was directly correlated with tongue pressure function and indirectly correlated with maximum occlusal force and lip-closing strength.
KEYWORDS

children, crowding, lip-closing strength, mixed dentition, occlusal force, tongue pressure

1|

I N T RO DU CT I O N

including larger interincisal angle, overjet, and overbite values, as well
as shorter mandibular and maxillary dental arches (Melo, Ono, &Crowding is one of the more frequent types of malocclusion in chil-

Takagi, 2001; Turkkahraman & Sayin, 2004). These results suggested

dren with mixed dentition (Sanin & Savara, 1973). In particular, man-that crowding of the mandibular anterior teeth could be related todibular anterior crowding is often found by the children's parents or

malocclusion, such as deep overbite.

dentists during the earliest stage of mixed dentition (Turkkahraman &Generally, the position of dentition is considered to be determined

Sayin, 2004). When there is a problem with the arrangement of a

by a combination of forces exerted by the buccinator muscles andpatient's anterior teeth, their parents often express concern about

tongue, termed the buccinator mechanism (Brodie, 1938). Muscle mass

aesthetic problems. Previous studies have identified several factors

and muscle function gains may be inadequate during developmental

related to mandibular anterior crowding in early mixed dentition,

stages, and failure to correct these developmental defects could beThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,provided the original work is properly cited.
© 2020 The Authors. Clinical and Experimental Dental Research published by John Wiley & Sons Ltd
Clin Exp Dent Res. 2020;1–8.

wileyonlinelibrary.com/journal/cre2

1

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OHNO ET AL.

related to impairment in the development of healthy oral function.

the head in the Frankfort plane, and body weight was measured toPreviously, the relationships between malocclusion and oral functionan accuracy of 0.1 kg (Takeshima et al., 2019). Subjects were

including tongue pressure and lip strength have been reported in

instructed to take off their shoes and socks before height measure-children with maxillary or mandibular protractions (Kurabeishi, Tatsuo,

ment, and to remove their closing before measurement of body

Makoto, & Kazunori, 2018; Thuer & Ingervall, 1986). Therefore, weweight.

hypothesized that mandibular anterior crowding is associated with poor
development of oral function. However, few studies have examined therelationship between mandibular anterior crowding and oral function.

2.3|

Maximum occlusal force

The purpose of this study was to investigate the relationshipbetween oral function and mandibular anterior crowding in earlyMaximum occlusal force was measured using a portable occlusal

mixed dentition. Additionally, we assessed the relationships between

force meter (GM10, Nagano Keiki Co., Ltd., Tokyo, Japan), which

oral function and dental and maxillofacial factors related to mandibu-

consisted of a strain gauge in the center of a biting element encasedlar anterior crowding in early mixed dentition.

in a plastic tube. Participants were examined in a relaxed sitting position. Participants were asked to place the element on the maxillary
first molar and to bite the element with maximal voluntary muscular2

MATERIALS AND METHODS|

effort for approximately 3 s. We determined whether participants bit
the device with maximum effort based on the findings of a previous

2.1|

Participants

study (Takeshima et al., 2019). Occlusal force was measured inkilonewtons (kN) by a pressure gauge built into the element and dis-This study was approved by the Human Investigation Committee ofplayed digitally. Maximum bite force was measured on each side,

Kyushu Dental University (Approval Number: 18–37), and all partici-

with a 30-s interval between bite measurements. The larger of thepants and their parents provided written informed consent to partici-

values recorded on the left and right sides was considered the maxi-

pate in the study. The participants were recruited following an initial

mum bite force and was used in subsequent analyses (Takeshima

examination at a single private dental clinic in Japan.

et al., 2019).

The results of a pilot study suggested that the mean value of maximum tongue pressure among children with early mixed dentition isapproximately 30 kPa (Fujita, Ichikawa, Hamaguchi, & Maki, 2018).

2.4|

Lip-closing strength

That pilot study involved multiple tests, and a significant difference
was observed between the two groups (Hellman's developmental

Lip-closing strength was measured using a Lippulekun® (SHOFU, Inc.,

stages III A vs. III B) when the mean maximum tongue pressure was

Kyoto, Japan). Participants were asked to insert a Lippule button®

greater than 6 kPa, additionally the response within each subject(SHOFU, Inc.) into the space between their incisors and lips, and togroup was normally distributed with SD 5 (Fujita et al., 2018). There-

hold it with minimal mouth opening in a relaxed sitting position. They

fore, if the true mean difference between two groups were 6 kPa,

were then asked to hold the button tightly in their mouths, and a

12 participants would be needed to be able to reject the null hypothe-

string approximately 10 cm long was attached to the center ofsis with a power of 0.8 and a Type I error probability of 0.05.

the button. Strength was measured by a strain force gauge attachedThe inclusion criteria were children with early mixed dentition (fourto the end of the string and was displayed digitally in Newtons (N).

fully erupted permanent mandibulae incisors, deciduous canines, decidu-

As the gauge was pulled parallel to the floor, it recorded the highest

ous molars, and permanent first molars) with a Class I or Class II molar

tension before the button was pulled from the mouth (Saitoh

relationship. The exclusion criteria were systemic disturbances, ingestion

et al., 2017).

of medicines that could interfere directly or indirectly with muscular
activity, and uncooperative behavior. In addition, children were excluded
if they had alterations in the form of oral tissues, structure or number of

2.5|

Maximum tongue pressure

teeth, negative overjet, or negative overbite, as well as if they had a history of orthodontic treatments or temporomandibular dysfunction

Maximum tongue pressure was measured using a tongue pressure(Takeshima, Fujita, & Maki, 2019; Turkkahraman & Sayin, 2004). Basedmanometer (JMS, Hiroshima, Japan). Participants were examined in aon these criteria, participants included 61 children (30 boys and 31 girls).

relaxed sitting position; they were asked to place a balloon on theanterior part of their palate and to close their lips, biting a hard ringwith the upper and lower incisors. Then, the participants were asked

2.2|

Anthropometry and intraoral examination

to raise their tongues and compress the balloon onto the palate withmaximal voluntary muscular effort for approximately 7 s. The pres-Height and body weight were measured in the consultation room

sure was measured (in kilopascals) using a digital voltmeter attachedof the clinic. Height was measured to an accuracy of ±0.1 cm using

to the tongue pressure manometer (Ichikawa, Fujita, Hamaguchi,

a portable digital stadiometer (AD-653, A&D, Tokyo, Japan) with

Chaweewannakorn, & Maki, 2016).

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OHNO ET AL.

2.6|

Dental cast analysis

considered very severe irregularity (Little, 1975). In this study, to compare the average values of each parameter, participants were divided

Plaster models were measured in 0.01 mm units using a digital caliper

into two groups using a cutoff of 3.5 mm. In total, 32 participants had(Mitutoyo, Kanagawa, Japan) for all measurements. The 13 measured

LII < 3.5 mm (15 boys, 17 girls), whereas 29 had LII ≥ 3.5 mm (15 boys,

items included the maxillary and mandibular intercanine widths, mandib-

14 girls).

ular intercanine perimeter distance, lateral arch lengths from the maxillary and mandibular central incisors to first molars, sum of the widths ofthe mandibular four incisors, available mandibular incisor space (inter-

2.7|

Cephalometry

canine perimeter distance minus the sum of the widths of the four incisors), maxillary inter-first molar width (distance from central sulcus to

Lateral cephalometric radiographs were taken with the Hyper-G/CM

central sulcus of the upper first molars), overjet, and overbite; these

NEO PREMIUM version (ASAHIROENTGEN Ind. Co., Ltd., Kyoto,items were presumably related to mandibular anterior crowding based

Japan) in accordance with the manufacturer's instructions and wereon the findings of previous studies (Figure 1; Louly, Nouer, Janson, &traced. Cephalometric reference points and measurements were

Pinzan, 2011; Schutz-Fransson, Lindsten, Bjerklin, & Bondemark, 2017).

assessed following the method of Schutz-Fransson et al. (2017). Ref-

Additionally, Little's irregularity index (LII) was measured; this was

erence lines and points are shown in Figure 3, and the following

defined as the sum of displacement of the anatomic contact points of

11 measurement items were recorded: 1. SNA angle; 2. SNB angle;

the mandibular anterior teeth (Figure 2). LII is divided into the following

3. ANB angle; 4. SN/ML angle; 5. ML/NL angle; 6. SN/NL angle;

five classifications based on the degree of crowding: LII < 0.5 mm

7. U1/NL angle; 8. L1/ML angle; 9. interincisal angle; 10. L1 to APg

is considered perfect alignment; 0.5 ≤ LII < 3.5 mm is considered mini-

distance; 11. Ar to B distance.

mal irregularity; 3.5 ≤ LII < 6.5 mm is considered moderate irregularity;
6.5 ≤ LII < 9.5 mm is considered severe irregularity; and 9.5 mm ≤ LII is

2.8|

Reliability of measurements

All examinations of oral function were performed in duplicate, separated by at a least 30-s period to allow subjects to rest and to indicate
they had recovered, and the mean values were used for subsequent
analyses. In addition, measurements using dental casts and lateralcephalograms were performed twice at an interval of 2 weeks, and themean values were used in the analysis. All examinations were performed by the same examiner. Data generated during sample collection
were assessed for reliability. Random error was characterized based on
F I G U R E 1 Variables measured on dental casts. 1. Mandibularintercanine width, 2. Mandibular intercanine perimeter distance,
3. Lateral arch length from the mandibular left central incisor to firstmolar, 4. Lateral arch length from the mandibular right central incisor
to first molar, 5. Maxillary intercanine width, 6. Lateral arch lengthfrom the maxillary left central incisor to first molar, 7. Lateral archlength from the maxillary right central incisor to first molar,
8. Maxillary inter-first molar width

intra-rater reliability, which was quantified using the intraclass correlation coefficient (ICC). The test–retest reliability results are expressed interms of ICC, with 0.90 ≤ ICC ≤ 1.00 corresponding to excellent reliability (Domholdt, 1993).

2.9|

Data analysis

The Shapiro–Wilk test was used to determine the normality of thedata. All data in each group (LII < 3.5 and LII ≥ 3.5) are expressed asmeans ± SDs. Statistical comparisons between two groups were performed using a two-tailed t-test. Group difference between variables
with p < .05 were considered significant. False discovery rates (FDR)
correcting for multiple testing were calculated using the Benjamini–
Hochberg correction (Benjaminiand & Hochberg, 1995). Pearson's
correlation coefficients were used to determine associations among
three types of oral function, LII, and other variables. Furthermore, partial correlations were used to assess the relationship between oral
function (maximum occlusal force and maximum tongue pressure) andF I G U R E 2 Little's irregularity index. Little's irregularity index (LII)
was defined as the sum of displacement of the anatomic contact
points of the mandibular anterior teeth. LII = A + B + C + D + E

variables related to mandibular anterior crowding, while controllingfor height and body weight. All data were analyzed using SPSS for
Windows (version 23.0; IBM Japan, Tokyo, Japan).

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OHNO ET AL.

F I G U R E 3 Reference lines
and points in lateralcephalometric analysis.
Measurement items were SNA ( ),SNB ( ), ANB ( ), SN/ML ( ),ML/NL ( ), SN/NL ( ), U1/NL ( ),L1/Apg (mm), L1/ML ( ),Interincisal angle ( ), and
Ar-B (mm)3|

RESULTS

Although lip-closing strength was not significantly correlated with
LII, it was significantly correlated with maximum tongue pressure,

The ICCs for height, body weight, maximum occlusal force, lip-closing

mandibular intercanine perimeter distance, and U1/NL angle; thesestrength, and maximum tongue pressure were all ≥0.95. The ICCs forvalues were significantly correlated with LII (p < .05 for all).

the items of dental casts analysis were all ≥0.91, whereas for the

The maximum tongue pressure was significantly negatively corre-

items of cephalometric analysis, they were all ≥0.90. No statisticallylated with LII (p < .05). The maximum tongue pressure was significantly

significant differences between the sexes were found in any parame-

correlated with mandibular intercanine width, mandibular intercanineter (data not shown).

perimeter distance, available mandibular incisor space, bilateral dental

The maximum tongue pressure in the LII ≥3.5-mm group tended

arch lengths from the maxillary central incisors to first molars, over-to be lower than that in the LII <3.5-mm group (p = .038, FDR = 0.020;

bite, U1/NL angle, interincisal angle, L1 to APg distance, and Ar to B

Table 1). There were significant differences between the two groups

distance; these values were significantly correlated with LII (p < .05

in 8 of 13 items by dental cast analysis (p < .02, FDR < 0.02 for all;for all).

Table 1). In lateral cephalometric analysis, the Ar to B distance was

The maximum occlusal force and maximum tongue pressure

significantly lower in the LII ≥ 3.5-mm than in the LII < 3.5-mm groupwere significantly positively correlated with height and body weight(p = .005, FDR = 0.007; Table 2).

(p < .05). Among the variables that were correlated with both LII and

The results of Pearson's correlations coefficient analysis of eachoral function (maximum occlusal force and maximum tongue pres-variable are shown in Tables 3 and 4. Maximum occlusal force wassure), the mandibular intercanine width, mandibular intercanine perim-

not significantly correlated with LII, but was significantly positively

eter distance, bilateral arch lengths from the maxillary central incisors

correlated with maximum tongue pressure, mandibular intercanine

to first molars, and Ar to B distance were significantly correlated withperimeter distance, available mandibular incisor space, bilateral archheight and body weight (p < .05 for all).

lengths from the maxillary central incisors to first molars, U1/NL

The partial correlations of these variables after controlling forangle, and Ar to B distance; these values were significantly correlated

height and body weight are shown in Table 5. The maximum occlusal

with LII (p < .05).

force exhibited significant correlations with maximum tongue

OHNO ET AL.

5

TABLE 1analysis

Comparison of the degree of mandibular anterior crowding on age, height, body weight, oral function, and parameters of dental cast

LII < 3.5 mm(perfect alignment or
minimal irregularity)
N = 32
Mean (SD)

LII ≥ 3.5 mm(moderate orsevere irregularity)
N = 29Mean (SD)p Value

FDR

NS

—

131.66 (9.32)

NS—

29.68 (5.95)

NS—

Age (years)

8.81 (0.99)

8.89 (0.95)

Height (cm)132.58 (8.35)

Body weight (kg)29.56 (4.86)

Maximum occlusal force (kN)

0.39 (0.15)

0.34 (0.12)

NS—

Lip-closing strength (N)10.66 (4.11)

9.47 (3.79)

NS—

Maximum tongue pressure (kPa)

35.85 (7.31)

31.61 (8.26)

.038

0.020

Mandibular intercranine width (mm)27.89 (1.58)

26.46 (2.43)

.010

0.013

Mandibular intercanine perimeter distance (mm)24.05 (1.79)

21.34 (1.81)

.000

0.005

Lateral arch length, mandibular left 1–6 (mm)32.15 (1.66)

30.84 (1.98)

.006

0.008

Lateral arch length, mandibular right 1–6 (mm)32.28 (1.83)

30.86 (2.20)

.009

0.012

Sum of the width of the mandibular four incisors (mm)23.08 (1.67)

23.44 (1.20)

NS—

Available mandibular incisor space (mm)0.76 (1.35)−2.14 (1.36)

.000

0.003

Little's irregularity index (mm)1.18 (1.14)

6.57 (2.69)

.000

0.002

Maxirally intercranine width (mm)34.97 (2.47)

32.47 (6.86)

NS-

Lateral arch length, maxirally left 1–6 (mm)36.39 (2.44)

34.50 (2.85)

.007

0.010

Lateral arch length, maxirally right1-6 (mm)36.26 (2.67)

34.41 (2.85)

.011

0.015

Maxirally inter-first molar width (mm)47.07 (2.73)

47.01 (3.27)

NS—

Overjet (mm)3.67 (1.70)

3.73 (1.57)

NS—

Overbite (mm)2.71 (1.21)

3.23 (1.82)

NS—

Abbreviations: FDR, false discovery rates; LII, Little's Irregularity Index; NS, not significance; SD, standard deviation.

TABLE 2Comparison of the degree of mandibular anterior crowding on the lateral cephalometry
LII < 3.5 mm(perfect alignment or
minimal irregularity)
N = 32
Mean (SD)

LII ≥ 3.5 mm(moderate orsevere irregularity)
N = 29Mean (SD)p Value

FDR

SNA angle ( )79.68 (3.77)

80.04 (2.71)

NS—

SNB angle ( )76.68 (3.32)

76.00 (2.38)

NS—

ANB angle ( )3.00 (2.02)

4.04 (2.39)

NS—

SN/ML angle ( )35.77 (4.78)

37.54 (5.38)

NS

—



ML/NL angle ( )27.55 (3.38)

29.93 (5.13)

.040

0.022

SN/NL angle ( )8.22 (2.92)

7.61 (2.95)

NS

—



U1/NL angle ( )116.78 (6.36)

112.32 (7.72)

.019

0.018

L1/ML angle ( )97.20 (5.30)

93.30 (6.93)

.019

0.017

Interincisal angle ( )120.32 (7.86)

125.98 (12.59)

.047

0.023

L1 to APg distance (mm)3.95 (2.04)

2.80 (2.25)

.047

0.025

Ar to B distance (mm)96.45 (5.23)

92.11 (6.07)

.005

0.007

Abbreviations: FDR, false discovery rates; LII, Little's Irregularity Index; NS, not significance; SD, standard deviation.

pressure (p < .05), mandibular intercanine perimeter distance (p < .01),

maximum tongue pressure was positively correlated with mandibularbilateral dental arch lengths from the maxillary central incisors tointercanine width (p < .05) and mandibular intercanine perimeter dis-first molars (p < .05), and Ar to B distance (p < .01). In addition, thetance (p < .01).

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OHNO ET AL.

TABLE 3

Pearson's correlation coefficients of the anthropometry and measurements of oral function and dental casts
Little's
irregularity
index

Maximum
occlusalforce

Lip-closing
strength

Maximum
tonguepressure

HeightBody
weightLittle's irregularity index−0.172−0.149

1

0.175

0.126

0.336**

Maximum occlusal force

0.352**

0.292*−0.1751

0.407**

0.387**

Lip-closing strength

0.115

0.126

−0.1210.407**

1

0.475**

Maximum tongue pressure

0.421**

0.336**−0.368**0.387**

0.475**

1

Mandibular intercranine width

0.437**

0.463**−0.382**

0.114

0.123

0.447**

Mandibular intercanine perimeter distance0.307*

0.270*−0.646**

0.398**

0.311*

0.573**

Lateral arch length, mandibular left 1–6

0.125

0.151

−0.359**

0.122

0.054

0.145

Lateral arch length, mandibular right 1–6

0.100

0.109

−0.424**

0.169

0.021

0.115

Sum of the width of the mandibular four incisors

0.160

0.299*

0.144

0.000

0.028

0.244

Available mandibular incisor space

0.261

0.125

−0.802**0.408**

0.250

0.429**

Maxirally intercranine width−0.079−0.174−0.297*

0.149

0.053

0.214

Lateral arch length, maxirally left 1–6

0.378**

0.436**−0.420**0.370**

0.108

0.260*

Lateral arch length, maxirally right 1–6

0.451**

0.502**−0.373**0.398**

0.140

0.325*

Maxirally inter-first molar width

0.456**

0.434**−0.077

0.108

0.161

0.345**

Overjet0.262*

0.325*

0.000

0.085

−0.098−0.085

Overbite

0.003

0.010

0.322*−0.037−0.243−0.518***p < .05.
**p < .01.

T A B L E 4 Pearson's correlation coefficients of the anthropometry, Little's irregularity index, and measurements of oral function and lateralcephalometry

SNA angle

HeightBody
weightLittle's
irregularity
index

Maximum
occlusalforce

Lip-closing
strength

Maximum
tonguepressure

0.152

0.172

−0.007−0.025−0.301*−0.049

SNB angle

0.265*

0.250−0.0890.410**

0.007

0.274*

ANB angle−0.121−0.0720.104−0.565**−0.446**−0.424**

SN/ML angle−0.362**−0.288*0.081−0.330*0.000−0.112

ML/NL angle−0.291*−0.1740.183−0.337**−0.051−0.123

SN/NL angle−0.192−0.239−0.136−0.0660.079−0.010U1/NL angle

0.246

0.226

−0.338**0.500**

0.325*

0.388**

L1/ML angle

0.249

0.159

−0.387*0.058−0.141

0.173

Interincisal angle−0.258−0.2350.410**−0.206−0.097−0.372**

L1 to APg
distance

0.132

0.075

−0.406**

0.143

0.101

0.281*

Ar to B distance

0.541**

0.570**−0.300*0.533**

0.158

0.293**p < .05.
**p < .01.

Finally, when height and body weight were controlled, LII and max-

(p < .05). LII exhibited a significant correlation with maximum tongue

imum occlusal force showed significant correlations with maximum

pressure (p < .05), and both LII and maximum tongue pressure were sig-

tongue pressure, mandibular intercanine perimeter distance, available

nificantly correlated with mandibular intercanine width, mandibular

mandibular incisor space, bilateral dental arch lengths from the maxil-intercanine perimeter distance, available mandibular incisor space, over-lary central incisors to first molars, U1/NL angle, and Ar to B distancebite, U1/NL angle, interincisal angle, and L1 to APg distance (p < .05).

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OHNO ET AL.

T A B L E 5 Partial correlation coefficients of the measurements ofthe maximum occlusal force and maximum tongue pressure, anddental and maxillofacial morphology

molar bite force in participants aged 18–27 years was higher when the
inclinations of both upper and lower incisors increased, consistent with
our findings (Alabdullah, Saltaji, Abou-Hamed, & Youssef, 2014).

Maximum
occlusal force

Maximum tongue
pressure

with LII, suggesting that maximum tongue pressure was directly corre-

Maximum occlusal force

10.318*

lated with crowding of the mandibular anterior teeth. Maximum

Maximum tongue pressure

0.318*

1

tongue pressure was also positively correlated with mandibular dental

Mandibular intercranine
width

0.001

0.338*

arch length in the anterior region (intercanine width, intercanine

Mandibular intercanine
perimeter distance

0.347**

0.517**

Lateral arch length,
maxirally left 1–6

0.327*

0.130

Lateral arch length,
maxirally right 1–6

0.343*

0.175

Ar to B distance

0.464**

0.110

Maximum tongue pressure was significantly negatively correlated

perimeter distance, available incisor space) and weakly correlated withlabial inclination of the upper and lower incisors (overbite, U1/NL
angle, interincisal angle, and L1 to APg distance).
In a previous study, the average maximum tongue pressures in
7-, 8-, and 9-year-old children were reported to be 32.46, 32.10,
and 35.33 kPa, respectively (Ichikawa et al., 2016). We found thatthe maximum tongue pressure in children with moderate or severe

Note: The control variables; height and body weight.
*p < .05.
**p < .01.

crowding (31.61 kPa) was lower than the average maximum value in 7-year-old children, although the maximum tongue pressure in children
with perfect alignment or minimal crowding (35.85 kPa) was slightly
higher than the average maximum value in 9-year-old children. We suggest that more severe mandibular anterior crowding was associated with

4|

DISCUSSION

inhibition of the acquisition of maximal tongue pressure according to age.
Although lip-closing strength was not directly correlated with mandib-

In this study, we evaluated the relationship between LII and factors

ular anterior crowding, it was significantly weakly correlated with mandib-

related to LII. Based on our results, we conclude that increased LII is asso-ular intercanine perimeter distance (r = .311) and U1/NL angle (r = .325).

ciated with shorter mandibular and maxillary dental arches, shorter man-

However, compared to the correlations with lip-closing strength, mandibu-

dibular bone length, and deeper overbite, although these parameters werelar intercanine perimeter distance and U1/NL were more strongly corre-

weakly correlated with LII, except for the mandibular anterior dental archlated with maximum occlusal force (r = .398 and r = .500, respectively)

length. However, the mandibular and maxillary dental arches and mandib-and maximum tongue pressure (r = .573 and r = .388, respectively). Addi-

ular bone length in children with moderate or severe crowding were sig-

tionally, positive correlations were found among lip-closing strength, maxi-nificantly smaller than those in children with perfect alignment or minimal

mum occlusal force, and maximum tongue pressure. Therefore, we

crowding. Overbite in children with moderate or severe crowding tended

consider that the lip-closing strength is determined by the cooperative

to be greater than in children with perfect alignment or minimal crowding.

effort of tongue pressure and occlusal force, rather than by the character-

We found that greater lingual inclination of the upper and lower incisors

istics of dental and maxillofacial morphology.

(interincisal angle and L1 to APg distance) was also significantly correlated

These results suggest that mandibular anterior crowding may bewith LII. Additionally, all values indicating lingual inclination of the upperassociated with abnormalities in the development and coordinationand lower incisors in children with moderate or severe crowding tendedof perioral muscles and occlusion. Alternatively, the lack of develop-

to be more severe than in children with perfect alignment or minimal

ment and coordination of perioral muscles and occlusion may induce

crowding. Together, some variables had weaker correlation coefficients

mandibular anterior crowding, although these causal relationships

with LII, but moderate anterior crowding was associated with mandibular

were not explored. However, crowding at an early mixed dentition

and maxillary dental arches, mandibular bone length, and lingual inclination

stage may improve when permanent canines erupt; therefore, at thisof the upper and lower incisors. These findings are consistent with previ-

stage, it is difficult to reliably assess the mandibular anterior crowding.

ous studies (Melo et al., 2001; Turkkahraman & Sayin, 2004).

Additionally, these causal relationships could not be proven. There-

Based on our results, although maximum occlusal force was notdirectly correlated with mandibular anterior crowding, the maximum

fore, longitudinal studies on the association between mandibular anterior crowding and oral function should be needed.

occlusal force was positively correlated with maximum tongue pres-A previous study reported that the lip-closing strength was not

sure and mandibular anterior dental arch length (intercanine perimetercorrelated with cephalometric variables, including relationships anddistance and available incisor space). Additionally, increased length ofinclinations of the incisors in children 7–13 years of age (Ingervall &mandibular bone (Ar to B distance), increased lateral length of the max-

Janson, 1981). Another study reported that lip-closing strength was

illa (lateral arch length from the maxillary central incisor to first molars),

negatively correlated with the maxillary incisor/Frankfort horizontal

and greater labial inclination of the upper central incisors (U1/NL

angle among young adult participants (Jung, Yang, & Nahm, 2003).

angle) were correlated with increased maximum occlusal force. TheseThus, if there was a correlation between lip-closing strength and theresults suggest that maximum occlusal force was indirectly correlated

U1/NL angle, labial inclination of the upper incisors could be advanta-

with mandibular anterior crowding. A previous study showed that thegeous for lip-closing strength, depending on the developmental stage

8

OHNO ET AL.

of dentition. Generally, labial inclination of the maxillary incisors during the early mixed dentition period is considered a normal dentition
development process (Higley, 1954), and participants in our study
were approximately 8 years of age with early mixed dentition.
In this study, children with Angle Class I or Class II molar relationships were included. Although no significant relationship with LII
was found, lip-closing strength was negatively correlated with the
ANB angle due to a smaller SNA angle; additionally, maximum occlusal
force and maximum tongue pressure were negatively correlated with
ANB angle due to a larger SNB angle, consistent with the findingsof previous studies (Kurabeishi et al., 2018; Lambrechts, De Baets,
Fieuws, & Willems, 2010; Thuer & Ingervall, 1986). These results suggest that maxillary protrusion can be related to a disadvantage during
the development of oral function.
This study has several limitations that should be addressed. First,
the dentition and maxillofacial measures (including soft tissue form)
related to oral function and mandibular anterior crowding could not beclarified. It is important to determine whether there is a relationshipbetween lip and tongue morphology, oral function, and mandibular
anterior crowding. Second, all participants, in this study, had a Class I or
Class II molar relationship without cross bite or open bite, so data could
not be compared between participants with malocclusion and thosewith normal occlusion. Comparisons of oral function and mandibular
anterior crowding between children with normal occlusion and children
with various types of malocclusion are required. An additional limitation
was the cross-sectional design, which precluded the determination of
causal relationships between the mandibular anterior crowding and oral
function. Longitudinal studies are required to investigate the effects offactors associated with mandibular anterior crowding on oral function.

5|

C O N CL U S I O N S

We demonstrated that crowding of the mandibular anterior teeth isdirectly correlated with inhibition of the tongue pressure function.
Additionally, our results suggest that poor oral function, including
occlusal force, and lip-closing strength, could be related to crowdingof the mandibular anterior teeth.
ACKNOWLEDGMENT
This work was supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan (17K11967).
CONF LICT OF IN TE RE ST
The authors have no financial conflict interests to disclose.

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ORCID
Yuko Fujita

https://orcid.org/0000-0002-5803-0742
How to cite this article: Ohno Y, Fujita Y, Ohno K, Maki K.

RE FE R ENC E S
Alabdullah, M. M., Saltaji, H., Abou-Hamed, H., & Youssef, M. (2014). The
relationship between molar bite force and incisor inclination: A prospective cross-sectional study. International Orthodontics, 12, 494–504.

Relationship between oral function and mandibular anterior
crowding in early mixed dentition. Clin Exp Dent Res. 2020;
1–8. https://doi.org/10.1002/cre2.306