Relationship between facial asymmetry and positional plagiocephaly analyzed by three-dimensional computed tomography

Journal Pre-proof Relationship between Facial Asymmetry and Positional Plagiocephaly Analyzed by Three-dimensional Computed Tomography Jun Sasaki, DDS, Shogo Hasegawa, DDS, PhD, Satoshi Yamamoto, DDS, PhD, Satoshi Watanabe, DDS, PhD, Hitoshi Miyachi, DDS, PhD, Toru Nagao, DDS, DMSc, PhD PII:

S1010-5182(19)31148-5

DOI:

https://doi.org/10.1016/j.jcms.2019.12.011

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YJCMS 3408

To appear in:

Journal of Cranio-Maxillo-Facial Surgery

Received Date: 8 September 2019 Revised Date:

15 November 2019

Accepted Date: 5 December 2019

Please cite this article as: Sasaki J, Hasegawa S, Yamamoto S, Watanabe S, Miyachi H, Nagao T, Relationship between Facial Asymmetry and Positional Plagiocephaly Analyzed by Threedimensional Computed Tomography, Journal of Cranio-Maxillofacial Surgery, https://doi.org/10.1016/ j.jcms.2019.12.011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd on behalf of European Association for Cranio-Maxillo-Facial Surgery.

Relationship between Facial Asymmetry and Positional Plagiocephaly Analyzed by Three-dimensional Computed Tomography

Author list: Jun Sasaki, DDS, Shogo Hasegawa, DDS, PhD, Satoshi Yamamoto, DDS, PhD, Satoshi Watanabe, DDS, PhD, Hitoshi Miyachi, DDS, PhD, and Toru Nagao, DDS, DMSc, PhD

Department of Maxillofacial Surgery, School of Dentistry, Aichi-Gakuin University, Aichi, Japan 2-11 Suemori-Dori, Chikusa, Nagoya, Aichi, 464-8651, Japan Telephone: +81-52-759-2160 Fax: +81-52-759-2160

Corresponding author: Shogo Hasegawa, DDS, PhD Aichi-Gakuin Dental Hospital 2-11 Suemori-Dori, Chikusa, Nagoya, Aichi, 464-8651, Japan Telephone: +81-52-759-2160 Fax: +81-52-759-2160 E-mail address: [email protected]

Financial Disclosure Statement: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Author contributions: All authors have viewed and agreed to the submission.

Relationship between Facial Asymmetry and Positional Plagiocephaly Analyzed by

Three-dimensional Computed Tomography

SUMMARY

Purpose: A relationship between positional cranial deformation and facial asymmetry has been

suggested; however, details regarding this relationship remain to be clarified. This study aimed

to elucidate the relationship between facial asymmetry and positional plagiocephaly using

three-dimensional computed tomography (3D-CT).

Methods: One-hundred-and-twenty-nine patients were included, and cranial vault asymmetry

index (CVAI) and cephalic index (CI) were used as indicators of positional cranial deformation.

Temporal muscle was constructed using 3D-CT data, and its volume was measured. VRL, Me

(vertical reference line (VRL)-anterior nasal spine (ANS) and menton (Me) line) angle and the

frontal occlusal plane (FOP) angle were measured.

Results: CVAI and VRL, Me angle (R2=0.31, P<0.0001), VRL, Me angle and temporal muscle

1

volume (R2=0.13, P<0.0001), and FOP angle and VRL, Me angle were significantly correlated

(R2=0.32, P<0.0001), but CVAI and FOP angle were not (R2=0.08). Multiple linear regression

analysis indicated that CVAI, FOP angle, and variable temporal muscle volume were significant

predictors of VRL, Me angle [(F (5, 123)=14.94, P<.0001, R2=0.38)].

Conclusions: Our results revealed that mandibular deviation was associated with contralateral

head slant and ipsilateral increase in temporal muscle volume. Positional plagiocephaly may be a

cause of facial asymmetry, and such deviations may occur in the temporal muscle.

Keywords: positional plagiocephaly, facial asymmetry, cranial vault asymmetry index, cephalic

index, temporal muscle, frontal occlusal plane angle

2

INTRODUCTION

Until recently, positional plagiocephaly in infants has not been a significant issue in medicine;

however, it has increasingly become the focus of medical interest over the past few years (Kluba

et al., 2011). Since 1992, the American Pediatric Society has advocated a supine sleeping

position for infants to prevent sudden infant death syndrome. Subsequently, there has been a

significant increase in the incidence of positional plagiocephaly among infants (Argenta et al.,

1996; Roby et al., 2012). Positional plagiocephaly occurs when external forces deform the shape

of the skull, such as those present when infants are placed in the supine position (Cummings,

2011). If left untreated, positional plagiocephaly may lead to delays in intellectual and motor

development (Dörhage et al., 2016). Deformities in the cranio-maxillofacial region are thought

to increase the risk of developmental delays.

While subclinical facial asymmetry is often observed in the general population, significant

facial asymmetry may represent both a functional and esthetic issue (Hossain et al., 2010). Facial

asymmetry is thought to reflect genetic and molecular development; however, it may be

influenced by environmental factors, nutrition, illness, and behavior (Thiesen et al., 2015; Linden

3

et al., 2018).

Previous studies have reported that plagiocephaly can cause significant posterior cranial

asymmetry, distortion of the cranial base, deformation of the forehead, and facial asymmetry

(Kubo et al., 1992; St. John et al., 2002; Moon et al., 2014). However, the pathogenesis of

plagiocephaly remains to be clarified.

In the present study, we aimed to elucidate the relationship between positional plagiocephaly

and facial asymmetry using three-dimensional computed tomography (3D-CT).

MATERIALS AND METHODS

Patients

Among 140 patients who visited the Department of Maxillofacial Surgery at Aichi-Gakuin

University School of Dentistry from 2012 to 2018 for orthognathic surgery, 129 patients (33

males and 96 females) who had 3D-CT examination prior to the surgery were included in the

present study. The exclusion criteria included hereditary and congenital diseases. The ethics

4

committee of Aichi-Gakuin University approved this retrospective study (approval No. 556), and

all patients provided written informed consent prior to participation in the study. All the

procedures in this study were conducted in accordance with the principles stated in the

Declaration of Helsinki “Ethical Principles for Medical Research Involving Human Subjects.”

Analysis of the posterior-anterior (P-A) cephalogram

P-A cephalograms obtained during the first visit were used for analyses. The photographs were

obtained with the head parallel to the floor and the skull in the anatomical position, which is

based on a plane passing through the inferior margin of the left orbit and the upper margin of

each ear canal or external auditory meatus (Frankfurt plane: FH plane). The mandibular position

was obtained at the permanent occlusal position. As with CT data, P-A cephalogram data were

saved in the DICOM format, transferred to a computer, and each reference point shown below

was measured using the graphics processing software A to Z (Yasunaga Computer Systems,

Fukui, Japan).

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In accordance with the methods described by Ricketts (Ricketss, 1960; 1961), we analyzed

measurement points and frontal cephalogram items as shown in Figure 1. Cephalometric

landmarks, parameters, and reference lines are shown in Table 1. The horizontal reference line

(HRL) was defined as a straight line connecting the left lateral orbitale (LOL) and right lateral

orbitale (LOR). This line passes through the restenosis portion of the neck of crista Galli (NC),

and a straight-line perpendicular to the reference line (HRL) was set as the facial midline (VRL).

The VRL-anterior nasal spine-Menton angle (VRL, Me angle) was used as an index of facial

asymmetry. The line joining the bilateral maxillary first molars was used to define the frontal

occlusal plane (FOP). The angle of a line parallel to the HRL and FOP was regarded as the FOP

angle (Figure 1). The FOP angle was used as an index of maxillary deviation.

Analysis of 3D-CT data

A 3D image of the bone and muscle was reconstructed from helical CT (Asterion Super 4;

Toshiba Medical Systems, Tochigi, Japan) data obtained with patients in the supine position. The

CT images were obtained during the patient’s first visit to facilitate surgical treatment planning,

6

and it was assumed that orthodontic treatment and orthognathic surgery had not been performed.

CT scans were performed with sequences obtained 0.5 mm apart for 3D reconstruction (120 kV;

average tube current, 150 mA; helical pitch, 3.5). The slice thickness of the reconstructed image

was 1.0 mm. CT data were transferred in Digital Imaging and Communication in Medicine

(DICOM) format to a workstation, following which they were reconstructed as 3D images using

Mimics software (version 19.0, Materialise, Leuven, Belgium). Both bone and muscle images

were produced in the same 3D space.

The 3D reference planes were set in accordance with methods described by Katsumata et al

(Katsuma et al., 2005). The same experienced examiner performed evaluations of all 3D

datasets.

For specific measurements, a further plane (i.e., measurement plane) was defined as a cranial

shift of the XY plane to the height of the maximum anterior-posterior expansion of the skull

(Kunz et al., 2019). The following symmetry-related variables were used to analyze head

shape—cephalic index (CI) and cranial vault asymmetry index (CVAI)—both of which were

calculated in accordance with the methods described by Loveday and de Chalain (Loveday and

7

de Chalain, 2001). The CI was calculated from measurements of head breadth and length

according to the following equation (Figure 2):

CI (cephalic index: calculated from the head breadth and head length) (%) = Head

breadth/head length ×100.

The CVAI was calculated from the dual cranial diagonal diameters (Kunz et al., 2019) as follows

(Figure 2):

CVAI (cranial vault asymmetry index: the difference between the longer and shorter

diagonals at the level of the measurement plane at a 30-degree angle to the Y-axis

concerning the length of the longer 30-degree diagonal) (%) = (A−B) ×100 / A or B

(whichever is greater).

Similarly, the temporal muscle was extracted from soft tissue CT data, and 3D construction was

performed to determine the volume of the temporalis muscle (Figure 3).

For convenience, right-sided displacement of the VRL, Me angle was represented as “+”, while

left-side displacement of the VRL, Me angle was represented as “-”. The upper right FOP angle

8

was defined as “+”, while the upper left FOP angle was defined as “-” (Figure 1). When A>B,

CVAI is displaced toward the left side (“+”). When A
side (“-”). Statistical analyses were performed to evaluate differences in temporal muscle volume

for the left and right directions (variable temporal muscle volume = right temporal muscle

volume - left temporal muscle volume).

Statistical analysis

Using the Shapiro-Wilk test, all parameters were analyzed and were found to be normally

distributed. Differences between groups were analyzed using Scheffe’s F-test. Correlations

among CVAI, CI, variable temporal muscle volume, and VRL, Me angle, as well as the

correlation between CVAI and FOP angle, were analyzed via simple regression. A multiple

linear regression model was used to evaluate predictions of VRL, Me angle. Five variables

related to VRL, Me angle (CVAI, variable temporal muscle volume, age, CI, sex) were used as

predictors. Differences were considered significant at P <.05. Data were statistically analyzed

using the JMP software program (version 13; SAS Institute, Cary, NC, USA).

9

RESULTS

Patient profiles and 3D-CT results are shown in Table 2. The average age was 25.50±9.32 years.

There was a correlation between CVAI and VRL, Me angle (R2 = 0.31, P <0.0001) (Table 3).

There was no correlation between CI and VRL, Me angle (R2 = 0.0001, P = 0.88). However, a

weak correlation between VRL, Me angle and variable temporal muscle volume was observed

(R2 = 0.13, P <0.0001). There was no correlation between CVAI and FOP angle (R2 = 0.08) (not

shown). A significant regression equation was determined based on the relationship between the

measured and predicted values for VRL, Me angle [(F (5, 123) =14.94, P <.0001, with an R2 of

0.38)]. CVAI, FOP angle, and variable temporal muscle volume were significant predictors of

VRL, Me angle as determined by multiple liner regression analysis (Table 4).

DISCUSSION

In the present study, we investigated the relationship between positional plagiocephaly and facial

10

asymmetry using three-dimensional computed tomography. Our results revealed that the cranial

vault asymmetry index (CVAI) assessed that cranial asymmetry was related to the facial

asymmetry. On the other hand, the cephalic index (CI) that defines the long and short heads, but

does not reflect plagiocephaly, was not related to the facial asymmetry. However, our findings

revealed a correlation between cranial asymmetry and variable temporal muscle variable volume.

Previous studies reported that muscle involvement is a cause of facial asymmetry (Dong et al.,

2008). In particular, the masseter muscle has been regarded as the main muscle associated with

facial asymmetry (Machida et al., 2003; Higashi et al., 2006).

However, no previous studies have mentioned the role of the temporalis muscle in facial

asymmetry. In the present study, we observed a significant difference in the volume of the

temporal muscle based on positional cranial deformation and facial asymmetry. Because the

temporal muscle is attached to the cranium, our findings suggest that positional cranial

deformities may cause crosswise differences in the area at which the temporal muscle attaches.

Our results indicate that there are two types of plagiocephaly and facial asymmetry. We

observed a correlation between CVAI and facial asymmetry, with right head slant accompanied

11

by mandibular deviation to the left (type 1) (Figure 4-1) and left head slant accompanied by

mandibular deviation to the right (type 2) (Figure 4-2). We also observed a correlation between

mandibular asymmetry and temporal muscle volume, in which type 1 was associated with

increases in left temporal muscle volume, while type 2 was associated with increases in right

temporal muscle volume.

Many patients with facial asymmetry present with an occlusal plane inclination caused by

unilaterally extruded maxillary molars or asymmetrical mandibular vertical development (Jeon et

al., 2006). The occlusal plane is an essential element in the position and adaptation of the

mandible (Ishizaki et al., 2010). Such inclination is usually associated with mandibular deviation

and vice-versa (Hashimoto et al., 2009; Kim et al., 2014). The degree of maxillary inclination is

proportional to the degree of mandibular deviation in both the hard and soft tissues (Kim et al.,

2014). In the present study, although CVAI and mandibular deviation also exhibited a correlation,

there was no correlation between CVAI and FOP angle. These findings suggested that positional

plagiocephaly does not affect the deviation of the maxilla, but instead is involved in the deviation

of the mandible.

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With regard to the involvement of the temporal muscle in facial asymmetry, our study indicated

that differences in the attachment site of the temporal muscle due to positional plagiocephaly and

suppression of mandible growth at the coronoid process are thought to cause changes in

muscular activity and force. Positional cranial deformities may lead to lateral imbalances in the

mandibular bone via the temporal muscle, thereby leading to facial asymmetry. Therefore, the

mechanisms underlying facial asymmetry due to positional cranial deformity are related to the

temporal muscle, and facial asymmetry can be predicted based on CVAI, FOP angle, and

temporal muscle volume.

The importance of correcting positional cranial deformities is also reflected in the application of

orthosis for patients with positional plagiocephaly to reduce facial asymmetry and any associated

developmental effects as early as possible. For instance, cranial molding orthosis (helmet)

therapy is recommended for infants with persistent moderate to severe plagiocephaly after a

course of conservative treatment (repositioning and/or physical therapy) (Tamber et al., 2016).

To our knowledge, our study is the first to demonstrate a relationship between cranial

asymmetry and facial asymmetry, FOP angle, and temporal muscle volume. However, as this

13

was a cross-sectional study, further investigation is required to confirm the extent of long-term

changes after plagiocephaly treatment and orthognathic surgery.

CONCLUSIONS

This study revealed that a head slant was accompanied by mandibular deviation on the other side,

and a mandibular deviation was associated with the increases in the temporal muscle volume on

the same side. Facial asymmetry due to positional cranial deformity might be related to changes

in the temporal muscle development.

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ACKNOWLEDGEMENTS

We would like to thank Editage (www.editage.com) for English language editing.

CONFLICT OF INTEREST

None.

SOURCES OF SUPPORT

This research did not receive any specific grant from funding agencies in the public, commercial,

or not-for-profit sectors.

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TABLES

Table 1. Parameters of frontal cephalometric and 3D construction image

VRL, Me angle (°)

angle formed by vertical reference line (VRL)-anterior

nasal spine (ANS) and menton (Me) line

FOP angle (°)

angle formed by parallel to the horizontal reference line

and frontal occlusal plane

CVAI (%)

cranial vault asymmetry index

CI (%)

cranial index

Variable temporal

right temporal muscle volume - left temporal muscle

muscle volume (mm3)

volume

21

Table 2. Participant characteristics and three-dimensional computed tomography results

Items

n=129

mean ± S.D.

Sex

male

33

female

96

Age (y)

25.50 ± 9.32

CVAI (%)

–0.69 ± 3.97

CI (%)

86.35 ± 5.20

VRL, Me angle (°)

–0.94 ± 3.85

FOP angle (°)

–0.24 ± 2.86

Variable temporal muscle volume (mm3)

−1,606.60 ± 11,382.41

22

Table 3. Correlation between VRL, Me angle and each independent variable

R2

Linear regression

P-value

CVAI

0.31

y=0.54x-0.57

<.0001

CI

0.0001

y=-0.001x-0.84

0.88

Variable temporal muscle volume

0.13

y=0.0001x-0.74

<.0001

FOP angle

0.32

y=0.76x-0.76

<.0001

23

Table 4. Multiple linear regression of VRL, Me angle

Log worth

P-value

CVAI

8.073

<.0001

FOP angle

7.911

<.0001

Variable temporal muscle volume

1.354

0.044

Sex

0.994

0.10

CI

0.753

0.18

Age

0.473

0.34

24

FIGURE LEGENDS

Figure 1. Analysis of symmetry based on cephalogram data

CVAI: cranial vault asymmetry index; CI: cephalic index; VRL, Me angle: index of facial

asymmetry; FOP angle: index maxillary deviation. ＋: VRL, Me angle with right-sided

displacement and upper right FOP angle. –: VRL, Me angle with left-sided displacement

and upper left FOP angle.

Figure 2. Analysis of symmetry based on 3D-construction image

CI (cephalic index) (%) = X/Y×100, CVAI (cranial vault asymmetry index) (%) =

(A−B)×100/A or B (whichever is greater). When A>B, CVAI is displaced toward the left

side (“+”). When A
Figure 3. Measurement of temporal muscle volume

The 3D temporal muscle was constructed from CD data and the volume was measured.

Statistical analyses were performed to evaluate differences in temporal muscle volume

for the left and right directions (variable temporal muscle volume = right temporal

25

muscle volume - left temporal muscle volume).

Figure 4-1. Relationship between plagiocephaly and facial asymmetry (type 1).

Right head slant accompanied by mandibular deviation to the left with increase in left

temporal muscle volume. The volume of the left temporal muscle (red) was larger than

that of the right temporal muscle (blue).

Figure 4-2. Relationship between plagiocephaly and facial asymmetry (type 2).

Left head slant accompanied by mandibular deviation to the right with increase in right

temporal muscle volume. The right temporal muscle volume (red) was larger than the

left temporal muscle (blue).

26

VRL LOR

LOL NC

HRL

ANS FOP angle FOP YY Me AA

Figure 1.

BB

XX

Y A

30° 30°

B

X

30° 30° Y A

Figure 2.

B

X

Y B

A

Y A

Figure 3.

B

X

Figure 4-1.

Figure 4-2.