RESEARCH ARTICLE Open Access

Occlusal force predicts global motioncoherence threshold in adolescent boysKensuke Kiriishi1,4, Hirokazu Doi2, Nobuaki Magata1,4, Tetsuro Torisu1,4, Mihoko Tanaka1,4, Makoto Ohkubo3,Mitsuhiro Haneda3, Masaki Okatomi3, Kazuyuki Shinohara2 and Takao Ayuse1,4*

Abstract

Background: Beneficial effects of mastication on cognitive abilities in the elderly have been shown in humanstudies. However, little is currently known about the effect of masticatory stimulation on cognitive and perceptualability in younger populations. The purpose of the present study is to investigate the influences of masticatorystimulation on perceptual ability in adolescent boys.

Methods: The present study examined the relationship between occlusal force (i.e., masticatory stimulation) andvisual perception ability in adolescent boys. Visual perception ability was quantified by measuring global motioncoherence threshold using psychophysical method. As an index of masticatory stimulation, occlusal force wasmeasured by pressure sensitive film. We also measured participants’ athletic ability, e.g. aerobic capacity and gripstrength, as potential confounding factor.

Results: The multiple regression analysis revealed a significant negative correlation between global motion coherencethreshold and occlusal force, which persisted after controlling for confounding factors such as age and aerobiccapacity.

Conclusions: This finding indicates that masticatory stimulation enhances visual perception in adolescent boys,indicating the possibility that beneficial effects of masticatory stimulation are observed not only in the elderlybut in developing population consistently with the findings of the previous animal studies.

Keywords: Masticatory stimulation, Occlusal force, Bite force, Global motion coherence, Aerobic capacity, Adolescence

BackgroundMasticatory stimulation, i.e., occlusal sensation inducedby chewing movements, transmits various informationto the central nervous system (CNS) [1]. After reachingthe periodontal membrane, masticatory stimulation istransmitted to the CNS via the mesencephalic trigeminalnucleus [2]. The CNS then uses this information for ob-ject identification through texture and hardness [1, 3].Recent studies have revealed functional linkage be-

tween mastication and cognitive function during thegrowth period [4–6]. These studies have also pointedout that neuronal development might be affected bymasticatory stimulation in the growth period as defined

by Scammon’s curve. For example, rodents fed only softfood during the growth phase have been reported toshow impaired performance in behavioral tasks such asconditioned avoidance learning and the maze learningtest, compared with rats fed solid food [7, 8]. Becausethese tasks require functionality of the hippocampus andprefrontal area, the results suggest that increased masti-catory stimulation induced by chewing solid foods facili-tates the development of the mnemonic and navigationalfunctions of these neural regions. Likewise, feeding onlyon soft foods has been shown to decrease hippocampalformation and to precipitate hippocampal neural cellapoptosis in mice [9–12].In humans, neuroimaging studies have shown in-

creased activation in several neural regions, includingthe primary/supplementary motor area, cerebellum,somatosensory areas, thalamus, insula, and hippocam-pus, during chewing movement compared with control

* Correspondence: ayuse@nagasaki-u.ac.jp1Department of Clinical Physiology, Graduate School of Biomedical Sciences,Nagasaki University, Nagasaki, Japan4Department of Prosthetic Dentistry, Graduate School of Biomedical Sciences,Nagasaki University, 1-7-1 Sakamoto, Nagasaki, JapanFull list of author information is available at the end of the article

© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Kiriishi et al. BMC Pediatrics (2018) 18:331 https://doi.org/10.1186/s12887-018-1309-2

movement [13–15]. These and results from other studiessupport the view that mastication also enhances CNSfunctions in humans [16–22]. However, previous humanstudies have only investigated associations between mas-ticatory stimulation and a limited variety of cognitivefunctions such as long-term memory [19] and arousalregulation [15, 22, 23]. Thus, whether masticatory stimu-lation also facilitates other important function such asvisual perception remains unclear. In addition, themajority of existing studies have focused on the effectsof mastication in the elderly [18, 20, 21], with few havingpaid attention to developing populations, despite accu-mulating evidence from animal studies that masticatorystimulation influences performance in learning and emo-tional response during the growth period as early as theearly postnatal stage [5].The goal of the present study is to investigate the

effects of masticatory stimulation on visual perceptionability in a human developing population. To achievethis, we examined the association between visual percep-tion ability and occlusal force in adolescent boys. We re-cruited adolescent boys belonging to an amateur footballclub. The reasons for recruiting this particular groupwere two-fold. First, acquisition of a sophisticated levelof cognitive and visual perception ability is adaptive forfootball players. Previous studies have reported that foot-ball players require flexibility in control of visual atten-tion and decision-making based on incoming sensoryinformation during a football match [24, 25] in order torespond to a dynamically changing, unpredictable andexternally paced environment [26, 27]. Second, Ohkawaet al. [28] revealed strong masticatory muscle activities dur-ing kicking in football players. Neuroimaging studies haveshown that brain structure is still immature and highlyplastic to external inputs during adolescence [29, 30]. Wethus surmised that prominent influences of mastication/oc-clusion on perceptual ability should be observed in boyswho routinely play football during this developmental stagedue to abundant input of masticatory stimulation into theCNS.Visual perception ability has been quantified by meas-

uring global motion coherence [31–33]. The reason formeasuring this particular ability of visual perception isthat perception of global motion information is of greatimportance when playing football. In judging game situ-ations, players have to rely on global patterns of the dir-ection of movement of other players. We thus thoughtthat we could quantify one aspect of visual perceptionability that is adaptively important for football players bymeasuring global motion coherence threshold.On the basis of previous studies linking masticatory

stimulation and enhanced CNS activation [13, 16–21]together with protracted development of global motionperception ability into late childhood [32, 34], we

hypothesized that adolescents with high occlusal force,whose CNS would supposedly receive abundant mastica-tory stimulation, should show superior global motion co-herence perception to adolescents with low occlusalforce.Children with greater aerobic capacity have been

shown to exhibit superior performance in cognitivefunctions such as learning, recognition, and memorycompared to those with lower aerobic capacity [35–37].We therefore measured aerobic capacity as a potentialconfounding factor. A spurious association betweenvisual perception ability and occlusal force might emergebecause both of them develop with age. Furthermore,even if there is actually relationship between these vari-ables, it remains unclear whether visual perceptionability is specifically linked to occlusal force, hence mas-ticatory stimulation; it remains possible that visual per-ception ability is associated with general physical abilityand not specifically linked to occulusal force. To excludethese possibilities, we also included age and the mea-sures of anaerobic aspect of athletic ability, i.e. shuttlerun record and grip strength, as confounding factors.

MethodsParticipantsIn total, 51 early- to middle-adolescent boys aged 9–15 years old (Mean (M) = 12.1 years; standard deviation(SD) = 1.7 years) were recruited from an amateur footballclub in Japan. All had normal or corrected-to-normal vis-ual acuity and none reported problems in observing thevisual stimuli used in the present study. Children with thehistory of psychiatric condition or currently taking medi-ation were excluded form the present study. All proce-dures performed in this study were in accordance with theethical standards of the institutional research committeeand with the 1964 Helsinki declaration and its lateramendments or comparable ethical standards.The guard-ians and children were provided with information aboutthe research and all gave written informed consent priorto enrolment.

ProcedureThe following measurements were taken from each par-ticipant: global motion coherence threshold, occlusalforce, and athletic ability. Dental development was alsoevaluated by observing current situation of number ofadult teeth and deciduous teeth.

Global motion coherence threshold measurementGlobal motion coherence threshold was measured usinga laptop computer with a 17-in. display viewed fromroughly 40-cm away. During each trial, after presenta-tion of the fixation cross for 250 ms, 100 white dotsmoving at a speed of 60 pixel/s appeared within an

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invisible square about 16.8 cm in width at the center ofthe screen for 400 ms, with p% of the 100 dots movingin the same direction, either to the right or left horizon-tally, while the remaining dots moved in random direc-tions. The lifetime of each dot was 200 ms, after whichthey were relocated to a random location within theinvisible square. Relocation of dots occurred asynchron-ously. The lifetime was imposed to prevent participantsfrom adopting the strategy of continuously tracking onlya handful of dots rather than focusing on the global pat-tern. The concept of stimulus display is schematicallyshown in Fig. 1a. Participants indicated the direction ofmotion by key-press after the disappearance of whitedots. When a participant did not respond within 10 s ofthe disappearance of a stimulus, an incorrect responsewas considered to have been made. The proportion p ofcoherently moving dots in each trial was determinedusing a staircase procedure [38]. The staircase startedfrom p = 100 to help participants grasp the concept ofthe task. This proportion decreased when the participantmade correct responses three times consecutively, andincreased when the participant made an incorrectanswer. Step size started from 32% and halved at everyreversal. The threshold was analyzed on the basis of thelast 6 reversals.

Occlusal force measurementOcclusal force (N) and occlusal contact area were evalu-ated using the Dental Prescale system (Fuji Film, Tokyo,Japan), in which a pressure-sensitive film (50H type)shows color variation depending upon the force of theapplied occlusal pressure within a range of 5–120 MPa.We measured the occlusal force of all participants usinga medium-sized dental prescale. Each subject was re-quested to bite a prescale film in the centric occlusalposition for 2 s at maximum force with deeply sittingposition. During measurement, his head was aimed at aposition in which Frankfurt plane was parallel to thefloor. Participants were taught by a dentist (K.K.) how to

bite in a centric occlusal position so that jaw positiondid not affect occlusal state. Films were scanned usingan Occluzer FPD-703 scanner™ (GC International,Tokyo, Japan) [39]. Examples of films with colorationdue to applied occlusal pressure, and the scanner usedare shown in Fig. 1b. The discolored area was recordedas the occlusal contact area. Maximum occlusal forcewas determined based on the degree of coloration andthe area of each contact point.

Occlusal supportA licensed dentist (K. K.) evaluated tooth occlusal sup-port for each participant based on Eichner’s classificationas a reference. Eichner’s classification is used as anevaluation of the occlusion support level widely andpopular in occlusal force studies [40–42]. Because itmight be inappropriate to adapt Eichner’s classificationto assessment of children’s mixed dentition, we decidedto evaluate occlusal support areas using modified classi-fication in which an occlusal support area is defined as aminimum of one upper and one lower tooth in maxillo-mandibular contact in the premolar or the molar regionon the left or right sides. According to the number ofocclusion support areas, maxillomandibular occlusalstates were divided into the following three groups inthis study (four supporting areas, three supporting areas,two supporting areas).

Evaluation of tooth developmentDental ages of participants were determined based onHellman’s dental development stages by a licensed den-tist (K.K.). Division of Hellman’s dental developmentalstages is based on dental age and clinical finding [43].Dental developmental stages were divided into groups asfollows: IA, deciduous tooth non-eruption phase; IC, de-ciduous tooth eruption starting phase; IIA, completionof primary occlusion; IIC, eruptive phase of permanentfirst molar or incisor; IIIA, eruption of permanent firstmolars or incisors completed; IIIB, exchange phase of

Fig. 1 a Schematic representation of stimulus in global motion coherence threshold measurement. Coherently moving dots are presented inwhite, while the remaining dots are in gray. Arrows indicate the direction of dot movement. Arrows and dotted line depicting the boundary ofdot movement were not shown in the actual experiment. b A sample of colored film (left) and the measurement system (right) used in occlusalforce measurement

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lateral teeth; IIIC, eruptive phase of permanent secondmolar; and IVA, eruption of permanent second molarcompleted.

Athletic ability measurement

Grip strength To measure grip strength, we used aSmedley-type hand dynamometer (T.K.K.5001, TakeiScientific Instruments Co.,Ltd., Nagoya, Japan). Weinstructed the participant to grip the hand dynamometeras firmly as possible with legs spread and arms lowerednaturally. We prohibited the participant from swingingthe hand dynamometer around. Grip strengths of bothleft and right hands were each measured twice, thenaveraged.

Aerobic capacity The aerobic capacity of each par-ticipant was measured using a maximal multistage20-m shuttle-run test [44–46]. This required the par-ticipant to run between parallel lines drawn on theground 20-m apart. The timing for participants toreach each line and turn was set by a beep soundplayed by a CD player. The interval between eachbeep was shortened so that running speed increasedby 0.5 km/h every minute. The test continued untilthe participant was unable to maintain the designatedrunning speed. We estimated maximum oxygenconsumption (VO2max) based on the total numberof turns made by referring to the “20-m shuttle run(round trip endurance running) maximum oxygenconsumption estimation table” issued by the Ministryof Education, Culture, Sports, Science and Technol-ogy of Japan.

50-m run We measured the individual record time forthe participant to run 50 m on the football ground forthe 50-m run for each participant. The record was mea-sured to the nearest 1/10th of a second, then roundedup to the next whole number.

Statistical analysisFirst, we tested whether the distribution of each variableconforms to normal distribution. Shapiro-Wilk normal-ity test revealed signfiicat deviation from normal distri-bution in age, grip strength and 50 m run record (Ws< .94, ps < .05). These variables were submitted to furtheranalyses after being transformed by Box-Cox transform-ation [47].In the second step of analysis, we examined bivariate

correlations between every pair of measurements inaddition to computing descriptive statistics. Due to the ex-planatory nature of this analysis, we did not control forthe family-wise error rate. The third set of analyses specif-ically focused on examination of our primary hypothesis,

that high masticatory stimulation would facilitate visualperception ability. To achieve this, we carried out multipleregression analysis with global motion coherence thresh-old as the dependent variable. Independent variables in-cluded occlusal force, age, grip strength, aerobic capacityand 50-m run record. We detected no multicollinearity bythe criteria of variance inflation factor (VIF) > 10. Thus,all the independent variables were entered into the model.All statistical analyses were conducted using R software(The R Foundation).

ResultsData from 3 participants were discarded from the finalanalyses owing to a failure to obtain measurements onocclusal force and athletic ability (n = 2), or broken teeth(n = 1). Data from a further 10 participants were ex-cluded from the final dataset because the data for globalmotion coherence threshold measurement was missingdue to equipment failure (n = 6), or the individualshowed poor performance due to inattention or misun-derstanding of instructions (n = 4). The final datasetincluded the data from 38 participants between 9 and15 years old (M = 12.2 years; SD = 1.8 years). The finalsample size is comparable to existing studies on the cor-relation between visual perception and motor skills inpediatric samples [48, 49]. Participants included in thefinal dataset were divided into three classes according toocclusal support:four supporting areas (n = 24), threesupporting areas, (n = 5), and two supporting areas (n =9). Participants in the final dataset comprised threegroups according to Hellman’s dental development stage:group IIIB (n = 16), group IIIC (n = 19), and group IVA(n = 3), indicating that the majority of participants werestill in the tooth eruption phase. Mean and standard de-viation of measured variables are summarized in Table 1.

Exploratory bivariate correlation analysisWe first investigated the relationship among variables inan exploratory manner by testing bivariate correlationsbetween every pair of variables. Results are summarizedin Table 2. As predicted, a significant correlation existed

Table 1 Means and standard deviations of measurements

Mean SD

Contact Area (mm2) 9.18 3.64

Mean Pressure (MPa) 63.39 78.46

Occlusal Force (N) 463.82 177.62

Grip Strength (kg) 21.99 7.78

Aerobic Capacity (ml/kg min) 48.29 4.91

50 m Run Record (sec) 8.07 0.91

GMC Threshold (%) 54.48 22.11

Note. SD Standard Deviation, GMC Threshold Global MotionCoherence Threshold

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between occlusal force and global motion coherencethreshold (r (36) = − 0.415, p = .0096). The scatter plotfor occlusal force and global motion coherence thresholdis shown in Fig. 2. Aside from this, 50-m run record cor-related negatively with age (r (36) = − 0.811, p < .001),grip strength (r (36) = − 0.859, p < .001) and aerobic cap-acity (r (36) = − 0.651, p < .001). Aerobic capacity showedsignificant positive correlations with age (r (36) = 0.756,p < .001) and grip strength (r (36) = 0.552, p = .0003). Asignificant positive correlation was also seen betweengrip strength and age (r (36) = 0.835, p < .001).

Multiple regression analysisTo examine the nature of the significant correlation be-tween occlusal force and global motion coherence thresh-old in greater detail, we conducted a multiple regressionanalysis with global motion coherence threshold as thedependent variable. Predictors included occlusal force, age,averaged grip strength, aerobic capacity, and 50-m run rec-ord. The standardized coefficient (β) of each predictor andassociated statistics are summarized in Table 3. Occlusalforce, but not aerobic capacity, correlated significantly withglobal motion coherence threshold after controlling forconfounding factors (β =− 0.424, t = − 2.72, p = 0.0104, df =32). The same analysis was run on a subset of data

excluding participants with only three or two support-ing areas. That analysis revealed a significant influenceof occlusal force on global motion coherence threshold(β = − 0.599, t = − 3.43, p = 0.003, df = 18) after control-ling for the effects of confounding variables.

DiscussionThe present study investigated associations between oc-clusal force as an indicator of masticatory stimulation,and visual perception ability in adolescent boys playingfootball. The main finding was that global motion coher-ence threshold correlated negatively with occlusal force,indicating that children with stronger occlusal force ex-hibited higher sensitivity to global motion coherence, inline with our hypothesis. The association even persistedafter controlling for confounding factors such as age andaerobic capacity by multiple regression analysis. The as-sociation between occlusal force (i.e., masticatory stimu-lation) and global motion coherence persisted whenconsidering only data from participants with four sup-porting areas, indicating that the observed relationshipwas not an artifact due to missing support zones. Chew-ing and masticatory stimulation have been suggested toenhance mnemonic function in middle-aged adults andthe elderly [15, 18, 20, 21]. To the best of our

Table 2 Correlation coefficients between each pair of observed variables

Occlusal Force Age Grip Strength Aerobic Capacity 50 m Run Record GMC Threshold

Contact Area 0.961** 0.014 0.135 0.023 −0.034 − 0.331*

Occlusal Force 0.028 0.165 0.038 −0.089 −0.415**

Age 0.835** 0.756** −0.811** −0.294

Grip Strength 0.552** −0.859** −0.251

Aerobic Capacity −0.651** −0.228

50 m Run Record 0.159

Note.*p < .05, **p < .01 without family-wise error rate control. GMC Threshold Global Motion Coherence Threshold

Fig. 2 Scatterplot showing relationship between occlusal force and global motion coherence threshold

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knowledge, this represents the first study to indicate afacilitatory effect of masticatory stimulation on percep-tual ability in developing humans.Although exactly how masticatory stimulation (occlu-

sal force) may enhance cognitive function in humansduring the developmental period has not been clarified,Fukushima-Nakayama et al. [4] indicated in an in vivostudy that neuronal functions underlying memory andlearning might be affected by changes in masticatorystimulation during the growth period. They further re-vealed that changes in masticatory stimulation canmodulate neurogenesis and neuronal activity in thehippocampus, functionally contributing to cognitivefunction in the growth period. Likewise, Nose-Ishibashiet al. [5] showed that poor masticatory stimulation in-duced by soft-food feeding during the post-weaningperiod led to weaker pre-pulse inhibition in mice, i.e., abehavioral indicator of increased vulnerability to psychi-atric conditions. Interestingly, this behavioral impair-ment was accompanied by reduced cell proliferation inhippocampal regions. Given these findings together withthe finding from a human neuroimaging study thatvisual perception of global motion stimuli recruits thehippocampal region [50, 51], one potential interpretationof the present results is that enhanced hippocampalfunction induced by masticatory stimulation led tohigher ability of global motion coherence perception inadolescent boys.At the same time, facilitatory effects of masticatory

stimulation on neural regions other than hippocam-pus may be equally plausible for explaining thepresent finding. Takeda and Miyamoto [14] haveshown increased activation in fronto-parietal regionswhile chewing gum than during chewing movementwithout masticatory stimulation. Likewise, Sakamotoet al. [52] revealed faster elicitation of event-relatedpotential peaks during auditory processing task afterchewing than control mouth movement (see also[22]). These studies indicate the possibility that masti-catory stimulation enhances functionality in wide-spread neural regions. Frota de Almeida et al. [6]

revealed that long-term reduction of masticatory stimula-tion leads to astrocyte hypoplasia in 3-month-old mice.Considering that astrocytes play pivotal roles in regulatingcerebral blood flow [53, 54], proliferation of astrocytesfacilitated by stronger masticatory stimulation duringgrowth period may induce better control of blood flowdistribution in CNS, thereby enhancing cognitive and per-ceptual abilities.Occlusal force as an index of masticatory function

has been shown to be affected by maxilla-mandibularocclusal state [55, 56]. Because the present study re-cruited adolescent children, developmental stage couldconceivably have influenced both occlusal state andperceptual ability, thereby yielding a spurious associ-ation between variables. In the occlusal force mea-surements of the present study, a dentist instructedeach participant to bite in the centric occlusal pos-ition so that jaw position did not affect occlusal state.In addition, dental age evaluation confirmed that allteeth grew normally in participants whose data wereincluded in the final analysis. Further, the correlationbetween occlusal force and global motion coherencethreshold was maintained after controlling for the ef-fect of age or when considering only participants withfour supporting areas. Based on these results, we arefairly confident that the observed correlation repre-sents a real effect, not an artifact of developmentalstage, jaw position, or occlusal state.Caution should be exercised in interpreting the data

due to the following limitations of the present study.First, the sample size in the present study was relativelysmall, so the present findings should be replicated in alarger sample size. Second, participants in the presentstudy comprised members of an amateur football clubteam. Football players experience intense contact andstrong occlusal force when performing headshots andshoots [28]. Frequent experience of strong occlusal forcemight thus have accentuated the effects of occlusal forceon visual motion perception in this particular group.Examining whether the present findings can be general-ized to a wider population including girls would be animportant issue for future study. Interestingly, a recentstudy reported a positive correlation between footballtechnique level and cognitive function [26]. Determiningwhether the observed link between visual motion per-ception and occlusal force is mediated by level of foot-ball technique would thus also be interesting. The thirdlimitation is that we did not collect data of DMFT(decayed, missing, and filled teeth) index and anthropo-metric variales, e.g. body weight and body height. Al-though they were not of the primary interest in thepresent study, it is still conceivable that these varialesmoderate the observed association between occlusalforce and global motion coherence threshold.

Table 3 Standardized coefficients and statistics in multipleregression analysis

β SE t-value p-value

Intercept <.0001 0.147 <.0001 >.999

Occlusal Force −0.424 0.156 −2.724 0.010*

Grip Strength −0.029 0.361 −0.08 0.937

Aerobic Capacity −0.031 0.245 −0.129 0.899

50 m Run Record −0.332 0.323 −1.027 0.312

Age −0.503 0.364 −1.383 0.176

Note. Grip Strength Averaged Grip Strength, Shuttle Run Shuttle-Run Record. df= 32, *p < .05. SE Standard error

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ConclusionThe present study revealed a negative correlation be-tween occlusal force and global motion coherencethreshold, which indicates that masticatory stimulationpossibly enhances visual perception ability not only inthe elderly, but also in adolescent children. At the sametime, it remains equivocal whteher the present findingcan be generalized to wider population including girls aswell. Thus, further study is required to establish the as-sociation between masticatory stimulation and percep-tual ability in developing population.

AbbreviationsCNS: Central nervous system; DMFT: Decayed, missing, filled teeth;GMC: Global motion coherence; M: Mean; SD: Standard deviation;SE: Standard error

AcknowledgementsThe authors would like to thank all the participants and their gurdians.

FundingNot applicable.

Availability of data and materialsThe data set to susport the conclusions of this manuscript is available onrequest from the corresponding author.

Authors’ contributionsHD and TA conceived of this study. KK, HD, NM, TT, MT, MO1, MH, MO2carried out the experiment. KK and HD analyzed the data. KK, HD, TA and KSwrote the manuscript. All authors read and approved the final manuscript.

Ethics approval and consent to participateAll procedures performed in this study were in accordance with the ethicalstandards of the institutional research committee (the Ethics Committee ofNagasaki University Graduate School of Biomedical Sciences) and with the1964 Helsinki declaration and its later amendments or comparable ethicalstandards. The guardians and children were provided with information aboutthe research and all gave written informed consent prior to enrolment.

Consent for publicationNot applicable.

Competing interestsThe authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.

Author details1Department of Clinical Physiology, Graduate School of Biomedical Sciences,Nagasaki University, Nagasaki, Japan. 2Graduate School of BiomedicalSciences, Nagasaki University, Nagasaki, Japan. 3Football club BRISTOL,Nagasaki, Japan. 4Department of Prosthetic Dentistry, Graduate School ofBiomedical Sciences, Nagasaki University, 1-7-1 Sakamoto, Nagasaki, Japan.

Received: 23 April 2018 Accepted: 10 October 2018

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