Physiognomy in the Classification of IndividualsWith a Lateral Preference in Mastication

David K,L, Tay. BDS, MS, FAMS300 Orchard Road #05-06The Promenade, Singapore 0923

Chronic habitual ciieivers usually present with a lack of passive lat-erotrusive range of mandibular motion at maximum intercuspa-tion, and many have some form of mandibular asymmetry. Thisstudy attempted to discover mechanisms that may be responsiblefor this association and proposes a system by which such personscan be classified based on physiognomy and patterns of mandihu-lar deviation. The influence of facial biotype and altered head pos-ture on the selection of a preferred chewing side was also analyzed.J OROFACIAL PAIN J994;g:61-72.

The decision to chew is voluntary; however, as masticationprogresses, ir becomes an involuntary act that is dependenton complex central and peripheral neural mechanisms.'

When bolus placement durmg mastication is performed consistent-ly on either the right or left side of the dentition, it is commonlyreferred to as masticatory lareral preference.'

Using an electronic jaw-tracking sysrem and computer softwarecapable of complicated graphic and statistical analyses, Neill andHowell' found thar 10% of their sample of 140 dentate Britishdental students exhibited a unilateral dominance to the same sidethroughout their mastication sequences. They defined "unilateraldominance" as a situation in which mastication was confined toone side of the mouth for more rhan 66,6% of the masricarionsequence.

In a quesrionnaire survey on a nonpatient population, Tay et al''found thar a similar proportion (9.45%) of the sample could becategorized as "chronic habitual unilateral chewers" (CHUCs), ie,as individuals who were very aware they masticated on one partic-ular side either "a lot of the time" or "almost all the time" for aslong as they could remember. In contrast, 53% of a temporo-mandibuiar disorder (TMD) patient population reported unilateralchewing at frequency levels corresponding to the CHUC category,'

A lateral preference in mastication could not be predicted fromhand laterality and was believed to be mainly an expression ofmotivational chewing behavior.- Since habitual behaviors are large-ly a product of functional convenience as well as the avoidance ofdiscomfort and pain, peripheral factors like the size and consisten-cy of the ingested food, rhe character of rhe occlusal interface (eg,missing teeth, presence of occlusal interferences/crossbite/open bite,steepness of the anterior guidance, etc), and the presence of noci-ception in the pulpal, periodontal, and articular tissues doubtlessplay important roles-'' Goodwin and Lushei" have also providedexperimental evidence for the role of central factors in masticatorylateral preference. Their creation of a lesion in the rrigeminal mes-encephalic tract of primates resulted in a srrong tendency for chew-

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ing on the opposite side. By comparison, not mtichis known about the role of head posture or facialform in the selection of a preferential chewing side.

In 1983, Lagaida and White' reported an inter-esting association between the high eye and thehabitual chewing side in children and speculatedthat unilateral mastication could influence dento-facial formation. The present author's clinicalobservations and investigations on CHUCs overthe past 7 years revealed that these individualsusually had some form of mandibular asymmetryat maximum intercuspation.

The following is a summary of an attempt tounravel the basic mechanisms responsible for thecurious association hetween masticatory lateralpreference and mandibular asymmetry. The paperdiscusses the possible roles of altered head postureand facial biotype in the selection of the preferen-tial chewing side and proposes a system to classifyCHUCs based oti physiognomy and patterns ofmandibular displacement.

Physiognomic Characteristics

If the postulation that a lateral preference in mas-ticarion in growing children could influencedentofacial formation* were true, then it should bepossible to characterize adult CHUCs physiog-nomically. A simple visual system was thus devel-oped to characterize these subjects based uponfacial biotype, the presence and degree of head tilt,pattern of chin deviation, and other distinguishmgfeatures such as a deviated nasal septum andaltered lip posture.

Facial Biotype

Little information on physiognomy could be foundin the dental literature; however, history teachesthat "Ming Xiang" (the art of face-reading) has itsroots m Chinese culture. For the Chinese, the facenot only revealed certain personality traits and thespirit of the person but also the past, present, andfuture.' Although the Chinese face reader describes10 different frontal face shapes, these can bebroadly grouped into the triangular, oval, andsquare forms, which roughly correspond to thedolichofacial, mesofacial, and brachyfacial moldsdescribed in orthodontic literature.

In addition to the frontal physiognomic classifi-cation, lateral and submentovertex (perpendicularto Frankfort plane) cephalometric views allow abetter conceptualization and definition of facialhiotype in three dimensions as follows:

1. Horizontal axis—dolichocephalic, mesocephalic,and brachycephalic

2. Vertical axis—Low, average, and high Frankfortmandihular angle (FMA)

3. Sagittal axis—Skeletal Class II, I, and Itl

From Fig 1 it can be clearly seen that the meso-facial (mesocephalic. Class I, average FMA),dolichofacial (dolichocephalic, Class II, highFMA), and brachyfacial (brachycephalic. Class III,low FMA) stereotypes represent only 3 of the pos-sible 27 basic facial biotypes. A dolichocephalicClass III facial biotype, for example, is not anuncommon finding since there are more doHcho-cephalics than brachycephalics and more Class IIImalocclusions than Class II malocclusions in theloca! immigrant Southern Chinese population thatwas studied.'""

Head Posture Evaluation

To assess standing orthoposture,"•" subjects weretold to spin around with their eyes tightly closeduntil they felt disoriented. They were then in-structed to alternately extend and flex their necksin progressively decreasing oscillations until theysteadied in a position of equilibrium with respectto the horizon.

The presence of any significant head and shoul-der tilt as well as the relation of the postural highereye to the side of the chin deviation were notedtogether with other distinguishing characteristicssuch as a crooked nose and/or an upper lip droop.Subjects were then instructed to very slowly openthe eyes, and the side with the narrower eye was

Fig 1 Classification of facial biotype.

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Figs 2a and 2b Measuring the passive laterotrusive ROM from frcnale: (kft¡ MI (note frénale is not at the NUS pointin this patient); (right) left maximum Iaterotrusion, (A = MI, B = NUS, C = frénale.)

noted. If the significantly narrower eye was oppo-site the side of chin deviation, it was referred to asexhibiting a positive "soft tissue drape" relation-ship. A negative "soft-tissue drape" relationshipexisted when the narrower eye was on the sameside ofthe chin deviation.

Laterotrusive Range of Motion

Since CHUCs frequently exhibited some form oflateral jaw deviation at maximum intercuspation(Mil, a novel technique for monitoring the medio-lateral coordinate was devised to better qualifyand quantif}' patterns of mandibular displacement.

Although previous researchers" measured theactive laterottusive range of motion (ROM) of themandible against a constant landmark on the max-illa (eg, the maxillary labial frenum or upper den-tal midline) in calculating research norms for vari-ous population samples, it was found to be moreclinically valuable to record the maximum left andright passive ROM from frénale (ie, the root of themandibular labial frenum] using a triangular mil-limeter ruler (TMJ Trimeasure, Clinitec Dental,Irvine, CA) (Figs 2a and 2b).""

The positions of frénale at the left and rightmaximum passive laterotrusive ROM, at MI, andat the iatrogenic superior medial close-packedposition (ISMCP; ie, centric relation as obtainedvia Dawson's bimanual jaw manipulation tecb-nique) were either marked on the labial surfaces ofthe maxillary teeth or on a transferable anteriorjig,"'" The point of laterotrusive symmetry (navi-gational usage of symmetry [NUS| point) was thencalculated and recorded. The details and diagnos-tic significance of this exercise have been describedelsewhere.' -"

Fig 3 Schematic representation of mandible, as seen inSMV cephalogram, depicted against frontal measure ofmaximum laterotrusive ROM from frénale, A - man-dibular horizot!Cal displacement asymmetry; B - truemandibular structural asymmetry.

It is important to realize that a mandibular asym-metry observed at MI in the frontal plane could bethe result of either a true mandibular structuralasymmetry, a mandibular displacement asymmetry,or a combination of both mechanisms,'^ Amandibular horizontal displacement asymmetrymay be diagnosed whenever tbe passive laterotru-sive symmetry improves from MI to ISMCP,whereas a true mandibular structural asymmetryshould be suspected whenever there is a lack ofpassive laterotrusive symmetry at ISMCP (Fig 3).'"

Classification of CHUCs

CHUCs could now be classified into five broaddescriptive categories based on pbysiognomy andcharacteristics of passive laterotrusive ROM (Fig 4):

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13 II

IV

Fig 4 Classifitanon of CHUCs (note: categories III and IV share the same physiognomictemplate).

1. Homo ocular, ipsilateral-type horizontal man-dibular displacement asymmetry: no significantdifference in the size of the eyes, chin deviationtoward the side of head tilt, passive laterotru-sive ROM symmetry at ISMCP

2. Hetero-ocular, ipsilateral-type horizontal man-dibular displacement asymmetry; significantdifference in the si e of the eyes, chin deviationopposite the narrower eye, narrower eye postu-rally higher, passive laterotrusive ROM sym-metry at ISMCP

3. Hetero-ocular, contralateral-type horizontalmandibular displacement asymmetry: signifi-cant difference in size of eyes, chin deviationopposite narrower eye, narrower eye posturallylower (head tilt may be insignificant), passivelaterotrusive ROM symmetry at ISMCP

4. Hetero-ocular, contralateral-type true mandib-ular structural asymmetry: significant differencein the size of the eyes, chin deviation oppositethe narrower eye, head tilt usually insignificant,lack of passive laterotrusive ROM symmetry atISMCP, usually some form of anterior/posteriorcross bite/op en-bite present

5. Hetero-ocular, reverse drape, true mandibularstructural asymmetry; significant difference insize of eyes, chin deviation toward side ofnarrower eye, passive laterotrusive symmetry,ROM worsening from MI to ISMCP, lack ofpassive laterotrusive ROM symmetry at ISMCP

Methods of Subclassification

These CHUCs can be further subdivided accordingto the magnitude of the total (left -F right) lat-erotrusive ROM. The mean total laterotrusiveROM of asymptomatic adult ethnic Chinese sub-jects was 17.9 mm (SD = 3.3 mm).'" It was thusreasonable to further divide the CHUCs into threegroups based on the magnitude of this measure:

1. < 15 mm2. 15 CO 21 mm3. > 21 mm

Electrognathics

Although "unilateral domitiance" has been definedas where mastication is confined to one side of rhemouth for more than GG.6°/a of a masticatorysequence, some individuals have been found inwhom mastication has been clectrognathicallyshown to be confined to one side of the mouth formore than 95% of the chewmg sequence when aver-aged over three different test foods.'" These individ-uals are referred to as obligate unilateral chewers.

SMV Analysis

Using the Lew-Tay SMV cephalometric analysis/

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Gothic Arcti Tracing

Lalerotrusive ROMat lSMCP

SMV Cephalogram

BrachytacialBiotype

V

7 7

A

MesotaciaiBiotype

A

DolichofaciaiBiotype

True StructuralAsymiTiBtry

v-

Fig 5 Typical laterotrusive ROM, graphic tracing, and condylar morphology in the differ-ent facial biotypes.

CHUCs can be further subdivided accorditig totheir condylar morphology and condylar interaxesangulation.

Graphic Tracings

Central bearing point tracings can also be useddiagnostically to subclassify these individuals. Thelength, shape, and symmetry of the gotbic archestraced by the sagittal pin in the different facialt)'pes are shown in Fig 5.

NUS Point

Further, it was important to note whether therewas a general improvetnent in facial symmetrywhen the mandible was repositioned such that fre-nal was at the NUS point.

Clinical Observations and Associations

Although it was never clarified whether the higheye described by the Lagaida and White" referredto an actual superior displacement of the bonyorhital cavity or merely to a higher eye positionsecondary to head orthoposture, the CHUCs m thepresent study were generally found to prefer tbe

side of the "postural" bigh eye whenever there wassignificant head tilt. Class III individuals and tbosewith dolicboccphalic features tended to exhibitmore head tilt compared to their Class II and/orbracbyccphalic counterparts.

The association hetween the postural high eyeand the preferential chewing side was weaker,however, in the structural asymmetry physiognom-ic categories where the presence and location ofthe malocclusion (eg, crossbite, open bite, non-working side interferences) and/or degenerativejoint disease seem to play a more important role inthe selection of the mastication side-

Chin deviation, wben present, can either be inan ipsilateral or contralateral relationship to thehead tilt. Hetero-ocular, ipsilateral-type mandibu-lar displacement asymmetry physiognomy wasfound to be more associated with dolichocephalicCHUCs, and brachycephalic biotypes were over-represented in tbe hetero-ocular contralateral-typemandibular displacement asymmetry and thehetero-ocular contralateral-type true mandibularstructural asymmetry categories.

Some CHUCs, especially those who reportedunilateral chewing since childhood, tended also toexhibit a noticeable difference in the size of theireyes. Alert orthodontists may have encounteredthis positive soft tissue drape phenomenon whentreating growing children with lateral orthopedicmandibular repositioning appliances."' The eye

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opposite the side ro which the jaw was iatrogeni-cally moved has been observed in certain cases tonarrow with respect to its counterpart—sometimeswithin a matter of weeks.

The presence of a deviated nasal septum wasanother common finding in subjects with signifi-cant head tilt. The development of the lower partof rhe nasal complex seemed to be intimately re-lared to rhe development of the masticatory sys-rem. The lower rhird of the nasal septum was oftenfound to be parallel with rhe plumb-line, and itsvertical alignment may prove to be a useful radio-graphic landmark in the ortliopostural orientationof frontal cephalograms,-'

Some CHUCs, especially those without a historyof extensive restorative work or orthodontic inter-vention, typically presented with a lack of passivelaterotrusive symmetry at MI and generally exhib-ited some form of dentofacial asymmetry.

The CHUCs exhibiting true mandibular struc-tural asymmetries (le, those with radiographie evi-dence of significant length discrepancies"'" in theramus and/or corpus) usuaily have asymmetricgothic arch tracings rhar reflecr the presence ofasymmerric kinematic rotational centers.

The CHUCs who exhibited total laterotrusiveROM greater than 21 mm tended to have dolicho-cephalic features, and those exhibiting total lat-erotruSLVc ROM less than 15 mm tended to havebrachycephalic characteristics. It was interesting,however, that CHUCs exhibiting total laterotru-sive ROM greater than 21 mm and those withdolichocephalic features both tended to masticateon the side with the larger maximum passiveROM, whereas subjects exhibiting a total lat-erotrusive ROM less than 15 mm and/or possess-ing brachycephalic features usually preferred theside with the shorrer maximum ROM,

In the case of obligare unilateral chewers, a dis-tincr difference in the morphology and angularion ofthe condyles with respect to the intermeatal planecan be observed in the submento-vertex cephalo-gram.-' One side (usually the mastication side) tendsto he longer, more barrel-shaped, and flatter in ori-entation, and the other might be more elliptical andmore anguiated. The significance of these remod-elling changes has been discussed previously,-"

For most CHUCs, even those in the hetero-ocu-lar reverse drape true mandibular structural asym-metry category, facial symmetry improved whenthe mandible was repositioned such rhat frénalewas at rhe NUS point. Only those CHUCs in rhehetero-ocular conrraiatcral-type true mandibuiarstructural asymmetry did nor symmetricalize ar thethe NUS poinr.

Discussion

Altered Head Posture and MandibularDeviation at Ml

The upright postural position of the mandible(UPPM) is influenced by many factors, one of themost important of whicb is head posture." Fromphysical therapy, it is learned that side flexion withthe neck in extension or flexion usually results inan ipsilateral and contralaterai displacement of themandible, respectively,-' Whether the jaw deviatesto the side of rbe head tilt therefore largely de-pends upon the craniocervical orthoposture of theindividual. Thus, values in rhe craniocervical angu-lation may be helpful in predicting the lateral dis-placement pattern of the mandible,"'"

Funakoshi" previously described different pat-terns of electromyographic (EMG) response toaltered head posture. In dentofacial development,the influences of postural activity of the soft tissuehave been found to be more important than theeffecrs of muscle conrraction and jaw movements,"It is speculated rhar the EMG caregories are some-bow linked to rhese physiognomic caregories. Theherero-ocular ipsilateral-type mandibular displace-menr asymmetry physiognomy, for instance, maybe the growth or skeletal unit'"* end resuir of a par-ticular parrern of postural muscle activity associ-ated with an unbalanced ipsilateral-type EMGresponse ro alrered head posture.

Funakoshi showed how the introduction ofexperimental occlusal prematurities affecred tbeEMG readings. He also elegantly demonsrraredthat simple occlusal therapy could symmetricalizeor convert an unbalanced EMG response patternof the postural masticatory and accessory mastica-tory musculature inro a balanced one. The implica-tions if such symmetricalization occurred during,as opposed to after, the growrh phase are unclear-

Others working in the field of complete dentureshave observed a tendency for the jaw to deviate tothe side of posrerior vertical dimension loss in skele-tal Class III and I patients and to the contralateralside in the skeletal class II patienr." They alsoreported a compensatory head tilt in the same direc-tion, as if to balance the lateral mandibular shift.This was usually marked in the large and heavyClass III ¡aw, moderate in the Class I jaw, andinsignificant in the smaller Class II jaw. Suchresearchers believe that the restoration of the (uni-lateral) loss in posterior vertical dimension and thecorrection of the midline alignment would automat-ically reestablish the original upright head posture,"

There is already some indirect evidence thar an

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iatrogenic change in mandibular posture couldalter existing head posture. The present author hasdocumented frontal plane changes in the head pos-ture of some patients, especially those withdolichocephalic and/or skeletal Class III features,after mediolateral ¡aw repositioning. Secondaryresponses in craniocervical orthoposture in thesagittal plane resulting from anreroposterior jawrepositioning have been described by others."There is, however, a need to scientifically verifywhether these craniocervicai postural changes arepredictable or as significant in all the differentphysiognomic categories.

Patterns of Mandibular Displacement

Altered Mandibular Poscure During Growth. Itis speculated that those CHUCs in the hetero-ocular ipsilateral- and contra lateral-type mandibu-lar displacement asymmetry physiognomic cate-gories who exhibired positive soft tissue draperelationships were individuals who developed analtered mandibular position in consequence ro asustained alteration of the lateral craniocervicalposture (eg, scoliosis or congenital torticollis-'-').The form-function interaction includes not onlythe effects of active movement but also the long-lasting effects of the soft tissues on the developingskeletal and dental structures. Soft-tissue stretchinghas been proposed as a possible control factor inmorphogenesis by Solow and Kreiborg.^

Mongini and Schmid'* believed that an uncor-rected mandibular displacement asymmetry duringgrowth secondary to occlusa! factors could devel-op into true mandibular structural asymmetry, butthey did not functionally differentiate these entitiesaccording to laterotrusivc symmetry or physiogno-my. The success of functional orrhopedic therapyin growing children is based upon such an assump-tion. The classification of these secondary adjus-tive variants is shown in Table 1.

There could he diagnostic problems sometimessince these patients share similar features withCHUCs in the hetero-ocular ipsilateral-type hori-zonral mandibular displacement asymmetry phys-iognomic category and present with passive lat-erotrusive ROM symmetry at ISMCP despite thefact that the length discrepancy between the ramimay be quite apparent on the orthopantomogram.It must be reemphasized that the present laterotru-sive ROM measurements at MI versus ISMCP canonly disclose horizontal displacements of themandible.

Defect in the Adjustive Condylar CrowthCapability During Childhood. Individuals diag-nosed with arthrosis deformans juvenile'' or earlycondylar fractures'" and certain congenital disor-ders (eg, hemifacial microsomia) usually developthe hetero-ocular reverse drape mandibular struc-tural asymmetry physiognomy.

In the two former conditions, the diminished"adjustive" growth of the ramus is probably relat-ed to the injury or destruction of rhe condylar car-tilage during childhood." Although ramus lengthdiscrepancies are commonly seen on the radio-graphs, corpus lengrh differences are evident onlyin cases where trauma or injury to the condylarcartilage had occurred in very early childhood.This is consistent with the idea that corpus devel-opment is derived from the posterior relocation ofthe ramus during growth remodelling. These truemandibular structural asymmetries are classified assecondary maladjustive variants. (Table 1).

Facia! symmetry, however, improved when themandible was repositioned to the point of passivelaterotrusive symmetry (the NUS poinr). This wasbecause essentially normal soft tissue was in-volved. The normalization of the patient's posturalmuscle vectors could be expected once the jaw wasencouraged to function from an UPPM condi-tioned by the reorganization of Mi at the NUSpoint. In the presence of occlusal stability, there is

Table 1 Classification of Mandibular Asymmetry

Presence oflaterotrusive

Radiographie symmetry arevidence MI

Presence of Improvementlaterotrusivc in facialsymmetry at symmetry at

ISMCP NUS point

Mandibular horizontaldisplacement asymmetry

True mandibular structural asymmetryPrimary (autonomous)Seoordary (adjustive)Secondary (maladjustive)

No No Yes Yes

YesYesYes

NoNoNo

NoYesNo

NoYesYes

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ample evidence''' ''" tbat tbe mandible can functionadequately even witbout a condylar fulcrum.

Hemifacial microsomia is an example of a con-genital condition attributed to the early loss ofneural crest cells whereby the primary disturbancelies in tbe soft tissue on tbe affected side of theface, usually in the area of the mandibular ramusand external ear. Nevertheless, functional appli-ance therapy instituted at an early age has beenshown to be effective in obtaining some develop-ment of tbe soft tissue or at least in sttetching thesoft tissues for a better surgical result.''•'

Hemimandibular Elongation. Fxcessivc and/oruncontrolled unilateral growth activity in the condy-lat fibtocartilage may also result in mandibuiardeviation in conjunction with contralateral cross-bites and/or ipsilaterai open-bites, (Hemimandibularhypetplasia is a rate occurrence in comparison tohemimandibular elongation,") Accordingly, onewould not expect an improvement in facial symme-try even if the mandible was repositioned to theNUS point. These true mandibular structural asym-metries bave been classified as primary autonomousvariants (Table I),

Severe cases should be managed orthognathical-!y. To maintain a symmetrical arch form foresthetic lip support, two-jaw surgery with the aimof shifting the pogonion to the NUS point must beconsidered,' Occlusal plane correction as weli asthe removal of the condylar growth center (if stillactive) are the other important considerations.''

Mandibular Displacement Occurring AfterCompletion of Growth. Tbe CHUCs with homo-ocular ipsilateral-type horizontal mandibular dis-placement asymmetry usually exbibit no signifi-cant difference in the size of their eyes. It isbelieved that these are individuals in whom there isno asymmetrical mandibular displacement duringgrowtb and facial development proceeded withoutevent. Thus, any alteration in head postute proba-bly occurred after maturity as a secondary re-sponse to, rather than a cause of, the lateral jawdisplacement. Craniocervical posture in this phys-iognomic category migbt therefore be moreamendable to correction, and it is believed that itsmaintenance may be facilitated by mandibulatrepositioning to the NUS point.

Lack of Passive Laterotrusive Symmetty at Ml

The most common mastication pattern is quad-riphasic or teardrop-shaped when viewed in thefrontal plane. In comrnon sequencing," themandible moves downward during the openingphase and then deviates to the side of the bolus

during tbe closing phase. This basic, repetitious,and ordered pattern of masticatory muscle con-tracrion is provided by a programmed neural pat-tern generatof" as well as reflexes that control themagnitudes of lateral and dorsoventral deviationsduring a cbewing cycle. If the influence of theocclusal interface was negated (eg, via the employ-ment of a flat full-coverage occlusal splint), beadposture and intrinsic factors (such as facial form,muscle quality, and characteristics of the individu-al's central pattern generator) become the primaryfactors determining the shape of the movementpattern in empty mouth mastication.

If head posture is kept constant, it can he seenthat there will be greater opportunity for toothcontact on the side of tbe larger lateral ROM bymerely coordinating MI asymmetrically (Fig 6).(Patients who were not originally CHUCs, buthave been "finished" more than 1,5 mm awayfrom the NUS point, will consistently favor theside with the larger laterotrusive range,) This canbe readily observed in dohchoccpbalic patientswitb skeletal Class III jaw relationsbips. Indirectsupport for tbis postulation could also be found inProschel's work.'"

This simplistic model does help to partly explainwhy CHUCs so often present with an absence ofsymmetry in their laterotrusive ROM at Ml,Interestingly, individuals who profess to be bilater-al chewers but present witb, for example, hetero-ocular reverse drape true mandibular structuralasymmetry were found to have their MI within 0,5mm of the NUS point.

BrachycephalJc vs Dolichocephalic CHUCs

The masticatory stroke comprises both choppingand grinding movements. Brachycephalics typically

Fig 6 Frontal view of qiiadriphasic chewing patterns;MI - Ml corresponds rn NUS point, MI' - Ml coordi-nated lateral to NUS point.

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masticate in a verticalized "chopping" fashion anddolichocephalics tend to exhibit a more horizontal"grinding" pattern. The orientation of the pterygo-masseteric sling and lateral pterygoid musculatureis significantly different in the above facial forms.''"The dolichocephalic individual is laterotrusivelyefficient whereas the brachycephalic subject is lat-erotrusively inefficient.

From the resolution of the medial pterygoid vec-tors, it can be appreciated that the vertical hitingforce in the dolichocephalic person is very muchweaker than that in the brachycephalic subject.Thus, in the presence of head tilt, the compensato-ry ipsilateral mandibular movement commonlyobserved in the dolichocephalic individual wouldfurther weaken the vertical vector on the ipsilateralside hur strengthen that on the contralareral side(Fig 7). It seems reasonable to believe the dolicho-cephalic child in such a situation would serendipi-tously select the side of the postural high eyewhenever he encounters a resistant bolus of food.This, as previously discussed, happens to be on theside with the latger laterotrusive range.

Brachycephalic CHUCs, on the other hand,shnw much less head tilt than their dolichocephaliccounterparts (Fig 4, III). Tbe typical contralateralmandibular displacement pattern exhibited bybrachycephalic CHUCs brings the side of the pos-tural higher eye into a more lateral position fromwhich it can more easily make grinding move-ments. The side selected by the brachycephalicCHUC exhibiting unbalanced conrralateral-typemandibular displacement asymmetry would usual-ly be that with the shorter laterotrusive range.

It is postulated that in the presence of alteredhead posture, the preferential chewing side in thedohchocephalic was selected because of the differ-ential improvement in crushing capability. In the

Fig 7 Relationship between postural high eye and pref-erential chewing side in dolichocephalic CHUCs.

laterotrusively inefficient hrachycephalic, one sidewas favored because of the comparative improve-ment in grinding capability.

True Structural Mandibular Asymmetries

Most CHUCs, particularly the obligate unilateralchewer, fall under this category. As was earlier dis-cussed, they usually possess identifiable morpholog-ic and positional adaptations in the TMJ complex."The difference in the condylar polar angulationsbetween the sides may be responsible for the lack oflaterotrusive symmetry at ISMCP.

Head posture did not seem to be as important afactor in the selection of the mastication side inthese categories as, perhaps, occiusal table factors(eg, crossbite/open bite/occlusa! interferences) orchildhood degenerative joint disease. Reversedsequencing" was commonly seen in CHUCs withhetero-ocular contralateral-type true mandibularstructural asymmetry. Patients classically diag-nosed as having "condylar hyperplasia" or, morespecifically, hemimandibular elongation will usual-ly come under the hetero-ocular contralateral-typemandibular structural asymrnetry category.

The lack of symmetry in the midsagittal gothicarch tracings is pathognomonic of individuals withtrue mandibular structural asymmetry and revealsthe presence of asymmetric kinematic rotation cen-ters. The clinical significance of the latter is dis-cussed elsewhere."

Association With TMD

It is a common clinical finding that patients withTMJ complaints usually prefer to masticate on themore painful side.™ Masticatory movements insubjects with severe TMJ impairment tend to devi-ate toward the lesion side and are generally morerestricted. These characteristic movement patternsappear to be adaptive responses that allow themasticatory task to be performed in the least dam-aging and painful manner." Some clues as to whythe dysfunctional side is selected may be found inresearch. Replicator studies" have shown that theworking-side condyle performed movements thatare considerably shorter than its nonworkingcounterpart, and the results of primate experi-ments" support the idea that rhe working-sidecondyle was less subjected to biomechanical loadsthan its counterpart.

In a questionnaire survey on a nonpatient popu-lation,' positive associations were found betweenthe subject's awareness of a unilateral chewinghabit and complaints of "noises from the joints,"

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"difficulty in opening the mouth wide," "anuncomfortable bite," "soreness in the teeth,""painful cheek muscles," "pain m the temples,""tension in the muscles of the face and ¡aws,"and/or "neck pains."

Although the observed alterations in the param-eters of mastication seen in TMD patients are hke-ly direct consequences of TMJ and/or muscle dys-function, this still does not eliminate the possibilitythat chronic habitual unilateral chewing duringgrowth may predispose these individuals to devel-op certain intracapsulat joint dysfunctions such asinternal derangements. Some bold the view thatTMDs are mere extensions of growth''' and postu-ral disturbances^" that began very early in life.

A clinical impression is gained that CHUCs willgenerally be asymptomatic so long as tbey can goon functioning on their habitual chewing side. Itmust be assumed that the condylar form thatdeveloped during growth was the best morpho-functional option for that particular pattern of jawfunction.''' Trouble usually begins wben circum-stances (eg, pulpitis, periodontitis, loss of posteriorteeth, or some form of iatrogenic orthodontic orprosthodontic intervention) force them to masti-cate on their uncharacteristic side. Thus, if anadult patient presents with a significantly narrowereye on the side of mandibular deviation, oneshould either rule out a hetero-ocular reversedrape true mandibular structural asymmetry or atleast be aware the lateral mandibular deviationcould be iatrogenically caused and tbat the lattermay have been a rather recent development.

A Reduction in the Posted or-Lacerai Joint BufferSpace. It is hypothesized that the maximum later-al range at MI is a convenient frontal plane mea-sure tbat reflected the amount of ftinctional poste-rior-lateral joint buffer space demanded by tbefunctional masticatory jaw movement patternhabitually exhibited by the individual duringgrowth.'* As a clinical rule of thumb, wheneverthere is an asymmetry in laterotrusive range at Mi,the CHUC patient should be advised not to switchfrom the side that has been preferred since child-hood or else to favor the longer side (ie, the sidewith the larger posterolateral TMJ buffer space).

The risky combination of lateroretrusive toothinclines*' and reduced posterolateral joint bufferspace found (usually on the side to which themandible deviates) in subjects exhibiting a widehorizontal mastication pattern or parafunctionalhabits may explain tbe close association reportedin tbe literature between posterior condylar posi-tion and TMJ internal derangements.'*""

Association With Lateral Centric Slides. The

association of lateral centric slides with TMD hasbeen frequently reported in the literature.""" Thepresent author and others" maintain that theseasymmetric centric slides are more likely tbe effectof cbildhood TMD rather than its cause.

CHUCs in the hetero-ocular reverse drape truemandibular structural asymmetry category usuallyhad some radiographie evidence of TMJ degenera-tion and more often than not experienced somediscomfort or pain when forcefully verified usingDawson's bimanual jaw manipulation tech-nique."'"'' Tbere was a lack of passive laterotrusiveROM at ISMCP although the affected condyieusually adopted a more anterior position at MIwhen compared to the opposite condyie. Thesevariations in condylar position can be easily appre-ciated on cephalometrically oriented lateral-oblique transcramai radiographs.

Weinberg"' was probably describing this verygroup of patients when he introduced the conceptof a Dysfunctional Centric (situations when thedocumented condylar position in TMJ radiographsdid not correlate with the mandibular deflectionfound clinically. Fig 3). It is believed that CR reha-bilitation techniques should be contraindicated,both from a biologic and convenience perspective,whenever tbere is a lack of passive laterorrusiveROM symmetry at ISMC."'"

Furthermore, some authorities" are of the viewthat TMJ degeneration is the principle cause ofbotb acquired facial skeleton remodelling andunstable occlusion in patients witb intact dentitionand without previous mandibular fracture. Per-baps it is time that tbe conspicuous lack of symme-try in jaw function and dentofacial morphologybe considered one of the key signs of TMD.

Conclusion

Postural and facial form factors may be responsi-ble for tbe observed association between dentofa-cial asymmetry and masticatory lateral preference.

Even though it is now possible to functiotiallydefine subgroups in the CHUC population andcategorize mandibular asymmetry variants accord-ing to physiognomic and laterotrusive ROM crite-ria, sample identification remains the biggestobstacle in research on masticatory function andcraniomandibular disorders. This latent 'noise' inthe data may partly explain why most reports onTMD*" in which morphofunctional factors wereinvolved have concluded with the general state-ment that "no significant difference could befound between control and test populations."

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Knowledge of subjects is important, for onlythrough tightly controlled prospective studies cana better understanding of the unilateral mastica-tion phenomenon be gained. Furrher researchis necessary ro accurately work out rhe specificroles played by peripheral and central factors ineach situation, identify rhose patrerns that aremodifiable, and/or predict, with some reliability,the extent ro which the latter can be accomplished.

Acknowledgments

The author wiîhes lo express his thanks to Miss Tan SiewHong for her assistance in the preparation of the illusrralionsand to Michael Lim, BDS, MDS for critically reviewing themanuscript.

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Resumen

La fisionomia en la clasificación de individuos cuya masli-caclón es preferentemente lateral.

Las personas que mastican habitual y crónioamente carecenusualrnente del radio de acciór laterotrusivo pasivo en losmovimientos mandibulares en la intercuspidación máxima, ypueden tener algún tipo de asimetra mandibular Este estudiointentó descjbrir mecanismos que puedan ser responsables deesta asooiación, y propone un sistema por medio del cual talespersonas pueden ser clasificadas basados en ia fisionomia y lospatrones de desviación mandibular. Se analiza también la influ-enoja del biotipo faoial y la posición oefálioa alterada en la selec-ción del lado preferido para efectuar la masticación.

Zusammenfassung

Die Bedeutung der Physiognomie in der Einteilung vonIndividuen mit einer bevorzugten Kauseite.

Die Kaubewegungen von Individuen, die habituell auf einer Seitekauen, haben oft einen fehlenden passiven laterotrusiven Anteil indie maximale i n te rku s pida tion. Viele dieser Individuen vueisenauch eine Unterkieferasymmetne auf Diese Studie versuchteden Mechanismus herauszufinden, der fur diese Zusammenhangeverantwortlich sein i<ónnle und schlagt ein System vor, mit Hilfedessen solche Personen aufgrund ihrer Physiognomie undMuster der Unterkieferbewegungsabvueicbungen kiassifiziertwerden konnten. Der Einfluss des facialen Biotypus und derveränderten Kopfhaltung auf die Wahl der bevorzugten Kauseitewird ebenso analysiert.

72 Volumes, Number 1. 1994