1988 Alavi. Facial and dental arch asymmetries in Class II subdivision malocclusion .pdf

7/28/2019 1988 Alavi. Facial and dental arch asymmetries in Class II subdivision malocclusion .pdf

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Facial and dental arch asymmetries in Class IIsubdivision malocclusionDebra G. Alavi, D.D.S., #I&,* EWn A.Bernard J. St+hnMw, D.D.S., MS,**Chicago, I l l.

, Ph.D.,** and

The purpose of this investigation was to determine if any significant C$#WWIWSexiated with regardto dental arch and facia l asymmett%s behnsen parsona wmw%lt-d%ftnedasymmetric character&tics, spec#kWy At-@&s C&as tl s usions, and personshaving nom-tat ocdusions. Setcwtdarily, it was sought to &&ermine the nature of the dWerer~~+s hatcontributed to the asymmetric occ lusal relationship of buccai segments observed in Class IIsubd ivision malocekafons. The sampte consfated of 28 subjects in each of thpossessed a full complement of permanent teeth itWu@ng fit@ n-&a%. The awein both groups was 17 years. lMasur8me#ts p&war& of a linear nature wereposteroanterior and lateral cephaiometric r in addition to dental mod&s. Smeasurements were summed to produce of asymmetry. Vawith multivariate discrimMnt analysis. A sigM dads b&wnvariab les descr ibing asymmetry of the derWM&ar region of the mandible appearad to be theprimary contributors to the difference obtained. Aayrmwtry of the maxWryappeared to make a secondary cont&uf~n. (AM J ORTW O DENTOFAC Owrwop 1

C ass II subdivision malocclusions withtheir asymmetric occlusal relationships often pose treat-ment difficult ies. The nature of the occlusal asymmetrymay be due to dentoalveolar or skeletal asymmetries,or a combination of these factors, and it is thought thatthese underlying factors complicate the attainment of asymmetric occlusion.The anthropologic investigations of Woo, Tildes-ley,* Pearson and Woo, Bji irk and Bj6rk,4 Gundara andZivanovic, and White6 have all shown craniofacialasymmetry to be common in most persons. Vazquez,Grostic, and Ponder7 compared indices of skul l asym-metry and malocclusion, and found asymmetry to berelated to severe malocclusion.Several studies showed a tendency for the maxil lato be more asymmetric than the mandible or dentoal-veolar regions.*- Vig and Hewitt, and Shah andJoshi have shown that in normal occlusion the den-

From the Department of Orthodontics, University of Illinois at Chicago, Col-lege of Dentistry.*This article is based on research submitted by Dr. Alavi in partial fu lfillme ntof the requirements for the degree of master of science in oxthadoatics, Uni-versity of Illiiis at Chicago.**Associate Professor of Biostatistics, Department of Orthodontics.***Professor, Department of Orthodontics.38

toalveolar region shows less asymmetry than the skel-etal areas of the face and these authors suggest that anadaptive mechanism may be present to explain this,Most studies of dentoalveolar asymmetry have useddental models and most often only the maxi llary arch.Using the median raphe as an axis of symmetry, anumber of studies reported some degree of dental arch

asymmetry even in persons with normal occlusion.1*-6He&~ter~ analyzed asymmetry of the dental arches innormal and malocclusion subjects, and reported greaterasymmetry in the mandibuhu arch for both groups. Inad&ion, he found an increase in asymmetry in botharches wben malocclusion was present. Using mathe-matic functions, investigations of dental arch form havealso demonstrated asymmetries. *-*OWith the advent of cephalometric radiography, stud-ies were conducted that attempted to relate occlusionto skeh%al asymmetry. Shore, by means of posteroan-terior cephalometric radiographs, compared normal andmalocclusion groups, and found that occiusion wasindependent of skeletal asymmetry. Letzer and Kron-man* also compared malocclusion and exceiient oc-clusion groups using posteroanterior cephalometric ra-cfi~hs and found no statistical evidence to corro-borate any relationship between the occlusion and facia lasymmetries. However, they found both the mandible


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Vo lume 93Number 1 Facial and dental arch asymm etries 39

Fig. 1. Diagram showing the linear and angular measureme ntsmade from bilateral landm arks to the constructed midline onthe PA cephalometr ic radiographs.

and anterior cranial base to be more symmetric in theexcellent occlusion group. It is not surprising that thesestudies failed to demonstrate differences betweengroups since the malocclusion groups studied were non-specific and only posteroanterior cephalometric radio-graphs were used. Will iamson and Simmons** used bothposteroanterior and submental-vertical cephalometricradiographs to study mandibular asymmetry as it relatesto pain dysfunction. In relating occlusal and skeletalcharacteristics, they found several subjects in their sam-ple to be more Class II dentally on the short side of themandible.The present study compared a group of subjectshaving normal occlusion with a group having well-defined asymmetric relationships of the buccal seg-ments, classically known as Angles Class II subdivi-sion malocclusion. The primary objective of the studywas to characterize asymmetries of the dental arch andfacial regions in both groups and to discriminate be-tween the groups on the basis of a set of measurements.Secondarily, it was sought to find which combinationof variables contributed most to the discrimination. Toaccomplish this, a multivariate statistical approach wasused that allowed complex interrelationships amongmany variables to be explored.METHODS AND MATERIALS

Two groups of 28 individuals were used in the study:a normal occlusion group and a malocclusion groupconsisting of persons having Class II subdivision mal-

Fig. 2. Diagram showing the linear measureme nts made frombilateral land marks to the constru cted vertical reference line onthe lateral cepha lometric radiographs.

occlusions. Mean ages were 17.1 years for both groupswith standard deviations of 6.4 and 6.7 years, respec-tively. Records consisted of posteroanterior and lateralcephalometric radiographs of good quality, and dentalmodels. These were obtained from duplicated recordsof the Child Research Council, University of ColoradoSchool of Medicine in Denver, for the normal group,and from the records of the University of Illinois Ortho-dontic Department for the malocclusion group. The cri -terion for selection was a full complement of permanentteeth through the first permanent molars. Additionalcriteria for the malocclusion group included (1) a fullClass I molar relationship on one side of the dental archwith a full Class II on the contralateral side, (2) theabsence of any severely malaligned or blocked ou tteeth, and (3) no apparent functional mandibular shiftsas reported in the clinica l history.Dental models

On the maxillary dental model, two points-namely, one at the distal aspect of the incisive papillaand the second at the posterior border of the raphe nearthe fovea central is-were used to define the medianraphe that was used as the dental midline. These twopoints were transferred to the occluded mandibularmodel with a device similar to that described by Hech-ter.17 Buccal cusp tips of the posterior teeth and themidincisal edges of the anterior teeth were then marked


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40 Alavi, BeCol e, and Schneider

_Fig. 3. Diagram showing the finear measurements made frombilateral tooth landmarks to the horkontai and vertical referencelines on the maxilla ry and mandiiular den&if mode ls.

on the casts with pencil. The models were photographedwith a 35 mm telephoto setup at a standard object fi lmdistance and life size prints of the models were thenproduced. The accuracy of the method has been pre-viously confirmed by BeGole.Al l points on the prints and an additional point be-tween the central incisors at the level of the incisal edgewere then digitized using a G&/Pen sonic instrumentto record coordinates in an X,Y system. From the co-ordinate data, a computer program of Cleall andChebib was used to calculate measurements.Linear measurements were made from each of theseven bilateral tooth landmarks in a transverse directionto the median raphe line. An anterior reference line wasconstructed perpendicular to the raphe line through themidpoint between the central inc isors and similar mea-surements in an anteroposterior direction were made tothis line. Fig. 1 shows the measurements that were madeon the dental models. Asymmetries were calculated bydetermining the absolute difference between homolo-gous measurements as related to both reference l ines.Measurements of transverse asymmetry for the incisorsand canines were combined to produce an index of

MAtOCClUSiON

Fig. 4. Diagram il lustrating the relationship between the normaland maloccWo n groups for the sk&eWen tal analysis.

transverse anterior segment asymmetry. An index oftransverse buccal segment asymmetry was formed as acombination of measurements from premolar and molarpoints. The same procedure was followed to computeanteroposterior indices of anterior and buccal segmentasymmetries. Dental midline deviations were assessedby making linear measurements from the midpoint be-tween the central incisors to the median raphe line orits analog on the mandibular model. Thus, a total often dental model variables resulted since the measure-ments were made on both maxillary and mandibularmodels.Lateral c Fadiogrephs

Cephalometric films were traced and landmarks des-ignated before digitization. For the lateral cephdomet-tic radiographs, right and left side landmarks were dis-tinguished with the aid of the Broadbent-Golden ori-entator.% It should be noted that while right and leftsides may be difficult to distinguish in asymmetriccases, this did not affect the outcome of the study be-cause only absolute values of the difference betweenthe sides as opposed to signed differences were usedin the group comparisons. In addition, reference wasmade to the original models when using the orientatorto identify left and right sides. The following landmarkswere digitized: sella, nasion, basion, the bilateral or-bitale, PTM, antegonia, gonia, articulare, points ofmaximum concavity on the anterior surface of the ra-mus, and the most posterior points on the distal surfacesof the maxillary and mandibular first permanent molars.A reference line was determined by constructing a per-pendicular to the selia-nasion line through badon. Lin-ear measurements were made in a horizontal directionfrom the bilateral orbitale and PTM, articulare, anteriorramal point, antegonion , and gonion , to the constructedvertical line. Fig. 2 shows measurements taken fromthe lateral radiograph. Skeletal asymmetry was evalu-


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Vo lume 93Number 1

Facial and dental arch asymmetries 41

Table 1. Means and standard deviations of asymmetry measurement for posteroanterior cephalometric films(in millimeters)

Sample

VariableTransverse maxillary asymmetryTransverse mandibular asymmetryIncision superior-midlineIncision inferior-midlineMenton-midlineANS-midlineAngular maxillary asymmetry*Angular mandibular asymmetry*

Normal group Malocclusion groupMean SD Mean SD

1.87 1.22 1.82 1.672.89 1.92 3.50 2.441.09 1.24 1.34 1.101.45 1.09 1.64 1.331.89 1.34 2.16 1.670.84 0.71 0.90 0.62

18.58 13.46 17.85 12.9218.21 13.81 20.59 15.45

*Measurements given in degrees.

ated by determining the absolute difference betweenmeasurements for right and left side landmarks. Indi-vidual measurements of asymmetry were then com-bined to produce indices of anteroposterior maxillaryand mandibular asymmetries, producing two skeletalvariables.Linear measurements were also made from the bi-lateral maxillary and mandibular molar points to thevertical reference line. Again, the absolute differencebetween right and left side landmarks was determinedto arrive at measurements of anteroposterior maxillaryand mandibular molar asymmetries, resulting in twoskeletodental variables.

and left sides for both linear and angular measurementsresulting in four skeletal measurements: angular man-dibular and maxillary asymmetries, and transverse man-dibular and maxillary asymmetries. Midline deviationsof the maxilla and mandible were measured by deter-mining the absolute distance from the two anatomicallydetermined midpoints, ANS and menton, to the con-structed facial midline, resulting in two additional skel-etal variables. Midline deviations of the dental archeswere measured by determining the absolute distancefrom incision superior and inferior to the constructedfacial midline, resulting in two skeletodental variables.Statistical analysesPosteroanterior cephalometric radiographs

On the posteroanterior cephalometric radiographs,as shown in Fig. 3, the landmark points-namely, rootof crista galli, ANS, incision superior, incision inferior,menton, and bilateral gonia, maxillare, and superiororbital points-were digitized. A reference midline wasdetermined geometrically so that it passed through theroot of crista galli perpendicular to a line connectingthe most superior points on the orbital contours. Thismethod of midline construction was chosen because itdid not rely upon the use of any maxillary or mandibularlandmarks and has been well documented in the liter-ature.

The 22 variables-ten dental models, eight skele-tal, and four skeletodental-were analyzed. For bothgroups means and standard deviations were calculatedfor each variable. To fulfill the major objective of thestudy, which was to characterize the groups, a multi-variate discriminant analysis was performed using allthe variables.Discriminant analysis is a multivariate statisticalmethod in which combinations of variables are used todistinguish between groups. It allows group differencesto be studied with respect to several variables simul-taneously, thus avoiding the type I errors common toseparate univariate analyses.

Linear measurements were made in a horizontal The first purpose in using the analysis is to deter-direction from the bilateral maxillary landmark max- mine the optimal subset of variables that best separateillare and the bilateral mandibular landmark gonion to or characterize the groups of interest. The method se-the constructed midline. Angular measurements were lects variables on the basis of abil ity to add to thealso made from the same bilateral landmarks to the discrimination until further variables no longer make aconstructed midline with the root of crista galli as the signif icant contribution. Once the variables have beenvertex of the angle. Skeletal asymmetry was evaluated selected, a discriminant function that maximizes groupby determining the absolute difference between right differences may be computed.


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42 Alavi, BeG ole, and Schneider Am. . I . Orthod. Den toja~ WhopJanuurv 1988Table II. Means and standard deviations of asymmetry measurements for lateral cephalometric films(in. millimeters)

Normal groupSample

Malocclusion group--

Variable Mean SD Mean SDAP maxillary asymm etry 1.77 0.98 2.41 2.23AP mandibular asym metry 3.93 3.21 8.05 6.87AP maxillary molar asymm etry 0.59 0.62 1.39 1.28AP mandibular molar asymm etry 0.87 0.72 2.10 1.34

Table HI. Means and standard deviations of asymmetry measurements for maxillary dental models(in millimeters)Sample

VariableTransverse buccal segment asymmetryTransverse anterior segment asymmetryAP buccal segment asymmetryAP anterior segment asymm etryMandibular midpoint-raphe

Normal group Malocclusion groupMean SD Mean SD3.95 2.77 5.66 3.554.48 3.17 4.99 4.807.85 5.25 6.95 5.762.72 1.98 3.23 2.950.94 0.56 1.03 1.09

The second phase of the discriminant analysis in-volves calculation of a discriminant score and proba-bility of group membership for each individual in thesample using the mathematic function. The subject isclassified into the group for which the probability ishighest and a table is printed showing the actual andtheoretical group membership based on the use of thefunction. These results indicate the percentage of casescorrectly classified based on the use of the function.The use of Wilks lambda, which is the multivariatetest statistic for group differences, indicates the dis-criminating power of the variables. In addition, the taustatistic, which is a part of the analysis, provides ameasure of improvement in classification over thatwhich might be expected through random assignmentof subjects to groups.The discriminant analysis also produces a set ofstandardized coefficients, which are used to computediscriminant scores in standard deviation units. Theseare useful in determining the contribution of each vari -able in the discriminating set. The larger the absolutemagnitude of the coefficient, the greater the contribu-tion of the variable to the discrimination.RESULTS

Means and standard deviations for all variables inboth groups are listed in Tables I through IV. Tables I

and II provide the statistics for measurements taken onthe posteroanterior and lateral cephafometric radio-graphs, respectively. Overall, the mean values forasymmetry tend to be greater for the malocclusiongroup as compared with those of the normal group.This was an anticipated finding since the malocciusiongroup was selected on the basis of asymmetric occlusalrelationships. The variabIes measured o&e lateral filmshowed the greatest asymmetry. This was especiallytrue for anteroposterior mandibular asymmetry, whichshowed a mean difference of 4.1 mm between thegroups. ANS-midline and transverse maxillary asym-metries, both of which were measured on posteroan-terior cephalometric radiographs, had group means thatwere nearly equal to the malocclusion group or, in thecase of transverse maxillary asymmetry, slightlysmaller by an insignificant amount. Although this couldbe interpreted as a function of unequal distances of theright and left sides of the structures from the cassetteon the lateral radiograph, such a condition should in-fluence computation of asymmetry equally for bothgroups under study. Thus, any reported difference be-tween the groups shouki be meaningful.Tables III and IV list statistics for dental modelmeasurements. Again, the mean values for asymmetrytended to be consistently larger for the malocclusiongroup as compared with the normal group with the


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Vo lume 93Numbrr I Facial and dental arch asymmetries 43

Table IV. Means and standard deviations of asymmetry measurements for mandibular dental models(in millimeters)

Sample

Variable

Transverse buccal asymmetryTransverse anterior segment asymm etryAP buccal segment asymmetryAP anterior se gment asymm etryMandibular midpoint-raphe

Normal group Malocclusion groupMean SD Mean SD4.79 2.84 6.59 5.614.66 4.09 8.31 6.645.81 5.09 9.60 6.601.95 1.33 3.42 2.351.11 0.80 1.56 1.36

Table V. Classification results ofdiscriminant analysisPredicted group membership

Actual grorrp Normal MalocclusionNormal 27 (96.4%)* 1 (3.6%)**Malocclusion 3 (10.7%)** 25 (89.3%)*

*Percent of total cases correctly classified: 92.868, tau = 0.857.**Percent of total cases ncorrectly classif ied: 7.14%.

exception of anteroposterior maxillary buccal segmentasymmetry. In the malocclusion group, mandibularmean asymmetry values were consistently larger thanthe maxillary arch asymmetry values. Within the nor-mal group, the values of anteroposterior asymmetry forboth buccal and anterior segments tended to be smallerfor the mandibular arch; mean values for transverseasymmetry differed only minimally between arches,with the values being slightly larger for the mandibulararch. For both groups, the anterior dental segment dis-played greater asymmetry in the transverse dimension.Most l ikely, this is related to the magnitude of tbedimensions involved since transverse measurementswere larger than anteroposterior measurements.

All measurements on both the cephalometric radio-graphs and dental models displayed consistently highstandard deviations. This indicates a large amount ofvariabili ty within the groups with regard to asymmetry.This was particularly true for the malocclusion group,whose standard deviations exceeded those of the normalgroup.Results of the discriminant analysis are listed inTables V and VI. The Wilks latnbda of 0.7273 for theanalysis was statistically significant. This shows thatthe combination of variables selected in the analysiswas highly effective in distinguishing between thegroups. Table V lists the classification results of thediscriminant analyses. The percentage of known cases

Table VI. Standardized coefficients ofdiscriminating variables for skeletodentaldiscriminant analysis

VariableAP mandibular molar asymmetry ( lat-eral cephalometric radiographAP maxillary molar asymmetry ( lateralcephalometric radiograph)Maxillary midpoint-raphe

(maxillary model)Incision superior-midline (PA cephalo-metric radiograph)Transverse anterior segment asymm etry(maxillary model)

AP buccal segment asymmetry (man-dibular model)ANS-midline (PA cephalometr ic ra-diograph)AP mandibular asymmetry ( lateralcephalometric radiograph)

Transverse buccal segment asymmetry(mandibular model)

Coeficient1.05700.85290.55940.5306

-0.5081

0.49510.40570.29600.2028

correctly classified based on the mathematic functionis important in evaluating group differences because itgives me probability of correct classification using thetheoretical model. It can be used together with theWilks lambda as a further means of indicating howwell the variables discriminated between groups. Usingthe mathematic function, 92.86% of all cases were cor-rectly classified; thus, the probability of misclassifica-tion using the model was 0.9286. The computer valueof tau was 0.857, which indicates that classificationbased on the function produced 85.7% fewer errors manwould be anticipated using random assignment. An il-lustration of this relationship is provided in Fig. 4.Table VI lists the standardized function coefficientsfor the nine variables of the original 22, which was theset chosen as the best discriminators in the discriminantanalysis. These coefficients indicate tbe relative im-


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44 Alavi, BeGole, and Schneider

portance of each of the variables and they are listed indecreasing order of importance on the basis of absolutemagnitude. Three skeletodental variables, four dentalmodel variables, and two skeletal variables comprisedthe set of discriminating measures, indicating that acombination of both skeletal and dental factors wasresponsible for the separation between the two groups.The highest ranked and first selected variable was an-teroposterior mandibular molar asymmetry. This is thesingle variable that produced the greatest separationbetween groups. The Wilks lambda based on this vari-able alone was 0.7473, with a significance level of0.003. Anteroposterior mandibular asymmetry as mea-sured in this study reflects asymmetry in spatial positionof the mandibular molar within the craniofacial com-plex, which could be due to both dentoalveolar andskeletal asymmetries. However, the former seems tomake a greater contribution since buccal segment asym-metry of the mandibular arch ranked above asymmetryof the mandible among the discriminating variables.Anteroposterior maxillary molar asymmetry also madea signif icant contribution, second in order to mandibularmolar asymmetry. Beyond this it is difficult to singu-larly evaluate the individual contribution of any variablebecause all variables following the first selected in thediscriminant analysis were chosen for the improvementthey added to the discrimination. It is the unique com-bination of al l the selected variables that is responsiblefor the total discrimination.DISCUSSION

As demonstrated by the means and standard devia-tions, asymmetry of both dental arches, maxil la, andmandible was a common finding in both normal andmalocclusion groups. This result i s not unique and hasbeen reported in the literature on numerous occasions.

The discriminant analysis results provided the basisfor this study. As might have been expected, the dis-criminant analysis demonstrated a highly significant dif-ference between normal and Class II subdivision mal-occlusion groups. A combination of skeletal and den-toalveolar variables was shown to be responsible forthe separation of the groups.The relative importance of specific variables in thediscrimination demonstrated a somewhat unexpectedresult. Since the two groups had been original ly selectedon the basis of dental model character istics, that is, therelationships of the first permanent molars, it wouldseem that variables relating to the dental arch shouldrank highest among the discriminating variables. Thiswas not the case, however, as AP buccal segment asym-metry ranked sixth among the discriminating variables,

while AP mandibular and maxillary molar asymmetries,as determined from the lateral cephalometric f ilms, bestcharacterized the difference between groups. Thesevariables accounted for most of the discrimination evenif no other variables had been considered. These vari -ables would not have been discernible on the PA ra-diographs that were used to evaluate asymmetries inmany previous studies.

It is of further interest that a malocclusion whoseclassification is made from study models on the basisof molar relationships is best represented by variablesdiscernible on lateral cephalometric films involving themolar teeth. Apparently, the us e of the posterior verticalreference line passing through basion on the lateralcephalometric films offers a reference away from thearea of the dental region and results in a more accurateevaluation of the spatial position of the maxillary andmandibular molar teeth.While the spatial position of the molar teeth. es-pecially the mandibular molars, proved to be the mostdiscriminating variable, what cannot be ascertained iswhether this was due to their position in their respectivejaws, or to the position of the jaws in the craniofacialcomplex, or a combination of both. There is yet oneother consideration affecting mandibular molar positionand position of the mandible that should be addressed--namely, that of mandibular repositioning incident todental relationships. Cephalometric films are typicallyexposed in the position of maximum intercuspation, beit in centric occlusion or centric relation. If occlusalinterferences are present, which is a possibi lity in thecase of malocclusions, these positions may not coin-cide. Thus, in certain cases he position of the mandible,and consequently that of the mandibular molar, couldbe affected by dental reIationships that create mandib-ular repositioning. Although the preceding would beimpossible to determine because the cephalometricfilms used in this study had been taken previously, thispossibil ity does deserve mention. If it is assumed thatno malpositioning of the mandible occurred, it may beconcluded that the dentition is a good indicator of asym-metry and that such asymmetry is perhaps best evalu-ated with references outside the dental region.

It should also be noted that while spatial positionof both permanent molars was important in distinguish-ing the groups, it was the position of the mandibularmolar that provided the maximum amount of discrim-ination, with the AP position of the maxillary molarserving to enhance this result. In fact, AP mandibularmolar asymmetry, as measured on the lateral cephalo-metric film, alone discriminated between groups at ahighly significant level. While it could be argued that


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Volume 93Number 1

this might be a function of measurement s ince, withthe radiographic technique, the mandibular molar isfurther away from the central ray through porion, thisfactor should not have affected the results because thediscriminant analysis is carried out between groups,which would make the effect of such measurementsinsignificant.It is generally accepted that shape, size, and positionof the jaws and dentoalveolar region are to a certainextent genetically programmed. Johnson,26 in analyzingStockards classic work on heritability in dogs, con-cluded that genetic constitution was a vital factor in thedevelopment of skull form and dental occlusion.Moore* thought that heredity was operative in facialsymmetries and found such asymmetry to be three tofour times as great in children whose parents were sim-ilarly asymmetric.The concept of a genetically predetermined asym-metric pattern of growth and development in Class IIsubdivision persons might offer some explanation as towhy such persons may be difficult to treat orthodonti-tally. Few reliable methods of delivering asymmetricorthopedic forces are available, especially forces to themandible, which according to this study was most re-sponsible for the existing asymmetric occlusal relation-ship. In the case of the dentoalveolar region, where itis often possible to use unilateral mechanics, the asym-metric growth pattern in these cases may tend to un-dermine or hinder treatment. Brodie2 suggested thateven into the postretention period, the supporting al-veolar bone continues to grow in its asymmetric form.Coincident midlines are also often difficult to achievebecause of the presence of such asymmetries. Thus, itcan be seen that subdivision malocclusion cases causedby the presence of increased skeletal and dentoalveolarasymmetries may often require compromises in treat-ment or an asymmetric extraction pattern as suggestedby Wertz3 and Cheney.As mentioned previously, it was concluded in thisstudy that skeletal and dentoalveolar asymmetries werefound to contribute to the asymmetric buccal segmentrelationship present in Class II subdivision malocclu-sions. Significant differences in skeletal asymmetryhave not always been reported in previous studiesof asymmetry comparing normal and malocclusiongroups, most notably those of Shore and Letzer andKronman2 The somewhat contrary finding of the pres-ent study can be accounted for by several factors.

First, the present study used a malocclusion groupwith well-defined asymmetric characteristics. The im-plications of using such a group as opposed to a non-specific malocclusion sample should be apparent. While

Facial and dental arch asymm etries 45

most malocclusions would be expected to display in-creased asymmetry of the individual dental units, mostoften they are of a bilateral nature with regard to buccalsegment relationships except perhaps in the case ofmutilation or blocked out teeth. It does not seemthat previous authors should have anticipated findingincreased skeletal asymmetry in such malocclusiongroups as compared with normals.Second, the use of only PA cephalometric films inprevious studies was limiting in that it allowed onlytransverse and vertical dimensions to be studied. Theuse of lateral cephalometric radiographs in the presentstudy allowed asymmetry to be studied in an antero-posterior direction, The use of these radiographs didinvolve the problem of accurate identification of rightvs. left side landmarks. However, using the Broadbent-Golden orientator and the dental models as a secondaryreference, identificat ion was reasonably wel l made. Itshould be noted that the use of submental vertical filmswould also have been quite appropriate for this type ofstudy. In their cephalometric study using submentalvertical films, Williamson and Simmons2* found thatsubjects displaying 3 mm or more of mandibular asym-metry had a tendency toward a Class II buccal segmentocclusion on the short side. This correlates well withthe results of the present study.

Third, the difference in statistical techniques be-tween previous studies and the present one must alsobe appreciated. Simple univariate analysis allows onlyfor the comparison of variables on a one-to-one basisand is lacking in the characterization of complex in-terrelationships among the variables. The multivariatestatistical approach of the discriminant analysis allowsfor perturbations of the data that are difficult to evaluatevisually and impossible to reveal by measurements ofsimple univariate analysis alone. Such an approach al-lowed the information contained in numerous variablesto be summarized, producing an expanded picture ofdentoalveolar and skeletal relationships.REFERENCESI.

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Will iamson EH, Simmons MD . Mandibular asymm etry and itsrelation to pain dysfunction. AM J ORT HOII 1979;76:612-7,Cleall JF , Chebib FS. Co-ordinate an alysis applied to orthodonticstudies. Angle Otthod 1971;41:214-8.Broadbent BH Sr, Broadbent BH Jr, Golden WH. Bolton stan-dards of dentofacial developmental growth. St. Louis: TheCV Mosby Company, 1975.Hellman M. Some facial features and their orthodonttc impli-cat ion. AM J ORTHODORAL SURG1939;25:927-51.Johnson AL. The constitutional factor in skull form and dentalocclusion. AM J Oarnon ORAL SURG 1940;26:627-63.Stockard CH . The genetic and endocrine basis for differencesin form and behavior. Philadelphia: Wistar Institute of Anatomyand Biology. 1941.Moore G R. Heredity as a guide in dentofacial orthopedics. A MJ ORTHODORAL SURG1944;30:548-54.Brodie AC . Anatomy and physiology of the head and neck mus-culature AM J ORTHO B1950;26:831-44.Wertz RA. Diagnosis and treatment planning of unilateral ClassII malocclusions. Angle Orthod 1975;45:85-94.Cheney EA. The influence o f dentofacial asym metries upon treat-ment procedures. AM J ORTHOD 952;38:934-45.For&erg CT , Burstone CJ, Hanley K J. Diagnosis and treatmentplanning of skeletal asymmetry with the submental-vett ica1 ra-diograph. AM J OKTHO D1984;85:224-37.

Reprint requests toDr. Ellen A. BeGoleUniversity of I l l inois at ChicagoCollege of DentistryDepartment of Orthodontics801 South Paulina St.Box 6998Chicago, IL 60680

Documents

1988 Alavi. Facial and dental arch asymmetries in Class II subdivision malocclusion .pdf