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Oral Maxillofac Surg5:1538-1543, 2007

Relationship Between the MasticatoryMuscles and Mandibular Skeleton inMandibular Prognathism With and

Without AsymmetryTae-Geon Kwon, DDS, PhD,* Ki-Ho Lee, DDS, MS,†

Hyo-Sang Park, DDS, PhD,‡ Hyun-Mo Ryoo, DDS, PhD,§

Hyun-Jung Kim, DDS, PhD,� and Sang-Han Lee, DDS, PhD¶

Purpose: The purpose of the present study was to evaluate the relationship between masticatorymuscle volume and mandibular skeletal measurements in patients with and without facial asymmetry.This was done in order to determine whether asymmetric mandibular prognathism is related tomasticatory muscle asymmetry.

Patients and Methods: The study was conducted with 40 adult patients with mandibular prognathismcomposed of 2 groups, the asymmetry group (n � 20) and the nonasymmetry group (n � 20). Using3-dimensional reformatted computed tomography (CT) images, the volume of masticatory muscle(masseter, temporal, medial, and lateral pterygoid muscles) and various skeletal measurements (hemi-mandibular volume, ramal height, body length, mandibular length, gonial angle) were evaluated andcompared. The right-left difference was expressed by an asymmetry index {(right � left)/left, %}.

Results: The results showed that in the asymmetry group, the longer mandibular side (contralateral sideof chin deviation) exhibited longer ramal and body length, a wider gonial angle, and more hemiman-dibular volume with less medial pterygoid volume, whereas the nonasymmetry group did not show astatistical bilateral difference between the skeletal and muscular measurements. The correlation analysisshowed that patients with facial asymmetry did not have similar patterns of muscle-bone relation ascompared with the symmetrical subjects.

Conclusion: In patients with mandibular prognathism, the bilateral difference in muscle volume wouldreflect the difference in the spatial anatomy of a skeletal structure and could not predict mandibularskeletal asymmetry.© 2007 American Association of Oral and Maxillofacial Surgeons

J Oral Maxillofac Surg 65:1538-1543, 2007

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he most common type of asymmetrical facial defor-ity is asymmetrical mandibular prognathism or de-

iated prognathism,1 which is characterized by anncrease in the unilateral growth of the mandible. Itad been shown that asymmetric masticatory func-

*Associate Professor, Department of Oral and Maxillofacial

urgery, School of Dentistry, Kyungpook National University,

aegu, Korea.

†PhD Student, Department of Oral and Maxillofacial Surgery,

chool of Dentistry, Kyungpook National University, Daegu, Korea.

‡Professor, Department of Orthodontics, School of Dentistry,

yungpook National University, Daegu, Korea.

§Professor, Department of Cell and Developmental Biology,

chool of Dentistry, Seoul National University, Seoul, Korea.

�Professor, Department of Pedodontics, School of Dentistry,

yungpook National University, Daegu, Korea. d

1538

ions2,3 or occlusion4 were also important in the de-elopment of facial asymmetry. Changes in the me-hanical strain in the muscle insertion area, which areaused by masticatory force, can influence the localkeletal structure.5 The exact contribution of the pre-

¶Professor, Department of Oral and Maxillofacial Surgery, School

f Dentistry, Kyungpook National University, Daegu, Korea.

This research was supported by the Kyungpook National Uni-

ersity Research Team Fund, 2003.

Address correspondence and reprint requests to Dr Kwon: De-

artment of Oral and Maxillofacial Surgery, School of Dentistry,

yungpook National University, Samduck 2 Ga, Jung Gu, Daegu,

00-421, Korea; e-mail: kwondk@mail.knu.ac.kr

2007 American Association of Oral and Maxillofacial Surgeons

278-2391/07/6508-0016$32.00/0

oi:10.1016/j.joms.2006.09.024

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KWON ET AL 1539

iously mentioned factors regarding the developmentf facial asymmetry, however, is still vague. Moreover,ue to the lack of 3-dimensional (3D) quantitative dataegarding muscular and skeletal asymmetry, there isnly a limited understanding regarding the relation-hip between masticatory muscle and adjacent bones.ccording to the previous studies with symmetricalubjects, large masseter and medial pterygoid muscleolume showed a flat mandibular angle and a smallonial angle.6 Thicker masseter muscle also showed aorrelation with a longer ramus.7,8 It has been re-orted that bony hypoplasia was related to mastica-ory muscle hypoplasia in patients with hemifacialicrosomia.9,10 However, in terms of asymmetry,emifacial microsomia is different from deviated man-ibular prognathism or hemimandibular elongationecause the former is an example of unilateral hy-oplasia and the latter is hyperplasia. Therefore,hether this muscle–bone relationship also exists in

ommon types of facial asymmetry has not been fullynvestigated. As mandibular prognathism is the mostopular dentofacial deformity in Koreans,11,12 it isasy to compare patients with and without facialsymmetry. An investigation of the morphological re-ationship between the asymmetry of skeleton and

uscle in mandibular prognathism would providensight into the etiological background of facial asym-

etry.The purpose of the present investigation was to

valuate the 3D volume of masticatory muscles inatients with and without facial asymmetry, in ordero determine whether muscular asymmetry is relatedo skeletal asymmetry of the mandible. An investiga-ion into these relationships would be helpful in un-erstanding the factors that contribute to facial asym-etry.

aterials and Methods

MATERIALS

Forty adult patients (19 males, 21 females), whoad undergone a 3D computed tomography (CT) ex-mination before orthognathic surgery for mandibularrognathism were investigated. The subjects wereivided into 2 groups: the asymmetry group (n � 20,verage age � 22.8 years) and the nonasymmetryroup (n � 20, average age � 21.9 years). The asym-etry group was composed of patients with a man-

ibular chin deviation of over 4 mm from the facialidline with a skeletal Class III relationship. Theonasymmetry group was composed of patientshose facial midlines nearly coincided with the chinidpoint (menton and pogonion) and who had a

keletal Class III relationship. Patients with any sign of

athology in masticatory muscles or craniofacial mal-

K2

ormation such as hemifacial microsomia or a cranio-acial cleft were excluded from the study. When usinghe morphological classification by Obwegeser andakek,13 the patients in the asymmetry group were

ubclassified as having either bilateral hemimandibu-ar elongation (n � 17) or unilateral hemimandibularlongation (n � 3).

METHODS

CT Scanning and the Evaluation of MuscularVolume and Mandibular MorphologyA spiral CT scan was carried out with a HiSpeed

T-1 (GE Co, Milwaukee, WI) under conditions of 140Vp, 300 mA, with a slice thickness of 3 mm betweenhe supraorbital and the chin, parallel to the FH plane.ll CT data were stored and transferred to a personalomputer running a 3D reconstruction and the mea-urement software program V-Works 4.0 (CyberMed,eoul, Korea). With the function of grayscale thresh-lding, the muscular or osseous area of each slice wasutomatically delineated and manually edited in detaily a digitizer (Intous 2; Wacom Co, Saitama, Japan),iewing the coronal and sagittal views at the sameime. The defined muscular or osseous regions ofach axial slice were then automatically added andultiplied. The 3D reconstructed images were then

isplayed with corresponding volumetric measure-ents (mm3) (Fig 1).In order to evaluate the mandibular structure, soft

issue was virtually removed from the 3D osseous

IGURE 1. 3D volumetric image reconstruction from CT. A, Delinea-ion of muscular area of each axial slice. B, Automatic multiplication ofxial slices and display of 3D-reconstructed images (view from theottom).

won et al. Mandibular Prognathism. J Oral Maxillofac Surg007.

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1540 MANDIBULAR PROGNATHISM

tructure. Inter-landmark distances and angles werealculated between the coordinates of the skeletaltructures in 3D space, according to a previous re-ort.14 Ramal height (condyle to gonion), body lengthgonion to menton), mandibular length (condyle toenton), coronoid length (gonion to coronoid tip),

nd the mandibular angle were measured with a Vurgery 1.0 program (CyberMed). The degree of man-ibular deviation was defined as the horizontal dis-ance from the facial midline to the menton, wherehin deviation to the left was set to positive, and tohe right was set to negative values.

Data AnalysisWhen evaluating the bilateral difference of each 3Deasurement and muscular activity, the mandibular

eviation side was set to the left side in the asymme-ry group for the convenience of comparison, and theight-left differences were analyzed by the paired test. The statistical difference between the 2 groupsas compared using the Student’s t test. As the rawata of the right-left difference cannot reflect individ-al variations in volume, the ratios of the contralateraluscular volume difference were also calculated and

xpressed as asymmetry index (AI). The AI was de-ned by the right-left difference of the measurementselative to the left side; AI (%) � {(right � left)/left} �00. The statistical significance of AI was evaluatedith one sample t test. The statistical significance was

et at P � .05.Pearson’s correlation coefficients were calculated

n order to detect a relationship between the muscu-ar volume and mandibular morphology. All statisticalnalyses were performed with SPSS PC 10.0 for Win-ows (SPSS Inc, Chicago, IL).

Error of the MethodThe reproducibility of the 3D volume of the mus-

les and skeletal measurements were verified by com-aring the differences between the original and re-eated examinations of 10 patients during a 2-week

nterval. The method error was defined as the repro-ucibility of double determination and it was calcu-

ated as Se � �(� d2/2n), where d is the differenceetween the double measurements and n is the num-er of paired double measurements.15

esults

The method error of the repeated measurements ofhe masticatory muscle volume was 260.93 mm3 (dif-erences ranged between 44.40 and 626.55 mm3),nd the skeletal linear and angular measurementsere 1.99 mm (differences ranged between 0.28 and

.08 mm) and 1.28° (differences ranged between

.39° and 4.25°). The error of AI was 3.21% (differ-

nces ranged between 1.70% and 6.26%) for the mus-

K2

ular volume, 1.07% (differences ranged between.85% and 2.25%) for the linear, and 1.16% (differ-nces ranged between 0.46% and 1.97%) for the an-ular measurements of the mandible. Statistically sig-ificant differences were not detected.As we divided groups depending upon chin asym-etry, the morphology of the mandibular angle didot show mirror images even in the nonasymmetryroup. However, the asymmetry group showed statis-ically significant right-left differences in various skel-tal measurements even though there was no bilateralifference found in the nonasymmetry group.The intergroup difference in the right-left asymme-

ry of the mandibular skeleton was remarkable inatients with facial asymmetry. The asymmetry groupxhibited a longer mandibular ramus and bodyength, a larger gonial angle, and a larger hemiman-ibular volume in the mandibular elongation side, asompared with the mandibular deviation side. Theoronoid length did not show any statistical bilateralr intergroup difference. The degree of mandibulareviation was proportional to the AI of hemimandibu-

ar volume, and the mandibular volume of the devi-ted side was relatively smaller than that of the con-ralateral side (Fig 2).

When comparing the bilateral difference of theasticatory muscle volume, there were significant

ifferences regarding the medial pterygoid muscleetween the groups. On the other hand, other masti-atory muscles such as the masseter, lateral pterygoid,nd temporalis did not show any significances. Theedial pterygoid volume on the longer mandibular

IGURE 2. Correlation between the degree of mandibular deviationnd the asymmetry index (AI) of hemimandibular volume. Positive valuen the x-axis indicates the mandibular deviation to the left side andegative value indicates deviation to the right side.

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KWON ET AL 1541

ide (contralateral side of chin deviation) was signifi-antly smaller than that of the deviated side of thehin (Table 1).The asymmetry of the masseter volume showedclose relation with the asymmetry of the gonial

ngle in both groups. In other measurements, how-ver, the 2 groups showed different correlation pat-erns between the volume of the masticatory musclesnd the skeletal measurements of the mandible. In thesymmetry group, the medial pterygoid volumehowed an inverse correlation with the mandibularkeletal measurements (medial pterygoid volume:ody length, r � �0.596, P � .01). In the nonasym-etry group, the medial pterygoid volume showed aositive correlation with mandibular body length (r �.468, P � .05). The hemimandibular volume was

Table 1. THE MEAN ASYMMETRY INDEX OF SKELETAL AAND ASYMMETRY GROUPS

Asymmetry Index (AI, %) �{(Right � Left)/Left} � 100

AI of NonasymmetryGroup (n � 20)

Mean SD Min

keletal measurementsRamal height 1.71 5.02 �6.72Body length 0.64 2.95 �3.78Mandibular length 0.58 2.97 �5.63Coronoid length 0.14 3.65 �7.22Gonial angle 0.79 2.52 �3.73Hemimandibular volume �0.01 4.57 �11.96uscular volumeMasseter volume 1.83 5.40 �8.06Temporalis 1.22 5.11 �8.04Medial pterygoid �2.43 5.34 �9.34Lateral pterygoid 1.20 8.34 �13.02

The deviated site was set to the left side in the asymmetry ground **. P† value indicates the intergroup difference.

won et al. Mandibular Prognathism. J Oral Maxillofac Surg 20

Table 2. CORRELATION BETWEEN THE ASYMMETRY INDMUSCULAR VOLUME

Nonasymmetry Group

Masseter TemporalisMe

Pter

I of skeletal measurementsRamal height 0.004 �0.425 �0.Body length �0.079 �0.077 0.Mandibular length 0.051 �0.160 �0.Coronoid length 0.077 �0.134 0.Gonial angle �0.504* 0.207 �0.Hemimandibular volume 0.564** 0.051 0.

Correlation is significant at the .05 (*) and .01 levels (**).

won et al. Mandibular Prognathism. J Oral Maxillofac Surg 2007.

ore increased in proportion to the masseter volumen the nonasymmetry group (Table 2).

iscussion

A volumetric assessment with a 3D CT scan isseful in evaluating masticatory muscles.9,10,16,17 Theethod error in our study was similar to that ofrevious reports.10 The degree of muscular asymme-ry in the nonasymmetry group was similar to that ofhe normal asymmetry of control patients in othereports,9,10 when calculating the AI in the presenttudy.

In our study, regarding the asymmetric adult pa-ients, the right and left ramus and body length andandibular angle were strikingly different, whereas

USCULAR MEASUREMENTS IN THE NONASYMMETRY

AI of AsymmetryGroup (n � 20)

IntergroupDifference

Mean SD Min Max P†

6 5.63** 7.84 �10.87 19.27 �.058 3.47** 2.77 �2.18 7.89 �.018 6.05** 3.68 �3.19 11.43 �.016 �2.41 5.74 �12.85 7.54 .1230 2.22* 4.06 �3.32 10.98 .3442 2.72* 4.92 �6.26 12.03 .176

5 �0.91 8.49 �15.95 20.56 .1606 3.37 9.22 �15.05 26.38 .2134 �7.96** 6.52 �16.99 8.83 �.050 �1.09 16.57 �32.39 21.62 .537

stically significant AI at the .05 and .01 levels were expressed as *

I) OF SKELETAL MEASUREMENTS AND

AI of Muscular Volume

20) Asymmetry Group (n � 20)

LateralPterygoid Masseter Temporalis

MedialPterygoid

LateralPterygoid

0.269 �0.081 0.047 �0.503* 0.0060.251 �0.388 0.221 �0.596** �0.1010.253 �0.356 0.187 �0.679** �0.034

�0.056 0.235 �0.216 0.468* �0.062�0.437 �0.573** 0.003 �0.672** �0.054

0.326 �0.093 0.004 �0.423 �0.048

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1542 MANDIBULAR PROGNATHISM

here was no significant bilateral skeletal asymmetryn the nonasymmetry group. In our patients withsymmetry, mandibular morphology was character-zed by the elongation of one half of the mandiblemandibular body and ramus) and an increase in theonial angle, which is a characteristic of hemiman-ibular elongation.13 The longer mandibular sidehowed larger mandibular volume in proportion tohe degree of chin deviation. It has been proposedhat these types of asymmetries are related to asym-etric condylar hyperactivity, that influence not only

he growth of the condyle itself, but also the ramusnd body of the mandible.13 However, the mecha-ism of influence of the condyle cartilage on the

ength or mass of the mandible has not yet beenroven.18

The results showed that the medial pterygoid mus-le had a significant right-left difference and inter-roup difference. However, the masseter, lateralterygoid, and temporalis did not show a right-leftifference. It is interesting that the medial pterygoidolume in the mandibular elongation side decreasedn the asymmetry group. As mandibular structures cane influenced by local environmental factors in theorm of tension and pressure forces on the musclesdjacent to the bony structure,19,20 we tried to deter-ine whether muscular factors can explain mandib-

lar asymmetry.The study of adult symmetric subjects showed that

eople with a large masseter and a medial pterygoiduscle showed that they had a short anterior face andsmall gonial angle. In addition to this, there was a

orrelation between mandibular body length or ramaleight and masticatory muscle size.8,21-23 An increase

n the function of the masticatory muscle is associatedith anterior growth rotation of the mandible andell-developed angular, coronoid, and condyle pro-

ess.24 It had been reported that there is a correlationetween a larger adult masseter and medial pterygoidolume and a small mandibular angle and a longeramal height.6 Our results showed that in the non-symmetry group, the larger masseter volume waslosely related to larger hemimandibular volume andmaller gonial angle. There was also a correlationetween the larger medial pterygoid volume and a

onger body length. If this relation does exist in pa-ients with facial asymmetry, it may be expected thatheir ramal or body length is relatively longer and thatheir gonial angle is smaller in the side that has largerasseter or medial pterygoid volume. However, inatients with asymmetry, larger masticatory muscleolume did not accompany longer skeletal measure-ents. In particular, the medial pterygoid volume

ecreased in proportion to the increase in the body oramal length. These findings are contrary to previous

eports on symmetrical adult subjects.6,8 i

Asymmetrical muscular volume can be explainedy the spatial anatomy of the origin-insertion site ofhe masticatory muscles. The masseter muscle is ver-ically oriented, and the craniolateral inclination tohe midsagittal plane is approximately 7 degrees,hereas the medial pterygoid muscle shows a 20-egree inclination toward the midsagittal plane.25

hen comparing the 3D coordinates of the right andeft gonion, in patients with asymmetry, the gonion ofhe deviated side is located more laterally than that ofhe opposite side in our study. Thus, it can be postu-ated that as the distance of the origin-insertion of the

edial pterygoid decreases, the volume also de-reases. However, the bilateral spatial difference ofhe masseter muscle might be smaller compared withhe medial pterygoid because its direction is nearlyertical. As van Spronsen et al25 reported, variation inhe spatial orientation of a masticatory muscle is as-ociated with variation in mandibular morphology.ence, the bilateral difference in muscle volumeould reflect the difference in the spatial anatomy ofskeletal structure.There had been few preceding reports concerning

he muscular asymmetry in the facial asymmetry sub-ects. According to research on hemifacial microso-

ia, the degree of masticatory muscle hypoplasia pre-icted the degree of hypoplasia in their osseous originnd insertion.4,5 However, there had been no reportsn whether these also can be applied to the commonype of facial asymmetry. Our result showed that theariables of the skeletal AI that correlated with theuscular AI in the asymmetry group did not show a

orrelation in the nonasymmetry group in our study.he result showed that mandibular prognathism with

acial asymmetry did not have similar muscle-bonenteraction in the skeletal development compared

ith symmetric subjects. Therefore, it is better tossume that medial pterygoid asymmetry in mandib-lar asymmetry is an adaptation of the asymmetricunctional condition, which is caused by existingandibular asymmetry rather than a reason for devel-

pment of mandibular asymmetry.There are fundamental limitations in our research. As

he muscle volume is dependent on the number and sizef the muscle fibers, muscular activity could be regardeds being in proportion with muscular volume. However,uscular volume is not the only factor that can scientif-

cally evaluate muscular function. In order to understandhe mechanism of muscle action and its influence thor-ughly, muscular activity (electromyographic analysis),ite force, and the status of occlusal contact should benalyzed with regard to 3D skeletal deformity. In addi-ion to this, in the present study, asymmetric patientsere not compared with normal subjects who do notave a dentofacial deformity. This is due to the difficulty

n subjecting normal subjects to radiation exposure dur-

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ng 3D CT scan. As this is a cross-sectional study, weope that a longitudinal study in the future would beble to clarify the contributing factors that influencesymmetrical growth patterns.

eferences1. Melnik AK: A cephalometric study of mandibular asymmetry in

a longitudinally followed sample of growing children. Am JOrthod Dentofacial Orthop 101:355, 1992

2. Pirttiniemi PM: Normal and increased functional asymmetriesin the craniofacial area. Acta Odontol Scand 56:342, 1998

3. Poikela A, Kantomaa T, Pirttiniemi P: Craniofacial growth aftera period of unilateral masticatory function in young rabbits. EurJ Oral Sci 105:331, 1997

4. Langberg BJ, Arai K, Miner RM: Transverse skeletal and dentalasymmetry in adults with unilateral lingual posterior crossbite.Am J Orthod Dentofacial Orthop 127:6, 2005

5. Kitai N, Fujii Y, Murakami S, et al: Human masticatory musclevolume and zygomatico-mandibular form in adults with man-dibular prognathism. J Dent Res 81:752, 2002

6. Gionhaku N, Lowe AA: Relationship between jaw muscle vol-ume and craniofacial form. J Dent Res 68:805, 1989

7. Kubota M, Nakano H, Sanjo I, et al: Maxillofacial morphology andmasseter muscle thickness in adults. Eur J Orthod 20:535, 1998

8. Weijs WA, Hillen B: Relationships between masticatory musclecross-section and skull shape. J Dent Res 63:1154, 1984

9. Huisinga-Fischer CE, Zonneveld FW, Vaandrager JM, et al: Re-lationship in hypoplasia between the masticatory muscles andthe craniofacial skeleton in hemifacial microsomia, as deter-mined by 3-D CT imaging. J Craniofac Surg 12:31, 2001

0. Kane AA, Lo LJ, Christensen GE, et al: Relationship betweenbone and muscles of mastication in hemifacial microsomia.Plast Reconstr Surg 99:990, 1997

1. Gok DB, Kyung HM, Kwon OW, et al: The distribution and

trend of malocclusion patients visited at department of ortho-dontics (Korean). Korean J Orthod 19:35, 1989

2. Yang WS: The study on the orthodontic patients who visiteddepartment of orthodontics, Seoul National University Hospital(Korean). Taehan Chikkwa Uisa Hyophoe Chi 28:811, 1990

3. Obwegeser HL, Makek MS: Hemimandibular hyperplasia-hemi-mandibular elongation. J Maxillofac Surg 14:183, 1986

4. Kwon TG, Park HS, Ryoo HM, et al: A comparison of craniofa-cial morphology in patients with and without facial asymmetry.Int J Oral Maxillofac Surg 35:43, 2006

5. Houston WJ: The analysis of errors in orthodontic measure-ments. Am J Orthod 83:382, 1983

6. Takashima M, Kitai N, Murakami S, et al: Volume and shape ofmasticatory muscles in patients with hemifacial microsomia.Cleft Palate Craniofac J 40:6, 2003

7. Marsh JL, Baca D, Vannier MW: Facial musculoskeletal asym-metry in hemifacial microsomia. Cleft Palate J 26:292, 1989

8. Obwegeser HL: Mandibular Growth Anomalies: Terminology,Aetiology, Diagnosis, Treatment. Berlin, Germany: Springer-Verlag, 2001, p 142

9. Moss ML: The functional matrix hypothesis revisited. 1. Therole of mechanotransduction. Am J Orthod Dentofacial Orthop112:8, 1997

0. van Limborgh J: A new view of the control of the morphogen-esis of the skull. Acta Morphol Neerl Scand 8:143, 1970

1. Lamey PJ, Burnett CA, Fartash L, et al: Migraine and masticatorymuscle volume, bite force, and craniofacial morphology. Head-ache 41:49, 2001

2. Raadsheer MC, Kiliaridis S, Van Eijden TM, et al: Massetermuscle thickness in growing individuals and its relation tofacial morphology. Arch Oral Biol 41:323, 1996

3. Benington PC, Gardener JE, Hunt NP: Masseter muscle volumemeasured using ultrasonography and its relationship with facialmorphology. Eur J Orthod 21:659, 1999

4. Kiliaridis S: Masticatory muscle influence on craniofacialgrowth. Acta Odontol Scand 53:196, 1995

5. van Spronsen PH, Koolstra JH, van Ginkel FC, et al: Relation-ships between the orientation and movement arms of thehuman jaw muscles and normal craniofacial morphology. Eur

J Orthod 19:313, 1997