342 J Formos Med Assoc | 2011 • Vol 110 • No 5

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Journal of the Formosan Medical Association

Journal homepage: http://www.jfma-online.com

J Formos Med Assoc 2011;110(5):342–346

Journal of the Formosan Medical Association

ISSN 0929 6646

Formosan Medical AssociationTaipei, Taiwan

Volume 110 Number 5 May 2011

XMRV: Not a mousy virusPrevalence of telomerase activity in human cancerIncreased risk of mortality in overweight adultsIschemic stroke with dural arteriovenous fistulas

Case Report

Craniofacial Skeletal Dysplasia of Opposite-sexDizygotic TwinsSzu-Ting Chou,1 Yu-Chuan Tseng,1 Chin-Yun Pan,1 Jenny Zwei-Chieng Chang,2 Hong-Po Chang1*

Craniofacial skeletal dysplasia can lead to different skeletal malocclusions. Both environmental factorsand heredity contribute to the formation of malocclusions. There are strong familial tendencies in the development of Angle’s Class II and III malocclusions. Cases such as opposite-typed (Class II and III) mal-occlusions with skeletal and dentoalveolar discordance in siblings or dizygotic (DZ) twins have seldombeen reported. We describe the rare case of a pair of opposite-sex DZ twins with completely different skeletalmalocclusions, and discuss the clinical considerations for treatment. The patients were twins aged 13 yearsand 4 months. The girl had mandibular prognathism and a Class III dentoskeletal relationship, whereasthe boy had skeletal Class II with mandibular retrusion. Several morphological traits have been implicatedwith hormonal effect. However, there was no evidence of whether the masculinization effect had any impact on jaw size in the female fetus or whether this effect lasted into adolescence. We suggest that, although DZ twins share the same growth environment, genetic or other unknown extrinsic factors canresult in discordance of characteristics of the craniofacial skeleton, dentition, and occlusion.

Key Words: dizygotic twins, malocclusion, opposite-sex twins, orthodontic treatment, skeletal dysplasia

Skeletal dysplasia of craniofacial structures can

lead to different types of malocclusion.1 Skeletal

components of Class II malocclusion include:

mandibular retrusion with a normally developed

midface,2 maxillary protrusion with a normally

positioned mandible, or a protrusive maxilla with

reduced total mandibular length.3 On the contrary,

mandibular prognathism, maxillary retrognathism,

or a combination of both might contribute to the

formation of Class III malocclusion.4

Both hereditary and environmental influences

have been suggested as etiological factors of skele-

tal dysplasia and its associated malocclusions.5–8

Kawala et al have used twin-method analysis to

examine malocclusions and have shown that the

differences in monozygotic (MZ) twins depend

©2011 Elsevier & Formosan Medical Association. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1Department of Orthodontics, Kaohsiung Medical University Hospital, and Faculty of Dentistry, College of Dental Medicine,Kaohsiung Medical University, Kaohsiung, and 2Department of Orthodontics, National Taiwan University Hospital, andSchool of Dentistry, College of Medicine, National Taiwan University, Taipei, Taiwan.

Received: August 7, 2008Revised: February 25, 2009Accepted: April 8, 2009

*Correspondence to: Dr Hong-Po Chang, Department of Orthodontics, Kaohsiung MedicalUniversity Hospital, 100 Tzyou 1st Road, Kaohsiung 80756, Taiwan.E-mail: hopoch@kmu.edu.tw

Skeletal dysplasia of dizygotic twins

J Formos Med Assoc | 2011 • Vol 110 • No 5 343

on environmental activity, whereas in dizygotic

(DZ) twin pairs, they can be because of genetic

or environmental factors.5

It is accepted that strong familial tendencies

exist in the development of Angle’s Class II and

Class III malocclusions,6 and there are highly sig-

nificant correlations between those parents and

offspring and between siblings.9 Cases such as

opposite-typed (Class II and III) malocclusions

with skeletal and dentoalveolar discordance in

siblings of the same family or in DZ twins have

seldom been reported. Harris and Kowalski even

have reported that it is rare to find a patient who

differs greatly from the rest of his or her family.10

We present the rare case of a pair of opposite-sex

(OS) DZ twins with opposite-typed skeletal mal-

occlusions, and discuss the clinical considerations

for treatment.

Case Report

OS DZ twins, aged 13 years and 4 months, were

first seen by a clinician for orthodontic consulta-

tion. Both patients were in permanent dentition.

The girl had a protruded mandible and anterior

crossbite since early childhood in primary denti-

tion (about 3–4 years old), whereas the boy had

protruded and crowded upper teeth and retruded

chin in mixed dentition at age 10 years. Past med-

ical history revealed no allergies or medical prob-

lems. A review of the family history revealed that

the parents’ skeletal patterns were not affected by

any osseous dysplasia, and were defined as skeletal

Class I relationship. The pedigree of the family with

regard to malocclusion is illustrated in Figure 1.

Clinical examination of the girl showed a

concave profile, with protruded chin (Figure 2),

Class III molar relationship, anterior crossbite

and deepbite (Figure 3). Cephalometric analysis

revealed skeletal Class III with maxillary deficiency

and mandibular prognathism, and orthodiver-

gent facial pattern. Angle’s Class III malocclusion

was diagnosed. Orthodontic treatment was per-

formed by maxillary protraction device combined

with chin cup traction (constructed of rapid

maxillary expander and high-pull chin cap), which

caused maxillary advancement and mandibular

clockwise rotation. This was followed by extrac-

tion of the upper second and lower third molars.

Then, full-mouth, fixed edgewise appliances and

Class III elastics were applied for precise tooth

movement and final settling of the dentition.

Total treatment time was 1 year and 8 months.

Class I Class I

Class II Class III

Figure 1. Pedigree of family regarding malocclusion.

A B

C D

Figure 2. (A, B) Pretreatment frontal and lateral photo-graphs of the Class III girl and (C, D) her Class II twinbrother at the same age.

S.T. Chou, et al

344 J Formos Med Assoc | 2011 • Vol 110 • No 5

The treatment results were satisfactory, including

well-aligned dentition, good intercuspation of

posterior teeth, and Class I canine and molar re-

lationships were achieved (Figure 3). The facial

profile also improved (from concave to straight)

(Figure 5).

For the boy, a convex profile was noted, with

retruded chin (Figure 2), protruding upper front

teeth, dental crowding, Class II molar relation-

ship, and anterior deepbite (Figure 4). Skeletal

Class II with mandibular retrusion and hyperdi-

vergent facial pattern were revealed by cephalo-

metric analysis. The patient was diagnosed with

Angle’s Class II malocclusion. The treatment

method included full-mouth, fixed appliance

therapy. Four bicuspids were extracted to relieve

anterior crowding and retract the upper front

teeth. An anterior bite plate combined with Class II

elastics were used to facilitate forward movement

of lower dentition. After 2 years and 5 months

of orthodontic treatment, anterior crowding and

deepbite were resolved and dental occlusion was

corrected to a Class I relationship (Figure 4). The

protruded lip profile also improved (Figure 5).

Discussion

MZ twins are genetically identical, whereas DZ

twins have different genotypes, which is also the

case with other siblings. One would normally ex-

pect OS twins to be more different than same-sex

(SS) DZ twins, because the former have different

sexes as well as genetic variations. In fact, OS twins

are more alike than SS DZ twins in several charac-

teristics, due to the effects of hormonal transfer.11

The reason for the discordance in OS DZ twins is

more complicated than in SS DZ twins because

prenatal sex hormone (androgen) transfer in utero

can masculinize the female fetus or feminize the

male fetus in a behavioral11 or morphological11,12

manner. Several morphological traits, such as in-

creased tooth crown size12 and smaller finger-

length ratios13 in female OS DZ twins, and

A B

C D

Figure 3. Intraoral photographs of the Class III twin girl: (A, B) pretreatment and (C, D) post-treatment.

Skeletal dysplasia of dizygotic twins

J Formos Med Assoc | 2011 • Vol 110 • No 5 345

resemblance to parents,11 have been implicated

with hormonal diffusion by maternal circulation,

or directly through the fetal and placental mem-

branes and amniotic fluid.14 In our patients, the

twin girl had a protruded mandible since very

early in life. However, there was no evidence of

whether the masculinization effect had any im-

pact on jaw size in the female fetus and whether

this effect lasted into adolescence. Further study

is needed to investigate this unknown field.

In our patients, the Class III girl had mandibu-

lar prognathism, whereas her twin brother was

skeletal Class II. According to Schulze,15 the con-

cordance for mandibular prognathism among

twin pairs was 81–83% for MZ but only 10.0–

13.3% for DZ twins, which suggests that, besides

genetic factors, environmental influences and

functional adaptations explain more than we

believed.5 Heredity dominates skeletal areas of

craniofacial structures,6,16 including mandibular

linear measurement,7 facial angles,17 and vertical

dimension of the craniofacial complex,18 whereas

A B

C D

Figure 4. Intraoral photographs of the Class II twin boy: (A, B) pretreatment and (C, D) post-treatment.

C D

BA

Figure 5. (A, B) Post-treatment frontal and lateral photo-graphs of the Class III girl and (C, D) her Class II twinbrother.

S.T. Chou, et al

346 J Formos Med Assoc | 2011 • Vol 110 • No 5

environmental factors dominate the dental areas

of craniofacial structures.16 It has also been re-

ported that a strong genetic component overlaid

by functional adaptation is most prominent in

the dental area.8

Skeletal pattern is believed to be more re-

lated to genetic factors, but significant differences

between Class II and Class III patients have been

found for SNA angle, gonial angle, Ar-Go length,

and U1-nasal floor angle, and are assumed to be

influenced by the environment.6 For example,

nasoairway obstruction (enlarged tonsils) results

in mouth breathing and anterior positioning of

the tongue, which leads to protrusion of the man-

dible in skeletal Class III;19 nasoairway obstruc-

tion (adenoids), thumb-and-finger sucking, and

lip biting habits also induce downward–backward

rotation of the mandible and overgrowth of the

maxillary alveolar process in skeletal Class II.20

Distinguishing the variations that result from

genetic and environmental factors in Class II and

III malocclusions is important for clinical ortho-

dontists, because it is the region that is affected

by environmental factors that can be improved

by orthodontic treatment.6

Baccetti et al21 have assessed the ideal treatment

timing in dentofacial orthopedics and have con-

cluded that Class II treatment is most effective dur-

ing the circumpubertal growth period (CVM stage:

CS3–CS4 interval), in which the mandible is at

maximum growth spurt. However, prepubertal

treatment of skeletal Class III is effective in both

jaws, but later into puberty, the treatment effect is

greater in the mandible only.21 We must keep in

mind that redirection of mandibular growth pat-

tern can only be modified within biologically tol-

erated limits,8 therefore, long-term follow-up of

maxillary and mandibular developmental changes

is important until the cessation of skeletal growth.

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