Mandibular prognathism

Mandibular prognathism Alex Jacobson, M.D.S., MS., C. 8. Preston, B.D.S., Dip.Orth., dohnm~eshurg, South Afrior

Ph.D.,* W. G. Evans, B.D.S., Dip.Orth., and P. 1. Sadowsky, B.D.S., Dip.Orth.

T

rue mandibular prognathism is a skeletal imbalance, usually accompanied bp a Class 111 dental occlusion. It should be distinguished from a pseudoprognathism, which is essentially the result of a mesial thrust of t,he mandible. Mandibular prognathism and Class III antcroposterior skeletal imbalances are usually attributable to one or more of the following component variables: 1. The mandible may he too large relative to t.he maxilla. 2. The maxilla may hc too small relative to the mandible. 3. The maxilla ma)’ be retropositioned relative to the mandible. 1. The mandible may be positioned too Ear forward relative to the maxilla. 5. A forward rotation of the mandible relative to the cranium will cause the chin point to move into a horizontally more protrusive positioll. A prognathous mandible map thereby result, together with a reduction in lower anterior facial height. The relative proportions of the faGa components have bern studied frequently by means of cephalometric* roentgenographp. Class III malocclusion has been divided into two basic morphologic types the divergent and the convergent types (Figs. 1 and 2). Typically characteristic features of the divergent Class I II pattern arc palatal, occlusal, and mandibular planes which diverge, an obtuse gonial angle, and an anterior open-bite in extreme cases. The convergent Class III pattern features l)alat,al, o~lusal, ancl mandibular planes which tend toward parallelism, an acute genial ant1 a deep anterior overbite. Sassouni’+. 25.si refers to the two extremes as “skeletal open-bite” and “skeletal deep-bite” cases. Schud.v”” introduced the terms hyperdive,~~/e)~t :~rrd hy~&i~eryesrt. \\‘alther”!’ and Sassouni and Nanda” list a number of features frcyucntly associated with BIIglC,

Yroni thr: Department

of

Orthodontics,

University

of

tht:

IVitwrtrrsrand,

Ikrktal

School. The first portion of first nnnu:~l meeting Nowmher ’ t 1972 . “Winner of the Elide

140

this of Gibbs

article was presented at the Northeastern Society Award,

1973.

the of

Kftaefrch

Orthodontists,

Setision

of tlw fiftyBoston, Mass.,

Volume 66 Nuna her 2

Fig.

1. Divergent

types

of Class

craniofacial

these morphologic extremes. The vast majority between these extremes.

skeletal

patterns.

of cases, however, lie somewhere

Incidence

Reports of the incidence of Class III malocclusion have varied widely. In 1946 Huber and Reynolds I4 found a 12.2 per cent occurrence among 500 university students, while Ast and associates,z in 1965, reported an incidence of only 1.6 per cent among 1,413 children 15 to 18 years of age. The results of some surveys of the incidence of Class III are summarized in Table I. Some of the variation in the observations presented in this table may be due to differences in criteria and method. Brash, McKeag, and Scott4 likewise compiled a table recording the incidence of Class III malocclusion as reported by various investigators. Their range extends from 1 per cent to 12.2 per cent (the latter figure being that of Huber and Reynolds), Most studies reflected an incidence below the level of 5 per cent. Inheritance

Serious study of human inheritance is complicated by the intricacy of man’s genetic complex. However, there appears to be a strong genetic influence determining the occurrence of Class III. characteristics,

142

Jacobson

et

d.

Fig. 2. Convergent

Table

I, Incidence

of Class

Investigator(S)

Massler

and

Goose et al. Ast et al.

Reynolds

of Cl1 3s~ III craniofacial

III malocclusions

I

Angle Ainsworth Huber and Seipel Seipel Krogman

types

Date

I

by

various

pattern.

investigators

Sample

1907 1925

Several thousand cases 4,170 school children

1946 1946

500 students 137 Swedes 21 years old 414 Swedes 12 years old Males Females 2,758 schoolchildren 14-18 years 2,956 children 7-15 years old 1,413 children 15-18 years old

1951 Frankel

as reported

skeletal

1951 1957 1965

Inddence (per cent) 4.2 1.35 12.2 4.0 2.7 2.2 1.2 9.4 2.91 1.6

Among the most famous examples of inheritance of physiognomy must surely be the familial characteristics of the Hapsburg family the famous prognathous Iower jaw, the protruding lower lip (the “Hapsburg lower lip”), and the wellknown “Hapsburg nose” with its prominent dorsal hump. Of forty members of the family of whom sufficient records remain, thirty-three showed pr0gnathism.l” According to Stiles and Luke,37 mandibular prognathism is inherited via a dominant gene with an unknown degree of reduced penetrance. Litton and

Mandiibulm

Fig. 3. Cephalometric in the study.

tracing

showing

the

conventional

anatomic

prognathism

points

and

planes

143

used

associates,18 who studied the familes of fifty-one probands, fully support the assumption that Class III characteristics are genetically transmitted. Purpose

The purpose of this article is to develop a fuller understanding and appreciation of the problem of mandibular prognathism from an orthodontic and, to no less extent, a surgical viewpoint. An endeavor is made to identify various types of skeletal Class III pattern by comparing lateral head-film tracings of Class III malocclusions with each other and with those of normal occlusions. In an effort to understand growth changes, prognathous skeletal patterns in children are compared with those of adults. Materials

and

methods

The data in this study were collected from pretreatment lateral cephalometric radiographs of 149 patients with Class III malocclusion. This sample comprised thirty-two adult males and thirty-four adult females and thirty male and fiftythree female children (age range, 6 to 16 years). Tracings of these head-films were compared with those of 112 persons with normal occlusion, the selection being based upon excellence of occlusion. This “normal” sample comprised twent,y-two adult males and twenty-five adult females and thirty-seven male and twenty-eight female children (age range, 6 to 16 years). All measurements were recorded from tracings of lateral head films taken

Jacobson

144

Fig. in the

4.

Am. J. Orthod.

et al.

Cephalometric

August1974

tracing

showing

various

linear

and

angular

measurements

used

study.

with the teeth in occlusion. The head of the condyle was traced from a film taken in the “mouth-open” position and was subsequently transferred to the tracing of the “teeth-in-occlusion” film. The following points, planes, and angles were used, and most are illustrated in Figs. 3 and 4: orbitale, porion, nasion, anterior and posterior nasal spine, subspinale (point A), supramentale (point B) , infradentale, pogonion, gnathion, menton, gonion, sella turcica, Frankfort horizontal plane, mandibular plane, occlusal plane, palatal plane, SNA, SNB, ANB, OM, and gonial angles, and angle of inclination of upper and lower incisor teeth. All of these have been identified previously on tracings according to well-known accepted definitions and criteria (for example, those of Down& 6 and Steiner”4-3”). For points not in the midline, averages between the two sides were used. The following additional measurements may require definition : UFH

-Linear

upper

nasion

LFH

-Linear from

ATFH PTFH Overbite

to

facial

height

lower nasion

facial to

plane]

measured

along

height

anterior

-Linear

posterior

-Linear

distance

between

and

mandibular

(palatal

plane

to

menton)

measured

total

facial

total

facial

height

from

the

line

(sella-gonion).

horizontal

incisors

projections

to the

of

nasion-menton

the

incisal

-Upper

facial

height

to total

facial

height,

expressed

as a percentage.

-Lower

facial

height

to total

facial

height,

expressed

as a percentage.

CL

-Linear

length

a perpendicular

along from

the

mandibular

pogonion

meets

plane this

edges

of

line.

L/T%

corpus

along

(nasion-menton).

height

U/T%

where

line

menton.

-Linear

maxillary

(nasion-palatal

menton.

plane.

from

gonion

to

a

point

MamWmlar RH

-Linear of the

TL Symph. Symph.

ramus ramus

height

MFD MFI UIH

point -Midfacial -Linear

edge.

LIH

menton incisal -Linear menton -Incisor -Linear

II

UMH LMH MI

the

along

intersection

u

with

the

line

tangent

to

mandibular

the

plane

145

posterior

border

to a perpendicu-

lar to the head of the condyle. -Linear total length, being the greatest distance between the outline of the condylar head and gnathion. H-Linear symphyseal height from infradentale to menton. D-Linear symphyseal depth, widest anteroposterior dimension of symphyseal outline. -Linear midfacial mandibular plane -Linear midfacial

MFH

measured

from

prognathism

plane -Linear plane -Molar

height from point midway along SN line to midway between gonion and menton. depth from a point midway along nasion-menton

midway along sello-gonion line. index, midface height/depth, expressed upper incisor height is measured as a line

between

the

lower

incisor

height

line index, upper

between upper molar

through lower through index,

palatal

plane

is measured

and as

perpendicular projection

the to

along

point

on

line

as a percentage. projection along a

a

a

the the

to

nasionupper nasion-

menton and a perpendicular to the lower incisal edge. to lower incisor height, expressed as a percentage. height measured as a perpendicular from the palatal

the mesiobuccal cusp molar height measured the mesiobuccal cusp upper to lower molar

to the occlusal plane. as a perpendicular from the mandibular to the occlusal plane. height, expressed as a percentage.

All linear measurements were read to the nearest 0.5 mm. on a standard millimeter rule. Degrees were likewise measured to the nearest 0.5 degree on a standard protractor. The nature of craniofacial form is a composite assembly pattern of the various skeletal components in this region. The craniofacial proportions within the skeletal framework differ markedly in normal occlusions as well as in skeletal malocclusions, such as that of mandibular prognathism. Even if the dimensions of the individual bones are within the range of normal, a prognathous appearance may still result from an unfavorable combination of the various components4” The profile of the face is thus dependent upon various combinations of these craniofacia1 skeletal components in a horizontal as well as a vertical dimension. Balanced facial profiles are achieved only when there exists a favorable combination of these components in both dimensions. The first phase of the present study is devoted to the determination of the resultant profile relationship of the upper and lower jaws to the cranium, regardless of any variables within the skeletal composite. Phase I: Range of prognathism To establish the normal range of prognathism in each jaw, two angular measurements (SNA and SNB) from the control sample were employed. One standard deviation on each side of the mean for each of the two measurements was regarded as the range of normal. In males the “normal range of prognathism” in the ma,xilla extended from 79.0 to 87.0 degrees (SNA mean of normal occlusion being 83.20 + 4.09 degrees)

146

Jacobson et al.

Table

II.

Incidence

Bm. J. Orthod. August 1974

of the

categories

of Class

III craniofacial

skeletal

Children

Adults

No.

A R c D E F G

13 9 7 2 1 0 0

- Totals

- 32

Male

Female

Male

Group

% 40.63 28.12 27.87 3.12 6.25 0 0 loo

profiles

YO

No.

55.89 23.53 5.88 5.88 5.88 2.94 0

4 3 18 0 2 3 0

NO.

19 8 2 2 2 1 0 34

loo

30

Female

/

%

NO.

13.33 10.00 tio.00 0 6.67 10.00 0

7 4 31 0 3 4 4

loo

3

/

% 13.31 7.55 58.48 0 5.66 7.55 7.55 loo

and in the mandible from 77.0 to 85.0 degrees (SNB mean of normal occlusions being 81.11 t 4.03 degrees). In females the “normal range of prognathism” in the maxilla extended from 78.5 to 86.0 degrees (normal SNA mean being 82.26 k 3.83 degrees) and in the mandible from 76.0 to 83.5 degrees (normal SNB mean being 79.66 i- 3.88 degrees). The adult and child Class III malocclusions were then divided into the following categories (Fig. ,5) : Group Group Group Group Group Group Group

8. Maxilla within the “normal range of prognathism,” mandible beyond the normal range. B. Maxilla below %ormal range of prognathism,” mandible within the normal range. C. Maxilla and mandible within the %ormal range of prognathism.” D. Maxilla below “normal range of prognathism,” mandible beyond normal range of prognathism. E. Maxilla and mandible beyond “normal range of prognathism.” F. Maxilla and mandible below %ormal range of prognathism.” ‘kormal range of prognathism,” mandible I)elolv normal G. Maxilla within

range. The incidence of the various groups is listed in Table Il. Findings. Adults. Group A comprised the largest percentage of Class III malocclusions, namely, 40.63 per cent and 55.89 per cent in males and females, respectively. In both sexes, group B comprised the second largest percentage of Class III cases, namely, 28.12 and 23.53 per cent in males and females, respectively. The largest sex difference occurred in (+roup C. In males, the incidence of maxillas and ma,ndibles within the range of “normal prognathism” is 21.87 per cent, whereas in females the incidence is considerably less (5.88 per cent). Groups 11 to (4 combined formed less than 15 per cent of the total sample of Class III malocclusions in both sexes. Children. In this sample the percentage incidence of the groupings is different from that of the adult series. The largest group in this series is Group C, in which both maxilla and mandible are within the range of “normal prognathism.” The remaining approximately 40 per cent in both sexes is randomly distributed among the remaining Groups A, B, E, E’, and G.

OF PROGNATHlSM

Fig.

5.

normal thism,”

The

four

range of mandible

most

common

prognathism.” beyond the

prognathism, mandible within normal range of prognathism. mandible beyond normal range

categories Group normal

of

mandibular

A: Maxilla within range. Group 6:

the normal range. Group D: Maxilla of prognathism.

Group below

prognathism

compared

the “normal Maxilla below C:

Maxilla normal

and range

range normal

to of

“the

prognarange of

mandible within of prognathism,

In Class III malocclusion, the forward growth of the mandible tends to exceed that of the maxilla. Angle, as early as 1907, noticed that Class III malocclusion, if allowed to develop, always progressed until the mesial occlusion became greater. In his cephalometric study of prognathism, Bjork” noted that over the long growth range the chin tended to become more prognathic in the face than the maxilla. Seipe132 and Lande17 commented on this phenomenon, as did Tweed,3s who contended that both categories of Class III malocclusion are accompanied by a “booming type C growth trend. In this the growth pattern of the lower face is growing downward and forward more rapidly than the middle face.” As a general rule, according to Graber,‘? a Class III face shows a dominant horizontal direction and tends to become “more Class III” or prognathic. This phenomenon of excessive mandibular growth explains the high incidence Discussion.

148

Fig. 6. occlusions at sella.

Ant. J. Orthod. 1974 Azwust

Jacobson et al.

Craniofacial skeletal are compared by

patterns of superimposing

adult the

male Class representative

III

malocclusions outlines on

SN,

and normal registered

of maloccIusions in the Group C category (maxilla and mandible within the range of “normal prognathism”) in children and the lesser incidence of this group in the adult series. With growth from childhood to adulthood, the proportions in the groups tend to alter. The high incidence of malocclusions in the Group C category in children (approximately 60 per cent in both sexes) is reduced to 22 per cent and 6 per cent in adult males and females, respectively. By the same token, the excessive forward growth of the mandible in this type of malocclusion accounts for the prctlominancc of the (:roup A category (maxilla within the “normal range of prognathism,” mandible beyond the normal range) observed in adults (40.6 per cent in males, 56 per cent in females), Sanborqz3 using a similar classification for the combined sexes in his Class III study, likewise reported the prevalence of Group A over Groups B and C. Phase

II: Craniofacial

parameters

The next phase of the study is devoted to the determination of skeletal craniofacial differences between normal occlusions and Class III malocclusions and between Class III malocclusions in children and in adults. The means, standard deviations, standard errors of the means, and coefficients of variation were calculated for all measurements. These are listed in Tables III to VIII.” Statistical differences between the means of the various groups were calculated by means of the Student’s t test, taking 0.05 as the standard level of probability, In an effort to gain a general impression of the basic and sex differences *Detailed

statistical

tables

are listed

in Tables

IX

to XIV.

Mandibdar

Fig. 7. Craniofacial skeletal patterns of occlusions are compared by superimposing

adult the

female Class representative

prognathism

III malocclusions outlines on

SN,

149

and normal registered at

sella.

b Fig. 8. Representative normal mandibles of symphysis.

outlines superimposed

MALE

of the male and female adult Class along the mandibular plane registered

III mandibles at posterior

and border

Am. J. O&hod. August 1974

Jacobson et al.

150 Table

Ill.

Craniofacial

malocclusion

in adult

and

dental

measurements

in

Normal Xean

Parameter Jnw

SNh SNH ANR

relationship angle angle angle

Craniofncial

occlusion

and

Class

III

Class III Mean

Difference 5% level

at

Sig.

83.20 81.11 2.09

80.63 86.35 -5.72

Sig. Sig.

78.50 59.00 74.00 133.00 87.59 44.41 55.59 112.50 76.59 68.14

74.12 56.09 76.29 132.40 85.32 42.45 57.55 107.90 72.50 67.35

Sig. Rig. Not Not Not Sig. Sig. Sig. Sig. Not

f-56.09 95.00 132.40 124.40 30.45 17.27 12.73 3 7.45 16.61

65.35 92.50 136.30 133.20 33.10 19.13 13.97 36.16 13.73

Not sig. Not sig. Sig. Sig. Sig. Not Not Not Sig.

21.52

29.93

Sig.

5.84

7.37

Sig.

25.52

16.59

Sig.

5.36 .79 30.45 47.00 64.87 25.05 37.05 67.88

4.89 1.25 30.65 46.26 66.37 25.41 31.68 80.87

Sig. Not Not Not Not Not Sig. Sig.

relationships

SN UFH LPH ATFH PTFH U/T% L/T% M.F.H. M.F.D. M.F. index Mandibular morphology R.H. CL. T.L. Gonial angle Mand. plane OM Oct. plane Symph. H Symph. D Tooth position and 1 to NA (degrees) i-to Nh r to NB

normal

males

(mm.) (degrees)

Tto NB (mm.) Overbite UIH LIH Tncisor index UMH LMII Molar index

sig. sig. sig.

sig.

asad planes

sig. sig. sig.

relationship

sig. sig. sig. sig. sig.

between adult normal occlusions and adult Class 111 malocclusions and between child and adult Class III malocclusions, representative outlines of the mean facial patterns of eaeh group were constructed. The various anatomic points were plotted from the calculatetl means of the various angles and dimensions. By locating the main bony landmarks, it was a reasonably simple procedure to draw a representative outline of the facial skeleton and teeth for each group (Figs. 6 to 16).

Volume Number

Mandibular

66

2

Table IV. malocclusion

Craniofacial and in adult females

dental

Normal Mean

Parameter Jaw SNA SNR ANR

relationship angle angle angle

Cmniofntial SN UFH LFH ATFH PTFH

in normal

occlusion Class IZI Mean

and

151 Class Ill

Diference tct 5% level

82.26 7936 2.60

79.91 85.25 -5.39

Rig. Sig. Sig.

72.00 52.92 64.28 117.20 76.16 45.19 54.81 98.36 70.52 71.85

68.58 53.26 69.08 122.30 75.68 43.62 56.38 98.89 68.61 69.56

Sig. Not Sig. Sig. Not Sig. Sig. Not Sig. Sig.

59.60 55.68 119.60 123.10 30.52 16.32 14.00 31.16 14.30

60.21 88.55 127.40 131.80 34.93 19.09 15.99 32.21 12.62

Not Sig. Sig. Sig. Sig. Sig. Sig. Not Not

relationships

U/T% L/T% M.F.H. M.F.D. M.F. index iZlandibular Morphology R.H. CL. T.L. Gonial angle Mand. plane OM Oct. plane Symph. H Symph. D Tooth position and 1 to NA (degrees) Ito NA (mm.) ‘i-to NB (degrees) -i-to NB (mm.) Overbite UIH LIH Incisor index UMH LMH Molar index

Differences occlusion and

measurements

progsnthism

and

sig.

sig.

sig.

planes sig.

sig. sig.

relationship 21.64 5.08 24.58 4.76 2.96 28.36 41.20 68.92 23.20 32.16 72.23

26.05 6.80 18.00 3.95 2.55 29.05 41.82 69.69 23.53 28.68 80.87

Sig. Sig. Sig. Not Not Not Not Not Not Sig. Sig.

sig. sig. sig. sig. sig. sig.

in craniofacial skeletal and dental patterns between normal Class Ill malocclusion in adults. Comparison of the data recorded

for the adult normal sample with that of the adult Class III sample revealed differences lying mainly in the skeletal parameters (Tables III and IV Figs. 6, 7, and 8). Skeletal diflerevlces. The most significant difference between the Class III adult samples and the normal occlusion sample was that found between the re-

152

Am. J. Orthod. August 1474

Jncobson et a’l.

IO /

l-

CO-

DEGREE5

110

I20

115

130

135 141

45

I20 125 IJO 13

1ILO-

is to 145

FEMALES

MALES GONIAL Fig. 9. Histograms showing in adult male and female occlusion sample occurred

lls- 120-125

ANGLE

the distribution and incidence of gonial angle measurements Class III malocclusions. The mean gonial angle in the normal within the shaded range (that is, 120 degrees to 125 degrees).

q&i\-c ANB angles. In normal adult females the mean ANB angle is 2.60 degrees, whereas in Class III adult females it is -5.39 degrees (t = 11.25). Male sample tlata for ANB measurements arc 2.09 degrees in normal occlusions and -5.72 degrees in Class III malocclusions (t = 11.73). These marked ANB angle differences reflect a forward positioning of the Class III mandible relative to the anterior cranial base. This relationship was recordetl by the mean SiW angle measurements, which are significantly larger in the Class III sample by about 5.5 degrees. A comparative maxillary rctrusion accentuates the typical Class III profile. This retropositioning may be partly associated with the shorter anterior cranial base found in the Class III sample, but it is also a result of a distal location of the maxilla relative to the anterior cranial base, as determined by measuring the SNA angles which arc slightly but nevertheless significantly smaller in the Class III sample. General morphologic differences in the mandibles of persons with Class III nm1occIusions and normal occlusions included significantly greater mean total effective lengths of the Class II1 mandibles, especially among females (Fig. 8). Further comparison revealed that there were no significant differences between mean ramns heights and mean corpus lengths, except in Class III adult fcmalcs who had a significantly greater mean corpus length (Table IV). The greater total cffecti\e length of the Class III mandible may be attributed to a “straightj litle” morphology in Class III mandibles. The gonial angles in the Class III samples arc significantly more obtuse than those of persons with normal occlusions (Fig. 8). The normal female mean gonial angle is 123.1 degrees, whereas the fcmalc Class III mean is 131.8 degrees, the “t” value being 5.49. The

Volume Nunlber

Mandibular

66 2

prognathism

normal male mean genial angle is 124.4 degrees, and the Class III 133.2 degrees (t = 5.49) (Fig. 9, Tahles III and IV).

153

male mean is

The mean mandibular pIane angle in the Class III sample is markedly steeper than that seen in normal occlusion. Males showed no significant differences in the occlusal plane and OM measurements between the Class III and normal occlusion samples. In females, however, both. occlusal plane and OM angles in Class III malocclusions are significantly larger than those corresponding readings in normal occlusions. The bony sgmphysis of the mandible is generally thicker in the male normal occlusion sample than in Class III malocclusions. The symphyseal heights of the Class III sample do not differ significantly from those of normal occlusions. The female Class III skeletal profile tends toward being more divergent than that of normal occlusion, The anterior facial height in Class III malocclusions is significantly greater than that of normal occlusions (normal female mean is 117.20 mm., Class III female mean is 122.30 mm., t = 2.76). The mean posterior facial heights in the two adult female groups are not significantly different from each other. Comparison of Class III and normal males revealed no significant differences between these facial dimensions. Lower facial heights, expressed as a percentage of anterior total facial height, are significantly greater in the Class III malocclusion sample than in the normal occlusion sample This difference is associated with the predominantly divergent type of Class III patterns recorded in the sample, namely, high mandibular planes, high OM angles, and obtuse gonial angles (Tables III and IV). Denfnl difereaces. Compensating for the apical base dysplasia in Class III malocclusion, the lower incisors are relatively upright (female : normal mean is 24.58 degrees, Class III mean is 18.00 degrees, t = 1.14 ; male: normal mean is 25.52 degrees, Class III mean is 16.59 degrees, t = 4.82). Class III upper incisors are relatively proclined (female : normal mean is 21.64 degrees, Class III mean is 26.05 degrees, t = 2.67 ; male : normal mean is 21.52 degrees, Class III mean is 29.93 degrees, t = 4.65). Except for the linear measurements relating the lower incisors to the NB lint, all the other readings for the teeth in both sexes are not significantly different in Class 111 malocclusion from the normal occlusions (Tables III and IV). Su?Hnlctry. In sum, Class III malocclusions differ from normal occlusions not only in the characteristic mandibular protrusion but also in having a relatively distally positioned maxilla associated with a shorter anterior cranial base. The more obtuse gonial angle and the steeper mandibular plane contribute to a relatively greater effective total mandibular length in Class III malocclusions as compared to the normal sample. The differences in the dental parameters appear to be compensatory responses to the anteroposterior dysplasias of the apical bases in Class III malocclusions. The general tendency in the Class III sample is toward an “open-bite” or divergent skeletal type pattern. Differences Class

III

in malocclusions.

craniofacial

and

dental

As anticipated,

patterns

practically

between

child

and

adult

all the mean linear param-

Am. J. Orthod. August 1974

Jacobson et al.

154

Table

V. Craniofacial

and

dental

measurements

in child

and

adult

Class

III malocclusions

in males Children mean

Paricmeter

Adult mean

Diference 5%

level sig.

at

Jnw relationship SNA SNB ANB

angle angle angle

~rnniofacial

80.63 86.35 -5.72

Not Sig. Sig.

G9.69 49.75 63.13 112.90 72.09 44.13 56.87 91.31 66.19 72.95

74.12 56.09 76.29 132.40 85.32 42.45 57.55 107.90 72.50 67.35

Sig. Sig. Sig. sig. Sig. Sig. Sig. Sig. Sig. Sig.

56.06 81.47 114.30 128.40 32.11 16.23 15.92 29.34 14.16

65.35 92.50 136.30 133.20 33.10 19.13 13.97 36.16 13.73

Sig. Sig. Sig. Sig. Not Sig. Sig. Big. Not

23.78

29.93

Sig.

4.70 19.64 3.73 2.02 26.31 39.34 67.11 20.28 27.63 73.47

7.37 16.59 4.89 1.25 30.65 46.26 66.37 25.41 31.68 80.87

Sig. Sig. Not Not Sig. Sig. Not Sig. Sig. Sig.

relationships

SN UFH LFH ATFH PTFH U/T% L/T% M.F.H. M.F.D. M.F. index IKandibzslar Morphology R.H. C.L. T.L. Gonial angle Mnnd. plane OM Oct. plane Symph. H Symph. D

Tooth position 1 to NA Zto Nk

80.41 82.09 -1.44

and (degrees) (mm.)

Tto NB (degrees) i-to NB (mm.) Overbite UIH LIH Incisor index UMH LMH Molar index

and, planes

sig.

sig.

relatiolzship

sig. sig.

sig.

etcrs arc statistically significantly larger in the adult group than the corresponding values in children (Tables V and VI). Only symphyseal depth did not show a significant increase. Marked differences were also observed between child and adult Class III malocclusions in the relative spatial orientation and proportions of the craniofacial skeletal structures (Figs. 10, 11, and 12). These differences are reflected as angular or ra.lculated ratio variations, The dominant growth of the mandible

Volunae66

Mandibuhr

Number 2

Table Vi. Craniofacial

and dental

measurements

155

prognathism

in child and adult Class Ill malocclusions

in females Children mean

Parameter

Adults mean

Difference at 5% level

Jazv relationship SNA SNB ANB

79.40 80.95

angle angle angle

Crnniofacial

U/T% L/T% M.F.H. M.F.D. M.F. index

Xandibdar

Morphology

Not Sig. Sig.

65.81 47.59 59.64 107.20 6’6.09 44.37 55.63 85.81 63.14 73.83

68.58 53.26 69.08 122.30 75.68 43.62 56.38 98.89 68.61 69.56

Sig. Sig. Sig. Sig. Sig. Sig. Sig. Not Sig. Sig.

51.28 77.66 108.80 128.30 33.90 15.96 17.99 27.02 12.53

60.21 88.55 127.40 131.80 34.93 19.09 15.99 32.21 12.62

Sig. sig. Sig. Sig. Not Sig. Sig. Sig. Not

25.60 4.51 20.16 3.74 1.35 24.52 36.78 66.81 18.57 26.31 70.58

26.05 6.80 18.00 3.95 2.55 29.05 41.82 69.69 23.53 28.68 80.87

Not Sig. Not Not Sig. Sig. Sig. Sig. Sig. Sig. Sig.

sig.

position

ati (degrees) (mm.)

i-to NB (degrees) T-to NB (mm.) Overbite UIH LIH Incisor index UMH LMH Molar index

sig.

and planes

R.H. C.L. T.L. Gonial angle Mand. plane OM Oct. plane Symph. H Symph. D Tooth

-1.55

85.25 -5.39

relationships

SN UFH LFH ATFH PTFH

1 to NA Zto NA

79.91

sig.

sig.

relationship sig. sig. sig.

is reflected by the significantly greater mean SNB angle in the adults as compared with that in the children. The mean SNB angle in male adults is 86.35 degrees, and in male children it is 82.09 degrees; the mean SNB angle in female adults is 85.25 degrees, and in fema.le children it is 80.95 degrees (Tables V and VI). SNA angle mean values, however, show little differences between the child and adult Class III groups, It follows, then, that the ANB angle must increase

156

Am J. Orthod. August 1974

et al.

Jacobson

\

Fig.

10.

superimposed

Fig. 11. compared

Representative along

n:

ADULT

cl:

CHILD

outlines the

mandibular

Craniofacial skeletal by superimposing

of

male plane

and

female registered

patterns of child the representative

adult at

and posterior

child border

and adult male Class III outlines on SN, registering

Class

III of

mandibles

symphysis.

malocclusions at S.

are

Voluflre Number

Handibulw

66 2

0 Fig. are

12. Craniofacial compared by

skeletal superimposing

propa

thim

157

: CHILD patterns the

of child and representative

adult femcle Class outlines on SN,

III malocclusions registering at

S.

as point B is carried forward by growth. (The mean ANB difference between male adults and children is 3.28 tlcgrees, whereas between female adults and children it is 3.84 degrees). Whim the mean mandibular plane angles (10 not indicate any significant intergroup differences, the occlusal plane in adult Class III malocclusions is significantly lower posteriorly (by about 2 degrees) than in children. These factors have the effect of significantly increasing the OM angles in adults (male adult, 19.13 degrees, and malt children, 1.623 deqws; female adult, 19.09 degrees, and female children, 15.96 degrees). Likewise, the upper to lower molar index from childhood to adulthood is also increased. In both sexes the height of the facial skeleton tends to increase proportionately more than the tlepth, as determinetl by the midfacial index, from childhood to adulthood. The height increases in both sexes are statistically significant (Tables V and VI). This appears to be a normal trend since, according to Hellman,l” the height of the face increases most rapidly, followed by the depth and then the width. With the increase in facial height, t.he lower to total height ratio becomes significantly greater in adults. Dental difierelbces. The maxillary incisors are more proclined in the adult sample than in the child sample (Figs. 11 and 12). The mandibular incisors, on the other hand, are generally tipped more lingually in adults than in children, although this effect reaches a significant level only in males (Fig. 10). This lower incisor retroclination may be the result of a restraining effect upon the crowns by the encircling orbieularis oris musculature while the roots are being carried forward by the active growth of the mandible.

158

Jacobson

Table

VII.

A?Ib. J. Ol’thod. Awust 1974

et al.

Sexual

dimorphism

in adult

III malocclusions Male mean

Parametei Jaw SNA SNB ANB

Class

relationship angle angle angle

Craniofaoiicl SN UFH LFH ATFH PTFH

Female mean

Difference at 5% level

80.63 86.35 -5.72

79.91 85.25 -5.39

Not Not Not

74.12 56.09 76.29 132.40 85.32 42.45 57.55 107.90 72.50 67.35

68.58 53.26 69.08 122.30 75.68 43.62 56.38 98.89 68.6’1 69.56

Sig. Sig. Sig. Sig. Sig. Sig. Sig. Sig. Sig. Sig.

65.35 92.50 136.30 133.20 33.10 19.13 13.97 36.16 13.73

60.21 88.55 127.40 131.80 34.93 19.09 15.99 32.21 12.62

Sig. Sig. Sig. Not Not Not Sig. Sig. Sig.

29.93 7.37 16.59 4.89 1.25 30.65 46.26 66.37 25.41 31.68 80.87

26.05 6.80 18.00 3.95 2.55 29.05 41.82 69.69 23.53 28.68 80.87

Sig. Not Not Not Not Sig. Sig. Sig. Sig. Sig. Not

sig. sig. sig.

relationships

U/T% L/T% M.F.H. M.F.D. M.F. index Xandibulnr Morphology R.H. C.L. T.L. Qonial angle Mand. plane OM Oct. plane Symph. H Symph. D Tooth position ancl Ito NA (degrees) Ito Nh (mm.)

and

planes

sig. sig. sig.

relationship

-i-to NB (degrees) i-to NB (mm.) Overbite UIH LIH Incisor index UMH LMH Molar index

sig. sig. sig. sig.

sig.

Summary. As growth in Class III malocclusions carries point B forward, the BNA angle is generally increased, the lower incisors upright, and the upper incisors procline. Little effective growth rotation appears to take place, since the mandibular planes do not change significantly. Sexual dimorpliism III malocclusion

in

the

craniofacial

skeletal

and

dental

patterns

in

adult

Class

The major difference between the means of the adult male and female Class III groups was in size (Table VII). The male linear measurements are generally

Man&h&w proynathtinz

------

Fig. 13. are

Craniofacial

compared

by

159

FEMALE

skeletal

patterns

superimposing

the

of

male

representative

and

female outlines

adult on SN,

Class

III

registering

malocclusions

at S.

greater than the corresponding measurements in females. The spatial proportions of the various craniofacial skeletal structures, however, are similar in both sexes. This can be illustrated diagrammatically by superimposing the composite tracings of male means on that of females along the SN line registered at sella (Fig. 13).

The male Class III mandible is significantly larger than the female Class III mandible in all the linear parameters-ramus height (M = 65.35, F = SO.Zl), corpus length (M = 92.5, F = 88.55)) and total length (M = 136.3, F = 127.4). The symphysis is significantly higher in males, but there is little difference between male and female mean symphyseal depths. Although the absolute linear facial measurements are significantly greater in the male, the facial ratios show a much lower degree of significant sexual dimorphism. Facial depth in males is, however, proportionately less than in females (midfacial index in males is 67.35 per cent; in females it is 69.56 per cent ) . On the composite tracing of mean measurements, the position of the condyle in males is located slightly lower (relative to SN plane) than it is in females. This might partly account for the more prominent chin point seen in males. However, on superimposing the mandibles on the mandibular plane registering on the posterior border of the symphysis, the larger over-all size of the male mean mandibular dimension is evident (Fig. 14). The upper incisors are more labially inclined in males than in females. (Axial inclination in males is 29.93 degrees ; in females it is 26.05 degrees).

160

Jncobson et al.

Fig. 14. imposed

Fig. are

Representative along mandibular

15. Craniofacial compared by

Sexucll

dimorphism

-

MALE

----

FEMALE

outlines of the male and female plane, registered at posterior

skeletal superimposing in

the

patterns the craniofacial

of male representative skeletal

and

and

adult border

female outlines dental

Class Ill mandibles of symphysis.

super-

child Class III malocclusions on SN, registering at S. patterns

in

child

Class

Ill

malocclusions

As in adult Class III malocclusion sex comparisons, the mean linear dimensions in male children arc larger than the corresponding dimensions in female children. The mean age of male children is 11.07 years, compared to the mean age of 9.94 years in Ccmalc children. The age difference probably accounts in part for the larger male dimensions.

Volunte Nunlber

Mam-libular

66 2

VIII. Sexual dimorphism

Table

Male mean

.Inu~ relationship

Morphology

-1.44

-1.55

69.69 49.75

65.81

Difference at 5% level Not Not Not

sig. sig. sig.

Tooth position

a& (degrees) (mm.)

Tto NJ3 (degrees) rto NB (mm.) Overbite UIH LIH Incisor index UMH LMH Molar index

47.59 59.64 107.20

66.09 44.37 55.63 85.81 63.14 73.83

Sig. Sig. Sig. Sig. Sig. Not Not Sig. Sig. Not

sig. sig.

sig.

and planes 56'.06 81.47 114.30 128.40 32.11 16.23 15.92 29.34 14.16

R.H. CL. T.L. Gonial angle Mand. plane OM Oct. plane Symph. H Symph. D NA NA

79.40 80.95

63.13 112.90 72.09 44.13 56.87 91.31 66.19 72.95

U/T% 10% M.F.H. M.F.D. M.F. index

1 to Ito

mean

relationships

UFH LFH ATFH PTFH

Mandildar

Female

82.09

80.41

angle angle angle

C~aniofncircl SN

161

in child Class III malocclusions

Parameter SNA SNB ANB

prognnthisnz.

15.96 17.99 27.02 12.53

Sig. Big. Sig. Not Not Not Sig. sig. Sig.

25.60 4.51 20.16 3.74 3.35 24.52 36.78 66.81 18.57 26.31 70.58

Not Not Not. Not Not Sig. Sig. Not Sig. Sig. Not

51.28 77.66 108.80 128.30

33.90

sig. sig. sig.

relationship 23.78 4.70

19.64 3.73 2.02 26.31

39.34 67.11 20.28 27.63 73.47

sig. sig. sig. sig. sig.

sig.

sig.

The differences in the craniofacial skeletal angular proportions between the sexes, however, arc minimal (Table VIII). The only significant angular difference bctwcen the sexes is that of the occlusal plane, which in males is approximately 2 degrees less than in females. As in adult Class III malocclusions, the skeletal craniofacial pattern is slightly less divcrgcnt in males than in females (Fig. 15). The relationship of the condyle to SN in male children, as in the adult series, is more inferior than in female children with Class III malocclusions. The larger mandible in the male is again evident when the tracings of both sexes are superimposed on the

162

Jncobson

Fig. 16. imposed

et

Representative on mandibular

mandibular

Am. J. Orthod. August 1974

aJ.

-

MALE

----

FEMALE

ouilines plane,

of the degistered

plane, registering

male and female child Class III on posterior border of symphysis.

on the posterior

mandibles

border of the symphysis

super-

(Fig.

16). Sw~mrnr~~. Even though a significant size discrepancy is shown between male and fcmalc children with Class III malocclusions, the relative craniofacial proportions tcntl to hc similar.

Discussion

The stud.v was designed to identify morphologic differences between Class III malocclusions and a selected group of %ormal” occlusions. Aspects of sexual dimorphism and growth changes were also investigated. In an endeavor to compare the average sizes of the mandible in Class III malocclusions and normal occlusions, Joffe15 used a planimeter to calculate the average areas of the mandibles on lateral cephalometric tracings in the two groups. He concluded that there was no difference in the actual area of the mandibles in the two groups but that they differed only in morphology. A limitation of the study is that, strictly speaking, two-dimensional cephalograms arc inappropriate for studying a three-dimensional bone such as the mandible. A method for measuring this bone in three dimensions has been described by Savara.28 It entails projecting cephalometric landmarks which appear on frontal and lateral ccphalograms into actual skull space by relating them to a threedimensional Cartesian coordinate system with its origin at the intersection of the two rays. The true distances between two landmarks can be readily obtained from their central coordinates by using the distance formulas which, in themselves, require extensive calculations. Calculations involving any large number of measurements would require the aid of a computer. From a practical stand-point, however, it seemed apparent that the conventional two-dimensional cephalogram is a valid indication of the relationship of the mandible to the face.

PoEunae 66 Number 2

Mandibular

prognathism

163

4.09

.87

4.92

4.96

.65

6.15

4.03

.86

4.97

4.26

.,3

4.93

1.82

.39

86.89

3.06

.52

3.45

.7&

4.39

3.2,

.58

4.41

2.65

.57

4.50

4.53

.,8

8.07

4.72 2.9, 1.29

-53.54

-2.00 4.54 - 11.73

5.36

l.l[i

7.25

7.72

a.32

10.13

6.12

1.30

4.60

9.34

1.60

7.05

.28

7.94

1.69

9.07

7.47

1.28

8.75

1.07 -2.78 -2.78

2.05

.4&

4.61

3.12

.54

7.36

2.05

.4‘

3.68

3.12

.53

5.42

4.95

1.06

4.40

7.32

1.26

6.79

2.58

.55

3.36

4.89

.84

6.75

3.01

,611

4.42

4.64

.80

6.89

2.75 3.99 .76

5.95

1.2,

9.01

6.35

1.09

9.72

.43

4.82

1.03

5.07

6.99

1.20

7.56

1.55

5.65

1.21

4.27

9.58

1.64

7.03

-1.88

4.37

.93

3.51

7.38

1.2,

3.54

5.lr9

5.19

1.11

17.04

5.48

.97

lb.50

2.

3.27

.70

18.92

4.48

.77

23.42

1.66

3.65

.78

28.68

4.50

.77

32.18

1.07

3.42

.73

9.13

3.22

.55

8.89

2.03

.43

12.21

1.82

,322

6.91

1.47

32.12

6.26

1.87

.40

31.96

4.48

5.85

1.25

22.93

2.62

.56

48.84

1.85

.39

3.55 3.61 6.73

l.‘l

13.30

3.90

1.07

20.92

.60

47.23

4.b5 2.08 -

7.20

1.23

43.40

4.82

2.34

.40

57.16

234.20

3.81

.65

305.00

-1.88 .5R

.76

11.67

3.35

.57

10.92

.21

.77

7.68

3.54

.61

7.64

.75

1.4?

10.37

6.69

1.15

10.08

.Bi

2.08

.44

8.31

2.8,

.49

11.30

.5”

3.42

.7:

9.22

3.84

.66

12.13

5.27

5.84

1.2)

8.61

9.87

1.69

12.20

6.06 -

Characteristic of Class III malocclusions is the prominence of the chin. Some authors consider that mandibular prognathism is an expression of continued growth beyond normal limits, while others believe that the deformity represents an alteration in pattern.33 Our findings indicate that the majority of Class III patients owe their prognathism to mandibular characteristics rather than to a relative maxillary deficiency. About one half of the adult sample (40 per cent males, 55 per cent females) had mandibles that were prognathic beyond the normal range, while only one fourth had maxillas that were retrusive below the normal range of prognathism. Approximately 60 per cent of the child Class III sample had mandibles which were within the normal range of prognathism (Category C) . It is the burgeoning

164

Jucobson et al.

growth of the Class III mandible which has the effect of producing the greater proportion of adults in Category A, that is, eases in which the mandible has developed beyond the range of normal prognathism. These results do not, however, necessarily indicate that the majority of Class III mandibles are, in fact, larger than normal mandibles. The present study compared measurements and tracings of Class III mandibles with those of average normal occlusions. Comparison of linear measurements of the ramus and the corpus revealed some minor differences, the only significant reading being a larger mean corpus length in the Class III adult female sample. The mean total effective lengths of Class III mandibles in both sexes, however, were significantly greater than those of average normals. This no doubt is due to the “straightening” of the angle between the body and the ramus. The

4.96

.a5

6.L5

3.64

.64

4.52

4.26

.73

4.93

3.6”

.64

4.39

4.24

3.06

.52

2.43

.43

159.20

6.07

3.27

.58

4.41

69.64

4.58

.84

6.57

4.53

.78

8.07

49.75

5.06

.a9

10.17

7.72

1.32

63.13

7.40

1.31

11.73

9.34

1.6”

7.06

112.9”

11.75

2.08

10.41

7.47

1.28

1.7,

13.39

8.75

72.09

9.65

.54

7.36

44.13

1.90

.34

3.*2

.53

5.42

56.87

1.90

.33

3.39

7.32

1.26

6.79

91.31

2.07

12.85

.84

6.75

66.19

6.56

I.16

9.91

4.64

.80

6.89

72.95

5.97

1.05

8.18

6.35

1.09

9.72

56.06

9.20

1.63

16.41

6.99

L.2”

1.56

81.41

10.18

1.80

12.49

9.58

1.64

7.03

114.30

13.88

2.45

12.15

7.38

1.27

5.54

128.40

5.41

.96

4.21

5.40

.92

16.30

32.11

5.36

.95

16.70

4.48

.77

23.42

16.23

4.48

.79

27.60

4.50

.77

32.18

15.92

4.76

.84

29.89

3.22

.55

8.89

29.34

3.95

.70

13.45

1.82

,332

13.26

14.16

2.25

.418

15.90

7.20

indicate

.60 1.23

20.92

23.78

7.71

47.23

4.70

2.72

43.40

19.64

6.81

1.36 .48 1.20

32.44

-3.57

57.91

3.45

34.67

1.77

.40

57.16

3.73

1.81

.32

48.45

0.69

3.81

.65

305.00

2.02

2.32

.41

114.90

0.99

3.35

.57

10.92

26.31

2.88

.51

10.94

5.63

3.54

.61

7.64

39.34

4.04

.71

10.28

10.08

67.11

6.22

9.27

-7.71 0.46

1.15

1.10

2.87

-49

11.30

20.28

3.99

.70

19.65

5.97

3.84

.66

12.13

27.63

4.09

.72

14.82

4.15

12.20

73.47

9.58

13.04

3.09

9.87

values

1.07

2.34

6.69

“t”

ll.73

4.30

4.89

3.48

underlined

IO.13

3.12

6.26

* The

-53.54

.20

e significant

1.69

difference

between

the

means

at

the

1.69

5% level.

gonial angle is significantly greater in both male and female Class III mandibles than in the average normals. These results confirm that the difference between average normal and Class III mandibles is not so much one of size but, rather, one of morphology. The characteristic straight-line morphology of the Class III mandible explains the increased total effective length, but this only partly accounts for the mandibular prognathism. Composite tracings of males and females with Class III malocclusion (Figs. 6 and 7) show that the position of the head of the condyle in these cases generally is located farther anteriorly than in average normal occlusions. This confirms the recent findings of Droel and Isaacson,i who observed that the glenoid fossa in Class III skeletal malocclusions tends to be positioned farther forward than in Class I malocclusions. This will naturally

166

Jacobson et al.

Am.

J. Orthod.

August

1974

locate the mandible forward in the craniofacial skeletal complex and will accentuate the prognathism. From a clinical point of view, however, it must be emphasized that a single morphologic feature does not necessarily produce a protrusive or Class III relationship, since a structural imbalance in any one area also affects the nature of balance in other areas. Consequently, a number of separate but nevertheless interrelated regional cause-and-effect factors tend to augment each other in a cumulative composite manner. 8, g Average normal occlusions are characterized by an effective compensatory balance between the contributions of the various craniofacial and skeletodental components. In skeletal Class III malocclusions there exists an imbalance of these intrinsic compensating factors. The influence of the soft tissues should also not be neglected. Quantitative

Volume Number

66 2

Mandibular

prognathism

167

studies of the more easily measured hard tissues have led to an oversimplification of the true complexity of the problem of growth and development, Moorrees and co-workerP express the view that bone exists in a milieu and, irrespective of its form at a given developmental horizon, it is subject to modification by adjacent tissues. Bones should not be studied as isolated anatomic specimens but should be recognized within the totality of the organism. These views are in accord with those of Moss,~~ who believes that bone exists within a matrix and its structure reflects the functioning of this matrix. The growth of a bone and its changing position in space are related to the growth of the matrix. These bone areas are undoubtedly originally intrinsically (genetically) determined. But their maintenance is related to extrinsic (environmental and functional) factors. Moorrees and his colleagueszl extend this concept by

stating that architectural aspects become subjugated to the underlying factors that create. Included in these factors are not only genes but morphogenesis of all Gssues (hard and soft) and their subsequent interaction. Also involved is the central nervous system, which guides voluntary and sensory-receptor-induced muscle activity. In addition to these theoretical considerations, a knowledge and understanding of the internal eraniofacial skeletal anatomy in Class III malocclusion are also important, particularly in surgical or surgical/orthodontic treatment technique. Treatment is aimed both at correcting the dentoalveolar relationships and improving the facial profile by reduction of the mandibular prognathism. While the dentoalveolar structures will yield to orthodontic treatment, it may be necessary in more severe cases to resort to orthopedic or surgical procedures to achieve an effective improvement in the skeletal relationships.

Volume Number

66 2

Mandibular

yrogunthism

169

The cephalometric profile analysis usually reflects the resultant profile patterns dependent upon the relative contributions of the skeletal and dental factors. Satisfactory treatment will demand an assessment of the degree of severity of the dysplasia, a localization of the aberrant pattern, and an appreciation of the biologic potentials. It has been our experience that, as a general rule, Class III malocclusions with ANB readings of up to -3.5 degrees tend to respond to orthodontic treatment while dgsplasias greater than this usually require surgical correction. Since variation is the rule in biology, it rnust be emphasised that the reading of -3.5 degrees should be regarded only as a guideline in the treatment of Class III malocclusion and must not be accepted as a hard and fast rule. Other factors, such as age, sex, open-bite, or deep-bite skeletal patterns and gross discrepancies in arch width must be evaluated in an over-all assessment in planning treatment. Summary

Cephalometrie tracings of 149 male and female adults and children having Class III malocclusions arc compared with those of 112 adults and children of both sexes with normal occlusions. The first phase of the study is devoted to determining the incidence of various Class III craniofacial skeletal patterns. The most common Class III pattern was found to be one in which the maxilla was within the “normal range of prognathism” while the mandible extended beFond this range. Approximately one fourth of the Class III sample showed a relative maxillary deficiency. The second portion of the study compares the characteristic craniofacial skeletal features of Class TII malocclusions with those of normal occlusions in adults. A further section compares Class III malocclusion in children with similar malocclusions in adults. Sexual dimorphic characteristics arc included in the investigation. The most significant differcncr between Class III malocclusion and normal occlusion is the ANB angle, which may bc attributetf principally to the Class I11 mandible being more prognathir. Also contributing to this difference is the generally shorter anterior cranial base in Class 111 malocclusion which, in turn, tcn(ls to effect a relative maxillary cleficicncy. A further contributoyv factor toward mandibular prognathism is a more obtuse genial angle, associated with a “straight-line morphology,” while the glenoid fossa is located relatively farther forward in the Class 111 craniofacial pattern. The dominance of mandibular growth was demonstrated by t,hc comparison of child and adult Class 111 cases. Incisor inclinations appear to change as a consequence of this burgeoning growth. Sesual tlimorphism is mainly reflected in the relatively larger malt Class III mandible. Although the craniofacial proportions of male and female Class III casts were similar, fcmalcs tentlctl to a slightly more tlivergent type of pattern. The senior author gratefully acknowledges the financial assistance granted by the University of the Witmatersrand and the Elida Gibbs Foundation. The authors express their appreciation to Mr. Hall of the National rnstitute for Personnel Kcsearch for kind assistance with the statistical analyses.

170

Jacobson ef al.

REFERENCES

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