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The shape of the dental arch
The shape of the dental arch C.
1. B. Lavelle,
Birminghrrnz,
M.D.S.,
Ph.D.
Ey$attd
T
hc shape of the dental arch is of primary concern to the orthodontist, especially as it relates to the ability to predict, future growth changes and results of treatment. On the basis of purely observational criteria, the shape of the tlcntal arch can bc segrcgatetl into a number of different categories.’ FTowever, since the definition of such categories is purely subjective, visual observation alone cannot provitlc reliable data. Alctrical tlefinition of arch shape is bedeviled by the lack of datum points, so that thcrc is inconsistency rcgartling the number of dimensions necessary to provitlc an adcquatc arch profile. One technique is to examine the curvature of the dental areadc which, in its iclcal form, follows a cdat,cnary curve.’ In practice, however, the decision as to whether a catcnary cul~c passes through the midline of the o~lusal surfaces of the dentition is purely arbitary. This probah1.v contrihutcs to the fact t,hat catcnary curve asscssmcnt has yet to receive universal usage. Other objective ant1 rclativcl,v simple techniques have been used in arch analysis (for csamplc, the computation of arch arc;~:’ or arch index4 and the determinat,ion of the length of the outer circumfercncc of the tlent,al arcade”). The present investigation was therefore undertaken to compare t hc results from each of these methods of arch analysis for examination of the dental arch age e1~1~gcs bctwccn Caucasoitls, Mongoloids, and h’egroids. Materials
and
methods
This cross-sectional investigation of dental arch growth was based on the mcasurcmcnt of study casts of the masillary and mandibular arches, derived from alginate.basc li~tlroc~olloitl impressions. A total of 2,010 sets of maxillary and mandibular casts was obtained from equal samples from three ethnic groups CaueasoitIs, Mongoloids, and Negroids, between the ages of -I and 20 years. There were sets of casts from twenty male and twenty female subjects within each yarl,v age gtmp in each population sample. The Caucasoid sample was derived from British patients resitlin g within a 20 mile radius of Birmingham, whereas From 176
the
Department
of Ornl
Pathology,
The
Dental
School,
Rirmingham,
England.
Shape of dental arch
177
both Negroid and Mongoloid samples were derived from immigrant populations to the United Kingdom from Africa and Hong Kong. These population samples were heterogenous rather than homogenous. The casts were not randomly selected, the criteria for selection being as follows : 1. There was an intact dental arch for the particular age group, with the exception of the third molar. 2. Only those arches exhibiting a parabolic arrangement of the dental arcade were included, which results in the elimination of approximately 4 per cent of the casts showing other configurations (for example, square or “V” forms). 3. There was a “normal” anteroposterior relationship between the maxillary and mandibular molars; thus, those with gross discrepancies were rejected. 4. There were no gross dental abnormalities or imbrication. 5. Casts were sufficiently accurate to permit their detailed measurement. Since discrepancies, such as tooth drifting, may be assumed to occur at all ages, the measurement of these casts did not preclude the detection of any overall changes in dental arch dimensions with advancing age. Measurements
The dimensions were selected on the basis that they could be determined on all the dental arches between the ages of 4 and 20 years and were recorded regardless of whether the teeth were from the deciduous or the permanent dentition. Hence, this investigation spanned the transition period between the two dentitions. The dimensions of the dental casts were measured by means of dial calipers, held vertical to the occlusal plane and reading to the nearest 0.1 mm. Dental trrch width. The dimensions of dental arch width were measured between the centers of corresponding teeth on each side of the arch. The following dimensions were measured: (E)5-5(E), (D)4-4(D), (C)3-3(C), (B)2-2(B), (A)l-l(A). Denta’l m-ch len.gth. The dimensions of dental arch length were measured as t’he minimum distance between the centers of adjacent teeth on the left side of the dental arch. The following dimensions were measured: (E)5-4 (D), (D)4-3(C), (C)3-2(B), (B)2-l(A). (In the listing of arch dimensions, the figures represent permanent teeth and the letters represent deciduous teeth.) As a check on the accuracy of the measurement technique, the dimensions of five male and five female sets of maxillary and mandibular casts were selected at random from each of the three population samples and were measured five times on five separate occasions. The mean error of the measurement technique was less than 2 per cent, which proved statistically insignificant from analysis of variance (P >0.2). Thus, any inconsistencies in the measurement technique are unlikely to have contributed in any material way to the outcome of the comparisons bet.ween either age groups or ethnic groups.
178
Lavelle
Table
I. Summary
of changes
between
the
1 i Sex
/ Arch
areas
Male
indices
Male Female
Dental
arc
lengths
Male Female
Number All
in sample,
mean
*c = t SE =
values
Mean
20 in each rounded
dimension
Standard
age
to the (cm.2
for
group
nearest dental
4 and
20
years
in dental
arch
areas,
3InxiZlary
Age (years)
I
of
-7
Female Arch
ages
Caucasoid
r /
/-----
arch
arch
-
Mon~goloid
P
SEt
4 20 4 20
4.6 14.7 4.6 13.0
0.07 0.08 0.03 0.08
~--s---F 4,s L-l.6 4.ti l? I .2
0.10 0.04 o.oi 0.08
4 20 4 20
78.7 89.4 87.2 107.4
0.65 0.49 0.52 0.74
79.5 90.6 88.3 108.9
0.58 0.89 0.99 0.76
4 20 4 20
10.4 11.5 10.4 11.2
0.07 0.07 0.04 0.02
10.6 11.7 10.6 11.1
0.06 0.05 0.04 0.03
and
sex.
0.1 unit arch
I
area,
for
simplification
percentage
for
of dental
the
table. arch
index,
and
cm.
error.
The effect of age on the dental arches derived from three ethnic groups was then assessed by three methods of computation. DESTAL ARCH AREA. The area of the dental arch was determined from the area enclosed within the datum points. Thus, the arch was divided into a number of trapezia between adjacent pairs of teeth on each side of the dental arch. The area of the arch between the second premolars (or deciduous predecessors) and the central incisors was then derived b;v summation of the areas of the component trapezia. DENTAL ARCH ISDEX. The dental arch index was determined as the ratio of arch width (taken between the canines) to arch length (taken between the central incisors and second premolars or deciduous predecessors), a distance derived from summation of all the distances of arch length measured between adjacent teeth, expressed as a percentage. LESGTH OF ARC OF DENTAL AR&IDE. All the dental arches used in this inr-estigation were parabolic in form. The mathematical advantages of a parabola arc essentially that all are alike and thus are directly comparable one with another. In this investigation, the parabola was selected to span the curvature &ending from the center of the second premolar (or deciduous predecessor) to the homologous region of its antimere, measuring an imaginary line passing through the fossae of the row of teeth between these two sites. Using the approximate formula for the length of the arc of a parabola, 2 length between t.he central incisors and second premolars (or deciduous predecessors) and “x” by arch width between the second premolars (or deciduous predecessors).
for
Volume Number
indices,
GT 2
and dental
S’hape
denttrl
179
arch
arc lengths
Negroid
dental
of
Camasoid
5
SE
z
4.4 14.0 4.2 12.3
0.07 0.10 0.05 0.08
77.2 88.8 84.2 101.8 10.3 11.1 10.3 10.9
Mongoloid 1
SE
:
4.8 13.8 3.4 12.5
0.07 0.12 0.10 0.09
0.65 0.99 0.85 0.52
78.5 88.5 87.4 103.9
0.04 0.06 0.05 0.05
9.1 9.9 9.1 9.8
Negroid /
SE
:
SE
5.0 13.7 4.0 12.9
0.07 0.03 0.07 0.10
4.7 13.0 3.1 12.0
0.03 0.10 0.09 0.09
0.42 0.84 0.31 0.63
SO.2 90.6 88.2 106.4
0.74 0.97 0.84 0.85
76.8 86.4 81.7 91.4
0.35 0.97 0.47 0.48
0.04 0.03 0.05 0.05
9.2 10.0 9.2 9.9
0.01 0.07 0.02 0.04
9.0 9.8 9.0 9.8
0.07 0.05 0.05 0.08
arc length).
Results
In order to reduce the tabular material to a minimum, the data derived from each of these methods of arch analysis are summarized in Table I. Arch area. The general trend was for. the maxillary and mandibular dental arch areas in both male and female subjects to increase with age and to be greatest for Negroids and least for Mongoloids, with those for Caucasoids being intermediat.e throughout the age range of 4 to 20 years. Also, between 4 and 20 years, the areas of the maxillary arch were always highly correlated with those of the mandibular arches, the correlation coefficient never being less than 0.98. Differences were apparent, however, at the various age groups. For instance, at 4 years, the mandibular arch area was greater than that for the maxillary arch by 4 per cent for Caucasoids and Negroids and 6 per cent for Mongoloids in males, whereas the maxillary arch was greater than that for the mandibular arch in females by 14 per cent for Negroids, 30 per cent for Mongoloids, and 31 per cent for Caucasoids. At 20 years, the maxillary arch areas were greater than those for the mandibular arch by 7 per cent for male Caucasoids and Negroids and 8 per cent for Mongoloids, whereas for females the percentage difference was 4 per cent for Caucasoids and 3 per cent for Negroids and Mongoloids. In all three ethnic groups, two spurts of growth in dental arch area were noted between 5 and 7 and 11 and 13 years (Fig. 1). The variation in the spurts of growth between the ethnic groups possibly contributed to the fact that whereas at 4 years of age the degree of sexual dimorphism for the maxillary arch area was 2 per cent in Caucasoids, 3 per cent in Negroids, and 4 per cent in ?ulongoloids and that for the mandibular arch area was 21 per cent in Negroids,
180
Lnvelle 3.6
-Caucasoid m---m Negroid *,...oMongoloid
3.2
6
4
a Age
Fig. 1. Rate
of change
of
mandibular
arch
area
10
12
14
(yr) in females
from
different
ethnic
groups.
36 per cent in Caucasoids, and 40 per cent in Mongoloids, at 20 years of age the degree of sexual dimorphism for the maxilla was 10 per cent for Negroids and 13 per cent for Mongoloids and Caucasoids, and for the mandible 6 per cent for Negroids, 8 per cent for Mongoloids, and 10 per cent for Caucasoids. Thus, it is concluded that the dental arch areas prored highly sensitive in ident.ifying the changes in dental arch dimensions with advancing age in the three ethnic groups. Arch index. Throughout the age range the dental arch index was greatest in Negroids and least in Mongoloids, with that for Caucasoids being intermediate. In addition, the arch index was greater in males than in females bctwecn 1 and 20 pears in all three ethnic groups for both the maxilla and the mandible,
Shape of dental arch
181
-Caucasoids m--a Negroids l . . ..O Mongoloids
1 4
6
8
10
12
14
Age(yr.1 Fig. 2. Rate
of change
of mandibular
arch
index
in females
from
different
ethnic
groups.
averaging 7 per cent for Mongoloids and 10 per cent for both Negroids and Caucasoids at the age of 4 years and 10 per cent for Mongoloids and 17 per cent for both Caucasoids and Negroids at 20 years. The changes in arch indices with advancing age showed varying degrees of growth spu.rts (Fig. 2). For instance, the indices for the maxillas of both males and females increased maximally between 6 and 12 years of age, there being little change thereafter, whereas the indices for the mandible showed two spurts at 6 to 8 years and 9 to 12 pears. 3. Arc length of dental arcade. The lengths of the arcs described by the dental arcades were greatest in Negroids, least in Mongoloids, and intermediate in Caucasoids. Also, the arc lengths were greater for the masillary arches than for the mandibular arches, the over-all difference centering approximately 14 per cent in all ethnic groups throughout the age range in both males and females. As illustrated in Fig. 3, whereas both Negroids and Mongoloids followed a similar growth pattern, there was considerable fluctuation in the spurts of growth in arc lengths in Caucasoids. In addition, this technique of dental arch analysis failed to illustrate a growth spurt in the early age groups.
182
Lnuelle s--+ *....a
0
f 4
I 6
I 8
I 10
Caucasoid Negroid Mongoloid
(MI (M) (MI
I 12
I 14
I 16
I 18
12
1L
16
18
of
maxillary
and
Age (yr) n za
0.24-
L
6
8
10 Age(yr)
Fig. 3. Rate of change in males from different
in length of arc ethnic groups.
of
parabola
mandibular
arches
Discussion
Studies of age changes in the human dental arcade are important since they provide (1) normative data for comparison of individual children with the population norm and so provide a basis for discriminating between the normal and abnormal; (2) growth norms for comparison of growth of a child with normal curves, detecting deviations at the earliest possible time, so that corrective procedures can be instituted; (3) data concerning growth spurts, thus
Shupe
of
dewtcd
arch
183
enabling treatment to bc planned and so take full atlvantage of the growth processes. Most annual growth changes are small, so that measurcm~nts have to be accurate if genetic and environmental factors arc to be dissociated. The data from this investigation showed that computation of dental arch areas and clental arch indices proved scnsitivc to the age changes in the arch dimensions in the three cthnie groups, whereas the lengths of the ares tlcscribcd by the dental arcades were relatively insensitive. Also, the first two methods of computation especially illustrated the marked changes in arc.11 dimensions in the 5 to 7- and lo- to 13-year age groups in each ethnic group. Some degree of hereditability affecting arch form has been established from family,” twin,: and population stutlies.x The size and shape of the dental arch are, however, sub&t to considerable variation, being dependent upon the form and position of the tooth crowns.!’ The position of the tooth crowns is, in turn, affected by tooth eruption*0 and migration,” the sixc of the adjacent tcetli,” and ,jaw growth.‘:’ Other factors affecting the form of the dental arch include tht degree of equilibrium between the surrounding orofacial musculaturc.‘4 Thus, although the present stud,v comparctl the performante of three methods of arch analysis for three ethnic groups, muc~h more data arc required bcforc genetic and environmental factors can be dissociated in relation to tlental arch form. Nevertheless, the presrnt study confirms that the use of ol)qjtctirc methods of arch assessment arc rclativclg easy to apply and enable growth spurts to bc reatlily identified-a Scaturc important for the prediction of future arch changes. Summary
The shape of the dental arcatlr was mtasurcd in a cross-sectional study of 2,040 sets of maxillary and mandibular study casts of Caucasoids, Mongoloids, and Negroids between I and 20 years of age. Thrcr methods of dental arcade analysis were used: dental arch arca, dental arch index, and length of’ the arc of the dental arcade. Each of thcsc c~omputational techniques showctl that a different growth pattern was shared by the three ethnic groups, an important factor to be taken into consideration in orthodontic treatment. REFERENCES
1. Thomsen, S.: Dental morphology and occlusion in the people of Tristan da Cunha; results of the Norwegian Scientific Expedition to Tristan da Cunha 1937-1938, No. 25, Oslo, 1955. 2. Williams, P. N.: Determining the shape of the dental arch, Dent. Cosmos 59: 695-708, 1917. 3. Lavelle, C!. L. B. : Age changes in dental arch shape, J. Dent. Res. 49: 1517.1521, 1970. 4. Martin, I;:.: Lehrbuch der Anthropologic, Jena, 1914, G. Fisher. 5. Mills, L. F., and Hamilton, P. M.: Epidemiological studies of malalignment, a method for computing dental arch circumference, Angle Orthod. 35: 244-248, 1965. 6. Bowden, D. E., and Goose, D. II.: The inheritance of pnlatal arch width in human families, Arch. Oral Biol. 13: 3293-1295, 1968. 7. Lundstrom, A.: An investigation of 202 pairs of t\vins rrgarding fundamental factors in the aetiology of malocclusion, Trans. Eur. Orthod. Roe., pp. 161.176, 1948. 8. Lavelle, C. L. B., Flinn, R. M., Foster, T. D., and Hamilton, M. C.: An analysis into age changes of the human dental arch by a multivarinte trchniqur, Am. J. Phys. Anthropol. 33: 403-412, 1970. 9. Black, G. V.: Descriptive anatomy of the human teeth, ed. 3, Philadelphia, 1894, Wilmington Dental Manufacturing Co.
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Premature
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When of
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[Translated Palm
und
1. B. Lavelle,
Birminghrrnz,
M.D.S.,
Ph.D.
Ey$attd
T
hc shape of the dental arch is of primary concern to the orthodontist, especially as it relates to the ability to predict, future growth changes and results of treatment. On the basis of purely observational criteria, the shape of the tlcntal arch can bc segrcgatetl into a number of different categories.’ FTowever, since the definition of such categories is purely subjective, visual observation alone cannot provitlc reliable data. Alctrical tlefinition of arch shape is bedeviled by the lack of datum points, so that thcrc is inconsistency rcgartling the number of dimensions necessary to provitlc an adcquatc arch profile. One technique is to examine the curvature of the dental areadc which, in its iclcal form, follows a cdat,cnary curve.’ In practice, however, the decision as to whether a catcnary cul~c passes through the midline of the o~lusal surfaces of the dentition is purely arbitary. This probah1.v contrihutcs to the fact t,hat catcnary curve asscssmcnt has yet to receive universal usage. Other objective ant1 rclativcl,v simple techniques have been used in arch analysis (for csamplc, the computation of arch arc;~:’ or arch index4 and the determinat,ion of the length of the outer circumfercncc of the tlent,al arcade”). The present investigation was therefore undertaken to compare t hc results from each of these methods of arch analysis for examination of the dental arch age e1~1~gcs bctwccn Caucasoitls, Mongoloids, and h’egroids. Materials
and
methods
This cross-sectional investigation of dental arch growth was based on the mcasurcmcnt of study casts of the masillary and mandibular arches, derived from alginate.basc li~tlroc~olloitl impressions. A total of 2,010 sets of maxillary and mandibular casts was obtained from equal samples from three ethnic groups CaueasoitIs, Mongoloids, and Negroids, between the ages of -I and 20 years. There were sets of casts from twenty male and twenty female subjects within each yarl,v age gtmp in each population sample. The Caucasoid sample was derived from British patients resitlin g within a 20 mile radius of Birmingham, whereas From 176
the
Department
of Ornl
Pathology,
The
Dental
School,
Rirmingham,
England.
Shape of dental arch
177
both Negroid and Mongoloid samples were derived from immigrant populations to the United Kingdom from Africa and Hong Kong. These population samples were heterogenous rather than homogenous. The casts were not randomly selected, the criteria for selection being as follows : 1. There was an intact dental arch for the particular age group, with the exception of the third molar. 2. Only those arches exhibiting a parabolic arrangement of the dental arcade were included, which results in the elimination of approximately 4 per cent of the casts showing other configurations (for example, square or “V” forms). 3. There was a “normal” anteroposterior relationship between the maxillary and mandibular molars; thus, those with gross discrepancies were rejected. 4. There were no gross dental abnormalities or imbrication. 5. Casts were sufficiently accurate to permit their detailed measurement. Since discrepancies, such as tooth drifting, may be assumed to occur at all ages, the measurement of these casts did not preclude the detection of any overall changes in dental arch dimensions with advancing age. Measurements
The dimensions were selected on the basis that they could be determined on all the dental arches between the ages of 4 and 20 years and were recorded regardless of whether the teeth were from the deciduous or the permanent dentition. Hence, this investigation spanned the transition period between the two dentitions. The dimensions of the dental casts were measured by means of dial calipers, held vertical to the occlusal plane and reading to the nearest 0.1 mm. Dental trrch width. The dimensions of dental arch width were measured between the centers of corresponding teeth on each side of the arch. The following dimensions were measured: (E)5-5(E), (D)4-4(D), (C)3-3(C), (B)2-2(B), (A)l-l(A). Denta’l m-ch len.gth. The dimensions of dental arch length were measured as t’he minimum distance between the centers of adjacent teeth on the left side of the dental arch. The following dimensions were measured: (E)5-4 (D), (D)4-3(C), (C)3-2(B), (B)2-l(A). (In the listing of arch dimensions, the figures represent permanent teeth and the letters represent deciduous teeth.) As a check on the accuracy of the measurement technique, the dimensions of five male and five female sets of maxillary and mandibular casts were selected at random from each of the three population samples and were measured five times on five separate occasions. The mean error of the measurement technique was less than 2 per cent, which proved statistically insignificant from analysis of variance (P >0.2). Thus, any inconsistencies in the measurement technique are unlikely to have contributed in any material way to the outcome of the comparisons bet.ween either age groups or ethnic groups.
178
Lavelle
Table
I. Summary
of changes
between
the
1 i Sex
/ Arch
areas
Male
indices
Male Female
Dental
arc
lengths
Male Female
Number All
in sample,
mean
*c = t SE =
values
Mean
20 in each rounded
dimension
Standard
age
to the (cm.2
for
group
nearest dental
4 and
20
years
in dental
arch
areas,
3InxiZlary
Age (years)
I
of
-7
Female Arch
ages
Caucasoid
r /
/-----
arch
arch
-
Mon~goloid
P
SEt
4 20 4 20
4.6 14.7 4.6 13.0
0.07 0.08 0.03 0.08
~--s---F 4,s L-l.6 4.ti l? I .2
0.10 0.04 o.oi 0.08
4 20 4 20
78.7 89.4 87.2 107.4
0.65 0.49 0.52 0.74
79.5 90.6 88.3 108.9
0.58 0.89 0.99 0.76
4 20 4 20
10.4 11.5 10.4 11.2
0.07 0.07 0.04 0.02
10.6 11.7 10.6 11.1
0.06 0.05 0.04 0.03
and
sex.
0.1 unit arch
I
area,
for
simplification
percentage
for
of dental
the
table. arch
index,
and
cm.
error.
The effect of age on the dental arches derived from three ethnic groups was then assessed by three methods of computation. DESTAL ARCH AREA. The area of the dental arch was determined from the area enclosed within the datum points. Thus, the arch was divided into a number of trapezia between adjacent pairs of teeth on each side of the dental arch. The area of the arch between the second premolars (or deciduous predecessors) and the central incisors was then derived b;v summation of the areas of the component trapezia. DENTAL ARCH ISDEX. The dental arch index was determined as the ratio of arch width (taken between the canines) to arch length (taken between the central incisors and second premolars or deciduous predecessors), a distance derived from summation of all the distances of arch length measured between adjacent teeth, expressed as a percentage. LESGTH OF ARC OF DENTAL AR&IDE. All the dental arches used in this inr-estigation were parabolic in form. The mathematical advantages of a parabola arc essentially that all are alike and thus are directly comparable one with another. In this investigation, the parabola was selected to span the curvature &ending from the center of the second premolar (or deciduous predecessor) to the homologous region of its antimere, measuring an imaginary line passing through the fossae of the row of teeth between these two sites. Using the approximate formula for the length of the arc of a parabola, 2 length between t.he central incisors and second premolars (or deciduous predecessors) and “x” by arch width between the second premolars (or deciduous predecessors).
for
Volume Number
indices,
GT 2
and dental
S’hape
denttrl
179
arch
arc lengths
Negroid
dental
of
Camasoid
5
SE
z
4.4 14.0 4.2 12.3
0.07 0.10 0.05 0.08
77.2 88.8 84.2 101.8 10.3 11.1 10.3 10.9
Mongoloid 1
SE
:
4.8 13.8 3.4 12.5
0.07 0.12 0.10 0.09
0.65 0.99 0.85 0.52
78.5 88.5 87.4 103.9
0.04 0.06 0.05 0.05
9.1 9.9 9.1 9.8
Negroid /
SE
:
SE
5.0 13.7 4.0 12.9
0.07 0.03 0.07 0.10
4.7 13.0 3.1 12.0
0.03 0.10 0.09 0.09
0.42 0.84 0.31 0.63
SO.2 90.6 88.2 106.4
0.74 0.97 0.84 0.85
76.8 86.4 81.7 91.4
0.35 0.97 0.47 0.48
0.04 0.03 0.05 0.05
9.2 10.0 9.2 9.9
0.01 0.07 0.02 0.04
9.0 9.8 9.0 9.8
0.07 0.05 0.05 0.08
arc length).
Results
In order to reduce the tabular material to a minimum, the data derived from each of these methods of arch analysis are summarized in Table I. Arch area. The general trend was for. the maxillary and mandibular dental arch areas in both male and female subjects to increase with age and to be greatest for Negroids and least for Mongoloids, with those for Caucasoids being intermediat.e throughout the age range of 4 to 20 years. Also, between 4 and 20 years, the areas of the maxillary arch were always highly correlated with those of the mandibular arches, the correlation coefficient never being less than 0.98. Differences were apparent, however, at the various age groups. For instance, at 4 years, the mandibular arch area was greater than that for the maxillary arch by 4 per cent for Caucasoids and Negroids and 6 per cent for Mongoloids in males, whereas the maxillary arch was greater than that for the mandibular arch in females by 14 per cent for Negroids, 30 per cent for Mongoloids, and 31 per cent for Caucasoids. At 20 years, the maxillary arch areas were greater than those for the mandibular arch by 7 per cent for male Caucasoids and Negroids and 8 per cent for Mongoloids, whereas for females the percentage difference was 4 per cent for Caucasoids and 3 per cent for Negroids and Mongoloids. In all three ethnic groups, two spurts of growth in dental arch area were noted between 5 and 7 and 11 and 13 years (Fig. 1). The variation in the spurts of growth between the ethnic groups possibly contributed to the fact that whereas at 4 years of age the degree of sexual dimorphism for the maxillary arch area was 2 per cent in Caucasoids, 3 per cent in Negroids, and 4 per cent in ?ulongoloids and that for the mandibular arch area was 21 per cent in Negroids,
180
Lnvelle 3.6
-Caucasoid m---m Negroid *,...oMongoloid
3.2
6
4
a Age
Fig. 1. Rate
of change
of
mandibular
arch
area
10
12
14
(yr) in females
from
different
ethnic
groups.
36 per cent in Caucasoids, and 40 per cent in Mongoloids, at 20 years of age the degree of sexual dimorphism for the maxilla was 10 per cent for Negroids and 13 per cent for Mongoloids and Caucasoids, and for the mandible 6 per cent for Negroids, 8 per cent for Mongoloids, and 10 per cent for Caucasoids. Thus, it is concluded that the dental arch areas prored highly sensitive in ident.ifying the changes in dental arch dimensions with advancing age in the three ethnic groups. Arch index. Throughout the age range the dental arch index was greatest in Negroids and least in Mongoloids, with that for Caucasoids being intermediate. In addition, the arch index was greater in males than in females bctwecn 1 and 20 pears in all three ethnic groups for both the maxilla and the mandible,
Shape of dental arch
181
-Caucasoids m--a Negroids l . . ..O Mongoloids
1 4
6
8
10
12
14
Age(yr.1 Fig. 2. Rate
of change
of mandibular
arch
index
in females
from
different
ethnic
groups.
averaging 7 per cent for Mongoloids and 10 per cent for both Negroids and Caucasoids at the age of 4 years and 10 per cent for Mongoloids and 17 per cent for both Caucasoids and Negroids at 20 years. The changes in arch indices with advancing age showed varying degrees of growth spu.rts (Fig. 2). For instance, the indices for the maxillas of both males and females increased maximally between 6 and 12 years of age, there being little change thereafter, whereas the indices for the mandible showed two spurts at 6 to 8 years and 9 to 12 pears. 3. Arc length of dental arcade. The lengths of the arcs described by the dental arcades were greatest in Negroids, least in Mongoloids, and intermediate in Caucasoids. Also, the arc lengths were greater for the masillary arches than for the mandibular arches, the over-all difference centering approximately 14 per cent in all ethnic groups throughout the age range in both males and females. As illustrated in Fig. 3, whereas both Negroids and Mongoloids followed a similar growth pattern, there was considerable fluctuation in the spurts of growth in arc lengths in Caucasoids. In addition, this technique of dental arch analysis failed to illustrate a growth spurt in the early age groups.
182
Lnuelle s--+ *....a
0
f 4
I 6
I 8
I 10
Caucasoid Negroid Mongoloid
(MI (M) (MI
I 12
I 14
I 16
I 18
12
1L
16
18
of
maxillary
and
Age (yr) n za
0.24-
L
6
8
10 Age(yr)
Fig. 3. Rate of change in males from different
in length of arc ethnic groups.
of
parabola
mandibular
arches
Discussion
Studies of age changes in the human dental arcade are important since they provide (1) normative data for comparison of individual children with the population norm and so provide a basis for discriminating between the normal and abnormal; (2) growth norms for comparison of growth of a child with normal curves, detecting deviations at the earliest possible time, so that corrective procedures can be instituted; (3) data concerning growth spurts, thus
Shupe
of
dewtcd
arch
183
enabling treatment to bc planned and so take full atlvantage of the growth processes. Most annual growth changes are small, so that measurcm~nts have to be accurate if genetic and environmental factors arc to be dissociated. The data from this investigation showed that computation of dental arch areas and clental arch indices proved scnsitivc to the age changes in the arch dimensions in the three cthnie groups, whereas the lengths of the ares tlcscribcd by the dental arcades were relatively insensitive. Also, the first two methods of computation especially illustrated the marked changes in arc.11 dimensions in the 5 to 7- and lo- to 13-year age groups in each ethnic group. Some degree of hereditability affecting arch form has been established from family,” twin,: and population stutlies.x The size and shape of the dental arch are, however, sub&t to considerable variation, being dependent upon the form and position of the tooth crowns.!’ The position of the tooth crowns is, in turn, affected by tooth eruption*0 and migration,” the sixc of the adjacent tcetli,” and ,jaw growth.‘:’ Other factors affecting the form of the dental arch include tht degree of equilibrium between the surrounding orofacial musculaturc.‘4 Thus, although the present stud,v comparctl the performante of three methods of arch analysis for three ethnic groups, muc~h more data arc required bcforc genetic and environmental factors can be dissociated in relation to tlental arch form. Nevertheless, the presrnt study confirms that the use of ol)qjtctirc methods of arch assessment arc rclativclg easy to apply and enable growth spurts to bc reatlily identified-a Scaturc important for the prediction of future arch changes. Summary
The shape of the dental arcatlr was mtasurcd in a cross-sectional study of 2,040 sets of maxillary and mandibular study casts of Caucasoids, Mongoloids, and Negroids between I and 20 years of age. Thrcr methods of dental arcade analysis were used: dental arch arca, dental arch index, and length of’ the arc of the dental arcade. Each of thcsc c~omputational techniques showctl that a different growth pattern was shared by the three ethnic groups, an important factor to be taken into consideration in orthodontic treatment. REFERENCES
1. Thomsen, S.: Dental morphology and occlusion in the people of Tristan da Cunha; results of the Norwegian Scientific Expedition to Tristan da Cunha 1937-1938, No. 25, Oslo, 1955. 2. Williams, P. N.: Determining the shape of the dental arch, Dent. Cosmos 59: 695-708, 1917. 3. Lavelle, C!. L. B. : Age changes in dental arch shape, J. Dent. Res. 49: 1517.1521, 1970. 4. Martin, I;:.: Lehrbuch der Anthropologic, Jena, 1914, G. Fisher. 5. Mills, L. F., and Hamilton, P. M.: Epidemiological studies of malalignment, a method for computing dental arch circumference, Angle Orthod. 35: 244-248, 1965. 6. Bowden, D. E., and Goose, D. II.: The inheritance of pnlatal arch width in human families, Arch. Oral Biol. 13: 3293-1295, 1968. 7. Lundstrom, A.: An investigation of 202 pairs of t\vins rrgarding fundamental factors in the aetiology of malocclusion, Trans. Eur. Orthod. Roe., pp. 161.176, 1948. 8. Lavelle, C. L. B., Flinn, R. M., Foster, T. D., and Hamilton, M. C.: An analysis into age changes of the human dental arch by a multivarinte trchniqur, Am. J. Phys. Anthropol. 33: 403-412, 1970. 9. Black, G. V.: Descriptive anatomy of the human teeth, ed. 3, Philadelphia, 1894, Wilmington Dental Manufacturing Co.
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