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RESEARCH AND EDUCATION
Maxillary lateral incisor agenesis and its relationship
to overall tooth size
Jane Wright, DDS, MS,a Jose A. Bosio, BDS, MS,b Jang-Ching Chou, DDS, MS,c and
Shuying S. Jiang, MSd
Prosthodontists, orthodontists,
ABSTRACT
and general dentists frequently
Statement of problem. Agenesis of the maxillary lateral incisor has been linked to differences in
encounter difficulties when
the size of the remaining teeth. Thus, the mesiodistal space required for definitive esthetic restoattempting to restore the ocration in patients with missing maxillary lateral incisors may be reduced.
clusion if unilateral or bilateral
Purpose. The purpose of this study was to determine whether a tooth size discrepancy exists in
maxillary lateral incisors are
orthodontic patients with agenesis of one or both maxillary lateral incisors.
congenitally missing. RestoraMaterial and methods. Forty sets of dental casts from orthodontic patients (19 men and 21
tion of the missing lateral
women; mean 15.9 years of age; all of European origin) were collected. All casts had agenesis of one
incisor using an implantor both maxillary lateral incisors. Teeth were measured with a digital caliper at their greatest
supported crown, a partial
mesiodistal width and then compared with those of a control group matched for ethnicity, age, and
fixed dental prosthesis, or
sex. Four-factor ANOVA with repeated measures of 2 factors was used for statistical analysis (a=.05).
mesial movement of the
Results. Orthodontic patients with agenesis of one or both maxillary lateral incisors exhibited
canine are treatment options.
smaller than normal tooth size compared with the control group. The maxillary arch had a larger
In order to establish the
tooth size difference between the control and test groups than the mandibular arch (there was a
optimal amount of space
significant Jaw×Group interaction [F=4.78, P=.032]).
required for an ideal restoraConclusions. Agenesis of one or both maxillary lateral incisors is significantly associated with
tion, the contralateral lateral
tooth size discrepancy, which may affect the space remaining for restoration of the remaining
incisor size can be used as a
teeth. (J Prosthet Dent 2016;115:209-214)
guide for determining the size
of the missing lateral incisor.1 However, the contralateral
where midlines are coincident, canines are in a class I
tooth is frequently peg shaped or also missing.2 In order
relationship, and an ideal vertical and horizontal overlap
to determine the appropriate size, 2 methods have been
is present, the space required to restore the maxillary
lateral incisor may still be insufficient. A tooth size
suggested: use of the golden proportion, which indicates
that the lateral incisor should be approximately 62% of
discrepancy in the mandibular, maxillary, or both arches
the width of the central incisor; and the Bolton analysis,
could explain this clinical situation.
which is used to determine the required space for a
Numerous studies have evaluated tooth size and
missing lateral incisor.3,4
malocclusion,4-11 but no firm conclusions have been
drawn as to whether tooth size can determine or affect
A minimum of 6 mm, but ideally 7 mm of space, is
malocclusions. One study found the mesiodistal dimenusually recommended for an implant in the area of the
sion of the maxillary lateral incisor to be the most
lateral incisor.5,6 However, in many clinical situations,
Supported, in part, by the American Association of Orthodontists and American Association of Orthodontists Foundation (to J.A.B.). This article is based on the
Master’s thesis (J.W.) presented in 2011 in partial fulfilment of requirements for the degree of Master of Science, Marquette University School of Dentistry, 2011.
a
Adjunct Professor, Department of Developmental Sciences, Marquette University School of Dentistry, Milwaukee, Wis.
b
Associate Professor, Department of Orthodontics, Rutgers School of Dental Medicine, Newark, N.J.
c
Assistant Professor, Department of Oral Health and Rehabilitation, University of Louisville School of Dentistry, Louisville, Ky.
d
Research Associate I, Department of Institutional Assessment and Quality Improvement, Rutgers School of Dental Medicine, Newark, N.J.
THE JOURNAL OF PROSTHETIC DENTISTRY
209
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Volume 115 Issue 2
Clinical Implications
Because maxillary and mandibular teeth are smaller
than normal in patients missing one or both
maxillary lateral incisors, the space created or
remaining for the definitive restorations may be
smaller than ideal. Thus, clinicians should plan
accordingly.
significant variable affecting tooth size arch size
discrepancy compared with the mesiodistal and buccolingual dimensions of other teeth.10
Research has demonstrated that genetic factors may
contribute to agenesis and tooth size discrepancies.12-19
Specifically, MSX1 and PAX9 mutations have been associated with tooth agenesis.12,15 PAX9 gene mutation has
also been associated with smaller than normal teeth.19
Although individuals of different ethnic origins
experience dental agenesis, those of European origin are
more often missing maxillary lateral incisors, with a
higher incidence in women.16,20,21 Men generally have
larger teeth than women within any given ethnicity. To
minimize ethnic variations within the sample population,
this study focused on the agenesis of maxillary lateral
incisors of individuals of European origin.
Few studies have discussed the relationship between
tooth size and agenesis.22-26 However, some studies have
suggested that newer implants require smaller spaces for
implant placement, leading to size reduction in tooth
replacement.27-30 Thus, the purpose of this study was to
evaluate whether tooth size discrepancy is observed in
white orthodontic patients with agenesis of one or both
maxillary lateral incisors. The null hypothesis was that orthodontic patients with unilateral or bilateral agenesis of
the maxillary lateral incisor have the same sized teeth in the
maxilla and mandible as those in a matched control sample.
The secondary hypothesis was that sex, arch, and tooth type
affect tooth size in the missing lateral incisor group.
MATERIAL AND METHODS
This research was approved by the Institutional Review
Board. Forty sets of dental casts (21 women, 19 men,
mean 15.9 years of age), with missing maxillary lateral
incisors (22 unilateral, 18 bilateral) (Fig. 1), were collected
from local orthodontic practices. For comparison, an
equal number of dental casts were collected from the
Graduate Orthodontics program at the Marquette University School of Dentistry to form a control group
matched for ethnicity, age, and sex. Inclusion criteria for
the test population were: white individuals with unilateral or bilateral agenesis of the maxillary lateral incisors,
with the other permanent teeth (except for third molars)
in both arches fully erupted; and with no evidence of
extreme wear, breakdown, or interproximal reduction of
any teeth. Pretreatment and post-treatment dental casts
were used for measurement. Post-treatment dental casts
were examined only to confirm that no interproximal
enamel reduction or enameloplasty had been conducted
during treatment. Dental casts with crowns or mesiodistal restored teeth were rejected because of the modification of tooth structure and size.
The mean age for the test group was 15.9 years
(SD=7.12), ranging from 11 to 47 years of age, and the
mean age for the control group was 15.9 years (SD=6.7).
The orthodontic program’s computerized charting system
(axiUm; Exan Group) was used to search for the matching
sample. Mesiodistal widths of each tooth were measured
with a high-precision digital caliper (Digital Calipers; Masel,
Henry Schein Orthodontics), with measurements rounded
Figure 1. A, Patient cast showing unilateral agenesis of left maxillary lateral incisor, retained left primary canine, left permanent canine in position of
lateral incisor, and peg-shaped right lateral incisor. B, Cast of patient with agenesis of both maxillary lateral incisors. Maxillary canines moved mesially
into lateral incisor space.
THE JOURNAL OF PROSTHETIC DENTISTRY
Wright et al
February 2016
Figure 2. Digital caliper measuring mesiodistal widths of maxillary teeth.
Teeth were measured at their widest points.
to the nearest hundredth of a millimeter (Fig. 2). One
investigator (J.W.) collected all dental cast measurements.
For each combination of arches and teeth number,
except for maxillary lateral incisor teeth, the left and right
tooth sizes were averaged and used as an outcome variable for the tooth size in the data analysis. Because the
maxillary lateral incisor was missing in one or both sides
of the arch, all lateral incisors were excluded from data
analysis. Tooth sizes were compared between the test
and control groups and also compared for tooth
numbers, both jaws, and sex. A 4-factor ANOVA with
repeated measures of 2 factors was used for data analysis.
Tooth number and jaw were within-subjects factors,
whereas group and sex were between-subjects factors.
Software (SAS v9.4; SAS Institute Inc) was used for data
analysis (a=.05). Descriptive statistics are shown in
Table 1, and test results from repeated measures ANOVA
are shown in Table 2. In order to calculate the reliability
of measurements, 2 casts from the test group were
measured at 3 different time points, 2 months apart. The
intrarater version of the Shrout-Fleiss statistic test was
used to evaluate the reliability of the investigator’s tooth
width measurements.
RESULTS
The results of this study reject the null hypothesis that no
differences exist in tooth size in patients with agenesis.
Orthodontic patients with agenesis of one or both
maxillary lateral incisors exhibited smaller than normal
tooth size than the control group. (F=4.01, P=.049)
(Table 2; Fig. 3A).
Men were found to have larger teeth than women.
(Table 2; Fig. 3B). Canine teeth presented the largest
tooth size differences between men and women (there
was a significant tooth type×sex interaction [F=5.51,
P=.003]) (Table 2). First premolars presented the smallest
tooth size differences between men and women.
Wright et al
211
As expected, tooth size in the maxillary arch was
larger than in the mandibular arch (F=680.84, P<.001),
and the maxillary arch had larger tooth size differences
between control and test groups than the mandibular
arch (there was a significant arches×group interaction
[F=4.818, P=.031]) (Table 2).
Central incisors presented the largest tooth size differences between the maxillary and mandibular arches (there
was a significant tooth number×jaw interaction
[F=1879.67, P<.001]). The mean width for maxillary central
incisors was 8.51 mm, and the mean for mandibular incisor
was 5.24 mm (Table 1). First premolars had the smallest
tooth size difference between maxillary and mandibular
arches (Fig. 3C). The Shrout-Winer intrarater reliability test
found a consistency of 0.994, confirming the excellent
reliability of the test.
DISCUSSION
Although clinicians discuss the minimum necessary
space for an implant when one or both maxillary lateral
incisors are missing, they should focus on determining
the appropriate space for the implant/restoration. The
golden proportion for the anterior teeth can be considered when they determine the size for the missing lateral
incisor.3 Patients with smaller than normal maxillary and
mandibular teeth, as shown in the test sample of this
study, may require a smaller than necessary 6.5-mm or
7-mm space for an implant-supported replacement.5,6
Although the size of the lateral incisors was eliminated from Table 1, the average size for maxillary lateral
incisors in the test group was 5.39 mm. This size is
appreciably less than that of the minimal 7-mm tooth-totooth distance traditionally advocated for implant placement.6 The 7-mm tooth-to-tooth distance allows for
placement of an implant approximately 4 mm in diameter, with 1.5 mm between the implant and adjacent
teeth.26 With the advent of platform switching and narrow diameter implants, there is evidence that less space
may be needed for implant placement.27,28 A minimal
tooth-to-tooth space of 5.5 mm may be acceptable if a
3.5-mm diameter platform-switched implant is placed
with 1 mm between the implant and adjacent teeth.29,30
This procedure would allow for an appropriate maxillary
lateral incisor replacement.
Even though the present study evaluated the mean
mesiodistal widths of each tooth within a group, the large
range of the size of the maxillary lateral incisor size in
patients with unilateral agenesis (2.9 mm to 6.95 mm) is
of note. This demonstrates a wide spectrum in tooth size
of the maxillary lateral incisor when the contralateral
incisor is congenitally missing.
Yaqoob et al25 found an association between
the agenesis of maxillary lateral incisors and tooth size.
Mirabella et al26 also found that agenesis of a maxillary
THE JOURNAL OF PROSTHETIC DENTISTRY
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Volume 115 Issue 2
Table 1. Descriptive statistics of populations studied
Maxillary Arch
Tooth
Central incisor
Factor
n
Mean
Mandibular Arch
SD
Mean
SD
Total
Mean
SD
Group
Control
40
8.62
0.64
5.34
0.34
6.98
0.45
Test
40
8.39
0.67
5.13
0.41
6.76
0.52
Female
42
8.42
0.62
5.19
0.37
6.81
0.46
Male
38
8.60
0.70
5.28
0.42
6.94
0.53
Total
80
8.51
0.66
5.24
0.39
6.87
0.50
Sex
Canine
Group
Control
40
7.77
0.50
6.74
0.44
7.25
0.45
Test
40
7.53
0.47
6.52
0.48
7.03
0.45
Female
42
7.47
0.37
6.47
0.32
6.97
0.31
Male
38
7.84
0.55
6.81
0.54
7.33
0.52
Total
80
7.65
0.49
6.63
0.47
7.14
0.46
Sex
First premolar
Group
Control
40
7.02
0.42
7.12
0.48
7.07
0.43
Test
40
6.80
0.48
6.93
0.45
6.86
0.45
Female
42
6.88
0.43
7.01
0.44
6.95
0.42
Male
38
6.94
0.50
7.04
0.51
6.99
0.49
Total
80
6.91
0.46
7.02
0.47
6.97
0.45
Sex
Second premolar
Group
Control
40
6.65
0.41
7.14
0.39
6.90
0.38
Test
40
6.48
0.40
7.10
0.45
6.79
0.40
Female
42
6.52
0.41
7.04
0.37
6.78
0.37
Male
38
6.62
0.41
7.21
0.46
6.91
0.41
Total
80
6.57
0.41
7.12
0.42
6.84
0.39
Control
40
10.19
0.60
10.92
0.74
10.56
0.62
Test
40
9.93
0.55
10.93
0.59
10.43
0.51
Sex
First molar
Group
Sex
All teeth
Female
42
9.95
0.47
10.72
0.50
10.34
0.44
Male
38
10.18
0.68
11.15
0.76
10.67
0.64
Total
80
10.06
0.59
10.93
0.67
10.49
0.57
Group
Control
40
8.05
0.43
7.45
0.40
7.75
0.40
Test
40
7.83
0.43
7.32
0.40
7.58
0.40
Female
42
7.85
0.38
7.29
0.33
7.57
0.34
Male
38
8.04
0.48
7.50
0.45
7.77
0.46
Total
80
7.94
0.44
7.39
0.40
7.66
0.41
Sex
Total
lateral incisor was a strong predictor for reduced overall
tooth size. On average, that study showed that the difference in mesiodistal width of the maxillary central incisor
was 0.47 mm and that of the mandibular incisors was 0.43
mm. However, they had no control for race and sex.26
The results of the present study, where race, sex,
and age were controlled, also demonstrated that the
mandibular and maxillary teeth of patients with agenesis
of one of both maxillary lateral incisors were smaller than
those who had all permanent teeth. However, this study
could not find a specific tooth or group of teeth that
THE JOURNAL OF PROSTHETIC DENTISTRY
would indicate the discrepancy, nor has the study found
significant differences between specific teeth (central
incisor, canines, first and second premolar or molars) in
the control and sample test.
Ballard8 discovered that 90% of teeth in his sample
were not symmetrically sized between right and left sides,
with differences as much as 0.25 mm. In the present study,
no statistical differences were found between teeth on the
right and left sides. Therefore, the teeth were grouped
together, and an average size was created for each tooth.
Because one or both maxillary lateral incisors were missing
Wright et al
February 2016
213
Table 2. Repeated measures ANOVA
Factor
F
4.01
.049*
8.1
Sex
5.13
.026*
8.0
Tooth number
2908.66
<.001*
Arch
680.84
<.001*
0.7
.406
Tooth number × group
1.04
.384
Tooth number × sex
5.51
<.001*
Arch × group
4.78
.032*
Arch × sex
0.30
.583
Tooth number × arch
1879.67
<.001*
Tooth Size, Mean (mm)
Group
Group × sex
Group
Control
Test
8.2
P
7.9
7.8
7.7
7.6
7.5
7.4
Tooth number × group × sex
2.24
.065
7.3
Arch × group × sex
0.38
.542
7.2
Tooth number × arch × group
2.00
.094
Tooth number × arch × sex
2.01
.093
Tooth number × arch × group × sex
1.72
.145
Maxillary
Mandibular
Arch
A
*Statistically significant at a level of .05.
Wright et al
Tooth Size, Mean (mm)
11
10
Sex
Female
Male
9
8
7
6
Central Canine 1st
2nd
1st
Incisor
Premolar Premolar Molar
Tooth
11.0
10.0
Tooth Size, Mean (mm)
from the test sample group, all lateral incisors were
excluded from the study to fit the statistical model.
Mandibular teeth were also affected by maxillary
agenesis. However, the trend found in the mandibular
arch was not as large as that found in the maxillary arch.
If the discrepancy in the maxillary arch is larger, then a
replacement space for the missing lateral incisor may
need to be smaller in the maxilla.
The differences found for sex are contradictory in that
some authors found a sex-linked genetic association
between agenesis and tooth size19 and others25 found
similar results for both sexes. The present study found
that men had larger teeth than women in both test and
control groups. The canine teeth of men had the largest
difference in size, whereas first premolars presented the
smallest difference (Fig. 3B); however, this study did not
find a statistical correlation in tooth size between sex and
group samples.
Although Kinzer and Kokich1 advocated using the
Bolton analysis to determine the appropriate size for
the replacement of the missing maxillary lateral incisor, the
Bolton analysis was not conducted in this study test group,
mainly because of the unilateral or bilateral maxillary lateral
incisor agenesis. In retrospect, if the guidelines described by
Kinzer and Kokich1 had been followed, then the sum of the
widths of the mandibular anterior teeth should have been
divided by the sum of the available maxillary anterior teeth
plus x (the missing tooth), making this ratio equal to the
ideal anterior Bolton ratio (0.78). This could then have been
solved for x. This procedure may work well in patients with
appropriately sized anterior teeth. However, in a population with a suspected tooth size discrepancy, using the ideal
anterior Bolton ratio of 0.78, would not characterize such a
discrepancy and could suggest that the missing lateral
incisor was larger or smaller than normal.7 Furthermore,
with this equation, x (the size of the missing lateral incisor)
may not match the existing contralateral maxillary lateral
B
Arches
Mandibular
Maxillary
9.0
8.0
7.0
6.0
5.0
Central Canine 1st
2nd
1st
Incisor
Premolar Premolar Molar
Tooth
C
Figure 3. A, Differences are shown for test versus control groups with
maxillary and mandibular arches as factors. B, Differences in tooth size
are shown with sex and tooth types as factors. C, Differences in tooth
size are shown with maxillary and mandibular arches and tooth type as
factors.
THE JOURNAL OF PROSTHETIC DENTISTRY
214
incisor. This could lead the orthodontic clinician to prepare
an incorrect amount of space for the implant and crown
restoration.
Results that are statistically significant may not be
clinically significant. A tooth size discrepancy of 1.5 to 2
mm within an arch (0.75 to 1 mm/side) is deemed clinically significant. Othman and Harradine30 used the
Bolton analysis to determine this clinical significance. The
study was conducted with a sample of orthodontic patients with a full complement of teeth, making it harder
to compare it with the present study of participants with
agenesis. Because the present study did not calculate the
tooth size discrepancy per participant (as with the Bolton
analysis), it is difficult to determine whether the results
from this sample are clinically significant. These tooth
size reductions may be clinically significant for some
patients but not for others.
The present study found significantly smaller teeth in
patients with agenesis of one or both maxillary lateral
incisor. However, these findings were not specific and may
have varied among individuals as well as being dependent
on specific clinical situations. It is thus advisable to
approach patients with maxillary lateral incisor agenesis on
a case-by-case basis, keeping in mind that these patients
can have varying patterns of smaller than normal teeth.
This study was limited to white orthodontic patients,
so generalizing tooth size discrepancy patterns across
races may not be prudent. The main finding of the present sample from a white population showed patients
missing one or both maxillary lateral incisors to have
smaller tooth size than patients with all teeth present.
However, possibly because the reduced sample size, this
study could not find any specific tooth or group of teeth
responsible for the difference. The sample size should be
increased in future studies. Furthermore, people of African, Hispanic, or Asian descent with agenesis of the
maxillary lateral incisor(s) may show different tooth size
discrepancies or none at all. Future studies are recommended to evaluate these populations.
CONCLUSIONS
Orthodontic patients with unilateral or bilateral agenesis of
the maxillary lateral incisor have smaller than average teeth
when compared with a control-matched group. The
maxillary arch has larger tooth size differences between the
control and test groups than the mandibular arch. Clinicians should consider the findings of this study when
preparing maxillary lateral incisor sites for restorations.
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Corresponding author:
Dr Jose A. Bosio
Rutgers School of Dental Medicine
110 Bergen Street, Rm C780
Newark, NJ 07103
Email: [email protected]
Acknowledgments
The authors thank Drs William Lobb, Gerard T. Bradley, and Dawei Liu, Marquette University School of Dentistry, for advice and suggestions; and Jessica
Pruszynski and Dr Raphael Benoliel for statistical analysis.
Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.
Wright et al