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Mental foramina variations: a retrospective analysis using cone-beam computed tomography

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Mental foramina variations: a retrospective analysis using cone-beam computed tomography

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Mental Foramina Variations: A Retrospective Analysis Using Cone-Beam Computed
Tomography

LUBNA HAMADAH
Craniofacial Biology
Masters of Science
AUGUST, 2017

2
Table of Contents

Table of Figures ....................................................................................................................... 3
Table of Tables ........................................................................................................................ 3
Acknowledgments ................................................................................................................... 4
Dedications ............................................................................................................................. 4
I. ABSTRACT: ....................................................................................................................... 5
II. INTRODUCTION ............................................................................................................... 6
III. MATERIALS AND METHODS ........................................................................................... 11
IV. RESULTS ......................................................................................................................... 20
A. Linear Distance from the root apex of the second premolar to the mental foramen. .............. 20
B. Horizontal relationship of the mental foramen in relation to the root apex of the second
premolar. ....................................................................................................................................... 21
C. Vertical relationship of the mental foramen in relation to the root apex of the second
premolar. ....................................................................................................................................... 22
D. Characteristics of the mental foramen ................................................................................... 23
D.1. Area ............................................................................................................................................ 23
D.2. Length (Apico-coronal dimension) ............................................................................................. 24
D.3. Width (Mesio -distal dimension) ................................................................................................ 25
V. DISCUSSION ................................................................................................................... 28
A. Mental foramen characteristics related to Age ....................................................................... 30
B. Mental foramen characteristics related to gender .................................................................. 30
C. Mental foramen and ethnicity ................................................................................................ 31
VI. CONCLUSION ................................................................................................................. 33
VII. REFERENCES .................................................................................................................. 34

3

Table of Figures

Figure 1. Flow Diagram Outlining Identification of Study Subjects ........................................... 12
Figure 2. Axial View of Mental Foramen Used for Width Measurement. .................................. 15
Figure 3. Coronal View of the Mental Foramen for Length Measurement ................................. 15
Figure 4. Horizontal Positions of the Mental Foramen in Relation to Root Apex of the Second
Premolar (A) Distal, (B)Mesial, (C) Along .......................................................................... 16
Figure 5. Vertical Positions: (A) Coronal to the Root Apex of the Second Premolar, (B) In Line
With the Root Apex of the Second Premolar, (C) Apical to the Root Apex of the Second
Premolar ............................................................................................................................... 17
Figure 6. Distance of the Apex to the Mental Foramen by Gender, Age and Race ..................... 21
Figure 7. Area by Gender, Age and Race .................................................................................... 24
Figure 8. Apico – Coronal Dimension of the Mental Foramen by Gender, Age and Race ......... 25
Figure 9. Mesio-Distal Dimension of the Mental Foramen by Gender, Age and Race ............... 26

Table of Tables

Table 1. Characteristics of the Study Population (n=790) ........................................................... 13
Table 2. Linear Distance of the Mental Foramen to the Apex of the Mandibular Second
Premolar, by Gender, Age and Race/Ethnicity among Study Subjects (n=790) .................. 20
Table 3. Mesio-Distal and Corono-Apical Position of the Mental Foramen Among Study
Subjects, by Gender, Age and Race ..................................................................................... 23
Table 4. Mental Foramen Size by Gender, Age and Race/Ethnicity among Study Subjects
(n=790) ................................................................................................................................. 27

4
Acknowledgments

I would like to acknowledge my committee members:

Dr. Rafael Roges & Dr. Ilan Rotstein for their exceptional support and guidance during my
enrollment in the advanced Endodontics residency program as well as in the Craniofacial
biology masters program.

Dr. Michael Paine who helped me throughout my masters in craniofacial biology.

Dr. Melissa Wilson from the department of preventive medicine at University of Southern
California for her contribution in the statistical analysis.

Also, I would like to thank the Redmond imaging center at Herman Ostrow School of Dentistry
of USC.

The Author denies any conflicts of interest related to the study.

Dedications

I would like to dedicate this work to my family:

My mother and father, Amal and Essam, who raised me up to become what I am today and for
their continuous support throughout my life.

My husband, Amro, who believed in me, stood by my side and supported me all the way.

My siblings; Muna, Reem, Sara and Khaled.

5

I. ABSTRACT:

Introduction: Identifying the mental foramen (MF) location is important to protect the neurovascular
bundle that runs through it while performing different surgical and non-surgical dental procedures
around its location to prevent idiopathic nerve injury. Variation in size and location of the MF have been
reported in the literature. The purpose of this study is to evaluate the diameter of the mental foramen, its
proximity and position to the root apex of the mandibular second premolar in a human population of
males and females from different ethnicities and age groups.
Methods: A total of 790 CBCT images were retrospectively evaluated from axial, coronal and sagittal
views to measure the height, width and area of the mental foramen. Both vertical and horizontal
positions of the MF in relation to the root apex of the second premolar, as well as, the linear distance
between those two anatomical structures were also evaluated. Age, race/ethnicity and gender of the
subjects were also evaluated.
Results: The average of the mental foramen height, width and area in our sample were 2.758 ±.576
mm, 2.758±.576 mm and 6.983±2.648 mm
2
respectively. Males showed greater measurements than
females p<.001. The mean of the linear distance from the apex of second premolar to the mental
foramen in our total sample is 3.84 ±1.9mm, the distance in Asians was 1.01mm (p-value < .001, 95%
CI [ .62,1.42]) greater than in whites. 60.38% of MFs were found to be located mesial to the root apex of
the second premolar. 87% of the MFs were located apical to the root apex of the second premolar. The
horizontal location of MF in African Americans is likely to be located distal to the second premolar six
times higher than mesial to it (p < 0.007, OR= 6.24, 95% CI [ 2.72,14.33]).The Vertical location of MF
in Asians had higher tendency to be located apical than being in line with the root apex 11.11 times (p<
0.007, OR= 0.09, 95% CI [ 0.02, 0.38].
6
Conclusion: Gender and ethnicity does affect the mental foramen size, location and distance to the
second premolar. However, age does not affect any of the previous variables.
Keywords: Mental foramen; Mental foramina; Lower second premolar; Cone beam computed
tomography.

II. INTRODUCTION

One of the anatomical landmarks that should be recognized while performing endodontic
apical surgery in the mandible is the mental foramen (1). The mental foramen contains one of the
terminal branches of the inferior alveolar nerve, which is a branch of the mandibular division of
the trigeminal nerve. It descends following the inferior alveolar artery inferior to the lateral
pterygoid muscle and finally between the sphenomandibular ligament and the ramus of the
mandible until it enters the mandibular foramen. The mental branch of the inferior alveolar nerve
runs through the MF. The incisive branch supplies the pulp of lower anterior teeth (2).

Injury to the mental nerve can be manifested as altered sensation, thermal and tactile
changes in the structures that are being innervated by the mental nerve (3). Such an incident may
affect the patient’s quality of life temporarily or permanently. Patients should be informed about
the risk of nerve damage in the informed consent before initiating dental treatment even if there
is a very small chance for it to occur (4).
Identifying the mental foramen location is crucial when performing multiple
maxillofacial procedures, including administrating local anesthetic for mental nerve block,
performing incision and drainage or surgical placement of implants. Surgical and non-surgical
endodontic therapy can cause injury to the inferior alveolar and mental nerve in case of failure to
7
identify and respect these structures while performing the procedure. Careful analysis of
anatomical structures and pre-surgical planning aids the clinician to perform safer surgical and
non-surgical endodontic treatment lowering the risk of iatrogenic complications (5).
Proper working length determination in performing non-surgical root canal treatment and
careful instrumentation, irrigation and obturation to the root canal system should be done to
prevent iatrogenic complications (6). The importance of verifying the working length in each
step becomes more important as the mental foramen gets closer to the periapical lesion or the
root apex itself (4).
Planning and preparation for surgical treatment requires the clinician to be
knowledgeable about mental foramen location and its variation. Such knowledge will aid the
clinician to avoid placing vertical incision in that area and protect it while retracting and
mobilizing the flap (7).
Radiographically, the appearance of the mental foramen can be challenging to the dentist
as it may be confused with an apical radiolucency related to lower bicuspids. Thus, knowledge
about the mental foramen position is important for diagnosis (8).
Different techniques had been reported in the literature to locate the MF. Hazani et al (9)
used the palpation method which is not precise. It was done on fourteen fresh cadaveric hemi-
faces were dissected with the aid of magnifying loupes. Also, Kqiku et al (10) 2011 studied the
location of mental foramen by dissecting 400 hemi-mandibles from human cadavers with
unknown demographic data. Radiographic images such as periapical, panoramic, computed
tomography, cone beam computed tomography, magnetic resonance imaging and ultrasound had
all been used to locate the MF (11). Periapical radiographic images were unable to detect the MF
8
if it was located below the apical edge of the film. Moiseiwitch (1) suggested taking the
radiograph from different angles, panoramic view and vertical bitewing to enhance the detection
of the mental foramen when using periapical radiograph technique. Also, the reliability of the
image is affected by magnification due to the angulation of the film (11,12). In 2002, Jacobs et al
(13) reported a 94% detection rate of mental foramen on panoramic radiographs but only 49%
were distinct. On the other hand, computed tomography has showed 100% detection rate of MF
but the subjects were exposed to large amount of radiation. MRI showed high reliability in MF
detection but it is an expensive and not easily accessible imaging modality (14,15).
Ludlow et al (16) showed that CBCT is an appropriate and accurate modality for
measuring mandibular anatomical landmarks. Thus, currently CBCT is considered the best
imaging modality to detect and locate the mental foramen until imaging technology further
advances in that field (11).
Variations in the size and position of mental foramen have been reported in the literature.
Some studies found that the size of the mental foramen may vary per gender (17). Also, certain
ethnic groups may have higher tendency of a certain position of mental foramen in the mandible.
The location of the MF had been studied in different ethnic groups (18). Fishel et al (5)
studied Caucasian subpopulation and found that in 70.4% of cases the mental foramen was
located between the apices of the two mandibular premolars while 18.9% were located apically
to the second premolar. A study of Chinese population reported that in 21% of cases the MF was
found between the apices of the mandibular premolars and in 59% of the cases it was located
apically to the second premolar (19). A study of Nigerian population showed similar results to
9
the ones found in Chinese populations; 26.88% were located between the apices of the
mandibular premolars and 55.63% apically to the second premolar (20).
In a cadaveric study done in the United States reported that the position of the MF
showed a small but significant difference in location between men and women and between
ethnicities (21).

The MF orientation was inferior to the crown of the second premolar and
approximately 60% of the distance between the buccal cusp tip of the second premolar and the
inferior border of the mandible (12).
Another study (22) investigated the position of mental foramen in a sample of cadavers
with known demographics. A total of 55 cadavers (53 Caucasian and 2 African-American) met
the study criteria. The two African American cadavers were excluded since it is too small to be
considered as a representative of that population. They found that the position of the MF did not
differ between males and females. Also, three common locations for the MF were noticed: (1)
between the first and second premolars; (2) between second premolar and the first molar; and (3)
aligned with the crown of the second premolar.

Carruth et al (17) studied 106 CBCT from three different views (Tangential, axial, and
coronal) to determine the size and position of the MF in relation to the mandibular second
premolar apex and cemento-enamel junction (CEJ). Distinguishing characteristics of sex, age
and ethnicities were also evaluated. No significant differences were found in the MF size
between the age groups. However, MF in males had significantly greater height compared to
females. Also, MF in African-Americans was located more distally to the CEJ of the mandibular
second premolar with a significant difference compared to Caucasians (11). In another study,
142 CBCT images were analyzed to determine the size of the MF (23).

A statistical significant
10
difference in the height of the MF was found between males and females. No difference in the
size of the MF size was found among the different age groups.
Apinhasmit et al (24) assessed the right and left mental foramen in sixty-nine dried
mandibles (45 male and 24 females) in Thais. Their aim was to find if there is a difference in the
size orientation and location of the mental foramen related to gender or side. Both direct
inspection and computerized imaging analysis was done to each skull. Measurements did not
vary per side. However, males showed significantly larger readings than females in all
measurements.
Recently in 2016, Thakare et al (25) analyzed the vertical and horizontal position of the
mental foramen in 200 digital panoramic radiographs with an equal number of male and female
subjects. They found that the position of the mental foramen both vertically and horizontally is
the same in both genders but significant variation was present in the distance of the superior and
inferior borders of the mental foramen to the lower border of the mandible.
Thus, the aim of our study is to assess if there are variations in the size of the mental
foramen, its position and distance to the root apex of the lower second premolar between males
and females from different ethnicities and age groups.

11
III. MATERIALS AND METHODS

This study was designed as a cross-sectional retrospective analysis of cone beam
computed tomography CBCT images taken with a “Sirona, Galiloes comfort plus” unit at the
Redmond imaging center at Herman Ostrow School of Dentistry of the University of Southern
California. In April 2016, the study was approved for institutional review board exemption by
the University Park Institutional Review Board (UPIRB), project ID: UP-16-00248. The IT
department at Ostrow conducted an electronic database query searching CBCT code (M70486)
in Axium. The query identified 3590 records from 2008 to 2016 and a list was generated. Patient
identifiers were removed and only age, gender and ethnicity were disclosed. Exclusion of 1655
scans was done because of the patient not having the minimum age, missing demographic data or
not meeting the inclusion criteria specified below (Figure 1). A total of 793 MF were analyzed
from 564 CBCT scans. 459 scans had both the right and left mental foramina were included, 181
scans with only the right mental foramen and 153 with only the left mental foramen. Last, three
cases that had multi-rooted second premolar were excluded as suggested by the statistician,
because of difficulty of establishing measurement points. A total of 790 CBCT scans were used
for analysis in this study. Characteristics of the study population is shown in table 1.
Inclusion criteria:
(1) CBCT voxel size 0.3mm or less
(2) Mandibular premolars and first molars are present
Exclusion Criteria:
(1) Presence of a root resorption at the apex of the mandibular second premolar
(2) Presence of previous apicectomy in the mandibular second premolar
(3) Presence of periapical radiolucency in the region of interest,
12
(4) Presence of dental implant in the region of interest,
(5) Edentulous mandibles,
(6) One of the mandibular premolars or first molars is absent
(7) multi-rooted second premolar.

Figure 1. Flow Diagram Outlining Identification of Study Subjects

3590 subjects
identified
Exclusions: (1655)
Missing age (n=14)
Missing sex (n=11)
Missing race (n=1454)
Age <20 (n=176)
1935 subjects
Excluded for not
meeting
inclusion/exclusion
criteria
*
(n=1371)
564 subjects
13
Table 1. Characteristics of the Study Population (n=790)
Characteristic
a
Count Percent (%)
Gender
Male 377 47.7
Female 413 52.3
Age (years)
20-29 95 12.0
30-39 142 18.0
40-49 146 18.5
50-59 171 21.6
60+ 236 29.9
Race/Ethnicity
African American 63 8.0
Asian 126 16.0
Hispanic 262 33.2
White 339 42.9
Side
Right 409 51.5
Left 381 48.5
a
Percentages not adding to 100% in any one characteristic are due to rounding error.

The reconstructed studies were exported in multi-file “Digital Imaging and
Communications in Medicine” (DICOM) format and examined in InVivo 5.4 Dental software
Anatomage, (San Jose, CA), three-dimensional volumetric imaging software using the
multiplanar reconstruction-viewing tool on Dell PC (OptiPlex 3040). 40% of the mental
foramina were measured twice to insure viewer intra-reliability.
Each scan was measured by one investigator for the following:
a. Shortest distance from the MF circumference to the mandibular second premolar root
apex
b. Height of the MF (corono-apically) CA
c. Width of MF (mesio-distal) MD
d. Area will be calculated with a mathematical formula:
Area= a (CA/2)*b (MD/2)*π
14
e. Relation of the root apex to the mental foramen in horizontal plane (mesial, distal or
along) the apex.
f. Relation of the root apex to the mental foramen in vertical plane (coronal, apical or in
line) the apex.
g. Data was divided per:
(1) Age: 20-29, 30-39, 40-49, 50-59, >60
(2) Gender: Female, Male
(3) Ethnicity/Race: Asian, Caucasian, African-American, Hispanic.
Measurements were done to all 561 DICOMs using the Invivo 5.4 software. Using the
axial view, the maximum width of the foramen was measured as shown in figure 2. To measure
the length, the maximum measurement was taken from the coronal view as shown in figure 3.
Using the sagittal section of the 3D reconstructed view the linear distance from the mental
foramen to the apex of the second premolar was measured by using the ruler tab in the software.
Steps:
First, the contrast of the image is adjusted until the view is clear. Next, the radiographical
apex of the premolar was marked. A Line was drawn running through the entire length of the PM
corono-apically from the cusp tip to the marked apex. Afterwards, the mental foramen is marked
and a line is drawn connecting the nearest point on the circumference of the mental foramen to
the marked point on the apex of the root. From the same view, both the vertical and horizontal
relationship between the root apex and the mental foramen was taken for each scan. Vertical
position was set to be “in line “with the apex if the foramen was located within the straight line
that connects the mental foramen to the apex of the root. Coronal if the root apex lies above the
mental foramen and Apical if the apex lies below the mental foramen (Fig.4). The horizontal
15
relationship between the root apex and the mental foramen was taken for each scan as following:
Mesial, distal or along. Along if it lies within the long access of the root. Mesial, if the foramen
is lies right to the root apex and Distal if it lies left to the root apex (Fig.5).

Figure 2. Axial view of mental foramen used for width measurement.

Figure 3. Coronal view of the mental foramen for length measurement

16

Figure 4. Horizontal Positions of the mental foramen in relation to root apex of the second
premolar (A) Distal, (B)Mesial, (C) Along

17

Figure 5. Vertical Positions: (A) Coronal to the root apex of the second premolar, (B) In line
with the root apex of the second premolar, (C) Apical to the root apex of the second premolar

18
All data obtained was entered excel sheet and statistical analysis was done by Dr.
Mellissa Wilson assistant professor from the department of preventive medicine at University of
Southern California, using the “Stata 14.0” (College Station, Texas) software.
Statistical analysis:
The objective of the current study is to determine if there are variations in the size of the
mental foramen and its position between males and females from different ethnicities and age
groups. Specifically, we will evaluate whether (1) mental foramen size (A-C , M-D, area), (2)
linear distance of mental foramen to the apex of the mandibular second premolar (mm), (3)
mesiodistal (mesial, distal or along the apex) and coronoapical (coronal, along, or inferior)
position of mental foramen in relation to apex of mandibular second premolar differ by (1) age
(20-29, 30-39, 40-49, 50-59 and >60), (2) gender (M/F) and (3) race/ ethnicity (Asian,
Caucasian, African-American and Hispanic). We hypothesize that there will be no differences in
mental foramen size and position by these factors. The primary outcomes of the study will be
the investigation of area and linear distance of mental foramen to the apex of the mandibular
second premolar (mm) and the other evaluations will be of secondary interest.
To address these questions, we conducted an electronic database query and identified
3590 records covering time between 2008 to 2016. Total sample of 793 mental foramina were
analyzed, three cases that had multi-rooted second premolar were excluded as suggested by the
statistician, leaving 790 for statistical analysis (Fig. 1).
For this cross-sectional study, we described the study population with respect to age,
gender, race/ethnicity and orientation (right/left) using counts and frequencies. Subsequently, we
investigated the independent effects of age, race and ethnicity on mental foramen size and
position using linear regression for outcomes such as A-C, M-D, area, distance from apex to
19
mental foramen, and depth. For subjects with two measurements for outcome variables (e.g., A-
C) models were fit in two ways. First, we fit the model using the main mandibular foramen as the
outcome. Next, we fit a second model using the average of the two measurements as the
outcome. Since there were no appreciable differences between these two models, we present the
averaged outcomes.
For outcomes that are categorical (e.g., mesio-distal position of mental foramen in relation
to apex), we used multinomial logistic regression to model the independent effects of age race and
ethnicity. We evaluated the fit of all models via inspection of residuals and evaluation of influential
points. A Bonferroni-corrected p-value of <.007 (.05/7) was considered statistically significant.
Power Calculation
The primary outcomes are linear distance of mental foramen to the apex of the
mandibular second premolar and area which were analyzed using multivariable linear regression.
Since the sample size is fixed at 790, we have calculated the effect size that can be detected with
80% power and a two-sided p-value of .007 and three predictor variables. Based on these
assumptions, we will be able to detect a rather small effect size of f
2
=0.016 between any two
groups in the mental foramen area and/or linear distance of mental foramen to the apex of the
mandibular second premolar.

20
IV. RESULTS

A. Linear Distance from the root apex of the second premolar to the mental foramen.
The mean of the linear distance from the apex of second premolar to the mental foramen
in our total sample is 3.84 ±1.9mm. Linear regression model showed that distance stays the same
and is not affected by age (Table 2, p > 0.05). However, gender had shown to influence the
distance as it increased in males by 0.68 mm compared to females (Table 2, p < 0.001, 95% CI [
.41,.95]). Also, ethnicity had shown a difference as the distance in Asians was 1.01mm (Table 2,
p < 0.001, 95% CI [ .62,1.42]) greater than in whites (Fig.6).
Table 2. Linear Distance of the Mental Foramen to the Apex of the Mandibular Second
Premolar, by Gender, Age and Race/Ethnicity among Study Subjects (n=790)
Characteristic Beta (95% CI) p-value
Gender
Female 0 (ref) --
Male 0.68 (0.41, 0.95) <.001
Age (years)
20-29 0 (ref) --
30-39 -0.14 (-0.65, 0.37) .60
40-49 -0.13 (-0.63, 0.38) .62
50-59 0.14 (-0.35, 0.63) .57
60+ -0.07 (-0.54, 0.40) .77
Race/Ethnicity
White 0 (ref) --
African American -0.03 (-0.55, 0.49) .91
Asian
1.01 (0.62, 1.42)
<.001
Hispanic 0.41 (0.09, 0.73) .01
a
Beta is interpreted as the change in linear distance (in mm) per unit change in the characteristic relative to the reference category,
mutually adjusted for all other characteristics in the linear regression model (e.g., gender is adjusted for age and race). The linear
regression equation can be used to predict the outcome for any given combination of characteristics. We used the following
equation: E(LD|x) = 3.26 + 0.68(Male) - 0.14(Age
30-39
) - 0.13(Age
40-49
) + 0.14(Age
50-59
) - 0.07(Age
60+
) - 0.03(Race
AA
) +
1.01(Race
A
) + 0.41(Race
H
).

21

Figure 6. Distance of the Apex to the Mental Foramen by Gender, Age and Race

B. Horizontal relationship of the mental foramen in relation to the root apex of the
second premolar.
Most of MF in our sample were found to be located mesial to the root apex of the second
premolar 60.38%. The next more common location was aligned with the long access of the
second premolar 33.29% and only 6.33% were located distal to the second premolar. Odds ratios
(OR) and 95% confidence intervals were calculated using multinomial logistic regression. All
characteristics are mutually adjusted for one another (e.g., gender is adjusted for age and race).
Age and gender did not increase the likelihood of certain position. The likelihood of the mental
foramen to be located distal to the second premolar is six times higher than mesial location in
African Americans (Table 3, p < 0.007, OR= 6.24, 95% CI[ 2.72,14.33]).

0 5 10 15 0 5 10 15
20-29 30-39 40-49 50-59 60+ 20-29 30-39 40-49 50-59 60+
20-29 30-39 40-49 50-59 60+ 20-29 30-39 40-49 50-59 60+
F M F M F M F M F M F M F M F M F M F M
F M F M F M F M F M F M F M F M F M F M
Afr/Am Asian
Hispanic White
Distance Apex to MF
Graphs by Race
22
C. Vertical relationship of the mental foramen in relation to the root apex of the second
premolar.
Eighty seven percent of the mental foramens were located apical to the root apex of the
second premolar while 9.24 % were found to be in line with the root apex of the same tooth.
Only 2.91% located coronal to the root apex of the second premolar. Odds ratios (OR) and 95%
confidence intervals were calculated using multinomial logistic regression. All characteristics
are mutually adjusted for one another (e.g., gender is adjusted for age and race). As the
horizontal position, age and gender did not affect the likelihood of certain vertical position.
However, Asians had higher tendency to be located apical than being in line with the root apex
11.11 times (Table 3, p< 0.007, OR= 0.09, 95% CI [ 0.02, 0.38]) and none were found to be
located coronal to it.

23
Table 3. Mesio-distal and Corono-apical Position of the Mental Foramen Among Study
Subjects, by Gender, Age and Race
Mesio-distal Position
OR (95% CI)
a

Corono-apical Position
OR (95% CI)
a

Mesial
(n=477)
Along
(n=263)
Distal
(n=72)
Apical
(n=697)
Inline
(n=73)
Coronal
(n=23)
Characteristic
Gender
Female 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref)
Male 1.0 (ref) 0.90
(0.66, 1.22)
1.34
(0.73 , 2.46)
1.0 (ref) 0.76
(0.46, 1.25)
0.60
(0.25, 1.44)
Age (years)
20-29 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref)
30-39 1.0 (ref) 1.15
(0.63, 2.10)
1.11
(0.29, 4.22)
1.0 (ref) 0.62
(0.25, 1.53)
0.66
(0.17, 2.58)
40-49 1.0 (ref) 1.84
(1.02, 3.33)
3.14
(0.97, 10.14)
1.0 (ref) 0.98
(0.42, 2.29)
0.62
(0.16, 2.44)
50-59 1.0 (ref) 1.77
(1.0, 3.14)
1.90
(0.58, 6.29)
1.0 (ref) 0.37
(0.14, 0.97)
0.45
(0.11, 1.77)
60+ 1.0 (ref) 1.85
(1.07, 3.19)
0.91
(0.26, 3.16)
1.0 (ref) 0.71
(0.31, 1.60)
0.25
(0.06, 1.06)
Race/Ethnicity
White 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref) 1.0 (ref)
African American 1.0 (ref) 1.72
(0.93, 3.18)
6.24
(2.72, 14.33)
b

1.0 (ref) 0.21
(0.05, 0.89)
0.29
(0.04, 2.28)
Asian 1.0 (ref) 1.67
(1.07, 2.61)
1.59
(0.64, 3.93)
1.0 (ref) 0.09
(0.02, 0.38)
b

inestimable
Hispanic 1.0 (ref) 1.48
(1.03, 2.13)
0.90
(0.40, 2.00)
1.0 (ref) 0.60
(0.34, 1.03)
0.35
(0.13, 0.94)
a
Odds ratios (OR) and 95% confidence intervals were calculated using multinomial logistic regression. All characteristics are
mutually adjusted for one another (e.g., gender is adjusted for age and race). The ORs can be be interpreted as the risk of having
a particular postion and characteristic (e.g., corono-apical, age 30-39) compared to being in referent category (e.g., corono-apical,
age 20-29) for a given position relative to the referent position (e.g., corono-apical vs. inferior). The multinomial logistic
regression equations can be used to predict the outcome for any given combination of characteristics. We used the following
equations: logit(p(Apex)) = -1.25 -0.11(Male) + 0.14(Age
30-39
) + 0.61(Age
40-49
) + 0.57(Age
50-59
) + 0.61(Age
60+
) + 0.54(Race
AA
)
+ 0.51(Race
A
) + 0.39(Race
H
); logit(p(Distal)) = -3.15 + 0.30(Male) + 0.11(Age
30-39
) + 1.14(Age
40-49
) + 0.64(Age
50-59
) –
0.10(Age
60+
) + 1.83(Race
AA
) + 0.46(Race
A
) – 0.11(Race
H
); logit(p(Along)) = -1.29 – 0.27(Male) – 0.48(Age
30-39
) – 0.02(Age
40-
49
) – 0.99(Age
50-59
) – 0.35(Age
60+
) – 1.57(Race
AA
) – 2.42(Race
A
) – 0.52(Race
H
); logit(p(Coronallp;oq)) = -1.85 + -0.50(Male) –
0.42(Age
30-39
) – 0.48(Age
40-49
) – 0.81(Age
50-59
) – 1.38(Age
60+
) - (1.22Race
AA
) + -15.24(Race
A
) + 1.06(Race
H
)
b
Statistically significant at p<0.007

D. Characteristics of the mental foramen
D.1. Area
The mean value of the mental foramen area in our sample was 6.983±2.648 mm
2
. Linear
regression model showed that age had no effect on the area as no significant change was between
individuals of different age groups(Fig.7). Mental foramen area in males were statistically
greater than females by 1.08 mm
2
, (Table 4, p < 0.001, 95% CI [ .73,1.43]). Hispanics showed
24
significantly smaller area by 0.57 mm
2
than whites (p <0.007) while African Americans were
2.15 mm
2
larger (Table 4, p <0.001, 95% CI [ 1.48,2.83]).

Figure 7. Area by Gender, Age and Race

D.2. Length (Apico-coronal dimension)
The average Apico-coronal (length) of the MF in our sample was 2.758 ±.576 mm. It
ranged from 0.83 to 4.92 mm. Linear regression model showed that age had no effect on the area
as no significant change was between individuals of different age groups (Fig.8). Males were
statistically greater than females by 0.28 mm
2
(Table 4, p < 0.001, 95% CI [ .20, .35]). African
Americans showed significantly higher readings (.35 mm) compared to whites (p-value <0.001,
95% CI [ .20, .50]). However, no significant difference was found between Asians and Hispanics
compared to whites (Table 4, p >0.05).
0 5 10 15 20 0 5 10 15 20
20-29 30-39 40-49 50-59 60+ 20-29 30-39 40-49 50-59 60+
20-29 30-39 40-49 50-59 60+ 20-29 30-39 40-49 50-59 60+
F M F M F M F M F M F M F M F M F M F M
F M F M F M F M F M F M F M F M F M F M
Afr/Am Asian
Hispanic White
area
Graphs by Race
25

Figure 8. Apico – coronal Dimension of the Mental Foramen by Gender, Age and Race

D.3. Width (Mesio -distal dimension)
The average width of the MF in our sample was 2.758±.576 mm. Linear regression
model showed that age had no effect on the area as no significant change was between
individuals of different age groups(Fig.9). Males were statistically greater than females by .17
mm
2
with (Table 4, p < 0.001, 95% CI [.07,.26]). African Americans showed significantly higher
readings .53 mm compared to whites (p-value <.001, 95% CI [ .20, .50]). However, no
significant difference was found between Asians and Hispanics compared to whites (Table 4, p
>0.05).

1 2 3 4 5 1 2 3 4 5
20-29 30-39 40-49 50-59 60+ 20-29 30-39 40-49 50-59 60+
20-29 30-39 40-49 50-59 60+ 20-29 30-39 40-49 50-59 60+
F M F M F M F M F M F M F M F M F M F M
F M F M F M F M F M F M F M F M F M F M
Afr/Am Asian
Hispanic White
A-C
Graphs by Race
26

Figure 9. Mesio-Distal Dimension of the Mental Foramen by Gender, Age and Race

0 2 4 6 0 2 4 6
20-29 30-39 40-49 50-59 60+ 20-29 30-39 40-49 50-59 60+
20-29 30-39 40-49 50-59 60+ 20-29 30-39 40-49 50-59 60+
F M F M F M F M F M F M F M F M F M F M
F M F M F M F M F M F M F M F M F M F M
Afr/Am Asian
Hispanic White
M-D
Graphs by Race
27
Table 4. Mental Foramen Size by Gender, Age and Race/Ethnicity among Study Subjects
(n=790)

a
Beta is interpreted as the change in AC/MD/Area (in mm) per unit change in the characteristic relative to the reference
category, mutually adjusted for all other characteristics in the linear regression model (e.g., gender is adjusted for age and
race). The linear regression equation can be used to predict the outcome for any given combination of characteristics. We
used the following equations: E(AC|x) = 2.54 + 0.28(Male) + 0.09(Age
30-39
) + 0.15(Age
40-49
) + 0.08(Age
50-59
) + 0.10(Age
60+
)
+ 0.35(Race
AA
) -0.02(Race
A
) -0.09(Race
H
); E(MD|x) = 2.99 + 0.17(Male) + 0.04(Age
30-39
) + 0.02(Age
40-49
) + 0.03(Age
50-59
)
+ 0.20(Age
60+
) + 0.53(Race
AA
) -0.05(Race
A
) -0.13(Race
H
); E(Area|x) = 6.11 + 1.08(Male) + 0.31(Age
30-39
) + 0.43(Age
40-49
) +
0.25(Age
50-59
) + 0.70(Age
60+
) + 2.15(Race
AA
) -0.19(Race
A
) -0.57(Race
H
)

AC MD Area
Characteristic Beta
a
(95% CI) p-
value
Beta (95% CI) p-value Beta (95% CI) p-value
Gender
Female 0 (ref) -- 0 (ref) --
Male 0.28
(0.20, 0.35)
<.001 0.17
(0.07, 0.26)
<.001 1.08
(0.73, 1.43)
<.001
Age (years)
20-29 0 (ref) -- 0 (ref) -- 0 (ref) --
30-39 0.09
(-0.06, 0.23)
.24 0.04
(-0.13, 0.22)
.63 0.31
(-0.35, 0.96)
.36
40-49 0.15
(0.01, 0.29)
.04 0.02
(-0.15, 0.20)
.79 0.43
(-0.22, 1.08)
.19
50-59 0.08
(-0.06, 0.21)
.26 0.03
(-0.15, 0.20)
.77 0.25
(-0.39, 0.88)
.45
60+ 0.10
(-0.03, 0.24)
.12 0.20
(0.04, 0.36)
.02 0.70
(0.10, 1.31)
.02
Race/Ethnicity
White 0 (ref) -- 0 (ref) --
African American 0.35 (0.20, 0.50) <.001 0.53
(0.35, 0.71)
<.001 2.15
(1.48, 2.83)
<.001
Asian -0.02
(-0.13, 0.09)
.73 -0.05
(-0.19, 0.09)
.49 -0.19
(-0.70, 0.33)
.48
Hispanic -0.09
(-0.18, 0.005)
.06 -0.13
(-0.24, -0.01)
.03 -0.57
(-0.99, -0.16)
.007
28
V. DISCUSSION

Several studies have been reported in the literature studying the position and diameter of
the mental foramen. Certain studies were done on specific ethnic groups. For example, Green
(26) and Wang et al (19) compared the position of the mental foramen in Chinese mandibles
(18,19). Olasoji et al (27) and Kekere-Ekun (20) did their studies on Nigerians. In 1998,
Moiseiwitsch studied the mental foramen position in North American white population (22).
However, other studies did not identify the ethnicity of the studied population as in Berge and
Bergman, 2001(28).
Different methods had been used to study the mental foramen characteristics in humans.
Apinhasmit et al (24) and Moiseiwitsch (22), used dry skulls and cadavers to directly visualize
the mental foramen. Different Radiographical images had been also used. Periapical and
panoramic radiographs, computed tomography, cone beam computed tomography, magnetic
resonance imaging and ultrasound. In 2015, Muinelo-Lorenzo et al (29) compared the anatomical
characteristics and visibility of mental foramen in panoramic radiography and CBCT. They
concluded that panoramic radiograph is not an adequate technique to properly identify the mental
foramen and accessory mental foramen and they recommended CBCT for that purpose.
The subjects in our study had age, gender and ethnicity identified. In addition, CBCT
images were used to study the mental foramen characteristics, position and distance to the root
apex of the lower second premolar. To our knowledge, no other study, that have used similar
methodologies, on CBCT images has a large sample size and statistical analysis of so many
variables as our study.
The mean distance of the mental foramen to the apex of the second premolar in our
sample was 3.84±1.9 mm. In 2016, Chong et al (4), using similar methodology found similar
29
results, they reported a mean distance of 3.70 ±1.57mm. They also reported that the second
premolar is the closest tooth to the mental foramen. Von Arx et al (23) reported a mean distance
to be 5 mm between the MF and the closest root without mentioning the specific root. This is
different from our study in that the measurements were taken in relation to the second premolar
only.
The most common horizontal location of the mental foramen in relation to the long
access of the second premolar in our sample was mesial in 60.38%. Our study corroborates this
mesial location reported by Von Arx et al (23). Von Arx et al (23) found the location of his
majority MF in his sample between the first and second premolars (56%). However, in 1990,
Philips et al (12) reported that the mental foramen was mesially located only 18% of the time and
62.7% of the time it was intersecting with the long access of the second premolar crown. This
difference in the results can be possibly explained by the different methodology used, cadaveric
versus CBCT measurements, since the apex of the second premolar was not visible and housed
in bone. In another study by Philips et al (30) where they used periapical radiographs in their
methodology on the same seventy-five adult human mandibles that were used in part I (12). They
assessed the position of the mental foramen in relation to the mandibular second premolar apex
both horizontally and vertically. The mesial position was the most common horizontal location.
Vertically, the mental foramen was located mostly below the apex of the second premolar which
is in line with our study results. Also, like our result Apinhasmit et al (24) reported the most
common position of the mental foramen to be below the second premolar.

30
A. Mental foramen characteristics related to Age
Based on our sample, the mean (apico-coronal dimension) length and ( mesio-distal
dimension) width of the mental foramen were not statistically affected by age. In 2013, Von Arx
et al. (23) reported same findings. They also found that age did not affect the likelihood of the
mental foramen to be positioned in either of the three positions (23). This agrees with our results.
The linear distance from the MF to the root apex of the second premolar in our sample as it did
not show significant increase or decrease in individuals of different age groups(p>.05). The
relative location of the inferior alveolar nerve canal, mandibular foramen and mental foramen
had been found to remain constant with increasing age (31). In contrast, greater height and width
of the mental foramen in older individuals has been reported when individuals less than twenty
years old were included in the study (32).
B. Mental foramen characteristics related to gender
Our sample showed that gender affects the mental foramen measurements. Males showed
statistically significant greater readings in length, width and area by .28mm, .17mm and
1.08mm
2
respectively compared to females (p<.001). In 2015, Carruth et al (17) statistically
greater averages for both the coronal and tangential height in their male sample by .55 mm and
.56mm respectively compared to females (p <0.002). Also, in 2012 Kalender et al (32), measured
total of 386 mental foramina on CBCT images using “Maxillim software”. They found a
significant difference in the size of mental foramen between males and females as males showed
greater values. The mean horizontal and vertical sizes in males were 3.9 mm and 3.6 mm
respectively, while in females mean horizontal size was 3.5 mm and mean vertical size was
3.3mm (p<0.05).
31
The root apex of the second premolar in our male subjects was found to be positioned
further away from the mental foramen in males by 0.68 mm than females (p-value < .001, 95%
CI [ .41,.95]). The likelihood of the mental foramen to be positioned in either of the three
positions both horizontally and vertically was not statistically affected by gender. Same findings
were reported in both vonArx et al (23) and Carruth et al (17).

C. Mental foramen and ethnicity
Race/ethnicity have shown some significant differences in mental foramen characteristics
in our study sample. The size of the MF in African Americans was significantly greater than in
whites (p<0.001). Apico-coronal, mesiodistal as well as the area were all greater by 0.35,0.53
and 2.15 mm in African Americans compared to whites, respectively (p<0.001). However, the
MF area was smaller by 0.57 mm in Asians compared to whites (p<0.007). This is opposite to
what was reported by von Arx et al (23) 2013 were no significant difference was found in MF
measurements related to ethnicity/race(p0.>05). This could possibly be due to the smaller sample
size of their study. Also, Linear Distance from the root apex of the second premolar to the MF in
our sample was significantly greater by 1.01 mm in Asians compared to whites (p<0.001). On
the other hand, the MF was closer to root apex of second premolar in African Americans
compared to whites but without statistical significance (p>0.91).
Ethnicity/race had shown to affect the MF position in our sample as higher tendency to
certain positions were found to be associated with specific ethnicity/race. The likelihood of the
MF in African Americans to be located distal to the second premolar apex is six times higher
than mesial position compared to whites (p < 0.007, OR= 6.24, 95% CI [ 2.72,14.33]). Similar
findings were reported when the horizontal relationship between the MF and CEJ of the second
32
premolar was studied in different ethnicities (p< 0.006). MF was positioned more distally to the
CEJ of second premolar in African Americans compared to whites (23). Also, a study done
cadavers showed that MF had higher tendency to be located distal to the second premolar 32% of
the time in African Americans compared to 10% in whites (21). The vertical position of the MF
in our sample was affected by ethnicity/race. MF in Asians compared to whites had 11.11 times
higher tendency to be located inferior to the apex of second premolar than being in the same
vertical plane (p<0.007).

33
VI. CONCLUSION

Gender and ethnicity does affect the mental foramen size, location and distance to the
second premolar. However, age does not affect any of the previous variables. These variations
highlight the importance of the dentist’s knowledge about what to expect when planning for a
surgical or non-surgical procedure near the area of the lower second premolar to prevent any
iatrogenic damage. This knowledge is especially important when there is no access to CBCT
imaging.
This study emphasizes the value of CBCT images, as long as ALARA and ALADA
guidelines are met, since they are an excellent tool to evaluate anatomical structures in three-
dimensions.

34
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Asset Metadata

Creator Hamadah, Lubna E. (author)

Core Title Mental foramina variations: a retrospective analysis using cone-beam computed tomography

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 06/30/2017

Defense Date 05/25/2017

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag cone beam computed tomography,lower second premolar,mental foramen,mental foramina,OAI-PMH Harvest

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Roges, Rafael (committee chair), Paine, Michael (committee member), Rotstein, Ilan (committee member)

Creator Email hamadah@usc.edu,lubna.hamadah@gmail.com

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