İnterforaminal bölgenin retrospektif olarak spiral bilgisayarlı tomografi ile radyolojik değerlendirilmesi: dental implant uygulaması için uygunluğun yaş ve dişsel durumlar ile ilişkisi

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Introduction
The anatomical position of the mental foramen
(MF) is important in many processes of dentistry,
including diagnosis, anesthesia, endodontics and
surgery. Locating mental nerve, identifying its loop
and measuring its length have great importance before replacement of missing teeth with endosseous
implants at interforaminal region (1,2). Surgical traumas to mental nerve are possible during the implant
therapy and cause altered sensation in lower lip and
chin. The location of the MF and the anterior loop of
mental neurovascular bundle determine the number
and the location of dental implant at interforaminal area (1-3). As the age advances, the direction of
aperture of the MF alters in direction from forwards
to upwards and backwards (1,4). Besides, after losing
lower bicuspids, MF approaches to the superior border of alveolar process owing to the alveolar atrophy,
shortens the length of dental implant to be located or
precludes implant therapy in this region (1-3). Some
studies have shown that the distance between MF
and lower cortex of mandible is also smaller in older
patients owing to basal bone atrophy (1,4-6).
There are evident differences among studies in the
location and available alveolar bone quantity around
mental nerve (7-15). Reports have shown that the
MF is not in a constant location, changing with age
(4,13), gender (10,14,15), dental status (5,6,13,14),
systemic conditions (10) and racial variety (1-3). In
previous reports the localization and the measurements related with MF was determined by panoramic
radiography (PR) (5,6,9,12-16), periapical radiography
(8,12,17), direct measurement during the surgery (3)
or dissection in anatomic cadavers (2,4,7,10,11).
PR is a commonly used diagnostic imaging technique, showing all oral anatomic landmarks in one
radiographic image. Its accuracy is limited owing to
difficulty in controlling the distortion and magnifi-
* Department of Periodontology, Dental Sciences Center, Gulhane Military
Medical Academy
** Department of Oral and Maxillofacial Surgery, Dental Sciences Center,
Gulhane Military Medical Academy
*** Dental Service, Turkish Military Academy
Reprint request: Dr. Hasan Ayberk Altuğ, Dental Service, Turkish Military
Academy, Dikmen-06540, Ankara, Turkey
E-mail: aybork@yahoo.com
Date submitted: January 07, 2010 • Date accepted: February 22, 201070 • June 2010 • Gulhane Med J Şahin et al.
cation; however, it has been commonly used before
implant surgery to estimate the quality and quantity
of the alveolar bone for appropriate implant length
and wide to avoid damage to anatomic structures during the surgery (1,3,9,15-18). Two-dimensional images such as periapical and PR have evident deficiencies, especially depending on patient position (19,20)
and corticalization quantity of the canal wall (20,21).
Besides, computed tomography (CT) scans locate
and measure more accurately with disadvantages of
more radiation exposure and financial cost (18-21).
Spiral computed tomography (SCT) shows the cortical margins of the neurovascular bundle more clearly
when compared with other tomographies and panoramic images even in poor bone quality (18,21). SCT
shows the boundaries of the mandibular canal, MF
and its anterior loop more clearly even in older ages
(18,21,22).
As far as we know, the position of MF has not yet
been investigated by SCT to date. The aim of this retrospective study was to investigate the changes in
anatomic landmarks at interforaminal region with
aging and tooth loss, using the radiodiagnostic advantages of SCT, which can directly affect dental
implant planning.
Material and Methods
Each patient was selected according to his/her dental
status, age and gender from a population of 638 subjects who attended the Department of Periodontology
and Department of Oral and Maxillofacial Surgery of
Gulhane Military Medical Academy between March
2006 and March 2008. The patients who had unerupted mandibular premolar, deciduous teeth and systemic diseases related to bone such as osteoporosis, diabetes, renal disease, thyroid disease, and who were
not a member of Caucasian race were not included in
the study. In addition, the patients with a history of
chronic or aggressive periodontitis were not included
in dentate group.
The study was carried out with SCT images of
100 selected patients. Fifty of the patients were younger than 40 years of age (Younger age group). In
both younger (mean 34.98±3.76) and older (mean
59.09±7.06) age groups, 25 of the subjects were fully
dentate up to mandibular bicuspids contralaterally
and the remaining 25 were completely edentate at interforaminal region including mandibular bicuspids.
The average age distribution of each group is shown
in Table I. SCT was performed with a sixteen channel
multidetector Philips MX8000 IDT (Philips Medical
Systems, Best, Netherlands). SCT was used at 120 kV
and 221 mAs, with 0.5 sec rotation time, with 1.6 x
0.75 cm rectangular collimator, 1 mm slice thickness.
The data were transferred to a network computer
workstation (Philips Extended Brillance Workspace
2.0.11, Philips Medical Systems, Best, Netherlands).
Processing conditions of the films were standardized by using Kodak-2180 (Eastman Kodak Company,
Rochester, NY, USA) automatic developer.
Radiographic measurement procedure: To ensure consistency, all radiographs were selected and measured
by one author (SS), with a previous experience in
SCT interpretation and dental implant planning and
treatment. Seven measurements were made in crosssectional images of SCT to evaluate interforaminal
region:
1- The shortest distance from the two lines passing
through the most superior point of mental foramen
and alveolar crest in edentulous or fully dentate interforaminal area (MF-AC) (Figure 1). In fully dentate
interforaminal areas, the periodontal ligament of tooth, same level with mental foramen, started to be
of equal width, was considered as the most superior
border of the alveolar crest (23).
2- The shortest distance from the two lines passing
through the most apical border of the mental foramen to the outer lower cortex of the mandible (MFLC) (Figure 1).
Table I. Average age distribution of the age groups
Group n Mean age
Young (≤40), fully dentate interforaminal region 25 34.85±4.34
Young (≤40), completely edentulous interforaminal
region
25 35.12±3.19
Older (>40), fully dentate interforaminal region 25 53.72±8.81
Older (>40), completely edentulous interforaminal
region
25 64.46±5.32
Figure 1. The evaluation of mental foramen in cross-sectional CT
images: MF-AC: Mental foramen to alveolar crest. MF-LC: Mental
foramen to lower cortex. HMF: The height of mental foramen. MFLEB: Mental foramen to lingual external borderVolume 52 • Issue 2 Retrospective radiographic evaluation • 71
3- The highest distance from the coronal and apical
border of mental foramina was evaluated to measure
its height (HMF) (Figure 1).
4- The shortest distance between the lingual border
of mental foramen and inferior part of lingual external border of the mandible (MF-LEB) (Figure 1).
5- The distance between the most anterior point of
contralateral mental foramens (MF-MF) (2). If there
was any loop, it was measured from the most mesial
point of the mental genu. MF-MF was measured in
axial section of CT images.
6- Distance from the first and the last image of
mental foramen (WMF).
7- If an anterior mental loop (ML) was observed at
SCT, the distance from the first and last images which
mandibular and mental canal were seen together or
attached like “figure 8” or elongated mandibular canal apperance (Figure 2) (18,24).
Intervals between the above mentioned landmarks
were measured by counting cross-sectional 1-mm
thick slices. During MF-MF measurement, care was
taken not to include the first and last slice of foramen mandible and MF, opening to the internal and
external cortex, respectively (18,24).
All examinations were performed on a standard
radiologic light box, under standardized viewing
conditions.
Statistical analysis: The data were computerized and
the statistical analysis was performed with a software
program (SPSS 9, SPSS Inc., Chicago, IL, USA). The
measured values from right and left sides were pooled in the same column for statistical analyses, displaying mean, standard deviations and range for all
parameters in each group. Two-tailed independent t
test was carried out to establish possible differences
between younger and older subjects and fully dentate
and edentate interforaminal regions. A p value greater than 0.05 was not considered significant.
Results
MF-AC: The values ranged from 5.5 to 22 mm, no
significant difference being found between younger
and older subjects (p>0.05); but when the same subjects were evaluated by their dental status, a significant difference was found (p<0.001) (Tables II,III).
MF-LC: The values ranged from 9.5 to 19 mm and
no significant difference was found between younger and older subjects (p>0.05), but when the same
subjects were evaluated by their dental status, a significant difference was found (p<0.001) (Tables II,III).
HMF: The values ranged from 2 to 6 mm, with a
significant difference between younger and older
subjects and between edentate and dentate groups
(p<0.01) (Tables II,III).
MF-LEB: The values ranged from 3 to 6.5 mm, with
no significant difference when the subjects were either evaluated by their age or dental status (p>0.05)
(Tables II,III).
Figure 2. Anterior loop of the mental nerve appears in cross-sections 61 through 63, nerve exits in cross-sections 64. Loop has a “figure 8”
shape with mandibular canal
Table II. Comparison of the value of average measurements (mm) on spiral computed tomography according to age
Younger (≤40) subjects* (n=50) Older (>40) subjects* (n=50) Significance
MF-AC (mm) 12.16±3.45 (8.5-22) 11.92±3.89 (5.5-21) Not significant
MF-LC (mm) 14.53±3.45 (10-19) 14.32±3.07 (9.5-18) Not significant
MF-MF (mm) 62.88±6.92 (45-89) 65.32±6.78 (48-90) p<0.001
WMF (mm) 3.62±1.12 (2-5) 2.53±1.31 (2-5) p<0.01
HMF (mm) 3.67±1.22 (2-5) 2.21±1.31 (2.5-6) p<0.01
MF-LEB (mm) 5.49±1.32 (3-6.5) 5.28±1.62 (3-6.5) Not significant
*: Values are given as mean±standard deviation (range)
MF-AC: Mental foramen to alveolar crest, MF-LC: Mental foramen to lower cortex, MF-MF: Between contralateral mental foramens, HMF: The height of mental foramen,
WMF: Width of mental foramen, MF-LEB: Mental foramen to lingual external border72 • June 2010 • Gulhane Med J Şahin et al.
MF-MF: The values ranged from 45 to 90 mm, with
a significant difference related to age and dental status (p<0.001) (Tables II,III).
WMF: The values ranged from 2 to 5 mm, with a
significant difference according to age and dental status (p<0.01) (Tables II,III).
ML: The prevalence and the length of ML is 30%
and 3.11±1.62 mm (range between 1 to 7 mm), respectively. The prevalence of mental loop in younger,
older, fully dentate and edentate groups was 28%,
32%, 31% and 29%, respectively, with no significant
differences among the groups (p>0.05). The average
length of mental loop in younger, older, fully dentate
and edentate groups was 3.05±1.51 mm, 3.16±1.73
mm, 3.14±1.56 mm and 3.08±1.68 mm, respectively.
There were no significant differences among the groups (p>0.05).
Discussion
During dental surgical procedures at interforaminal
region such as dental implant placement and harvesting bone graft from symphysial area, it is important
to know the exact location of MF and the amount of
surrounding alveolar bone, in order not to damage
mental nerve (1,2). To determine the location of MF
and to measure the quantity of available bone, periapical radiographs and PR are usually used but these
radiography techniques have magnification and distortion problems (8,9,19,20). MF displacements were
seen in periapical radiographs and PR, when compared with anatomical measurements mainly because
of the position of the X-ray and patient head, respectively (8,9). CT is a more precise imaging technique
and SCT has the capability of showing the cortical
margins of the neurovascular bundle more clearly
when compared with other tomographies and panoramic images even in poor bone quality (18,21). The
purpose of this study was to determine the exact location of anatomic landmarks and the available bone
varying with age and dental status at interforaminal
region. The present study is unique and significant
in that it has a large population evaluated in SCT as
entire measurements must be known before dental
implant placement at interforaminal region.
According to previous reports, there is significant
change in the location of MF according to the systemic conditions (10), gender (10,14,15) and race (1-
3). Depending on these previous reports, comparable
population of systemically healthy female and male
subjects were included in all groups which were kept
as large as possible, with no racial variety including
only Caucasian race. Some investigators stated that
the age-related bone resorption begins at about forties (11). In accordance with these studies the age groups were planned as younger (≤40) and older (>40) in
this study. There are clear distinctions between the
studies regarding the anatomic location of mental foramina in relation to dental status, race, age and gender of the subjects and diagnostic methods used for
evaluation (1-17). Comparison of this study with the
previous investigations is difficult, because the boundaries of anatomical landmarks were not defined
clearly (4,12) or selected differently (2,3,6-9,13,14).
Regarding the above mentioned distinctions between
the studies, we will discuss each measurement one by
one to eliminate confusion.
MF-AC: Prior to the eruption, the foramen is located closer to the upper margin alveolar crest, and
it descends lower than half way between the upper
and lower borders of mandible with time (4). Some
previous studies stated that in older ages with edentelous mandible, it is situated closer to the alveolar
crest owing to physiologic alveolar atrophy, sometimes top of the alveolar crest (1,4,5). In previous reports cusp tip (3,7), apex of lower bicuspid (8,9) or
cemento-enamel junction (CEJ) (2) superior to MF
was selected as a landmark, not allowing the comparison of edentulous patients with dentate ones.
Table III. Comparison of the value of average measurements on spiral computed tomography according to dental status of the subject
population
Edentate interforaminal region*
(n=50)
Fully dentate interforaminal region*
(n=50)
Significance
MF-AC (mm) 8.14±3.27 (5.5-17.5) 15.94±4.07 (11.5-22) p<0.001
MF-LC (mm) 12.65±3.82 (9.5-14.5) 16.20±2.70 (10-19) p<0.001
MF-MF (mm) 66.78±7.01 (45-90) 61.42±6.69 (45-90) p<0.001
WMF (mm) 2.39±1.15 (2-5) 3.76±1.28 (2-5) p<0.01
HMF (mm) 2.25±1.32 (2-6) 3.63±1.21 (2.5-6) p<0.01
MF-LEB (mm) 5.37±1.35 (3-6) 5.40±1.59 (3-6.5) Not significant
*: Values are given as mean±standard deviation (range)
MF-AC: Mental foramen to alveolar crest, MF-LC: Mental foramen to lower cortex, MF-MF: Between contralateral mental foramens, HMF: The height of mental foramen,
WMF: Width of mental foramen, MF-LEB: Mental foramen to lingual external borderVolume 52 • Issue 2 Retrospective radiographic evaluation • 73
Besides, alveolar crest around the extracted tooth
is important to determine the length of immediate
dental placement instead of CEJ especially in patients
with periodontitis. Previous studies have also shown
significant variation in MF-AC level, ranging between
0 to 22 mm (2,4,12,14). In our study mean length
were 11.92 mm and 12.16 mm in older and younger
groups, respectively, with no significant difference.
When the same patient population was evaluated according to their dental status the distance was 8.14
mm and 15.94 mm in edentulous and fully dentate
groups, respectively, being significantly higher in the
dentate group. Although different reference points
were used, comparable results were obtained by other
studies. Neiva et al. found this distance to be 15.52
mm in 22 Caucasian dentate mandibles at the mean
age of 79.1 (2). Oğuz et al. in a study of 34 dried young (30-40 years old) Turkish adult male mandibles,
found MF-AC to be 14.45 mm (12). Gershenson et al.
reported MF-AC, as 13.77 mm and 5.43 mm in dentate and edentate adults, respectively (4). Our findings
were similar with these studies, which supports the
idea that the dental status of the mandible can be
more important than the age in evaluating the distance of MF-AC. According to our study, there was
no significant difference between younger and older
groups, each of which has equal numbers of dentate and edentate subjects (p>0.05). Karaağaçlıoğlu et
al. have stated that the mandibular ridge decreases
more significantly in older age (>60); however all examined subjects were edentelous male subjects older
than 40 years of age (11). MF-AC can be affected by
age in edentelous mandibles, linearly decreasing with
the elapsed period of edentulism.
MF-LC: The mandibular canal shows a convex
downward contour when coursing through the body
of mandible, get closest to the lower edge of mandible at first molar level and then moves upwards when
approaching mental foramina (1,24). In previous reports MF-LC level varied between 5 to 21.5 mm (2-
6,12,13). There are conflicting results about the distance of MF-LC. Xie et al. stated that the basal bone
below the MF was not affected by dental status and
age in men but it was found to be significantly smaller in old edentulous women when compared with
old dentate or young dentate women (6). Soikonen et
al. found that mental foramina was situated 3.8 mm
lower in edentulous mandibles than in dentate jaws
and stated that the foramen is moving through the lower cortex (5). However, in both of these studies they
did not evaluate the MF-AC level, only investigating
basal bone atrophy and stating downwards movement of mental loop according to the age and dental
status (5,6). Nevertheless in our study we observed
resorption in both distances (MF-AC and MF-LC) according to the dental status but the results were not
related with age in the same subject population. This
agrees with the report of Kingsmill et al. who found
a correlation between alveolar bone and basal bone
height (25). They stated that the basal bone is not
in a constant position throughout lifetime and can
change with tooth loss as alveolar bone. Therefore,
it is unsafe to measure other landmarks from lower
cortex (25,26).
MF-MF: Although this distance has paramount importance in determining the number and width of
dental implants in interforaminal region, it was evaluated scarcely in studies on mental foramina (2). We
did not find any references in the literature comparing the age or dental status in relation to MF-MF. In
some studies it was observed that mental foramina
moves distally in jaws continuing to atrophy, which
is in accordance with our study (5,13,15).
WMF and HMF: Previous reports found the size
of mental foramina in a range of 0.75 to 6.5 mm
(1,4,7-9,11,12,16). Gershenson et al. found the diameter of round shaped MF to be 2.12 mm and 1.83
mm in dentulous and edentulous adult subjects, respectively, which is lower than our results (4). Neiva
et al. found WMF to be 3.59 mm in 22 old dentate
Caucasian skulls, which is similar to our dentate subjects (2). We found a significant difference related to
age and dental status, being wider and higher in both
dentate and young groups.
MF-LEB: We did not observe any significant differences related to age or dental status. Ulm et al. measured the distance between the mandibular canal and
lingual external border in 6 different locations in 43
edentelous mandibles showing different degrees of
alveolar ridge resorptions (27). Their results ranged
between 2.87 to 5.92 mm, not finding any significant difference even between the depressed ridge and
the alveolus after the extraction, similar to our study
(27). Kingsmill et al. have stated that the mandibular
changes related with dental status affects the alveolar height more when compared to its bucco-lingual
width, which is in accordance with our findings (25).
ML: The prevalence and the length of ML have
significant variations between the studies. This wide
variation in results may be related to racial variety,
bone quality and diagnostic techniques (1,18). We
did not found any difference related to the age and
dental status. In the review of MF, it is stated that
there must be a safety zone 2 mm above and mesial
to the mental foramina (1). However in our previous
report (18) and present study we found the length of 74 • June 2010 • Gulhane Med J Şahin et al.
ML to be longer than 3 mm, in accordance with other
studies (2,28). The authors of this study think that the
length of the safety zone for MF must be re-evaluated.
One unique feature of CT is its cross-sectional imaging, giving information not only about the buccolingual width of the alveolar bone but also about the
quality of alveolar bone and determining the anatomical landmarks like mandibular foramen, mandibular canal, MF, anterior loop of mental nerve, incisive
canal and lingual canal (18-20,22,24,29). Jacobs et al.
identified the MF in all cases, with good visibility in
97% of them in CT scans (22) but in their other report they observed only 49% of MF with good visibility in PR (30). SCT gave more accurate values not
only than PR, but also than hypocycloidal and spiral
tomography (21).
By the assistance of the superior features of SCT it
is concluded that MF-AC and MF-LC measurements
can change according to the dental status of the individual, not with age. When the patients are compared only according to their age, the difference related
with the dental status can be overlooked (Tables II,III).
Although shortened vertical distances of MF-AC and
MF-LC creates disadvantage by placing shorter dental
implant; longer MF-MF, narrower WMF, lower HMF
and constant MF-LEB can yield advantage for more or
wider dental implant placement in edentate and older
subjects at interforaminal region. The prevalence and
the length of the mental loop may not depend on age
or dental status. We found the length of mental loop
to range from 1 to 7 mm, and according to our findings the safety zone must be at least 3 mm.
Acknowledgment
The authors thank Mrs. Melike Bahcecitapar at the
Department of Statistics, Faculty of Science, Hacettepe
University for her statistical support in analyzing the
data.