Download Azadeh Amin Eslami, D.D.S., M.S. Saint Louis University in Partial Fulfillment

MENTAL FORAMEN LOCATION IN PATIENTS WITH MANDIBULAR SECOND
PREMOLAR AGENESIS
Azadeh Amin Eslami, D.D.S., M.S.
A Thesis Presented to the Graduate Faculty of
Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2014
COMMITTEE IN CHARGE OF CANDIDACY:
Professor D. Douglas Miley
Chairperson and Advisor
Associate Professor Ki Beom Kim
Associate Clinical Professor Donald R. Oliver
i
DEDICATION
This Master thesis is dedicated to my wonderful family, Mazyar and Rayan.
You have always encouraged and supported me along my journey through both
specialties at SLU, and I will always be thankful and loving you for that.
A special thank you goes out to my mom, Maryam, who guided me right from
the beginning throughout this long road of my career. I am forever grateful
towards that. I love my job and that makes me a happy person.
Finally, I am thankful to have enjoyed an exceptional faculty at Saint Louis
University, CADE. I am certain that they have provided me with a strong
foundation which will be the basis of my future practice.
ii
ACKNOWLEDGEMENTS
This thesis project was not possible without the support of the following
faculty:
Dr. D. Douglas Miley. Thank you for sparking my interest in the
interdisciplinary aspect of Periodontics and Orthodontics and helping me develop
this thesis topic. Your guidance during this thesis preparation and your
continuous support has meant a lot to me.
Dr. Rolf Behrents. Thank you for contributions to my thesis project and
allowing me to obtain an excellent orthodontic education at Saint Louis
University.
Dr. Donald Oliver. Thank you for always being there for me whether
discussing clinical patients, thesis, organizational aspects of orthodontics or
contributing to my general knowledge. Your attention to detail is not comparable
to anybody else.
Dr. Ki Beom Kim. Thank you for your guidance during my thesis preparation
and for enhancing my orthodontic skills clinically as well as in the classroom.
Your innovative ideas are a great enrichment to the program.
Dr. Eustáquio Araújo. Thank you for being the most dedicated teacher in
orthodontics. I consider myself very fortunate to have had the opportunity to learn
from one of the world class orthodontist.
iii
Dr. J. Martin Palomo. Thank you for providing me with the opportunity to use
CBCT records for this thesis from the orthodontic department at the Case
Western University in Cleveland, OH.
Dr. Heidi Israel. Thank you for your assistance with the statistical analysis for
this thesis.
iv
TABLE OF CONTENTS
List of Tables ........................................................................................................ vi
List of Figures ...................................................................................................... vii
CHAPTER 1: INTRODUCTION ............................................................................ 1
CHAPTER 2: REVIEW OF THE LITERATURE
Congenital absence of mandibular second premolars ................... 3
Formation of the permanent mandibular second premolar ............ 5
Treatment alternatives for permanent mandibular second
premolars agenesis ........................................................................ 6
Cone Beam Computed Tomography............................................ 13
Anatomy of the inferior alveolar nerve and the mental foramen ... 15
Facial growth and position of the mental foramen ........................ 19
When is the right time to place dental implants in postpubertal
orthodontic patients? ................................................................... 20
Surgical implant placement in the posterior mandible .................. 21
Short implants and success rates ................................................ 22
Statement of Thesis ..................................................................... 24
Literature Cited ............................................................................ 26
CHAPTER 3: JOURNAL ARTICLE
Abstract ....................................................................................... 36
Introduction .................................................................................. 37
Materials and Methods ................................................................ 39
Sample ................................................................................... 39
Cone Beam Computed Tomography (CBCT) Technique ....... 39
Measurement Repeatability Study .......................................... 42
Statistical Analysis ....................................................................... 43
Results ......................................................................................... 45
Discussion .................................................................................... 53
Conclusions ................................................................................. 58
Literature Cited............................................................................. 59
VITA AUCTORIS ................................................................................................ 63
v
LIST OF TABLES
Table 1:
Group Statistics, calculation of mean and standard deviation for
linear measurements (in mm) from AOP to MF, AC to MF and
LMM to MF in the congenitally missing mandibular second
premolar sites and controls on the right and left........................... 47
Table 2:
Independent Samples Test shows statistical difference in an
increased distance of the AOP to MF measurement on the left
missing mandibular second premolar sites compared to controls,
see highlight .................................................................................. 48
Table 3:
Group Statistics, calculation of mean and standard deviation for
linear measurements (in mm) from AOP to MF, AC to MF and
LMM to MF in the congenitally missing mandibular second
premolar sites and controls on the right and left in relation to
gender .......................................................................................... 49
Table 4:
Independent Samples Test shows no statistical significance in
terms of gender related to the position of MF ............................... 50
Table 5:
Group statistics, calculation of mean and standard deviation for
linear measurements (in mm) from AOP to MF, AC to MF and
LMM to MF in the congenitally missing mandibular second
premolar sites and controls on the right and left in
relation to age ............................................................................... 51
Table 6:
Independent Samples Test shows significant difference for
female individuals under 18 years of age where MF is positioned
more occlusally compared to controls ........................................... 52
vi
LIST OF FIGURES
Figure 1:
Example of patient’s anatomical occlusal plane. ........................... 41
Figure 2:
Example of landmark selections on a 3D image ........................... 41
Figure 3:
Example of vertical measurements from AOP to MF, MF to AC
and MF to LMM ............................................................................. 42
Figure 4:
Example of the three vertical lineaer CBCT measurements.......... 44
Figure 5:
Report of mean measurements in test and control groups............ 46
vii
CHAPTER 1: INTRODUCTION
The congenital absence of mandibular second premolars is the most
common finding after congenitally missing third molars. As the studies are
limited, we don’t know the exact incidence of lower second premolar agenesis,
but it has been reported to occur with a frequency of about 3.4%, with a higher
incidence than maxillary lateral incisor agenesis.1 In orthodontic treatment
planning, a decision needs to be made on how to handle that absence. Some
treatment options include:
A. Maintaining the deciduous second molar
B. Attempted orthodontic space closure
C. Prosthetic replacement with:
a. Removable partial denture
b. Fixed bridge
c. Dental implant
d. Autotransplantation
Studies have been performed to analyze the position of the mental
foramen and inferior alveolar nerve in subjects not missing the mandibular
second premolars and in partially and totally edentulous patients that are missing
teeth in addition to the second premolar, but there are no known published
studies of the location and variability of the mental foramen and mandibular canal
when only the second premolar is congenitally missing.2-4
Clinicians have observed that the congenital absence of the second
premolar is associated with a more crestal location of the mental foramen, which
1
may impact the placement as well as selection of the prosthetic implant. Dentist
performing implant surgery always need to be aware of critical anatomy. Special
attention may be needed for these patients since the vertical height of available
bone for implant placement may be limited.
Earlier publications have described the high degree of variability in
mandibular bone volume surrounding the inferior alveolar nerve. The use of CT
scans should be considered for surgical procedures in the posterior mandible
when there is risk of injury to the inferior alveolar nerve.4-7
After evaluating the existing literature, gaps remain regarding the
following question: “Does the mental foramen location vary in patients with
congenitally missing mandibular second premolars compared to patients with
mandibular second premolars present?” There are no known published studies
that have attempted to investigate this question. Knowing the location of the
mental foramen and its variation when the second premolar is congenitally
absent would aid the clinician in understanding the risks and importance of
treatment planning for these patients.
There are a number of studies that suggest multiple treatment options to
manage congenitally missing mandibular second premolars.8-14 The long term
goal of this study is to investigate whether the location of the mental foramen is
altered in patients with congenitally missing mandibular second premolars.
2
CHAPTER 2: REVIEW OF THE LITERATURE
Congenital absence of mandibular second premolars
For many years, dental anthropologists have studied and researched the
evolution of the human dentition. One theory that tries to explain the failure of
formation of some teeth versus others is the Butler’s theory. Butler hypothesized
that the mammalian dentition can be divided into three morphologic fields
corresponding to incisors, cuspids, and bicuspids/molars. There is one “key”
tooth within each field that is considered stable. Flanking teeth however within
each field become progressively less stable. Thinking about each quadrant
separately, the “key” tooth in the bicuspid/molar field would be the first molar.
This would place the second and third molar in this field at the distal end, and the
first and second bicuspids at the mesial end. Based on this scheme, the third
molar and the first bicuspid would be predicted to be most variable in size and
shape. Clinical epidemiology studies support this theory for the third molars, but
not for the first bicuspid. However, the very first mammals had 4 bicuspids per
quadrant, whereas some higher primates, such as human, have lost the first two
bicuspids on the mesial end. These teeth would have been farthest away from
the “key” tooth and in evolution considered unstable.15
Changes in the human masticatory apparatus originate from the development
of food processing from pre-historic to modern times. 16 According to the
literature, a decrease of mesio-distal width of human teeth has been observed
over time and generally hypodontia is associated with the reduction in tooth
size.17, 18
3
Congenitally missing permanent teeth are known to be of genetic origin,
which may be an explanation for racial or ethnic variations. Missing teeth may
occur in isolation, or as part of a syndrome. Genetic components present
themselves in many syndromes such as recessive X-linked syndromes, and thus
female carriers might remain undiagnosed.19 Arte et al. and Vastardis described
familial tooth agenesis as an autosomal dominant mutation in the MSX1 gene.
Other environmental factors and genetic defects may also have an important
influence in the variability of the phenotypic expression.20, 21
Some researchers have suggested that agenesis can be caused by factors
related either to the mucosal ectoderm, to the ectomesenchyme, or even to
innervation.22 In individuals who are congenitally missing a single tooth, it was
considered less likely that the occurrence of agenesis was caused by a
ectodermal factor, but more likely by a local factor in that area, such as local
innervation.23
Mattheeuws et al. questioned if hypodontia in Caucasians has increased
during the 20th century in a meta-analysis. They concluded that the second
mandibular premolars were most often congenitally absent, but the considered
period of time is too short and available data too limited to describe a possible
trend in the human dentition. However, the study confirms that hypodontia has
been diagnosed more often in recent studies.24
In multiple large population studies, it has been shown that agenesis of
permanent teeth occurs between 6.1% and 8% of the population, and it is more
prevalent in girls than boys.25, 26 One study has been able to report a female to
4
male ratio of 3:2 for mandibular second premolar agenesis, whereas others have
been unable to confirm this finding.25, 27
The permanent dentition is more frequently affected from tooth agenesis than
the primary dentition. The incidence of congenitally missing permanent teeth in
other studies has been found to range from 1.6% to 9.6% in the general
population excluding the third molars.26, 27 In the deciduous dentition tooth
agenesis ranges from 0.5% to 0.9%.28 Whenever a primary tooth is missing, it’s
permanent counterpart will be missing as well.29
Hypodontia of permanent teeth occurs with equal frequency in the maxillary
and mandibular arches and usually affects the third molars the most. However,
excluding the third molars, the frequently affected agenesis in the permanent
dentition is the second mandibular premolars.1, 30-32 Often times, one assumes
that the congenital absence of these teeth tends to be symmetrical or bilateral. 33
However, in a meta-analysis it was concluded that unilateral absence occurred
more frequently than bilateral agenesis in maxillary and mandibular second
premolars for both genders.19 Unilateral agenesis may be the result of a reduced
penetrance of a gene.34
Formation of the permanent mandibular second premolar
There is some individual variation in eruption ages in children. This is rather
extensive and endogenic, which implies the differences in chronological age.
Skeletal age, and amount of residual growth are parameters that must be
considered, in addition to the dental stage. 35 Van der Linden and Du Burl found
5
evidence that crypt formation of the permanent mandibular second premolar
starts as early as 9 months of age. Crown calcification begins at 3 years of age
and completes at 6 years of age.36 Proffit reports mandibular second permanent
premolar crown calcification at age 28 months. The crown is completed at 7.5
years of age and the eruption happens around 11.4 years. Root formation is not
completed until 15 years of age.37
Treatment alternatives for permanent mandibular second premolars agenesis
As clinicians who are always looking to best serve their patients, excellent
background evidence-based information on how to treat agenesis of the
mandibular permanent second premolars, is of great importance. Once it has
been diagnosed that the patient is missing one or two mandibular second
premolars, it is best to initiate an orthodontic analysis. Orthodontic treatment
planning for patients with agenetic teeth can be challenging. Direct clinical
implications can be attributed to congenital absence of permanent teeth. Early
evaluation of size and number of teeth remaining in both arches should aid the
orthodontist in planning and managing treatment. The type of malocclusion,
degree of crowding and facial profile is of prime concern in determining a final
treatment plan.38
The five alternatives dealing with mandibular missing premolars are
orthodontic space closure or space opening for a prosthetic replacement, dental
implants or autotransplantation. All of these can compromise aesthetics and
periodontal health and function.
6
Maintaining the deciduous second molar is a possible option, but usually the
interdigitation of teeth is compromised due to the larger size of the primary tooth.
Bjerklin and Bennett found that retained deciduous second molars undergo 60%
root resorption on their mesial root and 46% on the distal root between the ages
11 and 20. This poses a question of how predictable it is to anticipate the primary
molar to last long-term. A future loss of the tooth would leave a large edentulous
space that would have to be closed prosthetically. However, the rate of root
resorption of deciduous mandibular second molars diminishes with age, and
these teeth have been shown to be occlusally stable without further root
resorption beyond the age of 20. In fact, the authors found that neither
infraocclusion, tipping of adjacent teeth, nor root resorption increased very much,
after the age of 20.39 In a study of retained deciduous second mandibular molars
in adults with congenitally missing second premolar teeth, the authors state that
the potential for retaining the deciduous second molar for many years is excellent
and a viable treatment alternative. They conclude that the average length of time
they might be retained, rivals the lifespan of some prosthetic appliances.
Contrary to the above mentioned study, Sletten et al. found that shortening of the
retained deciduous second molar tooth and its’ submergence were negligible.40
A different approach is to close the edentulous space by performing a
hemisection of the primary mandibular second molar at an early age so that the
permanent first molar can erupt in a mesial direction without affecting the position
of the lower incisors. This alternative is attractive if the orthodontist observes the
patient regularly.41 Valencia et al found that sequential slicing followed by
7
hemisection of the second deciduous molars in cases of congenitally missing
mandibular second premolars showed a greater success rate compared with
extractions. Permanent molars showed a 80% bodily space closure within one
year. A higher success rate, up to 90% space closure was achieved when this
technique was applied at an early age between 8-9 years of age. The success
rate tended to decrease as the patients aged. After age 9 options become more
limited, and that’s where spontaneous space closure might no longer be a viable
option.42
An alternative treatment plan for a patient with agenetic mandibular second
premolars is to orthodontically close the space after extracting the primary
second molar. It will be advantageous to close the edentulous space if the patient
has severe crowding in the opposing arch or a protrusive facial profile. However,
an undesirable facial profile can be produced in a patient with no crowding in the
opposite dental arch and a retrusive profile, if one tried to close the edentulous
space.10
If the space is going to be left open for an eventual restoration, the key of
orthodontic treatment is going to be creating the appropriate amount of space
and leaving the alveolar ridge in an ideal condition for future restoration. If the
space is planned to be closed, the clinician must pay significant attention to not
creating any detrimental alterations to the occlusion and facial profile. These
early decisions might affect the patient’s dental health for a lifetime. Therefore a
correct decision needs to be made at the right time.10
8
A conventional fixed partial denture could be placed, although preparation of
the abutment teeth might need to be delayed in young individuals due to larger
pulp size. So again, space maintenance is the key until a fixed partial denture is
placed. Postinsertion, fixed partial dentures have proven to have an 84% survival
rate after 10 years. In these cases, caries is reported to be the major reason for
failure. A resin- bonded bridge can be another option of replacing congenitally
missing mandibular second premolars, but unfortunately there are disadvantages
such as the irreversibility of tooth preparation that is required, and the very
uncertain longevity of this type of prosthesis.11, 43
Autotransplantation is a popular procedure in some European countries; the
widespread lack of possibility of this procedure by North American orthodontists
and surgeons often causes this solution to be overlooked. Autotransplantation of
a premolar or possibly a very small third molar into the edentulous site is a viable
option if, a suitable tooth is available. Some authors have shown a high rate of
success with this procedure. Success rates are higher when root formation is not
complete, compared to teeth with closed apices. The main reasons for failure
include ankylosis, external root resorption, and microtrauma to the periodontal
membrane during the extraction of the donor tooth.44, 45
In growing patients, the placement of osseointegrated implants is contraindicated making autotransplantation a suitable choice for replacing missing.
Dental implant placement are most often deferred until growth of the jaws is
complete, typically in the late teens or early 20s.46-50 However, studies have
demonstrated that significant changes in craniofacial dimensions occur even in
9
adulthood, including changes in dento-alveolar height, indicating eruptive
movement of the teeth. 51-54 During the time between orthodontic treatment and
until the patient has finished growth and is ready to have a dental implant placed,
space maintenance is key and can be achieved in various ways. 13 The
cumulative survival rate of implants has been reported by Eckert and Wollan and
is approximately 95% after 10 years. The location of the implant was shown to
have no effect on the survival rate. The most common complication was shown
to be loosening of the abutment screw, which is usually an easy fix from a
prosthodontic point of view.55
It is important to evaluate bone levels in the area of a missing tooth even
when the primary mandibular second molar is still present. Should the bone level
be angled or oblique indicating the continued eruption of the adjacent permanent
teeth, the primary molar is most likely ankylosed. Extraction is recommended at
the earliest time.13 Ankylosis is not common in the permanent dentition, while it
has an incidence rate of 6-8% in the deciduous molar region, more often in the
mandible than in the maxilla. In the most severe cases it can cause
malocclusions and impaction of the underlying successor. Biavati et al. state a
greater incidence of ankylosis occurs with the mandibular second deciduous
molar.56 If the patient has little facial growth left, and the primary molar is
submerged only slightly, the tooth can be retained to preserve the alveolar ridge
for a future implant. However, if the patient still has significant growth remaining,
the deciduous molar has to be extracted to prevent any significant ridge defect. 10
10
If extraction of the ankylosed tooth is delayed, the alveolar ridge may be
compromised vertically and may complicate implant placement.57
Bone grafting for the purpose of achieving greater height is not predictable.
Therefore it is often recommended to extract the deciduous ankylosed tooth in
order to preserve the existing bone level while the face is still growing. Having
said that, should vertical bone loss exist, an attempt to achieve adequate bone
height can be tried surgically. 9 Vertical ridge augmentation can be attempted
before implant placement or at the time of implant placement, so that the platform
and healing abutment are positioned in the proper location for an ideal esthetic
restoration. 13 Another possibility to develop the alveolar ridge in an orthodontic
patient is to place the first premolar into the second premolar position, so that
space is created in the first premolar location for a future implant. However,
orthodontists are sometimes concerned about the insufficiency of the alveolar
ridge width in which the permanent premolar is moved into. Stepovich, Hom and
Turley have shown that a wider tooth can be moved through a narrower alveolar
ridge without compromising the eventual periodontal support around the
repositioned tooth. In fact, the alveolar width can be increased by orthodontic
treatment, more readily in younger than older patients. Spaces of 10 mm or more
could be closed in the posterior mandible by moving the lower first molar forward
into an edentulous space. Also, Strepovich et al. noted that when closing
edentulous spaces in the mandible, the young adults generally generated more
alveolar bone than adult patients.
8, 14
11
Ostler and Kokich evaluated the long-term alveolar ridge width changes after
the extraction of the mandibular primary second molar. Their results show that
the alveolar ridge resorbs about 25% during the first 4 years after the deciduous
tooth extraction. The ridge narrowed another 5% after 7 years, for a total
narrowing of the ridge of 30%. Unfortunately, the ridge reduced more on the
facial aspect than on the lingual, but still a dental implant could be placed
positioned more lingually, and later corrected with an angled prosthetic abutment
and crown.58 When the ridge width measures 4-5 mm, it may be possible to place
the implant even though the buccal aspect might have a dehiscence or
fenestration. The deficient area usually can be augmented at the time of implant
placement.59
Another option would be to retain the mandibular primary second molar until
the patient’s vertical facial growth has ceased and it is the appropriate time for
implant placement. Cessation of facial growth is usually determined by
comparing serial cephalometric radiographs. Fudalej et al. found that on average,
male’s facial growth is completed at 21 years of age, whereas the girls on
average continue to grow up until the age of 17. It is generally desirable to retain
the mandibular deciduous second molar until the end of growth.60 Primary molars
are mesiodistally wider than their permanent counterparts which could affect the
fit of posterior teeth. Thus it can be advantageous to reduce the width of the
primary second molar to the size of a permanent second premolar. Often
clinicians are concerned about interproximal reduction of a mandibular deciduous
second molar followed by space closure due to the diverging roots of the primary
12
tooth beyond the width of its’ crown, and therefore possibly preventing space
from closing. However, often times the diverging roots will resorb and be
replaced by bone as the permanent roots move closer in an attempt to close
space. This would be an ideal way to prepare the missing tooth site for a future
dental implant. 13 The average mandibular second premolar is between 7 and 8
mm wide. In cases where the premolar is congenitally absent, the space created
for the implant and crown will be determined primarily by the posterior occlusion.
If the edentulous space measures 8 mm and the implant platform will be 4mm, a
space of 2 mm will remain on the mesial and distal of the implant to the adjacent
teeth. This space will allow appropriate and healthy bone fill as well as papilla fill
around the implant.13
Cone Beam Computed Tomography
Cone Beam Computed Tomography (CBCT) is considered the gold standard
as a diagnostic resource in the dental and medical field. It has been in use for
decades and has been demonstrated to be highly effective in head and neck
imaging. In 1967, Hounsfield developed computerized tomography (CT) which
was the predecessor to CBCT imaging. The CT scan uses multiple rotations to
stack and combine the slices into an image, whereas the CBCT uses one
rotation only.61 The CBCT has shown several advantages over the conventional
CT method, such as improved data acquisition efficiency, spatial resolution and
uniformity, all of which have contributed to a significantly improved 3D imaging of
the bony structures.62 There is extensive research that has proven CBCT being a
13
valid diagnostic measurement tool. Studies have investigated the accuracy of
alveolar bone height measurements derived from a CBCT and have found it to
be highly correct, to a mean difference of 0.30 mm.63 Other authors have found
that when using a voxel size of 0.38 mm at 2 mA, the alveolar bone height could
be correctly measured to 0.6 mm. They also noted that root fenestrations could
be more readily detected than bony dehiscence.64 CT scans are more accurate
than conventional radiography. However, conventional radiographs can be used
if distortions are being taken into account. When it is difficult to locate structures
like the mental foramen or the mandibular canal, obtaining a CBCT is
recommended.65-69 Computed tomography imaging allows for an accurate
assessment of the position of the alveolar canal in all three dimensions. It has
been proposed that the mental foramen is easier to identify on CBCT imaging
than the mandibular canal since it is always located on the edge on a crosssectional image.69 Periago et al. performed direct measurements on human
skulls and compared those to CBCT measurements. They concluded that the
differences were clinically insignificant.70 Other studies are in agreement that the
CBCT will provide very close measurements compared to physical
measurements. In a follow-up study, investigators found that in comparison to 3D
imaging, 2D images are less reliable when comparing them to direct
measurements on human skulls.71 Zamora and coworkers concluded that CBCT
is highly reliable diagnostic tool with a correlation greater than 0.90 but tends to
underestimate the physical truth.72 It is generally accepted that CBCT offers an
undistorted view of the dentition and surrounding structures, features of roots and
14
spatial orientation of teeth, without any magnification error. This is all made
possible because CBCT imaging is recreated using a mathematical algorithm to
produce 3-dimensional high resolution images. 73
According to the American Dental Association Council, CBCT should be an
adjunct to conventional oral imaging modalities. CBCT may supplement
conventional dental radiographs for the diagnosis and treatment of oral
conditions, when the dentist determines that an anatomical structure may not be
imaged adequately. The American Association of Oral and Maxillofacial
Radiology states that the decision to obtain a CBCT image must be justified on
an individual basis showing that the benefits outweigh the risks. 74 Cone beam
computed tomography is the best available imaging technique at this time to
locate anatomic structures such as the mental foramen, but it has shortcomings
such as radiation and cost. In the future, magnetic resonance imaging and
ultrasound technology should be investigated more for the dental practice and
seem to provide promising noninvasive imaging techniques.75
Anatomy of the inferior alveolar nerve and the mental foramen
The mandibular nerve (V3) originates from the trigeminal nerve and enters
the mandibular foramen, which is located at the medial aspect of the ramus and
horizontally forward in the body, along with the inferior alveolar artery and vein.
The inferior alveolar nerve (IAN) proceeds obliquely downward and forward
through the mandibular canal, from the lingual to the buccal aspect of the
mandible. In the molar area, the IAN generally branches into an extraosseous
15
terminal mental nerve and the intraosseous terminal incisal nerves.76 Once in the
mental canal, the nerve proceeds upward and emerges from the mental foramen
located mostly next to the second premolar apex, along with blood vessels.
Moiseiwitsch found 90% of mental foramina being located either at the second
premolar apex or abruptly mesial or distal to this position.77 On average, the
mental nerve exiting the mental foramen gives off three branches deep to the
depressor anguli oris muscle, one of which innervates the skin of the mental
area, and the remaining two innervate the skin of the lower lip, mucous
membranes, and the gingiva as posterior as the second premolar.78 Mesial to the
mental foramen, studies were able to confirm the existence of an incisive canal,
which can be an extension of the mandibular canal.65, 78-80 It is usually hard to
identify, and its neurovascular structures may be running through the trabecular
network. A very small percentage of the population (1%) has bifurcating
mandibular canals, which proceed anteriorly into an inferior superior and medial
lateral plane.81, 82 Therefore, the bifurcated mandibular canal will exit in two
separate mental foramina. In these case clinicians have to be cautious since a
panoramic or perapical film may not reflect it. Furthermore, Dario et al. suggest
that the clinicians planning a surgical implant placement procedure above the
inferior alveolar canal, should obtain a CBCT prior to the surgery, to avoid any
possible nerve injury.81
The shape of the mental foramen was studied by Mbajiorgu et al. who found
it to be round in 43.7% and oval in 56.3% of the mandibles investigated.83 The
mental foramen size was analyzed in morphometric skull analyses which found
16
that the average height of the mental foramen is 3.47 mm and the average width
is 3.59 mm. Other researchers had similar findings.2
The foramen was found to be 28 mm from the midline of the mandible, and
15 mm from the inferior border of the mandible.2, 84 Studies investigating the
same measures found that the mental foramen was 13.2 mm and 12.4 mm
above the lower mandibular border. Distance did not vary by side, but showed
significantly higher measures for males than females. The location of the mental
foramen can vary, but it is usually found more coronal than the mandibular canal.
Ethnic differences in the location of the mental foramen have been published.
Since the mandibular canal ascends and curves crestally from its lowest part
below the first molar to the mental foramen, the mental foramen is located close
to root apices of adjacent teeth. In the vertical view, the mental foramen might
even be located more crestally to the apices of the adjacent teeth. 85, 86 A group of
researchers studied 100 patients using CT scans and evaluating the distance
from the mental foramen to the alveolar crest. They found the mean distance
being 14.2 mm, with side and gender showing no significant difference.87 Von Arx
et al. showed that the mean measure from the alveolar crest to the mental
foramen was 12.6 mm, however this distance might not have been consistent
since alveolar bone loss was not taken into consideration. They also reported
that there was a significant difference for gender regarding the vertical position of
the mental foramen, with male patients exhibiting greater values than females. 88
Some authors have suggested the use of the cementoenamel junction (CEJ) of
adjacent teeth as a more reliable landmark. The controversy remains that in
17
cases where those teeth are tipped, the CEJ might not present an accurate
reference point.2 Other studies have found the foramen being halfway between
the crest of bone and the inferior border of the mandible. However, this measure
can be influenced by the amount of crestal bone loss.78 The mental foramen
location in the horizontal plane could be race related, but is usually found to be
by the apex of the lower second premolar.89 Yet, Fishel et al. concluded that the
foramen’s location is not constant in the horizontal and vertical planes. When
performing immediate placement of dental implants, caution is advised since the
foramen may be coronal to the apex of the root of the second premolar. 86 The
mental foramen becomes closer to the alveolar crest as teeth are extracted and
remodeling of the alveolar ridge takes place.90, 91 In severe cases, the mental
foramen can even be in very close proximity with the alveolar ridge. In situations
like this, the inferior alveolar nerve can be surgically relocated to avoid damage
prior to osteotomies or oral surgical procedures. Transpositions would allow the
surgeons to place dental implants in cases with insufficient vertical amount of
bone. Some authors found these procedures to be highly predictable and
successful, while others noted a high rate of sensory dysfunction post
operatively.92, 93
Angel et al. studied the position of the mental canal and mental foramen in
adults with different age and gender, employing CBCT images. The authors
found that increasing age and sex dimorphism does not have any influence on
the position of the associated canal and foramen.4
18
Facial growth and position of the mental foramen
The position of the mental foramen can change during facial growth. 94 Kjaer
reported the prenatal location of the mental foramen is within the alveolar bone
between the deciduous canine and first molar. A lag in prenatal development can
alter the final location of the mental foramen.95 Previous studies have proven that
the mental foramen position and orientation tend to shift during postnatal dental
development.96, 97 From an anterior posterior aspect, the mental foramen moves
distally relative to the dentition, from the deciduous mandibular canine and first
molar region to the permanent second molar region in adults.98 The relative
posterior shift can be reasoned by a mesial drift of the developing teeth relative
to the mandible itself.98, 99 From a superior inferior view, the mental foramen is in
proximity of the alveolar crest before dental formation, and moves halfway
between the alveolar crest and the lower border of the mandible in adults with a
full dentition. In conclusion, the mental foramen’s superior inferior position
depends on dental and alveolar development.90, 100, 101 Differences in dental size
and root length have been reported as a potential factor to cause shift of the
mental foramen.102, 103
19
When is the right time to place dental implants in postpubertal orthodontic
patients?
Presently, dental implants have become an attractive alternative to replace
congenitally missing teeth. It is important for orthodontists to determine when
facial growth has ceased to avoid the implant from submerging relative to the
adjacent teeth. Very few investigators have studied growth after puberty. Most
studies have reported on growth changes in the second decade of life or in the
late adolescents. 104-106 Behrents studied post-adolescent growth in the Bolton
sample and found that growth continued even after adulthood.107 Later studies by
Bondevik and Bishara et al. confirmed those findings.108, 109 Statural growth
discontinues at some point, whereas vertical growth of the face and the alveolar
processes along with eruption of teeth continue past puberty. Fudalej et al.
concluded that facial growth continues post puberty; however the amount of
growth decreases constantly after the second decade of life. This finding seems
to have a clinical irrelevance. Comparing growth after age 20 between genders,
Fudalej found that the intergender growth difference slowly diminish. The
maxillary incisor eruption in females seems to be more than in males despite
greater growth of anterior facial height in males over the same time period. 60
20
Surgical implant placement in the posterior mandible
Over the last four decades, replacement of teeth with endosseous dental
implants has become an extremely viable and predictable evidence supported
part of dentistry. An in-depth knowledge of the anatomy of the mandibular canal
and mental foramen, as well as the quantity and quality of bone available, is of
great significance and may avoid nerve injury, when performing surgical implant
procedures in the mandibular second premolar region.6
Not all complications necessarily result in implant failure; however they are
the most common cause of a lawsuit for the clinician.110 To avoid any nerve injury
to the mandibular canal or mental foramen during surgical implant placement,
surgeons commonly measure the distance from the alveolar crest to the most
coronal part of the nerve on periapical, panoramic or CBCT images, and
accordingly plan their osteotomy to a certain depth avoiding any neurovascular
damage.111 Violating the mandibular canal or mental foramen during an
osteotomy can cause complications such as paresthesia, hypoesthesia,
dysthesia, or anesthesia of the teeth, lip or perioral tissues. Other complications
may be venous or arterial bleeding.3 Following implant placement in the
mandible, the occurrence of altered lip sensation has been reported by various
investigators to be as high as 8.5%, 11%, and 24% of patients. These patients
continued to manifest symptoms 4-16 months after treatment.112-114 Despite
exposing and protecting the mental foramen from any injury, as part of the
surgical protocol and placement of implants at least 3 mm away from the mental
foramen, altered sensation of the lower lip and surrounding tissues was
21
diagnosed.114 Greenstein et al. proposed a safety distance of 2 mm between the
implant and the coronal margin of the mental foramen. However, temporary and
permanent sensitivity changes of the perioral tissues caused by mental nerve
injury were still reported in multiple studies following that protocol.112-115 Normally,
an implant is 0.5 mm wider than the osteotomy site; therefore bone compression
occurs and can be transferred to the nerve. Dahlin et al. demonstrated in a rat
study that nerves compressed at 200-400 mm Hg for 2 hours showed
demyelination and axonal degeneration, lasting for 3 weeks after compression.116
Adell et al. reported that the mean bone loss around osseointegrated dental
implants was 1.5 mm during the healing period and the first year after connecting
the implant abutment. After year one, only 0.1 mm of marginal bone was lost per
year, in patients observed for 5 to 9 years.117
Short implants and success rates
The proximity of the mental nerve and mandibular canal to the alveolar crest
has to be considered when treatment planning patients for mandibular posterior
dental implants. Several bone grafting techniques, such as guided bone
regeneration, distraction osteogenesis, and inferior alveolar nerve transposition
have been performed to allow the placement of a longer and wider implant.
These techniques, in preparation for dental implants, have evidenced high
success rates. However, many patients are hesitant to undergo this type of
procedure due to high cost, need of multiple surgical procedures, poor general
health and/or discomfort, and greater risk of paresthesia.118 With the introduction
22
of short implants, less than 10 mm in length, dental implant placement has
become a less complicated and costly procedure in the resorbed posterior
mandible and maxilla.118-121 Longer implants however, have always been
considered more predictable due to a greater surface area for osseointegration
and an improved crown-to-implant ratio which both help to dissipate the imposing
occlusal forces.122
Multiple studies evaluated the success rates of short versus long implants.
Mean success rates were found to be 91.4% and 97% for short and long
implants respectively.123 Winkler argued that short implants are more likely to fail
than long implants, at second stage surgery and after loading, from occlusal
loading forces.124
In a literature review article, Galvão et al. concluded that implant diameter
seems to be more important than implant length for distributing the tension,
because the region receiving most forces is the alveolar crest. Short implants
have become more predictable in terms of biomechanics by splinting them,
creating a balance between crown/implant ratio, reducing the occlusal table, and
avoiding a cantilever mechanism. Furthermore, the bone quality and implant
surface treatment have become primary factors for short implant’s success.
Finally, a two stage surgical protocol is significant for a more predictable outcome
for treatment using short implants.125
23
Statement of Thesis
The question remains to be answered whether the congenital absence of
mandibular second premolars has an influence on the distance from the mental
foramen to the alveolar crest. Therefore, this study will conduct a retrospective
search of patients with mandibular second premolar agenesis and determine if
there is a difference in overall location of the mental foramen in relation to the
occlusal plane, alveolar crest and lower border of the mandible. Studies have
reported mean measure from the alveolar crest to the mental foramen being 12.6
mm, however this distance might not have been consistent since alveolar bone
loss was not taken into consideration in cases where premolars were extracted
and missing.88 There is evidence that position of the mental foramen can change
during facial growth.94 Kjaer reported the prenatal location of the mental foramen
is within the alveolar bone between the deciduous canine and first molar. A lag in
prenatal development can alter the final location of the mental foramen. 95 Also,
previous studies have proven that the mental foramen position and orientation
tend to shift during postnatal dental development. 96, 97 As a result, a congenitally
missing mandibular second premolar edentulous space might measure a shorter
distance between the mental foramen and the alveolar crest, and possibly
redirect the treating clinician towards an alternative treatment plan rather than
setting the patient up for dental implant in that region.
The alternative hypothesis is that the mental foramen is situated more
coronally in congenitally missing mandibular second molar cases vs. cases with
existent mandibular second premolars. The null hypothesis is that there is no
24
difference between the location of the mental foramen in patient with and without
second premolar agenesis. The sample size will be 52 for each group and come
from existing set of patient records located at Case Western Reserve University,
Orthodontic Department that include pretreatment CBCT images. The selection
criteria include: permanent mandibuar dentition that includes patients
congenitally missing mandibular second premolars with or without retained
mandibular deciduous second molars as the test group, and patient with a
complete permanent dentition for the control group. Initial records of patients
between the ages 11 to 65 will be evaluated for this purpose.
25
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105. Haavikko K, Rahkamo A. Age and skeletal type-related changes of some
cephalometric parameters in Finnish girls. Eur J Orthod. 1989;11(3):283-9.
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35
CHAPTER 3: JOURNAL ARTICLE
Abstract
Introduction: Among teeth that are missing, the congenital absence of
mandibular second premolars is a relatively common finding. In orthodontic
treatment planning, a decision needs to be made on how to treat a patient with
missing mandibular second premolars. The mental foramen is generally located
in this area. There are no known published studies of the location and variability
of the mental foramen and mandibular canal in these patients. Dental implants
are often used to replace these teeth. Determining the location of the mental
foramen and mandibular canal is critical prior to implant placement. Purpose:
The goal of this study is to analyze the location of the mental foramen in subjects
with congenitally missing mandibular second premolars. Materials and
Methods: Cone beam computerized tomography (CBCT) images from 52 test
subjects with mandibular second premolar agenesis and 52 control subjects with
a complete dentition were retrieved and analyzed. The position of the mental
foramen (MF) in relation to the anatomical occlusal plane (AOP), alveolar crest
(AC) and lower margin of the mandible (LMM) was measured and analyzed for
differences between the test and control groups. Results: No significant
difference was found between the test and control groups. Age as well as gender
did not have any influence on the anatomic position of the mental foramen.
Conclusions: The mental foramen was located in the same anatomic position in
patients with congenitally missing mandibular second premolars compared to
patients with a complete dentition. Although no difference in location was found
36
in this study, clinicians should always verify the location of the mental foramen
and mandibular canal prior to implant placement in the mandibular second
premolar region.
Introduction
The congenital absence of mandibular second premolars is the most
common finding after congenitally missing third molars. As the studies are
limited, we don’t know the exact incidence of lower second premolar agenesis,
but it has been reported to occur with a frequency of about 3.4%, with a higher
incidence than maxillary lateral incisor agenesis.1 In orthodontic treatment
planning, a decision needs to be made on how to handle that absence. Some
treatment options include:
A. Maintaining the deciduous second molar
B. Attempted orthodontic space closure
C. Prosthetic replacement with:
a. Removable partial denture
b. Fixed bridge
c. Dental implant
d. Autotransplantation
Studies have been performed to analyze the position of the mental
foramen and mandibular canal in subjects not missing the mandibular second
premolars and in partially and totally edentulous patients that are missing teeth in
addition to the second premolar, but there are no known published studies of the
37
location and variability of the mental foramen and mandibular canal when only
the mandibular second premolar is congenitally missing.2-4
Clinicians have observed that the congenital absence of the mandibular
second premolar is associated with a more crestal location of the mental
foramen, which may impact the placement as well as selection of the prosthetic
implant. Dentists performing implant surgery always need to be aware of critical
anatomy. Special attention may be needed for these patients since the vertical
height of available bone for implant placement may be limited.
Earlier publications have described the high degree of variability in
mandibular bone volume surrounding the inferior alveolar nerve. The use of CT
scans should be considered for surgical procedures in the posterior mandible
when there is risk of injury to the inferior alveolar nerve.5
After evaluating the existing literature, gaps remain regarding the
following question: “Does the mental foramen location vary in patients with
congenitally missing mandibular second premolars compared to patient with
existent mandibular second premolars?” Currently, there are no published
studies that have attempted to investigate this question. Understanding the
relationship of the mental foramen location in patients with missing mandibular
second premolars would aid the clinician in treatment planning.
There are a number of studies that suggest multiple treatment options to
manage congenitally missing mandibular second premolars. 6-12 The long term
goal of this study is to investigate whether the location of the mental foramen is
altered in patients with congenitally missing mandibular second premolars.
38
Materials and Methods
Sample
This study was designed as a retrospective analysis of CBCT images taken
at the Case Western Reserve University, Orthodontic Department (Cleveland,
Ohio) as part of the initial diagnostic protocol of patients who presented for the
purpose of initiating orthodontic treatment. Cases that were enrolled provided a
complete head and neck scan. This study was approved by Saint Louis
Univeristy Institutional Review Board.
52 CBCT test samples with lower second premolar agenesis, both unilaterally
or bilaterally, could be retrieved and analyzed. An additional 52 CBCT samples,
with a full set of dentition, were matched to the test samples by gender. The ages
were matched as close as possible. The mean age of the patients was 19.4
years. The youngest patient was 11 years old and the oldest was 65 years old.
Cone Beam Computed Tomography (CBCT) Technique
CBCT scans (3D images, Hitachi, Japan) were taken with basic voxel size of
0.08 mm and exposure setting of 5.0-7.0 mA and 80 kV. All CBCT images were
uploaded into Dolphin 11.5 3D Imaging software (Dolphin Imaging Systems LLC,
Chatsworth, CA). Using the Dolphin software, the CBCT images were reviewed
on a computer screen and oriented along the X axis which was determined to be
the anatomic occlusal plane (AOP) from the right sagittal view and along the
buccal cusps of the 1st molars in occlusion bisecting the overbite (vertical overlap
of the incisors anteriorly) (see Figure 1). The images were reformatted into
39
multiplanar reconstructions to obtain the most appropriate sections for the
measurements. Radiographic measurements were carried out using crosssectional images of CBCTs.
To examine the position of the mental foramen (MF) in relation to the AOP,
alveolar crest (AC) and lower margin of the mandible (LMM), the three
dimensional coordinates of selected landmarks were recorded using serial CBCT
imagery. The representative point of the MF was determined as the most
superior point on the MF rim (see Figure 2) When measuring from the CBCT
cross sectional images, the image showing the widest and sharpest crosssection of the mental foramen was selected. The mental foramina usually opened
superior anteriorly around or below the mandibular second premolar region.
Employing the Dolphin software program, slices were obtained at the region of
the mental foramen (MF) and used for measurements. The vertical distance
between the highest point of the upper marginal border of the mental foramen
and the AOP was measured perpendicular to the AOP. On the same
perpendicular line in a parallel manner, the vertical height between the upper
marginal border of the MF to the alveolar crest was measured, as well as the
vertical distance between the upper marginal border of the MF and the lower
marginal border of the mandible (see Figure 3 and 4).
40
Figure 1: Example of patient’s anatomical occlusal plane
Figure 2: Example of landmark selections on a 3D image
41
Figure 3: Example of vertical measurements from AOP to MF, MF to AC and MF to LMM
Measurement Repeatability Study
A single investigator performed all measurements. Ten percent of the total
sample was chosen to be reassessed randomly. Ten CBCT records were remeasured, including all previously mentioned values. Intra-class correlations
equal or greater than 0.80 are considered accurate.
42
Statistical Analysis
Statistical analysis using SPSS statistical analysis software (PASW
Statistics Version 18.0, SPSS, Inc.) was performed and means and standard
deviations were calculated for the measurements in both groups
All data were first analyzed descriptively. An independent-samples t-test
was conducted to compare vertical linear measurements from the AOP to MF,
AC to MF, and LMM to MF in patients with mandibular second molar agenesis
compared to those with a complete dentition. For analysis of age as an
influencing factor, the data were divided into 2 groups (18 ≤ years and ≥ 18
years). Another possible influencing factor that was examined was gender. Both
of these values were analyzed by employing an independent t-test.
The alternative hypothesis was described as the mental foramen being
situated more coronally in congenitally missing mandibular second molar cases
vs. cases with existent mandibular second premolars. The null hypothesis was
that there is no difference between the location of the mental foramen in patient
with and without second premolar agenesis.
Statistical significance of P < 0.05 is referred to in describing the results.
43
AOP-MF
AC-MF
LMM-MF
Figure 4: Example of the three vertical linear CBCT measurements
44
Results
The mental foramen was identified in 100% of the CBCT scans. The most
common position for the mental foramen relative to the teeth in these samples
was in line with the second premolar area for both right and left sides. 30 female
and 22 male patients were evaluated each in the test as well as in the control
groups. The mean age of the test group was 20.5 years and 17.4 years for the
control group.
On average, there was no significant difference in the linear scores for
AOP to MF, AC to MF, and LMM to MF between test and control groups (See
Figure 5) The only statistical difference that was found in this study, was an
increased distance of the AOP to the MF on the left missing mandibular second
premolar sites compared to controls (Tables 1 and 2).
No statistical significant difference was found regarding gender (Tables 3
and 4) and age in relation to the location of the MF (Table 5), except for female
individuals under 18 years of age where MF is positioned more occlusally
compared to controls (Table 6).
The statistics for repeated measures all resulted in intra-class correlations
greater than 0.90, which are considered highly accurate.
45
Mean measurements in test and control
groups
25
Millimeter (mm)
20
15
10
5
0
AOP-MF
(R)
AC-MF
(R)
LMMMF (R)
AOP-MF
(L)
AC-MF
(L)
LMMMF (L)
Bilaterally missing mandibular
2nd premolar group
20.22
11.70
14.45
20.41
11.61
14.39
Unilaterally missing mandibular
2nd premolar group
20.32
10.24
14.69
19.93
12.12
14.43
Control group with full
dentition
19.71
11.11
14.24
18.84
10.80
14.75
Figure 5: Report of mean measurements in test and control groups
46
Table 1: Group Statistics, calculation of mean and standard deviation for linear
measurements (in mm) from AOP to MF, AC to MF and LMM to MF in the congenitally
missing mandibular second premolar sites and controls on the right and left
Measured Distances
N
Mean
Std.
Deviation
Std.
Error
Mean
43
20.25
3.046
0.465
52
19.71
1.980
0.275
43
11.36
2.673
0.408
52
11.11
1.766
0.245
43
14.50
2.201
0.336
Miss lower R sec premolar
52
14.24
2.004
0.278
Bilat and unilat miss lower 2nd
premolar
42
20.30
2.772
0.428
Miss lower R sec premolar
52
18.84
2.283
0.317
Bilat and unilat miss lower 2nd
premolar
42
11.72
2.666
0.411
Miss lower R sec premolar
52
10.80
2.039
0.283
Bilat and unilat miss lower 2nd
premolar
42
14.40
2.197
0.339
Miss lower R sec premolar
52
14.75
1.675
0.232
Bilat and unilat miss lower 2nd
premolar
10
19.47
3.951
1.249
Groups
Bilat and unilat miss lower 2
premolar
AOP-MF (R)
nd
Miss lower R sec premolar
Bilat and unilat miss lower 2
premolar
AC-MF (R)
nd
Miss lower R sec premolar
Bilat and unilat miss lower 2
premolar
LMM-MF (R)
AOP-MF (L)
AC-MF (L)
LMM-MF (L)
AOP-MF (L)
AC-MF (L)
nd
a
Miss lower R sec premolar
0
.
.
.
Bilat and unilat miss lower 2nd
premolar
10
10.62
2.461
0.778
0
.
.
.
10
14.92
1.757
0.556
.
.
.
a
Miss lower R sec premolar
nd
Bilat and unilat miss lower 2
premolar
LMM-MF (L)
Miss lower R sec premolar
a.
a
0
Cannot be computed because at least one of the groups is empty.
47
Table 2: Independent Samples Test shows statistical difference in an increased distance of the
AOP to MF measurement on the left missing mandibular second premolar sites compared to
controls, see highlight
Sig. (2tailed)
Mean
Difference
Equal variances assumed
0.304
0.537
Equal variances not assumed
0.323
0.537
Equal variances assumed
0.592
0.247
Equal variances not assumed
0.606
0.247
Equal variances assumed
0.548
0.260
Equal variances not assumed
0.552
0.260
Equal variances assumed
0.006
1.466
Equal variances not assumed
0.007
1.466
Equal variances assumed
0.063
0.915
Equal variances not assumed
0.071
0.915
Equal variances assumed
0.38
-0.352
Equal variances not assumed
0.394
-0.352
Measured distances
Variance
AOP-MF (R)
AC-MF (R)
LMM-MF (R)
AOP-MF (L)
AC-MF (L)
LMM-MF (L)
48
Table 3: Group Statistics, calculation of mean and standard deviation for linear measurements
(in mm) from AOP to MF, AC to MF and LMM to MF in the congenitally missing mandibular
second premolar sites and controls on the right and left in relation to gender
Measured
distances
AOP-MF (R)
N
Mean
Std.
Deviation
Std. Error
Mean
19
20.89
3.455
0.793
22
19.64
1.958
0.417
19
11.80
3.343
0.767
22
10.82
1.857
0.396
19
15.23
2.189
0.502
22
14.90
1.487
0.317
Bilat and unilat miss lower 2
premolar
19
20.27
3.452
0.792
Miss lower R sec premolar
22
19.03
1.829
0.390
19
11.76
3.184
0.731
Miss lower R sec premolar
22
10.70
1.710
0.365
Bilat and unilat miss lower 2nd
premolar
19
15.06
2.383
0.547
Miss lower R sec premolar
22
15.03
1.671
0.356
6
19.18
4.136
1.689
Miss lower R sec premolar
0a
.
.
.
Bilat and unilat miss lower 2nd premolar
6
10.68
2.689
1.098
Miss lower R sec premolar
0a
.
.
.
6
15.42
1.941
0.792
0a
.
.
.
Groups
Bilat and unilat miss lower 2
premolar
nd
Miss lower R sec premolar
AC-MF (R)
Bilat and unilat miss lower 2
premolar
nd
Miss lower R sec premolar
LMM-MF (R)
Bilat and unilat miss lower 2
premolar
nd
Miss lower R sec premolar
nd
AOP-MF (L)
AC-MF (L)
LMM-MF (L)
AOP-MF (L)
Bilat and unilat miss lower 2
premolar
Bilat and unilat miss lower 2
premolar
nd
nd
AC-MF (L)
LMM-MF (L)
Bilat and unilat miss lower 2
premolar
nd
Miss lower R sec premolar
49
Table 4: Independent Samples Test shows no statistical significance in terms of gender related to
the position of MF
Sig. (2tailed)
Mean
Difference
Equal variances assumed
0.154
1.253
Equal variances not assumed
0.173
1.253
Equal variances assumed
0.244
0.982
Equal variances not assumed
0.265
0.982
Equal variances assumed
0.564
0.336
Equal variances not assumed
0.575
0.336
Equal variances assumed
0.15
1.242
Equal variances not assumed
0.171
1.242
Equal variances assumed
0.184
1.059
Equal variances not assumed
0.206
1.059
Equal variances assumed
0.962
0.031
Equal variances not assumed
0.963
0.031
Measured distances
Variance
AOP-MF (R)
AC-MF (R)
LMM-MF (R)
AOP-MF (L)
AC-MF (L)
LMM-MF (L)
50
Table 5: Group statistics, calculation of mean and standard deviation for linear measurements (in
mm) from AOP to MF, AC to MF and LMM to MF in the congenitally missing mandibular second
premolar sites and controls on the right and left in relation to age
Measured Distances
Age
N
Mean
Std.
Deviation
Std. Error
Mean
<18 yo
16
18.81
1.853
0.463
>18 yo
27
21.10
3.313
0.638
<18 yo
16
10.54
2.143
0.536
>18 yo
27
11.84
2.870
0.552
<18 yo
16
14.21
2.497
0.624
>18 yo
27
14.68
2.034
0.392
<18 yo
19
19.53
2.079
0.477
>18 yo
23
20.95
3.134
0.654
<18 yo
19
11.21
2.763
0.634
>18 yo
23
12.14
2.567
0.535
<18 yo
19
13.34
2.193
0.503
>18 yo
23
15.27
1.813
0.378
<18 yo
2
17.25
0.636
0.450
>18 yo
8
20.03
4.272
1.510
<18 yo
2
8.65
0.919
0.650
>18 yo
8
11.11
2.506
0.886
<18 yo
2
14.15
1.909
1.350
>18 yo
8
15.11
1.800
0.636
AOP-MF (R)
AC-MF (R)
LMM-MF (R)
AOP-MF (L)
AC_MF (L)
LMM-MF (L)
AOP-MF (L)
AC-MF (L)
LMM-MF (L)
51
Table 6: Independent Samples Test shows significant difference for female individuals under 18
years of age where MF is positioned more occlusally compared to controls.
t-test for Equality of Means
Measured distances
Variance
Sig. (2-tailed)
Mean
Difference
Equal variances assumed
0.015
-2.294
Equal variances not assumed
0.006
-2.294
Equal variances assumed
0.123
-1.307
Equal variances not assumed
0.097
-1.307
0.5
-0.475
Equal variances not assumed
0.525
-0.475
Equal variances assumed
0.099
-1.422
Equal variances not assumed
0.087
-1.422
Equal variances assumed
0.264
-0.934
Equal variances not assumed
0.268
-0.934
Equal variances assumed
0.003
-1.937
Equal variances not assumed
0.004
-1.937
Equal variances assumed
0.406
-2.775
Equal variances not assumed
0.117
-2.775
Equal variances assumed
0.225
-2.463
Equal variances not assumed
0.071
-2.463
Equal variances assumed
0.521
-0.963
Equal variances not assumed
0.604
-0.963
AOP-MF (R)
AC-MF (R)
Equal variances assumed
LMM-MF (R)
AOP-MF (L)
AC-MF (L)
LMM-MF (L)
AOP-MF (L)
AC-MF (L)
LMM-MF (L)
52
Discussion
There have been many studies in the literature describing the course of the
IAN and the location of the MF in relation to the second mandibular premolar.4, 5,
13-25
Along the same lines, there are many reports showing that of missing teeth,
the congenital absence of the mandibular second premolar is the most common
finding after congenitally missing third molars, and many treatment options
available to overcome this agenesis. In an attempt to relate these two finding,
this is the first study to evaluate the location of the mental foramen in patients
with mandibular second premolar agenesis compared to those without, by using
CBCT imaging in a relatively large test population. Understanding the
relationship of the mental foramen location and missing mandibular second
premolars may aid the clinician in understanding the critical anatomy for potential
dental implant placement.
Cone Beam Computed Tomography (CBCT) is considered the gold standard
as a diagnostic resource in the dental and medical field. It has been in use for
decades and has been demonstrated to be highly effective in head and neck
imaging. The CBCT has shown several advantages over the conventional CT
method, such as improved data acquisition efficiency, spatial resolution and
uniformity, all of which have contributed to a significantly improved 3D imaging of
the bony structures. There is extensive research that has proven CBCT to be a
valid diagnostic measurement tool. Studies have investigated the accuracy of
alveolar bone height measurements derived from a CBCT and have found them
to be highly accurate. Generally, when comparing CBCT data to data obtained
53
from other imaging techniques, these have to be cautiously translated because of
differences in methodology and diverse interpretation of results.
The present results are in agreement with previous studies reporting the most
common location of the MF being apical to the mandibular second premolar.
15, 17, 18, 21
14,
The results of this study report no difference in terms of the position of
the mental foramen relative to the AOP, AC and LMM in patients with mandibular
second molar agenesis compared to those without. Age as well as gender were
examined but did not have any influence on our findings.
According to earlier studies, high degrees of variability were found with the
measurements from the alveolar crest to the mental foramen in patients with
missing teeth and therefore often resorbed alveolar ridges, compared to those
with existing teeth.23, 26, 27 It is important to evaluate bone levels in the area of a
missing tooth even when the primary mandibular second molar is still present.
Should the bone level be angled or oblique indicating the continued eruption of
the adjacent permanent teeth, the primary molar is most likely ankylosed.
Extraction is recommended at the earliest time.11 If the patient has little facial
growth left, and the primary molar is submerged only slightly, the tooth can be
retained to preserve the alveolar ridge for a future implant. However, if the patient
still has significant growth remaining, the deciduous molar should be extracted to
prevent any significant vertical ridge defect.8 If extraction of the ankylosed tooth
is delayed, the alveolar ridge may be compromised vertically and may complicate
implant placement and or long term success.28
54
There are some limitations to this study. When evaluating retained
mandibular second primary molars in congenital missing mandibular second
premolar case samples, it was found that most of the individuals presented with
retained primary molars in those sites which were not ankylosed. This might have
helped to preserve the ridge height so that no difference in our linear
measurements could have been detected between test and control groups. For
follow-up studies, it might be important to differentiate whether a mandibular
second premolar agenetic case has a retained primary tooth, a retained
ankylosed primary tooth or is a case where the primary tooth has been lost
already. That way, one would be able to analyze more groups that would
represent different clinical presentations. In addition, although we were able to
find an appropriate number of the test samples, having a larger sample would
have created more accurate findings and reduced any possible error calculation
effects.
There is some individual variation in eruption ages in children.
Skeletal age and amount of residual growth are parameters that must be
considered, in addition to the dental stage. 29 Van der Linden and Du Burl found
evidence that crypt formation of the permanent mandibular second premolar
starts as early as 9 months of age. Crown calcification begins at 3 years of age
and completes at 6 years of age.30 Proffit reports mandibular second permanent
premolar crown calcification at age 28 months. The crown is completed at 7.5
years of age and the eruption happens around 11.4 years Root formation is not
completed until 15 years of age.31 From a superior inferior view, the mental
55
foramen is in proximity of the alveolar crest before dental formation, and moves
halfway between the alveolar crest and the lower border of the mandible in adults
with a full dentition. The mental foramen’s superior inferior position depends on
dental and alveolar development.15, 18, 32 Differences in dental size and root
length have been reported as a potential factor to cause shift of the mental
foramen.33, 34 The ages of our case samples ranged from 11-65 years, this might
not have allowed for a proper comparison within similar developmental stages in
terms of mental foramen location.
Moreover, errors in the CBCT measurements could arise from faulty
positioning of the patient’s head so that the cross-sectional images are not
absolutely perpendicular to the lower border of the mandible. In such cases the
distance to the mental foramen would increase. Another possible source of error
could be the way measurements were performed. In the present study we tried to
ensure that the measurements were performed parallel to the anatomical
occlusal plane. The use of only one observer in this study could also have
influenced the results in some way, although the CBCT measurements were
repeated in order to calculate the intra-observer error.
The two major alternatives to treating agenetic mandibular missing premolars
are orthodontic space closure or space opening for a prosthetic replacement with
dental implants or autotransplantation. All of these can compromise esthetics and
periodontal health and function. Maintaining the deciduous second molar is a
possible option, but usually the interdigitation of teeth is compromised due to the
larger size of the primary tooth. In growing patients, the placement of
56
osseointegrated implants is contra-indicated, and that’s where transplantation of
growing teeth remains a suitable treatment option. Dental implant placement
should be deferred until growth of the jaws is complete, typically in the very late
teens or early 20s.35-39
The proximity of the mental nerve and mandibular canal to the alveolar crest
has to be considered when treatment planning patients for mandibular posterior
dental implants. Several bone grafting techniques, such as guided bone
regeneration, distraction osteogenesis, and inferior alveolar nerve transposition
have been performed to allow the placement of a longer and wider implant.
These techniques, in preparation for dental implants, have evidenced high
success rates. However, many patients are hesitant to undergo this type of
procedure due to high cost, need of multiple surgical procedures, poor general
health and/or discomfort, and greater risk of paresthesia.40 With the introduction
of short implants, less than 10 mm in length, dental implant placement has
become a less complicated and costly procedure in the resorbed posterior
mandible and maxilla.40-43 Longer implants however, have always been
considered more predictable due to a greater surface area for osseointegration
and an improved crown-to-implant ratio which both help to dissipate the imposing
occlusal forces.44 Judging by our findings, patients with mandibular second
premolar agenesis could be orthodontically treatment planned the same way as
patients without agenesis.
57
Conclusions
The mental foramen is an important anatomic landmark; its location needs to
be taken into consideration prior to surgical procedures such as implant
placement. In this study, the foramen was not positioned more crestally in
patients with congenitally missing mandibular second premolars compared to the
controls. The results of this study need to be substantiated with larger and more
uniform samples.
58
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62
VITA AUCTORIS
Azadeh Amin Eslami was born in Tehran, Iran on March 21st, 1982. Her
family immigrated to Düsseldorf, Germany when she was 3 years old. She grew
up in Germany ever since, however still keeping close ties to her heritage.
Azadeh has two younger sisters who both still reside in Germany. She graduated
from LFSM Mülhausen (High School) in 2001, attended Julius Maximilians
University Würzburg, Germany shortly after and received a Doctor of Dental
Surgery degree in June 2007.
In 2008, Dr. Amin decided to follow her husband to the United States where
she soon started a Periodontic residency program at the Saint Louis University.
She then decided to be a double trained specialist in Periodontics and
Orthodontics and began the orthodontic program in 2012, also at Saint Louis
University. In the same year, Dr. Amin was honored to become a Diplomate of
the American Academy of Periodontics.
Azadeh has been married to her husband, Mazyar, since October 2007. They
both are proud parents of their son Rayan who is 5 and half years old (as old as
both residency programs together). Azadeh will complete her Master of Science
in Orthodontics degree in December 2014. Upon graduation, Dr. Amin and her
family plan to move to Cincinnati, OH where she will be practicing both
specialties on a part time basis.
63