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INTRODUCTION
he mental foramen is an oval/circular opening on
the anterior surface of the mandible, located
below or between the premolars, approximately
midwaybetweenthealveolarcrestandlowerborderofthe
mandible. Mental foramen holds strategic importance in
clinical dentistry and oral surgery. Its accurate
identificationdetermines the effectivenessofnerveblocks
and prevention of post operative neurovascular
complications in the mental region like neuro-sensory
disturbances, paralysis, hemorrhage, altered sensation,
orofacial pain, atypical neuralgia etc. Studies highlight a
clearracialtrendintheshape,size,antero-posteriormodal
position (ranging from sub-canine to sub-molar) and
number (multiple /absent) of the mental foramen. This
review aims to elucidate the normal morphology of the
MF and its commonly occurring variations with a view to
facilitatethe accurateidentificationofMFduringinvasive
procedures of the lower face, thereby limiting post
proceduralcomplications.
An extensivereview of published literaturewas done
through use of general and meta search engines (google
scholar, pubmed, ovid, science direct) to harvest
prominentmedical database(medline,embase,cochrane).
The search strategies used were 'Mesh'(key terms used
were :mandible, mental foramen, mandibular canal,
mental nerve, mental canal etc), 'text word' searching,
'reference list' harvesting and 'related articles' feature.
Strict inclusion and exclusion criteria were applied to
select 100 articles ranging from the year 2000 through
2011,basedoncontextrelevance.
Dry skulls macroscopic investigations on Mental
T
1
2
METHODOLOGY
Inclusioncriteria:
Lecturer, Department of Anatomy, Faculty of Medicine,
Jazan University
Correspondence: Tabinda Hasan<drtabindahasan@gmail.com>
OBJECTIVE:
METHODOLGY:
RESULTS:
DISCUSSION:
CONCLUSION:
KEYWORDS:
Howtocitethisarticle:
The mental foramen is an oval or circular opening on the anterior surface of the human mandible,
through which the inferior alveolar nerves and vessels, after passing through the inferior alveolar canal, exit as
mental nerves and vessels. It is a morphologically and clinically important landmark for anatomists, dentists and
orthognathic surgeons alike. This paper aims to elucidate the anatomy and clinics of mental foramen in human
mandibles General and meta search engines were employed to conduct an extensive review of published
medical literature with strict inclusion and exclusion criteria in order to highlight the morphologic characteristics
and commonly occurring variations of mental foramen in humans. 1000 representative studies ranging from the
year1956through 2011weresystematicallyanalyzed,based on contextrelevance.
Available literature indicates that the common position of the mental foramen is below or between
mandibular premolars .Variations in foramen number (multiple or absent foramen) and modal location (subcanine
tosubmolar)areoftenencountered and holdimportantneurovascularimplications.
The mental foramen has strategic importance in determining the effectiveness of nerve blocks in
dentistry and post operative success outcomes of invasive procedures involving the lower jaw. Mental foramen
morphologyand variations arelargelyinfluencedbydemographics,race and ethnicity.
Athorough knowledge of the mentalforamenanatomyanditscommonlyoccurringmorphologic
variations can significantly reduce the incidence of misjudgments in clinical dentistry and post procedural
parasthetic,paralytic and hemorrhagic complicationsofthementalregion.
Mandible,Mentalforamen,Mental nerve.
HasanT.Mentalforamenmorphology:amust knowinclinicaldentistry.JPakDentAssoc: 2012:03;00-00
MENTAL FORAMEN MORPHOLOGY: A MUST KNOW IN
CLINICAL DENTISTRY
Tabinda Hasan MBBS, MD
REVIEWARTICLE
00 JPDA Vol. 21 No. 03 Jul-Sep 2012
_
foramina, investigations with plane radiography,
periapical radiography and computed tomography
highlighting foramen gross morphology and
morphometry, retrospective analysis of mandibular and
mental regions, Evaluative studies to detect the
frequency, size, number and modal position of mental
foramina, reviews dealing with age and race related
variations of mental foramen or post operative
complicationsof the sub-mandibularregion.
Exclusion criteria: Studies on child skulls, animal
mandibles and dry mandible researches where race or sex
were unidentified ,Studies with exclusive or excessive
detailing on histology and development of the mental
foramenwereexcluded from the review.
Key journals from Jazan medical library were hand
searched(British dental journal, Saudi dental journal,
Americanjournalof dentistry,Journal of anatomy,Journal
of American dental association, American Journal of
orthotics, Archives of oral biology etc.); along with
consultation from experts of the field and directly
contacting an author in one case for further information.
Endnotereferencemanagement systemwasused.
MF represents the termination of the mental canal.
The inferior alveolar nerves and vessels, after traversing
the mandibular canal, exit through the MF as the mental
nervesand vessels. These formimportantinnervations for
the lower jaw, cheek, teeth and lip. Developmentally, MF
is an ideal model to study bone remodeling due to
presence of neurological, vascular and primordial
contents. It is the determinant of mandible maturity and
symmetry pattern of the mental triangle. There are
discrepancies in studies regarding the shape, size, modal
position and number of MF in human mandibles. These
discrepancies result from naturally occurring differences
in facial structure, jaw skeleton size and feeding habit
inducedboneremodelinginmandiblesof different human
races.Anotherreasonfor diversificationin observationsis
the difference in strategies used to record data about MF;
ranging from direct measurements to digital imaging
,conventional panoramic radiography, computerized
tomographicscansetc.
Morphometric studies reveal the height of MF to
range from 2.5 to 5.5 mm (average 3.5mm) and thewidth
rangingfrom2to5 mm (average 3.6 mm).
The shape of MF is predominantly oval and less
commonly circular. Different shapes have been observed
in different studies.(Table 1 ) The direction of exit of MF
on the buccal cortical plate of mandible is usually
posterosuperior, with an average inclination of about 97 ;
though other exit patterns exist, like superior, labial,
mesial and posterior. A continuous bony canal is not
always present between mandibular andmental foramina.
It may lack definite walls, or end abruptly into multiple
spread out canaliculi. Bilateral symmetry of the canal is
common,whereasduplications areinfrequent.
The most common location is below the second
premolar. Otherpositionsvaryfrombeingbelowthefirst
premolar, between the two premolars, below the first
molar or below canines. (Table2) The last two positions
are relatively rare. Clear racial trends have been observed
in the MF location in the horizontal plane; below the
second premolar inmongoloids, between the premolars in
Caucasians and between the second premolar and first
molar in blacks. Vertically, MF is placed slightly above
the midpoint of the line joining the mandibular lower
border and alveolar crest. The distance of MF from
symphysis mentii ranges between 22 to 31 mm and 10 to
15 mm from the lower cortex of the mandible. The
location of MF also changes with age. It islocated closer
to the lower border of mandible in adults, nearer to the
DISCUSSION
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5
o
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12
Size:
Shape:
Location:
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JPDA Vol. 21 No. 03 Jul-Sep 2012
Mental foramen morphology
Hasan T
Author Year Oval MF Round MF Race/ region
Gershenson 1986 65.5% 34.5% Israel
Mbarji orgu 1998 56% 44% Zimbabwean
Oguz O 2002 92% 8% Turkish
Igbigbi 2005 70% 30% Malawian
Prabodha 2006 67% 33% Srilanka
Fabian 2007 54% 46% Tanzania
Oliveria J 2009 72% 27% Japnese
Ilayperuma 2009 59% 41% Srilanka
Siddiqui 2010 70% 30% Western India
Singh 2011 10% 90% West India
Table No 1: Shape of mental foramen (MF) as observed in
various studies 6-11
alveolar marginin children and midway in adolescents .In
old age related bony resorption, it seems to move further
upwards, so that inextreme cases, it may open directlyat
the alveolar crest. These positional variations must be
respectedduringosteotomy procedurestoavoiddamage.
Though existenceof asingle MFis themost common
presentation, variations like supernumerary (accessory
MF) or absent foramen are also encountered. (Table3) In
f a c t , p r e s e n c e o f a c c e s s o r y f o r a m i n a
(double>triple>quadruple) is a much more common
phenomenon than absence. (Table4) This phenomenon
occurs due to splitting of the mental nerve into several
fasciculi before the development of MF during the 12th
week of intrauterine life. Such variations are often
accidently discovered during routine therapeutic or
diagnostic oral procedures. The incidence of accessory
foramen differs among races: being more frequent in
Caucasiansthan non Caucasians.
The presence of supplementary blood vessels or
nerves in assessory MF hold clinical significance as
portals of anesthetic and post operative neurovascular -
hemorrhagic /paralytic complications during implant
procedures. Rarely, they may play an unexplained role in
thespreadof tumors of the mental region.
Absence of MF is an extremely rare occurrence and
reports of the same are quite infrequent. The key
underlying reasons for absence may include congenital
agenesis, hyperplasic bony lesion, posttraumatic fibrosis
or age related bone resorption induced positional ascent
and loss of MF at the alveolar crest. Occasionally, false
reportings of apparent MF absence may arise from poor
quality imaging or patient position induced misalignment
and superimposition of the foramen with other bony
structures. The absence of MF is not related with the
perviousnes of inferior alveolar canal; in fact in all the
above reported cases of absence, authors have described
the inferior alveolar canal to be partially or completely
pervious. MF absence may cause sensory alterations in
the lower jaw because of absence of any exit portal for
mentalnerves.
Inferior alveolar anesthesia is administered routinely
in dental practice and unfortunately it is not always
successful. Thecommon cause of failure in this anesthetic
Number:
ClinicsofMF:
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Mental foramen morphology
Hasan T
Author Year Location of Mental
foramen
Percentage of
occurrence
Race / Region
Fish et al 1976 Between premolars
Below 2ndpremolar
70%
20%
Caucasian
Wang et al 1986 Below 2nd premolar 59% Chinese
Kekere Ekun et al 1989 Below 2nd premolar
Between premolars
56%
27%
Nigerian
Santini eta l 1990 Below 2nd premolar 53% Chinese
Santini eta l 1990 Between 1st & 2nd
premolar
65% British
Philips et al 1992 Below 2nd premolar 65% Unknown
Bergmane t al 2001 Below 2nd premolar
Between premolars
61%
36%
Unknown
Negow et al 2003 Below 2nd premolar 70% Malaysian
Olasoji etal 2004 Between apex of 1st
& 2nd premolar
65% Nigerian
Neiva et al 2004 Between premolars
Below 2ndpremolar
58%
42%
Caucasian
Apin Hasmit et al 2006 Below 2nd premolar 65% Thailandian
Gignor etal 2006 Between 1st &2nd
premolars
72% Turkish
Kim et al 2006 Below 2nd premolar 64% Korean
Fabian et al 2007 Below 2nd premolar 45% Tanzan ian
Location of Mental Foramen as observed in
various studies
Table No 2: 6-8
Popuilations Incidence of accessory
mental foramina
Freanch
American Whites
American Blacks
Greeks
Russians
Hungarians
Melanese
Egyptians
Japanese
Asian Indians
2.6%
1.6%
5.4%
3.3%
1.5%
3%
9.7%
3.6%
12%
1.5%
Table No 3: Occurrence of accessory mental foramina in
different populations.4
procedure is mis-placement of the hypodermic needle
because of improper evaluation of anatomic landmarks.
Theviolationof mentalforamenbyanimplantmayleadto
permanent injury to the mental nerve and permanent
paresthesia or anesthesia of the lower lip. Accurate
localization is therefore extremely important in order to
avoidsuchfailures.
Radiography is the only available noninvasive
medium for diagnosis and treatment planning of major
surgical procedures of the mandible. Panoramic
radiographs are commonly used for screening, diagnosis,
and for selecting the best surgical approach. Panoramic
radiography based appearance of MF has been classified
inTable5 .
Digital imaging was first introduced in dentistry for
intraoral radiography, but is now widely available for
panoramic radiography. The advantages of digital
techniques compared with film techniques are fast
communication of images, the small storage space needed
and lower contamination of the environment. However,
radiographic images often lack sufficient information in
the bucco-lingual direction and are prone to linear
distortions and superimpositions induced by patient
positioning, contrast, brightness & magnification values.
Delineation and compromised visibility of MF in relation
to other anatomical landmarks of the jaw as visualized on
panoramic radiographs can lead to in-correct prosthetic /
surgical planning in the mental region. Conventional
panoramic radiographs have been generally reported as
being more accurate than digital panoramic images while
computerized tomography(CT) scans yield the best
results; with minimum chances of errors. Clinicians
should keep in mind these measurement discrepancies.
Accurate localization of the mental and mandibular
foramens, with a measurement error less than 1mm is
important to avoid complications during periapical and
orthognathic procedures. Measuring a safety zone in mm
fromthealveolarcresttothecoronalmarginoftheMFcan
limitdamages.
Presence of a mesial loop of mental nerve poses
significant clinical challenges. It is an extension of the
inferior alveolar neurovascular bundle anterior to the MF,
which doubles up to enter the MF before finally exiting it.
Cadaveric dissection data on the prevalence or length of
mesialnerveloop doesnotcorrelatewitheitherpanoramic
/periapicalimages or CT scans.(Table6)
It is, however, a rare occurrence and predetermined
safety guidelines can reduce chances of accidental
damageto relatedstructuresinthisarea.
There is dire need for determining 'safety guidelines' and
'clinical recommendations' based on standardized
national and international criterion with special
consideration of race or age specific variability of MF in
variouspopulations.
Some relevant clinical recommendations are
suggestedbelow:
1-Mandatory radiographs (periapical or panoramic) and
preferably CT scans for viewing MF morphology and
accurately accessing its location prior to implants or any
otherinvasiveproceduresof thesub-mandibularregion.
2- Updated records in medical literature and routine
anatomy text books on MF morphology and
morphometry; with special reference to modal location,
shape and commonly occuring variations in major ethnic
groupsoftheworld.
3- Considering the possible presence of 'Mesial loop of
mental nerve' anterior to the MF, it is recommended to
leave a '2mm Safety zone' between the coronal aspect of
thenerveandthe intendedimplant.
4- Since paresthesia, bleeding and inadvertent nerve
damages are common in case of positional misjudgments
of MF, it is therefore recommended to accurately localize
the mental foramen radiographically prior to any attempt
ofadministrationof routine mentalnerve blocks.
The MF is an important landmark for implant
placement in the foraminal region. The position and
number can vary, radiographs can be misleading and a
mesial nerve loop may be present. These aspects must be
considered by doctors prior to invasive procedures of the
jaw, for safe clinical practice.An adequate knowledge of
the gross morphology and clinically relevant anatomic
features of MF in different populations holds strategic
importance for anthropologists, anatomists and dentists
alike
CONCLUSION
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Hasan T