mand anatomy

8/17/2019 Mand Anatomy

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THE MANDIBLE

The human mandible has no one design for life. Rather, it adapts and remodels through the seven stages of life, from the slim arbiter of things to

come in the infant, through a powerful dentate machine and even weapon in

the full flesh of maturity, to the pencil-thin, porcelain-like problem that we

struggle to repair in the adversity of old age.

D.E. Poswillo 1988

The cartilages and bones of the mandibular skeleton form from

embryonic neural crest cells that originated in the mid and hindbrain regions

of the neural folds. These cells migrate ventrally to form the mandibular

(and maillary! facial "rominences# where they differentiate into bones and

connective tissues.

The first structure to develo" in the region of the lower $aw is the

mandibular division of the trigeminal nerve that "recedes the

ectomesencymal condensation forming the first (mandibular! branchial arch.

The "rior "resence of the nerve has been "ostulated as re%uisite for inducing

osteogenesis by the "roduction of neurotro"hic factors. The mandible is

derived from ossification of an osteogenic membrane formed from

ectomesencymal condensation at &'&8 days of develo"ment. This

mandibular ectomesenchyme must interact initially with the e"ithelium of

the mandibular arch# before "rimary ossification can occur) the resulting

intramembranous bone lies lateral to *eckel+s cartilage of the first

(mandibular! branchial arch. , single ossification center for each half of the

mandible arises in the 'th week i.u. (The mandible and the clavicle are the


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first bones to begin to ossify! in the region of the bifurcation of the inferior

alveolar nerve and artery into mental and incisive branches. The ossifying

membrane is lateral to *eckel+s cartilage and its accom"anying

neurovascular bundle. -ssification s"reads from the "rimary centre below

and around the inferior alveolar nerve and its incisive branch# and u"wards

to form a trough for the develo"ing teeth. "read of the intramembranous

ossification dorsally and ventrally forms the body and ramus of the

mandible. *eckel+s cartilage becomes surrounded and invaded by bone.

-ssification sto"s dorsally at the site that will become the mandibular

lingula# from where *eckel+s cartilage continues into the middle ear. The

"rior "resence of the neurovascular bundle ensures formation of the

mandibular foramen and canal and the mental foramen.

The first branchial arch core of *eckel+s cartilage almost meets its

fellow of the o""osite side ventrally. /t diverges dorsally to end in the

tym"anic cavity of each middle ear# which is derived from the first

"haryngeal "ouch# and is surrounded by the forming "etrous "ortion of the

tem"oral bone. The dorsal end of *eckel+s cartilage ossifies to form the

basis of two of the auditory ossicles# vi0. the malleus and incus. The third

ossicle# the sta"es# is derived "rimarily from the cartilage of the second

branchial arch.

,lmost all of *eckel+s cartilage disa""ears (*eckel+s cartilage lacks

the en0yme "hos"hatase found in ossifying cartilages# thus "recluding its

ossification# and disa""ears by the 2th week i.u.!. Parts transform into the

s"henomandibular and anterior malleolar ligaments. , small "art of its

ventral end (from the mental foramen ventrally to the sym"hysis! forms

accessory endochondral ossicles that are incor"orated into the chin region of

the mandible. *eckel+s cartilage dorsal to the mental foramen undergoes


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resor"tion on its lateral surface at the same time# as intramembranous bony

trabeculae are forming immediately lateral to the resorbing cartilage. Thus#

the cartilage from the mental foramen to the lingula is not incor"orated into

ossification of the mandible.

The initial woven bone formed along *eckel+s cartilage is soon

re"laced by lamellar bone# and ty"ical haversian systems are already "resent

at the 3th month i.u. This earlier remodeling than other bones is attributed as

a res"onse to the early intense sucking and swallowing which stress the

mandible.

econdary accessory cartilages a""ear between the 14th and 12th

weeks of i.u to form the head of the condyle# "art of the coronoid "rocess#

and the mental "rotuberance. The a""earance of these secondary mandibular

cartilages is dissociated from the "rimary branchial (*eckel+s! and

chondrocranial cartilages. The secondary cartilage of the coronoid "rocess

develo"s within the tem"oralis muscle as its "redecessor. The coronoid

accessory cartilage becomes incor"orated into the e"anding

intramembranous bone of the ramus and disa""ears before birth. /n the

mental region# on either side of the sym"hysis# one or two small cartilages

a""ear and ossify in the 5th month i.u. to form a variable number of mental

ossicles in the fibrous tissue of the sym"hysis. The ossicles become

incor"orated into the intramembranous bone when the sym"hysis menti is

converted from a syndesmosis into a synostosis during the first "ostnatal

year.

The condylar secondary cartilage a""ears during the 14th week i.u. as

a cone sha"ed structure in the ramal region. This condylar cartilage is the

"rimordium of the future condyle. 6artilage cells differentiate from its

centre# and the cartilage condylar head increases by interstitial and


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a""ositional growth. 7y the 12th week# the first evidence of endochondral

bone a""ears in the condyle region. The condylar cartilage serves as an

im"ortant centre of growth (The nature of this growth# as "rimary (an initial

source of mor"hogenesis! or secondary (com"ensating for functional

stimulation!# is controversial# but e"erimental evidence indicates the need

for mechanical stimuli for normal growth! for the ramus and body of the

mandible. *uch of the cone sha"ed cartilage is re"laced with bone by the

middle of fetal life) but its u""er end "ersists into adulthood# acting as both a

growth cartilage and an articular cartilage. 6hanges in mandibular "osition

and form are related to the direction and amount of condylar growth. The

condylar growth rate increases at "uberty# "eaks between 1 1 and 12 years

of age# and normally ceases at about 4 years. owever# the continuing

"resence of the cartilage "rovides a "otential for continued growth# which is

reali0ed in conditions of abnormal growth such as acromegaly.

The sha"e and si0e of the diminutive fetal mandible undergo

considerable transformation during its growth and develo"ment. The

ascending ramus of the neonatal mandible is low and wide) the coronoid

"rocess is relatively large and "ro$ects well above the condyle) the body is

merely an o"en shell containing the buds and "artial crowns of the

deciduous teeth) the mandibular canal runs low in the body. The initial

se"aration of the right and left bodies of the mandible at the midline

sym"hysis menti is gradually eliminated between the 2th and 1th months

"ostnatally# when ossification converts the syndesmosis into a synostosis#

uniting the two halves.

,lthough the mandible a""ears in the adult as a single bone# it is

develo"mentally and functionally divisible into several skeletal subunits.

The basal bone of the body forms one unit# to which are attached the


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alveolar# coronoid# angular and condylar "rocesses and the chin. Each of

these skeletal subunits is influenced in its growth "attern by a functional

matri that acts u"on the bone: the teeth act as a functional matri for the

alveolar unit) the action of the tem"oralis muscles influences the coronoid

"rocess) the masseter and medial "terygoid muscles act u"on the angle and

ramus of the mandible) and the lateral "terygoid has some influence on the

condylar "rocess. The functioning of the related tongue and "erioral

muscles# and the e"ansion of the oral and "haryngeal cavities# "rovide

stimuli for mandibular growth to reach its full "otential. -f the facial bones#

the mandible undergoes the most growth "ostnatally and evidences the

greatest variation in mor"hology.

;imited growth takes "lace at the sym"hysis menti until fusion

occurs. The main sites of "ostnatal mandibular growth are at the condylar

cartilages# the "osterior borders of the rami# and the alveolar ridges. These

areas of bone de"osition account grossly for increases in the height# length

and width of the mandible. owever# su"erim"osed u"on this basic

incremental growth are numerous regional remodeling changes# sub$ected to

the local functional influences that involve selective resor"tion and

dis"lacement of individual mandibular elements.

The condylar cartilage of the mandible serves the uni%uely dual roles

of an articular cartilage in the tem"oromandibular $oint# characteri0ed by a

fibro cartilage surface layer# and as a growth cartilage analogous to the

e"i"hyseal "late in a long bone# characteri0ed by a dee"er hy"ertro"hying

cartilage layer. The subarticular a""ositional "roliferation of cartilage within

the condylar head "rovides the basis for growth of a medullary core of

endochondral bone# on whose outer surface a corte of intramembranous

bone is laid. The growth cartilage may act as a functional matri to stretch


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the "eriosteum# inducing the lengthened "eriosteum to form

intramembranous bone beneath it. The diverse histological origins of the

medulla and corte are effaced by their fusion. The formation of bone within

the condylar heads causes the mandibular rami to grow u"ward and

backward# dis"lacing the entire mandible in an o""osite downward# forward

direction. 7one resor"tion sub$acent to the condylar head accounts for the

narrowed condylar neck. The attachment of the lateral "terygoid muscle to

this neck# and the growth and action of the tongue and masticatory muscles#

are functional forces im"licated in this "hase of mandibular growth.

,ny damage to the condylar cartilages restricts the growth "otential

and normal downward and forward dis"lacement of the mandible#

unilaterally or bilaterally# according to the side(s! damaged. ;ateral

deviations of the mandible# and varying degrees of micrognathia and

accom"anying malocclusion result.

/n the infant# the condyles of the mandible are inclined almost

hori0ontally# so that condylar growth leads to an increase in the length of the

mandible# rather than increase in height. Due to the "osterior divergence of

the two halves of the body of the mandible (in a < sha"e!# growth at the

condylar heads of the increasingly more widely dis"laced rami results in

overall widening of the mandibular body# which# with remodeling# kee"s

"ace with the widening cranial base. =o interstitial widening of the mandible

can take "lace at the fused sym"hysis menti after the first year# a"art from

some widening by surface a""osition.

7one de"osition occurs on the "osterior border of the ramus# while

concomitant resor"tion on the anterior border maintains the "ro"ortions of

the ramus# and in effect# moves it backwards in relation to the body of the

mandible. This de"osition resor"tion etends u" to the coronoid "rocess#


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involving the mandibular notch# and "rogressively re"ositions the

mandibular foramen "osteriorly# accounting for the anterior overlying "late

of the lingula. The attachment of the elevating muscles of mastication to the

buccal and lingual as"ects of the ramus# and to the mandibular angle and

coronoid "rocess influences the ultimate si0e and "ro"ortions of these

mandibular elements.

The "osterior dis"lacement of the ramus converts former ramal bone

into the "osterior "art of the body of the mandible. /n this manner# the body

of the mandible lengthens# the "osterior molar region relocating anteriorly

into the "remolar and canine regions. This is one means by which additional

s"ace is "rovided for eru"tion of the molar teeth# all three of which originate

in the ramus body $unction. Their forward migration and "osterior ramal

dis"lacement lengthen the molar region of the mandible.

The forward shift of the growing mandibular body changes the

direction of the mental foramen during infancy and childhood. The mental

neurovascular bundle emanates from the mandible at right angles or even a

slightly forward direction at birth. /n adulthood the mental foramen (and its

neurovascular content! is characteristically directed backward. This change

may be ascribed to forward growth in the body of the mandible# while the

neurovascular bundle > drags along+. , contributory factor may be the

differential rates of bone and "eriosteal growth. The latter# by its firm

attachment to the condyle and com"aratively loose attachment to the

mandibular body# grows more slowly than the body# which slides forward

beneath the "eriosteum. The changing direction of the foramen has clinical

im"lications in the administration of local anaesthetic to the mental nerve: in

infancy and childhood# the syringe needle may be a""lied at right angles to


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the body of the mandible to enter the mental foramen# whereas in the adult

the needle has to be a""lied obli%uely from behind to achieve entry.

The location of the mental foramen also alters its vertical relationshi"

within the body of the mandible from infancy to old age. ?hen teeth are

"resent the mental foramen is located midway between the u""er and lower

borders of the mandible. /n the edentulous mandible# lacking an alveolar

ridge# the mental foramen a""ears near the u""er margin of the thinned

mandible.

The alveolar "rocess develo"s as a "rotective trough in res"onse to the

tooth buds# and becomes su"erim"osed u"on the >basal bone+ of the

mandibular body. /t adds to the height and thickness of the body of the

mandible# and is "articularly manifest as a ledge etending lingually to the

ramus to accommodate the third molars. The alveolar bone fails to develo" if

teeth are absent# and resorbs in res"onses to tooth etraction. The

orthodontic movement of teeth takes "lace in the labile alveolar bone# of

both mailla and mandible# and fails to involve the underlying >basal bone+.

The chin# formed in "art of the mental ossicles from accessory

cartilages and the ventral end of *eckel+s cartilage# is very "oorly develo"ed

in the infant. /t develo"s almost as an inde"endent subunit of the mandible#

influenced by seual as well as s"ecific genetic factors. e differences in

the sym"hyseal region of the mandible are not significant until other

secondary se characteristics develo". Thus# the chin becomes significant

only at adolescence from develo"ment of the mental Protuberance and

tubercles. ?hereas small chins are found in adults of both sees# very large

chins are characteristically masculine. The skeletal >unit+ of the chin may be

an e"ression of the functional forces eerted by the lateral "terygoid


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muscles that# in "ulling the mandible forward# indirectly stress the mental

sym"hyseal region by their concomitant inward "ull. 7one buttressing to

resist muscle stressing# which is more "owerful in the male# is e"ressed in

the more "rominent male chin. The "rotrusive chin is a uni%uely human trait#

lacking in all other "rimates and hominid ancestors.

The mental "rotuberance forms by osseous de"osition during

childhood. /ts "rominences are accentuated by bone resor"tion in the

alveolar region above it# creating the su"ramental concavity known as >Point

7+ in orthodontic terminology. @nderdevelo"ment of the chin is known as

microgenia.

, genetically determined eostosis on the lingual as"ect of the body

of the mandible# the torus mandibularis# develo"s# usually bilaterally# in the

canine "remolar region. These tori are unrelated to any muscle attachments

or known functional matrices.

During fetal life the relative si0es of the mailla and mandible vary

widely. /nitially# the mandible is considerably larger than the mailla# a

"redominance lessened later by the relatively greater develo"ment of the

mailla) by about 8 weeks i.u. the mailla overla"s the mandible. The

subse%uent relatively greater growth of the mandible results in

a""roimately e%ual si0e of the u""er and lower $aws by the 11th week.

*andibular growth lags behind maillary between the 1&th and 4th weeks

i.u.# due to a change over from *eckel+s cartilage to condylar secondary

cartilage as the main growth determinant of the lower $aw. ,t birth# the

mandible tends to be retrognathic to the mailla# although the two $aws may

be of e%ual si0e. This retrognathic condition is normally corrected early in

"ostnatal life by ra"id mandibular growth and forward dis"lacement to

establish orthognathia# or an ,ngle 6lass / maillomandibular relationshi".


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/nade%uate mandibular growth results in an ,ngle 6lass // relation# and

overgrowth of the mandible "roduces a 6lass /// relation. The mandible can

grow for much longer than the mailla.

,nomalies of develo"ment:

The mandible may be grossly deficient or absent in the condition of

agnathia which reflects a deficiency of neutral crest tissue in the lower "art

of the face. ,"lasia of the mandible and hyoid bone (first and second arch

syndrome! is a rare lethal condition with multi"le defects of the orbit and

mailla. ?ell develo"ed# albeit low set# ears and auditory ossicles in thissyndrome suggest ischaemic necrosis of the mandible and hyoid bone occurs

after formation of the ear.

*icrognathia# a diminutive mandible is characteristic of several

syndromes# including Pierre Aobins and the catcry (6ri du chat! syndromes#

mandibulofacial dysostosis (Treacher 6ollins syndrome!# "rogeria Down+s

syndrome (trisomy 1!# oculomandibulodysce"haly (the allermantreiff

syndrome! and Turner+s syndrome (B- sechromosome com"lement!.

, central dysmor"hogenic mechanism of defective neural crest

"roduction# migration# or destruction may be res"onsible for the hy"o "lastic

mandible common to these conditions. ,bsent or deficient neural crest tissue

around the o"tic cu" causes a >vacuum+ so that the develo"ing otic "it#

normally ad$acent to the second branchial arch# moves cranially into first

arch territory and the ear becomes located over the angle of the mandible.

Derivatives of the deficient ectomesenchyme# s"ecifically the 0ygomatic#

maillary and mandibular bones# are hy"o"lastic# accounting for the ty"ical

facies common to these syndromes.


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/n Pierre Aobin+s syndrome# the underdevelo"ed mandible usually

demonstrates catchu" growth in the child) in mandibulofacial dysostosis#

deficiency of the mandible is maintained throughout growth) in unilateral

agenesis of the mandibular ramus# the deformity increases with age. emi

facial microsomia (Coldenhar+s syndrome! also becomes more severe with

retarded growth.


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between two "remolar teeth# or the second "remolar# is the mental foramen#

from which emerge the mental nerve and vessels) its "osterior border is

smooth accommodating the dorsolaterally emerging nerve (?arwick 1934!.

, faint obli%ue line ascends backwards from each mental tubercle# swee"s

below the foramen# and then becomes more marked as it continues into the

anterior border of the ramus.

The body+s lower border# the base# etends "osterolaterally from the

sym"hysis in to that of the ramus behind the third molar tooth. =ear the

midline# on each side# is a rough digastric fossa# behind which the base is

thick# rounded# with a slight antero"osterior conveity. ,s the ramus is

a""roached# this changes to a gentle concavity) thus# in "rofile# the whole

base is sinuous.

The u""er border# the alveolar "art# contains 1' alveoli for roots of

teeth# varying in si0e and de"th# some being multi"le.

The internal surface is divided by an obli%ue mylohyoid line# shar"

and distinct near the molars# faint in front and etending from behind the

third molar# a centimeter from the u""er border# to the mental sym"hysis

between the digastric fossae. 7elow this line is the slightly concave

submandibular fossa) the area above it widens anteriorly into a triangular

sublingual fossa. ,bove the latter and etending back to the third molar# the

bone is covered by oral mucosa. ,bove the anterior ends of the mylohyoid

lines# the "osterior sym"hyseal as"ect has a small elevation# often divided

into u""er and lower "arts# the mental s"ines (genial tubercles!. Posteriorly

the mylohyoid groove etends down and forwards from the ramus below the

mylohyoid lines "osterior "art. u"erior to the mental s"ines most mandibles

dis"lay a median "it o"ening into a canal. ,s yet its develo"ment and

contents are uncertain but it is a useful radiological landmark the name


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genial foramen has been "ro"osed. ,bove the mylohyoid line# medial to the

molar roots# a rounded torus mandibularis sometimes a""ears.

The mandibular ramus is %uadrilateral# with two surfaces# four borders

and two "rocesses. The flat lateral surface has obli%ue ridges in its lower

"art) the medial "resents a little above centre# an irregular mandibular

foramen# leading into the mandibular canal# curving down and forwards into

the body to its mental foramen. ,nteromedially the foramen is overla""ed

by a thin# triangular lingula. The mylohyoid groove descends forwards from

behind the lingula) short ridges mark the surface behind it. The inferior

border# continuous with mandibular base# meets the "osterior border at the

angle. This is ty"ically everted# but in females fre%uently incurved. The thin

u""er border bounds the mandibular incisure surmounted in front by the

somewhat triangular# flat# coronoid "rocess# behind by a strong condylar

"rocess. The "osterior border thick and rounded etends from condyle to

angle# being gently conve backwards above# and concave below) it is in

contact with the "arotid gland. The anterior border is thin above and

continuous with that of the coronoid "rocess# and thicker below and

continuous with the obli%ue line.

The coronoid "rocess "ro$ects u" and slightly forward. /ts "osterior

border bounds the mandibular incisure# its anterior continues into that of the

ramus. /ts margins and medial surface are attachments for most of the

tem"oralis.

The condylar "rocess is a"ically enlarged as a head of condyle#

covered by fibrocartilage. /t articulates with the tem"oral bones mandibular

fossa# with an articular disc between. /t is conve in all directions# its

transverse dimension greater. /ts lateral as"ect is a blunt "ro$ection# "al"able

in front of the auricular tragus. ,s the mouth o"ens the condyle descends


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forwards# admitting a finger ti" towards its vacated fossa. 7elow the head is

the narrower neck# slightly flattened from before backwards# its anterior

as"ect overla""ed laterally by the mandibular incisures# margin medial to

which the neck+s anterior surface bears a rough "terygoid fovea.

The mandibular canal descends obli%uely forwards in the ramus from

the mandibular foramen# then hori0ontally forwards in the body below the

alveoli# with which it communicates by small canals. /t contains the inferior

alveolar nerve and vessels from which branches enter dental roots#

"eriodontal sockets and se"ta. 7etween the roots of the first and second

"remolars# or below the second# the canal divides into mental and incisive

"arts the mental canal swerves u"# back and laterally to the mental foramen)

the incisive canal continues below the incisor teeth.

The mandibular body bears a small shallow incisive fossa below the

incisors# an attachment for mentalis and "art of orbicularis oris. To the

anterior ends of the obli%ue lines are attached de"ressor labii inferioris and

anguli oris# and "latysma to bone below and backwards beyond them.

,d$oining the alveolar border bone is covered by oral mucosa and# below

this in the molar region# the buccinator has a linear attachment etending

medially behind the last molar to the "terygomandibular ra"he.

To the mylohyoid line is attached the mylohyoid muscle and# above its

"osterior end the su"erior "haryngeal constrictor# some retro molar fascicles

of the buccinator# and the "terygomandibular ra"he behind the third molar.

,lthough usually described se"arately# here the constrictor# buccinator and

the ra"he are blended and $ointly attach to the mandibular "eriosteum. The

lingual nerve reaches the tongue above the mylohyoid line# closely related to

bone near its "osterior end often the nerve is accommodated in a shallow#

curved groove. To the su"erior mental s"ines are attached the genioglossi# to


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the inferior the geniohyoids. The submandibular fossa ad$oins

submandibular lym"h nodes as well as the salivary gland) the facial artery

usually descends here to curl round the base of the mandible# sometimes

making a shallow groove. The digastric fossa is for attachment of the muscle

anterior belly.

The mandibular ramus and its "rocesses "rovide attachment for

muscles of mastication# much of its lateral surface to masseter# ece"t

"osterosu"eriorly# where it is covered by the "arotid gland) the medial

surface receives the medial "terygoid on the roughened area "osteroinferior

to the mylohyoid groove. To the lingula the s"henomandibular ligament is

attached# "osterior to which the mylohyoid nerve and vessels enter the

mylohyoid groove# reaching the mandibular body below the mylohyoid

groove# reaching the mandibular body below the mylohyoid line) they then

"ass su"erficial to mylohyoid. 7elow the lingula# but above the roughened

attachment mentioned above# the medial surface of the ramus is related to

the medial "terygoid# the lingula nerve being between muscle and bone. The

lowest attachment of tem"oralis descends beyond the coronoid "rocess to

the anterior ramal border and "articularly its ad$oining medial surface. To

the area "osterosu"erior to the mandibular foramen the maillary artery and

its inferior alveolar branch are related# and lateral "terygoid to the area near

the mandibular incisure. The mandibular incisure transmits the masseteric

nerve and vessels form the infratem"oral fossa.

The coronoid "rocess is covered laterally by the anterior "art of

master descending to its attachment on the ramus. /ts anterior border is

"al"able below the 0ygoma) this is most evident during mouth o"ening. The

condylar "rocess "ro$ects more at its medial "ole. /ts articular surface

descends only a little on its anterior surface# covers the whole of its su"erior


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as"ect and descents 3 mm "osteriorly. /ts "ro$ecting lateral "art is se"arated

from the cartilaginous eternal acoustic meatus by the "arotid. ;aterally on

its neck the $oint+s lateral ligament is attached# covered by "arotid glad. The

"terygoid fovea# anterior on the neck# receives the lateral "terygoid. The

neck+s medial surface is related to the ,uriculotem"oral nerve above and

maillary artery below.

The "arotid gland is below the eternal acoustic meatus and lies

between the ramus and mastoid "rocess# with the styloid "rocess medial) but

it etends forwards lateral to the tem"oromandibular $oint and to the e"osed

ramus behind the masseter. /t also curls round the "osterior border to the

medial as"ect of the ramus above the attachment of the medial "terygoid.

,ccessory foramina of the mandible are usually unnamed and

infre%uently described. et a study of &44 mandibles yielded a count of

229 accessory foramina. ince many transmit auiliary nerves to teeth

(from facial# buccal# transverse cutaneous others!# their occurrence is

significant in dental blocking techni%ues. Further mandibular variants

include lingual de"ressions# molar or canine# variable "osition of mental

foramen# multi"le mental foramina# lingual fenestrations of molar sockets#

retromolar foramina and condylar defects.

Aspects of mandibular structure. /n addition to variable mandibular

canals# numerous analyses have been made of the structure of surface tables

and buttresses of com"act bone and the geometry of trabeculation in

attem"ts to relate these to habitual functional stresses. ologra"hic

interferometry has been used to study surface strains induced by orthodontic

forces.


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Age !anges in t!e Mandible

,t birth the two halves of the mandible are united by a fibrous

sym"hysis menti. ,nterior ends of both rudiments are covered by cartilage#

se"arated only by a sym"hysis. @ntil fusion occurs new cells are added to

each cartilage from sym"hyseal fibrous tissue# ossification on its mandibular

side "roceeding towards the midline) when the later "rocess overtakes the

former# etending into median fibrous tissue# the sym"hysis fuses but details

are uncertain. ,t this stage the body is a mere shell# enclosing im"erfectly

se"arated sockets of deciduous teeth. The mandibular canal is near the

lower border# the mental foramen o"ens below the first deciduous molar and

is directed forwards. The coronoid "rocess "ro$ects above the condyle.

/n the first to third "ostnatal years the two halves $oin at their

sym"hysis from below u"wards) se"aration near the alveolar margin may

"ersist into the second year. The body elongates# es"ecially behind the

mental foramen# "roviding s"ace for three additional teeth. During the first

and second years# as a chin develo"s# the mental foramen alters direction

from anterior to "osterosu"erior# and then almost hori0ontally "osterior# as

the adults# accommodating a changing direction of the emerging mental

nerve. The "roimal 0one of the conical condylar cartilage "ersists as an

e"i"hyseal "late. /ts "roliferation contributes to "ersist as an e"i"hyseal

"late. /ts "roliferation contributes to vertical increase in the ramus and to

general mandibular growth# which is essentially downwards and forwards.

?ith its antimere it also ada"ts the intercondylar distance to the widening

cranial base. The condylar cartilage is covered on its articular as"ect by

self"er"etuating fibrous tissue# dee"# to which a "roliferating intermediate

0one is res"onsible for ramal growth. 7eneath this are hy"ertro"hic

chondrocytes and then bone. ,s de"th of the body increases# alveolar


18/44

growth makes room for roots of teeth# the subalveolar region becoming

thicker and dee"er. ,fter eru"tion of "ermanent teeth the mandibular canal

is a little above the mylohyoid line# and the mental foramen occu"ies its

adult "osition. ,s the mandible increases in si0e# bone is added at "osterior

borders of ramus and coronoid "rocess# absor"tion occurring at their anterior

borders. This remodeling is continuous until adult si0e is reached# allowing

alveolar "arts to accommodate "ermanent molar teeth.

/n adults alveolar and subalveolar regions are about e%ual in de"th# the

mental foramen midway between u""er and lower borders) the mandibular

canal nearly "arallels the mylohyoid line. The angle between the lower

border of body and a "lane touching the "osterior surface of condyle above#

and ramus below# diminishes as ramal height increases with age) but Bray

"hotogra"hs at different ages show that its contour remains unaltered.

/n old age the bone is reduced in si0e# as teeth are lost and the alveolar

region absorbed) the mandibular canal and mental foramen are nearer the

su"erior border. 7oth may even disa""ear# e"osing the inferior alveolar

nerve. The ramus becomes obli%ue# the angle about 124o# and the neck

inclined backwards. ,bsor"tion affects chiefly the thinner alveolar wall and#

after com"letion# a linear alveolar ridge is left at the su"erior border of the

mandible. /n the mandible the labial wall is thinner in incisor and canine

regions# the lingual wall in the molar. The mandibular alveolar ridge hence

is within the line of teeth in the former but outside it in molar regions#

forming a curve wider "osteriorly than that of the line of the teeth# but

intersecting it near the "remolars. /n the mailla# however# the labial wall is

ridge entirely within the line of teeth.


19/44

7;--D @PP;

7lood su""ly to the mandibular corte is a decisive factor of bone

growth bone re"air. Disturbances of


20/44

Gone ///: -ne or two branches of masseteric artery one or two branches

of Transverse facial artery.

End "eriosteal blood su""ly

Gone /: /nferior alveolar mental arteries

Gone //: Transverse facial *assetric arteries

Gone ///: Transverse facial *assetric arteries

Periosteal blood su""ly

Gone /: *ental# submental ;ingual arteries

Gone //: Transverse facial Dee" tem"oral arteries

Gone ///: u"erficial tem"oral# Dee" tem"oral facial arteries.

"#$%IAL ANATOMY O& THE MANDIBLE

/=TA-D@6T/-=:

The mandible is basically long bone# which is bent into a blunt


21/44

develo"ing teeth and then su""orts their roots after eru"tion. The form of

the alveolar "rocess is entirely de"endent u"on the "resence (or absence! of

the teeth and the functional forces transmitted through them.

The mandible differs from all other long bones in two im"ortant

res"ects:

1. ,ny movement inevitably causes both condyles to move with res"ect to

the skull bases.

. ,lthough anatomically the condyles are the articular surfaces of the

mandible# functionally the occlusal surface of the mandibular teeth sub

serves this role. /n a functional sense then# the oral cavity is analogous to a

$oint s"ace.

The mandible is subcutaneous or submucosal in most of its etent# the

only "art inaccessible to "al"ation being the u""er and "osterior "ortion of

the ascending ramus. The anterior edge of the ascending ramus may be

readily "al"ated intraorally# but the fibres of the tem"oralis muscle clothe

its u""er half.

TE TEET

The "resence of the teeth in the mandible and mailla is the most im"ortant

anatomical factor# which makes fractures of these bones entirely different

from# fractures elsewhere. The occlusion of the teeth is a delicately

balanced mechanism and any disturbance resulting from malunion of a

fracture leads to a reduction in masticatory efficiency and comfort# and so

restoration of the occlusion is the "rime aim in the treatment of fractures of

the mandible. 6onversely# the "resence of the teeth is etremely hel"ful in

the reduction and fiation of mandibular fractures) the teeth may be regarded

as a row of bone"ins offering direct control of attached fragments of bone


22/44

without any of the "roblems associated with surgically introduced metal

"ins. During reduction of mandibular fractures it is im"erative to determine

the original occlusion of the teeth and to restore it. This will obviously

re"osition the toothbearing fragments of the mandible to their original

relationshi" with the mailla.

6om"lete fractures of the body of the dentate mandible must of

necessary involve the roots or "eriodontal membrane of ad$acent teeth# only

very rarely "assing through interradicular bone. The root of an involved

tooth may be fracture# but this is unusual. The most im"ortant "oint to note

in such fractures is that the alveolar "rocess is invested over about half its

tears in all cases directly over the fracture both buccally and lingually. uch

fractures are thus o"en (com"ound! into the oral cavity and e"osed to

"ossible infection. The tooth contained in the socket through which the

fracture "asses may also undergo "ul"al necrosis as a result of damage to the

a"ical blood su""ly# which leaves the "ul" chamber of the tooth filled with

necrotic tissue and ultimately tissue fluid. This is a "otential nidus for

infection since it is beyond the reaches of the defensive mechanisms of the

body. The muco"eriosteum of the edentulous mandible is# by contrast# an

intact sleeve and is less fre%uently ru"tured in association with underlying

fractures. /n conse%uence these remain closed and the muco"eriosteum

limits their dis"lacement.

The mandible is commonly fractured because of its "rominent

"osition. Forward falls in the unconscious "atient will result in the "oint of

the chin striking the ground and the chin and body of the mandible form an

inviting landmark in fights.


23/44

TE TAE=CT -F TE *,=D/7;E

There have been numerous investigations into the resistance of the

mandible to a""lied forces with "articularly valuable contributions by

uelke (19'1! and odgson (19'5!. The ma$or "oints to note are that bones

fracture at sites of tensile strain, since their resistance to compressive forces

is greater.

uelke (19'1! has shown that the isolated mandible is liable to

"articular "atterns of distribution of tensile strain when forces are a""lied to

it. ,nterior forces a""lied to the sym"hysis menti# over one mental foramen

or over the mandibular body# lead to strain at the condylar necks and along

the lingual "lates in the o""osite molar region.

lightly different strain "atterns result from im"ortant variables#

including whether the condyles are fied or mobile# but to %uote:

In its response to loading the mandible is quite similar to an

architectural arch in that it distributes the applied force along its length.

The mandible, however, is not a smooth curve of uniform cross-section... as

a result there are parts of the mandible which develop greater force per unit

area parts and consequently, tensile strain is concentrated in these locations.

There is a marked correlation between these sites of tensile strain and

the results of e"erimental fractures# which occur in 52H of cases at sites of

high stress. The mandible is a strong bone# the energy re%uired to fracture it

being of the order of 22.'52.2 kgm# which is about the same as the 0ygoma

and about half that for the frontal bone.

-FT T/@E F,6T-A

The "eriosteum of the mandible a""ears to "lay little "art in

influencing the site of mandibular fractures or subse%uent dis"lacement#


24/44

ece"t in the edentulous "atient. The muscles attached to the mandible eert

considerable influence over the dis"lacement of fragments after fracture)

they "robably "lay a significant but "oorly defined role in the actual

locali0ation of fracture lines both directly and indirectly# and will be

discussed in connection with in$uries at "articular sites.

TE /=FEA/-A DE=T,; =E@A-


25/44

THE ONDYLA$ $E%ION

1. ;ocali0ation

The 0ygomatic arch gives some measure of "rotection to the condyle from

direct trauma# so that the im"act# which causes condylar in$uries# is usually

an indirect one# through the body of the mandible# with the ece"tion of

"enetrating in$uries.

,n im"act transmitted through the neck of the condyle may fail to

cause a fracture but may contuse the ca"sular ligaments# causing a ca"sulitis.

This results in an effusion of inflammatory eudate into the $oint cavity itself

or bleeding into the give a haemarthrosis. The lateral side of the ca"sule is

thickened to form the tem"oromandibular ligament so that# if ru"ture of the

ca"sule occurs# it is more likely to be on the weaker medial as"ect#

encouraging dis"lacement to this side. The ca"sule is less well develo"ed in

children than in adults# making ru"ture more likely.

=ormally the eminentia limits the etent of forward translatory

movement of the condyle. -n some individuals# with a la ca"sule allowing

hy"ermobility# subluation or dislocation over the eminentia occurs %uite

fre%uently. ?hether this ecessive range of movement is due "rimarily to

the synovial laity or whether it is the causal factor which leads to

subse%uent dislocation is uncertain. /t seems that the latter is more likely#

since such individuals fre%uently demonstrate hy"ermobility in other $oints.

,fter dislocation# s"asm in the lateral "terygoid# with its insertion into the

neck of the condyle# may make reduction difficult and this factor also affects

the tem"oralis muscle.

The usual site for a fracture of this region is not through the

anatomical neck but obli%uely downwards and backwards from the sigmoid

notch to a "oint somewhat above the middle of the "osterior border of the


26/44

ascending ramus. This fracture is etraca"sular and is commonly referred to

as a subcondylar fracture. Fracture may occur through the anatomical neck

of the condyle or# %uite ece"tionally# the head is fractured within the $oint

cavity (intraca"sular! and may often be comminuted.

The subcondylar fracture is invariably "roduced indirectly as a result

of violence to the mental "rominence or contra lateral body of the mandible.

*ost authorities agree that it occurs with the teeth a"art and the elevator

muscles relaed. The line of fracture# very significantly# lies $ust above the

"osterosu"erior insertion of the masseter muscle. ,s mentioned "reviously#

the condylar neck is a site of maimum tensile strain with anterior and

anterolateral a""lied forces. The fact that the fracture occurs "recisely

where it does may be related to the muscular res"onse to in$ury# which# with

"osterolateral condylar dis"lacement along the ais of the lateral "terygoid

muscle# would initiate a stretch refle in that muscle. Fractionally later#

contraction of the masseter and medial "terygoid muscles would im"ose an

area of maimum strain $ust above the masseteric insertion. This >active+

ty"e of fracture would not occur in unconscious "atients.

The im"ortance of the meniscus is becoming increasingly recogni0ed

in tem"oromandibular $oint in$ury. ;oss if this structure leads to eventual

degenerative changes in the condylar articulation tearing or dis"lacement of

the meniscus may be im"ortant re%uirement for ankylosis after condylar

fracture.

/n coordination of translatory movement of the condyle and meniscus

under the influence of the lateral "terygoid muscle can result in clicking# or

locking in the tem"oromandibular $oint "aindysfunction syndrome# Trauma

may initiate such sym"toms# "articularly if a tear is created in the mensical

attachments to the ca"sule.


27/44

The nerve su""ly to the $oint is "rinci"ally from the articular branches

of the ,uriculotem"oral nerve (T$ilander 19'1!# which run medial to the

condylar neck# with additional "ossible innervation from branches of the

nerve to the masseter and dee" as"ect of the tem"oralis muscles.

timulation of these nerves# for eam"le by trauma# may cause refle

changes in the muscles of mastication accounting for the s"asm and

restricted mobility seen after in$ury. The condyle is in close relationshi" to a

number of other nerves# "articularly the facial and long buccal# so that it is

understandable that trauma to these could occur after in$ury with resultant

motor or sensory loss# although neurological loss in these circumstances is

com"aratively uncommon.

Paraesthesia of the mental nerve may# rarely# occur following fracture

dislocation of the condyle in an anteromedial direction as the result of

trauma to or com"ression of the mandibular nerve as it emerges from the

foramen ovale.

Dis"lacement

The word >dis"lacement+ always refers to the disturbed relationshi"

between the fractured bone ends and in the case of condylar fractures this is

usually slight. The im"ortant abnormality here relates to the "osition

assumed by the condylar head. The bone ends may be deviated due to

anteromedial dis"lacement of the condylar head# which is still contained by

the $oint ca"sule# or alternatively the ca"sule may ru"ture# leading to

se"aration of the articulating surfaces thus# by definition# being dislocated.

There is no doubt that this anteromedial dis"lacement or dislocation is

"roduced by violent contraction of the lateral "terygoid muscle at# or

immediately after# the moment of fracture. ,n anterior ca"sular tear#


28/44

associated with ru"ture of the fibres of the lateral "terygoid muscle inserted

into the disc# would have the "otential for causing "eriodic inability to close

the $aw fully# due to the disc being dis"laced "osteriorly# while a "osterior

tear without ru"ture of these muscle fibres could result in e"isodes

interference with o"ening due to anterior dis"lacement of the disc.

, "atient with a unilateral subcondylar fracture may "resent without

the ty"ical disturbance of occlusion# but during function all demonstrate a

common characteristic. This is deviation of the mandible to the side of the

in$ury on o"ening. This is due to the insertion of the lateral "terygoid no

longer influencing the main fragment of the mandible. Thus only rotary

movements (at the fracture site! occur on the side of in$ury with no anterior

translation of the condylar head on o"ening. The net effect is that the centre

of rotation of the in$ured side is moved from its normal "osition u"wards

and backwards into the fracture line.

The thin tym"anic "late constitutes "art of the "osterior nonarticular

"ortion on the glenoid cavity. The cushioning effect of the "ostcondylar soft

tissues tends to "rotect this area# as does the restraining effect of the

tem"oromandibular ligament# but this "late may be fractured# with distortion

of the bony wall. This may# very rarely# occur without condylar fracture#

whereas a fracture generally absorbs most of the im"act.

6entral dislocation of the condyle into the middle cranial fossa is a

wellrecogni0ed although a very rare entity when the mandible receives a

blow in the mental region with the mouth o"en# the medial and lateral "oles

of its condyles are im"acted against the medial and lateral elevated margins

of the glenoid fossa# thus "reventing the dislocation of the condyles into the

cranial cavity through the thin roof of the fossa. -nly a small rounded


29/44

condyle would be likely to im"inge on the centre of the fossa with a

"ossibility of "enetration. This ty"e of condylar configuration is

uncommon# accounting for about .8H of all eisting condylar sha"es. /t is#

indeed# fortunate that the neck of the condyle breaks so readily# since it

limits the force a""lied to the cranial base# thus reducing the likelihood of

dis"lacement of the head of the condyle through the thin roof of the glenoid

fossa into the middle cranial fossa.

$AM#" AND O$ONOID '$OE""

There is usually only minimal dis"lacement of the coronoid "rocess#

since the fragment is s"linted by the tendinous insertion of the tem"oralis

muscle. -ccasionally# as the result of considerable violence# this insertion is

ru"tured and elevation of the ti" of the coronoid "rocess occurs. uch

in$uries are "robably caused when there is a combination of forces in

o""osite directions arising from an im"act on the chin de"ressing the

mandible# which simultaneously invokes a "owerful contraction of the

tem"oralis muscle# elevating the mandible as an associated com"onent of the

"hysiological $aw reflees. /n rare cases the fracture may etend from the

sigmoid notch downwards and forwards to the retro molar fossa.

THE AN%LE O& THE MANDIBLE

,fter the condylar neck# the angle region is the commonest site of

mandibular fracture. /t is im"ortant to distinguish between:

The clinical angle# which is the $unction between the alveolar bone

and the ramus at the origin of the internal obli%ue line.

The surgical angle# which is the $unction between the body of the

mandible and the ramus at the origin of the eternal obli%ue line.


30/44

The anatomical angle or gonion where the lower border meets the

"osterior border of the ramus.

Fractures in this region have# in common# involvement of the $unction

of the "osterior end of the alveolar "rocess and body of the mandible with

the ascending ramus. From this "oint on the u""er border they etend

downwards# but only rarely do they involve the anatomical angle.

/n most cases the fracture line etends from the surgical angle

downwards and backwards# terminating at the lower border anterior to the

masseter muscle. ?hen a third molar tooth is "resent the fracture commonly

etends through its cry"t or socket# but occasionally it "asses in front of or

behind the wisdom tooth. The fracture usually results from a blow over the

same side of the mandible between the canine and second molar regions# but

may result from violence to the chin "oint on the o""osite side. /t is

"robable that this occurs# in common with body fractures# when a force is

a""lied with the teeth clenched but there is no direct evidence to su""ort this

theory.

1. ;ocali0ation

,s mentioned earlier# the lingual surface of the mandible in the region

of the second and third molars is one site of maimum tensile strain

resulting from anterolateral a""lication of force on the same side. The

weakness of the angle is "roduced by the abru"t change in direction between

the body and ascending ramus in two "lanes. /n the vertical "lane a change

of direction is almost 44# while in the hori0ontal "lane it is about 544 at the

u""er border. This due to the lack of conformity between the curve of the

alveolar "rocess# which is @sha"ed# and the rami of the mandible# which


31/44

diverge. /n addition to the sha"e of the bone# two other factors are

im"ortant.

a! , "artly eru"ted or uneru"ted wisdom tooth occu"ies a s"ace# which

would otherwise contribute materially to the strength of the mandible and

thus weakens it. The strength of the mandibular body in this region lies in

the u""er border as evidenced by the thickness of the cortical "late.

b!. The insertions of the masseter and medial "terygoid muscles com"rise a

great source strength to the ascending ramus# and the anterior limit of their

insertion lies $ust behind the third molar. The common dis"osition of the

fracture is through the anterior root socket of the third molar and associated

buccal "late# but on the lingual side it etends backwards# leaving the distal

surface of the distal root socket and then "assing through the lingual "late.

The whole fracture line inclines downwards from the wisdom tooth to the

lower border.

. Dis"lacement

, fracture at the angle "revents the elevator muscles attached to the

ascending ramus (the "roimal or "osterior fragment! from having any direct

effect on the remainder of the mandible (the tooth bearing fragment!. There

is thus a tendency for the "osterior fragment to ride u"wards# forwards and

inwards since the medial "terygoid muscle eerts its action medially at about

&44 to the vertical ais. 6linical evidence indicates that the bulk of the

dis"lacement occurs at the time of in$ury and is "robably due to activation of

the stretch reflees in the "terygomasseteric sling by the in$uring force. The


32/44

"osterior fragment is held in its dis"laced "osition by the refle s"asm

im"osed u"on in the muscles by "ain. The toothbearing fragment is thus

secondarily dis"laced in an anterior and contralateral direction.

Favourable and unfavourable fractures

These traditional but not very "ractical terms relate to the line of

fractures at the angle as observed along the vertical and hori0ontal "lanes.

The terms are defined# from the view"oint of the observer# as vertical or

hori0ontal# and with res"ect to the "otential for dis"lacement ehibited by

the "osterior fragment) favourable if the dis"lacement is limited by thedis"osition of the fracture line and unfavourable if otherwise. Thus a

vertically favourable fracture runs from the buccal "late anteriorly and

backwards through the lingual "late "osteriorly while a vertically

unfavourable fracture runs from the lingual "late anteriorly. ;ikewise#

hori0ontally unfavourable fractures etend from the u""er border

downwards and backwards whereas hori0ontally favourable fractures etend

from the u""er border downwards and forwards.

Dis"lacement of the "osterior fragment is only marked if the fracture

line is unfavourable in both "lanes or if there is comminution# which

automatically reduces the stability of the "osterior fragment.

These fractures involve the tooth bearing area and so are normally

o"en (com"ound! into the oral cavity. /f# however# the wisdom tooth is

uneru"ted the laity of the muco"eriosteum in this region fre%uently means

that the fracture remains closed (sim"le!.


33/44

&$AT#$E" O& THE BODY O& THE MANDIBLE

1. ;ocali0ation

*ost fractures in this area result from direct violence and tend to be

concentrated in the first molar or canine regions# in the molar region because

it is the site of recei"t of the blow and the canine region because it is a site of

high strain resulting from a""lied forces# together with a "oint of maimum

weakness associated with the length of the canine root.

. Dis"lacement

The further forward the site of fracture# the more is the u"ward

dis"lacement of the elevators counteracted by the downward "ull of the

mylohyoid muscle attached to the mylohyoid ridge on the lingual as"ect of

the mandible. ,t the same time# the medial or lingual dis"lacement tends to

be increased# whilst the factors "reviously mentioned with regard to the

favourability or otherwise of the lines of fracture remain unaltered. 7ecause

the lesser and greater fragments bear teeth which are "artially controlled by

their maillary o""onents# dis"lacement is limited and the occlusion may be

minimally disturbed.

These fractures must of necessary cause a tear of the overlying

attached muco"eriosteum and be o"en into the mouth. Fractures in the

lower incisor region are fre%uently obli%ue# with the fragment from which

the genial muscles arise dis"laced lingually. The two fragments tend to

rotate medially due to the medial "ull of the mylohyoid muscles# and the

incisor teeth ad$acent to the fracture overla". /t is# in fact# rare for the

fracture line to "ass through the anatomical sym"hysis# which is the thickest

and strongest area of the mandible. /n clinical "ractice it will be found that


34/44

the fracture "asses through the "arasym"hyseal region# to one side or the

other of the genial tubercles.

Multiple fractures of t!e mandible

1. Fracture of the body and opposite angle or condyle. These are common

combinations# since a blow directed anterolaterally to the mandible may

result in a direct fracture at the site of in$ury and an indirect fracture of the

condylar or angle region on the o""osite side. 6onversely# a blow may

result in i"silateral angle fracture and contralateral fracture of the body

through the canine or "remolar region. /n general the second fracture

im"lies a greater force and thus the likelihood of increased dis"lacement.

owever# the "resence of occluding teeth will minimi0e this effect# whereas

the absence of teeth# constituting an edentulous "osterior fragment# increases

the degree of dis"lacement.

. ilateral subcondylar fractures are common and usually due to an im"act

on the "oint of the chin with the teeth a"art. /f the teeth are in occlusion at

the moment of im"act much of the force will be absorbed# usually resulting

in fracture of the teeth# and the condyle may esca"e in$ury. owever# with a

greater degree of force# a condylar fracture will occur# Cross anteromedial

deviation or dislocation of the condyles is usual and the tooth bearing

fragment is sub$ect to marked rotation in the vertical "lane about the last

standing teeth with the "roduction of a gross anterior o"en bite.

&. ilateral angle fractures are not common but result in gross dis"lacement

of all three fragments. The two "osterior fragments are drawn u"wards and

forwards and the anterior toothbearing fragment is rotated downwards by

the inframandibular musculature.


35/44

2. ilateral body fractures are also infre%uent. The anterior fragment is

driven downwards and backwards and the "osition is maintained by the

uno""osed traction of the genial and inframandibular muscles. The two

"osterior fragments are "revented from medial dis"lacement by the anterior

fragment and from u"ward dis"lacement by occlusal contact and thus retain

a relatively normal "osition. /n the edentulous case the lack of occlusal

contact results in serve dis"lacement of the fragments.

3. Three or more fractures. *andibular fractures at more than two sites are

not uncommon and the "ermutation of combinations is large.

,PP;/ED ,=,T-* -F TE @AA-@=D/=C -FT T/@E

TE 6-=D;,A AEC/-=

The mandibular condyle and its ca"sule are covered by the "arotid

gland# which# in this region# is termed the glenoid lobe. This lobe lies within

the "arotid fascia# glenoid lobe. This lobe lies within the "arotid fascia#

which is derived from the divergent and u"ward etension of the dee"

cervical fascia# and encloses the su"erficial tem"oral artery and vein with the

tem"oral and 0ygomatic branches of the facial nerve. The fascia fuses with

the "erichondrium and "eriosteum of the eternal auditory meatus# and also

the tem"oral fascia behind the $oint ca"sule at the root of the 0ygomatic

arch. dissection to e"ose the $oint# therefore# is carried out in close contact

with# and following the direction of# the "erichondrium and "eriosteum

covering the anterior wall of the eternal and auditory meatus. , surgical

cleft is thus created along an almost avascular "lane which leads naturally to

the "osterior as"ect of the $oint ca"sule behind and beneath the glenoid lobe

of the "arotid gland and its contained arteries# veins and nerves. The general

direction of the meatus is downwards# forwards and inwards and dissection


36/44

must "roceed in the manner. Failure to a""reciate this fact# leading to an

in$udicious attem"t to dee"en the incision at right angles to the surface# will

result in transection of the cartilaginous anterior wall of the meatus and#

"ossibly# in$ury to the tym"anum.

The tem"oral fascia is attached to the u""er border of the 0ygomatic

arch and blends with the "eriosteum overlying that structure. The branches

of the facial nerve crossing the arch on their way to su""ly the frontalis and

orbicularis occuli muscles lie immediately su"erficial to the "eriosteum.

,ny dissection in a forward direction in this area must "roceed su"erficially

to the bone and dee" to the "eriosteum if in$ury to the nerves is to be

avoided. The maillary artery will be close to medial "roimity to the

condylar neck. This relationshi" is "articularly im"ortant in cases of

ankylosis characteri0ed by massive bone formation in cases of relation to the

medial "ole of the condyles. The medial "ole of the condyle is situated

within a""roimately 4.3 cm of the "haryngotym"anic tube from which it is

se"arated by the down tuned edge of the tegmen tym"ani. /t also lies a

similar distance from the carotid canal lying "oster medially# which is#

however# se"arated by the tym"anic "late of the tem"oral bone.

TE ,=C;E ,=D 7-D

,s well as "roviding an ideal a""roach to the angle of the mandible#

the natural skin creases of the neck run in the correct direction for avoiding

the im"ortant underlying anatomical structures. ,n incision made in one of

these will immediately e"ose the underlying "latysma. 6areful dissection

through the "latysma "revents damage to any of the im"ortant underlying

structures# although care should be taken to avoid the eternal $ugular vein

running from the angle of the mandible downwards and "osteriorly.


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/mmediately beneath the "latysma the dee" cervical fascia invests the

muscles of the neck. /t consists of fine fibroareolar tissue# which occu"ies

all the intervals# which would otherwise eist between the muscles and the

vessels of the neck. /n certain situations the fascia assumes the form of a

thin fibrous sheet or layer# but elsewhere the tissue is loosely arranged and is

easily broken down. /t becomes condensed around the blood vessels#

"roviding them with fibrous sheaths# which here# as elsewhere in the body#

bind the arteries and their accom"anying vessels closely together.

,long the "osterior border of the sternomastoid it divides to enclose

the muscle# and at the anterior margin again forms a single lamella# which

covers the anterior triangle of the neck and reaches forwards to the median

"lane# where it is continuous with the corres"onding lamella from the

o""osite side. /n the median "lane the fascial is fied to the sym"hysis

menti and the body of the hyoid bone. u"eriorly# from its attachment to the

su"erior nuchal line of the occi"ital bone and the mastoid "rocess of the

tem"oral bone# the fascia etends forwards and downwards to the attached to

the whole length of the base of the mandible. Posterior to the angle of the

mandible it is very strong and binds the anterior edge of the sternomastoid

firmly to that bone# ensheaths the "arotid gland.

The loose areolar "ortion of the dee" cervical fascia immediately

anterior to the sternomastoid muscle forms an easy "lane of dissection#

which is avascular. ,fter division of the overlying "latysma# this "lane is

sought and a dissection commenced which may# if necessary# be etended

u"wards or downwards as re%uired. /mmediately anterior to this "lane# the

denser layer of fascia investing the submandibular gland forms an im"ortant

landmark. Division of this investing layer achieves a""roach to the lower

border of the mandible.


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/mmediately below lie the branches of the facial nerve. Dingman

Crabb (19'! investigated the distribution of the mandibular branches of the

facial nerve in relationshi" to the angle of the mandible and the overlying

dee" cervical fascia.

Following the dissection of the mandibular branch of the facial nerve in 144

cases# the "rinci"al conclusions were as follows.

1. /n 8H of cases the nerve# "osterior or "roimal to the "oint where the

facial artery crossed the mandible# "assed above the inferior border.

. /n the remaining 19H the nerve described a downward arc# the lower

"oint of which was / cm below the inferior border.

&. ,nterior or distal to the where the facial artery crossed the mandible#

144H of the branches of the facial nerve which innervated the de"ressors of

the lower li" "ass above the inferior border.

2. ,ll branches anterior or distal to the facial artery# which were situated

below the mandible# were innervating the "latysma and not the de"ressors of

the lower li". owever# since the anterior fibres of the "latysma were

fre%uently in continuity with the lower fibres of the %uadratus labii

inferioris# these muscles contracted as one unit so that a false inter"retation

of the "recise innervation might be made.

3. /n 98H of cases the mandibular branch "assed over the su"erficial surface

of the "osterior facial vein and could readily be identified at this site.

144Hof cases showed that the nerve "assed su"erficial to the anterior facial

vein. The nerve lay su"erficial to the facial artery# being situated

immediately anterior or "osterior to the vessel# and not infre%uently u"on the

artery. The submandibular lym"h node lay immediately "osterior to the

artery# was constant in "osition and was a useful landmark.

'. The mandibular and buccal branches anastomosed in only 3H of cases.


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5. The mandibular ramus consisted of two branches in '5H of s"ecimens) a

single branch in 1H) a tri"le branch in 8H and four or more in &H of the

dissections "erformed.

The facial artery lies immediately beneath the dee" cervical fascia and

can be observed "ulsating beneath this layer. /n a""roimately 4H of cases

the mandibular branch of the facial nerve turns u"wards and accom"anies

the vessel at this "oint and should if "ossible be dissected away and

retracted. Fre%uently the anterior branch of the "osterior facial vein may

also be seen traversing this "articular area and may re%uire isolation#

division and ligation.

BIOMEHANIAL ON"IDE$ATION"

The outer corte of the body of *andible has an average thickness of

&.&mm is "articularly strong offers a good anchorage for the

osteosynthesis screws. The cortical bone is thicker in the chin region is

reinforced laterally by the obli%ue line# which runs from the coronoid

"rocess to the molar region. /n the sym"hysis region# cross sections of the

mandible show the thickest corte to be at the lower border) behind the third

molar# it is stronger at the u""er border.

=ear the alveolar "rocess the thickness of the bone is variable) the

anatomy of the tooth roots the structure of the bone do not allow screw

fiation in this region. To avoid damaging the root a"ices it is safe to "lace

the screws away from the occlusal "lane by a distance of at least three times

the length of the crown of the tooth.

The inferior alveolar nerve runs in the mandibular canal# from the

lingula to the mental foramen# on a concave course. *easurements show

that# from back to front# it runs even closer to the outer corte! and to the


40/44

lower border . ,t its lowest "oint it is 8 to 14mm away from the basilar

border of mandible. ,lthough the average thickness of the corte in that

region is 3mm# it may be less than &mm in some cases.

,bout 1 cm before the mental foramen the canal turns u"wards

forwards. The foramen lies a""roimately at the middle of the distance

between the alveolar crest lower border of mandible on a vertical line

corres"onding to the /st or nd "remolar. /t is im"ortant to remember that

the mental foramen sometimes lies higher than the canine a"e. Therefore

osteosynthesis in this region involves a certain risk of a"ical in$ury.

/n most cases the mandibular canal surrounds the neurovascular

bundle as a bony tunnel# but sometimes its bony structure is "oorly

develo"ed. Ae"eated tests in freshly "re"ared mandibles have shown that

the intrusion of a screw into canal does not usually cause nerve in$ury#

because the nerve moves away from the instrument (CEA7EA# 1953!.

Drilling the holes a""ears to be more dangerous to the nerve than inserting

screws.

/t should be noted that# with ageing# the alveolar bone atro"hies the

structure of the mandible is reduced to cortical layers. /n Edentulous

"atients the flat u""er border of mandible is com"osed of sclerous bone#

giving "oor anchorage for the screws.

During the first year of life# the blood# su""ly of the mandible de"ends

on the inferior dental artery. ;ater "eriosteal


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should be avoided to "reserve the blood su""ly. For this reason or

transmucosal a""roach is "referred than a Transcutaneous a""roach.

Inowledge of *asticatory stresses Eerted on the mandible is

fundamental because these stresses determine the rational design

"ositioning of osteosynthesis "lates. 7y means of strain gauges connected to

a wheatstone bridge# maimal biting forces in young men with healthy teeth

were measured. The following values were obtained:

/ncisor region: 94 =

6anine region: &44 =

Premolar region: 284 =

*olar region: ''4 =

/t is im"ortant to understand the distribution of strains created within

the mandible as a result of these *,T/6,T-A F-A6E.

P/-;-C/6,;; 6--AD/=,TED *@6;E F@=6T/-=

PA-D@6E TE=/-= F-A6E ,T TE @PPEA 7-ADEA and

6-*PAE/


42/44

DEF/=/T/-= -F /DE,; -TE-=TE/ ;/=E -F *,=D/7;E

/t corres"onds to the course of a Tension line at the base of the

alveolar "rocess inferior to the root a"ices. /n that region a "late can be

fied with monocortical screws# as follows:

• 7ehind the mental foramen the "late is a""lied immediately below the

dental roots above the /nferior alveolar nerve.

• ,t the angle of the $aw the "late is "laced ideally on the inner broad

surface of the Eternal obli%ue line) if it has been destroyed# the "late

is fied on the eternal corte as high as "ossible.

•/n the anterior region# between the mental foramina in addition to the

sub a"ical "late# another "late near the lower border is necessary to

neutrali0e Torsion forces.

The result of such a monocortical Tensionbanding osteosynthesis is the

neutrali0ation of the distraction Torsion band strains Eerted on the

fracture site# while "hysiological selfcom"ression strains are restored.

/n the edentulous mandible the correct "osition of the "late is on the outer

corte of the mandible where the biting forces "roduce Tension forces at the

u""er border of mandible. The "late should never be fied on the u""er flat

surface where the bone is sclerous.

"#$%IAL A''$OAHE" TO MANDIBLE

1. *,=D/7;E /=TA,-A,;


43/44

down through the submucosa# muscles "eriosteum. 6are must be taken to

avoid in$ury to the mental nerve. The nerve has to be identified during

sub"eriosteal dissection. ,fter dissection of the neurovascular bundle

mandible can be e"osed. The dissection should be carried out /nferiorly

enough to allow ade%uate a""lication of the fiation system. The

"eriosteum should be handled with care should not be Elevated

Ecessively. ?ound is closed in layers or in single ;ayer.

. *,=D/7;E /=TA-A,; *,AC/=,; A/* /=6//-=

For e"osure of the alveolar crest or Treatment of Traumati0ed teeth# then

"rotection of lacerated mucosa is "referred. This can be achieved by using

the marginal rim incision# which allows direct Elevation of the underlying

"eriosteum without involving incisions into the overlying mucosa#

submucosa# muscle "eriosteal layers. ?ound healing is Ecellent without

visible scarring (IEA6EA# IAE@/6 199&!.

&. *,=D/7@;,A 6-*7/=ED /=TA,-A,; ,=D TA,=7@66,;

,PPA-,6

This a""roach is needed in case of fiation of screw+s or "lates at the

angle of mandible by an intraoral Transbuccal a""roach. First the mandible

is e"osed by an intraoral a""roach. For Transbuccal drilling# ta""ing

screw insertion a stab incision is made three the skin overlying the "late.

The incision should follow the relaed skin tension lines. =et the

Transbuccal trocar is inserted a transbuccal tunnel is established. The

Trocar is removed a check retractor inserted.


44/44

2. *,=D/7;E EBTA,-A,; @7*,=D/7@;,A ,PPA-,6

(A/D-=+!

The incision should be made in a natural skin crease at a level

a""roimately finger breadth+s below the inferior border of mandible.

kin subcutaneous tissues is incised to the level of "latysma muscle#

which is incised at right angle to the muscle fibers. The marginal

mandibular branch of facial nerve is closely related must be identified

immediately beneath the "latysma muscle.

,lternatively the dissection can be develo"ed through the dee" cervical

fascia at the level of submandibular gland. The ca"sule of the gland is

identified 5 the overlying facial vein artery are ligated. The marginal

branch of facial nerves lies su"erior to these vessels is therefore not

endangered by this a""roach.

E6IE;T (1991! develo"ed techni%ue for lag screw osteosynthesis of

condylar fractures. , skin incision is made 1cm su"erior to the inferior

border of mandible Etended to the level of "latysma muscle. Then the

facial nerve is identified using nerve stimulation. The masseter is incised

su"erior to the facial nerve reflected inferiorly# which avoids risk of

damage to the facial nerve.

mand anatomy

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