口腔解剖生理學之國外下頜第三磨牙課件

DESCRIPTIVEANATOMY

OFMANDIBULAR

THIRDRMOLA.Prof.

A.

Els1ahnInitial

calcification8

–

10

yearsCompletion

of

crown12

–

16

yearsEruption17

–

21

yearsCompletion

of

roots18

–

25

yearsProf.

A.

2ElsahnCHRONOLOGY.The

lower

third

molar is

the

lasttooth

in

the

dental

arch

and

theeighth

from

the

middle

line.GENERAL

CHARACTERISTICSIt

is

the

shortest

ofthe

mandibular

teeth.431

28765.Prof.

A.

3ElsahnGENERAL

CHARACTERISTICSThis

tooth

is

a

very

variable

tooth,but

less

so

than

upper

third

molar.It

shows

variationsin

size,

form,

numberof

cusps

and

numberand

form

of

roots..Prof.

A.

4ElsahnThe

crown

of

this

tooth

in

most

casesresembles

that

of

the

lower

secondmolars

in

the

number

of

cusps

(fourcusps)

and

sometimes

resembles

thecrown

of

the

lower

first

molar

(fivecusps)

or

it

may

bear

little

resemblancetoeither.GENERAL

CHARACTERISTICS.Prof.

A.

Elsah5nIn

many

cases,

it

is

largerthan

the

lower

second

molar.GENERAL

CHARACTERISTICSThe

third

molar

maybesmall

or

large..Prof.

A.

Elsahn6Prof.

A.

ElsahnBUCCAL

ASPECTTHE

CROWNM

DThe

mesial

and

distal

outlinesof

the

crown

are

generallyasthose

of

all

lower

molars.The

distal

outline

is

markedlyconvex..7The

crest

of

contour

mesially

and

distallyis

located

a

little

more

than

half

thedistance

from

cervical

line

to

cusp

tips.BUCCAL

ASPECTTHE

CROWNThe

crown

is

wider

at

thecontact

area

mesio-distallythan

at

the

cervix.M

DProf.

A.

Elsahn.8In

the

four-cusp

type;

the

molar

issmall

and

shows

two

buccal

cuspsonly.

Prof.

A.

ElsahnBUCCAL

ASPECTTHE

CROWNThe

buccal

cusps

areshort

and

rounded..9The

mesio-buccal

cusp

is

thewidest

and

usuallyhighest

ofthe

two

or

three

buccal

cusps.BUCCAL

ASPECTTHE

CROWNThe

buccal

surface

is

very

convex..Prof.

A.

Elsah1n0This

molar

usually

showtworoots,

one

mesial

and

one

distal.BUCCAL

ASPECTTHE

ROOTSM

D.Prof.

A.

Elsahn11Prof.

A.

ElsahnBUCCAL

ASPECTTHE

ROOTSThese

roots

are

usuallyshorter

than

thoseoffirst

or

second

molars.111.81.6::.The

root

to

crown

ratio

is

smallerthan

that

of

first

and

second

molar(1.6

to

1

compared

to

1.8

to

1).12The

roots

are

more

inclineddistally

than

those

of

the

firstand

second

molars.Prof.

A.

ElsahnBUCCAL

ASPECTTHE

ROOTSM

D13M

D.The

roots

may

be

separated

orfused

for

all

or

part

of

their

length.BUCCAL

ASPECTTHE

ROOTS.Prof.

A.

Elsahn14If

the

roots

are

separated,

theroottrunk

is

long

and

the

roots

areusually

more

pointed

at

theapexthan

on

other

molars.BUCCAL

ASPECTTHE

ROOTS.Prof.

A.

Elsahn15The

proximal

sides

of

the

crownconverge

slightly

lingually.LINGUAL

ASPECTTHE

CROWN.Prof.

A.16ElsahnThe

lingual

surface

is

also

very

convex,

butmore

than

the

buccal

surface.LINGUAL

ASPECTTHE

CROWNThe

lingual

cuspsare

longerthan

the

buccal

cusps.Themesio-lingual

is

largerthan

thedisto-lingual.D

M.Prof.

A.17ElsahnFrom

the

mesial

aspect,thetooth

resembles

the

secondmolar

except

in

dimensionsand

the

more

bulbous

crown.Prof.

A.18ElsahnMESIAL

ASPECT.The

mesial

rootis

taperingmorefrom

cervix

to

apex

and

its

apexis

usually

more

pointed

thanthatof

the

second

molar.Prof.

A.

ElsahnMESIAL

ASPECTThe

buccal

cusp

tips

are

closerto

the

lingual

cusp

tips

than

onfirst

and

second

molars,

givinga

small

occlusal

table.LB.19From

the

aspect,

the

tooth

isalsosimilar

to

that

of

second

molarexcept

for

size,

absence

of

the

distalcontact

areaand

the

more

pointedapex

of

the

distal

root.Prof.

A.

ElsahnDISTAL

ASPECTsecond

molar.20The

occlusal

aspect

is

similar

tothatof

the

second

or

first

mandibularmolar

according

to

the

type

of

thethird

molar

(four-cusp

type

orfive-cusp

type).Prof.

A.

ElsahnOCCLUSAL

ASPECT.21The

occlusal

outline

is

rectangularor

oval

mesio-distally

but

morerounded

than

that

of

lower

first

orsecond

molars

and

the

distal

side

ismuch

smallerbucco-lingually.OCCLUSAL

ASPECTM

DLB.Prof.

A.

E2l2sahnThe

occlusal

surface

has

irregulargroove

pattern

with

numeroussupplemental

grooves

and

pits

thatproduce

a

wrinkled

appearance,which

is

a

constant

finding.Prof.

A.

ElsahnOCCLUSAL

ASPECT.23VARIATIONSIt

is

very

variabletooth

but

less

so

thanupper

third

molar.Crown

may

show

three,

four,

five

or

multi-cusped

variations.Most

of

the

anomalies

have

largecrownportio