Lecture # 7

Morphology Of Permanent Molars

,In this lecture we will talk about permanent 1st molarbut at first we will talk about molars in general.

slide #2:-Class traits

1) molars have 3 or more cusps but sometimes there is a possibility to have a premolar with 3 cusps , so how can we

distinguish between the two? - by counting the # of cusps buccally; premolars have

only one cusp buccally , but molars have at least two .

2) lingually, molars have one or two cusps , but the most common to have two.

3) molars in general have 2 or 3 roots.

Slide #3:-Arch traits ( to distinguish between upper & lower molars):

1 )Roots:Upper molars have 3 roots; 2B and 1L but lower molars have 2 roots; 1M & 1D.

2 )Crown: In upper molars the crown is wider BL than MD (BL = 11.0mm , MD=10.0mm)Lower molars the MD>BL (BL=10.0mm , MD=11.0mm).

3 )Cusps:In upper molars there're 4 unequal cusps but lower molars have 4 equal sized cusps.

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- When we look to the occlusal surface of upper molar we find:

*Three major cusps belong to 1 unit (ML, MB &DB) which are arranged in a tricuspid-triangular pattern (Trigon).

*DL cusp which doesn't belong to the Trigon & this cusp is small in size and sometimes missing especially in 3rd molar.

4) In upper molars there's an oblique ridge which comes from ML to DB cusps , but in lower molars there's no oblique ridge.

5) In upper molars the cusps are unequal in size (MB>DB) (ML > DL) -Remember in lower molars cusps are equal in size.

Slide #5:

- Type trait ( to distinguish between 1st , 2nd & 3rd molars):

1) When we go posteriorly the DL cusp reduces in size and may be missing in 3rd molar "as we mentioned previously"

2) 1st molar is the largest & shows the least morphological variation .

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U should know that the 4 equal sized cusps in lower molar belong to 1 unit

Great Apes like Gorilla and Chimpanzee have the oblique ridges but monkeys don't have it , they've transverse ridges & their lower molars look like upper

molars .

1st molar2nd molar3rd molar

3) Cusp of carabelli which present on the ML cusp of the 1st molar .

- Sometimes we've a cusp of carabelli in the max. 2nd molar in a very very low % , but of course we don't have in the 3rd molar .

4) Roots :As we go posteriorly the roots become distally inclined & much closer to each other and there's possibility to fuse in 3rd molar .

5) Line angles ( this point isn't mentioned in the slides ) As we go posteriorly the acute angles become more acute & the obtuse angles become more obtuse & the tooth become more rhomboidal .

Slide # 6:Now we will talk about max. permanent 1st molar

Buccal aspect :- MB & DB cusps are equal in height but the MB cusp is wider mesio-distally & the 2 cusps are separated by B groove that terminates halfway.- Slopes of DB are steeper than MB- The apex of DB cusp is sharper than MB

- tip of ML cusp can be seen between the 2 B cusps bcz

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Remember that 80% of molars have cusp of carabelli that means U may find a 1st molar without this cusp, sooooo if U get a molar without cusp of carabelli that doesn't mean this molar isn't the 1st molar .

the ML cusp is big.

Slide #7:

-M profile is highly convex in occlusal 2/3s & flat or concave in the cervical 1/3. On the other side D profile is entirely convex.-M HOC is 3/4 the distance from cervical line to marginal ridge, & the D HOC is 3/5 the distance from CL to MR …......... so M HOC is higher than D HOC ( higher : closer to O surface).

-CL has 2 slightly curved segments separated by apical peak.

Slide #8: Buccal surface :

- When we look to the buccal surface we can see part of D surface bcz the angle between B &D surfaces is obtuse < on the other hand we can't see any part of the M surface bcz the angle between B & M surfaces is acute .

- Cervical 1/3 is convex .- Occlusal 2/3s are flat .

Roots )buccal surface)- 3 roots are visible - B bifurcation is at junction between cervical & middle

1/3s.- The area before bifurcation is called " Root Trunk "

where we find a shallow vertical groove in the middle of it .

- MB & DB roots are narrow and get closer to each other in the apical 1/3 .

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In all teeth the M HOC is higher than D HOC except the mand. 1st premolar in which the M &D HOCs are nearly in the same level .

- If we draw a line from the MB root apex it will pass through the MB cusp tip and this is a type trait bcz when we go posteriorly this line will pass distally to the MB cusp tip why ???? bcz the root inclines distally.

- The longest L root is visible between the B roots .

Slide # 9 :

Lingual aspect :

-2 cusps are unequal in size : ML ( bigger )DL ( smaller ) .

- ML cusp makes 3/5s of MD width of the crown and this cusp is prominent & blunt .- DL cusp is rounded , shorter & narrower .

- The 2 cusps are separated by L groove which terminate at the midway of L surface.

Slide # 10:

- From the lingual aspect the M profile is convex except in the cervical 1/3 , and the D profile is entirely convex

- L surface : Evenly convex OC . L groove divides L surface into M &D segments Cusp of carabelli is on the M segment ( some races

don't have this cusp ).- Roots :

3 roots are visible . Proximal outline of B roots are visible .

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Remember that DL cusp in 1st molar is larger than 2nd &3rd molars .

About the L root @ Tapered with blunt apex .@ Has shallow vertical depression from CL to 2/3s@ If U draw a line from the L root apex it should pass through the mid portion of the crown .

Slide # 11 :

Measial aspect :

- Maximum MD dimension at the cervix of the crown.- ML cusp is higher than MB.- B cervical 1/3 is strongly convex .- B outline continues lingually in a straight line .- L outline is uniformly convex .- L HOC is at the mid point , B HOC is very close to

the cervical 1/3.

- CL is slightly convex occlusaly.

Slide # 12 :

Mesial surface :- Mesial marginal ridge is marked by several tubercles &

supplementary grooves ( type trait )- Cervical 1/3 is flat or concave .- Occlusal 2/3s are convex .- Contact point is between middle & occlusal 1/3s

Roots )Measial aspect):- 2 roots : MB & L.- MB is broad bucco-lingually .- L root is narrow & banana – shaped , curves L then

B at the apex.

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- Both MB & L roots project beyond the crown profile ( outside the crown's limits ) and it's a type trait bcz in the 2nd & 3rd molars the roots become closer to each other .

Slide # 13 :

Distal aspect :

- DB cusp is more prominent than DL .- Only small portions of M cusps are visible from

distal aspect and it's a type trait bcz in 2nd & 3rd

molars greater amounts of M cusps can be seen as the DMR becomes lower when we go posteriorly .

- DMR is shorter than MMR that's why we can seepart of the O surface from the D aspect while we can't see it from M aspect .

- DMR rarely has tubercles .- B & L profiles are similar to those of M aspect .- CL is nearly straight .

Slide # 14 :

- Distal surface : uniformly convex.- Roots ( distal aspect )

3 roots are visible ( palatal , DB , MB ). DB root is shorter & narrower than MB soooo we

can see part of MB root .

Slide # 15: Occlusal aspect ) most important)

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- Square or rhomboidal shape but in lower molars is square or rectangular .- MB & DL are acute angles .- ML & DB are obtuse angles .- 1/3 of B surface & 1/2 of L are visible .- In the B outline : D part is more lingually & the M part

is more buccally sooo the maximum distance bucco-lingually should be in the mesial half .

Slide #16 :

- Transverse ridge joins the tips of MB & ML cusps .- Oblique ridge joins the tips of ML & DB cusps

- Transverse ridge Oblique ridge D ridge of MB cusp M ridge of DB cusp

- Trigon consists of 3 cusps :

MB ,.ML ,.DB . DL isn't included in trigon .

- Talon Bears DL cusp & DMR . Located disto-lingually to the trigon . Well-developed in 1st molar & reduces in size in

2nd & 3rd molars .

Slide # 17 & 18 :

- Cusps in order of decreasing size: ML> MB> DB > DL

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Make a triangle ) trigon)

- M MR is longer & more prominent than D MR.

- Major fossae :1) Central fossa :

- Located between the transverse & oblique ridges - The maximum pit of the central fossa is the central pit .

2) Distal fossa : located distal to the oblique ridge( don't confuse between this fossa & the D triangular Fossa ).

- Minor fossae :1) M triangular fossa

- Located between MMR & transverse ridge .- It contains M pit , MB & ML supplementary

grooves 2) D triangular fossa : located next to the D MR .

The ML cusp has an M cusp ridge goes mesially but the D cusp ridge isn't going distally it goes disto-buccaly to make the oblique ridge .

( This point may be an exam question )The oblique ridge is composed by 2 things :1) the triangular cusp ridge of DB cusp 2) the D cusp ridge of ML cusp

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Each cusp has 2 ridges 1 M & 1 D except the ML cusp .

Slide # 19:

§ Pulp- MD section

• 2 horns, MB is higher• Pulp chamber, roof & floor• Canals, narrow• Canal orifice

- BL section• Pulp chamber is wider• 2 horns of equal height

- X-section• 3 canals

* This point isn't mentioned in the slides :60% of population have 4 canals in this tooth that means one root contain 2 canals which is the ML root .

THE END

DONE by : Ghadeer Swaqed & Areej Al-lama'

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