Prostho 3rd lecture,Mandibular Edentulous Anatomy

8/3/2019 Prostho 3rd lecture,Mandibular Edentulous Anatomy

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In the name of Allah most graceful most merciful

Today we are going to discuss the anatomy of the mandibular ridge ,

and how its being different from that of the maxillary ridge .

Although theres some degree of similarity between the upper and lowerarches . for example , both have the labial and buccal mucosa , there are

a lot of differences between them from many aspects such us : type of

mucosa , the underline mucosa ,type of bone , the surface area , the

bone and muscle attachment on the lingual surface . the presence of

tongue greatly affects the way that the mandibular denture is made in

comparison to the maxillary denture .

Last week in the lab, we have completed the first clinic , we have takenthe primary impression , then we have fabricated the custom trays .

This week in the lab, we took how to make the secondary impression

and did a demonstration for pouring up the secondary impression to

have a final or secondary cast , so this week well complete the second

clinic and part of the second lab .

Next week,we will complete the second lab by making the record

blocks.

The maxillary denture usually has a greater degree of success than the

mandibular denture ,we are going to see why is that during this lecture.

In the mandible , there is no palate . instead we have the tongue with

very active groups of muscles , the extrinsic and intrinsic muscles and

they will greatly affect the borders which we extend from the complete

denture.

As we know , for a good support we require the maximum denture

bearing surface area , the presence of the tongue limits this extension ,

which reduces the support .


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And because we have greater active border in the mandible due to the

presence of those muscles , they are going to cause displacement of the

denture in the mandible . while in the maxillary arch , muscles affectingthere such as the baccinator , orbicularis oris and others cause less

effect , they are not that strong . tongue is a very powerful muscle and

its much more active than the buccinator .

So in the mandible , we have less surface area and more activemuscles , that is why the mandibular denture is less stable.

In the previous lecture , we said that after teeth extraction the alveolar

bone ( or the residual ridge ) will continue to resorb . it atrophiesaccording to a certain path , not randomly .

The direction of resorption is related to the angulation of the alveolar

bone , which is related to the angulation of the teeth were there before

extraction . and almost everywhere in the mouth , the direction of the

teeth is to the outward , proclined posteriorly and anteriorly in the

upper arch and anteriorly in the lower arch . one exception to that is the

lower posterior mandible . in the premolar area the direction is straight

up .

As the resorption occurs , different shaped ridge results . there is a

difference in the amount of resorption between the types of ridges ,

usually residual ridge resorption occurs most within the first year after

extraction. it continue to resorb throughout life , but at the beginning it

is always faster than at the end , logically so because the amount of

bone left becomes less at time goes on.

An important thing to remember , another reason why the mandibular

ridge is not well suited to support the denture , is that the mandibular

residual ridge resorbs faster than the maxillary residual ridge .


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During the first year of resorption , the mandibular residual ridge

resorbs twice as fast as the maxillary residual ridge . After that time , it

resorbs four times faster , that means that within the first year both

residual ridges are resorbing quickly . This creates imbalance between

both arches .

To summary, there are three reasons why the mandibular denturecreates problems ; less surface area , more active tongue muscles

and faster resorption of the residual ridge .

Anatomical landmarks of the mandible is usually divided into static

area and dynamic area; the denture bearing area which is the surface of

the ridge , and the border where the muscle attachment occurs which

are the dynamic structures limiting the borders of the denture ,

respectively .

(The doctor is viewing a picture)In a sagittal view of the mouth , if welook at the anterior portion of the upper residual ridge , at resorption

occurs , we go from the yellow to the blue to the green and finally down

to the basal bone ( direction of resorption from up to down ) . You will

notice that the center of the ridge progressively moves from facial ( or

labial ) towards palatal . In the lower anterior portion is the same thing, after extraction the bone tends to go in a lingual direction .

In all cases there will be reduction in height (vertical direction ofbone resorption ) wherever the bone is in both arches(

) .

however, horizontal bone resorption differs depending on theregion( ) , usually it goes from the

anterior to the posterior so the arch becomes smaller , and this islogical because you can see the face of an old patient losing its

support.


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Posteriorly in the upper arch the molars are proclined , having a buccal

angulation . In the lower posterior, they have a lingual angulation , and

the bone follows. So when teeth are extracted . ( from a picture ) if the

center of the ridge was A and B, as time goes on we notice that the

center moves further palately in the upper posterior and furtherbuccally in the lower arch . and this is the main exception and it's really

important when we set the teeth according to where they originally

were .

Without the understanding of where the ridge is and where it was

before extraction it is difficult to put the teeth in the correct position ,

and the occlusion will be in reverse by putting the upper teeth too much

toward the palatal and the lower teeth directed too buccally , and crossbite results.

From the fact that the resorption in the lower arch is pretty fast , vital

structures which are normally buried in the bone , become exposed

affecting the borders of the denture .

So borders of the lower denture is not only affected by muscleattachment around the peripheries , but also by vital structures

within the bone ; nerves and bony prominences .

(The doctor is viewing another picture)Notice the position of the mental

foramen in relation to the premolar area , and how it gets close to the

surface at time goes on , due to resorption .

The bone sometimes gets so thin to a point that you will be able tosnap it between your fingers, or to cause cracks in it especially

while taking a forceful impression !

The doctor is showing a radiograph in the symphysis region ( in the

middle of the jaws ) showing a jaw with a new denture , and another

radiograph showing the same jaw after a while of having the denture in

, the denture appears radiopaque in both of the radiographs due to a

certain material , just to see what happens to the denture as time goes


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on . in the first one , the denture is really large , and the internal

surface of the denture is concave setting on the top of the ridge , notice

what happens when time goes on , in the other picture , I have a flat

surface setting on a flat surface, wheres the stability !

Its difficult to compare natural teeth to the mucosa after extraction ,

the reason is that this mucosa was not designed to withstand the force

of a denture , technically its a scar tissue formed at the junction

between the buccal and lingual gingiva after healing , so its a not

normal healthy mucosa . it was designed to take a tension force not

compression . normally when we chew , food passes across the ridge or

the buccal mucosa and come on contact with it ( tension) but not

compressing on it , we dont use our mucosa to smash food .

In natural teeth , the roots set within the bone through a very

specialized attachment that is the Periodontal Ligament , and the

direction of the fibers is very well designed for Biting force , it

transmits the compression to tension , the oblique fibers ( ) are

stretched when the patient bites , only at the apex this does not happen

because the force it takes is compression , but usually along the

periodontal ligament fibers are well designed to take an occlusal force .

so the direction of fibers( and the receptors they have ) in the PDLis much better designed for axio- occlusal force than normal

mucosa , though its total thinner but there is muchbetter sensory

perception in PDL .

from book (page 9 ) : periodontal ligament provides the means bywhich force exerted on the tooth is transmitted to the bone that

supports it , the two principle function of the PDL are supportand positional adjustment of the tooth , together with the

secondary and dependent function of sensory perception .


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But even if we take this point aside >>> which is that 1 square cm of

PDL is not equal to 1 square cm of mucosa by means of effectiveness (

) , its less than edentulous mucosa . however , if we stretch the

PDL of all teeth forming a flat surface area , it has been found that the

dentulous maxilla has a PDL surface area of about 45 square cm andthe same in the mandible . however the edentulous mucosa in the

maxilla is double that in the mandible , and the maxilla is half that of

natural dentition . so the mandibular edentulous surface area is about

the original surface area of the PDL . we conclude that the support

from the edentulous mucosa is not very effective . we can tell from these

numbers that the mandible is very bondable and its not well suited to

support the denture .

Stress bearing area :

Theres a very important part of the anatomy of the mandible and that

is the Buccal shelf area ; it is located in the posterior buccal sulcus , the

borders of this area are laterally : the external oblique ridge at the

attachment of the buccinator , medially : the crest of the residual ridge ,

anteriorly : the buccal frenum and posteriorly it extends to the

pterygomandibular raphe which is located posterior to the retromolar

pad.

The buccal shelf area is important because it is the area of the residual

ridge that is compact and have strong bone and instead of being sharp

at the tip , it is horizontal . if we take a look at the residual ridge we

will find that the type of bone on the crest is Cancellous bone(not very

strong). the crest of the ridge can take force but not to the best degrees

due the type of tissue and bone there . and some types of ridges are very

thin and very narrow .

The pterygomandibular raphe has to some degree pressure fromthe masseter and buccinator muscles and forms the posterior

border of the buccal shelf area .


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So this area is good for the denture support , because :1-It is more horizontal than vertical .2-It has a compact type of bone .

Even when resorption occurs , this area does not resorb very much . as

ridge resborbs the buccal shelf tends to look like its larger . so we have

two shelfs at the sides of the jaw that regardless of what happens to the

rest of the ridge provides us with some support to the mandibular

denture .

External oblique ridge is the attachment of the buccinator muscle. We

can go slightly beyond this attachment because there are horizontal

fibers there where we can press without affecting the movement of the

denture .

Pterygomandibular raphe is the point where the superior constrictor

muscle meets the buccinators ( this is what the doctor was showing in a

horizontal section of the patients head ).

After the teeth are extracted the depth of the sulcus also changeswith the atrophy of the ridge.

The fibers of the buccinators inferiorly tends to be more horizontal than

diagonal , this means that the fibers will contract anterio-posteriorly

not superior inferiorly , when they contract they will have less affect on

waving the denture . so to get more surface area , its found by

experience that we can encroaches on the buccinator attachment by a 1-

2 mm to cover more area of the mandible .

the mylohyoid ridge :

it is on the buccal surface , it is the attachment of the mylohyoid muscle

, which runs from the internal surface of the mandible to the hyoid bone

. its function is to raise the tongue when we swallow , when we swallow

the floor of the mouth rises (and so does the tongue) this is due to


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contraction of the mylohyoid muscle . the ridge of the attachment will

effect the borders of the muscle , the ridge itself creates a problem .

when extending the lingual flange , we can only go to the muscleattachment , beyond that we will go over the bony ridge , and if the

border of the denture reaches there it will hurt the patient because its

passing over this ridge .

So the location of the mylohyoid ridge and the contraction of themylohyoid muscle has a significant affect on the lingual border of

the mandibular denture .

so if we take a look from underneath the mandible, we can see the

mylohyoid is on the right and left , and that the attachment of this

muscle is different anteriorly than posteriorly ; anteriorly the mylohyoid

attachment is closer to the lower border of the mandible , whereas

posteriorly it is closer to the crest of the residual ridge .


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The main effect of this muscle on the border of the denture isposteriorly more than anteriorly , in the middle of the lingual

flange region . in the anterior part the main effect is from the

lingual frenum , and we can extend it anteriorly as long as we

dont encroaches the lingual frenum . however , as the flangego posteriorly it should be raised due to the presence of the

mylohyoid ridge .

The premylohyoid fossa :

It is the region infront of this rising of the lingual flange . it is located

posteriorly to the mylohyoid muscle . it is like a pocket between the

tongue and the bone infront of the premylohyoid eminence .

The retromylohyoid fossa : is the area behind the premylohyoideminence.

And these structures are important for the border molding and the

border of the denture, they affect the peripheral seal and stability of the

lower denture .


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There is this pocket between the tongue and the ridge , the floor of this

pocket is the submandibular gland . we have to go all the way back to

reach the muscles ; the superior constrictor and the palatoglossal whichattaches the tongue to the palate .

This pocket area has no muscles ( It is behind the mylohyoid muscle

and in front of the palatoglossal) and in this area the flange goes down

and it is free to move there .

There are other structures within the bone ( not only the mylohyoid

ridge ) which are normally buried in the bone like nerves and blood

vessels , and that when the bone resorbs might get exposed and becomemore apparent . the mental foramen is a common one , which is located

in the canine, 1 & 2nd premolar area .

As the bone gets down, the nerve coming out of that foramen in the

bone becomes near the surface . so to a patient who is wearing denture ,

when such thing occurs , it will become very annoying that everytime he

bites it presses on the nerve . this is also a reason why a lower denture

is less comfortable than an upper one !

In some cases when this continues , numbness from the nerve,parasthesia and sometimes anesthesia may result.

So at the extreme cases the mental foramen becomes on the top (the doctor is showing a picture of that ) .

We have to relief this area and provide alternative ways of support ,

benefiting from the buccal shelf area and sometimes a bone

compensation procedure is needed or by using dental implants to raisethe support away from the bone towards something else .

In very sever cases , the inferior dental nerve becomes exposed atthe top , it appears as a grey line on the surface of the residual

ridge in the radiograph.


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Torus mandibularis :

it is present in only 1 out of 5 people , it varies in size , located in the

lingual surface of the 1st and 2nd premolars . this structure even it is a

compact bone with irregular shape it is covered by a very thin layer of

mucous membrane , and that creates problems because lingual flange of

the denture can not be extended to that area .

If the size of torus mandibularis is large , they have to be surgically

removed , if they are small , we have to work around them and relief

under this area . the doctor showed a picture of a patient having tora

mandibularis ( more than one ) extending all the way to from premolarsto molars area , this is an extreme case .

Rememberunderstanding the anatomy of the borders of the denture

will affect the way we border mold , the way we do tracing around the

denture to take the impression correctly .

Now talking about the sections of the anatomy of the edentulous

mandible , we have the buccal shelf area which extends from the buccal

frenum to the retromolar pad .

The muscles that affects the denture are the buccinator , and superficial

to the buccinator we have the masseter muscle located behind the

ramus .

How does the master muscle affects the denture ?The masseter does not come in a direct contact with the borders of the

denture , but its located outside the buccinators . and when it contracts

it compresses through the buccinators and press on the denture .

particularly in patients where the direction of the masseter is more


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medial . just to remind you , the masseter muscle runs from the angle

of the mandible to the zygomatic arch , and if it was directed more

medial than vertical or towards the back , then its effect on the

posterior part of the denture is greater . and that varies between a

patient and another depending on the anatomical shape of his face .

The labial frenum ; in the lower anterior area is a very common area that

causes trauma if the denture was not fitted carefully in that area .

The buccal frenum : is significant like that in the maxilla because it is

attached to an area where the modiolus is there ( its a notch of muscles

, a point where muscles of facial expression meet ) so I need the denture

to get a free movement in that area in all directions ; anterioposteriorly

and superioinferiorly .


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buccal frenum buccal frenum

.

So when we do the border molding , we need to move the buccalfrenum backwards and forwards , upwards and downwards .

Lingual frenum ; sometimes its more prominent in some patients than

others , sometimes it extends all the way to the crest of the ridge when

we have excessive resorption.

The doctor showed a picture of that case , if we stretch the tongue the

tension goes all the way from the tongue to the lip as if they were

attached together , because a little of the residual ridge is remained due

to extreme resorption .

Mentalis muscle : is a particularly active muscle in the lower labial region

; when it contracts it extends the lip upward ( the lip becomes longer )

and it makes the flange thinner.


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Both mentalis and orbicularis oris affect the shape of the labialvestibule . orbicularis oris is located in the body of the lip.

An important picture >> if we look horizontally to a patients head , itslike if there are circles of muscles around the patients mouth , from the

mouth all the way back to behind the esophagus . we have the

orbicularis oris attached to the buccinator through the modiolus , the

buccinator goes around the cheek , then when it reaches the body of the

mandible it cant go any further back , it has to go medially and inside

to reach the pterygomandibular raphe which attaches it to the superior

constrictor .

So these three muscles are the main effector muscles on themovement of the lower denture .

These muscles are like a curtain , behind this curtain there are agroup of strong muscles that effect the denture through it .

Examples of these are :

1) Masseter , It acts to raise the jaw and clench the teeth , when it

contracts powerfully it compresses on the buccinator as mentioned

previously and makes a notch in the distobuccal part of the flange in the

back end of the buccal shelf area . this is called masseteric notch .2) Lingually , on the inside of the mandible , there is the medial

pterygoid muscle ( the counterpart of the masseter muscle at the inside

of the ramus ) , its a very powerful muscle . it elevates the mandible

and closes the jaw . it affects the lingual flange ( )

through the superior constrictor .

So Masseter through the buccinator , and medial pterygoidthrough the superior constrictor . Remember that !


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3)Now in the very back area of the mouth , there is the palatoglossal

muscle , forming with the mucous membrane covering its surface two

arches)on both sides that narrows the space when the patient swallows

.

To summary , there are two groups of muscles ; one that affect theborder directly and the other one indirectly.

Retromolar pad area :the mucosa in this area is non keratinized so its is not designed to take

an occlusal force , though it provides some support.

The posterior part of the retromolar pad contains glandular tissue (

minor salivary glands ) , it is also the attachment of : the

pterygomandibular raphe , the buccinator , the superior constrictor and

some of the fibers of the temporalis muscle .

The retromolar pad has a pear shaped ,we cover only half or two thirds

of its height . we cant go further behind because we will weaken the

muscles there (mentioned above ).

The doctor is reviewing some structures :Lingual to the retromolar pad we have the premylohyoid fossa ,then

mylohyoid muscle goes upward , where the premylohoid eminence is , (

the highest point where the muscle ends ) ,So the flange is shaped by

the lingual frenum and the mylohyoid muscle .

Behind the premylohyiod eminence the flange can go down again ,

because of an area called the retromylohyoid fossa ,the boundaries of

this fossa aremedially : the tongue , laterally : the mandible , anteriorly: the posterior border of the mylohyoid muscle ,inferiorly : the

mucolingual fold (no muscles ) . deep to this fold there are

submandibular glands . thats why we should be careful not to go too

deep in that area while taking impression , these glands are soft and if


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the flange compresses on them they cause pain and less stability to the

denture .

The retromylohyoid fossa provides lateral stability , because its like apocket that is deep and vertical . we should not forget that the buccal

shelf area is also a stabilizing structure for the denture .

Now Posteriorly : postolaterally , the superior constrictor . postomedially

, the palatoglossal .

The palatoglosssal is like an arch and the superior constrictor islike a curtain from behind .

With this we concludethe anatomy of the maxillary and mandibular

denture , the doctor apologizes about the slides ( he did not give them to

us ) , he wants us to read these topics from the textbook and to set

according to the oral pathologys seat numbers from now on .

Best of luck to all of you ,Aya Qassem Alali


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Prostho 3rd lecture,Mandibular Edentulous Anatomy

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