Prostho 3rd Lecture Mandibular Edentulous Anatomy Modified

8/3/2019 Prostho 3rd Lecture Mandibular Edentulous Anatomy Modified

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

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

***Before we start there are some important terms that you need to

know there specific meaning from a prosthodontic point of view:

**denture foundation area: the surfaces of oral structures available tosupport a denture.*

**denture impression surface: the inner surface of denture base that isin direct contact with the denture foundation area ex. alveolar ridge.


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**denture polished surface :the outer surface of the denture that is indirect contact with the tongue.

**denture base :the part of the denture which rests on the foundationtissue and to which teeth are attached.*

**denture border :the margin of the denture base at the junction of thepolished surface and the impression surface.*

**denture flange: the part of the denture base that extends from thecervical area of the acrylic denture teeth to the denture border.*

**Denture stability:1.the resistance of the denture to move on itstissue foundation especially to lateral (horizontal forces) 2.a quality of a

denture that permits it to maintain a state of equilibrium in relation to

its tissue foundation.*

**support :.with respect to dental prostheses the resistance todisplacement away from the basal tissue or underlying structure .***denture retention :1.the resistance in the movement of a dentureaway from its tissue foundation especially in vertical direction.2. a

quality of a denture that holds it to the tissue foundation.*

*glossary of prosthodontic terms .

^^^Good retention needs primarily support and stability also helps.Although theres some degree of similarity between the upper and lower

arches . for example , both have the labial and buccal mucosa , there are

lots of differences between them from many aspects such us :

1. type of mucosa ,2. the underlying submucosa ,3. type of bone ,


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4. the surface area ,5.

the bone and muscle attachment on the lingual surface of thealveolar ridge ,

6. the presence of tongue which greatly affects the way that themandibular denture is made in comparison to the maxillary

denture .

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.

A)In the mandible , there is no palateinstead we have the tongue1)The tongue has very active groups of muscles , the extrinsic and

intrinsic muscles and they will greatly affect the flangeswhich extend

from the denture base.

**in the maxilla the muscles are less active.

2)As we know , for good support we require the maximum denturebearing surface area , the presence of the tongue limits this extension ,which reduces support .

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

An important thing to remember , another reason why the mandibularridge is not well suited to support the denture ,

B) the mandibular residual ridge resorbs faster than the maxillary

residual ridge .

During the first year of resorbtion , the mandibular residual ridge

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


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resorbs four times faster , that means that within the first year bothresidual ridges are resorbing quickly . This creates imbalance between

both arches .

To sum it up, reasons why the mandibular denture creates problems

;

less surface area more active tongue muscles. the mandibular residual ridge resorbstwice as fast as the

maxillary residual ridgeIn the previous lecture , we said that after teeth extraction the alveolarbone ( or the residual ridge ) will continue to resorb . it atrophies

according to a certain path , not randomly .

The direction of resorbtion is related to the angulation of the alveolarbone , which is related to the angulation of the teeth were there beforeextraction . and almost everywhere in the mouth , the angulation of the

teeth is outward (buccally or facially),

posteriorly and anteriorly in the upper arch teeth are proclined(outward)

anteriorly in the lower arch teeth are proclined . one exception to that is the lower posterior teeth are Lingually

angulated (inward).

in the premolar area the direction is straight up .As the resorbtion occurs , different shaped ridges result .

there is a difference in the amount of resorbtion between the types of

ridges , usually residual ridge resorbtion occurs mostly within the firstyear after extraction. it continues to resorb throughout life , but at the


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beginning it is always faster , logically so because the amount of bone

left becomes less at time goes on.

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

) .

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

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

support.

in the lower anterior portion , after extraction the bone tends to goin a lingual direction .

in the upper anterior region bone also move palately afterextraction.

Posteriorly in the upper arch the molars areproclined , having abuccal angulation .

In the lower posterior region, molars have 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 of the ridge moves further palately in the upper

posterior region

but in the lower arch the center of the ridge moves furtherbuccally , and this is the main exception and it's really


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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:

A)it is difficult to put the teeth in the correct position ,B)and the occlusion might become reversed by putting

the upper teeth too much palataley and the lower teeth

directed too buccally , so cross bite results.*cross bite :is when teeth bite in reverse to normal (upperteeth bite lingual to lower).

Anatomical landmarks of the mandible are usually divided into static

area and dynamic area;

the denture bearing area which is the surface of the ridge(static area),

the borders where the muscle attachments occur which arethe dynamic structures limiting the borders of the denture

,(dynamic area)

Another reason why mandibular denture is harder than maxillary

denture

C)From the fact that the resorbtion in the lower arch is pretty fast , vitalstructures which are normally buried in the bone , become exposedaffecting the borders of the denture .


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So borders of the lower denture are 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 as time goes on , due to resorbtion .

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 ( the mid

line of the lower jaw ) showing a jaw with a new denture , and another

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

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

on .

A)in the first one , the denture is really large , and the internalsurface of the denture is concave resting on the top of the

ridge .

notice what happens when time goes on ,

B)in the other picture , I have a flat surface setting on a flatsurface, wheres the stability !

another reason why maxilla is better than mandible in denture

support :D) the surface area of edentulous mucosa of the mandible is less than

that of maxilla so the support is less .


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

normal healthy mucosa .

**The main point is that the mucosa was designed to take tensionforce which is the type of force applied when teeth are in place .**And it was not designed to withstand compression force which is theforce applied by denture.

normally when we chew , food passes across the ridge or the buccal

mucosa and come in 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 thedirection of fibers is very well designed to withstand Biting force , ittransmits the compression to tension , the oblique fibers( )

along the whole length of the root are stretched when the patient bites ,

only at the apex its not stretched because it takes compression force,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 , although its very thin but thePDL has much bettersensory perception in .

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


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supports it , the two principle functions of the PDL are support

and positional adjustment of the tooth , together with the

secondary and dependent function of sensory perception .PDL versusedentulous mucosa:

Althought th PDL has less surface area than edentulous mucosa

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

) PDL is more effective, however, if we stretchthe PDL of all teeth forming a flat surface area , it has been found that

the edentulous maxilla has a PDL surface area of about 45 square cm

and the same in the mandible ,but in fact the PDL isnt stretched so:i. the edentulous mucosa in the maxilla is double that

in the mandible , andii. the maxillary mucosa surface area is half that ofnatural dentition .

iii. so the mandibular edentulous surface area is about the original surface area of the PDL .

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 the buccal shelf area are

i. laterally : the external oblique ridge at the attachmentof the buccinator muscle ,

ii. medially : the crest of the residual ridge ,


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iii. anteriorly : the buccal frenumiv.

and Posteriorly it extends to the pterygomandibularraphe which is located posterior to the retro molar pad.

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

ridge that:

has compact dense 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 boneon 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 degreepressure from the masseter and buccinator muscles

and forms the posterior border of the buccal shelf area

.

Even when resorbtion occurs , the buccal shelf area does not resorb very

much . as ridge resborbs the buccal shelf tends to look like its larger.

so we have two shelves 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:it is the attachment of the buccinator muscle,We extend our denture slightly beyond this attachment because the

buccinator fibers are horizontal fibers parallel to the denture surface

there so they wont affect the movement of the denture .


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

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

not superio -inferiorly , when they contract they will have less affect onmoving the denture .

so to get more surface area , its found by experience that we can

encroach press on the buccinator attachment by a 1-2 mm to covermore area of the mandible .

*Pterygomandibular raphe:is the line 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 ridgethe sulcus becomes shallowerbecause the ridge becomes closer to the sulcus.

*the mylohyoid ridge :

it is on the lingual surface of the residual alveolar ridge , it is theattachment of the mylohyoid muscle , which runs from the internal

surface of the mandible to the hyoid bone , its function is to raise thefloor of the mouth when we swallow , when we swallow the floor of themouth rises (and so does the tongue) this is due to contraction of the

mylohyoid muscle . the ridge of the attachment will effect the borders of

the denture , the ridge itself creates a problem why?????

when extending the lingual flange , we can only extend it to the muscleattachment , beyond that we will go over the bony ridge , and if theborder of the denture reaches there it will hurt the patient because itspassing over this ridge of bone.


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

the mandibular denture .

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

mylohyoid attached to right and left of the lingual surface of the

mandible ,

** the attachment of mylohyoid 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 .

The main effect of the mylohyoid muscle on the border of thedenture is Posteriorly more than anteriorly , in the middle ofthe lingual flange region :


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in the anterior part the main effect is from the lingualfrenum , and we can extend it anteriorly as long as wedont encroach the lingual frenum .

as the flange goes posteriorly it should be raised due tothe presence of the mylohyoid ridge .

*The premylohyoid fossa :

It is in the anterior region posterior to the lingual frenum . it is like a

pocket bony depression between the tongue and the bone in the area

of thepremylohyoid eminence.

In the premylohyoid fossa the denture flange becomes larger as itextends to the level below the mylohyoid ridge .

*Premylohyoid eminence:is a bony tubercle present on thelingual surface of the mandible anteriorly almost in the area where

mylohyoid fibers start to raise their attachment.

*The retromylohyoid fossa : is the area behind the mylohyoid muscleposteriorly where the mylohyoid attachment ends posteriorly, This area


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has no muscles ( It is behind the mylohyoid muscle and in front of the

palatoglossus) and in this area the denture flange goes down and it is

free to move there .

---In retromylohyoid area the denture flange is no more affected by theaction of mylohyoid muscle so the flange turns laterally toward theramus to fill the fossa .

And these structures are important for the border molding and the

borders of the denture, they affect the peripheral seal and stability ofthe lower denture .

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

ridge ) these structures are normally buried in the bone like nerves and

blood vessels , when the bone resorbs they might get exposed and

become more apparent .

*the mental foramen is a common oneit 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 every time

he bites it presses on the nerve .

D) 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,paraesthesia and sometimes anesthesia may result.

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


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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.

*Torus mandibularis :

They are bony masses , present in only 1 out of 5 people (20%) , it varies in

size , projecting from the lingual surface of the mandible in 1st and 2nd

premolars area.

this structure even though 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 on

that area .

If the size of torus mandibularis is large , they have to besurgically removed

if they are small , we have to work around them and reliefthis area .

the doctor showed a picture of a patient having tora mandibularis (

more than one ) extending all the way to from premolars to 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 .


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The muscles that affect the denture are the buccinator , and superficialto the buccinator we have the masseter muscle located lateral to theramus of the mandible .

How does the masseter muscle affect the denture ?just to remind you , the masseter muscle runs from the zygomatic

arch to the angle of the mandible, 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 masseter does not come in a direct contact with the borders of the

denture , but its located outside the buccinator on it(superficial orlateral to it) .

i. When the masseter contracts it compresses on the buccinatorii. So the buccinator which is close to the denture border presses

on the denture .

And this is more evident in patients where the direction of themasseter is more medial .

*The labial frenum

in the lower anterior area this frenum a very common area of trauma

if the denture was not fitted carefully on it .

*The buccal frenum :

It is significant like that in the maxilla because it is attached to an

area where the modulus is ( the modulus: a coalescence of muscle fibers


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

to get a free movement in that area in all directions ; anterior-

posteriorly and superior-inferiorly .

buccal frenumbuccal frenum

.

So when we do the border molding , we need to move the buccal

frenum 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 resorbtion.

The doctor showed a picture of that case , ifwe stretch the tongue thetension goes all the way from the tongue to the lip as if they wereattached together , because a little of the residual ridge is remained due

to extreme resorbtion .


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**Muscles effect on the mandibular denture:*Mentalis muscle :

is a particularly active muscle in the lower labial region ; when it

contracts it extends the lip upward ( the lip becomes longer )so we need

to make the flange of the denture thinner in its area.

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 from above to a patients head in

cross section, we can see like there are circle of muscles surrounding the

patients mouth , from the mouth all the way back to behind the

esophagus .

we have the orbicularis oris attached to the buccinator through themodulus ,

the buccinator goes around the cheek , then when it reaches the bodyof the mandible it cant go any further back , it has to go medially and

inside to reach the pterygomandibular raphethe pterygomandibular raphe attaches it to the superior constrictor .the superior constrictor attaches to the superior constrictor from theother sidethe superior constrictor on the other side attaches to thepterygomandibular raphe on its side.

and the circle repeats its self


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Now there are three muscles that mainly affect the movement ofthe 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 itcontracts powerfully it compresses on the buccinator as mentioned

previously and makes a notch in the disto-buccal part of the flange inthe back end of the buccal shelf area . this is called masseteric notch .2) Lingually , on the inside of the mandible , there is the medialpterygoid muscle ( the counterpart of the masseter muscle at the insideof the ramus it opposes the masseter from inside) , 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 !

3)Now in the very back area of the mouth , there is the palatoglossusmuscle , forming two arches of soft tissue with the mucous membranecovering its surface on both sides so that narrows the space when the

patient swallows .

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

Retro molar 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.


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The posterior part of the retro molar pad contains glandular tissue (

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

i. pterygomandibular raphe ,ii. the buccinator ,

iii. the superior constrictoriv. and some of the fibers of the temporalis muscle .

The retro molar pad is pear shaped ,we cover only half or two thirds of

it(in anteroi-posterior direction) . we cant go further behind because wewill press on the muscles there (mentioned above ).

to review some structures :

Lingual to the retro molar pad we have the retromylohyoid fossa ,then

mylohyoid muscle goes downwards as we go anteriorly from there , until

we reach the premylohyoid eminence .

So the flange is shaped by the lingual frenum and the mylohyoid

muscle

Anterior to the premylohyoid eminence the flange can go down again ,

because of an area called the premylohyoid fossa ,the boundaries of this

fossa aremedially : the tongue , laterally : the mandible, anteriorly :but a little bit away from it, is the lingual frenum ,inferiorly : the mucus-

lingual fold (no muscles ) deep to this fold there are sublingualglands .thats why we should be careful not to go too deep in that area while

taking impression , these glands are soft and if the flange compresses

on them they cause pain and less stability to the denture .


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The premylohyoid fossa provides lateral stability , because its like a

pocket that is deep and vertical . we should not forget that the buccal

shelf area is also a stabilizing structure for the denture .

***in the middle of the lingual flange that is the area extending from

the premylohyoid fossa to the distal end of mylohyoid ridge the flange is

made to slope medially (toward the tongue) and this gives me three

advantages :

1. The tongue rests on the top of the flange and aids in stabilizingthe lower denture .

2. The slope of the lingual flange provides a space for the floor of themouth to be raised during function without displacing the lower

denture.

3. The seal of the lower denture is maintained during thesemovements because the lingual flange remains in contact in the

mucolingual fold.

**therefore in this area the flange doesnt rest on bone but it rests onsoft tissue .**when the mylohyoid muscle is relaxed there is a space between the

flange and the floor of the mouth but contact is reestablished when the

floor of the mouth is raised.

NowPosteriorly

: Posterio-laterally , the superior constrictor . Posterio-medially , the palatoglossus.


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The palatoglossus 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 end


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Documents

Prostho 3rd Lecture Mandibular Edentulous Anatomy Modified