Cavity Prepartion2[1]

7/31/2019 Cavity Prepartion2[1]

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Principles of Cavity Preparation II

The doctor talks about the exam which will be in the first 6 lectures (to the end of the amalgam)

so the lecture of next Thursday will be included in the exam.

Last time we talked about the initial stage of cavity preparation which is outline form and

resistances form and now we will talk about other forms of cavity preparation:

Primary Retention Form:

The shape or form of the conventional preparation that resist displacement or removal of the

restoration from tipping or lifting forces.

We seed last time that the restoration inside the oral cavity will be subjected to a number of

forces from different directions which may cause displacement; the retention form is to resistcompression forces (forces that are directed at the long axis of the tooth) when the patient

occludes the restoration will be subjected to the force and we want to prevent dislocation of

the restoration.

The Principles of retention depends on the restorative material that means that the amalgam has

different retention form than composite, in composite we have chemical retention while in amalgam we

have mechanical retention; we need to do something geometry inside the cavity so the amalgam stay

retentive to the tooth structure.

How we can get a retention form???

1- Amalgam:

In Class I and Class II, walls should converge occlusally so the cavity has wider base, and when

we add amalgam it will be one unit inside the cavity which will prevent it from dislocation.

In class II we make what called dovetail to aid in retention and prevent the restoration proximal

dislocation.

In Class V, walls diverge outward to provide strong enamel margin, retention obtained by

grooves in the dentinal walls.

2- Composite:

Micromechanical bond by acid etching & bonding.


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Convenience form:

The form or shape of the preparation that provides adequate observation, accessibility and

ease of operation in preparing & restoring the tooth that means I should be able to see where

I am working and be able to do operation, it should not be too wide nether too narrow it should

be convenient.

Final Tooth Preparation Stage:

Removal of any remaining enamel pit or fissure, infected dentin or old restorative material if

indicated, that mean we should remove any carious part of the tooth and any defective old

restorative material.

Infected dentin: it is the carious part of dentin (invaded by bacteria) and its soft and we should

remove it, especially at the DEJ (dentino- enamel junction) because the spread of the caries at

the DEJ is faster than anywhere in the tooth.

Affected dentin:its the non carious part of the dentin and its hard so we can keep it to beconservative.

Pulp protection:

Dentin is the best isolator against irritation to the pulp, but if you expose the pulp you can

isolate it by a liner or a base; there should be 2.0 mm of bulk (remaining tooth structure, liner,

and/or base),the base provide protection against mechanical, thermal and chemical irritants.

Secondary resistance and retention forms :

When we have large cavity preparation ( we have lots of walls ) so we lose the retention and

resistance forms ( the ordinary ones ) so we have to have the secondary forms of retention and

resistance .

and it can be applied by

a. Mechanical preparation features:

examples ( you should know just the names of them ) :

1. Retention locks, grooves, and coves

2. Groove extension

3. Skirts

4. Pins, slots,, steps, & amalgam pins


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b. Placement of etchant, primer, or adhesive on prepared walls:

1. Enamel wall etching

2. Dentin treatment

*** you should remember that " Placement of etchant, primer, or adhesive on prepared walls

"

considered as primary retention and resistance form for composite and its considered as

secondary retention and resistance form for amalgam .

*** amalgam adhesives was considered as way to have resistance and retention but lately they

found that it doesn't help in resistance and retention so it's no longer used !!!

Procedures for Finishing the External Walls of the Tooth Preparation:

this Procedures is to make sure that we don't have under cusp or unsupported enamel and its

the final check of my cavity .

and it's The further development, when indicated ( we don't always do it ), of a specific

cavosurface design & degree of smoothness or roughness that produces the maximum

effectiveness of the restorative material being used .

so that when I put the restoration it will not be fractured or there will be no micro gap between

the tooth surface and the restorations .

Objectives:


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1. Create the best marginal seal possible between the restorative material & the tooth

structure.

2. Afford a smooth marginal junction.

3. Provide maximum strength for both the tooth structure & restorative material.

Final Procedures: Cleaning, Inspecting, and Sealing:

the purpose of this procedures is to prepare the cavity to receive the restoration .

* Cleansing & dryness of the cavity for inspection.

* Sometimes, certain medicaments can be applied to the cavity prior to the restorative

step.

Sequence of Caries Removal

1. Entry is made in conventional manner

with a high speed fissure bur.

The first thing you should have access

to the caries so we start with removal

of the enamel we cannot remove


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we use low speed round bur and it should be

large that fit the cavity because if we

used the small one we may have pulp

exposure .

First stip I will do my out line first , intial depth with high speed fissure bur , then I will change and use a

round bur in order to remove caries in carious dentin

4. The caries removal process should begin peripherally in the DEJ areas

its mean I will not move all the way into the pulp removing first form the pulp. First step of removing

the caries is to remove form the DEJ peripheral, lateral wall we clean the lateral wall

for example in class I, Ill clean mesial , lingual , buccal ,distal walls in leave pulpal wall this dont mean

we will leave caries in the pulpal floor I will remove caries but this will be the final step .

Why ?

Because of two reason

A] To make sure to remove caries from the most important place the DEJ dentino enamel junction


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B] And if I I am too deep pulb exposure may occur although I made all procedure to avoid pulp

exposure , but if it occur I will know how to deal with it , we can avoid

Some time ican avoid rct treatment I leave it like this in order to have pulbal recovery

So fo these thing I shold clean the lateral walls then go to the pulb

Clean the lateral wall then go pulbaly minute 33:00

In class ii for example I will clean buccal lingual gingival then I go axialy

Caries in areas involving potential exposures, such as the axial and pulpal walls should be removed

last

How much dentin should I remove ? as we said we have something call infected dentin and

something called affected dentin ,,,,,, in order to be more conservative I should not remove all dentin

colored dentin

We will learn to differentiate between sound dentin and carious dentin ,,,,, some time the color is not

indicator when I look for example I see brownish discoloration of the dentin or yellowish discoloration of

the dentin Other than the normal color of dentin this dose not mean it might be carious

Primary criteria is the hardnessand for this we have an instrument call spoon excavator its for removal

of caries

Spoon excavator is used also for detecting if this dentin is soft or not .

If the dentin can be removed by excavator it means its soft we have to remove it ,,if its hard to spoon

excavator then we have to leave it its affected dentin not infected dentin

So the color is not usually our criteria for removal of dentin our indicator of carious dentin is

softness of the dentin how we can check this with the spoon excavator

Only those areas that are soft should be removed


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6. After all caries have been removed ,the preparation is re-evaluated for any undermined enamel ,

resistance for ,retention form .

This is under mind enamel the dentin below the enamel was removedbecause its carious leaving me

with under mind enamel , so the final stage I have to remove this under mind enamel because its un

supported by dentine it will fracture

When I remove the caries a large part of the tooth will be removed when I remove caries so I should

reevaluate my cavity preparation do is still have a flat floor , I have retention form , resistance form and

if I have any under mind enamel for example this pic below

usually what we do we do it step by step . ya3ni I open the axis as we said , start to remove the caries

out peripheral at the DEJ with a round bur the I reevaluate if I have undermid enaml I will remove it

To remove the under minde enamel I have to return back to the high speed fissure bur and remove any

un supported enamel and make if I need ,retention , resistance , adjustment of the wall I should do

them with high speed fissure bur

Always when we make class I we emphasis on the box shape and flat pulpal floor here in this pic we can

end up with this un flat pulpal floor


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So dose this mean I have to reduce the pulpal floor to this level to make it flat ? in this way we might

endanger the pulp and we loss unnecessary tooth structure we want to conserve more tooth structure

not to loss it .. when we remove more of the tooth structure we lose more of the resistance of the tooth

so the tooth may fracture later ! so in this case NO its not necessary to make it flat

Some time we have to put some liner here liner material in the deepest area to be away from the

pulp and to make it little pit flat

Done by:- Abdallah Zireeni and Mohammad F. elwir

Cavity Prepartion2[1]

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