Dental materials sheet # 4 - Mo7ammad Abu kar

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Dental materials sheet # 4 - Mo7ammad Abu kar

Post by Shadi Jarrar on 24/10/2011, 11:49 pm

Subject: Dental Materials (week 5)
Date of Lecture: 16-10-2011
Dr: Mohammad Alrababah

Regarding the article, the Dr. said we are not supposed to know that trade names but we have to know the ideas and principles.
Here is the "debate" that was between Valeria Manfalouti and Mohammad Abukar in the beginning of the lecture.
Now the etching duration depends whether it is only enamel or enamel and dentin. The duration of rinsing should be at least equal to the duration of etching

*Dr. comment: when there is a book published in a year.. its information will be lagging about 5 years because editing after writing. You should know that books are not as beneficial as new articles. The dr. offers for anyone who searches for an article but didn't get an access to it to send him the link of the article and he will return it back as a full article.
About drying.. Valeria mentioned that the cavity that includes dentin should be a little moist.. it should not be overwetted. If it was overwetted then the operator has to redo etching. If the operator overdried the surface, then there are rewetting substances such as cotton roll with a little water and HEMA (Hydroxyethyl methacrylate).
Regarding the wetting of the surface, most of the studies stated that the surface should be a bit moist so that dentinal collagen will not collapse.
If you logged in to the university main library you will find many (about 250) dental journals. in the database including very fine articles.
If the surface was overwetted then the bonding agent components (hydrophilic and hydrophobic) will separate so the properties of the bonding agent will deteriorate.
Regarding another research that was done on Optibond (a bonding system).. the study was to see the differences if the drying was made for 5 seconds or for 15 seconds.
There were no signs of overwetting or failure because of microleakage or collagen collapse after using electron microscopy. The difference was in the ultrastructure, this may give a hint that till 15 seconds it is okay to dry. So, 15 seconds is good but 5 is better. Another research stated that 30 seconds is too much for drying.
The hybrid layer is the interaction between and resin particles in the primer mainly with the collagen network in the outermost part of the dentinal tubules. It is from 0.2 to 1 micron and not more than that.. other than that becomes resin tags. The reliability and durability of the bond strength comes from the hybrid layer.
Another experiment was done to find the difference between bonding on dry and moist dentin . there was no significant difference anywhere but the gingival margins.
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Now Dr. AlRababa'a speech starts
Don't be the average dentist. don't be the penguins
Now, most of you used composite. Did you count how many seconds you did washing and etching.. did you count 15 seconds while etching?
For athletes in 100m race, a fraction of a second may make a difference and applying the same principle, a second may make the difference for a collagen fiber to collapse or to not.
*YouTube videos uploaded by "someone" are not scientific sources.
"Respectable" acid etches have thixotropic properties so that its flow will reach the whole cavity.
Thixotropic: Becoming a fluid when agitated but solid or semi-solid when allowed to stand.- Wiktionary
You can't say that you made a good bonding just because the bonding agent didn't separate after you applied it because a good bonding depends on both reliability and durability. If you hit the margin and the bonding didn't break then it is reliable.. but this doesn't mean that the bonding will be durable. So, you can't say it was a good bonding.
Composite is your butter and bread in dentistry because amalgam is going down and many patients do not accept it but when the patient comes to do a composite restoration he doesn't want to have a postoperative sensitivity after one or two weeks.
Sensitivity depends on how long you rinse how long you dry after rinsing. Rinsing for only 5 seconds was acceptable in 1998 but it is not anymore. Rinsing should be AT LEAST for the same duration of etching if not more because if this acid remained it will go directly to the pulp as it is a low molecular weight molecule.
If you did overdrying, collagen collapse will occur and when collapse occurs you can't bring the collagen fibers to their previous status.
You should know the properties of a bonding agent generations because you are gonna pay to buy them. If you paid to buy a newer product without knowing its properties then you will be like a person who buys a smartphone only to send messages! You should know if this material worths this money or not and you should know what bonding agent you bring from the dispensary.
*Rewetting has its indications that will be discussed later.
One thing that is important to know about bonding agents in particular is that every company has its own chemistry. The manufacturer's instruction is something very important. You should read the manufacturer's instruction and know the difference between the 5th, 6th and 7th generations.
Instead of calling them as generations now we name them as two bonding systems:
1- Total etch technique
2- Self etch technique
Ideal requirements for bonding:
1- High consistent bond strength for enamel and dentin because we are bonding to both.
2- Quick and easy application.. it shouldn't need 16 steps for example!
3- Consistent product quality.
4- no postoperative sensitivity.
5- Universal use with direct and indirect fillings (no degradation).
*You should wait for about 18 months to decide that this really is a good restoration.
*A high percentage of our patients will experience postoperative sensitivity or pain
and this is one of the main problems because till now we didn't find one sole strategy for rinsing and drying.
Postoperative sensitivity may be related to acid etching, something related to impregnation (as there was not enough impregnation of the primer into the dentinal tubules so they remained exposed), salivary contamination or post polymerization stress that may lead to failure in the bond, occlusal loads or direct trauma to the pulp by the preparation.
When a patient comes to your clinic suffering from postoperative a redo you may cause more damage, so it is better to wait and think in a logical. You should know what the reason is.
You should know whether the sensitivity is transient or not.. is it because of acid etching while there is fresh organ dentinal tubules because not all of them will have good bonding? or is this because of direct hit from the monomer into the pulp? sometimes we need to wait. There is no one direct answer.. you should assess each case by its own.
How to assess?
You should know the degree of pain, the mechanism of bonding you did, the duration of acid etching and bonding. If you know your process exactly then the sensitivity is related to the biology of the pulp itself and you can do much about it but if you don't know how long you did acid etching and bonding and don't know whether you left the cavity dry or not………
After all it is all related to the clinical situation.
If dentin was overdried collapse of the collagen fibrils will occur and the bond will not be strong enough to hold and thus more stresses are there.
As we said we have two bonding mechanisms:
1- etch and rinse (removal of smear layer)
2- Self etch (it modifies the smear layer.. nowadays it is either two bottles or one bottle)
Etch and rinse strategy:
we etch with organic acids such as the phosphoric acid with the concentration of 36% (in our university hospital)
Rinsing and drying..
do we dry? And how wet is wet dentine?? If you searched about wet dentin VS dry dentin you will find many articles
When we talk about bonding generations we talk about dentin. With enamel we have only one bonding generation we have reliable and durable bonding with enamel. Many of our cavities contain mainly dentin we need to have proper bonding to dentine.
1st and 2nd generations were very premature. In the 80s dentists used to isolate dentine and did not acid etch and bond it. They only etched enamel and coated dentine with Vaseline because they thought that acid etching can't be used with a living structure.
In 90s we started talking about total etch strategy, in the new mellinium we started talking about the self etch strategy.
The aim of primer is that it does wet the surface
Self etching adhesives wet the surface and make a contact with the hydrophilic structure because it is a bifunctional molecule and increase the permeability of the smear layer because it has an acidic monomer
It provides micromechanical retention and provides chemical bonding and this is the ongoing research in 2010 and 2011.. we are trying to find molecules that can have direct chemical bonding to dentin collagen fibers (smart molecules) – can interact with tooth structure in a very smart way.
BisGMA is the main molecule and found in about 99% of the adhesives and bonding agents that we use.
Belgian scientist Van Meerbeek is the one who made most researches on the bonding agents and he still prefers the etch and rinse technique (it gives the most reliable bonding).
You need to etch then to apply your bonding after rinsing, but the main problem is the postoperative sensitivity
The problem when doing a lengthy acid itching is that some dentinal tubules will remain opened we said that in the hybrid layer we need a thickness from .2 micron to 1 micron.
Using the phosphoric acid etching we may make more emptying of the tubules than the impregnation of the resin è in the total etch technique there will be some areas that are not impregnated by resin.
In self etch technique:
We don't rinse so we don't dry
à so there is less postoperative sensitivity and now we won't be worrying about the duration of dryness 5 or 10 seconds and we won't be worrying about rewetting.
Faster application because with total etch technique you apply the acid etch then you wait then you rinse it then you dry it.
*There should not be a very wet surface.. you need to use air syringe or cotton, etc.
*when we were talking about generations the self etching adhesives were the 6th generation..
Generations of adhesives:
1st and 2nd generations were so premature.
3rd generation: Etching with strong acid & bonding agent (without a primer).
4th generation: Etching with strong acid, primer & bonding agent.
*The most reliable bonding is the 4th generation.
5th generation: Etching and (primer and bonding agent) together.
6th generation: Self etching primers (primer with weak acid) + separate bonding agent.
7th generation: All in one (self etching adhesives)
We don't use generations anymore.. now we use the terms self-etch and total-etch.
The 5th generation is the total etch technique.. we do acid etching separately then primer with adhesive
The 4th generation which is the most reliable bonding.. we use acid etch alone, rinse, primer alone, bond in dentine..
On enamel in the 4th generation we used acid etch alone then rinse then dry for how long? The Dr. dunno you need just to figure it out.. then we do bonding on enamel…
The components of self etching primers are four:
1- Acidic monomer (makes modification of the smear layer)
2- Cross linking monomer (so that it will bind to the adhesive. It binds by one side to the acidic monomer that is hydrophilic because it is acidic and thus it binds to the dentin and on the other side the cross linking monomer binds to the adhesive we are to use).
3- Photoinitiator stabilizers: for light cure, otherwise it will be set without use.
4- Solvent (it is not to prevent collapse.. one reason is the ease of application.. The solvent is either water-based or alcohol-based (ethanol or acetone based).. collagen fibers have water in between.. this solvent comes to replace water.. if there is no solvent we can't replace water.. so it displaces water and comes in its place.. but we don't want the solvent to stay there.. so we use a solvent that can evaporate.. in the past they mostly used acetone.. otherwise no good bonding will take place.. because after the primer we want the bonding agent to be in.. the bonding agent is a resin material (hydrophobic material).. so the main idea from having the solvent is just to displace the water.. so we can put either acetone-based or ethanol-based.. these two could be dried.. but if we dry them we will let other things insert into the space.. these things are the cross-linking monomer and the acidic monomer..
With the acidic monomer.. simultaneous acid etching and resin infiltration occur
The main aim of the cross linking monomer is to stabilize the 3D polymer network.. the polymer network exists because of the structure of collagen.
We said that the reliable bonding tensile strength is 20 megapascal.. this is the reliable bonding with enamel..
With enamel we have a tensile strength of 20 megapascal after acid etch, drying and bonding.. with dentin it may be from 0-36 megapascal if not more.. now, the tensile strength may be higher because now we have materials that do chemical bonding
When we create a 3D polymer network around collagen or infiltrating the collagen fibrils we may reach 36 megapascals so the tensile strength in dentin sometimes may exceed that of enamel.. You will always hear that bonding with dentin is not good.. this is not true.. it is about our strategy.. if we're following the right strategy, it's okay.. and it should be chemically stable and undegradable.. the bonding agents that has acetone were not very stable..
….
After using the primer, we should dry it or not dry it but to evaporate the solvent.. we don't want the solvent.. we said what we wanted the solvent for.. if the solvent remained the bond will be degraded very quickly.. so it should evaporate.
All in one: we mentioned it.. it contains: acidic monomer, primer and the bond itself all in one.. its problem is that you need to apply it 2-4 times (layers)..
It is less durable because it contains acidic monomer that is hydrophilic (not all hydrophobic.. not all resin) so any water contamination that may occur later will lead to.. any hydrophilic material with water will undergo degradation with time..
You will read in the article "don't use aggressive self-etching primers"
If the structure was enamel you can't use a mild self-etching primers this applies also to sclerotic dentine.. if you are having sclerotic dentine and you want to do self etch strategy you have to acid-etch it before..
*If the cavity is big we use mild or moderate acid etch.. (because it is on dentine)
The main problem that still faces us is the degradation of our bond.. in acetone-based primers .. nowadays, we don't use the acetone based primers because of degradation that much.
Any polymeric material with time even it is somewhat elastic undergoes more hardness this hardness with repeated cyclic loading may lead to plasticization in bond and this reduces elastic recoil leading to bond degradation.
We always want the resin molecules to penetrate into the collagen but this doesn't always happen so degradation of the bond may occur
How does capillary attraction affect bond strength and durability à this was assigned for a colleague to speak about
AMALGAM BONDING
The materials to bind with amalgam should contain 4 META or 10 MDP in their chemistry

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Before deciding to choose a treatment, think..
If this tooth was yours, will you choose the same treatment?


This sheet was of October 16th,
It was not assigned to me at first.. I got it in exchange with Anas Moghrabi on Friday (October 21st)
I am usually prepared to write my sheets in several aspects, surely I did none of them during the lecture as, till that moment, it was not mine.. I finished writing the sheet on the same day.. but as I was not prepared, there were many information that I did not understand.. thus I waited till Sunday (October 23rd) to ask Dr. Mohammad Alrababah some related questions……


Good Luck!

Mohammad M. Abukar
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Shadi Jarrar
مشرف عام

عدد المساهمات : 997
النشاط : 12
تاريخ التسجيل : 2009-08-28
العمر : 26
الموقع : Amman-Jordan

http://jude.my-rpg.com

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