シャドーイング練習: Pascal Magne Part #2 - 動画で英語スピーキングを学ぶ

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if you take these basic material properties of enamel, like the elastic modulus, the thermal extension,

expansion coefficient, the ultimate tensile strength, first you will see that enamel is very stiff,

about 80 gigapascal of stiffness.

Compared to dentin, it is much more.

It is almost five times.

Enamel is very brittle and very low resistance in tensile strength.

And this is very important.

It's a major difference between enamel and dentin.

Dentin is much tougher and can resist more in tensile stress in crack propagation.

For enamel, what is the closest substitute?

What do you think is the closest substitute?

There are not many possibilities.

We have a material that is a similar elastic modulus.

We have a material that has almost the exact same thermal expansion.

And in this class of material, the weakest that we found is still two to four times stronger than enamel.

And what is this material?

Of course it is porcelain.

And what porcelain?

The weakest porcelain we have on the market.

It is feldspatic porcelain.

Still I told you two to four times stronger than enamel in tensile strength.

For dentin, as you guessed, dentin is flexible, is bendable.

What do we have that is five times less rigid than enamel, that is in the same range of elastic modulus.

Unfortunately, there is no material in that range that has really a good thermal expansion.

It is composite resins.

As you see, composite resins are also a little less tough than dentin.

And that's not a big problem in that case because as you can see,

feldspatic porcelain have like a reserve of strength that we can spend to compensate on the weakness of the composite resin.

So now we just need a dentine enamel substitute and what is the dentine enamel junction substitute, excuse me, it is dentine bonding.

And I must tell you I am very confident in the way we bond to dentin today.

Because, unlike the dentin enamel junction, current dentin bonding agent, especially fourth generation,

not the sixth, not the fifth, the sixth and the seventh,

the fourth generation dentin bonding agent act very much as an interphase,

exactly like the dentin enamel junction we have inter diffusion between

two materials on one side the collagen fibers of dentin on the other side the resin component

and this is not a simple interface it is an interface

I hope you see the difference in the pronunciation the S versus the C it's an interface

and I have to tell you I could spend an hour on dentin bonding.

I'm not going to do that of course, but I want to tell you that for good dentin bonding there are four steps required.

And these four steps you cannot go around.

These are separate, let's call them bottles or products and separate steps that must be applied to dentin.

And the first one is etching, deneralizing the collagen surface to obtain this porous layer.

The second step is using a primer, a hydrophilic resin that is able to penetrate the the deminolized collagen.

It has to be hydrophilic and you cannot mix too much the hydrophilic and the hydrophobic.

The hydrophobic is going to be the next layer which is the bonding resin.

Now you can't mix these three in the same bottle.

Of course you can, But what you will get is kind of a salad dressing

that is not going to work as well as if you use each of these products independently and separately.

And there is a fourth step, I told you fourth step.

What is the fourth step to bonding to successful bonding it

is the use of light pre-curing the adhesive before you apply the restorative material

when we do direct restorations we always do

that we we pre-cure the adhesive before we actually place the restoration for indirect techniques it is not popular yet

but I I will show you, we have now a number of works showing that the adhesive should be placed and pre-cured and before you take your impression,

which is the only way to solve the problematic that is specific to indirect restoration.

I will share that with you later.

So, can we do dentin bonded veneers?

The answer is yes.

and I will not say three times, yes, yes, yes, we can do dentin bonded veneer, but we need to know how to bond to dentin.

And this requires the technique I mentioned just before.

You see a case here, I will show you more details just after, but with a large exposure of dentin,

we have actually enamel only remaining at the margins in the interproximal and the incisal.

If you note carefully, what you will see is that dentin has been sealed immediately and before taking the impression.

It is the technique called immediate dentin sealing.

And it is absolutely fundamental when you are doing indirect bonded restoration.

I will talk to you about that a little more extensively later.

So you see the same case.

You see the picture taken just before taking the impression, before making the impression.

And you see that dentin is sealed already before that, and I will show you the step by step.

This case was also using a very nice layering technique with the simulation of the root portion of the tooth to simulate,

to maintain the good width by length ratio in that case.

It was a a case of severe erosion of the dentine, of enamel, excuse me, that led to major dentine exposures.

You see the case five years later.

So I will talk more about this problem of dentine

and let's go now in a general description of what can be done with bonded porcelain restoration.

I like to call them bonded porcelain restoration instead of veneers

because you will see you can do much more than a simple fingernail type restoration on an incisor.

It is much more comprehensive than that.

As I said,

bonded porcelain restorations by definition allow you to be biomimetic allows you to let's say abandon the principles of prosthodontics

that we've all learned at dental school stabilization

and retention this is now replaced by adhesion

and it is kind of a i don't want to say a leap of faith

but once once you see what you can do with

that you will do fantastic and and you have to believe first

and i'm here today making maybe and hopefully to make you believe even more in

that approach so we need dentin-like and enamel-like materials now we understand

that and what are these materials we know they are simple traditional materials

that we are going to use in an advanced way

but the technique themselves itself is not very advanced

because the composites we need are micro hybrids

and the ceramics we need are simple feldspatic How long have we had these materials on the market?

Micro hybrids for 15 years, feldspatic, porcelain now for even more than that.

And so these are very traditional materials.

But together, together with dentin bonding, which is maybe the only thing that was really refined recently,

then we can have a fantastic approach to biomimetics.

And dentin boning, I tell you, is maybe the only thing I changed in the technique recently.

The rest was available to us and has been available to us for almost 20 years now.

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So composites will give us adhesion and economy of tooth substrate.

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Adhesion will allow us to forget about stabilization and retention.

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Porcelain will give us durability and aesthetics.

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This is very important.

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We know that composite materials for this type of restoration are not able to fulfill these criteria of durability

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and aesthetics in the long term.

シャドーイングとは？英語上達に効果的な理由

シャドーイング（Shadowing）は、もともとプロの通訳者養成プログラムで開発された言語学習法で、多言語習得者として知られるDr. Alexander Arguelles によって広く普及されました。方法はシンプルですが非常に効果的：ネイティブスピーカーの英語を聞きながら、1〜2秒の遅延で声に出してすぐに繰り返す——まるで「影（shadow）」のように話者を追いかけます。文法ドリルや受動的なリスニングと異なり、シャドーイングは脳と口の筋肉が同時にリアルタイムで英語を処理・再現することを強制します。研究により、発音精度、抑揚、リズム、連音、リスニング力、そして会話の流暢さが大幅に向上することが確認されています。IELTSスピーキング対策や自然な英語コミュニケーションを目指す方に特におすすめです。

シャドーイング練習: Pascal Magne Part #2 - 動画で英語スピーキングを学ぶ

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シャドーイングとは？英語上達に効果的な理由