This comprehensive course will give you the fundamental knowledge needed to begin performing cataract surgery by phacoemulsification and extracapsular removal. Each step from preoperative evaluation to postoperative care will be covered to help prepare you for the operating room.
02.04 - Viscoelastics
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Introduction to Cataract Surgery
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Unit 2: In the Operating Room and Getting to the Cataract
This unit begins in the operating room after the patient is prepped and draped. It covers each step needed for you to access the lens. Once you begin operating, you will realize that each step of surgery has a domino effect. If the beginning steps are not done skillfully, it will make the rest of the case more difficult.
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Elizabeth Du, M.D.Clinical Assistant Professor
Department of Ophthalmology & Visual Sciences, Kellogg Eye Center
Hi everybody, I am Dr. Tina Turner.
I am a comprehensive ophthalmologist in Grosse Pointe with
the Henry Ford Health System.
I did my residency at U of M, go blue.
Today we're going to talk about viscoelastics.
Most of what I'm going to tell you is from this excellent textbook.
There's a newer edition of the text book, which is also wonderful.
My other reference is going to be this brain.
This textbook is an excellent reference.
This brain is a work in progress and prone to malfunction so bear with me.
Let's first start talking about the function
of viscoelastic devices in cataract surgery.
We basically use them to maintain the intraocular space so
that you've got room to move when inside the eye.
How did we do that in the past?
Does anybody know?
We basically used air which was less than ideal.
And we also use OVDs to protect the endothelium.
Why is that important?
Because we're born with about 3500 endothelial cells per square millimeter.
And we have around 2400 of those by the time we're an adult and we continue
to lose them throughout the rest of our life and they don't repopulate.
So the more protection, the better.
We also need to talk a little bit about rheologic properties in order to
understand the function of OVDs and how they do their job.
The first word we need to understand in rheology is viscoelasticity, and
that's a substance's ability to return to its original state after being stressed.
Just like these balls when they're compressed,
they return to their original round state.
We also need to understand viscosity.
Viscosity is a substance's resistance to flow.
Let's take a look at one of my all time favorite substances, ketchup.
Can you believe that ketchup is more viscous than honey?
Which partially explains this phenomenon, but only partially.
Ketchup is ultra resistant to flow.
However, it is also highly pseudoplastic, and
pseudoplasticity refers to a substance's ability to transform under pressure.
So, something that's thick at rest and thinner at higher shear rates.
So if you take that bottled ketchup and put it in a squirt bottle and
place it under a super high shear rate, it behaves like a low viscosity substance and
comes out a whole lot easier.
Don't you just love science, getting your ketchup out of the bottle quicker?
One of the other very lovable attributes of ketchup is its surface tension,
or coatability.
Surface tension is determined by a substance's contact angle and
that angle is determined by how a substance sits on a surface.
The substance with low surface tension has a low contact angle.
It has a better ability to spread and a better ability to coat.
Just like ketchup, unlike water, which has a high surface tension and
beads up and has a high contact angle on a surface.
But ketchup, super super coatable.
Excellent on your burger.
Now that we understand some of these fancy rheologic terms,
let's talk about the physical characteristics of some of the compounds
which determine their properties.
One of the characteristics that determine a viscoelastic property
is its chain length.
So, these are polymers that usually have pretty long chain lengths.
And if you increase the chain length,
will obviously increase the interactions that the molecule has within itself.
And so that can increase viscosity and
elasticity by increasing intrachain reactions.
And if you increase a substance's concentration of all these molecules,
you'll increase interchain molecular interactions and that also increases
viscosity and increases elasticity and decreases pseudoplasticity.
Enough rheology, let's talk actually about some clinically useful descriptors
with respect to viscoelastics.
So a viscocohesive substance is a highly viscous substance, basically
through intermolecular interactions, entanglement and intramolecular bonds.
A viscodispersive substance has a low molecular weight, so it would have lower
intra and interchain reactions and a lower surface tension and low pseudoplasticity.
A viscoadaptive viscoelastic is going to have both of those properties.
And at low flow or low shear rates, it's going to behave as a viscuscohesive OVD,
but at high shear rates, you're going to get fracturing of the molecules
into smaller pieces and it will mimic the properties of a dispersive viscoelastic.
So it will do both.
So take a look at the picture to your right.
Which of those is a cohesive viscoelastic and which is a dispersive viscoelastic?
Yes, this one is a cohesive and this one is dispersive.
You're experts already.
So I'm going to have you link to this video.
It basically is just an ophthalmologist preparing to do cataract surgery and
squirting a little of each viscoelastic onto the surface of the cornea, so
you can see what it looks like.
See what the cohesive looks like.
And it'll look like a little ball up on top of the cornea and
the dispersive actually coats.
And is actually a pretty good way of maintaining lubrication of the corneal
surface during cataract surgery.
All viscoelastic compounds are made of three basic substances.
Sodium hyaluronate, chondroitin sulfate, and hydroxypropylmethylcellulose.
Sodium hyaluronate is a bipolymer that's ubiquitous throughout the body.
Chondroitin sulfate is found naturally in the cornea.
HPMC is actually a plant material.
It's cellulose in plant fibers and it is not naturally found in human beings.
So a little bit about sodium hyaluronate.
It's found in the eye in the aqueous and the vitreous.
It's a long chain of bipolymers and those long chains coil in solution.
If you increase concentration,
you obviously increase viscosity because of the intermolecular interactions.
And the molecular weight of sodium hyaluronate molecules is between 2 and
5 million daltons.
Where do we get sodium hyaluronate to use in our viscoelastics?
[NOISE] If you guessed rooster combs, you'd be correct.
And fortunately, however, we also get it from [SOUND] bacterial fermentation.
Let's move on to chondroitin sulfate.
Like I said, it's naturally occurring in the cornea.
It's a repeating disaccharide molecule or polymer.
And its molecular weight is only 50,000 daltons, so
clearly this is a lower molecular weight substance.
This actually would be used in a dispersive agent because of its lower
molecular weight.
Whats the source for chondroitin sulfate?
[MUSIC]
If you guessed shark fins, you'd be correct.
But that's not, I guess, the usual source.
And folks usually turn to cow trachea and pig ears and snouts.
And yes, that really is true.
All of these polymers come from naturally occurring substances and
aren't manmade in a lab.
On to HPMC.
This is synthesized from purified cellulose, which like I said is
a structural substance in plant fibers, and is a polymer of D-glucose molecules.
So these molecules were actually used in OVDs several years ago.
And actual vegetable fibers were found in some of the samples
of OVDs that were used in the early 80s.
So these aren't commonly used any longer,
just an interesting fact to make you look smart.
And the source of that is actually cotton wool fibers and wood pulp.
I should have said the source of that was, because like I said,
they're not really used very much in the United States anymore.
So let's talk about sodium hyaluronate derived OVDs.
Like I said, usually sodium hyaluronate is used to fashion viscocohesive OVDs.
Healon was one of the first OVDs to be created, or
actually was the first OVD to be created in the 1970s by Pharmacia.
Healon GV was then fashioned because they wanted to
make something that was greater viscosity, and so you can see what they did.
They actually increased the concentration and increased the molecular weight,
so both of those things led to an increase in viscosity.
They came up with Healon 5 because they wanted something that had
all the best properties of Healon GV.
And yet it was retained in the anterior chamber throughout the entire procedure at
low flow rates, making it a cohesive.
And then fractured, and was easily removed from the anterior
chamber at high flow rates, so then it became dispersive.
So that was the first viscoadaptive viscoelastic.
Healon Endocoat's actually a newer viscoelastic and
it's advertised as being a viscodispersive
which is interesting because sodium hyaluronate is used in its formulation.
And, like I said,
usually the viscodispersive substances are created with chondroitin sulfate.
Amvisc was created in 1983 by Bausch & Lomb and
is a little bit less viscous than Healon.
And that can make sense to you, because look at its molecular weight.
Which it's lower than Healon, but still it's viscohesive.
Amvisc Plus is a bit more cohesive and
was sort of Bausch & Lomb's answer to Healon GV.
And Provisc was created by Alcon.
They wanted to get into the game and it is very similar to Healon.
So all of these, again, pretty similar to one another with respect to Healon,
Amvisc and Provisc.
And it's just your personal preference, which one you enjoy using.
Let's move onto chondroitin sulfate and which viscoelastics are created with it.
Viscoat is a very common viscodispersive agent, notice its low molecular weight,
and a combination of sodium hyaluronate and chondroitin sulfate.
So this is an excellent coater because it's a dispersive.
DisCoVisc was Alcon's answer to Healon 5.
And it has both cohesive and dispersive properties, so it is viscoadaptive.
It's dispersive and cohesive.
So, it's viscoadaptive.
Several of the companies that make viscoelastics use dual packs.
DuoVisc is one of those.
It's Viscoat and Provisc together, so that you don't have to open two separate
packages for your viscocohesive and your viscodispersive agent.
And Healon got into the game and made Healon Duet, because now remember,
they have their own visodispersive agent, so it's Healon and Healon EndoCoat.
And then Healon Ultimate, which is just Healon and
their viscoadaptive agent, Healon 5.
HPMC, like I said, is really not used in the United States any longer.
But to make you look smart, I'm going to tell you a little bit about Ocucoat and
Cellugel.
These were the viscodispersive and
viscocohesive agents that were made with hydroxypropylmethylcellulose.
Bausch & Lomb created Ocucoat, notice its very low molecular weight,
so it is a coater.
It is dispersive.
And then Cellugel was created by Alcon.
Notice its higher molecular weight, so that means it's a cohesive agent.
And neither of those are available in the United States any longer.
So let's take a quiz.
Can you tell me what type of OVD this is?
Is it cohesive, dispersive or viscoadaptive?
If you said cohesive, you're correct.
Now Healon GV?
You are correct, that is cohesive.
Let's move on, what about DisCoVisc?
[SOUND] Correct, that's adaptive.
That's got your cohesive properties and
your dispersive properties depending on your flow rate.
And DuoVisc, if you guessed dual pack you're right
on because that's got your dispersive agent and your cohesive agent and
you don't have to have your staff open two separate boxes.
And now on to Amvisc, cohesive or dispersive?
If you said cohesive, you're right on.
Now, Healon Duet, this is harder because it's a newer product.
However, if you look right here, it gives you the answer.
It's Healon and Healon Endocoat.
So it's a dual pack.
[SOUND] If you said dual pack, you're right.
Healon 5, what one is that one?
Superviscous, it says.
But remember, it is [SOUND] viscoadaptive.
It does both.
It's cohesive and dispersive, dependent on your flow rates.
You're doing a great job.
Now let's talk about my favorite combo, what I use every week in the OR, Viscoat.
Cohesive or dispersive?
If you said dispersive, you're right.
And look right here, it's made with chondroitin sulphate.
It also has a little sodium hyaluronate.
And Amvisc, cohesive or dispersive?
[SOUND] Absolutely, cohesive.
These are the two viscoelastics that I use to do the soft-shell technique.
Which we'll talk about here in just a second.
Okay, so let's continue with the quiz, but use video.
So tell me what type of viscoelastic agent you believe this is that's being injected
into the anterior chamber?
If you said [SOUND] dispersive, you'd be correct.
Notice how it goes in, in like a squiggly chainlike, and not big glob of stuff.
As opposed to, what type do you think this is?
Notice how that goes in in one big glob [SOUND], very cohesive.
And, like I said, usually a dispersive and
cohesive agent are used to do the soft-shell technique.
Which means you use a cohesive agent injected after the dispersive
agent to push the dispersive agent up next to your corneal endothelium because
the main object of the use of the dispersive is to coat and protect.
And then the main object of the cohesive is to flatten the capsule and
we'll get to that later when it comes to creating capsulorhexis.
And make a space for you to work with all of your instruments inside the eye.
And like I said, I use Viscoat and Amvisc to accomplish that.
So let's just watch a little video of me doing the soft-shell technique,
which basically is the two videos you watched two seconds ago.
Here goes my dispersive agent, my Viscoat.
And here comes my cohesive agent, pressing all of my dispersive agent up
next to the endothelium and making a nice space for me to maneuver.
What are some of the complications of viscoelastic devices?
Well, there aren't many, really, it's elevated intraocular pressure.
And this occurs when the viscoelastic material isn't
completely removed from the anterior chamber.
It is transient but it can be exquisitely painful,
because pressures can go very high.
And the viscoelastic substance basically decreases the outflow of aqueous.
It clogs up your drainage system and sometimes for quite sometime.
If it's a dispersive agent, those are shorter molecules and
things usually exit in a shorter period of time.
But if you've got a highly cohesive agent, made up of a long chain polymer,
it can take a long time for that to make its way out of the eye.
So that's it for viscoelastics, feel free to contact me if you have any questions.
Let's go blue.
[MUSIC]
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