|
|
EyeNet
Magazine >> Cataract
Cataract
New Viscoelastic May Stretch Your
Practice
By Lori Baker Schena, Contributing Writer
There once was a time when viscoelastics could be
easily divided into one of two distinct categories:
higher-viscosity cohesives and lower-viscosity dispersives. In
a nutshell, the higher-viscosity cohesives were excellent for
creating and maintaining spaces during cataract surgery while
the lower-viscosity dispersives were very good for assisting
in the management of complications.
But if surgeons
wanted to take advantage of these two distinct classes of
viscoelastics, they had to keep both on hand and know when to
use each type. To address this situation, Canadian
ophthalmologist Steve A. Arshinoff, MD, introduced the "dispersive-cohesive
viscoelastic soft shell technique," which utilizes both
dispersive and cohesive viscoelastics together, sequentially.
And then along came Healon5, which is touted as
combining the beneficial properties of dispersive and cohesive
viscoelastics. With its approval by the FDA in November,
Healon5 has created a new category of its own–"viscoadaptive,"
a term coined by manufacturer Pharmacia & Upjohn to
reflect its ability to provide dual functions at different
flow rates. So does it live up to its promise as the answer to
the cohesive/ dispersive conundrum?
A Viscoelastics Primer In
order to better answer this question, a quick "viscoelastics
primer" is helpful. For the uninitiated, viscoelastics are
very viscous, elastic solutions, made of hyaluronic acid (with
or without chondroitin sulfate or hydroxypropyl
methylcellulose) and used to stabilize and protect ocular
structures during anterior segment surgery.
Dr.
Arshinoff, a surgeon in private practice in Toronto, explained
that long-chain ophthalmic viscoelastic molecules tend to
entangle in solution, causing them to aggregate. They are
therefore referred to as cohesive. As zero-shear viscosity
declines below 100,000 mPs (milliPascal-seconds), molecular
chain entanglement becomes a far less significant factor.
Consequently, less-viscous viscoelastics tend to be easily
broken up, dispersing across their containing solution;
accordingly, they are referred to as dispersive.
Pasta
provides an excellent analogy to illustrate the difference
between cohesive and dispersive viscoelastics, noted David F.
Chang, MD, a cataract surgeon in Los Altos, Calif., and
clinical professor of ophthalmology at the University of
California, San Francisco. "Cohesives are composed of
long-chain molecules that are all tangled up, like spaghetti
noodles. Dispersives are short- chain molecules that, like
macaroni noodles, remain untangled. When you serve spaghetti
with tongs, it's hard to remove a small amount without instead
getting a large entangled bolus. However, you can spoon out a
small serving of macaroni without disturbing the surrounding
noodles in the pot. This is why dispersives are harder to
remove and remain in the eye much longer during phaco," Dr.
Chang explained. "Unlike macaroni, a heap of spaghetti noodles
forms a mound on a plate, without spreading out. Thus,
cohesives are better able to maintain space and displace
tissues, while dispersives are better at coating structures,"
he added.
Advantages and
Disadvantages These different properties result in
advantages and disadvantages:
Cohesives (Healon, Healon GV, ProVisc, Biolon,
Amvisc and Amvisc Plus)
Advantages
Moving tissue to create working
space. "The capsulorhexis is facilitated by flattening
the dome of the anterior lens capsule," noted Dr. Chang.
"Cohesives maintain a mass effect when displacing tissue and
are therefore optimal for pushing the lens-iris diaphragm
posteriorly."
Ease of removal. "It's
much easier and faster to aspirate a cohesive viscoelastic,"
Dr. Chang said. "Although it's harder to completely remove a
dispersive viscoelastic, its smaller molecular size causes
less of a tendency to raise the IOP, compared to the larger
molecular weight cohesives."
Disadvantage
Suboptimal protection.
Dr. Arshinoff noted that while cohesives are easy to remove by
irrigation and aspiration at the end of surgery, the same
cohesive behavior results in cohesives being rapidly washed
out of the anterior chamber during phacoemulsification. Some
surgeons have thought that this may lead to diminished
endothelial protection during prolonged or difficult cases.
However, the presence of specific hyaluronic acid binding
sites on endothelial cells–and research suggesting the least
endothelial cell loss in routine cases when Healon GV is
used–would refute this contention. The issue has never been
resolved to satisfy all surgeons.
Dispersives (Viscoat, Occucoat, Vitrax)
Advantage
Protection.
Viscoelastics should protect the corneal endothelium. "Here,
the dispersives are generally better because they coat the
cornea and remain in place for a longer time during
phacoemulsification," Dr. Chang said. Dr. Arshinoff added that
dispersives allow the surgeon to partition the anterior
chamber, letting the surgeon work in one part of the anterior
chamber while at the same time protecting other parts of the
area from the fluid flow. Dispersives haven't been
conclusively demonstrated to be associated with less
endothelial cell loss than cohesives, but they do "appear" to
be retained better, and that is sufficient to convince many
surgeons to use them.
Disadvantages
Inability to maintain
space. The major drawback of dispersives is that their
relatively low viscosity and elasticity don't allow them to
maintain or stabilize spaces as well as cohesives, said Dr.
Arshinoff.
Obscured view.
Dispersives tend to be aspirated in small fragments during
phacoemulsification and irrigation and aspiration. This leads
to an irregular viscoelastics-aqueous interface that is
optically poor and can obscure the surgeon's view of the
procedure and posterior capsule. Dr. Chang added that
dispersives tend to trap small bubbles more frequently,
further obscuring the surgeon's view.
Difficult removal. Dr.
Arshinoff noted that dispersives, because of their low
cohesion, are more difficult to remove at the end of the
surgical procedure. The additional manipulation and aspiration
required to completely remove dispersive viscoelastics may
actually increase the likelihood of complications such as
endothelial damage or puncturing of the posterior capsule. The
question of whether the effort required to remove dispersives
significantly offsets any benefit derived from their use is
always in the back of the minds of many cataract
surgeons.
Who Uses Which One
and Why
Given the wide range of viscoelastics
available–and considering all their strengths and
weaknesses–it isn't surprising that cataract surgeons have
developed highly individualized techniques that mirror their
priorities:
Corneal protection. For
example, Dr. Chang uses the dispersive Viscoat for routine
cases because of its superiority in corneal endothelial
protection. "I use higher aspiration flow rates for phaco
chop, and I find that under these conditions Viscoat stays in
the eye longer than the cohesive viscoelastics and even the
viscoadaptive Healon5."
Flexibility. Mark L.
McDermott, MD, professor of ophthalmology at Wayne State
University in Detroit, performs low to moderate volumes of
cataract surgery. He is a fan of Dr. Arshinoff's "soft shell
technique," and his viscoelastics of choice when using this
technique are Viscoat and Healon. "I can vary how much Viscoat
and Healon5 use, giving me maximum flexibility, and this
technique can be used in virtually every single cataract
surgery patient."
Cost. Richard L.
Lindstrom, MD, a managing partner in Minnesota Eye
Consultants, performs between 800 and 1,000 cataract surgeries
annually. "In the high-volume ambulatory setting, when costs
are a factor, we find that Occucoat (a dispersive) is a nice
viscoelastic that is relatively inexpensive, maintains corneal
clarity and is best for lubricating the IOL injector systems,"
Dr. Lindstrom said. "Of course, it is still necessary to have
a more premium viscoelastic such as Amvisc available when
doing difficult cases. But Occucoat performs very well in
routine surgeries."
And Now
Comes Healon5
Healon5 is a "viscoadaptive" that
takes on different properties depending on the flow rate in
the eye. At lower flow rates, with the consequent low
turbulence in the anterior chamber, it behaves like a
"super-viscous" cohesive. At a higher flow rate, it becomes
fracturable and mimics some of the properties of dispersives,
and so it is referred to as pseudo-dispersive.
So does
Healon5 truly represent the best of both worlds in a single
syringe? It depends on whom you ask.
"While Healon5 is
designed to incorporate the advantages of both cohesive and
dispersive viscoelastics," Dr. McDermott noted, "I don't think
any viscoelastic can accomplish this. When I have approaches
such as the soft shell technique available, and I don't have
to change the way I do surgery, I'm not sure there is that
much to be gained by using Healon5."
Dr. Chang noted
that Healon5 is dramatically different from all previous
viscoelastics. "Therefore, it requires a learning curve to
understand all of the properties," he said, adding that
Healon5 is excellent at maintaining the anterior chamber and
thus will be of benefit in any case in which there is
difficulty performing the capsulorhexis. "Healon5 is
particularly helpful for shallow chambers and small pupils."
Dr. Chang also likens Healon5 to working with
"transparent wet cement" because it has a greater tendency to
push and displace tissue compared with previous viscoelastics.
"Unlike other viscoelastics, Healon5 is so retentive that it's
difficult to burp out of the eye. As a result, one has to be
very careful not to overfill the chamber and not to inject it
in such a way that it pushes a capsular tear radially. So
while it is a great tool, the extremely high viscosity of
Healon5 can create some problems if it is used incorrectly."
Dr. Chang said that he still prefers Viscoat for his routine
cases.
Dr. Arshinoff, who has worked with Pharmacia on
developing Healon5, has come up with a slightly modified
version of the soft shell technique to take advantage of the
unique properties of Healon5 (see box). Thus, while Healon5
has some remarkable features–and Dr. Arshinoff continues the
quest for improvement–cataract surgeons seem to be embracing
this new viscoelastics cautiously. The bottom line: The choice
of a viscoelastic remains a matter of individual
preference.
Note: In
some journals, the term OVD (ophthalmic viscosurgical device)
is used instead of viscoelastic.
Soft Shell Technique
Original Soft Shell Technique
In the soft shell technique, the dispersive and
cohesive viscoelastics are used sequentially, concentrically
placed in adjacent spaces within the anterior chamber, without
mixing during surgery. A cohesive in a confined space will
transmit pressure through adjacent fluids. This will
pressurize the entire space, thus transmitting the beneficial
properties of cohesives throughout the entire area. A
dispersive will maintain its characteristics when subjected to
moderate pressure from an adjacent cohesive fluid, thus
preserving the properties of dispersives in the area where
needed.
However, it requires two separate syringes of
viscoelastics instead of a single syringe, thus increasing
cost and inconvenience.
Ultimate Soft Shell Technique
This uses Healon5 and balanced salt solution (BSS).
"This takes advantage of the extremely high viscosity of
Healon5 to permit the use of BSS as the dispersive
viscoelastic," Dr. Arshinoff explained.
According to
Dr. Arshinoff, the advantage of the ultimate soft shell
technique is that Healon5 "creates greater stability for
[performing] a capsulorhexis and [implanting] an IOL, compared
to other ophthalmic viscoelastic devices, thus decreasing the
incidence of errant capsular tears; and the BSS layer just in
front of the lens creates a low-viscosity environment to
facilitate movement of instruments in the anterior chamber,
making the capsulorhexis easier than with any other
viscoelastic system."
Dr.
Arshinoff is a paid consultant to many viscoelastic
manufacturers, including Pharmacia; Dr. Chang has no financial
interest in any of the products mentioned; Dr. McDermott has
received support in the past from Alcon; Dr. Lindstrom is a
consultant for Bausch & Lomb.
top
| |