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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.

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