The Basics of Choosing and Using a Viscoelastic

Types, behaviors and tips on using OVDs.

BY LESLIE GOLDBERG, ASSOCIATE EDITOR

Since their introduction in the early 1970s, ophthalmic viscosurgical devices (OVD) have revolutionized cataract surgery as well as IOL implantation. OVDs serve to maintain a deep anterior chamber during anterior segment surgery, allowing reduced trauma to the corneal endothelium and surrounding ocular tissues. They also help to push back the vitreous face in the event of a posterior capsule tear or a rent in the zonules and prevent formation of a flat chamber during surgery. This article will discuss the different types of viscoelastics, their behaviors and clinical tips for using viscoelastics appropriately.

Important Properties of Viscoelastics

There are essentially four different properties to mention when discussing OVDs, says Kevin Miller, M.D., professor of clinical ophthalmology at the Jules Stein Eye Institute at the University of California, Los Angeles. They are:

► Elasticity, or the tendency of a viscoelastic to go back to its original shape after it is deformed or stretched.

► Viscosity, which is the measure of resistance to flow (resistance depends on the rate of flow, or shear rate).

► Pseudoplasticity, or the ability of a viscoelastic to transform under pressure from a gel to a liquid substance (agents that are very pseudoplastic tend to have a very low viscosity when they are being forced through a cannula at high speeds; however, this property is not very important to surgeons unless they are using hydroxypropyl methyl cellulose, a substance with very low pseudoplasticity).

► Cohesiveness/dispersiveness, or whether the agent adheres to itself or to surrounding tissues.

Cohesives vs. Dispersives

Dr. Miller prefers to evacuate his viscoelastic quickly once his case is done. He says that when removing a dispersive, "it just nibbles and fractures off and the surgeon needs to chase after it. When you grab one little strand of cohesive, everything just comes to you."

Additionally, he does not want to worry about IOP spikes. "You are more likely to get all of the OVD out and avoid IOP spikes with a cohesive," he says. "My corneas are not as pristine at the end of a case as they are when I have used a dispersive. Surgeons who choose to use a dispersive tend to have patients whose eyes see a little bit better the next day but whose IOPs are higher. They spend more time in the eye cleaning their OVD out, but probably worry less about causing trauma to the cornea."

Types of Viscoelastics

Dr. Miller says that almost all surgeons start out using a basic viscoelastic like Healon (Advanced Medical Optics [AMO], Santa Ana, Calif.), a cohesive OVD, or Viscoat (Alcon, Fort Worth, Texas), a dispersive. "Healon creates good space for surgeons and comes out of the eye very quickly. In clean cases, where there is not a lot of phaco time or turbulence and the cornea looks healthy, the majority of surgeons will use a cohesive agent like Healon, because these cases tend to go quickly," he says.

Dr. Miller says that, while Viscoat takes a little longer than Healon to remove from the eye, many surgeons prefer Viscoat because it protects the cornea better. "However, Viscoat does not maintain space as well as Healon and the clarity is not as good during the surgery," he says. "Dispersives, because they stick to things, also trap air bubbles. This results in the air bubbles obstructing the view during surgery."

When choosing an OVD, Dr. Miller says that it is up to the surgeon to decide whether he/she wants (1) a cornea that that is not as clean at the end of the case, but has no IOP or visibility issues, as with a cohesive, or (2) a cleaner cornea postop day 1, but with possible IOP elevation. Additionally, a dispersive OVD may make surgery more difficult because visibility is not as good as with a cohesive. He says for basic surgeries, either a cohesive or dispersive is fine — it's the special cases that require a specific choice.

Special Cases

For more difficult cases, Dr. Miller recommends a more careful approach when choosing an OVD. Such cases include:

■ Compromised corneas. Surgeons should choose a dispersive OVD for maximum corneal protection.

■ Very shallow interior chambers. A cohesive OVD is best for such cases because it is better at maintaining space. Dr. Miller recommends using Healon GV or Healon 5 (AMO).

■ Small pupils/floppy irises. For opening up the eyes, a highly cohesive viscoelastic works best, such as Healon 5.

■ Dense cataracts. A dispersive agent, such as Viscoat, works best for these cataracts. The OVD must be reapplied throughout the case to keep the cornea coated. Reapplication should be utilized for compromised corneas as well.

DisCoVisc (Alcon), a viscous dispersive OVD, provides space maintenance for cataract surgery and easy removal during aspiration. "Currently, I use DisCoVisc when I might otherwise have to use two separate vials of viscoelastics such as Viscoat and Healon GV. In that scenario, one vial of DisCoVisc is cheaper, even at the current higher price," says Dr. Miller.

Clinical Pearls for OVDs

Below is a list of practical tips from Dr. Miller to consider when using an OVD.

► When preparing to coat the eye with a viscoelastic, the surgeon should let some of the viscoelastic out of the cannula beforehand to keep air bubbles from entering the eye. Dr. Miller recommends that air always be ejected from the cannula before entering the eye.

► When first filling the eye with OVD, surgeons should move all the way across the anterior chamber and start injecting from the opposite side and backfill the anterior chamber. This will push all of the aqueous humour out of the eye.

What some surgeons do, says Dr. Miller, is go into the incision and start injecting OVD. This fills at the incision and the OVD starts spreading across the eye. The aqueous humour gets trapped in the eye and the IOP rises at the paracentesis and the viscoelastic starts flowing out.

► Surgeons should fill the anterior chamber completely to eliminate the "squiggly lines" seen in the viscoelastic. This is truer of the dispersives than the cohesives, says Dr. Miller.

► For infants and children, surgeons should overfill the anterior chamber to flatten the anterior surface of the lens. This will provide increased control over the capsulorrhexis.

► When using Healon GV during the hydrodissection, take note of the subincisional iris, says Dr. Miller. As the fluid is filling the capsule, IOP may rise and iris prolapse can occur. In a case of iris prolapse, stop the hydrodissection and start the phaco. This will remove a lot of the viscoelastic, making it possible to go back and restart the hydrodissection, says Dr. Miller.

► With Healon 5, before hydrodissection is started, create a tunnel through the viscoelastic so that the aqueous injected for hydrodissection has a way to get out. Otherwise the pressure will increase greatly and the iris may prolapse.

"Healon 5 is in a category all its own, because it is so highly cohesive," says Dr. Miller. "Healon 5 works really well if you need a lot of space and you want a big pupil, but it has a huge learning curve and is really meant for special cases."

► For Viscoat, if the bubbles get to be too much, evacuate the bubbles with an I/A probe and recoat the endothelium to improve clarity.

► If the surgeon is losing control of the capsulorrhexis, the best thing to do is stop and refill the anterior chamber or overfill the chamber with more OVD. This will help the surgeon to gain control, says Dr. Miller.

"Lastly, a special mention should be made of Steve Arshinoff, M.D.'s contribution," says Dr. Miller. "He uses a technique that combines a dispersive and a cohesive viscoelastic agent called the soft-shell technique. This is really effective for difficult cases. The technique helps to achieve good corneal protection and good space at the same time."

Lesson Learned

Dr. Miller says that OVDs have revolutionized cataract surgery because they have leveled the playing field. "It has made mediocre surgeons look like great surgeons," he says. He points to Healon, the first OVD on the market, in particular.

Dr. Miller says that many surgeons reach a comfort level with the product they are using. "It is easy to resist change, especially since newer agents tend to cost more," says Dr. Miller. However, he advises that surgeons should try newer OVDs to determine what works best for each individual case. OM

Kevin Miller, M.D., is professor of clinical ophthalmology at the Jules Stein Eye Institute at the University of California, Los Angeles. Dr. Miller has no financial ties to any of the products mentioned in this article.