Insights on Viscoelastics Use

Factors to consider when selecting an OVD

By Samantha Stahl, Assistant Editor

Back in the 1960s, ophthalmic surgeons had few options to protect patients' eyes during cataract surgery; corneal edema was almost a given. Endre Balazs, MD, however, came along in the 1970s and changed the conversation. After patenting a process for purifying hyaluronic acid, a substance he called Healon, Dr. Balazs sent a few vials over to Dr. David Miller, who used the gel-like material to protect rabbits' corneal endothelium while implanting IOLs. The rest, as they say, is history. The FDA approved Healon in 1980.

The ophthalmic viscosurgical devices (OVDs) of today bear little resemblance to the Balazs-era viscoelastics. When Balazs's original patent expired in the late 1990s, manufacturers jumped at the chance to give the product a facelift, introducing profoundly helpful new features. This article will highlight the options on the market today and offer advice on how to select the right viscoelastics.

Prospective Properties

There are a few important physical properties to understand when it comes to viscoelastics. An OVD's molecular weight, measured in Daltons, changes its cohesion and clarity. As the weight increases, the viscoelastic will have higher cohesion and do a better job at maintaining space. The catch, however, is that clarity drops. Inversely proportional, as the viscoelastic's weight drops its clarity increases.

Each product has two viscosities—one at zero shear and one when it is under shearing force. A high or low aspiration flow rate can influence how a viscoelastic acts. At a high shearing force, a cohesive product will be pulled out of the eye within a minute.

Other factors to consider when selecting a product include storage, bottle size, cost and availability. Most viscoelastics require constant refrigeration, but practices that can't commit to investing in cooled storage should try a product like Bausch + Lomb's OcuCoat, a low-weight dispersive. Alan Aker, MD, of Boca Raton, Fla. recommends OcuCoat for soft cataracts with low concern for corneal damage. If the patient requires endoscopic cyclophotocoagulation, he'll also use Bausch + Lomb's Amvisc. The product comes in 0.5-ml vials, an ideal size for this purpose. Meanwhile, he appreciates the 1-ml vials of Alcon's Discovisc for pseudophakic patients who require a larger amount of product to maintain the anterior chamber during surgery.

For routine cases, Robert Weinstock, MD, of Largo, Fla. says a single 0.8-ml cohesive vial like Bausch + Lomb's Amvisc Plus is adequate for the entire case. “It coats the endothelium well, has great clarity, maintains the chamber during capsulorhexis, fills the bag well prior to lens insertion, and evacuates easily and completely at the end of the case. It is also very cost-effective and efficient in the OR.”

Figure 1. A cohesive viscoelastic provides tremendous clarity and allows the surgeon to see the capsule clearly for the capsulorhexis.

Since many cases require a combination of viscosities, Dr. Aker finds that Alcon's Duovisc is a cost-effective product, as it provides both a dispersive (Viscoat) for tissue protection and a cohesive (Provisc) for capsular bag expansion and easy cleanout. He uses this for all his premium lens implantations and also for patients with miotic pupils, compromised corneas and those on Flomax. Dr. Aker says that in these cases, it is important to select a viscoelastic that “minimizes risk to the patient and provides a greater level of protection to the endothelium.”

Cohesives or Dispersives? That is the Question

Michael Colvard, MD, of Encino, Calif., says that a surgeon should be able to look at the molecular weight of a viscoelastic and be able to predict how it's going to perform. Products with longer molecular chain length and higher molecular weight tend to behave as a cohesive agent, while products with shorter chain length and lower weight behave as a dispersive agent.

Figure 2. A dispersive viscoelastic can be used to break posterior synechiae.

Cohesives, explains Dr. Colvard, behave like a jelly when they're under zero shear. Excellent for maintaining space, they work well within the eye for IOL implantation. “Dispersives tend to be like honey at zero shear. They're a little runnier and don't maintain space as well. However, under high shear during irrigation and aspiration, they tend to stay in the eye, coat tissues and protect the cornea,” Dr. Colvard says. “For a dense nucleus or eyes with endothelial dystrophy, I like to use Alcon's Viscoat to protect the corneal endothelium.”

Dr. Colvard uses Healon for routine cases because the easily-removed cohesive is less likely to be associated with pressure spikes postop and is ideal for IOL implantation. He's a fan of Healon 5 for difficult procedures because he believes it is the most highly cohesive product on the market. He uses it when he encounters floppy iris to maintain pupillary dilation, but warns that use of the cohesive requires a low flow rate and vacuum, and the nucleus must be disassembled gently or you run the risk of the product leaving the eye too soon.

Rosa Braga-Mele, MD, associate professor at the University of Toronto and director of the cataract unit and surgical teaching at Mount Sinai Hospital, Toronto, says that not every viscoelastic is right for every case. “I think it is important to have a variety of products on call and be versatile in what you use because not every case is the same,” she says.

For general ophthalmology, Dr. Braga-Mele says she turns to a higher-end cohesive, like AMO's Healon GV because it maintains space well for IOL implantation and reduces pressure spikes, since the product is easy to remove. “For small pupil, shallow chambers or IFIS eyes, I like a supercohesive viscoelastic like Healon 5. It compartmentalizes the anterior segment, so I can control the iris and get the largest and most stable pupil possible with a significant deepening of the anterior chamber over other viscoelastics,” she says. “I will use Healon 5 on its own or with a dispersive viscoelastic for a donut effect—I'll lay the Healon 5 over the iris and then place a small amount of dispersive viscoelastic in the center of the pupil to help keep the Healon 5 stable.” This technique, she reports, was described initially by Robert Osher and Steve Arshinoff.

Combating Complications

Yara P. Catoira-Boyle, MD, an assistant professor of clinical ophthalmology at the Indiana University School of Medicine, frequently uses Alcon's Ex-Press mini glaucoma shunt for her trabeculectomies. During these cases, she doesn't see a need for viscoelastics because, unlike traditional filtering procedures with large sclerotomies, the small, controlled Ex-Press tube size allows the right amount of fluid to flow out of the eye. However, if she finds that eye pressure dips too low or the anterior chamber goes flat during the post-op period, she fills with viscoelastic.

“AMO's Healon GV flows well through the Ex-Press if there is excessive flow that needs to be reduced in the early postoperative period. You probably will get about three days of fill. Healon 5, in my opinion, can cause IOP spikes due to higher viscosity,” says Dr. Catoira-Boyle.

Perhaps the most common cataract surgery complication is rupture of the posterior capsule. Satish Modi, MD, of Poughkeepsie, NY, says that when this occurs, you don't want to immediately take the phaco tip out of the eye. Instead, he recommends that you maintain pressure and use a dispersive like Alcon's Viscoat that will tamponade the hole and push the vitreous back.

He says Viscoat also comes in handy for toric implants, which must be inserted at a specific angle. Once the capsular bag is emptied of OVD, the IOL still has to be positioned, an uncomfortable sensation for patients. “With the lens close to the iris, patients tend to feel the pressure of moving the IOL. They could squeeze their eye and have a posterior capsular rupture,” he says. “If an implant is not moving easily, put Viscoat anterior to the haptic. Just that little dollop will allow you to move it without fear of capsular rupture or postoperative IOP spikes.”

Will Femtosecond Change the Game?

As femtosecond laser cataract surgery increases in promise and allows for procedures with less phaco energy, should viscoelastic manufacturers be worried about becoming obsolete? Dr. Modi says no. Traditional phacoemulsification “is like a microscopic jackhammer going into hard mud with pieces flying around,” making a protective agent crucial for maintaining clarity and protecting the corneal endothelium. However, because femtosecond lasers can soften a nucleus, the needed amount of viscoelastic will drastically reduce. But Dr. Modi insists that it will still be just as important to protect the corneal endothelium during nuclear removal and to open up the capsular bag when inserting the implant.

He argues that surgeons will still need two types of OVDs, and compares each viscosity to pasta. For maintaining space, he likens a cohesive OVD to “a bowl of spaghetti—you can stick a fork into it, twist it around, and take the whole bowl out with one swoop.” Protective dispersives, on the other hand, are like macaroni. “With one fork you can only pick up about eight pieces at a time. A dispersive fragments when you try to remove it, but protects the endothelium,” he says.

So which will be on the menu for you, spaghetti or macaroni? OM