Spotlight: ON TECHNOLOGY AND TECHNIQUE
The Best of Both Worlds
Ideally, viscoelastics should behave in different ways at different times. Now, they can.
BY CHRISTOPHER KENT, SENIOR ASSOCIATE EDITOR AND STEVE A. ARSHINOFF, M.D., F.R.C.S.C.,TORONTO, ONTARIO

As you know, ophthalmic viscosurgical devices (OVDs), more commonly known as viscoelastics, are a key tool in many kinds of ophthalmic surgery, especially cataract removal. Replacing the aqueous in the anterior chamber with a viscoelastic serves a number of purposes, including space maintenance, pressurization, stabilization and protection of tissue from surgical trauma.

Viscoelastics are commonly categorized as either "cohesive" or "dispersive." The key rheologic agent in a cohesive viscoelastic is almost invariably sodium hyaluronate, which consists of long-chained molecules. This type of viscoelastic is called cohesive because hydrogen bonds tend to form between the molecular chains as they entangle. The result is a liquid substance that seems to cling to itself as a springy mass. Dispersives, on the other hand, usually consist of shorter-chained molecules that don't entangle as much. (Some people compare the former to spaghetti, and the latter to macaroni.)

Depending on the conditions you create, Healon5, from Pharmacia, can display the desirable characteristics of a cohesive or dispersive viscoelastic.

The cohesive/dispersive dilemma

The differences between cohesive (higher viscosity) and dispersive (lower viscosity) viscoelastics make them best suited for different aspects of surgery. For example, when removing viscoelastic from the eye, it's an advantage to have it behave like a cohesive. (Once a cohesive viscoelastic begins to move, it tends to stick together. This makes it very easy to remove, compared to a dispersive.) Unfortunately, this advantage becomes a disadvantage if the viscoelastic isn't well retained in the anterior chamber during surgery; once fluid flow is started, it tends to keep coming out.

Generally, the properties of a cohesive viscoelastic are most useful when you're inserting or removing the substance from the eye, creating and maintaining space, and inducing or counteracting pressure. Its properties become problematic during prolonged or difficult cases because the viscoelastic may not last as long as the surgery. On the other hand, the characteristics of a dispersive viscoelastic have the opposite benefits and liabilities. Dispersives tend to be most useful for managing certain types of complications.

Resolving the dilemma

Although it's possible to derive the benefits of both types of OVD by using two different viscoelastics during surgery, the simplest solution to the viscoelastic dilemma is a substance that displays different characteristics under different conditions that you can control during surgery.

Healon5, the new viscoelastic from Pharmacia, does exactly that. Depending on the flow rate of your phaco machine, it behaves like a cohesive viscoelastic or a fracturable pseudo-dispersive. (Because of this ability to change behavior under varying conditions, Pharmacia refers to Healon5 as a "viscoadapative.")

How is it possible for a substance to behave differently without an external environmental change such as temperature? The answer has to do with the concentration of sodium hyaluronate in Healon5.

Healon5 differs from Pharmacia's earlier Healon product primarily in that it contains a higher concentration of sodium hyaluronate. (The molecular weight is the same.) This increased concentration causes the high viscosity liquid to display some of the properties of a solid -- including becoming fracturable under stress. (This is the same effect seen when chocolate pudding is placed in the refrigerator.)

As a result, the degree of turbulence in the fluid environment of the OVD changes the characteristics it displays. If turbulence is low, it behaves like a cohesive; if turbulence is high, it behaves like a dispersive.

	The long molecular chains and high concentration of sodium hyaluronate in Healon5 result in excellent visibility during surgery.

Clinical advantages

According to Pharmacia, in addition to making it possible to change characteristics, the unusually high viscosity of Healon5 has numerous beneficial effects during surgery:

•  Increased stability. Because it becomes supercohesive in the eye:

  • It stays in the anterior chamber throughout surgery (as long as turbulence remains low).

  • It stabilizes tissue more effectively than other viscoelastics. For example, it reduces the risk of iris prolapse by preventing the iris from moving.

  • It maintains inflation of the bag.

• Greater control. Healon5's high viscosity:

  • A results in a firmer globe with more resistance, causing a crisper, more controlled incision and wound construction

  • A flattens the anterior capsule, which increases control
    during capsulorhexis and helps to reduce the risk of peripheral extension

  • A holds the nucleus in place, giving you greater control if you use the "phaco flip" technique

  • A enhances control during IOL placement.

• Less waste. You need smaller quantities of Healon5 than of most viscoelastics because of its high viscosity and rigidity.

• Excellent visibility. The longer molecular chains and higher concentration of sodium hyaluronate result in greater entanglement and superior transparence. Lens particles and bubbles also move more freely, so they don't become trapped and interfere with visibility.

• Less cellular damage. Using Healon5 results in minimal loss of endothelial cells following surgery. That's because its high viscosity and molecular weight give the fluid elasticity. It absorbs energy by distorting its shape when impacted, and then resumes its previous shape (sort of like a rubber bumper), protecting the surfaces inside the eye. This minimizes damage from instrument or nuclear fragment contact, cushions the shock of the unfolding lens, and protects surfaces from fluid turbulence.

Safety first

Healon5 is FDA approved. However, because of its unusual characteristics -- including its high viscosity and high concentration of sodium hyaluronate, which can cause postoperative transient elevations of intraocular pressure if left in the eye -- Healon5 requires a modified removal technique. (In fact, in cases where complete removal of the viscoelastic is likely to be difficult, Healon5 may not be the best option.) For this reason, Pharmacia requires that you take a special CME course before using Healon5. The company considers this a prudent way to introduce the product.

The CME course provides basic information about Healon5 and instruction in removal techniques. It also describes other procedures and techniques that are enhanced by the use of Healon5, such as "viscomydriasis" (using it to expand small pupils) and the phaco flip technique.

Taking the next step

Healon5 is available in 0.6-ml syringes. (According to the company, the increased viscosity and rigidity of Healon5 makes this amount more than sufficient for routine cataract surgery.) Refrigeration is required.

For more information about Healon5, or to arrange to take Pharmacia's CME course, contact your local Pharmacia representative.

Steve Arshinoff, M.D., FRCSC, practices at Humber River Regional Hospital in Toronto, where he specializes in cataract and refractive surgery. He coined the viscoelastic terms "cohesive" and "dispersive," and is a consultant to Pharmacia.