Several years ago, the Balazs Healon patent expired. Since then, manufacturers have made numerous innovations in the field. In this article, we'll discuss the different viscoelastics that are now available, and the characteristics and advantages each one offers.
Maintaining a deep chamber
Akorn's BioLon has been used in more than 1 million eyes worldwide since 1993. It's a highly cohesive viscoelastic with a molecular weight of 3 million daltons, containing 1.0% sodium hyaluronate 10 mg/ml.
Indications as a surgical aid include cataract extraction (it protects the corneal endothelium and surrounding tissues), intraocular lens (IOL) implantation and anterior segment surgery. When introduced in the anterior segment of the eye during these surgical procedures, BioLon serves to maintain a deep anterior chamber.
Additionally, BioLon helps to push back the vitreous face and prevent formation of a postoperative flat chamber.
To reduce waste and keep costs down, BioLon comes in two economical sizes: 1.0 ml, to help you avoid starting a second syringe, and 0.5 ml, when a smaller quantity is required.
Akorn touts several features of the BioLon syringe. First, its finger grip allows you to rotate it in the anterior chamber, meaning you don't need to withdraw and re-insert the cannula to change its position. Second, its shaped cannula eliminates air bubbles, giving you a clear view. Third, to help you ensure sterility, BioLon's packaging holds the syringe firmly in place.
BioLon is also available from Aller-gan, a licensed distributor.
"In my hands," says Richard Fich-man, M.D., Farmington, Conn., "the adhesive viscoelastic AMO Vitrax has a few properties I enjoy. Number one: it has great clarity, great visualization; it doesn't have a spaghetti-like quality. Number two: It doesn't need to be refrigerated, which makes it easier to work with. And number three: It comes in just the right quantity -- 0.65 ml. So it's very rare that I have to request a second vial of Vitrax."
AMO Vitrax's low molecular weight is designed for phacoemulsification and small-incision surgery. Its viscosity and adhesiveness maintains anterior chamber depth to give you safe and ample working room.
Says Henry Alvarez, M.D., Miami, Fla., "Vitrax slows down my haptics, so I can watch them go into the capsular bag, and I'm sure where they are. Addition- ally, Vitrax protects the capsular bag while those haptics are expanding.
"And should you run into a situation where you need to place a little bit of viscoelastic behind the nucleus during phacoemulsification -- in other words, a 'sandwiching technique' -- it'll stay there during that 'sandwiching technique,' and won't be aspirated out."
One criticism of AMO Vitrax concerns the difficulty in removing it during aspiration. But Paul Mitchell, M.D., Marietta, Ga., sees advantages to this characteristic. "Vitrax adheres well to the corneal endothelium. At the end of phacoemulsification and irrigation/aspiration (I/A), you'll still find the layer of the viscoelastic in the anterior chamber angles, protecting the entire cornea."
Dr. Fichman disregards this criticism as well. "Evacuating all of the Vitrax takes less than a minute."
In the February 1999 issue of Ophthalmology Management, the cover story, "Picturing the Worst," offered insights into how clear cornea surgeons overcame surgical mistakes.
In that article, R. Bruce Wallace III, M.D., suggested adding a second viscoelastic, such as Bausch & Lomb's Amvisc Plus, during procedures with patients who exhibit more eye movements than most. Dr. Wallace said Amvisc Plus helps to control the advancement of the capsular tear under topical anesthesia.
But this isn't the major advantage to Amvisc Plus. According to Bausch & Lomb, Amvisc Plus is the only viscoelastic in the current market to work well through all phases of the phacoemulsification process. Amvisc Plus is made with a moderate chain length polymer, 1.5 daltons, which produces moderate cohesion. Thus, Amvisc Plus will resist
complete removal during phaco.
Although more Amvisc Plus material will be removed than that of a dispersive agent, an amount sufficient for protection remains. During clean up, however, high levels of vacuum are used. In this environment, Amvisc Plus is easily removed.
This moderate cohesion offers other benefits, as well:
- A relatively high pseudoplastic curve for easy IOL implantation.
- High viscosity, which holds back forward vitreous pressure, facilitating capsulorhexis.
- The viscoelastic stays in the chamber throughout phacoemulsification, providing consistent endo- thelial cell protection.
Also, Amvisc Plus is available in two syringe sizes: 0.5ml and 0.8ml.
Alcon's market research shows that 50% of physicians prefer a cohesive viscoelastic while 50% prefer a dispersive. That's why Alcon offers DuoVisc, a viscoelastic system to satisfy both worlds. DuoVisc contains one sterile syringe (0.40 ml) of Viscoat and one sterile syringe (0.40 ml) of ProVisc.
Viscoat, a dispersive, is used at the beginning of surgery to maintain space and protect the corneal endothelium. It has three noteworthy properties:
- low surface tension
- low degree of pseudoplasticity
- low-to-medium molecular weight.
At the other end of the spectrum is the cohesive ProVisc. Obviously, this viscoelastic material adheres to itself. Its properties include:
- high molecular weight
- high pseudoplasticity
- high surface tension.
ProVisc also possesses the advantages of manipulating tissue and facilitating IOL implantation.
One ophthalmologist who can attest to the advantages of DuoVisc is Donald Serafano, M.D., Los Alamitos, Calif.
He's been using DuoVisc for several years and prefers it for phacoemulsification procedures.
"First, I apply Viscoat near the endothelium, then I put ProVisc on the anterior capsule; this pushes the Viscoat closer to the endothelium. A continuous curvilinear capsulorhexis is then performed under the ProVisc.
"Prior to beginning ultrasound, I can aspirate some of the ProVisc with the phaco tip. This establishes good flow in the tip and helps to prevent the possibility of the tip heating up, causing a corneal burn. The aspiration of the ProVisc doesn't disturb the Viscoat and leaves the Viscoat against the endothelium during phaco."
Alcon suggests DuoVisc should be allowed to attain room temperature 20 to 40 minutes prior to use, depending on your desired quantity.
Back in 1979, Healon splashed onto the viscoelastic scene. Since then, Pharmacia & Upjohn has taken cohesives to the next level.
Healon GV has a molecular weight of 5 million daltons, compared to its predecessor's 4 million daltons. Consequently, Healon GV has the highest molecular weight of any viscoelastic currently on the U.S. market.
This extremely high viscosity rate gives you optimum protection, thus enhancing space creation and maintenance. Also, Healon GV expands the anterior chamber, flattens the anterior capsule to facilitate capsulorhexis, protects endothelial cells by binding to hyaluronic acid binding sites on the corneal endothelium and -- despite Healon GV's high viscosity -- provides enhanced optical clarity for a clear, unobstructed view of the surgical field.
Obviously, Healon GV is designed for foldable IOL surgery. Its advantages include ease of injection and greater shock absorption.
A word of caution, though. At the end of the procedure, you must be aggressive in removing Healon GV. The potential for high-pressure spikes is likely, due to Healon GV's high viscosity. When removing it, you need to get behind the lens and push down on all four quadrants.
Meanwhile, Pharmacia & Upjohn is currently working toward introduction of its next product into the U.S. viscoelastic market, Healon 5.
I. Howard Fine, M.D., of Eugene Ore., has been working with the material. "I'm part of the investigational study for Healon 5. This viscoelastic promises to be both cohesive and dispersive."
Ophthalmology Management, Issue: April 2000