Choosing a Viscoelastic for Implantation of Phakic IOLs
A cohesive product is recommended, but consider these rationales for just how cohesive your choice should be.
BY ROCHELLE NATALONI, CONTRIBUTING EDITOR

Surgeons agree that the use of a cohesive -- as opposed to a dispersive -- viscoelastic is the optimum choice for phakic IOL implantation, but just how cohesive the viscoelastic needs to be remains a matter of debate.

The range of available viscoelastics from dispersive to cohesive includes Viscoat, Ocucoat, Provisc, Amvisc, Amvisc Plus, Healon, Healon GV and Healon 5, with Viscoat being the most dispersive, and Healon 5 being the most cohesive. Cohesiveness is just one of the physiochemical properties that govern the behavior of viscoelastics in ophthalmic surgery. Others are elasticity, pseudoplasticity, surface tension and, perhaps most importantly with respect to phakic IOL implantation, molecular weight and viscosity, which affect the viscoelastic's ability to maintain space.

The Case for High Viscosity

Kerry K. Assil, M.D., likes to use a high-viscosity cohesive viscoelastic, such as Healon GV, for phakic IOL surgery. The main rational for using a higher-viscosity viscoelastic, Dr. Assil pointed out, is the need for the chamber to be sufficiently formed throughout the procedure. "You're doing all the surgery in the anterior chamber as opposed to cataract surgery where we have much more working space," he said. "When all the surgery is done in the anterior chamber it's more challenging to stay away from the endothelium. That's why the high-viscosity viscoelastic makes sense." Dr. Assil is the medical director of the Assil Sinskey Eye Institute in Santa Barbara, Calif.

He has used the whole spectrum of viscosity levels ranging, using one product line as an example, from as low as Healon to as high as Healon 5. To put that range in perspective, he says Healon 5 is like "papa bear," and Healon is like "baby bear." Healon GV, he says, is like "momma bear." "It's just right."

Healon GV offers sufficient viscosity to maintain the shape of the eye without the problems associated with Healon 5, Dr. Assil pointed out. "If you were to use Healon 5 and any of it remained behind in the eye it would cause tremendous problems with pressure spikes," he said.

Another reason a highly viscous viscoelastic is the right choice, says Dr. Assil, is that in phakic IOL surgery, the viscoelastic is solely responsible for maintaining the shape of the eye. "When we're doing cataract surgery the chamber stays formed more readily than it does during phakic IOL surgery because in cataract surgery we're infusing liquid into the eye, he said. "A highly viscous viscoelastic will maintain the shape of the eye, whereas a low-viscosity viscoelastic tends to squirt out," he explained.

Other Options

John D. Hunkeler, M.D., prefers a lower-viscosity cohesive viscoelastic for phakic IOL implantation. His preference, he says, is based on reducing the likelihood of postoperative pressure spikes that could arise if any higher-viscosity formula remains in the eye. Further, he recommends using temporary 8-0 silk suture of the 6-mm incision to help maintain anterior chamber depth throughout the procedure. "Rather than using a higher molecular weight viscoelastic, I use two temporary sutures to close the primary incision while I secure the implant to the iris with the technique called enclavation," he said. "The temporary suture reduces viscoelastic escape from the anterior chamber. There is a trade-off if you use a higher molecular weight. It is going to manipulate the tissue greater and create some increase in the space, but you can achieve a similar result using the temporary silk sutures just as well." Dr. Hunkeler is head of the Hunkeler Eye Institute in Kansas City.

While operating in the anterior chamber, one must achieve a delicate balance, Dr. Hunkeler pointed out. "With the normal crystalline lens in place, the anterior segment is more crowded than what we are accustomed to with implantation of a pseudophakic IOL in cataract surgery. The iris is still there and the implant and the instruments that we are using to enclave the iris to the implant," he said. "Instead of having the large space to work in cataract surgery, you're working in a much more confined area. Dispersive viscoelastics are difficult to remove from the anterior chamber angle and you run the risk of having an elevated intraocular pressure postoperatively. Besides being uncomfortable for the patient and blurring vision, a dramatic rise in intraocular pressure can lead to iris atrophy. The bottom line is that you need to get the viscoelastic out, and it's a lot easier to irrigate and aspirate and remove the vast majority of the material when you're working with a cohesive viscoelastic," he said.

The question of how cohesive the viscoelastic should be comes down to molecular weight, according to Dr. Hunkeler. "Those with the heaviest molecular weight, such as Healon 5 and Healon GV, will create a huge amount of space for you and give you a lot of working room, but those 'big molecules' tend to hide out in the angle and because of the limited amount of space in the anterior chamber it's much more difficult to get the material out. The smaller-chain viscoelastics are probably better and should be sufficient to do the procedure," he said. Dr. Hunkeler has used Amvisc, Amvisc Plus, Healon and Vitrax. "They all seem to work very well and so far I have no significant preference when used in conjunction with the temporary sutures."

The temporary sutures, he says, enhance the effectiveness of the viscoelastic. "The sutures close down the size of the 5.5-mm to 6-mm incision so the viscoelastic won't come out," he said. "I hold onto the implant optic and gather the iris and enclave it to the haptic of the lens implant. This is where the viscoelastic helps prevent damage to the cornea, the iris and the crystalline lens."

Additional Considerations

Dr. Hunkeler recommends the use of bimanual irrigation and aspiration after the IOL is implanted. "A bimanual procedure ensures that you have a good fluid flow into the anterior chamber and can remove the viscoelastic as completely as possible," he said. "I think this is a particularly effective place for bimanual irrigation and aspiration."

Another pearl from Dr. Hunkeler is to avoid moving to topical anesthesia in phakic IOL implantation until you feel your technique will allow you to do so. "Certainly for the beginning srugeon, following the protocol I described is the safest way to move forward with the procedure," Dr. Hunkeler said. "Doing this under peribulbar anesthesia to minimize the possibility of iris and corneal trauma may be in your best interest. Until you become very experienced, trying to do the procedure under topical anesthesia is fraught with some potential difficulty."

Dr. Hunkeler said the novice would do well to stick with a lower molecular weight viscoelastic in combination with enclavation, and perhaps graduate to the use of a more viscous formula once he or she has conquered the learning curve. "I would discourage using a higher molecular weight viscoelastic until you are very comfortable implanting these IOLs and ready to try something a little different," he said.

Once the implant is secured to the iris, Dr. Hunkeler closes the primary incision with permanent polypropylene sutures after having removed the viscoelastic with the temporaty sutures in place. "Hydrating the two paracentesis sutures is adequate to get excellent self-sealing closure," he said. "Three sutures seem adequate to close the incision satisfactorily. Attention needs to be paid to the tension on the sutures to control astigmatism."

Dr. Assil formerly employed the enclavation technique, as well, but has since changed his approach. "The problem with that approach is that sometimes by the time you finish manipulating everything you've stretched the sutures lose again, so you end up suturing the wound closed twice," he said.

Rather than use enclavation to avoid the possibility of IOP rise associated with higher-viscosity viscoelastics, Dr. Assil uses a two-port irrigation and aspiration system. "This helps to ensure that I get all the viscoelastic out at the end of my surgery," he said. "After I've closed the wound, I go into the two paracentesis sites sticking in an irrigation port through one and an aspiration port through the other, and I aspirate out all of the viscoelastic."