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Two kinds of viscoelastics play pivotal role in phacoemulsification
Syrupy coating of dispersive OVDs helps to protect the corneal endothelium, while thicker cohesive OVDs are able to maintain space and pressurize the eye.
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Viscoelastics, also referred to as ophthalmic viscosurgical devices, or OVDs, are viscous substances that allow us to make phacoemulsification easier and safer. Once the first incision is made into the eye, it has a tendency to collapse as the aqueous leaks out. The larger the incision, the greater the propensity for the eye to collapse and the greater the risk to the patient. By replacing the aqueous with a thicker viscoelastic, we can prevent the collapse of the eye.
Two main classes
 Uday Devgan |
There are two main classes of viscoelastics, dispersive and cohesive, and they behave differently.
Dispersive OVDs have the consistency of syrup or molasses, and they are able to flow like thick liquids. This gives dispersive OVDs the ability to coat ocular structures well, and this coating is not easily washed away by the flow of balanced salt solution during surgery. This coating of dispersive OVD is helpful to protect the corneal endothelium from the ultrasonic waves during surgery.
Cohesive OVDs are more solid than liquid, and they have the consistency of gelatin, which means that they cannot coat or flow well. However, because they are much thicker, they are able to maintain space and pressurize the eye well. This is useful to keep the anterior chamber formed, to keep the anterior capsule flat during capsulorrhexis creation, to move and manipulate iris or other tissues and to keep the empty capsular bag open for IOL insertion.
Ideal characteristics
It is important to understand that there is a spectrum of viscoelastics, and that a moderate OVD may have some dispersive properties as well as some cohesive properties. For many surgeons, using a moderate OVD has the best of both and they are able to use it as their exclusive viscoelastic for the entire surgery. Other surgeons may prefer having two viscoelastics, one cohesive and one dispersive, for each surgery.
At the beginning of surgery, when the viscoelastic is placed into the eye, the goal is to perform an exchange: Inject the OVD while the aqueous is forced out of the eye. This is accomplished by placing the cannula across the anterior chamber and injecting distally, thereby allowing the aqueous to be released out of the same incision.
Ideal viscoelastic characteristics during surgery:
- Maintain anterior chamber depth during capsulorrhexis creation.
To maintain space and keep the anterior lens capwsule flat, the cohesive viscoelastics are the most helpful during this step. - Protect corneal endothelial during phacoemulsification.
Because they have the ability to coat, the dispersive viscoelastics work well to keep the corneal endothelium protected during phaco. - Prevent iris prolapse during surgery.
The ability to pressurize and maintain space is best with a cohesive; therefore prolapse of iris tissue can usually be treated with it. - Lubricate the IOL injector system.
The thinner dispersive OVDs can lightly coat the IOL injector system and allow lubrication that will facilitate IOL delivery. - Expand the empty capsular bag for IOL insertion cohesive.
Again, to maintain space and keep the empty capsular bag open, the cohesive OVDs work well. Avoid the super-cohesive OVDs here, as they may be so solid that they can deflect the IOL.
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Images: Devgan U |
Removing the viscoelastic
At the end of surgery, it is important to thoroughly remove the viscoelastic from the eye. Otherwise it can block the trabecular meshwork, and the patient will experience high IOP. The dispersive viscoelastics can be harder to remove because they have a tendency to spread out and coat the ocular structures. The cohesive viscoelastics tend to stick together as a single mass and are therefore usually easier to fully remove.
Use of a viscoelastic can make phacoemulsification easier for the surgeon as well as safer for the patient. It is for this reason that is has become an integral part of our surgeries.
For more information:
- Uday Devgan, MD, FACS, is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills, and Newport Beach, California. Dr. Devgan is Chief of Ophthalmology at Olive View UCLA Medical Center and an Associate Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine. Dr. Devgan can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax: 310-388-3028; e-mail: devgan@gmail.com; Web site: www.DevganEye.com. Dr.Devgan is a consultant to Abbott Medical Optics and Bausch & Lomb, and is a stockholder in Alcon Laboratories and formerly in Advanced Medical Optics.