Viscoelastics, also referred to as OVDs (ophthalmic visco-surgical devices), are viscous substances that allow us to make phaco-emulsification easier and safer. While there are many viscoelastics available on the market, there are two main classes: dispersive and cohesive. What are the differences and when do you use each? Which is better for each step of the surgery?
There are two main classes of viscoelastics, dispersive and cohesive, and they behave differently. There are also combination OVDs and visco-adaptive OVDs, but their use is not as common for routine cataract cases.
Dispersive OVDs have the consistency of honey, 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. Since it is more liquid than solid, it is also a good choice for lubricating the lens injector cartridge. The downside to a dispersive OVD is that it is not as good at maintaining a space within the eye and it is more difficult to remove at the end of the case. Examples of dispersive OVDs include: VisCoat (Alcon), EndoCoat (J&J), and OcuCoat (B&L) — note that these all have the word “Coat” in the name since dispersive viscoelastics coat the eye like honey.
Cohesive OVDs are more solid than liquid, and they have the consistency of gelatin, which means that they cannot coat or flow as well. However, because they are much thicker, they are able to maintain space and pressurize the eye quite 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. These agents are easier to remove from the eye at the end of surgery because the entire bolus of OVD is cohesive and once part of it is pulled from the eye via the suction tip, the rest tends to follow. Examples of cohesive OVDs include: ProVisc (Alcon), Healon / Healon GV (J&J), and AmVisc (B&L).
There is a spectrum of viscoelastics beyond simply cohesive and dispersive OVDs. Different OVDs will fall at different spots along this spectrum which some combo agents being in the middle.
Combination OVDs tend to be in the middle of the spectrum of and may have some dispersive properties as well as some cohesive properties. For many surgeons, using a moderate OVD has the best of both as they are able to use it as the exclusive viscoelastic for the entire surgery. Examples of combo OVDs include DisCoVisc (Alcon) and AmVisc Plus (B&L).
Visco-Adaptive OVDs act differently under various fluidic parameters on our phaco platforms: at low flow they stay together and act as a cohesive viscoelastic while at high flow rates they will fracture and come out in pieces much like a dispersive viscoelastic. An example of a visco-adaptive OVD is Healon 5 (J&J) — with this agent be very certain to fully remove it from the eye since it can cause high pressure spikes which may necessitate a return trip to the OR for aspiration.
Dual OVD Packages: Other surgeons may prefer having two viscoelastic syringes, one cohesive and one dispersive, for each surgery. Examples of this include: DuoVisc (VisCoat + ProVisc, Alcon) and Healon Duet (EndoCoat + Healon, J&J). Since having two OVDs packaged together in one box is actually a patent which is owned by Alcon and has been licensed to AMO/J&J, we may not see other examples of this in the near future.
Ideal viscoelastic characteristics during each step of cataract surgery:
- Keep AC maintained and pressurized during main incision creation
- cohesive is best, dispersive works well also (I use dispersive)
- Maintain anterior chamber depth and keep the anterior lens capsule flat during capsulorrhexis creation.
- cohesive is best, but dispersive works well also as long as surgeon pivots within the incision (I use dispersive)
- Protect corneal endothelial during phacoemulsification
- dispersive is best because it will coat and protect the corneal endothelium
- Lubricate the IOL injector system
- dispersive is thinner and can lightly lubricate and coat the IOL injector
- Expand the empty capsular bag for IOL insertion cohesive
- cohesive works well to maintain space and keep the empty capsular bag open
- Removability at the end of the case
- cohesive agents are much easier to remove from the eye
We can see from this list that a good option is to fill the eye with dispersive viscoelastic at the beginning of the case and then use cohesive viscoelastic for IOL insertion and ease of removal.
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.
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.
We can even do the soft-shell technique where both a dispersive and a cohesive OVD are used to protect the cornea, flatten the anterior lens capsule, and keep the AC formed during surgery.
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. Even more great information is in the video below.