The large variety of available OVDs allows the surgeon to tailor the surgery to meet any specific needs a patient may have.
Phacoemulsification presents several opportunities for endothelial cell damage, including fluidic friction, localized temperature increases, contact or collision of lens nucleus fragments during the irrigation process, inadvertent instrument/IOL contact with the endothelium, air bubbles, and free radical formation. The ultrasonic waves that are generated during phaco leave behind large quantities of free radicals (that is, highly reactive electrons), submitting the surrounding cells to oxidative stress and disrupting vital cell processes.1
Because human corneal endothelial cells do not naturally regenerate, cell damage forces the endothelial cells to rearrange themselves to cover the corneal surface,1 a process that can present as diminished endothelial density. A significant portion of the protection afforded by an OVD to the corneal endothelium is due to its free-radical scavenging ability, which is directly related to the degree of retention during and after phaco.
Although easily removed, highly viscous OVDs may hinder IOL insertion due to deflection forces. When surgeries are performed using low-flow or slow-motion irrigation/aspiration (I/A), some of the OVD will remain in place, regardless of the viscosity. More normal, higher-flow I/A, however, decreases the retention of the cohesive OVDs.1
When high levels of endothelial protection are of concern, lower-viscosity OVDs, such as the 4% chondroitin sulfate/3% sodium hyaluronate formulation (VisCoat, Alcon Laboratories), provide superior coverage and better absorb free radicals. Lower-viscosity (dispersive) OVDs also can lubricate the IOL injector system, can better coat and protect the corneal endothelium, and selectively isolate and partition spaces.
Correspondingly, they also can be more difficult than cohesives to remove at the end of surgery. Lower-viscosity dispersive agents tend to be aspirated in fragments during phaco and I/A, leading to an irregular viscoelastic-aqueous interface that can obscure the view of the posterior capsule and limit overall visibility during phaco.2 This removal time, which may be up to seven times longer than with higher-viscosity OVDs, involves additional manipulation and aspiration, increasing the likelihood of endothelial damage or posterior capsule puncture.2
Note that the time required to remove the newer OVDs that are classified as either viscoadaptive or viscous dispersive agents via I/A is similar to that of a highly viscous (cohesive) agent and significantly less than a more dispersive agent.3
The newest generation of OVDs, specifically the 2.3% sodium hyaluronate formulation (Healon 5, Abbott Medical Optics) and the 4% chondroitin sulfate/1.65% sodium hyaluronate formulation (DisCoVisc, Alcon Laboratories), have been manufactured with the goal of producing a single, ideal OVD.