The addition of chondroitin sulfate to ophthalmic viscosurgical devices has been a big plus, because it positively affects zero-shear viscosity.
"The physical properties of an OVD are directly correlated with its performance during surgery," said Dr. Modi, assistant clinical professor of ophthalmology, Albert Einstein College of Medicine, New York. "As clinicians, it behooves us to know how changes in molecular weight, concentration, and the constituent elements within the OVD will change the behavior of the OVD during surgery."
Using the example of the simple transition from longitudinal to torsional phacoemulsification with all variables (flow, vacuum, and incision size) remaining the same, he said he had to change the way he used the OVD.
In this animal study performed using New Zealand white rabbits, one surgeon evaluated the in vivo performance of chondroitin sulfate and hyaluronate using a phaco system (Legacy, Alcon Laboratories) with 300 mm Hg of vacuum, flow rate of 40 ml/minute, ultrasound power of 60%, and a 1-mm incision.
When considering molecular weight, he said that when the variable increases from 1.5 million to 3 million daltons, the OVD viscosity increases exponentially. When considering the concentration of hyaluronate versus viscosity, an increase from 1.6% to 1.8% shows a trend toward increased viscosity.
OVD clarity, protection from shear
"For uncomplicated cataract surgery, maintaining the depth of the anterior chamber is not an issue," Dr. Modi said. "For me, the clarity of the OVD and the protection from shear are more important. When there is an increase from 0.8 million to 2.8 million daltons, the anterior chamber is well maintained once 1.7 million daltons or more is reached.
"Interestingly, at both ends of the spectrum, the clarity of the OVD decreases," he said. "The clarity was optimal at 1.7 million daltons, and this has clinical implications."
Dr. Modi and colleagues also evaluated the ratio of hyaluronate to chondroitin sulfate in relation to zero-shear viscosity.