Using OVDs for glaucoma surgery

We have reviewed currently available OVDs and their classification. Their use in unusual problems of cataract surgery demonstrates that generally a variant of Soft Shell techniques is optimal for unusual problems and complications. This differs in glaucoma surgery, where space creation and maintenance is the usual goal, making highly viscous cohesive OVDs, and sometimes viscoadaptives the usual agents of choice. J Emmetropia. 2013; 4: 161-169 Investigationally used in the 1970’s, but first marketed in 1980 (Healon), ophthalmic viscosurgical devices (OVDs referred to as viscoelastics before 2000), are pharmacologically inactive, clear, highly viscous and elastic fluids that have properties bordering upon gels and solids1. The prototype rheologic component of an OVD material is sodium hyaluronate (HA), however chondroitin sulfate, and hydroxypropylmethylcellulose have also become commonly used. Their physical properties, which collectively are the determinants of OVD behavior in surgery, include viscosity, elasticity, pseudoplasticity, and cohesion, which in turn depend on the rheologic polymeric mean chain length (molecular weight) and concentration. Consequently saying that two OVDs are both 1% HA solutions is insufficient to claim equivalence, because they may have different polymeric chain lengths, molecular weight distribution, UPDATE/REVIEW 1York Finch Eye Associates and Humber River Regional Hospital, Toronto, Ontario, Canada. 2Department of Ophthalmology and Vision Sciences, University of Toronto, Toronto, ON, Canada. 3Department of Ophthalmology, McMaster University, Hamilton, ON, Canada. 4Eye Foundation of Canada, Toronto, ON, Canada. 5University of Sherbrooke, Sherbrooke, Quebec, Canada. Disclaimer: Steve A. Arshinoff has consulted for most major manufacturers of OVDs worldwide. The other author and contributors have no financial interest in anything discussed herein. Acknowledgements: The authors would like to thank Sourabh Arora, BSc3, and Nir Shoham, MD2 for their contributions to this paper. Corresponding Author: Steve A. Arshinoff. York Finch Eye Associates. 2115 Finch Ave. W. #316. Toronto, Ontario, Canada M3N 2V6. E mail: [email protected] etc. Therefore, unlike with pharmaceuticals, OVDs must be referred to by their trade names to be accurate about their properties and behavior. Generic names are usually misleading and inadequate descriptions of an OVD preparation. OVDs serve many purposes in ophthalmic surgery: space creation & maintenance, balancing pressure in the anterior chamber and posteriorly, tissue stabilization, anterior chamber space partitioning, and protection of the corneal endothelial cells. OVDs have been classified based upon their behavior in surgery. The first groups, prior to 1990 were higher viscosity cohesives and lower viscosity dispersives. Viscoadaptives, a class based upon the properties of Healon5 were added in 1998, and the subsequent advent of DisCoVisc in 2004, a higher viscosity dispersive, required the classification to become two dimensional and more complicated. The current classification is illustrated in Table 12. Surgeons should select different OVDs for different tasks based on rheologic properties. All materials found to be useful as OVDs to date, are pseudoplastic polymeric aqueous solutions. They are most viscous when stationary, i.e. at zero shear rate, and their viscosity decreases as shear rate increases. Plastics also exhibit decreased viscosity with increasing shear rates, but their viscosity goes to infinity at shear rate zero, making them solid when stationary. Unlike true plastics, OVDs reach a finite or limiting viscosity as shear rates approach zero and always remain in a fluid state. Thus they are referred to as pseudoplastic. The zero-shear viscosity at a given temperature is one of the few ways to reliably compare OVDs, without requiring a more complex graphical description of properties under different conditions (Figure 1). A simplified, surgically useful classification can be created based upon viscosity and cohesive properties of OVDs (Figure 2).