Understanding, retaining, and removing dispersive and pseudodispersive ophthalmic viscosurgical devices

PURPOSE: To analyze and explain the rheologic behavior of pseudodispersive viscoadaptive ophthalmic viscosurgical devices (OVDs) and compare it with that of dispersive OVDs. SETTING: York Finch Eye Associates and Humber River Regional Hospital, Toronto, Ontario, Canada. METHODS: Poiseuille's law was used as a basis of understanding fluid aspiration. Each OVD class was analyzed to determine how well it fit the law and why. RESULTS: Higher viscosity cohesive OVDs are removed in a manner predicted by Poiseuille's law. Lower viscosity dispersive and viscoadaptive OVDs are not. CONCLUSIONS: Lower viscosity dispersive OVDs possess inadequate cohesion to maintain continuous contact with the aspiration port during irrigation/aspiration, whereas viscoadaptive OVDs are too rigid to permit scrolling around obstacles in the eye (the intraocular lens), also resulting in interrupted contact with the aspiration port. Dispersion and pseudodispersion represent opposite ends of a spectrum of increasing cohesion and rigidity correlating with increasing zero-shear viscosity and are not similar.