Quantitative method to determine the cohesion of viscoelastic agents by dynamic aspiration

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Abstract
Purpose: To quantitatively determine the cohesion of ophthalmic viscoelastic agents using an in vitro method based on dynamic aspiration kinetics. Setting: Alcon Laboratories, Inc., Fort Worth, Texas, USA. Method: Five viscoelastic agents were tested: Healon GV® (sodium hyaluronate 1.4%); Provisc® (sodium hyaluronate 1.0%); Healon® (sodium hyaluronate 1.0%); Amvisc® Plus (sodium hyaluronate 1.6%); Viscoat® (sodium hyaluronate 3.0%-chondroitin sulfate 4.0%). Samples were placed into a tared polypropylene container using a positive displacement pipette. Calibrated vacuum was applied for 2 seconds to a polypropylene tip (inner diameter 0.5 mm) lowered into the viscoelastic sample. The quantity of viscoelastic agent remaining in the container after aspiration was determined gravimetrically. The procedure was repeated at various vacuum levels between 100 and 700 mm Hg. The percentage of viscoelastic agent aspirated was plotted against vacuum pressure. The slopes of these curves indicate the relative cohesion of the viscoelastic sample. Results: The cohesion-dispersion indices (percentage viscoelastic agent aspirated/100 mm Hg) were Healon GV (72.3) > Provisc (46.0) > Healon (31.2) = Amvisc Plus (21.4) > Viscoat (3.4). Conclusion: The method provided a quantitative basis for the clinical classification of viscoelastic materials as cohesive or dispersive. The aspiration kinetics profile (curve shape), maximum rate of removal, and vacuum levels at which the bolus removal of the viscoelastic agent started (break point) were useful in characterizing the viscoelastic agents. Because the results agree with the clinical impression of cohesion/dispersion, this method may be used to predict the surgical performance of viscoelastic agents.