Abstract
Purpose: :
All OVDs are designed to prevent mechanical damage to the corneal endothelium, manipulate delicate tissues, and maintain anterior chamber depth. Viscosurgical products have been used extensively for many years in cataract surgery. One complication is transient postoperative elevation of intraocular pressure (IOP). When developing new OVDs, the ISO standard 15798, "Ophthalmic Implants–Ophthalmic Viscosurgical Devices" mandates studying a rabbit model of IOP development after aqueous exchange. The purpose of our study was to determine the correlation between the rheological profiles of various marketed OVDs and the development of IOP.
Methods: :
Concentration and molecular mass of the rheologically active substance (sodium hyaluronate or hydroxypropyl methylcellulose) and zero shear rates were measured for Ocucoat, Viscoat, Ivisc, Provisc, Biolon, Ophthalin, Healon, Healon GV, and Healon5 OVDs. New Zealand White albino rabbits were used for the IOP study. Approximately 25% of the total aqueous volume, (50 µl aqueous humour), was exchanged with the OVD tested. A minimum of 8 eyes was used for each OVD. The Healon OVD was injected as a control in the fellow eye. IOP was measured every second hour up to 12 hours, followed by measurements at 24, 48, and 72 hours using a Pneuma-Tonometer (Bio-Rad). Data were evaluated by multivariate analysis.
Results: :
All OVDs tested caused postoperative increases in IOP, with peak pressures occurring between 2 (Ophthalin OVD) and 8 hours (Healon GV OVD). At 24 hours post-op, values had returned to normal. The maximum IOP (mm Hg) of the products tested ranged from 34 (Provisc OVD) to 52 (Viscoat OVD), and correlated positively (r = 0.91) to the concentration of the rheologically active substance, while peak time correlated positively (r= 0.71) to the average molecular mass of the product.
Conclusions: :
The IOP of products with dispersive low molecular mass peaks earlier than products with cohesive high molecular mass. The height of the peak correlated strongly to the concentration of the rheologically active substance of the OVD. The results from this ISO 15798 test may provide a useful guideline for assessing post operative IOP in OVD clinical evaluations.
Keywords: intraocular pressure • anterior chamber