Healon 5 was the first viscoadaptice OVD to be designed rheologically. Their physical properties are different from both cohesive and dispersive OVDs. Viscoadaptive OVDs are substances with high molecular weight and high concentration that contain fragile long-chain molecules. At high shear rates, these OVDs separate and disperse providing excellent coating but being less prone to conglomerate and come out of the eye (similar to macaroni).ĭispersive agents include OcuCoat (Bausch + Lomb), Viscoat (Alcon), and Healon Endocoat (Abbott Medical Optics).
Kinematic viscosity definition wikipedia plus#
However, during conditions of turbulence (high shear), the long-chained cohesive OVDs tend to entangle and extrude from the eye as a single mass (similar to spaghetti).Ĭohesive agents include Healon, Healon GV (Abbott Medical Optics, Santa Ana, CA) Amvisc, Amvisc Plus (Bausch + Lomb, Rochester, NY) and Provisc (Alcon, Ft Worth, TX).ĭispersive OVDs are materials that contain short-chain molecules that display low surface tension, which results in low molecular weight and low viscosity.
These agents tend to maintain space well when there is minimal movement. Ĭohesion is the tendency of a material’s constituent molecules to adhere to one another rather than to disperse.Ĭohesive OVDs are substances that contain long-chain molecules and are therefore characterized by high molecular weight and high viscosity. The relative cohesive or dispersive behaviour of an OVD is important as it relates directly to how it is used in surgery. A low surface tension and a low contact angle result in high coatability which means the OVD is good at coating tissue but is difficult to aspirate from the eye. Contact angle is the angle that a drop of substance forms with a surface. Surface tension refers to the forces acting at the interface between a viscoelastic agent and a surface. The higher the elasticity, the better the OVD is in maintaining space.Ĭoating ability is determined mainly by surface tension and contact angle. All OVDs will lead to reformation of corneal shape and anterior chamber after insertion or withdrawal of an instrument. If an OVD shows pseudoplasticity it means at zero shear rate it will have high viscosity and coat tissue well but at high shear rate it will lower its viscosity and more readily extrude from the anterior chamber.Įlasticity describes the ability of a substance to return to its original shape after deformation. Shear rate is the speed at which 2 plates are moved in relation to one another with a solution between them. Pseudoplasticity indicates the facility with which a material can change its viscosity according to shear rate. This means the OVD will be effective at moving tissue and will be difficult to displace from the anterior chamber. It is determined principally by molecular weight and concentration, so that substances with high molecular weight and high concentration have high viscosity. Viscosity of a substance is characterized by its resistance to flow. Surgical applications of the different OVDs are determined mainly by their physical properties which are a consequence of their molecular chain length and interactions both within chains and between chains and eye tissue. OVDs are unique in that they own characteristics of both solids and fluids. Many other hyaluronates followed, differing from one another in molecular weight, concentration, and viscosity.īecause of the multitude of important functions that viscoelastic substances serve in intraocular surgery they have been renamed ophthalmic viscosurgical devices (OVDs), suggesting that these agents are now to be considered essential surgical tools, not just corollary products to be used during surgery. The development of the first viscoelastic agent which was made of sodium hyaluronate helped to revolutionize the way cataract surgery was performed. The term viscosurgery was introduced almost 30 years ago.
7.5 Intraoperative Floppy Iris Syndrome (IFIS).
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