Ophthalmic viscosurgical devices (OVDs) or viscoelastic agents are essential tools for the anterior segment surgeon. Although we use some type of
viscosurgical device routinely for intraocular surgery, the number of available products continues to increase and improve, and therefore it is worthwhile to review the various agents and their advantages.
OVDs are transparent, gel-like substances that have viscous and elastic properties. Composed of sodium hyaluronate, chondroitin sulfate, and methylcellulose, they vary in molecular weights and viscosities, and can be classified into two general categories: cohesive and dispersive. OVDs are used during surgery in order to maintain and preserve space, displace and stabilize tissue, and coat and protect tissue. Examples of these functions during cataract surgery include: maintaining the anterior chamber during the capsulorhexis and IOL insertion, preventing iris prolapse and trapping nuclear fragments, and protecting the corneal endothelium from turbulence, lens material, and ultrasound energy, respectively.
OVDs have properties of both fluids and solids and vary among each other with respect to viscosity, pseudoplasticity, viscoelasticity, and coatability. The clinical applications of an individual agent depend upon its characteristics:
- Viscosity, or resistance to flow, is primarily determined by the molecular weight and concentration, so that substances with high molecular weight and high concentration have the highest viscosity. The higher the viscosity, the better the OVD at displacing tissue and staying in place.
- Pseudoplasticity refers to changes in viscosity with different shear rates. Some OVDs (chondroitin sulfate) do not have pseudoplasticity because their viscosity is constant regardless of shear rate. Other OVDs (sodium hyaluronate and methylcellulose) demonstrate pseudoplasticity with lower viscosity at higher shear rates. This property enables easy injection and removal of an agent at increasing flow rates.
- Elastic materials return to their original shape after being deformed, and therefore are excellent substances for maintaining space. The viscoelasticity of an OVD, determined by its viscosity and molecular structure, allows it to protect ocular tissue from forces such as ultrasound energy and fluid irrigation.
- Coatability is inversely proportional to surface tension and contact angle. Thus, an OVD with low surface tension and contact angle is better at coating tissue (i.e., chondroitin sulfate has better coatability than sodium hyaluronate).
Cohesive OVDs, such as Healon, ProVisc, and Amvisc, are long chain, high molecular weight, high viscosity substances that act like “spaghetti”. These agents maintain space well at no or low shear rates, while at high shear rates they are easily displaced. The advantage is that they are easier to remove from the eye since they stick together and are aspirated as long pieces. Thus, there is less chance of retention and risk of an IOP spike. However, they have minimal coating ability and therefore afford less tissue protection during surgery.
Dispersive OVDs, like Viscoat, Ocucoat, and Vitrax, are short chain, low molecular weight, low viscosity substances with low surface tension that act like “macaroni”. These properties produce excellent coating and protection at high shear rates; however, they are more difficult to remove from the eye since they don’t stick together and are aspirated in short fragments. Therefore, they have an increased risk of elevated IOP.
A new viscoadaptive product, Healon 5, is a long fragile chain, high molecular weight, super viscous substance that breaks at high shear rates. Therefore, it has properties of both cohesive and dispersive agents, mimicking cohesive OVDs at low flow and dispersive OVDs at high flow conditions. It is well retained in the eye and maintains space during high shear manipulations, but is also easily fractured so that it can coat well. It must be completely removed since it causes elevated IOP if left in the eye. DisCoVisc, a viscous dispersive agent, is another new OVD that is very good for stabilizing tissue, maintaining space at high shear rates, and coating the cornea.
It is important to understand the properties of the various OVDs and their advantages and disadvantages so that the most appropriate product can be selected for the particular job. Since the cohesive and dispersive agents have different strengths and weaknesses, it is sometimes necessary to use both types of OVDs, either for different steps of a surgery or in combination for a single maneuver (i.e., soft shell technique for capsulorhexis or IOL insertion in which two different OVDs are layered one below the other to maximize the advantages of their different properties). Because the newer OVDs combine the advantages of the traditional agents, they may be more useful for a greater range of clinical applications.
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