The physical properties of OVDs are not necessarily due to their specific biopolymer composition; rather, they are the result of chain length and molecular interactions both within chains and between chains and ocular tissue. OVDs have 4 general physical properties:
Viscosity describes resistance to flow and can be thought of as the “thickness” or “thinness” of a fluid. It is determined primarily by 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 is at displacing tissue and staying in place.
Elasticity refers to the ability of a material to return to its original shape after being stressed. It describes the OVD’s ability to re-form after an external force is applied to the anterior chamber and then removed. A highly elastic substance is excellent for maintaining space.
Pseudoplasticity refers to the ease with which a material can change from being highly viscous at rest to being watery at increasing rates of shear stress. This property is found in certain everyday substances such as toothpaste; when squeezed out of a tube, toothpaste flows easily, but it retains its shape when it is at rest on the toothbrush. In clinical terms, at zero shear force an OVD is a lubricant and coats tissues well, but when forced through a small-gauge cannula it functions like a liquid.
Surface tension describes how the surface of a fluid tends to stick to another surface. This property is also referred to as coatability, which is inversely proportional to surface tension. Thus, an OVD with low surface tension is better at coating tissue but is harder to remove from the eye.
Given the different combinations of all of these properties, OVDs can be classified into 2 general categories:
Cohesive OVDs are long-chain, high-molecular-weight, high-viscosity substances. These agents maintain space well at no or low shear rates, whereas at high shear rates they are easily displaced. Cohesive OVDs are easier to remove from the eye because they stick together and are aspirated as long pieces (similar to spaghetti). However, they have minimal coating ability and therefore afford less tissue protection during surgery.
Dispersive OVDs are short-chain, low-molecular-weight, low-viscosity substances with low surface tension. These agents provide excellent coating and protection at high shear rates; however, they are more difficult to remove from the eye because they do not stick together and are aspirated in short fragments (similar to macaroni). They are more likely to be retained in the eye after cataract surgery, increasing the likelihood of angle obstruction with reduced outflow and subsequent IOP elevation.
Figure 7-7 Behavior of ophthalmic viscosurgical devices (OVDs).
Cohesive agents include Healon, Healon GV (J&J Surgical); Amvisc, Amvisc Plus (Bausch + Lomb); and Provisc (Alcon). Dispersive agents include OcuCoat (Bausch + Lomb), Viscoat (Alcon), and Healon Endocoat (J&J Surgical). Discovisc (Alcon) combines qualities of dispersive and cohesive agents. Figure 7-7 shows the full spectrum of OVDs.
Some additional OVDs, such as the viscoadaptive agent Healon5 (J&J Surgical), may need separate classification. Healon5 is a long, fragile chain with high molecular weight that changes its behavior at different flow rates. The lower the flow rate, the more viscous and cohesive the OVD is, and the higher the flow rate, the more the chains fracture. As a result, this OVD acts as a pseudodispersive agent. However, it must be carefully removed at the end of surgery because it can cause extremely elevated IOP if left in the eye.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.