Elasticity refers to the ability of a solution to return to its original shape after being stressed. The rheologic property of viscoelasticity is the essence of the usefulness of these materials as surgical tools in ophthalmology. Elasticity allows the anterior chamber to reform after deformation by depression on the cornea when external forces are released. A nonelastic solution such as balanced salt solution (BSS) will show no such reformation after release of forces.

The terms viscosity and viscoelasticity are not synonymous. Viscosity, viscoelasticity, and pseudoplasticity are, however, interrelated. The amount of elasticity of an elastic compound increases with increasing molecular weight and greater chain length of the molecules. Unfortunately, comparison of the different OVDs with regard to elasticity is not easily made because of the different ways and nonuniform expression of values by the various manufacturers.

Viscosity (Table 9-1) reflects a solution’s resistance to flow, which is in part a function of the molecular weight of the substance. Viscosity of viscoelastics is measured in centipoise (cP) or centistokes (cSt), which are measures of the resistance to flow relative to a given shear force. Liquid solutions are generally considered to have viscosities of <10,000 cSt at rest, whereas solutions with resting viscosities >100,000 cSt are gel-like. The higher the solution’s molecular weight, the more it resists flow. Molecular weight, on the other hand, reflects the size of the solution’s molecules. Viscosity is dependent on the degree of movement of a solution, which is also known as the shear rate and varies inversely with temperature. The viscosity of a solution can be increased by increasing either the concentration or the molecular weight of the solution.

To facilitate optimal intraocular manipulation, an OVD should maintain space and protect tissues (possess a high viscosity at low shear rates), allow movement of instruments, aid in intraocular lens (IOL) implantation (possess a moderate viscosity at medium shear rates), and allow easy introduction into the eye through a small cannula (possess a low viscosity at high shear rates).² At the present time, no OVD fulfills all of these requirements.

Pseudoplasticity refers to a solution’s ability to transform when under pressure, from a gel-like substance to a more liquid substance. The more pseudoplastic a material is, the more rapidly it changes from being highly viscous at rest to a thin, watery solution at high shear rates. A change in molecular structure accompanies this pseudoplastic behavior. In clinical terms, a high-molecular-weight, high-viscosity OVD at rest (zero shear force) acts as an excellent lubricant, and coats tissues and maintains space very well. When under the influence of stress (i.e., a high shear rate), however, the OVD will become an elastic molecular system behaving as an excellent shock-absorbing gel. The highest shear rates occur when a solution is passed through a cannula, and viscosity becomes independent of molecular weight. When the molecules align themselves in the direction of flow, the viscosity is determined solely by the concentration. Pseudoplastic solutions, therefore, have a low viscosity at high shear rates and can be extruded easily through a small-diameter cannular (27 or 30 gauge) (Fig. 9-1). It is important to emphasize that the viscosity of a viscoelastic substance at rest (0 shear rate) is a function of concentration, molecular weight, and the size of the flexible molecular coils of the material (Figs. 9-2A and B). At high shear rates, the viscosity is independent of molecular weight and is determined mainly by the concentration.³

The coating ability of an OVD is determined not only by the surface tension of the material itself but also by the surface tension of the contact tissue, surgical instrument, or IOL. By measuring the angle formed by a drop of the OVD on a flat surface (the contact angle), the coating ability of a substance can be estimated. Lower surface tension and lower contact angle indicate a better ability to coat. In this respect a solution of sodium hyaluronate (HA) has a significantly higher surface tension and contact angle than does a solution of chondroitin sulfate, sodium hyaluronate/chondroitin sulfate in combination, or hydroxypropyl methylcellulose (HPMC), thus indicating these latter solutions provide superior coating.⁴

A comparison of the various physical properties of OVDs is summarized in Tables 9-1 and 9-2.

Sodium hyaluronate (NaHa) is a biopolymer occurring in many connective tissues throughout the body, including both the aqueous and vitreous humors. Its basic structural unit is a disaccharide, joined by a beta one-4 glucosidic bond, which is linked in a repeating fashion with beta glucosidic bonds to form a long unbranched chain. This mucopolysaccharide chain subsequently forms a random coil when placed in a solution such as physiologic saline. As the concentration of large sodium hyaluronate molecules is increased (>0.5 mg/mL), individual molecular coils start to overlap and are compressed (Fig. 9-2). This crowding of the chains increases the chances for various noncovalent chain–chain interactions. This, in turn, causes a considerable increase in the viscosity of the solution. For example, the kinematic viscosity of a 2-mg/mL concentration of sodium hyaluronate in physiologic buffer is only in the 100-cSt range: At 10 mg/mL, it is in the 100,000-cSt range. Therefore, a fivefold increase in the concentration causes a 1,000-fold increase in the viscosity of the solution. With this increase in viscosity, the elastic properties of the solution also increase. This forms the rationalization of Amvisc Plus and Healon GV. The elastic behavior of a concentrated (> 0.5 mg/mL) sodium hyaluronate solution is greatly dependent on the mechanical energy (shear force) applied to the solution. On a molecular level, this means that under the imposed strain, the polysaccharide coils slip by each other, and conformational and configurational rearrangements occur while the solution exhibits viscous flow (Fig. 9-2). The hyaluronate acid fraction (NIF-NaHa)⁶ used for ophthalmic procedures has a high molecular weight (2–5 million Da), a low protein content (<0.5%), and carries a single negative charge per disaccharide unit.