Viscoelastic substances

The rheological characteristics of AB cements are complex. Mostly, the unset cement paste behaves as a plastic or plastoelastic body, rather than as a Newtonian or viscoelastic substance. In other words, it does not flow unless the applied stress exceeds a certain value known as the yield point. Below the yield point a plastoelastic body behaves as an elastic solid and above the yield point it behaves as a viscoelastic one (Andrade, 1947). This makes a mathematical treatment complicated, and although the theories of viscoelasticity are well developed, as are those of an ideal plastic (Bingham body), plastoelasticity has received much less attention. In many AB cements, yield stress appears to be more important than viscosity in determining the stiffness of a paste. 

Mucus exists in the form of a viscous solution or a gel and is a sticky viscoelastic substance with water as its major constituent, accounting for 95 to 99.5 percent.101 Mucins are large (MW range 1000 to 40,000... 

These materials exhibit both viscous and elastic properties. In a purely Hookean elastic solid, the stress corresponding to a given strain is independent of time, whereas for viscoelastic substances the stress will gradually dissipate. In contrast to purely viscous liquids, on the other hand, viscoelastic fluids flow when subjected to stress, but part of their deformation is gradually recovered upon removal of the stress. 

Hyaluronate sodium is also combined with sodium chondroitin sulfate (in Viscoat) as a corneal transplant preservation medium. Sodium chondroitin sulfate and other viscoelastic substances protect the corneal endothelium during intraocular surgery. 

The major adverse effect associated with the use of viscoelastic substances, such as hyaluronate sodium, is a transient rise in intraocular pressure in the immediate postoperative period, attributed to its viscoelastic nature, resulting in coating and plugging of the trabecular mesh-work. For this reason, it is advisable to dilute hyaluronate sodium at the end of the surgical procedure with a balanced salt solution. Chondroitin sulfate is relatively less likely to precipitate such extreme rises in intraocular pressure because it is cleared rapidly from the trabecular meshwork. However, with any technique or chemical used in surgery there is always the potential for an unexpected adverse effect, and the risks of Viscoat include subepithelial calcium deposition and keratopathy (3). 

It should be noted that viscoelastic behavior varies widely among materials. The magnitudes of the instantaneous elastic modulus, the apparent viscosity and the elastic recovery, and especially the time scales involved, vary widely. Some viscoelastic substances, like cheese, seem on the face of it to be solids, but they are observed to flow over longer time scales. Others, like egg white, appear to be liquids but show elasticity on closer inspection. Closer inspection may involve sudden acceleration of the liquid, for example by rapidly giving a turn to the beaker containing it. If the liquid then shows oscillatory behavior, best seen in the motion of a few enclosed air bubbles or small particles, it also has some elastic property. 

Harooni M, Freilich JM, Abelson M, Refojo M. Efficacy of hyaluronidase in reducing increases in intraocular pressure related to the use of viscoelastic substances. Arch Ophthalmol 1998 116 1218-1221. 

A definitive advance was made with the introduction of viscoelastic substances in anterior segment surgery (Balazs, 1983 Balazs, 1984 Balazs, 1986). Viscoelastic substances (ophthalmic viscosurgical devices, OVD), a class of nonactive surgical implants with viscous and/or viscoelastic properties were intended for use during surgery in the anterior segment of the eye. 

The effects of viscoelastic substances in maintaining the anterior chamber space and the protection offered the vulnerable corneal endothelium from me-... 

Georg Eisner (1980, 1981) explored surgical viscoelastic substance use from the more practical aspects of tissue surface application and space creation. His impressive representations of viscoblockade, -tamponade and viscospatula (Eisner, 1983) are a tribute to his efforts. 

Based on our own study results, we propose a partial revision of the viscoelastic substance grouping/classification proposed by Arshinoff, to be based on the rheologic and physicochemical characteristics of viscoelastic substances obtained in cooperation with the Max-Planck Institute for Polymer Research, Department of Rheology, in Mainz, Germany. Furthermore, a new product, the so-called "viscous cohesion dissociated high molecular weight viscoelastic", better known by its product name, Healon 5 (Fig. 2), is now at our disposal. Healon 5 is formulated to fulfill specific requirements for use in phacoemulsification (i.e., with both cohesive and dispersive characteristics). 

Fig.l. A selection of commercially available viscoelastic substances in Germany to this date... 

Fig. 5. Percentage distribution of the number of ophthalmic surgeons, who apply a second viscoelastic substance during phacoemulsification obtained during a survey among 1370 members of the American Society of Cataract and Refractive Surgery (ASCRS modified according to Learning, 1997)...

Fig. 7. Percentage distribution of main classes of viscoelastic substances routinely applied during cataract operations. Data for the German speaking areas (modified according to Wenzel et al., 1999)...

To understand the mode of action of viscoelastic substance in ocular surgery, the following determinants are particularly important viscosity, elasticity, viscoelasticity, pseudoplasticity and coatability. 

Although flow mechanics are determined by their physical surroundings in more watery fluids, it is a substance s rheologic character which essentially determines flow characteristics among the more viscous and viscoelastic substances (Fig. 8). 

Molecular reformation, following deformation by an external force, is time-de-pendent (Fig. 9). An external force exerted briefly and quickly, causes a viscoelastic substance to go out of shape for a short time only, allowing the substance to regain its shape as soon as the force is removed. If force is applied more slowly, the viscoelastic molecules gradually adapt to the new form. This relationship is easily seen in the following two models which also take into account the direction of the external force ... 

Fig. 9. Schematic picture showing the time and force-dependent behaviour of a viscoelastic substance.

The viscoelastic substance is placed in a chamber with an opening... 

The viscosity at rest, i.e., zero shear rate, is clinically referred to as zero shear viscosity (unit Pas = Pascal seconds = 1000 cp = 1000 centipoise). This value can not be accurately determined by a rheometer, but is rather extrapolated mathematically, using various computation methods. Viscosity at rest is occasionally employed as the single measure for the assessment and classification of behavioral properties of viscoelastic substances. However, viscosity at rest merely reflects one aspect of the varied biophysical characteristics of viscoelastic substances which merit differentiation. 

Very low shear rates (-lO sec ) occur when ocular tissue is at rest and the viscoelastic substance occupies space during the operation. 

Very high shear rates ( 10 sec ) occur during viscoelastic substance injection through a cannula. Viscoelastic substances need very high viscosity at very low shear rates to maintain intraocular stability. Ophthalmic surgeons require low viscosity in order to move the lOL or instruments with the least possible resistance. 

The term viscoelasticity itself named the substance group. Viscoelastic substances exert a minimal resistance to deformation, as opposed to elastic substances. They neither immediately regain their shape, nor does their shape restore completely (Hysteresis-effect, Fig. 12). Viscoelastic substances have a relatively consistent form and do not regain their original shape following a deforming force, which differentiates them from plastic substances such as plasticine. 

A viscoelastic substance behaves in both an elastic and viscous manner, as can best be described by the simplified mechanical model shown in Figure 13. The elastic element is represented by a coil and the viscous element by a plunger in oil (Bothner8cWik,1989). 

Fig. 13. Mechanistic model to imitate the behaviour of a viscoelastic substance, combining elasticity and viscosity The spring imitates the elastic behaviour which dominates if a force is brought upon the substance with a high frequency or high velocity. Meanwhile the piston represents the viscous flow behaviour at low frequencies...

At low frequencies, viscous components prevail for all available viscoelastic substances used in ophthalmic surgery. With some viscoelastic substances, the elastic character increases and predominates with higher frequencies, while other solutions show mostly viscous properties at all frequencies. 

Due to their relatively consistent form, viscoelastic substances are suitable as space-maintaining, surgical aids and, additionally, do not flow out of the anterior chamber easily following eye manipulation. 

Fig. 15. Diagram showing the coherence between viscosity and shear rate The viscosity of viscoelastic substances declines with increasing velocity gradient (shear rate). This behaviour characterises pseudoelastic fluids...

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