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6.4Using Ophthalmic Viscosurgical Devices with Smaller Incisions

Steve A. Arshinoff1

Core Messages

ßUnderstanding the rheology of cataract surgery steps greatly facilitates micro incision surgery,

whether coaxial or biaxial.

ßBefore using an ophthalmic viscosurgical device (OVD) in any situation, a clear idea of

the method of removing it at the end of the case, is necessary.

ßOVD techniques need only minor modification to accommodate microincision surgery. Aware-

ness of the actual purpose of each OVD in a given situation is a critical factor.

ßGenerally speaking, smaller incisions seal better, making all OVD techniques more stable

and easier to perform.

ßWhile a single OVD may be excellent in uncomplicated routine cataract surgery, varia-

tions of soft shell and ultimate soft shell techniques make difficult cases much easier.

ßIt is never too early or too late in the cataract procedure to alter or correct an OVD strategy.

6.4.1 Introduction

Routine cataract surgery, by phacoemulsification and intraocular lens implantation, is regarded as a quick, mature and relatively simple procedure. However, when the sequential steps of the procedure are

analyzed, it is understood that each sequential maneuver is the result of many years of painstaking research and trial and error, and consists of an initial subtask of stabilization of the surgical environment, followed by a specific surgical task in that environment. Ophthalmic viscosurgical devices (OVDs) have become the primary surgical tool of the ophthalmologists to create the environment needed to perform intraocular maneuvers in a controlled environment. When considering the effect of the smaller incisions on the use of OVD, all of the above must be reevaluated in all aspects.. Since the introduction of Healon® in 1979 [1], OVDs have proliferated and become essential tools in anterior segment surgery for space creation, balancing pressure in the anterior and posterior chambers, tissue stabilization and protection of the corneal endothelial cells from surgical trauma, free radicals, and other surgical hazards [2]. An understanding of the factors that need to be controlled in surgery, and the properties of the OVD tools available, allows the surgeon to perform at a higher level, and makes his/her surgery, in the created controlled environments optimized for each step, appear simpler and smoother than it really may be conceptually. The changes in phacoemulsification cataract surgery, over the past decade, have basically been a gradual movement toward more controlled environments and smaller incisions. Before addressing the specific spatial problems encountered in some difficult situations, and the method of dealing with them with smaller incisions, some understanding of the properties of the variety of OVDs available, and how they may integrate into modern cataract surgery, is important. The goal should always be to create an environment in which a given task can be performed easily, rather than learning to perform difficult and complex maneuvers to achieve the same goal in an uncontrolled environment.

6.4.1.1 The Nature of OVDs: Rheology

OVDs are pseudoplastic solutions of biopolymers. Pseudoplasticity means that when zero shear viscosity (the viscosity of the OVD at rest) is plotted against shear rate (a measure of the stress to which the viscoelastic is exposed in a standard rheometer), the viscosity of a pseudoplastic solution falls dramatically as the shear rate rises, but has a limiting value with declining shear rate. There are four types of behavior recognized for rheologic solutions when performing these measurements (Fig. 6.40). Newtonian fluids possess

Log Shear Rate (sec–1)

Fig. 6.40 Rheometric patterns of behavior of fluid viscosity in response to increasing rates of shear. The varying patterns of rheologic behavior of fluids

constant viscosity independent of shear rate. Plastics have a viscosity which increases to infinity with declining shear rate (thus making them behave as solids at zero or very low shear rates) whereas pseudoplastics have “pseudoplasticity curves” similar to plastics, but possess a limiting viscosity at low shear, above which viscosity does not increase as shear rate declines toward zero, thus remaining as fluids at very low shear rates, unlike plastics. The fourth type, dilatant fluids, have increasing viscosity as shear rates increase (the opposite of pseudoplastics). OVDs useful in ophthalmic surgery must have low viscosity at high shear rates in order to be deliverable through small bore cannulas, and should have high viscosity at low shear rates to maintain surgical spaces and stabilize the anterior chamber (AC), permitting delicate surgical maneuvers. All OVDs that have been found to be useful, to date, possess pseudoplastic rheologic behavior.

OVDs differ in their rheologically active polymeric substance(s) (hyaluronic acid, chondroitin sulfate and hydroxypropylmethylcellulose (HPMC) have been used to date), concentration(s), and chain length(s). These factors determine the viscosity, elasticity, and cohesion of the OVD, and so, significantly affect other physical and chemical properties [3].

6.4.1.2 The Classification of OVDs

The classification of OVDs is essential to develop surgical techniques which optimize their use. Initially, all cataract viscosurgery was done using Healon®. A few