33   Using Ophthalmic Viscosurgical Devices to Manage Problems in Cataract Surgery

The developments and innovations that have occurred in the world of viscosurgery over the past 30 years have greatly enhanced our abilities and reduced our complication rates in cataract surgery. The use of ophthalmic viscosurgical devices (OVDs) in cataract surgery was first described by Balazs, Miller, and Stegmann¹ in 1979, forever revolutionizing anterior-segment surgery. Up until that point, corneal endothelial damage had been an inevitable consequence of intraocular surgery, but the unique space-creating abilities of OVDs made protection of the endothelium possible while simultaneously facilitating intraocular lens (IOL) implantation. OVDs now represent an indispensable part of even routine cataract surgery, but the diverse range of currently available OVDs and their varying chemical and physical (rheological) properties afford the skilled and knowledgeable cataract surgeon an unprecedented level of adaptability to manage virtually any intraoperative complication that may arise, as well as the ability to design techniques to facilitate management of cases previously viewed as difficult and entailing high risk.

This chapter discusses OVDs and their use in cataract surgery, with special emphasis on techniques for managing difficult cases and intraoperative complications. The chapter classifies and groups OVDs according to their rheological properties, relating each group to intraoperative strengths and weaknesses, and discusses the “soft-shell” techniques, focusing on their adaptability to different intraocular surgical situations.

The optimal use of OVDs in intraocular surgery will enhance every procedure, but there is no single preferred or universally used OVD. Understanding the rheological basis of classification of OVDs, the strengths and weakness of different groups, and how they may be combined in the most efficacious manner can provide the surgeon with an infinitely adaptable viscosurgical armamentarium.

Classification of Ophthalmic Viscosurgical Devices

Background

A viscoelastic preparation intended for intraocular use must chemically be aqueous based, isotonic, and pH balanced. The range of physical properties is governed by the molecular nature, chain length, and concentration of the rheologically active polymer constituent(s). To devise surgically useful classification criteria, the rheological parameters that best apply to OVDs in relation to their function in surgery must be identified. Currently, the two most useful laboratory-measurable OVD parameters for such a classification are (1) zero-shear viscosity (V₀), and (2) the cohesion-dispersion index (CDI).

V₀ is defined as the viscosity of the OVD at rest, or more specifically, when a shear stress equaling zero is applied to it (no turbulence). It is read off the extreme left axis of pseudoplasticity curves where shear rate is 10⁻³, which is as near zero as is generally measured (Fig. 33.1). In the context of intraocular surgery, this would occur when neither phacoemulsification (phaco) nor irrigation and aspiration (I/A) are being performed. An OVD’s V₀ correlates positively with both the molecular weight and concentration of its rheologically active viscoelastic component(s).

The other clinically important laboratory OVD parameter, CDI, is an objective, quantitative measure of its cohesive-dispersive behavior (physical opposites) under vacuum aspiration analogous to our use of OVDs in surgery (Fig. 33.2). Cohesive OVDs tend to aspirate in a bolus, as the molecules tend to entangle, are supple enough to flow around obstacles, and cohere sufficiently to avoid fragmentation when exposed to the usual vacuum aspiration forces in phaco. Dispersive OVDs tend to fragment under even mild aspiration force, and consequently a lesser volume of OVD is aspirated per unit of time when exposed to the same vacuum level compared to a cohesive OVD.

Taken together, these two parameters, V₀ and CDI, can be used to establish a classification of OVDs (Fig. 33.3). Long-chain ophthalmic viscoelastic molecules tend to entangle in solution, causing them to aggregate. The OVDs containing the longest chains of hyaluronic acid are therefore referred to as “higher-viscosity cohesive” and they have higher V₀ and usually higher CDI values as well. As V₀ declines below 100,000 millipascal-seconds (mPas), molecular chain entanglement becomes a far less significant factor. Consequently, these lower viscosity OVDs generally tend to be easily broken up and have lower CDI values, and are accordingly referred to as “lower-viscosity dispersives.”

In the vast majority of currently marketed OVDs, the CDI correlates with the V₀ (but not in a simple linear relationship), as well as with the molecular weight and concentration of the rheologically active constituent(s) (which is hyaluronic acid in the vast majority of OVDs); thus, the degree of cohesion generally falls neatly into a classification based on only V₀. DisCoVisc (Alcon, Fort Worth, TX), however, was specifically designed with viscosity and cohesion somewhat dissociated from each other, thus giving it the V₀ of Healon (Abbott Medical Optics [AMO], Abbott Park, IL), that is, viscous-cohesive, but a CDI close to that of Viscoat (Alcon), that is, lower viscosity dispersive. Thus, the classification of OVDs had to be expanded from one- to two-dimensional in 2005, and it has been updated since.²

The surgical tasks for which OVDs are utilized are generally facilitated more by higher V₀. Highly viscous OVDs tend to be more cohesive (less dispersive). However, in certain situations, dispersion rather than cohesion may be desirable (e.g., to retain a thick protective layer of OVD adjacent to the corneal endothelium to protect it from the turbulence of phaco or I/A). Consideration of this issue and the nature of the steps involved in phacoemulsification soon make it apparent that soft shell techniques (SSTs) are preferable for almost all tasks in cataract surgery, as SSTs were designed to combine the best attributes of both cohesive and dispersive OVDs, while simultaneously overcoming the drawbacks of each in different surgical situations.^3,4 SSTs have been updated to tri-soft shell techniques, permitting almost endless possibilities of partition of the anterior chamber (AC) during surgery with selective protection of chosen areas of the AC.⁵ The following subsections discuss each class of OVD and highlight its strengths and weaknesses.

Fig. 33.1 Pseudoplasticity curves and representative cohesion-dispersion indexes (CDIs) of ophthalmic viscosurgical devices (OVDs). Graphs of log viscosity versus log shear rate are plotted for common ophthalmic viscosurgical devices sold by Alcon, Abbott Medical Optics, Bausch and Lomb, and iMed Pharma. All OVDs useful for ophthalmic surgery are pseudoplastic, which means that they exhibit low viscosity when exposed to high shear stress (right side of curve). Viscosity increases going toward the left as shear stress on the molecule decreases. iVisc phaco (top curve) is almost plastic in that the graph does not level off much on the left side of the graph. The continuously rising graph indicates that iVisc phaco will have the physical properties of a solid at zero shear. All of the other OVDs exhibit flattening of their curves to the left, indicating that there is a limit to how high their viscosities become, even at low shear, thus making them pseudoplastic. The box inset lists the cohesion-dispersion indices of four different OVDs with the lower numbers indicating dispersive behavior, and higher numbers cohesive behavior.

The higher viscosity-cohesive OVDs are best used to create space and stability where it is otherwise inadequate. A practical example of this can be recognized when surgery is performed using topical anesthesia, when the nonparalyzed extraocular muscular tone can cause shallowing of the AC. In this setting, a successful continuous curvilinear capsulorrhexis can be facilitated by using a highly viscous-cohesive OVD to pressurize the AC equal to that of the posterior pressure, thus flattening the anterior convexity of the anterior lens capsule. This neutralizes the centrifugal vector of the force generated by internal (or transmitted posterior) pressure on the anteriorly convex surface of a spheroid (the lens), encouraging the capsulorrhexis to tear toward the equator, and permits the creation of a round capsulorrhexis of the desired size and shape in the now pressure-equalized environment. A corollary of this principle pertains to the management of a rhexis that has already begun to tear toward the periphery. After injection of a superviscous-cohesive OVD into the AC (anywhere in the AC that is most convenient) to increase the pressure in the AC, thereby flattening the anterior capsule, external sources of excessive posterior pressure such as a tight lid speculum or drapes should be alleviated, and the capsulorrhexis can be rescued by gently pulling the flap centrally when it is folded over to create a shearing force at the point of tearing (Fig. 33.4).

Fig. 33.2 New classification of OVDs, 2005, modified and updated to 2015. The advent of DisCoVisc, the first OVD that claimed to be viscous-dispersive, required that the presumed correlation between zero-shear viscosity and cohesion-dispersion behavior of OVDs be abandoned, and the classification of OVDs be changed from a one- to a two-dimensional table, because viscosity and cohesion may behave reasonably independently. For rheologists, the most interesting parts of the table are the red boxes where we have no OVDs available. This encourages consideration of how OVDs with behavior of those descriptions could be helpful in surgery. If the thinking is productive, designing can begin. (Modified from Arshinoff SA, Jafari M. A new classification of ophthalmic viscosurgical devices (OVDs). J Cataract Refract Surg. 2005;31:2167–2171.)

Fig. 33.3 Viscoadaptive behavior of Healon5 to turbulence. Healon GV displays cohesive behavior throughout the range of aspiration flow settings (and therefore induced turbulence) commonly used in phacoemulsification surgery. Similarly, Viscoat displays dispersive behavior across this normal range of fluid turbulence encountered in phacoemulsification surgery. These two products are therefore appropriately classified as cohesive and dispersive, respectively. Healon5 was designed to have the unique property of becoming a fracturable solid at flow rates above 25 cc/mm, and therefore exhibiting typical viscous cohesive behavior at settings below 25 cc/min, and fracturable “pseudo-dispersive” behavior at flow rates above 25 cc/min. Healon5 is therefore appropriately referred to as viscoadaptive, because its rheological behavior adapts as the flow rate is increased above 25 cc/min for some parts of the phaco procedure (e.g., phacoemulsification of the lens and viscoelastic removal). It follows that to keep Healon5 in the eye during phaco, all that has to be done is to lower the flow rate to ∼20 cc/min or lower.

Fig. 33.4 Pressure equalized cataract surgery. The concept of pressure-equalized cataract surgery is used to prevent anterior capsulorrhexis tear-outs when performing surgery on white hypermature cataracts. There is always posterior pressure due to the tension of the extraocular muscles. This is transmitted through the posterior structures of the eye to the lens, which has a convex-anterior curvature, much like a balloon. If the posterior pressure is not exceeded with anterior pressure, the tear will tend to go outward toward the equator. The use of a highly viscoelastic and cohesive OVD permits the anterior pressure on the anterior capsule to be increased until it exceeds the posterior pressure, which is noticed by the surgeon as central flattening or even concavity of the anterior capsule. Once this is done, the capsulorrhexis can be completed without centrifugal vector forces favoring extension. It will actually want to tear inward.

The high cohesion of viscous-cohesive and superviscous-cohesive OVDs results in their easy removal as a bolus by I/A at the end of the surgical procedure. However, this same desirable cohesive behavior results in these OVDs being relatively rapidly aspirated out of the AC compared to dispersive OVDs during the turbulence caused by phacoemulsification or I/A. Although an invisible, thin layer of hyaluronan bound to endothelial cell membrane specific binding sites remains behind, this may not be sufficient to protect the corneal endothelium in all cases.⁶ In a similar vein, this cohesive bolus-like behavior during aspiration makes these OVDs unable to partition the AC into two adjacent fluid spaces: one zone to remain stable and to protect, and the other where the surgeon can evacuate the OVD and work in with active flow. It is impossible to use a cohesive OVD to protect and sequester a structure by coating it with OVD (e.g., protruding vitreous, frayed iris, etc.) in the AC and simultaneously work with the phaco or I/A tip in an adjacent area; the OVD and the structure that it is supposed to be protecting will be aspirated in a bolus together by the instrument.

Lower Viscosity-Dispersive OVDs

Lower viscosity-dispersives include all current OVDs with V₀ values below 100,000 mPas. Molecular chain entanglement is less prevalent in these OVDs, and so cohesion tends to be significantly weaker, resulting in dispersive behavior when exposed to the turbulence of intraocular surgery (the extreme of dispersive behavior in an aqueous solution is a grain of salt dissolving in the water). Two subgroups make up this OVD class. The first is the medium-viscosity dispersives, possessing V₀ values between 10,000 and 100,000 mPas (1 log unit less than regular viscous cohesives). They include Viscoat (Alcon), Cellugel (Alcon), Rayvisc (Rayner, Kansas City, MO), Opelead [Shisheido Co. Ltd., Tokyo], and Healon EndoCoat (AMO), to name a few. The second subclass, the very low viscosity dispersives, consists of all the unmodified hydroxypropyl methylcelluloses, including Eyefill [Croma Pharma GMBH, Vienna (purchased by B&L 2014)], iCell (San Antonio, TX), OcuVis (Toomac Ophthalmic, Auckland, New Zealand), OcuCoat (Bausch and Lomb), Adatocel [Adatomed GMBH, Munich], Acrivisc [Acri.Tec GMBH, Henningsdorf, Germany], Visilon [Shah and Shah, Calcutta], and many others.

Areas of Strength

Surgically, the most useful properties of dispersive OVDs are their resistance to aspiration and their ability to partition spaces. Dispersive nature, negative electrical charge, and the presence of hyaluronic acid to bind to specific endothelial-binding sites are the three factors that have been demonstrated by Poyer et al⁷ to improve an OVD’s retention in the AC during phaco and I/A. This enables dispersives to remain adjacent to the corneal endothelial cells and protect them.

Equally important, dispersive OVDs are highly useful as surgical tools in complex situations where delicate ocular structures are exposed in the AC. It may become necessary in these settings to selectively isolate or move such a structure out of harm’s way until the surgery is completed (e.g., holding back vitreous at an area of zonular disinsertion, moving aside a strand of frayed iris, plugging a small posterior capsular hole, etc.). Dispersive OVDs can accomplish these tasks by partitioning the AC into two separate fluid workspaces without the two mixing: a viscoelastic-occupied space containing the delicate structure, and a surgical zone in which phaco or I/A can be safely performed under low-flow conditions. The OVD encases the protected area in an aspiration-resistant viscoelastic shell while the surgery proceeds in an adjacent area.

Areas of Weakness

The major drawbacks of lower viscosity-dispersive OVDs is their inability to maintain or stabilize spaces as well as higher viscosity-cohesive OVDs, due to their relatively low viscosity and elasticity (e.g., to facilitate capsulorrhexis creation, as described above). In addition, lower viscosity-dispersives tend to be aspirated in small fragments during phaco and I/A, as opposed to in a bolus. This gives these OVDs their inherent resistance to aspiration, but also leads to an irregular viscoelastic-aqueous interface, which may obscure the surgeon’s view of the posterior capsule during phaco. Furthermore, these OVDs tend to trap particulate matter and microbubbles generated during phaco, further impairing the surgeon’s view. Finally, dispersive OVDs’ aspiration resistance makes them more difficult to remove at the end of the surgical procedure. Assia et al⁸ demonstrated in a controlled in vitro study that lower viscosity-dispersive OVDs such as Viscoat, Ocucoat, and Orcolon [Optical Radiation Corporation, Azusa, CA], may take more than seven times longer to remove than highly viscous-cohesive OVDs such as Healon and Healon GV. The additional manipulation and aspiration required to completely remove dispersive OVDs may actually increase the likelihood of complications such as endothelial damage or puncturing of the posterior capsule, thus offsetting the benefit derived from their use in surgery.

Viscoadaptive OVDs

As discussed above, higher viscosity-cohesive and lower viscosity-dispersive OVDs both have areas of strength and weakness. In an effort to address the weaknesses of each, the search for a single OVD that could perform satisfactorily in all aspects of cataract surgery began, and in 1998 a new class of OVDs was born with the release of Healon5, the first OVD with “viscoadaptive” properties. The method of Healon5’s development is interesting because the desired rheological parameters were determined based on what was thought to be optimal for modern phacoemulsification surgery. Candidate formulations were then extensively tested, and the desired result was the creation of a highly viscous and cohesive OVD that not only possessed the best properties of Healon GV (considered to be the model of the superviscous-cohesive OVDs) but also was highly retentive in the AC throughout phacoemulsification, similar to the best dispersive OVDs. Healon5 was therefore the first OVD designed in a rheological laboratory to meet preselected rheological criteria. Another viscoadaptive OVD, iVisc Phaco (Bohus BioTech), has since been developed, but our discussion of viscoadaptives will use Healon5 as the class example because it was the first.

Viscoadaptives possess the highest V₀ values of all currently available OVDs (≥ 7 million mPas). Like Healon GV and Healon, Healon5 is also very pseudoplastic; the injection of Healon5 into the AC through a 25- or 27-gauge cannula is similar, with respect to required force and feedback sensation, to that of Healon GV. The molecular weight average is 4 million daltons, the same as Healon (Healon GV is 5 million). However, its hyaluronic acid concentration is 2.3%, higher than both Healon (1.0%) and Healon GV (1.4%). It is the increased concentration at the same high molecular weight hyaluronan as Healon that enables Healon5 to display its unique characteristics. Increasing concentration increases V₀. Also increasing concentration yields more dispersive behavior (because each polymer molecular chain will occupy a smaller domain), and therefore increased retention in the AC during surgery. This is then complicated by the fact that Healon5’s rheological behavior straddles the line between fluids and solids, which makes it viscoadaptive, that is, a viscous-cohesive fluid at low shear, and a pseudo-dispersive solid at higher shear rates (above 25 cc/min).

During phaco and I/A, it is the fluid turbulence that determines the character of an OVD’s response; the ultrasonic energy itself has little effect at the OVD surface, which is distant from the phaco probe in molecular terms. Lower viscosity-dispersives and higher viscosity cohesives behave in dispersive and cohesive manners, respectively, across the entire range of fluid turbulence and aspiration forces typically encountered in the AC during cataract surgery (Fig. 33.2). Both of these OVD classes are therefore appropriately named; their surgical behavior is consistent.

“Viscoadaptives” are so named because they adapt their behavior in surgery to the environment that the surgeon creates around them, namely, the level of fluid turbulence. Under conditions of low turbulence, Healon5 behaves just like a viscous-cohesive fluid, whereas in increased turbulence Healon5 becomes a fracturable solid, breaking into smaller pieces and therefore mimicking the behavior of a dispersive, hence the descriptive term pseudodispersive (Fig. 33.2). Interestingly, what actually happens is that as OVDs are made ever more viscous and cohesive from Ocucoat to Viscoat to Healon to Healon GV to Healon5, they begin to approach the properties of solids and start to become brittle. This is analogous to what takes place when warm chocolate pudding is placed in the refrigerator to cool. If evaluated after increasingly longer periods of cooling, the pudding will become more and more viscous. It finally reaches a point when it appears to be almost solid, and a spoon inserted into it can easily fracture its structure and be removed with a relatively solid mound of pudding on its surface.

Like the chocolate pudding, the unique fracturability of visco-adaptives like Healon5 makes them “know” whether to display cohesive or dispersive properties based on the fluid turbulence in the immediate AC environment. Therefore, during capsulorrhexis when there is no turbulence, Healon5 acts as a superviscous-cohesive fluid, whereas during the high turbulence of phaco the viscoadaptive adopts pseudodispersive solid properties, and will come out in pieces. This quality allows the surgeon to use Healon5 to maintain space throughout surgery as if it were a viscous-cohesive OVD like Healon GV. During phaco, if low flow is used (flow rates below 20 to 25 cc/min), the Healon5 mass can be broken at the iris plane, allowing evacuation of the OVD from the capsular bag while simultaneously retaining the OVD in front of the iris in a thick layer, protecting the endothelium. Later, when removal of the Healon5 is desired, we need only turn up the flow rate and direct the irrigation ports into the mass of Healon5, fracturing it and causing the pieces to move in the fluid turbulence toward the aspiration tip (Fig. 33.2).

OVD Removal Techniques

The “rock ‘n’ roll” and “two-compartment” techniques were shown to be the most efficient ways to consistently remove OVDs at the end of surgery, prior to the advent of the ultimate soft shell technique. Healon5 is best removed using one of these three techniques. The aspiration flow rate is set at 28 to 30+ cc/min, the vacuum is set at 350 to 500 mm Hg, the bottle height is set at 70 to 100 cm above the patient’s eye, and a 0.3-mm I/A tip aspiration port size is used. These settings cause sufficient turbulence to achieve easy fracturing of the Healon5 matrix. In the “rock ‘n’ roll” technique, the I/A tip is placed on the surface of the IOL and the foot pedal is fully depressed.^9,10 The I/A tip is now rolled back and forth across the surface of the IOL, allowing irrigation fluid to crack the Healon5 matrix with aspiration of the pieces. Simultaneously, slight posterior pressure from the I/A tip on the surface of the lens and alternately tilting the IOL ∼ 45 degrees gently to the left and right mobilizes the mass of OVD posterior to it. The I/A tip is not placed behind the IOL; the IOL serves as a barrier between the I/A tip and posterior capsule, protecting it while the vacuum level is high. This part of the procedure is rapid, usually lasting less than 30 seconds, but the end point is complete removal of the OVD regardless of the time elapsed, which is usually much less than 30 seconds.

In the two-compartment technique, the IOL is placed into the capsular bag and not centered, but rather displaced remote from the incision. The I/A tip is then inserted behind the IOL into the capsular bag, and I/A is commenced with settings similar to those in the “rock ‘n’ roll” technique discussed above, except that the aspiration port is kept aimed toward the IOL. Once the capsular bag is evacuated, the I/A is placed in front of the IOL and is used to center the IOL, and the remainder of the OVD is aspirated. The two-compartment technique is slightly quicker than the rock ‘n’ roll technique but involves placing the I/A between the IOL and the posterior capsule, and so slightly increases the risk of snagging the posterior capsule.¹⁰ Our preference is the ultimate soft shell technique, discussed below.

Soft Shell Viscosurgical Techniques

Background

The term soft shell technique (SST) refers to a group of viscosurgical techniques that use both dispersive and cohesive OVDs in a predetermined order and positioning method to take optimal advantage of the benefits of both OVD classes while eliminating the drawbacks of each. The SSTs are useful in all case types, but have outstanding advantages in complex surgeries. Generally, the OVDs should not mix in the eye during the period of the surgery, but rather occupy adjacent spaces within the AC. The SSTs rely on four key principles:

1. A cohesive OVD (i.e., a pressurized elastic device) in a confined space transmits pressure through adjacent fluids within the confined space. This pressurizes the entire space, transmitting the beneficial properties of cohesive OVDs throughout the entire space.
   
2. A dispersive OVD maintains its dispersive characteristics when subjected to moderate pressure from an adjacent cohesive fluid, thus preserving the properties of a lower viscosity dispersive OVD in the area where it is wanted.
   
3. Two rheologically dissimilar but transparent OVDs can be placed in adjacent spaces in the AC without significant mixing during the time period of the surgery. This utilizes, to maximum advantage, the unique properties of each, in the exact location where each is most needed. Their combined use does not blur the surgeon’s view as the cohesive OVD forces the adjacent dispersive to adopt a smooth boundary, optimizing its clarity.
   
4. It is rheologically impossible to control the surgical environment with any single OVD as well as with a specifically placed combination of rheologically different OVDs. The more different the rheological properties of the OVDs, the greater the possibilities of creating an optimized partitioned surgical environment. This reaches the current optimal limit in tri-soft shell techniques, where three different OVDs are used together: a lower viscosity dispersive, a viscoadaptive, and an aqueous solution.⁵

The Dispersive-Cohesive Viscoelastic Soft Shell Technique (Fig. 33.5)

The first SST, the dispersive-cohesive OVD SST, was described in 1999.³ After the paracentesis is made, through which intracameral lidocaine or lidocaine-phenylephrine solution is injected, the main phaco incision is created, through which a lower viscosity-dispersive viscoelastic is injected on the anterior lenticular surface to create a mound. It is important to note that in all soft shell techniques the OVDs are injected through the main cataract incision, not the side port, as this permits greater facility in positioning the OVD more accurately within the AC, and also in inflating the AC to the pressure desired. A higher viscosity-cohesive OVD is then injected onto the anterior capsule surface, below the dispersive mound, so that the incoming viscous-cohesive fills the center of the eye and pushes the lower viscosity-dispersive up and out, forming a smooth, even, pressurized layer of dispersive OVD adjacent to the corneal endothelium. This protective “soft shell” of dispersive OVD, from which all soft shell techniques derived their name, will remain in place even after the cohesive OVD is aspirated out during phaco and I/A, ensuring enhanced protection of the corneal endothelium.

After phaco and I/A, but before IOL implantation, the second step of SST can be performed if IOL folding forceps are to be used.³ This step is rarely used currently because IOL injector cartridges seal the incision as the IOL is injected, making step 2 of the SST unnecessary. However, the second, pre-IOL implantation, step of the ultimate soft shell technique (see below) is greatly advantageous in modern IOL implantation.

Several studies have experimentally validated the dispersive-cohesive OVD SST, finding less increase in postoperative central corneal thickness and less endothelial cell loss, even with dense cataracts and Fuchs’s endothelial dystrophy, than with the use of a single OVD.

Ultimate Soft Shell Technique

With the advent of viscoadaptives, new SSTs became possible. The more rheologically dissimilar the properties of the two OVDs being used, the more effective the technique. Thus arose the ultimate soft shell technique (USST), extending this principle to its practical limit by pairing a viscoadaptive OVD with balanced salt solution (BSS), which has a viscosity of only 1 mPas, the same as water.⁴ This dramatic rheological difference creates two adjacent but completely different physical environments within the AC. In the precapsulorrhexis step, an outer shell of viscoadaptive OVD is used to coat the corneal endothelium and block the incision, ensuring good AC pressurization. BSS, often containing a pharmacological agent (commonly trypan blue, lidocaine, phenylephrine, or a combination of lidocaine and phenylephrine), is then injected beneath it onto the lenticular surface to create a low viscosity workspace (Fig. 33.6a–c). The capsulorrhexis is then performed in a low viscosity aqueous environment, while the AC is highly pressurized by the blockage of the incision with the viscoadaptive OVD.

A similar soft shell arrangement is created during IOL insertion, with an outer layer of viscoadaptive OVD filling the AC entirely or partially (blocking the incisional area and extending partially into the AC) and an inner layer of BSS filling the capsular bag. The leading IOL haptic unfolds easily once it has traversed the viscoadaptive layer and enters the low viscosity BSS environment of the bag. The trailing haptic remains folded, encased in viscoadaptive near the incision. After the IOL injector is removed, the I/A device is promptly inserted into the AC, before the IOL unfolds, and when irrigation is turned on the IOL begins to fall backward into the BSS-filled capsular bag, due to increased pressure anterior to the IOL (Fig. 33.6d–g) Gentle nudging of the still-folded trailing haptic with the I/A tip positions the implant in the bag as aspiration is turned on to remove residual OVD. Because the IOL is inserted under an OVD shell into BSS, minimal OVD remains behind the IOL, eliminating the need to place the I/A behind the IOL, or to perform much rocking and rolling, to remove residual OVD. The two-compartment technique can be used, but it is rarely needed.¹¹

Modifications to the Dispersive-Cohesive OVD SST and the USST for Special Cases

White, Intumescent, or Brunescent Cataracts (Fig. 33.7)

In cases of white or brunescent cataracts, the USST can be modified to enhance anterior capsule visualization by painting trypan blue into the BSS zone directly adjacent to the anterior lens surface only. This provides targeted staining of the anterior capsule with only an extremely small quantity of dye for full effect, avoiding AC clouding and reducing the potential for toxicity. The enhanced pressurization of the AC with the viscoadaptive ensures flattening of the capsule, overcoming any posterior pressure and permitting a pressure-equalized capsulorrhexis to be easily performed.

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