where contact may be absolutely unavoidable. I like to see lenses unfolded in the bag and absolutely kept off the corneal endothelium. The same is the case with any instrumentation inside of the eye.

Knowing that pseudophakic bullous keratopathy, even today, is one of the leading diagnoses for penetrating keratoplasty, how can we avoid phaco corneal damage in a meaningful way? Let me propose a few suggestions:

1.The profound endothelial loss of the past was in the previscoelastic era, and viscoelastics can now make a huge difference! Some, however, are better than others, and there is a case to be made for dispersive viscoelastics that persist throughout the procedure and provide a buffer for inadvertent contact with the corneal endothelium. I like to use a thin layer of viscoelastic throughout the procedure. The only negative associated with this is a scalloped posterior edge that can trap bubbles and debris. DuoVisc is a small bubble of cohesive viscoelastic that can be inserted inside of the dispersive viscoelastic such that as it is removed it leaves a thin, even layer of dispersive viscoelastic (in this case, Viscoat) on the endothelial surface. Studies have demonstrated variable endothelial protection afforded by different viscoelastics; however, we clearly showed by our air bubble model that Viscoat could completely prevent this damage in association with phacoemulsification,14 and I am sure that it is a protection that under some circumstances can make the difference between endothelial survival and failure.

2.A deep chamber for all of our maneuvers is another way to be certain we are not working on or near the corneal endothelium. Some surgeons try to insert phacoemulsifiers with a totally collapsed chamber, but doing so may destroy a large swath of endothelium. If we are not comfortable that we can make any maneuver inside of the eye without contacting the corneal endothelium, there is no excuse in this age of viscoelastics why we cannot expand the space to safely enter and exit or otherwise maneuver inside of the eye. Where expense may be a concern about viscoelastics, particularly in other parts of the world, an irrigating cannula with running solution attached to the anterior chamber is another way to maintain space.

3.A deep chamber or working space is of no value if we work near the corneal endothelium. Although I do not believe we have to do everything inside the capsular bag, it makes no sense to crowd the wound, as mentioned, or to be working on pieces with emulsification energy near the

corneal endothelium. Nuclear fragments can be trapped next to the endothelium; however, simple aspiration should move them a safe distance away before we try to use emulsification energy to eliminate these pieces.

4.Phacoemulsification time should be minimized because it is directly correlated with endothelial damage and swelling. I advocate the phaco chop, which can substantially decrease phaco time, as has been documented.7 I think this difference is even more apparent in very hard nuclei where minimal phaco time is possible with multiple small chops with very clear corneas where in the past I have never been able to get a clear cornea where I know my phaco time is in the 8- to 10minute range or longer.

POSTOPERATIVE CORNEAL EDEMA

How do we handle a case with significant corneal edema on the first postoperative day? Keep in mind that early endophthalmitis can often result in unusual corneal edema often associated with unusual inflammation. The TASS syndrome certainly needs to be eliminated as a possibility, and if the edema appears to be more severe inferiorly in further postoperative visits, then we look for a nuclear chip sitting in the angle as a cause. Guttattaless Fuchs’ is another consideration, and careful slit-lamp examination of the opposite eye will usually alert us, although we have had some cases in which we did not make the diagnosis until corneal transplantation. Old trauma with profound endothelial loss can sometimes result in unexpected corneal edema. Fortunately, the usual corneal endothelial reserves are usually quite profound and the situation will get better.

There is no medical treatment that makes any difference for endothelial survival, although symptomatically with minimal levels of epithelial edema topical hypertonic agents can sometimes help. Prescribing frequent topical 5% sodium chloride, which is not helping the patient visually, however, makes absolutely no sense and is not going to help cells that are dead somehow come back to life.

I am often asked how long one waits to know whether the cornea is going to clear. I have always been pleased with the 6-week/3-month rule. This rule for me is that a corneal endothelial problem, which has not improved within 6 weeks after the time of surgery, is not going to get better. This is fairly uncommon, but I have seen this in some TASS cases and these very unhappy patients are anxious to take the next step. I am prepared to do it after 6 weeks if they look no better than they did on the first postoperative day. The majority of cases, however, will show some

clearing, in which case I use the 3-month rule, which is that at 3 months after surgery what you see is what you get. This at least gives the surgeon and the patient some specific guidelines for making the decision. Pseudophakic bullous keratopathy is an iatrogenic disease, and there is a lot we can and should be doing to prevent this uncommon but significant problem! Fortunately, corneal transplantation is very successful for this specific indication.

Prevention and appropriate adaptation is best in all of our techniques during surgery in the face of preexistent corneal pathology and to avoid irreparable damage. Nothing ruins a perfect day quite as much as a demolished cornea first noted postoperatively!

ACKNOWLEDGMENT

This work is supported in part by a grant from Research to Prevent Blindness, Inc., New York, NY, to the Department of Ophthalmology, University of Utah.

REFERENCES

1.Mamalis N, Anderson LW, Kreisler KR, Lundergan MK, Olson RJ. Changing trends in the indications for penetrating keratoplasty. Arch Ophthalmol 1992;110: 1409–1411.

2.Jones LT, Beeh MJ, Wirtschafter JD. The cornea and limbus. In: Ophthalmic Anatomy: A Manual with Some Clinical Applications. Rochester, MN: American Academy of Ophthalmology and Otolaryngology; 1970:96–103.

3.Olsen BR, McCarthy MT. Molecular structure of the sclera, cornea, and vitreous body. In: Albert DM, Jakobiec FA, eds. Principles and Practice of Ophthalmology: Basic Sciences. Philadelphia: WB Saunders; 1994: 47–49.

4.Farrell R. Corneal transparency. In: Albert DM, Jakobiec FA, eds. Principles and Practice of Ophthalmology: Basic Sciences. Philadelphia: WB Saunders; 1994:67–78.

5.Miller CA, Krachmer JH. Endothelial dystrophies. In: Kaufman HE, Barron BA, McDonald HB, Waltman SR, eds. The Cornea. New York: Churchill Livingstone; 1998:425–440.

6.Beesley RD, Olson RJ, Brady SE. The effects of prolonged phacoemulsification time on the corneal endothelium. Ann Ophthalmol 1986;18:216–222.

7.DeBry P, Olson RJ, Crandall AS. Phaco chop and divide and conquer cataract extraction: a prospective comparison of phacoemulsification energy. J Cataract Refract Surg 1998;24:689–692.

8.Olson RJ. Air and the corneal endothelium: a cat in vivo study. Arch Ophthalmol 1980;98:1283–1284.

9.Kremer I, Stiebel H, Yassur Y, Weinberger D. Sulfur hexafluoride injection for Descemet’s membrane detachment in cataract surgery [see comments]. J Cataract Refract Surg 1999;23:1449–1453.

10.Gault JA, Raber IM. Repair of Descemet’s membrane detachment with intracameral injection of 20% sulfur hexafluoride gas. Cornea 1996;15:483–489.

11.Monson MC, Mamalis N, Olson RJ. Toxic anterior segment inflammation following cataract surgery. J Cataract Refract Surg 1992;18:184–189.

12.Richburg FA, Reidy JJ, Apple DJ, Olson RJ. Sterile hypopyon secondary to ultrasonic cleaning solution. J Cataract Refract Surg 1986;12:248–251.

13.Olson RJ, Apple DJ. Unexplained ocular toxicity after cataract-IOL surgery. J Cataract Refract Surg 1987;13: 688–689.

14.Monson MC, Tamura M, Mamalis N, Olson RJ, Olson RJ. The protective effects of Healon and Occucoat against air bubble endothelial damage during phacoemulsification. J Cataract Refract Surg 1991;17:613– 616.

Two distinct methodologies for the use of ophthalmic viscosurgical devices (OVDs)1 (viscoelastics) in dealing with problems in cataract surgery have been developed recently. They both considerably enhance our ability to manage previously difficult cases. These are the dispersive-cohesive viscoelastic soft shell technique, and a new viscoadaptive OVD, Healon5. This chapter discusses these two viscoelastic strategies sequentially. It is the author’s belief that presenting the strategies in this way gives the reader a clearer understanding of how to approach specific problems that are likely to occur in the operating room.

The soft shell technique was developed prior to the advent of Healon5. Because Healon5 use represents a new and different viscoelastic technique, the discussion in the soft shell section below refers to viscoelastics prior to Healon5. The Healon5 section, which follows the soft shell discussion, outlines methods for management of similar problems with Healon5. The advent of Healon5 does not make the soft shell technique obsolete. It will, however, divide ophthalmologists into two groups based on their preference for a “universal,” viscoelastic system. The first group, the soft shell group, are those surgeons who want the benefits of the use of two classes of viscoelastics, each with distinct properties, in an effort to obtain the maximum rheologic potential from each class. They recognize that the two classes are

different, with different advantages and disadvantages. The second group, the Healon5 group, are ophthalmologists who feel most comfortable working in a high viscosity, stable surgical environment. They have generally preferred highly viscous-cohe- sive viscoelastics in the past. However, the advent of the soft shell technique has made them aware of the deficiencies of highly viscous cohesive OVDs when used alone. As they recognize the benefits of this category of viscoelastic, but also desire some of the properties of lower viscosity viscoelastics, these surgeons will gravitate toward the unique potential of the viscoadaptive Healon5, perceiving it as the universal option.

The best advice is to study and understand both approaches to viscosurgery for a better perspective on the options now available with the OVDs.

DISPERSIVE AND COHESIVE

VISCOELASTICS

BACKGROUND

There is a large variety of ophthalmic viscosurgical devices now available for use in intraocular surgery (Table 24–1). Each possesses different chemical and physical rheologic properties.2 The question of which is the best viscoelastic for cataract surgery has

led the author to investigate those rheologic properties that are most important in anterior segment surgery. In this investigation no general consensus on the ideal viscoelastic has emerged. This is confirmed by the essentially unchanged market share distribution of vastly different viscoelastics over the past decade. The lack of consensus among surgeons on what viscoelastic properties are best suggests that different viscoelastic properties may be useful to ophthalmic surgeons in different surgical situations. Therefore, rather than looking for the “best” viscoelastic, it might be best to analyze their different properties in an effort to develop a new surgical approach that uses both cohesive and dispersive viscoelastics sequentially to avoid mixing them. Thus, the surgeon fully takes advantage of the positive attributes of each type of viscoelastic while minimizing drawbacks.

CLASSIFICATION OF OPHTHALMIC

VISCOELASTICS

A viscoelastic preparation intended for intraocular use must be an isotonic, pH balanced, aqueous solution. The acceptable range of physical properties is governed by the molecular nature, chain length, and concentration of the rheologically active polymer constituent. In an effort to devise criteria for a surgi-

cally useful classification system, it is essential to identify the rheologic parameters that best apply to the viscoelastic in relationship to its behavior in surgery. Currently the two most useful parameters for such a classification are (1) zero-shear viscosity (V0), and (2) cohesive or dispersive behavior (relative degree of cohesion).3,4 Zero-shear viscosity correlates with both the molecular weight of the rheologically active viscoelastic component and its concentration. It can be used to establish a ranking of ophthalmic viscoelastics for the purpose of classification (Table 24–1 and Fig. 24–1). Long-chain ophthalmic viscoelastic molecules tend to entangle in solution, causing them to aggregate. They are therefore referred to as cohesive. As zero-shear viscosity declines below 100,000 mPs (millipascal-seconds), molecular chain entanglement becomes a far less significant factor. (The unit mPas was previously centipoise or cps, where 1 mPas = 1 cps, the dynamic viscosity of water.) Consequently, less viscous viscoelastics tend to be easily broken up, and are accordingly referred to as dispersive.

Because cohesion of all currently marketed viscoelastics correlates with zero shear viscosity, as well as with the molecular weight of the rheologically active constituent (but not in a simple, linear relationship), the degree of cohesion falls neatly into the same classification with zero shear viscosity, above (Fig. 24–2). Future viscoelastics may be manufac-

FIGURE 24–1 Pseudoplasticity curves of common commercial ophthalmic viscoelastics. Pseudoplasticity curves of common commercial ophthalmic viscosurgical devices (OVDs) are usually plotted as shown here with log dynamic viscosity (millipascal-seconds) vs. log shear rate (seconds 1). The zero-shear or stationary viscosity is the level at the far left of each curve, and is indicative of the ability of the OVD to maintain space when stationary. The viscosity level in the mid–shear rate ranges (100 to 101) represents the resistance offered by the OVD to the normal rate of movement of surgical instruments through the eye during surgery. The viscosity at high rates of shear (about 103) is indicative of the resistance offered by the OVD to flow through a small cannula. Different laboratories tend to report these data with significant variation. It is therefore wise to consider differences of less than 10% in any given point as equivalent.

tured with viscosity and cohesion somewhat dissociated from each other, especially if a rheologic component other than hyaluronic acid is used as the basic constituent. Then these parameters may not correlate as well. Therefore, one of these parameters alone is insufficient for classification.

The surgical tasks for which ophthalmic viscoelastics are utilized are generally facilitated by higher zero-shear viscosity. Highly viscous OVDs tend invariably to be more cohesive (less dispersive). However, in certain situations dispersive rather than cohesive behavior may be desirable (e.g., to retain a thick layer of viscoelastic adjacent to the corneal endothelium to protect it from the trauma of phaco or irrigation). In such circumstances it is often better to accept lower viscosity to gain greater dispersion, rather than the reverse. It is helpful, therefore, when considering surgical use, to classify ophthalmic viscoelastics based on zero-shear viscosity as well as the cohesive or dispersive nature.

HIGHER VISCOSITY-COHESIVE

VISCOELASTICS

All current OVDs in this category possess, as their active rheologic agent, various concentrations and chain lengths of non–cross-linked long-chain hyaluronic acid. In addition all have zero-shear dynamic viscosities greater than 100,000 mPas. Super–viscous- cohesive viscoelastics, a subcategory including MicroVisc Plus(iVisc Plus in Canada) and Healon GV, have zero-shear viscosities exceeding 1,000,000 mPas. Viscous-cohesive viscoelastics, including, among others, MicroVisc, Allervisc Plus (Viscorneal Plus), ProVisc, Healon, Biolon, Allervisc (Viscorneal), Amvisc, and Amvisc Plus (Table 24–1) possess zeroshear viscosities between 100,000 and 1,000,000 mPas. Clinical trials have shown that the members of

FIGURE 24–2 Cohesive and dispersive viscoelastics. Relative cohesion of ophthalmic OVDs correlates with zero shear viscosity and molecular weight. When OVDs are plotted on a graph where log zero shear viscosity is on the ordinate and log molecular weight on the abscissa, cohesive and dispersive viscoelastics cluster in their respective groups.

each of these subcategories share similar physical properties, behave in a similar fashion intraoperatively, and lead to comparable surgical results. However, the super–viscous-cohesive viscoelastics appear to have an advantage over the regular viscous-cohe- sive viscoelastics in facilitating surgical maneuvers, ease of removal, and endothelial protection.5–8

Advantages (Table 24–2)

Zero-shear viscosity correlates well with elasticity.9 Highly viscous-cohesive OVDs are the best for creating space with their viscosity, and preserving it by their elasticity. Elasticity allows the OVD to contract and expand in the presence of an ocular pulse or externally applied force, without being expelled from the eye. These OVDs can displace and stabilize tissues in the surgical environment. Balanced intraocular pressure (i.e., the maintenance of a deep anterior chamber after the creation of the cataract incision), is best accomplished with an elastic and viscous substance. Only the highly viscous-cohesive elastic OVDs are capable of neutralizing posterior positive pressure. I have referred to this OVD-induced pres- sure-neutral situation as “pressure equalized cataract surgery.”

As a result, the higher viscosity-cohesive OVDs are best used in problem cases to create space and stability where it is otherwise inadequate. This benefit can be recognized during surgery when topical anesthesia is utilized. There is often shallowing of the anterior chamber during capsulorrhexis. This is due to the tone of the nonparalyzed extraocular muscles. CCC is enhanced by using viscoelastic to pressurize the anterior chamber equal to that of the posterior pressure. The effect of flattening the anterior convexity of the cataractous lens is to reduce the vector force compelling the tear to extend toward the equator. A round CCC of the appropriate size is therefore more easily accomplished in a pressure equalized environment. A corollary of this principle pertains to the management of a tear that begins to extend toward the periphery. External sources of ex-

cessive posterior pressure such as a tight lid speculum or drapes should be alleviated after injection of a superviscous cohesive OVD to flatten the cataractous lens surface, and the CCC should be completed as described in Chapter 5.

Other uses include expanding a shallow anterior chamber in hyperopia, facilitating the insertion of the phacoemulsification tip where there is positive posterior pressure or a flaccid iris, enlarging a small pupil, dissecting iris-lens capsule adhesions, and assisting during the implantation of foldable intraocular lenses (IOLs) so that the haptic of the lens cannot catch on a fold in the posterior capsule, resulting in a torn capsule (Table 24–2).

Disadvantages

The high cohesion of viscous-cohesive and super– viscous-cohesive OVDs results in ease of removal by irrigation and aspiration at the end of the surgical procedure. However, the same cohesive behavior results in cohesive OVDs being rapidly washed out of the anterior chamber during phaco. Although an invisible thin layer of hyaluronate bound to endothelial cell membrane specific binding sites remains behind,10 there may be insufficient protection of corneal endothelium. Furthermore, cohesive viscoelastics are unable to partition fluid spaces within the anterior chamber (AC). It is therefore impossible, using a cohesive viscoelastic, to sequester an OVD-coated and protected structure in the AC, while working on the other side of the AC with the irrigation and aspiration (I&A) tip, without aspirating the OVD and the structure that should have been sequestered (vitreous, frayed iris, etc.) during phaco or I&A.

LOWER-VISCOSITY DISPERSIVE

VISCOELASTICS

Lower-viscosity dispersive viscoelastics include all current OVDs with zero-shear viscosities of less than 100,000 mPs (to date all dispersives marketed have zero shear viscosities less than 50,000 mPs). In these

To deepen the anterior chamber in hyperopic eyes to insert phaco

To stabilize a difficult capsulorrhexis

To enlarge small pupils

To dissect adhesions

To implant foldable intraocular lenses

To protect corneas with Fuchs’ endothelial dystrophy

To isolate a piece of frayed iris from aspiration by the phaco tip

To isolate an area of disinserted zonules

To isolate vitreous behind a small hole in the posterior capsule

To partition the anterior chamber

viscoelastics, molecular chain entanglement is less prevalent. Cohesion tends to be significantly weaker, resulting in a tendency for the OVD to disperse when injected into the anterior chamber.

There are two subgroups. The first, the medium viscosity dispersive viscoelastics, possess zero-shear viscosities between 10,000 and 100,000 mPas. They include Viscoat, Cellugel, and Vitrax. The second, the very low viscosity dispersive viscoelastics, consisting of all the unmodified hydroxypropyl methylcelluloses, includes iCell, Ocuvis, Ocucoat, Adatocel, Hymecel, Visilon, Celoftal, and many others (Table 24–1).

Advantages (Table 24–2)

Surgically, the most useful properties of dispersive viscoelastics are their resistance to aspiration and their ability to partition spaces. A 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 al11 to improve the retention of OVDs in the anterior chamber in the presence of I&A, allowing dispersive OVDs to remain adjacent to the corneal endothelium throughout both phacoemulsification and I&A.

Equally important, dispersive viscoelastics are useful as surgical tools in situations where it may become necessary to selectively move or isolate a structure in the anterior chamber (e.g., holding back vitreous at an area of zonule disinsertion, moving a strand of frayed iris, plugging a small hole in the posterior capsule, etc.). They are capable of partitioning the anterior chamber into two separate spaces—a viscoelastic-occupied space and a surgical zone in which phaco or I&A can be continued— without the two areas mixing.

Disadvantages

The major drawback of lower viscosity-dispersive viscoelastics is that their relatively low viscosity and elasticity do not allow them to maintain or stabilize spaces as well as higher viscosity-cohesive OVDs (e.g., in the performance of a capsulorrhexis). In addition, lower viscosity-dispersive viscoelastics tend to be aspirated in small fragments during phaco and I&A. This leads to an irregular viscoelastic-aqueous interface, which, to some extent, obscures the surgeon’s view of the procedure. Furthermore, these OVDs tend to trap particulate matter and microbubbles generated during phacoemulsification, partially obscuring the surgeon’s view of the posterior capsule during surgery. Lower viscosity-dispersive viscoelastics, because of their low cohesion, are more difficult to remove at the end of the surgical procedure. Assia et al12 demonstrated, in a controlled in

vitro study, that lower viscosity-dispersive viscoelastics such as Viscoat, Ocucoat, and Orcolon, may take more than seven times longer to remove than highly viscous-cohesive viscoelastics such as Healon and Healon GV. The additional manipulation and aspiration required to completely remove dispersive viscoelastics may actually increase the likelihood of complications such as endothelial damage or puncturing of the posterior capsule, and may partially offset any benefit derived from their use in surgery.

THE DISPERSIVE-COHESIVE

VISCOELASTIC SOFT SHELL TECHNIQUE

The dispersive-cohesive viscoelastic soft shell technique is a method that uses both dispersive and cohesive viscoelastics sequentially to derive the benefits of both viscoelastic types and eliminate the drawbacks of each. The technique is beneficial in all types of cases. However, it has outstanding advantages in complicated cases. Generally, the viscoelastics should not mix in the eye, but should occupy adjacent spaces within the anterior chamber. The technique relies upon three principles:

1.A cohesive viscoelastic (e.g., a pressurized elastic device) in a confined space will transmit pressure through adjacent fluids within the confined space. This will pressurize the entire space, thus transmitting the beneficial properties of viscous cohesive viscoelastics throughout the entire space.

2.A dispersive viscoelastic will maintain its dispersive characteristics when subjected to moderate pressure from an adjacent cohesive fluid, thus preserving the properties of lower viscosity dispersive viscoelastics in the area where needed.

3.Two rheologically dissimilar but transparent viscoelastics can be placed in adjacent spaces within the anterior chamber, without significant mixing during the time period of the surgery. This utilizes, to maximum advantage, the unique properties of each viscoelastic, in the exact location where it is most needed. Their combined use will not result in blurring of the surgeon’s view.

METHOD (FIG. 24–3)

After incision and paracentesis, a lower viscositydispersive viscoelastic, such as Viscoat, is injected on the anterior surface of the lens in such a manner as to create a mound. Next, a high viscosity-cohesive viscoelastic, such as Provisc or Healon GV, is injected between the anterior capsule and the mound so that the incoming viscous-cohesive viscoelastic fills the center of the eye and pushes the lower viscosity-

dispersive viscoelastic up and out, thus eventually forming a smooth, even, pressurized layer of dispersive viscoelastic adjacent to the corneal endothelium. The high viscosity-cohesive viscoelastic permits creation of space and pressurization of the anterior chamber. The deep and stable anterior chamber will facilitate capsulorrhexis in a manner that could never be achieved with a lower viscosity-dispersive alone. Simultaneously, the lower viscosity-disper- sive forms an even, protective layer adjacent to the corneal endothelial cells. It will remain in place, even after the cohesive viscoelastic is irrigated and aspirated out during phaco, to protect the endothelium from the cavitation wave, microbubbles, nuclear chips, and irrigation fluid.

After phaco and I&A, but before implantation of the IOL, the OVDs are injected in reverse order. The high viscosity-cohesive viscoelastic is injected first, followed by the lower viscosity-dispersive viscoelastic into its center. The high viscosity-cohesive viscoelastic will pressurize the anterior chamber expanding the capsular bag. This will prevent wrinkles in the posterior capsule and facilitate foldable lens implantation. Its presence in the periphery of the anterior chamber stabilizes the iris and lens capsule so that they do not move when the IOL is introduced. It also protects the delicate structures of the eye against the sometimes rapid unfolding of silicone IOLs. The low viscositydispersive viscoelastic, occupying the center of the anterior chamber and capsular bag, allows for easier movement of instruments and the IOL through the viscoelastic. Because of the concentric partitioning of the anterior chamber into two different viscoelasticfilled spaces, the unfolding of the IOL in the central lower viscosity-dispersive–filled space transmits far less force into the surrounding high viscosity-cohesive OVD–filled space than would occur with either type of OVD alone.

After lens implantation, the OVDs are aspirated from the eye. By virtue of the lower viscosity-disper- sive viscoelastic being enveloped by the high viscos- ity-cohesive viscoelastic, removal of both viscoelastics is quick and easy, similar to the experience of removing a high viscosity-cohesive OVD alone.

Viscoelastic combinations for the soft shell technique are Healon or Healon GV with Viscoat or DuoVisc (Alcon, consisting of Provisc and Viscoat). Similarly, Healon GV or iVisc (MicroVisc) can be used with HPMC (iCell, or other).

APPLICATION OF THE SOFT SHELL

TECHNIQUE TO SPECIFIC PROBLEMS

Fuchs’ Endothelial Dystrophy

In cases of Fuchs’ endothelial dystrophy, the soft shell technique is performed in the usual fashion, ex-

cept that at the completion of surgery no attempt is made to remove the Viscoat layer residing adjacent to the endothelium. Instead, the patient is treated prophylactically with intraocular pressure–reducing agents to prevent a viscoelastic-induced unacceptable postoperative intraocular pressure rise.13

Broken Zonules

The soft shell technique must undergo slight modification in the management of cases of broken zonules, whether traumatic or congenital. The objective is to isolate a specific area of the anterior chamber from the turbulence of the irrigation stream during phaco or I&A. In this situation the dispersive OVD is injected to cover the area of absent or disinserted zonules. Cohesive viscoelastic is then injected behind the dispersive. This will force the dispersive OVD into the area of zonular absence, pushing exposed vitreous back behind the capsular bag equatorial plane. The area of weak or absent zonules and vitreous is sequestered from the remaining procedure. A capsular tension ring, if available, can then be inserted into the capsular bag to support the capsular bag equator and further enhance the safety of the procedure. Once the phaco is begun, the cohesive viscoelastic is aspirated, but the dispersive remains in position, to isolate and protect the vitreous surface in the area of the broken zonules (Fig. 24–3F).

A Small Hole in the Posterior Capsule

Occasionally, during phaco, a small hole is punched in the posterior capsule. When recognized, the surgeon must first examine the rent to be sure that there is a smooth round edge. If not, a posterior capsulorrhexis should be completed under viscoelastic, using the soft shell technique. This is accomplished by first covering the posterior capsule in the area of the hole with a dispersive OVD. Next, the AC is gently filled with a cohesive OVD. Excessive pressure is to be avoided as this might extend the tear. A posterior capsulorrhexis is then completed with forceps. Once the edge of the hole is determined to be continuous, more cohesive viscoelastic is gently injected to slightly pressurize the AC. This serves to push any protruding vitreous back through the hole. With the AC and capsular bag now stabilized, phaco and I&A, using low-flow parameters, or IOL insertion can be completed.

Frayed Iris During Phaco

Unfortunately, when the iris is damaged by the phaco tip, it will fray. The iris strands then are repeatedly aspirated by the phaco tip almost as if the two were magnetic. This will produce progressive and eventually severe damage to the iris. The result may be so significant as to create postoperative refractive problems. In addition progressive damage to the blood–aqueous barrier will give rise to pro-

longed postoperative inflammatory changes. The soft shell technique is therefore utilized to segregate the damaged iris from the phaco tip. The area of frayed iris is covered with the dispersive OVD, and the cohesive OVD is injected behind it to pressurize the AC and force the strands of iris out of harm’s way. When the phaco is resumed, the cohesive viscoelastic comes out with the fluid flow. The dispersive remains behind to isolate and protect the area of frayed iris strands.

Miscellaneous

With a bit of creativity, the surgeon may find many situations where the soft shell technique can be utilized to either protect or sequester tissue during the procedure, enhancing surgical safety.

HEALON5

The viscoelastic soft shell technique was introduced to avoid having to accept the detriments of either cohesive or dispersive OVDs when a single OVD is chosen for use because of one of its beneficial characteristics. Using OVDs that have divergent physical properties together allows the surgeon to exploit the benefits of both OVD classes and to avoid the pitfalls encountered with the use of either class of OVDs alone.14,15 The disadvantage of the viscoelastic soft shell technique is that it requires the use of two separate syringes of viscoelastic creating increased inconvenience and cost. Additionally, there is the risk of using the viscoelastics in the wrong order, not only negating the benefits but creating a problem that may require additional steps to correct.

In an effort to address the above problems with a single OVD, Healon5, an OVD with “viscoadaptive” properties, has been recently designed. The method of development of this OVD is interesting because the desired rheologic parameters were determined based on what was thought to be optimal for modern phacoemulsification surgery. Candidate formulations were then extensively tested. The desired result was the creation of a highly viscous viscoelastic that not only possessed the best properties of Healon GV (considered to be the model of the super–viscous- cohesive viscoelastics) but also was highly retentive in the anterior chamber throughout phacoemulsification similar to the best dispersive OVDs. As noted previously, viscous-cohesive viscoelastics generally have poorer retention than dispersive viscoelastics.11

BIOMECHANICAL PROPERTIES

It can be seen from Table 24–3 that Healon5 has the highest zero shear viscosity (7 million mPs) of any viscoelastic yet marketed. Like Healon GV and

TABLE 24–3 COMPARATIVE PROPERTIES OF

HEALON5

Healon, Healon5 is also very pseudoplastic (Fig. 24–4). The pseudoplasticity curve of Healon5 makes injection into the anterior chamber through a 25or 27-gauge cannula similar, with respect to required force and feedback sensation, to that with Healon GV. The molecular weight average is 4 million daltons, similar to that of Healon. However, its concentration is 2.3%, compared to Healon at 1.0% and Healon GV at 1.4%. It is the increased concentration of hyaluronic acid at this high molecular weight that allows Healon5 to display its unique characteristics. Increasing the concentration of hyaluronic acid will effect zero shear viscosity, and, if the molecular mass remains constant, will impart the property of increased retention by behaving in a more dispersive fashion than a lower concentration, similar molecular mass OVD.

The essence of Healon5 then is the rheologic idiosyncrasy that makes it unique in being both viscouscohesive and retentive. Figure 24–5 illustrates the outstanding characteristics of Healon5.

During phaco and I&A, fluid turbulence is the factor that determines the mode of OVD response. The ultrasonic phaco energy itself has little effect distant from the phaco. Lower viscosity-dispersive OVDs behave as dispersives over the entire range of fluid turbulence and aspiration forces normally encountered in the AC during cataract surgery. Similarly, viscous-cohesive viscoelastics behave as viscous devices over the entire range of turbulence and aspiration normally encountered in intraocular surgery. Both of these viscoelastic classes are therefore appropriately named. Their behavior, across the range of conditions we expose them to, is consistent.

Healon5 is referred to as “viscoadaptive” because under conditions of low turbulence it behaves just like a viscous-cohesive device. In contrast, when the turbulence is increased, Healon5 fractures into smaller pieces and therefore mimics the behavior of dispersive OVDs. Interestingly, what actually happens is that, as viscoelastics 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 be found to have become more and more viscous. Finally it reaches a point when it appears to be almost solid, as a spoon inserted into it can easily fracture its structure and be easily removed with a relatively solid mound of pudding sitting on its surface.

Like the chocolate pudding, the unique fracturable quality of Healon5 makes it “viscoadaptive.” A viscoadaptive viscoelastic is one that spontaneously adapts its rheologic behavior to the fluid environment created by the surgeon. The viscoadaptive “knows” whether to behave as a cohesive or dispersive device based on a predetermined clue signaled

by the surgeon. The clue chosen is one that is appropriate to the task at hand: the fluid turbulence in the environment of the viscoadaptive. Therefore, during capsulorrhexis, when there is no turbulence, the viscoadaptive acts as a super–viscous-cohesive, whereas during phacoemulsification, when fluid turbulence is relatively high, the viscoadaptive adopts pseudodispersive properties, due to its fracturability. This quality allows the surgeon to use Healon5 to maintain space throughout surgery as if it were a viscouscohesive OVD like Healon GV. During phaco, however, if low flow is used, 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, similar to a dispersive OVD. On the other hand, 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 it to move in the fluid turbulence toward the aspiration tip.

USES

The author acted as a consultant to Pharmacia and Upjohn for the development of Healon5. During the process of development, we wanted to see if this type of viscoadaptive device was really superior to conventional viscous-cohesive and lower viscositydispersive viscoelastics during all phases of cataract surgery. The world’s leading cataract surgeons were therefore asked to take part in masked handling tests on eye bank eyes in simulated phaco surgery, using the Miyake-type preparation of eye bank eyes. The cumulative results of these tests are shown in Figure 24–6. It can be seen from the figure that although Healon5 was not judged as absolutely the best at every stage of phacoemulsification, it was judged either best, or insignificantly different from the best, at each and every stage of phaco surgery, whereas cohesive and dispersive viscoelastic groups both had decidedly weak performance in some aspects of phaco surgery. In fact, Healon5 is unique in having been judged as excellent for every surgical task.

USES IN COMPLICATED SURGERY

The fracturable characteristic of Healon5 allows it to be substituted for the soft shell technique. Therefore, rather than sequential use of two viscoelastics, one viscoelastic is all that is necessary to (1) isolate endothelium in Fuchs’ endothelial dystrophy, (2) isolate vitreous in congenital or traumatic zonular dehiscence, (3) isolate strands of frayed iris, and (4) provide support to perform posterior capsulorrhexis and isolate small holes in the posterior capsule. Healon5 can therefore be considered a substitute viscoelastic for soft shell technique in that it can be adapted to any intraocular surgical circumstance, as long as the concept of fracturability is understood and applied by the operating surgeon.

REMOVAL

Like all viscoelastics, Healon5 must be removed as completely as possible from the eye at the completion of surgical procedure. This is necessary to avoid unacceptable postoperative elevations of intraocular pressure. The “rock ’n’ roll” technique of viscoelastic removal had been shown to be the most efficient