Ординатура / Офтальмология / Английские материалы / Modern Cataract Surgery_Kohnen_2002

50Patel BCK, Clinch TE, Burns TA, Shomaker ST, Jessen R, Crandall AS: Prospective evaluation of topical versus retrobulbar anesthesia: A converting surgeon’s experience. J Cataract Refract Surg 1998;24:853–860.

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Roberto Bellucci, MD, Via Degli Abeti 17, I–25087 Salò/BS (Italy) Tel. 39 036 543 678, Fax 39 036 543 678, E-Mail robbell@tin.it

Results and Discussion

In order to understand why a new OVD is needed in modern cataract surgery, we must first understand the OVDs we had before viscoadaptives, and their limitations. Let us begin by looking at the classification of OVDs, and discuss the relative merits and drawbacks of each class, before viscoadaptives.

In order to classify OVDs we must first decide which parameters to use as the basis of the classification. There are numerous chemical and physical properties that could be chosen, but the two that play the most important roles in determining utility in cataract surgery are zero shear viscosity (i.e. the viscosity of the material when it is at rest) and its relative cohesion or dispersion [1]. Table 1 illustrates the classification of OVDs based upon these two parameters, dividing OVDs into two broad groups: the higher viscosity cohesives and the lower viscosity dispersives. We can see that viscoadaptives fit into the higher viscosity cohesive group, but we will initially confine our discussion to the subgroups that existed prior to the advent of viscoadaptives.

If we ignore the viscoadaptive subgroup and concentrate on the choices we had available before viscoadaptives, we see two groups, each divided into two subgroups. In each group, the subgroup on the top, with higher zero shear viscosity, performs better in cataract surgery than the lower viscosity subgroup below it. However, when looked at by major grouping, the higher viscosity cohesives and lower viscosity dispersives have opposite surgical benefits and drawbacks (table 2).

We, as surgeons, therefore find ourselves in a difficult position without viscoadaptives. If we want to obtain maximum anterior chamber (AC) depth and stability in a shallow chambered hyperope, to allow easier insertion of the phaco tip, we must accept the fact that if we choose a higher viscosity cohesive OVD, and our surgery is prolonged or difficult, the OVD chosen may not be retained in the AC long enough to yield optimal protection for the corneal endothelial cells. On the other hand, if we choose a lower viscosity dispersive OVD, because the AC is shallow and we are concerned for the endothelial cells, we may not be able to deepen the AC sufficiently to allow safe insertion of the phaco at all. Furthermore, the irregular fracture boundaries seen with dispersives may obscure our view of the posterior capsule to the extent that in our zeal to overly protect the endothelium, we expose the posterior capsule to increased risk of rupture due to impaired visibility.

It is precisely for these reasons that I devised the dispersive-cohesive viscoelastic soft shell technique a number of years ago. This method uses a dispersive and cohesive viscoelastic together, sequentially, in order to take advantage of the best characteristics of both types, and neutralize the drawbacks of each [2]. The viscoelastic soft shell technique gets around the problems

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Table 2. Best uses and drawbacks of OVD groups

Higher viscosity cohesives

Best uses

Create and preserve spaces

Displace and stabilize tissues

Pressurize the AC

Clear view of posterior capsule during phaco

Disadvantages

Leave AC too quickly during I/A or phaco

Suboptimal endothelial protection

Unable to partition spaces

Lower viscosity dispersives

Best uses

Remain adjacent to corneal endothelium throughout phaco

Selectively move and isolate

Partition spaces

Disadvantages

Do not maintain spaces or stabilize as well Irregular fracture boundaries obscure view of posterior capsule

More difficult to remove at the end of the procedure

encountered in selecting a single viscoelastic, as described above, in cataract surgery. But this technique demands that the surgeon bear the cost and inconvenience of two OVDs for each procedure, and further requires that the surgeon learn to use the two OVDs in the proper sequence and relative amounts in every possible circumstance. Unfortunately, this leaves the technique prone to improper use. This makes the soft shell technique, although excellent, less than ideal.

In September of 1996, I was retained by Pharmacia & Upjohn to work with them to develop a new OVD that would possess all of the best properties of Healon®GV, and yet be retained in the AC, at least as well as the best dispersive OVDs throughout the phaco procedure, and permit partitioning of the AC into an OVD-protected space and a surgical zone, where phaco or I/A could be carried out. Healon®5 is the result of this effort, and the unique properties, which make it viscoadaptive, are best described by explaining the behavior of Healon®5 when compared to previous OVDs [3, 4]. MicroVisc® Phaco was recently introduced as the second viscoadaptive in the world.

‘Viscoadaptive’ means that somehow the rheologic molecule adapts its behavior to the intended surgical task, without the surgeon having to do anything,

Flow rate (cm3/min)

Fig. 1. Viscoadaptive response to fluid turbulence. The rheologic behavior of ophthalmic viscosurgical devices is dependent upon the environmental stress that the molecule is exposed to. In phacoemulsification, the ultrasonic tip is too remote from most of the OVD to have any effect, but the fluid turbulence, which is a function of flow rate, induces stress upon the OVD, which may break it up. We can see from the illustration that under all commonly encountered flow rates in phacoemulsification, Viscoat, a typical dispersive OVD, behaves in a dispersive fashion, whereas Healon GV behaves in a cohesive fashion. Healon5, on the other hand, changes its behavior from cohesive to fracturable, or ‘pseudo-dispersive’ at flow rates around 25 cm3/min, enabling the surgeon to select the behavior wanted by altering the BSS flow rate.

except perform the task. Figure 1 explains how this is achieved. Viscoadaptivity was achieved by increasing the concentration of the rheologically effective polymer (sodium hyaluronate) to the point that the OVD solution became so viscous that its properties began to approach those of a solid, hence becoming fracturable. This is analogous to the behavior of chocolate pudding. When it is initially made, it is watery, and a child will spill most of it over their clothes when trying to eat it. But, after placing the pudding in the refrigerator for a while, it becomes sufficiently rigid for a piece to be fractured out of it with a spoon. Similarly, in a viscoadaptive, the interchain entanglement strength exceeds the intrachain carbon to carbon bond strength, to the point that imposed stress will cause the chains to fracture, disassembling the mass, instead of untangling to reshape the mass.

During ophthalmic surgery, the behavior of the OVD depends upon its response to the fluid turbulence (the ‘stress’) in its environment. The ultrasonic energy of the phaco tip is felt only in the immediate vicinity of the needle tip, whereas the OVD in the entire AC is exposed to the fluid turbulence of irrigation, and it is this turbulence that affects its behavior. As we can see in figure 1, created to demonstrate the viscoadaptivity of Healon®5, Viscoat® demonstrates dispersive behavior over the entire range of fluid turbulence (proportional to flow rate) normally encountered in phaco surgery. Similarly, Healon®GV demonstrates cohesive behavior over this entire range of fluid flow rates. Healon®5, however, is different. When exposed to low flow rates, it behaves like a super viscous cohesive device, as if it were super Healon®GV, demonstrating zero shear viscosity three and a half times greater than Healon®GV. However, as flow rates increase above 25 cm3/min, Healon®5 begins to fracture into smaller pieces, making its behavior appear to mimic some of the properties of dispersive viscoelastics, behavior we have come to describe as pseudodispersive. It is this duality of function that allows viscoadaptives to be effective replacements for both cohesive and dispersive OVDs in cataract surgery. This is at first surprising when the pseudoplasticity curves of the two available viscoadaptives are reviewed (fig. 2).

Figure 2 shows the pseudoplasticity curve of Healon®5 and iVisc® Phaco (MicroVisc® Phaco). Most of the work done quantifying the behavior of viscoadaptives has been done on Healon®5, and inferred to also apply to iVisc® Phaco. In the description below, I will therefore discuss the information that we already have on Healon®5, and generalize in the instances that the same has been shown to apply to iVisc® Phaco.

Because of high pseudoplasticity, viscoadaptives are easily injected through small-bore cannulas, but one gauge size larger bore is usually preferred in comparison to super viscous cohesives like Healon®GV. They behave like super Healon®GV to yield excellent pressurization of the AC and stability for capsulorhexis. During the phaco part of the procedure, viscoadaptives can be fractured at the pupillary plane, because the turbulence behind the iris exceeds that in front of the iris, thus emptying the capsular bag of OVD, while the AC remains full of immobile viscoelastic, effectively preventing any irrigation of the corneal endothelial cells. For IOL implantation, viscoadaptives maintain the stability of the capsular bag and AC like super Healon®GV, while removal requires some special considerations (see below under removal).

What ultimately determines the utility of a new OVD is not just that it can be used to perform familiar tasks as well as currently available OVDs, but rather that it enables ophthalmic surgeons to do things they could not do before. It was the ability to safely implant IOLs while protecting the endothelium that encouraged us to first use Healon®, despite worrisome reports of severe postoperative IOP spikes during the first few months after the release of Healon® in 1980.

log shear rate (s 1)

Fig. 2. Pseudoplasticity curves of viscoadaptives. The pseudoplasticity curves of viscoadaptives are compared to other common ophthalmic viscosurgical devices. Both Healon5 and iVisc (MicroVisc) Phaco have higher zero shear viscosities than Healon GV, and are very pseudoplastic as demonstrated by their low viscosity at log shear rate 3 (shear rate 1,000). An examination of this curve makes it clear that there is more to the story of viscoadaptivity than is evident in pseudoplasticity curves. Indeed their behavior demonstrated in figure 1 must also be taken into account.

Similarly, it is the new things we can do with viscoadaptives that will encourage surgeons to change over from their current favorite OVD. To date, surgeons that have moved over to Healon®5 rarely go back. Enough experience with iVisc® Phaco has not yet been accumulated to draw conclusions, but it also seems very promising.

To date, the following new things that can be done with viscoadaptives have been identified:

(1)Dissect the anterior hyaloid face from the posterior capsule in pediatric cataract surgery. I first heard this described at conferences by Robert Stegmann of South Africa. He claims that the use of Healon®5 for this makes the performance of a posterior capsulorhexis in young children considerably easier.

(2)Perform phaco tumble techniques better. David Brown of Florida has popularized phaco tumble techniques. Healon®5 greatly stabilizes the vertically tilted nucleus and preserves the depth of the AC, making this technique easier and safer.

(3)The use of trypan blue (Vision blue®) in mature cataracts is much easier with Healon®5, making phacoemulsification of mature white cataracts a simple matter. See below (Ultimate Soft Shell Technique).

(4)Do viscosurgery with only partial filling of chamber with OVD. With all OVDs prior to viscoadaptives, the OVD was ineffective, unless the entire target space (usually the AC) was filled with the OVD. This is because, in an incompletely filled space, the OVD will just float around with aqueous surrounding it, and therefore the AC would behave as if it was filled with aqueous (it is only the OVD effect that is present at boundaries with the surrounding or contained structures that has any rheologic effect), yielding no effect of the OVD. Viscoadaptives, for the first time, are so viscous and rigid that a column of OVD can be formed in the center of the AC, contacting the endothelium above and the capsule below, and yielding viscoadaptive properties in the intervening space, even if the viscoadapive column is surrounded by a ring of aqueous. This means that for the first time, we do not need to inject the OVD in the angle, making later removal easier. It also means that we will need less viscoadaptive to do a given case than conventional viscoelastic.

(5)Viscomydriasis. Because of their increased zero shear viscosity, viscoadaptives allow the attainment of unparalleled viscomydriasis.

(6)Central delling of capsule for capsulorhexis. Capsulorhexis is performed on an anteriorly convex surface with posterior pressure behind it from the tension in the extraocular muscles. This posterior pressure causes the tendency for the capsulorhexis to go errantly toward the periphery. If the AC pressure can be made to equal or exceed this posterior pressure, a pressure equalized capsulorhexis can be done, which makes the whole thing much easier. Viscoadaptives are so viscous that not only can the anterior capsule be flattened, but it can be indented forming a central dell, thus making the creation of a small round capsulorhexis very easy. The first users of Healon®5 actually repeatedly described the tendency of the capsulorhexis to be too small, rather than too large. In other words the increased AC pressure tended to make the capsulorhexes run inward rather than outward, the opposite of the experience with other OVDs.

(7)Prevention of leakage of cortex out of capsular bag during capsulorhexis. The extremely high zero shear viscosity of viscoadaptives prevents the leakage of cortex out of mature cataracts during capsulorhexis, when the AC is adequately pressurized, thus improving visibility.

(8)Greater stability for trauma cases, corneal transplants. The high zero shear viscosity of viscoadaptives makes them ideal for repair of lacerated eyes, and for preservation of the AC during corneal transplantation.

(9)Facilitate AC IOL implantation. Again, because of increased zero shear viscosity, the AC stability during AC IOL implantation is enhanced with viscoadaptives.

(10)Enhanced corneal contact lens stability for vitrectomy. Retinal surgeons have commented that the corneal contact lens remains much more stable when placed in position with Healon®5.

(11)Wound blockade. It is important to observe whether or not wound blockade is achieved with a viscoadaptive, and to do it only intentionally. When lower viscosity dispersive OVDs are used, irrigation of BSS in the OVD-filled AC dilutes the dispersive OVD and it exits the eye gradually diluted with the irrigant. When a higher viscosity cohesive OVD, like Healon®GV is used, the irrigating BSS does not dilute the cohesive OVD mass, but instead may get behind it and cause wound blockade. This blockade results in elevation of the intraocular pressure as the irrigation of BSS continues, and eventually with the burping out of the mass of Healon®GV. The zero shear viscosity of viscoadaptives is so high that they may not burp out, but instead may permit the IOP to increase to a level high enough to dislocate the cataract posteriorly. It is therefore important to create an exit path for BSS when injecting it into an AC full of viscoadaptive in order to prevent causing viscoadaptive wound blockade unintentionally. However, wound blockade can be used to tremendous advantage, if done intentionally under appropriate conditions (see Ultimate Soft Shell Technique below).

The Ultimate Soft Shell Technique [5]

The idea of adapting the soft shell technique to Healon®5 came to me while performing a capsulorhexis with Healon®5, and noticing the increased resistance, and then again when using Vision Blue® with Healon®5 in a mature cataract case. The ultimate soft shell technique works equally well with Healon®5 and iVisc® Phaco.

In the soft shell technique, the idea is to use two OVDs with very disparate properties [2]. When I began to use Healon®5, it came to me that the ultimate disparate combination would be to use a viscoadaptive and BSS. So I tried that combination, and the BSS leaked out of the AC. But there is always a way with small modifications.

In the ‘ultimate soft shell technique’ the viscoadaptive is used with BSS as illustrated in figure 3. The AC is filled about 60–80% full of OVD. Then the remainder of the AC is filled with BSS by advancing the injection cannula to the angle opposite the incision before commencing injection. Injection of BSS is then done slowly. A curious thing happens. Since the AC was not completely filled with OVD, the viscoadaptive mass is reasonably free to move around in the remaining space in the AC as the BSS is injected. If the BSS is injected just over the lens surface into the angle remote from the incision, the viscoadaptive