years later, when Viscoat® and HPMC OVDs appeared, surgeons divided themselves into one group that preferred to work with higher viscosity cohesives (Healon®, and later others), and another that preferred lower viscosity dispersives (Viscoat® or HPMCs).

The staggering number of OVDs marketed since 1990 made OVD classification important for the surgeons to have a logical basis to understand their mechanisms of action, based on which, the one for a specific purpose could be selected. Optimal classification of OVDs for usage in ophthalmic surgery should be based upon the physical properties that are most important in cataract surgery, which are viscosity and cohesion. It is only for these reasons that these two properties have been used to classify OVDs. There is a high degree of correlation between the zero shear viscosity of a hyaluronate-based OVD and it’s cohesion. Most ophthalmic OVDs have hyaluronic acid as their rheologic polymer. Therefore the initial classifications of OVDs were based solely on zero shear viscosity.

After reviewing the various physical and chemical properties of OVDs, and assessing the most pertinent one to be used in ophthalmic surgery, Arshinoff devised a classification scheme based upon zero-shear viscosity, and noting the high correlation between zero-shear viscosity and the relative degree of cohesion or dispersion [4, 5]. Higher viscosity cohesives were excellent at creating spaces and sustaining pressure, whereas lower viscosity dispersives were capable of partitioning spaces and coating tissues. Each group was poor at performing the tasks in which the other group excelled, and so surgeons were forced to choose an OVD based on the type of complication they felt they might encounter in a particular case. The appearance of viscoadaptives in 1998 required expansion of the scheme (Table 6.1) [6], and the recent appearance of another new OVD, DisCoVisc™, which does not fit into the previous classification, required a further, more major, modification of the scheme, from a simple list to a two dimensional table (Table 6.2) [7].

Table 6.1 The classification of OVDs (Arshinoff 1990–2000) (primary parameter is zero shear viscosity)

SA

Research

fluids of disparate properties than that can be achieved with any single fluid [8, 9]. Attempts are being made to design a single OVD that can replace multiple OVD techniques (e.g., DisCoVisc®), but, despite the success achieved for routine cataract cases, a single OVD can never replace the ability of SSTs to create physically different environments in adjacent spaces, separated only by the rheological characteristics of the two OVDs, for complicated cases. It can be argued that with progressively smaller incisions, the space creation role of OVDs can be, at least, partially replaced by pressurization of the AC with an irrigating hand piece, or that a lower viscosity OVD will tend to leak out of a smaller incision less than out of a larger incision, thus reducing the advantage of a higher viscosity cohesive OVD over a lower viscosity dispersive. Although this is true, it is easier to perform the surgery in an environment of low turbulence, but pressurized irrigation increases the turbulence. Lower viscosity OVDs will still leak out of smaller incisions, reducing AC stability for delicate maneuvers. Consequently, most OVD techniques designed to deal with difficult situations or complications are variations of the SST and USST and actually work with greater stability, with smaller incisions, and so should be modified only slightly for micro incisions as noted below.

6.4.1.3Soft Shell and Ultimate Soft Shell Technique (SST & USST)

Since the evolution of the “viscoelastic dispersivecohesive SST,” and the “USST,” it has been recognized that more physical effects can be achieved with two

6.4.2Routine, Special and complicated Cases

In routine phacoemulsification/PC IOL cases, in which complications do not occur, any marketed OVD will suffice. In conditions where, the patient is not squeezing, the AC is sufficiently deep, there is no posterior

pressure and the capsulorhexis does not run, the endothelial cell count and cellular morphology are excellent, and the zonules are complete and strong, there is no need to worry. SST and USST Techniques were designed to overcome any possible problem encountered in cataract surgery, and remain excellent choices for routine cases. However, many surgeons prefer to use only a single OVD for routine cases. This is a reasonable alternative as long as SST and USST OVDs are routinely available in the OR, on short notice, if a complication arises in routine cases. Newer OVDs are being designed that give reasonable zeroshear viscosity to overcome potential space creation and maintenance problems in routine cases, while at the same time being dispersive enough to protect the endothelial cells. DisCoVisc is an example of one of these efforts. The makeup of DisCoVisc is shown in Table 6.3, where it is compared to Viscoat. Alcon had to increase the zero shear viscosity of DisCoVisc in

order to make it suitable to maintain spaces, while it remained dispersive. This was done by lowering the concentration and increasing the molecular weight of the NaHa in DisCoVisc, compared to Viscoat, while

Table 6.3 The nature of DisCoVisc

DisCoVisc™

DisCoVisc™

•1.6% NaHa - MMW (1.7MD)

•4.0% CDS – 25KD

Viscoat™

• 3.0% NaHa – LMW (500KD)

• 4.0% CDS – 25KD

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Research

A New Viscous Dispersive OVD: DisCoVisc™ (1.6% MMW [1.7MD] NaHa + 4% CDS)

LOG SHEAR RATE ( sec-1)

the CDS concentration and molecular weight was kept the same. The result was an OVD with the zero shear viscosity of Healon, and the dispersive nature of Viscoat (Fig. 6.41a, b).

6.4.2.1 Phakic and Anterior Chamber IOLs

In recent years, special needs for OVDs have arisen. Two similar cases, as far as OVD selection goes, are phakic and AC IOLs. To implant an AC IOL, the surgeon needs a deep AC, despite an open incision,

preferably with a flat iris, so that the iris is not snagged by the incoming IOL. Healon5 sounds terrific for this use, except that its removal is often problematic in AC IOL cases lacking an intact posterior capsule. Healon GV, the next most viscous and rigid OVD after Healon5, is a much better choice as it is easily fracturable by injecting BSS with a hockey stick cannula, thereby removing it easily. Similarly, many surgeons prefer HPMCs for phakic IOLs because the low turbulence needed for their removal reduces the likelihood of lens touch and possible cataract induction during this delicate maneuver.

6.4.2.2Trabeculectomy

and Phaotrabeculectomy

Another special use is the maintenance of the AC after phaco-trabeculectomy, or trabeculectomy alone. Lower viscosity OVDs disappear through the created filter in less than 24 h, thus creating a need for an alternate means to maintain a deep AC (suture, etc.). Healon5 can be used to fill the AC, and if left in place, will not induce elevated intraocular pressure, but will keep the AC formed for over 5 days, because it mixes so slowly with the aqueous.

6.4.2.3 Fuchs’ Endothelial Dystrophy

Fuchs Endothelial Dystrophy cases are best handled with SST or USST variations. The idea is to first place a dispersive OVD on the lenticular surface, and then pressurize it up against the corneal endothelium by injecting a cohesive (SST) or viscoadaptive (USST) OVD below it. In USST, a further layer of BSS is then injected over the lenticular surface, below the OVDs. During phaco, the viscoadaptive layer can be preserved by a skilled surgeon experienced with USST, whereas the cohesive OVD (SST) will likely be aspirated. At the end of the case, residual viscoadaptive or cohesive OVD is removed, while the dispersive is left in the eye to protect the endothelium. The eye is best treated with a cholinergic ocular hypotensive agent, either intracameral carbachol (commercial preparation), or topical carbachol (0.2% topically) to prevent postoperative IOP spikes. If needed, a topical prostaglandin analogue may be added, but all the agents which reduce aqueous production by the ciliary body, delay OVD washout and are completely ineffective in reducing or preventing post op IOP spikes caused by OVDs [10].

6.4.2.4 Zonular Deficiency

It is not uncommon to encounter a post-traumatic situation, or a patient with Marfan’s syndrome, or another reason to be missing a significant portion of the zonular ring. SSTs have greatly simplified these cases. First, the area of zonular deficiency is covered with a dispersive OVD. Then a cohesive (SST) or viscoadaptives (USST) is injected behind the dispersive to pressurize the AC and force most of the bulging vitreous, along with some dispersive OVD to return behind the cataractous lens. BSS is added on the lenticular surface (USST), and phaco is

commenced. At the earliest sign of instability, a capsular tension ring, Cionni ring, or Ahmed segment is inserted and Grieshaber, or other hooks may be used as indicated. As there is no vitreous in the AC, these cases usually progress relatively routinely, with the capsular tension ring in place (Cionni variation or Ahmed segment may be needed in the presence of a larger zonular defect).

6.4.2.5Capsular Staining for White & Black Cataracts

The first technique proposed for capsular staining with trypan blue, as described by Melles was simply to fill the AC with Vision Blue®, leave it in for 1 min, and then wash it out and continue the surgery with OVD injection followed by capsulorhexis.

Because of the concern for the amount of dye used in the Melles technique, many have used Vision Blue® under an air bubble, thus dramatically decreasing the amount needed, and endothelial contact with the dye.

Trypan blue, which dyes the anterior lens capsule beautifully, as it reduces its elasticity, has emerged as the leader among capsular dyes [11, 12]. Since the time the

author first began using trypan blue (Vision Blue®) in late 1999, he also used a variation of the USST (below) and has addressed its use at many meetings [13]. Marques et al. published their three-step technique, which differs from the USST technique in that the last two steps are reversed. The author, however, still prefers the USST method. Trypan blue is listed as a carcinogen in the Merck Index. Yetil et al. [15] were able to successfully stain the anterior capsule with as little as 0.1mL of 0.0125% trypan blue (1/4 commercial concentration), and the general trend is to use as little of any foreign substance as possible for capsular staining, since it may later prove to be hazardous, as in the case of ICG.

The ultimate soft shell capsular dye technique (USSCDT) is as follows:

1.The AC 80–90% should be filled with viscoadaptive OVD, being careful not to inject initially at the wound, which will blockade the incision and cause retention of aqueous behind the viscoadaptive blockade.

2.Trypan blue should be painted over the capsule, using only a tiny drop ejected onto the capsular surface through a 27 gauge hockey stick cannula, attached to a tuberculin syringe containing trypan blue (Fig. 6.42a). (Vision Blue® is now supplied in its own syringe. This syringe is considerably more difficult to use than a tuberculin syringe, but after a bit of practice, it