Ophthalmic Viscosurgical Devices

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Introduction

The term viscosurgery was introduced almost 30 years ago.[1] 

The development of the first viscoelastic agent which was made of sodium hyaluronate [Healon􏰀] helped to revolutionize the way cataract surgery was performed.[2] Many other hyaluronates followed, differing from one another in molecular weight, concentration, and viscosity.

Because of the multitude of important functions that viscoelastic substances serve in intraocular surgery they have been renamed ophthalmic viscosurgical devices (OVDs), suggesting that these agents are now to be considered essential surgical tools, not just corollary products to be used during surgery.[3]  

Physical Properties

OVDs are unique in that they own characteristics of both solids and fluids. Surgical applications of the different OVDs are determined mainly by their physical properties which are a consequence of their molecular chain length and interactions both within chains and between chains and eye tissue.[4] They have 4 main physical properties[5][6] :

Viscosity

Viscosity of a substance is characterized by its resistance to flow. It is determined principally by molecular weight and concentration, so that substances with high molecular weight and high concentration have high viscosity. This means the OVD will be effective at moving tissue and will be difficult to displace from the anterior chamber.

Pseudoplasticity

Pseudoplasticity indicates the facility with which a material can change its viscosity according to shear rate. Shear rate is the speed at which 2 plates are moved in relation to one another with a solution between them. If an OVD shows pseudoplasticity it means at zero shear rate it will have high viscosity and coat tissue well but at high shear rate it will lower its viscosity and more readily extrude from the anterior chamber.

Elasticity

Elasticity describes the ability of a substance to return to its original shape after deformation. All OVDs will lead to reformation of corneal shape and anterior chamber after insertion or withdrawal of an instrument. The higher the elasticity, the better the OVD is in maintaining space.

Coatability

Coating ability is determined mainly by surface tension and contact angle. Surface tension refers to the forces acting at the interface between a viscoelastic agent and a surface. Contact angle is the angle that a drop of substance forms with a surface. A low surface tension and a low contact angle result in high coatability which means the OVD is good at coating tissue but is difficult to aspirate from the eye.

Cohesive vs Dispersive OVDs

The relative cohesive or dispersive behaviour of an OVD is important as it relates directly to how it is used in surgery.[7]

Cohesion is the tendency of a material’s constituent molecules to adhere to one another rather than to disperse.

Cohesive OVDs are substances that contain long-chain molecules and are therefore characterized by high molecular weight and high viscosity. These agents tend to maintain space well when there is minimal movement. However, during conditions of turbulence (high shear), the long-chained cohesive OVDs tend to entangle and extrude from the eye as a single mass (similar to spaghetti).

Cohesive agents include Healon, Healon GV (Abbott Medical Optics, Santa Ana, CA); Amvisc, Amvisc Plus (Bausch + Lomb, Rochester, NY) and Provisc (Alcon, Ft Worth, TX).

Dispersive OVDs are materials that contain short-chain molecules that display low surface tension, which results in low molecular weight and low viscosity. At high shear rates, these OVDs separate and disperse providing excellent coating but being less prone to conglomerate and come out of the eye (similar to macaroni).

Dispersive agents include OcuCoat (Bausch + Lomb), Viscoat (Alcon), and Healon Endocoat (Abbott Medical Optics).

Viscoadaptive OVDs

Viscoadaptive OVDs are substances with high molecular weight and high concentration that contain fragile long-chain molecules. Their physical properties are different from both cohesive and dispersive OVDs.[8]

Healon 5 was the first viscoadaptice OVD to be designed rheologically. This OVD is highly retentive and maintains the AC shape during surgical manipulation better than cohesive substances. The term viscoadaptive relates to the different behaviours they display during surgery according to flow rate. At low flow rate, a viscoadaptive OVD is highly retentive and maintains the AC shape during surgical manipulation better than cohesive substances. At high flow rate, its chains fracture and coat the endothelium similar to dispersive substances, being also difficult to fully aspirate.[9] Thus this OVD acts as a pseudodispersive agent.

Higher viscosity dispersive OVDs

The appearance of Discovisc (Alcon) established another category for OVDs.

This substance has an intermediate to high viscosity but is still highly dispersive. Combining qualities of dispersive and cohesive agents, it prompted a new expanded classification that considers cohesion- dispersion independently from zero-shear viscosity.[10] Discovisc behaves like a cohesive OVD with respect to ease of removal from the anterior chamber while providing endothelial protection similar to a dispersive substance.

Cataract Surgery

Cataract surgery is highly dependent on fluid flow. Understanding the physical properties of OVDs and their behaviour in different surgical environments is therefore key to comprehending phacoemulsification fluidics and to performing better surgery.[11]

1. Fill the anterior chamber (AC)

The first surgical step in which an OVD is used , both in extracapsular cataract extraction (ECCE) and phacoemulsification, is the filling of the AC. The shear rate at which the material is expelled through a cannula is high and the viscosity for it to move through the cannula should be low. Therefore a highly pseudoplastic OVD is desirable so that only a relatively gentle force of injection is required and ocular inflation pressure can be judged and modulated by the surgeon’s tactile sensitivity.

2. Capsulorhexis

During capsulorrhexis the OVD will mantain the corneal dome and anterior chamber depth and provide stability to the anterior capsule surface reducing the probability of it "running out" peripherally. At a shear rate close to 0 (instruments are manually moved) the ideal OVD should have high viscosity and high elasticity at low shear.

3. Removal of nucleus

Removal of the nucleus by ECCE happens at a low shear rate which means the ideal OVD will have high pseudoplasticity to decrease resistance.

In phacoemulsification during nucleus emulsification, the AC depth will be mantained by the infusion pressure of the intraocular irrigant and not by the OVD. The cornea, however, is vulnerable to damage by ultrasonic energy and fluid turbulence. The ideal OVD in this step will therefore have high coatability (to protect the endothelium) and high elasticity (to absorb vibration).

A cohesive OVD will rapidly extrude the eye whereas a dispersive OVD will tend to remain within the corneal concavity. However, a dispersive OVD will also tend to entrap air bubbles and thus block the surgeon’s view. This problem has been assessed in two ways: the use of the soft-shell technique and the development of a viscoadaptive OVD.

4. Irrigation and Aspiration (I/A)

Irrigation and aspiration (both in ECCE and phacoemulsification) requires the same characteristics of an OVD as the previous step. Although the endothelial cells are still susceptible to damage from fluid turbulence, there is no longer risk of trauma from ultrasonic energy or free radicals. 

5. IOL Implantation

In this step the OVD has two main roles: to protect the corneal endothelium from compression that might occur if the lens were to approach the endothelium too closely and to keep the capsular bag inflated to facilitate the introduction of the IOL and to lower the risk of touching the capsule with and instrument.

At low shear rate, such as when the IOL is static, a high viscous agent between the lens and the cornea is better at protecting the endothelium from compression by the IOL and also provides excellent cushioning to the opening of foldable IOLs inside the eye. On the other hand, when the IOL moves and the shear rate increases, a less viscous material will let the IOL slide through and will not transmit drag forces to the endothelium that would put it in danger. The ideal OVD for this step is therefore highly pseudoplastic. 

6. Removal of OVD

The last step in both techniques (ECCE and phacoemulsification) is removal of the OVD. This will be facilitated by a highly cohesive OVD because its molecules will entangle and extrude the eye as a single mass, presenting little risk for postsurgical intraocular pressure spikes. A dispersive OVD will, under conditions of turbulence, scatter and increase the risk of angle outflow obstruction.[12]

Practical Indications in Cataract Surgery


Cohesive OVDs

High viscosity increases the stability of the surgical environment and high cohesion eases surgical removal. A cohesive OVD is therefore doubly advantageous both during and at the end of the procedure.

This kind of OVD may be particularly useful when there is a need to deepen the anterior chamber such as in hyperopic eyes to make space for the phaco tip. In small-pupil cataract surgery, it can expand an inadequatelly dilated pupil (viscomydriasis)[13] and break synechiae if injected at the pupil-anterior capsule junction. It can also be used to stabilize the anterior capsule surface during capsulorrhexis reducing the probability of it "running out" peripherally. Moreover it may be used temporarily to hold back prolapsed iris tissue at the incision site or blood from the iris or the angle.

Dispersive OVDs

Low viscosity and highly dispersive behaviour are associated with excellent coating potential and ability to compartmentalize segments within the eye. 

A dispersive OVD should therefore be used whenever the corneal endothelium requires extra protection such as in endothelial dystrophies, shallow anterior chambers (which often lead to lens fragments and/or instruments touching the endothelium), dense cataracts (which demand considerable amounts of ultrasound energy) and long operations.[14]

A dispersive OVD may also be used to selectively isolate an area of zonule desinsertion keeping the vitreous out of the operative field or a piece of frayed iris from the phaco tip and the fluid flow. After a capsular tear a dispersive OVD will work as a barrier between the anterior and posterior segments of the eye, preventing prolapse of vitreous anteriorly and fall of lens fragments posteriorly.

Soft Shell Technique [15]

The coating ability of a dispersive OVD could be detrimental if the surgeon’s operative view is diminished by the layer it forms against the corneal endothelium.[16] The soft-shell technique uses one OVD from each of the two major classes sequentially to take advantage of the best properties of each type, while avoiding the problems associated with each, when used alone. 

At the beginning of the surgery, a dispersive OVD is injected into the AC, forming a mass on the anterior surface of the lens. Then, a cohesive OVD is injected into the posterior centre of the dispersive OVD mass, pushing it upwards and outwards to form a smooth layer against the corneal endothelial cells. During phacoemulsification and I/A, the high-viscosity cohesive OVD will promptly extrude the eye, whereas the low-viscosity dispersive OVD will remain intact in its layer against the endothelium, protecting it from ultrasonic energy and fluid turbulence.

After the nucleus and cortex have been removed, the cohesive OVD is injected first and the dispersive OVD is injected secondly, forming a mass at the centre that allows freer movement of the incoming IOL. The cohesive OVD is therefore pushed against the periphery of the AC, stabilizing the iris, capsule, and AC depth. Because the dispersive OVD is encircled by the cohesive OVD both are easily aspirated from the eye together at the end of surgery.

This technique is efficient in reducing corneal endothelial cell loss after phacoemulsification, especially in eyes with dense cataracts, when compared with outcomes using one cohesive or dispersive OVD only.[17][18]

Ultimate Soft-Shell Technique [19]

The soft shell technique involves the use of two different OVDs, in correct sequence and proper positioning, thus rising cost and inconvenience.

The Ultimate Soft-Shell Technique technique uses balanced salt solution (BSS) underneath a viscoadaptive OVD with which the anterior chamber is filled to the desired extent.

The viscoadaptive OVD blocks the incision wound, while BSS above the lens surface reduces the resistance to surgical manoeuvres such as the capsulorhexis. To perform hydrossection, the cannula is wiggled to break out the piece of viscoadaptive OVD that is blocking the incision wound so that the IOP does not rise too much. Before implanting the IOL, the viscoadaptive OVD is injected across the capsular bag until it begins to enter it. BSS is then injected into the capsular bag and the IOL is inserted. [11] The viscoadaptive OVD is now fully anterior to the IOL, in the AC, and is easily aspirated in about 5–10 seconds, with gentle “Rock ‘n’ Roll”.[20]

Intraoperative Floppy Iris Syndrome (IFIS)

IFIS is a well known complication resulting from the use of α-blockers for prostate treatment. A viscoadaptive OVD may help to mechanically expand the pupil (viscomydriasis) and stabilize the iris, preventing it from prolapsing to the incisions.[21] In addition to the use of a single OVD, the combination of the soft-shell and ultimate soft-shell techniques in which a viscoadaptive OVD (Healon 5) is associated with a dispersive OVD (Viscoat) was reported.[22]

 
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