IOL Implantation
The main consideration in selecting a viscoelastic for lens implantation is to maintain the anterior chamber depth and corneal dome. Thus surgeons look for a viscoelastic solution that is relatively cohesive in order to maintain the chamber while at the same time protecting the corneal endothelium during lens insertion. However, surgeons are also concerned with how difficult it is to move the lens and instruments through the viscoelastic, so they look for high pseudoplasticity to prevent drag. The common uses of viscoelastic solutions in lens insertion are included below.

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Uses of Viscoelastic Substances in Intraocular Lens Insertion and Removal

• Expand the capsular bag for lens insertion
• Expand ciliary sulcus for insertion
• Cushion anterior chamber angle for anterior lens insertion
• Coal IOL
• Coat and cushion corneal endothelium
• Create adequate space for insertion of instruments and implants
• Temponade opening through which air, aqueous, or vitreous may be lost
• Push back vitreous or anterior hyaloid membrane during IOL implantation
• Tamponade break in posterior capsule for placement of implant

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Features of an ideal product are listed below:

• Ease of Infusion: Surgeons desire a viscoelastic that is relatively easy to infuse into the eye. The rheological property that affects ease of infusion is pseudoplasticity, or the shear rate of the viscoelastic. Some viscoelastic solutions have a very high viscosity at high shear rates, which make them difficult to infuse into the eye, and also make it more difficult for the surgeon to move instruments or an IOL through the solution.
  
• Retention During Positive Pressure in the Eye: Surgeons do not want a viscoelastic that leaks from the incision during surgery. They instead desire a product that is retained during the various stages of surgery, and particularly when there is positive pressure in the anterior chamber.
  
• Retention During Phacoemulsification: One of the challenges for the phaco surgeon is maintaining viscoelastic on the posterior capsule during phacoemulsification. This is not an easy challenge because the surgeon is constantly aspirating lens material for the eye, thus the likelihood of also removing most or all of the viscoelastic. Cohesive viscoelastic solutions can be more difficult to maintain in the eye during phacoemulsification because they are more easily aspirated. Dispersive viscoelastic solutions are retained better, but can also be difficult to remove once the lens has been removed and the IOL implanted.
  
• Easy Removal or No Removal Required: Ideally, the surgeon would not have to be concerned with removing a viscoelastic. Or, if it does need to be removed, this can be accomplished easily.
  
• Does Not Interfere with Instruments or IOL Placement: Surgeons desire a viscoelastic solution that does not interfere with the maneuvering of instruments in and out of the eye and the insertion and placement of the IOL. Thus, look for a substance that has low viscosity at high shear rates.
  
• Protects Corneal Endothelium: Since one of the most important reasons for using a viscoelastic is the protection of the corneal endothelium, surgeons want a viscoelastic that coats and protects during the entire procedure, but at the same time does not interfere with the maneuvering of instruments, or the placement of the IOL.
  
• Clear: Because the surgeon must be able to visualize the tissues in the eye during surgery, particularly the posterior capsule, it is important that a viscoelastic is clear and does not in any way interfere with visualization.
  
• Non-Toxic: Since it is not always possible for the surgeon to completely remove all of the viscoelastic substance from the eye following surgery, it is important that the solution does not cause inflammatory response or unacceptable increase in IOP.
  
• Does Not Obstruct Aqueous Outflow: While virtually every viscoelastic causes transient IOP increase immediately following surgery, it’s critical that large particles in the solution do not clog the trabecular meshwork, which would cause persistent and unresolved IOP increase.

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Sodium Hyaluronate
The most common viscoelastic solution used today is sodium hyaluronate (HA). HA is the natural biologic lubricating and shock absorbing molecule of the musculoskeletal system and also of the eye. In the eye, HA is found in high concentrations in the aqueous humor and covering the endothelium.

HA is extracted from a number of biological sources, and cultures of streptococci. While the purified HA from each of these sources is similar in structure, the molecular weight varies and is a critical factor in the physical properties of the viscoelastic solution.

For more information, there are numerous websites that include:
www.who.int
(World Health Organization)