Viscoelastic substances play an essential role in intraocular surgery. They are used in phacoemulsification cataract surgery to protect the corneal endothelium from mechanical damage and to deepen the anterior chamber, providing space for surgical manipulation. These substances, however, have a wide array of functions including injection at the pupil-anterior capsule junction to break synechiae in small-pupil cataract surgery and placement underneath nuclear fragments to prevent loss in the event of posterior capsule rupture.In addition, viscoelastic substances are used to minimize further trauma to the zonules in cases of zonular dehiscence and to increase control during insertion of foldable intraocular lenses. Because of the myriad of important functions that viscoelastic substances serve in intraocular surgery they have been renamed ophthalmic viscosurgical devices (OVDs).
II. HISTORICAL PERSPECTIVE
A number of surgical techniques were originally developed to protect the corneal endothelium and provide room for manipulation of intraocular tissues.Initially, balanced salt solution and air were used but both were quickly lost from the anterior chamber during procedural movements. Sodium hyaluronate (Healon) was the first viscoelastic substance introduced commercially and was used in human intraocular surgery starting in 1979. Amvisc, released in 1983, is also a sodium hyaluronate product but is slightly less viscous than Healon. Methylcellulose (Methocel) was originally used in 1979 to coat implants prior to introduction into the anterior chamber. This OVD has since been mainly replaced by other substances because it requires increased infusion pressure and use of a large-bore cannula for injection. Viscoat, a combination of chondroitin sulfate and sodium hyaluronate, was developed by Cooper Vision-Cilco and has traditionally provided useful tissue coating but is less effective at maintaining space and separating tissue because of its low viscosity.
The clinical applications for the different OVDs are determined in large part by their rheologic properties, including viscosity, pseudoplasticity, visco- elasticity, and coatability.1,8 Rheology is the study of the relationship between the deformation of physical bodies and the forces generated within them. In this regard, OVDs have some properties of fluids and some of solids and vary among each other with respect to many of these characteristics.
The viscosity of a solution is defined as its resistance to flow. This property is determined mainly by a composite of molecular weight and concentration; substances with high molecular weight and high concentration will have the highest viscosity. This is an important property of OVDs because high- viscosity solutions will be difficult to displace from the anterior chamber and will move tissues more effectively than those of low viscosity.
Substances have the property of pseudoplasticity when their viscosity changes with shear rate. Shear is the friction that occurs when a plate is made to move in relation to another plate with a solution between them. Shear rate is the speed at which the plate is moved in relation to the second plate. OVDs such as chondroitin sulfate do not have the property of pseudoplasticity; their viscosity is constant regardless of how fast the plate is moving. Substances such as sodium hyaluronate and methylcellulose, however, do show pseudoplasticity. Therefore, the faster the plate is moving, or the higher the shear rate, the lower the viscosity for these OVDs. This behavior is important in intraocular surgery because a substance with greater pseudoplasticity will move more easily through a small cannula as flow continues but will also more readily extrude from the anterior chamber during the turbulence of phacoemulsification.
Elasticity is the ability of a material to return to its original shape after deformation. Substances without elasticity such as balanced salt solution will be forced into an adjoining space or out of the wound when the cornea is deformed by outside pressure. In contrast, all OVDs will promote reformation of corneal shape following stress such as occurs with corneal retraction or insertion or withdrawal of instrumentation. Therefore, anterior chamber volume will be maintained despite manipulation to varying degrees depend- ing on the viscoelasticity of the OVD. Viscoelasticity also allows for ocular tis- sue protection from high frequency mechanical deformations that occur secondary to phacoemulsification vibrations or irrigating streams. The amount of viscoelasticity that a particular OVD displays is dependent on its viscosity, molecular length, and configuration.
The coating ability of an OVD is determined mainly by surface tension and contact angle. Surface tension relates to the forces acting at the interface between a viscoelastic substance and a tissue or surgical instrument. The contact angle is the angle that a drop of substance forms with a flat surface. Lower surface tension and lower contact angle result in superior coatability. Substances that contain chondroitin sulfate such as Viscoat tend to show better coatability and are used, for example, to coat an implant prior to placement into the eye. Sodium hyaluronate tends to have less coatability.
*Dikutip dari Buku Essentials Of Cataract Surgery 2nd Ed, halaman 81-83