IV. COHESIVE VERSUS DISPERSIVE
In general, ophthalmic surgeons group OVDs into 2 classes: cohesive and dispersive.Cohesive OVDs contain long chains of sodium hyaluronate molecules and include Healon, Provisc, and Amvisc. They are high molecular weight, high-viscosity substances that tend to maintain space well when there is minimal movement within the anterior chamber. However, during conditions of turbulence (phacoemulsification) or other surgical manipulation, the long-chained cohesive OVDs tend to entangle and come out of the eye as a bolus. This results in suboptimal tissue protection during surgery. However, this type of OVD is easily removed from the eye at the end of the surgery and therefore presents little risk for unintentional retainment and postsurgical intraocular pressure (IOP) spikes.
Dispersive OVDs such as Viscoat, a combination of short-chained chondroitin sulfate and sodium hyaluronate, are low-viscosity, low molecular weight substances that also display low surface tension. This combination of properties allows these OVDs to separate and disperse in the anterior chamber thereby providing excellent coating and protection for the corne- al endothelium during phacoemulsification. Because of their molecular structure, dispersive OVDs are also less likely to conglomerate and extrude from the eye than the cohesive substances. However, this property makes the dispersive OVDs more difficult to aspirate from the anterior chamber at the end of a procedure and this results in an increased risk for postoperative IOP spikes.
V. NEW OPHTHALMIC VISCOSURGICAL DEVICES
A new formulation of sodium hyaluronate has been recently introduced called Healon 5. This OVD has both a high molecular weight and high concentration and is inherently different in its physical properties when compared with both cohesive and dispersive substances.Healon 5 has been termed viscoadaptive because it displays a combination of behaviors during intraocular surgery.Because of its high molecular weight and concentration, this OVD is highly retentive and maintains the anterior chamber shape during surgical manipulation better than cohesive substances such as traditional Healon. However, Healon 5 can also be fractured and compartmentalized and therefore coats the endothelium similar to dispersive substances. Healon 5 must be fully aspirated from the anterior chamber at the end of a procedure because, if left behind, it may cause IOP spikes with greater frequency than the other OVDs.
The appearance of a new OVD, DisCo Visc, has established another category for viscoelastic substances. DisCo Visc (hyaluronic acid 1% and chondroitin sulfate 4%) is classified as a higher viscosity dispersive OVD.This substance has an intermediate to high viscosity but is still highly dispersive. DisCo Visc behaves like a cohesive OVD with respect to ease of removal from the anterior chamber. In addition, it provides endothelial protection similar to a dispersive substance. Therefore, this OVD has rheological characteristics that make it suitable for the entire surgical procedure.
VI. THE IDEAL OPHTHALMIC VISCOSURGICAL DEVICE
The ideal OVD is biocompatible within the eye, maintains space during surgical manipulation, and coats and protects intraocular tissues. It should also be easily removed from the anterior chamber at the end of a procedure and should have little effect on IOP. In addition, the ideal OVD should be low cost. These attributes are not entirely met by the OVDs currently available, but new developments and improvements continue to be made.
*Dikutip dari Buku Essentials Of Cataract Surgery 2nd Ed, halaman 83-84