Physicians must be aware of options, know how to use different categories of viscoelastic
Senior ophthalmologists with years of surgical experience have performed thousands of cataract surgeries and often know what they want and need from an ophthalmic viscosurgical device (OVD).
Commercial OVDs, even those with the same polymer constituent(s) present at similar concentrations, have unique biomechanical properties, and thus have different advantages and disadvantages during individual stages of surgery.
The reality of OVD rheologic diversity has led some surgeons to begin to use more than one OVD during their procedures, catering their choice of viscoelastic to different stages of surgery and possible complications. To follow the growing use of two separately packaged products and provide surgeons with a more convenient alternative, OVD manufacturers began to offer dual-syringe products, beginning in the late 1990s with a combination system (DuoVisc, which is 1% sodium hyaluronate [ProVisc] and 4% sodium chondroitin sulfate–3% sodium hyaluronate [Viscoat], Alcon Laboratories).
One way to maximize the potential of each type of OVD and facilitate efficient and successful surgery is to understand the basis of how OVDs are classified.
Modern OVD classification is based on the zero-shear viscosity (the viscosity of a liquid at rest) and the cohesion-dispersion index (CDI). CDI is the percentage of viscoelastic solution aspirated per 100 mmHg vacuum pressure (typically measured at a 80º phaco tip insertion angle)1 and can range from less than 5% for the most dispersive OVDs to more than 70% for the most cohesive OVDs.
The appeal of two-syringe products is that they package a high-CDI cohesive agent, such as 1% sodium hyaluronate (Healon, Abbott Medical Optics [AMO]; CDI = 31.2 or ProVisc, Alcon Laboratories; CDI = 46),1 with a low-CDI dispersive agent, such as 4% sodium chondroitin sulfate–3% sodium hyaluronate (Viscoat, Alcon Laboratories; CDI = 3.4), to enhance the accessibility of complementary viscoelastics during a single operation.
In the past few years, however, the viscoadaptive 2.3% sodium hyaluronate (Healon5, AMO; CDI = 22) and the viscous dispersive 4% sodium chondroitin sulfate–1.6% sodium hyaluronate (DisCoVisc, Alcon; CDI = 12) have challenged the versatility of dual-syringe systems by offering many of the features of both types of OVDs in a single syringe.
Case for dual-syringe systems
Experienced surgeons are naturally poised to take full advantage of dual-syringe systems that offer expanded surgical options.
Aside from the relatively simple technique of using each syringe of a dual-syringe system individually during different phases of surgery, the dispersive and cohesive agents can be used adjacently with what is known as the dispersive-cohesive soft-shell technique. The soft-shell technique was designed by Dr Steve A. Arshinoff, to maximize the benefits of cohesive and dispersive OVDs by using them simultaneously during phacoemulsification.2
In the soft-shell technique, the dispersive agent is first injected into the anterior chamber beneath the corneal endothelium and then the cohesive agent is injected behind the dispersive agent, so that the dispersive agent is pushed against the corneal endothelium. This, in effect, gives the surgeon the best of both worlds: the cohesive OVD provides space maintenance and pressurization of a deep anterior chamber for capsulorhexis, while the dispersive agent provides a protective layer over the endothelial cells.
In a study of patients with hard lens nuclei, the soft-shell technique using 1% sodium hyaluronate (Healon) and 4% sodium chondroitin sulfate–3% sodium hyaluronate (Viscoat) resulted in significantly greater endothelial cell count (ECC) 3 months after surgery (6.4%) compared with 1% sodium hyaluronate alone (Healon) (16.3%).3 Although the soft-shell technique may be difficult for the inexperienced surgeon to master, a more skilled surgeon can use it to maximize successful outcomes in high-risk patients.
In a head-to-head comparison of 4% sodium chondroitin sulfate–1.6% sodium hyaluronate (DisCoVisc) and the combination system of 1% sodium hyaluronate and 4% sodium chondroitin sulfate–3% sodium hyaluronate (DuoVisc) using the soft-shell technique, eyes that were operated on using the combination system showed significantly greater 3-month postoperative endothelial cell density and lower central corneal thickness than those that were operated on with 4% sodium chondroitin sulfate–1.6% sodium hyaluronate (DisCoVisc), although the differences were not statistically significant.4
Furthermore, during phacoemulsification of dense nuclei in patients with Fuchs' dystrophy, the dispersive agent of the dual-syringe system can be injected repeatedly to help minimize endothelial loss.