Indentation gonioscopy with Posner Gonioscope

In the four-mirror variety, all four mirrors point toward the iridocorneal angle. This allows physicians to quickly look at all segments of the angle without much rotation of the lens, significantly cutting down on examination time. Compared to three-mirror lenses, the four-mirror lenses have a relatively small and flat area of contact. Some physicians, especially those who are still perfecting their technique, may find that this smaller area of contact creates difficulties. Pushing too firmly on the lens can artificially open the angle. While this may be desired, as in indentation gonioscopy, it can also fool the practitioner into thinking that the angle is more open than it actually is. Makes of the four-mirror variety include the Zeiss, Volk G, Posner, and Susmann lenses. Volk’s popular G-4 series is available with a variety of options, including large and small rings to fit a variety of facial features, adjustable handles, and flanges for stability. Another difference between the three- and four-mirror lenses is the requirement of a viscous coupling solution. In general, three-mirror lenses actually create a vault over the cornea rather than coming into contact with it; as a result, they require the use of a methylcellulose viscous coupling solution such as Goniosol. This can be time consuming and may slow down the exam. In addition, the fluid can make subsequent visualization of the fundus, and other parts of the exam such as visual fields and fundus photography, difficult. However, in some cases, the benefits of the three mirror lens outweigh the cumbersome aspects of the use of a coupling solution. In addition, some manufacturers, such as Volk, are now offering designs that eliminate the need for a coupling solution altogether with their Advanced No Fluid (ANF+) contact option.