Peripheral corneal staining (PCS) is probably the most common adverse event that affects lens wearers. Although never a good sign, not all PCS, also called 3/9 staining, is dangerous. Breaks in the epithelial layer make it possible for bacteria to enter the cornea, which increases the risk of vision-threatening infections.
While many factors can play a role in 3/9 staining, the gas-permeable lens parameters that may be responsible for PCS include lens material, lens centration, peripheral curve system (edge lift, edge thickness, edge profile and radius, and width of intermediate and peripheral curve system) lens wearing time and lens-to-cornea relationship or sagittal depth of the lens.
The Power of a Wet Lens
High-Dk materials have allowed for lenses that greatly reduce corneal hypoxia. These new materials allow multifocal lenses to be designed with more add power, and high-powered lenses to be made thinner. But, in some patients, the advantages of high Dk and high index of refraction are negated by poorer wetting characteristics of the lenses.
Wetting angles have improved greatly with each new generation of high-Dk lenses, and I expect the same will happen as the high index of refraction lenses continue to improve. Lenses with lower wetting angles in the laboratory should provide better wetting on the eye. But, each patient’s tear chemistry can significantly alter this measurement, and lens materials that wet poorly on the eye will likely induce some corneal desiccation.
We all have our favorite GP materials that we feel allows the tears to sheet across the lens and has a high tear break-up time on the lens. A clean, wet lens is less likely to create an environment for staining than a soiled lens with dry surfaces.
A lens that positions at or near the superior limbus, one that is tucked beneath the upper lid, will minimize the lens-lid interaction. This relationship allows for a normal blinking pattern. Conversely, a lens that positions low causes increased lens-lid interaction and will hinder the normal blink reflex. The more normal blink pattern with the superior lens position will induce less corneal desiccation.
The peripheral curve system on the lens is designed to provide a reservoir of tears to exchange during a blink. The peripheral curves must be designed so that the lens clears the limbus on lateral gaze. Edge lift is affected by not only the peripheral curve system, but also the overall lens diameter, the back optical zone diameter, the radii and width of intermediate and peripheral curves. Excessively thick lens edges tend to inhibit blinking, which means that the wearer will most frequently not have a full blink. Partial blinking does not cause the lens to move sufficiently to get maximal tear circulation and does not clean the front of the lens. Poor tear circulation and soiled lenses will increase the likelihood of corneal staining.
Determining the ideal edge lift is an art that involves exact precision: Too little edge lift will cause an insufficient tear reservoir to exchange tears beneath the lens, which could lead to central corneal staining, peripheral corneal staining or both. Excessive edge lift may cause the tears to break apart near the edge and/or interrupt proper blinking due to strong lid-lens interaction—again, discomfort and staining may result. The best way to determine the proper edge lift is through the fluorescein evaluation.
As a general rule, extended wear patients will have more PSC. With extended wear, the lenses are cleaned less often, resulting in more soiling; and while sleeping, there is no blinking to circulate tears beneath the lens, which can increase the likelihood of corneal staining.
Lastly, the lens-cornea fitting relationship can create a situation that is conducive to corneal staining. Excessive apical clearance with insufficient peripheral clearance can cause staining because of insufficient edge lift, as discussed earlier. A lens fit that is excessively flat over the apical cornea will most likely be excessively flat in the periphery. This can cause an interruption in normal blinking due to excessive lens-lid interaction.
Fine-Tune the Fit
We must have a deep understanding of all of the factors that could induce corneal staining and the skills necessary to balance all of these material, tear and fitting characteristics to provide our patients long-term lens comfort and safety.