Utilizing Topography in Fitting Irregular Corneas
BY GREGORY W. DENAEYER, OD, FAAO
Corneal topography is an essential tool for managing patients who have corneal irregularity. Videokeratography provides qualitative and quantitative data that you can use when fitting GP lenses to successfully improve visual function for patients who have corneal disease or who have suffered anterior segment trauma.
Help With Initial Lens Selection
Placido-based topographers can measure thousands of points across the corneal surface. The most common way to analyze this data is by looking at a curvature map, which can be axial or tangential. Axial maps, sometimes referred to as sagittal maps, assume that the cornea is spherical, making them less accurate for peripheral measurements. Tangential maps use instant radius of curvature for each data point, and thus will be more precise throughout the data set.
A curvature map is useful for making or confirming a diagnosis, such as keratoconus, but it’s also useful for determining the severity of any irregularity as well as the geometry of the corneal surface. Judging the amount of irregularity will help with choosing a lens modality. For example, a scleral lens design may be best for fitting a cornea that has severe irregularity. Knowing the geometry of the cornea is also important in determining initial design selection. For example, an aspheric back-surface corneal GP would be a good initial selection for a keratoconic cornea that has a steep central nipple; a reverse geometry design should be considered for a corneal transplant button resulting in an oblate anterior surface.
The simulated central keratometry reading from the curvature map can help in selecting the initial diagnostic lens for a corneal GP based upon suggestions from the fitting guide of the particular design chosen.
For scleral lenses, Shornack et al (2010) concluded that there was only a weak predictive measurement when using topography for choosing a base curve selection for a scleral lens design. However, some topographers can measure a sagittal height value (z-value), which is used in choosing an initial diagnostic lens for certain scleral lens designs.
Tweaking the Fit, Following Up
Some topographers are equipped with a contact lens module that allows you to virtually fit specific corneal GP designs. Simulated fluorescein patterns can show how parameter changes will affect the lens fitting characteristics. Sindt et al (2011) found that the accuracy of these simulated patterns was 95 percent when quality ring images were produced. The accuracy drops if the ring images are distorted because of tear film breakup or severe irregularity. The simulated fluorescein patterns don’t take into account other factors that can significantly affect lens fit such as lid position, lid tension, and aperture size.
Lastly, topographical measurements are useful for tracking corneal changes that might inherently occur from progressive disease or secondary to lens wear.
Although other instruments, such as optical coherence tomographers, are gaining in popularity for evaluation and assessment when fitting irregular corneas, topography remains an essential tool for success. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #211.
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio, and a consultant to Visionary Optics, B+L, and Aciont. You can contact him at email@example.com.