RGP insights

The Practicality of Corneal Topography in RGP Fitting

BY LORETTA SZCZOTKA,OD,MS
JUNE 1998

I've noticed that the most frequently asked corneal topography question has changed over time. What was once "Why do I need a corneal topographer?" is now "Which one should I buy?" Here are a few features that may support your decision to buy a topographer and the accompanying RGP fitting software.

Increased Efficiency

I use computerized fitting programs to trial fit RGP lenses on normal corneas. Titrating the lens fit on the screen rather than behind the slit lamp decreases chair time, increases patient comfort and eliminates the need to sterilize diagnostic lenses. We compared diagnostic RGP fits to topographic RGP fits on the same patients and found an identical success rate between the two, yet the topographic fit was performed in half of the time. I recommend that the dynamic RGP fitting process still be performed by the practitioner because he knows each patient's lens history and has the data required to fit each patient individually.

Accuracy

Most second-generation contact lens programs use peripheral topographic data or corneal eccentricity derived from topography to generate a suggested RGP lens fit and resultant fluorescein pattern. These programs are more accurate in initial lens selection and fluorescein pattern simulations than the first-generation programs, which used preprogrammed manufacturer's nomograms based on simulated keratometry readings. One study found that programs that display fluorescein patterns with discrete contour lines are more difficult to interpret than those that use smooth simulations (Fig. 1).

Flexibility

Since the examiner is the only one who knows the clinical characteristics that influence the RGP fit, the ability to manipulate any RGP parameter and update the resultant fluorescein simulation is crucial. Lens movement, tilt simulations and the ability to review the topographic color map at any time without exiting the contact lens program are excellent features. RGPs always position over the steepest portion of a normal cornea, so the map review option will help you to decide how to manipulate the RGP position with the click of a button.

Complicated Designs

I don't use RGP topographic software programs to fit irregular corneas or complex RGP designs. Since the programs were originally designed and validated on normal corneas, the tear layer clearance and lacrimal lens calculations are usually variable beneath the optical zone over an irregular surface, resulting in inaccurate RGP power estimations. Although baseline topographic data is critical to initial lens selection in cases of keratoconus, post-penetrating keratoplasty, post-refractive surgery and other irregular corneas, diagnostic lens fitting is still required. CLS

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