A Go-To Option for Fitting Keratoconus Patients
BY ANN LAURENZI, OD, FAAO
There are many fitting strategies and many different lens designs available when fitting keratoconus patients. When asked what is the best way to fit a keratoconus patient, I often want to answer, “With whatever works,” (while maintaining corneal health and integrity, of course). But to achieve a successful fit, I do think you need to have a variety of lenses in your “toolbox” to manage the many different needs for this challenging set of patients.
I prefer fitting keratoconus lenses that have straightforward and easily comprehensible parameters. One such lens is the Keratoconus Bi-Aspheric (KBA) lens from Essilor/Precision Technology Services. This lens has a large overall diameter of 10.2mm. The bi-aspheric design and larger diameter help center the lens, allowing for lid attachment and good stability. The KBA lens also has a thicker center thickness to help reduce flexure.
KBA Fitting Strategies
The KBA lens should vault the cone and align on the more normal peripheral cornea (Figure 1). You can achieve this fit by varying the two main parameters: base curve/sagittal height and the eccentricity (rate of flattening from center to periphery).
You can determine the parameters of the first diagnostic lens using keratometry readings or topography. With K readings, determine the base curve of the initial diagnostic lens by subtracting 0.6mm from the flat K value. Adjust the base curve until you achieve apical clearance. You can also modify the peripheral lens-to-cornea fit by adjusting the base curve. Look for midperipheral alignment and good edge clearance in the horizontal meridian.
The KBA design is available with an eccentricity of either 0.98e or 1.3Oe. Most keratoconus patients have low, temporally positioned cones, which do well with the 0.98e for proper alignment of the peripheral cornea. Smaller, more central cones tend to have a larger change in curvature from apex to periphery and need the larger 1.3 e to align the periphery.
Customizing the Fit
You can customize a KBA lens by diagnostically finding the base curve that optimally vaults the cone and the best base curve to align in the periphery, then contacting a fitting consultant who can input this information into the KBA software to design a custom lens with the appropriate sagittal depth and edge lift. You can also install the KBA software on any computer, which would enable you to calculate these parameters on your own.
Medmont topographers also offer software that allows you to review simulated fluorescein patterns of diagnostic lenses based on patient topography. You can then adjust the lens parameters until you achieve an ideal simulated fluorescein pattern. However, it is not necessary to have this topographer or this topography software to fit the KBA lens.
Figure 1. KBA lens on-eye.
A Lens of Choice
I believe the KBA is a go-to lens because it facilitates keratoconus fitting. You need to adjust only two main parameters and you can customize the final lens to create a stable, centralized fit with good tear exchange and comfort. The KBA diagnostic fitting set is available in base curves of 5.80mm to 7.70mm in the 0.98e for the peripheral fit and in base curves of 5.14mm to 6.88mm in the 1.3e for the peripheral fit. CLS
Dr. Laurenzi is currently appointed a staff position at NorthShore University Healthsystems Glenbrook Hospital to start October, 2009.