contact lens case reports
Determining Overall Sagittal Height of the Cornea
BY PATRICK J. CAROLINE, FCLSA, FAAO, & MARK P. ANDRÉ, FCLSA, FAAO
In our last three columns, we described how corneal diameter contributes to the overall sagittal height of the cornea and influences optimum soft lens parameters. We demonstrated that larger corneas often require steeper base curve radii, while smaller corneas often require flatter radii.
Over time we have learned that the inability of more rigid soft lens materials, like silicone hydrogel and ballasted toric designs, to "mold" to the shape of the underlying cornea makes them more sensitive to anterior segment sagittal height variations.
Effective K and First Fit Success
We reviewed the files of 64 patients that had successfully worn CooperVision Preference Toric soft lenses for six months or more. The Preference Toric is a 43 percent water content lens with base curves of 8.4mm and 8.7mm.
At the time of this study, the Preference Toric fitting guide recommended using the 8.7mm base curve if the flat K was less than 45.00 diopters and an 8.4mm base curve if the flat K was greater than 45.25 diopters.
In our retrospective evaluation of 64 successful patients, 45 had Ks flatter than 45.00 diopters, suggesting that the flatter 8.7mm base curve would be indicated. However, 20 of these patients (44 percent), actually required the steeper 8.4mm base curve. Of these 20, 19 had larger than normal horizontal visible iris diameters (HVID) of greater than 12.0mm (Figure 1).
In addition, 19 patients had Ks of 45.25 diopters or greater, suggesting that the steeper 8.4mm base curve would be indicated. But seven (37 percent) required the flatter 8.7mm base curve design. All seven had HVIDs less than 11.5mm (Figure 2).
|Figure 1. Patients with flat Ks less than 45.00 diopters.|
Of the 64 patients successfully wearing Preference Toric lenses, 40 percent required base curve radii outside of the fitting guide recommendations. If we had compensated for corneal diameter by first calculating the effective K, our empirical first fit success with the current fitting guide would have been 95 percent instead of 60 percent. This translates into increased patient success as well as significant savings in chair time.
Figure 2. Patients with flat Ks greater than 45.25
It is clear that a number of anatomic features interact to form the overall sagittal height of the cornea: central radius of curvature, mid-peripheral asphericity and overall corneal diameter.
Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.
Mark André is director of contact lens services at the Oregon Health Sciences University.