contact lens case reports

The Effect of Corneal Diameter On Soft Lens Fitting, Part 2

contact lens case reports
The Effect of Corneal Diameter On Soft Lens Fitting, Part 2

Last month's column described how misleading K readings can be in soft lens fitting. This is especially true when the corneal diameter is outside of the normal range of approximately 11.8mm. In our horizontal visible iris diameter (HVID) study of 200 prospective lens patients, 52 patients measured smaller than 11.6mm, 98 patients were between 11.6mm and 12.0mm, and 50 were larger than 12.0mm.

Clinically, we have found the soft lens fitting guides can be used with a great deal of confidence when the HVID is between 11.6mm and 12.0mm. Outside of those parameters, we need to adjust our base curve selection instincts to accommodate the influence of corneal diameter on overall sagittal height.

We refer to this adjustment as the effective K reading. Effective K incorporates both the central K and corneal diameter to assist in the appropriate selection of the base curve radius. This is accomplished by the following;

Step 1: Obtain central Ks

Step 2: Measure the (HVID)

Step 3: For every 0.2mm larger than 11.8mm, add one diopter to the horizontal K

For every 0.2mm smaller than 11.8mm, subtract one diopter from the horizontal K

When this formula is applied to patients with corneal diameters that are larger or smaller than normal, we significantly improve our chance of selecting the appropriate base curve radius. As an example, let's calculate the effective K adjustments for the two patients that we discussed last month in Part 1.

Figure 1. The 8.9/15.0mm soft lens suggested by the manufacturer's fitting guide. Note the decentration secondary to an inadequate sagittal depth.

Smaller Diameter Corneas

Patient MC had K readings of 46.25 @ 180/ 46.50 @ 90 with a HVID of 10.2mm. Therefore, her corneal diameter was 1.6mm smaller than 11.8mm. This requires that eight diopters be subtracted from the horizontal K of 47.00D., making her effective K reading 39.00 diopters. In this situation, a flatter base curve will be necessary to provide the appropriate sagittal height.

Larger Diameter Corneas

Patient DP had K readings of 40.00 @ 180/40.50 @ 90 with a HVID of 13.0mm. Therefore, his corneal diameter was 1.2mm larger than 11.8 mm. This measurement requires that six diopters be added to the horizontal K of 40.00, making his effective K reading 46.00 diopters. Figure 1 illustrates the flat (8.9mm) base curve radius suggested by the manufacturer's fitting guide. Figure 2 shows the steeper (8.3mm) base curve radius required to accommodate the patient's increased sagittal height.

Figure 2. The steeper 8.3/15.0mm soft lens with perfect lens centration and movement.

In both of these patients, the ultimate base curve required was significantly different from the base curve suggested by a manufacturer's traditional, keratometric-based fitting guide. By incorporating corneal diameter and K readings into our soft contact lens fitting process, we can better estimate the true sagittal height of the cornea, thereby improving our initial base curve selection.

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.