contact lens case reports
Corneal Topography Changes with Aspheric GP Lenses
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO

The high eccentricity (0.7 to 1.0) on the posterior surface of aspheric GP multifocal lenses requires that you fit the base curve 2.00 diopters to 4.00 diopters steeper than flat K. This creates the necessary tear volume beneath the center of the lens to generate both the distance and near add powers and allows the lens to land appropriately in the mid-periphery of the cornea. However, the excessive apical clearance can often result in unwanted topographic steepening.

Figure 1. Tear film forces beneath a spherical, single vision GP lens (top) and an aspheric multifocal lens (bottom).

Patient JZ was a 48-year-old male who we successfully fit with posterior aspheric multifocal lenses. We selected base curves that were approximately 3.50 diopters steeper than K OU resulting in add powers of +1.75D. JZ wore the lenses for two years and recently presented with symptoms of decreased acuity at near.

Simulated Tear Film Forces

The OrthoTools software from EyeDeal Software can simulate the location and volume of tear film forces beneath GP lenses. For purposes of comparison only, we used the software to design a spherical, single vision GP lens for the patient's right eye and an aspheric multifocal GP lens for his left eye. We designed the spherical base curve OD on flat K, which resulted in 15µm of apical clearance. We designed the aspheric base curve OS 3.50 diopters steeper than flat K, resulting in an apical clearance of 60µm. (Figure 1 shows that both lenses "land" on the corneal surface mid-peripherally approximately 4.25mm from center.)

	Figure 2. Topographical maps of JZ's right eye, pre-fitting (left), post-fitting (right).

The software shows the uniform tear forces beneath the right lens and the increased volume of tears beneath the center of the left lens. In this example the tear film forces beneath the right lens will induce no topographical changes. But beneath the left lens we expect the corneal topography to steepen centrally secondary to the corneal tissue being drawn towards the area of greatest negative pressure.

In our patient, the post-fitting keratometric readings steepened only 0.75D. However, the power at the corneal apex steepened OD 1.75D and OS 1.25D.

Fighting the Effect

Today GP lens manufacturers are effectively combating this problem by reducing posterior surface asphericity and incorporating some asphericity onto the anterior surface.

Patrick Caroline is an associate professor of optometry at Pacific University and is 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 and serves as an adjunct assistant professor of optometry at Pacific University.