topography topics

Evaluating On-Eye Optical Performance of Hydrogel Bifocals

BY KENNETH A. LEBOW, OD, FAAO
December 2000

It's one thing to fit a soft bifocal, but another to understand how and why the patient sees with the lens on the eye. You can appreciate the on-eye lens optics by comparing the ocular surface topography with that of the topography of the lens on the eye.  Using a difference map on these two surfaces can help you understand the optical characteristics and pupillary dynamics achieved with these simultaneous vision lenses.

Sunsoft Additions Lenses are center-near design aspheric hydrogel bifocals. The curvature of these lenses is steeper centrally and progressively flattens from the apex to the periphery. This variance in lens curvature generates plus power, creating a bifocal effect. Figure 1 shows axial and numerical maps of the right and left eyes of a patient wearing Additions C lenses. The right eye curvature changes from 46.2 diopters centrally to 41.9 diopters peripherally (nasal), demonstrating an aspheric flattening factor of approximately 4.2 diopters.

Interpreting the Difference Map

The difference map (Figure 2) shows a central green to red zone approximately 3.5 mm in diameter that corresponds to the strongest plus power of the lens. The lens periphery appearing primarily in blue represents the distance power of the lens. The patient's spectacle correction was ­1.75D sphere with a 2.25D add. The lens placed on the eye was a ­1.75C, which corresponded to the appropriate distance and near correction for this patient. Note that the lens add power is geometrically centered over the pupil and the patient's line of sight. This is the proper lens position for an aspheric design.

If we evaluate the power differences achieved with this lens, we would expect the central near power to be approximately +1.00D (-1.75D distance with a +2.75D effective near add power). The distance power in the periphery should be approximately ­1.75D. This is almost exactly what the difference map shows, indicating an ideal physical fit and optimum lens performance. The patient reported 20/20 distance and J-2 near vision.

While this analysis is not exact, it does present a clearer picture of how the lens fits on the eye and whether the power relationship is appropriate. Had the central difference been plano, for example, that would infer inadequate add power and the patient would require reading glasses. Conversely, should the difference in the periphery have been ­1.00D, it could explain why the patient would report improved near vision with poor distance vision.

Using corneal topography to understand and visualize the optical relationships achieved with hydrogel lenses takes much of the guesswork out of the process. 

Dr. Lebow is a member of the AOA and a Fellow of the AAO. He is in private practice in Virginia Beach, Virg.