contact lens case reports
Add Power Success with Aspheric GP Multifocals
BY PATRICK J. CAROLINE, FCLSA, FAAO, & MARK P. ANDRÉ, FCLSA, FAAO
Practitioners who fit aspheric GP lenses for presbyopia are aware of two facts related to overall lens performance: achieving an effective add power can be a difficult endeavor, and a 0.25D additional add power can significantly impact the success of the modality. The ultimate add power of a posterior aspheric GP multifocal lens is governed by the following parameters.
A normal single vision GP lens incorporates an asphericity of approximately 0.35 to provide a gradual flattening effect to the lens. This degree of asphericity yields a minimal add power of about +0.25D. To create a multifocal effect, a higher degree of asphericity is required often between 0.60 to 1.00. The higher the asphericity, the higher the potential add power. However, a higher asphericity often results in diminished distance acuity; therefore, there is often a limit to their effectiveness.
Figure 1. Parameters for a typical aspheric multifocal
Material Index of Refraction
One of the often overlooked lens characteristics that can influence the final add power is the materials index of refraction. For example, a patient presents to your office with a spectacle Rx of 3.00 1.00 x 180 and Ks of 42.00 @ 180 / 43.00 @ 90. An aspheric multifocal lens is designed with a posterior asphericity of 0.80, necessitating a base curve radius of 44.75 diopters, 7.55mm, 2.75D steeper than flat K and a power of 5.75D (Figure 1).
Table 1 lists a number of common GP materials, their Dk values, material index and potential add power for the lens described above. If an experimental material were to be developed with an index of refraction of 1.550, the potential add power for that material (in the design described above) would be +2.25D.
It is clear from the information in Figure 1 that as the index of refraction of the contact lens material increases, so does the potential add power of the lens (with asphericity constant). Therefore, when selecting a material for aspheric multifocal contact lenses, it may be important to consider the lens material especially in patients who require add powers beyond +1.25D.
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.