prescribing for presbyopia
Know Thy Aspheric Multifocals
BY CRAIG W. NORMAN, FCLSA
Simultaneous vision multifocals are by far the most common type of presbyopic contact lens design. Almost all soft lens multifocals feature this design, and while there are many RGP alternating vision alternatives, simultaneous designs remain more popular. In fact, the Rigid Gas Permeable Lens Institute's searchable database of RGP lenses (www.rgpli.org) lists more than 80 variations of this concept. While different design approaches exist, manufacturers usually employ some type of posterior surface aspheric optics to generate add power.
What makes these designs different from one another? More importantly, what does that mean to us when choosing, fitting or problem solving an aspheric multifocal lens?
Figure 1. Eccentricity affects the fit of aspheric
The amount of eccentricity can vary greatly among aspheric posterior surface RGP multifocals, depending on design philosophy. Figures 1a and 1b both present RGP aspheric multifocals that generate approximately 2.00D of add power. Note the significant difference in their fluorescein patterns. Figure 1a shows a bi-aspheric lens design with a low eccentricity optic zone combined with an adjacent hyperbolic curve. Figure 1b shows a high eccentricity design that rapidly flattens from the central apical radius towards the periphery. At first glance it is hard to believe that they are from same category of lens design, yet both are worn successfully by patients with similar presbyopic needs.
Fit low eccentricity designs like Figure 1a just slightly steeper than the flat K usually 0.50D to 0.75D. Figure 1b is fit 4.00D steeper than flat K. The fitting relationship is significantly different between the two, but they both produce similar add powers. One shows edge clearance that appears to border on excessive, but actually provides improved tear circulation under the lens. The other seems grossly steep centrally, but notice how well the lens aligns to the mid-peripheral cornea. Different designs, different clinical appearances both with great patient results.
Generally, all aspheric RGP multifocals need to position centrally to superior-centrally on the cornea, regardless of the amount of eccentricity. The lens provides adequate distance vision when it positions over the pupil as much as possible in primary distance gaze. Avoid excessive superior decentration, which causes the greater plus power mid-peripheral lens portion to align near the patient's distance visual axis, decreasing distance acuity. Increased minus power is then necessary to improve vision with the net result including decreased reading vision.
Keep in mind the lens position when incorporating over-refraction results. If the lens decenters too much superiorly, fix the fit first. Then worry about determining the final lens power.
Most importantly, all aspheric RGP multifocals must translate slightly during near gaze. This allows the reading optics to move over the pupil during reading tasks, which improves visual performance.
Craig W. Norman is Director of the Contact Lens Section at the South Bend Clinic in South Bend, Indiana. He is a fellow of the Contact Lens Society of America and an advisor to the RGP Lens Institute.