Prescribing for Presbyopia
Comparing Aspheric GP Multifocals
BY CRAIG W. NORMAN, FCLSA
Researchers Chad Rosen, OD; Josh Lotoczky, OD; and Trevor Fosso at Michigan College of Optometry’s Vision Research Institute recently performed a pilot study analyzing the optical properties of six proprietary multifocal GP lens designs.
The lenses were ordered in the same parameters: –3.00D distance power, 7.50mm (45.00D) base curve, and +2.00D add power, and all were fabricated in the Boston XO (Bausch + Lomb) material.
The lenses were measured with the Nimo TR1504 contact lens power mapper and wavefront analyzer. Power measurements were generated within the central 2.5mm of each lens. Another measurement determined the distance from the center of the lens to the point at which maximum add power (2.00D) was reached (Figure 1).
Figure 1. Distance from center at which maximum add power was reached.
Power profiles were created along the 180° meridian to provide a visual interpretation of each distance zone size along with how rapidly the power changed from distance to near (Figure 2).
Figure 2. Power profiles of each lens along the 180º meridian.
Although all lenses were ordered similarly, there definitely were differences among manufacturers in distance zone sizes, power graduation from distance to near zone, positioning of distance optics, and optical clarity.
What Does This Mean?
First, the full add power is generated in each design, but the distance from the lens center to achieve this varies significantly.
This variance means that different designs have to translate more or less than others do to achieve full add power. This distance varied from 2.53mm to 3.78mm among the test designs. Lens #10 needs to move vertically almost 4mm or approximately 3mm over the superior limbus to achieve maximum add. Or, a higher add could achieve maximum add with less translation.
Finally, every design is different, so practitioners need to work with their suppliers to understand the subtleties of each design. CLS
Craig Norman is Director of Research, Michigan College of Optometry at Ferris State University. He is a fellow of the Contact Lens Society of America and is an advisor to the GP Lens Institute. You can reach him at CraigNorman@ferris.edu.