Progressive addition lenses (PALs) are one of optometrys mainstay workhorses. Patients generally prefer them to segmented multifocals, and they generate healthy growth for most practices. Still, there is plenty of room for improvement in how we prescribe these lenses, especially for patients who work on computers for prolonged periods. New research may improve how optometrists prescribe lenses to match each patients needs.
In the past, we relied on three major sources of information in fitting PALs: manufacturers claims, patient feedback and our own anecdotal experience. What we have lacked is peer-reviewed scientific data.
All that has changed with a series of studies by James E. Sheedy, O.D., Ph.D, dean of Pacific University College of Optometry.1-3 The studies, published during the last two years, offer an objective source of scientific data on the optics of different PALs.
This article will discuss one important aspect of these studies: relations among key optical parameters in PAL design, and how this information can help match the right lens with the right patient.
Always a Tradeoff
Currently, there is no such thing as a perfect PAL, which would offer large distance, intermediate and near zones; a short corridor from distance to near; and minimal or no peripheral astigmatism. Lens technology has not advanced to the point at which all these are possible. When one optical characteristic is enhanced, another is diminished.
There is an almost unlimited number of ways to design a progressive lens. Dr. Sheedy and his colleagues measured 28 lens designs, using a Rotlex Class Plus lens analyzer to assess horizontal cross sections in 1mm vertical steps with respect to the fitting cross. They calculated the widths and areas of distance, intermediate and near viewing zones based on these measurements and determined unwanted astigmatism, minimum zone width and maximum power rate in the corridor. They then combined and analyzed this data to determine how these attributes relate to each other.
Not surprisingly, the relationships among the three viewing zones are all negative. In other words, making one zone larger or wider tends to make the other two smaller and narrower.
There is a clear design trade-off between distance and intermediate zones. Larger distance zone widths and areas generally result in narrower intermediate widths and areas and vice versa. However, this relationship is not overly strong, and Dr. Sheedy found that individual lens design can somewhat mitigate this inverse association.
The most statistically significant comparison between these two measurements occurs between the distance width at 1mm and the intermediate width at +0.75D. But elsewhere there is considerable variation. On average, the variance in one of these measurements accounts for only 16.7% of the variance in the other, according to Dr. Sheedys research.
A similar relationship exists between distance and near zones. This results in a design tradeoff between distance width and area and the height at which the full add can be attained. Thus, a design with a larger distance and width area usually means the wearer must turn his or her gaze farther down to achieve the near add. Again, this relationship is not overly strong; the variance in one accounts for only 18.6% in the other.
The strongest inverse relationship among the three vision zones exists between the intermediate and near zones. Wider, larger zones in one area mean markedly smaller, narrower zones in the other. The variance in one accounts for 42% to 80% in the other. There is also less opportunity here to minimize this tradeoff with design innovations, according to Dr. Sheedys data.
Optical Limit Values
Power rate and unwanted astigmatism are fundamental to PALs. Each of these values has a high degree of correlation with itself at different locations on the lens. But, according to Dr. Sheedys research, these are independent of one another. We know this because the correlation between their maximum values and locations is very low.
However, power rate and unwanted astigmatism both have a significant relationship, individually, with minimum zone width. An increase in the power rate or unwanted astigmatism results in a decrease of the minimum zone width. In other words, the locations and magnitudes of maximum power rate and maximum unwanted astigmatism are independent of each other. But, the location and magnitude of the minimum zone width depends on both values.
Zone width at different levels is not correlated across different brands of lenses. This suggests that zone width is not a fundamental value for a given lens. So, a lens designer should be able to vary zone width at different levels, unlike power rate and unwanted astigmatism, which are more basic values.
Also, power rate and zone width, although not related to each other, are both strongly correlated to intermediate and near viewing zones. These four relationships appear to be the strongest in PALs, according to Dr. Sheedys data. Unwanted astigmatism appears to be related to the intermediate zone and to the upper portion of the distance zone, but not to the size of the near viewing zone.
Overall, Dr. Sheedy concludes that the most fundamental optical relation is likely between power rate and zone width.
This is only a small portion of the research that Dr. Sheedy and his team have published. Those of you interested in fine-tuning your PAL fits should consult these studies for further information.
Dr. Glasser is in private practice in Washington.
1. Sheedy JE. Correlation analysis of the optics of progressive addition lenses. Optom Vis Sci 2004 May;81(5):350-61.
2. Sheedy JE. Progressive addition lensesmatching the specific lens to patient needs. Optometry 2004 Feb;75(2):83-102.
3. Sheedy JE, Campbell C, King-Smith E, Hayes JR. Progressive powered lenses: the Minkwitz theorem. Optom Vis Sci 2005 Oct;82(10):916-22.
New research may improve how optometrists prescribe lenses to match each patients needs.