Dr. Sheedys scientific approach to analyzing PALs is good indeed, but if the results cant be made intelligible, its not going to impact patient care very much.
Usable PAL Data Needed
Dr. Stephen Glassers article on PALs (A Scientific Approach to PAL Options, December 2006) tantalized us but it failed to provide practical application for the clinical prescription of progressive lenses for our patients. He said, Power rate and zone width, although not related to each other, are both strongly correlated to intermediate and near viewing zones. Such statements may be useful for someone designing a series of lenses, but how do they help the doctor know which lenses to choose for a given patient? Dr. Sheedys scientific approach to analyzing PALs is good indeed, but if the results cant be made intelligible, its not going to impact patient care very much.
I published an in-house study of PALs in Optometry in 2001 that mentioned some data a clinician can use.1 I had done a pilot study in 1987, from which I learned my chops. There were some design flaws in that study, and I didnt even attempt publication. But, those earlier measurements did allow some comparison to 2001 data.
Here are some pearls I can share about PALs:
PALs measured in 2001 had higher levels of quality and consistency compared with those measured in 1987. Specifically, the drop distances had decreased, and the zone widths became more consistent in the 2001 measurements.
Hard design lenses had peripheral astigmatism (PA) measurements of up to -4.00D, while soft design lenses had PA measurements of -2.00D or less. The hard design lenses did not have any discernable payoff, as zone widths were about the same as soft design lenses. The bottom line: There is no good reason to prescribe hard design lenses.
Compressed design lenses (e.g., AO Compact) had a reduced drop distance (which is good) but had reduced zone width and small intermediate zones. Nonetheless, they are quite useful when prescribing for small frames.
Polycarbonate PALs had reduced zone sizes and more aberrational dispersion. Use them cautiously.
A subtle difference between an Essilor lens and a lower-cost competitor: Both lenses had equal amounts of PA and similar zone sizes. However, the Essilor lenses showed a more graduated astigmatism buildup as one moved away from the optical center, while the competitor showed maximal PA closer to the reading zone. This subtle difference made the Essilor lens slightly easier to adjust to, in my opinion.
Patients measurements correlated to their complaints. Some lenses had small zone sizes, and under-powering of the add was not unusual. Such dud lenses often resulted in patient complaints.
Thats the wisdom that Ive culled from a sea of disinformation about PALs. I invite your comments and wish Drs. Sheedy and Glasser well.
Gary R. Bell, O.D., M.S.Ed.,
Dr. Glasser responds:
Dr. Bell is absolutely right when he states that if information disseminated is not useful to the practitioner, then it does not help in patient care.
My purpose for writing the article, A Scientific Approach to PAL Options, was intended to be just that: a scientific approach. It is important that practitioners understand not only the practicality but also the science behind the products that we use. In this way, we have a better understanding of their abilities and capabilities.
That being said, Dr. Sheedy, in his article on progressive lenses, does in fact include listings of progressives by name and how they rank as to particular aspects of design.2 They are ranked by reading area, distance area, peripheral induced cylinder, etc. If anyone would like more practical information on progressives, I would encourage you to seek out Dr. Sheedys article and save it as a reference for care.
Quest for Perfect PAL
I agree with the points of the article by Dr. Glasser titled A Scientific Approach To PAL Options. He states: 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. That couldnt be more true. However, it is something that is difficult for staff or a doctor to convey to patients.
I use a Web-based program to configure the lens design and prescription that is custom-tailored for each individual. This program utilizes key factors such as age, height and monitor distance. Once the general use prescription and add are determined, those numbers are run through the program, and it will determine the necessary power distribution and whether a multifocal or single-vision design is best suited. My staff and I have had great success with the program.
If you are interested, go to www.crystaleyesrx.com. There is no fee to register, and your practice will be listed in the provider search tool open to the public.
Larry Tarrant, O.D., President, CrystalEyes Rx Computer Lenses, L.L.C.,
Dr. Glasser responds:
While I compliment Dr. Tarrant for his quest to help design the perfect PAL, I am concerned that what he has presented in the way of an online progressive lens may be lacking some basic ingredients.
It would be difficult, if not impossible, to design the proper lens for a computer user without asking about such things as the type of work performed, type of monitor used, angle of the user to the monitor, screen colors and available lightinglet alone such prescribing basics as monocular PD measurements or the height from the bottom of the frame to the pupil, which is impossible to determine online.
As a writer and lecturer on computer vision syndrome for more than 20 years, I have learned that when dealing with a computer-using patient, every piece of information about the user and his or her environment lends clues to help in the choice of the proper prescription and its design.
Editors note: Dr. Tarrant provided a recent update. We have made the decision to not sell our lenses directly to patients over the Internet, he writes. We will only sell the lenses to providers and to employers. I felt that it was in the best interest of the eye care providers and patients to make this change.
1. Bell GR. Verifying and evaluating progressive addition lenses in clinical practice. Optometry 2001 Apr;72(4):239-46.
2. Sheedy J, Hardy RF, Hayes JR. Progressive addition lensesmeasurements and ratings. Optometry 2006 Jan;77(1):23-39.
