By Thomas H. Clark O.D.
Ever heard this? “Seems like I need to rotate my head to read clearly through one of my lenses.”
I’ve been annoyed and frustrated when I thought that the lenses I dispensed were measured and fit correctly and yet the patient, the real expert, says, they aren’t. If these lenses were correctly placed, patients would have good binocularity and comfort at near, not the stereoscopic disruption and discomfort that they are experiencing.
Stereoscopic disruption is one of the main causes of visual discomfort in progressive addition lenses. It results in unexplained redo’s, non-adapts and for some, a general dissatisfaction with progressives. This seems counter-intuitive since new progressive designs and digital processing and measurement techniques have generally improved patient reactions to new lenses. However, there is still a fundamental fitting error that reduces a progressive add lens wearer’s satisfaction. The issue is that the monocular PDs used as a visual axis measurement are incorrectly defined and measured. And, will not determine the correct fitting points in a patient’s RX.
This error is the result of the expectation that the eye’s visual axis and optical axis are sufficiently close, and that the corneal reflex indicates the path of the visual axis. This results in the use of the corneal reflex as a good approximation of a person’s PD.
However, a known biometric artifact, called angle kappa, the angle between the pupillary axis and the visual axis affects the accuracy of PD’s measured. Termed positive when the pupillary axis is nasal to the visual axis, and negative when the pupillary axis is temporal to the visual axis, angle kappa can make a patient say, “seems like I need to rotate my head to read clearly through one of my lenses”.
This author believes that progress in the area of reducing patient complaints, redo’s and non- adapts with progressive add lenses has slowed, first because designs have improved but also that there is still an unknown measurement error. Even with the introduction of free-form lens designs that improve overall satisfaction and new “high tech” digital devices that are supposed to make measuring for the PAL segment placement more accurate (including the frame’s affect on monocular PD), these rates haves changed only a small amount.
No one would ever willingly order a prescription from a lens laboratory with misaligned fitting points. The most accepted method for ordering PAL placement uses the distance monocular PD only (PD rule or CRP). The lens laboratory then uses various software algorithms that take the distance Rx into account as well as design targets, the PD, height and the expected reading distance to create the path of the corridor and its angle. This creates a positioning of the reading centers of the lens that may or may not be accurate. As a result, an incorrect PAL segment inset, in the final patient’s eyewear, is possible.
A November 2012 survey conducted by GFK research determined consumer attitudes towards PAL lenses. At that time, they found that only 43% of respondents were completely satisfied with their progressive lens eyewear. 6% or 3.3 million could not adapt to their PALs and 38% might not “repurchase “ PALs again. (1) Not addressing the biometric issue of angle Kappa, I claim, is one reason for this low success rate.
Not being aware of or ignoring this misalignment issue concerning angle Kappa not only causes unnecessary patient visual discomfort but also makes the process of resolving fitting problems very difficult if not impossible. Remember, troubleshooting a problem pair of glasses is most often the responsibility of the optician or para-optometric in the office. With the variety of levels of expertise, some of these problems are not understood and never get solved.
This series of articles will address the issue of incorrect placement of a patient’s progressive lens reading area in a patient’s frame. This incorrect placement is a result of interpupillary distance (PD) measurements using a technique, that relies on either pupil center or corneal reflection but do not take into account the biometric effect of Angle Kappa. It shifts the true location of the visual axis. That results in a location where the visual axis intercepts the patient’s lens away from correct the horizontal fitting point.