By Thomas H. Clark O.D.
In Part 1, I introduced angle Kappa, the angle between the pupillary axis and the visual axis, as a reason for patient dissatisfaction with PALs. Part 2 addresses why angle Kappa is important.
TROUBLESHOOTING THE PAL RX
Typically, when a patient presents with complaints about their new PAL lens, the first action by the dispenser is to test to see if the lenses were made according to the lab order, rechecking the patient’s Rx, segment height, and distance PD measurements.
Validating that the Rx, seg height and segment placement according to PD measurements are correct is a fairly simple process. Yet when everything seems right, why is the patient complaining about available reading width or that head rotation is required to see clearly out of either eye but not together? In some cases, the patient can’t articulate what is wrong, only that there is discomfort.
My research suggests that the mistake here is with the method of PD measurement. It is incorrect to assume that the distance PD measured at the patient’s corneal apex will result in the determination of the correct fitting points for that patient’s lenses. When the accuracy of these basic elements are (mistakenly) verified, the practitioner would now move on to other issues that may be the cause of patient dissatisfaction.
Unfortunately, assuming that the horizontal segment separation of the patient’s PALs is correct when using a PD measurement device can be a very serious mistake. This will cause unnecessary difficulties in the discovery process needed to re-adjust the patient’s new spectacle RX for optimum results.
IT’S TIME FOR A PARADIGM SHIFT
The optical community, in general, believes that a PD measuring device will give accurate information for the correct placement of PAL segments in a patient’s frame. Unfortunately, there is enough error contributed by angle Kappa to make this assumption wrong enough times, and that has cost the optical industry millions in chair and dispensing time, money and the loss of dissatisfied patients.
It’s time to measure the true actual visual axes for patients for correct PAL lens placement. External PD, CRP and algorithm measurements as the information for PAL placement in a patient’s frame are not accurate enough of the time.
This same issue affected the success rate of IOLs. Ophthalmologists, who have performed Lasik and cataract surgery, placing multifocal IOLs using optical axes measurements, have had poor results because of angle Kappa effects. Ophthalmologists now understand the importance of angle Kappa and accurate visual axes measurements. This has greatly improved IOL results where the exact alignment of these optical devices with the patient’s visual axes is critical for success.
In Part 3, we’ll take an in depth look at angle Kappa and its effect on PAL fitting.