If you ask any laboratory or lens manufacturer, they’ll tell you that the No. 1 reason for progressive addition lens (PAL) rejections by patients is not the lens design, it’s the fitting measurements. After about 40 years of PAL availability, that seems pretty surprising. There is, however, a simple reason why obtaining accurate fitting measurements is so difficult.
Parallax is the major problem in fitting height measurement inaccuracy. This occurs when you view along the wrong line which happens when the measurer (optician) sits higher or lower than the patient.
Figure 1 illustrates a patient and an optician sitting across from each other at the dispensing table. The optician instructs the patient to gaze into the center of her open left eye with the patient’s right eye. Notice that in this figure, the patient and the optician are sitting at the same eye level and the optician’s eye is exactly at the same height as the patient’s above the dispensing table top. This ensures that the optician and the patient are avoiding parallax. In this viewing relationship, the optician has the potential for obtaining the right fitting measurements because both people are viewing along the correct line—parallel to the dispensing tabletop.
If the optician sits lower than the patient (as in Figure 3), the optician will obtain a lower measurement because once again, she’ll be viewing along the wrong line.
Amount of Error
While aligning to the patient may seem trival to some, sitting at the same eye level avoids huge errors. Figure 4 tells us that for each inch the optician sits higher or lower than the patient during measuring, a 1.2mm error is induced. This means that an optician sitting only 3 in. higher than his patient will obtain a 3.6mm fitting height error. That’s a huge measuring error and one that’s easily avoided.
To avoid this fitting height error, position yourself at the same eye level as your patient across the dispensing table. If you are positioned higher than your patient, lean forward and pick up your chin until you are at the same eye level. If you are shorter, position your chair higher if possible or position your patient’s chair lower. If that is not possible, find something to sit on that can position you higher. Adjustable height furniture is valuable in adjusting your eye level position too.
Here are some other tips for avoiding fitting height errors:
• Pre-adjust the frame
• Don’t move once you start measuring
• No head turning
• No moving between each eye’s measurement
• If you break alignment at any time during measurement, start over again
• Keep the ruler steady
• Take the reading twice using this technique
Paying careful attention to fitting height measurements will pay off in more accurate measuring, which can improve your patients’ satisfaction with their multifocal lenses. Make sure you give your multifocal patients the right fit for their lenses by properly measuring them.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.