ask the labs
Fitting PALS
Susan P. Tarrant
Progressive lenses serve most presbyopic patients well, and are ever-evolving in design to provide clearer and wider fields of vision and seamless transition between zones. But proper fitting of PALs is essential for patient adaption and success. Here, we bring you tips and opinions of lab experts regarding the specifics of proper PAL fitting.
CORRIDOR CONSIDERATIONS
Q What factors should go into choosing corridor length in progressive lenses?
A Before fitting any progressive, try to evaluate whether the patient is a good candidate.
■ PATIENT PATIENCE. Patients with issues with motion sickness or vertigo, or those with limited range of motion issues, may face challenges if fitted with PALs. The same is true for emmetropes. The closer they are to plano in the distance, the more they notice the unwanted aberrations inherent in a progressive, and the harder it is for them to adapt. The quality of modern PALs alleviates much of this, but look carefully at these patients before fitting.
■ SIZE MATTERS. Avoid frames with a B measurement of less than 25mm. A 25mm B measurement with a short corridor progressive fit at 14 will give the patient 11mm of distance area, and this should be as narrow as you go. A deeper B measurement would make your life easier with fewer patient complaints and non-adapts. Always pre-adjust the frame before measuring for lenses.
■ RESPECT THE SEGS. Manufacturers want as many patients as possible wearing their lenses, so their minimum seg height should not be violated. My rule of thumb is to get a seg height 1mm to 2mm above the minimum specified; but never violate the manufacturer's minimum.
■ MEASUREMENT TIPS. Take monocular PDs, preferably with a digital pupillometer. Without them, the patient's pupils may not center up properly in the transition corridor, resulting in distortion and non-adapts.
Measure seg heights at the center of the pupil. The actual power change from distance to intermediate does not begin at that fitting cross, so don't “fudge” the fitting height thinking the seg will be too high. The power changes will start anywhere from 2mm to 6mm below the fitting cross.
— Wes Palmer, director of sales and marketing, Optic Blue, Lubbock, Texas
| OC Roles |
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| Q Do I always have to take optical center measurements for PALs? Why? A All PALs are designed to perform the best if you measure monocular PDs and monocular “fitting heights” (pupil center to lowest part of lens). Depending on the manufacturer, this may or may not be the optical center (OC)/Prism Reference Point (PRP) of the lens. All PALs have distance, intermediate, and near vision zones. If a PAL was made without a fitting height, the zones would not be in the optimum position. If the fitting height is too low, the patient would have to look too far down into the lens before they entered the intermediate and near zones; if too high, they would be looking through the intermediate zone when trying to see something in the distance. — Robert Hughbanks, ABOC, HFOAA, manager of training and development, Expert Optics, Inc., Shorewood, Ill |
If you have a question you'd like to have answered in Ask the Labs, send it to Susan P. Tarrant. Email: Susan.Tarrant@Springer.com. An archive of past Ask the Labs columns can be found on the Eyecare Business website at eyecarebusiness.com.
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