TACKLING THE TOUGH RX
National Federation of Opticianry instructors and directors answer what's, why's, and how's of some of the toughest lens questions to help you hone your opticianry skills and give patients a wow visual experience
By Karlen McLean, ABOC, NCLC
Illustrations by Bruce MacPherson
Knowing when to recommend a slab-off, how to best handle a high plus- or -minus-Rx, and other tricky prescriptions like dissimilar segs and low-minus distance power with a high-plus add, can help set your practice above those that only handle the everyday easy stuff and create an expertise that many patients may not be able to find anywhere else in today's retail environment of cookie cutter lenses.
National Federation of Opticianry Schools instructors address tough lens questions in their curriculums and regularly help students solve tough lens problems. Here, they help you do the same.
DISSIMILAR SEGS: Randall Smith, program director, Baker College, Jackson, Mich.
When a patient is wearing regular multifocal lens to read, he will experience a prismatic effect from the distance correction and the add power of the segment. If the patient has anisometropia of over 2.0D, he will notice the prismatic difference between the two lenses and will have a hard time forming a clear image at near. Correct this by using two different types of segment styles for each eye. Each bifocal segment has its optical center at a different distance from the segment line. They range from on the segment line (FT45) to 19 below in an Ultex. If you have a patient wearing a +2.00 add, and you use an FT35 and FT45, you can eliminate 0.9 diopters of prism.
Steps to finding the correct dissimilar segments to use include the following recommendations:
1 Calculate the vertical imbalance at the reading level for each eye. Determine the amount of imbalance that must be corrected.
2 Using Prentice rule: Determine the distance that the optical centers of the segments must be apart to create the amount of prism you calculated in step one.
3 The lens that has the most minus or least plus power in the 90th meridian should have the segment with the highest OC placement.
4 Match the segment tops in height. Fitting the segments low will reduce cosmetic differences.
| TOUGH RX TAKE-AWAYS |
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| What defines a tough Rx? At what point in the Rx does it require more dispenser and lab know-how? Jesse Arndt, vice president of Epic Labs, Inc. in St. Cloud, Minn., a lab known for expertise in processing tough Rx's, answers: HIGH MINUS To most labs, a minus Rx really becomes categorized as a high minus Rx when some part of the lab process has to be modified to accommodate the job. On average, Rx's that have minus powers of -8.00 or higher are high minus. HIGH PLUS Similar to high minus Rx's, high plus Rx's are generally defined by what cannot be processed in the lab without a modification of some kind in the overall lab process. High plus Rx's are a bit more complicated to define, due to lens variations based on frame selections. In general, Rx's that are +6.00 or higher are tagged as high plus. SLAB-OFF/REVERSE SLAB An Rx that requires a slab-off is any that will create a significant amount of vertical imbalance for the wearer. If an Rx has more than 1.50D of difference in overall power between eyes, a slab-off should be considered. A reverse slab should be considered under similar circumstances, but may only be an option on certain materials and seg styles. DISSIMILAR SEGS For the most part, employing dissimilar segs is not a good way to attack an Rx, unless there is an underlying reason for doing so. They often create confusion for the lab and generally do not achieve desired results without creating more problems. NORMAL DISTANCE WITH HIGH ADD Usually, high adds are problematic due to the availability from lens manufacturers. In cases where the distance power is relatively low, the best strategy is to employ plastic materials. Remember that there are alternatives to stock lenses. Labs can custom-make or use specialty manufacturers for unique add powers. PALS WITH SLAB-OFF If processed correctly, a PAL slab-off can provide the wearer with the advantages of the add progression as well as balancing vertical prism for comfortable near vision. On the flip side, PAL slabs processed purely with cosmetics in mind (such as blended line slabs) may introduce additional unwanted swim effects. |
SLAB-OFF: Richard Palumbo ABOM, director of Vision Care Technology, Essex County College, Newark, N.J.
Vertical imbalance is found in monocular pseudophakic patients or patients who have had cataract surgery in one eye only. This can often result in a large difference in lens power at the reading level in the 90th meridian. The difference can lead to diplopia or double vision.
Typically, the patient can fuse the two images well until she reaches 2.0D to 2.5D of vertical prism. Beyond this point, the patient starts to experience diplopia. In order to correct this optical error, displace the image to a more convenient place so that the patient is able to once again fuse the image as one.
Equalizing the imbalance is accomplished with slab-off or bicentric grinding. If there is a vertical imbalance of 2.0D of prism base up in the OD, then slabbing off the OS 2.0D base up will equalize the imbalance. In glass lenses, it is only possible to grind base up prism. In plastic, you can go base up, base down, or reversed slab-off; leave the slab-off up to the lab surfacing the lenses.
When calculating the vertical imbalance, here's a shortcut method: Calculate the total power of each eye in the 90th meridian, then calculate the dioptric difference in each eye.
If the total power of the OD is -3.0D and the total power of the OS is -0.50D, the dioptric difference is 2.50 diopters. Apply this difference to the prism formula where total power (dioptric difference) × decentration (reading level 10) divided by 10 is equal to the vertical imbalance.
The base direction is always in the stronger eye; therefore, it is base down in the OD. VI = 2.5D BDOD (base down). Slab-off would be 2.5D BDOS or 2.5D BUOD (base up).
HIGH-MINUS: Lynn Coffman, AAS opticianry, LDO, faculty, School of Opticianry, Seattle Central Community College, Seattle, Wash.
For a high minus, choose something as small and round as possible, since larger and more rectangular frames will add lens edge thickness.
Verify the vertex distance of the current frame and match it with the new frame choice and the base curve measurement. The fitting height and use of a monocular PD will give the lenses the best possible fit. The lens material needs to be aspheric high index. Anti-reflective coatings are a must. The lens thickness in the horizontal meridian will affect the nosepad adjustment capabilities, as well as the ability to close the temples.
To minimize the edge thickness, some high-minus wearers opt for rolled edges and some also like the edges polished. Another option is to have the lens bevel placed so that a third of the lens is in front of the bevel and the rest is behind the bevel. This is known as a hide-a-bevel.
NORMAL DISTANCE Rx WITH HIGH ADD POWER: Michael Szczerbiak, program director/instructor, Durham Technical Community College, Durham, N.C.
A typical low vision patient needs better reading performance with additional add power over the distance Rx. What makes them different than the average presbyope is that their near vision is dependent on a strong plus power incorporated into the add.
Anything over a +3.50 add is considered a high add. The typical lens style recommended is either a flattop segment or a round segment.
Which segment you use is dependent on the add power. The higher the add power, the larger the seg ledge, making it more conspicuous to the wearer. When using a round seg bifocal, the edge does not protrude from the surface but it has a greater vertical drop from seg top to segment center.
Progressives are rarely recommended for higher adds. Also, if the add is less than +4.00D, base-in prism may be needed for binocularity.
High add segs should be fit higher than a standard power seg. There will be less vertical prism induced because the visual axis through the seg will be closer to the distance optical center and a higher seg height may allow the patient to view beneath it for mobility. Suggest frames with adjustable nosepads.
For older individuals, a second pair of reading glasses may be the best choice rather than a multifocal with an add over +3.50D. The readers will help them with orienting to the food on their plate or items on a table or shelf if not used for reading.
HIGH-PLUS: Maureen Irey, LDO, ABOC, NCLC, School of Opticianry, Seattle Central Community College, Seattle, Wash.
With the current use of intraocular transplants, lenses for high plus patients are less common. High index aspheric is best, but often available only in 1.50 plastic in higher powers.
If the prescribed power is +10.00 OU and the patient wants half eyes, measure monocular PDs and MRP fitting heights. With spheric lenses, the optics aren't as good as with aspheric lenses. Remember to calculate base in prism for convergence using the Kestenbaum formula; always 2D more of base in prism than the diopter amount. With +10.00D, that would be 12 diopters base in prism OU.
By going with a monocular lens instead of a binocular lens, you could eliminate the prism for convergence, but you'd need to frost over the nondominant eye lens. You could also use Fresnel prisms to reduce the weight factor. New, so-called "invisible" readers are available from low vision aid companies. The lenses are thin and made of aspheric material with integrated diffractive structures. They are available for binocular or monocular vision. EB
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