L&T: RxPertise

Oct
2013

Prisms and Progressives

Is What You Order, What You Really Want?



By Palmer R. Cook, OD

“You have three wishes and three wishes only,” rumbles the Genie. “So tell me, what is your desire?” We have all read such fairy tales, and inevitably the wishes are squandered in such a way that the wisher is no better off, and often much worse off, than he was originally.

“Bring a rock group to entertain me” might yield obnoxious, gyrating musicians. The second wish is used to relieve the wisher of the annoying musicians. After some thought, the wisher becomes more specific with his final wish, “Bring me England’s most widely recognized and famous rock group,” only to be crushed when Stonehenge falls upon him. When ordering complex prescriptions to be fabricated with progressive addition lens (PAL) technology, you can unwittingly be playing the part of that hapless wisher.

Although patients usually won’t land on you like 1,200 tons of old English rock, it is never pleasant to delay delivery of ordered eyewear because it took several attempts to get it right. Even worse, it’s possible for an order to be correctly filled as you ordered it, yet it can be needlessly difficult to adapt to, or impossible to wear.

Each of the following four patients want PAL lenses, and they have prescriptions that require more than copying the prescribed lens powers onto an order form if progressives are ordered. Progressive designs separate the distance MRP (that point giving the prescription) into two points. The prescribed refractive power for distance (the DRP) is placed several millimeters higher than the PRP, the point that gives the optical center or the correct prism if prism is Rxed. For most patients, this works rather well. For others, this can make adaptation difficult or impossible.

When ordering any PAL, you should know the distances from the DRP to the Fitting Cross, PRP and the NRP. These distances vary. They are not standard for all designs. For the design illustrated here, the PureSite Ultima from Diversified Ophthalmics (Fig. 1), the DRP is 10 mm above the PRP. The drop from the Fitting Cross to the PRP is 4 mm, and from the Fitting Cross to the NRP can be 14, 16 or 18 mm. Shorter corridors, smaller eye sizes and B measurements, higher Abbe lens materials, anti-reflective lenses and consultation with the experts at your lab can be key factors in achieving the best possible outcomes for patients with challenging prescriptions.

Fortunately, patients who are negatively affected by dividing the MRP into two separate points can often wear PAL designs if certain compensations are made when ordering. These compensations, as with so many optical trade-offs, solve some issues and create others. Making these compensations tends to increase the thickness and weight of the lenses, and the use of the intermediate powers in the corridors will be limited. The patients who need these compensations are primarily those with a vertical imbalance, oblique astigmatism or prescribed prism.

UNEQUAL LENS POWERS AND PRISM IMBALANCE
If your patient has a stronger lens in one eye than the other, as in Fig. 2, a prism imbalance occurs when the lines-of-sight are not passing through the optical centers. Although tolerance varies, patients may experience difficulty when as little as 0.5Δ of vertical imbalance is created. These difficulties can include adaptation problems. Intermittent or constant diplopia may result with greater amounts of vertical imbalance. Because we constantly converge and diverge our eyes in the horizontal meridian, horizontal imbalance seldom causes problems in maintaining single, binocular vision. In Fig. 2, if the patient looks 1 cm to his right, he will experience a yoked prism effect of 1ΔBI in his right eye and 3ΔBO in his left eye. This imbalance would be fairly well-tolerated by most patients.

OBLIQUE ASTIGMATISM RAISES THE ANTE
The power of lenses with cylinder varies according to the meridian you are considering. In the axis meridian, a cylinder adds no power to the lens, and the power meridian contributes the full amount of the cylinder power to the lens. The power meridian and the axis meridian are always at right angles to each other. For a simple cylinder such as plano -2.00 x 090 the power in the axis meridian (090) is plano, and the power in the power meridian (180) is -2.00. From a practical standpoint, it is most important to watch for differences in power of about 1.5 diopter or greater between the vertical meridians when ordering PAL lenses. When the axis of the cylinder is oblique, you can estimate the power and the prism effect at any point on the vertical meridian. It’s important to remember that an oblique cylinder contributes both lateral and vertical prism to the vertical meridian.

YOKED PRISMS AND ADAPTATION
Spherical prescriptions with prism present few complications especially if the power is about the same in both eyes. If both lenses have a +2.00DS power, the adaptation to the prism at the DRP will be fairly easy. The amount of prism for the design in Fig. 3 is calculated by multiplying the lens power by the distance from the PRP to the DRP (1 cm), which gives 2ΔBD. If the other eye has the same +2.00 power, the patient will be looking through 2ΔBD in each eye when he looks through his lenses at the DRP. This combination of prisms is termed yoked prisms because each eye is “yoked,” like oxen pulling a cart, with each eye moving upward the same amount in order to see straight ahead.

In Fig. 3, the eye moves about 0.3 mm per prism diopter (in the direction of the apex of the prism). Here the fitting cross should be raised from its measured position by about 0.5 mm (0.3 x 2, rounded to 0.5 mm for practicality) to assure the best lens performance at distance. If the prism at the DRP was BU, the fitting cross should have been dropped 0.3 mm per prism diopter.

Yoked prisms work well up to about 2 to 2.5 prism diopters. Over that amount, adaptation becomes more difficult especially for distance vision, which is one of the reasons why patients who have stronger lenses have more difficulty with adaptation when they graduate from single vision lenses to PALs.

NATURE’S MONOVISION
A 47-year-old maintenance man used single vision lenses for constant wear for many years. For the past “year or so” he says he has lifted up his safety glasses for detailed near work such as sharpening drill bits. At distance the refractive error is about the same as the correction in the old single vision lenses (OD +1.00 DS, OS -3.00 DS). PALs are prescribed with a +2.00 add. A day following dispensing, he returns saying he can’t see with the new eyewear. The lenses are rechecked and found to be as ordered, but the optician noticed that there was a vertical imbalance of 2Δ at the distance reference point and a little over 3.5Δ at the reading level. The lab claims the Rx was filled as ordered.

Our “Nature’s Monovision” patient has tried to compensate for presbyopia by lifting his glasses and using his left eye for close work. The doctor did not prescribe prism, but if the patient looks above the PRP he has BD prism for the right eye and BU prism for the left. If you order the Rx without allowing for this, he will have about 1ΔBD OD and 3ΔBU OS (a 4Δ imbalance) at the point in each lens that gives the prescribed powers for distance seeing. If you use the Ultima, 14 mm corridor (14 mm from the Fitting Cross to the NRP), there will be a 10 mm drop from the optical centers of the lenses to the NRP, giving a 4Δ imbalance.

If you order slab-off, there will be no imbalance at the NRPs, but if only a 4Δ slab-off is used, the imbalance at distance will remain. The easy way to handle this order is to ask your lab to: “give no vertical imbalance at the DRP” (about 1ΔBU and 3
ΔBD will be needed), “and slab-off for no vertical imbalance at the DRP.” The best solution will probably be to move the optical centers to the DRP, but you should follow your lab’s recommendation for the best outcome. 

A NEW PRESBYOPE
A 43-year-old teacher is given her first Rx with an add. It is OD +2.00 -2.25 x 013 3ΔBI, OS +2.00 -3.75 x 150 3ΔBI with a +1.25 add. She absolutely wants “those lenses without lines” when you explain about the need for an add. Her old single vision prescription is nearly the same as the new distance prescription, and she has worn it for a couple of years without difficulty. At dispensing she complains that the new eyewear “doesn’t seem right.” You reassure her by telling her that her visual system must adapt to the new eyewear. She calls at the end of each day for the next three days, finally admitting on the third day that there is still some strain. You see her at the grocery store a few months later, and she is wearing glasses that you did not dispense.

For this “New Presbyope” with lateral prism, the ordering is a bit more complicated. Using the Ultima and with a 10 mm drop from the DRP to the PRP, the power in the right vertical meridian is about -0.14, and in the left eye it is about -1.00. This can be estimated using an off-axis cylinder power chart and combining that power with the sphere power. The vertical imbalance will only be 0.86Δ at the DRP. This may not be a problem, especially if you use a lens design in which the power gain in the corridor is greater toward the lower end of the corridor. If she does not encounter much additional plus at near the fitting cross, she will have the option of raising her chin to look a bit below the DRP where the imbalance is less. Her distance PD was 33/34.

The 16 mm corridor was specified and so the drop from the PRP to the NRP is 12 mm, and the prism in the vertical meridian is about 0.17ΔBD OD and about 1.2ΔBD OS, giving a vertical imbalance of 1.03Δ. Because these imbalances at distance and near are relatively small, you should consult the prescribing doctor. He may want to do further testing before lenses are ordered. One way to minimize this imbalance is to use a lens design with a shorter corridor, a shorter drop from the DRP to PRP and a corridor with a fast power gain low in the corridor. Your lab can calculate the prism imbalance at the DRP and NRP for whatever lens design you are ordering.

This prescription ordered “conventionally,” will have the lateral prism somewhat in error in both eyes at the DRP because of the oblique cylinders. Unless the patient’s fusional range is narrow, this may not cause problems. When ordering, it would have been better to ask the lab to: “calculate the prism that will be found at the DRP,” consult the doctor as needed and then order. By asking the lab to give “no vertical and only the prescribed lateral prism at each DRP,” the imbalance at near would increase to about 1.89Δ, and a slab-off would likely be needed.

The 3ΔBI OU will cause the patient’s eyes to move outward 0.3 mm/prism diopter OU, so the patient’s monocular PDs would have to be increased by 1 mm (0.3 x 3 = 0.9 round to 1 mm) from 33/33 to 34/34.

VETERAN WEARER—NEW LIFESTYLE
A 54-year-old widow has worn D-28 lenses with a slab-off successfully for many years. She wants fashion frames with lenses without lines this time. Since her old frames had the same B measurement, and there was not much change in the distance Rx, your order (OD +2.00 -3.00 x 180, OS +2.75DS) had specified a 3.5Δslab-off, which was the same as her previous eyewear. She returns after five days complaining of problems with both distance and near vision. You return the lenses to the lab and are told the lenses are “as ordered” but that more slab-off is needed. You order a remake with increased slab-off, and she reports that this improves her near vision, but her distance vision is uncomfortable. She is 20/15 OD and OS at distance. She says that her vision indoors is OK, but outside and for night driving her distance vision is a problem.

The “New Lifestyle” patient has worn slab-off in D-seg bifocals and knows that her eyewear has one level for clear, comfortable distance seeing and another for clear, comfortable near seeing. That will be the same in a PAL slab-off, but she will have a longer drop from the distance to the near level. Bifocal wearers with slab-off are usually comfortable with about a 10 mm drop from distance to near. If you fit an Ultima with a long corridor (because you have room in the frame and to reduce peripheral distortion in her first PAL), the drop will be 24 mm and the amount of slab-off will be greater. If you use the 14 mm corridor, the drop will be 20 mm. A 20 mm drop would require a 7Δ slab-off, and the optical centers should be moved to the DRP.

When ordering, specify: “move the optical centers to the DRP” and then “slab-off for no imbalance at the NRP.” Because the lab should be able to place the slab-off line as you specify it, you can drop it a bit to get the biprism effect out of her way. Most labs will place the slab-off line at the PRP unless directed otherwise.

DON’T LET THE  YOKE BE ON YOU
This 48-year-old male chemical engineer has had several pairs of PAL lenses over the past “3.75 years,” as he phrases it. He has a sales position with a major company and is highly concerned about eyewear appearance and performance. His Rx is OD plano -3.50 x 045 1ΔBU, OS plano -1.00 x 135 1ΔBD. He is familiar with the term slab-off and has worn it in the last two sets of eyewear. None of the previous PALs had been “satisfactory,” and although he knows his lens prescription is complicated, he wants the best possible outcome. The optician felt that there was an error in the prism for distance seeing. She wisely ordered a remake to correct the problem before dispensing.

The “Yoked Engineer” has about -1.75 in his right vertical meridian and about -0.50 in his left vertical prism. This will give him 2.75ΔBU at his DRP OD, and 0.5ΔBD OS at his DRP if you simply specify the 1ΔBU OD and 1ΔBD OS, and he will have unwanted lateral prism (BO OU) at the DRPs. Because you are giving him a long corridor, the lab will probably want to use yoked prisms at the DRP to save on thickness and weight. Using a standard plastic lens (lower index = lower reflectance) will help. He resisted your advice to get anti-reflective lenses because he “had some in 1978 and they crazed.” He also refused photochromic lenses with some residual tint as well as refusing a light tint, commenting that he “didn’t want any tint in his way.” The low power of these lenses combined with the prisms almost cries out for AR lenses, and a light 8 percent to 10 percent tint would help even more.

The optician was very perceptive in noting the unwanted lateral and incorrect vertical at the DRP level when the order was first placed conventionally. Many ECPs are under the impression that the patient looks through the Fitting Cross for distance seeing. Perhaps this is because the Fitting Cross is placed at the center of the pupil when the optician’s eye is at the patient’s eye level. You must measure the distance Rx at the DRP, for the same reason the patient must seek the DRP for best distance seeing. This is because just a bit below the DRP, the ray bundle of the lensometer and the ray bundle of the eye begins to impinge on increasing plus power. The re-order correctly specified: “yoke prisms to maintain the 2Δ imbalance at distance and slab-off to maintain that imbalance at near.”

USE TECHNOLOGY TO THE BEST ADVANTAGE

For patients with challenging prescriptions, it is usually best to favor the distance vision. For prolonged near work, reading lenses can be prescribed. For some patients, contact lenses may be the best solution, or a combination of contacts with a spectacle over-correction may offer a good alternative. Fortunately digital surfacing offers a less visible slab-off line and a greater range of available slab-off power.
To serve your patient well, you can’t always just copy the power formula onto the order pad or select a design arbitrarily. PAL designs, especially those that must be produced digitally, are complex and should be chosen and ordered in a way that their excellent technology is used to the best advantage.

If you have the time and inclination to calculate how to place the exact power and prism needed to give your patient what the prescription calls for, you can do it. However, it’s much more efficient to partner with your lab by specifying what you want and allowing them to do the calculations. In some cases, you may need those calculations to make the best ordering decisions.■

Clinical Tips for Complicated Ordering
  1. Lenses incorporating prism should be anti-reflective because AR will improve their performance, especially with relatively lower powers.
  2. Prism always exacerbates the negative effects of chromatic aberration especially when the prism is in the distance portion of the lens. The more prism, the more the problems with chromatic aberration—always and especially with people who work out-of-doors.
  3. By letting your lab consultant calculate the amount of overall prism needed to move the optical center and also by giving a reading level instead of an amount of slab-off, the lab will have to foot the cost if an error in calculations is made.
  4. Shorter corridors shorten the drop from the DRP to the NRP, so they require lower power slab-offs.
  5. Most labs place the slab-off line at the prism reference point unless you specify otherwise. With PALs you can move the line down a little. People with long-term vertical imbalances often learn to suppress the vision from one eye when they can't fuse. For that reason alone, the decision to use slab-off or not isn't always clear-cut and requires careful, professional consideration. These patients also tend to have a greater vertical fusional range, so you may get fusion in the upper portion of the corridor when looking above the slab-off line, which allows better use of the corridor.


L&T contributing editor Palmer R. Cook, OD, is director of professional education at Diversified Ophthalmics, Cincinnati, Ohio.

 

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