By Palmer R. Cook, OD
You order spectacle lenses in pairs all the time, so when you have to order just one lens, you might think that order has only half the potential for trouble. Unfortunately it works the other way. The potential for problems for you, for your patient and for your lab is far greater when only one lens is ordered, especially if the lens you are replacing is missing. The reasons for this are rooted in the physiology of the visual system and in the increased potential for human error all along the trail from the power formula to the finished eyewear.
The patient arrives with one lens missing, the left one and a written Rx that is nine months old. The lens fell out on a fishing trip and “didn’t float.” You call the prescribing doctor’s office and you are told that replacement of the lens as written nine months ago is acceptable. The doctor’s tech comments that the new Rx is about a diopter more minus than the patient’s previous Rx from five years earlier. The Rx is OD -3.00 -0.75 x 180 and OS -3.50 -1.50 x 005. A +2.25 add is specified. The remaining lens is a flat-top 28 set 19 high with a +6.00 base curve in standard plastic.
As you dispense the completed job, the patient inquires about the pink tint that he “always has in his lenses.” You look more closely at the old lens and realize there is a pink tint, faded with age, but definitely there. You fortunately have in-office tinting capabilities, so you remove the lens from the frame and add the needed tint. After reinsertion and re-fitting the frame, the patient complains somewhat bitterly that “it doesn’t seem right.”
The patient grumbles and receives a short lecture on getting used to new lenses. He heads for the mirror and says, “Hey, I can’t see myself in the mirror very well unless I cover my left eye.” You pop the eyewear into your lensometer and discover the old right lens is -2.00 -0.25 x 015 with a +2.00 add. This is less minus than his new prescription calls for and it gives him a fairly clear view at the distance he was standing from the mirror. Upon questioning the patient admits he never had the nine-month-old Rx filled.
A Closer Look
Humans are predators. At least that’s how our visual system is designed. We have two eyes and for maximum functioning, we must have the ability to aim both eyes at objects from close (convergence) to far away (lines-of-sight parallel). The images formed on the retinas should be similar in brightness, size and shape. These retinal images are similar, but they are not the same due to the fact that one eye is 55 to 75 millimeters to the right (or left) of the other. Our brains can use the slight differences in these images to give us a 3D percept. For predators this 3D advantage can be very important. For the patient above, although he tolerated approximately a diopter of distance blur with the old lenses, the new left lens has now caused his retinal images to be quite dissimilar. Not only that, he now has approximately a 2.00 difference in power between the vertical meridians of the right and left lens. As a result he will experience about 2 prism diopters of unwanted vertical prism, and possibly diplopia, just 10mm above or below his MRP level. He will almost
certainly notice this and express himself in a lively fashion to you.
We make convergence and divergence movements all the time because we are constantly looking from near to distant objects and back. Because of this, most of our patients have the ability to adapt to moderate amounts of unwanted lateral prism. Adaptation to unwanted vertical prism is much more difficult because most of us seldom have a need to make vertical corrections to our lines-of-sight. As a result, relatively small errors in the vertical placement of the MRP can cause reduced lens performance and poor patient tolerance. Know it or not, your lab is following certain rules of their own for the vertical placement of MRPs when you fail to specify them.
The Lab Does It If You Don’t
The present day rule for PALs is simple and industry-wide. You tell the lab the heights of the center of the patient’s pupils and that’s where the distance MRP goes. But for bifocals and trifocals your lab will place the MRP several millimeters (different labs have different standards for “several”) above the seg lines unless you specify MRP positions (see above illustration). For single-vision prescriptions, it is fairly universal to place the MRP at the mounting line, unless the MRP heights are specified. These practices for MRP placement work fairly well for ECPs who neglect to supply MRP heights with every job. Hint: If you specify the MRP heights for every job you will have happier and more satisfied patients, and you will avoid a lot of problems—it’s just as important as taking the PD and in many cases it is more important. Discrepancies in vertical placement of the MRPs lead to a lot of problems when only one lens is ordered, because a millimeter or two or three or more can induce a serious amount of vertical prism, depending on the power of the Rx.
The power, the thickness and the index of a lens are among the factors that determine how much the lens magnifies or minifies. If the size of one retinal image is inappropriately larger or smaller than the other in some or all meridians, adaptation may be difficult or impossible. For base curves, follow the lens manufacturer’s recommendations in most cases. The same material should be used for both lenses and the thicknesses should be as close to the same as practical.
Tints & AR
When replacing a single lens, it is important to check for any tint by placing the existing lens on a white background. If the existing lens is anti-reflective, your lab should identify the AR and match it. The patient should be told that an exact match may not be possible unless both lenses are changed.
The Human Factor
In the above case, either a better case history or an analysis of the existing lens would have revealed that disaster was looming. People aren’t always forthcoming with needed information because they don’t realize it’s needed or they may withhold needed information purposefully. They may be in your office, rather than in the office of the original provider because of an unpaid balance. Up-front payment is a good policy, especially with a patient who “can’t recall” where he got the eyewear or if he makes deprecating remarks about the original supplier.
For the most part only one lens is ordered because only one eye requires a new Rx, or just one lens is lost, broken or abraded. Occasionally patients will have problems adapting and one lens or the other will need to be changed. Whatever the reason for ordering a single lens, you will find the following checklist helpful in avoiding trouble.
Single Lens Ordering Checklist:
» Case History – Get all the information you can. Ask why the patient only wants to change one lens.
» Lens Material – Identify the lens material carefully. If you are not sure of its identification, send the eyewear to the lab. If the patient objects that he can’t get along without the eyewear, suggest having new eyewear made that could subsequently be used for spares or
tinted for sunwear.
» Segs – The seg height should be carefully measured and the seg line should fall at the same position relative to the pupil in each eye.
» Lens Identification – The lens design (PALs) must be correctly matched. The OLA publishes a Progressive Identifier that should be available through your lab at nominal cost. If you don’t have one, you should get a copy and use it.
» Size & Shape – If you have a frame tracer, use it. If not, take a circumferential measurement of the existing lens and, for zyl frames, carefully compare the shape shown in Frames with the actual shape of the frame in hand. Too often lenses are forced into a frame of another shape when the original frame breaks leaving you with a disaster waiting to happen when you replace one or both lenses.
» Tint & AR – Take a moment to check for a tint in the existing lens before ordering and send the existing lens to your lab unless you are absolutely certain of the type of AR needed.
» Thickness – Use calipers to determine the center thickness of minus lenses and the minimum edge thickness of plus lenses. Try to approximate these thicknesses in the lens you are ordering.
» Base Curve – Use your lens clock to determine the base curve of the existing lens. Ask your lab what base curve is recommended for that prescription by the manufacturer of the lens you are ordering for the other eye. If it is not the same, ask what brand of lens would have that curvature and order that brand for the replacement lens.
» MRP – Knowing where the MRP is placed in the remaining lens relative to the pupil and then matching that placement is a pretty good rule. This means both lateral and vertical placement. Just taking the PD is not enough. If the patient has a hyper eye, try to determine whether the missing lens compensated for the condition by calling the ECP who fabricated the original job.
Every lens order should include the horizontal (i.e. PD) and the vertical measurements for correctly positioning the MRP. If you are only providing PD information, you may be depriving your patient of the best possible eyewear performance. This is important for lenses ordered in pairs, and it is exponentially important when ordering one lens only. An informal survey of several ophthalmic laboratories indicates that PDs, seg heights, center of pupil (for PALs) and even half PD measurements are commonly included in lens orders, but MRP heights are neglected by the vast majority of ECPs. Ophthalmic labs “fudge” the MRP's vertical locations as best they can, but your patients deserve this one extra measure of quality eyecare.
Palmer R. Cook, OD, is director of professional education for Diversified Ophthalmics in Cincinnati.