L&T: RxPertise

Dec
2013

Measuring for Individualized Lenses


Screen shots of The m'eyeFit System courtesy of Essilor


By Palmer R. Cook, OD

The owner and sole salesman for a men’s clothing store greeted me as I entered his store to buy a perfectly fitting suit for a special occasion. I started to ask him if he could help me find a light gray suit, but I paused.

“A light gray suit you started to ask me for? It’s the eye patches of course. You are wondering how I could possibly fit you with a suit with both of my eyes patched shut,” the salesman said.

“Well, it did occur to me,” I admitted.

“I’ve run this store for 30 years,” he said. “And I know it frontwards and backwards. You think I would let a little thing like eye infections get in my way? Think again, I’ve got lots of experience and besides, we’ve got averaged measurements.”

I excused myself and found a store that tailored suits from scratch. The result was a quality suit that fit perfectly.

In the above example I was a typical consumer, much like patients that ECPs see every day. Patients want to see as well as possible, and they assume that is a certainty because the doctor wrote a “prescription” for them. In the past, we only had lenses that were the equivalent to off-the-rack suits. Technology would not let us individualize, or tailor, lenses to meet our patients’ needs. However, today’s digital lathes and computers allow labs to produce digital lenses that give the prescribed power, even though the lenses are tilted (for pantoscopic effect) and wrapped (rotated around a vertical axis), which changes both their refractive power and prismatic effect. They also are curved to more accurately minimize peripheral aberrations.

Getting the best performance from most individualized lenses generally requires position of wear measurements to be taken in addition to the traditional PD and seg height. Some lens designs also require biometric measurements such as refracting vertex and stop distance. Other lens designs include averaged values to replace those measurements that the ECP has not taken. These lens designs utilize the averaged values for the final lens curvature calculations. An unknown factor is the sensitivity of each individual patient. For example, a diopter of marginal astigmatism would probably be universally unacceptable, but marginal astigmatism of 0.01, 0.06 or even 0.25 diopters may be of little consequence depending on the tolerance of the particular patient. By carefully following patient outcomes, ECPs will gain skill in matching the various individualized designs and methods of measuring to the needs of each of their patients. Among the values that are included in calculating lens curvatures are refracting distance, pantoscopic tilt, wrap and stop distance (a combination of vertex distance and the distance from the corneal apex to the center of rotation).

MEASURING TOOLS
Individual measurements, whether taken with handheld devices or with an electronic measuring system, are required to get the best performance from digitally designed and manufactured lenses. There is an array of tools to help you take the needed measurements. They range from very simple and inexpensive to complex and costly. The simplest tools are easy to use. The more complex require a bit more training, and they offer the intangible value of impressing the patient. You should understand that even though a value is intangible, it can be a very real and important part of your patient’s attitude toward the care they receive.

TILT AND WRAP CREATE ERRORS
Try clamping a -6.00D spherical lens in your lensometer and verify the power, then release the clamp and tilt the lens toward you as though you are giving it some pantoscopic tilt, and re-check the power. You will find that you now measure a sphero-cylinder power with the axis around 180. The same effect occurs when a patient is tested and then fitted with lenses that have pantoscopic tilt and wrap (or face form). The power is correct as measured in the lensometer, but when worn, it does not provide the power the doctor wants nor is it what the patient needs—but unfortunately, it is what he gets.

So why not fit the lenses without pantoscopic tilt or wrap? Pantoscopic tilt is needed because our eyes are not fixed in a permanent, straight-ahead position. Pantoscopic tilt allows us to look downward for close work and still have undistorted vision through our lenses. Wrap is sometimes used for cosmetic reasons, to limit interference by visible light and UV from the lateral periphery, or to reduce annoying lens reflections from light sources located behind the patient. Changing tilt and/or wrap changes the performance of any ophthalmic lens, and digital technology now allows us to compensate for those changes.

COMPENSATED PRESCRIPTIONS
If a -6.00D poly is prescribed with a +2.50 add, and if it is to be mounted in a frame with a 9-degree pantoscopic tilt and a 5-degree wrap, then a compensated lens should to be created with a power of -5.50 -0.41 X 074 and with a +2.14 add. This lens, when tilted 9 degrees and wrapped 5 degrees, will have the -6.00 spherical power and the 2.50 add power that the doctor prescribed. Before lenses could be so accurately produced on digital lathes, it was neither practical nor economically possible to provide these power refinements for vision through the center of lenses. 

VISION WHEN THE EYE TURNS

When the patient looks away from the center of the lens, aberrations arise. Until the advent of digital lathes that can cut complex curves, our best alternatives were corrected curve, or molded aspheric, or atoric lenses. These only approximated the patients’ needs because they were made with spherical peripheral curves (in the case of corrected curve designs), or they were cast from molds with complex curves calculated for “average” positions of wear. With either corrected curves or aspheric molded curves, prescriptions are grouped into “recommended base curves” in about two or three diopter steps.

Today’s digital lenses can be individualized or tailored for improved performance for both straight ahead viewing and for viewing away from the center of the lens. To do this, measurements must be taken. If you don’t take those measurements, your lab becomes like the salesman with both eyes patched.

REFRACTING VERTEX DISTANCE
The first needed measurement is the refracting vertex distance. Lens designers assume a refracting distance, and the manufacturer should make that distance available to you. For example, PureSite Digital lenses from Diversified Ophthalmics are based on a 13.5 mm refracting distance. The easiest way to handle this is to refract at 13.5 mm if you want your patient to have PureSite digital lenses. You can supply the refracting distance to your lab when ordering, but unless you do (and perhaps even if you do) the lenses will be produced using calculations based on whatever refracting distance was used in the lens design—whether it’s the one the doctor used or not. Even if you refract at the recommended vertex distance (or supply the refracting vertex distance), you should also supply the spectacle vertex distance with the order.

WRAP
Wrap is the angle the line-of-sight makes with the lens plane when the lens is rotated around a vertical axis. This changes the effective power and prismatic effect of the lenses, and it also causes the PD to increase. Wrap can be easily and accurately measured using a wrap chart. The wrap angle is the one measurement that can be taken by the lab if the fitted frame is supplied with the order. It’s a good practice, however, to measure the wrap and supply it on the order form. Otherwise the lab might inadvertently use an “averaged” wrap angle.

The wrap angle should be checked before dispensing. If the wrap angle is changed from what the order specified, the lenses will not perform optimally. Most labs do their best to return each frame to its specified wrap angle after the lenses are inserted, but that isn’t always possible. Care should be taken to use frames designed to accept either unusually flat or steep lens curvatures if your patient’s lenses will be either. If you don’t, the wrap angle that looked good with demo lenses may be cosmetically unacceptable with the prescribed lenses.

Individualized lens designs generally compensate for the induced prism caused by wrap in their calculations. If in doubt about this, ask your lab rep whether this is done for the designs you use. The increased PD that results from wrap is sometimes, perhaps frequently, ignored unless you specify that you want this compensation included. This PD change can be significant. For example, a patient with a 66 mm PD, in a 14-degree wrap frame must be laid out for a 68 mm PD if the MRPs or distance reference points are to be intersected by the lines-of-sight for straight ahead distance seeing. 

PANTOSCOPIC TILT
Pantoscopic tilt is the vertical angle the line-of-sight makes with the lens plane when the line-of-sight is level and the patient’s head is in his preferred postural position (Fig. 1 and 1A). The pantoscopic tilt is not the angle the temple makes with the front. If it were, patients with one ear higher than the other would have differing pantoscopic measurements between the right and left side of the eyewear.



Although the pantoscopic tilt does not affect the distance PD, it does affect the effective power of the lens when worn just as the wrap does. The near PD does change, becoming wider as the pantoscopic angle is increased.

To accurately measure the pantoscopic tilt, the patient’s line-of-sight must be level. Ask the patient to view his own eye in a vertical, flat mirror (Fig. 1B). As long as the patient is viewing his own eye in this manner, his line-of-sight will be level regardless of his head tilt or turn. To accurately measure the pantoscopic angle (and MRP, fitting cross and seg locations), his line-of-sight must be level, and he must maintain his preferred head position when you are measuring. A handheld inclinometer can be used to establish the patient’s chin-up, chin-down tilt both before and after you have taken your measurements. Patients frequently assume a more chin-up tilt when measurements are being made. Care should be taken to avoid this chin-up movement because it can significantly affect your measurement data.

VERTEX DISTANCE
The vertex distance is the distance from the back of the lens to the corneal apex. From a practical standpoint, it is measured from the back of the demo lenses rather than from the back of the Rx lenses, so there a small difference occurs when the prescription lens is in place. A bigger problem is that the traditional measuring from the side with a PD rule builds in a parallax error that can cause as much a 1.5 mm or more reduction in measured versus the actual vertex distance. You can measure the vertex distance with a slit lamp that has a scale for measurement fore and aft, or you can use a distometer (cost is around $80) if the system you are using does not include vertex measurement for both eyes.

CENTER OF ROTATION
The center of rotation of a tennis ball rolling across a floor is the point around which the ball rotates as it rolls. It is easy to imagine that the eye rotates around such a point, however the human eye does not have a fixed center of rotation. “The eyeball corresponds to a ball in a pocket of fat whose position is controlled by muscle tendons pulling on it from six different directions and also by check ligaments. There appears to be nothing, therefore, in the nature of the mechanism of the movement which would insure rotation around an axis fixed with respect to the head and with respect to the eye.” (Center of Rotation of the Eye, Fry et al. American Journal of Optometry and Archives of the American Academy of Optometry, Nov. 1962, Vol. 39, 581-595)

STOP DISTANCE
The stop distance for a spectacle lens is the distance from the back surface of the lens to the center of rotation of the eye. The stop distance is the sum of the vertex distance (the distance from the posterior lens surface to the apex of the cornea) and the distance from the corneal apex to the center of rotation of the eye (see Fig. 2).

The stop distance is used to determine the best curvature for viewing away from the optical center of the lens. For less complicated measuring systems, an average center of rotation distance from the corneal apex is used and added to the measured vertex distance. In some designs an axial refractive error is assumed, and an increased center of rotation distance is used for myopic eyes. For more complex systems, a center of rotation location is measured at some peripheral angle of view, and that location is probably used for all viewing angles for most lens designs.

CHECK MEASUREMENT OUTCOMES

It is easy to find and dot both the distance reference points (DRPs) and near reference points (NRPs) as you check the fitting cross location on a centration chart to be sure the patient will have full benefit of the technology he is paying for. If you put red dots on the DRPs on the demo lenses, you can ask the patient to view his own right eye (cover his left eye) in a vertical, plano mirror that is parallel with his facial plane and with his head in its preferred position. (When the patient is looking through a red dot close to his eye, there will be a reddish appearance to the object he is viewing.) If he is not looking through the red dot, ask him to move his head a bit until he is looking through the red dot and ask him if that is still a comfortable head position. If it is, move the cover to his right eye and ask if he is looking through that red dot. If he is, your DRP locations are correct. If he is not looking through the red dots with both eyes, you can ask him to turn his head slightly to the right and left. If a position is found in which he is looking through the red dots with both eyes as you alternately occlude, and that head position is comfortable for him, your DRP locations are correct. If such a location can’t be found, you need to re-measure.

To check the near PD, place a red dot at the locations of the NRPs. Then cover the patient’s left eye and ask him to move his head until he is looking through the red dot at a letter located on his midline on a page of reading material. Then move the cover to the right eye and ask if he is looking through the red dot as he views the same letter. The frame must be completely adjusted to do this near PD check, and neither the material nor the patient’s head should move during this test. If he is not looking through the red dots with both eyes, change the pantoscopic tilt or the distance to the reading material so the red dots are before each eye. Retake the measurements if the dots cannot be lined up properly.

CONCLUSIONS

Lenses that are tailored to your patient’s anatomy and to the fit of the frame give better than ever vision and comfort. These “individualized” or “as worn” or “personalized” lenses are not likely to give the best results unless measurements are taken. If you don’t supply measurements when ordering, the lab will use averaged measurements. If you can take a PD, you should also be able to use the very basic measurement tools, or the more sophisticated measuring devices. In either case, your outcomes are likely to be better than if you rely on averaged values for your patient. All of the systems currently on the market as of this writing will probably be improved as clinicians gain patient feedback and more research is done. ■


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

 

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