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IOL parameters no longer pass the accuracy test

Pin-point IOL power selection is now becoming essential. Refractive recalibration is an industry responsibility, too.

Issue: September 1, 2005

ByWilliam F. Maloney, MD

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This column has been focused in recent months on the refractive recalibration necessary for today’s refractive cataract surgery.

Although it began with increased expectations for uncorrected distance vision, the shift to this higher refractive standard is even more crucial within the 2+ D range of presbyopia correction, in which a refractive deviation of just 0.5 D can represent a highly significant error.

This new metric raises significantly the threshold for refractive accuracy. Surgeons are responding to the challenge. Astigmatic keratotomy is increasingly employed. The IOLMaster from Carl Zeiss Meditec is now used by a rapidly rising 35% of respondents to the most recent survey of members of the American Society of Cataract and Refractive Surgery. Although this new technology comes at significant expense, the recalibration will continue because the contact A-scan is inadequate in this new refractive era. It simply does not pass today’s refractive accuracy test.

Many surgeons do not know that an even greater degree of error potentially exists within the labeling of every IOL manufactured in the United States today.

For any given patient, the labeling introduces a potentially significant discrepancy between the targeted and the labeled IOL power. The IOL label itself is now likely to be the greatest source of refractive error in your surgery. That’s right: When it comes to power, what you see is definitely not what you get. The tolerance for error in most IOL labeling is now obsolete, and it too fails today’s refractive accuracy test. Until that changes, your efforts at refractive recalibration will be hostage to this outdated industry standard.

This is an untenable risk for surgeons who are expected to deliver uncorrected vision. Yet, as we will see, it is completely unnecessary. I am not alone when I call for our partners in industry to recalibrate along with us, by updating IOL power identification to meet today’s clinical requirements.

Standards inadequate

The current standards for refractive tolerances were established by the International Standardization Organization and the American National Standards Institute. The accompanying chart lists the IOL power tolerances published by the ISO.

The band of error tolerance starts at ±0.3 D and widens with increasing IOL power. Some IOL manufacturers use a slightly tighter tolerance of ±0.25 D, and eyeonics has recently attempted to mitigate this risk by introducing power increments of 0.25 D. Nevertheless, the artificial system of imposed error tolerance remains an anachronism in need of change. Here’s why.

First, deviation of ±0.3 D immediately introduces a potential error approaching twice that number, or 0.6 D. Because the most commonly used implant powers lie between 15 D and 25 D, surgeons are most often dealing with an error range of ±0.4 D — far above the tolerance required by current refractive expectations.

Second, this increasingly wider tolerance for error in higher power implants misses completely the main premise of all presbyopia correction. Regardless of the amount of distance ametropia to be corrected, the reading focus provided must be “added” to an accurate emmetropic distance correction. As I have repeated here ad nauseam, exact emmetropia is now our requisite refractive starting point, yet it cannot be achieved except by pure luck while these IOL labeling standards remain in place.

Consider this case. The IOL Master calculates the exact IOL power needed to achieve the target of emmetropia as 19.24 D. The closest IOL power is labeled 19 D. But the current error tolerance of ±0.4 D means that the actual power is almost certainly not 19 D but somewhere between 18.6 D and 19.4 D. If the IOL selected has an actual power of 18.6 D, the result will be a hyperopic “surprise” of 0.64 D and a likely presbyopic failure despite all best efforts by the surgeon including perfect surgery and biometry.

As I said, this risk is untenable. It is also unnecessary.

Why is industry slow to act?

The actual measured IOL power that we need is already known and readily available. It is measured to within one or two hundredths of a diopter, recorded by serial number and kept on file for every IOL manufactured. Yet despite appeals by leading surgeons for more than a year, no U.S. IOL manufacturer has acted.

Why? After all, Technomed in Germany moved to exact power labeling for its EasyCare 600 hydrophilic acrylic IOL months ago, and users have reported positively on their experience. (See “Exact refraction labeling fills niche in IOL industry”) The resistance by U.S. manufacturers may be a simple matter of an overly myopic fixation on their bottom line, but I think there may be another factor at work here. Perhaps manufacturers need a paradigm shift in their perspective, such as the one I describe below.

Moment of recognition

Imagine a patient, a candidate for presbyopic lens surgery, comes to you for an evaluation. Suppose he is skeptical about the need for so many tests, for such precise refractive preop evaluation. What would you tell him?

“Look,” the patient says, “I just don’t believe that this really matters all that much in the real world. When I sit here for a refraction, I am often not able to tell the difference between No. 1 and No. 2.”

Indeed, I reply, it is true his 20/20 acuity is hardly affected by each 0.25 D click of the phoropter as I refine his distance correction.

“Now for your reading,” I say, dialing in his 2 D add. He easily reads J1 at 16 inches. Then I incrementally reduce the add with three 0.25 D clicks down to 1.25 D. His near vision drops noticeably with each click, and by the third it is essentially useless. From perfect to useless reading vision in three clicks of the phoropter.

In that moment of recognition — a true paradigm shift is always this stunning — that patient “got it.” His point of view had been instantly recalibrated. He was now in our world of presbyopia correction where a couple of those 0.25 D clicks can mean the difference between a fully satisfactory reading vision result and failure: perfect to useless.

Here is our message for industry: The degree of error inherent in IOL labeling lies within those same three clicks that changed the patient’s mind. This is the paradigm shift that industry must undergo.

Eyes, not iPods

The frustration level among many surgeons is mounting, myself included. I recently commented to a surgeon friend and colleague, “These guys act as though the IOL power is theirs to divulge or not. This data belongs to the profession and ultimately to our patients. This is a quality-of-care issue, not some internal marketing matter. They sell products that we insert into human eyes, not I-pods, and they have responsibilities here.”

Industry responsibility

In the end we physicians must insist that industry meet its responsibility to recalibrate just as we are meeting ours. This is, after all, our profession, and the patient will always be our patient. But no individual can move toward something that he does not yet see as relevant.

We surgeons know that there is only one right outcome here. We know that the inaccuracy of this arbitrary power bracket must be realigned with the refractive demands we face today. To us, the solution is as obvious as it is simple. The measured power of each IOL ought to be identified, allowing surgeons to set our own error brackets according to the refractive demands of each individual case.

This will happen because our present standard of refractive accuracy requires it. Perhaps the most persuasive thing that we can do for industry representatives who still can’t see that is to offer each of them a good refraction.

Next column

“This procedure is not for you”: How to identify and advise marginal candidates against presbyopia correction.

For Your Information:

• William F. Maloney, MD, is head of Eye Surgery Associates of Vista, Calif., and a well-known teacher of cataract and lens-based refractive surgery techniques. He can be reached at 2023 West Vista Way, Suite A, Vista, CA 92083; 760-941-1400; fax: 760-941-9643; e-mail: maloneyeye@yahoo.com. In the interest of objectivity, Dr. Maloney has no financial interest in any ophthalmic product and has no financial relationship with any ophthalmic company.