From cataract surgery to vision correction: Ready to cross the 20/20 threshold?

This month, we answer the questions posed in last month’s column.

This is where we ended our last column: “Even the most experienced cataract surgeon must take nothing for granted when planning to step across the 20/20 threshold. Although the skills are the same as in state-of-the-art cataract surgery, there is much greater demand placed on the accuracy of results over the 20/20 threshold. The margin of error is narrowed significantly and this difference is immediately apparent to the surgeon. It is one thing to discuss the possibility of eliminating glasses in a patient who has a cataract. It is quite another when that cataract — no matter how marginal its impact on the vision — is removed from the clinical picture. Performing cataract surgery without the cataract suddenly makes vision correction the only reason for surgery, which instantly shifts the outcome equation toward results that consistently must be very near perfect.

It requires every surgeon to ask, “Have I prepared enough to be sure I can deliver results at this level on a consistent basis”? We must each answer this question for ourselves. In order to cross into 20/20 territory responsibly, we need to be certain of the answer. You will know that you are ready only when you no longer have to ask yourself the question. What mileposts must we pass along the path? I can tell you those that guided me. I felt a need to have a firm conviction that I could consistently satisfy each of the following five criteria.”

In this column I will outline the specific methods I chose as we look at each criterion. We’ll look at them all now and then treat each separately in more detail in future columns.

Prerequisites

1. The capsule must remain intact. The incidence of capsular rupture in cataract surgery today remains about 3%. When the cataract is removed from the equation, this incidence must be zero. The refractive error that inevitably results from the altered IOL position, and more important the increased incidence of cystoid macular edema and retinal detachment, are not acceptable risks for refractive patients.

It is possible to completely eliminate capsule rupture by switching from the endocapsular approach and moving phaco away from the fragile capsular environment. Relocating the lens outside the capsular bag before phaco, using one of several supracapsular approaches, will dramatically reduce the chance of capsular rupture, quickly eliminating it altogether.

This change in location dramatically shifts the complication equation away from the capsule. Is this advantage accomplished at the expense of the cornea? Actual corneal edema does not occur, owing to the endothelial protective properties of routine viscoelastic use and the very low phacoaspiration time required to remove a clear lens.

It is not necessary to ‘flip’ the lens out of the capsular bag. In fact, this maneuver is often not possible due to the softness of the lens. It is easier simply to rotate the lens out of the capsulorrhexis after hydrodissection. The lens is then returned to the posterior chamber to a position above the capsular bag for phacoaspiration.

The transition to supracapsular phaco should be completed with cataract surgery before it is applied to refractive surgery. I have used this approach since 1996 in my cataract surgery and am completely comfortable with it now in my refractive patients. It is the only way I know to achieve absolute confidence there will be no capsular rupture.

2. The final postop refraction will be within 0.25 D of my target (0.5 D for ametropia greater than 8 D). Much has been written about axial length measurements and other means to achieve the greatest possible accuracy in IOL calculation. I suspect that still not many believe this order of accuracy is readily achieved today, but it is, thanks to the work of Warren Hill, Jack Holladay and many others who foresaw how crucial IOL calculation would be to achieve success in the refractive arena.

As for my own experience with this essential “accuracy adjustment” of the IOL selection process, I have had the good fortune to have Warren Hill on the faculty of our Refractive Cataract Surgery Course. Before Warren’s arrival I thought I had very likely been achieving near the maximum accuracy possible in IOL calculation, which was almost always within 1 D of the target. When Warren then told us how it was possible to sharpen that accuracy to within 0.25 D, I was skeptical to say the least. No longer. Now I am a true believer, anxiously waiting for IOLs in quarter-diopter increments to be approved here in the United States. I hope to have Warren write a column dedicated to this topic in the near future. For now, these are the essential elements of highly accurate IOL calculations that he advocates:

Keratometry:

• Use the same manual keratometer for all measurements.
• Calibrate this dedicated keratometer at least monthly.
• Use corneal topography backup, especially for poor mires or unusual readings.

Axial length:

• Use a recent-generation ultrasound machine with solid probe and fixation light.
• Use immersion (noncontact) technique.
• Reset default velocities to 1532 m/sec at all gates (ACD, LT, VIT), then add 0.32 mm to measured axial length.
• Or use optical coherence biometry (IOL Master)

IOL formula:

• Use third-generation formulas.
• Use the Holladay IOL Consultant.

These are the changes Warren recommended to us in that first lecture some 4 years ago. I used each of them and very quickly added one of my own that I also recommend to you: do all of these measurements and calculations yourself for those patients (cataract or refractive) requesting vision corrective lens implant surgery.

I can’t overemphasize how important this has been for me. Like most of you, I had not done an A-scan measurement in years, but I felt I had to get my own ‘feel’ for this process again. My ‘sixth sense’ now alerts me immediately of anything unusual that might introduce error. It is a very rare technician who can duplicate that. Any future technicians I might entrust with this crucial process will be under my watchful (and now experienced) eye. This was well worth my time.

3. There will be no surgically induced astigmatism. A temporal, single-plane clear corneal incision, 3 mm or less, is astigmatically neutral. I use this incision in all cases to ensure against induced astigmatism.

4. Existing astigmatism will be surgically reduced to less than 1 D on a long-term basis. Limbal astigmatic keratotomy has been a significant improvement over the traditional astigmatic keratotomy located at the 5-mm optical zone that I began using in 1984. It is a powerful and highly accurate tool to correct astigmatism. In the presbyopic age group it can readily correct up to 3.5 D with no induced irregular astigmatism. Because 1 D or less of residual astigmatism is typically the threshold for uncorrected vision, this approach can address up to 4 D of existing corneal astigmatism safely and accurately. I use Louis Nichamin’s nomogram with one difference: blade depth is reduced from 6 mm to 5.5 mm in patients over 55 years old and 5 mm over 70.

5. Presbyopia will be addressed to the patient’s complete satisfaction on a long-term basis. Restoration of reading vision is an essential element of this surgery. Accommodative IOLs are awaited with much anticipation, and I think they may ultimately become a useful tool in the lens-based correction of presbyopia.

But this will take time, possibly more time than most currently anticipate. It took more than 20 years for the multifocal IOL concept to become clinically viable, and they still require limited patient selection. In the meantime, I intend to continue to use the “blended reading vision” provided by near-far IOL pairing that I have used for several years. Unfortunately, this approach has been likened to the IOL equivalent of monovision contact lens fitting. This a costly misunderstanding of this method of reading vision restoration.

First, the name “monovision” is confusing. I have been using the term ‘blended reading vision’ to describe this approach to my patients for several years now. They immediately understand the concept and the blended vision they experience is exactly the result they expected. I am glad to see that the conductive keratoplasty advocates have also begun using this useful term. It is much more suitable than monovision, which creates daunting images of separate, not blended vision.

A key benefit is that this approach can be completely customizable according to each patient’s lifestyle, reading needs and prior refractive error. After a detailed discussion, the appropriate blended-vision implant pair is selected with significant input from the patient.

The full visual spectrum is divided into five equal focus zones:

1. Newspaper print
2. Computer text, menu print and playing cards
3. Indoor distance
4. Daytime outdoor distance
5. Nighttime outdoor distance.

When I am sure that what will and will not be in focus within each zone is clearly understood, the patient is asked to select which three contiguous zones they prefer for the uncorrected portion of their vision correction. An avid reader will likely select zones 1 to 3, while the golfer will tend toward zones 3 to 5. Most commonly the average patient selects 2 to 4. The patient who persists in asking why he can’t have uncorrected vision for all 5 zones does not yet understand the process, or may be a noncandidate.

Here are two examples:

A 55-year-old man with active athletic lifestyle.

Preop refraction both eyes: +2.75 spherical equivalent.

Postop target: Right eye –0.25 (dominant eye), left eye –1.50.

Vision spectrum zones selected: 3 to 5.

Result: excellent full distance and reading vision for all but stock charts.

A 47-year-old woman novelist who reads often and writes on a computer.

Preop refraction: Right eye –6.50, left eye –7.25 spherical equivalent.

Postop target: Right eye –2.75 (dominant eye), left eye –0.75 (note that the dominant eye is used for near in this myopic patient).

Vision spectrum zones selected: 1 to 3.

Results: excellent reading and working vision. Wears glasses only for night driving, when she anticipated wearing them for all outdoor distance. Results exceeded her expectation (which is fairly typical, since this approach typically allows four zones of uncorrected vision).

Both of these patients were very happy with their uncorrected vision. The particular blend of the selected near-far implant pair was customized to their specific needs. I think of this almost infinitely variable combination of near-far as being similar to blending colors to achieve a specific, completely unique custom shade of paint. I could probably add 20 patients to this list without duplicating a specific corrective blend. This customized correction is the unique (and for me, indispensable) benefit of blended reading vision implants. We left the era of “one-size-fits-all” distance implants 25 years ago; we have the ability to do the same for reading vision with a better appreciation of the art of blended vision implants.

There is much more to say on this. Significant misunderstanding is widespread, such as the concept that a contact lens trial is a useful predictor of success with this method. It my experience this test is very unhelpful. We’ll discuss this and several other misconceptions about contact lens monovision that in my experience have little or no bearing on blended vision implants.

Perhaps the only disadvantage to this approach is that is does not depend upon any specific new IOL technology, so there is no strong incentive for industry to promote it. That, I suspect, is why it has been the most underused approach to presbyopia correction in the area of lens-based refractive surgery.

That brings me to the topic for the next column. I plan to begin a more detailed description of specific surgical techniques, but something perhaps more pressing has been on my mind as I have begun writing this column. It has to do with the difference between professional progress and jumping on a bandwagon.

Next column: September 1

LASIK lessons: What we can learn from the LASIK experience.