TORIC LENSES

Introduction to Toric Lenses

Techniques and strategies to guide your initial experiences.

By Robert H. Osher, MD

I've been teaching surgeons how to reduce preexisting astigmatism at the time of cataract surgery for almost three decades now, so I'll share some of the techniques and strategies that I have developed over time.

First, you should understand that no ophthalmologist would refract a patient, find a large amount of astigmatism, and prescribe a pair of eyeglasses that only corrects sphere. He would prescribe spectacles that would neutralize both sphere and cylinder. Why should surgery be any different? Shouldn't we offer the patient their full correction? The patient should be capable of achieving the ultimate outcome in cataract surgery—emmetropia or unaided vision.

I initiated the battle against pre-existing astigmatism by introducing astigmatic keratotomy (AK) with cataract surgery in 1983. AK was certainly effective, but AK was an art—it depended on surgical technique and patient healing. In contrast, toric lenses are a precise science. They're much more reliable and accurate. We have this technology, which I believe will become the standard in the future. So, the question becomes how do you get started, particularly since many surgeons aren't exposed to these lenses during their training.

Well, there are a number of ways you can get started: I've made an hour-long CD-ROM about toric lenses (ask your Alcon representative for a copy of the CD-ROM); you can read any number of articles, attend courses or observe a colleague. The bottom line is you have to get started. The actual surgery is basically the same as standard cataract surgery. It's very safe and there's little downside to performing it.

Approaching the Topic of Astigmatism With a Patient

When a patient enters my exam room, I've already reviewed his K readings and other tests, so I know if he is a toric lens candidate. After introducing myself, I ask, “Do you know you have astigmatism?” I explain that the cornea should be round like a marble, but because of astigmatism, it is more warped and looks like a spoon. I explain that, “it's not like a basketball, but like a football.”

Using the analogy of an island surrounded by an ocean, I review the topography image with the patient and point out the red mountains. This is very graphic. I explain that there are three choices: monofocal, toric and multifocal lenses and I emphasize that the toric option is best to correct the astigmatism. Once a patient understands the issue, he is usually sold. I never promise a patient that he won't have to wear glasses. I find by being honest and genuinely concerned, it's easy to lead the patient to choose the IOL choice that's best for his eyes.

Measuring Power

First, you should know that the naming convention for toric lenses is a little bit crazy—the T3 corrects 1D of refractive astigmatism, the T4 corrects 1.5D and the T5 corrects 2D—but naming aside, what is most important is making the proper lens selection for your patients. When reducing astigmatism, you have to understand the tests that will help to determine the optimal axis and power to be corrected. Moreover, you need a way to measure and account for the astigmatism that's surgically induced, because this modifies the power.

There are a number of ways to measure power. Some, such as looking at a patient's glasses or performing a refraction, are basically worthless in a cataract patient. I recommend careful manual keratometry, which has been the gold standard, but it's critical that you have a skilled technician. This measurement can be compared to optical keratometry. There are two technologies that measure optical keratometry: the IOL Master (Carl Zeiss Meditec) and the LenStar (Haag Streit). Both accurately measure the amount of astigmatism.

Another technology is corneal topography. This yields essential information because it not only measures the amount of astigmatism; it also confirms that the astigmatism is regular and susceptible to correction.

The last and newest technology that I use is wavefront aberrometry (iTrace, Tracey Technologies). This technology not only quantifies the astigmatism, but it also provides a wavefront portrait. Moreover, it separates the origin of the cylinder into the corneal component and that which originates from the internal optics of the eye.

Each of these technologies measures the amount of astigmatism a patient has. It's my opinion that we should use automated keratometry, corneal topography, plus one other technology as a “check and balance” against measurement error. The more data we collect, the more confident we become in selecting the best lens power.

Surgically induced astigmatism also must be considered. If you construct small incisions, 2.2 mm or less, which the Alcon Infiniti permits, minimal astigmatism will be induced. I induce 0.3D of astigmatism depending if I am correcting with the rule or against the rule cylinder. I prefer a temporal incision, so I have to add or subtract that 0.3D of surgically induced astigmatism. These astigmatic vectors can be confusing to the less experienced surgeon, which is why Alcon introduced the toric IOL calculator.

I input the surgically induced astigmatism and the amount of astigmatism that was measured, then the calculator selects the power of the toric IOL to use. Another concept that I believe is valid pertains to selecting a power that reduces the greatest amount of pre-existing astigmatism. For example, if the amount of cylinder is 1.4D, I will use a T5 rather than a T4, which some surgeons would select. Regardless of which strategy makes sense to you, a 100% correction cannot be achieved in every patient. However, the patient will be delighted with any significant reduction—and that is the ultimate goal—to satisfy the patient.

Calculating the Axis

The same tests used above for measuring power provide the information you need to select the axis. Alignment is critical because for every degree of error, you lose 3.3% of the intended effect. So, if you're off 10°, you've lost one-third of the effect.

When I calculate the axis, I consider each of the measurements and weigh them. I give manual keratometry a great deal of weight—which, again, you can trust if you have a good technician. I also give substantial weight to optical keratometry obtained with either the IOLMaster or LenStar. I am still learning about the iTrace, but I have been very impressed to date.

In my experience, corneal topography is the least accurate for measuring the axis. As previously stated, I recommend you average your measurements rather than depend on one technology. Most offices do not have all of these technologies, but you should be fine with manual keratometry, corneal topography and one other measurement. If you are just beginning with toric lenses, the axis can be computed for you using the toric calculator.

Making Your Mark

Most people use ink and a handheld instrument to mark the major meridians, which are intended to help identify the target axis. Be aware that ink diffuses and may even disappear in some cases.

Studies independently conducted by Rudy Nuijts, MD, of Belgium and myself have shown that it's easy to be off 7 degrees or more using ink. If you multiply that by 3.3%, you may be under-correcting the amount of astigmatism.

Pre-op patient information can be entered on this screen within the toric calculator to obtain an automated calculation of the required lens power.

I've developed and recommend any of three different approaches for surgeons who don't prefer ink to mark the major meridians. The first is iris fingerprinting, which uses high-resolution photography (Micron Imaging Systems and Haag Streit). During the initial exam, an image of the iris is obtained after the pupil is dilated. Software will then generate the major meridians or the exact location in degrees of any landmark (a nevus, crypt, stromal pattern, etc.) for increased accuracy. So, when entering the OR, I know from the photograph where the important meridians are located.

The second technique is based upon iris registration. A device registers the iris prior to surgery and produces the image as an overlay on the video monitor while you're operating. You can easily see where the axis is located on any of the available iris registration devices (Sensory Motor Instruments, Carl Zeiss Meditec and TrueVision Systems, Inc.) The overlay guides toric lens rotation until the axis is reached.

The third technique is intraopertive aberrometry (Wavetec Vision and Clarity Medical Systems), which may also allow you to rotate the lens until the cylinder is neutralized. However, the jury is out on this new approach.

While all of these technologies are in their infancy, it is necessary to state that our current methods are producing very good outcomes with consistent results.

My Routine in the OR

In the preop area, I still require my nurse to mark the patient while he's sitting up and looking in the distance. The nurse makes the mark at 6 o'clock. In the OR, I use iris fingerprinting to check the mark for accuracy from my preop image; I line up my reference points. I use a protractor dial for identifying the target meridian which is marked by placing two fine cautery dots 180° apart. Most surgeons use a circular dial, but I prefer a semi-lunar marker (Crestpoint Management, Bausch + Lomb and Geuder), which allows me to snuggle the scale right up to the limbus, which is very accurate. Then I confirm the axis with a modified Mastel ring. The cataract is removed by micro-coaxial phacoemulsification with torsional ultrasound. I inject the toric IOL and leave it a few degrees counterclockwise from the final axis, which can be aligned after the OVD is removed from behind the IOL. Then the incision is hydrated and the remaining OVD is removed. Remember that the cataract surgery is practically the same and the lens won't rotate once it is in place—it's very stable.

Toric lenses are here to stay and they will become the accepted standard in the future—so the time to start is now. nMD