TORIC LENSES

Predictable, Precise Results Crucial for the New Doc on the Block

Correcting corneal astigmatism with a toric IOL during cataract surgery benefits both you and your patients.

By Anna F. Fakadej, MD, FACS, FAAO

When you're the “new doc on the block,” you may feel as if some of your patients are looking at you and wondering whether you're old enough to have graduated high school, let alone medical school—especially those of typical cataract surgery age, for whom your youthful appearance may really stand out. You may have other patients who fully expect that you're “up” on the latest technologies and techniques because you've recently completed your ophthalmology education and training. Either way, your goal is to earn or keep their trust by providing them with a stellar surgical experience, which is ultimately how you will build your practice. This doesn't necessarily mean you have to provide every possible surgical option right away. You can expand your skill set gradually. The key is to be confident you can achieve your desired outcomes with whatever options you choose to utilize.

Correcting Astigmatism at the Time of Cataract Surgery

In my opinion, the standard of care in cataract surgery is to attain for patients the best visual outcome, which requires addressing all of the eye's optical imperfections. Therefore, my aim is to correct not only as much sphere as possible, but also as much cylinder as possible when astigmatism is an issue. If we don't address preexisting corneal astigmatism at the time of cataract surgery, patients will have decreased acuity without eyeglasses postoperatively. Depending on where it is located, just 1.0 D of corneal astigmatism can reduce Snellen acuity by as much as three lines. With 2.0 D of corneal astigmatism, acuity drops precipitously.

Many studies have shed light on the prevalence of corneal astigmatism in the U.S. population and suggest that a substantial number of patients may benefit from corneal astigmatism correction at the time of cataract surgery. For example, data (unpublished) on 6,000 patients compiled by Warren E. Hill, MD, indicate that approximately 28% of patients have between 1.0 D and 2.0 D of corneal astigmatism, and approximately 53% have 0.75 D or more.

As you know, other than making our incision on the steep axis, we have two options for correcting corneal astigmatism at the time of cataract surgery: limbal-relaxing incisions (LRIs) and toric IOLs. While both options are useful tools in the cataract surgery armamentarium, toric IOLs are a good way for surgeons just starting out to offer patients a premium cataract procedure that has a low learning curve for the doctor and a high likelihood that the desired refractive targets will be hit consistently.

LRIs vs. Toric IOLs

Several studies conducted outside the United States have shown that toric IOLs produce more consistent results and provide patients with better visual acuity than LRIs. Mingo-Botin and colleagues,¹ for example, compared toric IOL implantation to LRIs in eyes with cataract and corneal astigmatism between 1.0 D and 3.0 D. The study authors concluded that although refractive astigmatism was reduced in both groups, toric IOL implantation reduced astigmatism more effectively and predictably and resulted in greater spectacle independence.¹ At 3 months postoperatively, mean residual refractive astigmatism was 0.61 D ± 0.41 D in the toric IOL group and 1.32 D ± 0.60 D in the LRI group (p<.01). Mean logMar uncorrected distance visual acuity (UDVA) was 0.13 ± 0.10 in the toric IOL group and 0.19 ± 0.12 in the LRI group. Also, 15% of patients in the toric IOL group required eyeglasses for distance vision after surgery compared with 45% in the LRI group.

Figures 1-2. Preliminary results from a study comparing a toric IOL and limbal-relaxing incisions for the correction of preexisting corneal astigmatism at the time of cataract surgery suggest that uncorrected and corrected distance visual acuity is better in eyes that received the toric IOL. Preliminary results from a study comparing a toric IOL and limbal-relaxing incisions for the correction of preexisting corneal astigmatism at the time of cataract surgery suggest more K value variability than expected in eyes that received the LRI option.

I recently was part of the first study in the United States comparing a toric IOL to LRIs, and I'm in the process of verifying the final results. This is a preliminary look at a prospective, randomized, contralateral study that enrolled 69 patients who had up to 2.50 D of corneal astigmatism preoperatively. One eye of each patient received either an AcrySof IQ Toric IOL (Alcon) or an AcrySof IQ IOL (Alcon) and LRIs. Each patient was randomly assigned to one of the two treatment options for the first eye and then received the other treatment in the second eye.

Based on the preliminary 6-month results, mean refractive residual astigmatism is lower among patients who received the toric IOL. Also, uncorrected and corrected distance visual acuity appear to be better with the toric IOL by at least one line and sometimes by more than two lines (Figure 1). Another interesting preliminary finding is that while we would expect variability in K values with LRIs (an LRI is supposed to affect K values; a toric IOL does not), the variability seems to be wider than would be expected (Figure 2).

In clinical practice, a number of variables can affect outcomes with LRIs. Some are patient-related, such as age and corneal rigidity. Others are surgeon-dependent, such as incision depth and the instrumentation used. Several LRI nomograms, such as those developed by Louis “Skip” Nichamin, MD, or Eric Donnenfeld, MD, are available online. While these nomograms can greatly improve the accuracy and predictability of LRIs, they don't eliminate the need for each surgeon to compile his own data for personalizing the nomogram or the need to work on perfecting his surgical technique and to know his surgically induced astigmatism factor (SIA). Furthermore, they cannot control for the individual patient's healing response, which remains an inherent unknown. Depending on the healing response, the cylinder may end up being over- or under-corrected or the effect of the incisions may regress. Also of note, in particular for surgeons just starting out, is the higher risk of certain complications with LRIs compared with toric IOLs, such as, although rare, epithelial defect or corneal perforation.

In contrast, using a toric IOL to correct corneal astigmatism at the time of cataract surgery takes the unknown of individual patient healing out of the equation. Instead of counting on the cornea to react in a predictable manner, we're utilizing an optic inside the eye to correct the astigmatism. An additional advantage is the shorter and lower learning curve compared with LRIs and the increased precision toric IOLs offer. If you can successfully implant and rotate an AcrySof IQ IOL, you can successfully implant an AcrySof IQ Toric IOL, as long as you use the AcrySof IQ Toric Calculator (www.acrysoftoriccalculator.com), accurately mark the cornea at the intended axis of implantation and ensure the lens is precisely placed at that axis at the end of the case.

Implanting the AcrySof IQ Toric IOL

Before recommending a toric IOL for a patient, it's necessary to use topography to ensure the corneal astigmatism is regular.

Steps for Implanting a Toric IOL

Implanting a toric IOL in an eye with irregular astigmatism is likely to result in undercorrection and could also induce vision-degrading higher-order aberrations. Once you make this determination, decide on the right spherical power of the IOL using your usual methods.

The spherical IOL power is one of the parameters you will then enter into the AcrySof IQ Toric IOL Calculator. Other key parameters include flat K and axis, steep K and axis, incision location and surgically induced astigmatism. If you've not yet figured out how much astigmatism your cataract incision induces, the calculator will estimate it to be 0.50 D. Based on the parameters entered, the calculator recommends the appropriate AcrySof IQ Toric IOL model (SN6AT3, SN6AT4 or SN6AT5, which provide 1.03 D, 1.55 D and 2.06 D of cylinder correction at the corneal plane, respectively). The calculator also recommends the optimal axis for IOL placement and the magnitude and axis of anticipated residual astigmatism. If the expected residual astigmatism is too low or too high, you can choose to implant one of the other three models.

Accurately marking the cornea at the intended axis of IOL implantation is a crucial step. If the mark is incorrect and you implant the IOL at the wrong axis, the amount of astigmatism corrected will be reduced. If the IOL is 10º off axis, for example, one-third of your intended effect is lost. Misalignment of more than 30º results in worsening of the astigmatism.

Prior to taking the patient into the OR, while he is sitting upright and focusing on a distance target, make reference marks on the cornea at 3 and 9 o'clock. Several instruments are available to help you do this accurately, including one that contains a level (Bakewell Reference Marker, Mastel). Once the patient is under the microscope, use the reference marks to mark your incision location and axis of implantation. Several useful fixation rings and meridian markers are available for this step.

After removing the cortical material and inflating the capsular bag with viscoelastic, insert the IOL. Once it attaches to the capsule, a bioadhesive reaction helps to keep it on-axis. However, most surgeons rotate the IOL at this stage to a point that is 10º to 20º counterclockwise from the intended axis so that if it rotates during viscoelastic removal, it will not rotate past the intended axis. Surgeons use a variety of strategies to prevent lens over-rotation during visco removal. I usually place the I/A handpiece on the optic to hold the lens in place. Other surgeons use a second instrument to stabilize the lens. When all of the viscoelastic has been removed from the eye, the IOL can be carefully rotated to its final intended axis.

Once you've done your homework on how to implant the toric IOL, you'll be ready for your first patients. Patients who have at least 1 D of pre-existing corneal astigmatism are ideal first candidates. They'll be pleased with the quality of their uncorrected distance vision postoperatively, which may be further improved by the AcrySof IQ Toric IOL's aspheric optic.

Patients with latent astigmatism are good first candidates as well. This is a group who have corneal astigmatism but never needed to correct it with eyeglasses or contact lenses because it was mitigated by the crystalline lens. When the natural lens is removed, however, their corneal astigmatism remains and may noticeably affect their vision. If you implant a spherical IOL, these patients will likely need correction for distance vision. So, in their minds, you haven't really improved their vision. If you take advantage of the AcrySof IQ Toric IOL option, you not only restore the pre-cataract quality of vision, you also add the benefits of an aspheric optic, and the patient still doesn't have to wear eyeglasses to see at distance.

Foundation for Your Future

Because many patients with astigmatism have never had good vision at any distance, the AcrySof IQ Toric IOL makes them some of the happiest patients in my practice. Once you see the results in your practice, I have no doubt it will inspire you to continue expanding the options you offer for giving your patients their best possible vision. Whatever new technologies the future brings, the AcrySof IQ Toric IOL can be your low-stress, high-satisfaction launch pad to success. nMD

REFERENCE

1. Mingo-Botín D, Muñoz-Negrete FJ, Won Kim HR, et al. Comparison of toric intraocular lenses and peripheral corneal relaxing incisions to treat astigmatism during cataract surgery. J Cataract Refract Surg 2010;36(10):1700-1708.