Focus on Cataract Surgery

Achieving more accurate results with toric IOLs

The key is to accurately calculate astigmatism and adjust IOL selection accordingly.

By Uday Devgan, MD

A satisfied cataract patient with a toric IOL can be among the happiest patients in a practice, but implanting toric IOLs has been challenging for many clinicians. While the femtosecond laser can produce more precise corneal relaxing incisions than a diamond blade, toric IOLs surpass them both in the ability to predictably address astigmatism at the time of cataract surgery. Toric calculators are available to assist in toric lens selection, but they do not eliminate the surgeon’s role in predicting and managing postoperative surgically induced astigmatism (SIA). Here I’ll describe my approach to calculating toric IOL power.

MARK AND MEASURE

Mark the meridians

The steep axis of astigmatism needs to be marked accurately to align the toric IOL. You can accomplish this with manual ink markers or electronic image guidance, or by performing corneal stromal puncture with a cystotome. When I perform laser-assisted cataract surgery, I also use the laser to intrastromally mark the astigmatic axes. This noninvasive approach does not violate the epithelium and has minimal astigmatic effect because the marks are only at a depth of 30% to 50%. Their arc length is 10°, the smallest arcuate incision you can make today, but I anticipate with newer software we will be able to make even smaller ones.

We can mark the cardinal meridians at 0°, 90°, 180° and 270°, but we also need to mark the actual steep or flat axes. The key here is accuracy. One clock hour represents 30°. If your meridian is off by 30°, that’s 100% of effective toric lens power lost. If you’re off by just one minute, or 6º, that’s a loss of 20% of effective toric lens power.

Measure the entire cornea

Douglas Koch, MD, at Baylor College of Medicine and his colleagues reported on the importance of posterior corneal astigmatism when calculating overall corneal astigmatism.¹ They used tomography in 715 corneas of 435 patients and found the average posterior corneal astigmatism was -0.30 D. The steep corneal meridian was aligned vertically (60° to 120°) in 52% of eyes for the anterior surface and around 86% for the posterior surface.

The steep anterior corneal meridian tended to shift from vertical to horizontal with age while the steep posterior corneal meridian did not change. They concluded that anterior corneal measurements underestimated total corneal astigmatism by 0.22 at 180° and exceeded 0.50 D in 5% of eyes.

Figure 1 illustrates a typically satisfied patient postoperatively. The blue boxes show refractions at plano with no cylinder, but the orange boxes show 0.75 D of cylinder. How can this be if the refraction is zero astigmatism if a monofocal, non-toric IOL was used? It’s because keratometry reads the anterior corneal power, not power of the entire cornea, including posterior corneal.

Figure 1. The light blue boxes below show refractions at plano with no cylinder. However, the orange boxes show 0.75 D of cylinder.

You can certainly measure the posterior corneal astigmatism with a tomographer, such as the Galilei (Ziemer USA, Alton, Ill.) and Pentacam (Ouclus, Arlington, Wash.), which can show the total corneal power matrix (Figure 2). However, not everyone has access to this type of imaging. An alternative is to use intraoperative aberrometry to measure the entire cornea when the eye is in an aphakic state.

Figure 2. Tomography measures the posterior cornea astigmatism and shows the total corneal power matrix, here circled in red.

IOL SELECTION

The Koch/Baylor nomogram

You can also use the typical posterior keratometry values. Dr. Koch’s recommendations are:

• Target up to 0.40 D with-the-rule astigmatism postoperatively. For most eyes, the anticipated amount is much lower. This is one way to account for the against-the-rule shift with age. I had a patient who was perfectly spherical and happy post-cataract surgery. When I saw him 10 years later, he had 1.50 D against-the-rule astigmatism because, as people age, they develop more against-the-rule astigmatism.

• Account for posterior corneal astigmatism. Dr. Koch’s estimated mean values for posterior corneal astigmatism are slightly different for with-the-rule vs. against-the-rule eyes, but pretty close; that is, up to 0.50 D in with-the-rule astigmatism and up to 0.30 D in against-the-rule astigmatism.

Four toric IOLs are available in the United States: Alcon AcrySof, Bausch + Lomb Trulign, Staar Plate and AMO Tecnis. The Alcon AcrySof Toric has the widest range as well as consistent 0.5 D of cylinder at the cornea in incremental steps. AcrySof Toric IOLs have a “T” power from T3 to T9, each step providing more toric correction at the corneal plane. The magic T formula is T minus one, and half of that is the actual cylinder power. An AcrySof T6 IOL corrects about 2.5 D of astigmatism at the corneal plane (Table).

A simplified approach

The Koch/Baylor Toric Nomogram modifies that approach (Figure 3). I have a simplified version: If the patient has with-the-rule, you want to leave them a little with-the-rule on the anterior keratometry measurements, so you “wimp” down a T step. If the patient is against-the-rule, you want to leave them with a little bit of net with-the-rule, so you add up a T step.

Figure 3. The simplified approach to the Koch/Baylor Toric Nomogram involves dropping down one T step if the patient has with-the-rule astigmatism and adding up one step for against-the-rule cylinder.

So if your calculations call for a T6 lens but the patient has with-the-rule cylinder, you would “wimp” it down to T5. Conversely, if the patient has against-the-rule cylinder and your calculations still call for a T6 lens, you would add up to a T7. This will be confirmed in 95% or more of patients when you look at their tomography.

Other roads to accurate calculations

Intraoperative aberrometry can be especially helpful for calculating cylinder for toric IOLs. I’ve incorporated this into my protocol, and it has given me a way to accurately predict IOL power.

To add another level of accuracy, you can use the approach Dr. Jack Holladay developed, which uses two factors — anterior chamber depth and IOL power — to determine the toric lens power.

INCISION AND SIA

Balanced incision is best

Minimizing astigmatism after surgery depends on how and where you make the incisions. A great incision achieves a balance between the roof and floor (Figure 4). The four most used incision types — single plane, two-plane, three-plane and three-plane hinged — all achieve this balance.

Figure 4. A well-executed incision is a matter of balance. Excellent symmetry between the floor and roof eliminates the risk of a chevron-shaped wound can have a greater astigmatic effect, among other problems.

I nick the limbal vessels with every incision, which I’ve also learned to do with the femtosecond laser. The symmetry between the floor and roof eliminates the risk of a chevron-shaped wound that is too close to the visual axis, can have a greater effect on SIA and take longer to seal.

In making the incision, the appropriate width and tunnel length are key in minimizing astigmatism. Good form involves radial entry of the blade, straight in and straight out, along with proper incision width and tunnel length. Avoid an oblique blade entry with varying entry and exit paths, which can create a tunnel that is too wide, thus increasing the astigmatism. An ideal incision will have an almost square appearance with the tunnel length and width being similar. For the single piece acrylic toric IOLs, this would be a tunnel width of 2.2 to 2.4 mm and a tunnel length of about the same.

Incision placement matters

Incision placement plays a significant role in determining SIA, which tends to have a spread among patients unless you can be very consistent with your incision. Even then, all corneas are different sizes. A superior incision may cause twice the flattening of the meridian than a temporal one because of the effect of position. So the temporal incision is preferred. A superior incision is closer to the visual axis, which can magnify its flattening effect.

Calculating SIA is not difficult. You need three factors: the preoperative and postoperative keratometric measurements and the location of the incision of a dozen or so patients. The math is easy to do if the incision is on the axis of the astigmatism. Otherwise, you will need to do some vector math or use an online calculator, such as Dr. Warren Hill’s Surgically Induced Astigmatism Calculator (www.sia-calculator.com).

THREE PRINCIPLES OF MANAGING SIA

Managing SIA relies on three principles:

• Make the phaco incisions away from the visual axis. Keep the structure of the incision balanced and nick the limbal vessels. A temporal incision is more predictable and causes less SIA than a superior incision.

• Calculate your own SIA using preoperative and postoperative K readings and incision locations for about a dozen patients.

• Follow serial topography and wait for the eye to stabilize after poorly executed incisions. These corneas heal slowly, often taking a year or more. OM

REFERENCE

About the Author

Uday Devgan, MD, FACS, FRCS(Glasg), is in private practice at Devgan Eye Surgery in Los Angeles and partner at Specialty Surgical in Beverly Hills, Calif. He is also chief of ophthalmology at Olive View UCLA Medical Center where he teaches the UCLA Jules Stein Eye Institute residents. His e-mail is devgan@gmail.com. Disclosures: Dr. Devgan is a consultant for Alcon and Bausch & Lomb, and formerly for AMO and Staar. He has no direct financial interests in the products mentioned.