Cataract surgery is our most powerful refractive procedure based on the spherical power that we can correct. Anything from –30 D of myopia to +20 D of hyperopia can be brought down to a spherical equivalent of plano by virtue of the IOL power that we calculate and select. Addressing the astigmatism at the same sitting is an added bonus and results in better visual clarity for our patients. While we can use a toric IOL for higher degrees of astigmatism, up to 4 D in the U.S., for lower amounts of astigmatism, corneal incisional procedures may be a better option.
The traditional method of using incisions to treat corneal astigmatism involves partial-depth arcuate incisions in the form of astigmatic keratotomies or limbal relaxing incisions. These incisions are typically about 90% depth, and the placement and arc length depend on the nomogram being used. The risk in these procedures is inadvertent perforation and subsequent leak of aqueous. Because these incisions are perpendicular to the corneal surface, any perforation is challenging to seal and will likely require sutures for wound closure. In addition, the surgeon must purchase adjustable-depth blades, such as diamond keratomes, which can cost thousands.
Another viable option is to make the phaco incision on the steep corneal meridian and then pair it with a second identical full-thickness incision directly opposite, on the other side of the cornea. These paired phaco incisions are on the same corneal meridian, and they will increase the flattening effect. Because we are using the same keratome to make the main phaco incision and the additional incision, there is no extra cost per case for instrumentation.
The best time to make the additional phaco incision is near the end of the cataract surgery, when the IOL has been placed in the capsular bag and before the viscoelastic has been aspirated from the anterior segment. Using the fixation ring, we move the eye so that we can accurately place the keratome opposite the main phaco incision, and then we perform the additional incision the same way as the original incision. By performing the second incision with the eye full of viscoelastic, we maintain a much more stable anterior chamber without the risk for leakage or instability. At the end of the cataract surgery, both the main phaco incision and the additional incision are sealed in the traditional manner using mild stromal hydration.
There is some variation in the degree of astigmatic effect depending on multiple factors such as incision size, incision placement, incision position, incision tunnel length, patient age, corneal diameter and corneal pachymetry. For most of our cataract patients, the effect of these factors is limited, and we can come up with a reasonable and predictable nomogram to treat astigmatism.
A wider incision will induce more corneal flattening, as will more central placement, inside the limbal vessels. A shorter tunnel length will also cause more corneal flattening, but it may not seal as well as an incision with a longer tunnel length. Patient age can affect corneal rigidity, and we expect more effect in older patients. Small anterior segments with shorter white-to-white corneal diameters may have more effect from the same width incision as a larger eye.
Using paired phaco incisions is not a new idea, and it has been described many times over the past two decades. Original publications referenced incisions of 3 mm to 3.5 mm in width and a corneal refractive flattening effect of 1.25 D to 2 D. In modern-day cataract surgery, we use much smaller incisions, typically between 2.2 mm and 2.8 mm in width, which will have less of an astigmatic effect.
We performed a small study in our clinic to judge the astigmatic benefit of paired phaco incisions using a 2.8-mm keratome and single-plane architecture with a consistent tunnel length and placement right at the edge of the limbal vessels. We divided these patients into two groups: ATR and WTR. ATR is against-the-rule astigmatism in which the steep meridian was at or close to the 180° mark, which is most common in cataract patients. WTR is with-the-rule astigmatism, having a steep meridian at or near the 90° mark, and is more common in younger patients and myopic eyes.
We found that for patients with ATR astigmatism, placement of paired phaco incisions on the steep meridian gave an astigmatic benefit of 0.7 D of treatment, while those having WTR astigmatism, the effect was larger at about 1 D of flattening. We have since used this nomogram for many more patients and have found the results to be relatively consistent. I encourage you to try using paired phaco incisions in your own patients to address astigmatism at the time of cataract surgery.
My nomogram for astigmatism correction with paired 2.8 mm-wide incisions is:
Click below for the video about paired phaco incisions to treat astigmatism: