Surgeons strive for refractive stability with no astigmatism after cataract surgery
The effect of the posterior cornea needs to be accounted for, and leaving a small degree of with-the-rule cylinder will help patients over time.
With the fields of cataract surgery and refractive surgery merging, we are increasingly looking at addressing corneal astigmatism in order to provide excellent vision without glasses for our patients. Our goal is to provide a refractive state with no astigmatism, which may not exactly correspond to standard keratometry readings, and to give refractive stability over the years. Recent work by ophthalmologists at Baylor College of Medicine in Houston has helped to create guidelines to address this goal to optimize refractive outcomes after cataract surgery.
Corneal astigmatic changes over time
A patient who had cataract surgery 10 years ago returned for consultation, noting that his distance vision had slowly declined over the years. His original surgery was state-of-the-art at the time, with a 2.8-mm temporal corneal incision placed on the steep axis to address corneal astigmatism, an aspheric IOL and a postop refraction of essentially plano in both eyes. But now, a decade later, I was surprised to see that he had developed against-the-rule (ATR) corneal astigmatism and now required a refraction with 1.5 D of astigmatism correction.
This is a classic example of the ATR shift of corneal astigmatism that happens with age. Note that in our case the patient even had a phaco incision that was made at the temporal position, on the steep axis. This was effective in neutralizing the mild astigmatism that was present at the time of the original cataract surgery, resulting in a plano postoperative refraction. This change with age would be even more pronounced if we had made a superior incision, which would induce flattening at the 90° meridian.
Using a postoperative goal of a small amount of with-the-rule (WTR) corneal astigmatism is helpful in giving our patients a longer duration of excellent uncorrected vision as time starts to lessen the WTR cylinder and then begins to transform the cornea into ATR cylinder. If you are measuring only the anterior keratometry, aiming for a goal of about 0.5 D of WTR astigmatism will serve your patients well.
Posterior corneal astigmatism
A recent patient had the fortunate postoperative result of near perfection: –0.25 D spherical in the right eye and absolute plano in the left eye. This gave him a visual acuity of 20/20 in each eye without glasses and a high level of satisfaction. But curiously, although his overall refraction had no astigmatism, his keratometry reading showed 0.75 D of WTR astigmatism (Figure 1). This is due to the nature of our measurements: The keratometer is only measuring the anterior corneal astigmatism and not the entire cornea. In this case, the anterior corneal astigmatism is neutralized by the posterior corneal astigmatism, and as a result, the postoperative refraction of the eye is without cylinder.
Images: Devgan U
Although our keratometers measure only the anterior corneal curvature, there are devices such as tomographers that can accurately measure the posterior cornea before cataract surgery. In addition, intraoperative aberrometers can give us a reading of the cornea as a whole once the cataract is removed and the eye is in an aphakic state before IOL implantation. The Baylor group has shown us that an average eye with WTR astigmatism will have about 0.5 D of posterior corneal astigmatism acting as a negative lens cylinder WTR. This means that an eye that measures 2 D WTR on keratometry likely has only 1.5 D of total corneal WTR astigmatism that needs to be addressed. For eyes that have ATR astigmatism, the typical posterior corneal astigmatism is 0.3 D acting as a negative cylinder WTR. In an eye that measures 2 D ATR on keratometry, there is even more total corneal ATR astigmatism, likely close to 2.3 D.
Currently, four manufacturers make U.S. Food and Drug Administration-approved toric IOLs for cataract surgery: Abbott Medical Optics, Alcon, Bausch + Lomb and STAAR. For simplicity, let’s examine the Alcon AcrySof toric IOLs, which come in the widest range and have even steps of 0.5 D of toricity at the corneal plane. In doing our toric IOL calculations, if we calculate that an eye has 2.5 D of corneal astigmatism WTR as measured by our standard keratometers, we know that we need to treat about 0.5 D less than this, so we drop from the calculated T6 IOL (2.5 D of toricity) to the T5 IOL (2 D of toricity). We can remember that for WTR astigmatism, we should drop down one T step (Figure 2).
For another patient, if we calculate that the eye has 2.5 D of corneal astigmatism ATR as measured by standard keratometers, we should treat for slightly more than this, so we add up from the calculated T6 IOL (2.5 D of toricity) to the T7 IOL (3 D of toricity). We can remember that for ATR astigmatism, we should add up one T step (Figure 3). Note that for any toric IOL calculation, the surgeon should take into account the effect of the phaco incision and even the spherical IOL power and effective lens position for best accuracy.
For the next cataract surgery patient that you encounter with significant corneal astigmatism, take into account the effect of the posterior cornea and plan for the future by leaving a small degree of WTR cylinder. When our surgery patients achieve a great refractive result in addition to correction of the cataract, their satisfaction is high and their freedom from spectacles is maximized.