As cataract surgery and refractive surgery become more closely integrated, we will focus more on the management of corneal astigmatism in order to get patients off the lens and have excellent vision after surgery. The goal is to obtain a refractive state free of astigmatism (which may not exactly match a standard keratometer) and hopefully this refractive state will remain stable for many years. Recent studies by ophthalmologists at Baylor College of Medicine in Houston have served as a guide to achieving these goals and optimizing refractive outcomes after cataract surgery. Corneal astigmatism over time A patient who had cataract surgery 10 years earlier returned to the office for a follow-up appointment and notably showed a year-to-year trend of decreasing distance vision. His surgical procedure was near perfect, with a 2.8mm temporal corneal incision to manage preoperative retroconformity astigmatism, implantation of an aspheric IOL, and a postoperative refractive status of orthokeratology in both eyes. But now, 10 years later, I was very surprised to find that the patient had progressive corneal retro-regular astigmatism (ATR) and received 1.5D astigmatism correction.
This is a classic example of the increase in corneal antegrade astigmatism with age. It is important to note that in our case, the patient had a phaco incision performed on the temporal side, the steepest meridian. This was done to counteract the mild astigmatism and achieve orthokeratology after the initial cataract surgery. This age-related refractive change would become more pronounced if the incision were made superiorly, as the superior incision would flatten the 90-degree direction.
Therefore, cataract surgery preserves a small amount of corneal cis-regular astigmatism (WTR), which will allow patients to have excellent naked eye vision for a longer period of time after surgery, as corneal cis-regular astigmatism will gradually decrease and progress to retro-regular astigmatism as we age. If you can only measure the anterior corneal surface curvature, then setting the target refraction at 0.5D cis-gauge astigmatism can result in higher patient satisfaction.
Posterior corneal surface astigmatism recently had a patient who was lucky enough to have a near-perfect postoperative result: -0.25D spherical lens in the right eye and complete orthokeratology in the left eye. This resulted in a very high satisfaction level of 20/20 naked eye vision in both eyes. Curiously, however, although he had no astigmatism in the total refraction of the entire eye, the keratometer degree showed a 0.75 WRT cis-regular astigmatism (Figure 1). This is due to the nature of our measurement: the keratometer only measures astigmatism on the anterior surface of the cornea, not the entire cornea. In this case, the anterior corneal surface astigmatism was just offset by the posterior corneal surface astigmatism, which ultimately resulted in no postoperative astigmatism in the eye.
Although our keratometer only measures anterior corneal surface curvature, there are other devices (such as corneal topography) that can accurately measure posterior corneal surface curvature prior to cataract surgery. In addition, intraoperative aberrometry can provide corneal astigmatism in the aphakic state before IOL implantation after the cloudy lens is removed.
Studies by Baylor’s group have shown that eyes with cis-gauge astigmatism have an average of approximately 0.5 D posterior corneal surface astigmatism, which is equivalent to 0.5 D negative lens WTR. This means that if the keratometer measures 2.0 D cis-gauge astigmatism, only 1.5 D cis-gauge astigmatism needs to be addressed.
For toric astigmatism, the posterior corneal surface astigmatism is approximately 0.3D, which is equivalent to the same degree of negative lens WTR. If the keratometer measures 2.0D toric astigmatism, the total corneal toric astigmatism to be treated is higher, closer to 2.3D. Currently, four toric astigmatism IOLs are FDA-approved: AMO, Alcon, Phalen and STAAR. let’s Let’s take Alcon AcrySof toric IOLs as an example, which is the type of lens with the largest range of diopters and has an incremental step of 0.5 D of astigmatism. When doing the toric IOL calculation, if the corneal astigmatism measured with a standard keratometer is 2.5 D of toric astigmatism, it should be clear that the astigmatism to be treated is 0.5 D less than the measured diopters. IOL (2.5 D of toricity) down to T5 IOL (2D of toricity). We should keep in mind that for cis-regular astigmatism, one step should be lowered (Figure 2).
For another patient, if our corneal astigmatism measured by a standard keratometer is 2.5 D of retrogressive astigmatism, our treatment should be slightly higher than the above, so we increase from the calculated T6 IOL (2.5 D of toricity) to T7 IOL (3D of toricity).
Similarly, we should keep in mind that for retrogressive astigmatism, one step should be added (Figure 3). It is important to note that for any kind of toric IOL calculation, the surgeon should take into account the effects of the phaco incision, the IOL spherical lens prescription and the effective position of the lens to achieve the most accurate result.
For the next cataract patient encountered with large corneal astigmatism, you should consider the effect of posterior corneal surface astigmatism and allow for a small amount of cis-gauge astigmatism for long-term results. When our surgical patients achieve a perfect refractive result in addition to resolving their cataract, they will achieve a very high level of satisfaction by being maximally independent of their glasses.