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Deeper understanding of toric IOL calculations can help avoid postoperative errors
The Holladay IOL Consultant & Surgical Outcomes Assessment Program pinpointed the problem in a case of residual astigmatism.
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At the Hawaiian Eye meeting earlier this year, I presented a case in which a patient who underwent cataract surgery with a toric IOL had residual astigmatism postoperatively, even though the IOL was perfectly positioned at the intended axis. One of my mentors, Jack Holladay, MD, MSEE, FACS, explained to me that the error was likely in the toric calculation and that the HICSOAP software — Holladay IOL Consultant & Surgical Outcomes Assessment Program, www.hicsoap.com — would help pinpoint the issue.
The first step was to double check all data from the preoperative measurements. The patient had consistent corneal power readings from three different devices, including topography, tomography and automated keratometry. The data were entered correctly, without any transcription errors, into the manufacturer’s online toric calculator. On the day of surgery, the patient’s cardinal meridians were correctly marked and the IOL was precisely placed at the correct axis.
At the postoperative visit, the toric IOL was not tilted or significantly decentered, both of which could induce an astigmatic effect. To make sure that no iatrogenic corneal changes occurred during surgery, the keratometry, topography and tomography were repeated in the postop period, and they showed values that were the same as the preop data. As for positioning, the marks on the toric IOL were aligned with the steepest meridian of corneal astigmatism. I then went to the HICSOAP software and used the “toric postop back calculator” function, which showed me that because my patient had an unusually small, hyperopic eye, the actual calculations from the online website were flawed.
The online calculator uses an approximation of the toric power at the corneal plane equal to the toric power of the IOL divided by 1.45 for the typical eye. This ratio of 1.45 to 1 works reasonably well if the IOL power is average and the anterior chamber depth, which affects effective lens position, is normal. But for eyes that have a more anterior or posterior effective lens position and eyes that require a very low or very high IOL power, this changes dramatically. For a highly hyperopic eye with an IOL power above 30 and a short effective lens position of 4 mm, this ratio is 1.2 to 1 instead. And for a very myopic eye with an IOL power less than 10 and a long effective lens position of 6 mm, this ratio becomes 1.75 to 1 (Figure 1). This range can give a variance of up to 0.75 D at the corneal plane, depending on the eye.
Source: Devgan U
Figure 2. The HICSOAP software gives more accurate toric IOL calculations, particularly in eyes that are unusual, such as this patient with a shallow anterior chamber, short axial length and high IOL power.
Image: Devgan U
My patient had a very shallow anterior chamber, which gives a more anterior effective lens position, and a short axial length, which gives a higher IOL power. Both of these issues altered the toric IOL calculation and made a significant difference compared with the standard online calculator. Newer generations of online toric calculators will likely incorporate this higher level of accuracy, but for now I rely on Dr. Holladay’s program.
The patient was informed that because her eye was so unusual her calculations were less accurate, and a second procedure to enhance the astigmatic correction would be required to maximize her freedom from glasses for driving, which was her original request. Because she had worn glasses full time for so many decades, she was understanding. Her original refraction was +4.50 +2.50 × 180, which then improved to –0.50 +1.00 × 180 after the cataract surgery. Paired limbal relaxing incisions with a 550 µm diamond keratome were performed, and that resulted in a plano outcome.
Refractive cataract surgery is a highly mathematical field, and a deeper understanding of the calculations that are involved is helpful in improving our outcomes. Using more exact methods for toric IOL calculations can result in better vision for our patients, particularly those who have unusual eyes with a high degree of refractive error before surgery. Hyperopic eyes will get more astigmatic correction from the toric IOL and myopic eyes will get less, when compared with an average emmetropic eye.
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Uday Devgan, MD, is in private practice at Devgan Eye Surgery, Chief of Ophthalmology at Olive View UCLA Medical Center and Clinical Professor of Ophthalmology at the Jules Stein Eye Institute, UCLA School of Medicine. He can be reached at 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; email: devgan@gmail.com; website: www.DevganEye.com.Disclosure: Devgan has no relevant financial disclosures.