October 2019


What ophthalmologists should be doing today
Precision refractive cataract surgery
Biometry, astigmatism control, and enhancements

by Chiles Samaniego EyeWorld Contributing Writer

“Enhancements are a critical part of a refractive cataract surgery practice’s success.”
—Tal Raviv, MD

Dr. Koch said the choice between an LRI or toric IOL depends on a surgeon’s “comfort level with both technologies.” LRIs (above) can be successful for low amounts of astigmatism, Dr. Hill said, but toric IOLs are preferred for 0.5 D or more of ATR astigmatism and 1 D or more of WTR.
Source: Rex Hamilton, MD

Today, surgeons are able to achieve desired refractive outcomes with greater precision than ever before. EyeWorld corresponded with three experts on how to best utilize biometry and astigmatism management to achieve emmetropia, improving patient satisfaction and ensuring surgeons’ success in refractive cataract surgery.

Biometry: Due for an upgrade?

The right biometer can help better achieve desired refractive outcomes. “I think that the [two] key elements of state-of-the-art biometry are (1) optical measurement of axial length, by optical low-coherence reflectometry [OLCR] or preferably swept-source OCT, which has the advantage of being able to measure axial through much more dense cataracts than OLCR can, and (2) measurement of corneal curvature with at least 18 LEDs, preferably in more than one ring,” said Douglas Koch, MD. “Lacking these, an upgrade will dramatically improve outcomes.”
The latest biometers have certainly improved keratometry readings, said Tal Raviv, MD, noting that “both the LENSTAR [Haag-Streit] and IOLMaster 700 [Carl Zeiss Meditec] take multiple simultaneous measurements on multiple points of the central and paracentral cornea to derive their K1 and K2.” They also indicate whether the tear film is unstable, requiring optimization—a critical factor for refractive outcomes.
“If a surgeon is using an early generation optical biometry with old software, then moving to a newer biometer with the most recent IOL power selection methods would allow for better outcomes,” said Warren Hill, MD.
The technology, however, isn’t without limitations. “No one should trust a single measurement, in my view,” Dr. Koch said. “I can cite plenty of examples where my biometer gave incorrect readings, almost always due to inaccurate corneal measurements.”
As such, he recommended performing topography. “Topography is essential (1) to confirm the meridian and to a lesser extent the magnitude of the astigmatism, (2) to detect abnormal topography such as keratoconus, and (3) if one has a Placido device, to evaluate the ocular surface by examining the quality of the mires,” he said.
Dr. Raviv agreed that “every refractive surgeon needs topography.” He explained: “Biometers only give two Ks and a steep axis and it’s critical to see the whole picture. Is the topography regular? Irregular? Flat in one area from a small Salzmann’s nodule or scar or EBMD? Placido disc-based topographers are best in my hands for cataract purposes, where Scheimpflug are critical for corneal refractive.”

IOL power calculation and formulas

Even with the best keratometers, measurements should be validated for IOL power calculations. “In general, IOL power calculations are best carried out using high-quality autokeratometry with the application of validation guidelines for the specific instrument being used,” Dr. Hill said. Regarding the formulas used for these calculations, he recommends the Barrett and Hill-RBF formulas.
Dr. Raviv agreed, noting “the published literature has demonstrated the superiority of the Barrett Universal II and Hill-RBF over all older formulas.”1,2 
The Barrett formulas—with variants for post-LASIK/PRK/RK cases and for toric IOLs—were created by Graham Barrett, MD, and are available on the ASCRS website at ascrs.org/barrett-toric-calculator, among other websites.
Meanwhile, the Hill-RBF Calculator is available via rbfcalculator.com, where it is described as “an advanced, self-validating method for IOL power selection employing pattern recognition and sophisticated data interpolation.” “The current version 2.0 is based on 12,419 implantations,” Dr. Hill said. “This works for biconvex and meniscus IOLs from +32.00 D down to –5.00 D.
“As the database increases in size, the depth and accuracy of the Hill-RBF method will advance,” he added. “Version 3.0, which will be released in the future, will add lens thickness, white-to-white and the central corneal thickness as input parameters. I anticipate that the calculation range for version 3.0 will be the same as for version 2.0.”
The Hill-RBF artificial intelligence calculator is licensed to Haag-Streit and optimized for use with the LENSTAR LS 900.
Dr. Koch recommends these formulas but makes allowances for surgeon experience. “I think that ophthalmologists should use formulas that they have optimized with their experience, if they have tracked their outcomes, plus one or more of the newer or more sophisticated formulas: Barrett Universal II, Hill-RBF, Holladay 2, and Olsen,” he said. “These latter [four] are typically going to outperform older formulas in eyes at the extremes: short/long axial length, shallow anterior chamber, unusual corneal power.”
Dr. Raviv cited the Ladas Super Formula (LSF) as “promising, but more data is forthcoming.” Initially developed by John Ladas, MD, the current iteration, according to the website (iolcalc.com), improves on the original LSF “using two major studies of more than 4,000 eyes” and introduced artificial intelligence “to improve performance and harness the power of machine learning.”

Astigmatism control

Astigmatism control can spell the difference between a satisfied and an unsatisfied patient. Surgeons can opt for either relaxing incisions or a toric IOL.
“LRIs can be successfully used for low amounts of astigmatism. In our practice, we sometimes use femtosecond laser-assisted AKs for small amounts of astigmatism,” Dr. Hill said. “However, we prefer toric IOL placement for ATR astigmatism of 0.50 D or more and WTR astigmatism of 1.00 D or more.”
Generally, Dr. Raviv prefers toric IOLs. “Studies that compare relaxing incisions to toric IOLs show a greater accuracy and stability to the toric IOLs,”3,4 he said. “Therefore, we should use a toric IOL whenever we can. Of course, in the U.S. the lowest power toric IOL is either 1.25 D (Envista and Trulign Toric
[Bausch + Lomb]) or 1.50 D (AcrySof [Alcon] or Tecnis Toric [Johnson & Johnson Vision]), which translates to about 0.90 D or 1.00 D of corneal cylinder, respectively.” 
Due to a tendency for posterior corneal astigmatism and drift to ATR cylinder, Dr. Raviv said that they, on average, “place more toric IOLs around the 180-degree axis and more relaxing incisions at the 90-degree axis or oblique axis.”
Dr. Koch again made allowance for individual surgeon experience, saying that the choice between relaxing incisions and toric IOLs “depends on one’s comfort level with both technologies. In my practice, I perform relaxing incisions if there is –0.3–0.5 D of against-the-rule astigmatism, 0.3–0.7 D of oblique astigmatism (depending in part on the orientation of the astigmatism versus my incision location), and 0.8–1.5 D of with-the-rule astigmatism. Anything above that is treated with a toric IOL.”


Despite surgeons’ best efforts, they are sometimes unable to achieve the desired refractive outcome. In these cases, Dr. Hill said that it is generally better to exchange the incorrect IOL for the correct one, for any type of IOL.
“This is not difficult, especially if the surgery was done recently,” he explained. “If the refractive miss is small, LASIK would certainly be one option. However, if the refractive miss is significant, LASIK may induce higher order aberrations, with a loss of contrast at larger pupil sizes. Surgeons who elect this option are most likely not comfortable with a lens exchange.”
For Dr. Koch, the decision depends on the surgeon’s comfort level and the situation. “I perform PCRIs when the spherical equivalent is within 0.25 D of plano,” he said. “For myopia down to around –1.5 and hyperopia up to 1 D, I usually go with LASIK or PRK. IOL exchange is reserved for greater amounts of ametropia. However, there can be mitigating factors, e.g., the cornea might not be a healthy substrate for excimer laser treatment. For residual astigmatism after implanting a toric IOL, I will rotate the IOL in the lane on day 1 if it is misaligned, and later on manage residual refractive errors with rotation, exchange, occasionally PRK or LASIK, or quite often, relaxing incisions to eliminate small but symptomatic amounts of astigmatism.” 
Dr. Koch recognized that these techniques can be challenging and prevent some ophthalmologists from using advanced technology IOLs. “Not all of us have been trained or have a lot of experience with these techniques. Partnering with a colleague who can assist in managing these problems can be reassuring to the ophthalmologist and facilitate optimal patient care.”
They may be a challenge, but Dr. Raviv said that “enhancements are a critical part of a refractive cataract surgery practice’s success. If the surgeon can’t offer them, they will face unhappy patients.”
According to Dr. Raviv, zonular or capsular compromise make a corneal approach favorable while IOL exchange is better for high myopic post-LASIK eyes, eyes with forme fruste keratoconus, and patients dissatisfied with their outcomes due to IOL effects such as glare and halo. Both corneal refractive procedures and IOL exchange are thus “critical in a surgeon’s armamentarium,” he said, rounding out and ensuring a successful refractive cataract practice.

At a glance

• The latest biometry devices are essential but must be validated according to instrument-specific guidelines and are best paired with topography.
• The use of multiple IOL calculation formulae is recommended; the Barrett and Hill-RBF are generally acknowledged as having the best results.
• Astigmatism control can be achieved using relaxing
incisions for lower, toric IOLs for higher levels of astigmatism.
• Enhancements, whether by corneal procedures or IOL exchange, require additional skills or partnering with other surgeons but are an essential component of refractive cataract surgery.

About the doctors

Warren Hill, MD
Medical director
East Valley Ophthalmology
Mesa, Arizona

Douglas Koch, MD
Professor and Allen, Mosbacher, and Law Chair in Ophthalmology
Cullen Eye Institute
Baylor College of Medicine

Tal Raviv, MD
Associate clinical professor
of ophthalmology
New York Eye & Ear Infirmary
of Mount Sinai
Icahn School of Medicine
at Mount Sinai
New York


1. Kane JX, et al. Accuracy of 3 new methods for intraocular lens power selection. J Cataract Refract Surg. 2017;43:333–339.
2. Roberts TV, et al. Comparison of Hill-radial basis function, Barrett Universal and current third generation formulas for the calculation of intraocular lens power during cataract surgery. Clin Exp Ophthalmol. 2018;46:240–246.
3. Leon P, et al. Correction of low corneal astigmatism in cataract surgery. Int J Ophthalmol. 2015;8:719–24.
4. Lee J, et al. Comparison of toric foldable iris-fixated phakic intraocular lens implantation and limbal relaxing incisions for moderate-to-high myopic astigmatism. Yonsei Med J. 2016;57:1475–81.

Relevant financial interests

: Haag-Streit
Koch: Alcon, Johnson & Johnson Vision, Carl Zeiss Meditec
Raviv: Johnson & Johnson Vision

Contact information

Hill: hill@doctor-hill.com
Koch: dkoch@bcm.edu
Raviv: tal.raviv.md@gmail.com

Precision refractive cataract surgery Biometry, astigmatism control, and enhancements Precision refractive cataract surgery Biometry, astigmatism control, and enhancements
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