August 2019

REFRACTIVE

Yes Connect
IOL power calculations for the young eye surgeon


by Liz Hillman EyeWorld Senior Staff Writer

In the era of refractive cataract surgery, intraocular lens technologies continue to evolve toward maximizing spectacle independence. The accuracy of intraocular lens power calculations has never been so relevant as in the current times, when patients are invested both financially and emotionally in maximizing spectacle independence, and the intraocular lens technologies demand precise and predictable outcomes. Yet, as always, our expertise is warranted to discern what is best for each individual patient among the overwhelming amount of options, and intraocular lens calculation formulas and biometry measurements are not exempt. Here we offer some pearls on intraocular lens calculations from Dr. Khandelwal and Dr. Holladay to maximize refractive outcomes after cataract surgery.

Claudia Perez-Straziota, MD,
YES Connect Co-editor
 

There are many factors that affect the accuracy of IOL powers chosen for patients. Which formula to use? What variables to consider for IOL calculations? Which biometer to use? When to adjust calculations for specific situations? Physicians also have to consider effective lens position estimates and more.
Jack Holladay, MD, and Sumitra Khandelwal, MD, shared their thoughts on this topic, offering young eye surgeons insights into effective IOL power calculations that can help lead to more accurate outcomes.
The two biggest mistakes in IOL power calculations Dr. Holladay sees young eye surgeons making today are: (1) not looking at the quality of optical keratometry and axial length measurements and (2) not personalizing the lens constant for the primary IOL. Dr. Khandelwal expressed a similar sentiment.
“The biggest mistake I see is young surgeons sticking to what they were taught in training rather than what the updated recommendations are,” she said. “I also see young surgeons by default choosing –0.50 as their target. Once a surgeon is in practice, one priority should be to optimize their surgeon factor so that they can start aiming closer to plano and be able to reach the goals of the patient.”
Quality calculations start with quality measurements, paying special attention to keratometry, axial length, and anterior chamber depth. Both Drs. Khandelwal and Holladay emphasized the importance of accurate and repeatable keratometry. Obtaining this, however, can be difficult in some patients.
“These include dry eye patients, cornea pathology like anterior basement membrane dystrophy or keratoconus, or contact lens warpage,” Dr. Khandelwal said. “The key with keratometry is to recognize any issues, pretreat the patient, and counsel them that the irregular keratometry may affect their outcome. If that is not mentioned before surgery, any issues after including residual refractive errors will feel like something the surgeon did rather than how the patient’s eye always was.”
Dr. Khandelwal pays special attention to eyes with long axial lengths. In these eyes, a hyperopic outcome would be a worst-case scenario because these patients have always been myopic. Meanwhile, she added, anterior chamber depth cannot be overemphasized, especially if the patient has a shallow anterior chamber. These patients’ effective lens position is often tough to determine so the refractive error can be unpredictable especially with higher diopter lenses, she said.
Dr. Holladay said that for eyes longer than 24.0 mm, one should use either the Wang-Koch linear regressions or the Holladay non-linear equations to avoid hyperopic errors.
“The most likely sources of error in IOL calculations were shown by Sverker Norrby,”1 Dr. Holladay said. “In order of the error in normal patients is: effective lens position, [postop] spectacle refraction, axial length, keratometry, and pupil size. These are the sources of the prediction error, and choosing the correct IOL formula and looking at the quality of axial length and keratometry can make the most significant improvement in results.”
Dr. Holladay said standard deviation for three measurements of optical keratometry would be <0.20 D (or 0.030 mm or 30 µm), and axial length should have a signal-to-noise ratio of >2.0. He said that symmetry between eyes is also good for screening measurements. If the axial length is >0.3 mm, the Ks >1.0 D, or the IOL powers >1.0 D, the measurements must be repeated.
“The values are on every optical biometer and identify the patients who are most likely to be outliers. Repeating the measurements and go-to topography for the cornea when irregular reduces the chance of a refractive surprise,” he explained.
With more modern biometry systems, Dr. Khandelwal said she is using immersion biometry less. She currently uses the IOLMaster 700 (Carl Zeiss Meditec), which reads through posterior subcapsular cataracts and long axial lengths. However, both she and Dr. Holladay see a continued role for immersion biometry, such as for white cataracts, nuclear sclerotic cataracts, and eyes with staphylomas and retinal detachments for combination retinal cases where the anatomy is not clear.
In terms of which formula to use, Dr. Khandelwal said she compares at least two formulas with two different biometries, when possible.
“I most often use the more modern formulas because they have been shown to work well in most eyes, including outliers like longer axial length and shallow anterior chamber depth. For me, the easiest to access with my biometry are the Barrett and Holladay on the IOLMaster 700 and Olsen and Hill-RBF on the LENSTAR [Haag-Streit],” Dr. Khandelwal said.
Certain scenarios might require specific adjustments. Dr. Holladay said that recent studies have shown the Barrett Universal II, Olsen II, and Holladay 2 to perform between 78% and 80% within ±0.50 D. Dr. Khandelwal said she will use the Holladay 1 with the Wang-Koch axial length modification, the Barrett formula, and the Olsen or Hill-RBF, if available, for long eyes. For shorter eyes and those with shallow anterior chambers, she likes to use three formulas “because it can be difficult to decide which lens.”
“I aim plano or a little hyperopic for these eyes, especially if they are already hyperopic prior to surgery. They are, ironically, the one group that does not like to be myopic after surgery,” Dr. Khandelwal said.
Post-refractive surgery eyes and patients with nanophthalmos can be difficult to calculate due to poor effective lens position predictions, resulting from unusual measurements, Dr. Holladay said. In these cases, he emphasized corneal topography and tomography with repeated measurements. However, he noted, the variability in outcomes is still greater than that of a normal patient.
As for final pearls for IOL calculations for those who are still relatively new in practice, Dr. Holladay reiterated implementing the quality checks on axial length and keratometry, as he already described, and of personalizing your lens constant.
“It can improve the percentage of cases within ±0.50 D by 5%,” Dr. Holladay said.
Dr. Khandelwal recommended picking two easy-to-access formulas and repeating them with the same pattern for 30–40 cases. Then look back at these outcomes to determine your surgeon factor and see which formula works best for you.
“In addition, it is important to take note of refractive errors and surprises even in patients who are happy with the outcome. Even if that patient is accepting of residual refractive error, your next patient may not be,” Dr. Khandelwal said.

About the doctors

Jack Holladay, MD
Clinical professor
of ophthalmology
Baylor College of Medicine
Houston

Sumitra Khandelwal, MD
Assistant professor of
ophthalmology
Baylor College of Medicine
Houston

Reference

1. Norrby S. Sources of error in intraocular lens power calculation. J Cataract Refract Surg. 2008;34:368–76.

Financial interests

Holladay
: Holladay Consulting
Khandelwal: None

Contact information

Holladay
: holladay@docholladay.com
Khandelwal: Sumitra.Khandelwal@bcm.edu

IOL power calculations for the young eye surgeon IOL power calculations for the young eye surgeon
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