The Right Formula for Success

Dermot McGrath Reports.

Choosing a modern IOL formula in tandem with accurate preoperative biometry can increase IOL power calculation accuracy and reduce the risk of refractive surprise after surgery, according to Professor Filomena Ribeiro.

“It is possible to get 90% of our patients to within less than 0.50 D of target refraction,” she said. “For that, we need very good validation of our measurements and ensure they fit well in the range of the population. We also need to select a modern IOL formula and regularly analyse our results to improve them.”

Discussing the development of modern IOL formulas, Prof Ribeiro said the choice has evolved from outdated regression formulas to more sophisticated methods incorporating additional biometric variables, such as anterior chamber depth, lens thickness, and white-to-white measurements, among others. These formulas increasingly embrace different optical approaches and techniques - such as exact ray-tracing and artificial intelligence - in trying to improve prediction accuracy.

“The probability of success with a more modern IOL formula such as Kane, Olsen, Barrett, or EVO is more than 80% of cases will end up within 0.50 D of target and 98% within 1.0 D,” she explained. “However, this could be even better as several publications show different results for the same formulas.”

By comparison, she noted that data from 100 cataract surgery clinics in the EUREQUO database in 2014 and 2015 showed 73% of patients within 0.5 D of target refraction and 93% within 1.0 D.

“If we look at the ESCRS survey of preferred lens formulas over different years, we see that up until around 2019, most surgeons were using SRK/T,” she said. “Whereas in 2021, other, more modern formulas were chosen more.”

Rather than trying to compare the advantages and drawbacks of each formula, Prof Ribeiro recommended surgeons take advantage of the ESCRS IOL calculator (https://iolcalculator.escrs.org), which provides results for up to seven different modern formulas – including the Barrett Universal II, Cooke K6, EVO, Hill-RBF, Hoffer QST, Kane, and Pearl GDS.

“You only need to input the data once, and with this approach, it is easy to assess the range of residual refractive error predicted, which can be small in some cases but much larger in extreme eyes,” she said.

The key improvements in modern formulas derive from factors such as better effective lens position (ELP) estimation, correction of the keratometer (K) index error, optimization of optical biometry measurements of axial length, and empirical adjustments using linear regression and machine learning.

Although most modern formulas perform well for normal eyes, Prof Ribeiro said accuracy might be affected when performing biometry in long and short eyes and those with flat and steep corneas. For instance, a comparative study by Melles [et al.] found the Haigis formula was most affected by variations in lens thickness, while the Hoffer Q was impacted by varying anterior chamber depth. Another study showed that SRK/T was particularly sensitive to eyes with very flat or steep corneas.

Surgeons can potentially improve IOL power predictions by using second eye adjustment.

“It has been shown that second eye adjustment can provide better results if there is good symmetry between both eyes and regression factors we need to incorporate are related to the ELP estimation,” Prof Ribeiro said. “Some modern formulas like the Pearl-DGS allow us to perform this calculation directly in the online calculator.”

Industry also has a role in improving outcomes, she said, by disclosing basic information about the IOL design that is important for accurate IOL power calculation.

A recent JCRS article noted the need for more specific information concerning IOL design—including the refractive index, the central optic thickness, the anterior and posterior curvature radii, the toricity location, the spherical aberration, and haptic angulation.ⁱ

“With that data, we can really personalise the IOL power calculation with exact ray tracing and improve our outcomes even more,” she concluded.

Prof Ribeiro gave this presentation as part of a recent ESCRS eConnect Webinar, available through the QR code above.

Filomena Ribeiro MD, PhD, FEBO is Head of the Department of Ophthalmology at the Hospital da Luz, Lisbon, Portugal. filomenajribeiro@gmail.com

ⁱ Olsen et al. JCRS. 2023 Feb 6. doi: 10.1097.