4 minute read
New Biometry
More data from new devices and IOL power formulae improve refractive outcomes. Howard Larkin reports from ASCRS in Las Vegas, USA
Before 2000, the ZEISS IOLMaster and Haag-Streit Lenstar revolutionised cataract surgery by introducing laser interferometry technology for more accurately measuring ocular axial length.
“It took a lot of the guesswork out of axial length measurement,” Robin R Vann MD told a session at Refractive Surgery Day.
But the demands of today’s cataract refractive surgery require even more from biometry devices and IOL power formulae, he said.
“We have to go beyond just axial length measures and look at other aspects of the eye to get good outcomes.”
Over the past decade or so, that has meant incorporating tomography and topography into axial length measuring devices. Early examples include the Pentacam® AXL, NIDEK AL-Scan, and GALILEI G6. Hybrid designs incorporating a swept-source OCT engine—such as the ZEISS IOLMaster 700, Alcon Argos® Movu, Haag-Streit EyeStar 900, Heidelberg ANTERION, and Tomey OA-2000—are now available.
IMPROVE WORKFLOW By combining a range of refractive cataract diagnostics into a single machine, these devices improve workflow, reducing the amount of clinic space and time it takes to work with patients, Dr Vann observed.
“The patient doesn’t have to move from machine to machine,” he noted.
Tomography and/or topography can help visualise the posterior cornea, measure higher-order aberrations, and assess corneal shape changes—all critical to successfully implanting modern presbyopia-correcting and toric lenses.
Incorporation of swept-source OCT has been a crucial step forward, he emphasised, “We’ve seen from the retinal world how it can revolutionise the fidelity and resolution of some of the captures in the retinal field. So too, it can do this for us in cataract surgery planning.”
Advantages of swept-source OCT over laser interferometry include longer wavelengths for greater penetration, making it possible for devices such as the ZEISS IOLMaster 700 to image through 97% of mature cataracts and 98% of advanced cataracts, Dr Vann said. Faster engines enable the capture of 2,000 to 3,000 scans per second, enabling better imaging—advantages observed in recent comparative studies.
Swept-source OCT devices allow for visualisation of the anterior segment and the macula in the same scan, Dr Vann noted. Individual devices offer features such as segmental axial length, pachymetry, and anterior and posterior corneal topography. Software packages such as the Alcon SMART Suite and ZEISS FORUM/CALLISTO are beginning to integrate these rich data sets with surgical microscopes and other intraoperative devices to support better-informed decisions before and during surgery.
IOL FORMULAE Just as important as better biometry is IOL power formulae capable of transforming the extra data into better lens choices. In the late 1990s, Jack Holladay MD identified nine categories of eyes based on axial length and anterior segment size, Dr Vann said. These different eyes, ranging from nanophthalmia to large eyes with axial myopia, and different formulae may be better for different eye types.
Traditional formulae only captured differences in the anterior segment portion of axial length, so new formulae were developed to better model effective lens position based on other variables, Dr Vann noted. Newer IOL formulae fall into three categories: vergence/ray tracing, AI formulae, and hybrids. The Holladay II and Barrett Universal vergence and Olsen C ray tracing fall under vergence/ray tracing, Hill RBF AI and PEARL-DGS under AI formulae, and LADAS Superformula and the Kane formula under hybrids.
“Many of these have come out in the last four to five years, and there is not a lot of data to look at the success rates.”
That said, studies since 2015 have consistently shown the Barrett Universal as one of the best performing formulae, Dr Vann said. The Kane formula has shown good performance in the last two years, with one review of 958 articles finding it has the best performance across all axial lengths. (Kane J et al. doi: 10.1016/j. ophtha.2020.08.010). Another 2020 study found the Kane was best for eyes with axial lengths under 22 mm (Wendelstein J et al. doi: 10.1136/bjophthalmol-2020-318272) and eyes over 30 mm (Cheng H et al. Am J Opthalmol. 2021; 223: 100–107).
Given the lack of articles on newer formulae outcomes, Dr Vann advised surgeons look to meetings and new publications to validate accuracy. Newer generation formulae are improving accuracy on extreme axial lengths, with the Barrett Universal II still excellent for eyes over 22 mm, and the Kane excellent at all lengths, he said. New formulae are constantly being released, such as the Hill RBF III, which has more data for smaller and larger eyes that should improve its accuracy.
But good biometry is critical, Dr Vann said. “Please, please keep in mind that formulas are only as good as the data captured.” He recommended surgeons review their own outcomes to determine the best method to proceed.
