It can take a great deal of time to select the best intraocular lens (IOL) power, since there are many formulas available — and even more time to run measurements through a couple of formulas to find a reliable average. Surgeons are largely staying with the popular third generation formulas Hoffer Q, Holladay 1, SRK/T, and fourth generation Haigis, which return accurate predictions for most patients, according to a study posted in the British Journal of Ophthalmology.
The combination of axial length (AL) with corneal power (K) should be considered when choosing a formula to calculate an IOL, according to researchers. They add that formula is even more critical with unusual biometry, such as eyes with very long axial lengths or steep corneal power.
Axial length was defined as short (<22.46 mm), average (22.46–24.64 mm), and long (>24.64 mm). K was described as flat, average, or steep in subgroups equivalent to <41.7 D and >44.44 D, keratometer index of 1.332. Subjects’ visual acuity was assessed using a Snellen chart at 24 or more days following surgery.
Based on the lowest median absolute error (MedAE) for each formula, and percentage of eyes with an absolute prediction error of 0.5 D or less, statistical analysis pairs particular corneal measurements with the most appropriate formula:
- Eyes with short AL and average K: Hoffer Q attained the lowest MedAE (.30), while Holladay 1 and SRK/T the most eyes at 0.5 D of less (71.4% each)
- Eyes with short AL and steep K: lowest MedAE was found in Holladay 1 (.24) and Hoffer Q (.25), while Haigis showed the most eyes at 0.5 D or lower (90%)
- Eyes with average AL and flat K: Haigis had the lowest MedAE (.24), and SRK/T the most eyes at 0.5 D or lower (80.5%)
- Eyes with average AL and average K: Haigis achieved lowest MedAE (.32), while SRK/T the most eyes with 0.5 D or lower (68.1%)
- Eyes with average AL and steep K: Haigis had the lowest MedAE (.25) and most eyes at 0.5 D or lower (73.8%)
- Eyes with long AL and flat K: Holladay 1 had the lowest MedAE (.33), and Haigis the most eyes at 0.5 D or lower (70%)
- Eyes with long AL and average K: SRK/T had the lowest MedAE (.29), while Haigis and Hoffer Q the most eyes at 0.5 D or lower (69.7% each)
“Overall, the Haigis formula shows accurate results in most subgroups,” according to the study. There were no subjects in the rarer subgroup of short AL–flat K, and only 2 in the long AL–steep K subgroup. Also, newer formulas such as Olsen, Kane, or Barrett were not evaluated.
In the study, investigators reviewed records for 611 eyes after uncomplicated cataract surgery at the Medical University of Vienna from 2015–2019. The 2 IOLs used were assessed in separate groups.
“We strongly advise against relying on 1 formula without considering biometric measurements,” the researchers explain. “Furthermore, our findings show that the results of 2 IOLs can be different, emphasizing the difficulty in comparing results between studies.”
Röggla V, Langenbucher A, Leydolt C, et al. Accuracy of common IOL power formulas in 611 eyes based on axial length and corneal power ranges. Bri J Ophthalmol. Published online September 30, 2020, doi: 10.1136/bjophthalmol-2020-315882.