The goal of any cataract surgery is to achieve near perfect vision in the postoperative period considering the only issue to be addressed in preoperative period is the cataractous lens. Over a period of time, many formulas were used to measure the appropriate power of the intraocular lens to be used. There are theoretical formulas, which are based on the mathematical principles revolving around the “schematic eye,” and there are regression formulas, which are based on retrospective outcomes. There are many variables to be considered for calculating the intraocular lens, such as anterior chamber depth (ACD), axial length (AL), keratometry readings, etc.,[1] Both theoretical formulas and regression formulas use these variables in their calculations.
In eyes having AL within the range of 22.50–24.00 mm and central corneal powers ranging from 42.00 D to 45.00 D with a normal ACD, most advanced IOL power calculating formulas will give good results. However, for eyes not falling in this range, most of the studies have clearly proven that Haigis (with properly optimized a0, a1, and a2 constants) and the newer generation formulas, like Hill RBF, Kane’s, Holladay 2, Olsen, Barrett Universal – II give edge over other old generation IOL calculating formulas.[234]
The IOL calculating formulas like Holladay 1, SRK/T, Haigis, and Holladay 2 require an adjustment for the eyes having AL of more than 25.00 mm, and this can be easily modified at the following link[3]: http://www.doctor-hill.com/iol-main/extreme_axial_myopia.htm.
The universally acceptable mode of IOL calculating formula is been shown in the below image [Fig. 1].
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Figure 1: Accuracy range of commonly used formulas by axial length
This study by the accepted article Mieno et al.[5] has found that the BU II formula had significantly lower absolute error in patients with long AL. A study conducted by Mohan et al. found that the Barrett universal II formula predict IOL power accurately in eyes with extremely long AL and even in eyes where negative power IOLs are used.[5] No adjustments were necessary even if the implantation of meniscus lens is planted.[6] Yichi Zhang et al. in their study supported the use of newer Barrett Universal II formula to enhance predictability in high myopic eyes.[6] Kei Iijima et al. stated that the absolute error (0.38 ± 0.38 D) using SRK/T formula was significantly larger that of (0.33 ± 0.27 D) using Barrett Universal II formula (P = 0.0006).[7]
However ironically, we surgeons are most of the time being judged by our fellow colleagues and patients by our refractive outcomes only. So, it is always an extraexpensive investment for every ophthalmologist to buy the latest biometry module equipped with the most advanced IOL calculating formulas. Although to achieve emmetropia in every eye is still an enigma, so as a researcher we must keep looking for a near perfect solution.[4]
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Conflicts of interest
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References
1. Dong J, Zhang Y, Zhang H, Jia Z, Zhang S, Wang X Comparison of axial length, anterior chamber depth and intraocular lens power between IOLMaster and ultrasound in normal, long and short eyes PLoS One 2018 13 e0194273
2. Iijima K, Kamiya K, Iida Y, Shoji N Comparison of predictability using Barrett Universal II and SRK/T formulas according to keratometry J Ophthalmol 2020 2020 5 doi:10.1155/2020/7625725
3. Wang L, Shirayama M, Ma XJ, Kohnen T, Koch DD Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm JCRS 2011 37 2018 27
4. Morya AK Commentary Indian J Ophthalmol 2021 69 584 5
5. Mohan P, Chakrabarti A Intraocular lens power calculation in 2019:The cutting edge Kerala J Ophthalmol 2019 31 191 201
6. Barrett GD An improved universal theoretical formula for intraocular lens power prediction J Cataract Refract Surg 1993 19 713 20
7. Iijima K, Kamiya K, Iida Y, Shoji N Comparison of Predictability Using Barrett Universal II and SRK/T Formulas according to Keratometry Journal of Ophthalmology 2020 Article ID 7625725, 5 pages, 2020
