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How to choose the short-axis IOL calculation formula? Professor Li Chaohui helps you clarify your thoughts

Posting time:2022-12-02 06:55:13

How to choose the short-axis IOL calculation formula? Professor Li Chaohui helps you clarify your thoughts

Editor's note: With the continuous advancement of cataract surgery technology, cataract patients have higher and higher requirements for postoperative visual quality. How to make some special types of patients also obtain better postoperative visual quality is an issue that ophthalmologists are very concerned about. Short-axis patients are patients with certain specialities. If the power of the intraocular lens (IOL) to be implanted is calculated according to the normal axial length (AL) of the patient, it is likely to lead to poor postoperative visual quality of the patient. So how can we choose the appropriate degree of IOL for short-axis patients? "International Ophthalmology News" specially invited Professor Li Zhaohui of PLA General Hospital to share his experience on this issue. Professor Li answered this question in detail based on his own experience and relevant research results at home and abroad. Now let us find out together. Why should we pay attention to the calculation of short-axis IOL? Short axial axis usually refers to AL. Now that cataract surgery has entered the era of refractive surgery, the accuracy of IOL power calculation is getting higher and higher. The study found that within the normal axis range, the percentage of postoperative refractive error less than ±0.5 D in patients can reach 85-92%, but in patients with short axis, this proportion is less than 75%, of which only 28.3%. ~48.7% achieved satisfactory results (±0.25 D). The results calculated by different formulas vary greatly, and the postoperative refractive error of some patients can be >1.0 D, or even nearly 2.0 D. The correct choice of IOL degree is related to accurate preoperative biological measurement and appropriate IOL calculation formula. Optical measurement devices such as Lenstar based on Optical Low Coherence Reflection (OLCR) and IOL MASTER based on Partial Coherent Interferometry (PCI) have good accuracy and repeatability, and are more accurate than A-mode measurements. There are many calculation formulas of IOL, but because its accuracy is easily affected by eyeball parameters (ACD, K, AL, LT, etc.), there is currently no calculation formula suitable for all patients. Compared with normal eyes, the structure of short-axis eyes does not change proportionally, which increases the difficulty of biological measurement. At the same time, due to the large degree of implanted IOL, it is of great significance to choose an accurate calculation formula of IOL degree. What are the formulas for calculating IOL? The traditional IOL calculation formula classification method is divided into five generations, and the accuracy is gradually improved: The first generation formula: such as SRK I, Fyodorov, Binkorst, Colenbrander, etc., is derived using the principle of geometric optics, based on the thin lens theory, obtained through regression analysis ; The second generation formula: such as SRK II, Binkhors II, etc., there are modified formulas of theoretical formulas, and there are formulas derived from the combination of theory and experience; the third generation formulas: such as SRK/T, Hoffer Q, Holladay I, The basis of the theoretical formula + the correction of postoperative refractive data, using AL and K to predict the effective lens position (ELP) after surgery, in the abnormal eye axis, the error is large The A constant and AL are not simply linearly related, and need to be optimized; Fourth-generation formulas: such as Haigis, Holladay II, etc., the reference variables are increased; fifth-generation formulas: such as Barrett Universal II, Olsen, Okulix, Hill-RBF, Kane, T2, Ladas Super Formula, etc., the accuracy is further improved. The new classification method also divides the IOL calculation formula into five categories (Figure 1). The traditional formula is based on Gaussian optics, which improves the accuracy of ELP prediction results by incorporating different biological measurement indicators. In recent years, the accuracy of artificial intelligence and ray tracing formulas has been greatly improved. The artificial intelligence formula uses refractive results from a large database, and provides accurate predictions through the results of a sufficient number of structurally similar eyes in the database. The ray tracing method is based on the thick lens theory, and Snell's law is used to calculate the ray trajectory, providing a real simulation of the human eye. included in the analysis. Figure 1. How to evaluate the accuracy of the new classification of IOL calculation formulas? Common metrics for evaluating the accuracy of the IOL calculation formula include: Prediction Error (PE) - Negative values ​​represent partial myopia Mean Prediction Error (ME) - Average of PE, used to optimize A constant Mean Absolute Error (MAE) - PE The mean of absolute values, avoiding positive and negative offsets Median Absolute Error (medAE) - Median of PE absolute values Constant optimization of the equivalent spherical diopter in visual acuity to reduce surgical-induced error Selection of short-axis IOL calculation formula from an evidence-based perspective Melles et al. published in Ophthalmology in 2018 The results of a study involving 18,501 surgical eyes showed [1] that the accuracy of most of the formulas performed well in the normal axial range, but as the axial length decreased, the PE increased (Fig. 2). Figure 2. Melles et al. found that with decreasing axial length Small, increased PE. Other studies on short-axis populations have found [2,3] that the commonly used SRK/T, Barrett Universal II, Hoffer Q, Holladay 1, and Holladay 2 formulas after surgery are often myopic. Refractive error after Haigis formula is often farsighted (Fig. 3). Fig. 3. Refractive condition of short-axis population after applying each formula to calculate IOL power. A meta-analysis in 2018 compared Haigis, Holladay 2, Hoffer Q, Holladay 1. Mean absolute error (MAE) of several formulas of SRK/T and SRK II in the short eye axis. The results show [4] that in the traditional formula, the accuracy of Haigis formula is better than that of Hoffer Q, SRK/T and SRK II, the reason may be that the optimized short-axis A constant-A1 was used, and the preoperative ACD was used to predict ELP more accurately (Figure 4). Figure 4. Meta-analysis results Several other studies showed that [5,6], the smaller the ACD, the greater the postoperative PE, and the more obvious changes in ACD before and after surgery in patients with short axial length, which increases the difficulty of ELP prediction. Therefore, for people with shallow anterior chambers, SRK/T and Hoffer should be avoided. Q formula, it is recommended to choose Barrett Universal II and Haigis formula. Regarding the performance of new formulas such as artificial intelligence and ray tracing, the conclusions drawn by different studies are not completely consistent. Generally speaking, the accuracy of the first report is better because it can be Correction and optimization are carried out according to their own database, but the accurate evaluation still needs to comprehensively analyze the results of multiple studies to reach a conclusion (Figure 5). Figure 5. The performance of the new formula in different studies Professor Li Zhaohui's team has analyzed the published results in the past ten years. A systematic review and meta-analysis of the literature data were carried out [7], and 14 literatures comparing the short-axis IOL calculation formula were finally included, including 13 formulas, which summarized and compared the postoperative refractive error within ± Results within 0.25 D, 0.5 D, and 1.0 D (Figure 6), and the corresponding findings have been published in this year's International Ophthalmology Journal. Figure 6. Distribution of PE at ±0.25 D, ±0.5 D, ±1.0 D. The results of this study show that for short-axis patients, new formulas based on artificial intelligence or ray tracing are more accurate, such as Pearl-DGS and Okulix . It is recommended that doctors refer to various formulas to calculate the results before surgery to avoid large refractive errors. Summary Regarding how to better calculate the short-axis IOL, Professor Li summed up the following experience: accurate biological measurement before surgery is the foundation; according to different situations, pay attention to avoid abnormal values; during binocular surgery, adjust the contralateral side in time according to the optometry results Eye surgery strategies; refer to multiple formulas to calculate results to avoid large refractive errors; new formulas for artificial intelligence or ray tracing are more accurate, such as Pearl-DGS and Okulix. References (swipe up and down to view)[1] Melles RB, Holladay JT, Chang WJ. Accuracy of Intraocular Lens Calculation Formulas. Ophthalmology. 2018 Feb;125(2):169-178.[2] Voytsekhivskyy OV, Hoffer KJ, Savini G, Tutchenko LP, Hipólito-Fernandes D. Clinical Accuracy of 18 IOL Power Formulas in 241 Short Eyes. Curr Eye Res. 2021 Dec;46(12):1832-1843.[3] Shrivastava AK, Behera P, Kumar B , Nanda S. Precision of intraocular lens power prediction in eyes shorter than 22 mm: An analysis of 6 formulas. J Cataract Refract Surg. 2018 Nov;44(11):1317-1320.[4] Wang Q, Jiang W, Lin T, Wu X, Lin H, Chen W. Meta-analysis of accuracy of intraocular lens power calculation formulas in short eyes. Clin Exp Ophthalmol. 2018 May; 46(4): 356-363.[5] Eom Y, Kang SY , Song JS, Kim YY, Kim HM. Comparison of Hoffer Q and Haigis formulae for intraocular lens power calculation according to the anterior chamber depth in short eyes. Am J Ophthalmol. 2014 Apr;157(4):818-824.e2. [6] Ning X, Yang Y, Yan H, Zhang J. Anterior chamber depth - a pred ictor of refractive outcomes after age-related cataract surgery. BMC Ophthalmol. 2019 Jun 25;19(1):134.[7] Luo Y, Li H, Gao L, Du J, Chen W, Gao Y, Ye Z, Li Z. Comparing the accuracy of new intraocular lens power calculation formulae in short eyes after cataract surgery: a systematic review and meta-analysis. Int Ophthalmol. 2022 Jan 26.

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