To evaluate and compare published methods of calculating intraocular lens (IOL) power following myopic laser refractive surgery.

We performed a retrospective review of the medical records of 69 patients (69 eyes) who had undergone myopic laser refractive surgery previously and subsequently underwent cataract surgery at Samsung Medical Center in Seoul, South Korea from January 2010 to June 2016. None of the patients had pre-refractive surgery biometric data available. The Haigis-L, Shammas, Barrett True-K (no history), Wang-Koch-Maloney, Scheimpflug total corneal refractive power (TCRP) 3 and 4 mm (SRK-T and Haigis), Scheimpflug true net power, and Scheimpflug true refractive power (TRP) 3 mm, 4 mm, and 5 mm (SRK-T and Haigis) methods were employed. IOL power required for target refraction was back-calculated using stable post-cataract surgery manifest refraction, and implanted IOL power and formula accuracy were subsequently compared among calculation methods.

Haigis-L, Shammas, Barrett True-K (no history), Wang-Koch-Maloney, Scheimpflug TCRP 4 mm (Haigis), Scheimpflug true net power 4 mm (Haigis), and Scheimpflug TRP 4 mm (Haigis) formulae showed high predictability, with mean arithmetic prediction errors and standard deviations of −0.25 ± 0.59, −0.05 ± 1.19, 0.00 ± 0.88, −0.26 ± 1.17, 0.00 ± 1.09, −0.71 ± 1.20, and 0.03 ± 1.25 diopters, respectively.

Visual outcomes within 1.0 diopter of target refraction were achieved in 85% of eyes using the calculation methods listed above. Haigis-L, Barrett True-K (no history), and Scheimpflug TCRP 4 mm (Haigis) and TRP 4 mm (Haigis) methods showed comparably low prediction errors, despite the absence of historical patient information.

A significant number of patients have undergone laser refractive surgery over the past decade. Furthermore, millions of postrefractive surgery patients are currently at an age at which senile cataracts can develop [

The three main reasons for this IOL calculation inaccuracy are error in corneal radius measurement, keratometric (K) index error, and error in the IOL power calculation [

With this in mind, our purpose in the current study was to evaluate and compare previously published methods with methods that employ biometric data measured using a Scheimpflug (Pentacam HR; Oculus, Wetzlar, Germany) system with respect to the efficacy of IOL power calculations after myopic laser refractive surgery. In addition, we sought to provide guidelines for selecting the best method for IOL calculation in eyes without clinical history data.

We sought to compare several K values or corneal radii from the Scheimpflug system to find the most accurate K value for IOL power calculations. Additionally, we sought to compare the accuracy of various IOL calculation methods in the same patients to determine which formula is the most accurate for calculating IOL power in cataract patients after corneal refractive surgery.

We performed a retrospective review of the medical records of 69 patients (69 eyes) who underwent myopic laser refractive surgery and subsequent cataract surgery at Samsung Medical Center in Seoul, South Korea from January 2010 to June 2016. The study was completed in accordance with the principles of the Declaration of Helsinki and was approved by the institutional review board of Samsung Medical Center (2018-11-140) and informed consent was waived by the board.

All surgeries were carried out by one of two experienced surgeons (TYC and ESC) using the same technique of clear corneal incision, phacoemulsification, and implantation of the IOL(s) in the capsular bag. The patients had no pre-refractive surgery biometric data and were examined prior to cataract surgery, undergoing a thorough ophthalmologic inspection including slit-lamp examination, visual acuity, manifest refraction, potential acuity meter, and optical biometry. Biometry measured on the date closest to cataract surgery was used to calculate IOL power. Partial coherence interferometry (PCI) (Zeiss IOLMaster; Carl Zeiss Meditec, Dublin, CA, USA) measurements were used in this study. All patients underwent biometry of axial length (AL), anterior chamber depth, and K measurements with PCI. Haigis-L, Shammas, WKM, and Barrett True-K (no history) formulae were used to calculate IOL power using online calculators provided by the ASCRS (

In addition, K data for IOL calculations for cataract surgery were obtained using values obtained from Scheimpflug system's total corneal refractive power (TCRP) map, true net corneal power (TNP) map, and total refractive power (TRP) map. K values were obtained for several different zones (3, 4, and 5 mm) in each map (TCRP, TNP, and TRP). PCI was used for AL calculations. SRK/T and the Haigis formula was used to calculate IOL power.

One of two different types of IOLs was implanted: 26 patients received a Tecnis 1-piece monofocal IOL and 30 patients received an AcrySof IQ monofocal IOL (Alcon, Fort Worth, TX, USA).

In total, 20 formulae (Haigis-L; Shammas; WKM; Barrett True-K; TCRP 3 and 4 mm [SRK-T, Haigis]; TNP 3, 4, and 5 mm [SRK-T, Haigis]; and TRP 3, 4, and 5 mm [SRK-T, Haigis]) were used to estimate corneal power or adjusted IOL power in patients who had previously undergone laser refractive surgery. Manifest refraction after cataract surgery was obtained for each subject by an examination three months after surgery. Then, prediction error (PE) was calculated by subtracting the predicted refraction from the power of the actual refraction. Mean numeric error, mean absolute error, and percentage of eyes within a refractive PE of ±0.5 and ±1.0 diopters (D) were calculated for each method. To assess whether the mean numeric and absolute PEs produced by various methods were significantly different, the variances of the mean numeric and absolute PEs were tested using the F-test. Bonferroni correction was applied for multiple tests. Statistical analyses were performed using Microsoft Excel 2007 (Microsoft Corp., Redmond, WA, USA) and IBM SPSS Statistics ver. 22 (IBM Corp., Armonk, NY, USA). Statistical significance was defined as

Cases were divided into two groups according to their axial length (≤26.00, >26.00 mm), anterior chamber depth (≤3.5, >3.5 mm), or K reading (≤39.73, >39.73). In each subgroup, mean numeric error, mean absolute error, and the percentage of eyes within a refractive PE of ±0.5 and ±1.0 D were calculated for each method.

Of the 69 patients selected, 56 patients (56 eyes) who underwent myopic laser refractive surgery and subsequent cataract surgery were included in this study. Mean age of the patients included was 54.6 ± 9.37 years; 21 (37.5%) were male and 35 (62.5%) were female. Among the 56 eyes, 30 (53.6%) eyes were right eyes and 26 (46.4%) eyes were left eyes. Mean best-corrected visual acuity before cataract surgery was 0.34 ± 0.23 according to the logarithm of the minimum angle of resolution (logMAR) scale. Mean spherical equivalent before cataract surgery was −2.83 ± 3.52 D. Mean AL as measured by PCI was 27.04 ± 2.36 mm, mean K value using PCI before cataract surgery was 39.73 ± 2.33 D, and mean anterior chamber depth was 3.63 ± 0.34 mm (

Mean IOL power was 19.63 ± 2.67 D. Mean uncorrected visual acuity and best-corrected visual acuity 3 months after cataract surgery were 0.26 ± 0.33 and 0.05 ± 0.08 log-MAR, respectively. Mean spherical equivalent and astigmatism 3 months after cataract surgery were −1.32 ± 1.32 and −0.61 ± 0.49 D, respectively (

Comparison of the 20 different formulas revealed that the accuracy of the IOL calculation was more precise in cases using the Haigis formula with K values of the Scheimpflug system rather than those using the SRK/T formula (

In terms of absolute PEs and standard deviations of the Haigis-L, Shammas, Barrett True-K (no history), WKM, TCRP 4 mm K (Haigis), TNP 4 mm K (Haigis), and TRP 4 mm K (Haigis) formulae, values were 0.51 ± 0.44, 0.92 ± 0.74, 0.00 ± 0.88, 0.94 ± 0.74, 0.82 ± 0.7, 1.16 ± 0.77, and 0.89 ± 0.86 D, respectively (

The percentage of individuals in which each formula combination predicted between ±0.5 and ±1.0 D relative to the target refraction was also evaluated (

Cases were divided into two groups according to axial length (≤26.00, >26.00 mm), K (≤39.73, >39.73), or anterior chamber depth (≤3.5, >3.5 mm). Each subgroup was also evaluated in terms of numeric PEs, absolute PEs, and the number of eyes for which each formula's prediction was within ±0.5 D, and ±1.0 D of the actual refraction after surgery. Among subgroups, only the subgroup designated based on axial length showed a significant difference in numeric and absolute PEs. Haigis-L (

Calculation of corneal power and IOL power for patients who have undergone laser refractive surgery remains a challenge. In an effort to address this, the British National Health Service (NHS) proposed two benchmark standards for refractive outcomes following cataract surgery: that 55% of normal eyes be within ±0.5 D and 85% of normal eyes be within ±1.0 D of the targeted spherical equivalent [

The Maloney and WKM methods recalculate postoperative K values to the exact power present at the anterior corneal surface and then add an average negative power value for the posterior corneal surface [

Unlike previous studies that predominantly calculated IOL formulae using single corneal power (TNP) measured with the Pentacam system, we evaluated various different corneal K readings such as TCRP, TNP, and TRP, and included several zones (3, 4, 5 mm) [

Kim et al. [

In our study, the IOL calculation accuracy of 20 formulae (Haigis-L, Shammas, Barrett True-K [no history], WKM, TCRP 3 and 4 mm [SRK-T, Haigis], TNP 3, 4, and 5 mm [SRK-T, Haigis], and TRP 3, 4, and 5 mm [SRK-T, Haigis]) were analyzed and compared. The best K readings as measured using the Pentacam system were with TCRP 4 mm and TRP 4 mm. Especially, the TCRP (4 mm, Haigis) and TRP (4 mm, Haigis) methods provided comparably accurate IOL predictions for numeric PE (0.00 ± 1.09 and 0.03 ± 1.25 D, respectively), absolute PE (0.82 ± 0.7 and 0.89 ± 0.86 D, respectively), and the percentage of eyes within ±0.5 D (42.6% and 31.5%, respectively) and ±1.0 D (70.4% and 72.2%, respectively) of the refractive PE. Furthermore, there were no significant differences between the Haigis-L and Barrett True-K methods (

In our study, additional subgroup analysis revealed that using the same formula, IOL calculations for eyes with relatively long axial length and low corneal power (K) tended to be less accurate. This is important for especially long eyes, which are more likely to undergo refractive surgery because of myopia.

Limitations of this study include the use of different IOL models, specifically the Tecnis and AcrySof IQ monofocal IOL. However, as can be seen from the mean numeric and absolute PE calculations, there was no statistically significant differences between the two different IOLs (

We analyzed 20 formula combinations in our study. The most accurate method was the Haigis-L, with 85.7% of eyes within ±1.0 D of target refraction. TCRP (4 mm, Haigis) and TRP (4 mm, Haigis) predicted between 70.4% and 72.2% of eyes within ±1.0 D of target refraction without the use of prior clinical data. Only the Haigis-L and Barrett True-K (no history) methods were more accurate than TCRP and TRP among the 20 formulae. With regard to measurement of corneal power using the Pentacam, TCRP 4 mm (Haigis) and TRP 4 mm (Haigis) may possibly be useful for corneal power calculations and IOL power calculations in post-refractive surgery patients with no pre-operative clinical data available. Additional research is needed to improve the accuracy of IOL calculation methods based on regression formulae to modify corneal power using the Pentacam system (TCRP and TRP) with the Haigis formula.

PE = prediction error; D = diopter; TCRP = total corneal refractive power; TNP = true net power; TRP = total refractive power.

^{*}Mean arithmetic PE; ^{†}Mean absolute PE; ^{‡},^{§}Percentages of eyes within ±0.5 and ± 1.0 D of the target refraction.

PE = prediction error; D = diopter; TCRP = total corneal refractive power; TRP = total refractive power.

^{*}Mean arithmetic PE; ^{†}Mean absolute PE; ^{‡},^{§}Percentages of eyes within ±0.5 and ± 1.0 D of the target refraction.

PE = prediction error; D = diopter; TCRP = total corneal refractive power; TRP = total refractive power.

^{*}Mean arithmetic PE; ^{†}Mean absolute PE; ^{‡},^{§}Percentages of eyes within ±0.5 and ± 1.0 D of the target refraction.

PE = prediction error; D = diopter; TCRP = total corneal refractive power; TRP = total refractive power.

^{*}Mean arithmetic PE; ^{†}Mean absolute PE; ^{‡},^{§}Percentages of eyes within ±0.5 and ± 1.0 D of the target refraction.

Values are presented as mean ± standard deviation (range) or number.

OD = right eye; OS = left eye; UCVA = uncorrected visual acuity; logMAR = logarithm of the minimum angle of resolution; BCVA = best-corrected visual acuity; D = diopter; PCI = partial coherence interferometry; ACD = anterior chamber depth; K = keratometry.