Cataract surgery is rapidly shifting toward a refractive procedure. Surgeons have increased patients' expectations by offering intraocular lenses (IOLs) able to correct any kind of refractive error, including presbyopia. This approach requires more accurate measurements of biometric variables for IOL power calculation. For this reason, a large number of manufacturers have entered the optical biometry market. Since the IOLMaster (Carl Zeiss Meditec, Jena, Germany) was first introduced in 1999, several other instruments have been produced for clinical use: the Lenstar LS900 (Haag-Streit, Köniz, Switzerland), the Aladdin (Topcon Europe, Visia Imaging, San Giovanni Valdarno, Arezzo, Italy), the AL-Scan (Nidek Co. Ltd., Gamagori, Japan), the Argos (Movu, Santa Clara, CA), the Galilei G6 (Ziemer, Port, Switzerland), the OA-2000 (Tomey, Nagoya, Japan), and the Pentacam AXL (Oculus Optikgeräte, Wetzlar, Germany). These devices rely on different technologies to measure axial length, keratometry, anterior chamber depth (from corneal epithelium to lens) and other values including lens thickness, central corneal thickness, and corneal diameter. Surprisingly, notwithstanding the great technological efforts invested, only a few studies investigated the clinical performance of most of these instruments, the only exceptions being the IOLMaster and the Lenstar LS900, which were the first to be marketed.1–9 The majority of studies have thus far focused on measurement agreement between instruments or their repeatability and reproducibility, but the main purpose for which they were developed (ie, IOL power calculation) has received less attention and has been analyzed in few cases for the remaining biometers.10–12
This study was primarily designed to assess the accuracy of the measurements by the OA-2000 optical biometer for IOL power calculation. As a secondary outcome, we compared the outcomes of IOL power calculations based on the measurements of this instrument to those based on IOLMaster 500 measurements.
Patients and Methods
This was a multicenter interventional study. Before being included in the study, all patients were informed of its purpose and gave their written consent. The study methods adhered to the tenets of the Declaration of Helsinki for the use of human participants in biomedical research. The research protocol was reviewed and approved by the Ethics Committee of G.B. Bietti Foundation IRCCS, Rome, Italy.
Patients and Surgery
Consecutive patients having cataract surgery were enrolled between April 2015 and April 2016 in six centers: Studio Oculistico d'Azeglio (Bologna, Italy), Clínica Begitek (San Sebastian, Spain), Aarhus Hospital (Aarhus, Denmark), St. Mary's Eye Center (Santa Monica, California), Shammas Eye Medical Center (Los Angeles, California), and the Eye Hospital of Wenzhou Medical University (Wenzhou, Zhejiang, China). Phacoemulsification was performed through a temporal near-clear 2.2- to 2.4-mm incision under topical anesthesia. Different IOL models were used and recorded according to the surgeon's preference and patient needs. However, because constant optimization should be performed separately for each IOL model,13 only the IOL model used in the largest sample of patients was selected and the eyes implanted with that IOL were subsequently analyzed. In patients undergoing bilateral surgery, only the first eye was enrolled and subsequently analyzed.
Exclusion criteria were: prior corneal or intraocular surgery, keratoconus and any other corneal disease, contact lens use during the past month, capsular rupture during surgery, and postoperative corrected distance visual acuity worse than 0.5 (Snellen 20/40) for any reason.
Preoperatively, all patients underwent optical biometry with the OA-2000 (software version 1.0R). This instrument combines an optical biometer, based on swept-source optical coherence tomography (SSOCT), and a Placido-ring topographer. It can measure axial length, keratometry, anterior chamber depth, lens thickness, corneal diameter, central corneal thickness, and pupil diameter. Its repeatability and reproducibility have already been assessed by Huang et al.14 The SS-OCT uses a wavelength of 1,060 nm. Placido disk corneal topography can simultaneously measure the radius of curvature of the cornea at diameters of 2.5 and 3 mm and corneal power is calculated by using the keratometric index of n = 1.3375. For the purposes of this study, the 2.5-mm diameter was selected and the mean (Km) of the flattest (Kf) and steepest (Ks) meridian was recorded.
In patients from two centers, optical biometry was also performed using a second device, the IOLMaster 500 (software version 5.2.1). It uses partial coherence interferometry with a 780-nm laser diode infrared light to measure axial length, whereas anterior chamber depth is measured through a lateral slit-illumination. The keratometry readings are calculated by analyzing the anterior corneal curvature at six reference points in a hexagonal pattern at approximately the 2.3-mm diameter central cornea.
IOL Power Calculation and Constant Optimization
Preoperatively, IOL power was calculated using the Hoffer Q, Holladay 1, and SRK/T formulas.15–17 A final evaluation was performed by assessing the subjective spherical equivalent refractive outcomes at least 1 month postoperatively, which is when refractive stability can be expected with small-incision clear cornea surgery and this type of IOL.18 The distance from the patient's eye to the acuity chart was 6 m. To calculate the prediction error in refraction, the spherical equivalent refraction was subtracted from the predicted refraction based on the IOL power actually implanted according to each formula. The mean prediction error, the median absolute error, and the mean absolute error were calculated, as well as the percentage of eyes with a prediction error of ±0.50 diopters (D) or less.13
Predictions made using the Hoffer Q, Holladay 1, and SRK/T formulas were optimized in retrospect by adjusting each formula's lens constants to give a prediction error of zero for the series, according to the method described by Hoffer et al.,15 Hoffer,19 and Olsen.20 As a result, it was possible to evaluate the statistical error as representing the optimum prediction error, rather than offset errors related to incorrect lens constants or systematic errors in the measuring environment. Optimization was performed using Hoffer Programs software (version 2.5).
All statistical analyses were performed using Instat (software version 3.1; Graphpad Software Inc., La Jolla, CA). Normal distribution of data was assessed by the Kolmogorov–Smirnov test. A paired t test was used to compare the axial length and keratometry values in the subsample of patients where measurements were taken by the two optical biometers. Because of the non-normal distribution of the absolute prediction errors, the Wilcoxon test was used to compare the prediction error of each formula with the two biometers. A P value of .05 was considered statistically significant.
Based on power and sample size calculations performed using the PS program (version 3.0.12; Dupont WD, Plummer WD Jr. PS: Power and Sample Size Calculation, version 3.0. Nashville, TN, Department of Biostatistics, Vanderbilt University, 2012. Available at: http://biostat.mc.vanderbilt.edu/twiki/bin/ view/Main/PowerSampleSize), it was estimated that a sample size of 21 eyes would be necessary to detect a difference in median absolute error of 0.05 D with a power of 95% at a significance level of 5%, given a within-subject standard deviation for simulated keratometry equal to 0.06 D.14
Accuracy of the OA-2000 Over the Whole Sample
A total of 841 consecutive eyes (573 patients) with both preoperative and postoperative measurements were investigated. Because of bilateral surgery, 268 eyes were excluded. Moreover, 26 eyes were excluded because of postoperative corrected distance visual acuity worse than 20/40 (due to retinal or corneal pathologies), 12 because of previous corneal refractive surgery, 6 because of dense cataract precluding axial length measurement, 4 because of keratoconus, and 2 because of capsular rupture.
In the remaining 523 eyes, 17 different IOL models had been implanted. The largest sample of one IOL model was with the AcrySof SN60WF (Alcon Laboratories, Inc., Fort Worth, TX), implanted by 6 surgeons in 249 eyes of 249 patients (mean age: 73.4 ± 8.6 years; range: 51 to 91 years; females: 144 [58%]). The primary analysis was done using this sample. For this group, the mean axial length was 23.67 ± 1.23 mm (range: 21.11 to 28.28 mm); the mean keratometry value was 43.71 ± 1.59 D (range: 39.50 to 48.24 D), and the mean power of the implanted IOL was 21.03 ± 3.05 D (range: 8.50 to 28.00 D).
The optimized constants were 5.58, 1.81, and 118.97 for the Hoffer Q, Holladay 1, and SRK/T formulas, respectively. The lowest median absolute error was obtained with the Holladay 1 (0.33 D), followed by the Hoffer Q (0.34) and the SRK/T (0.35 D) formulas. Figure 1 shows the distribution of the absolute prediction error, which ranged between 0 and 1.41 with the Holladay 1 formula, between 0 and 1.45 with the Hoffer Q formula, and between 0 and 1.84 with the SRK/T formula. The Friedman test failed to detect any statistically significant difference among the three formulas (P = .0765). The rate of eyes with a prediction error of ±0.50 D or less was 71.5% with the Hoffer Q, 69.1% with the Holladay 1, and 67.1% with the SRK/T formula.
Distribution of the absolute prediction error in refraction with the three intraocular lens power calculation formulas and the OA-2000 (Tomey Corporation, Nagoya, Japan) measurements. D = diopters
Comparison Between the OA-2000 and IOLMaster 500
Eighty-seven eyes of 87 patients (mean age: 73.2 ± 8.6 years, females: 48 [55.1%]) underwent optical biometry with both the OA-2000 and IOLMaster 500. The mean axial length and keratometry values for the OA-2000 were 23.40 ± 1.09 mm and 44.01 ± 1.59 D, respectively, and 23.42 ± 1.08 mm and 43.99 ± 1.60 D, respectively, for the IOLMaster. These differences were not statistically significant (P = .5418 and .1749, respectively, for axial length and keratometry). In this sample, the mean power of the implanted IOL was 21.0 ± 2.71 D (range: 11.50 to 26.50 D).
The absolute prediction error was lower with the OA-2000 than with the IOLMaster 500 (Table 1); the difference was statistically significant with the Hoffer Q (P = .0377), Holladay 1 (P = .0191), and SRK/T (P = .0087) formulas.
Comparison Between the Refractive Outcomes of IOL Power Calculation Based on the Measurements Provided by the Two Optical Biometers in 87 Eyes
Figure 2 shows the distribution of the absolute prediction error with each formula for both devices. Accordingly, all formulas provided a higher percentage of eyes with a prediction error of ±0.50 D or less when the calculation was based on the measurements of the OA-2000 as opposed to the IOLMaster 500.
Distribution of the absolute prediction error in refraction with the three intraocular lens power calculation formulas and the measurements from the OA-2000 (Tomey Corporation, Nagoya, Japan) and IOLMaster 500 (Carl Zeiss Meditec, Jena, Germany). D = diopters
The data from this multicenter study suggest that the axial length and keratometry measurements provided by the OA-2000 can produce clinically accurate IOL power calculations when entered into standard IOL power formulas. Across the whole sample, the prediction error was within ±0.50 D in approximately 70% of cases, which is well above the 55% value established as the benchmark standard by the National Health Service of the United Kingdom.21 Results might have been even better with other formulas, such as those by Barrett22 and Olsen and Hoffmann,23 because these have recently been shown to be accurate in calculating IOL power.7,9 However, it should be highlighted that this study did not aim to investigate formula accuracy, but rather to assess the accuracy of the measurements by this newer SS-OCT optical biometer for IOL power calculation.
The comparison between the IOLMaster and the OA-2000 reveals that the latter gives a lower median absolute error and higher rate of eyes with a prediction error of ±0.50 D or less, thus suggesting a higher accuracy of the SS-OCT technology. Given that the formulas tested in this study are based on keratometry and axial length measurements only and that agreement between the two devices is higher for axial length than for keratometry,14 we can logically assume that the different outcomes depend on the different technologies used to measure the anterior corneal curvature (the formula used to convert the curvature into power is the same and uses the 1.3375 keratometric index). We can suppose that the OA-2000 takes advantage of the Placido disk corneal topography, which provides much more data compared to the six points measured by the IOLMaster 500. However, a comparison based on a larger sample is necessary to confirm our results.
With respect to previously published studies, our data show that the OA-2000 leads to results similar to those provided by the IOLMaster 500 and Lenstar (Table 2), although the comparison in most cases is difficult because the authors did not report the median absolute error but only the mean absolute error in refraction prediction, which is no longer considered appropriate because absolute errors do not follow a normal Gaussian distribution.13 Our outcomes are close to those reported in other studies using the IOLMaster, which found a prediction error of ±0.50 D or less in between 56% and 77% of eyes and a mean absolute error between 0.34 and 0.46 D.2–5,7 In a sample of more than 3,000 eyes, Kane et al. used the IOLMaster 500 and reported a median absolute error of 0.35 D for the Hoffer Q, 0.33 D for the Holladay 1, and 0.33 D for the SRK/T formula.9 The rate of eyes with a prediction error of ±0.50 D or less was 67.2%, 69.4%, and 69.6%, respectively. These values are close to ours. With respect to the Lenstar, our data lie between those reported by Hoffmann and Lindemann8 and Hoffer et al.5
Accuracy of Biometric Measurements for IOL Power Calculation With Different Optical Biometers and Formulas
The optimized lens constants for the OA-2000 in the whole sample (Hoffer Q = 5.58; Holladay 1 = 1.81; SRK/T = 118.97) are close to those reported by the ULIB website ( http://ocusoft.de/ulib/c1.htm) (Hoffer Q = 5.64; Holladay 1 = 1.84; SRK/T = 119.00, accessed April 13, 2017) for the same IOL model and the IOLMaster. The similarity of constants reflects the previously reported high agreement between the measurements (keratometry and axial length) provided by the two devices.14
The main limitations of this study are the fact that we used only one other optical biometer to calculate the IOL power in the same sample and that the comparison was performed on a subset of patients and not across the whole sample. A comparison on a larger sample would be necessary to confirm whether the results of the two biometers are different. Moreover, we did not investigate two additional measurements provided by the OA-2000 (ie, lens thickness and anterior chamber depth). These would have enabled us to use other formulas (eg, those described by Barrett,22 Olsen and Hoffmann,23 and Haigis et al.24), but, as mentioned previously, a comparison of formulas was not the purpose of the current study. Finally, we did not investigate corneal topography–derived corneal asphericity, which has been shown to influence IOL power calculation.25
Our data show that the OA-2000 optical biometer is a reliable instrument for accurately calculating IOL power. Compared to the IOLMaster, it seems to provide surgeons with measurements leading to more predictable results.
- Aristodemou P, Knox Cartwright NE, Sparrow JM, Johnston RL. Formula choice: Hoffer Q, Holladay 1, or SRK/T and refractive outcomes in 8108 eyes after cataract surgery with biometry by partial coherence interferometry. J Cataract Refract Surg. 2011;37:63–71. doi:10.1016/j.jcrs.2010.07.032 [CrossRef]
- Findl O, Drexler W, Menapace R, Heinzl H, Hitzenberger CK, Fercher AF. Improved prediction of intraocular lens power using partial coherence interferometry. J Cataract Refract Surg. 2001;27:861–867. doi:10.1016/S0886-3350(00)00699-4 [CrossRef]
- Olsen T. Improved accuracy of intraocular lens power calculation with the Zeiss IOLMaster. Acta Ophthalmol Scand. 2007;85:84–87. doi:10.1111/j.1600-0420.2006.00774.x [CrossRef]
- Srivannaboon S, Chirapapaisan C, Chirapapaisan N, Lertsuwanroj B, Chongchareon M. Accuracy of Holladay 2 formula using IOLMaster parameters in the absence of lens thickness value. Graefes Arch Clin Exp Ophthalmol. 2013:251:2563–2567. doi:10.1007/s00417-013-2439-8 [CrossRef]
- Hoffer KJ, Shammas HJ, Savini G. Comparison of 2 instruments for measuring axial length. J Cataract Refract Surg. 2010;36:644–648. Erratum in: J Cataract Refract Surg. 2010;36:1066. doi:10.1016/j.jcrs.2009.11.007 [CrossRef]
- Olsen T. Use of fellow eye data in the calculation of intraocular lens power for the second eye. Ophthalmology. 2011;118:1710–1715. doi:10.1016/j.ophtha.2011.04.030 [CrossRef]
- Cooke DL, Cooke TL. Comparison of 9 intraocular lens power calculation formulas. J Cataract Refract Surg. 2016;42:1157–1164. doi:10.1016/j.jcrs.2016.06.029 [CrossRef]
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- Kaswin G, Rousseau A, Mgarrech M, Barreau E, Labetoulle M. Biometry and intraocular lens power calculation results with a new optical biometry device: comparison with the gold standard. J Cataract Refract Surg. 2014;40:593–600. doi:10.1016/j.jcrs.2013.09.015 [CrossRef]
- Savini G, Hoffer KJ, Barboni P, Balducci N, Schiano-Lomoriello D, Ducoli P. Accuracy of optical biometry combined with Placido disc corneal topography for intraocular lens power calculation. PLoS One. 2017;12:e0172634. Correction: PLoS One. 2017;12:e0175145. doi:10.1371/journal.pone.0172634 [CrossRef]
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- Huang J, Savini G, Hoffer KJ, et al. Repeatability and interobserver reproducibility of a new optical biometer based on swept-source optical coherence tomography and comparison with IOLMaster. Br J Ophthalmol. 2017;101:493–498. doi:10.1136/bjophthalmol-2016-308352 [CrossRef]
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Comparison Between the Refractive Outcomes of IOL Power Calculation Based on the Measurements Provided by the Two Optical Biometers in 87 Eyes
|Optimized Constant||PE (D)||MedAE (D)||≤ ±0.50 D||Optimized Constant||PE (D)||MedAE (D)||≤ ±0.50 D|
Accuracy of Biometric Measurements for IOL Power Calculation With Different Optical Biometers and Formulasa
|Study||Instrument||IOL Model||Sample Size||Formula||MedAE (D)||MAE (D)||PE ≤ ±0.50 D|
|Current study||OA-2000||Acrysof SN60WF||249||Hoffer Q||0.34||0.39||71.5%|
|Findl et al. (2001)2||IOLMaster||N/A||77||Holladay 1||N/A||0.44||N/A|
|Olsen (2007)3||IOLMaster||AcrySof MA60AC||461||Olsen||N/A||0.43||62.5%|
|Srivannaboon et al. (2013)4||IOLMaster||Hoya PY-60AD||163||Hoffer Q||0.32||0.40||68%b|
|Hoffer et al. (2010)5||IOLMaster||Acrysof SN60WF||50||Haigis||N/A||0.46||56%|
|Olsen (2011)6||Lenstar||AcrySof SA60AT||Unknown||Olsen||N/A||0.42||N/A|
|Cooke & Cooke (2016)7||IOLMaster||Acrysof SN60WF||1,079||Barrett||0.25||0.31||80.6%|
|Hoffmann & Lindemann (2013)8||Lenstar||3 IOL modelsc||308||Holladay 1||0.26||0.31||79.2%|
|Lenstar||Acrysof SN60WF||82||Holladay 1||0.25||0.31||N/A|
|Kane et al. (2016)9||IOLMaster||Acrysof SN60WF||3,241||Barrett||0.30||0.38||72.3%|