Comparison of intraocular lens power calculation using a standard ultrasonic biometer and a new optical biometer

Faruk Kaya, Ibrahim Ko?ak,2, Ali Aydin, Hakan Baybora, Koray Karadayi

1Department of Ophthalmology, School of Medicine, Medipol University, cilar, Istanbul 34214, Turkey

2Department of Ophthalmology, Nisa Hospital, Fatih Cad, Bah?elievler, Istanbul 34196, Turkey

3Department of Ophthalmology, Near East University, Nicosia, Mersin 33000, Turkey

Correspondence to:Ibrahim Ko?ak. Nisa Hospital, Fatih Cad. Bah?elievler, Istanbul 34196, Turkey. ibrahimkocak@msn.com

Received： 2015-09-29　　Accepted： 2016-02-22

比較標準超聲波測量儀和新型光學生物測量儀計算人工晶狀體屈光度的準確性

Faruk Kaya1, Ibrahim Ko?ak1,2, Ali Aydin1, Hakan Baybora1, Koray Karadayi3

(作者單位：1土耳其，伊斯坦布爾大學，醫學院，眼科；2土耳其，伊斯坦布爾 34196，Bah?elievler，Fatih Cad，Nisa醫院，眼科；3土耳其，梅爾辛33000，尼科西亞，Near East大學，眼科)

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Comparison of intraocular lens power calculation using a standard ultrasonic biometer and a new optical biometer

Faruk Kaya1, Ibrahim Ko?ak1,2, Ali Aydin1, Hakan Baybora1, Koray Karadayi3

2Department of Ophthalmology, Nisa Hospital, Fatih Cad, Bah?elievler, Istanbul 34196, Turkey

3Department of Ophthalmology, Near East University, Nicosia, Mersin 33000, Turkey

Correspondence to:Ibrahim Ko?ak. Nisa Hospital, Fatih Cad. Bah?elievler, Istanbul 34196, Turkey. ibrahimkocak@msn.com

Received： 2015-09-29Accepted： 2016-02-22

比較標準超聲波測量儀和新型光學生物測量儀計算人工晶狀體屈光度的準確性

Faruk Kaya1, Ibrahim Ko?ak1,2, Ali Aydin1, Hakan Baybora1, Koray Karadayi3

Abstract

?AIM:To compare the intraocular lens (IOL) power calculations and refractive outcomes obtained with a new optical biometer and standard ultrasonic biometer in phacoemulsification surgery.

?METHODS:Thirty-seven eyes of 37 cataract patients who underwent phacoemulsification with IOL implantation were included in this prospective comparative study. The same operator performed biometer measurements in eyes with cataract using a new optical biometer (Aladdin) and a standard ultrasonic biometer (Sonomed AB 5500). Biometric parameters;axial length (AL), keratometric (K) readings, anterior chamber depth (ACD) and IOL power obtained by two devices were recorded. Postoperative actual refractive errors and errors predicted by two devices according to SRK/T formula were analyzed. The mean estimation error (EE), mean absolute estimation error (AEE) and the biometric parameters obtained by two biometers were compared.

?RESULTS:The AL measured by Aladdin (23.45±0.73 mm) was significantly longer than AL by ultrasonic biometer (23.2±0.75 mm) (P=0.01). The mean EE and AEE values obtained by Aladdin were significantly smaller than the values by ultrasonic biometer (P=0.0006 and 0.03 respectively). The higher percentage of eyes within ±0.5 and ±1.00 D of target refraction was also found by using Aladdin (67% and 97%).

?CONCLUSION:The Aladdin optical biometer showed better accuracy and yielded better refractive outcomes compared with ultrasonic biometer.

KEYWORDS:?Aladdin optical biometer;ultrasonic biometer;axial length;intraocular lens power calculation;postoperative refractive error

Citation:Kaya F;Ko?ak I;Aydin A;Baybora H;Karadayi K. Comparison of intraocular lens power calculation using a standard ultrasonic biometer and a new optical biometer.GuojiYankeZazhi(IntEyeSci) 2016;16(5):807-810

INTRODUCTION

Correct intraocular lens power calculation is important in cataract surgery for successful postoperative refractive outcomes. In cataract surgery ocular biometer is essential for intraocular lens (IOL) power calculation. Chosen IOL formula defines the result of IOL power. For ultrasonic biometers, most of IOL formulas require keratometer (K) values, and axial length (AL)[1-2]. AL measurement is important for predicting the accuracy of the IOL calculation and it is source of the largest proportion of inaccurate measurements. Immersion A-scan technique is presumably more accurate than contact A- scan ultrasonography as it needs no indentation of the cornea[3].

Optical biometer devices can measure some other variables. They are fast, non invasive and independent from technician experience. The built in software in these devices gives us more accurate IOL power calculation and multiple choices of IOL formulas[4-5]. Several new optical biometers can also perform ocular biometry and IOL power calculation as accurate as the first standard optical biometer such as IOL Master (Carl Zeiss Meditec AG, Jena, Germany)[6]. The Aladdin (Topcon, Tokyo, Japan) is one of the most recently released optical biometer. The device is an optical low coherence interferometer (OLCI) which can measure six variables;K value, AL, anterior chamber depth (ACD), white to white (WTW) diameter, pupil size, corneal topography. AL is measured using OLCI with 820 nm super-luminescent diode. ACD is measured using light emitting diode (LED) making horizontal slit projections across the anterior chamber, similarly to the IOL Master. Corneal topography and keratometer measurements are based on 24 Placido disk reflection[7]. The IOL power is calculated by five different formulas built into the device.

The purpose of this study is to evaluate and compare the difference of variables K value,AL, ACD and IOL power measured using ultrasonic biometer and optical biometer.

SUBJECTS AND METHODS

Subjects enrolled in this prospective comparative study were patients with cataract who were candidates for phacoemulsification with IOL implantation surgery at Ophthalmology Department of Nisa Hospital, Istanbul, Turkey between Nov. 17thand Dec. 29th, 2014. The study project was approved by Institutional Ethical Board of Istanbul Medipol University. All research and data collection adhered to the tenets of the Declaration of Helsinki. The study was explained to each patient and written informed consent was obtained.

Preoperatively all patients had a complete examination including manifest refraction,best-corrected visual acuity (BCVA) testing, intraocular pressure (IOP) measurements with applanation tonometer, slit lamp, and dilated fundus examinations. Ocular biometer was first performed by Aladdin optical biometer followed by ultrasonic biometer (Sonomed AB 5500, Lake Success, NY, USA). Patients with good quality Aladdin biometer measurements were included in the study.

Exclusion criteria were history of traumatic or uveitic cataracts, previous intraocular or corneal surgery (e.g. refractive surgery or glaucoma surgery), intraoperative complications (e.g. anterior or posterior capsule ruptures, vitreous loss or zonule dehiscence), or postoperative complications (e.g. tilted or decentered IOL).

Each patient underwent biometric measurement on Aladdin optical biometer by the same examiner (Kaya F). After carefully positioning of patient, Aladdin biometer was focused as determined by a clear view of anterior segment and the display of a ‘green eye’ quality control image. Six AL measurements, three K values and three ACD readings were obtained. The goal in IOL power selection was a value that would provide a postoperative refraction nearest to plano, staying on the side of myopia. The power selection of implanted IOLs was determined based on the SRK/T formula. Another experienced ophthalmologist (Ko?ak I) performed applanation ultrasound biometer after application of one drop topical anesthetic (proparacaine 0.5%). K values requested for ultrasonic biometer were measured using an autorefractometer (Topcon KR 8800, Tokyo, Japan). Mean of five AL and K measurements was used to calculate IOL power based on the SRK/T formula.

All phacoemulsification and IOL implantations were performed under topical anesthesia by one of three experienced surgeons (Kaya F, Ko?ak I, Aydin A). A standard phacoemulsification was performed through a 2.8 mm temporal clear corneal incision. The monoblock foldable hydrophobic acrylic IOL (Focus force F260, A-constant of 118.4, Zarracom, Turkey) was inserted into the capsular bag using an injector system.

By the end of first postoperative month, ophthalmological examination was carried out for all patients. Postoperative objective refractive error was measured by using autorefractometer. Uncorrected visual acuity (UCVA) and BCVA were also evaluated.

The estimation error (EE) was defined as the difference between the postoperative objective refractive error (spherical equivalent) and the preoperatively predicted refractive errors by two different biometers using SRK/T formulas for the power of IOL implanted. The absolute estimation error (AEE) was defined as the absolute value of the EE. For example, if postoperative objective error is -0.75D and preoperative predicted error is -0.12D, the EE is calculated as -0.75-(-0.12) =-0.63D. The AEE (the absolute value of EE) is [-0.63]=0.63D.

Paired studentt-test and Wilcoxon test are used for comparison of parameters obtained from two devices.We used studentt-test to compare K value, AL, ACD, EE and AEE as the groups had normal distribution. We used Wilcoxon test to compare percentages of eyes within target refraction for two devices as the groups did not have normal distribution. Statistical analysis was performed using the Statistical Package for Social Sciences (SPSS) version 12.0 (SPSS Inc., Chicago, Illinois, USA).P<0.05 were considered to be statistically significant.

Table 1Clinical characteristics of patients

ParameterMean±SDRangeAge(a)67.9±7.647to81Sex,n(%) M15(40%)- F22(60%)-Laterality,n(%) R19(51%)- L18(49%)-ImplantedIOLpower(D)21.8±2.0514.5to25

Table 2Comparison of biometric parameters measured by two devices

ParametersOpticalbiometerUltrasonicbiometer(Kvaluebyautoref.)aPK(D),Mean±SD(Range)43.3±1.4(40.4to46.7)43.4±1.5(40.2to47)P=0.78AL(mm),Mean±SD(Range)23.4±0.7(21.7to24.7)23.2±0.7(21.7to24.6)P=0.01ACD(mm),Mean±SD(Range)3.1±0.2(2.4to3.6)3.0±0.3(2.4to3.6)P=0.08

K:Keratometry;AL:Axial length;ACD:Anterior chamber depth;aStudentt-test.

Table 3The results concerning the estimation error, absolute estimation error, and percentages of eyes within target refraction for two devices

ParametersOpticalbiometerUltrasonicbiometerPEE(D)Mean±SD(Range)0.13±0.56(-1.16to0.97)0.36±0.59(-0.73to1.49)P=0.0006aAAE(D)Mean±SD(Range)0.42±0.30(0.02to1.16)0.59±0.35(0to1.49)P=0.03aEyeswithin±0.50D(SE)67%58%P=0.18bEyeswithin±1.00D(SE)97%88%P=0.14b

EE:Estimation error;AAE:Absolute estimation error;SE:Spherical equivalent;aStudentt-test;bWilcoxon test.

RESULTS

Forty eyes of 40 patients were included in the study. Three eyes were excluded due to the intra-operative complications such as posterior capsule rupture, vitreous loss and zonule dehiscence. Characteristics of patients are shown in Table 1.

Biometric parameters including K value, AL and ACD are shown on Table 2. The AL measured by Aladdin (23.4±0.7 mm) was significantly longer than AL by ultrasonic biometer (23.2±0.7mm) (P=0.01). The mean ACD measurement by Aladdin was also longer than the ACD by ultrasonic biometer, however the difference was not statistically significant (P=0.08). There was no statistically significant difference between two devices in measuring the mean K value (P=0.78).

The postoperative mean refractive error (spherical equivalent) was 0.13±0.66 D (range -1.50 to+1.25D). The results concerning the estimation error (EE), absolute estimation error (AEE), and percentages of eyes within target refraction for two devices are shown in Table 3. The mean EE obtained by using Aladdin was significantly smaller than EE by ultrasonic biometer (0.13±0.56 Dvs0.36±0.59 D) (P=0.0006). Also, the mean AEE obtained by using Aladdin was significantly smaller than AEE by ultrasonic biometer (0.42±0.30 Dvs0.59±0.35 D) (P=0.03). Aladdin optical biometer predicted more eyes with EE within ±0.50 and ±1.00 D of target refraction compared to ultrasonic biometer, however the differences were not statistically significant (P=0.18 andP=0.14).

DISCUSSION

Today’s cataract patients have greater expectations and request irreproachable results, especially about refractive subjects.IOL power calculation is essential for assessing the success of cataract surgery. Incorrect lens power calculation is the main cause of dissatisfaction and lens exchange in cataract surgery, especially in eyes with short or long axial lengths[8-11]. Ultrasound biometer is still used for IOL calculation;it is more favorable than optical biometer, especially in cases with dense cataract, media or corneal opacity and vitreous opacities. Optical biometer is a newer technology and has some advantages such as high accuracy, non contact and non invasive measurements, higher speed, higher patient comfort. As light has a very short wavelength compared to sound, the laser light has better resolution, because resolution improves as wavelength decreases. Therefore the accuracy of AL measured with ultrasound biometer is approximately 0.10-0.12 mm compared to 0.012 mm for measurement with optical biometer. Optical biometer is recommended to be used especially in cases those premium IOLs such as multifocal or toric IOLs will be used. But high cost of equipment, inability to measure dense cataracts, some corneal abnormalities and eyes with poor fixation are disadvantages of optical biometers[7].

Previous studies have compared two devices[3,12-21]. Fontesetal[3]found that both methods had high precision and reproducibility but there was a trend toward a subtle improvement in the prediction for postoperative refraction with optical biometer. Rajanetal[12]showed that optical biometer improved the predictive value for postoperative refraction in pseudophakic eyes. Kissetal[13], Packeretal[14], and Haigisetal[15]proved similar improvement in the prediction for postoperative refraction with optical biometer and US biometer.Bhattetal[16]showed similar results. Optical biometer was a beter predictor of postoperative refraction than US biometer, particularly within close ranges. In two study of Gantenbeinetal[17-18]and a study of Kutschan and Wiegand[19];the accuracy of postoperative refraction compared to the preoperative target was beter with US biometer compared to the optical biometer. In Gantenbeinetal’s[17-18]study optical biometer measured AL longer than US biometer and measured keratometry lower than Javal keratometer. Landers and Goggin[20]found that optical biometers predict more correct refractive outcomes compared to US biometer. In optical biometer, AL measurements were longer than US biometer, but they had similar results for ACD measurements. In another study, Nemethetal[21]showed that, AL and ACD was measured longer by optical biometer compared to US biometer.

Because of optical biometers’ high success in prediction of postoperative refraction, there is a widespread use of them for premium IOLs such as multifocal or toric IOLs. As we use commonly monofocal IOLs in our clinic, we wanted to evaluate and compared success of Aladdin and US biometer for monofocal IOLs.

In our study the Aladdin provided shorter ACD, but longer AL than US biometer. Difference in AL was statistically significant, but difference in ACD was not significant statistically. Most of previous studies also predicted longer ALs[17-18,20-21]. While US biometers use corneal epithelium and internal limiting membrane as reference points in measuring AL, optical biometers use the second principal plane of the cornea (0.05 mm deeper than the corneal apex) and photoreceptor layer (0.25 mm deeper than ILM). This may explain longer AL measurements with optical biometers. The power selection of implanted IOLs was determined based on the SRK/T formula, because it was one of the most common used formulas and it was found as one of the most successful formulas in determining postoperative refractive error[22]. Although there was no statistically significant difference, the Aladdin optical biometer obtained better refractive outcomes correlated with some previous studies[3,16,20]and predicted more eyes with EE within target diopters when compared US biometer. Also Aladdin optical biometer predicted smaller EE and AEE than ultrasonic biometer and the difference was statistically significant.

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摘要

關鍵詞：Aladdin光學生物測量儀；超聲波測量儀；眼軸長；人工晶狀體屈光度數；術后屈光不正

DOI:10.3980/j.issn.1672-5123.2016.5.04

通訊作者：Ibrahim Ko?ak. ibrahimkocak@msn.com

目的：比較超乳手術時應用新型光學生物測量儀和標準超聲波測量儀計算人工晶狀體的屈光度及屈光結果。

方法：前瞻性研究。研究包含37例37眼白內障患者接受白內障超聲乳化聯合人工晶狀體植入術。同一測量人員分別使用新型光學生物測量儀(Aladdin)與標準超聲波測量儀(Sonomed AB 5500)對白內障患者進行檢測。通過這兩種設備記錄生物測定參數，包括眼軸長度、角膜曲率、前房深度及人工晶狀體屈光度數。分析術后實際屈光不正與兩臺設備根據SRK/T公式計算的誤差，比較兩臺設備檢查結果的平均估計誤差(EE)、平均絕對估計誤差(AEE)及生物測定參數。

結果：Aladdin測量儀(23.45±0.73 mm)較超聲波測量儀(23.2±0.75 mm)檢測出的眼軸顯著較長(P=0.01)。Aladdin測量儀的EE與AEE均明顯小于超聲波測量儀(P=0.0006與0.03)。應用Aladdin測量的大多數眼與目標屈光度相差在±0.5(67%)及±1.00(97%)以內。

結論：Aladdin光學生物測量儀較超聲波測量儀更精確，屈光結果更準確。

引用：Kaya F, Ko?ak I, Aydin A, Baybora H, Karadayi K. 比較標準超聲波測量儀和新型光學生物測量儀計算人工晶狀體屈光度的準確性.國際眼科雜志2016;16(5):807-810

·Original article·