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Agreement of anterior ocular biometric measurements with a new optical biometer and a Scheimpflug tomographer
ARTICLE
Agreement of anterior ocular biometric measurements with a new optical biometer and a Scheimpflug tomographer Colm McAlinden, MB BCh, PhD, Fangjun Bao, MD, Yan Guo, MD, Xinxin Yu, MD, Weicong Lu, MD, Hao Chen, MD, Qinmei Wang, MD, Jinhai Huang, MD
PURPOSE: To evaluate the agreement between a new optical biometer (AL-Scan) and an anteriorsegment Scheimpflug tomographer (Pentacam HR). SETTING: Eye Hospital of Wenzhou Medical University, Zhejiang, China. DESIGN: Prospective randomized comparative case series. METHODS: Three consecutive scans of the right eye of each subject were obtained using each device in a random order. The following common parameters were recorded: central corneal thickness (CCT), anterior chamber depth (ACD), aqueous depth, and keratometry (K) readings (steep, flat, and mean). Bland-Altman limits of agreement (LoA) were calculated for the common device parameters. RESULTS: The study comprised 121 healthy subjects (57 men, 64 women) with a mean age of 25.41 years G 6.86 (SD) (range 18 to 54 years). The mean difference in CCT was 1.61 mm with LoA of 6.29 to 9.52 mm. The mean difference in the aqueous depth and ACD was 0.02 mm with LoA of 0.10 to 0.14 mm and 0.09 to 0.14 mm, respectively. For the 2.4 mm K readings from the new biometer versus the standard K readings from the Scheimpflug tomographer, the mean difference for flat K, steep K, and mean K was 0.10 diopter (D), 0.05 D, and 0.08 D, respectively. The LoA were 0.17 to 0.38 D, 0.29 to 0.39 D, and 0.19 to 0.34 D, respectively. The CCT, ACD, and all K readings were assessed to be clinically interchangeable. CONCLUSION: There was good agreement between the 2 devices for CCT, ACD, aqueous depth, and K readings. Financial Disclosure: None of the authors has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2016; 42:679–684 Q 2016 ASCRS and ESCRS Supplemental material available at www.jcrsjournal.org.
Accurate biometry is essential to ensure accurate intraocular lens (IOL) power calculation before cataract surgery.1–3 Although advantageous in certain clinical situations, ultrasound (US) has been largely superseded by optical biometers such as the IOLMaster (Carl Zeiss Meditec AG),4 Lenstar LS 900 (Haag-Streit AG),5 Galilei G6 (Ziemer Ophthalmic Systems),6 OA1000 (Tomey Corp.),7 Aladdin (Topcon Corp.),8 and AL-Scan (Nidek Co., Ltd.).9 These noncontact devices are better than US in terms of the ease of use, speed of acquisition, patient comfort, and reduced risk for Q 2016 ASCRS and ESCRS Published by Elsevier Inc.
trauma and infection. They also provide improved prediction of IOL power requirements.10 The AL-Scan is a relatively new optical biometer implementing both partial coherence interferometry (PCI) and Scheimpflug imaging.9 It imparts the following 6 measurements: axial length (AL), corneal curvature (at 2.4 mm and 3.3 mm), anterior chamber depth (ACD), central corneal thickness (CCT), whiteto-white (WTW) distance, and pupil size. Initial studies of this device9,11,12 show good agreement with the longstanding IOLMaster in terms of AL, http://dx.doi.org/10.1016/j.jcrs.2016.01.043 0886-3350
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ACD, and keratometry (K) readings; however, poor agreement has been reported for WTW measurements.11,12 Studies11–14 also show the new biometer has good repeatability and reproducibility, with the exception of WTW12,14 and pupil size12 in healthy eyes and steep K measurements in eyes with keratoconus.14 The Pentacam HR (Oculus Optikger€ ate GmbH) is an anterior segment tomographer capable of providing several measurements throughout the anterior segment of the eye. It has been found to be highly precise in terms of repeatability and reproducibility.15,16 Akin to the AL-Scan biometer, it uses Scheimpflug imaging and provides the same measures with the exception of AL and WTW. Intraocular lens formulas, which are subject to errors and assumptions, use device measurements to estimate the required IOL at the time of cataract surgery. Accordingly, it is imperative that these measurements are as precise as possible. Aside from IOL calculations, the new biometer provides 6 measurements that could be used in other clinical situations, such as CCT for keratoconus management and glaucoma assessment. New optical devices should thus be compared with existing robust devices to assess agreement. The present study assessed the agreement between the new AL-Scan biometer and the Pentacam HR Scheimpflug tomographer for the common measurement parameters in a group of normal healthy eyes.
Submitted: October 7, 2015. Final revision submitted: January 19, 2016. Accepted: January 22, 2016. From the School of Ophthalmology and Optometry (McAlinden, Bao, Guo, Yu, Lu, Chen, Wang, Huang), Wenzhou Medical University, and the Key Laboratory of Vision Science (Bao, Lu, Chen, Wang, Huang), Ministry of Health P.R. China, Wenzhou, Zhejiang, China; the Abertawe Bro Morgannwg University Health Board (McAlinden), Swansea, United Kingdom; Flinders University (McAlinden), Adelaide, South Australia, Australia. Drs. McAlinden and Bao contributed equally to this work. Supported in part by the National Natural Science Foundation of China (81300807, 81300804), Foundation of Wenzhou City Science & Technology Bureau (J20140014, Y20150076), National Science and Technology Major Project (2014ZX09303301), the Health Bureau of Zhejiang Province (2016RCB013), and the Science and Technology Planning Project of Zhejiang Province (206C33082). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Corresponding author: Jinhai Huang, MD, Eye Hospital of Wenzhou Medical University, 270 West Xueyuan Road, Wenzhou, Zhejiang, 325027 China. E-mail: [email protected].
SUBJECTS AND METHODS Subjects This prospective study comprised eyes of healthy subjects. Subjects with a history of corneal pathology, contact lens use, previous ocular surgery, or other anterior segment abnormalities were excluded from this study. The study was approved by the Eye Hospital Review Board of Wenzhou Medical University and was performed according to the tenets of the Declaration of Helsinki for research involving human subjects. Informed consent was obtained from all participating subjects.
Instruments The AL-Scan biometer (version 1.03) uses a 970 nm lightemitting diode (LED) in the assessment of K readings and pupil size and a 525 nm LED in the assessment of WTW. Corneal power is measured from the reflection of double mires projected onto the cornea at 2.4 mm and 3.3 mm diameters. The flattest K reading and steepest K reading for each of the 2 diameters were averaged and recorded as the mean K reading. Scheimpflug imaging with a 470 nm monochromatic light from an LED is used to determine the CCT and ACD. The CCT is reported with respect to the pupil center. The WTW and pupil size are determined by fitting the best circle with the lowest error square to the detected edge. The optical path length measurements are aligned with the visual axis for each measurement. This biometer uses PCI to measure AL.12 The Pentacam HR tomographer (version 1.20) also uses the Scheimpflug principle with a 475 nm monochromatic slit of light to illuminate the cornea and a 1.45 megapixel camera for photography that rotates around the fixation line during the scan. The 25-picture scan was chosen for this study. The device calculates a 3-dimensional model of the anterior segment from up to 138 000 true elevation points.15 The Scheimpflug tomographer can produce a CCT estimate at any point on the cornea. It also provides the CCT at the pupil center, corneal apex, and thinnest point. Because the AL-Scan biometer reports the CCT at the pupil center only, the pupil center CCT produced by the Scheimpflug tomographer was used in this study. The K readings produced by the Scheimpflug tomographer are from a 15-degree ring centered on the corneal apex.
Measurement Technique The right eye of each subject was chosen to avoid statistical bias. The measurement order for each device was random. The random sequence was generated using Medcalc Statistical Software (version 13.0, Medcalc Software, Inc.). Three consecutive scans were performed with each device by the same experienced examiner (X.Y.). All measurements were performed without pupil dilation. Subjects were seated and positioned comfortably with a chinrest and headrest. Subjects were instructed to fixate on the internal target within each device. Fine adjustments of the chinrest were made to ensure the lateral canthus of the eye was in line with the side markers on the holding bars of the headrest. Subjects were asked to perform a rapid, complete blink just before measurements were taken to ensure that an optically smooth tear film would be spread over the cornea. All measurements were performed between 10 AM and 5 PM, and each set of measurements from an individual patient
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was completed within 20 minutes. Scans that were substandard because of blinking or eye movements were discarded and performed a second time. Both devices can measure aqueous depth, which is the distance from the corneal endothelium to the anterior lens capsule, and the ACD, which is the distance from the corneal epithelium to the anterior lens capsule.17
Statistical Analysis The mean G standard deviation (SD) for each of the common parameters reported for each device was calculated. The range for each parameter was also recorded. The Bland-Altman technique was used to assess agreement between the 2 devices.18 The agreement was summarized by determining the bias, which is estimated by the mean difference d, and the SD of the differences s. Most differences were expected to lie within d 1.96s and d C 1.96s. Provided the differences within d G 1.96s were not clinically significant, the 2 measurements could be used interchangeably. Bland-Altman plots (the difference between each parameter against their mean) were also produced displaying the 95% limits of agreement (LoA) between the 2 devices for the common parameters and the 95% confidence intervals (CIs) [LoA G (2.26 standard error)]. Calculations were performed using Excel software (version 2010, Microsoft Corp.) and SPSS software (version 21.0, International Business Machine Corp.). Bland-Altman plots were produced using Medcalc Statistical Software.
RESULTS The study comprised 121 healthy subjects (57 men and 64 women). The mean age of the subjects was 25.41 years G 6.86 (SD) (range 18 to 54 years). The mean spherical equivalent refraction was 4.67 G 2.36 diopters (D) (range 0.50 to 11.75 D). Table 1 shows the mean and range of values for each parameter by device. Table 2 shows the mean differences, SD of the differences, and upper and lower LoA (with 95% CIs) of the 2 devices. Figures 1 to 3 show the Bland-Altman plots, as do
online-only Figures S1 to S6 (available at http:// jcrsjournal.org). The differences in the CCT and LoA between devices (Table 2) were assessed to be clinically interchangeable. The mean differences in the aqueous depth and ACD (Table 2) were assessed as clinically interchangeable. Considering the LoA with their 95% CIs (Table 2), all K readings were assessed to be clinically interchangeable. DISCUSSION Precise estimates of the parameters used within IOL formulas are necessary to ensure good visual and refractive outcomes as well as improved patientreported outcomes after cataract surgery.19–21 The present study compared measurements from the AL-Scan biometer with those obtained with the commonly used Pentacam HR Scheimpflug tomographer to assess agreement. Beginning with the CCT, the mean difference was 1.61 mm and the LoA were 6.29 to 9.52 mm (95% CI, 7.53 to 10.76). The repeatability limit (defined as 1.96O2 within-subject SD) provides the likely limits within which 95% of measurements are expected to occur.18 The previously reported repeatability limit for the Pentacam HR Scheimpflug tomographer for CCT measurements was 111.6 mm.15 The reason for this poor repeatability limit is probably because of the CCT being reported with respect to the pupil center and because in undilated eyes, the pupillary hippus is likely to have slightly altered the exact pupil center; in such cases, the CCT measurement would be at a different location on the cornea. This might be less of an issue with dilated pupils. The present study found an extremely small mean difference between the 2 devices and narrow LoA, even with the aforementioned limitations of pupil
Table 1. Mean G SD, maximum and minimum values for the parameters reported by the new biometer and Scheimpflug tomographer. AL-Scan Parameter CCT (mm) ACD (mm) Aqueous depth (mm) Kf (F Z 2.4) (D) Ks (F Z 2.4) (D) Km (F Z 2.4) (D) Kf (F Z 3.3) (D) Ks (F Z 3.3) (D) Km (F Z 3.3) (D)
Scheimpflug Tomographer
Mean G SD
Range
Mean G SD
Range
537.44 G 27.46 3.16 G 0.30 3.70 G 0.30 42.77 G 1.43 43.88 G 1.44 43.33 G 1.41 42.74 G 1.43 43.84 G 1.43 43.29 G 1.40
454.00, 592.67 2.44, 4.05 2.98, 4.59 38.93, 46.77 39.79, 47.60 39.73, 47.00 38.85, 46.81 39.71, 47.67 39.62, 47.03
535.82 G 26.47 3.14 G 0.29 3.67 G 0.29 d d d 42.67 G 1.43 43.83 G 1.44 43.25 G 1.40
459.67, 594.00 2.44, 3.91 2.98, 4.46 d d d 38.67, 46.57 39.83, 47.60 39.48, 46.73
F Z diameter; ACD Z anterior chamber depth; CCT Z central corneal thickness (over pupil center); Kf Z flattest keratometry; Km Z mean keratometry; Ks Z steepest keratometry
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Table 2. The mean difference G standard deviation, 95% LoA with 95% CIs for differences between the new biometer and Scheimpflug tomographer. Parameter CCT (mm) Aqueous depth (mm) ACD (mm) Kf (F Z 2.4) (D) Ks (F Z 2.4) (D) Km (F Z 2.4) (D) Kf (F Z 3.3) (D) Ks (F Z 3.3) (D) Km (F Z 3.3) (D)
Mean Difference G SD
P Value
1.61 G 4.03 0.02 G 0.06 0.02 G 0.06 0.10 G 0.14 0.05 G 0.17 0.08 G 0.14 0.06 G 0.12 0.01 G 0.18 0.04 G 0.13
!.001 !.001 !.001 !.001 !.001 !.001 !.001 .663 .003
Lower LoA (95% CI) 6.29 ( 0.10 ( 0.09 ( 0.17 ( 0.29 ( 0.19 ( 0.17 ( 0.34 ( 0.22 (
7.53 to 0.11 to 0.11 to 0.22 to 0.34 to 0.23 to 0.20 to 0.39 to 0.26 to
5.04) 0.10) 0.08) 0.13) 0.24) 0.15) 0.13) 0.29) 0.18)
Upper LoA (95% CI) 9.52 (8.27 to 10.76) 0.14 (0.12 to 0.16) 0.14 (0.12 to 0.16) 0.38 (0.34 to 0.42) 0.39 (0.33 to 0.44) 0.34 (0.30 to 0.38) 0.30 (0.26 to 0.33) 0.35 (0.30 to 0.41) 0.29 (0.25 to 0.34)
F Z diameter; ACD Z anterior chamber depth; CCT Z central corneal thickness (at pupil center); Kf Z flattest keratometry; Km Z mean keratometry; Ks Z steepest keratometry
center CCT measurements. Based on this, the 2 devices can be considered interchangeable for pupil center CCT; however, caution is advised, in particular when measuring eyes with undilated pupils. McAlinden et al.15 showed the repeatability limit for the corneal apex CCT with the Pentacam HR Scheimpflug tomographer as 10.4 mm and even more robust for the thinnest CCT (9 mm). For the ACD, the mean difference for both the aqueous depth and ACD was very small at 0.02 mm, with narrow LoA of 0.10 to 0.14 mm (95% CI, 0.11 to 0.16) and LoA of 0.09 to 0.14 mm (95% CI, 0.11 to 0.16), respectively, which again suggests these parameters are clinically interchangeable. In terms of K readings, the new biometer reports these at 2.4 mm and at 3.3 mm. The 2.4 mm diameter is similar to that of the IOLMaster biometer (Carl Zeiss Meditec AG) (2.5 mm), and the 3.3 mm diameter matches that of Nidek keratometers. The flat and steep
K readings were very similar at the 2 diameters. At 2.4 mm and 3.3 mm, the flat K readings were 42.77 D and 42.74 D, respectively. At 2.4 mm and 3.3 mm, the steep K readings were 43.88 D and 43.84 D, respectively. The Scheimpflug tomographer in our study reports K readings from a 15-degree ring centered on the corneal apex; the mean flat and steep K readings were 42.67 D and 43.87 D, respectively. The mean difference between the new biometer and Scheimpflug tomographer was small. The smallest mean difference was in the steep K readings at 3.3 mm with the new biometer and largest for the flat K reading at 2.4 mm. The LoA were small for all K readings, including the mean K readings. The reported repeatability limit for flat K, steep K, and mean K with the Scheimpflug tomographer was 0.25 D, 0.36 D, and 0.33 D, respectively. The LoA with the aforementioned 95% CI found in the present study are not significantly greater than the repeatability limits of the
Figure 1. Bland-Altman plots show the agreement between the new biometer and the Scheimpflug tomographer for measurements of the CCT. The solid line indicates the mean difference (bias), and the dotted lines indicate the 95% LoA.
Figure 2. Bland-Altman plots show the agreement between the new biometer and the Scheimpflug tomographer for measurements of the aqueous depth from the corneal endothelium to the anterior lens. The solid line indicates the mean difference (bias), and the dotted lines indicate the 95% LoA.
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Figure 3. Bland-Altman plots show the agreement between the new biometer and the Scheimpflug tomographer for measurements of the ACD from the corneal epithelium to the anterior lens. The solid line indicates the mean difference (bias), and the dotted lines indicate the 95% LoA.
Scheimpflug tomographer, and despite exact measurement location in terms of optical zone diameter differing between devices, we conclude that the K readings of the 2 devices are interchangeable. To our knowledge, this is the first study to compare the AL-Scan biometer and the Pentacam HR Scheimpflug tomographer; however, recent studies have compared the AL-Scan biometer and other devices. Kaswin et al.9 studied the agreement between the AL-Scan biometer and the IOLMaster 500 biometer in 50 eyes. They found a mean difference in the 2.4 mm mean K readings of 0.17 D between the 2 devices, which is significantly greater than the value in the present study (0.04 D). They found a mean difference of 0.13 D for ACD, which is also significantly greater than the mean difference of 0.02 D in the present study. However, they did not report the LoA for these parameters. Can et al.22 studied the effects of pupil dilation (with cyclopentolate) on the parameters of the AL-Scan biometer in 72 eyes. The AL and ACD were compared before and after pupil dilation. Only the ACD readings were significantly different before and after dilation, and only 2 cases in the study showed changes in IOL power higher than 0.5 D. The study advised that increases in ACD after dilation should be taken into account when performing refractive surgeries in which the ACD is very important, such as in anterior chamber phakic IOL implantation.22 Srivannaboon et al.11 also compared the AL-Scan biometer and the IOLMaster 500 biometer. They assessed the precision (repeatability and reproducibility) of the AL-Scan biometer in 137 eyes and found excellent repeatability and reproducibility as assessed by intraclass correlation coefficients (ICCs) and LoA. However, they did not use the recommended within-
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subject SD method,18 which would have permitted comparison with the precision of the Pentacam HR Scheimpflug tomographer.15 However Srivannaboon et al.11 found a mean difference and LoA for the same parameters reported in the present study. For 2.4 mm K readings, they found a mean difference and LoA of 0.09 D and 0.26 D to 0.44 D, respectively, which is very similar to the results in the present study. For 3.3 mm K readings, they found a mean difference and LoA of 0.08 D and 0.32 D to 0.48 D, respectively, which is also similar but slightly larger than in the present study (mean difference 0.04 D, LoA, 0.22 D to 0.29 D). For ACD, they found a mean difference of 0.02 D and LoA of 0.24 to 0.19 D, respectively, which is almost identical to our findings. Moon et al.23 compared the AL-Scan biometer and applanation US in 104 eyes. Biometry was performed with both devices in 104 eyes, and the SRK/T formula24 was used to calculate the required IOL power. Automatic refraction was performed 1 month postoperatively. There was no statistically significant difference in the AL measurement or postoperative refractive error between the 2 techniques. Huang et al.12 studied the precision of the AL-Scan biometer, and its agreement with the IOLMaster biometer. They found the AL-Scan was highly repeatable and reproducible, except for WTW and pupil diameter measurements. Good agreement between the devices was found for AL, ACD, and K readings. Compared with the IOLMaster biometer, the AL-Scan–derived K readings using a diameter of 2.4 mm showed narrower LoA than those obtained with a diameter of 3.3 mm. This might be because the IOLMaster biometer measures the K readings at a very similar diameter (2.5 mm). In comparison with the present study, there was no obvious difference in the LoA in IOL power calculations between the AL-Scan biometer using the 2.4 mm or 3.3 mm K readings and the Pentacam HR tomographer. Dervis¸o gulları et al.25 studied ACD measurements with the AL-Scan biometer and the Galilei dual Scheimpflug analyzer (Ziemer Ophthalmic Systems AG) in 63 normal eyes. Again, the ICC was used to assess the ACD measurements between 2 observers on each device (AL-Scan 0.996 and Galilei 0.968). However, the LoA were calculated between each observer for each device rather than the LoA between the 2 devices, limiting the usefulness of their study results. The main limitations of the present study are that the results are only applicable to healthy adult eyes with myopia. The results might not apply to patients with anterior segment eye disease, such as keratoconus, or eyes with previous surgery, such as refractive procedures, corneal crosslinking, and corneal transplantation. They might not apply to patients with
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hyperopia or to pediatric patients. Future research should address these limitations. In conclusion, the present study compared the AL-Scan biometer and the Pentacam HR tomographer and found good agreement between all common parameters. WHAT WAS KNOWN Scheimpflug imaging is widely used to measure the corneal thickness, ACD, and keratometry. An optical biometer was recently introduced that can measure these parameters. However, it is not known whether the values obtained with the 2 devices are comparable and can be used interchangeably. WHAT THIS PAPER ADDS The mean CCT, ACD, and K measurements were highly comparable between the new ocular biometry device and the validated Scheimpflug anterior segment tomographer. REFERENCES 1. Ianchulev T, Hoffer KJ, Yoo SH, Chang DF, Breen M, Padrick T, Tran DB. Intraoperative refractive biometry for predicting intraocular lens power calculation after prior myopic refractive surgery. Ophthalmology 2014; 121:56–60 2. Grulkowski I, Liu JJ, Zhang JY, Potsaid B, Jayaraman V, Cable AE, Duker JS, Fujimoto JG. Reproducibility of a long-range sweptsource optical coherence tomography ocular biometry system and comparison with clinical biometers. Ophthalmology 2013; 120:2184–2190 3. Eom Y, Kang S-Y, Song J-S, Kim HM. Use of corneal powerspecific constants to improve the accuracy of the SRK/T formula. Ophthalmology 2013; 120:477–481 4. Bang S, Edell E, Yu Q, Pratzer K, Stark W. Accuracy of intraocular lens calculations using the IOLMaster in eyes with long axial length and a comparison of various formulas. Ophthalmology 2011; 118:503–506 5. Huang J, McAlinden C, Su B, Pesudovs K, Feng Y, Hua Y, Yang F, Pan C, Zhou H, Wang Q. The effect of cycloplegia on the Lenstar and the IOLMaster biometry. Optom Vis Sci 2012; 89:1691–1696. Available at: http://journals.lww.com/optvissci/Fulltext/2012/12000/ The_Effect_of_Cycloplegia_on_the_Lenstar_and_the.6.aspx. Accessed February 6, 2016 6. Huang J, Ding X, Savini G, Pan C, Feng Y, Cheng D, Hua Y, Hu X, Wang Q. A comparison between Scheimpflug imaging and optical coherence tomography in measuring corneal thickness. Ophthalmology 2013; 120:1951–1958 7. Goebels SC, Seitz B, Langenbucher A. Reproducibility of the optical biometer OA-1000 (Tomey). Biomed Res Int 2014:814761. Available at: http://downloads.hindawi.com/journals/bmri/2014/ 814761.pdf. Accessed February 6, 2016 8. Mandal P, Berrow EJ, Naroo SA, Wolffsohn JS, Uthoff D, Holland D, Shah S. Validity and repeatability of the Aladdin ocular biometer. Br J Ophthalmol 2014; 98:256–258 9. 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
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Agreement of anterior ocular biometric measurements with a new optical biometer and a Scheimpflug tomographer Colm McAlinden, MB BCh, PhD, Fangjun Bao, MD, Yan Guo, MD, Xinxin Yu, MD, Weicong Lu, MD, Hao Chen, MD, Qinmei Wang, MD, Jinhai Huang, MD
PURPOSE: To evaluate the agreement between a new optical biometer (AL-Scan) and an anteriorsegment Scheimpflug tomographer (Pentacam HR). SETTING: Eye Hospital of Wenzhou Medical University, Zhejiang, China. DESIGN: Prospective randomized comparative case series. METHODS: Three consecutive scans of the right eye of each subject were obtained using each device in a random order. The following common parameters were recorded: central corneal thickness (CCT), anterior chamber depth (ACD), aqueous depth, and keratometry (K) readings (steep, flat, and mean). Bland-Altman limits of agreement (LoA) were calculated for the common device parameters. RESULTS: The study comprised 121 healthy subjects (57 men, 64 women) with a mean age of 25.41 years G 6.86 (SD) (range 18 to 54 years). The mean difference in CCT was 1.61 mm with LoA of 6.29 to 9.52 mm. The mean difference in the aqueous depth and ACD was 0.02 mm with LoA of 0.10 to 0.14 mm and 0.09 to 0.14 mm, respectively. For the 2.4 mm K readings from the new biometer versus the standard K readings from the Scheimpflug tomographer, the mean difference for flat K, steep K, and mean K was 0.10 diopter (D), 0.05 D, and 0.08 D, respectively. The LoA were 0.17 to 0.38 D, 0.29 to 0.39 D, and 0.19 to 0.34 D, respectively. The CCT, ACD, and all K readings were assessed to be clinically interchangeable. CONCLUSION: There was good agreement between the 2 devices for CCT, ACD, aqueous depth, and K readings. Financial Disclosure: None of the authors has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2016; 42:679–684 Q 2016 ASCRS and ESCRS Supplemental material available at www.jcrsjournal.org.
Accurate biometry is essential to ensure accurate intraocular lens (IOL) power calculation before cataract surgery.1–3 Although advantageous in certain clinical situations, ultrasound (US) has been largely superseded by optical biometers such as the IOLMaster (Carl Zeiss Meditec AG),4 Lenstar LS 900 (Haag-Streit AG),5 Galilei G6 (Ziemer Ophthalmic Systems),6 OA1000 (Tomey Corp.),7 Aladdin (Topcon Corp.),8 and AL-Scan (Nidek Co., Ltd.).9 These noncontact devices are better than US in terms of the ease of use, speed of acquisition, patient comfort, and reduced risk for Q 2016 ASCRS and ESCRS Published by Elsevier Inc.
trauma and infection. They also provide improved prediction of IOL power requirements.10 The AL-Scan is a relatively new optical biometer implementing both partial coherence interferometry (PCI) and Scheimpflug imaging.9 It imparts the following 6 measurements: axial length (AL), corneal curvature (at 2.4 mm and 3.3 mm), anterior chamber depth (ACD), central corneal thickness (CCT), whiteto-white (WTW) distance, and pupil size. Initial studies of this device9,11,12 show good agreement with the longstanding IOLMaster in terms of AL, http://dx.doi.org/10.1016/j.jcrs.2016.01.043 0886-3350
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ACD, and keratometry (K) readings; however, poor agreement has been reported for WTW measurements.11,12 Studies11–14 also show the new biometer has good repeatability and reproducibility, with the exception of WTW12,14 and pupil size12 in healthy eyes and steep K measurements in eyes with keratoconus.14 The Pentacam HR (Oculus Optikger€ ate GmbH) is an anterior segment tomographer capable of providing several measurements throughout the anterior segment of the eye. It has been found to be highly precise in terms of repeatability and reproducibility.15,16 Akin to the AL-Scan biometer, it uses Scheimpflug imaging and provides the same measures with the exception of AL and WTW. Intraocular lens formulas, which are subject to errors and assumptions, use device measurements to estimate the required IOL at the time of cataract surgery. Accordingly, it is imperative that these measurements are as precise as possible. Aside from IOL calculations, the new biometer provides 6 measurements that could be used in other clinical situations, such as CCT for keratoconus management and glaucoma assessment. New optical devices should thus be compared with existing robust devices to assess agreement. The present study assessed the agreement between the new AL-Scan biometer and the Pentacam HR Scheimpflug tomographer for the common measurement parameters in a group of normal healthy eyes.
Submitted: October 7, 2015. Final revision submitted: January 19, 2016. Accepted: January 22, 2016. From the School of Ophthalmology and Optometry (McAlinden, Bao, Guo, Yu, Lu, Chen, Wang, Huang), Wenzhou Medical University, and the Key Laboratory of Vision Science (Bao, Lu, Chen, Wang, Huang), Ministry of Health P.R. China, Wenzhou, Zhejiang, China; the Abertawe Bro Morgannwg University Health Board (McAlinden), Swansea, United Kingdom; Flinders University (McAlinden), Adelaide, South Australia, Australia. Drs. McAlinden and Bao contributed equally to this work. Supported in part by the National Natural Science Foundation of China (81300807, 81300804), Foundation of Wenzhou City Science & Technology Bureau (J20140014, Y20150076), National Science and Technology Major Project (2014ZX09303301), the Health Bureau of Zhejiang Province (2016RCB013), and the Science and Technology Planning Project of Zhejiang Province (206C33082). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Corresponding author: Jinhai Huang, MD, Eye Hospital of Wenzhou Medical University, 270 West Xueyuan Road, Wenzhou, Zhejiang, 325027 China. E-mail: [email protected].
SUBJECTS AND METHODS Subjects This prospective study comprised eyes of healthy subjects. Subjects with a history of corneal pathology, contact lens use, previous ocular surgery, or other anterior segment abnormalities were excluded from this study. The study was approved by the Eye Hospital Review Board of Wenzhou Medical University and was performed according to the tenets of the Declaration of Helsinki for research involving human subjects. Informed consent was obtained from all participating subjects.
Instruments The AL-Scan biometer (version 1.03) uses a 970 nm lightemitting diode (LED) in the assessment of K readings and pupil size and a 525 nm LED in the assessment of WTW. Corneal power is measured from the reflection of double mires projected onto the cornea at 2.4 mm and 3.3 mm diameters. The flattest K reading and steepest K reading for each of the 2 diameters were averaged and recorded as the mean K reading. Scheimpflug imaging with a 470 nm monochromatic light from an LED is used to determine the CCT and ACD. The CCT is reported with respect to the pupil center. The WTW and pupil size are determined by fitting the best circle with the lowest error square to the detected edge. The optical path length measurements are aligned with the visual axis for each measurement. This biometer uses PCI to measure AL.12 The Pentacam HR tomographer (version 1.20) also uses the Scheimpflug principle with a 475 nm monochromatic slit of light to illuminate the cornea and a 1.45 megapixel camera for photography that rotates around the fixation line during the scan. The 25-picture scan was chosen for this study. The device calculates a 3-dimensional model of the anterior segment from up to 138 000 true elevation points.15 The Scheimpflug tomographer can produce a CCT estimate at any point on the cornea. It also provides the CCT at the pupil center, corneal apex, and thinnest point. Because the AL-Scan biometer reports the CCT at the pupil center only, the pupil center CCT produced by the Scheimpflug tomographer was used in this study. The K readings produced by the Scheimpflug tomographer are from a 15-degree ring centered on the corneal apex.
Measurement Technique The right eye of each subject was chosen to avoid statistical bias. The measurement order for each device was random. The random sequence was generated using Medcalc Statistical Software (version 13.0, Medcalc Software, Inc.). Three consecutive scans were performed with each device by the same experienced examiner (X.Y.). All measurements were performed without pupil dilation. Subjects were seated and positioned comfortably with a chinrest and headrest. Subjects were instructed to fixate on the internal target within each device. Fine adjustments of the chinrest were made to ensure the lateral canthus of the eye was in line with the side markers on the holding bars of the headrest. Subjects were asked to perform a rapid, complete blink just before measurements were taken to ensure that an optically smooth tear film would be spread over the cornea. All measurements were performed between 10 AM and 5 PM, and each set of measurements from an individual patient
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was completed within 20 minutes. Scans that were substandard because of blinking or eye movements were discarded and performed a second time. Both devices can measure aqueous depth, which is the distance from the corneal endothelium to the anterior lens capsule, and the ACD, which is the distance from the corneal epithelium to the anterior lens capsule.17
Statistical Analysis The mean G standard deviation (SD) for each of the common parameters reported for each device was calculated. The range for each parameter was also recorded. The Bland-Altman technique was used to assess agreement between the 2 devices.18 The agreement was summarized by determining the bias, which is estimated by the mean difference d, and the SD of the differences s. Most differences were expected to lie within d 1.96s and d C 1.96s. Provided the differences within d G 1.96s were not clinically significant, the 2 measurements could be used interchangeably. Bland-Altman plots (the difference between each parameter against their mean) were also produced displaying the 95% limits of agreement (LoA) between the 2 devices for the common parameters and the 95% confidence intervals (CIs) [LoA G (2.26 standard error)]. Calculations were performed using Excel software (version 2010, Microsoft Corp.) and SPSS software (version 21.0, International Business Machine Corp.). Bland-Altman plots were produced using Medcalc Statistical Software.
RESULTS The study comprised 121 healthy subjects (57 men and 64 women). The mean age of the subjects was 25.41 years G 6.86 (SD) (range 18 to 54 years). The mean spherical equivalent refraction was 4.67 G 2.36 diopters (D) (range 0.50 to 11.75 D). Table 1 shows the mean and range of values for each parameter by device. Table 2 shows the mean differences, SD of the differences, and upper and lower LoA (with 95% CIs) of the 2 devices. Figures 1 to 3 show the Bland-Altman plots, as do
online-only Figures S1 to S6 (available at http:// jcrsjournal.org). The differences in the CCT and LoA between devices (Table 2) were assessed to be clinically interchangeable. The mean differences in the aqueous depth and ACD (Table 2) were assessed as clinically interchangeable. Considering the LoA with their 95% CIs (Table 2), all K readings were assessed to be clinically interchangeable. DISCUSSION Precise estimates of the parameters used within IOL formulas are necessary to ensure good visual and refractive outcomes as well as improved patientreported outcomes after cataract surgery.19–21 The present study compared measurements from the AL-Scan biometer with those obtained with the commonly used Pentacam HR Scheimpflug tomographer to assess agreement. Beginning with the CCT, the mean difference was 1.61 mm and the LoA were 6.29 to 9.52 mm (95% CI, 7.53 to 10.76). The repeatability limit (defined as 1.96O2 within-subject SD) provides the likely limits within which 95% of measurements are expected to occur.18 The previously reported repeatability limit for the Pentacam HR Scheimpflug tomographer for CCT measurements was 111.6 mm.15 The reason for this poor repeatability limit is probably because of the CCT being reported with respect to the pupil center and because in undilated eyes, the pupillary hippus is likely to have slightly altered the exact pupil center; in such cases, the CCT measurement would be at a different location on the cornea. This might be less of an issue with dilated pupils. The present study found an extremely small mean difference between the 2 devices and narrow LoA, even with the aforementioned limitations of pupil
Table 1. Mean G SD, maximum and minimum values for the parameters reported by the new biometer and Scheimpflug tomographer. AL-Scan Parameter CCT (mm) ACD (mm) Aqueous depth (mm) Kf (F Z 2.4) (D) Ks (F Z 2.4) (D) Km (F Z 2.4) (D) Kf (F Z 3.3) (D) Ks (F Z 3.3) (D) Km (F Z 3.3) (D)
Scheimpflug Tomographer
Mean G SD
Range
Mean G SD
Range
537.44 G 27.46 3.16 G 0.30 3.70 G 0.30 42.77 G 1.43 43.88 G 1.44 43.33 G 1.41 42.74 G 1.43 43.84 G 1.43 43.29 G 1.40
454.00, 592.67 2.44, 4.05 2.98, 4.59 38.93, 46.77 39.79, 47.60 39.73, 47.00 38.85, 46.81 39.71, 47.67 39.62, 47.03
535.82 G 26.47 3.14 G 0.29 3.67 G 0.29 d d d 42.67 G 1.43 43.83 G 1.44 43.25 G 1.40
459.67, 594.00 2.44, 3.91 2.98, 4.46 d d d 38.67, 46.57 39.83, 47.60 39.48, 46.73
F Z diameter; ACD Z anterior chamber depth; CCT Z central corneal thickness (over pupil center); Kf Z flattest keratometry; Km Z mean keratometry; Ks Z steepest keratometry
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Table 2. The mean difference G standard deviation, 95% LoA with 95% CIs for differences between the new biometer and Scheimpflug tomographer. Parameter CCT (mm) Aqueous depth (mm) ACD (mm) Kf (F Z 2.4) (D) Ks (F Z 2.4) (D) Km (F Z 2.4) (D) Kf (F Z 3.3) (D) Ks (F Z 3.3) (D) Km (F Z 3.3) (D)
Mean Difference G SD
P Value
1.61 G 4.03 0.02 G 0.06 0.02 G 0.06 0.10 G 0.14 0.05 G 0.17 0.08 G 0.14 0.06 G 0.12 0.01 G 0.18 0.04 G 0.13
!.001 !.001 !.001 !.001 !.001 !.001 !.001 .663 .003
Lower LoA (95% CI) 6.29 ( 0.10 ( 0.09 ( 0.17 ( 0.29 ( 0.19 ( 0.17 ( 0.34 ( 0.22 (
7.53 to 0.11 to 0.11 to 0.22 to 0.34 to 0.23 to 0.20 to 0.39 to 0.26 to
5.04) 0.10) 0.08) 0.13) 0.24) 0.15) 0.13) 0.29) 0.18)
Upper LoA (95% CI) 9.52 (8.27 to 10.76) 0.14 (0.12 to 0.16) 0.14 (0.12 to 0.16) 0.38 (0.34 to 0.42) 0.39 (0.33 to 0.44) 0.34 (0.30 to 0.38) 0.30 (0.26 to 0.33) 0.35 (0.30 to 0.41) 0.29 (0.25 to 0.34)
F Z diameter; ACD Z anterior chamber depth; CCT Z central corneal thickness (at pupil center); Kf Z flattest keratometry; Km Z mean keratometry; Ks Z steepest keratometry
center CCT measurements. Based on this, the 2 devices can be considered interchangeable for pupil center CCT; however, caution is advised, in particular when measuring eyes with undilated pupils. McAlinden et al.15 showed the repeatability limit for the corneal apex CCT with the Pentacam HR Scheimpflug tomographer as 10.4 mm and even more robust for the thinnest CCT (9 mm). For the ACD, the mean difference for both the aqueous depth and ACD was very small at 0.02 mm, with narrow LoA of 0.10 to 0.14 mm (95% CI, 0.11 to 0.16) and LoA of 0.09 to 0.14 mm (95% CI, 0.11 to 0.16), respectively, which again suggests these parameters are clinically interchangeable. In terms of K readings, the new biometer reports these at 2.4 mm and at 3.3 mm. The 2.4 mm diameter is similar to that of the IOLMaster biometer (Carl Zeiss Meditec AG) (2.5 mm), and the 3.3 mm diameter matches that of Nidek keratometers. The flat and steep
K readings were very similar at the 2 diameters. At 2.4 mm and 3.3 mm, the flat K readings were 42.77 D and 42.74 D, respectively. At 2.4 mm and 3.3 mm, the steep K readings were 43.88 D and 43.84 D, respectively. The Scheimpflug tomographer in our study reports K readings from a 15-degree ring centered on the corneal apex; the mean flat and steep K readings were 42.67 D and 43.87 D, respectively. The mean difference between the new biometer and Scheimpflug tomographer was small. The smallest mean difference was in the steep K readings at 3.3 mm with the new biometer and largest for the flat K reading at 2.4 mm. The LoA were small for all K readings, including the mean K readings. The reported repeatability limit for flat K, steep K, and mean K with the Scheimpflug tomographer was 0.25 D, 0.36 D, and 0.33 D, respectively. The LoA with the aforementioned 95% CI found in the present study are not significantly greater than the repeatability limits of the
Figure 1. Bland-Altman plots show the agreement between the new biometer and the Scheimpflug tomographer for measurements of the CCT. The solid line indicates the mean difference (bias), and the dotted lines indicate the 95% LoA.
Figure 2. Bland-Altman plots show the agreement between the new biometer and the Scheimpflug tomographer for measurements of the aqueous depth from the corneal endothelium to the anterior lens. The solid line indicates the mean difference (bias), and the dotted lines indicate the 95% LoA.
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Figure 3. Bland-Altman plots show the agreement between the new biometer and the Scheimpflug tomographer for measurements of the ACD from the corneal epithelium to the anterior lens. The solid line indicates the mean difference (bias), and the dotted lines indicate the 95% LoA.
Scheimpflug tomographer, and despite exact measurement location in terms of optical zone diameter differing between devices, we conclude that the K readings of the 2 devices are interchangeable. To our knowledge, this is the first study to compare the AL-Scan biometer and the Pentacam HR Scheimpflug tomographer; however, recent studies have compared the AL-Scan biometer and other devices. Kaswin et al.9 studied the agreement between the AL-Scan biometer and the IOLMaster 500 biometer in 50 eyes. They found a mean difference in the 2.4 mm mean K readings of 0.17 D between the 2 devices, which is significantly greater than the value in the present study (0.04 D). They found a mean difference of 0.13 D for ACD, which is also significantly greater than the mean difference of 0.02 D in the present study. However, they did not report the LoA for these parameters. Can et al.22 studied the effects of pupil dilation (with cyclopentolate) on the parameters of the AL-Scan biometer in 72 eyes. The AL and ACD were compared before and after pupil dilation. Only the ACD readings were significantly different before and after dilation, and only 2 cases in the study showed changes in IOL power higher than 0.5 D. The study advised that increases in ACD after dilation should be taken into account when performing refractive surgeries in which the ACD is very important, such as in anterior chamber phakic IOL implantation.22 Srivannaboon et al.11 also compared the AL-Scan biometer and the IOLMaster 500 biometer. They assessed the precision (repeatability and reproducibility) of the AL-Scan biometer in 137 eyes and found excellent repeatability and reproducibility as assessed by intraclass correlation coefficients (ICCs) and LoA. However, they did not use the recommended within-
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subject SD method,18 which would have permitted comparison with the precision of the Pentacam HR Scheimpflug tomographer.15 However Srivannaboon et al.11 found a mean difference and LoA for the same parameters reported in the present study. For 2.4 mm K readings, they found a mean difference and LoA of 0.09 D and 0.26 D to 0.44 D, respectively, which is very similar to the results in the present study. For 3.3 mm K readings, they found a mean difference and LoA of 0.08 D and 0.32 D to 0.48 D, respectively, which is also similar but slightly larger than in the present study (mean difference 0.04 D, LoA, 0.22 D to 0.29 D). For ACD, they found a mean difference of 0.02 D and LoA of 0.24 to 0.19 D, respectively, which is almost identical to our findings. Moon et al.23 compared the AL-Scan biometer and applanation US in 104 eyes. Biometry was performed with both devices in 104 eyes, and the SRK/T formula24 was used to calculate the required IOL power. Automatic refraction was performed 1 month postoperatively. There was no statistically significant difference in the AL measurement or postoperative refractive error between the 2 techniques. Huang et al.12 studied the precision of the AL-Scan biometer, and its agreement with the IOLMaster biometer. They found the AL-Scan was highly repeatable and reproducible, except for WTW and pupil diameter measurements. Good agreement between the devices was found for AL, ACD, and K readings. Compared with the IOLMaster biometer, the AL-Scan–derived K readings using a diameter of 2.4 mm showed narrower LoA than those obtained with a diameter of 3.3 mm. This might be because the IOLMaster biometer measures the K readings at a very similar diameter (2.5 mm). In comparison with the present study, there was no obvious difference in the LoA in IOL power calculations between the AL-Scan biometer using the 2.4 mm or 3.3 mm K readings and the Pentacam HR tomographer. Dervis¸o gulları et al.25 studied ACD measurements with the AL-Scan biometer and the Galilei dual Scheimpflug analyzer (Ziemer Ophthalmic Systems AG) in 63 normal eyes. Again, the ICC was used to assess the ACD measurements between 2 observers on each device (AL-Scan 0.996 and Galilei 0.968). However, the LoA were calculated between each observer for each device rather than the LoA between the 2 devices, limiting the usefulness of their study results. The main limitations of the present study are that the results are only applicable to healthy adult eyes with myopia. The results might not apply to patients with anterior segment eye disease, such as keratoconus, or eyes with previous surgery, such as refractive procedures, corneal crosslinking, and corneal transplantation. They might not apply to patients with
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hyperopia or to pediatric patients. Future research should address these limitations. In conclusion, the present study compared the AL-Scan biometer and the Pentacam HR tomographer and found good agreement between all common parameters. WHAT WAS KNOWN Scheimpflug imaging is widely used to measure the corneal thickness, ACD, and keratometry. An optical biometer was recently introduced that can measure these parameters. However, it is not known whether the values obtained with the 2 devices are comparable and can be used interchangeably. WHAT THIS PAPER ADDS The mean CCT, ACD, and K measurements were highly comparable between the new ocular biometry device and the validated Scheimpflug anterior segment tomographer. REFERENCES 1. Ianchulev T, Hoffer KJ, Yoo SH, Chang DF, Breen M, Padrick T, Tran DB. Intraoperative refractive biometry for predicting intraocular lens power calculation after prior myopic refractive surgery. Ophthalmology 2014; 121:56–60 2. Grulkowski I, Liu JJ, Zhang JY, Potsaid B, Jayaraman V, Cable AE, Duker JS, Fujimoto JG. Reproducibility of a long-range sweptsource optical coherence tomography ocular biometry system and comparison with clinical biometers. Ophthalmology 2013; 120:2184–2190 3. Eom Y, Kang S-Y, Song J-S, Kim HM. Use of corneal powerspecific constants to improve the accuracy of the SRK/T formula. Ophthalmology 2013; 120:477–481 4. Bang S, Edell E, Yu Q, Pratzer K, Stark W. Accuracy of intraocular lens calculations using the IOLMaster in eyes with long axial length and a comparison of various formulas. Ophthalmology 2011; 118:503–506 5. Huang J, McAlinden C, Su B, Pesudovs K, Feng Y, Hua Y, Yang F, Pan C, Zhou H, Wang Q. The effect of cycloplegia on the Lenstar and the IOLMaster biometry. Optom Vis Sci 2012; 89:1691–1696. Available at: http://journals.lww.com/optvissci/Fulltext/2012/12000/ The_Effect_of_Cycloplegia_on_the_Lenstar_and_the.6.aspx. Accessed February 6, 2016 6. Huang J, Ding X, Savini G, Pan C, Feng Y, Cheng D, Hua Y, Hu X, Wang Q. A comparison between Scheimpflug imaging and optical coherence tomography in measuring corneal thickness. Ophthalmology 2013; 120:1951–1958 7. Goebels SC, Seitz B, Langenbucher A. Reproducibility of the optical biometer OA-1000 (Tomey). Biomed Res Int 2014:814761. Available at: http://downloads.hindawi.com/journals/bmri/2014/ 814761.pdf. Accessed February 6, 2016 8. Mandal P, Berrow EJ, Naroo SA, Wolffsohn JS, Uthoff D, Holland D, Shah S. Validity and repeatability of the Aladdin ocular biometer. Br J Ophthalmol 2014; 98:256–258 9. 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
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