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ORIGINAL ARTICLE
Year : 2017  |  Volume : 29  |  Issue : 1  |  Page : 35-40

Comparison of ocular biometry parameters between IOL Master and applanation A-scan in eyes with short, medium, long, and very long axial lengths


Department of Ophthalmology, Little Flower Hospital and Research Centre, Angamaly, Kerala, India

Date of Web Publication19-Jun-2017

Correspondence Address:
Sanitha Sathyan
Department of Ophthalmology, Little Flower Hospital and Research Centre, Angamaly - 683 572, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/kjo.kjo_43_17

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  Abstract 


Aim: The aim of this study is to compare the ocular biometric parameters between intraocular lens (IOL) master and applanation A-scan in eyes with different axial lengths (ALs).
Materials and Methods: A cross-sectional observational study was conducted 211 patients scheduled for cataract surgery at a tertiary care eye hospital. AL and anterior chamber depth (ACD) measurements were obtained both by IOL master and applanation A-scan under standard operating conditions and analyzed.
Results: The study sample comprised 211 eyes of 211 patients. Mean age of patients was 60.04 ± 7.90 years (range 40–84 years), 81% were male and 19% were female. Eyes were divided into four groups depending on their AL measurements as extremely long >27.0 mm, long 25.0–27.0 mm, medium 22.0–25.0 mm, short <22.0 mm. The ALs measured by IOL master was significantly higher than those measured by A-scan (P < 0.001). Mean ACD with both methods was 3.23 ± 0.45 and 3.20 ± 0.46, respectively. In the subgroup analysis, there was good agreement between AL and ACD between IOL master and ultrasound A-scan in extremely long, long, medium, and short eyes.
Conclusion: AL and ACD showed good agreement between IOL master and applanation A-scan measurements at all range of ALs.

Keywords: Applanation A scan, IOL master, ocular biometry


How to cite this article:
Gopi R, Sathyan S. Comparison of ocular biometry parameters between IOL Master and applanation A-scan in eyes with short, medium, long, and very long axial lengths. Kerala J Ophthalmol 2017;29:35-40

How to cite this URL:
Gopi R, Sathyan S. Comparison of ocular biometry parameters between IOL Master and applanation A-scan in eyes with short, medium, long, and very long axial lengths. Kerala J Ophthalmol [serial online] 2017 [cited 2020 Apr 1];29:35-40. Available from: http://www.kjophthal.com/text.asp?2017/29/1/35/208482


  Introduction Top


Precise biometry is one of the key factors for obtaining satisfactory refractive outcomes after cataract surgery. Visual outcomes after cataract surgery depend on the minimization of errors associated with the measurement of ocular parameters. Of these parameters, axial length (AL) is the most responsible for postoperative prediction errors. Literature reports that 54% of errors in predicted refraction after intraocular lens (IOL) implantation was due to variability in AL measurement and 38% were due to inaccurate estimation of anterior chamber depth (ACD).[1]

A-scan ultrasonic biometry is the most common method used for measuring AL and ACD. IOL master provides an alternative technique for the measurement of ocular biometry parameters based on partial coherence interferometry. Precise AL and ACD measurements are required to determine IOL power and position and to prevent endothelial cell damage in cataract surgery and phakic IOL implantation. Studies show that errors in the prediction of effective lens position (ELP) may account for 20% to 40% of the total refractive prediction errors.[2] New generation formulas such as Haigis, Holladay require preoperative ACD to predict ELP. Different modalities, including ultrasound (US), optical, and photographic methods are available for measuring ACD. The most common method for ACD measuring is US biometry. In IOL master, ACD is determined by calculating the distance between corneal and lens surface through lateral slit illumination, with 0.01 mm resolution for ACD measurements.[2] A 1-mm error in AL measurement results in a refractive error of approximately 2.35 D error of IOL power in an average eye of 23.5 mm, an error of 3.75 D in a 20 mm eye, and much more in the very short eye.[2] This can lead to greater prediction errors in short or long ALs.

There are only a few previous reports comparing AL and ACD between IOL master and ultrasound A-scan in relation to the different ALs. Hence, our aim was to determine the agreement of AL and ACD measurements between the two instruments in our population.

Aim

The aim of this study is to determine the agreement of ocular biometry parameters measured using IOL master and applanation A-scan, in eyes with different ALs.


  Materials and Methods Top


A total of 211 subjects were recruited in this cross-sectional observational study, conducted from January 2015 to June 2016, through simple random sampling. The study followed the tenets of the Declaration of Helsinki, and informed consent was obtained from all subjects after explanation of the study protocol. The study protocol was approved by the Institutional Ethics Committee of Little Flower Hospital, Angamaly.

Those patients with history of trauma/previous ocular surgery, ocular infective pathology, patients who could not be positioned satisfactorily for accurate measurement, those with nystagmus and poor fixation were excluded from the study.

Preoperative measurements of AL and ACD were measured by both optical biometry (Carl Zeiss IOL Master 500) and applanation A-scan ultrasound biometry (Sonomed AB 5500+) by a single experienced observer. Optical biometry was always first performed followed by US measurements. This order was considered to maintain the integrity of the corneal epithelium which may be compromised by contact with the US probe.

Optical biometry was performed according to standard protocol provided by the manufacturers. After entering patient data, fixation lights and illumination lights were switched on, and the patient was asked to fixate on the fixation target. A crosshair with a circle in the middle appears in the display. Keratometry measurements taken when the six peripheral points appear optimally focused on the display. After that, AL was measured according to AL protocol. Four reliable scans within 0.02 mm of ideal waveform, signal ratio more than 10, and displayed as green were taken. The ACD measurement was measured through image analysis of the distance between the anterior corneal pole and anterior surface of crystalline lens illuminated by an optic section. Measurements of ACD were automatically generated, and average reading was recorded.

Applanation A-scan was performed after the instillation of one drop of topical anesthetic proparacaine hydrochloride 0.5% on the lower conjunctiva. The patient data were entered, and the same keratometric readings obtained from IOL master were used. The patient was seated comfortably in an upright position and asked to look straight. A scan unit was equipped with a 10 MHz transducer probe, and velocities were set by device per medium. US biometric sound velocities of 1532 m/s were taken for the aqueous and the vitreous humor and 1641 m/s for lens. Gain was kept at minimum level that allows the proper resolution of the spikes. The probe was brought forward to touch the cornea without indenting it. The probe should be properly aligned along the visual axis to optimize the five high amplitude spikes on the screen. Five measurements were taken to meet the standard deviation of 0.1 based on manufacturer's recommendations.

The machines were calibrated, and intraoperator repeatability was determined to reliable levels, before the commencement of the study.

Statistical analysis

Statistical Package for Social Science version: 20 (IBM, United States) was used for the analysis of data. Bland–Altman plots were used to evaluate the agreement in AL and ACD between devices with 95% confidence intervals.


  Results Top


The study sample comprised 211 eyes of 211 patients (171 males and 40 females). The mean age of patients was 60.04 ± 7.90 years (range 40–84 years), 81% were male, and 19% were female.

Eyes were divided into four groups depending on their AL measurements as extremely long >27.0 mm, long 25.0–27.0 mm, medium 22.0–25.0 mm, and short <22.0 mm. [Table 1] shows the group-wise distribution based on their ALs.
Table 1: Distribution based on axial length

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In the total sample, the mean AL with A-scan and IOL master was 25.21 ± 3.37 mm and 25.32 ± 3.45 mm, respectively. The AL measured by IOL master was significantly higher than that measured by A-scan (P < 0.001, unpaired test). Mean of the difference in AL measured by both methods was 0.11 ± 0.36 mm. Bland–Altman plot indicates that 97.15% of the differences in the readings between the instruments is between −0.607 and 0.820. [Figure 1] shows the agreement between A-scan and IOL master for AL measurements in the total sample.
Figure 1: Agreement between A-scan and intraocular lens master for axial length measurements in the total sample

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Mean ACD with both methods was 3.23 ± 0.45 and 3.20 ± 0.46, respectively. Mean difference in the ACD measured by both methods was −0.03 ± 0.23 mm. The statistically significant differences in ACD measured with IOL master, and A-scan was found with higher values obtained in A-scan (P = 0.026, unpaired t-test). Bland–Altman plot indicates that 95.73% of the differences in the readings between two instruments is between −0.486 and 0.417. [Figure 2] shows the agreement of ACD measurements between A-scan and IOL master in the total sample.
Figure 2: Agreement of anterior chamber depth measurements between A-scan and intraocular lens master in the total sample

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Subgroup analysis

Extremely long eyes

Mean AL measured in extremely long eyes with A-scan and IOL master was 30.02 ± 1.96 mm and 30.31 ± 1.94 mm, respectively. Mean ACD values with two methods were 3.44 ± 0.43 mm and 3.47 ± 0.31 mm. The AL and ACD values measured with IOL master were higher than those with A-scan, with a mean difference of 0.27 ± 0.55 mm and 0.04 ± 0.29 mm, respectively. The interdevice difference in the AL was 0.27 ± 0.55 (P = 0.001). The interdevice difference in the ACD was 0.04 ± 0.29 (P = 0.36).

Bland–Altman plot for AL indicates that 99.52% of the differences in the reading between them are between −0.830 and 1.368. [Figure 3] shows the agreement of AL between A-scan and IOL master on extremely long eyes.
Figure 3: Agreement of axial length between A-scan and intraocular lens master in extremely long eyes

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Bland–Altman plot for ACD indicates that 99.05% of the differences in the readings are between –0.534 and 0.609. [Figure 4] shows the agreement of ACD between A-scan and IOL master on extremely long eyes.
Figure 4: Agreement of anterior chamber depth between A-scan and intraocular lens master in extremely long eyes

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Long eyes

For long eyes, mean AL measured with A-scan, and IOL master was 26.05 ± 0.92 mm and 26.16 ± 0.90 mm, and ACD values with two methods were 3.37 ± 0.42 mm and 3.42 ± 0.43 mm. The AL and ACD values measured with IOL master were higher than A scan with a mean difference of 0.10 ± 0.32 mm and 0.05 ± 0.13 mm, respectively.

The Bland–Altman plot for AL indicates that 99.05% of the differences in the reading between them lay between −0.545 and 0.748 and 97.63%; for ACD lay between −0.204 and 0.306. [Figure 5] shows the agreement of AL between A-scan and IOL master on extremely long eyes. [Figure 6] shows the agreement of ACD between A-scan and IOL master on extremely long eyes.
Figure 5: Agreement of axial length between A-scan and intraocular lens master in long eyes

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Figure 6: Agreement of anterior chamber depth between A-scan and intraocular lens master in long eyes

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Medium eyes

For medium eyes, mean AL measured with A-scan, and IOL master was 23.18 ± 0.60 mm and 23.21 ± 0.64 mm. ACD values with two methods were 3.20 ± 0.33 mm and 3.09 ± 0.33 mm. The AL and ACD values measured with IOL master were higher than A-scan with a mean difference of 0.03 ± 0.24 mm and −0.11 ± 0.20 mm, respectively.

Bland–Altman plot for AL indicates that 98.58% of the differences in the reading between them lay between −0.442 and 0.498, and for ACD, it was 99.05% lay between −0.506 and 0.282. [Figure 7] shows the agreement of AL between A-scan and IOL master on medium eyes. [Figure 8] shows the agreement of ACD between A-scan and IOL master on medium eyes.
Figure 7: Agreement of axial length between A-scan and intraocular lens master in medium eyes

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Figure 8: Agreement of anterior chamber depth between A-scan and intraocular lens master in medium eyes

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Short eyes

For short eyes, mean AL measured with A-scan and IOL master were 21.34 ± 0.89 mm and 21.37 ± 0.84 mm; ACD values with two methods were 2.85 ± 0.40 mm and 3.09 ± 0.33 mm. The AL and ACD values measured with IOL master were higher than A scan with a mean difference of 0.03 ± 0.15 mm and −0.14 ± 0.22 mm, respectively.

Bland–Altman plot for AL indicates that 99.05% of the differences in the reading between them lay between −0.270 and 0.338, and for ACD, it was 98.58% lay between −0.574 and 0.299. [Figure 9] shows the agreement of AL between A-scan and IOL master on short eyes. [Figure 10] shows the agreement of ACD between A-scan and IOL master on short eyes.
Figure 9: Agreement of axial length between A-scan and intraocular lens master in short eyes

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Figure 10: Agreement of anterior chamber depth between A-scan and intraocular lens master in short eyes

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  Discussion Top


In our study, AL measured by IOL master was 0.11 ± 0.36 mm longer than that of A-scan and was statistically significant (P < 0.001, unpaired t-test). Hitzenberger et al. found that AL measured by optical biometry were 0.18 mm longer than AL measured by immersion technique and 0.47 mm longer than measured by applanation technique.[3] The differences in AL between IOL master and A scan was found in other studies also.[4],[5],[6],[7],[8],[9],[10] In the study by Goyal et al. found a difference of 0.2 mm between A-scan US and IOL master.[4] Rose and Moshegov reported a difference of 0.15 mm between the devices.[6] In a prospective randomized clinical trial, of 100 patients by Rajan et al. found that the preoperative mean AL was 23.47 ± 1.1 mm in the PCLI group (range 20–27.6 mm) and 23.43 ± 1.2 mm in the US group with a range of 20.1–27 mm (P > 0.05).[7] Similarly, in a study involving 100 eyes, Eleftheriadis estimated that the AL obtained by IOL master was significantly longer by 0.47 mm than applanation US.[11]

The possible reason for this difference in AL is due to the difference in the starting point of measurement between the two modalities. Ultrasound A-scan measures AL from the anterior surface of the corneal apex to the internal limiting membrane (ILM) of the fovea, whereas optical biometry measures AL from the second principal plane of the cornea (0.05 mm deeper than the corneal apex) to photoreceptor layer (0.25 mm deeper than ILM) of the fovea.[9] Another point that is resolution improves with the decrease in wavelength. The laser light has better resolution, and the accuracy of US AL is approximately 0.10–0.12 mm compared to 0.012 mm optical AL. Another is due to corneal indentation possible during contact US measurements leading to the shortening of AL by an average of 0.1–0.3 mm.[9]

In our study, statistically significant difference in ACD measured with IOL master and A-scan was found, with lower values in A-scan (P = 0.026, unpaired t-test). According to Santodomingo-Rubido et al., ACD measured with the IOL master was significantly shorter by −0.06 ± 0.25 mm than that measured by applanation US.[5] This is in agreement with our results. The explanation for this difference is indentation of the cornea which is responsible for shorter values with US probe. In addition, the IOL master does not measure the axial ACD because the slit source always comes from the temporal side and results in deeper ACD.[11] Pupil diameter and accommodative state of the lens can also be responsible for difference in ACD values measured between both devices. According to Nemeth et al., the difference in ACD measurements between IOL master and ultrasound A-scan was due to lack of pupil dilatation which causes US measurements values to be smaller than true values in many cases.[9] The reason was that the distance between the anterior corneal surface and the iris could be erroneously measured as the ACD.[9] Another source of error is off-axis measurement.[12] Kriechbaum et al. found that off-axis measurements occurring during US ACD evaluation results in shallower ACD readings.[13]

There was good agreement between the AL and ACD in all the subgroups of AL in our study. Previous studies have reported good agreement in short eyes and long eyes.[2],[12] Study conducted by Shen et al. in highly myopic eyes with an AL longer than 25 mm concluded that optical biometry provided more precise measurements of biometric parameters, including AL and ACD than applanation US biometry in highly myopic eyes.[12] However, no such significant difference was obtained in high myopic eyes included in our study.

The merit of our study is its large sample size and subgroup analysis, justifying the interchangability of IOL master, and applanation A-scan across all the ALs. However, the accuracy of the two instruments was not compared with postoperative refraction. Longitudinally designed studies are needed for such comparisons.


  Conclusion Top


AL and ACD showed good agreement between IOL master and applanation A-scan measurements at all range of ALs. This indicates that both the instruments can be used interchangeably for accurate preoperative biometry at all range of ALs.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

1.
Bai QH, Wang JL, Wang QQ, Yan QC, Zhang JS. The measurement of anterior chamber depth and axial length with the IOL Master compared with contact ultrasonic axial scan. Int J Ophtalmol 2008;1:151-4.  Back to cited text no. 1
    
2.
Moschos MM, Chatziralli IP, Koutsandrea C. Intraocular lens power calculation in eyes with short axial length. Indian J Ophthalmol 2014;62:692-4.  Back to cited text no. 2
[PUBMED]  [Full text]  
3.
Hitzenberger CK, Drexler W, Dolezal C, Skorpik F, Juchem M, Fercher AF, et al. Measurement of the axial length of cataract eyes by laser Doppler interferometry. Invest Ophthalmol Vis Sci 1993;34:1886-93.  Back to cited text no. 3
    
4.
Goyal R, North RV, Morgan JE. Comparison of laser interferometry and ultrasound A-scan in the measurement of axial length. Acta Ophthalmol Scand 2003;81:331-5.  Back to cited text no. 4
    
5.
Santodomingo-Rubido J, Mallen EA, Gilmartin B, Wolffsohn JS. A new non-contact optical device for ocular biometry. Br J Ophthalmol 2002;86:458-62.  Back to cited text no. 5
    
6.
Rose LT, Moshegov CN. Comparison of the Zeiss IOLMaster and applanation A-scan ultrasound: Biometry for intraocular lens calculation. Clin Exp Ophthalmol 2003;31:121-4.  Back to cited text no. 6
    
7.
Rajan MS, Keilhorn I, Bell JA. Partial coherence laser interferometry vs. conventional ultrasound biometry in intraocular lens power calculations. Eye (Lond) 2002;16:552-6.  Back to cited text no. 7
    
8.
Verhulst E, Vrijghem JC. Accuracy of intraocular lens power calculations using the Zeiss IOL master. A prospective study. Bull Soc Belge Ophtalmol 2001; 281:61-5.  Back to cited text no. 8
    
9.
Németh J, Fekete O, Pesztenlehrer N. Optical and ultrasound measurement of axial length and anterior chamber depth for intraocular lens power calculation. J Cataract Refract Surg 2003;29:85-8.  Back to cited text no. 9
    
10.
Carkeet A, Saw SM, Gazzard G, Tang W, Tan DT. Repeatability of IOLMaster biometry in children. Optom Vis Sci 2004;81:829-34.  Back to cited text no. 10
    
11.
Eleftheriadis H. IOLMaster biometry: Refractive results of 100 consecutive cases. Br J Ophthalmol 2003;87:960-3.  Back to cited text no. 11
    
12.
Sheng H, Bottjer CA, Bullimore MA. Ocular component measurement using the Zeiss IOLMaster. Optom Vis Sci 2004;81:27-34.  Back to cited text no. 12
    
13.
Kriechbaum K, Findl O, Kiss B, Sacu S, Petternel V, Drexler W. Comparison of anterior chamber depth measurement methods in phakic and pseudophakic eyes. J Cataract Refract Surg 2003;29:89-94.  Back to cited text no. 13
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
 
 
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