|Year : 2020 | Volume
| Issue : 2 | Page : 39-45
Intraocular lens Master optical biometry vs conventional ultrasonic biometry in intraocular lens power calculations in highly myopic vs emmetropic eyes
Mina N Gad1, Mervat S Mourad2, Rafaat A Rehan2, Mouamen M Mustafa2
1 Imbaba Ophthalmic Hospital, Egypt
2 Ministry of Health, Cairo, Egypt
|Date of Submission||05-Mar-2020|
|Date of Acceptance||08-Apr-2020|
|Date of Web Publication||10-Jul-2020|
MSc, MD, PhD Mervat S Mourad
Department of Ophthalmology Ain Shams University, Cairo, 11471
Source of Support: None, Conflict of Interest: None
Aim The aim was to compare between the preoperative intraocular lens (IOL) power calculations accuracy measured by IOL Master vs applanation ultrasonic biometry in highly myopic and emmetropic eyes regarding postoperative refractive error.
Patients and methods The IOL powers of 32 eyes of 32 patients prepared for phacoemulsification with IOL implantation were calculated preoperatively by IOL Master and A-scan. Postoperative refractive outcomes and spherical equivalent were compared with the predicted refractive error of each method.
Results The mean±SD age of the 32 recruited patients was 53.91±10.21 years. The mean axial length (AXL) measured by IOL Master was higher (25.8±2.65 mm) than that with A-scan (25.46±3.43 mm), with a mean difference of 0.34±1.04 mm (P=0.2112). The mean predicted IOL power was 13.50±7.80 D with IOL Master vs 13.63±9.05 D with A-scan (P=0.930). However, no statistically significant difference was found regarding average K readings and predicted postoperative refraction (P=0.05 and 0.564, respectively). Further subgroup analysis revealed a statistically significant difference in AXL between both devices only in emmetropic group, with mean difference of 0.99±0.485 mm (P=0.00068). The mean absolute errors measured by IOL Master was 0.19±0.1417, whereas that of A-scan was 0.561±0.623. The mean difference between the two methods was −0.371 (P=0.0385), which was statistically significant.
Discussion and conclusion The IOL Master AXL measurements were accurate, allowing more accurate calculation of the (IOL) power. It is user friendly, fast, with no contact with the corneal surface and with no possibility of damage to the surface or transmitting a disease.
Keywords: A-scan, axial length, biometry, cataract, intraocular lens Master, phacoemulsification
|How to cite this article:|
Gad MN, Mourad MS, Rehan RA, Mustafa MM. Intraocular lens Master optical biometry vs conventional ultrasonic biometry in intraocular lens power calculations in highly myopic vs emmetropic eyes. J Egypt Ophthalmol Soc 2020;113:39-45
|How to cite this URL:|
Gad MN, Mourad MS, Rehan RA, Mustafa MM. Intraocular lens Master optical biometry vs conventional ultrasonic biometry in intraocular lens power calculations in highly myopic vs emmetropic eyes. J Egypt Ophthalmol Soc [serial online] 2020 [cited 2020 Aug 15];113:39-45. Available from: http://www.jeos.eg.net/text.asp?2020/113/2/39/289489
| Introduction|| |
Cataract and intraocular lens (IOL) implantation is a major concern for all ophthalmologist, as it deals with the first cause of preventable blindness. Axial length (AL), keratometry, and lens formulas are among many factors affecting the refractive state after cataract removal and IOL implantation. The preoperative axial length (AXL) measurement is the most important for the calculation of IOL power . An error of 1 mm in AL measurement can give a postoperative error of refraction of 2.35 D in an eye whose AXL is 23.5 mm. This refractive error declines to an error of 1.75 D/mm in an eye whose AXL is 30 mm but rises to an error of 3.75 D/mm in an eye whose AXL is 20 mm .
Ultrasonic biometry requires that its transducer must contact the cornea directly in the applanation technique or indirect contact in the immersion one. Studies show that errors from AL measurement with ultrasonic biometry can account for 54% of the postoperative refractive errors . Applanation ultrasonic (AUS) technique can result in corneal epithelial injury, infection, patient discomfort, and errors owing to corneal indentation. It is also confounded by certain clinical problems such as globe deformities, myopic staphylomas, and silicone oil tamponade .
The principle of partial coherence laser interferometry (PCLI) resembles that of the optical coherence tomography. It has been developed to overcome ultrasound limitation, where it does not need contact with the patient giving the advantage of less discomfort of the patient and a low error coming from the observer . PCLI has the advantage of allowing evaluation of the AXL along the visual axis, decreasing the effect of staphylomas in high myopic eyes .
However, in nuclear sclerotic cataract (+4) and white cataract, PCLI cannot get a reading. This is because the patient must be able to achieve fixation, and the light coming from the instrument has to be able to reach the fovea and return to the detector. The technology will not work with cataracts that reduce the patient’s visual acuity to 3/60 to finger counting. As PCLI needs adequate foveal fixation, eyes with opacification of the cornea, lens opacities, eccentric fixation, or macular degenerations fail to obtain reliable results with this technology .
The principles of ocular ultrasound do not differ from other applications of this same technology. The sound waves whose frequency is greater than 20 kHz travel along the tissues and then are reflected back to the transducer, where a piezoelectric crystal in the transducer vibrates, giving electrical impulses. which are translated into an image or other data .
Optical biometers that integrate swept source ocular coherence tomography (SS-OCT) technology for ocular biometry measure the central corneal thickness, keratometry readings, anterior chamber depth (ACD), anterior aqueous depth, pupil size, lens thickness (LT), AL, and horizontal white-to-white corneal diameter. They can also scan the central 1.0 mm zone of the retina to check fixation and calculate the IOL power using several IOL formulas that are built-in .
The potential advantages of SS-OCT are significant. It obtains the depth profile of the eye for ocular biometry and enables long-range OCT imaging of posterior segment ocular structures with a single instrument. A big advantage is that all needed parameters for modern IOL power formulas (i.e. AL, ACD, central corneal thickness, K1, K2, and white-to-white distance) can be obtained from a single data set using a single instrument .
The aim of this study was to compare between the preoperative IOL power calculations accuracy measured by IOL Master vs AUS biometry in highly myopic and emmetropic eyes regarding postoperative refractive error.
| Patients and methods|| |
This study was conducted on 32 eyes of 32 patients (8 men and 24 women) during the period between November 2016 and February 2017. The study patients were recruited from patients scheduled for phacoemulsification and IOL implantation after undergoing routine ophthalmological examination.
The patients included in this study were clearly informed about the purpose of the study, the steps of examination and surgery, and had to sign an informed consent before inclusion. Data collection was done following the laws of Egypt and was compliant with the principles of the Declaration of Helsinki.
Eyes that are free from any ocular pathology apart from age-related cataract or complicated cataract to silicone injection or myopia were included in the study. A total of 32 eyes were divided into two groups: group 1 included 16 eyes that underwent preoperative IOL Master examination, where eight eyes of group 1 were highly myopic with a equivalent (SE) greater than or equal to −6 and or AXL greater than or equal to 26.00 mm and the other eight eyes were emmetropic, and group 2 included 16 eyes that underwent A-scan examination with the same criteria for group 1.
The study excluded patients having ocular pathology such as retinal detachment and proliferative diabetic retinopathy and also if the AXL of the eye could not be measured with IOL Master, because of dense ocular media opacities such as opacities of the cornea or cataracts dens enough, corneal astigmatism greater than 2 D, or patients who underwent previous corneal surgery as grafting or refractive surgery. Patients were also excluded from the study whenever intraoperative complications occurred such as rupture of the posterior capsule, vitreous loss, weak zonule or its rupture, with failure of safe in the bag implantation of the IOL.
Group 1 patients underwent biometry using the Zeiss IOL Master (Carl Zeiss Meditec AG, Jena, Germany) and group 2 patients underwent biometry using ultrasound biometry (PAC Scan; Sonomed, Carleton Optical, Buckinghamshire, UK), at the Outpatient Clinic, Embaba Ophthalmic Hospital.
One drop of topical anesthesia was instilled before the AXL measurements were done by AUS. For each eye, five AXL measurements were taken by AUS, and the mean of at least three valid measurements (having the difference between them ≤0.2 mm) was used as the AXL.
Keratometry readings (K) were measured by the manual keratometer, and the A-scan ultrasound was used in IOL calculation.
Measurements were taken while the patient was sitting upright and the transducer held taking in consideration that the ultrasound beam was perpendicular to the globe.
Keratometry readings (K1 and K2) were taken by manual keratometer and were added to the ultrasound program before AXL measurement were done, and then the instrument calculated the IOL power automatically according to the used formula.
Intraocular lens Master biometry
Swept-source biometry applies optical B-scan technology to measure the biometric data. Cross-sectional visualization of structures along the visual axis was done using the optical B-scan technology. In this way, we could be sure that the ocular interfaces were detected correctly by the algorithm. Moreover, the keratometry mode was used for calculation of the Ks by projection of reflected light spots on the surface of the cornea. To get the K readings, three average Ks, each one consisted of five single measurement, were taken, and the final average K readings were calculated. Measurement of the AL was done by taking the average values of three scans in each of six meridians. SD values of the ACD, LT, and AL measurements were calculated. If the results were of low quality, the machine gave warning to the operator, which occurs when the SD for ACD greater than 0.021 mm, for LT greater than 0.038 mm. and for AL greater than 0.027 mm.
Then, we activated the mode of measurement and tuned a fine alignment, instructing the patient to keep looking at the red fixation point all the time of examination. We recorded 18 full-eye-length B-scans and 15 K measurements simultaneously. We repeated any step that is not of accepted quality in the quality check. To measure the keratometry, our patients were instructed to blink to distribute a homogenous tear film all over the ocular surface, to ensure a good reflective corneal surface. Eighteen peripheral measuring points were focused on the cornea as demonstrated by a green light.
All patients underwent phacoemulsification from a stepped 2.4 mm temporal self-sealing clear corneal incision, using a stop-and-chop technique. A foldable IOL was implanted in the capsular bag with the injector. All surgeries were performed by the same experienced surgeon. The IOL power used aimed for emmetropia ranging from −0.50 to 0.00 D.
On examination, one month after surgery, SE using autorefraction (AutoRef-Keratometer RM8800; Topcon, Tokyo, Japan) and subjective manifest refraction were done. A Snellen chart was used for estimating the best-corrected visual acuity (BCVA).
Data were coded and entered using the statistical package SPSS version 21 (IBM, NewYork, USA). Data were summarized using mean±SD, minimum and maximum in quantitative data, and using frequency (count) and relative frequency (percentage) for categorical data. Numerical error (NE) was calculated as the difference between measured error and the predicted error, and absolute error was calculated as the absolute difference between measured error and the predicted error for each method. Comparisons between variables measured by IOL Master and A-scan were done using paired t-test in normally distributed data, whereas nonparametrical Wilcoxon test was used for non-normally distributed data. Correlations were assessed using linear regression. P values less than 0.05 were considered as statistically significant.
| Results|| |
A total of 32 eyes of 32 patients (24 female and 8 male), with a mean years of age 53.91 (SD: 10.21) (range: 34–72 years) were recruited in this study. Preoperative BCVA ranged from 6/18 to 3/60. The mean preoperative SE was −6.73 (SD±8.03) D. (range −22.00 to +3.50 D).
The types of cataract were subgrouped into posterior subcapsular cataract in 13 (41%) eyes, nuclear cataract grades I and II in 11 (34%) eyes, and cortical cataract in eight (25%) eyes ([Figure 1]).
The mean KAV measured by the keratometer used with the A-scan was 45.18±1.41 D (range: 42.25–46.95 D), but the mean KAV measured by IOL Master was 43.92±2.12 D (range: 39.73–47.56 D). The mean difference between the two methods was −1.258 (P=0.059) ([Table 1]).
|Table 1 Difference between K values as measured by intraocular lens Master and keratometry|
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Axial length measurement
In all patients
The mean AXL using the IOL Master was 25.8±2.56 mm (range: 22–30.53 mm). This was longer than the mean AXL using the A-scan, which was 25.46±3.43 mm (range: 21.16–30.8 mm). The mean difference between the two methods was 0.34 mm, which was statically insignificant (P=0.2112). There was a high correlation between the AXL by IOL Master and AXL by ultrasound, with correlation coefficient of 0.974 (95% confidence interval: 0.935–0.984) ([Figure 2]).
|Figure 2 Relation between the intraocular lens Master vs ultrasound axial length measurements (Pearson correlation coefficient=0.974).|
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[Table 2] shows the AXL in emmetropic and myopic eyes, as measured by the IOL Master and the A-scan.
|Table 2 Comparison between axial length in emmetropic and myopic eyes measured by intraocular lens Master and ultrasound|
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Intraocular lens power calculations
The mean IOL power calculated by IOL Master was +13.50D±7.80 (ranging from +2 D to +21 D). IOL power measured by A-Scan was +13.63±9.05 D (ranging from +1.00 to +27.00 D). ([Table 3]). The mean difference between the two methods was −0.125 (P=0.930), which was statistically insignificant. A comparison between the powers calculated by the IOL Master and the A-scan in the high myopic group is shown in [Table 4].
|Table 3 Comparison between intraocular lens powers calculated by intraocular lens Master and A scan|
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|Table 4 Comparison between the powers calculated by the intraocular lens Master and A-scan in the high myopic group|
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The predicted error as measured by IOL Master was −0.35±0.16 (ranging from −0.65 to −0.12). The predicted error as measured by A-Scan was −0.40±0.21 (ranging from −0.71 to −0.11). The mean difference between the two methods was 0.0475 (P=0.564), which was statistically insignificant ([Table 5]).
|Table 5 Comparison between predicted error as measured by intraocular lens Master and A scan|
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Postoperative BCVA reached 6/9 in 87.50% of cases, whereas it was 6/12 in 12.5% of cases. The mean SE was −0.34±0.61 D postoperatively (ranging from −2.00 to +1.25 D).
The mean numerical error
The mean numerical error (MNE) as measured by IOL Master was 0.05±SD 0.236 (ranging from − 0.29 to 0.51). The MNE measured by A-Scan was −0.005±0.851 (ranging from −1.6 to 1.96). The difference in the mean estimated by two instruments was 0.0463 (P=0.850), being statically insignificant.
The mean absolute error in all patients
The mean absolute error (MAE) as measured by IOL Master was 0.19±SD 0.1417 (ranging from 0.01 to 0.51). The MAE as measured by AS was 0.561±SD 0.623 (ranging from 0.02 to 1.96). The mean difference between the two methods was −0.371 (P=0.0385), which was statistically significant. [Figure 3] and [Table 6] show the MAE in emmetropic and myopic groups as measured by both methods.
|Figure 3 Comparison between mean absolute errors (MAE) measured by IOL Master and A scan.|
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Postoperative SE in 23 patients ranged from −0.50 to 0.00 D, whereas in nine patients, it was outside this range. Estimation of the overall accuracy proved to be 71.875%. Furthermore, on a questionnaire, not a single patient found applanation biometry more comfortable compared with the IOL Master, mainly because the IOL Master does not touch their corneas.
| Discussion|| |
The IOL Master 700 (using Swept-source OCT) provides a measurement that is based on an image, allowing viewing a complete longitudinal section of the eye by the operator. Based on that fact, the operator who will be following the fovea’s image will be alerted that the patient is not fixating sufficiently. He/she will be alerted and can notice irregular eye geometries such as lens tilt. Consequently, more accurate calculations of the IOL power would be done, and hence better postoperative refractive state. It is replacing the ultrasound in measuring the AXL. Moreover, it provides very fast data acquisition and the possibility of measuring the AL along six different axes .
Many clinical implications come from the differences between ultrasound biometry and optical biometry. The first difference is that improvement in the resolution when the wavelength decreases. As the wavelength of the light is shorter than that of the sound, consequently there will be a better resolution, so measuring the AXL with ultrasound is nearly 0.10–0.12 mm, whereas it is 0.012 mm for optical AXL, being more accurate. The second difference is that on measuring the AXL using the ultrasound, it occurs from the anterior corneal apical surface to the internal limiting membrane of the fovea, whereas measuring it with the optical biometry, it is done from the second principal plane of the cornea, which is 0.05 mm deeper than the corneal apex, to the photoreceptor layer, which is 0.25 mm deeper than internal limiting membrane of the fovea. Therefore, it is considered theoretically that ultrasonic biometry reads shorter than optical AXL. Lastly, the ultrasound measurements are done by measuring the anatomic axis as AXL, whereas the optical biometry measures the AXL through the visual axis. Knowing that the visual axis is shorter than the anatomical axis, so optical measurements read a shorter AXL compared with ultrasound measurements .
In our study, we found the AXL that was measured by the IOL Master was 0.34 mm longer than that measured by the A- scan, which was statically insignificant (P=0.2112).
Eleftheriadis  reported the results of 100 consecutive cases of AXL that were measured by both IOL Master and A-scan, where he found the AXL measured by the IOL Master was 0.47 mm longer (P<0.001).
An explanation for why we get shorter AXLs on measuring with ultrasound is owing to the possibility of corneal indentation on placement of the probe, which reduces the AXL. It is possible that the effect of eye contact is responsible for the difference in AXLs as measured by the two methods. However, Rajan et al.  found no significant difference in the AXL (0.04 mm) measured with contact ultrasound and that measured with optical biometry.
In the emmetropic group, the mean difference between the two methods was 0.99 mm, which was statically significant (P=0.00068).
Nakhli  in 2014 reported statistically significant differences when comparing measurements between devices for emmetropic eyes (P=0.033).
This may be explained by the fact that the anatomy of the posterior pole of the eye is comparably small, and the minimal misalignment might cause misdirection of an ultrasound signal from the fovea.
Moon et al.  in 2014 reported that eyes with an AXL shorter than 24.4 mm showed 0.05 mean difference between the two methods, which was statistically insignificant.
In the high myopic group, we found that the mean difference between the two methods was 0.311 mm, which was statically insignificant (P=0.433). The results of this study support the literature in the lack of a statistical difference in long eyes.
The mean average K reading (Kav) measured by keratometer used with A-scan was 45.18±1.41 D (range: 42.25–46.95 D), but the mean Kav measured by the IOL Master was 43.92±2.12 D (range: 39.73–47.56 D). The mean difference between the two methods was −1.258 (P=0.059), which was statistically insignificant.
This may be explained by the fact that the cornea is bell shaped, and it flattens in the periphery. Although manual keratometer uses a ring at a 3 mm on the central cornea to do the measurements, the IOL Master uses a six-point measurement on a 2.5 mm circle, allowing for a measurement that is more central, giving a corneal power for IOL calculation, which yields better clinically and more relevant results. Moreover, the IOL Master gives multiple sets in very small time, giving the patient ease to cooperate well.
In our study, the mean±SD IOL power calculated by IOLM was +13.50±7.80 D (range: +2 to +21 D) which was smaller than that calculated by A-Scan, which was +13.63±9.05 D (range −1.00 to +27.00 D). The mean difference between the two methods was 0.125 (P=0.930), which was statistically insignificant.
Goyal et al.  also found that the A-scan gave higher IOL powers than IOL Master, with a mean difference of 1.01 D (SD 0.96 D) (P<0.01), which was statistically significant.
MNE measured by IOLM was 0.05±0.236 (range − 0.29 to 0.51). MNE measured by A-Scan was −0.005±0.851 (range −1.6 to 1.96). The mean difference between the two methods was 0.0463 (P=0.850), which was statically insignificant.MAE measured by IOLM was 0.19±0.1417 (range: 0.01–0.51). MAE measured by AS was 0.561±0.623 (range: 0.02–1.96). The mean difference between the two methods was −0.371 (P=0.0385), which was statistically significant.
The MNE difference was not found to be statistically significant.
However, MNE has the unfavorable defect of taking the mean of the negative and positive errors. In considering the ultrasound and the IOL Master technologies, we will find the difference MAE, a better helpful measure to find the true estimation of the error, being statistically significant to improve from a 0.561 D. error (MAE as measured by A-scan) to only a 0.19 D error (MAE as measured by IOL Master). This denotes an improvement of 66% in absolute postoperative refractive error measured by IOL Master in comparison with that measured by AUS .
Rose et al.  reported similar results where they found statistically insignificant MNE difference whereas they found statistically significant MAE difference between IOL Master and the ultrasound, being improved from a 0.65 D error to 0.42 D. Such finding represents an improvement of 35% in postoperative absolute refractive error with IOL Master in comparison with AUS.
Moreover, GabAlla et al.  reported that the MAE difference between the two methods showed a statistically significant improvement from an error of 0.37 D to an error of 0.26 D, which represents an improvement of 30% in absolute postoperative refractive error with the IOL Master in comparison with the AUS.
Ocular biometry has definitely become simpler by the IOL Master. Its use does not need to contact the cornea, nor needs instilling surface anesthesia, allowing the patient to be comfortable with no fear of corneal abrasions or disease transmission. In addition, it is much more accurate compared with ultrasound biometry, as AXL measurement is done along the visual axis, whereas in the ultrasound biometry, the misalignment between the visual axis and the axis that has been measured might lead to markedly longer AL measurements. If a posterior staphyloma is present, it can be a real problem in ultrasonic biometry, whereas the IOL Master is very precise in locating the fovea. All beside that, the instrument is much easier while use.
However, ultrasonic biometry is not completely excluded, as a significant number of eyes keep needing ultrasound biometry, from 8 to 10% of cases, such as in opaque ocular media as corneal scarring and opaque cataracts, which prevent the acquisition of optical AXL measurements. Eyes with nonoptimal fixation (age-related macular degeneration) can lead to inaccurate AXL measurements as the measurements are not on the visual axis. In addition, disable patients will be hard enough for them to be positioned on the IOL Master machine.
| Conclusion|| |
In conclusion, the IOL Master is quick and friendly to use and does not need an eye contact with no risk of disease transmission and most patients are comfortable with its use. It makes accurate AXL measurement, and hence a more precise calculation of the power of the IOL, yielding better postoperative refractive state.
However, our study is limited by the few number of patients, but it may be a nidus for future research.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]