Validation of orbscan ii posterior corneal curvature measurement for intraocular lens power calculation

ABSTRACT AIMS To validate the use of Orbscan II slit-scanning topography for measuring posterior corneal curvature, by comparing corneal power calculations using this value with the standard

keratometric method of corneal power calculation. METHOD Corneal measurements were taken from both eyes of 15 normal subjects using the Javal–Schiotz keratometer and the Orbscan II

topographer. Corneal power was calculated using standard keratometric indices of 1.337.5 or 1.331.5 and Javal–Schiotz keratometry. Corneal power was then recalculated using the thick-lens

formula, with anterior corneal curvature from the Javal–Schiotz or Orbscan II and posterior measurements from the Orbscan II; in addition, the 3.0 mm ‘Mean Power’ value from the Orbscan II

software was noted. Six comparisons were then made using mean–difference plots. RESULTS The smallest difference and therefore the most predictable agreement was between Javal–Schiotz

keratometry using a refractive index of 1.331.5 and the thick-lens formula using Javal–Schiotz anterior curvature and Orbscan II posterior curvature. The mean difference was 0.27 D with a

confidence interval of 0.02–0.52 D. CONCLUSIONS In normal eyes, data on posterior corneal curvature from the Orbscan II can be used to calculate corneal power in close agreement with the

standard keratometric method. This suggests the use of the Orbscan II in eyes that have previously undergone refractive surgery, for calculation of intraocular lens power prior to cataract

surgery. SIMILAR CONTENT BEING VIEWED BY OTHERS COMPARISON OF CORNEAL MEASUREMENTS USING TWO DIFFERENT SCHEIMPFLUG ANALYZERS IN SIRIUS AND PENTACAM DEVICES Article Open access 07 October

2023 ASSESSMENT OF CORNEAL BIOMECHANICS, TONOMETRY AND PACHYMETRY WITH THE CORVIS ST IN MYOPIA Article Open access 04 February 2021 COMPARISON OF CORNEAL TOPOGRAPHY MAPS OBTAINED USING THE

IOLMASTER 700® AND THE ANTERION® IN CANDIDATES FOR TORIC IOL IMPLANTATION Article 23 April 2024 INTRODUCTION Intraocular lens (IOL) power calculation for cataract surgery is problematic in

patients who have previously undergone corneal refractive surgery. The difficulty arises from the inability to measure posterior corneal curvature. Instead, corneal power is estimated from

the anterior corneal curvature alone using an approximation that considers the cornea as a single refracting surface, with two parameters, radius and refractive index. This assumes a

relationship between the anterior and posterior corneal curvatures. Following excisional refractive surgery, for example, excimer laser photorefractive keratectomy (PRK) and laser _in situ_

keratomileusis (LASIK), the anterior surface is flattened, while the posterior surface is unchanged or may even steepen slightly.1 Clearly, the relationship between the anterior and

posterior surfaces is altered by such surgery. Laser refractive surgery for myopia reduces the converging power of the anterior surface while retaining the diverging power of the posterior

surface. Thus, calculation of the total corneal power from anterior surface measurements alone underestimates the reduction in power. The net effect of this is to underestimate the IOL power

required, resulting in relative postoperative hyperopic refraction.2 The Orbscan II (Orbtek, Salt-Lake City, UT, USA) is a scanning slit-beam topographer that measures posterior corneal

elevation and thereby curvature. If this measurement were to be validated, then determination of corneal power after refractive surgery would be greatly simplified. This study aimed to test

the validity of slit-scan topography measurement of posterior corneal curvature in normal eyes by using it to calculate corneal power, and comparing this result to the corneal power

calculated using Javal–Schiotz keratometry. METHODS Fifteen normal subjects with no history of corneal disease or ocular surgery underwent Javal–Schiotz keratometry and slit-scan topography,

performed by the author. Javal–Schiotz keratometry was recorded to the nearest 0.05 mm. Slit-scan topography-simulated keratometry was recorded to 0.01 mm. CALCULATION OF CORNEAL POWER

SINGLE REFRACTING SURFACE ASSUMPTION Corneal power may be estimated considering the cornea as a single refracting surface of negligible thickness. Refractive indices of 1.3375 (the standard

keratometric index (SKI) designated by Javal) and 1.3315 (the refractive index recommended by Gullstrand in his schematic eye and independently by Olsen3) are used in this study, designated

‘Javal 337.5’ and ‘Javal 331.5’, respectively. THICK-LENS FORMULA CALCULATION The total optical power of the cornea may be calculated according to the formula for thick lenses:4 where with

_n_1 being the refractive index of air (1.0), _n_2 the refractive index of cornea (1.376), _n_3 the refractive index of aqueous (1.336), and _t_ the corneal thickness (m). For this study,

power is calculated using slit-scan topography data for _r_1, _r_2 and corneal thickness (‘Orbscan Power’), or using Javal–Schiotz keratometry for _r_1 and slit-scan topography for _r_2 and

corneal thickness (‘Orbscan/Javal Power’). The Orbscan II proprietary software package also calculates the total corneal power averaged across the central 3 mm of cornea; this result is

presented as ‘Orbscan Mean Power’. DATA COMPARISON This study compares two methods of measuring a parameter for which the true result is unknown. Assessment of agreement between the two

methods is therefore best assessed using Bland–Altman mean–difference plots5 (Figure 1a–f). For each plot, mean total corneal power, calculated as the arithmetic mean of the two methods

being assessed is plotted (_x_-axis) against the difference between the two methods (_y_-axis). This allows analysis of any systematic difference between the methods (from the mean of the

differences) and the consistency of any difference (the spread of the differences), including altered relationship between methods as the mean is varied. Six comparisons were made: ‘Javal

337.5’ was compared with ‘Orbscan Power (Figure 1a), ‘Orbscan/Javal Power’ (Figure 1b), and ‘Orbscan Mean Power’ (Figure 1c). ‘Javal 331.5’ was compared with ‘Orbscan Power’ (Figure 1d),

‘Orbscan/Javal Power’ (Figure 1e), and ‘Orbscan Mean Power’ (Figure 1f), all as defined above. Data from both eyes are presented in the plots; only right eye data were used for calculations

of the mean keratometry, simulated keratometry, and mean and standard error of differences between methods. A two-tailed paired _t_-test was used to compare keratometry and simulated

keratometry. All calculations and data plots were performed on Excel 97 (Microsoft, Seattle, WA, USA). RESULTS PATIENTS In all patients, both keratometry and slit-scan topography were

possible. The mean keratometry was 7.92 mm (SD 0.28) and topographic simulated keratometry was 8.08 mm (SD 0.28) (_P_<0.001). The mean topographic simulated posterior curvature was 6.67 

mm (SD 0.26). MEAN–DIFFERENCE PLOTS The six mean–difference plots are shown in Figure 1. Mean and standard deviations of power and difference between powers are given in Table 1. The

smallest confidence interval of differences (0.02–0.52 D) and therefore most predictable agreement was between the ‘Javal 331.5’ and the ‘Orbscan/Javal Power’ (Figure 1e). The ‘Javal 331.5’

power was a mean of 0.27 D less than the ‘Orbscan/Javal Power’. DISCUSSION A number of methods have been described to calculate corneal power following refractive surgery.6,7 The

peroperative data method8 requires knowledge of the preoperative keratometry and refraction, and stable post-excimer laser refraction prior to the development of cataract. If pre- and

post-laser keratometry is known, then an estimate of corneal power can be made using the thick-lens formula assuming that the posterior corneal curvature does not change.6,9 If these

preoperative data are unavailable, as is often the case, the hard contact lens method10 must be used. The difference between unaided and plano-contact lens over-refraction spherical

equivalent refractions is subtracted from the base curve power of the contact lens to determine effective corneal power. This requires reasonable visual acuity for accurate subjective

refraction, which may not be present in the precataract surgery situation, and may be rendered inaccurate by irregularities of corneal shape and/or disruption to the tear lens under the

contact lens. The difficulties inherent in these methods result in the recommendation to perform all possible calculations and then use the lowest corneal power that results in order to err

on the side of a myopic outcome.9 Unfortunately, any significant postoperative error is likely to result in dissatisfaction in a group of patients who have previously undergone the risks and

financial costs of surgery in order to achieve emmetropia. A more accurate method of corneal power calculation is therefore required. Hamed _et al_9 estimated posterior corneal curvature in

eyes prior to LASIK, assuming that posterior curvature was related to anterior curvature by a factor of 6.8/7.7. Assuming no change in the posterior corneal power, they calculated

post-LASIK power, and compared it with that determined by the refractive history method. They found reasonable agreement (SD of differences 0.66). This method does, however, like the

refractive history method, require knowledge of pre-LASIK corneal curvature. In addition, it assumes stability of the posterior corneal shape following LASIK, which is disputed.1 Clearly,

direct measurement of the posterior corneal curvature would be preferable to making the assumptions described above. The keratometer uses the tear film on the anterior corneal surface as a

convex mirror.4 At a fixed working distance, the object size is adjusted to form the first Purkinje image of a constant size, object size is inversely proportional to curvature. For, an

average eye, only the central 3.2 mm of cornea contributes to the reading. Owing to the small difference in refractive indices between the cornea and aqueous, the image from the posterior

cornea is too faint to allow measurement of curvature. The Orbscan II calculates corneal elevation by triangulating between the edge of an illuminating beam of known (calibrated) position

and the back-scattered light emitted perpendicular to the instrument. The corneal elevation between adjacent points is interpolated as a section of a sphere and local curvature calculated as

the average curvature in all directions at that point. Curvature over the central 3 mm is averaged to generate simulated keratometry, and is possible for both anterior and posterior corneal

surfaces. Direct validation of slit-scan topography posterior corneal measurement has not been possible so far. Slit-scan topography-calculated total optical power change has been

demonstrated to correlate well with refractive change following myopic LASIK (correlation coefficient=0.835, _P_<0.04).11 From this study, it can be stated that slit-scan topography

measurement of posterior corneal curvature and subsequent calculation of corneal power is in close agreement with that calculated using the Javal–Schiotz keratometer and the single

refracting surface approximation. The best agreement was obtained using a refractive index of 1.3315 for the keratometric calculation, and Javal–Schiotz anterior corneal keratometry in

conjunction with Orbscan posterior corneal curvature for the thick-lens formula calculation. IOL power calculation with the SRK/T formula using either method is clinically interchangeable

(approximately 0.3 D difference in IOL power selection for emmetropia in an average eye). Using the ‘Mean Power’ calculated by the Orbscan II software led to worse agreement than calculating

power using the thick-lens formula as described above. It is not clear from the Orbscan II technical data how the ‘Mean Power’ is calculated, and the manufacturers were unable to provide

information as to the reason for the disparity. Use of slit-scan topography-simulated anterior corneal curvature in the thick-lens formula results in less good agreement with the standard

Javal–Schiotz measurement of corneal power than substituting the keratometric anterior corneal curvature. Javal–Schiotz keratometry has been demonstrated to be highly accurate and repeatable

in normal corneas. 12 Following refractive surgery, loss of sphericity of the central cornea reduces its reliability, and automated keratometry and videokeratoscopy have been suggested to

be more accurate. 13 Orbscan II consistently measured flatter keratometry than did the Javal–Schiotz keratometer, used in IOL power calculation after refractive surgery, which would tend

towards myopic error. For these two reasons, following refractive surgery, slit-scan topography might generate a better value for anterior corneal curvature than the Javal–Schiotz

keratometer. CONCLUSIONS Orbscan II measurement of posterior corneal curvature may be used to calculate corneal power in normal eyes, using the thick-lens formula. The same methodology could

be applied to corneal power calculation after refractive surgery, comparing the perioperative data method of corneal power calculation with Orbscan II power calculation. If good agreement

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Scholar  Download references AUTHOR INFORMATION Author notes * The author has no proprietary interests in any of the instruments described AUTHORS AND AFFILIATIONS * Royal Berkshire

Hospital, Reading, UK M Leyland Authors * M Leyland View author publications You can also search for this author inPubMed Google Scholar ADDITIONAL INFORMATION Poster presented at the Royal

College of Opthalmologists, Annual Congress, Manchester, 21–23 May 2002 RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Leyland, M. Validation of Orbscan

II posterior corneal curvature measurement for intraocular lens power calculation. _Eye_ 18, 357–360 (2004). https://doi.org/10.1038/sj.eye.6700659 Download citation * Received: 06 June

2002 * Accepted: 15 January 2003 * Published: 07 April 2004 * Issue Date: 01 April 2004 * DOI: https://doi.org/10.1038/sj.eye.6700659 SHARE THIS ARTICLE Anyone you share the following link

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content-sharing initiative KEYWORDS * keratometry * topography * LASIK * intraocular lens * biometry