Journal of Refractive Surgery

Original Article 

Repeatability of a Commercially Available Adaptive Optics Visual Simulator and Aberrometer in Normal and Keratoconic Eyes

Rohit Shetty, FRCS, DNB, PhD; Shruti Kochar, MS, DNB; Tushar Grover, MS, DNB; Pooja Khamar, MBBS, MS; Pallak Kusumgar, MD; Kanchan Sainani, MD; Abhijit Sinha Roy, PhD

Abstract

PURPOSE:

To evaluate the repeatability of aberration measurement obtained by a Hartmann–Shack aberrometer combined with a visual adaptive optics simulator in normal and keratoconic eyes.

METHODS:

One hundred fifteen normal eyes and 92 eyes with grade I and II keratoconus, as per the Amsler–Krumeich classification, were included in the study. To evaluate the repeatability, three consecutive measurements of ocular aberrations were obtained by a single operator. Zernike analyses up to the 5th order for a pupil size of 4.5 mm were performed. Statistical analyses included the intraclass correlation coefficient (ICC) and within-subject standard deviation (SD).

RESULTS:

For intrasession repeatability, the ICC value for sphere and cylinder was 0.94 and 0.93 in normal eyes and 0.98 and 0.97 in keratoconic eyes, respectively. The ICC for root mean square of higher order aberrations (HOARMS) was 0.82 in normal and 0.98 in keratoconic eyes. For 3rd order aberrations (trefoil and coma), the ICC values were greater than 0.87 for normal eyes and greater than 0.92 for keratoconic eyes. The ICC for spherical aberration was 0.92 and 0.90 in normal and keratoconic eyes, respectively.

CONCLUSIONS:

Visual adaptive optics provided repeatable aberrometry data in both normal and keratoconic eyes. For most of the parameters, the repeatability in eyes with early keratoconus was somewhat better than that for normal eyes. The repeatability of the Zernike terms was acceptable for 3rd order (trefoil and coma) and spherical aberrations. Therefore, visual adaptive optics was a suitable tool to perform repeatable aberrometric measurements.

[J Refract Surg. 2017;33(11):769–772.]

Abstract

PURPOSE:

To evaluate the repeatability of aberration measurement obtained by a Hartmann–Shack aberrometer combined with a visual adaptive optics simulator in normal and keratoconic eyes.

METHODS:

One hundred fifteen normal eyes and 92 eyes with grade I and II keratoconus, as per the Amsler–Krumeich classification, were included in the study. To evaluate the repeatability, three consecutive measurements of ocular aberrations were obtained by a single operator. Zernike analyses up to the 5th order for a pupil size of 4.5 mm were performed. Statistical analyses included the intraclass correlation coefficient (ICC) and within-subject standard deviation (SD).

RESULTS:

For intrasession repeatability, the ICC value for sphere and cylinder was 0.94 and 0.93 in normal eyes and 0.98 and 0.97 in keratoconic eyes, respectively. The ICC for root mean square of higher order aberrations (HOARMS) was 0.82 in normal and 0.98 in keratoconic eyes. For 3rd order aberrations (trefoil and coma), the ICC values were greater than 0.87 for normal eyes and greater than 0.92 for keratoconic eyes. The ICC for spherical aberration was 0.92 and 0.90 in normal and keratoconic eyes, respectively.

CONCLUSIONS:

Visual adaptive optics provided repeatable aberrometry data in both normal and keratoconic eyes. For most of the parameters, the repeatability in eyes with early keratoconus was somewhat better than that for normal eyes. The repeatability of the Zernike terms was acceptable for 3rd order (trefoil and coma) and spherical aberrations. Therefore, visual adaptive optics was a suitable tool to perform repeatable aberrometric measurements.

[J Refract Surg. 2017;33(11):769–772.]

Aberrations are unique features of a patient's eye that arise from refractive interfaces (eg, cornea and lens).1 Aberrations can affect the quality of vision1 and are known to increase in keratoconus and other ectatic disorders.2–4 Adaptive optics technology could play an important role in determining the best combination of aberrations to maximize the quality of vision. When corrected with adaptive optics, visual quality and contrast sensitivity can be improved beyond the limits of spectacles and contact lenses in normal eyes.5 A new prototype device that combined the Hartmann–Shack aberrometer and deformable mirror was demonstrated in 2002.6 The device was capable of measuring and modifying the ocular aberrations for functional vision testing of the patient.6,7 Subsequently, the device was modified to replace the deformable mirror with a liquid crystal on silicon light modulator to perform specific modulation of aberrations.8–10 The test–retest variability of the aberrometer in the commercial device was assessed in normal eyes.11 However, repeatability of this device on keratoconic eyes needs to be assessed. This is particularly important for surgical treatments such as wavefront-guided surgery, where ablation volume is computed based on reliable measurement of ocular aberrations. Therefore, the purpose of the current study was to analyze intrasession repeatability of the device for lower order aberrations and higher order aberrations (HOAs) in normal and keratoconic eyes.12

Patients and Methods

This was a prospective study, approved by the Narayana Nethralaya Eye Hospital Ethics Committee, Bengaluru, India. The research followed the tenets of the Declaration of Helsinki and all participants gave written informed consent. One hundred fifteen normal eyes of 115 patients and 92 keratoconic eyes (grade I and II severity on Amsler–Krumeich classification13,14) of 92 patients were recruited for the study. For the normal population, only normal eyes with a corrected distance visual acuity of 20/20 or better, spherical error less than −5.00 diopters (D), and astigmatism less than 4.00 D were included. Keratoconus was diagnosed on the basis of clinical signs such as scissoring of the red reflex or an abnormal retinoscopy reflex, Fleischer's ring, Vogt's striae, and topographic evidence.15 Exclusion criteria included the presence of progressive myopia, advanced keratoconus, active ocular disease, diabetic retinopathy, contact lens wear, or any other ocular diagnosis that may alter the optical quality. All participants underwent a complete ocular examination before the test.

Measurements of ocular aberrations were obtained using the Visual Adaptive Optics simulator (Voptica S.L., Murcia, Spain). The device measured the refraction in diopters and the root mean square of HOAs (HOARMS) in microns for a pupil size of 4.5 mm. The device software provided an output up to 8th order Zernike decomposition of the aberrations. However, the current study was limited up to 5th order to maintain clinical relevance because most Zernike coefficients of 5th order and higher were generally small in magnitude compared to 4th order and lower. After proper focus and alignment, three measurements were obtained by the same experienced operator (SK) using the same method for normal and keratoconic eyes. Defocus (sphere), astigmatism, trefoil (Z6 and Z9), coma (Z7 and Z8), spherical aberration (Z12), and the total HOAs (HOARMS) were assessed using the device.

Statistical Analysis

Statistical analyses were performed using MedCalc statistical software (version 16.8; MedCalc Software, Inc., Mariakerke, Belgium) and included within-subject standard deviation. The standard deviation was calculated as the square root of the mean square error.16 An intraclass correlation coefficient (ICC), a measure of the repeatability of measurements, was also obtained.17 This correlation measured the relative homogeneity within groups (between the repeated measurements) in relation to the total variation. The ICC approached 1.0 when the variability within repeated measurements was zero. Low intra-observer repeatability is generally assumed when the ICC is below 0.75. The three measurements for each participant were considered to account for intra-session repeatability.

The sample size of normal and keratoconic eyes for this study was sufficient (within 10% confident limit).18

Results

The mean age of the normal eyes was 27.3 ± 4.3 years (range: 16 to 40 years). Three measurements each for sphere (defocus), cylinder (astigmatism), HOARMS, and Zernike polynomials trefoil (Z6 and Z9), coma (Z7 and Z8), and spherical aberration (Z12) are summarized in Table 1. The mean value with 95% confidence interval and the mean ± SD across participants for a given measurement were assessed. Table 1 also shows ICCs for all of the parameters. An ICC above 0.8 was obtained for many parameters, suggesting good repeatability in normal eyes (sphere: 0.99, cylinder: 0.99, HOARMS: 0.80). Further, lower order aberrations (sphere and cylinder) were associated with better repeatability than HOAs in normal eyes. The ICC of axis of cylinder was low (0.71). Therefore, the ICC was calculated for the subgroup of eyes based on magnitude of cylinder. For eyes with cylinder less than 0.50 D, the ICC was only 0.49. For eyes with cylinder greater than 1.00 D, the ICC improved to 0.84. For eyes with cylinder greater than 2.00 D, ICC was 0.83. Thus, magnitude of cylinder was critical in obtaining repeatable measurements of axis.

Parameters in Normal Eyes (n = 115)a

Table 1:

Parameters in Normal Eyes (n = 115)

Relevant parameters for keratoconic patients (51 women and 41 men) with a mean age of 26.7 ± 10.8 years (range: 14 to 42 years) are summarized in Table 2. An ICC greater than 0.9 was obtained for all parameters (sphere: 0.94, cylinder: 0.96, HOARMS: 0.95), suggesting high repeatability in grade I and grade II keratoconus. In this cohort, the ICC was comparable for both lower order aberrations and HOAs. In patients with keratoconus, the ICC of the axis was greater than the ICC of the axis in normal eyes (0.95) due to greater magnitude of aberrations and abnormal curvature.

Parameters in Keratoconic Eyes (n = 92)a

Table 2:

Parameters in Keratoconic Eyes (n = 92)

Discussion

The Hartmann–Shack aberrometer (Irx3; Imagine Eyes, Orsay, France; Keratron, Optikon, Rome, Italy) can give excellent results for total ocular aberrations.19 A highly repeatable measurement of aberrations will help tremendously in designing optical corrections using wavefront-guided corneal surgery,20 aberration-correcting contact lenses,21 and wavefront-based custom intraocular lenses.22 In this study, overall the device achieved high and similar repeatability in both normal and keratoconic eyes.

Another study evaluated the inter-session and intra-session repeatability of aberrations in normal individuals using visual adaptive optics.11 They reported ICC values of 0.99 and 0.97 for sphere and cylinder, respectively. However, the ICC for HOARMS was 0.61, which implied that lower order aberrations had better repeatability than HOAs. Overall, better results were obtained with RMS values than with some of the individual Zernike coefficients (Tables 12). Other commercial instruments based on Hartmann–Shack sensors were comparable to our study. In a study using the Zywave (Bausch & Lomb, Rochester, NY), the ICC of total HOAs and second-order terms were greater than 0.94. The ICC for 3rd-order terms was high (ICCs > 0.87).23 Using the iDesign aberrometer (Abbot Medical Optics, Inc., Santa Ana, CA), the ICC for sphere and cylinder was 0.999 and 0.995 D, respectively.24 Another study evaluated the repeatability of Topcon KR-1W (Topcon, Inc., Tokyo, Japan).25 For intra-session repeatability, excellent ICC values were obtained (ICC > 0.87), except for internal primary coma (ICC = 0.75) and 3rd order (ICC = 0.72) HOA.25

Studies suggested that measurement errors may occur in keratoconic eyes with Hartmann–Shack devices.26 A study used the IRX3 (Imagine Eyes, Paris, France) to compare the aberrometry-derived refractive error in patients with keratoconus and normal eyes (n = 12, age: 33.6 ± 4.2 years). The lower order aberrations and HOAs measured in keratoconic eyes showed high variability compared to measurements in normal eyes.27 In the current study, most of the parameters had comparable repeatability in normal and keratoconic eyes, although keratoconic eyes had somewhat better repeatability. However, whether a similar trend will remain for higher grades of keratoconus is unknown at this point. Another study reported better repeatability (good to moderate) in keratoconic eyes than in normal eyes (moderate to poor).28 However, the current study established that visual adaptive optics demonstrated good performance in terms of single-session intra-user repeatability for refractive and wavefront parameters. A future study comparing this device with other Hartmann–Shack aberrometers could be worthwhile. The study conclusions may not apply to eyes with more advanced stages of the disease.

References

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Parameters in Normal Eyes (n = 115)a

ParameterExam 1Exam 2Exam 3ICC
Sphere (D)−1.82 ± 2.05 (−2.20, −1.44)−1.81 ± 2.07 (−2.20, −1.43)−1.82 ± 2.09 (−2.21, −1.44)0.99
Cylinder (D)−0.72 ± 0.94 (−0.90, −0.55)−0.76 ± 0.99 (−0.94, −0.57)−0.73 ± 0.98 (−0.92, −0.55)0.99
Axis (degrees)92.94 ± 61.29 (81.62, 104.26)92.23 ± 59.75 (81.20, 103.27)90.16 ± 60.85 (78.92, 101.40)0.71
HOARMS0.18 ± 0.09 (0.17, 0.20)0.17 ± 0.08 (0.15, 0.18)0.18 ± 0.10 (0.17, 0.20)0.80
Z6−0.05 ± 0.06 (−0.06, −0.04)−0.05 ± 0.06 (−0.06, −0.04)−0.05 ± 0.07 (−0.06, −0.04)0.85
Z7−0.02 ± 0.10 (−0.04, 0.0)−0.02 ± 0.10 (−0.03, 0.0)−0.02 ± 0.10 (−0.04, 0.0)0.95
Z80.0 ± 0.065 (−0.02, 0.0)0.0 ± 0.063 (−0.01, 0.01)0.0 ± 0.065 (−0.017, 0.01)0.81
Z90.0 ± 0.07 (−0.01, 0.01)0.02 ± 0.06 (0.0, 0.03)0.0 ± 0.07 (−0.01, 0.02)0.86
Z120.02 ± 0.05 (0.01, 0.025)0.01 ± 0.05 (0.0, 0.02)0.02 ± 0.06 (0.0, 0.03)0.91

Parameters in Keratoconic Eyes (n = 92)a

ParameterExam 1Exam 2Exam 3ICC
Sphere (D)−1.77 ± 2.53 (−2.32, −1.22)−1.88 ± 2.48 (−2.42, −1.34)−1.82 ± 2.48 (−2.36, −1.28)0.94
Cylinder (D)−3.51 ± 2.79 (−4.11, −2.90)−3.42 ± 2.76 (−4.02, −2.83)−3.43 ± 2.51 (−3.98, −2.88)0.96
Axis (degrees)93.02 ± 53.7 (81.36, 104.68)90.70 ± 54.0 (78.99, 102.42)90.62 ± 53.8 (78.96, 102.28)0.95
HOARMS0.72 ± 0.51 (0.61, 0.83)0.72 ± 0.51 (0.61, 0.83)0.72 ± 0.50 (0.62, 0.83)0.98
Z6−0.01 ± 0.39 (−0.10, 0.07)−0.03 ± 0.41 (−0.12, 0.06)−0.02 ± 0.35 (−0.10, 0.06)0.89
Z7−0.37 ± 0.33 (−0.45, −0.30)−0.35 ± 0.40 (−0.44, −0.26)−0.40 ± 0.40 (−0.48, −0.31)0.88
Z80.01 ± 0.33 (−0.04, 0.06)0.03 ± 0.40(−0.02, 0.07)0.03 ± 0.40 (−0.02, 0.08)0.90
Z90.07 ± 0.34 (0.0, 0.14)0.03 ± 0.32 (−0.04, 0.10)0.02 ± 0.26 (−0.04, 0.08)0.95
Z12−0.08 ± 0.16 (−0.12, −0.05)−0.09 ± 0.18 (−0.12, −0.05)−0.07 ± 0.18 (−0.11, −0.03)0.87
Authors

From Corneal and Refractive Services (RS, SK, TG, PKhamar, PKusumgar, KS), and Imaging, Biomechanics and Mathematical Modeling Solutions (ASR), Narayana Nethralaya Foundation, Bangalore, India.

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (RS, ASR); data collection (SK, TG, PKhamar, PKusumgar, KS); analysis and interpretation of data (SK, TG, PKhamar, PKusumgar, KS); writing the manuscript (SK, TG, PKhamar, PKusumgar, KS); critical revision of the manuscript (RS, ASR)

Correspondence: Abhijit Sinha Roy, PhD, Narayana Nethralaya, #258A Hosur Road, 34 Narayana Health City, Bommansandra, Bangalore 560099, India. E-mail: asroy27@yahoo.com

Received: January 20, 2017
Accepted: July 14, 2017

10.3928/1081597X-20170718-02

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