Journal of Refractive Surgery

Original Article Supplemental Data

Repeatability of OCT Anterior Surface and Bowman's Layer Curvature and Aberrations in Normal and Keratoconic Eyes

Himanshu Matalia, MD; Nandini Chinnappaiah, MD; Rachana Chandapura, MTech; Sugaranjini Galiyugavaradhan, MD; Rohit Shetty, MD, PhD, FRCS; Abhijit Sinha Roy, PhD

Abstract

PURPOSE:

To study the repeatability of anterior surface and Bowman's layer curvature in normal and keratoconic eyes using optical coherence tomography (OCT).

METHODS:

In this study, 96 normal and 96 keratoconic eyes underwent corneal imaging using Pentacam (Oculus Optikgeräte, Wetzlar, Germany) and OCT (Triton, Topcon Corporation, Tokyo, Japan). The elevation data from segmented air–epithelium (A–E) and epithelium–Bowman's layer (E-B) interfaces in OCT scans were used to quantify curvature and aberrations. The wavefront aberrations were evaluated with the ray tracing method and 6th order Zernike polynomials. The intraclass correlation coefficient (ICC), within-subject standard deviation (Sw), and coefficient of variation (CoV) were used to assess repeatability.

RESULTS:

For curvatures, the Sw was less than 0.25 diopters (D) for the normal and keratoconic eyes. The Sw was highest for root mean square of lower order aberrations (0.14 µm) in keratoconic eyes. The CoV for curvatures was well below 0.5% for both groups. For some aberrations irrespective of groups, the CoV was greater because some individual aberrations (mean of three successive measurements) tended to be smaller in magnitude and even a small Sw resulted in a high CoV. For all variables, the ICC ranged between 0.80 and 0.99 for both the OCT and Pentacam measurements. Most variables were similar between the A–E and E-B interfaces (P > .05) for both groups. However, both differed significantly from all Pentacam variables (P < .05) in normal and keratoconic eyes.

CONCLUSIONS:

The repeatability of OCT curvatures and aberrations compared well with the Pentacam indices for normal and keratoconic eyes.

[J Refract Surg. 2020;36(4):247–252.]

Abstract

PURPOSE:

To study the repeatability of anterior surface and Bowman's layer curvature in normal and keratoconic eyes using optical coherence tomography (OCT).

METHODS:

In this study, 96 normal and 96 keratoconic eyes underwent corneal imaging using Pentacam (Oculus Optikgeräte, Wetzlar, Germany) and OCT (Triton, Topcon Corporation, Tokyo, Japan). The elevation data from segmented air–epithelium (A–E) and epithelium–Bowman's layer (E-B) interfaces in OCT scans were used to quantify curvature and aberrations. The wavefront aberrations were evaluated with the ray tracing method and 6th order Zernike polynomials. The intraclass correlation coefficient (ICC), within-subject standard deviation (Sw), and coefficient of variation (CoV) were used to assess repeatability.

RESULTS:

For curvatures, the Sw was less than 0.25 diopters (D) for the normal and keratoconic eyes. The Sw was highest for root mean square of lower order aberrations (0.14 µm) in keratoconic eyes. The CoV for curvatures was well below 0.5% for both groups. For some aberrations irrespective of groups, the CoV was greater because some individual aberrations (mean of three successive measurements) tended to be smaller in magnitude and even a small Sw resulted in a high CoV. For all variables, the ICC ranged between 0.80 and 0.99 for both the OCT and Pentacam measurements. Most variables were similar between the A–E and E-B interfaces (P > .05) for both groups. However, both differed significantly from all Pentacam variables (P < .05) in normal and keratoconic eyes.

CONCLUSIONS:

The repeatability of OCT curvatures and aberrations compared well with the Pentacam indices for normal and keratoconic eyes.

[J Refract Surg. 2020;36(4):247–252.]

Corneal epithelium masks irregularities of the stromal surface.1,2 Thus, irregular epithelium could be an additional tool for diagnosing keratoconus.3–5 The curvature of keratoconic eyes differs significantly between the stroma and the anterior corneal surface.2,6 Further, a treatment plan based on anterior corneal surface topography may leave the irregularities on the stromal surface partially uncorrected due to the masking effect of the epithelium thickness.1,2,7 A few cases of treatment guided by stromal surface elevation showed a significant reduction in stromal irregularities and improvement in visual quality, while preserving valuable stromal tissue.1,8

Several corneal tomographers can assess the anterior and posterior surface of the cornea. Recently, curvature of the Bowman's layer was analyzed using clinically approved high-resolution optical coherence tomography (OCT) devices.9–11 Also, OCT-based epithelium thickness distribution could be used to detect corneal diseases.3,5 Recent studies have demonstrated a non-contact method to quantify curvature and aberrations of the anterior corneal surface and Bowman's layer using OCT.10,11 However, the repeatability of Bowman's layer curvature is yet to be investigated. The purpose of this study was to test the repeatability of three consecutive measurements from anterior segment OCT and Pentacam (Oculus Optikgeräte, Wetzlar, Germany). This would establish the potential use of Bowman's layer curvature in clinical practice.

Patients and Methods

This was a prospective, observational, cross-sectional study. This study was approved by the ethics committee of the Narayana Nethralaya Eye Hospital, Bangalore, India. The study adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all participants.

The study included 96 normal (96 patients) and 96 clinically keratoconic (96 patients) eyes. The normal eyes were selected at random by a coin toss. In keratoconic patients, the more severely affected eye was chosen for analyses. The screening of all patients was performed by a single physician (HM) over a period of 6 months. Three consecutive measurements on Pentacam and OCT (Triton, Swept Source 100 kHz; Topcon Corporation, Tokyo, Japan) were obtained for each participant. Eyes with no clinical signs of keratoconus such as Fleischer ring, Vogt striae, scissoring of the red reflex, an abnormal retinoscopy, thin corneas, and curvature asymmetry causing abnormal corneal astigmatism were classified as normal. These eyes were from patients who subsequently underwent laser in situ keratomileusis (LASIK) in our hospital. Eyes with corneal steepening, stromal thinning, asymmetric astigmatism, corneal scarring, and abnormal topography (asymmetric astigmatism and focal steepening) were classified as keratoconus.

Exclusion criteria were any prior corneal surgery, ocular surgery, contact lens wear, autoimmune disorders, pregnancy, corneal scarring, and allergy. Also, all scans with significant blinking errors were excluded.

The 25 radial scan (50 semimeridians) mode of Pentacam was used. The curvature of the anterior corneal surface was derived from the elevation data, which was exported from Pentacam as a comma separated value file.12 The aberrations were computed using ray tracing on the same elevation data.12 Further, Zernike polynomials up to the 6th order were used to analyze aberrations. A net refractive index of 1.3375 was used for all calculations. The analysis zone was restricted to a 6-mm diameter because the size of the OCT scans was limited to 6 mm.

In this study, clinical OCT was used to acquire 12 radial frames (24 semi-meridians). Only OCT scans with a signal strength of 40 and above were included in the analyses. Further, all scans with visually significant deviation from the center of the cornea were excluded. The distortion-corrected OCT B-scans were then used to segment the air–epithelium (A–E) and epithelium–Bowman's layer (E-B) interfaces.10,11,13 The distortion-corrected scans were provided by the manufacturer. The curvature was calculated using the elevation data of the segmented OCT interfaces. Using graph search, the anterior edge and E-B interface from each B-scan were detected. The pixel coordinates along the edges were converted to radius (r) and elevation (z) coordinates using the digital pixel to micron resolution of the B-scan. These coordinates were fitted with a 6th order polynomial. The tangential and axial curvature at a specific radial location (r) on each edge were calculated using Equations 1 and 2, respectively:

Tangential Power(r)=(n−1)/[(1+(dz/dr)2)1.5/|d2z/dr2|]
Axial Power(r)=∫0.0rTangential Power(r)drr

Here, n was the net refractive index of the cornea and dz/dr and d2z/dr2 were the first and second order gradients, respectively.

Also, the elevation data were used to quantify aberrations using the method similar to the one followed for Pentacam.12 A net refractive index of 1.3375 was used for all of the above calculations. In the OCT scans, the A–E interface formed the anterior surface of the cornea. The OCT A–E interface was in concept the same as the anterior corneal surface obtained from Pentacam. Further, air was used as the incident medium for the analyses of both the A–E and E-B interfaces simply to enable direct quantitative comparison between the shape of the two surfaces (A–E and E-B). A time interval of 30 seconds was set between repeat OCT and Pentacam measurements. The Pentacam only provided the elevation of the anterior corneal surface. The OCT provided the elevation of two surfaces (ie, A–E and E-B). Both curvature and aberrations were derived offline using custom scripts.10–13

Statistical Analyses

All variables were reported as mean and 95% confidence interval (CI) of the mean. The curvature (diopters [D]) and aberrations (µm) from OCT (A–E and E-B interface) and Pentacam were analyzed. The axial curvature variables were steep axis curvature (K1), flat axis curvature (K2), and maximum curvature (Kmax). The aberration variables were defocus, spherical aberration, vertical and horizontal coma, astigmatism of 0° and 45°, and root mean square (RMS) of lower and higher order aberrations (LOA and HOA, respectively). The intraclass correlation coefficient (ICC) was used to test the repeatability of the devices for each surface. An ICC equal to or close to 1 indicated excellent repeatability (ie, the repeat measurements had the same magnitude). The within-subject standard deviation (Sw) and coefficient of variance (CoV = 100 × Sw / Mean) were analyzed between the three successive measurements. The smaller their magnitude, the better is the repeatability. The differences between the variables obtained from the OCT and Pentacam were analyzed using the paired t test. MedCalc v18.5.0 software (MedCalc, Inc., Mariakerke, Belgium) was used to perform all analyses. A P value of less than .05 was considered to be statistically significant.

Results

The mean age was 26.53 ± 6.98 years for the 96 normal eyes and 26.06 ± 5.51 years for the 96 kerato-conic eyes (P = .92). The sample size of 96 eyes was adequate for repeatability.14 The mean central corneal thickness (CCT) was 537.38 ± 3.34 µm for the normal eyes and 467.25 ± 5.03 µm for the keratoconic eyes (P < .001). Table 1 lists the ICC of all indices derived from the repeat measurements on OCT and Pentacam. In the normal eyes, each analyzed variable had an ICC between 0.80 and 0.99 for both the OCT A–E and E-B interfaces. The keratoconic eyes had an excellent repeatability with an ICC of more than 0.90 (Table 1). For the Pentacam anterior corneal surface, the ICC was 0.90 and above for both normal and keratoconic eyes, suggesting excellent repeatability (Table 1). Table 2 summarizes the mean of the three measurements from both the OCT (A–E and E-B interface) and Pentacam for normal eyes. Paired comparisons of the mean measurements between the surfaces are also shown in Table 2. For all variables, the A–E and E-B interfaces differed significantly from Pentacam (P < .05). However, K1, K2, Kmax, coma, astigmatism 0°, and RMS of HOA were similar between the A–E and E-B interfaces (P > .05).

ICC of All OCT Indices Derived From A–E and E-B and All Pentacam Variables of the Anterior Corneal Surface

Table 1:

ICC of All OCT Indices Derived From A–E and E-B and All Pentacam Variables of the Anterior Corneal Surface

Mean (95% CI) of 3 Consecutive Scans in Normal Eyes (n = 96)

Table 2:

Mean (95% CI) of 3 Consecutive Scans in Normal Eyes (n = 96)

Table 3 summarizes the mean of three measurements from keratoconic eyes. K1 and K2 were significantly different (P < .0001) between all interfaces (Pentacam, OCT A–E, and OCT E-B interface). The Kmax of the OCT E-B interface was significantly greater than the Kmax of the OCT A–E interface and Pentacam (P < .0001). Among the aberration variables, defocus, vertical coma, spherical aberration, and RMS of HOA differed (P < .0001). Among the keratoconic eyes, the OCT E-B interface had the greatest aberrations. The Sw and CoV for both the OCT and Pentacam variables are reported in Table A (available in the online version of this article). The Sw was less than 0.25 D for all curvature variables. The RMS of LOA had the highest Sw, but it did not exceed 0.14 µm. The aberration variables, such as defocus, spherical aberration, coma, astigmatism 0°, and RMS, had a CoV of not more than 16%. However, these differences between repeat measurements were not clinically significant.

Mean (95% CI) of 3 Consecutive Scans in Keratoconic Eyes (n = 96)

Table 3:

Mean (95% CI) of 3 Consecutive Scans in Keratoconic Eyes (n = 96)

Within-Subject Standard Deviation (Sw) and Coefficient of Variation (CoV) for 3 Measurements of Curvature and Aberrations from OCT and Pentacam

Table A:

Within-Subject Standard Deviation (Sw) and Coefficient of Variation (CoV) for 3 Measurements of Curvature and Aberrations from OCT and Pentacam

Discussion

Several intraoperative studies revealed that the epithelium masked the true stromal surface.2,4,5,7,10 However, a non-contact method of evaluation is a clinical requirement. Recent studies used spectral-domain and swept-source OCT devices to evaluate A–E and E-B interfaces non-invasively.10,11 However, these studies did not investigate variability between the measurements. The repeatability of measurements is an important clinical need while diagnosing or monitoring a disease. Therefore, this study assessed the repeatability of curvature and surface aberrations of the anterior surface and Bowman's layer using high-speed OCT for the first time. Previous studies reported repeatability of curvature and aberrations in several corneal topographers.15–19 The ICCs for Pentacam curvature were above 0.90 in both normal and keratoconic eyes.16,19 Also, the RMS of aberrations showed high repeatability.15,19 These results were consistent with the current study. Also, repeatable anterior surface curvature was obtained from OCT.20,21 A study reported excellent repeatability of anterior curvature using OCT (CASIA SS-100; Tomey Corporation, Nagoya, Japan) with ICC greater than 0.90 for the normal eyes.20–22

Several studies comparing curvature measured by different devices reported significant differences between them.20,21,23 However, the measurements were repeatable within the instrument.20,21 The CoV was numerically higher (Tables 23) for some of the variables because some individual aberrations (mean of three successive measurements) tended to be small in magnitude and even a small Sw resulted in a high CoV. A study reported significant differences in curvature between OCT (CASIA) and Pentacam in the normal20,21 and keratoconic20 eyes. Also, studies suggested that OCT provided reliable and repeatable evaluations.21,24 Further, similar results were observed in patients who underwent penetrating keratoplasty.21 In this study, a CoV of less than 0.5% also was observed in Pentacam simulated keratometry (terminology borrowed from Placido devices)18,25 and OCT. A study on keratoconic eyes showed a high ICC (0.99) and Sw of 0.32 to 0.36 D for Pentacam curvature.18 However, the current study showed that the Sw was less than 0.10 D for Pentacam and 0.25 D for OCT. The current study showed significant differences between the OCT A–E interface and Pentacam anterior corneal surface for all variables. The RMS of HOA was lower in Pentacam compared to OCT A–E, which was comparable to a previous study.10 The epithelium thickness profile could be used for earlier detection of ectatic disorders despite a clinically normal anterior topography.4 The curvature of the A–E and E-B interfaces were generally similar in normal eyes, whereas these were significantly different in the keratoconic eyes. These findings matched the results of the previous study.10 The OCT A–E interface indices were significantly different from the OCT E-B interface indices of the PRK eyes.6 In the keratoconic eyes, the OCT E-B interface was significantly steeper than the OCT A–E interface by 2.00 D, which was consistent with the conclusions from intraoperative studies.2,7

Prolonged corneal exposure to ambient air after removal of epithelium might result in stromal edema, which may influence intraoperative measurement.6 Also, the contact between the tear film and the Bowman's layer might increase steepening and thus the curvature.2 These could be avoided by using a non-contact method to quantify Bowman's layer curvature with high-resolution OCT images. The advantages of high acquisition speed and greater penetration depth of swept-source OCT enabled excellent detail in different layers of the cornea. A recent study showed excellent agreement between curvatures derived from OCT/Scheimpflug and Placido imaging.25 Thus, tomography curvatures and Placido keratometry may be considered equivalent despite dissimilar techniques of measurement and calculation. However, a broader consensus among the clinical fraternity is required. Although the Triton OCT had a fast A-scan scanning speed, it provided only 24 semi-meridians (12 B-scans), and the Pentacam acquired up to 50 semi-meridians (25 two-dimensional cross-sections). In fact, the acquisition time of the Triton was only approximately half a second for 12 B-scans and significantly better than Pentacam. The new generation CASIA SS-1000 acquires 16 B-scans at a speed of 50 kHz and time of 0.3 second to reconstruct corneal curvature and thickness. The new MS-39 anterior segment OCT (Costruzione Strumenti Oftalmicihas, Firenze, Italy) allows the number of B-scans to be changed from 12 to 25 with oversampling while keeping the acquisition time to less than 1 second. Thus, there is tremendous scope to increase the number of semi-meridians scanned with current OCT devices without increasing the imaging time beyond what is used by the Pentacam in the clinical domain.

Further, the corneal diseases involving the periphery can be assessed with larger scan diameters. To ensure accurate measurement of the change in variables between subsequent visits, it is crucial for a clinician to establish variability between the measurements. Because this study showed excellent repeatability of curvature and aberrations, both OCT A–E and E-B interface curvature could be employed in clinical practice for better disease management and surgical planning.26 It would be interesting to study the changes occurring in the stroma through prospective studies of corneal cross-linking in the near future. This would require longitudinal follow-up and adequate sample size for the different cross-linking protocols. This study established the excellent repeatability of Bowman's layer curvature and aberrations in both normal and keratoconic eyes.

References

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ICC of All OCT Indices Derived From A–E and E-B and All Pentacam Variables of the Anterior Corneal Surface

ParameterNormal Eyes (n = 96)Keratoconic Eyes (n = 96)


A-E ICCaE-B ICCaPentacam ICCaA-E ICCaE-B ICCaPentacam ICCa
K1 (D)0.980.990.990.990.990.99
K2 (D)0.930.930.980.970.970.99
Kmax (D)0.970.980.990.990.990.99
Defocus (µm)0.900.920.960.990.980.99
Horizontal coma (µm)0.940.930.950.980.970.99
Vertical coma (µm)0.850.880.960.990.980.99
Spherical aberration (µm)0.870.880.900.980.950.99
Astigmatism 0° (µm)0.920.930.990.970.960.99
Astigmatism 45° (µm)0.920.890.980.970.970.99
RMS of LOA (µm)0.920.930.970.970.970.99
RMS of HOA (µm)0.840.820.910.940.940.99

Mean (95% CI) of 3 Consecutive Scans in Normal Eyes (n = 96)

Parameter1: A–E Interface OCT2: E-B Interface OCT3: Anterior Surface PentacamP (1 vs 3a)P (1 vs 2a)P (2 vs 3a)
K1 (D)44.66 (44.39 to 44.93)44.69 (44.40 to 44.98)44.35 (44.06 to 44.64)< .0001b.50< .0001
K2 (D)40.76 (40.31 to 41.22)40.85 (40.39 to 41.31)43.60 (43.31 to 43.89)< .0001b.15< .0001
Kmax (D)44.95 (44.66 to 45.24)45.01 (44.71 to 45.30)44.56 (44.26 to 44.85)< .0001b.13< .0001b
Defocus (µm)−0.77 (−0.93 to −0.61)−0.99 (−1.14 to −0.84)1.52 (1.40 to 1.63)< .0001b< .0001b< .0001b
Horizontal coma (µm)−0.31 (−0.42 to −0.19)−0.30 (−0.41 to −0.18)0.04 (−0.003 to 0.07)< .0001b.12< .0001b
Vertical coma (µm)0.49 (0.45 to 0.54)0.50 (0.45 to 0.55)0.09 (0.05 to 0.13)< .0001b.68< .0001b
Spherical aberration (µm)−0.21 (−0.25 to −0.16)−0.26 (−0.30 to −0.21)0.29 (0.27 to 0.31)< .0001b< .0001b< .0001b
Astigmatism 0° (µm)−2.07 (−2.26 to −1.88)−2.09 (−2.30 to −1.87)−0.65 (−0.78 to −0.51)< .0001b.99< .0001b
Astigmatism 45° (µm)0.24 (0.08 to 0.40)0.33 (0.16 to 0.49)−0.02 (−0.12 to 0.09).009b.008b.0005b
RMS of LOA (µm)2.48 (2.29 to 2.67)2.59 (2.38 to 2.79)1.76 (1.61 to 1.91).0001b.009b< .0001b
RMS of HOA (µm)1.94 (1.83 to 2.04)1.93 (1.82 to 2.03)0.45 (0.43 to 0.47)< .0001b.53< .0001b

Mean (95% CI) of 3 Consecutive Scans in Keratoconic Eyes (n = 96)

Parameter1: A–E Interface OCT2: E-B Interface OCT3: Anterior Surface PentacamP (1 vs 3a)P (1 vs 2a)P (2 vs 3a)
K1 (D)51.43 (50.27 to 52.58)52.89 (51.61 to 54.17)51.29 (50.18 to 52.41)< .0001b< .000 b< .0001b
K2 (D)43.78 (42.87 to 44.69)44.61 (43.61 to 45.62)46.62 (45.87 to 47.37)< .0001b< .0001b< .0001b
Kmax (D)51.97 (50.78 to 53.16)53.64 (52.31 to 54.96)52.25 (51.03 to 53.46).75< .0001b< .0001b
Defocus (µm)−2.21 (−2.65 to −1.76)−3.38 (−4.00 to −2.76)0.35 (−0.16 to 0.86)< .0001b< .0001b< .0001b
Horizontal coma (µm)−0.36 (−0.59 to −0.14)−0.33 (−0.60 to −0.06)0.06 (−0.09 to 0.21)< .0001b.33.0001b
Vertical coma (µm)1.58 (1.29 to 1.86)1.85 (1.54 to 2.16)1.40 (1.10 to 1.69)< .0001b< .0001< .0001b
Spherical aberration (µm)−0.57 (−0.69 to −0.46)−0.89 (−1.05 to −0.73)−0.27 (−0.39 to −0.15)< .0001< .0001< .0001
Astigmatism 0° (µm)−1.83 (−2.19 to −1.46)−1.68 (−2.14 to −1.22)−2.56 (−3.04 to −2.09).12.93.05
Astigmatism 45° (µm)0.19 (−0.16 to 0.54)0.26 (−0.15 to 0.66)0.19 (−0.29 to 0.67).29.12.31
RMS of LOA (µm)3.83 (3.45 to 4.20)4.97 (4.45 to 5.48)4.28 (3.79 to 4.75).06< .0001b.02b
RMS of HOA (µm)3.01 (2.81 to 3.22)3.61 (3.31 to 3.91)1.98 (1.73 to 2.24)< .0001b< .0001b< .0001b

Within-Subject Standard Deviation (Sw) and Coefficient of Variation (CoV) for 3 Measurements of Curvature and Aberrations from OCT and Pentacam

ParameterNormal (n = 96)Keratoconic (n = 96)


A–E Interface OCTE-B Interface OCTAnterior Surface PentacamA–E Interface OCTE-B Interface OCTAnterior Surface Pentacam
K1 (D)
  Sw0.060.050.030.090.120.05
  CoV (%)0.130.110.070.170.220.09
K2 (D)
  Sw0.180.170.040.210.230.10
  Cov (%)0.440.420.090.480.510.21
Kmax (D)
  Sw0.060.060.040.080.130.05
  CoV (%)0.130.130.090.150.240.10
Defocus (µm)
  Sw0.070.040.050.080.100.05
  CoV (%)8.505.403.203.502.9013.00
Horizontal coma (µm)
  Sw0.030.040.010.040.040.01
  CoV (%)11.2012.5013.9010.0013.3015.50
Vertical coma (µm)
  Sw0.020.020.010.040.040.01
  CoV (%)4.004.007.102.402.400.72
Spherical aberration (µm)
  Sw0.020.020.0050.020.030.01
  CoV (%)8.705.801.603.902.922.03
Astigmatism 0° (µm)
  Sw0.070.070.010.080.090.03
  CoV (%)3.203.302.104.405.400.99
Astigmatism 45° (µm)
  Sw0.060.070.010.080.090.03
  CoV (%)2.461.961.173.904.501.20
RMS of LOA (µm)
  Sw0.080.080.040.100.140.05
  CoV (%)3.223.092.272.602.821.17
RMS of HOA (µm)
  Sw0.070.070.010.070.100.01
  CoV (%)3.613.632.222.322.770.70
Authors

From the Department of Cornea and Refractive Surgery, Narayana Nethralaya, Bangalore, India (HM, NC, SG, RS, ASR); and Imaging, Biomechanics and Mathematical Modeling Solutions Lab, Narayana Nethralaya Foundation, Bangalore, India (RC).

Supported in part by the SIBAC research grant, Indo-German Science and Technology Center, Government of India, and the Topcon Research Foundation, Tokyo, Japan.

Drs. Shetty, Chandapura, and Sinha Roy have a pending patent application on Bowman's topography through the Narayana Nethralaya Foundation, Bangalore, India. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (HM, NC, RC, SG, RS, ASR); analysis and interpretation of data (HM, NC, RC, SG, RS, ASR); writing the manuscript (HM, NC, RC, SG, RS, ASR); statistical expertise (HM, NC, RC, SG, RS, ASR)

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

Received: February 05, 2019
Accepted: January 20, 2020

10.3928/1081597X-20200121-02

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