Journal of Refractive Surgery

Original Article Supplemental Data

Accuracy of OCT Curvature and Aberrations of Bowman's Layer: A Prospective Comparison With Physical Removal of Epithelium

Pooja Khamar, MD, PhD; Rohit Shetty, MD, PhD, FRCS; Prerna Ahuja, MD; Rachana Chandapura, MTech; Raghav Narasimhan, MTech; Rudy M.M.A. Nuijts, MD, PhD; Abhijit Sinha Roy, PhD

Abstract

PURPOSE:

To compare optical coherence tomography (OCT) and Scheimpflug curvature and aberrations of the Bowman's layer before and after removal of the epithelium.

METHODS:

Bowman's layer was mapped with OCT (Optovue Inc., Irvine, CA) before and after removal of the epithelium in normal eyes undergoing photorefractive keratectomy (n = 14) and keratoconic eyes undergoing corneal cross-linking (n = 25). The anterior corneal surface before removal and the underlying Bowman's layer after removal of the epithelium were also mapped with Pentacam (Oculus Optikgeräte, Wetzlar, Germany), and the surface aberrations with ray tracing were computed.

RESULTS:

The agreement of OCT curvatures before and after removal of the epithelium was excellent (intraclass correlation coefficient [ICC] = 0.9). A similar trend was seen between OCT and Pentacam after removal of the epithelium. The agreement of surface wavefront aberrations of the Bowman's layer before and after removal of the epithelium was excellent (ICC = 0.9) between the devices for keratoconic eyes. However, this agreement was relatively inferior in normal eyes (ICC < 0.5).

CONCLUSIONS:

The virtual OCT curvature and aberrations of the Bowman's layer agreed well with its actual magnitudes on removal of the epithelium in the keratoconic eyes. In normal eyes, the agreement was inferior for aberrations but not for curvature.

[J Refract Surg. 2020;36(3):193–198.]

Abstract

PURPOSE:

To compare optical coherence tomography (OCT) and Scheimpflug curvature and aberrations of the Bowman's layer before and after removal of the epithelium.

METHODS:

Bowman's layer was mapped with OCT (Optovue Inc., Irvine, CA) before and after removal of the epithelium in normal eyes undergoing photorefractive keratectomy (n = 14) and keratoconic eyes undergoing corneal cross-linking (n = 25). The anterior corneal surface before removal and the underlying Bowman's layer after removal of the epithelium were also mapped with Pentacam (Oculus Optikgeräte, Wetzlar, Germany), and the surface aberrations with ray tracing were computed.

RESULTS:

The agreement of OCT curvatures before and after removal of the epithelium was excellent (intraclass correlation coefficient [ICC] = 0.9). A similar trend was seen between OCT and Pentacam after removal of the epithelium. The agreement of surface wavefront aberrations of the Bowman's layer before and after removal of the epithelium was excellent (ICC = 0.9) between the devices for keratoconic eyes. However, this agreement was relatively inferior in normal eyes (ICC < 0.5).

CONCLUSIONS:

The virtual OCT curvature and aberrations of the Bowman's layer agreed well with its actual magnitudes on removal of the epithelium in the keratoconic eyes. In normal eyes, the agreement was inferior for aberrations but not for curvature.

[J Refract Surg. 2020;36(3):193–198.]

Mapping of the corneal layers with optical coherence tomography (OCT) imaging to derive their curvature is a new dimension of high-resolution corneal imaging.1–3 The epithelium, Bowman's layer, and stroma are altered in disease and after surgery.1,2 Therefore, the three-dimensional shape of these layers can be indicative of early disease (eg, keratoconus). The underlying Bowman's layer can be imaged intraoperatively after removal of the epithelium from patients.4–6 However, this technique can be used only in patients undergoing surgery involving removal of the epithelium.4–6 Imaging with OCT is noninvasive and does not require the physical removal of epithelium.1,2 Nonetheless, it is necessary to establish the accuracy of curvature of the air–epithelium (A–E) and epithelium–Bowman's layer (E–B) interfaces derived from OCT with other established methods (eg, Scheimpflug imaging). The A–E surface is the same as the anterior corneal surface reported by the current clinical Scheimpflug imaging devices. Therefore, the aim of this study was to establish the agreement of OCT-derived Bowman's layer curvature and surface ablations before and after removal of the epithelium in patients undergoing photorefractive keratectomy and corneal cross-linking. Scheimpflug imaging was also performed simultaneously on the same eye before and after removal of the epithelium.

Patients and Methods

This was a prospective, interventional, cross-sectional study that was approved by the Narayana Nethralaya Ethics Committee, Bangalore, India. The study adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from the patients. Patients undergoing photorefractive keratectomy to correct myopic refractive error and corneal cross-linking to manage keratoconus were included in the study.

Topical proparacaine 0.5% ophthalmic solution (Sunways Pharma, Mumbai, India) was applied. Imaging with OCT (RTVue; Optovue Inc., Irvine, CA) and Scheimpflug (Pentacam; Oculus Optikgeräte, Wetzlar, Germany) devices was performed prior to removal of the epithelium. The epithelium was removed manually with EpiClear (Orca Surgical, Caesarea, Israel) in the central 8-mm zone. Scheimpflug and OCT imaging were immediately redone before tissue was ablated in eyes undergoing photorefractive keratectomy and before soaking of keratoconic corneas with riboflavin began. The exposed surface of the Bowman's layer was not washed with sterile saline or balanced salt solution prior to repeat imaging.

For each eye, 50 (25 cross-sections) and 16 (8 B-scans) semi-meridian two-dimensional scans were acquired with the Pentacam and RTVue, respectively. The methods to calculate the surface curvatures and aberrations are briefly described below.1,2 The comma separated value files of elevation data were exported from the Pentacam. The curvature and aberrations of the anterior surface and exposed Bowman's layer after removal of the epithelium were computed from the comma separated value file. The A–E interface or A–E and E-B interfaces were detected using a graph search from the OCT B-scans.1,2 The interpolation along the interfaces was performed with a 6th order polynomial.1,2 The axial curvature and aberrations of the A–E and E-B interfaces were computed before removal of the epithelium. After epithelium removal, the A–E and E-B interfaces were essentially the same “conceptually.” A net equivalent refractive index of 1.3375 was used for all calculations. The zone size was a 6-mm diameter for both OCT and Pentacam analyses.

Statistical Analyses

The normality of distribution was confirmed with the Kolmogorov–Smirnov test. All variables were represented by their mean and standard error of mean. The paired t test was used for comparison of group means. The intraclass correlation coefficient (ICC) and within-subject deviation were used to assess the agreement between the groups. Linear regression analyses were used to establish the correlations between salient A–E and E-B interface indices. A P value of less than .05 was considered statistically significant. MedCalc software (version 18.7, MedCalc Inc., Mariakerke, Belgium) was used for statistical analyses.

Results

The mean age of the patients with normal and keratoconic eyes was 28.78 ± 1.63 and 20.96 ± 1.2 years, respectively. Table 1 shows the curvatures and root mean square (RMS) of aberrations for the normal eyes. In the normal eyes, all indices were similar between the A–E and E-B interfaces before removal of the epithelium (P > .05). Further, the indices of both A–E and E-B interfaces were significantly different from the indices of Pentacam (P < .05) except for the spherical aberration (P > .05), when evaluated in pairs with the Pentacam measurements (Table 1). In general, the OCT detected greater RMS of higher order aberrations and lower RMS of lower order aberrations than the Pentacam for normal eyes before removal of the epithelium (Table 1).

Mean ± Standard Error of Keratometry and Aberration Indices From OCT and Pentacam of Normal Eyes (n = 14)

Table 1:

Mean ± Standard Error of Keratometry and Aberration Indices From OCT and Pentacam of Normal Eyes (n = 14)

The ICC of OCT curvatures before and after removal of the epithelium (3 vs 4 in Table 1) was generally excellent (0.9 and above). The same was observed between OCT before removal of epithelium and Pentacam after removal of epithelium (4 vs 5 in Table 1). The coefficient of variation of curvature variables was less than 4% for both 3 versus 4 and 4 versus 5 (Table 1). However, the ICC was low for almost all aberration indices between 3 versus 4 and 4 versus 5 (Table 1). The coefficient of variation for the aberration indices was also high (significantly greater than 4%), confirming the low ICC and proportionally high within-subject deviation (Table 1). Interestingly, the RMS of lower order aberrations from the OCT was greater than the Pentacam by a factor of 2.05 (Table 1). However, the RMS of higher order aberrations from OCT was lower than the Pentacam by a factor of 2.22 (Table 1).

Table 2 shows the curvatures and RMS of aberrations for the keratoconic eyes. In the keratoconic eyes, all indices except the spherical aberration (P = .07) differed significantly between the A–E and E-B interfaces (P < .05). Only the defocus (P = .61) and RMS of 4th order aberrations (P = .93) were similar between the A–E and Pentacam interfaces before removal of the epithelium. Further, steep axis curvature at 3 mm (P = .18), maximum axial curvature (P = .25), spherical aberration (P = .93), RMS of lower order aberrations (P = .45), and RMS of 3rd order aberrations (P = .88) were similar between the E-B and Pentacam interfaces before removal of the epithelium. The ICC of OCT curvatures and all aberration indices before and after removal of the epithelium (3 vs 4 in Table 2) were 0.92 or greater, except for the ICC of RMS of astigmatism (ICC = 0.86) and 4th order aberrations (ICC = 0.77). Similarly, the coefficient of variation for keratoconic eyes was significantly better than the same for normal eyes. The ICC of curvatures and all aberration indices were 0.92 or greater between the OCT and Pentacam after removal of the epithelium (4 vs 5 in Table 2) except for the RMS of astigmatism (ICC = 0.65) and lower order aberrations (ICC = 0.83). In general, within-subject deviation of all indices for the normal eyes (Table 1) was lower in magnitude than the same for keratoconic eyes (Table 2).

Mean ± Standard Error of Mean of Keratometry and Aberration Indices From OCT and Pentacam of Keratoconic Eyes (n = 25)

Table 2:

Mean ± Standard Error of Mean of Keratometry and Aberration Indices From OCT and Pentacam of Keratoconic Eyes (n = 25)

Linear correlations between some of the A–E and E–B indices were evaluated (Table A, available in the online version of this article). There were excellent correlations between the indices of the A–E and E-B interfaces. Table B (available in the online version of this article) lists the mean central corneal and epithelium thicknesses measured by the two devices before and after removal of the epithelium. Immediate swelling of the stroma after removal of the epithelium was evident from the data in Table B, although none of the increases in the thicknesses were significant (P > .05). Figure 1 shows an example of OCT curvatures of a patient with keratoconus before and after removal of the epithelium. Figures 1A–1B show the A–E and E–B interface curvature, respectively. Figure 1C shows the curvature of the Bowman's layer after removal of the epithelium. The corresponding figures from Pentacam (Figures 1D–1E) are also shown. In Figure 1, the RMS of higher order aberrations increased from the OCT A–E to E-B interface (from 7.13 to 8.73 µm or 22.44%). The same was observed from the A–E interface after removal (intraoperative) of the epithelium (from 7.13 to 7.96 µm or 11.64%). In Pentacam, this increase was only 2.08% (from 7.68 to 7.84 µm). Note the significantly greater flattened regions of the cornea diametrically superior to the cone in the OCT maps (A, B, and C) compared to the Pentacam (D and E) maps.

Linear Correlations Between Columns 3 and 4 of Table 1 (Normal Eyes) and Table 2 (Keratoconic Eyes)

Table A:

Linear Correlations Between Columns 3 and 4 of Table 1 (Normal Eyes) and Table 2 (Keratoconic Eyes)

Mean ± Standard Error of CCT and CET in the Normal and Keratoconic Eyes

Table B:

Mean ± Standard Error of CCT and CET in the Normal and Keratoconic Eyes

Sample optical coherence tomography (OCT) topography (axial curvature in diopters) of (A) air–epithelium interface (A–E); (B) epithelium–Bowman's layer interface (E-B); and (C) exposed Bowman's layer after removal of epithelium. For the same eye, sample Pentacam (Oculus Optikgeräte, Wetzlar, Germany) topography (D) before and (E) after removal of epithelium. The table lists the maximum curvature (Kmax) and root mean square of higher order aberrations (RMS HOA). D = diopters

Figure 1.

Sample optical coherence tomography (OCT) topography (axial curvature in diopters) of (A) air–epithelium interface (A–E); (B) epithelium–Bowman's layer interface (E-B); and (C) exposed Bowman's layer after removal of epithelium. For the same eye, sample Pentacam (Oculus Optikgeräte, Wetzlar, Germany) topography (D) before and (E) after removal of epithelium. The table lists the maximum curvature (Kmax) and root mean square of higher order aberrations (RMS HOA). D = diopters

Discussion

Bowman's layer is a critical layer for biomechanical stability of the cornea. This was shown recently because transplantation of the Bowman's layer was effective in stabilizing corneas suffering from advanced keratoconus.7 Thus, a virtual assessment of the E-B interface curvature could prove useful for assessing disease severity and planning therapeutic treatments.8 We refer to recent publications on curvature of the Bowman's layer in patients undergoing refractive surgery or corneal cross-linking. In young patients (mean age of 28.9 years) measured with Placido topography, Gatinel et al.4 found the mean central keratometry decreased by 1.33 diopters (D) after removal of the epithelium, the mean power decreased by 0.96 D, and the mean corneal astigmatism increased by 0.46 D. In relatively older patients (mean age of 34.81 years), Salah-Mabed et al.6 found the trend was partly reversed and the mean curvature at the 3-mm zone increased by 0.57 D. A possible explanation for this discrepancy is the difference in the mean age of the study populations. The mean age of our patients was closer to that of Gatinel et al., which explained the flatter curvature of the E-B interface and change in corneal astigmatism (Table 1). Further, the strong correlation between the indices before and after removal of the epithelium (Table A) indicated a somewhat uniform epithelial thickness profile in normal eyes. Thus, the spatial increase in the thickness of the cornea from the center to the periphery was equivalent to the spatial increase in the thickness of the stroma. This increase in thickness did not alter the curvature of the E-B interface differentially from the A–E interface in the normal eyes.

In keratoconus, the underlying Bowman's layer was thinner than usual.9 Keratoconus is also associated with reduction in the elastic modulus. Thus, the Bowman's layer and stroma undergo adverse changes that can cause steepening. This steepening is masked by the epithelium.1,5Table 2 clearly shows this effect, which also agreed with the earlier studies.1,5 In the keratoconic eyes, ICC 3 versus 4 (Table 2) was excellent (> 0.9) for all indices, unlike the normal eyes (Table 1). Similarly, ICC 4 versus 5 was excellent for all indices in the keratoconic eyes (Table 2) compared to the normal eyes (Table 1). The results from the ICC analyses also matched the outcomes of the linear regression analyses, where keratoconic eyes had much tighter correlation coefficients (r = 0.9). Thus, greater magnitude of the surface aberrations improved the agreement between epithelium “on” and “off” measurements.

An earlier study proposed a combination of ultrasound and OCT for the best imaging of the corneal layers.4 However, OCT alone is capable of performing segmental imaging of the corneal layers and their curvature. Further, the linear regressions showed that it may be possible to predict curvature and surface aberrations of the Bowman's layer from the A–E interface alone. These correlations provide an alternate option to those who do not have access to OCT imaging in their clinic. The alterations in the three-dimensional distribution of epithelium thickness can detect keratoconus early.10 The rate of change of elevation from the center to the periphery of the Bowman's layer may also be influenced by this three-dimensional distribution.

While performing the edge detection, we noticed a practical problem with the Pentacam analyses. Figures AA–AD (available in the online version of this article) show a schematic cross-section of the cornea before and after removal of the epithelium, respectively. In the Pentacam, the edge detection continued beyond the central 6 mm and the “step” change in elevation due to epithelium removal may have led to inaccuracies in the central curvature estimation (Figure AC). In OCT, the edge detection was restricted to the central 6 mm, which was the size of the B-scan (Figure AD). Thus, the OCT image analyses did not suffer similar to those of the Pentacam. In the normal eyes, the smaller magnitudes of aberrations in general and inaccuracy of the edge detection may explain the lower ICC of aberration variables (Table 1). Nonetheless, the close agreement between the Pentacam and OCT indices of keratoconic eyes lend significant credence to the results of this study. Table B showed similar central corneal thickness between the normal and keratoconic eyes because the normal eyes were thinner than usual. Hence, the choice of photorefractive keratectomy for these eyes. The central epithelial thickness was also similar between the groups because it was measured at the center of the map and may not coincide exactly with the disease location, which is usually decentered inferiorly in a keratoconic eye.

Schematic cross-section of the cornea (A) before and (B) after removal of epithelium. Note the detected edge did not capture the “step” change in epithelium elevation in Pentacam (Oculus Optikgeräte, Wetzlar, Germany) because (C) the edge detection extended beyond the central 6-mm zone. (D) In optical coherence tomography (OCT), edge detection was limited to the central 6-mm zone.

Figure A.

Schematic cross-section of the cornea (A) before and (B) after removal of epithelium. Note the detected edge did not capture the “step” change in epithelium elevation in Pentacam (Oculus Optikgeräte, Wetzlar, Germany) because (C) the edge detection extended beyond the central 6-mm zone. (D) In optical coherence tomography (OCT), edge detection was limited to the central 6-mm zone.

Placido imaging could also be used in future studies.4–6 However, Placido imaging would also be affected by lack of a normal tear film after removal of the epithelium.4–6 On the other hand, the Pentacam and OCT used in this study did not have the axial resolution to detect the tear film. Further, small local changes in elevation due to variability in tear film thickness could also be missed by the devices. Therefore, both were less susceptible to tear film thickness.6 However, it was obvious that only the OCT can be used for routine assessment of Bowman's layer curvature because the method is completely non-invasive and does not require prior removal of epithelium. Another limitation of the study was that the true refractive index of the epithelium was not factored in the calculations. However, the OCT provided distortion-corrected images for a refractive index of 1.3375 only and recorrection of these images with a different index would benefit from prior knowledge of the optical design of the OCT.

A limitation of the detection of the E-B interface was that as the epithelium became thinner, the interface became closer to the A–E interface and its detection was problematic. However, this occurred in advanced stages of keratoconus. Such keratoconic eyes did not meet the inclusion criteria. Another limitation of the OCT scanner was that the size of the B-scan was restricted to 6 mm only. Hence, the mid to peripheral epithelial changes were not captured with the study device. Segmental imaging of the corneal layers may improve in sensitivity further with this layer-specific information. Newer technology can potentially increase the scan diameter and axial resolution to perform wide-field topography of the Bowman's layer.11 Therapeutic treatments of highly aberrated corneas could benefit further from the implementation of OCT curvature of corneal layers.12 The elevation and curvature of the Bowman's layer could help distinguish between the epithelial thickness irregularity and stromal surface aberrations.13 This study established the accuracy of deriving Bowman's curvature “virtually” with OCT segmental imaging.

References

  1. Matalia H, Francis M, Gangil T, et al. Noncontact quantification of topography of anterior corneal surface and Bowman's layer with high-speed OCT. J Refract Surg. 2017;33(5):330–336. doi:10.3928/1081597X-20170201-01 [CrossRef]
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  3. Salomão MQ, Hofling-Lima AL, Lopes BT, et al. Role of the corneal epithelium measurements in keratorefractive surgery. Curr Opin Ophthalmol. 2017;28(4):326–336. doi:10.1097/ICU.0000000000000379 [CrossRef]
  4. Gatinel D, Racine L, Hoang-Xuan T. Contribution of the corneal epithelium to anterior corneal topography in patients having myopic photorefractive keratectomy. J Cataract Refract Surg. 2007;33(11):1860–1865. doi:10.1016/j.jcrs.2007.06.041 [CrossRef]
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  6. Salah-Mabed I, Saad A, Gatinel D. Topography of the corneal epithelium and Bowman layer in low to moderately myopic eyes. J Cataract Refract Surg. 2016;42(8):1190–1197. doi:10.1016/j.jcrs.2016.05.009 [CrossRef]
  7. van Dijk K, Parker JS, Baydoun L, et al. Bowman layer transplantation: 5-year results. Graefes Arch Clin Exp Ophthalmol. 2018;256(6):1151–1158. doi:10.1007/s00417-018-3927-7 [CrossRef]
  8. Reinstein DZ. Therapeutic refractive surgery: state of technology and a call to action. J Refract Surg. 2018;34(5):294–295. doi:10.3928/1081597X-20180402-01 [CrossRef]
  9. Abou Shousha M, Perez VL, Fraga Santini Canto AP, et al. The use of Bowman's layer vertical topographic thickness map in the diagnosis of keratoconus. Ophthalmology. 2014;121(5):988–993. doi:10.1016/j.ophtha.2013.11.034 [CrossRef]
  10. Reinstein DZ, Archer TJ, Urs R, Gobbe M, RoyChoudhury A, Silverman RH. Detection of keratoconus in clinically and algorithmically topographically normal fellow eyes using epithelial thickness analysis. J Refract Surg. 2015;31(11):736–744. doi:10.3928/1081597X-20151021-02 [CrossRef]
  11. Pircher N, Beer F, Holzer S, et al. Large field of view corneal epithelium and Bowman's layer thickness maps in keratoconic and healthy eyes. Am J Ophthalmol. 2020;209:168–177. doi:10.1016/j.ajo.2019.05.025 [CrossRef]
  12. Reinstein DZ, Gobbe M, Archer TJ, Youssefi G, Sutton HF. Stromal surface topography-guided custom ablation as a repair tool for corneal irregular astigmatism. J Refract Surg. 2015;31(1):54–59. doi:10.3928/1081597X-20141218-06 [CrossRef]
  13. Guglielmetti S, Kirton A, Reinstein DZ, Carp GI, Archer TJ. Repair of irregularly irregular astigmatism by transepithelial phototherapeutic keratectomy. J Refract Surg. 2017;33(10):714–719. doi:10.3928/1081597X-20170721-04 [CrossRef]

Mean ± Standard Error of Keratometry and Aberration Indices From OCT and Pentacam of Normal Eyes (n = 14)

ParameterBefore Epithelium RemovalAfter Epithelium Removal3 vs 44 vs 5




1: OCT A–E Interface2: Pentacam Anterior Surface3: OCT E-B Interface (Epi-on)4: OCT (Epi-off)5: Pentacam (Epi-off)ICCCOV (%)SwICCCOV (%)Sw
K1 (D)44.24 ± 0.5345.12 ± 0.5144.14 ± 0.5244.74 ± 0.5246.14 ± 0.490.961.530.680.970.990.44
K2 (D)42.21 ± 0.4343.83 ± 0.4141.55 ± 0.4242.38 ± 0.3944.51 ± 0.400.832.340.980.963.581.56
Kmax (D)44.62 ± 0.5545.29 ± 0.5244.42 ± 0.5445.33 ± 0.4546.39 ± 0.470.941.930.870.942.050.94
Axis of K1 (D)93.21 ± 3.2189.68 ± 3.9594.02 ± 4.9581.3 ± 3.7091.09 ± 4.900.5617.4415.290.6215.7013.5
Defocus (µm)0.78 ± 0.141.67 ± 0.170.93 ± 0.131.05 ± 0.232.08 ± 0.440.4521.510.960.3117.471.78
Spherical aberration (µm)0.21 ± 0.040.23 ± 0.020.24 ± 0.140.37 ± 0.05−0.03 ± 0.040.6521.880.260.68−42.300.11
RMS of coma (µm)0.27 ± 0.040.27 ± 0.040.28 ± 0.050.34 ± 0.050.63 ± 0.070.3652.680.160.6454.540.26
RMS of astigmatism (µm)0.66 ± 0.101.51 ± 0.180.55 ± 0.070.71 ± 0.171.78 ± 0.21−0.2778.900.500.5873.950.92
RMS of 3rd order aberrations (µm)0.54 ± 0.050.32 ± 0.030.53 ± 0.070.62 ± 0.070.7 ± 0.070.1143.490.250.7824.290.16
RMS of 4th order aberrations (µm)0.53 ± 0.070.25 ± 0.010.52 ± 0.050.81 ± 0.110.32 ± 0.030.3151.880.350.4575.100.42
RMS of LOA (µm)1.08 ± 0.152.26 ± 0.251.10 ± 0.131.19 ± 0.193 ± 0.360.2149.260.56−0.5181.811.71
RMS of HOA (µm)1.00 ± 0.090.41 ± 0.031.01 ± 0.101.40 ± 0.150.9 ± 0.070.2941.790.500.6540.370.47

Mean ± Standard Error of Mean of Keratometry and Aberration Indices From OCT and Pentacam of Keratoconic Eyes (n = 25)

ParameterBefore Epithelium RemovalAfter Epithelium Removal3 vs 44 vs 5




1: OCT A-E Interface2: Pentacam Anterior Surface3: OCT E-B Interface (Epi-on)4: OCT (Epi-off)5: Pentacam (Epi-off)ICCCOV (%)SwICCCOV (%)Sw
K1 (D)56.60 ± 1.2057.68 ± 1.1358.25 ± 1.2757.89 ± 1.1859.12 ± 1.160.982.071.200.992.241.31
K2 (D)48.38 ± 0.7251.13 ± 0.7349.79 ± 0.8650.13 ± 0.8652.64 ± 0.830.962.261.130.944.392.26
Kmax (D)57.66 ± 1.2659.18 ± 1.2359.68 ± 1.3359.27 ± 1.2461.18 ± 1.270.991.610.960.992.641.59
Axis of K1 (D)92.40 ± 2.6690.62 ± 2.8888.80 ± 1.4289.18 ± 2.3091.03 ± 3.520.578.137.240.1815.4113.90
Defocus (µm)−3.52 ± 0.54−3.20 ± 0.81−4.54 ± 0.76−4.03 ± 0.64−3.84 ± 0.960.95−25.631.100.85−51.642.03
Spherical aberration (µm)−0.89 ± 0.14−1.14 ± 0.18−1.15 ± 0.20−1.03 ± 0.16−1.64 ± 0.200.95−25.550.280.93−39.910.53
RMS of coma (µm)2.80 ± 0.253.39 ± 0.293.15 ± 0.283.13 ± 0.283.57 ± 0.290.9512.870.400.9713.870.46
RMS of astigmatism (µm)2.49 ± 0.284.03 ± 0.272.79 ± 0.292.66 ± 0.343.79 ± 0.260.8628.490.780.6541.571.34
RMS of 3rd order aberrations (µm)2.97 ± 0.253.43 ± 0.303.41 ± 0.293.34 ± 0.283.69 ± 0.300.9612.130.410.9613.150.46
RMS of 4th order aberrations (µm)1.43 ± 0.131.42 ± 0.141.87 ± 0.181.74 ± 0.151.96 ± 0.170.7727.770.500.9218.450.34
RMS of LOA (µm)4.62 ± 0.516.21 ± 0.475.89 ± 0.645.49 ± 0.526.60 ± 0.650.9220.091.140.8328.601.73
RMS of HOA (µm)3.53 ± 0.253.88 ± 0.304.18 ± 0.344.04 ± 0.294.36 ± 0.300.9412.930.530.9610.920.46

Linear Correlations Between Columns 3 and 4 of Table 1 (Normal Eyes) and Table 2 (Keratoconic Eyes)

ParameterNormal Eyes (Table 1)Keratoconic Eyes (Table 2)


3 vs 4rP3 vs 4rP
K1 (D)y = 0.92x+3.070.91< .001ay = 1.04x−2.110.96< .001a
K2 (D)y = 0.79x+7.800.83< .001ay = 0.94x+2.780.93< .001a
Kmax (D)y = 1.09x−4.850.91< .001ay = 1.05x−2.510.98< .001a
RMS of LOA (µm)y = 0.08x+1.00.12.67y = 1.06x+0.070.87< .001a
RMS of HOA (µm)y = 0.12x+0.840.13.54y = 1.04x−0.040.90< .001a

Mean ± Standard Error of CCT and CET in the Normal and Keratoconic Eyes

ParameterScheimpflug CCT (µm)OCT CCT (µm)OCT CET (µm)



NormalKeratoconicNormalKeratoconicNormalKeratoconic
Before epithelium removal465.63 ± 8.71475.55 ± 21.57464.77 ± 9.73463.81 ± 28.1849.87 ± 0.9049.32 ± 295.00
After epithelium removal485.70 ± 8.80473.19 ± 28.18472.78 ± 8.17454.56 ± 27.18
Pa> .05> .05> .05> .05
Authors

From the Department of Cornea and Refractive Services, Retina Care Clinic, Ahmedabad, India (PK); the Department of Cornea and Refractive Surgery, Narayana Nethralaya, Bangalore, India (PK, RS, PA); Imaging, Biomechanics and Mathematical Modeling Solutions Lab, Narayana Nethralaya Foundation, Bangalore, India (RC, RN, ASR); and University Eye Clinic Maastricht, Maastricht University Medical Center, Maastricht, the Netherlands (RMMAN).

Drs. Sinha Roy and Shetty have a pending patent application on the use of OCT for Bowman's layer imaging through the Narayana Nethralaya Foundation. Dr. Nuijts serves as a consultant for Alcon, Asico, Chiesi, and Theapharma, and a speaker for Abbott, Alcon, Bausch & Lomb, Carl Zeiss, Chiesi, HumanOptics, Ophtec, Oculentis, and Gebauer. The remaining authors have no financial or proprietary interest in the materials presented herein.

Supported in part by the Indo-German Science and Technology Center, India.

AUTHOR CONTRIBUTIONS

Study concept and design (RS, RMMAN, ASR); data collection (PK, RS, PA, RC, RN); analysis and interpretation of data (PK, ASR); writing the manuscript (PK, PA, RC, RN, ASR); critical revision of the manuscript (RS, RMMAN); statistical expertise (ASR); administrative, technical, or material support (PK)

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

Received: March 25, 2019
Accepted: January 20, 2020

10.3928/1081597X-20200122-01

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