The hallmark characteristic of Keratoconus (KC) is the development of localized, cone-shaped ectasia accompanied by corneal thinning in the area of the cone. This leads to irregular astigmatism and steeper corneal curvature over the area of the cone [
There are several tomographic criteria for KC diagnosis. They can be divided into three main subgroups: curvature-based, elevation-based, and pachymetry-based. Unfortunately, none of them is 100% sensitive and specific. Some authors believe that elevation maps are better than axial curvature maps for KC screening [
The rotating Scheimpflug camera “Pentacam” (Oculus GmbH, Wetzlar, Germany) can generate various indices within each of the three indices subgroups.
Pentacam allows for measuring local elevation points by fitting the corneal shape to a best-fit sphere (BFS) reference surface with variable diameters or to a best-fit toric ellipsoid surface (BFTE), which simulates more the corneal shape than the BFS [
More recently, there have been other proposed indices; one of the most valuable KC indices is the Ambrósio’s Relational Thickness (ART) [
This study aims at comparing the accuracy of each of the tested indices at various best-fit reference surfaces, in cohorts of KC and normal corneal cases who were followed up for six years, and to evaluate the accuracy of various indices in relation to Topographic KC (TKC) grading.
To the best of our knowledge, our study is innovative in evaluating the accuracy of KC indices using different reference surfaces of various diameters when applicable. Moreover, novelty lies in comparing the tested indices against time evidence, through our six-year follow-up interval (stability even after LASIK surgeries of normal corneas and spontaneous progression over years in KC cases). Time evidence is a definite proof of diagnosis not relying on subjective assessment.
A retrospective study was conducted using the data from consecutive patients’ files that were imaged in the time interval between June 2008 and December 2009, using the Pentacam branded as Allegro Oculyzer (WaveLight, GmbH, Erlangen, Germany) [
Every eye was scanned at least thrice by the Allegro Oculyzer, in a dark room and according to the recommendations stated in the device manual. Each scan included 25 Scheimpflug images. Despite good repeatability, data were collected from the most reliable scan as stated by the “QS” pop-up box (i.e., largest analysed area, valid data percent, and good alignment). The data were collected from the automatically calculated indices or generated by manually changing the reference surface shape (BFS or BFTE) and calculation area (7, 8, or 9 mm) and getting the elevation values on mouse click. The investigated indices were as follows.
Curvature indices are as follows: Mean eccentricity of the anterior corneal surface in the central 3 mm. Mean radii of curvature of anterior and posterior corneal surfaces in the central 3 mm (in mm). Determinants of BFS:
radii of curvature of the 8 and 9 mm anterior BFS (in mm), radii of curvature of the 7, 8, and 9 mm posterior BFS (in mm). Determinants of posterior BFTE:
eccentricity of the 7, 8, and 9 mm posterior BFTE, flattest and steepest radii of curvature of the 7, 8, and 9 mm posterior BFTE (in mm).
Elevation indices are as follows: Elevation of the thinnest point from the 7 mm posterior (PE) BFS (in Elevation of the thinnest point from the 8 and 9 mm anterior (AE) and posterior (PE) BFS and difference between corresponding anterior and posterior values (in Elevation of the thinnest point from the 7, 8, and 9 mm posterior (PE) BFTE (in Maximum elevation, minimum elevation, and the difference between them (in Maximum elevation, minimum elevation, and the difference between them (in Maximum elevation, minimum elevation, and the difference between them (in
Pachymetry indices are as follows: Pachymetry at corneal apex, corneal center, and thinnest point (in Pachymetry Progression Index-Average (PPI-Avg), Minimum (PPI-Min), and Maximum (PPI-Max) and the difference between PPI-Max and PPI-Min (PPI-Max minus PPI-Min). Ambrósio’s Relational Thickness-Average (ART-Avg), Minimum (ART-Min), and Maximum (ART-Max). Horizontal and vertical decentration of the thinnest point from the apex and the resultant diagonal decentration [=
Patients were classified into two groups according to TKC staging adopted by the device software;
The study adhered to the Tenets of the Declaration of Helsinki and to the local ethics standards.
Data were collected and verified, and the compound indices were calculated using Microsoft Excel 2010 (Redmond, Washington, USA). Statistical analyses were performed using IBM SPSS Statistics (v19; Armonk, NY, USA) and Medcalc v11.1.1.0 (MedCalc, Belgium).
The following tests were performed: calculation of the mean, standard deviation (SD), unpaired
Average patients’ age at the time of the initial Pentacam imaging was
The mean, SD, and 95% confidence interval (CI) of each parameter are listed in Table
The mean, SD, and 95% confidence interval (CI) of all tested KC detecting parameters. Radii of curvature are in mm, elevations in
Indices | Normal | Keratoconus stages 1 to 4 | ||||
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Mean | SD | 95% CI | Mean | SD | 95% CI | |
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Mean radius of curvature of anterior 3 mm corneal surface | 7.725 | 0.2305 | 7.323–8.149 | 6.954 | 0.6640 | 5.296–8.124 |
Mean radius of curvature of posterior 3 mm corneal surface | 6.425 | 0.256 | 6.040–6.809 | 5.624 | 0.685 | 4.145–6.777 |
Mean eccentricity of the anterior corneal surface | 0.519 | 0.132 | 0.210–0.730 | 0.799 | 0.3312 | −0.034–1.310 |
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Radius of curvature of the 8 mm anterior BFS | 7.786 | 0.2252 | 7.413–8.200 | 7.350 | 0.445 | 6.191–8.230 |
Radius of curvature of the 9 mm anterior BFS | 7.833 | 0.2225 | 7.462–8.269 | 7.488 | 0.398 | 6.535–8.284 |
Radius of curvature of the 7 mm posterior BFS | 6.390 | 0.2147 | 5.991–6.758 | 5.935 | 0.4148 | 5.109–6.700 |
Radius of curvature of the 8 mm posterior BFS | 6.464 | 0.3544 | 6.031–6.790 | 6.125 | 0.3842 | 5.465–7.022 |
Radius of curvature of the 9 mm posterior BFS | 6.548 | 0.3426 | 6.101–6.899 | 6.280 | 0.3041 | 5.765–6.981 |
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Flattest radius of curvature of the 7 mm posterior BFTE | 6.644 | 0.2495 | 6.143–7.152 | 5.972 | 0.7057 | 4.312–6.947 |
Steepest radius of curvature of the 7 mm posterior BFTE | 6.175 | 0.2317 | 5.741–6.609 | 5.273 | 0.6972 | 3.971–6.630 |
Eccentricity of the 7 mm posterior BFTE | 0.181 | 0.3173 | −0.441–0.765 | 0.618 | 0.5282 | −0.447–1.350 |
Flattest radius of curvature of the 8 mm posterior BFTE | 6.655 | 0.2507 | 6.143–7.169 | 5.972 | 0.7057 | 4.312–6.947 |
Steepest radius of curvature of the 8 mm posterior BFTE | 6.175 | 0.2317 | 5.741–6.609 | 5.278 | 0.6936 | 3.971–6.630 |
Eccentricity of the 8 mm posterior BFTE | 0.181 | 0.3173 | −0.441–0.765 | 0.618 | 0.5282 | −0.447–1.350 |
Flattest radius of curvature of the 9 mm posterior BFTE | 6.658 | 0.2665 | 6.143–7.169 | 5.972 | 0.7057 | 4.312–6.947 |
Steepest radius of curvature of the 9 mm posterior BFTE | 6.175 | 0.2317 | 5.741–6.609 | 5.287 | 0.7092 | 3.971–6.630 |
Eccentricity of the 9 mm posterior BFTE | 0.181 | 0.3173 | −0.441–0.765 | 0.618 | 0.5282 | −0.447–1.350 |
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Elevation of the thinnest point from the 7 mm posterior BFS | 1.5 | 3.13 | −4.0–9.9 | 46.2 | 32.7 | 2.3–115.4 |
Maximum elevation from the 7 mm posterior BFS | 32.8 | 15.12 | 17.1–72.6 | 79.9 | 34.3 | 27.7–153.7 |
Minimum elevation from the 7 mm posterior BFS | −34.4 | 15.4 | −75.0–−14.1 | −76.5 | 33.5 | −160.9–−29.3 |
Maximum minus minimum elevation from the 7 mm posterior BFS | 67.2 | 29.2 | 34.0–142.8 | 156.48 | 64.2 | 60.6–296.0 |
Elevation of the thinnest point from the 8 mm anterior BFS | 2.6 | 1.5 | 0.0–6.0 | 26.7 | 17.7 | 2.0–68.7 |
Elevation of the thinnest point from the 8 mm posterior BFS | 4.1 | 4.7 | −2.0–17.8 | 58.4 | 39.0 | 6.3–143.4 |
Elevation of the thinnest point from the 8 mm posterior BFS minus that of anterior BFS | 1.5 | 4.5 | −5.0–14.0 | 31.8 | 22.5 | 1.0–77.0 |
Maximum elevation from the 8 mm posterior BFS | 51.4 | 23.3 | 21.0–107.9 | 89.6 | 33.3 | 35.0–159.4 |
Minimum elevation from the 8 mm posterior BFS | −43.0 | 18.3 | −93.9–−22.1 | −90.9 | 41.1 | −198.9–−32.0 |
Maximum minus minimum elevation from the 8 mm posterior BFS | 94.5 | 37.9 | 49.2–188.6 | 180.5 | 71.4 | 67.6–351.8 |
Elevation of the thinnest point from the 9 mm anterior BFS | 4.5 | 2.0 | 1.0–9.0 | 34.0 | 21.5 | 6.0–90.7 |
Elevation of the thinnest point from the 9 mm posterior BFS | 9.9 | 6.8 | 2.0–31.9 | 72.7 | 43.9 | 11.0–171.8 |
Elevation of the thinnest point from the 9 mm posterior BFS minus that of anterior BFS | 5.3 | 6.2 | −2.0–24.9 | 38.7 | 24.0 | 2.3–89.0 |
Maximum elevation from the 9 mm posterior BFS | 76.5 | 28.16 | 43.1–145.9 | 112.836 | 41.9722 | 48.7–212.7 |
Minimum elevation from the 9 mm posterior BFS | −57.2 | 21.2 | −124.0–−33.2 | −104.8 | 61.0 | −248.4–−40.3 |
Maximum minus minimum elevation from the 9 mm posterior BFS | 133.7 | 45.7 | 77.6–284.5 | 217.6 | 89.23 | 87.3–428.0 |
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Elevation of the thinnest point from the 7 mm posterior BFTE | 3.5 | 2.8 | −1.0–10.0 | 37.6 | 24.9 | 0.0–97.01 |
Maximum elevation from the 7 mm posterior BFTE | 18.0 | 10.7 | 6.1–40.0 | 71.3 | 54.8 | 17.7–225.6 |
Minimum elevation from the 7 mm posterior BFTE | −25.8 | 8.6 | −47.8–−12.1 | −72.8 | 39.6 | −187.4–−18.3 |
Maximum minus minimum elevation from the 7 mm posterior BFTE | 43.8 | 16.3 | 23.0–80.3 | 144.1 | 88.3 | 39.3–393.1 |
Elevation of the thinnest point from the 8 mm posterior BFTE | 4.3 | 3.9 | −2.0–13.0 | 39.5 | 26.45 | −3.0–104.0 |
Maximum elevation from the 8 mm posterior BFTE | 43.9 | 22.2 | 11.1–114.8 | 103.1 | 84.1 | 23.0–360.0 |
Minimum elevation from the 8 mm posterior BFTE | −33.4 | 11.7 | −62.9–−16.1 | −89.9 | 40.9 | −185.1–−29.0 |
Maximum minus minimum elevation from the 8 mm posterior BFTE | 77.3 | 28.2 | 32.1–141.7 | 192.9 | 114.2 | 68.0–557.4 |
Elevation of the thinnest point from the 9 mm posterior BFTE | 3.6 | 5.6 | −5.0–16.9 | 38.7 | 30.8 | −18.34–111.8 |
Maximum elevation from the 9 mm posterior BFTE | 89.9 | 39.5 | 35.2–174.6 | 170.8 | 152.9 | 39.6–665.5 |
Minimum elevation from the 9 mm posterior BFTE | −42.1 | 15.5 | −88.3–−21.1 | −114.5 | 50.2 | −217.5–−32.7 |
Maximum minus minimum elevation from the 9 mm posterior BFTE | 132.0 | 43.3 | 65.2–231.4 | 285.2 | 177.1 | 107.3–859.2 |
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Apex thickness | 545.3 | 35.4 | 471.7–624.3 | 482.5 | 39.4 | 384.3–573.4 |
Central corneal thickness | 545. 5 | 35.5 | 471.7–623.4 | 491.4 | 35.3 | 408.9–573.4 |
Thinnest point thickness | 543.5 | 35.7 | 471.7–622.3 | 465.1 | 68.1 | 361.8–566.7 |
PPI-Min | 0.543 | 0.1512 | 0.300–0.800 | 1.504 | 0.8265 | 0.500–3.570 |
PPI-Avg | 0.826 | 0.1365 | 0.600–1.100 | 2.104 | 1.0454 | 0.900–4.967 |
PPI-Max | 1.077 | 0.1716 | 0.800–1.400 | 2.800 | 1.3960 | 1.232–6.705 |
PPI-Max minus PPI-Min | 0.534 | 0.189 | 0.300–0.992 | 1.296 | 0.763 | 0.400–3.835 |
ART-Min | 1109.9 | 430.9 | 613.0–2044.2 | 423.3 | 272.5 | 94.0–1045.4 |
ART-Avg | 677.6 | 131.4 | 395.7–975.2 | 276.6 | 138.2 | 75.7–616.9 |
ART-Max | 517.8 | 90.6 | 326.6–697.0 | 207.1 | 100.6 | 54.6–427.5 |
Horizontal decentration of the thinnest point from the apex | 0.026 | 0.569 | −0.948–1.019 | −0.087 | 0.574 | −0.940–0.827 |
Vertical decentration of the thinnest point from the apex | −0.273 | 0.208 | −0.639–0.214 | −0.519 | 0.347 | −1.394–0.110 |
Diagonal decentration of the thinnest point from the apex | 0.618 | 0.240 | 0.202–1.088 | 0.799 | 0.288 | 0.366–1.525 |
SD: standard deviation, 95% CI: 95% confidence interval, BFS: best-fit sphere, BFTE: best-fit toric ellipsoid, PPI-Min: Pachymetry Progression Index-Minimum, PPI-Avg: Pachymetry Progression Index-Average, PPI-Max: Pachymetry Progression Index-Maximum, ART-Min: Ambrósio’s Relational Thickness-Minimum, ART-Avg: Ambrósio’s Relational Thickness-Average, and ART-Max: Ambrósio’s Relational Thickness-Maximum.
Table
AUROC of all indices when differentiating all grades of KC collectively from normal corneas.
Indices | AUROC | SEM | 95% CI |
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Mean radius of curvature of anterior 3 mm | 0.885 | 0.03 | 0.828 to 0.928 |
Mean radius of curvature of posterior 3 mm | 0.882 | 0.0307 | 0.824 to 0.925 |
Mean eccentricity of the anterior surface | 0.811 | 0.039 | 0.745 to 0.866 |
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Radius of curvature of the 8 mm anterior BFS | 0.812 | 0.035 | 0.747 to 0.867 |
Radius of curvature of the 9 mm anterior BFS | 0.776 | 0.0374 | 0.707 to 0.835 |
Radius of curvature of the 7 mm posterior BFS | 0.841 | 0.0323 | 0.779 to 0.892 |
Radius of curvature of the 8 mm posterior BFS | 0.784 | 0.037 | 0.712 to 0.856 |
Radius of curvature of the 9 mm posterior BFS | 0.746 | 0.038 | 0.671 to 0.821 |
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Flattest radius of curvature of the 7 mm posterior BFTE | 0.829 | 0.0344 | 0.765 to 0.882 |
Steepest radius of curvature of the 7 mm posterior BFTE | 0.895 | 0.0298 | 0.840 to 0.936 |
Eccentricity of the 7 mm posterior BFTE | 0.757 | 0.0402 | 0.687 to 0.818 |
Flattest radius of curvature of the 8 mm posterior BFTE | 0.834 | 0.034 | 0.770 to 0.885 |
Steepest radius of curvature of the 8 mm posterior BFTE | 0.895 | 0.0298 | 0.840 to 0.936 |
Eccentricity of the 8 mm posterior BFTE | 0.757 | 0.0402 | 0.687 to 0.818 |
Flattest radius of curvature of the 9 mm posterior BFTE | 0.833 | 0.034 | 0.769 to 0.885 |
Steepest radius of curvature of the 9 mm posterior BFTE | 0.884 | 0.0313 | 0.827 to 0.927 |
Eccentricity of the 9 mm posterior BFTE | 0.757 | 0.0402 | 0.687 to 0.818 |
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Elevation of the thinnest point from the 7 mm posterior BFS | 0.971 | 0.0137 | 0.934 to 0.990 |
Maximum elevation from the 7 mm posterior BFS | 0.916 | 0.0202 | 0.865 to 0.953 |
Minimum elevation from the 7 mm posterior BFS | 0.908 | 0.0219 | 0.855 to 0.946 |
Maximum minus minimum elevation from the 7 mm posterior BFS | 0.923 | 0.0193 | 0.873 to 0.957 |
Elevation of the thinnest point from the 7 mm posterior BFTE | 0.953 | 0.0214 | 0.910 to 0.979 |
Maximum elevation from the 7 mm posterior BFTE | 0.944 | 0.0161 | 0.899 to 0.973 |
Minimum elevation from the 7 mm posterior BFTE | 0.91 | 0.0264 | 0.858 to 0.948 |
Maximum minus minimum elevation from the 7 mm posterior BFTE | 0.949 | 0.0174 | 0.905 to 0.976 |
Elevation of the thinnest point from the 8 mm anterior BFS | 0.968 | 0.018 | 0.930 to 0.989 |
Elevation of the thinnest point from the 8 mm posterior BFS | 0.979 | 0.00837 | 0.945 to 0.995 |
Maximum elevation from the 8 mm posterior BFS | 0.837 | 0.0304 | 0.775 to 0.889 |
Minimum elevation from the 8 mm posterior BFS | 0.89 | 0.0255 | 0.834 to 0.932 |
Maximum minus minimum elevation from the 8 mm posterior BFS | 0.875 | 0.0272 | 0.817 to 0.920 |
Elevation of the thinnest point from the 8 mm anterior BFS minus that of posterior BFS | 0.961 | 0.0139 | 0.920 to 0.984 |
Elevation of the thinnest point from the 8 mm posterior BFTE | 0.933 | 0.0264 | 0.885 to 0.965 |
Maximum elevation from the 8 mm posterior BFTE | 0.806 | 0.0342 | 0.739 to 0.861 |
Minimum elevation from the 8 mm posterior BFTE | 0.923 | 0.022 | 0.873 to 0.958 |
Maximum minus Minimum elevation from the 8 mm posterior BFTE | 0.912 | 0.0225 | 0.861 to 0.950 |
Elevation of the thinnest point from the 9 mm anterior BFS | 0.979 | 0.0118 | 0.945 to 0.995 |
Elevation of the thinnest point from the 9 mm posterior BFS | 0.977 | 0.0098 | 0.942 to 0.994 |
Maximum elevation from the 9 mm posterior BFS | 0.772 | 0.0364 | 0.703 to 0.832 |
Minimum elevation from the 9 mm posterior BFS | 0.865 | 0.0294 | 0.805 to 0.912 |
Maximum minus minimum elevation from the 9 mm posterior BFS | 0.832 | 0.0324 | 0.768 to 0.884 |
Elevation of the thinnest point from the 9 mm anterior BFS minus that of posterior BFS | 0.952 | 0.0171 | 0.909 to 0.979 |
Elevation of the thinnest point from the 9 mm posterior BFTE | 0.904 | 0.0309 | 0.851 to 0.943 |
Maximum elevation from the 9 mm posterior BFTE | 0.671 | 0.0451 | 0.597 to 0.740 |
Minimum elevation from the 9 mm posterior BFTE | 0.934 | 0.0204 | 0.886 to 0.966 |
Maximum minus minimum elevation from the 9 mm posterior BFTE | 0.887 | 0.0264 | 0.831 to 0.930 |
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Apex thickness | 0.897 | 0.0268 | 0.842 to 0.938 |
Central corneal thickness | 0.878 | 0.0288 | 0.820 to 0.922 |
Thinnest point thickness | 0.915 | 0.0241 | 0.863 to 0.952 |
PPI-Min | 0.939 | 0.0208 | 0.893 to 0.969 |
PPI-Avg | 0.978 | 0.00973 | 0.944 to 0.994 |
PPI-Max | 0.987 | 0.00563 | 0.958 to 0.998 |
PPI-Max minus PPI-Min | 0.903 | 0.0246 | 0.850 to 0.943 |
ART-Min | 0.949 | 0.0183 | 0.905 to 0.976 |
ART-Avg | 0.976 | 0.0101 | 0.942 to 0.993 |
ART-Max | 0.987 | 0.00605 | 0.957 to 0.998 |
Horizontal decentration of the thinnest point from the apex | 0.558 | 0.0444 | 0.481 to 0.633 |
Vertical decentration of the thinnest point from the apex | 0.737 | 0.0397 | 0.666 to 0.801 |
Diagonal decentration of the thinnest point from the apex | 0.686 | 0.0403 | 0.612 to 0.754 |
AUROC: area under the receiver operating characteristic curve. SEM: standard error of the mean. 95% CI: 95% confidence interval of the AUROC. BFS: best-fit sphere, BFTE: best-fit toric ellipsoid, PPI-Min: Pachymetry Progression Index-Minimum, PPI-Avg: Pachymetry Progression Index-Average, PPI-Max: Pachymetry Progression Index-Maximum, ART-Min: Ambrósio’s Relational Thickness-Minimum, ART-Avg: Ambrósio’s Relational Thickness-Average, and ART-Max: Ambrósio’s Relational Thickness-Maximum.
The indices with highest AUROC.
Indices | AUROC | 95% CI | Criterion | Sensitivity | Specificity | LR+ | LR− | AUROC compared to that of PPI-Max |
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PPI-Max | 0.987 | 0.958 to 0.998 | >1.4 | 91.78 | 98.06 | 47.27 | 0.084 | |
ART-Max | 0.987 | 0.957 to 0.998 | ≤412 | 97.26 | 93.2 | 14.31 | 0.029 |
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Elevation of the thinnest point from the 9 mm anterior BFS | 0.979 | 0.945 to 0.995 | >7 | 95.89 | 92.23 | 12.35 | 0.045 |
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Elevation of the thinnest point from the 8 mm anterior BFS | 0.968 | 0.930 to 0.989 | >5 | 91.78 | 96.12 | 23.63 | 0.086 |
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Elevation of the thinnest point from the 8 mm posterior BFS | 0.979 | 0.945 to 0.995 | >20 | 86.3 | 100 | N/A | 0.14 |
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Elevation of the thinnest point from the 7 mm posterior BFS | 0.971 | 0.934 to 0.990 | >10 | 87.67 | 100 | N/A | 0.12 |
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Elevation of the thinnest point from the 9 mm posterior BFS | 0.977 | 0.942 to 0.994 | >22 | 89.04 | 96.12 | 22.93 | 0.11 |
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PPI-Avg | 0.978 | 0.944 to 0.994 | >1.1 | 87.67 | 98.06 | 45.15 | 0.13 |
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Elevation of the thinnest point from the 7 mm posterior BFTE | 0.953 | 0.910 to 0.979 | >10 | 87.67 | 99.03 | 90.3 | 0.12 |
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ART-Avg | 0.976 | 0.942 to 0.993 | ≤496 | 94.52 | 94.17 | 16.23 | 0.058 |
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All other tested indices |
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AUROC: area under the receiver operating characteristic curve. 95% CI: 95% confidence interval of the AUROC. LR+: likelihood ratio of positive results. LR−: likelihood ratio of negative results.
Among the examined patients of both groups, pachymetry progression indices showed the highest AUROC accuracy among the evaluated parameters, where the Pachymetry Progression Index-Maximum (PPI-Max) and Ambrósio’s Relational Thickness-Maximum (ART-Max) had the same highest AUROC (0.987). The best cut-off for PPI-Max was (>1.4 with sensitivity 91.78% and specificity 98.06%) and for ART-Max was (≤412 with sensitivity 97.26% and specificity 93.2%).
The further analysis of the indices according to patients’ grouping (based on TKC grading) showed that the mean values of all indices were significantly different when comparing group 2c corneas and normal corneas
Scheimpflug imaging indices differentiating between GROUP 2a KC and normal corneas.
Indices | AUROC | SEM |
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PPI-Max | 0.690 | 0.038 | 0.615 to 0.765 | 0.018 |
ART-Max | 0.690 | 0.038 | 0.615 to 0.764 | 0.019 |
PPI-Max minus PPI-Min | 0.687 | 0.055 | 0.579 to 0.796 | 0.020 |
Central corneal thickness | 0.683 | 0.048 | 0.590 to 0.776 | 0.023 |
Diagonal decentration of the thinnest point from the apex | 0.674 | 0.062 | 0.552 to 0.795 | 0.031 |
AUROC: area under the receiver operating characteristic curve, SEM: standard error of the mean, 95% CI: 95% confidence interval of the AUROC, PPI-Max: Pachymetry Progression Index-Maximum, ART-Max: Ambrósio’s Relational Thickness-Maximum, and PPI-Min: Pachymetry Progression Index-Minimum.
Placido-based systems rely on the analysis of a reflected image. This cannot provide data from the posterior corneal surface [
With the appearance of many KC indices and suggestion of various best-fit reference surfaces [
The inclusion of every single possible index is out of the scope of this study. The included indices are those mentioned individually in other studies or those we thought they might be significant. On evaluating BFTE and smaller best-fit surface (7 mm), we focused on the posterior rather than the anterior corneal surface, being the site for primary subclinical tomographic changes, preceding the anterior surface [
Curvature-based indices derived from both anterior and posterior surfaces were evaluated and analysed. According to the ROC curves, all evaluated curvature indices had statistically significant less AUROC for diagnosing KC than most of elevation and pachymetry indices. Previous reports demonstrated similar results [
In this study, where we only analysed data from good quality scans with corneal surface area more than 9 mm, analysis of elevation indices showed that posterior surface elevation (PE) from 8 mm BFS and anterior surface elevation (AE) from 9 mm BFS had the highest accuracy, with AUROC (0.979) for both, followed by PE from 7 and 9 mm BFS (AUROC = 0.977 and 0.971, resp.). AE and PE from reference best-fit surfaces did not differ in their accuracy with various diameters (7, 8, and 9 mm). Correia et al. reported that PE from both 8 mm BFS and BFTE had highest AUROC (0.983 and 0.986, resp.) [
We enrolled in our work the elevation indices of the thinnest point from both BFS and BFTE. The latter was evaluated in literature and showed higher predictive accuracy in diagnosing KC and forme fruste KC compared to BFS, especially with a fixed eccentricity. A possible explanation for its diagnostic superiority in ectatic corneas is that its central vaulting can determine whether the corneal pattern is associated with an atypical condition, a true corneal disease, or an artifact of alignment or processing [
According to our study results, parameters that had the highest AUROC for diagnosing KC were the PPI-Max at cut-off value of >1.4 and the ART-Max at cut-off value of ≤412 (0.987 for both). These results were comparable to those of Ambrósio Jr. et al. [
Our study investigated the accuracy of various tomographic indices in relation to TKC grading [
Furthermore, Kamiya and his colleagues [
Our findings justify the modifications in the newer software versions that incorporated “ART-Max” and “diagonal decentration of the thinnest point from the apex,” being sensitive indices for early KC detection. As regards the hardware changes, we also recommend similar studies on Pentacam HR, because the current one was on Allegro Oculyzer I Pentacam. Randleman et al. [
The accuracy of different curvature, elevation, and pachymetry-based indices using various reference shapes of different diameters was evaluated, in relation to TKC grading. To the best of our knowledge, this is the first study to confirm clinical and topographical observations by a long follow-up period of 6 years to ascertain the diagnosis by time evidence. Generally, to discriminate all KC grades from normal, ten pachymetry and elevation-based indices were significantly more accurate than other indices, having higher AUROC. Five of them had statistically significant high AUROC when comparing early KC to normal corneas.
There are no competing interests of any of the authors with any establishment having a relation to this present work.
Sherine S. Wahba and Maged M. Roshdy received travel support by Alcon Laboratories Inc., USA, to attend ESCRS Congress.