Intrasubject repeatability in keratoconus-eye measurements obtained with a new Scheimpflug photography-based system

Differences in anterior chamber depth in keratoconus patients with binocular very asymmetry ectasia

BACKGROUND

To evaluate the difference in anterior chamber depth (ACD) between two eyes among keratoconus patients with binocular very asymmetric ectasia (VAE) and to explore the influencing factors.

METHODS

The corneal curvature and ACD in both eyes of patients with VAE were measured by Sirius (version 3.2, CSO, Italy) at the following points: corneal vertex, maximum curvature (apex), and the 1.5 mm, 2.5 mm, and 3.5 mm superior-, inferior-, nasal-, temporal-paracentral from center. The mean pupil power (MPP) and corneal morphology parameters were also measured. Correlations between ACD and curvature and morphology parameters were analyzed by linear regression.

RESULTS

172 eyes of 86 patients (9 to 45 years) were classified into the VAE-N (n = 86) group and the VAE-E group (n = 86) based on the corneal morphology. The central (3.32 ± 0.27 mm versus 3.43 ± 0.29 mm, P < 0.001) and paracentral ACDs increased significantly in the VAE-E group, and the corneal morphology parameters were also significantly higher. The central ACD was significantly correlated with the MPP (r = 0.465), KVf/b (Keratoconus Vertex front/back) (r = 0.306, r = 0.327), and BCVf/b (Baiocchi Calossi Versaci front/back) (r = 0.356, r = 0.416). Linear regression showed good relationships between △ACD and △MPP (R2 = 0.429) and △KVf/b (R2 = 0.504, R2 = 0.536).

CONCLUSIONS

The ACD was larger in the VAE-E group. The difference in ACD between the VAE-E and VAE-N groups was significantly correlated with corneal curvature and the extent of corneal elevation, indicating the influences of both the corneal magnification effect and corneal ectasia on ACD.

Novel Multivariable Evolutionary Algorithm-Based Method for Modal Reconstruction of the Corneal Surface from Sparse and Incomplete Point Clouds

Three-dimensional reconstruction of the corneal surface provides a powerful tool for managing corneal diseases. This study proposes a novel method for reconstructing the corneal surface from elevation point clouds, using modal schemes capable of reproducing corneal shapes using surface polynomial functions. The multivariable polynomial fitting was performed using a non-dominated sorting multivariable genetic algorithm (NS-MVGA). Standard reconstruction methods using least-squares discrete fitting (LSQ) and sequential quadratic programming (SQP) were compared with the evolutionary algorithm-based approach. The study included 270 corneal surfaces of 135 eyes of 102 patients (ages 11-63) sorted in two groups: control (66 eyes of 33 patients) and keratoconus (KC) (69 eyes of 69 patients). Tomographic information (Sirius, Costruzione Strumenti Oftalmici, Italy) was processed using Matlab. The goodness of fit for each method was evaluated using mean squared error (MSE), measured at the same nodes where the elevation data were collected. Polynomial fitting based on NS-MVGA improves MSE values by 86% compared to LSQ-based methods in healthy patients. Moreover, this new method improves aberrated surface reconstruction by an average value of 56% if compared with LSQ-based methods in keratoconus patients. Finally, significant improvements were also found in morpho-geometric parameters, such as asphericity and corneal curvature radii.

Best indices of dual Scheimpflug/Placido tomographer for keratoconus detection

PURPOSE

To evaluate the accuracy of different corneal parameters in detecting keratoconus using a dual Scheimpflug/Placido system (Sirius, CSO, Italy).

METHODS

Sixty-eight eyes of 68 keratoconus patients and 77 eyes of 77 normal subjects were prospectively assessed in a diagnostic test study. The mean differences of the corneal parameters were compared using the independent t-test. The accuracy of curvature, elevation, pachymetry, and aberrometry indices, aqueous depth, and corneal volume in 10 mm was evaluated using the area under the curve (AUC), and the DeLong method was used for the comparison of AUCs. Multiple tests in a parallel manner and multiple logistic regression analysis were applied to determine the best predictor indices.

RESULTS

All indices except aqueous depth and corneal volume were significantly different between the two groups (P < 0.001). Back keratoconus vertex (KVb) and front Baiocchi-Calossi-Versaci index (BCVf) had the highest sensitivity (for both 98.53%) followed by front symmetry index (SIf) (96.12%) and thinnest point of the cornea (88.24%) in elevation, aberrometry, curvature, and pachymetry parameters, respectively. The highest diagnostic ability was observed in KVb (AUC 0.993) and BCVf index (AUC 0.992) (DeLong > 0.05). Multiple test analysis showed a combination of indices with the highest accuracy that was similar to the performance of each one individually, and keratoconus was diagnosed correctly in 98.5% of the cases (R² = 93%).

CONCLUSION

The parameters extracted from Sirius can differentiate keratoconus from normal corneas with high accuracy without the need for complex computational algorithms. Elevation-based and combined aberrometry indices had the highest diagnostic power.

Logistic Regression Model Using Scheimpflug-Placido Cornea Topographer Parameters to Diagnose Keratoconus

Purpose

Diagnose keratoconus by establishing an effective logistic regression model from the data obtained with a Scheimpflug-Placido cornea topographer.

Methods

Topographical parameters of 125 eyes of 70 patients diagnosed with keratoconus by clinical or topographical findings were compared with 120 eyes of 63 patients who were defined as keratorefractive surgery candidates. The receiver operating character (ROC) curve analysis was performed to determine the diagnostic ability of the topographic parameters. The data set of parameters with an AUROC (area under the ROC curve) value greater than 0.9 was analyzed with logistic regression analysis (LRA) to determine the most predictive model that could diagnose keratoconus. A logit formula of the model was built, and the logit values of every eye in the study were calculated according to this formula. Then, an ROC analysis of the logit values was done.

Results

Baiocchi Calossi Versaci front index (BCV_f) had the highest AUROC value (0.976) in the study. The LRA model, which had the highest prediction ability, had 97.5% accuracy, 96.8% sensitivity, and 99.2% specificity. The most significant parameters were found to be BCV_f (p=0.001), BCV_b (Baiocchi Calossi Versaci back) (p=0.002), posterior rf (apical radius of the flattest meridian of the aspherotoric surface in 4.5 mm diameter of the cornea) (p=0.005), central corneal thickness (p=0.072), and minimum corneal thickness (p=0.494).

Conclusions

The LRA model can distinguish keratoconus corneas from normal ones with high accuracy without the need for complex computer algorithms.

Topographic, elevation, and keratoconus indices for diagnosis of keratoconus by a combined Placido and Scheimpflug topography system

PURPOSE

To determine the accuracy of various corneal parameters in keratoconus diagnosis using Scheimpflug camera combined with Placido disk corneal topography (Sirius, CSO).

METHODS

One hundred and fifteen keratoconic eyes (group1) and a 111 normal eyes (group2) were assessed prospectively between March 2018 and July 2019 for: corneal keratometric indices (K1, K2, sim K, apex curvature) at different corneal rings of both corneal surfaces, central corneal thickness (CCT), thinnest corneal thickness (TL), corneal asphericity (Q), elevation at thinnest point, root mean square (RMS), and root mean square per area (RMS/A) in spherical, aspheric and aspherotoric reference for both corneal surfaces and keratoconus summary parameters; surface asymmetry index of front and back (SIf, SIb respectively), elevation at keratoconus vertex front and back (KVf, KVb respectively), Baiocchi Calossi Versaci front and back index (BCVf, BCVb) and its vector summation (BCV) and convergence radius and cutoff value for each was calculated.

RESULTS

All studied indices were significantly different between the two groups. The highest predictive accuracy "Area under receiver operating characteristic curve (AUROC)" of 0.999 was observed for BCVf, KVb, RMS and RMS/A at 6 mm aspherotoric reference posterior surface. Keratoconus summary indices had high AUROC (0.986, 0.984, 0.948, 0.999, 0.999, 0.998 respectively). Curvature indices had lower AUROC than elevation indices, except for curvature of corneal apex at anterior (0.98) and posterior surface (0.99). Higher AUROC was noted with elevation at thinnest point especially at aspherotoric reference surface.

CONCLUSION

Sirius topography showed high predictive accuracy in detection of keratoconus. Elevation indices and keratoconus summary parameters have the highest diagnostic ability.

Repeatability of Zone Averages Compared to Single-Point Measurements of Maximal Curvature in Keratoconus

PURPOSE

To evaluate the repeatability of curvature zone averages centered on the point of maximum curvature (Kmax) compared to that of the single-point Kmax.

DESIGN

Comparative reliability analysis.

METHODS

Setting: American University of Beirut Medical Center, Beirut, Lebanon.

STUDY POPULATION

Sixty-five eyes of 65 adult keratoconus patients. Patients with other ocular disease, history of ocular surgery or trauma, and contact lens wear within 2 weeks of image acquisition were excluded.

OBSERVATION PROCEDURES

Eyes were evaluated with 3 consecutive scans using the Galilei dual Scheimpflug-Placido system.

MAIN OUTCOME MEASURES

Repeatability of axial and instantaneous Kmax single points, and zone averages with radii of 0.1-2.0 mm, centered on them. Repeatability was assessed by within-subject standard deviations, repeatability limits (r), and intraclass correlation coefficients.

RESULTS

Axial curvature zone averaging yielded clinically acceptable repeatability only in eyes with Kmax ≤50 diopters (D), for radii of 1.5 mm and 2.0 mm (r = 0.87 D and r = 0.76, respectively, vs r = 0.91 for the single-point axial Kmax). Compared to instantaneous Kmax, clinically acceptable repeatability was achieved with instantaneous zone averages of at least 1.5 mm radius in eyes with Kmax ≤50 D (r = 0.99 D and r = 0.70 D, respectively) and 2.0 mm radius in eyes with Kmax >50 D (r = 2.28 D and r = 0.87 D, respectively). For all eyes, the repeatability limit of the location of Kmax was 0.82 mm and 0.80 mm for axial and instantaneous curvature, respectively.

CONCLUSIONS

Instantaneous curvature zone averages centered on Kmax yielded a greater improvement in repeatability than axial zone averages and reached clinical adequacy with radii of at least 1.5 mm, for eyes with Kmax ≤50 D, and with a 2.0 mm radius for eyes with Kmax >50 D.

Repeatability and reliability of measurements obtained by the combined Scheimpflug and Placido-disk tomography in different stages of keratoconus

OBJECTIVE

The objective of this study is to evaluate the repeatability and reliability of corneal parameters in different stages of keratoconus patients using a combined Scheimpflug-Placido disc analysis system.

MATERIALS AND METHODS

In this prospective study, three consecutive measurements were performed by the same observer using Scheimpflug-Placido disc anterior segment analysis device in keratoconus patients. Flattest and steepest simulated keratometry and corneal volume, corneal aberrations, thinnest corneal thickness, symmetry index, keratoconus vertex and Baiocchi-Calossi-Versaci index were recorded. Keratoconic eyes were divided into four stages using the Amsler-Krumeich classification. Repeatability was evaluated using within the subject standard deviation, repeatability index (Ri) and coefficient of variation; reliability was evaluated by intraclass correlation coefficient (ICC). Pearson correlation coefficients were used to assess the correlation between the parameters evaluated.

RESULTS

Two hundred sixty-one eyes of 261 keratoconus patients were included in the study. The repeatability for all corneal curvature parameters decreased as the keratoconus severity increased, and there is a positive correlation between keratometry of the apex and corneal curvature parameters (p < 0.05) except mean simulated keratometry. The corneal aberrations were repeatable in all keratoconus subgroups (Ri < 0.34 μm). There are also positive correlations between keratometry of apex and corneal aberrations (p < 0.05) except total high-order root mean square and spherical aberration. The reliability was excellent (ICC > 0.90) for all indices except keratoconus vertex back.

CONCLUSION

The parameters used in the diagnosis and follow-up of keratoconus in the Sirius corneal tomography system may differ more than expected at stages 3 and 4 of the disease. Awareness of this situation may be helpful in planning follow-up and treatment.

[Intraocular correction of ametropia in patients with keratoconus]

This review was conducted due to the growing number of patients with keratoconus requiring cataract surgery and the complexity of surgical planning. The article reviews preoperative planning, intraoperative options, and postoperative management, which can help obtain high functional results. Treatment of cataracts in keratoconic eyes requires a multifaceted approach. In some cases, in the preoperative period, interventions such as crosslinking and implantation of intrastromal segments or corneal ring may be required to ensure stable keratometry values for accurate intraocular lens (IOL) calculations. The use of stabilizing procedures prior to optical biometrics can assist in preoperative lens selection and provide predictable surgical outcomes. To achieve targeted refraction and reduce unwanted optical effects, it is necessary to take a critical approach when choosing the type of lens (toric, monofocal). This is due to the appearance of aberrations of different order after preliminary surgical interventions on the cornea. The intraoperative decision plays important role in choosing the position of the corneal incisions, how to use sutures for wound adaptation, choosing scleral approaches to minimize possible postoperative complications. In the postoperative period, the degree of irregular astigmatism and the need for rigid gas-permeable or scleral lenses should be assessed.

Morphogeometric analysis for characterization of keratoconus considering the spatial localization and projection of apex and minimum corneal thickness point

This work evaluates changes in new morphogeometric indices developed considering the position of anterior and posterior corneal apex and minimum corneal thickness (MCT) point in keratoconus. This prospective comparative study included 440 eyes of 440 patients (age, 7-99 years): control (124 eyes) and keratoconus (KC) groups (316 eyes). Tomographic information (Sirius®, Costruzione Strumenti Oftalmici, Italy) was treated with SolidWorks v2013, creating the following morphogeometric parameters: geometric axis-apex line angle (GA-AP), geometric axis-MCT line angle (GA-MCT, apex line-MCT line angle (AP-MCT), and distances between apex and MCT points on the anterior (anterior AP-MCTd) and posterior corneal surface (posterior AP-MCTd). Statistically significant higher values of GA-AP, GA-MCT, AP-MCT and anterior AP-MCTd were found in the keratoconus group (p ≤ 0.001). Moderate significant correlations of corneal aberrations (r ≥ 0.587, p < 0.001) and corneal thickness parameters (r ≤ -0.414, p < 0.001) with GA-AP and AP-MCT were found. Anterior asphericity was found to be significantly correlated with anterior and posterior AP-MCTd (r ≥ 0.430, p < 0.001). Likewise, GA-AP and AP-MCT showed a good diagnostic ability for the detection of keratoconus, with optimal cutoff values of 9.61° (sensitivity 85.5%, specificity 80.3%) and 18.08° (sensitivity 80.5%, specificity 78.7%), respectively. These new morphogeometric indices allow a clinical characterization of the 3-D structural alteration occurring in keratoconus, with less coincidence in the spatial projection of the apex and MCT points of both corneal surfaces. Future studies should confirm the potential impact on the precision of these indices of the variability of posterior corneal surface measurements obtained with Scheimpflug imaging technology.

Posterior corneal features in patients with Down syndrome and their relation with keratoconus

AIMS

To characterise posterior corneal surface features in patients with Down syndrome (DS) and to compare them with healthy and mild keratoconus corneas.

METHODS

This restrospective, comparative, non-randomised, clinical study included 123 eyes, divided into three groups (37 eyes of patients with DS, 46 with mild keratoconus and 40 controls), and took place at Vissum Alicante. Only patients with no previous ocular surgery, no corneal scars and no active ocular disease other than keratoconus were included. The Sirius System topographer (CSO, Firenze, Italy) was used in order to analyse posterior corneal surface keratometry, shape and keratoconus screening indices, posterior corneal aberrations, corneal volume and pachymetry.

RESULTS

Patients with DS, when compared with healthy controls, have a steeper (mean keratometry 7 mm (KM): -6.30±0.44 vs -6.15±0.22; p<0.05) and more irregular (root mean square per unit of area: 4.5 mm 0.22±0.22 vs 0.09±0.03, p<0.001; posterior vertex of the ectatic area: 33.22±44.29 vs 10.63±2.88, p<0.001) posterior corneal surface, with higher aberrations (high-order aberrations (HOAs): 1.07±1.43 vs 0.15±0.06, p<0.001; coma-like: 0.88±1.09 vs 0.13±0.07, p<0.001) and thinnest pachymetry (497.68±26.88 vs 538.95±31.67, p<0.001). At the same time, no statistically significant difference was found between patients with DS and patients with mild keratoconus (p>0.05) in KM (-6.38±0.34), HOA (0.56±0.36), coma-like (0.51±0.34) and pachymetry (500.56±36.83).

CONCLUSIONS

Posterior corneal surface of patients with DS is steeper, more irregular and shows more higher order aberrations, as well as reduced volume and thinner pachymetry than patients with healthy corneas. Additionally, posterior corneal surface in patients with DS shows similar characteristics to those found in mild keratoconus.

Repeatability of Anterior Curvature Metrics in Healthy and Keratoconic Eyes with a Portable Handheld Topographer

SIGNIFICANCE

This is a prospective observational investigation that studies the repeatability of several outcomes obtained with the Keratron Scout in healthy and keratoconic corneas. In addition, we have been able to determine the "noise" values of the instrument, which can be very useful when evaluating possible progression in keratoconus (KCN).

PURPOSE

The purpose of this study was to evaluate the repeatability of several anterior curvature metrics provided by a portable and handheld Placido disk-based topographer in healthy and keratoconic eyes.

METHODS

One eye from 50 keratoconic patients and 50 eyes from healthy patients were included in this prospective observational study. Two consecutive sessions (with three scans per session) were performed, and the following parameters were analyzed with a Keratron Scout topographer: flat keratometry, steep keratometry, flat corneal axis, corneal astigmatism, and flat and steep asphericities. In addition, in the KCN cohort, indices derived from the Cone Location and Magnitude Index were also recorded. Intrasession (first session, all three measurements) and intersession (one measurement from each session, selected randomly) statistics were calculated. The following variables were calculated: within-subject standard deviation, coefficient of repeatability (R), coefficient of variation, and intraclass correlation coefficient (ICC).

RESULTS

In healthy eyes, the intrasession and intersession ICCs were high (0.947 to 0.999), and for both flat keratometry and steep keratometry, the within-subject standard deviation was 0.08 and R was 0.24 diopters. Repeatability was slightly lower in the KCN group, but the ICC was greater than 0.95, whereas the R for flat keratometry was 0.41 diopters.

CONCLUSIONS

The Keratron Scout provides repeatable measures for the studied metrics in healthy and keratoconic eyes.

Comparison of Simulated Keratometry and Total Refractive Power for Keratoconus According to the Stage of Amsler-Krumeich Classification

This study was aimed to assess the simulated keratometry (Sim K) and the total corneal refractive power (TCRP) in eyes with keratoconus with respect to the Amsler-Krumeich classification. We enrolled 100 eyes of 100 keratoconic patients and 25 age-matched normal eyes. The Sim K and TCRP were measured with a rotating Scheimpflug system (Pentacam HR, Oculus). The differences between Sim K and TCRP in the keratoconus group were significantly larger than those in the control group (p < 0.001). The differences between Sim K and TCRP became larger in the progressive stages of the disease (p = 0.191 for stage 1, p = 0.008 for stage 2, p < 0.001 for stage 3, p < 0.001 for stage 4). We found a significant correlation of Sim K with the differences between Sim K and TCRP in keratoconic patients (r = 0.497, p < 0.001). The differences between Sim K and TCRP for keratoconus were significantly larger than those for normal eyes, and the differences between Sim K and TCRP tended to become larger in the progressive stages of the disease. It is suggested that the Sim K readings overestimate the TCRP, especially in advanced keratoconus, and that this discrepancy is a possible source of a hyperopic refractive error after cataract surgery.

Time Course of Changes in Simulated Keratometry and Total Corneal Refractive Power after Corneal Collagen Cross-Linking for Progressive Keratoconus

Purpose

To assess the simulated keratometry (Sim K) and the total corneal refractive power (TCRP) in eyes undergoing conventional corneal cross-linking (CXL).

Methods

This study comprised 20 eyes of 20 keratoconic patients (14 men and 6 women; median age (25th and 75th percentile), 26.5 (21.8, 38.0) years) who underwent CXL. The Sim K and TCRP were measured with a rotating Scheimpflug system (Pentacam HR, Oculus), preoperatively and 1, 3, 6, and 12 months postoperatively.

Results

The values of Sim K were 52.65 (46.00, 55.70), 52.45 (45.85, 56.88), 51.70 (45.78, 55.83), 51.40 (45.68, 56.80), and 51.25 (46.08, 56.15) D preoperatively and 1, 3, 6, and 12 months postoperatively, respectively. The corresponding figures of TCRP were 52.10 (45.48, 55.08), 51.30 (45.18, 55.20), 50.95 (45.15, 54.50), 50.00 (45.18, 55.08), and 49.80 (45.48, 54.15) D, respectively. The variances of the Sim K and TCRP data were not statistically significant (p=0.994 and p=0.970, respectively, Kruskal-Wallis test). The Sim K was significantly larger than the TCRP before CXL and at 1, 3, 6, and 12 months after CXL (p<0.001, Wilcoxon signed-rank test).

Conclusions

Not only the Sim K but also TCRP was decreased by approximately 1 D after CXL. The Sim K readings may overestimate the TCRP, even after CXL for progressive keratoconus.

Repeatability of corneal elevation maps in keratoconus patients using the tomography matching method

To assess repeatability of corneal tomography in successive measurements by Pentacam in keratoconus (KC) and normal eyes based on the Iterative Closest Point (ICP) algorithm. The study involved 143 keratoconic and 143 matched normal eyes. ICP algorithm was used to estimate six single and combined misalignment (CM) parameters, the root mean square (RMS) of the difference in elevation data pre (PreICP-RMS) and post (PosICP-RMS) tomography matching. Corneal keratometry, expressed in the form of M, J₀ and J₄₅ (power vector analysis parameters), was used to evaluate the effect of misalignment on corneal curvature measurements. The PreICP-RMS and PosICP-RMS were statistically higher (P < 0.01) in KC than normal eyes. CM increased significantly (p = 0.00), more in KC (16.76 ± 20.88 μm) than in normal eyes (5.43 ± 4.08 μm). PreICP-RMS, PosICP-RMS and CM were correlated with keratoconus grade (p < 0.05). Corneal astigmatism J₀ was different (p = 0.01) for the second tomography measurements with misalignment consideration (−1.11 ± 2.35 D) or not (−1.18 ± 2.35 D), while M and J₄₅ kept similar. KC corneas consistently show higher misalignments between successive tomography measurements and lower repeatability compared with healthy eyes. The influence of misalignment is evidently clearer in the estimation of astigmatism than spherical curvature. These higher errors appear correlated with KC progression.

Evaluation of the Cornea and Anterior Chamber Morphologic Changes After Penetrating Keratoplasty in Patients With Keratoconus

OBJECTIVES

To evaluate changes of the cornea and anterior chamber after penetrating keratoplasty in eyes with keratoconus.

METHODS

Medical records of 68 eyes of 68 patients with keratoconus who experienced penetrating keratoplasty at the Eye Clinic of Dr. Lütfi Kirdar Kartal Research and Training Hospital between 2010 and 2012 were studied. Records were retrospectively evaluated with regard to anterior chamber depth, iridocorneal angle, central corneal thickness, thinnest corneal thickness, maximum keratometric value, and mean keratometric value. Patients who developed complications and required additional surgery were excluded.

RESULTS

A total of 68 eyes of 68 patients were included in this study. There were 40 (58.8%) male and 28 (41.2%) female patients. The mean age was 24.5±8.19 years (range: 11-42 years). The graft diameter was 7.5 mm and the recipient bed diameter was 7.0 mm in all patients. The preoperative anterior chamber depth decreased from 3.92±0.47 mm to 3.01±0.55 mm (23% decrease). No significant postoperative change was observed in the mean iridocorneal angle values. Keratometric values decreased by 30% and corneal thickness increased by 50% (P<0.001 for both).

CONCLUSION

Anterior chamber depth decreased significantly, but no change was observed in iridocorneal angle after penetrating keratoplasty in patients with keratoconus. Also, a decrease in the keratometric values and an increase in corneal thickness were observed. Understanding, estimating, and managing changes in the anterior chamber after penetrating keratoplasty, which induces significant changes in anterior chamber morphology, might increase the success of surgery.

Intrasession repeatability of ocular anatomical measurements obtained with a multidiagnostic device in healthy eyes

Background

To evaluate the intrasession repeatability of anterior chamber depth (ACD), central (CCT) and peripheral corneal thickness (PCT), white-to-white diameter (WTW), and irido-corneal angle (IA) measurements obtained with a multidiagnostic device in healthy eyes.

Methods

A total of 107 eyes of 107 patients ranging in age from 23 to 65 years were examined with the VX120 system (Visionix-Luneau Technologies). Three consecutive measurements were obtained with this device to assess the intrasession repeatability of ACD, CCT, PCT at different nasal and temporal locations, WTW, and nasal and temporal IA. Data analysis included the calculation of within-subject standard deviation (S_w), intrasubject precision (1.96xS_w), coefficient of variation (CV) and intraclass correlation coefficient (ICC).

Results

The S_w and CV for ACD was 0.03 mm and 1.16%, respectively, with an ICC of 0.992. The S_w values for central and peripheral pachymetric measurements were below 9 μm, with CV of less than 1.6% and ICC of 0.976 or higher. For IA measurements, S_w values of 0.84 or lower were found, with a CV between 1 and 2%, and an ICC of more than 0.970. The S_w for WTW was 0.24 mm and the CV was 1.95%. No statistically significant correlations were found between any anatomical parameter evaluated and their S_w and CV values associated (−0.220 ≤ r ≤ 0.204, p ≥ 0.125).

Conclusions

The VX120 system is able to provide repeatable measurements of anatomical parameters in healthy eyes. Inter-observer repeatability should be evaluated in future studies.

Repeatability and agreement of five imaging systems for measuring anterior segment parameters in healthy eyes

Purpose:

The purpose of this study is to assess the repeatability and agreement of five imaging devices, namely, the Pentacam (Oculus), Sirius (CSO), Orbscan IIz (Bausch and Lomb), Corvis (Oculus), and ultrasound pachymetry (UP, Tomey) in measuring steep keratometry (sKm), flat keratometry (fKm), central corneal thickness (CCT), thinnest corneal thickness (TCT), and anterior chamber depth (ACD) in healthy individuals.

Design:

This was prospective, comparative study.

Subjects:

Forty-six healthy Indian patients.

Materials and Methods:

Forty-six eyes of 46 healthy participants underwent three consecutive scans on each device by a single examiner. Within-subject standard deviation, test–retest repeatability (TRT), and coefficient of variation (COV) for assessing repeatability and Bland–Altman plots for the agreement between the mean measurements of each machine were analyzed.

Main Outcome Measures:

The repeatability and agreement between the five devices for the measurements of sKm, fKm, CCT, TCT, and ACD.

Results:

The TRT of sKm measurements ranged between 0.23 diopter (D) (with Pentacam) and 0.83 D (with Orbscan). The same of fKm, TCT, ACD, and CCT measurements ranged between 0.28 D (with Pentacam) and 0.74 D (with Sirius), 7.78 μm (Sirius) and 19.81 μm (Orbscan), 0.05 mm (Orbscan) and 0.07 (Sirius), and 7.36 μm (Sirius) and 18.02 μm (Orbscan), respectively. The TRT of sKm and fKm measurements with Pentacam was significantly lower than those with Orbscan and Sirius. The TRT of TCT measurement with Sirius was significantly lower than that with Pentacam (4.53 μm) and Orbscan (7.15 μm). There were statistically significant differences in the mean measurements of all parameters between the devices. The 95% limit of agreement on the Bland–Altman analysis was wide for the measurement pairs with all the devices. Significant proportional bias in the agreement was detected for TCT measurements with all the device pairs and for the ACD measurements between Sirius and Pentacam.

Conclusions:

The repeatability estimates of sKm, fKm, TCT, ACD, and CCT measurements with Pentacam, Orbscan, Sirius, Corvis, and UP in Indian eyes were good. However, the differences in the measurements between the devices were statistically significant and the same cannot be used interchangeably for anterior segments measurements.

A new approach to keratoconus detection based on corneal morphogeometric analysis

Purpose

To characterize corneal structural changes in keratoconus using a new morphogeometric approach and to evaluate its potential diagnostic ability.

Methods

Comparative study including 464 eyes of 464 patients (age, 16 and 72 years) divided into two groups: control group (143 healthy eyes) and keratoconus group (321 keratoconus eyes). Topographic information (Sirius, CSO, Italy) was processed with SolidWorks v2012 and a solid model representing the geometry of each cornea was generated. The following parameters were defined: anterior (A_ant) and posterior (A_post) corneal surface areas, area of the cornea within the sagittal plane passing through the Z axis and the apex (A_apexant, A_apexpost) and minimum thickness points (A_mctant, A_mctpost) of the anterior and posterior corneal surfaces, and average distance from the Z axis to the apex (D_apexant, D_apexpost) and minimum thickness points (D_mctant, D_mctpost) of both corneal surfaces.

Results

Significant differences among control and keratoconus group were found in A_apexant, A_apexpost, A_mctant, A_mctpost, D_apexant, D_apexpost (all p<0.001), A_post (p = 0.014), and D_mctpost (p = 0.035). Significant correlations in keratoconus group were found between A_ant and A_post (r = 0.836), A_mctant and A_mctpost (r = 0.983), and D_mctant and D_mctpost (r = 0.954, all p<0.001). A logistic regression analysis revealed that the detection of keratoconus grade I (Amsler Krumeich) was related to A_post, A_tot, A_apexant, A_mctant, A_mctpost, D_apexpost, D_mctant and D_mctpost (Hosmer-Lemeshow: p>0.05, R² Nagelkerke: 0.926). The overall percentage of cases correctly classified by the model was 97.30%.

Conclusions

Our morphogeometric approach based on the analysis of the cornea as a solid is useful for the characterization and detection of keratoconus.

Comparison of corneal measurements in keratoconus using swept-source optical coherence tomography and combined Placido-Scheimpflug imaging

PURPOSE

The aim of this study was to provide a comprehensive comparison of reliability of corneal topographic measurements in keratoconic eyes using swept-source optical coherence tomography (OCT) and a combined Placido-Scheimpflug imaging.

METHODS

A total of 30 eyes of 30 patients were included. The mean age was 31.2 ± 8.4 years. Two consecutive topographic measurements were obtained for one eye of each patient using swept-source OCT (CASIA) and combined Placido-Scheimpflug imaging (TMS-5). Test-retest reliability of CASIA and TMS-5 measurements including central corneal thickness (CCT) and thinnest corneal thickness (TCT), keratometry at steep (Ks) and flat (Kf) axes, average keratometry (Avg K), cylinder, and, best-fit spheres (BFS) of the anterior and posterior corneal surfaces were evaluated.

RESULTS

There was no systematic or scaling bias in any parameter in both devices. Systematic differences between CASIA and TMS-5 were found in posterior corneal Kf, Avg K and BFS, CCT and TCT (p ≤ 0.002); scaling differences between CASIA and TMS-5 were also found in CCT and TCT (p ≤ 0.002). Both machines illustrated adequate reliability. Intraclass correlation coefficients (ICC) ≥0.952 was recorded for all parameters measured with CASIA and ICC ≥ 0.914 was recorded for all parameters on TMS-5. CASIA showed significantly higher ICCs in CCT and TCT, and posterior corneal BFS (p < 0.001).

CONCLUSIONS

This study showed significant differences in posterior corneal surface and corneal thickness measurements between swept-source OCT and combined Placido-Scheimpflug imaging in eyes with keratoconus. Swept-source OCT might be preferred over Placido-Scheimpflug imaging owing to better repeatability of measurements.

Repeatability and Agreement of Orbscan II, Pentacam HR, and Galilei Tomography Systems in Corneas With Keratoconus

PURPOSE

To assess the repeatability and agreement of keratometry and pachymetry measurements obtained using 3 tomographers in eyes with keratoconus.

DESIGN

Reliability analysis.

METHODS

setting: Institutional.

STUDY POPULATION

Fifty eyes of 50 participants with keratoconus. observational procedure: Steep keratometry, flat keratometry, central corneal thickness (CCT), and thinnest corneal thickness (TCT) measurements using Galilei, Orbscan II, and Pentacam HR.

MAIN OUTCOME MEASURES

Repeatability was assessed using within-subject standard deviation (S_W), coefficient of variation (CV), and intraclass correlation coefficient (ICC). Bland-Altman plots and 95% limits of agreement (LoA) were used to evaluate agreement between device pairs.

RESULTS

For all studied parameters, ICC was >0.97 with the least repeatable measurements obtained using Orbscan II. Mean steep keratometry values were similar while mean flat keratometry values were significantly different between all devices. The Galilei and Pentacam HR had the lowest 95% LoA for both CCT and TCT. There were no significant differences in mean CCT between Galilei and Pentacam HR. Mean Orbscan II CCT measurements were not significantly different overall but had wide 95% LoA with Pentacam HR (-47.95 to 58.09 μm) and Galilei (-43.70 to 53.91 μm). Mean Orbscan II CCT measurements were significantly lower when an acoustic factor of 0.92 was applied (-33.6 μm vs Pentacam HR, P < .001; -33.6 μm vs Galilei; P < .001).

CONCLUSIONS

Keratometric and pachymetric measurements of keratoconic eyes obtained by Galilei, Orbscan II, and Pentacam were disparate. Measurements were less repeatable with Orbscan II compared with Pentacam HR and Galilei, although overall repeatability was high for all instruments.

Corneal Topographic and Aberrometric Measurements Obtained with a Multidiagnostic Device in Healthy Eyes: Intrasession Repeatability

Purpose. To evaluate the intrasession repeatability of corneal curvature, eccentricity, and aberrometric measurements obtained with a multidiagnostic device in healthy eyes. Methods. This study enrolled 107 eyes of 107 patients ranging in age from 23 to 65 years. All of them underwent a complete anterior segment examination with the VX120 system (Visionix-Luneau Technologies, Chartres, France). Three consecutive measurements were obtained. The within-subject standard deviation (S_w), intrasubject precision (1.96 × S_w), and intraclass correlation coefficient (ICC) were calculated. Results. All S_w for corneal power measurements were below 0.26 D, with ICC above 0.982. The S_w for corneal astigmatism at different areas (3, 5, and 7 mm) was below 0.21 D, with ICC above 0.913. Concerning the axis of astigmatism, its S_w was below 11.27°, with ICC above 0.975. The S_w and ICC for corneal eccentricity were 0.067 and 0.957, respectively. The S_w and ICC for high-order aberration root mean square (RMS) were 0.048 µm and 0.901, respectively. For 3rd- and 4th-order aberrometric parameters, all S_w were below 0.037 µm and all ICC were higher than 0.84, except for quadrafoil RMS (ICC: 0.689). Conclusions. The multidiagnostic device evaluated is able to provide consistent measurements of corneal power, eccentricity, and third- and fourth-order aberrations in healthy eyes.

Repeatability of Keratometry Measurements Obtained With Three Topographers in Keratoconic and Normal Corneas

PURPOSE

To assess the repeatability of the corneal topography functions of Orbscan II (Bausch & Lomb, Rochester, NY), OPD-Scan III (Nidek, Gamagori, Japan), and iTrace (Tracey Technologies, Houston, TX) in keratoconic eyes and in a control group of normal patients.

METHODS

In this prospective cohort study, patients were recruited between November 2011 and May 2012. Measurements were performed with a combined Placido-scanning slit system (Orbscan II) and two combined Placido-aberrometer systems (OPD-Scan III and iTrace). Repeatability limit and intraclass correlation coefficients (ICCs) of keratometric readings were calculated.

RESULTS

Fifty-nine keratoconic eyes of 34 patients and 54 normal eyes of 27 patients were included. Three groups were evaluated: all stage I-IV keratoconic eyes (59 eyes), a subgroup consisting of stage I-II keratoconic eyes (41 eyes), and normal eyes (54 eyes). For almost all parameters studied, the repeatability limit was higher in the two groups of keratoconic eyes compared to normal eyes with all three topographers, indicating lower repeatability. For the maximum keratometry measurement, repeatability limit was 1.73, 1.49, and 1.41 diopters (D) in the stage I-IV keratoconic eyes group, 1.11, 1.02, and 0.98 D in the stage I-II keratoconic eyes group, and 0.61, 0.37, and 1.02 D in the normal eyes group with Orbscan II, OPD-Scan III, and iTrace, respectively.

CONCLUSIONS

Topographies performed in keratoconic eyes are less repeatable than those performed in normal eyes. Threshold values of keratometric changes used to ascertain keratoconus progression should be carefully considered. Caution should be taken when interpreting the topographies of such patients. The higher variability should be taken into account before performing any treatment.

Corneal thickness measurements with Scheimpflug and slit scanning imaging techniques in keratoconus

PURPOSE

To determine the repeatability of corneal thickness measurements with Scheimpflug (Pentacam) and slit scanning (Orbscan) imaging techniques in different grades of keratoconus.

METHODS

This study was conducted as a cross-sectional research. Imaging with Orbscan and Pentacam was performed on patients with different grades of keratoconus. With each device, 3 measurements were taken at 10 min intervals. Repeatability indices in different grades of keratoconus were calculated for each device.

RESULTS

Seventy-four eyes of 42 keratoconus patients were enrolled. Repeatability index (RI) of central corneal thickness (CCT) measurements in keratoconus grade 1, 2, and 3, were 12.8, 9.9, and 24.2 with Pentacam, and 23.6, 26.3, and 59.3 with Orbscan, respectively. For the thinnest point, these figures were 9.6, 8.0, and 35.7 with Pentacam and 19.5, 16.6, and 26.8 with Orbscan, respectively. The 95% limit of agreement (LOA) between Pentacam and Orbscan in measuring CCT and thinnest point in grade 1 were -25.5-47.7 mic and -33.3-32.8 mic, respectively. These results for grade 2 were -9.8-50.6 mic and -26.2-43.7 mic, respectively. In grade 3, 95% LoA were -20-64.6 mic and -31.4-60.5 mic, respectively.

CONCLUSIONS

The results of this study showed that although repeated measurements of the CCT with Orbscan and Pentacam are strongly correlated, repeatability values of CCT measurements significantly decrease at more advanced grades of keratoconus. In all keratoconus grades, repeatability of CCT measurements was better with Pentacam than Orbscan. These findings indicate that corneal thickness readings have less validity in patients with advanced keratoconus.

Assessment of Anterior, Posterior, and Total Central Corneal Astigmatism in Eyes With Keratoconus

PURPOSE

To investigate the magnitudes and the axis orientations of anterior, posterior, and total central corneal astigmatism in eyes with keratoconus.

DESIGN

Retrospective case series.

METHODS

This study comprised 137 eyes of 137 keratoconic patients (97 men and 40 women; mean age ± standard deviation, 36.9 ± 12.2 years). The magnitude and the axis orientation of each corneal astigmatism were determined with a rotating Scheimpflug system.

RESULTS

The mean magnitudes of anterior, posterior, and total central corneal astigmatism were 3.93 ± 2.74 diopters (D), 0.93 ± 0.64 D, and 3.90 ± 2.75 D, respectively. With-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism of the anterior corneal surface was found in 90 eyes (65.7%), 33 eyes (24.1%), and 14 eyes (10.2%), respectively, whereas the corresponding astigmatism of the posterior corneal surface was found in 14 eyes (10.2%), 15 eyes (10.9%), and 108 eyes (78.8%), respectively. We found a significant correlation between the magnitudes of anterior and posterior corneal astigmatism (Pearson correlation coefficient r = 0.769, P < .001).

CONCLUSIONS

The mean magnitudes of anterior and posterior corneal astigmatism were approximately 4 D and 1 D, respectively, in eyes with keratoconus. Approximately 65% and 80% of eyes showed that WTR anterior astigmatism and ATR posterior astigmatism, respectively. The presence of posterior corneal astigmatism is not necessarily negligible for the accurate astigmatic correction of toric intraocular lens implantation or rigid gas-permeable contact lens wear for keratoconus.

Posterior Corneal Surface Stability after Femtosecond Laser-Assisted Keratomileusis

The purpose of this study was to evaluate posterior corneal surface variation after femtosecond laser-assisted keratomileusis in patients with myopia and myopic astigmatism. Patients were evaluated by corneal tomography preoperatively and at 1, 6, and 12 months. We analyzed changes in the posterior corneal curvature, posterior corneal elevation, and anterior chamber depth. Moreover, we explored correlation between corneal ablation depth, residual corneal thickness, percentage of ablated corneal tissue, and preoperative corneal thickness. During follow-up, the posterior corneal surface did not have a significant forward corneal shift: no significant linear relationships emerged between the anterior displacement of the posterior corneal surface and corneal ablation depth, residual corneal thickness, or percentage of ablated corneal tissue.

Comparison of Anterior Segment Measurements with Scheimpflug/Placido Photography-Based Topography System and IOLMaster Partial Coherence Interferometry in Patients with Cataracts

Purpose. To assess the consistency of anterior segment measurements obtained using a Sirius Scheimpflug/Placido photography-based topography system (CSO, Italy) and IOLMaster partial coherence interferometry (Carl Zeiss Meditec, Germany) in eyes with cataracts. Methods. A total of 90 eyes of 90 patients were included in this prospective study. The anterior chamber depth (ACD), keratometry (K), corneal astigmatism axis, and white to white (WTW) values were randomly measured three times with Sirius and IOLMaster. Concordance between them was assessed by calculating 95% limits of agreement (LoA). Results. The ACD and K taken with the Sirius were statistically significantly higher than that taken with the IOLMaster; however, the Sirius significantly underestimated the WTW values compared with the IOLMaster. Good agreement was found for Km and ACD measurements, with 95% LoA of -0.20 to 0.54 mm and -0.16 to 0.34 mm, respectively. Poor agreement was observed for astigmatism axis and WTW measurements, as the 95% LoA was -23.96 to 23.36° and -1.15 to 0.37 mm, respectively. Conclusion. With the exception of astigmatism axis and WTW, anterior segment measurements taken by Sirius and IOLMaster devices showed good agreement and may be used interchangeably in patients with cataracts.

Geometrical custom modeling of human cornea in vivo and its use for the diagnosis of corneal ectasia

AIM

To establish a new procedure for 3D geometric reconstruction of the human cornea to obtain a solid model that represents a personalized and in vivo morphology of both the anterior and posterior corneal surfaces. This model is later analyzed to obtain geometric variables enabling the characterization of the corneal geometry and establishing a new clinical diagnostic criterion in order to distinguish between healthy corneas and corneas with keratoconus.

METHOD

The method for the geometric reconstruction of the cornea consists of the following steps: capture and preprocessing of the spatial point clouds provided by the Sirius topographer that represent both anterior and posterior corneal surfaces, reconstruction of the corneal geometric surfaces and generation of the solid model. Later, geometric variables are extracted from the model obtained and statistically analyzed to detect deformations of the cornea.

RESULTS

The variables that achieved the best results in the diagnosis of keratoconus were anterior corneal surface area (ROC area: 0.847, p<0.000, std. error: 0.038, 95% CI: 0.777 to 0.925), posterior corneal surface area (ROC area: 0.807, p<0.000, std. error: 0.042, 95% CI: 0,726 to 0,889), anterior apex deviation (ROC area: 0.735, p<0.000, std. error: 0.053, 95% CI: 0.630 to 0.840) and posterior apex deviation (ROC area: 0.891, p<0.000, std. error: 0.039, 95% CI: 0.8146 to 0.9672).

CONCLUSION

Geometric modeling enables accurate characterization of the human cornea. Also, from a clinical point of view, the procedure described has established a new approach for the study of eye-related diseases.

New approach for correction of error associated with keratometric estimation of corneal power in keratoconus

PURPOSE

The aim of this study was to obtain the exact value of the keratometric index (nkexact) and to clinically validate a variable keratometric index (nkadj) that minimizes this error.

METHODS

The nkexact value was determined by obtaining differences (ΔPc) between keratometric corneal power (Pk) and Gaussian corneal power ((Equation is included in full-text article.)) equal to 0. The nkexact was defined as the value associated with an equivalent difference in the magnitude of ΔPc for extreme values of posterior corneal radius (r2c) for each anterior corneal radius value (r1c). This nkadj was considered for the calculation of the adjusted corneal power (Pkadj). Values of r1c ∈ (4.2, 8.5) mm and r2c ∈ (3.1, 8.2) mm were considered. Differences of True Net Power with (Equation is included in full-text article.), Pkadj, and Pk(1.3375) were calculated in a clinical sample of 44 eyes with keratoconus.

RESULTS

nkexact ranged from 1.3153 to 1.3396 and nkadj from 1.3190 to 1.3339 depending on the eye model analyzed. All the nkadj values adjusted perfectly to 8 linear algorithms. Differences between Pkadj and (Equation is included in full-text article.)did not exceed ±0.7 D (Diopter). Clinically, nk = 1.3375 was not valid in any case. Pkadj and True Net Power and Pk(1.3375) and Pkadj were statistically different (P < 0.01), whereas no differences were found between (Equation is included in full-text article.)and Pkadj (P > 0.01).

CONCLUSIONS

The use of a single value of nk for the calculation of the total corneal power in keratoconus has been shown to be imprecise, leading to inaccuracies in the detection and classification of this corneal condition. Furthermore, our study shows the relevance of corneal thickness in corneal power calculations in keratoconus.

Impaired corneal biomechanical properties and the prevalence of keratoconus in mitral valve prolapse

Objective. To investigate the biomechanical characteristics of the cornea in patients with mitral valve prolapse (MVP) and the prevalence of keratoconus (KC) in MVP. Materials and Methods. Fifty-two patients with MVP, 39 patients with KC, and 45 control individuals were recruited in this study. All the participants underwent ophthalmologic examination, corneal analysis with the Sirius system (CSO), and the corneal biomechanical evaluation with Reichert ocular response analyzer (ORA). Results. KC was found in six eyes of four patients (5.7%) and suspect KC in eight eyes of five patients (7.7%) in the MVP group. KC was found in one eye of one patient (1.1%) in the control group (P = 0.035). A significant difference occurred in the mean CH and CRF between the MVP and control groups (P = 0.006 and P = 0.009, resp.). All corneal biomechanical and topographical parameters except IOPcc were significantly different between the KC-MVP groups (P < 0.05). Conclusions. KC prevalence is higher than control individuals in MVP patients and the biomechanical properties of the cornea are altered in patients with MVP. These findings should be considered when the MVP patients are evaluated before refractive surgery.

Repeatability of aberrometric measurements in normal and keratoconus eyes using a new Scheimpflug-Placido topographer

PURPOSE

To evaluate the repeatability of the anterior and posterior corneal wavefront aberrations using the Sirius Scheimpflug-Placido topographer in normal eyes and keratoconus eyes.

SETTING

Bozok University Faculty of Medicine, Yozgat, Turkey.

DESIGN

Evaluation of diagnostic test.

METHODS

In eyes of healthy subjects and eyes of keratoconus patients, 3 repeated measurements were obtained using the Scheimpflug-Placido topographer. Repeatability of the corneal aberrometric data using a 7th-order Zernike expansion (6.0 mm pupil) and central corneal power (3.0 mm zone) in the anterior and posterior corneal surfaces were analyzed. The within-subject standard deviation (Sw) and the intraclass correlation coefficient (ICC) were calculated.

RESULTS

For all modal pairs, the Sw was 0.08 μm or less for anterior and posterior corneal aberrations in both groups. The ICC of the anterior corneal surface ranged from 0.607 (pentafoil) to 0.988 (primary coma) in keratoconus eyes (n = 41) and from 0.568 (quadrifoil) to 0.856 (primary coma) in normal eyes (n = 30). The ICCs for posterior corneal surface aberrometry were 0.656 to 0.873 and 0.592 to 0.824, respectively. For anterior and posterior corneal curvatures, the Sw was 0.12 or lower and the ICC values were more than 0.93 in all cases except the posterior corneal surface reading at the 3.0 mm corneal area in keratoconus eyes (ICC 0.875).

CONCLUSIONS

The intraexaminer repeatability of most anterior corneal aberrations with the Scheimpflug-Placido system was moderate to high in normal eyes and keratoconus eyes. The system showed moderate repeatability for the posterior corneal surface.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Comparative analysis of the relationship between anterior and posterior corneal shape analyzed by Scheimpflug photography in normal and keratoconus eyes

BACKGROUND

To analyze and compare the relationship between anterior and posterior corneal shape evaluated by a tomographic system combining the Scheimpflug photography and Placido-disc in keratoconus and normal healthy eyes, as well as to evaluate its potential diagnostic value.

METHODS

Comparative case series including a sample of 161 eyes of 161 subjects with ages ranging from 7 to 66 years and divided into two groups: normal group including 100 healthy eyes of 100 subjects, and keratoconus group including 61 keratoconus eyes of 61 patients. All eyes received a comprehensive ophthalmologic examination including an anterior segment analysis with the Sirius system (CSO). Antero-posterior ratios for corneal curvature (k ratio) and shape factor (p ratio) were calculated. Logistic regression analysis was used to evaluate if some antero-posterior ratios combined with other clinical parameters were predictors of the presence of keratoconus.

RESULTS

No statistically significant differences between groups were found in the antero-posterior k ratios for 3-, 5- and 7-mm diameter corneal areas (p ≥ 0.09). The antero-posterior p ratio for 4.5- and 8-mm diameter corneal areas was significantly higher in the normal group than in the keratoconus group (p<0.01). The k ratio for 3, 5, and 7 mm was significantly higher in the keratoconus grade IV subgroup than in the normal group (p<0.01). Furthermore, significant differences were found in the p ratio between the normal group and the keratoconus grade II subgroup (p ≤ 0.01). Finally, the logistic regression analysis identified as significant independent predictors of the presence of keratoconus (p<0.01) the 8-mm anterior shape factor, the anterior chamber depth, and the minimal corneal thickness.

CONCLUSIONS

The antero-posterior k and p ratios are parameters with poor prediction ability for keratoconus, in spite of the trend to the presence of more prolate posterior corneal surfaces compared to the anterior in keratoconus eyes.