Comparative evaluation of refractive surgery candidates with Placido topography, Orbscan II, Pentacam, and wavefront analysis

Corneal topometric indices and proclivity toward corneal ectasia in vernal keratoconjunctivitis

PURPOSE

To investigate changes in topometric corneal indices and proclivity toward corneal ectasia, as well as keratometric indices and anterior chamber dimensions in palpebral vernal keratoconjunctivitis (VKC).

METHODS

This study included 80 patients with clinically established symptoms of grade 0 or grade 1 palpebral VKC (group 1) and 66 healthy participants (group 2). After a comprehensive ophthalmic examination including best-corrected visual acuity, intraocular pressure measurements and slit-lamp biomicroscopy, pachymetric indices and anterior chamber dimensions were measured using the Pentacam HR rotating Scheimpflug device. Topometric indices, which are particularly useful for determining proclivity toward corneal ectasia, were extracted from a topometric map.

RESULTS

In groups 1 and 2, the mean ages were 13.11±5.22 and 16.45±5.09 years, respectively. The mean age at disease onset in group 1 was 10.09±5.03 years, and the mean disease duration was 36.23±8.43 months. Group 1 had significantly higher mean topometric indices than group 2, particularly the index of surface variance (P=0.001), index of vertical asymmetry (P=0.007), center keratoconus index (P=0.050), and Belin/Ambrosio enhanced ectasia total deviation value (P=0.032). Mean posterior corneal astigmatism differed significantly between groups 1 and 2 (P=0.003).

CONCLUSIONS

Significantly higher mean topometric indices in VKC indicate a proclivity for corneal ectasia, which could be attributed to general changes in the corneal ultrastructure caused by persistent itching-induced eye rubbing.

Characterization of postoperative LASIK ectasia features on higher-order aberration excimer ablation maps

BACKGROUND

To characterize anterior corneal higher-order aberration (HOA) excimer ablation map patterns in postoperative LASIK ectasia (POE) and to examine correlations between newly identified corneal HOA ablation map features of POE and known topographic indices.

METHODS

Prospective multicenter non-interventional descriptive study. A total of 28 eyes from 22 POE patients were enrolled. The postoperative HOA ablation map was derived from Topolyzer Vario corneal imaging at the time of POE diagnosis. Features that recurred were identified and then analyzed. Correlations to Orbscan indices were studied.

RESULTS

An arrangement of two elliptical paracentral ablation islands, deep inferior and shallow superior, in direct mirror-like opposition to each other, were identified on all HOA maps. The paracentral islands were accompanied by peripheral ablation crescents. The deep paracentral inferior island 'hot spot' coincided with the topographical apical POE cone and was highly reproducible in angular position (249.3 ± 17.3°). There was significant variation in ablation depth (shallow superior island: 11.5 ± 6.9 μm and deep inferior island: 32.5 ± 18.8 μm). The superior crescents had high variability in depth (34.8 ± 18.9 μm). Strong correlations were found between the corneal irregularity index and the ablation depth difference between the deep and shallow paracentral islands (R = 0.96; P < 0.0001).

CONCLUSION

The corneal HOA excimer ablation map revealed a recurring, distinct, easily recognizable pattern in POE eyes. Validated Orbscan POE indices and HOA ablation map islands showed a strong correlation. It is possible to extract useful information from the corneal HOA ablation map, potentially making it suitable for diagnosing and monitoring POE although more studies are needed.

Differentiating highly asymmetric keratoconus eyes using a combined Scheimpflug/Placido device

PURPOSE

To determine the ability to differentiate between normal eyes and clinically unaffected eyes of patients with highly asymmetric keratoconus (AKC) using a Scheimpflug/Placido device.

SETTING

Tel Aviv Sourasky Medical Center and Enaim Medical Center, Israel.

DESIGN

Retrospective case-control.

METHODS

Imaging from a combined Scheimpflug/Placido device (Sirius, C.S.O.) was obtained from 26 clinically unaffected eyes of patients with frank keratoconus in the fellow eye, and 166 eyes from 166 patients with bilaterally normal corneal examinations that underwent uneventful corneal refractive surgery with at least 1 year of follow-up. Receiver operating characteristic curves were produced to calculate the area under the curve, sensitivity, and specificity of 60 metrics, and finally a logistic regression modeling was used to determine optimal variables to differentiate populations.

RESULTS

The most predictive individual metric able to differentiate between 26 eyes in the case group to 166 eye in the control group was the posterior wall inferior-superior (I-S) ratio, with an receiver operating characteristics (ROC) of 0.862. A combination model of 4 metrics (posterior wall I-S ratio in the central 3 mm, thinnest pachymetry coordinate on the x horizontal axis, posterior asymmetry and asphericity index, corneal volume) yielded an ROC of 0.936, with a sensitivity/specificity pair of 92.3%/87%. Variables related to elevation were not found significant.

CONCLUSIONS

Using a combination of metrics from a combined Scheimpflug/Placido device, a practical model for discrimination between clinically normal eyes of patients with highly AKC and normal eyes was constructed. Variables related to pachymetry and posterior cornea asymmetry were the most impactful.

Keratoconus Features on Corneal Higher-Order Aberration Ablation Maps: Proof-of-Concept of a New Diagnostic Modality

PURPOSE

To assess the potential application of corneal higher-order aberration (HOA) excimer ablation map imaging in identifying reproducible keratoconus (KC) features and to explore if newly derived map metrics correlate to Pentacam KC indices.

METHODS

Case series of 12 eyes with KC ≥ grade 2. Topolyzer Vario corneal imaging with its resultant HOA ablation map islands were analyzed for their centroid, distance from center, angular position, orientation, sphericity, diameter, area, and maximal ablation depth. Correlations to Pentacam indices were studied.

RESULTS

All eyes showed recurrent features with an arrangement of two elliptical paracentral ablation islands, one deep inferotemporal and one shallow superonasal, in direct mirror-like opposition to each other. These were always accompanied by superior peripheral ablation crescents. The two paracentral islands had highly reproducible distance from center (1.2 ± 0.1 mm and 1.3 ± 0. 2 mm) and angular positions (246.8 ± 15.9° and 76.7 ± 7.7°), with greater variation in ablation depth (68.3 ± 33.2 µm and 17.6 ± 12.1 µm). Distance from center of the peripheral superior crescents was highly reproducible (3.3 ± 0.1 mm), with a larger range of depth (74.5 ± 37.2 µm). The deep paracentral inferotemporal island "hot spot" was coincident with the topographical apical cone. Strong correlations were found between the depth of the inferotemporal island and Pentacam indices of posterior radius curvature (PRC: R = -0.74) and Belin/Ambrosio enhanced ectasia total deviation (BAD-D: R = 0.71).

CONCLUSION

The corneal HOA ablation map revealed a recurring, distinct, easily recognizable pattern in KC eyes. There was a strong correlation between the depth of novel HOA ablation map metrics and validated Pentacam KC indices. Novel information can be extracted from the corneal HOA ablation map giving it the potential to be a new modality to diagnose and grade KC.

Biometric Measurement of Anterior Segment: A Review

Biometric measurement of the anterior segment is of great importance for the ophthalmology, human eye modeling, contact lens fitting, intraocular lens design, etc. This paper serves as a comprehensive review on the historical development and basic principles of the technologies for measuring the geometric profiles of the anterior segment. Both the advantages and drawbacks of the current technologies are illustrated. For in vivo measurement of the anterior segment, there are two main challenges that need to be addressed to achieve high speed, fine resolution, and large range imaging. One is the motion artefacts caused by the inevitable and random human eye movement. The other is the serious multiple scattering effects in intraocular turbid media. The future research perspectives are also outlined in this paper.

Assessment of Visual Quality in Eyes with Forme Fruste Keratoconus and Mild and Moderate Keratoconus Based on Optical Quality Analysis System II Parameters

Purpose

The study aimed to evaluate the visual quality of forme fruste keratoconus (FFK) and mild and moderate keratoconus by using an optical quality analysis system II (OQAS-II) and to explore the correlation between optical quality parameters and the disease progression.

Methods

Twenty-one normal eyes, twenty-one FFK eyes, twenty-one mild keratoconus eyes, and twenty-one moderate keratoconus eyes were included in this prospective study. The optical quality parameters, such as object scatter index (OSI), modulation transfer function cutoff (MTF cutoff), strehl ratio (SR), and OQAS-II values at contrasts of 100% (OV-100), 20% (OV-20), and 9% (OV-9), were measured by OQAS-II. The repeatability of these parameters was analyzed by intraclass correlation coefficient (ICC), repeatability coefficient (RC), and coefficient of variation (CV_w). Correlations between optical quality parameters and mean central keratometry readings (K_mean) were evaluated. The sensitivity and specificity of the parameters were analyzed using the receiver operating characteristic (ROC).

Results

All the optical quality parameters among four groups showed good repeatability (all ICC≥0.75). The MTF cutoff, SR, OV-100, OV-20, OV-9 in FFK, mild and moderate keratoconus eyes were significantly lower than those in the normal group (all P < 0.05). The ROC analyses of the MTF cutoff, SR, OV-100, OV-20, and OV-9 showed significant area under the curve (AUC) in discriminating FFK form normal, mild keratoconus from FFK, and moderate keratoconus from mild keratoconus (all P < 0.05). The ROC analyses of the MTF cutoff, SR, OV-100, OV-20, and OV-9 showed significant area under the curve (AUC) in discriminating FFK form normal, mild keratoconus from FFK, and moderate keratoconus from mild keratoconus (all P < 0.05). The ROC analyses of the MTF cutoff, SR, OV-100, OV-20, and OV-9 showed significant area under the curve (AUC) in discriminating FFK form normal, mild keratoconus from FFK, and moderate keratoconus from mild keratoconus (all P < 0.05). The ROC analyses of the MTF cutoff, SR, OV-100, OV-20, and OV-9 showed significant area under the curve (AUC) in discriminating FFK form normal, mild keratoconus from FFK, and moderate keratoconus from mild keratoconus (all P < 0.05). The ROC analyses of the MTF cutoff, SR, OV-100, OV-20, and OV-9 showed significant area under the curve (AUC) in discriminating FFK form normal, mild keratoconus from FFK, and moderate keratoconus from mild keratoconus (all K_mean) were evaluated. The sensitivity and specificity of the parameters were analyzed using the receiver operating characteristic (ROC). r = −0.710, P < 0.05). The ROC analyses of the MTF cutoff, SR, OV-100, OV-20, and OV-9 showed significant area under the curve (AUC) in discriminating FFK form normal, mild keratoconus from FFK, and moderate keratoconus from mild keratoconus (all

Conclusion

The repeatability of OQAS-II is good in measuring visual quality of normal as well as FFK, mild, and moderate keratoconus. The visual quality of the FFK, mild, and moderate keratoconus is worse than that in normal eyes. The OQAS-II has the potential value in screening FFK from normal eyes and might be a useful tool for evaluating the progression of keratoconus.

Distinguishing Highly Asymmetric Keratoconus Eyes Using Dual Scheimpflug/Placido Analysis

PURPOSE

To identify the best metrics or combination of metrics that provide the highest predictive power between normal eyes and the clinically unaffected eye of patients with highly asymmetric keratoconus using data from a Dual Scheimpflug/Placido device.

DESIGN

Retrospective case-control study.

METHODS

Combined Dual Scheimpflug/Placido imaging was obtained from the Galilei G4 device (Ziemer Ophthalmic Systems AG, Port, Switzerland) in 31 clinically unaffected eyes with highly asymmetric keratoconus and 178 eyes from 178 patients with bilaterally normal corneal examinations that underwent uneventful LASIK with at least 1 year follow-up. Receiver operating characteristic (ROC) curves were generated to determine area under the curve (AUC), sensitivity, and specificity for 87 metrics, and logistic regression modeling was used to determine optimal variable combinations.

RESULTS

No individual metric achieved an AUC greater than 0.79. A combined model consisting of 9 metrics yielded an AUC of 0.96, with 90.3% sensitivity and 92.6% specificity. Among those 9 metrics included, 5 related to corneal pachymetry: Opposite Sector Index and Anterior Height BFS Z from the anterior surface, Asphericity and Asymmetry Index, Posterior Height BFS Z, and Posterior Height BFS X from the posterior surface. The strongest variable in the model was the thinnest point location on the horizontal (x) axis.

CONCLUSION

While individual metrics performed poorly, using a combination of metrics from the combined Dual Scheimpflug/Placido device provided a useful model for differentiating normal corneas from the clinically normal eyes of patients with highly asymmetric keratoconus. Pachymetry values were the most impactful metrics.

Long-term Outcomes of a New Surgical Technique for Corneal Remodeling in Corneal Ectasia

PURPOSE

To analyze the outcomes of a new technique (corneal remodeling) to treat corneal ectasia.

METHODS

Sixty-nine cases that underwent corneal remodeling were analyzed. Anesthetic drops were instilled and a femtosecond laser platform was used to perform an 8-mm diameter keratectomy (180°, 270°, or 360°). Once ablation was completed, the edges of the resection were sutured with 8 to 12 interrupted stitches.

RESULTS

Three-year follow-up data are presented. The age of the population was 30.83 ± 12.65 years (range: 16 to 48 years). At 36 months postoperatively, 57.2% presented with uncorrected distance visual acuity (UDVA) of 20/100 or worse and 42.8% achieved between 20/80 and 20/40. After performing photorefractive keratectomy in 3 cases, 14.3% presented with UDVA of 20/100 or worse, 57.2% achieved between 20/80 and 20/40, and 28.5% achieved 20/30 or better in 180° arc length keratectomy. Most parameters compared were moderately to statistically significant (P < .05 to < .0000001).

CONCLUSIONS

Corneal remodeling is a safe technique that produces corneal flattening, reduction of anterior chamber depth, and decreased optical aberrations. It also offers a possibility to perform complementary refractive procedures. [J Refract Surg. 2019;35(4):261-267.].

Corneal elevation changes after forced eyelid closure in healthy participants and in patients with keratoconus

BACKGROUND

To assess pointwise corneal elevation changes after forced eyelid closure test (FECT) in the eyes of healthy subjects and in eyes with keratoconus.

METHODS

Twenty-nine subjects with keratoconus and 31 healthy volunteers were evaluated. Patients with keratoconus who had corneal hydrops, apical scarring, corneal thickness ≤ 400 μm, ocular surface disease, contact lens wear on the examination day and a history of corneal cross-linking were excluded. Exclusion criteria for healthy participants were spherical error > +3.00 D and < -3.00 D, corneal astigmatism > 1.50 D, corneal curvature > 47 D, ocular allergy, clinical findings and family history of keratoconus. Pentacam was performed before and after 20 seconds of FECT and raw data were extracted from the built-in software. Pointwise anterior and posterior elevation changes in the central 8 mm cornea were assessed using paired samples t-test and heat maps were constructed to reflect mean changes and statistically significant data points. Statistical significance was assumed at p < 0.01.

RESULTS

Age and gender were similar between healthy subjects (24.5 ± 1.6 years, 46.4 per cent female) and subjects with keratoconus (28.6 ± 9.2 years, 46.4 per cent female, p = 0.19, 0.61, respectively). Healthy eyes displayed posterior depression clustering in the inferotemporal and inferonasal areas (mean change: -4.5 ± 7.8 μm and -5.2 ± 9.8 μm, respectively, all p < 0.01). In contrast, keratoconus eyes exhibited a wider area of posterior elevation clustering in the inferior cornea (mean change: 8.1 ± 14.5 μm, all p < 0.01) with a small extension in the inferotemporal cornea (mean change: 12.1 ± 22.3 μm, all p < 0.01).

CONCLUSION

FECT elicits corneal elevation changes mainly in the inferior cornea with the change being more pronounced and wider in eyes with keratoconus.

Anterior segment parameters in normal dogs using the Pentacam® HR Scheimpflug system

OBJECTIVE

To describe and compare normative anterior segment parameters between canine age groups using the Pentacam^® HR Scheimpflug camera (Pentacam).

ANIMALS STUDIED

Thirty-six sedated dogs (60 eyes) of varying ages and breeds were imaged with the Pentacam; only nondiseased anterior segments were included.

PROCEDURES

Dogs were divided into three age groups: Group 1 (1-5 years), Group 2 (6-10 years), and Group 3 (11-15 years). Values assessed included central corneal thickness (CCT), anterior and posterior corneal elevation (ACE/PCE), anterior and posterior corneal curvature metrics, corneal volume (CV), anterior and posterior corneal astigmatism (AA/PA), anterior chamber depth (ACD), anterior chamber volume (ACV), and anterior chamber angle (ACA). Tukey-adjusted pairwise comparisons were performed.

RESULTS

Overall CCT (mean ± SD) was 631.07 ± 59.91 µm. Central corneal thickness was 608.60 ± 48.63 µm for Group 1, 648.57 ± 51.06 µm for Group 2, and 635.37 ± 73.71 µm for Group 3. Anterior corneal elevation (ACE) measured 9.08 ± 0.58 mm, PCE measured 8.04 ± 0.50 mm, and CV was 58.13 ± 5.39 mm³ . Astigmatism values were 1.34 ± 0.94 D for AA and 0.46 ± 0.44 D for PA. Anterior chamber values were 3.76 ± 0.56 mm for ACD, 383.68 ± 66.24 mm³ for ACV, and 23.62 ± 29.33˚ for ACA. Significant differences were found between Groups 1 and 2 for CV (55.08 ± 4.08 mm³ and 60.32 ± 4.19 mm³ , respectively), (P = 0.02).

CONCLUSIONS

Corneal volume significantly increased between Group 1 and Group 2. Central corneal thickness increased from Group 1 to Group 3, but was not significant with the current sample size. There were no other differences between age groups.

Pentacam Accuracy in Discriminating Keratoconus From Normal Corneas: A Diagnostic Evaluation Study

OBJECTIVES

This study aims to determine the diagnostic ability of Pentacam indices for keratoconus and identifying the best index for differentiating diseased from normal cases.

METHOD

In this study, 150 keratoconus patients and 150 refractive surgery candidates with a definitive diagnosis of normal healthy corneas were enrolled. Initially, the placido disk topography imaging was performed. The keratoconus and normal corneas were defined based on placido disk topographic data from Rabinowitz-McDonnell. After complete eye examinations for all participants, they underwent Pentacam imaging, and corneal surface topographic indices were extracted. Multiple logistic regression was used to determine the best indices for differentiating diseased from healthy corneas, and the receiver operating curve was calculated to determine the diagnostic capability of each index.

RESULTS

Among the studied indices, the keratoconus index (KI), index of vertical asymmetry (IVA), thinnest point (TP), and maximum keratometry (Kmax) were found capable of detecting keratoconus. Among these, IVA was the best index, with an area under curve (AUC) of 95.24%. The best cutoff point for IVA was 0.20 μm, and its sensitivity and specificity were 87.50% and 96.30%, respectively. Comparison of the AUC of different indices showed that only TP and IVA significantly differed (P=0.002). The combination of KI, IVA, Kmax, and TP indices leads to correct detection in 78% of cases.

CONCLUSION

Measuring corneal topographic indices using Pentacam can be helpful in the diagnosis of keratoconus. According to the results of this study, IVA is the best diagnostic index for keratoconus. However, it is recommended to use a combination of Pentacam indices for more accurate differentiation of keratoconus from normal cases.

Repeatability of Ophtha Top topography and comparison with IOL-Master and LenstarLS900 in cataract patients

AIM

To determine the repeatability of Ophtha Top topography and assess the consistency with intraocular lens (IOL)-Master and LenstarLS900 (Lenstar) in measuring corneal parameters among cataract patients.

METHODS

Totally 125 eyes were enrolled. Corneas were successively measured with Ophtha Top, IOL-Master and Lenstar at least three times. The flattest meridian power (Kf), the steepest meridian power (Ks), mean power (Km), J0 and J45 were recorded. Intra-class correlation coefficients (ICCs), the coefficient of variance (COV), within subject standard deviation (Sw), and test-retest repeatability (2.77Sw) were adopted to determine the repeatability. The 95% limit of agreement (95%LOA) and Bland-Altman plots were used to assess comparability.

RESULTS

Repeatability of Ophtha Top topography for measuring corneal parameters showed the ICCs were all above 0.93, 2.77Sw was lower than 0.31, and the COV of the Kf and Ks was lower than 0.25. The keratometric readings with Ophtha Top topography were flatter than with the IOL-Master and Lenstar devices, while the Pearson correlation coefficients were over 0.97. The J0 and J45 with Ophtha Top topography were smaller compared with Lenstar and IOL-Master, while was comparable between Lenstar and IOL-Master.

CONCLUSION

Ophtha Top topography shows excellent repeatability for measuring corneal parameters. However, differences between the Ophtha TOP topography and Lenstar, IOL-Master both in cornea curvature and the astigmatism should be noted clinically.

Comparison between corneal elevation maps using different reference surfaces with Scheimpflug-Placido topographer

The purpose of this study was to compare the anterior and posterior elevation measurements using different reference surfaces (spheric, aspheric, and aspherotoric) with Scheimpflug-Placido topography in simple myopic and keratoconus patients. 600 eyes of 600 patients undergoing screening for keratorefractive surgery (500 simple myopic, 100 keratoconus stage 1 and 2) in Sohag refractive center, Egypt, were examined by Scheimpflug-Placido topography (Sirius, CSO, Italy) for both the anterior and posterior corneal elevation maps using the spheric, aspheric, and aspherotoric reference surfaces. 100 keratoconic eyes showed higher discriminating power using the aspherotoric reference surface in both the anterior and posterior elevation maps. The use of aspherotoric reference surface gives more data for eyes with keratoconus and its use is more informative in screening.

Anterior segment characteristics in normal and keratoconus eyes evaluated with a combined Scheimpflug/Placido corneal imaging device

Purpose

To assess the anterior segment parameters of keratoconus (KC) eyes at different stages of the disease by a new Scheimpflug camera combined with Placido disk corneal topography (Sirius, CSO, Italy) in a sample of the Iranian population.

Methods

A total of 225 eyes of 225 individuals comprising 41 suspect KC, 40 mild KC, 71 moderate KC, 48 severe KC and 25 normal eyes were assessed for the following parameters: corneal thickness at the apex (CTA), thinnest corneal thickness (TCT), anterior chamber depth (ACD), corneal volume (CV), corneal keratometry (K), corneal asphericity (Q), and corneal elevation in the anterior and posterior surface. Also, the Zernike coefficients for the corneal aberrations including total root mean square (RMS), RMS Coma, RMS spherical aberration (SA), RMS Astigmatism, Baiocchi Calossi Versaci front index (BCV_f), and BCV back index (BCV_b) were noted for all eyes. Data was analyzed using analysis of variance (ANOVA) and post hoc Bonferroni test for comparison of the means of the five groups. P-value was considered significant if it was <0.05.

Results

The TCT, CTA and posterior corneal elevation were significantly different between all comparison groups (P < 0.05). ACD values showed inconsistent differences between groups. Mean value of CV in comparing normal eyes vs suspect KC group, normal eyes vs mild KC, and normal eyes vs moderate KC revealed statistically significant change (P < 0.05). Also, weak non-significant positive correlation was demonstrated between K and CV (r = 0.08). There were statistically significant differences in total RMS, RMS coma, BCV_f, and BCV_b for most groups (P < 0.05).

Conclusion

Posterior corneal elevation, corneal thickness and high order aberrations are important indices that need to be considered to diagnose different grades of keratoconus.

Predictive Ability of Galilei to Distinguish Subclinical Keratoconus and Keratoconus from Normal Corneas

Purpose:

To determine the predictive ability of different data measured by the Galilei dual Scheimpflug analyzer in differentiating subclinical keratoconus and keratoconus from normal corneas.

Methods:

This prospective comparative study included 136 normal eyes, 23 eyes with subclinical keratoconus, and 51 keratoconic eyes. In each eye, keratometric values, pachymetry, elevation parameters and surface indices were evaluated. Receiver operating characteristic (ROC) curves were calculated and quantified by using the area under the curve (AUC) to compare the sensitivity and specificity of the measured parameters and to identify optimal cutoff points for differenciating subclinical keratoconus and keratoconus from normal corneas. Several model structures including keratometric, pachymetric, elevation parameters and surface indices were analyzed to find the best model for distinguishing subclinical and clinical keratoconus. The data sets were also examined using the non-parametric “classification and regression tree” (CRT) technique for the three diagnostic groups.

Results:

Nearly all measured parameters were strong enough to distinguish keratoconus. However, only the radius of best fit sphere and keratometry readings had an acceptable predictive accuracy to differentiate subclinical keratoconus. Elevation parameters and surface indices were able to differentiate keratoconus from normal corneas in 100% of eyes. Meanwhile, none of the parameter sets could effectively discriminate subclinical keratoconus; a 3-factor model including keratometric variables, elevation data and surface indices provided the highest predictive ability for this purpose.

Conclusion:

Surface indices measured by the Galilei analyzer can effectively differentiate keratoconus from normal corneas. However, a combination of different data is required to distinguish subclinical keratoconus.

Evaluation of corneal higher order aberrations in normal topographic patterns

Purpose

This study reports the characteristics of corneal higher order aberrations (HOAs) in eyes with normal topographic pattern using the Pentacam scheimpflug system.

Methods

In this prospective, observational, comparative study, 165 eyes of 97 patients separated into five groups based on corneal topographic patterns were enrolled. All eyes received a comprehensive ophthalmologic examination including corneal tomographic analysis with the Pentacam system. Keratometry, corneal cylinder, and corneal aberrometric data were recorded and analyzed. Root mean square values (RMS) were calculated for corneal HOAs up to the 6th order, total coma, total trefoil, total spherical aberration, total tetrafoil, and higher order astigmatism. Evaluation of these data to discriminate between the five groups was assessed using the analysis of variance test by Generalized Estimation Equation Linear Model.

Results

Corneal HOAs were found to be significantly higher for Asymmetric Bow Tie and Irregular groups than other groups (p = <0.001). RMS of total coma aberration (Z3−1, Z31, Z5−1, Z51) were significantly greater in the Asymmetric Bow Tie pattern than others, and RMS of total Spherical aberration (Z40, Z60) was significantly higher in the Irregular pattern than other groups (p = <0.001). The results of our study demonstrate that a tendency toward significant higher values of trefoil, tetrafoil, and higher order astigmatism in Irregular pattern (all p < 0.05). Significantly higher amounts of 3rd order RMS in Asymmetric Bow Tie group and 4th to 6th order RMS in Irregular pattern group were other outcomes of our study (p = <0.001).

Conclusions

Based on results in this study, there were a good correlation between corneal topographic pattern and corneal HOAs in normal eyes. These results indicate that the corneal HOAs values are largely determined by the topographic patterns. A larger sample size would perhaps have been beneficial to yield in more accurate outcomes.

Posterior Corneal Elevation after Small Incision Lenticule Extraction for Moderate and High Myopia

Purpose

To investigate the changes of posterior corneal elevation after small incision lenticule extraction (SMILE) for moderate and high myopia.

Methods

In this prospective study, fifty consecutive eyes of thirty patients (10 male, 20 female) who underwent SMILE for myopia and myopic astigmatism were included. Eyes were divided in two groups based on the preoperative spherical equivalent refraction: high myopia group (32 eyes, range -6.25D to -10.00D) and moderate myopia group (18 eyes, range -3.00D to -6.00D). Posterior corneal surfaces were measured by a Scheimpflug camera (Pentacam, Oculus Germany) preoperatively and 1 month, 3 months, 6 months and 12 months postoperatively. Posterior central elevation (PCE) and posterior mean elevation (PME) at 17 predetermined points in the central-4mm area above the best-fit sphere were analyzed.

Results

No significant difference in the amount of posterior corneal elevation changes in the high myopia group was noted over time (P = 0.23 and P = 0.94 for PCE and PME, respectively). Similarly, the changes in the moderate myopia group before and after SMILE were not significant either (P = 0.34 and P = 0.40 for PCE and PME). A statistically significant correlation was found between the residual bed thickness and the shift of the PCE in the high myopia group at 12 months postoperatively (r = 0.53, P = 0.01).

Conclusions

The results of this study suggest that the posterior corneal surface remain stable within one year after SMILE for both moderate and high myopia. The changes of PCE correlate to the residual bed thickness for high myopia. Long-term changes of posterior corneal surface need further investigation.

Keratoconus: an inflammatory disorder?

Keratoconus has been classically defined as a progressive, non-inflammatory condition, which produces a thinning and steepening of the cornea. Its pathophysiological mechanisms have been investigated for a long time. Both genetic and environmental factors have been associated with the disease. Recent studies have shown a significant role of proteolytic enzymes, cytokines, and free radicals; therefore, although keratoconus does not meet all the classic criteria for an inflammatory disease, the lack of inflammation has been questioned. The majority of studies in the tears of patients with keratoconus have found increased levels of interleukin-6 (IL-6), tumor necrosis factor-α(TNF-α), and matrix metalloproteinase (MMP)-9. Eye rubbing, a proven risk factor for keratoconus, has been also shown recently to increase the tear levels of MMP-13, IL-6, and TNF-α. In the tear fluid of patients with ocular rosacea, IL-1α and MMP-9 have been reported to be significantly elevated, and cases of inferior corneal thinning, resembling keratoconus, have been reported. We performed a literature review of published biochemical changes in keratoconus that would support that this could be, at least in part, an inflammatory condition.

Staging of keratoconus indices regarding tomography, topography, and biomechanical measurements

PURPOSE

To derive limits of metric keratoconus indices for classification into keratoconus stages.

DESIGN

Validity and reliability analysis of diagnostic tools.

METHODS

A total of 126 patients from the keratoconus center of Homburg/Saar were evaluated with respect to Amsler criteria, using Pentacam (Keratoconus Index [KI], Topographic Keratoconus Classification [TKC]), Topographic Modeling System (Smolek/Klyce, Klyce/Maeda), and Ocular Response Analyzer (Keratoconus Match Probability [KMP], Keratoconus Match Index [KMI]). Mean value, standard deviation, 90% confidence interval, and the Youden J index for definition of the thresholds were evaluated.

RESULTS

For separation of keratoconus stages 0/1/2/3/4 we derived the following optimum thresholds: for KI 1.05/1.15/1.31/1.49 and for KMI 0.77/0.32/-0.08/-0.3. For Smolek/Klyce and Klyce/Maeda high standard deviations and overlapping confidence intervals were found; therefore no discrete thresholds could be defined. Nevertheless, for them we still found a good sensitivity and specificity in discriminating between healthy (stage 0) and keratoconus (stages 2-4) eyes in comparison with the other indices.

CONCLUSIONS

We derived thresholds for the metric keratoconus indices KI and KMI, which allow classification of keratoconus stages. These now need to be validated in clinical use. Smolek/Klyce and Klyce/Maeda were not sufficiently sensitive to allow classification into individual stages, but these indices did show a good specificity and sensitivity in discriminating between keratoconus and healthy eyes.

Can retinoscopy keep up in keratoconus diagnosis?

PURPOSE

To evaluate the diagnostic potential of retinoscopy in comparison with Amsler-grading, Pentacam and Ocular Response Analyzer (ORA) in classifying keratoconus stages.

METHODS

Clinical examination, retinoscopy, Pentacam and ORA were performed in 126 patients. Data of Amsler, retinoscopy, topographic keratoconus classification (TKC) of Pentacam and keratoconus match probability (KMP) of ORA were analyzed. Each of these four classification techniques quotes keratoconus into stage 0 (normal) to 4 (severe). Descriptive analysis and cross tables were used to compare the different devices.

RESULTS

For retinoscopy the distribution in the five keratoconus grades normal/suspect/mild/moderate/severe (in numbers) was 34/33/34/17/8. For Amsler it was 37/36/35/12/4, for TKC 38/17/34/31/4, for KMP 32/34/32/15/9. The cross tables show large classification differences of all devices. Overall, classification of retinoscopy and Amsler/TKC/KMP is congruent in 51.6%/36.3%/39.8% of the cases. Of all eyes, Amsler was congruent with TKC/KMP in 54.0%/48.4%, and TKC and KMP were congruent in 53.3%. In a binary decision (normal vs. any stage of mild/moderate/severe) matching between retinoscopy and Amsler/TKC/KMP was 98.6%/88.8%/82.4%. Sensitivity/specificity for retinoscopy and Amsler, TKC, KMP was 98.8%/94.0%, 84.4%/100% and 80.0%/79.1%.

CONCLUSIONS

The congruence of keratoconus classification was very poor of all the techniques tested in our study. This applies to objective measures such as TKC, KMP as well as clinical classification techniques such as Amsler and retinoscopy. Compared to TKC and KMP, retinoscopy underestimates keratoconus stages. In contrast, the performance of binary decisions (normal vs. keratoconus) shows a high sensitivity and specificity. Retinoscopy, however, showed a clear clinical use in confirming the diagnosis of keratoconus.

Comparison of multimetric D index with keratometric, pachymetric, and posterior elevation parameters in diagnosing subclinical keratoconus in fellow eyes of asymmetric keratoconus patients

PURPOSE

To compare the multimetric D index and other keratoconus-screening parameters in patients with clinical keratoconus in 1 eye and subclinical keratoconus in the fellow eye.

SETTING

Medipol University Hospital and Birinci Eye Hospital, Istanbul, Turkey.

DESIGN

Retrospective case-control study.

METHODS

Patients with clinical keratoconus in 1 eye and subclinical keratoconus in the fellow eye and eyes of normal subjects were evaluated with a rotating Scheimpflug imaging system (Pentacam). Parameters included anterior curve analysis, keratometry (K) values, minimum corneal thickness, pachymetric progression index, Ambrósio relational thickness, posterior elevation, back difference elevation, and D-index values. The receiver operating characteristic (ROC) curves were analyzed to evaluate the area under curve (AUC), sensitivity, and specificity of each parameter.

RESULTS

Forty-five patients and 67 normal subjects were evaluated. The pachymetric progression indices, posterior elevation, and the D-index measurements were statistically significantly higher whereas corneal thickness and Ambrósio relational thickness measurements were significantly lower in eyes with keratoconus or subclinical keratoconus than in eyes of normal subjects (P < .05). Using the ROC analysis, the AUC values of the mean steep K, minimum corneal thickness, pachymetric progression index minimum, Ambrósio relational thickness maximum, posterior elevation, back difference elevation, and D index to distinguish between subclinical keratoconus from control subjects were 0.52, 0.64, 0.71, 0.72, 0.71, 0.76, and 0.83, respectively.

CONCLUSION

The new multimetric D index seems to be better than other single-metric parameters in diagnosing keratoconus and subclinical keratoconus with good specificity. However, the sensitivity levels of all parameters were relatively limited in the diagnosis of subclinical keratoconus.

FINANCIAL DISCLOSURE

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

Prediction of residual astigmatism after cataract surgery using swept source fourier domain optical coherence tomography

PURPOSE

To compare corneal measurements obtained by a swept source fourier domain OCT (CASIA SS-1000), an autokeratometer (Haag-Streit Lenstar), a hybrid topographer (Tomey TMS-5), a Placido topographer (Tomey TMS-5 in Placido mode) and a Scheimpflug tomographer (Oculus Pentacam) to manifest subjective refraction.

METHODS

One hundred and four pseudophacic patients with non-toric IOLs were measured at least 6 months after surgery. Corneal astigmatism as measured on the anterior corneal surface as well as total corneal astigmatism including posterior surface data was compared to manifest refractive cylinder (cross-cylinder strategy) by computing difference vectors and correlation analysis of power vectors.

RESULTS

The OCT (0.43 ± 0.25 D) and the hybrid topographer (0.44 ± 0.25 D) yielded the smallest difference vector to subjective cylinder and by far the lowest percentage of outliers >0.75 D (≈10%). The rotating Scheimpflug camera showed the largest (0.70 ± 0.41 D) difference vector. The best predictive precision (0.37 ± 0.22) could be achieved by vector averaging Lenstar keratometry and OCT.

CONCLUSIONS

Autokeratometry yielded the least measuring noise but OCT as well as hybrid topography had better predictive precision due to posterior curvature data. Scheimpflug tomography suffered from high measuring noise. Combination of keratometry and OCT data yielded the best precision for planning of toric IOL implantation. To get a reliable target cylinder for TIOL calculation, accuracy of the measuring device is crucial. Keratometry and Placido topography lack the information of the posterior corneal curvature while Scheimpflug devices suffer from higher measuring noise. In this paper, a combination of ssOCT with autokeratometry yielded the best predictive quality.

Assessment of anterior segment parameters of keratoconus eyes in an Australian population

PURPOSE

To assess anterior segment parameters of eyes with keratoconus (KC) at different clinical stages of disease.

METHODS

KC and non-KC patients were recruited from public and private clinics in Melbourne, Australia. Axial length (AL), mean front corneal curvature (Front Km), mean back corneal curvature (Back Km), central corneal thickness (CCT), corneal thickness at the apex (CTA), corneal thickness at the thinnest point (CTT), anterior chamber depth (ACD), and corneal volume were noted for all the eyes.

RESULTS

A total of 181 individuals comprising 44 (24.3%) subclinical KC, 118 (65.2%) clinical KC, and 19 (10.5%) control subjects were analyzed. Significant differences were noticed between the subclinical KC and control group for ACD and CTT, whereas between clinical and control groups, significant differences were obtained for AL, CCT, CTA, CTT, and ACD (p < 0.05). In the case of mild, moderate, and severe KC groups, Back Km, CCT, CTA, and CTT were significantly associated (p < 0.001) with increasing disease severity. We further did receiver operating characteristic analysis to confirm the importance of pachymetric parameters in differentiating between control and KC eyes. The area under the receiver operating characteristic curve value of CTT for subclinical and clinical KC was 0.68 and 0.82, which showed that it may be a potential marker for the early detection and prevention of KC.

CONCLUSIONS

This study identified the anterior chamber parameters that differ between subclinical and clinical KC as well as the severity of KC. There is a significant reduction in CTT between control and subclinical eyes, although there are no significant alterations in Front and Back Km or AL between the two groups. Also, a progressive reduction in the pachymetric readings at the pupil center, apex, and thinnest corneal point was identified when comparing mild to severe KC groups. Thus, corneal thickness represents an important parameter that needs to be considered in monitoring KC disease severity.

Pentacam Scheimpflug tomography findings in topographically normal patients and subclinical keratoconus cases

PURPOSE

To evaluate Pentacam ectasia detection indices in topographically normal patients and in subclinical keratoconus cases.

DESIGN

Prospective, observational case series.

METHODS

setting: Institutional. patients: Group 1 comprised 1 eye from 189 patients with unremarkable topography and Groups 2 and 3 included the better and worse eyes, respectively, of 55 keratoconic patients. Group 2 eyes with normal topography (n = 37) were considered subclinical keratoconus cases. observation procedure: Pentacam Scheimpflug tomography. main outcome measures: Eleven Pentacam ectasia detection indices.

RESULTS

All Pentacam ectasia indices significantly differed between Groups 1 and 2 and were correlated with keratoconus grade. Only 99 eyes (52%) in Group 1 had normal values for every index, whereas 7 subclinical keratoconus eyes (19%) showed 2 or fewer abnormal indices. Standardized relational thickness and overall deviation indices had 73% and 89% sensitivity for subclinical keratoconus, respectively. Both average and maximum pachymetric progression indices offered 84% sensitivity while maximum relational thickness index showed 78% sensitivity for subclinical keratoconus. Optimized cutoff values for subclinical keratoconus increased the sensitivity of the standardized and maximum relational thickness indices.

CONCLUSION

Pentacam Scheimpflug tomography can detect most subclinical keratoconus cases with unremarkable topography, but performance is not as good as reported and varies considerably for each index. The overall deviation, average and maximum pachymetric progression, and maximum relational thickness indices offer the highest sensitivity, which can be improved by using optimized cutoff values. Specificity constitutes an issue for some indices and up to 10% of subclinical keratoconus cases may go undetected by this technology.

Technologies for anatomical and geometric characterization of the corneal structure and anterior segment: a review

Corneal and anterior segment imaging techniques have become a crucial tool in the clinical practice of ophthalmology, with a great variety of applications, such as corneal curvature and pachymetric analysis, detection of ectatic corneal conditions, anatomical study of the anterior segment prior to phakic intraocular lens implantation, or densitometric analysis of the crystalline lens. From the Placido-based systems that allow only a characterization of the geometry of the anterior corneal surface to the Scheimpflug photography-based systems that provide a characterization of the cornea, anterior chamber, and crystalline lens, there is a great variety of devices with the capability of analyzing different anatomical parameters with very high precision. To date, Scheimpflug photography-based systems are the devices providing the more complete analysis of the anterior segment in a non-invasive way. More developments are required in anterior segment imaging technologies in order to improve the analysis of the crystalline lens structure as well as the ocular structures behind the iris in a non-invasive way when the pupil is not dilated.

[Difference and distance between the central and thinnest points of the cornea: impact of refractive state, age and ocular side]

BACKGROUND

The aim of the study was to quantify the difference in corneal thickness between the central and thinnest points (∆PachyZ-PachyD), the distance between the center of the cornea and its thinnest point (vector length PachyD) and to explore the impact of refractive state, age and ocular side.

PATIENTS AND METHODS

This was a multicenter, retrospective, cross-sectional study and medical records of 16,872 eyes were reviewed. The Orbscan® (Bausch and Lomb) procedure was used for pachymetry and keratometry.

RESULTS

The results showed that ∆PachyZ-PachyD and vector length PachyD were higher in hyperopic eyes (∆PachyZ-PachyD: 11.99 ± 12.08 µm, vector length PachyD: 0.85 ± 0.44 mm) compared to myopic eyes (∆PachyZ-PachyD: 9.2 ± 7.86 µm, vector length PachyD: 0.7 ± 0.37 mm; p < 0.001). Refractive state, age and ocular side demonstrated an independent, statistically significant impact on ∆PachyZ-PachyD and vector length PachyD.

CONCLUSIONS

As a result of the significant impact of refractive state, age and ocular side on ∆PachyZ-PachyD and vector length PachyD, these variables should be considered in a normative data collection.

Repeatability, reproducibility, and agreement of corneal power measurements obtained with a new corneal topographer

PURPOSE

To study the repeatability and reproducibility of corneal power measurements obtained with a new corneal topographer (Keratograph 4) and assess their agreement with those obtained by a rotating Scheimpflug camera (Pentacam HR) and an automated keratometer (IOLMaster).

SETTING

Eye Hospital, Wenzhou Medical University, Wenzhou, China.

DESIGN

Observational cross-sectional study.

METHODS

One eye of normal subjects was measured 3 times using all devices. Another operator performed an additional 3 consecutive scans using the corneal topographer. One week later, the first operator repeated the corneal power measurements using the corneal topographer. Parameters were flat meridian power, steep meridian power, average power, J0, and J45. The repeatability and reproducibility of measurements were assessed by the within-subject standard deviation (Sw), repeatability (2.77 Sw), coefficient of variation (CoV), and intraclass correlation coefficient (ICC). Agreement between devices was assessed using 95% limits of agreement (LoA).

RESULTS

Intraoperator repeatability and interoperator and intersession reproducibility of all measured parameters showed a CoV of less than 0.3%, a 2.77 Sw of 0.32 diopter or less, and an ICC of more than 0.97. No significant differences in the parameters were noted between the corneal topographer and Scheimpflug camera. Statistically significant differences existed between the parameters analyzed by the corneal topographer and the automated keratometer, except J45. The mean differences between the corneal topographer and the other 2 devices were small, and the 95% LoA were narrow for all measurements.

CONCLUSION

The new corneal topographer had excellent reliability and high agreement with the other 2 devices in corneal power measurements in normal subjects.

FINANCIAL DISCLOSURE

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

Posterior corneal elevation and back difference corneal elevation in diagnosing forme fruste keratoconus in the fellow eyes of unilateral keratoconus patients

PURPOSE

To evaluate posterior corneal elevation and back difference corneal elevation in patients with keratoconus in 1 eye and forme fruste keratoconus in the fellow eye.

SETTING

Kudret Eye Hospital, Ankara, Turkey.

DESIGN

Case-control study.

METHODS

This study retrospectively reviewed patients with keratoconus in 1 eye and forme fruste keratoconus in the fellow eye and eyes of normal subjects. All subjects were evaluated with a rotating Scheimpflug imaging system (Pentacam), including sagittal and tangential anterior curve analysis, keratometry, and posterior elevation. The back difference elevation values were extrapolated from the difference maps of the Belin-Ambrosió enhanced ectasia display of the Scheimpflug system. The receiver operating characteristic (ROC) curves were analyzed to evaluate the sensitivity and specificity of the parameters.

RESULTS

The corneal power, pachymetric progression index, and posterior corneal elevation (posterior elevation and back difference elevation) measurements were statistically significantly higher in eyes with keratoconus or forme fruste keratoconus than in eyes of normal control subjects (P<.05). Using ROC analysis, the area under the curve values of mean keratometry, steepest point on the tangential curve, minimum corneal thickness, pachymetric progression index, Ambrósio's relational thickness, posterior elevation, and back difference elevation to distinguish forme fruste keratoconus from control subjects were 0.51, 0.84, 0.65, 0.81, 0.72, 0.68, and 0.76, respectively.

CONCLUSIONS

Back difference elevation was better than posterior elevation in diagnosing forme fruste keratoconus. However, as sole parameters, both had limited sensitivity and specificity to differentiate between forme fruste keratoconus eyes and normal control eyes.

Characterization of corneal structure in keratoconus

The increasing volume of patients interested in refractive surgery and the new treatment options available for keratoconus have generated a higher interest in achieving a better characterization of this pathology. The ophthalmic devices for corneal analysis and diagnosis have experienced a rapid development during the past decade with the implementation of technologies such as the Placido-disk corneal topography and the introduction of others such as scanning-slit topography, Scheimpflug photography, and optical coherence tomography, which are able to accurately describe not only the geometry of the anterior corneal surface but also that of the posterior surface, as well as pachymetry and corneal volume. Specifically, anterior and posterior corneal elevation, corneal power, pachymetry maps, and corneal coma-like aberrometry data provide sufficient information for an accurate characterization of the cornea to avoid misleading diagnoses of patients and provide appropriate counseling of refractive surgery candidates.

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.

Topographic and Tomographic Indices for Detecting Keratoconus and Subclinical Keratoconus: A Systematic Review

Purpose

To provide an overview of the topographic and tomographic indices developed for detecting keratoconus (KC) and subclinical keratoconus.

Methods

Literature review of studies describing and testing KC indices as well as indices developed for improving the sensitivity of subclinical KC detection.

Results

Several indices, based on anterior and posterior curvature measurements, corneal spatial distribution or posterior corneal elevation have been developed for improving the detection of KC and subclinical KC. However, to date, none of them could reach, alone, sufficient discriminating power for differentiating the mildest forms of the disease from normal corneas. New detection programs, based on a combination of corneal indices, and generated using artificial intelligence emerged recently and helped to significantly improve the subclinical KC detection.

Conclusion

The combination of topographic and tomographic corneal indices has helped to significantly improve the sensitivity of subclinical KC detection. However, combining these morphological indices to wavefront and biomechanical analyses of the cornea will certainly further improve the sensitivity of the future screening tests.

How to cite this article

Smadja D. Topographic and Tomographic Indices for Detecting Keratoconus and Subclinical Keratoconus: A Systematic Review. Int J Kerat Ect Cor Dis 2013;2(2):60-64.

The thinnest, steepest, and maximum elevation corneal locations in noncontact and contact lens wearers in keratoconus

PURPOSE

To assess the relationship between the thinnest corneal location and the steepest and maximum elevation corneal locations in subjects with keratoconus and the effect of gas permeable contact lens wear on the location of these points.

METHODS

Sixty-one consecutive subjects (98 eyes) with keratoconus. Thirty-one (49 eyes) and 30 (49 eyes) subjects were gas permeable contact lens (CL-W) and non-contact lens wearers (N-CL), respectively. Thinnest, steepest, and maximum elevation corneal locations were evaluated from topographies collected with Pentacam Eye Scanner.

RESULTS

In the entire sample and in N-CL and CL-W, the thinnest location does not overlap with the steepest or maximum elevation corneal locations (all P > 0.05). The thinnest and maximum tangential curvature locations were found to be located further away from the geometric center of the cornea in CL-W versus N-CL (P < 0.05).

CONCLUSION

The thinnest corneal location does not overlap with maximum axial and tangential curvatures or with the front and back elevation locations in keratoconus subjects. Contact lens wear does not affect this lack of overlapping.

International Values of Central Pachymetry in Normal Subjects by Rotating Scheimpflug Camera

PURPOSE

In corneal refractive surgery, postoperative ectasia risk assessment routinely includes pachymetric analysis at the apex and thinnest point. We examined whether these data differ worldwide and constructed preliminary country-specific normative thresholds.

DESIGN

This was a multicenter, cross-sectional study.

METHODS

Using the Pentacam Eye Scanner (OCULUS GmbH, Wetzlar, Germany), we examined 1 randomly selected eye from each of 555 normal adults (8 countries, 6 continents), measuring the thinnest point location, central pachymetry (thinnest point, pupillary center, and apex), and the apex-thinnest point difference. International differences were assessed by 1-way analysis of variance. Normative thresholds were defined by 2- and 3-SD gates or Tukey method.

RESULTS

The thinnest point averaged 0.44 mm temporal and 0.29 mm inferior to the apex. Individual thinnest points located more than 1.0 mm inferior represented fewer than 0.5% of normal corneas (>0.76 mm in the US subgroup). The mean thinnest-point pachymetry was 536 μm overall, and values less than 469 or 435 μm (-2 or -3 SD, respectively) would be expected in less than 2.5% or 0.15% of normal corneas, respectively. The thinnest-point pachymetry averaged 2 to 3 μm thinner than apical (range, 0-32 μm). Differences of greater than 8.5 μm would be expected in less than 0.5% of normal corneas overall.

CONCLUSIONS

International differences were largely clinically insignificant. Nevertheless, it remains preferable to establish racial/geographic-specific normative values. We defined preliminary generalized and country-specific thresholds useful to the corneal refractive surgeon for identifying potentially abnormal corneas based on common pachymetric parameters, particularly the thinnest point by tomography.

Precision analysis of posterior corneal topography measured by Visante Omni: repeatability, reproducibility, and agreement with Orbscan II

PURPOSE

To evaluate the repeatability and reproducibility of posterior corneal curvature and posterior corneal elevation best-fit sphere (BFS) obtained with the Visante Omni (Carl Zeiss Meditec) and to compare the results with the Orbscan II (Bausch & Lomb).

METHODS

Thirty eyes from 30 healthy volunteers were included in this study. All patients were examined 5 times with the Visante Omni and Orbscan II by 2 independent operators. The posterior corneal curvature (3- and 6-mm zone) and posterior corneal elevation BFS (5- and 8-mm zone) were generated for each system. Intraoperator repeatability and interoperator reproducibility and agreement between the systems were evaluated using the intraclass correlation coefficient (ICC) and the Bland-Altman method.

RESULTS

The repeatability of posterior corneal curvature and posterior corneal elevation BFS measured by the Visante Omni was high for all analysis zones (ICC, 0.99 to 1.00). The reproducibility also showed similar results (ICC, 0.99 to 1.00). Agreement between the Visante Omni and Orbscan II was high for posterior corneal curvature (ICC, 0.94 to 0.97) and posterior corneal elevation BFS (ICC, 0.96 to 0.98) with 95% limits of agreement at -0.26 to 0.22 diopters for posterior corneal curvature and 0.11 to 0.69 mm for posterior corneal elevation BFS.

CONCLUSIONS

The Visante Omni provides good repeatability and reproducibility of posterior corneal topography. Overall agreement with the Orbscan II system was high.

An indirect method to compare the reference centres for corneal measurements

PURPOSE

Corneal measurements are commonly presented with respect to a specific reference centre, but the location of the reference centre on the corneal surface could vary from one diagnostic modality to another. This study aimed to develop a method for comparing reference centres used by corneal measurement systems.

METHODS

An indirect method was developed to compare reference centres by making use of the pupil centre and its offset from the reference centre. Reference centres in a Scheimpflug imaging system, the Pentacam HR, and a Placido-ring corneal topography system, the ATLAS Corneal Topography System, were compared for the right and left eyes of 30 subjects. The subjects all had similar pupil sizes when measured by the two systems. Differences and correlations of the pupil centre offsets between the two systems were statistically tested and compared by Bland-Altman analyses.

RESULTS

There were no significant differences in mean pupil offsets between the two systems for either the right or left eyes (p > 0.05). There were strong correlations of the pupil centre offsets between the two systems for each eye (right eye x-axis: r = 0.95, p < 0.0001; right eye y-axis: r = 0.98, p < 0.0001; left eye x-axis: r = 0.96, p < 0.0001, left eye y-axis: r = 0.93, p < 0.0001). Bland-Altman analyses revealed no significant differences in pupil centre offsets between the two systems.

CONCLUSIONS

The Pentacam HR system and the ATLAS system have very similar reference centres. Thus it is possible to directly analyze data from the Pentacam HR and other instruments using the corneal vertex or the pupil centre as the reference centres due to the similarity in the reference centre settings between the two systems.

Evaluation of Scheimpflug imaging parameters in subclinical keratoconus, keratoconus, and normal eyes

PURPOSE

To determine the efficacy of different Scheimpflug-imaging parameters in discriminating between subclinical keratoconus, keratoconus eyes, and normal eyes.

SETTING

Department of Ophthalmology, Ankara University Faculty of Medicine, Ankara, Turkey.

DESIGN

Comparative case series.

METHODS

Pentacam Scheimpflug measurements were taken in eyes with mild to moderate keratoconus, subclinical keratoconus, or with myopic astigmatism (normal eyes). Several parameters provided by the software or derived from the elevation maps were evaluated and compared between the groups. The Mann-Whitney test, receiver-operating-characteristic (ROC) curves, and logistic regression analysis were used to compare the mean measurements and to evaluate the sensitivity and specificity of the parameters or constructed models.

RESULTS

The corneal power and elevation measurements were statistically significantly higher and the cornea was significantly thinner in eyes with subclinical or clinical keratoconus than in normal eyes (P < .05). The ROC curve analysis showed high overall predictive accuracy of various elevation and thickness indices in discriminating ectatic corneas from normal corneas. Logistic regression analysis showed that the goodness of fit of a model using a combination of corneal power, thickness, and anterior elevation parameters was best in discriminating keratoconus eyes from normal eyes, whereas that of a model using a combination of corneal power, thickness, and posterior elevation was best in discriminating subclinical keratoconus eyes from normal eyes.

CONCLUSION

Combined analysis of anterior and posterior corneal power, elevation, and thickness data provided by the Scheimpflug device effectively discriminated between ectatic corneas and normal corneas.

FINANCIAL DISCLOSURE

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