Reliability and repeatability of the Pentacam on corneal curvatures

Intra- and Intersession Repeatability of an Optical Quality and Intraocular Scattering Measurement System in Children

Purpose

To evaluate intra- and intersession repeatability of objective optical quality and intraocular scattering measurements with a double-pass system in children.

Methods

Forty-two eyes of 42 children were included in the study. An optical quality analysis system (OQAS) was used to measure optical quality parameters, including modulation transfer function cutoff frequency (MTF_cutoff), Strehl ratio (SR), OQAS values (OV) at 3 different contrasts and objective scatter index (OSI). Three measurement sessions with 10-min intervals were operated by the same technician, and in each session four consecutive measurements were obtained.

Results

Mean values for MTF_cutoff, SR and OSI were 46.85 ± 7.45cpd, 0.27 ± 0.06 and 0.34 ± 0.22 respectively. 1) The intraclass correlation coefficients were ranged from 0.89 to 0.97 and coefficients of variation from 0.06 to 0.16 for all the parameters in the first session; the relative repeatability were 11.1% (MTF_cutoff), 22.5% (SR), 10.9% (OV100%), 16.6% (OV2%), 22.4% (OV9%) and 56.3% (OSI). Similar results were found in the second and third sessions. 2) Bland-Altman analysis showed that narrow 95% confidence intervals (compared between the first and second sessions) ranged from -5.42 to 5.28 (MTF_cutoff), -0.05 to 0.07 (SR), -0.18 to 0.18 (OV100%), -0.26 to 0.29 (OV20%), -0.33 to 0.39 (OV9%) and -0.11 to 0.09 (OSI); the comparison between any two of the three sessions showed similar results.

Conclusion

Measurements of optical quality and intraocular scattering in children by the double-pass system showed good intra- and intersession repeatability. Retinal image quality is high and intraocular scattering is low in children.

Corneal Power Measurement Obtained by Fourier-Domain Optical Coherence Tomography: Repeatability, Reproducibility, and Comparison With Scheimpflug and Automated Keratometry Measurements

PURPOSE

To assess the repeatability and reproducibility of corneal power values obtained by a Fourier-domain optical coherence tomography (FD-OCT) system (RTVue) and to compare them with the values obtained by a Scheimpflug camera system (Pentacam HR) and by automated keratometry (IOL Master).

METHODS

Thirty-two eyes from 32 healthy subjects were included in this prospective study. Two experienced observers measured each eye 3 consecutive times with the Pentacam, IOLMaster, and RTVue centered on either the pupil or corneal vertex. The conventional keratometry equivalent (CKE) and anterior (Ka), posterior (Kp), and net (Kn) corneal power values were determined.

RESULTS

The corneal power values obtained by the RTVue showed high repeatability (all intraclass correlation coefficient >0.96) and reproducibility (coefficient of variation <1.0%). Pupil-centered FD-OCT performed slightly better than corneal vertex-centered FD-OCT. Mean corneal values had higher reproducibly than any of the individual values. CKE, Ka, Kp, and Kn obtained by FD-OCT were 0.62 to 0.68 diopters (D), 0.70 to 0.76 D, 0.11 to 0.13 D, and 0.93 to 0.94 D higher than those obtained by the Pentacam HR, respectively. CKE and Ka obtained with the RTVue were also 0.60 to 0.74 D higher than those obtained with the IOLMaster, respectively.

CONCLUSIONS

The corneal power measurements obtained by the RTVue FD-OCT system showed high repeatability and reproducibility. Measurements obtained by FD-OCT with pupil centration were more reproducible than those obtained by FD-OCT with corneal vertex centration. We recommend that pupil-centered FD-OCT be used in clinical applications. Neither RTVue versus Pentacam HR nor RTVue versus IOLMaster can be used interchangeably.

Repeatability and inter-device agreement for three different methods of keratometry: Placido, Scheimpflug, and color LED corneal topography

PURPOSE

To determine the repeatability of a color LED corneal topographer (Cassini; iOptics, The Hague, The Netherlands) and compare it with Placido and Scheimpflug based devices (EyeSys 2000; EyeSys Laboratories, Houston, TX, and Pentacam HR; Oculus Optikgeräte GmbH, Wetzlar, Germany).

METHODS

This prospective study involved 20 healthy volunteers (20 eyes) recruited from the staff of the Antwerp University Hospital. For each eye, three measurements were taken using each device, from which eight parameters describing keratometry and astigmatism were derived. Repeatability was assessed using intrasession, within-subject analysis of variance. Agreement was evaluated between pairs of devices with Bland- Altman plots and 95% confidence intervals.

RESULTS

The repeatability values were good for all three devices, although slightly worse for the Cassini than for the other two devices for steep, flat, and mean keratometry. The EyeSys showed worse repeatability for the astigmatism axis. EyeSys and Pentacam agreed well with each other in terms of mean keratometry, whereas the Cassini gave consistently higher values by 0.52 diopters (D) when compared with the EyeSys (P < .05, paired t test) and by 0.38 D when compared with the Pentacam (P < .05, paired t test). The Cassini provided similar repeatability values (2.31°) for the astigmatism axis to the Pentacam (2.22°), in contrast to the EyeSys, which produced much more variable axes (9.0°) (P < .05, paired t test).

CONCLUSIONS

Overall, the three devices display comparable repeatability. The one exception is the astigmatism axis measurement of the EyeSys. The Cassini provides higher keratometry values than the other two devices, but the astigmatism axis agrees well with that provided by the Pentacam.

Evaluation of posterior astigmatism measured with Scheimpflug imaging

PURPOSE

Our aim was to assess the corneal power, axis, and age dependence of the anterior and posterior corneal surfaces with Scheimpflug imaging.

METHODS

Patients older than 10 years without a history of ocular surgery, corneal diseases, contact lens wearing, or severe dry eye were enrolled. Data regarding the anterior and posterior cornea were analyzed by means of Scheimpflug imaging (Pentacam HR).

RESULTS

The median age was 46.8 years (range: 10.0-90.3), and involved 827 eyes of 827 patients. Posterior corneal astigmatism was a median of -0.30 diopters (D) and exceeded 0.50 D in 12.56%. The ratio of the anterior-to-posterior corneal radius was a median of 1.217 (range: 1.021-1.402). The correlation between this ratio and age was significant (r = -0.219; P < 0.001). The correlation between anterior and posterior corneal astigmatism and age was significant. At the anterior surface, the ratio of with-the-rule astigmatism was 80.0% for the age range 10 to 20, decreasing to 36.0% for those aged over 81 years, whereas the ratio of against-the-rule astigmatism increased from 7.1% to 44.0%. At the posterior surface, the ratio of with-the-rule astigmatism was 94.3% for the age range 10 to 20 years, decreasing to 84.0% for those aged over 81 years, whereas the ratio of against-the-rule astigmatism increased from 1.4% to 8.0%.

CONCLUSIONS

The posterior cornea significantly influences corneal astigmatism. Its power value and orientation are much more stable with advancing age compared with anterior surface values.

Analysis of surgically induced astigmatism on the posterior surface of the cornea

PURPOSE

To measure surgically induced astigmatism (SIA) on the posterior surface of the cornea using Scheimpflug-based keratometry on eyes with with-the-rule astigmatism.

METHODS

Repeated Scheimpflug-based keratometry was obtained preoperatively and a mean of 8.65 weeks postoperatively following superior clear corneal incisions. Differences in the keratometric values and SIA were determined on the anterior and posterior surfaces separately and the data were analyzed.

RESULTS

The study included 88 eyes of 88 patients (age range: 46.4 to 87.5 years), with the steepest total corneal meridian between 75° and 105°. Preoperatively, the magnitude of the anterior and posterior corneal astigmatism was 0.74 ± 0.44 and 0.29 ± 0.14 diopters (D), respectively, which was greater than 0.5 D in 10 eyes (11.36%). The difference between preoperative and postoperative keratometric data on the posterior surface was significant, as was the difference between the preoperative and postoperative posterior astigmatism. The mean magnitude of posterior corneal SIA was 0.32 ± 0.29 D, which was 0.5 D or greater in 22 eyes (25.00%). A statistically significant correlation was observed between the diopter of the preoperatively measured posterior corneal astigmatism and the postoperative SIA on the posterior surface (r = 0.34, P = .001). The correlation was not significant between SIA on the anterior and posterior surfaces (r = 0.10, P = .36).

CONCLUSIONS

SIA on the posterior surface of the cornea may have a significant role, especially in cases of toric intraocular lens implantation. Posterior corneal astigmatism and SIA may have a significant clinical impact on more precise planning of cataract surgeries.

Scheimpflug imaged corneal changes on anterior and posterior surfaces after collagen cross-linking

AIM

To compare the anterior and posterior corneal parameters before and after collagen cross-linking therapy for keratoconus.

METHODS

Collagen cross-linking was performed in 31 eyes of 31 keratoconus patients (mean age 30.6±8.9y). Prior to treatment and an average 7mo after therapy, Scheimpflug analysis was performed using Pentacam HR. In addition to corneal thickness assessments, corneal radius, elevation, and aberrometric measurements were performed both on anterior and posterior corneal surfaces. Data obtained before and after surgery were statistically analyzed.

RESULTS

In terms of horizontal and vertical corneal radius, and central corneal thickness no deviations were observed an average 7mo after operation. Corneal higher order aberration showed no difference neither on anterior nor on posterior corneal surfaces. During follow-up period, no significant deviation was detected regarding elevation values obtained by measurement in mm units between the 3.0-8.0 mm-zones.

CONCLUSION

Corneal stabilization could be observed in terms of anterior and posterior corneal surfaces, elevation and higher order aberration values 7mo after collagen cross-linking therapy for keratoconus.

Corneal biomechanical assessment using corneal visualization scheimpflug technology in keratoconic and normal eyes

Purpose. To compare the corneal biomechanical properties of keratoconic patients and age-matched controls using corneal visualization Scheimpflug technology (Corvis ST). Methods. Sixty keratoconic eyes from 47 keratoconus patients and 60 normal eyes from 60 controls were enrolled in this prospective study. Tomography and biomechanical parameters of all eyes were obtained with the Pentacam and Corvis ST, respectively. Intraocular pressure was measured using a Goldmann applanation tonometer. Results. The tomography and biomechanical parameters of the keratoconic corneas were significantly different from those of the normal corneas except for the anterior chamber angle, first applanation length, the highest concavity time, and peak distance. The deformation amplitude was the best predictive parameter (area under the curve: 0.882), with a sensitivity of 81.7%, although there was a significant overlap between keratoconic and normal corneas that ranged from 1.0 to 1.4 mm. In both the keratoconus and control groups, the deformation amplitude was negatively correlated with intraocular pressure, central corneal thickness, and corneal volume at 3 and 5 mm. Conclusions. Corvis ST offers an alternative method for measuring corneal biomechanical properties. The possibility of classifying keratoconus based on deformation amplitude deserves clinical attention.

Corneal shapes of Chinese emmetropes and myopic astigmats aged 10 to 45 years

PURPOSE

Myopia and astigmatism are highly prevalent in the Hong Kong Chinese. This study aimed to determine the effects of age and myopic astigmatism (MA) on the corneal shape factors in the Hong Kong Chinese.

METHODS

One hundred subjects with compound MA or emmetropia (EM) were recruited from three age groups: 10 to 15 years (n = 32), 20 to 25 years (n = 37), and 40 to 45 years (n = 31). Refractive errors were measured by noncycloplegic subjective refraction. Corneal astigmatism and corneal shape factors were measured by the Scheimpflug-based Pentacam. The effects of age and refractive errors on the whole corneal shape (mean-P) and the semimeridian corneal shapes (semi-Ps) at the nasal, temporal, superior, and inferior corneal quadrants (from corneal apex to 3 mm peripheral cornea) were analyzed.

RESULTS

Age had significant effects on the mean-P and semi-Ps (both p < 0.001), with both EM and MA showing less prolate corneal shapes in older age groups. Partial correlation analyses adjusted for age showed that mean-P and semi-Ps were correlated with multiple refractive-error components (Pearson r = -0.30 to -0.78, all p < 0.05), with higher correlations found along the horizontal semi-Ps in MA (Pearson r = +0.37 to -0.78, all p < 0.01). Compared with EM, MA had more prolate temporal semi-Ps in all the three age groups (p < 0.05). Strikingly, age and refractive errors also had significant impacts on the asymmetry of the corneal shape along the horizontal meridian.

CONCLUSIONS

Corneal shapes were influenced by age and MA in the Hong Kong Chinese. These results highlight the importance of controlling these factors when designing a study on corneal shape.

Comparison of corneal curvature and anterior chamber depth measurements using the manual keratometer, Lenstar LS 900 and the Pentacam

Purpose:

The purpose of this study was to compare keratometry and anterior chamber depth (ACD) measurements from the Lenstar LS 900 (Haag-Streit AG, Switzerland) and the Pentacam (Oculus, Weltzar, Germany), and compare the keratometry readings of these two systems to a manual keratometer (MK), (Haag-Streit, Switzerland).

Materials and Methods:

In this prospective study, keratometry and ACD measurements were obtained in 50 eyes of 50 normal subjects with the Lenstar and the Pentacam. Keratometry was also measured using a MK. Correlation, comparison, and interdevice agreement was evaluated using intraclass correlation analysis, Wilcoxon test, and Bland-Altman plots.

Results:

The keratometry and ACD measurements obtained from the Lenstar and the Pentacam showed excellent correlation. The mean interdevice differences in mean keratometry (Km) for the Lenstar and the Pentacam, the Lenstar and the MK, and the Pentacam and the MK were 0.39 D, 0.10 D, and 0.30 D respectively; and the 95% limits of agreement (LoA) for Km were 0.04-0.82 D; −1.90-2.10 D; and −2.30-1.70 D, respectively. The mean interdevice difference in ACD for the Lenstar versus the Pentacam was 0.09 mm, with 95% LoA of 0.23-0.05 mm.

Conclusions:

The ACD measurements obtained using the Lenstar and the Pentacam seem to be interchangeable, whereas, the keratometry measurements obtained using the Lenstar, Pentacam, and MK differ considerably, and are not interchangeable.

The influence of soft contact lens wear and two weeks cessation of lens wear on corneal curvature

INTRODUCTION

Accurate corneal measurements are crucial in corneal refractive surgery (CRS) to ensure successful outcomes. Soft contact lens (SCL) wear may result in changes to corneal curvature and structure. United States Food and Drug Administration (FDA) pre-operative guidelines recommend that prior to CRS, SCL wearers cease SCL wear for "at least two weeks before examination and treatment" [1]. Corneal curvature changes induced by SCL wear may take longer than two weeks to resolve.

PURPOSE

To examine the effect of SCL wear on corneal curvature before and following two weeks SCL wear cessation. To explore the possible impact of different SCL materials and years of SCL wear.

METHODS

Retrospective data analysis, between a group of SCL wearers (SCL: n=45); and a non-contact lens control group (NCL: n=45). Corneal curvature parameters were measured using the Pentacam (Oculus, Germany), before and following two weeks cessation of SCL wear.

RESULTS

No significant differences in keratometry or Sagittal radius of curvature between SCL and NCL groups prior to or following SCL cessation. Tangential radius of curvature showed significant inferior steepening for the SCL group prior to SCL cessation (SCL vs. NCL; 7.77±0.30mm vs. 7.90±0.30mm; p=0.04). Following two weeks cessation of SCL wear this appeared to have resolved.

CONCLUSIONS

Two weeks cessation of SCL wear appears sufficient for resolution of corneal curvature changes with modern SCL materials and years of SCL wear. However, further studies with longer lens deprivation periods are required to ensure stability for all SCL wearing patients.

Statistical guidelines for the analysis of data obtained from one or both eyes

PURPOSE

Measurements obtained from the right and left eye of a subject are often correlated whereas many statistical tests assume observations in a sample are independent. Hence, data collected from both eyes cannot be combined without taking this correlation into account. Current practice is reviewed with reference to articles published in three optometry journals, viz., Ophthalmic and Physiological Optics (OPO), Optometry and Vision Science (OVS), Clinical and Experimental Optometry (CEO) during the period 2009-2012.

RECENT FINDINGS

Of the 230 articles reviewed, 148/230 (64%) obtained data from one eye and 82/230 (36%) from both eyes. Of the 148 one-eye articles, the right eye, left eye, a randomly selected eye, the better eye, the worse or diseased eye, or the dominant eye were all used as selection criteria. Of the 82 two-eye articles, the analysis utilized data from: (1) one eye only rejecting data from the adjacent eye, (2) both eyes separately, (3) both eyes taking into account the correlation between eyes, or (4) both eyes using one eye as a treated or diseased eye, the other acting as a control. In a proportion of studies, data were combined from both eyes without correction.

SUMMARY

It is suggested that: (1) investigators should consider whether it is advantageous to collect data from both eyes, (2) if one eye is studied and both are eligible, then it should be chosen at random, and (3) two-eye data can be analysed incorporating eyes as a 'within subjects' factor.

Evaluation of femtosecond laser clear corneal incision: an experimental study

PURPOSE

To evaluate the changes of corneal topography following femtosecond laser and manual clear corneal incision (CCI).

METHODS

Inflation testing was performed in 14 human eye globes to evaluate the topographic response of the cornea to CCIs. In seven samples (femtosecond laser group), a 2.75-mm three-plane CCI was created using the iFS femtosecond laser (Abbott Medical Optics, Inc., Santa Ana, CA); the remaining seven samples (control group) received a 2.75-mm manual CCI using disposable angled knives. Topographic maps of the anterior and posterior cornea were acquired using a Scheimpflug topographer Pentacam HR (Oculus Optikgeräte GmbH, Wetzlar, Germany). Keratometric data were used to analyze the curvature changes of the cornea. The changes of corneal astigmatism were analyzed by vector analysis.

RESULTS

After CCI, the mean change of the anterior keratometric power was 0.04 ± 0.39 and 0.05 ± 0.51 diopters (D) (analysis of variance, P > .05) in the femtosecond laser and control groups, respectively. The mean change (P > .05) of the posterior corneal keratometric power was 0.16 ± 0.19 and 0.15 ± 0.18 D, respectively. The average change of the anterior and posterior corneal astigmatism vector magnitude was 0.17 D or less in both groups (P > .05). A slight against-the-rule astigmatic change of the anterior and posterior corneal interfaces was found after both CCI techniques.

CONCLUSION

The 2.75-mm three-plane CCI created with femtosecond laser showed minimal changes of the anterior and posterior corneal topography, comparable with those of single-plane angled manual incision.

Repeatability and comparability of anterior segment biometry obtained by the Sirius and the Pentacam analyzers

To assess the repeatability and comparability of six anterior segment biometry parameters obtained with a novel Scheimpflug camera with a Placido disc topographer (Sirius) and slit-scanning tomography with a Scheimpflug camera (Pentacam), in a sample of 16 unoperated eyes of healthy subjects. The anterior segment was analyzed by a single examiner using the Sirius and the Pentacam analyzers. Mean simulated keratometry (Sim K), flat and steep axis keratometry (K f and K s ), central and thinnest corneal thicknesses (CCT and TCT), and anterior chamber depth (ACD) measurements were evaluated. Repeatability of three sets of measurements from each device was assessed using the coefficient of variation (CV), within-subject standard deviation, and intraclass correlation coefficient (ICC). Bonferroni-adjusted t-tests, and Bland and Altman plots were used to establish agreement between devices. For both devices the CV of repeated measurements was <0.79 %. The ICC was >0.95 in all measurements except for Sirius K s (ICC = 0.869). For all parameters evaluated, the Pentacam systematically yielded higher values, although differences were statistically significant in only three parameters-0.31 diopters for K s , 10.1 μm for CCT and 12.4 μm for TCT. In the assessment of normal corneas both devices showed overall high repeatability. Although good agreement was found in three parameters (Sim K, K f and ACD) these devices do not seem to be interchangeable for pachymetric determination.

[Scheimpflug photography of acute angle closure]

PURPOSE

In acute angle-closure glaucoma a laser iridotomy (IO) is performed to balance the intraocular pressure between the anterior and posterior chambers. The changes induced in the anterior chamber architecture were analyzed using Scheimpflug photography.

MATERIAL AND METHODS

Scheimpflug photography was performed in eyes with acute angle closure and in the contralateral eyes before and after IO.

RESULTS

In a group of 10 patients (8♀, 2♂, average age71.2 ± 5.3 years) 10 eyes with acute angle closure and 10 contralateral eyes were analyzed. The anterior chamber depth, central corneal thickness and anterior chamber angle did not change significantly after IO. The mean anterior chamber volume showed a statistically significant increase in the 10 eyes with acute angle closure and in the 10 contralateral eyes.

CONCLUSION

Anterior chamber volume increases due to IO but not anterior chamber depth, central corneal thickness and anterior chamber angle in eyes with acute angle closure.

Cohort profile: Shahroud Eye Cohort Study

The Shahroud Eye Cohort Study was set up to determine the prevalence and incidence of visual impairment and major eye conditions in the 40-64-year-old population of Shahroud as a Middle Eastern population. The first phase of the study was conducted in 2009-10. Using random cluster sampling, 6311 Shahroud inhabitants were invited for ophthalmologic examinations; of these, 5190 participants completed phase 1 (participation rate of 82.2%). All participants were interviewed to collect data on participants' demographics, occupation status, socioeconomic status, history of smoking, and medical and ophthalmic history, as well as history of medication, and the quality and duration of their insurance. DNA and plasma samples, as well as four dots of whole blood were collected from participants. Extensive optometric and ophthalmologic examinations were performed for each participant, including lensometry of current glasses, testing near and far visual acuity; determining objective and subjective refraction; eye motility; cycloplegic refraction; colour vision test; slit-lamp biomicroscopy and intraocular pressure measurement; direct and indirect fundoscopy; perimetry test; ocular biometry; corneal topography; lens and fundus photography; and the Schirmer's (1008 participants) and tear breakup time tests (1013 participants). The study data are available for collaborative research at Noor Ophthalmology Research Center, Tehran, Iran.

A comprehensive assessment of the precision and agreement of anterior corneal power measurements obtained using 8 different devices

Purpose

To comprehensively assess the precision and agreement of anterior corneal power measurements using 8 different devices.

Methods

Thirty-five eyes from 35 healthy subjects were included in the prospective study. In the first session, a single examiner performed on each subject randomly measurements with the RC-5000 (Tomey Corp., Japan), KR-8000 (Topcon, Japan), IOLMaster (Carl Zeiss Meditec, Germany), E300 (Medmont International, Australia), Allegro Topolyzer (Wavelight AG, Germany), Vista (EyeSys, TX), Pentacam (Oculus, Germany) and Sirius (CSO, Italy). Measurements were repeated in the second session (1 to 2 weeks later). Repeatability and reproducibility of corneal power measurements were assessed based on the intrasession and intersession within-subject standard deviation (Sw), repeatability (2.77Sw), coefficient of variation (COV), and intraclass correlation coefficient (ICC). Agreement was evaluated by 95% limits of agreement (LoA).

Results

All devices demonstrated high repeatability and reproducibility of the keratometric values (2.77Sw<0.36D, COV<0.3%, ICC>0.98). Repeated-measures analysis of variance with Bonferroni post test showed statistically significant differences (P<0.01) among mean keratometric values of most instruments; the largest differences were observed between the EyeSys Vista and Medmont E300. Good agreement (i.e., 95%LoA within ±0.5D) was found between most instruments for flat, steep and mean keratometry, except for EyeSys and Medmont. Repeatability and reproducibility of vectors J₀ and J₄₅ was good, as the ICCs were higher than 0.9, except J₄₅ of Medmont and Pentacam. For the 95% LoAs of J₀ and J₄₅, they were all ≤ ±0.31 among any two paired devices.

Conclusions

The 8 devices showed excellent repeatability and reproducibility. The results obtained using the RC-5000, KR-8000, IOLMaster, Allegro Topolyzer, Pentacam and Sirius were comparable, suggesting that they could be used interchangeably in most clinical settings. Caution is warranted with the measurements of the EyeSys Vista and Medmont E300, which should not be used interchangeably with other devices due to lower agreement.

Trial Registration

ClinicalTrials.gov NCT01587287

Measurement of central corneal thickness by high-resolution Scheimpflug imaging, Fourier-domain optical coherence tomography and ultrasound pachymetry

PURPOSE

To compare the repeatability and reproducibility of central corneal thickness (CCT) measurements by high-resolution (HR) rotating Scheimpflug imaging and Fourier-domain optical coherence tomography (FD-OCT). CCT measurements were compared to those determined by ultrasound pachymetry (UP).

METHODS

In 35 healthy eyes, intra-observer repeatability for HR Scheimpflug (Pentacam) and FD-OCT (RTVue) systems was determined in consecutive images taken by an observer in the shortest time possible. Imaging was repeated again by a second observer to evaluate inter-observer reproducibility. The CCT measurements were compared among Scheimpflug, FD-OCT and UP images.

RESULTS

Mean coefficients of repeatability were 0.48% for Scheimpflug and 0.26% for FD-OCT. For Scheimpflug, the coefficient of inter-operator reproducibility was 0.87%. For FD-OCT, the coefficient of inter-operator reproducibility was 0.45%. The CCT measurements by Scheimpflug, OCT and UP images were (mean ± standard deviation) 521.7 ± 27.6 μm, 510.8 ± 28.6 μm and 516.5 ± 27.6 μm, respectively. The differences between instruments were statistically significant. The 95% limits of agreement in CCT were -0.7 to 22.5 μm for Pentacam-OCT, -13.4 to 24.0 μm for Pentacam-UP and -26.7 to 15.4 μm for OCT-UP. There was a high degree of correlation between CCT measured by all 3 methods.

CONCLUSION

Noncontact measurements of CCT with HR Scheimpflug and FD-OCT systems yielded excellent repeatability and reproducibility and can be used interchangeably. Although both devices were comparable with UP; in clinical practice, the measurements acquired by optical modalities are not directly interchangeable with UP measurements.

Corneal power measurement with Fourier-domain optical coherence tomography

PURPOSE

To study the accuracy and repeatability of anterior, posterior, and net corneal power measured by Fourier-domain optical coherence tomography (OCT).

SETTING

Doheny Eye Institute, Los Angeles, California, USA.

DESIGN

Cross-sectional study.

METHODS

A Fourier-domain OCT system (RTVue) was used to scan normal eyes, eyes after myopic laser in situ keratomileusis (LASIK), and keratoconic eyes. After the corneal surfaces were delineated, the system calculated anterior and posterior corneal powers by curve fitting over the central 3.0 mm diameter area. Net corneal power was calculated using a thick-lens formula. The repeatability of the calculations was evaluated by the pooled standard deviation of 3 measurements from the same visit. The net corneal power values were compared with standard automated keratometry measurements (IOLMaster).

RESULTS

The repeatability of Fourier-domain OCT net corneal power was 0.19 diopters (D), 0.26 D, and 0.30 D in the normal, post-LASIK, and keratoconus groups, respectively. The Fourier-domain OCT net corneal power was significantly lower than keratometry by a mean of -1.21 D, -2.89 D, and -3.07 D, respectively (P<.001). The anterior-posterior curvature ratio was lower in post-LASIK and keratoconic eyes than in normal eyes (P<.001).

CONCLUSIONS

Corneal power measured by Fourier-domain OCT achieved good repeatability in all 3 groups. The repeatability was better than slower time-domain OCT systems. Because Fourier-domain OCT directly measures both anterior and posterior corneal surfaces, it may produce more consistent results than standard keratometry in post-LASIK and keratoconic eyes in which the anterior-posterior corneal curvature ratios are altered by surgery or disease.

Corneal volume, pachymetry, and correlation of anterior and posterior corneal shape in subclinical and different stages of clinical keratoconus

PURPOSE

To evaluate the corneal volume, pachymetry, and correlation of anterior and posterior corneal shape in subclinical and clinical keratoconus.

SETTING

Vissum Corporation, Alicante, Spain.

METHODS

Eyes were placed into 1 of 4 groups as follows: keratoconus 2 (grade II), keratoconus 1 (grade I), subclinical (subclinical keratoconus), and control (normal eyes). All eyes had an ophthalmologic examination including corneal evaluation (curvature, elevation, asphericity, pachymetry, corneal volume) by rotating Scheimpflug imaging (Pentacam). The posterior-anterior corneal power ratio was also calculated.

RESULTS

Seventy-one eyes (51 patients; aged 16 to 64 years) were evaluated. Astigmatism and keratometry of both corneal surfaces were statistically significantly higher in the keratoconus 1 and 2 groups (P<or=.02). Posterior astigmatism was statistically significantly higher in the subclinical group than in the control group (P = .01). A strong correlation (r>or=-0.81) was found between anterior and posterior curvature in the normal and subclinical groups; the correlation was weaker in clinical keratoconus cases (r<or=-0.56). The correlation in astigmatism between the anterior and posterior surface was good in all keratoconus groups (r>or=0.81). The posterior-anterior corneal power ratio was significantly higher in the keratoconus 2 group than in the other groups (P<or=.01). Pachymetric readings were progressively lower in eyes with subclinical, early, or moderate keratoconus (P<.01). The corneal volume was statistically significantly lower in the keratoconus 2 group than in the other groups (P = .04).

CONCLUSION

The correlation between anterior and posterior corneal curvature was lower in keratoconus, although the correlation between anterior and posterior astigmatism was maintained.