Total Corneal Power Research Articles

Comparative evaluation of dual Scheimpflug imaging parameters in keratoconus, early keratoconus, and normal eyes

To determine the efficacy of various parameters measured by dual Scheimpflug imaging technology in differentiating eyes with keratoconus or early keratoconus from normal eyes. Cornea Service, Wills Eye Institute, Philadelphia, Pennsylvania, USA. Comparative case series. A retrospective evaluation was performed of the parameters provided by the Galilei dual Scheimpflug analyzer in keratoconus, early keratoconus, and normal eyes. Logistic regression and receiver operating characteristic curve analysis were used to compare the mean values and to calculate the sensitivity and specificity of these parameters. Many parameters were statistically significantly different between keratoconus and normal eyes compared with early keratoconus eyes (P<.05). Total cornea power-steep and posterior curvature-steep keratometry had the highest area under the curve (AUC) score (0.99) for differentiating keratoconus eyes from normal eyes. All anterior curvature parameters were statistically significant in differentiating keratoconus eyes from normal eyes, whereas only the anterior curvature-steep was statistically significant in differentiating early keratoconus eyes from normal eyes. The central pachymetry and thinnest pachymetry were statistically significant in differentiating keratoconus and early keratoconus eyes from normal eyes. Third-order root mean square (RMS) and total RMS had the highest AUC scores (0.83 and 0.82, respectively) for differentiating early keratoconus eyes from normal eyes. Total corneal power, anterior curvature, posterior curvature, pachymetry, and corneal aberration data generated from the dual Scheimpflug analyzer showed promising results in differentiating keratoconus and early keratoconus eyes from normal eyes. No author has a financial or proprietary interest in any material or method mentioned.

Comparison of Keratometric Changes After Myopic Ablation: Ray Tracing Versus Simulated Keratometry

To compare the changes in corneal power (ΔK) induced by myopic ablations estimated by Placido-disk and total corneal power (TCP) ray tracing methods to the refractive change (ΔSE). Manifest refraction, simulated keratometry from a Placido-disk based system, and TCP from a dual-Scheimpflug analyzer were obtained from 35 patients (58 eyes) before and 3 months after a myopic ablation. The change in the manifest refraction that occurred after surgery was then vertex distance corrected and compared to the changes observed in corneal power with the various systems analyzed. The Placido-based simulated keratometry overestimated the corneal power after the procedure by 0.50 ± 0.53 diopters (D) when compared to refractive change in the corneal plane induced by the laser surgery (ΔSE-ΔK). The ray tracing method showed the opposite trend, with the TCP simulated keratometry showing an underestimation of the corneal power of -0.25 ± 0.48 D. The Placido system showed a direct correlation between the overestimation of the corneal power and the level of myopia, whereas the ray tracing method showed an underestimation, which was more pronounced in higher levels of myopia. Ray tracing methods underestimate corneal power as opposed to the overestimation in corneal power after refractive surgery, directly related to the level of myopia, observed in Placido-based systems. They have the potential to overcome the errors in calculations based on anterior curvature alone. However, ray tracing methods need to be validated and optimized before it can be used routinely in IOL calculation.

Femtosecond Laser Correction of Presbyopia (INTRACOR) in Emmetropes Using a Modified Pattern

To evaluate functional results and corneal changes after femtosecond laser correction of presbyopia (INTRACOR, Technolas Perfect Vision GmbH) in emmetropes using a modified treatment pattern over a 12-month period. Twenty eyes from 20 emmetropic patients were treated with a modified intrastromal INTRACOR pattern consisting of 5 central rings and 8 radial cuts in a prospective, nonrandomized, uncontrolled, open, single-center, clinical study. Refraction, visual acuity, endothelial cell density, corneal pachymetry, total corneal power, and stray light were evaluated preoperatively and 1 (except endothelial cell density and stray light), 3, 6, and 12 months postoperatively. Patients filled out a subjective questionnaire at 12 months postoperatively. Comparison of preoperative versus 12-month postoperative median values revealed a significant improvement in uncorrected near visual acuity (UNVA) from 0.60 (20/80) to 0.10 logMAR (20/25) (P<.0001) and a significant decrease in corrected distance visual acuity (CDVA) from -0.10 (20/16) to 0.00 logMAR (20/20), which equals a median loss of one line (P=.0005). Fifteen percent of patients lost two lines of CDVA in the treated eye. Subjective spherical equivalent refraction remained unchanged at 0.00 diopters (D) (P=.194). After INTRACOR treatment, significant corneal steepening of 1.40 D and midperipheral flattening of 0.50 D occurred (both P<.0001). Corneal pachymetry at the thinnest point and endothelial cell density did not change significantly (P=.829 and P=.058, respectively). After 12 months, the modified INTRACOR pattern improved UNVA in emmetropic patients without inducing a myopic shift or significant changes in endothelial cell density or pachymetry.

Intrastromal Corneal Ring Segment SK for Moderate to Severe Keratoconus: A Case Series

To report clinical results and safety profiles of symmetrical intrastromal corneal ring segment (ICRS; Intacs SK, Addition Technology Inc) implantation in eyes with moderate to severe keratoconus. Interventional case series of 24 eyes from 22 patients implanted with Intacs SK using the femtosecond laser. Evaluation included uncorrected (UDVA) and corrected distance visual acuity (CDVA), manifest refraction, and Placido-based Scheimpflug imaging. Postoperative follow-up was 1 day, 1 week, and 1, 3, and 6 months. Mean patient age was 28±7 years (range: 16 to 41 years). All eyes had keratoconus. Twenty-one of 24 eyes had 2, 450-μm symmetrical segments inserted. Mean UDVA and CDVA were significantly better at 1, 3, and 6 months postoperatively (UDVA: 20/340 preoperatively vs 20/70 at 6 months [P<.001]; CDVA: 20/70 preoperatively vs 20/40 at 6 months [P<.001]; UDVA analysis of variance [ANOVA]: F=6.46, P=.0057; CDVA ANOVA: F=5.24, P=.0138). Pre- and 6-months postoperative clinical results were: mean spherical equivalent refractive error -9.60±4.86 diopters (D) and -3.05±2.05 D (P<.001), respectively; mean refractive cylinder, -3.20±1.50 D and -1.55±1.10 D, respectively (P<.001); and mean average total corneal power 52.03±4.49 D and 46.95±4.61 D, respectively (P<.001). No complications occurred, and no patients were lost to follow-up. Eyes with steeper preoperative corneal keratometry readings had the greatest flattening in corneal curvature and thus the greatest decrease in myopic refraction (r=-0.4, P<.001). No correlation was found between preoperative thinnest corneal pachymetry and postoperative curvature flattening. Implantation of Intacs SK for moderate to severe keratoconus is safe and effective, leading to significant decrease in myopia and appreciable decrease in astigmatism with improvement in CDVA.

Dual versus single Scheimpflug camera for anterior segment analysis: Precision and agreement

To assess the repeatability, reproducibility, and agreement of the Pentacam HR single-camera and Galilei G2 dual-camera Scheimpflug devices in anterior segment analysis. Begitek Clínica Oftalmológica, San Sebastián, Spain. Prospective randomized observational study. Healthy young individuals had 3 consecutive tests by 2 examiners. Analyzed parameters were anterior and posterior cornea simulated keratometry (K), K flat, K steep, astigmatism magnitude and axis, J(0) and J(45) vectors, asphericity, total corneal higher-order wavefront aberrations (root mean square [RMS], coma, trefoil, spherical aberration), central cornea and thinnest-point thicknesses, and anterior chamber depth. Repeatability and reproducibility were evaluated by calculating the within-subject standard deviation (S(w)), some derived coefficients, and the intraclass correlation coefficient. Agreement was assessed with the Bland-Altman method. The single-camera device reproducibility (S(w)) was simulated K, 0.04 diopter (D); J(0), 0.03D; J(45), 0.04 D; total power, 0.04 D; spherical aberration, 0.02 μm; higher-order aberrations (HOAs), 0.02μm; central corneal thickness (CCT), 3.39 μm. The dual-camera device S(w) was simulated K, 0.07 D; J(0), 0.13 D; J(45), 0.04 D; total power, 0.08 D; spherical aberration, 0.02 μm; HOAs, 0.11 μm; CCT, 1.36 μm. Agreement was good for most parameters except total corneal power (mean difference 1.58 D ± 0.22 (SD) and HOA RMS (mean difference 0.48 ± 0.19 μm) (both P<.00). Repeatability and reproducibility were good for all parameters. The single-camera device was more precise for curvature, astigmatism, and corneal wavefront error measurements and the dual-camera device for pachymetry measurements. Agreement was good with some relevant exceptions. Dr. Aramberri is consultant to Costruzione Strumenti Oftalmici, Firenze, Italy. No other author has a financial or proprietary interest in any material or method mentioned.

Comparison of corneal astigmatism and axis location in cataract patients measured by total corneal power, automated keratometry, and simulated keratometry

To compare the corneal astigmatism (magnitude and axis location) derived by total corneal power (TCP), automated keratometry, and simulated keratometry. Siriraj Hospital, Mahidol University, Bangkok, Thailand. Prospective comparative study. Eyes with previous ocular surgery or abnormalities were excluded. All patients were examined with the ARK 730A autokeratometer and the Galilei analyzer. The steepest and flattest corneal power along with the steepest axis of the TCP, automated keratometry, and simulated keratometry were recorded. Vector analysis (J0 and J45) was calculated. Analysis of variance with Bonferroni correction was performed for multiple comparisons. Outcome measures were the magnitude and axis location of astigmatism. One hundred eyes of 100 cataract patients were randomly selected. There was no statistically significant difference in the mean steepest axis between TCP (93.31 ± 68.75 [SD]), automated keratometry (94.24 ± 64.78), and simulated keratometry (92.42 ± 64.30). However, the mean magnitude of astigmatism measured by TCP (1.23 ± 0.75) was significantly higher than that measured by automated keratometry (0.93 ± 0.68) (P=.01) but not than that measured by simulated keratometry (1.08 ± 0.68) (P=.43); there was no statistically significant difference in J0 or J45. Twenty two (40%) of 54 eyes with more than 1.00 diopter of TCP astigmatism had more than 10 degrees of axis difference from automated keratometry. The magnitude of TCP astigmatism was higher than that of automated keratometry. The axis location was similar. However, there was more than 10 degrees of axis difference between automated keratometry and TCP in patients with high astigmatism. No author has a financial or proprietary interest in any material or method mentioned.

Assessments of total corneal power and intraocular lens power in post-LASIK eyes

To compare the different evaluations of corneal power in post-laser-assisted in situ keratomileusis (LASIK) eyes and assess the accuracy and differences of intraocular lens (IOL) power prediction with corneal powers through different calculation methods. For this prospective study, 71 eyes of 39 candidates undergoing LASIK and 3 months later returning for reexamination were included. All subjects were divided into 2 groups according to the axial length: Group A (23.46 - 25.97 mm) 44 eyes, Group B (26.00 - 28.52 mm) 27 eyes. The ranges of spherical equivalents were -1.75 - -9.50 D with an average of (-4.82 ± 1.98) D. Subjective refraction and Pentacam Scheimpflug analyzer were performed at pre-operation and subjective refraction, IOLMaster and Galilei II dual Scheimpflug analyzer at over 3 months post-operation. The following values were obtained: SimK by Galilei II dual Scheimpflug analyzer, Kn calculated with Gaussion thick lens formula, Kray with ray tracing method, Kchm calculated with clinical history method and Km by IOLMaster. Repeated measures analysis of variance analyze the differences between SimK, Kn, Kray, Km and Kchm. Repeated measurement analysis of variance was employed to analyze the differences between SimK, Kn, Kray, Km combining intraocular lens power formula and Kchm combining intraocular lens power formula. Pearson's analysis was performed to analyze the correlations between SimK, Kn, Kray, Km and Kchm. Those combining with formulas for IOL power calculations were also analyzed. P < 0.05 indicated statistical significance. The values of SimK, Kn, Kray, Km and Kchm were (39.02 ± 1.8), (37.8 ± 1.51), (36.8 ± 1.84), (39.18 ± 1.75) and (38.55 ± 1.83) D respectively. The differences between SimK, Kn, Kray, Km and Kchm had statistical significances (P < 0.01). High correlations existed between SimK, Kn, Kray, Km and Kchm (r > 0.9, P < 0.01). In the same IOL formula, Kray had the highest IOL power value, Kn followed, Kchm the third, SimK and Km the smallest. Among the IOL formulas, Haigis-L and Haigis predicted the highest IOL power, HofferQ followed, Holladay the third and SRK/T the smallest. Kn/Kray combining with Haigis formula may be a new and effective method for predicting the IOL power in post-LASIK eyes.

Total Corneal Power Estimation: Ray Tracing Method versus Gaussian Optics Formula

To evaluate with the use of corneal topographic data the differences between total corneal power calculated using ray tracing (TCP) and the Gaussian formula (GEP) in normal eyes, eyes that previously underwent laser in situ keratomileusis/photorefractive keratectomy (LASIK/PRK), and theoretical models. TCP and GEP using mean instantaneous curvature were calculated over the central 4-mm zone in 94 normal eyes, 61 myopic-LASIK/PRK eyes, and 9 hyperopic-LASIK/PRK eyes. A corneal model was constructed to assess the incident angles at the posterior corneal surface for both refracted rays and parallel rays. Corneal models with varying parameters were also constructed to investigate the differences between mean TCP and GEP (4-mm zone), and an optical design software validation was performed. The TCP values tended to be less than GEP in normal and myopic-LASIK/PRK eyes, with the opposite relationship in some hyperopic-LASIK/PRK eyes having the highest anterior surface curvature. The difference between TCP and GEP was a function of anterior surface instantaneous radii of curvature and posterior/anterior ratio in postrefractive surgery eyes but not in normal eyes. In model corneas, posterior incident angles with parallel rays were greater than those with refracted rays, producing an overestimation of negative effective posterior corneal power; differences in magnitude between TCP and GEP increased with decreasing ratio of posterior/anterior radii of curvature, consistent with clinical results. In eyes after refractive surgery, calculating posterior corneal power using the Gaussian formula and its paraxial assumptions introduces errors in the calculation of total corneal power. This may generate errors in intraocular lens power calculation when using the Gaussian formula after refractive surgery.

Estimation of preoperative corneal power in patients with myopic LASIK based on postoperative corneal data measured from Orbscan II

Compare the change of pre- and post-operative posterior corneal curvature measurements in peripheral fitting zones using the Orbscan II topographer in patients undergoing myopic laser in situ keratomileusis (LASIK). To establish a new ratio of the anterior and posterior corneal curvature for the calculation of preoperative total corneal power using postoperative corneal data alone in patients after LASIK. Retrospectively analyzed the changes of posterior corneal curvature in peripheral 7 to 10 mm fitting zones in 151 right eyes (151 individuals) 3 month after LASIK as measured by Orbscan II. The new ratio between the anterior and posterior corneal curvature was measured. The new ratio was used to estimate pre-operative total corneal refractive power in 30 eyes. The estimated total corneal refractive power was compared with the value measured from Orbscan II preoperatively. The accuracy of intraocular lens power estimated in 2 cases underwent cataract surgery after corneal refractive surgery was studied. The mean difference in estimated pre- and post-operative power of the posterior cornea was (-0.005 ± 0.154) mm (t = 0.417, P = 0.677). The ratio between the preoperative anterior central (0 - 7 mm) corneal curvature and the posterior peripheral (7 to 10 mm) corneal curvature was 1.167 ± 0.030. Estimated mean pre-operative total corneal power was (43.49 ± 1.79) D, whereas the value measuring from Orbscan II preoperatively was (43.77 ± 1.53) D. The mean difference between these two measurements was (-0.28 ± 1.00) D (t = -1.523, P = 0.139). The spherical equivalent refraction after cataract extraction of the two cases undertaken cataract surgery after corneal refractive surgery was 0.54 D and 0.69 D, respectively. The difference in posterior corneal curvature measurement following myopic LASIK using the peripheral fitting zone with the Orbscan II, as compared to the preoperative values, is clinically insignificant. The pre-LASIK corneal power can be estimated using post-LASIK data together with the new calculated ratio. This will be useful in post-LASIK patients requiring cataract surgery but without the availability of pre-LASIK corneal data for the estimation of the intraocular lens power.

Repeatability of automatic measurements performed by a dual Scheimpflug analyzer in unoperated and post-refractive surgery eyes

To assess the repeatability of the anterior segment measurements performed with a dual Scheimpflug analyzer (Galilei) in unoperated and post-refractive surgery eyes. Private practice. Evaluation of diagnostic test. Three consecutive scans were acquired in unoperated eyes and in eyes that had excimer laser surgery for myopia. Unoperated eyes were enrolled from 3 subgroups: younger than 50 years, aged between 50 and 70 years, and older than 70 years. The following parameters were evaluated: simulated keratometry, posterior corneal power, total corneal power, anterior and posterior best-fit sphere radius, mean and thinnest central corneal thickness, anterior chamber depth and volume, horizontal and vertical corneal diameter, iridocorneal angle in the 4 quadrants, and corneal spherical aberration. Repeatability was assessed using analysis of variance (ANOVA), the coefficient of variation (COV), intraclass correlation (ICC), and test-retest variability. The study evaluated 45 unoperated eyes (n = 45) and 15 post-refractive surgery eyes (n = 15). Each age subgroup in the unoperated group comprised 15 eyes. The ANOVA did not detect significant differences between the 3 measurements for any parameter. The COV was less than 0.5% for corneal power measurements and less than 3.5% for all remaining parameters except spherical aberration (16.68%). The ICC was more than 0.99 for corneal power measurements and more than 0.94 for all remaining parameters. The anterior segment measurements provided by the dual Scheimpflug analyzer were highly repeatable. Repeatability did not change with age or after myopic photorefractive keratectomy or laser in situ keratomileusis.

Accuracy of a dual Scheimpflug analyzer and a corneal topography system for intraocular lens power calculation in unoperated eyes

To evaluate the accuracy of the corneal power measurements by a dual Scheimpflug analyzer (Galilei) and a corneal topography system (Keratron) for intraocular lens (IOL) power calculation in unoperated eyes. Private practice. Case series. The IOL power was calculated for eyes having phacoemulsification and in-the-bag IOL implantation by entering into the Hoffer Q, Holladay 1, and SRK/T formulas 3 corneal power measurements: (1) simulated keratometry (K) by corneal topography, (2) simulated K derived by anterior corneal curvature only using the dual Scheimpflug analyzer, (3) total corneal power derived by anterior and posterior corneal curvatures through ray tracing using the dual Scheimpflug analyzer. The prediction error was calculated as the difference between the predicted and the measured refraction 1 month postoperatively. Forty-three consecutive patients were enrolled. The mean arithmetic error was zero for all combinations of measurements due to constant optimization. Using the Hoffer Q formula, the mean absolute prediction errors were 0.23 diopters (D) ± 0.22 (SD) for the corneal topographer simulated K, 0.21 ± 0.18 D for the Scheimpflug simulated K, and 0.27 ±0.20 D for the Scheimpflug total corneal power (P>.05). No statistically significant different results (compared with the Hoffer Q formula) were obtained with the Holladay 1 and SRK/T formulas. Both devices provided corneal power measurements that led to accurate IOL power calculation. The total corneal power by the dual Scheimpflug system is the first corneal power measurement shown to lead to accurate IOL power calculation without using the standard keratometric index. Drs. Savini, Barboni, and Carbonelli have no financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.

Effects of Pterygium on Ocular Aberrations

To compare high-order aberration between the eyes with pterygium and normal fellow eyes. Ophthalmic Research Center and Department of Ophthalmology Labbafinejad Medical Center Shahid Beheshti University of Medical Sciences Tehran, Iran. This comparative cross-sectional study comprised 31 patients with unilateral pterygium. NIDEK OPD-Scan wavefront aberrometry was performed in all patients in both eyes. Then, aberometric parameters were compared between the eyes. Sixty-two eyes of 31 patients (23 men and 8 women) with a mean age of 42.5 ± 5.9 years were included in the study. Root mean square (RMS) of total higher-order aberration increased statistically significantly in the involved eyes compared with normal fellow eyes (1.85 ± 2.22 and 0.36 ± 0.44 μm; P = 0.001). All RMS of Zernike orders and all RMS of different aberrations were higher in the involved eyes group and were statistically significant except for spherical aberration. The most significant differences belonged to RMS of total trefoil (1.37 vs. 0.25 μm) and RMS of total coma (0.37 vs. 0.14 μm) (P = 0.0001). With decreasing the uninvolved optical zone (increasing the size of pterygium), RMS of all aberrations increased and the differences were statistically significant except for spherical aberration. Pterygium and its size have significant influence on high-order aberrations of the eye especially on measured total coma and total trefoil in compared with normal fellow eye.

Repeatability of corneal power and wavefront aberration measurements with a dual-Scheimpflug Placido corneal topographer

To evaluate the repeatability of the Galilei dual-Scheimpflug analyzer in measuring corneal curvature, wavefront aberrations, pachymetry, and anterior chamber depth (ACD). Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. Three consecutive measurements were performed in 1 eye of each subject. The following were evaluated: (1) mean total corneal power at the central, paracentral, and peripheral zones (0.0 to 4.0 mm, 4.0 to 7.0 mm, and 7.0 to 8.0 mm, respectively) and posterior corneal power (K(avg)); (2) corneal higher-order wavefront aberrations (6.0 mm pupil); (3) mean pachymetry at the central, paracentral, and peripheral zones; and (4) ACD. Repeatability was assessed by calculating the within-subject standard deviation (SD), coefficient of variation (COV), and intraclass correlation coefficient (ICC). The study enrolled 20 subjects. The SD was 0.09 diopter (D), 0.05 D, and 0.19 D for central, paracentral, and peripheral total corneal power, respectively, and 0.03 D for posterior K(avg). The COV ranged from 0.10% to 0.35%, and the ICC was 0.996 or more (P<.001). For 3rd-order coma and trefoil, the SD was 0.08 mum and 0.09 mum, respectively. For 4th-order spherical aberration, astigmatism, and tetrafoil, the SDs were lower (0.02 mum, 0.04 mum, and 0.09 mum, respectively). The SD was 1.68 mum, 1.98 mum, and 2.82 mum for central, paracentral, and peripheral pachymetry, respectively, and 0.04 mm for ACD. Dual-Scheimpflug measurements of corneal power, pachymetry, ACD, and corneal aberrations for Zernike terms in the middle of the Zernike tree showed excellent repeatability.

Effect of the shape of the endothelial graft on the refractive results after Descemet's stripping with automated endothelial keratoplasty

Objective: To investigate the magnitude and the possible causes of the refractive changes after Descemets stripping with automated endothelial keratoplasty (DSAEK). In this article we describe the changes in corneal power after DSAEK and correlate them with the shape, diameter, and thickness of the endothelial graft obtained. Study Design: Retrospective chart review. Participants: 17 consecutive patients who underwent DSAEK at the Rand Eye Institute in 2007. Methods: The preoperative and post-DSAEK corneal power was determined using Gaussian optics and correlated with the DSAEK graft diameter and central, paracentral (P1), and peripheral (P2) graft thickness using images obtained from the Pentacam Scheimpflug. Results: During DSAEK there is addition of corneal tissue in the posterior layers of the cornea, which reduces the radius of curvature of the posterior layer of the cornea and also the total corneal power. The mean anterioposterior corneal radius of curvature ratio after DSAEK is 72.3% (SD 4.63%). In consequence, the keratometry values calculated after DSAEK using the Gaussian optics method are less than those measured by manual keratometry after surgery. A strong correlation was observed between the postoperative Gaussian keratometric power and the ratio of the central corneal DSAEK graft thickness (C) to the mean P2 measured at 7 mm optical zone (C:P2) (r 2 = 0.63, p < 0.01 ). Conclusions: DSAEK induced a mean hyperopic shift of 1.05 (SD 0.76) D. The mean anterioposterior corneal radius of curvature ratio decreased to 72.3%. Despite a great variability, the ratio between the central DSAEK graft thickness and the graft thickness at the 7 mm optical zone (C:P2 ratio) was correlated with the magnitude of the change in corneal power and induced hyperopia, which produces a steeper posterior corneal surface. No correlation between the DSAEK graft diameter and change in corneal power was noted. Customization in the shape of the donor graft lenticle might reduce the hyperopic shift noted after DSAEK.

Introducing a New Ratio Useful in the Estimation of Preoperative Corneal Power in Patients With Myopic LASIK Based on Postoperative Corneal Data

The aim of this study was to establish a new constant ratio for the calculation of preoperative total corneal power using postoperative corneal data alone in patients after laser in situ keratomileusis (LASIK). Preoperative anterior and posterior corneal radii from 192 pre-LASIK eyes were analyzed to derive a constant ratio in the first part of the study. In the second part of the study, with a set of post-LASIK patients (98 eyes), this ratio was used to estimate preoperative total corneal power. This estimated total corneal power was compared with the true value obtained from direct preoperative measurement. The ratio (R(roc)) between the preoperative anterior central 10-mm corneal curvature and the posterior peripheral 7- to 10-mm corneal curvature was 1.166 +/- 0.049. Estimated mean preoperative total corneal power was 43.42 +/- 2.21 D, whereas the true preoperative mean keratometric value (SimK) was 43.40 +/- 2.64 D. The mean difference was 0.02 +/- 2.65 D (P = 0.942). The pre-LASIK corneal power can be estimated using post-LASIK data together with R(roc). This will be useful in post-LASIK patients requiring cataract surgery but without the availability of pre-LASIK corneal data for the estimation of the preoperative keratometric power when the double-K technique was used to calculate the intraocular lens power.

Repeatability, reproducibility, and agreement characteristics of rotating Scheimpflug photography and scanning-slit corneal topography for corneal power measurement

To evaluate the repeatability, reproducibility, and agreement in anterior, posterior, and in particular the total corneal power of 2 topography devices, rotating Scheimpflug photography and scanning-slit topography. Department of Ophthalmology and Visual Science, Kitasato University Graduate School of Medical Sciences, Sagamihara, Japan. Seventeen eyes of 17 subjects (mean age 24.7 years +/- 4.1 [SD]) were included in the study. The corneal shapes within the central 3.0 mm were measured with rotating Scheimpflug photography (Pentacam) and scanning-slit corneal topography (Orbscan II). The within-rater repeatability and reproducibility of 2 raters and the overall between-instrument agreement of the measurements were evaluated using intraclass correlation coefficients (ICCs) and the Bland-Altman method. The repeatability of Scheimpflug photography and scanning-slit corneal topography was high (ICC, 0.70 to 0.99). Scheimpflug photography outperformed scanning-slit corneal topography for anterior power, posterior power, and total corneal power. The reproducibility results were similar, with limits of agreement (LoA) consistently narrower for Scheimpflug photography. The between-instrument agreement was moderate, with LoA around the mean value of total corneal power of 0.46 diopter ranging from 0.032 to 0.889. The results suggest that repeatability and reproducibility are higher in Scheimpflug photography than in scanning-slit topography. The agreement between rotating Scheimpflug photography and scanning-slit topography for total corneal power was moderate.

Validity of the keratometric index: Evaluation by the Pentacam rotating Scheimpflug camera

To determine the keratometric index based on actual measurements of the anterior and posterior corneal surfaces using a rotating Scheimpflug camera (Pentacam, Oculus) and evaluate the accuracy of this keratometric index in estimating total and posterior corneal powers. Departments of Ophthalmology, Taipei Medical University Hospital and Taipei City Hospital, Taipei, Taiwan. The right eye of 221 subjects was measured with the Pentacam system. The radius of the best-fit sphere for the anterior corneal surface (rant) and posterior corneal surface (rpost), mean radius of simulated keratometry (rsimK), and central corneal thickness were obtained. The ratio of rant to rpost (AP ratio) and keratometric index were calculated in each eye. The means for rant, rpost, rsimK, and AP ratio were 7.75 mm +/- 0.28 (SD), 6.34 +/- 0.28 mm, 7.75 +/- 0.27 mm, and 1.223 +/- 0.034 mm, respectively. These parameters were normally distributed. The mean calculated keratometric index (Ncal) was 1.3281 +/- 0.0018. Using the keratometric indices of 1.3281 (Ncal), 1.3315 (Gullstrand schematic eye), and 1.3375 (conventional), the mean arithmetic and absolute estimation errors for the total corneal power were, 0.00 +/- 0.24 diopter (D) and 0.17 +/- 0.17 D, 0.43 +/- 0.23 D and 0.45 +/- 0.21 D, and 1.21 +/- 0.24 D and 1.21 +/- 0.24 D, respectively. The total corneal power was predicted to within +/-0.50 D of the actual value in 95.0%, 60.2%, and 0.9% of eyes, respectively. The mean arithmetic and absolute estimation errors for the posterior corneal power using an AP ratio of 1.223 (this study) or 1.132 (Gullstrand schematic eye) were 0.00 +/- 0.17 D and 0.13 +/- 0.12 D and 0.47 +/- 0.18 D and 0.47 +/- 0.17 D, respectively. The posterior corneal power was estimated to within +/-0.50 D of the actual value in 97.7% and 60.2% of eyes, respectively. Using the Pentacam-derived keratometric index improved the prediction accuracies of total and posterior corneal powers.

Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography

To measure total corneal power using optical coherence tomography (OCT). Refractive surgery practices at 2 academic eye centers in Cleveland, Ohio, and Los Angeles, California, USA. Thirty-two eyes of 17 patients having myopic laser in situ keratomileusis (LASIK) were enrolled in a prospective observational study. Manifest refraction, OCT, and Placido ring corneal topography with the Atlas 995 (Carl Zeiss Meditec, Inc.) were performed preoperatively and 3 months after laser in situ keratomileusis (LASIK). A high-speed (2000 axial scans/second) corneal and anterior segment OCT prototype was used. The total corneal power was calculated by summation of the anterior and posterior surface powers, and the value was compared with that determined by simulated keratometry. Two methods of measuring total corneal power were tested: the direct method, which used OCT to measure both corneal surfaces directly, and the hybrid method, which combined OCT with anterior corneal topography. The repeatability (pooled standard deviation) of measuring total corneal power using the hybrid method was 3 times better than that using the direct method. It was 0.23 diopter (D) before LASIK and 0.26 D after LASIK. Preoperative total power was 1.13 D (2.6%) lower than the simulated keratometry. Compared to the LASIK-induced change in spherical equivalent refraction, the change in total corneal power was equivalent, while the change in simulated keratometry power was significantly smaller (-18.8%) (P<.001). Keratometry using the traditional index of 1.3375 overestimated the total power in preoperative corneas and underestimated LASIK-induced refractive change. Measuring both corneal surfaces using a combination of OCT and Placido ring topography provided a better measure of total corneal power that closely tracked the refractive change in post-LASIK eyes.

O cálculo do poder das lentes intra-oculares e o Orbscan-II. Parte 1: O poder óptico da córnea normal

PURPOSE: To evaluate the dioptric power of the normal central cornea. METHODS: Retrospective analysis of combined slit-scanning and Placido-ring topography in 30 normal eyes. RESULTS: Keratometric-mean, anterior-mean, thickness-mean, and total-mean powers; keratometric-optical and total-optical, keratometric-axial and total-axial, and total-tangential powers; equivalent power and the addition of anterior and posterior mean powers, were similar (p>0.8361) in the four analyzed central areas with 0.04, 1.0, 3.0, and 5.0 mm diameter. The posterior-mean power decreased (p=0.0030) from -6.62 D to -6.35 D. The average thickness increased (p=0.0231) from 524 µm to 554 µm, representing 0.13 D of total-mean corneal power. Keratometric-optical (p 0.4907), and to the addition of anterior-mean and posterior-mean powers (p>0.3868). Anterior-mean power was about 7.5 greater than posterior-mean power. CONCLUSIONS: The real power of the cornea calculated using both surfaces, its thickness, and the physiologic refraction indices is smaller (less positive) than the power usually accepted and determined by the anterior corneal surface radius and the keratometric index of refraction (F<K). Anterior-mean/posterior-mean power ratio is not ten-to-one as conventionally assumed. Power of posterior surface and thickness vary according to the size of the analyzed area. However, power of anterior surface and total power of the cornea remain the same. Components of total corneal power and pachymetry seem to be independent of each other.

Corneal power after refractive surgery for myopia: contact lens method

Purpose To clarify the theoretical background of the rigid contact lens overrefraction (CLO) method to determine corneal power after corneal refractive surgery. Setting University Eye Clinic, University of Würzburg, Würzburg, Germany. Methods Using paraxial geometrical optics, the measurement situation for the contact lens method was analyzed and the definitions of corneal refractive power were reviewed. Based on the theoretical Gullstrand eye, model eyes were constructed, representing 1 emmetropic and 2 myopic eyes (primary refraction –5.21 diopters [D] and –10.25 D, respectively) before and after photorefractive keratectomy and laser in situ keratomileusis. In these eyes, the application of the CLO was mathematically simulated using Gaussian thick-lens optics and commercial ray-tracing software. Results The CLO method measured neither the equivalent (total) power nor the vertex (back) power of the cornea but rather the quantity 336/ R 1C ( R 1C = anterior corneal radius). Based on these results and the Gullstrand eye, new formulas are proposed to derive the equivalent power and vertex power of the cornea by the CLO method. Conclusions Depending on whether intraocular lens calculation formulas are based on equivalent (total) corneal power or vertex corneal power, the respective new formulas for the CLO method should be applied in patients after corneal refractive surgery. An increase in prediction accuracy of the refractive outcome is expected.