The shape of the anterior and posterior surface of the aging human cornea

Ocular and optical coherence tomography-based corneal aberrometry in keratoconic eyes treated by intracorneal ring segments

PURPOSE

To analyze corneal and total aberrations using custom-developed anterior segment spectral optical coherence tomography (OCT) and laser ray tracing in keratoconic eyes implanted with intracorneal ring segments (ICRS).

DESIGN

Evaluation of technology. Prospective study. Case series.

METHODS

Nineteen keratoconic eyes were measured before and after ICRS surgery. Anterior and posterior corneal topographic and pachymetric maps were obtained pre- and postoperatively from 3-dimensional OCT images of the anterior segment, following automatic image analysis and distortion correction. The pupil center coordinates were used as reference for estimation of corneal aberrations. Corneal aberrations were estimated by computational ray tracing on the anterior and posterior corneal surfaces. Total aberrations were measured using a custom-developed laser ray tracing aberrometer. Corneal and total aberrations were compared in 8 eyes pre- and postoperatively for 4-mm pupils.

RESULTS

Total and corneal aberrations were highly correlated. Average root mean square of corneal and total high-order aberrations (HOAs) were 0.78 ± 0.35 μm and 0.57 ± 0.39 μm preoperatively, and 0.88 ± 0.36 μm and 0.53 ± 0.24 μm postoperatively (4-mm pupils). The anterior corneal surface aberrations were partially compensated by the posterior corneal surface aberrations (by 8.3% preoperatively and 4.1% postoperatively). Astigmatism was 2.03 ± 1.11 μm preoperatively and 1.60 ± 0.94 μm postoperatively. The dominant HOA aberrations both pre- and postoperatively were vertical coma (Z3(-1)), vertical trefoil (Z3(-3)), and secondary astigmatism (Z4(4)). ICRS decreased corneal astigmatism by 27% and corneal coma by 5%, but on average, the overall amount of HOA did not decrease significantly with ICRS treatment.

CONCLUSIONS

OCT is a reproducible technique to evaluate corneal aberrations. OCT-based corneal aberrations and ocular aberrations show a high correspondence in keratoconic patients before and after ICRS implantation. ICRS produced a decrease in astigmatism, but on average did not produce a consistent decrease of HOAs.

Numerical study on human cornea and modified multiparametric correction equation for Goldmann applanation tonometer

Glaucoma, a chronic eye disease, is quoted to be a second leading cause of blindness with 66.8 million people suspected to be victimised by the same in year 2000 (Morrison, 2003), and is caused due to an elevated intraocular pressure (IOP). Goldmann Applanation tonometer (GAT), a gold standard (Organization, 2001), suffers from inefficacies to measure intra-ocular pressure (IOP) independent of the biomechanical properties of an eye. Thus, the purpose of this paper is to study the importance of wide range of corneal parameters and to propose a multiparametric correction equation for GAT based on a numerically simulated comprehensive finite element model (Dohadwala et al., 1998; Morrison, 2003; Organization, 2001). Towards this end, with the help of numerical model, the parameters considered in correction equation proposed elsewhere (Elsheikh et al., 2011) were extended to include combined influence of central corneal thickness (CCT), central anterior curvature (R), peripheral corneal thickness (PCT), corneal asphericity (Pa), age, measured IOP itself and, was separately studied for role played by corneal support orientation (θ°) in this aspect. Findings indicate that for a wide range of parameters considered, while increased age is related with gain in corneal stiffness, the maximum (average) individual effects of variations in PCT, Pa, CCT, age and θ° on IOPG were estimated at 0.25mmHg/100µ of PCT, 0.073mmHg/0.1 of Pa, 1.9mmHg/100µ of CCT, 24.3%/decade relative of IOPT and 0.95mmHg/5° increase in θ°. The multiparametric correction equation has been modified accordingly. The GAT correction equation can consider the combined effect of PCT, Pa, CCT, and age on IOPG. Separately the non-linear effect of θ° on IOPG cannot be ignored for reasons of precision.

Correlation of the corneal toricity between anterior and posterior corneal surfaces in the normal human eye

PURPOSE

To evaluate the correlation of the magnitude of corneal toricity and power vector components of both corneal surfaces measured with a Scheimpflug photography-based system.

METHODS

A total of 117 healthy normal eyes of 117 subjects selected randomly with ages ranging from 7 to 80 years were included. All eyes received an anterior segment and corneal analysis with the Sirius system (CSO) evaluating the anterior and posterior mean toricity for 3 and 7 mm (aAST and pAST). The vector components J0 and J45 as well as the overall strength blur (B) were calculated for each keratometric measurement using the procedure defined by Thibos and Horner.

RESULTS

The coefficient of correlation between aAST and pAST was 0.52 and 0.62 and the mean anteroposterior ratio for toricity was 0.46 ± 0.39 and 0.57 ± 0.75 for 3 and 7 mm, respectively. These ratios correlated significantly with aAST, anterior corneal J0, and manifest refraction J0 (r ≥ 0.39, P < 0.01). The coefficient of correlation was 0.69 and 0.81 between anterior and posterior J0 for 3 and 7 mm, respectively. For J45, the coefficients were 0.62 and 0.71, respectively. The linear regression analysis revealed that the pAST and power vectors could be predicted from the anterior corneal data (R2 ≥ 0.40, P < 0.01).

CONCLUSIONS

The toricity and astigmatic power vector components of the posterior corneal surface in the human healthy eye are related to those of the anterior and therefore can be predicted consistently from the anterior toricity and astigmatic power vectors.

Postkeratoplasty Anterior and Posterior Corneal Surface Wavefront Analysis: Descemet's Stripping Automated Endothelial Keratoplasty versus Penetrating Keratoplasty

Purpose. To compare the higher-order aberrations (HOAs) due to the anterior and posterior corneal surfaces in patients that underwent either Descemet-stripping-automated-endothelial-keratoplasty (DSAEK) or penetrating keratoplasty (PK) for endothelial dysfunction and age-matched controls. Methods. This retrospective, observational, case series included 28 patients after PK, 30 patients after DSAEK, and 30 healthy controls. A Scheimpflug imaging system was used to assess the HOAs due to the anterior and posterior corneal surfaces at 4 mm and 6 mm optical zones. Total, 3rd and 4th order HOAs were considered. Intra- and intergroup differences were assessed using the Friedman and the Kruskal-Wallis tests, respectively; paired comparisons were performed using Duncan's multiple range test. Results. Total, 3rd and 4th order HOAs due to both corneal surfaces at 4 mm and 6 mm optical zones were significantly higher in the PK group, intermediate in the DSAEK group, and lower in controls (P < 0.05). The most important HOAs components in both PK and DSAEK groups were trefoil and coma from the anterior corneal surface (P < 0.05) and trefoil from the posterior corneal surface (P < 0.05). Conclusions. The optical quality of both corneal surfaces appeared significantly higher after DSAEK than after PK, which can increase the postoperative patient's quality of vision and satisfaction.

A ray tracing approach to calculate toric intraocular lenses

PURPOSE

To quantify the precision of astigmatic correction in routine cataract surgery with toric intraocular lenses (IOLs) and to evaluate the predictability of keratometric and anterior/posterior topographic measurement for the improvement of the overall accuracy.

METHODS

Seventy-eight eyes of 56 patients were implanted with toric IOLs. Data acquired by the Lenstar optical biometer (Haag-Streit, Bern, Switzerland) and TMS5 topography (Tomey, Nagoya, Japan) were processed with the ray tracing software Okulix (Tedics, Dortmund, Germany) to predict the residual refraction. Four different inputs were examined: keratometry only, anterior topography, anterior and posterior topography/ tomography, and combination of keratometry only and anterior and posterior topography/tomography. Four weeks postoperatively, the spherical prediction error and the cylindrical prediction error (difference vector between predicted and achieved cylindrical refraction) were determined.

RESULTS

Mean absolute error of spherical prediction error was 0.27 diopter (D). Cylindrical prediction errors were 0.57 D (keratometry only), 0.56 D (anterior topography), 0.56 D (anterior and posterior topography/ tomography), and 0.50 D (combination of keratometry only and anterior and posterior topography/tomography). Differences between intraocular lens groups were statistically significant (Friedman test, P < .05).

CONCLUSION

The combination of keratometry and anterior and posterior topography/tomography of anterior and posterior surface yielded the best results for toric IOL power calculations.

Minimizing the IOL power error induced by keratometric power

PURPOSE

To evaluate theoretically in normal eyes the influence on IOL power (PIOL) calculation of the use of a keratometric index (nk) and to analyze and validate preliminarily the use of an adjusted keratometric index (nkadj) in the IOL power calculation (PIOLadj).

METHODS

A model of variable keratometric index (nkadj) for corneal power calculation (Pc) was used for IOL power calculation (named PIOLadj). Theoretical differences (ΔPIOL) between the new proposed formula (PIOLadj) and which is obtained through Gaussian optics ((Equation is included in full-text article.)) were determined using Gullstrand and Le Grand eye models. The proposed new formula for IOL power calculation (PIOLadj) was prevalidated clinically in 81 eyes of 81 candidates for corneal refractive surgery and compared with Haigis, HofferQ, Holladay, and SRK/T formulas.

RESULTS

A theoretical PIOL underestimation greater than 0.5 diopters was present in most of the cases when nk = 1.3375 was used. If nkadj was used for Pc calculation, a maximal calculated error in ΔPIOL of ±0.5 diopters at corneal vertex in most cases was observed independently from the eye model, r1c, and the desired postoperative refraction. The use of nkadj in IOL power calculation (PIOLadj) could be valid with effective lens position optimization nondependent of the corneal power.

CONCLUSIONS

The use of a single value of nk for Pc calculation can lead to significant errors in PIOL calculation that may explain some IOL power overestimations with conventional formulas. These inaccuracies can be minimized by using the new PIOLadj based on the algorithm of nkadj.

Predicting ocular residual astigmatism using corneal and refractive parameters: a myopic eye study

PURPOSE

To investigate the nature of ocular residual astigmatism (ORA) in myopic patients and to identify indicators to predict a patient's ORA from corneal and ocular refractive data.

METHODS

In total, 356 myopic eyes from 178 patients (M:F = 41:137) were examined using a Scheimpflug keratometer (Pentacam) and a wavefront analyzer (WaveScan). Vertex distance-adjusted total ocular astigmatism and corneal astigmatism [anterior corneal power (ACP) and true net power (TNP)] were used to calculate ORA, respectively. A power vector system (J0 and J45) was adopted for the astigmatism analysis.

RESULTS

The mean age of the study eyes was 27.6 ± 5.0 years, and mean refractive spherical equivalent, sphere and cylinder was -4.94 ± 1.90 diopter, -4.42 ± 1.87 diopter, and -1.05 ± 0.82, respectively. Corneal astigmatism measured by TNP was more representative of total ocular astigmatism than ACP (p < 0.001). Both the J0 and J45 components of ORA showed significant negative correlation with corneal astigmatism measured by ACP and TNP (p < 0.001). ORA (J0 and J45) was negatively correlated with total ocular spherical equivalent and positively correlated with total ocular astigmatism.

CONCLUSIONS

We verified the general compensatory effect of corneal astigmatism by internal optics in myopic patients. TNP was closer to refractive astigmatism than ACP was. Total ocular spherical equivalent and total ocular astigmatism showed significant correlation with ORA. Therefore these can be possible indicators to predict ORA.

Modelling the impact of spherical aberration on accommodation

PURPOSE

To understand how primary and secondary spherical aberrations affect focusing of the retinal image and the measurement of refractive state in the accommodating eye.

METHODS

A computational eye model was constructed from published anatomical dimensions of the eye's refractive elements for a range of accommodative states. Two strategies for controlling accommodation were implemented, one in which paraxial rays are always perfectly focused and the other in which paraxial accommodative lag grew larger as target vergence increased. Multiple configurations of the model were achieved by selecting various combinations of pupil size and aberration structure. Refractive state was defined as optimum target vergence for maximizing retinal image quality according to several scalar metrics.

RESULTS

When accommodation optimally focuses paraxial rays, retinal image quality is sub-optimal for metrics of image quality sensitive to non-paraxial rays. This loss of image quality can be recovered by optimizing target vergence computationally, which indicates the presence of real accommodative error according to the non-paraxial metric even though the eye is accurately focused paraxially. However, such errors are spurious if non-paraxial refractive state is misinterpreted as paraxial refractive state. Accommodative errors may indicate lag or lead, but in general the slope of the stimulus-response function is less than 1 for non-paraxial measures of image quality. These results depend strongly on pupil size and its variation due to accommodative miosis.

CONCLUSIONS

spurious accommodative errors can appear when the eye focuses the retinal image optimally according to one metric of image quality (e.g. paraxial) while ocular refractive state is measured by another (e.g. non-paraxial). Spurious errors are small compared to real accommodative lag for small, photopic pupils but can be of the same order of magnitude as real lag for large, mesopic pupils.

Analysis of age-dependence of the anterior and posterior cornea with scheimpflug imaging

PURPOSE

To assess keratometric and higher-order aberrations of the anterior and posterior cornea and their age-related changes.

METHODS

This study investigated one healthy eye of 227 patients (mean age: 55.15 ± 21.2 years; range: 16 to 90 years; 135 right eyes, 92 left eyes). Images were captured from each eye with Pentacam HR (Oculus Optikgeräte GmbH, Wetzlar, Germany) using automatic mode. Keratometric, astigmatism data, and corneal higher-order aberrations were analyzed.

RESULTS

With respect to laterality, no deviance was found in any of the parameters (P > .05). Mean refractive error was 0.52 ± 0.23 diopters. The level of astigmatism decreased significantly with advancing age for both the anterior and posterior corneal surfaces (P < .05). The overall root mean square of the higher-order aberration increased continuously with age (r = 0.517; P < .01), which can be explained by the combined effect of the increased in both the anterior and posterior corneal root mean square higher-order aberrations. Of the higher-order aberrations, the constant increase of the primary and secondary spherical aberration with aging (P < .01) is caused by the spherical aberration growth of the anterior surface. Apart from these, only the vertical coma aberration of the posterior surface and the vertical trefoil aberrations of both the anterior and posterior surfaces showed a significantly positive correlation with aging (P < .05).

CONCLUSIONS

Corneal astigmatism showed a significant decrease with aging. Of the higher-order aberrations, primary and secondary spherical aberrations, vertical coma, and vertical trefoil significantly increase with age, whereas other higher-order aberrations show no correlation with aging.

Full OCT anterior segment biometry: an application in cataract surgery

In vivo three-dimensional (3-D) anterior segment biometry before and after cataract surgery was analyzed by using custom high-resolution high-speed anterior segment spectral domain Optical Coherence Tomography (OCT). The system was provided with custom algorithms for denoising, segmentation, full distortion correction (fan and optical) and merging of the anterior segment volumes (cornea, iris, and crystalline lens or IOL), to provide fully quantitative data of the anterior segment of the eye. The method was tested on an in vitro artificial eye with known surfaces geometry at different orientations and demonstrated on an aging cataract patient in vivo. Biometric parameters CCT, ACD/ILP, CLT/ILT Tilt and decentration are retrieved with a very high degree of accuracy. IOL was placed 400 μm behind the natural crystalline lens, The IOL was aligned with a similar orientation of the natural lens (2.47 deg superiorly), but slightly lower amounts (0.77 deg superiorly). The IOL was decentered superiorly (0.39 mm) and nasally (0.26 mm).

Refractive error assessment: influence of different optical elements and current limits of biometric techniques

PURPOSE

To identify and quantify sources of error on refractive assessment using exact ray tracing.

METHODS

The Liou-Brennan eye model was used as a starting point and its parameters were varied individually within a physiological range. The contribution of each parameter to refractive error was assessed using linear regression curve fits and Gaussian error propagation analysis. A MonteCarlo analysis quantified the limits of refractive assessment given by current biometric measurements.

RESULTS

Vitreous and aqueous refractive indices are the elements that influence refractive error the most, with a 1% change of each parameter contributing to a refractive error variation of +1.60 and -1.30 diopters (D), respectively. In the phakic eye, axial length measurements taken by ultrasound (vitreous chamber depth, lens thickness, and anterior chamber depth [ACD]) were the most sensitive to biometric errors, with a contribution to the refractive error of 62.7%, 14.2%, and 10.7%, respectively. In the pseudophakic eye, vitreous chamber depth showed the highest contribution at 53.7%, followed by postoperative ACD at 35.7%. When optic measurements were considered, postoperative ACD was the most important contributor, followed by anterior corneal surface and its asphericity. A MonteCarlo simulation showed that current limits of refractive assessment are 0.26 and 0.28 D for the phakic and pseudophakic eye, respectively.

CONCLUSIONS

The most relevant optical elements either do not have available measurement instruments or the existing instruments still need to improve their accuracy. Ray tracing can be used as an optical assessment technique, and may be the correct path for future personalized refractive assessment.

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.

Effect of corneal aberrations on intraocular lens power calculations

PURPOSE

To use ray tracing to determine the influence of corneal aberrations on the prediction of the optimum intraocular lens (IOL) power for implantation in normal eyes and eyes with previous laser in situ keratomileusis (LASIK).

SETTING

Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.

DESIGN

Case series.

METHODS

The optimum IOL power was calculated by ray tracing using a patient-customized eye model in cataract surgery cases. The calculation can be performed with or without inclusion of the patient's corneal aberrations. Standard predictions were also generated using current state-of-the-art IOL power calculation techniques. The results for all predictions were compared with the optimum IOL power after cataract surgery.

RESULTS

For patients without previous LASIK (n = 18), the standard approaches and the ray-tracing procedure gave a similar mean absolute residual error and variance. The incorporation of corneal aberrations did not improve the accuracy of the ray-tracing prediction in these cases. For post-LASIK patients (n = 10), the ray-tracing prediction incorporating corneal aberrations generated the most accurate results. The difference between the prediction with and without considering corneal aberrations correlated with the amount of corneal spherical aberration (r(2) = 0.82), resulting in a difference of up to 3.00 diopters in IOL power in some cases.

CONCLUSIONS

Ray tracing using patient-customized eye models was a robust procedure for IOL power calculation. The incorporation of corneal aberrations is crucial in post-LASIK eyes, primarily because of the elevated corneal spherical aberration.

FINANCIAL DISCLOSURE

Mrs. Canovas and Dr. Artal hold a provisional patent application on the ray-tracing procedure. Mrs. Canovas is an employee of Abbott Medical Optics Groningen B.V. No other author has a financial or proprietary interest in any material or method mentioned.

Scheimpflug photography-based clinical characterization of the correlation of the corneal shape between the anterior and posterior corneal surfaces in the normal human eye

PURPOSE

To evaluate the correlation of the mean curvature and shape factors between both corneal surfaces for different corneal diameters measured with a Scheimpflug photography-based system.

SETTING

Vissum Corporation, Alicante, Spain.

DESIGN

Case series.

METHODS

Randomly selected healthy normal eyes had a comprehensive ophthalmologic examination including anterior segment analysis with the Sirius system as follows: anterior and posterior mean corneal radius for 3.0 mm, 5.0 mm, and 7.0 mm; anterior and posterior mean shape factor for 4.5 mm and 8.0 mm; central (CCT) and minimum corneal thickness; and anterior chamber depth (ACD).

RESULTS

The study enrolled 117 eyes (117 subjects; aged 7 to 80 years). The mean anterior mean corneal radius:posterior mean corneal radius ratio was 1.19 (range 1.12 to 1.27) for all corneal diameters (P = .86). The correlation coefficient between the anterior and posterior mean corneal radius was 0.85 or more for all corneal diameters. The anterior mean shape factor:posterior mean shape factor ratio for the 2 corneal diameters analyzed was approximately 1 (range 0.45 to 4.03). The correlations between anterior and posterior mean shape factors were extremely poor and not significant. Multiple regression analysis showed that the central posterior mean corneal radius was significantly correlated with the anterior mean corneal radius, CCT, and spherical equivalent (R(2) = 0.77, P<.01).

CONCLUSIONS

Central posterior corneal curvature could be predicted from the anterior corneal curvature, pachymetry, and the refractive status of the eye but not from the corneal shape factor.

FINANCIAL DISCLOSURE

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

Repeatability of the Sirius imaging system and agreement with the Pentacam HR

PURPOSE

To assess measurement repeatability of corneal curvature, minimal corneal thickness, and anterior chamber depth obtained with the Sirius imaging system (Costruzioni Strumenti Oftalmici) and to assess its agreement with the Pentacam HR imaging system (Oculus Optikgeräte GmbH).

METHODS

Healthy individuals were prospectively recruited. To assess repeatability, eight consecutive measurements were performed in the right eye of healthy individuals with the Sirius. A single measurement was then performed consecutively with both systems. The anterior and posterior corneal radii (antR and posR, respectively), anterior chamber depth, and minimal corneal thickness were evaluated. Repeatability of Sirius was evaluated by calculating coefficients of variation (CoV). Agreement between Sirius and Pentacam was assessed by calculating 95% limits of agreement (LoA) and plotting Bland-Altman graphs.

RESULTS

Forty-five eyes from individuals (21 men, 24 women) aged 20 to 61 years were evaluated. The mean CoV was 0.37% and 1.32% for antR and posR at 3 mm, respectively, and 0.36% and 1.28% for antR and posR at 7 mm, respectively. For anterior chamber depth and minimal corneal thickness, the CoV was 0.56% and 1.69%, respectively. Calculated 95% LoA were -0.1 to 0.12 mm (mean difference: 0.018 mm) and -0.54 to 0.33 mm (mean difference: 0.1 mm) for antR and posR at 3 mm, respectively. For anterior chamber depth, 95% LoA was -0.23 to 0.09 mm (mean difference: 0.068 mm) and -9.61 to 33.44 μm (mean difference: 11.91 μm) for minimal corneal thickness.

CONCLUSIONS

The Sirius showed good to excellent repeatability for all measured parameters. Agreement analysis suggests that Sirius and Pentacam should not be used interchangeably.

Role of the human lens gradient-index profile in the compensation of third-order ocular aberrations

The open question regarding the compensation of the ocular aberrations between the cornea and the lens is currently being investigated. We report additional insights considering the role of the lens gradient-index (GRIN) profile in third-order ocular aberrations, since this profile changes through life. Thus, we have calculated the contribution of that profile to the ocular aberrations with aging by applying the Seidel third-order theory of tilted and decentered elements, and by using a schematic-eye model. The results show the GRIN profile is needed to account for the decoupling of the aberrations between the cornea and the lens because the geometrical changes of the ocular surfaces with aging are not enough. Therefore, the current developments of aging human-eye models, as well as the experimental studies, cannot neglect the changes of the lens GRIN structure through life when modelling mechanisms of the compensation of ocular aberrations.

Comparison of retinal image quality with spherical and customized aspheric intraocular lenses

We hypothesize that an intraocular lens (IOL) with higher-order aspheric surfaces customized for an individual eye provides improved retinal image quality, despite the misalignments that accompany cataract surgery. To test this hypothesis, ray-tracing eye models were used to investigate 10 designs of mono-focal single lens IOLs with rotationally symmetric spherical, aspheric, and customized surfaces. Retinal image quality of pseudo-phakic eyes using these IOLs together with individual variations in ocular and IOL parameters, are evaluated using a Monte Carlo analysis. We conclude that customized lenses should give improved retinal image quality despite the random errors resulting from IOL insertion.

Scheimpflug camera measurement of anterior and posterior corneal curvature in eyes with previous radial keratotomy

PURPOSE

To compare the anterior and posterior corneal curvature in eyes with previous radial keratotomy (RK) to normal unoperated eyes.

METHODS

In this retrospective observational case series, 29 eyes from 29 consecutive patients were analyzed and compared to a control group of 71 unoperated eyes. Corneal imaging was obtained by a rotating Scheimpflug camera (Pentacam, Oculus Optikgeräte GmbH). Anterior and posterior corneal curvature radii were measured at the 3-mm zone.

RESULTS

The mean anterior and posterior corneal radii were 9.54 ± 0.89 and 8.54 ± 1.01 mm, respectively, both values being significantly higher than in the control group (7.81 ± 0.28 and 6.40 ± 0.24 mm, respectively, P<.0001). The mean anterior-to-posterior corneal curvature ratio was 1.12 ± 0.07, a value significantly lower than in the control group (1.22 ± 0.03, P<.0001). Mean corneal flattening was more evident in the posterior (33.44%) than in the anterior (22.15%) corneal curvature. The mean keratometric index, as calculated with the Gullstrand equation for thick lenses, was 1.3319 ± 0.0026, a value significantly higher than in the control group (1.3281 ± 0.0011, P<.0001). Linear regression detected a significant and directly proportional relationship between the number of radial incisions and flattening of both corneal surfaces (P<.0001).

CONCLUSIONS

After RK, both corneal surfaces flatten but do not deform in parallel as commonly accepted, as shown by the fact that the anterior-to-posterior corneal curvature ratio decreases. This finding invalidates the standard keratometric index and thus has relevant implications for intraocular lens power calculation in RK eyes.

Anterior segment dimensions in Asian and Caucasian eyes measured by optical coherence tomography

BACKGROUND AND OBJECTIVE

To compare Asian and Caucasian anterior segment dimensions measured by optical coherence tomography (OCT).

PATIENTS AND METHODS

Anterior segment OCT images were obtained in normal subjects. Four line scans were acquired at the 90°, 45°, 0°, and 135° meridians of each eye. Computer calipers acquired anterior segment dimensions of corneal diameter, anterior chamber width, corneal vault, and anterior chamber depth on OCT images. Univariate and multivariate analyses were performed to assess correlations.

RESULTS

Corneal diameter was 0.5 mm narrower (P < .01), anterior chamber width was 0.46 mm narrower (P < .01), and corneal vault was 0.22 mm lower (P < .01) in Asian eyes. All anterior segment dimensions decreased with age.

CONCLUSION

Asian eyes had smaller anterior segments compared to Caucasian eyes. Regardless of race, anterior segment dimensions were smaller in older subjects. Age-related changes may affect the tolerability of long-term implants such as phakic intraocular lenses.

Characteristic higher-order aberrations of the anterior and posterior corneal surfaces in 3 corneal transplantation techniques

PURPOSE

To investigate the corneal higher-order aberrations (HOAs) of the anterior and posterior corneal surfaces in eyes that underwent penetrating keratoplasty (PK), deep anterior lamellar keratoplasty (DALK), and Descemet stripping automated endothelial keratoplasty (DSAEK).

DESIGN

Retrospective, case-control study.

METHODS

study population: Twenty-four eyes underwent PK, 28 eyes underwent DALK, and 19 eyes underwent DSAEK; 29 normal eyes served as controls. observation procedures: The anterior and posterior corneal heights and pachymetric data were obtained with a Scheimpflug-based corneal topographer. Corneal HOAs for 4-mm pupils were calculated from the height data and were expanded with normalized Zernike polynomials. The HOAs resulting from the anterior and posterior corneal surfaces were compared among the procedures. main outcome measures: Anterior and posterior corneal HOAs (root mean square).

RESULTS

Control eyes had significantly lower total HOAs and Zernike vector terms of the anterior and posterior surfaces than the other groups, except for spherical aberration. The mean anterior corneal surface total HOAs in the PK, DALK, DSAEK, and control groups were 1.38 ± 0.67 μm, 1.19 ± 0.57 μm, 0.61 ± 0.33 μm, and 0.21 ± 0.07 μm, respectively. The anterior corneal HOAs in the DSAEK group were significantly less than those in the PK group (P < .001) and DALK group (P < .001). The mean posterior corneal surface total HOAs were, respectively, 0.20 ± 0.09 μm, 0.24 ± 0.11 μm, 0.27 ± 0.15 μm, and 0.07 ± 0.02 μm. There were no significant differences in the posterior corneal HOAs among the treatment groups.

CONCLUSIONS

Because the refractive indices between the anterior and the posterior surfaces differed greatly, eyes that undergo DSAEK have lower anterior corneal HOAs compared with PK or DALK eyes. However, the anterior and posterior corneal HOAs in DSAEK eyes still were greater than those in control eyes.

Comparison of anterior segment measurements by 3 Scheimpflug tomographers and 1 Placido corneal topographer

PURPOSE

To compare the anterior segment measurements provided by 3 Scheimpflug tomographers and a Placido corneal topographer.

SETTING

Private clinical ophthalmology practice.

DESIGN

Evaluation of diagnostic test or technology.

METHODS

In a sample of 25 consecutive patients having either refractive or cataract surgery, the anterior eye segment was analyzed by means of a rotating Scheimpflug camera (Pentacam), 2 devices with a Scheimpflug camera combined with a Placido disk (Sirius and TMS-5), and a Placido disk corneal topographer (Keratron). Measurement results were compared using analysis of variance. Agreement was assessed using Bland-Altman plots.

RESULTS

The mean simulated keratometry (K) was different between the 4 instruments (P<.0001), with Keratron providing the highest value (44.43 diopters [D] ± 1.28 [SD]). The Pentacam and Sirius provided the lowest values (44.05 ± 1.21 D and 44.05 ± 1.27 D, respectively), without statistical difference (posttest). The mean posterior corneal power and minimum corneal thickness were statistically different between the 3 Scheimpflug cameras (P<.0001 and P=.0210, respectively); 95% limits of agreement, however, were narrow for posterior corneal power and large for corneal thickness. The only 2 devices measuring the distance between the corneal endothelium and the anterior lens surface showed a statistically but not clinically significant difference (2.90 ± 0.48 mm and 2.94 ± 0.47 mm, respectively). There were no statistically significant differences in anterior corneal asphericity between the 4 instruments.

CONCLUSION

Although the measurements of some parameters by different instruments were similar, caution is warranted before using them interchangeably.

Estimation accuracy of surgically induced astigmatism on the cornea when neglecting the posterior corneal surface measurement

PURPOSE

To evaluate the accuracy of corneal surgically induced astigmatism (SIA) estimation when neglecting the posterior corneal surface measurement.

METHODS

Fifty right eyes undergoing phacoemulsification were measured with a rotating Scheimpflug camera (Pentacam; Oculus Inc., Wetzlar, Germany) both before and after surgery. Clear corneal incisions with one suture were used in the phacoemulsification surgery. The keratometric corneal SIA (KSIA) was derived using the anterior corneal surface measurement and the keratometric index (1.3375) while neglecting the posterior corneal surface measurement. The Pentacam-derived total corneal SIA (PSIA) was derived by vergence tracing and polar value analysis [KP(135) and KP(180)] of the measurements on both corneal surfaces.

RESULTS

The mean arithmetic estimation errors of the KSIA for the PSIA were 0.16 ± 0.32 (-0.52 to 1.14) D for the KP(135), and -0.02 ± 0.30 (-0.75 to 1.29) D for the KP(180). There was a significant difference between the KP(135) components of the KSIA and PSIA. Bivariate analysis revealed a statistically significant difference between the combined means of the KSIA and PSIA. Overall, 24% had either a KP(135) component of the KSIA that differed by > 0.50 D from that of the PSIA or a KP(180) component of the KSIA that differed by > 0.50 D from that of the PSIA. The blurring strength caused by neglecting the posterior corneal measurement was > 0.50 D in 24% of eyes.

CONCLUSION

Neglecting the posterior corneal surface measurement may lead to significant deviation in the corneal SIA estimation after phacoemulsification in a proportion of eyes.

Corneal elevation topography: best fit sphere, elevation distance, asphericity, toricity, and clinical implications

PURPOSE

To describe the effect of the corneal asphericity and toricity on the map patterns and best fit sphere (BFS) characteristics in elevation topography.

METHODS

The corneal surface was modeled as a biconic surface of principal radii and asphericity values of r1 and r2 and Q1 and Q2, respectively. The apex of the biconic surface corresponded to the origin of a polar coordinates system. Minimization of the squared residuals was used to calculate the values of the radii of the BFSs and apex distance (A-values: z distance between the corneal apex and the BFS) of the modeled corneal surface for various configurations relating to commonly clinically measured values of apical radius, asphericity, and toricity.

RESULTS

Increased apical radius of curvature and increased prolateness (negative asphericity) led to an increase in BFS radius but had opposite effects on the A-value. Increased prolateness resulted in increased BFS radius and A-value. Increasing toricity did not alter these findings. Color-plot elevation maps of the modeled corneal surface showed complete ridge patterns when toricity was increased and showed incomplete ridge and island patterns when prolateness was increased.

CONCLUSIONS

High A-values in patients with corneal astigmatism may result from steep apical curvature and/or high prolateness (negative asphericity). The BFS radius may help in distinguishing between these 2 causes of increased A-values. Increased prolateness and decreased apical radius of curvature (often seen in keratoconus) have opposite effects on the BFS radius but similar effects on the apex distance.

Customized eye models for determining optimized intraocular lenses power

We have developed a new optical procedure to determine the optimum power of intraocular lenses (IOLs) for cataract surgery. The procedure is based on personalized eye models, where biometric data of anterior corneal shape and eye axial length are used. A polychromatic exact ray-tracing through the surfaces defining the eye model is performed for each possible IOL power and the area under the radial MTF is used as a metric. The IOL power chosen by the procedure maximizes this parameter. The IOL power for 19 normal eyes has been determined and compared with standard regression-based predictions. The impact of the anterior corneal monochromatic aberrations and the eye's chromatic aberration on the power predictions has been studied, being significant for those eyes with severe monochromatic aberrations, such as post-LASIK cataract patients, and for specific IOLs with low Abbe numbers.

Extended scan depth optical coherence tomography for evaluating ocular surface shape

Spectral domain optical coherence tomography (SD-OCT) with extended scan depth makes it possible for quantitative measurement of the entire ocular surface shape. We proposed a novel method for ocular surface shape measurement using a custom-built anterior segment SD-OCT, which will serve on the contact lens fitting. A crosshair alignment system was applied to reduce the misalignment and tilting of the eye. An algorithm was developed to automatically segment the ocular surface. We also described the correction of the image distortion from the segmented dataset induced by the nontelecentric scanning system and tested the accuracy and repeatability. The results showed high accuracy of SD-OCT in measuring a bicurved test surface with a maximum height error of 17.4 μm. The repeatability of in vivo measurement was also good. The standard deviations of the height measurement within a 14-mm wide range were all less than 35 μm. This work demonstrates the feasibility of using extended depth SD-OCT to perform noninvasive evaluation of the ocular surface shape.

Characterization of age-related variation in corneal biomechanical properties

An experimental study has been conducted to determine the stress-strain behaviour of human corneal tissue and how the behaviour varies with age. Fifty-seven well-preserved ex vivo donor corneas aged between 30 and 99 years were subjected to cycles of posterior pressure up to 60 mm Hg while monitoring their behaviour. The corneas were mechanically clamped along their ring of scleral tissue and kept in physiological conditions of temperature and hydration. The tissue demonstrated hyper-elastic pressure-deformation and stress-strain behaviour that closely matched an exponential trend. Clear stiffening (increased resistance to deformation) with age was observed in all loading cycles, and the rate of stiffness growth was nonlinear with bias towards older specimens. With a strong statistical association between stiffness and age (p < 0.05), it was possible to develop generic stress-strain equations that were suitable for all ages between 30 and 99 years. These equations, which closely matched the experimental results, depicted corneal stiffening with age in a form suitable for implementation in numerical simulations of ocular biomechanical behaviour.

Effects of aging on anterior and posterior corneal astigmatism

PURPOSE

To evaluate age-related changes in astigmatism of both corneal surfaces and the whole cornea.

METHODS

The right eyes of 370 subjects were measured with a rotating Scheimpflug camera (Pentacam). Astigmatisms of the anterior and posterior corneal surfaces were determined. The total corneal astigmatism was derived using power vector summation and vergence tracing. Age-related changes to corneal astigmatism were evaluated using polar value analysis (both in diopter and millimeter).

RESULTS

For the anterior and total cornea, the proportion of with-the-rule astigmatisms decreased and those of oblique and against-the-rule astigmatisms increased with age. For the posterior cornea, most eyes displayed against-the-rule astigmatisms in all age groups. There was a significant trend toward against-the-rule astigmatism associated with increasing age for both anterior and total corneal astigmatisms (mean changes of -0.18 and -0.16 diopters/5 years, respectively), and toward with the rule in posterior corneal astigmatism (a mean change of 0.022 diopters/5 years). Regarding shape changes, a "flat meridian toward a more vertical orientation" trend with increasing age for both the anterior and posterior corneal surfaces was observed (mean changes of 0.0295 and 0.0224 mm/5 years, respectively). The posterior corneal surface compensated for the astigmatism arising from the anterior corneal surface in 91.4% and 47.7% of eyes in the 21-30 and > or =71 years groups, respectively.

CONCLUSIONS

There were age-related shifts toward against-the-rule and with-the-rule astigmatisms for the anterior and posterior corneal surfaces, respectively. The compensating effects of the posterior corneal surface on anterior corneal astigmatism decreased with advancing age.

Physical human model eye and methods of its use to analyse optical performance of soft contact lenses

A bench-top physical model eye that closely replicates both anatomical and optical properties of an average human eye was designed and constructed. The cornea was sourced from a flouro-polymer with refractive index (RI) of 1.376 and crystalline lenses were made of Boston RGP polymers, EO and Equalens II, with an equivalent RI of 1.429 and 1.423 respectively. These materials served to make crystalline lens components of different age groups and accommodative states. De-Ionized water, with RI of 1.334 represented both aqueous and vitreous humor. The complementary metal-oxide sensor of a PixelLink digital camera with a resolution of 5MP and a 2.2 microm pixel pitch, hosted on a motor-base, served as the 'acting' retina. The translation and rotary functions of the motor-base facilitated the simulation of different states of ametropia and assessment of peripheral visual function, respectively. We validated one of its configurations to suit normal viewing conditions and results from the on and off-axis optical quality measurements are presented. As a demonstration of potential practical uses, several corrective soft contact lenses were placed on the model eye and their optical performance evaluated.

Origins of the keratometer and its evolving role in ophthalmology

The keratometer, or ophthalmometer as it was originally known, had its origins in the attempt to discover the seat of accommodation in the eye. Since that early beginning, it has been re-invented a number of times, with improvements and modifications made in the original principles of its design for new applications that arose as ophthalmology advanced. The cornea is not only responsible for the majority of the refraction in the eye, but is also readily accessible for measurement and modification. The keratometer's ability to measure the cornea has allowed it to play a central role in critical advances in ophthalmic history. This review describes the origins and principles of this instrument, the novel applications that led to the keratometer's continued resurgences over its nearly 250-year history, and the modern devices that have borrowed its basic principles and are beginning to replace it in common clinical practice.

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.