Posterior Corneal Elevation Research Articles

Correlation of Corneal Elevation With Severity of Keratoconus by Means of Anterior and Posterior Topographic Analysis

To investigate the severity of keratoconus in terms of corneal elevation differences with 2 different reference surfaces using topographic analysis. Eighty-six eyes of 50 patients with keratoconus of various clinical stages (61 male and 25 female eyes) with mean age of 37.3 ± 11.6 years were evaluated. Anterior and posterior corneal elevations were measured using topographic analysis (using a rotating Scheimpflug camera), and the imaging data were analyzed to estimate elevation differences as differential values from conventional best-fit-sphere and enhanced best-fit-sphere (exclusion of a 4-mm-diameter area from the most thinned cornea). The data correlated with the keratoconus severity index and Amsler-Krumeich classification of keratoconus. There was a significant positive correlation between elevation differences and keratoconus severity index in both anterior and posterior surfaces (Pearson correlation coefficient, r = 0.66; P < 0.001; r = 0.74; P < 0.001). Cases of larger elevation differences in the anterior and posterior corneal surfaces were staged higher in Amsler-Krumeich classification (1-way analysis of variance, P = 0.040; P < 0.001). Cases of lower staging in the Amsler-Krumeich classification had a larger area under the receiver operating characteristic curve in the posterior elevation differences than in the anterior elevation differences, suggesting a greater diagnostic value of the posterior elevation measurement. Anterior and posterior corneal surface height data obtained by elevation-based tomography provide useful information in improving keratoconus diagnostic accuracy and in grading the severity of keratoconus.

Refractive, Tomographic and Biomechanical Outcomes after Implantation of Ferrara ICRS in Keratoconus Patients

ABSTRACT Background Nowadays, ICRS are a step in the treatment of keratoconus. The purpose of this study was to evaluate the refractive effect and the tomographic and biomechanical parameters in keratoconus patients implanted with Ferrara ICRS, and their stability after 18 months. Materials and methods Twenty eyes of 20 keratoconus patients implanted with ICRS were evaluated. The average follow-up was 18 months. UDVA, CDVA, biomicroscopy, tomography (Pentacam) and biomechanics (ORA) of the cornea were evaluated before and after surgery. For the comparison of groups, the Wilcoxon test was used. Results The mean UDVA improved from 1.00 to 0.30 and the mean CDVA improved from 0.51 to 0.12; both were statistically significant (p = 0.0001). The average keratometry decreased from 50.7D to 47.5D (p = 0.0003), and the average astigmatism decreased from 5.5D to 3.5D (p = 0.0058). The mean CCT did not change significantly after surgery, but the mean TPP increased from 441.2 to 455.2 µm (p = 0.004). There was a significant reduction in the anterior cornea elevation, both the central (from 16.2-8.8; p = 0.0066) and the minimum (from–43.2 to −57.1; p = 0.0228). No significant change was found for posterior corneal elevation and for biomechanical parameters (hysteresis or CRF). Discussion There was a significant improvement of UDVA and CDVA after ICRS implantation, in keratoconic eyes. There was a significant and stable corneal flattening, and a decrease of the astigmatism. Corneal biomechanic parameters did not change. How to cite this article Salgado-Borges JM, Costa-Ferreira C, Monteiro M, Guilherme-Monteiro J, Torquetti L, Ferrara P, Ambrósio R Jr. Refractive, Tomographic and Biomechanical Outcomes after Implantation of Ferrara ICRS in Keratoconus Patients. Int J Keratoco Ectatic Corneal Dis 2012;1(1):16-21.

Posterior Corneal Curvature Assessment after Epi-LASIK for Myopia: Comparison of Orbscan II and Pentacam Imaging

PurposeTo compare the changes in posterior corneal curvature using scanning slit topography (Orbscan II) and Scheimpflug imaging (Pentacam) before and after Epi-laser in situ keratomileusis (LASIK) for myopia.MethodsIn a prospective observational case-series study, 20 myopic patients having undergone Epi-LASIK were examined serially with two different devices, Orbscan II and Pentacam, preoperatively and one month postoperatively. Posterior central elevation (PCE) and posterior maximal elevation (PME) were compared between the two devices, and the changes in parameters after Epi-LASIK were analyzed using a difference map.ResultsAll parameters (preoperative and postoperative PCE and preoperative and postoperative PME) that were measured using the Orbscan II were significantly greater compared to those of the Pentacam (for all p < 0.001). PCE and PME were significantly increased one month postoperatively in the Orbscan II measurements (p < 0.05) but were not significantly increased in the Pentacam measurements. Also, ΔPCE and ΔPME, in the difference map obtained by each serial scanning, were significantly greater in the Orbscan II measurements than with the Pentacam (p = 0.012, p = 0.016).ConclusionsThe Pentacam measurements displayed significantly reduced values in all parameters related to posterior corneal elevation compared to those of the Orbscan II. The Pentacam showed no significant change in posterior corneal curvature after Epi-LASIK, based on the difference map.

Precision Analysis of Posterior Corneal Topography Measured by Visante Omni: Repeatability, Reproducibility, and Agreement With Orbscan II

To evaluate the repeatability and reproducibility of posterior corneal curvature and posterior corneal elevation best-fit sphere (BFS) obtained with the Visante Omni (Carl Zeiss Meditec) and to compare the results with the Orbscan II (Bausch & Lomb). Thirty eyes from 30 healthy volunteers were included in this study. All patients were examined 5 times with the Visante Omni and Orbscan II by 2 independent operators. The posterior corneal curvature (3- and 6-mm zone) and posterior corneal elevation BFS (5- and 8-mm zone) were generated for each system. Intraoperator repeatability and interoperator reproducibility and agreement between the systems were evaluated using the intraclass correlation coefficient (ICC) and the Bland-Altman method. The repeatability of posterior corneal curvature and posterior corneal elevation BFS measured by the Visante Omni was high for all analysis zones (ICC, 0.99 to 1.00). The reproducibility also showed similar results (ICC, 0.99 to 1.00). Agreement between the Visante Omni and Orbscan II was high for posterior corneal curvature (ICC, 0.94 to 0.97) and posterior corneal elevation BFS (ICC, 0.96 to 0.98) with 95% limits of agreement at -0.26 to 0.22 diopters for posterior corneal curvature and 0.11 to 0.69 mm for posterior corneal elevation BFS. The Visante Omni provides good repeatability and reproducibility of posterior corneal topography. Overall agreement with the Orbscan II system was high.

Anterior and Posterior Corneal Elevation After Orthokeratology and Standard and Customized LASIK Surgery

To quantify the changes in the elevation topography of the front and back corneal surfaces after three different refractive treatments for correcting myopia with standard and custom laser in situ keratomileusis (LASIK) and orthokeratology using corneal refractive therapy. We evaluated 20 eyes undergoing orthokeratology for correction of myopia spherical equivalent (mean±SD=-3.41±0.76 D), 18 eyes undergoing custom LASIK surgery (mean±SD=-4.14±0.89 D), and 23 eyes undergoing standard LASIK (mean±SD=-3.61±0.67 D). The values of front and back corneal surfaces were derived by using Pentacam (Oculus, Inc. GmbH, Wetzlar, Germany) before and at least 3 months after each treatment, in the center of the cornea and 4 points to each side of the horizontal meridian at intervals of 1 mm. Corneal elevation data before treatment were not statistically different between patients in either group (P>0.070, for back and front elevation). After treatment, both surgical procedures significantly increased the positive value of the front elevation beyond an area of 6 mm. The opposite trend was found within the central 5 mm of the cornea, presenting a statistically significant decrease in elevation (P<0.001). In the case of orthokeratology, the elevation experienced a minor but a statistically significant reduction in the central region (P<0.001). On the back surface, the elevation did not undergo statistically significant alterations in any of the procedures and none of the items discussed (P>0.285). Differences in front corneal elevation changes between LASIK and orthokeratology reveal a much different mechanism for producing corneal power subtraction. The back corneal surface does not suffer significant changes after surgical and nonsurgical treatments for the correction of myopia.

Corneal topographic analysis in patients with keratoconus using 3-dimensional anterior segment optical coherence tomography

To assess a new corneal topographer based on 3-dimensional anterior segment optical coherence tomography (OCT-based corneal topographer) for analyzing corneal topography in patients with keratoconus. Osaka University Graduate School of Medicine, Osaka, Japan. Evaluation of diagnostic test or technology. Keratoconic eyes and normal control eyes were assessed using a rotating Scheimpflug-based corneal topographer and an AS OCT-based corneal topographer. The OCT-based corneal topographer, a 1310 nm swept-source device, captures 8192 points during 0.34 second for 16 radial scans of the corneal topography. The success rate of precisely digitizing the corneal surfaces, patterns of the color-coded maps, central corneal thickness (CCT), and central axial power were compared between the 2 devices. Forty keratoconic eyes and 30 control eyes were assessed. The OCT-based corneal topographer measured corneal configurations despite corneal opacities. The success rate of precisely digitizing the corneal surfaces in keratoconic eyes was 95% using OCT and 80% using the Scheimpflug-based corneal topographer. The color-coded maps of the anterior and posterior corneal elevations, CCT, and corneal axial power maps generated by the 2 devices agreed well. The correlations between the CCT values and between the central corneal axial power values measured by the 2 devices were well correlated in both groups. The OCT-based corneal topographer, with its faster acquisition time, provided more consistent measurements than the Scheimpflug-based corneal topographer. The OCT-based corneal topographer seems promising for evaluating highly irregular corneas, as in cases of advanced keratoconus. No author has a financial or proprietary interest in any material or method mentioned.

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

To determine the efficacy of different Scheimpflug-imaging parameters in discriminating between subclinical keratoconus, keratoconus eyes, and normal eyes. Department of Ophthalmology, Ankara University Faculty of Medicine, Ankara, Turkey. Comparative case series. Pentacam Scheimpflug measurements were taken in eyes with mild to moderate keratoconus, subclinical keratoconus, or with myopic astigmatism (normal eyes). Several parameters provided by the software or derived from the elevation maps were evaluated and compared between the groups. The Mann-Whitney test, receiver-operating-characteristic (ROC) curves, and logistic regression analysis were used to compare the mean measurements and to evaluate the sensitivity and specificity of the parameters or constructed models. The corneal power and elevation measurements were statistically significantly higher and the cornea was significantly thinner in eyes with subclinical or clinical keratoconus than in normal eyes (P < .05). The ROC curve analysis showed high overall predictive accuracy of various elevation and thickness indices in discriminating ectatic corneas from normal corneas. Logistic regression analysis showed that the goodness of fit of a model using a combination of corneal power, thickness, and anterior elevation parameters was best in discriminating keratoconus eyes from normal eyes, whereas that of a model using a combination of corneal power, thickness, and posterior elevation was best in discriminating subclinical keratoconus eyes from normal eyes. Combined analysis of anterior and posterior corneal power, elevation, and thickness data provided by the Scheimpflug device effectively discriminated between ectatic corneas and normal corneas. No author has a financial or proprietary interest in any material or method mentioned.

Patient-Specific Modeling of Corneal Refractive Surgery Outcomes and Inverse Estimation of Elastic Property Changes

The purpose of this study is to develop a 3D patient-specific finite element model (FEM) of the cornea and sclera to compare predicted and in vivo refractive outcomes and to estimate the corneal elastic property changes associated with each procedure. Both eyes of a patient who underwent laser-assisted in situ keratomileusis (LASIK) for myopic astigmatism were modeled. Pre- and postoperative Scheimpflug anterior and posterior corneal elevation maps were imported into a 3D corneo-scleral FEM with an unrestrained limbus. Preoperative corneal hyperelastic properties were chosen to account for meridional anisotropy. Inverse FEM was used to determine the undeformed corneal state that produced <0.1% error in anterior elevation between simulated and in vivo preoperative geometries. Case-specific 3D aspheric ablation profiles were simulated, and corneal topography and spherical aberration were compared at clinical intraocular pressure. The magnitude of elastic weakening of the residual corneal bed required to maximize the agreement with clinical axial power was calculated and compared with the changes in ocular response analyzer (ORA) measurements. The models produced curvature maps and spherical aberrations equivalent to in vivo measurements. For the preoperative property values used in this study, predicted elastic weakening with LASIK was as high as 55% for a radially uniform model of residual corneal weakening and 65% at the point of maximum ablation in a spatially varying model of weakening. Reductions in ORA variables were also observed. A patient-specific FEM of corneal refractive surgery is presented, which allows the estimation of surgically induced changes in corneal elastic properties. Significant elastic weakening after LASIK was required to replicate clinical topographic outcomes in this two-eye pilot study.

Comparison of Anterior and Posterior Corneal Surface Irregularity in Descemet Stripping Automated Endothelial Keratoplasty and Penetrating Keratoplasty

To evaluate the irregularity of the anterior and posterior cornea after Descemet stripping automated endothelial keratoplasty (DSAEK) and penetrating keratoplasty (PK). This clinical study comprised 39 eyes: 13 consecutive eyes after DSAEK, 13 consecutive eyes after PK, and 13 age-matched normal eyes. Corneal elevation data were acquired using a rotating Scheimpflug camera 1 and 3 months after DSAEK and PK. Anterior and posterior corneal elevation data were decomposed into a set of Zernike polynomials up to eighth order. Total higher-order root mean square (RMS) and RMS from third to eighth order were calculated. The astigmatism and irregularity of the anterior and posterior surfaces were compared between DSAEK and PK. The regular astigmatism and tilt components of the anterior surface were significantly lower after DSAEK than after PK at 1 and 3 months (P < 0.001), whereas there was no difference in astigmatism of the posterior surface between the groups (P = 0.07, 0.22). The higher-order RMS and RMSs of third- to eighth-order components of the anterior surface were significantly larger after PK than those after DSAEK at 1 and 3 months (P < 0.01), whereas there were no significant differences between DSAEK and PK in higher-order aberration RMS and RMSs of third- to eighth-order components of the posterior surface. Postoperative corneal irregularity of the anterior surface was greater after PK than after DSAEK, whereas there was no significant difference in posterior surface irregularity. DSAEK is superior to PK in terms of the higher-order irregularity of the anterior surface.

Posterior Corneal Elevation After LASIK with Three Flap Techniques as Measured by Pentacam

To evaluate and compare posterior corneal changes using elevation data obtained from Pentacam (Oculus Optikgeräte GmbH) Scheimpflug imaging in eyes undergoing LASIK with three different modes of flap creation: IntraLase femtosecond laser FS60 (Abbott Medical Optics) (femtosecond group), Amadeus (Ziemer Group AG) mechanical microkeratome (keratome group), or flap formation using 20% alcohol laser epithelial keratomileusis (LASEK) (LASEK group). Ninety myopic patients (90 eyes) undergoing refractive surgery were recruited. The change in posterior corneal elevation at 21 predetermined points in the central 5-mm area was measured using exported elevation data from the Pentacam before LASIK and 18 months postoperative and was compared among and within three modes of flap creation. Mean change in posterior elevation in the central 5-mm area was 5.13±4.16 μm for the femtosecond group, 5.78±4.42 μm for the keratome group, and 6.68±4.72 μm for the LASEK group and was similar among groups (P=.59). Change in posterior elevation before and after LASIK was not significant within any group (P=.342, P=.232, and P=.321 for the femtosecond, keratome, and LASEK groups, respectively). Preoperative spherical equivalent, central corneal thickness, ablation depth, and estimated residual bed thickness did not correlate with change in posterior corneal elevation for the femtosecond, keratome, or LASEK groups (P>.05). Using Pentacam elevation data, there were no significant changes in posterior corneal elevation following LASIK among or within the three methods of flap creation. At 18 months after LASIK, the posterior corneal surface is not displaced anteriorly significantly and is equally stable using these three surgical techniques.

Anterior chamber characteristics of keratoconus assessed by rotating Scheimpflug imaging

To compare anterior chamber parameters in normal eyes and eyes with mild to moderate keratoconus corneas using rotating Scheimpflug imaging and evaluate trends in corneal protrusion progression. Department of Ophthalmology, Semmelweis University, Budapest, Hungary. The anterior chamber depth (ACD) at the corneal center, thinnest point of the cornea, and 1.0 mm, 2.0 mm, and 3.0 mm paracentrally; central pachymetry; and posterior elevation were measured by Scheimpflug imaging (Pentacam) in eyes with keratoconus and eyes of refractive surgery candidates. A generalized estimating equation (GEE) was used to identify predictors of keratoconus. Correlations between posterior elevation and ACD at minimum pachymetry were analyzed by linear and piecewise linear regression. The keratoconus group comprised 41 eyes (24 patients) and the control group, 70 eyes (41 patients). The ACD was significantly deeper in the keratoconus group than in the control group (P<.05) and was significantly correlated with posterior elevation. In the multivariable GEE model, posterior elevation (odds ratio [OR], 1.46; P<.001) and ACD at minimum pachymetry (OR, 7.54; 95% confidence interval, 1.35-42.05) were significant predictors of keratoconus. Segmented analysis of the correlation between posterior elevation and central ACD showed a better fit to data than linear regression and identified a 40 mum threshold level for posterior elevation. The GEE determined 450 mum as a similar threshold for central corneal thickness. There was a threshold level of posterior corneal elevation and corneal thickness values beyond which the level of corneal protrusion in keratoconus accelerated. No author has a financial or proprietary interest in any material or method mentioned.

The Shape of Posterior Corneal Surface in Normal, Post-LASIK, and Post–epi-LASIK Eyes

PURPOSE. To evaluate the shape of the posterior corneal surface in eyes with normal myopia or myopic astigmatism and the changes after LASIK and epi-LASIK. METHODS. A total of 152 eyes were included in the study. The posterior corneal elevation was measured with Scheimpflug imaging (Pentacam; Oculus, Wetzlar, Germany), before LASIK or epi-LASIK and at 1 and 6 months after surgery, along the four optical zones (center and 2-, 4-, and 6-mm diameters) as a function of the meridian. RESULTS. In normal eyes, the heights were lower in the vertical meridian than in the horizontal meridian and were also lower in the superior hemisphere than in the inferior hemisphere. In post-LASIK eyes, almost all regions protruded 1 month after surgery, except for the 6-mm diameter, which exhibited a slight but not significant (P = 0.731) backward displacement. The various elevations returned to original levels 6 months after LASIK. In the post-epi-LASIK group, there was a significant backward displacement in the center and 2-mm regions after surgery. Moreover, there was no shift at the 4-mm diameter, and the 6-mm diameter showed a progressive forward shift. CONCLUSIONS. The shape of the normal posterior corneal surface exhibited obvious regional disparities and radial asymmetry. The displacement after LASIK was observed as time-dependent changes that protruded at an early stage but then returned to original levels 6 months after surgery. On the other hand, the displacement after epi-LASIK appeared as region-dependent changes that could be divided into central and peripheral parts that displayed backward and forward shift trends, respectively.

The Evaluation of Enhanced Ectasia Display Mode in Screening for Keratoconus

Purpose: To estimate the utility of the enhanced ectasia display mode of Pentacam in discriminating keratoconus and keratoconus suspect from normal cornea. Methods: Corneal topography was measured using the Pentacam in keratoconus, keratoconus suspect and a normal control group. A best-fit sphere (BFS) and an enhanced best-fit sphere (EBFS) were used as reference surfaces for corneal elevation measurements, and measured values from both the anterior and posterior surfaces were compared among the three groups. Receiver operating characteristic (ROC) curves were used to identify the optimal posterior corneal elevation cutoff points for maximal sensitivity and specificity in discriminating keratoconus and keratoconus suspect from the normal control group. Results: Mean anterior and posterior corneal elevation were statistically higher in keratoconus than in keratoconus suspect and normal corneas. The optimal cutoff point of posterior elevation was 23 m for the keratoconus group, and this value was associated with a sensitivity and a specificity of 96.7% and 98.6%, respectively for keratoconus. The optimal cutoff point of enhanced posterior elevation was 43 m for the keratoconus group, and this value was associated with a sensitivity and a specificity of 96.7% and 95.5%, respectively. Conclusions: The enhanced ectasia display mode showed similar diagnostic power to that of the conventional elevation map, and the former could be more useful in a clinical setting due to the pronounced visualization of corneal elevation.

Eficacia del Orbscan II y Pentacam en la evaluación de los mapas de elevación en candidatos a cirugía refractiva mediante un análisis de repetibilidad

Anterior and posterior corneal elevations are measurements used to detect keratoconus suspects. To determine the efficacy of Orbscan II and Pentacam when assessing their elevation maps. The efficacy of the Orbscan II and Pentacam measuring the anterior and posterior corneal elevations were evaluated in a sample of 68 eyes. The concordance between the two devices and the coefficient of repeatability were measured following the parameters of the British Standard Institution by the Bland-Altman concordance analysis and the Lin concordance correlation coefficient. The coefficient of repeatability at the point of maximum anterior elevation was 68.29% with the Orbscan and 24.20% with the Pentacam. The concordance correlation coefficient was 0.64 (CI 95%: 0.48-0.76) with the Orbscan and 0.94 with the Pentacam (CI 95%: 0.91-0.96). The coefficient of repeatability at the point of maximum posterior elevation was 38.7% with the Orbscan and 68.0% with the Pentacam. The concordance correlation coefficient was 0.69 with the Orbscan (CI 95%: 0.55-0.80) with a precision of 0.71 and an accuracy of 0.97, and 0.24 with the Pentacam (CI 95%: 0.00-0.45) with a precision of 0.24 and an accuracy of 0.99. Measurement of the point of maximum posterior elevation is better with the Orbscan II and less precise with the Pentacam. The random error can be reduced by using the mean of three assessments and can serve as a guide in the search of diagnostic devices with minimum absolute relative error in all measurements.

Keratoconus With High Hyperopia

To report the case of a patient with high hyperopia and bilateral keratoconus. A high hyperopic patient was evaluated using Orbscan corneal topography and slitlamp biomicroscopy to confirm suspected keratoconus. A male patient (32 years old) with a long history (10 years) of daily hydrogel contact lens wear with severe hyperopia was examined (RE +8.25-5.00 x 070 degrees , visual acuity [VA] 0.7 and LE +8.00-3.00 x 100 degrees , VA 0.9). The patient's Orbscan topography was compatible with bilateral asymmetric keratoconus (asymmetry of central dioptric power, irregular astigmatism, high anterior (110 and 63 microm for RE and LE, respectively) and posterior (200 and 128 microm for RE and LE, respectively) corneal elevation values; and corneal thinnest point (450 and 471 microm for RE and LE, respectively, in the cone). Slitlamp biomicroscopic findings included Vogt striae in RE. High Dk rigid gas-permeable CLs with keratoconus design (Z-B4P, Menicon, Japan) were fitted in both eyes (in RE the back optic zone radius [BOZR] was 7.70 mm, the total diameter was 10.00 mm, and the power was +4.00D, which provides a VA of 1.0; the LE BOZR was 7.65 mm, the total diameter was 10.00 mm, and the power was +4.25D, which provides a VA of 1.0). Although rare, keratoconus may occur in cases of high hyperopia. Management of hyperopic keratoconus with rigid gas-permeable contact lens fitting may be similar to that applied with other cases of keratoconus.

Effect of anterior and posterior corneal surface irregularity on vision after Descemet-stripping endothelial keratoplasty

To evaluate irregularity of the anterior and posterior cornea before and after Descemet-stripping endothelial keratoplasty (DSEK) and its effect on visual acuity. Department of Ophthalmology, Keio University School of Medicine, Tokyo, Japan. Corneal data were acquired using a rotating Scheimpflug camera before and 1 month and 3 months after DSEK. Anterior and posterior corneal elevation data were decomposed into a set of Zernike polynomials up to the 8th order within a 4.0 mm diameter region. Total higher-order root mean square (HO-RMS) and RMS from the 3rd to 8th order were calculated. The effects of anterior and posterior surface irregularity on visual acuity were evaluated. This clinical study comprised 13 consecutive eyes of 12 bullous keratopathy patients. The mean best corrected visual acuity (BCVA) was 1.11 logMAR +/- 0.5 (SD) preoperatively, 0.49 +/- 0.49 logMAR 1 month postoperatively, and 0.27 +/- 0.32 logMAR at 3 months. The HO-RMS of the anterior surface 1 month (P = .040) and 3 months (P = .048) postoperatively was significantly lower than preoperatively. There were no significant differences in posterior surface HO-RMS between preoperatively and 1 month (P = .45) and 3 months (P = .054). The postoperative BCVA was significantly correlated with HO-RMS (P<.001), but not with posterior surface HO-RMS, at 3 months (P = .354). Postoperative BCVA correlated with irregularity of the anterior surface but not the posterior surface. In addition to corneal transparency, regularity of the anterior surface is an important factor in visual acuity after DSEK.

Pentacam and Orbscan II Measurements of Posterior Corneal Elevation Before and After Photorefractive Keratectomy

To compare the measurements of posterior corneal elevation above the best-fit sphere before and after photorefractive keratectomy (PRK) using two different technologies--Scheimpflug imaging using Pentacam (Oculus) and scanning slit combined with Placido imaging using Orbscan II (Bausch & Lomb). In a retrospective observational case series study, 60 eyes measured using Orbscan II and Pentacam before or after PRK were evaluated. The data were analyzed as three groups: preoperative, early postoperative (from 1 to 3 months after surgery), and late postoperative (> 3 months postoperatively but not exceeding 1 year). Anterior chamber depth, posterior best-fit float sphere, posterior central elevation (PCE) above the best-fit sphere, and posterior maximum elevation (PME) above the best-fit sphere in the 6.0-mm zone were analyzed. Posterior central elevation and PME values measured using Orbscan II were greater than those measured using Pentacam (P < .001). Anterior chamber depth values measured using Orbscan II were smaller than those measured using Pentacam in all three groups (P < .001). Using Orbscan II, PCE and PME values for the two postoperative groups were greater than for the preoperative group. Using Pentacam, the PCE values for the three groups were similar, and the PME values for the preoperative and late postoperative groups were similar. These findings indicate that Orbscan II measurements of posterior corneal elevation before and after excimer laser refractive corneal surgery may not be valid. Pentacam measurement showed no significant change in posterior corneal elevation from before to after excimer laser corneal surgery.

Corneal collagen crosslinking using riboflavin and ultraviolet-A light for keratoconus: One-year analysis using Scheimpflug imaging

To evaluate changes in corneal curvature, corneal elevation, corneal thickness, lens density, and foveal thickness after corneal collagen crosslinking with riboflavin and ultraviolet-A (UVA) light in eyes with progressive keratoconus. Grewal Eye Institute, Chandigarh, India. Subjective refraction, best corrected visual acuity (BCVA), Scheimpflug imaging, and optical coherence tomography were performed preoperatively and 1 week, 1, 3, and 6 months, and 1 year after crosslinking. There were no significant differences (P > 0.05) in mean values between preoperatively and 1 year postoperatively, respectively, in BCVA (0.22 +/- 0.10 and 0.20 +/- 0.10), spherical equivalent (-6.30 +/- 4.50 diopters (D) and -4.90 +/- 3.50 D), or cylinder vector (1.58 x 7( degrees ) +/- 3.8 D and 1.41 x 24( degrees ) +/- 3.5 D). There was no significant difference in mean measurements between preoperatively and 1 year postoperatively, respectively, for central corneal thickness (458.9 +/- 40 microm and 455.2 +/- 48.6 microm), anterior corneal curvature (50.6 +/- 7.4 D and 51.5 +/- 3.6 D), posterior corneal curvature (-7.7 +/- 1.2 D and -7.4 +/- 1.1 D), apex anterior (P = .9), posterior corneal elevation (P = .7), lens density (P = .33), or foveal thickness (175.7 +/- 35.6 microm and 146.4 +/- 8.5 microm; P = .1). Stable BCVA, spherical equivalent, anterior and posterior corneal curvatures, and corneal elevation 1 year after crosslinking indicate that keratoconus did not progress. Unchanged lens density and foveal thickness suggest that the lens and macula were not affected after UVA exposure during crosslinking.

Comparison of laser in situ keratomileusis and photorefractive keratectomy results: Long-term follow-up

To compare the results of photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) over a 4-year follow-up. Miyata Eye Hospital, Miyazaki, Japan. This comparative retrospective study comprised 22 eyes (22 patients) that had PRK and 18 eyes (18 patients) that had LASIK. To be included, the patient had to have completed a 4-year follow-up. Uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), percentage of eyes within +/-0.5 diopter (D) of the targeted refraction, central corneal thickness, and the anterior and posterior corneal elevations were compared between the PRK and LASIK groups. The mean UCVA was significantly better in the LASIK group than in the PRK group at 6 months (P = .0043) and 1 year (P = .0044). At 2 years, there was no significant difference in the mean UCVA between the 2 groups. The mean BSCVA was significantly better in the LASIK group than in the PRK group at 6 months (P<.0001), 1 year (P<.0001), and 2 years (P = .0083). At 3 and 4 years, there was no significant difference in the mean BSCVA between the 2 groups. The percentage of eyes within +/-0.5 D of the targeted refraction was not significantly different between groups at any time. The superiority of LASIK over PRK in short-term efficacy was not retained 4 years after surgery. The main reasons were a myopic shift and a decline in UCVA at the last follow-up in the LASIK group.

The effect of the flap thickness on forward shift of posterior corneal surface in excimer laser surgery correcting high myopia

To investigate the influence of flap thickness of laser in situ keratomileusis (LASIK) and subepithelial keratomileusis on the forward shift of the posterior corneal surface in high myopia It was a case-control study. In this study, 151 right high myopic eyes of patients [spherical equivalent range - 6.00 to - 10.00 diopter, 46 subepithelial keratomileusis eyes (epithelial sheet group), 64 LASIK eyes with 110 microm flap thickness (thin flap group) and 41 LASIK eyes with 130 microm flap thickness (normal flap group)] were evaluated preoperatively and postoperatively with the Pentacam to determine the shape change in the posterior corneal surface. Changes in posterior shape were calculated by comparing refraction and elevation of posterior corneal surface in the part corresponding to the pupil center preoperatively and postoperatively. The mean estimated residual bed thickness (RBT) of epithelial sheet eyes, thin flap eyes and normal flap eyes was (362 +/- 23), (320 +/- 25) and (300 +/- 21) microm, respectively. The RBT difference between epithelial sheet group and thin or normal flap group was statically significant (F = 85.685, P <0.001) and the difference between 2 LASIK groups was also significant (P < 0.05). There was no significant difference between three groups in ablation depth (P > 0.05). The mean posterior corneal refraction and elevation change of epithelial sheet eyes, thin flap eyes and normal flap eyes in the pupil center was (-0.12 +/- 0.14)D and (0.9 +/-1.8) microm, ( -0.20 +/- 0.22)D and (1.3 +/- 1.6) microm, (-0.21 +/- 0.18)D and (1.5 +/- 1.5) microm, respectively. A significant correlation between posterior elevation and refraction change was present in all 3 groups. The amount of posterior corneal elevation change correlated significantly with the ablation depth in thin flap group and normal flap group (r = 0.265, 0.311; P <0.05). In treating high myopia, both subepithelial ablation and LASIK surgery induce tiny forward shift of posterior corneal surface when the RBT was thick enough. Deeper ablation or thicker corneal flap may increase the trend of forward shift of posterior corneal surface.