Corneal Elevation Research Articles

A Comparative Study of the Performance of Different Corneal Topographers on Children with Respect to Orthokeratology Practice

The purpose of this study was to evaluate and compare the performance of three topographers, Medmont E300, Keratron Scout, and Humphrey Atlas 991, for the measurement of corneal parameters of interest in orthokeratology practice on young children. Apical radius (R0), flattest corneal curvature (flat k), eccentricity (e) value, and elevation at 9-mm chord were measured twice on 22 healthy subjects of mean +/- standard deviation (SD) age 11.2 +/- 2.2 years using the topographers under investigation. The repeatability of the Medmont and Humphrey were good for the measurements of R0 and flat k. The repeatability of the Humphrey was also good for the measurement of e-value. However, for both topographers, the measurement of elevation was not precise enough for orthokeratology empirical lens design. The Keratron showed poor repeatability for all the parameters evaluated. The agreement of the parameters of interest between these instruments was poor. The Medmont and the Humphrey showed good repeatability on children, but the precision of elevation measurements was not good enough to be used for designing empiric orthokeratology lenses on children. The Keratron is not recommended to be used on children in orthokeratology fitting and management. Interinstrumental agreements were significantly different and therefore measurements from different topographers should not be used interchangeably.

Optical Quality and Depth-of-field of Eyes Implanted With Spherical and Aspheric Intraocular Lenses

To compare experimental optical performance in eyes implanted with spherical and aspheric intraocular lenses (IOLs). Corneal, total, and internal aberrations were measured in 19 eyes implanted with spherical (n=9) and aspheric (n=10) IOLs. Corneal aberrations were estimated by virtual ray tracing on corneal elevation maps, and total aberrations were measured using a second-generation laser ray tracing system. Corneal and total wave aberrations were fit to a Zernike polynomial expansion. Internal aberrations were measured by subtracting corneal from total wave aberrations. Optical performance was evaluated in terms of root-mean-square (RMS) wavefront error and Strehl ratio (estimated from the modulation transfer function). Depth-of-field was obtained from through-focus Strehl estimates from each individual eye. Corneal aberrations increased after IOL implantation, particularly astigmatism and trefoil terms. Third and higher order RMS (and the corresponding Strehl ratio) were significantly better in eyes with aspheric IOLs than with spherical IOLs; however, this tendency was reversed when astigmatism was included. Spherical aberration was not significantly different in eyes with aspheric IOLs, whereas it was significantly positive in eyes with spherical IOLs. Third order aberrations were not significantly different across groups. Depth-of-field was significantly larger in eyes with spherical IOLs. Spherical IOLs showed better absolute optical quality in the presence of negative defocus >1.00 D. Our study shows a good degree of compensation of the corneal spherical aberration in eyes implanted with aspheric IOLs, as opposed to eyes implanted with spherical IOLs. Other sources of optical degradation, both with aspheric and spherical IOLs, are non-symmetric preoperative corneal aberrations, incision-induced aberrations, and third order internal aberrations. Although best corrected optical quality is significantly better with aspheric IOLs, tolerance to defocus tended to be lower.

High Interocular Corneal Symmetry in Average Simulated Keratometry, Central Corneal Thickness, and Posterior Elevation

The purpose of this study was to assess interocular corneal symmetry in average simulated keratometry, corneal thickness, and posterior corneal elevation. This retrospective analysis included data from scanning slit topography (Orbscan II; Bausch and Lomb, Rochester, NY) on 242 eyes from 121 consecutive patients undergoing standard evaluation for consideration of elective laser vision correction. The symmetry between the right and left eye in average simulated keratometry, minimum central corneal thickness, and posterior corneal elevation was assessed by comparative data analysis. Simulated keratometry ranged from 39.9 to 48.6 D. The interocular difference in average simulated keratometry was 0.47 D (standard deviation [SD] 0.43). The interocular Pearson correlation coefficient for average simulated keratometry was 0.90 (p < 0.001). The range of minimum corneal thickness was 432 to 628 microm. The interocular Pearson correlation coefficient for minimum central corneal thickness was 0.95 (p < 0.001). Right and left eye minimum corneal thickness differed by an average 8 microm (SD 7). The range of posterior elevation was -4 to 54 microm. The average difference in posterior corneal elevation between the right and left eye was 6 microm (SD 5). The interocular Pearson correlation coefficient for posterior corneal elevation was 0.72 (p < 0.001). The average posterior elevation was 19 microm (SD 11). Although a wide range of values exists in simulated keratometry, minimum corneal thickness, and posterior corneal elevation, interocular symmetry in all these parameters was very high in this group of consecutive patients. Asymmetry of these interocular parameters may warrant repeat clinical testing for accuracy and may predict corneal abnormalities. Normative data on posterior cornea elevation is presented. This study points out potentially clinically important high interocular corneal symmetry data in simulated keratometry, corneal thickness, and posterior corneal elevation.

Preliminary Results of Neural Networks and Zernike Polynomials for Classification of Videokeratography Maps

Our main goal in this work was to develop an artificial neural network (NN) that could classify specific types of corneal shapes using Zernike coefficients as input. Other authors have implemented successful NN systems in the past and have demonstrated their efficiency using different parameters. Our claim is that, given the increasing popularity of Zernike polynomials among the eye care community, this may be an interesting choice to add complementing value and precision to existing methods. By using a simple and well-documented corneal surface representation scheme, which relies on corneal elevation information, one can generate simple NN input parameters that are independent of curvature definition and that are also efficient. We have used the Matlab Neural Network Toolbox (MathWorks, Natick, MA) to implement a three-layer feed-forward NN with 15 inputs and 5 outputs. A database from an EyeSys System 2000 (EyeSys Vision, Houston, TX) videokeratograph installed at the Escola Paulista de Medicina-Sao Paulo was used. This database contained an unknown number of corneal types. From this database, two specialists selected 80 corneas that could be clearly classified into five distinct categories: (1) normal, (2) with-the-rule astigmatism, (3) against-the-rule astigmatism, (4) keratoconus, and (5) post-laser-assisted in situ keratomileusis. The corneal height (SAG) information of the 80 data files was fit with the first 15 Vision Science and it Applications (VSIA) standard Zernike coefficients, which were individually used to feed the 15 neurons of the input layer. The five output neurons were associated with the five typical corneal shapes. A group of 40 cases was randomly selected from the larger group of 80 corneas and used as the training set. The NN responses were statistically analyzed in terms of sensitivity [true positive/(true positive + false negative)], specificity [true negative/(true negative + false positive)], and precision [(true positive + true negative)/total number of cases]. The mean values for these parameters were, respectively, 78.75, 97.81, and 94%. Although we have used a relatively small training and testing set, results presented here should be considered promising. They are certainly an indication of the potential of Zernike polynomials as reliable parameters, at least in the cases presented here, as input data for artificial intelligence automation of the diagnosis process of videokeratography examinations. This technique should facilitate the implementation and add value to the classification methods already available. We also discuss briefly certain special properties of Zernike polynomials that are what we think make them suitable as NN inputs for this type of application.

Topographic and biomechanical differences between hyperopic and myopic laser in situ keratomileusis

To evaluate the size, shape, and uniformity of the videokeratographic functional optical zone (FOZ) after laser in situ keratomileusis (LASIK) in 2 cohorts of patients with equivalent amounts of preoperative myopic or hyperopic astigmatism. Pepose Vision Institute, St. Louis, Missouri, USA. Eyes with myopic or hyperopic astigmatism (n=27 in each group) that had LASIK with the Visx Star S3 laser were retrospectively selected to match for level of preoperative refractive error. Slit-scanning videokeratography was performed preoperatively and 6 months postoperatively and analyzed using custom software. The FOZ was calculated by analyzing refractive power maps using a region-growing algorithm. Difference maps were generated from slit images and compared for interval change in corneal elevation, tangential curvature, and refractive power. The difference maps were also averaged (mean difference maps) for each target population. A Zernike decomposition of corneal first-surface elevation was performed to compare postoperative values with baseline parameters. The mean postoperative refractive sphere at 6 months was -0.17 diopter (D) +/- 0.66 (SD) and +0.25 +/- 0.85 D in the myopia group and hyperopia group, respectively, and the mean postoperative astigmatism, -0.49 +/- 0.32 D and -0.65 +/- 0.52 D, respectively (P=.11). Based on the refractive power maps, the mean preoperative and postoperative myopic FOZ was 33.09 +/- 7.30 mm(2) and 30.94 +/- 5.43 mm(2), respectively, and the mean hyperopic FOZ, 33.19 +/- 7.96 mm(2) and 37.99 +/- 6.88 mm(2), respectively. After LASIK, there was an increase in magnitude of negative anterior corneal surface spherical-like Zernike values in the myopia group (P<.0001) and an increase in magnitude of positive spherical-like Zernike values in the hyperopia group. Postoperatively, significant induction of corneal surface horizontal coma was noted in hyperopic eyes (P<.0001). Hyperopic eyes, on average, had larger topographic FOZs after LASIK, but with less uniformity of curvature and power change than myopic eyes. Hyperopic LASIK, which involves more transition points along the ablation diameter, produced a less uniform topographic FOZ than typical myopic treatments. Less predictable biomechanical changes from the circumferential release of tension on collagen bundles after midperipheral hyperopic ablation and greater variation in beam centration and the angle of incidence may contribute to the greater variability in corneal curvature and power in hyperopic LASIK than in myopic LASIK.

Orbscan computerized topography: Attributes, applications, and limitations

An extensive electronic search was undertaken in January 2004 to identify all relevant peer-reviewed publications on Orbscan slit-scanning/Placido computerized topography. Ninety-one publications were identified. These address elevation topography and best-fit sphere, accuracy and repeatability of anterior and posterior corneal elevation and keratometric maps, comparison of Orbscan-acquired data and Placido-based computerized videokeratography instruments, pachymetry measurement and correlation with ultrasound, screening eye-bank corneas, detection of keratoconus, identifying corneal ectasia after refractive surgery, and miscellaneous applications. Studies were analyzed and critically compared in relation to attributes, applications, and limitations of Orbscan corneal topography. The review highlights advantages of this technique in assessing the cornea in health and disease and after surgery and identifies specific aspects that require further investigation and clarification.

Model for nonectatic increase in posterior corneal elevation after ablative procedures

To evaluate the response of the posterior cornea after uneventful laser in situ keratomileusis (LASIK) with scanning-slit videokeratography using the change in elevation as a measure of biomechanical remodeling. Hong Kong Sanatorium and Hospital, Hong Kong, China. Three schematic models of possible posterior corneal response mechanisms to laser refractive surgery were proposed: model A, isolated central bulging; model B, forward shift over a large region of the posterior surface; model C, backward peripheral movement into the anterior chamber. A retrospective study was performed to determine which model was consistent with posterior corneal behavior in 2380 eyes of 1255 patients after myopic LASIK performed with the Technolas 217 laser (Bausch & Lomb). Patients received preoperative and 6-month postoperative Orbscan I (version 2.0, Bausch & Lomb) examinations. Preoperative posterior elevation maps were subtracted from postoperative maps of 2380 eyes using 3 fitting protocols. Difference maps were averaged and the results compared to the proposed schematic models. All topography data processing was done using the Ohio State University Corneal Topography Tool. Significant central increases in posterior elevation corresponded with significant peripheral decreases in posterior elevation. This pattern is consistent with schematic model C, backward peripheral movement of the posterior cornea, possibly due to a differential pattern of swelling. Increases in central posterior corneal elevation after LASIK appear to be dominated by backward peripheral corneal swelling into the anterior chamber rather than forward "bulging" of the central posterior cornea. This is consistent with stable remodeling of the corneal shape due to a surgically induced change in structure and fluid balance, rather than an ectatic event.

Corneal Thickness and Elevation Maps Computed From Optical Rotary Scans

Recently, the authors presented a technique that allows corneal thickness measurements along any meridian from optical sections obtained using a rotary scanning system. This paper presents three-dimensional mapping of the corneal thickness and topography of both corneal surfaces, obtained with the rotary system. Corneal thickness and topography are computed from optical sections obtained by illumination with a collimated beam expanded in a fan by a small cylindrical lens. This lens is provided with motor driven rotation to perform automated rotary scanning of the whole cornea. Two cameras are used to capture the images of the optical sections. With this system, it is possible to obtain measurements of corneal thickness, as well as corneal topography. Corneal thickness and elevation maps are shown. Although still under development, this new optical system allows measurement of the thickness of the whole cornea as well as topographical mapping of both corneal surfaces.

Corneal topographic changes following retinal surgery

BackgroundTo study the effect of retinal/ vitreoretinal surgeries on corneal elevations.MethodsPatients who underwent retinal/ vitreoretinal surgeries were divided into 3 groups. Scleral buckling was performed in 11 eyes (Group 1). In 8 (25%) eyes, vitreoretinal surgery was performed along with scleral buckling (Group 2). In 12 eyes, pars plana vitrectomy was performed for vitreous hemorrhage (Group 3). An encircling element was used in all the eyes. The parameters evaluated were best-corrected visual acuity (BCVA), change in axial length, and corneal topographic changes on Orbscan topography system II, preoperative and at 12 weeks following surgery.ResultsThere was a statistically significant increase in anterior corneal elevation in all the three groups after surgery (p = 0.003, p = 0.008 & p = 0.003 respectively). The increase in posterior corneal elevation was highly significant in all the three groups after surgery (p = 0.0000, p = 0.0001 & p = 0.0001 respectively). The increase in the posterior corneal elevation was more than the increase in the anterior elevation and was significant statistically in all the three groups (group I: p = 0.02; group II: p = 0.01; group III: p = 0.008).ConclusionsRetinal/ vitreoretinal surgeries cause a significant increase in the corneal elevations and have a greater effect on the posterior corneal surface.

Temporal changes in optical quality of air-tear film interface at anterior cornea after blink.

To examine temporal changes in the optical quality of the air-tear film interface at the anterior cornea after a blink. Corneal aberrations were determined in fifteen healthy subjects at 1 second time intervals after a blink, up to a total elapsed time of 15 seconds. Corneal aberrations were obtained from corneal elevation maps measured using a Tomey TMS-2N topographer and custom software. All data were decomposed using Zernike polynomials to yield the root mean square (RMS) wavefront deviations, in micrometers, for two pupil diameters (3 and 7 mm). Total wavefront aberration decreased slightly with time in the first few seconds after a blink for both pupil diameters, reaching a minimum after approximately 6 seconds. Thereafter aberrations increased steadily, exceeding the immediate postblink level after approximately 10 seconds. In normal subjects, the contribution of the anterior cornea to the overall ocular aberration remains reasonably stable over the normal interblink interval (approximately 4 seconds) but rises to levels which could perceptibly degrade retinal image quality under circumstances where the interblink interval is increased to exceed 10 seconds, as may occur during the use of visual display screens or when performing difficult tasks.

Comparison of real and computer-simulated outcomes of LASIK refractive surgery

Computer simulations of alternative LASIK ablation patterns were performed for corneal elevation maps of 13 real myopic corneas (range of myopia, -2.0 to -11.5 D). The computationally simulated ablation patterns were designed with biconic surfaces (standard Munnerlyn pattern, parabolic pattern, and biconic pattern) or with aberrometry measurements (customized pattern). Simulated results were compared with real postoperative outcomes. Standard LASIK refractive surgery for myopia increased corneal asphericity and spherical aberration. Computations with the theoretical Munnerlyn ablation pattern did not increase the corneal asphericity and spherical aberration. The theoretical parabolic pattern induced a slight increase of asphericity and spherical aberration, explaining only 40% of the clinically found increase. The theoretical biconic pattern controlled corneal spherical aberration. Computations showed that the theoretical customized pattern can correct high-order asymmetric aberrations. Simulations of changes in efficiency due to reflection and nonnormal incidence of the laser light showed a further increase in corneal asphericity. Consideration of these effects with a parabolic pattern accounts for 70% of the clinical increase in asphericity.

Contact lens fitting over intrastromal corneal rings in a keratoconic patient

Intrastromal corneal ring segments have been used as a refractive surgery method for treatment of myopia. More recently, attempts have been made to use this surgical procedure in patients with keratoconus. A 51-year-old man with keratoconus came to the clinic for a contact lens refit. He had undergone surgical insertion of intrastromal corneal rings in his right eye 7 months earlier in an attempt to obtain acceptable vision without contact lenses. At the time of presentation, the patient was not satisfied with his vision in the right eye and requested an attempt to improve his vision with a contact lens. The resulting corneal shape after surgery presented some complications, but the patient was able to achieve 20/25-vision and adequate comfort in his right eye with a piggyback contact lens system. Insertion of intrastromal corneal rings may be beneficial in patients with early keratoconus or who are contact lens-intolerant. However, patients with more-advanced keratoconus are not likely to achieve adequate vision after the surgery and will require a contact lens. In addition, changes in the corneal anatomy created by the intrastromal rings may make contact lens fitting more challenging. The corneal irregularity may be exacerbated due to drastic variations in corneal elevation over the intrastromal ring and directly adjacent to the ring. This irregularity in elevation leads to difficulty in centering the lens on the eye and problems with bubbles forming under the contact lens over areas of corneal depression. When successfully fitted, contact lenses over intrastromal corneal rings can improve vision significantly.

Zernike polynomial fitting fails to represent all visually significant corneal aberrations.

It is assumed that wavefront error data arising from aberrometry are adequately described by a Zernike polynomial function, although this assumption has not been extensively tested. Inaccuracies in wavefront error may compromise clinical testing and refractive correction procedures. The current retrospective study correlates visual acuity with corneal wavefront error and with the residual surface elevation error after fitting with the Zernike method. Corneal topography maps were obtained from 32 keratoconus cases, 27 postoperative penetrating keratoplasty cases, and 29 postoperative conductive keratoplasty cases (88 total). The best spectacle-corrected visual acuity (BSCVA) for each case ranged from -0.2 to 1.3 logarithm of the minimum angle of resolution (logMAR) units (20/12.5-20/400). Topography was analyzed to determine wavefront error and the elevation fit error for a 4-mm optical zone. The 4th and 10th expansion series were analyzed with the 0th-order (piston) and 1st order (tip and tilt) removed. Linear regression analysis was performed. The difference in root mean square (RMS) error between the 4th- and 10th-order analyses was assessed for both wavefront and elevation fit error. The correlation of BSCVA to wavefront error for 4th-order terms was moderately strong and significant (R2=0.581; P<0.001). The 10th-order correlation for wavefront error had a similar result (R2=0.565; P<0.001), but the regression was not significantly different from the 4th-order result. The correlation of BSCVA to the elevation fit error was strong and significant for the 4th order (R2=0.658; P<0.001). The 10th-order data had a similar result (R2=0.509; P<0.001), and there was no significant difference between the two regressions. Only 72% of the cases showed a shift toward increased wavefront error with the 10th-order series, whereas 18% lost wavefront error. All cases showed a shift toward improved elevation fit with the 10th-order expansion. The wavefront error correlation to acuity was moderately strong, but the corneal elevation fit error also strongly correlated with visual acuity, indicating that Zernike polynomials do not fully characterize the surface shape features that influence vision and that exist in postsurgical or pathologic eyes. In addition, the change in wavefront error when using a larger expansion series was found to increase or diminish somewhat unpredictably. The authors conclude that Zernike polynomials fail to model all the information that influences visual acuity, which may confound clinical diagnosis and treatment.

Corneal forward shift after excimer laser keratorefractive surgery

The excimer laser keratorefractive surgery inevitably compromises structural integrity of the cornea by the surgical tissue subtraction and loss of integrity of Bowman’s membrane. Forward shift of the cornea is commonly seen after both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). Antero-posterior movement of the cornea is evaluated by measuring the posterior corneal elevation with the scanning-slit corneal topography (Orbscan). Eyes with thinner cornea, higher intraocular pressure, and higher myopia requiring greater laser ablation were more predisposed to forward shift of the cornea. After PRK, there was a trend toward progressive forward shift of the cornea, but the progression stabilized 6 months after surgery. Because progressive thinning and expansion of the cornea were not observed during the one-year observation period after PRK, the forward shift of the cornea does not represent true ectasia. Forward shift of both corneal surfaces would add to the tendency toward myopic regression after excimer laser surgery.

Corneal forward shift after excimer laser keratorefractive surgery

The excimer laser keratorefractive surgery inevitably compromises structural integrity of the cornea by the surgical tissue subtraction and loss of integrity of Bowman’s membrane. Forward shift of the cornea is commonly seen after both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). Antero-posterior movement of the cornea is evaluated by measuring the posterior corneal elevation with the scanning-slit corneal topography (Orbscan). Eyes with thinner cornea, higher intraocular pressure, and higher myopia requiring greater laser ablation were more predisposed to forward shift of the cornea. After PRK, there was a trend toward progressive forward shift of the cornea, but the progression stabilized 6 months after surgery. Because progressive thinning and expansion of the cornea were not observed during the one-year observation period after PRK, the forward shift of the cornea does not represent true ectasia. Forward shift of both corneal surfaces would add to the tendency toward myopic regression after excimer laser surgery.

Diagnostic use of a rigid contact lens to show corneal topography abnormalities after laser refractive surgery

We describe 2 cases in which evaluation of rigid contact lens fluorescein patterns were used to delineate and characterize topography irregularities. Contact lens analysis confirmed and localized topography findings of an elevated central island in 1 patient and a semicircular pattern in the other patient. To determine a therapeutic strategy to correct topography irregularities after laser refractive surgery, it is critical to document a corneal elevation and delineate its location.

Posterior corneal topographic changes after retreatment LASIK

Purpose To evaluate posterior corneal topographic changes after retreatment after myopic laser in situ keratomileusis (repeat LASIK). Design Retrospective nonrandomized comparative self-controlled trial. Participants Thirty-three eyes of 23 patients who underwent repeat LASIK for residual myopia. Intervention Retreatments were performed. Slit-scanning corneal topography was performed before and at 1, 3, and 6 months after repeat LASIK. Main outcome measures Posterior corneal topographic changes before and after repeat LASIK were correlated with central corneal pachymetry (preoperatively, before repeat LASIK, after repeat LASIK), residual bed thickness (RBT) and ablation depth (after primary laser in situ keratomileusis (LASIK) and repeat LASIK). On the basis of the amount of posterior corneal elevation after repeat LASIK, the eyes were divided into two groups: group 1 (>66 μm) and group 2 (≤66 μm). Student’s t test/Mann–Whitney rank-sum test was used to determine the significant difference of mean level of each variable between the two groups. Results After primary LASIK, an increase in posterior corneal elevation had significant positive correlation with attempted correction ( P = 0.02), ablation depth ( P = 0.008), and significant negative correlation with preoperative central pachymetry ( P = 0.0003), RBT ( P = 0.0003), and postoperative central pachymetry ( P = 0.00008). After repeat LASIK, the mean increase in posterior corneal elevation had significant negative correlation with preoperative central pachymetry ( P = 0.03). However, its correlation with the ablation depth ( P = 0.43) during repeat LASIK and RBT after repeat LASIK ( P = 0.11) was statistically insignificant. On multiple linear regression analysis, the attempted correction ( P < 0.01) and RBT after primary LASIK ( P < 0.001) were two independent significant determinants of an increase in posterior corneal elevation after primary LASIK. However, for increase in posterior corneal elevation after repeat LASIK, preoperative central pachymetry ( P < 0.01) and posterior corneal elevation increase after primary LASIK ( P < 0.05) were the two significant determinants. Compared with group 2, group 1 had significantly high values of posterior corneal elevation both after primary LASIK ( P = 0.0037) and after repeat LASIK ( P = 0.0000). This group also had significantly low values of central pachymetry preoperatively ( P = 0.0003) and after primary LASIK ( P = 0.0001) and repeat LASIK ( P = 0.0001) surgeries. The mean RBT after primary LASIK ( P = 0.0006) and after repeat LASIK ( P = 0.001) was also lower in group 1. Conclusions Posterior corneal elevation increases after repeat LASIK. Eyes with an increase in posterior corneal elevation after primary LASIK and with thinner cornea are more predisposed.

Corneal and total optical aberrations in a unilateral aphakic patient

Purpose To measure corneal and total optical aberrations in the normal and treated eye of a unilateral aphakic patient to (1) cross-validate techniques in an eye in which corneal and total aberrations should be almost identical (aphakic eye) and (2) compare the interactions of corneal and internal aberrations in the normal eye with those in the aphakic eye. Setting Instituto de Óptica, Consejo Superior de Investigaciones Científicas, Madrid, Spain. Methods Aberrations in both eyes of a unilateral aphakic patient were measured using laser ray tracing. Corneal aberrations were obtained from corneal elevation data measured with a corneal videokeratoscope (Humphrey Instruments) using custom software that performs virtual ray tracing on the measured front corneal surface. Results There was a 98.4% correspondence between the total and corneal aberration pattern in the aphakic eye (6.5 mm pupil). In the normal eye, the total spherical aberration was much lower than the corneal spherical aberration; this did not occur in the aphakic eye. Conclusions The posterior corneal surface contributed slightly to the aberrations in the normal cornea (2% at most). The crystalline lens appears to play a compensatory role in the total spherical aberration in normal eyes.

Time course of changes in corneal forward shift after excimer laser photorefractive keratectomy.

Excimer laser refractive surgery has been reported to induce forward shift of the cornea, but its long-term sequelae remain unknown. To prospectively investigate the time course of changes in corneal elevation after excimer laser photorefractive keratectomy (PRK). We performed PRK on 65 eyes of 34 patients with refractive errors of -1.25 to -10.0 diopters. The anterior/posterior corneal elevation and corneal thickness were measured with a scanning-slit corneal topography system before and 1 week and 1, 3, 6, and 12 months after surgery. Twenty eyes of 10 healthy control subjects underwent similar measurements at 3-month intervals. The posterior corneal surface displayed a mean +/- SD forward shift of 36.6 +/- 25.3 microm 1 week after PRK, which gradually increased to 55.1 +/- 46.1 microm at 1 year. All postoperative values were significantly larger than those of healthy controls (2.4 +/- 8.9 microm; P<.001, Mann-Whitney test). The largest forward shift occurred within the first postoperative week. The progression thereafter was most pronounced from 1 to 6 months, and nearly stabilized at 6 months. The variance of postoperative data was statistically significant (P<.001, repeated-measures analysis of variance). Multiple postoperative comparisons demonstrated significant differences between measurements at 1 week and 6 months (P =.002, Tukey Honestly Significant Difference), at 1 week and 1 year (P<.001), at 1 and 6 months (P<.001), and at 1 month and 1 year (P<.001). Progression of forward shift was more prominent in eyes with less preoperative corneal thickness and greater myopia that required larger laser ablation. We observed no progressive thinning and expansion of the cornea during the 1-year follow-up, which refuted the occurrence of true ectasia. A statistically significant correlation was found between the amount of myopic regression and the forward shift of the cornea (Pearson correlation coefficient, r = -0.37; P =.005). Photorefreactive keratectomy induced forward shift of the cornea, which is not true corneal ectasia. The largest forward shift occurred within the first postoperative week. Changes were progressive up to 6 months postoperatively, but became almost stable thereafter. Eyes with thinner cornea and higher myopia, requiring greater photoablation, are more predisposed to progression. Forward shift of both corneal surfaces added to the tendency toward myopic regression after PRK.

Standardized color-coded scales for anterior and posterior elevation maps of scanning slit corneal topography

Purpose To find the most appropriate color-coded scales for the anterior and posterior elevation maps of scanning slit topography in the screening of abnormal corneas such as keratoconus. Design Retrospective case-control study. Participants Eighty eyes of 40 normal subjects and 175 eyes of 95 patients with keratoconus. Intervention Anterior and posterior corneal elevations were assessed using Orbscan 2. Best-fit sphere maps were drawn with several color-coded scales: 2-, 5-, 10-, and 20-μm height per each color interval. Main outcome measures The maps were judged to be abnormal when more than three colors (discriminant number) were found within the central 3-mm area. For each color-coded scale, sensitivity, specificity, positive predictive value, negative predictive value, and sensitivity + specificity were calculated. After determining the most appropriate color-coded scales for the anterior and posterior elevation maps, validity of the discriminant number was assessed. By varying the discriminant number from two to eight, receiver operator characteristic (ROC) curves were created using the sensitivity and specificity for each threshold number. Results The highest sensitivity + specificity values and highly balanced predictive values were obtained with the 10- and 20-μm scales for the anterior and posterior elevation maps, respectively. The ROC curve analyses showed that the best discriminant color number is three, indicating that maps with four or more colors within the central 3-mm area are judged abnormal in screening. Conclusions The 10- and 20-μm interval color scales are most appropriate for the anterior and posterior elevation maps of the scanning slit topography, respectively.