Corneal Topography Measurements Research Articles

The Individual Virtual Eye: a Computer Model for Advanced Intraocular Lens Calculation

Purpose To describe the individual virtual eye, a computer model of a human eye with respect to its optical properties. It is based on measurements of an individual person and one of its major application is calculating intraocular lenses (IOLs) for cataract surgery. Methods The model is constructed from an eye's geometry, including axial length and topographic measurements of the anterior corneal surface. All optical components of a pseudophakic eye are modeled with computer scientific methods. A spline-based interpolation method efficiently includes data from corneal topographic measurements. The geometrical optical properties, such as the wavefront aberration, are simulated with real ray-tracing using Snell's law. Optical components can be calculated using computer scientific optimization procedures. The geometry of customized aspheric IOLs was calculated for 32 eyes and the resulting wavefront aberration was investigated. Results The more complex the calculated IOL is, the lower the residual wavefront error is. Spherical IOLs are only able to correct for the defocus, while toric IOLs also eliminate astigmatism. Spherical aberration is additionally reduced by aspheric and toric aspheric IOLs. The efficient implementation of time-critical numerical ray-tracing and optimization procedures allows for short calculation times, which may lead to a practicable method integrated in some device. Conclusions The individual virtual eye allows for simulations and calculations regarding geometrical optics for individual persons. This leads to clinical applications like IOL calculation, with the potential to overcome the limitations of those current calculation methods that are based on paraxial optics, exemplary shown by calculating customized aspheric IOLs.

Assessing Visual Quality With the Point Spread Function Using the NIDEK OPD-Scan II

To present the use of the point spread function (PSF) metric pre- and postoperatively for the assessment of visual quality in cataract and refractive surgery. Case examples of cataract and refractive surgery and the effect of accommodation are presented. All PSF measurements were obtained using the NIDEK OPD-Scan II, and corneal internal and total aberrations were simulated using the NIDEK OPD-Station software. All eyes underwent corneal topography, wavefront aberrometry, autorefraction, keratometry, and pupillometry measurements pre- and postoperatively using the OPD-Scan II. The PSF was used to assess visual quality. Four case examples including refractive surgery, aspheric multifocal intraocular lens (IOL) implantation, toric IOL implantation, and the difference in PSF due to accommodation are presented. The PSF in refractive and cataract surgery is a clinically valuable metric to evaluate the patient's visual quality pre- and postoperatively. The OPD-Scan II can measure the effect of accommodation and distant fixation providing an assessment of changes in visual quality.

Aberrations and Pupil Location under Corneal Topography and Hartmann-Shack Illumination Conditions

This study was conducted to determine the magnitude of pupil center shift between the illumination conditions provided by corneal topography measurement (photopic illuminance) and by Hartmann-Shack aberrometry (mesopic illuminance) and to investigate the importance of this shift when calculating corneal aberrations and for the success of wavefront-guided surgical procedures. Sixty-two subjects with emmetropia underwent corneal topography and Hartmann-Shack aberrometry. Corneal limbus and pupil edges were detected, and the differences between their respective centers were determined for both procedures. Corneal aberrations were calculated using the pupil centers for corneal topography and for Hartmann-Shack aberrometry. Bland-Altmann plots and paired t-tests were used to analyze the differences between corneal aberrations referenced to the two pupil centers. The mean magnitude (modulus) of the displacement of the pupil with the change of the illumination conditions was 0.21+/-0.11 mm. The effect of this pupillary shift was manifest for coma corneal aberrations for 5-mm pupils, but the two sets of aberrations calculated with the two pupil positions were not significantly different. Sixty-eight percent of the population had differences in coma smaller than 0.05 microm, and only 4% had differences larger than 0.1 microm. Pupil displacement was not large enough to significantly affect other higher-order Zernike modes. Estimated corneal aberrations changed slightly between photopic and mesopic illumination conditions given by corneal topography and Hartmann-Shack aberrometry. However, this systematic pupil shift, according to the published tolerances ranges, is enough to deteriorate the optical quality below the theoretically predicted diffraction limit of wavefront-guided corneal surgery.

Corneal elevation and thickness in relation to the refractive status measured with the Pentacam Scheimpflug system

To prospectively evaluate the elevation and thickness data in patients with different types of refractive errors. Department of Ophthalmology, Ankara University Faculty of Medicine, Ankara, Turkey. After the refractive errors in 215 consecutive patients were determined, corneal topography measurements with the Pentacam Scheimpflug system were taken in the right eye of all patients and the right eye of 31 healthy emmetropic volunteers. The eyes with refractive errors were assigned to 1 of the following 4 groups: myopia, myopic astigmatism, high myopia, and hyperopia. The means of the parameters of 3 Pentacam measurements were evaluated and compared. Eyes with high myopia had significantly lower mean corneal thickness and volume measurements and higher mean anterior chamber depth (ACD) and anterior chamber volume (ACV) measurements than eyes in the other groups. The mean ACD, ACV, and anterior chamber angle were significantly lower in hyperopic eyes than in the other groups. The mean keratometry readings were statistically significantly flatter in the hyperopia group than in the other 4 groups. Eyes with high myopia had thinner corneas and deeper anterior chambers than emmetropic eyes and eyes in the other ametropic groups. Excluding eyes with hyperopia, which had significantly flat anterior and posterior elevation measurements, the elevation measurements in eyes with myopic refractive errors did not differ from each other or from those in emmetropic eyes. These findings may help clinicians and refractive surgeons using the Pentacam to better define normal from abnormal in the clinical setting.

Structural and optical changes in the eye after soft contact lens wear

Abstract Purpose To investigate the magnitude and aetiology of structural and optical changes in the cornea after short‐term hydrogel contact lens wear and to explore the effect of these changes on visual performance. Methods Sixteen healthy subjects were fitted with a low‐Dk hydrogel contact lens (CH) in one eye and a silicone hydrogel (SH) lens in the contralateral eye. Subjects wore the lenses for 1 month in daily wear (DW) and 1 week in extended wear (EW), with at least 1 week ‘wash out’ in between. All lenses were plano powered and were manufactured in matched designs with the same specifications. Measurements of corneal topography, corneal pachymetry (apical and peripheral), and corneal and ocular aberrations were performed at baseline and after lens wear using a Scheimpflug imaging system (Oculus Pentacam) and a Hartmann‐Shack aberrometer (IRX3). Best corrected visual acuity (BCVA, logMAR) and contrast sensitivity measurements (CS, Pelli‐Robson) were also performed. Results The optical and structural changes in the cornea after 1 month of DW and 1 week of EW for both the CH and SH materials were of low magnitude. The topographic and pachymetric changes observed were not found to be significant (p>0.05, repeated measures ANOVA). After 1 month of DW, however, the corneal root mean square of quatrefoil aberrations was found to be significantly different after CH contact lens wear (mean difference ‐0.021 µm; 95% confidence interval for difference: ‐0.037 to ‐0.06 µm; p=0.003). BCVA and CS did not change significantly after lens wear. Conclusion Our data showed that the CH and SH lenses tested do not significantly alter the structure or the optical performance of the cornea after one month of daily wear and 1 week of extended contact lens wear.

Objective Assessment of the Quality of Vision Before and After Repositioning of a Dislocated Iris-fixated Aphakic Anterior Chamber Lens

To document severe visual symptoms due to the dislocation of an iris-fixated aphakic anterior chamber lens using wavefront aberrometry. A patient complaining of severe visual blur and distortion due to a decentered Artisan iris-fixated anterior chamber lens was evaluated pre- and postoperatively after recentration. The patient underwent complete ophthalmic evaluation including the measurement of corneal topography and wavefront aberrations using the NIDEK OPD-Scan. OPD Station software was used to correlate and simulate the patient's symptoms. After the repositioning procedure, the visual symptoms resolved, and high magnitudes of tilt (7.536 microm) and high order aberrations (5.129 microm) were dramatically reduced by simple lens repositioning (4.40-mm pupil). The cylinder magnitude was reduced from 6.25 to 0.75 diopters. These are the first diagnostic images of this kind in a highly aberrated eye with iris-fixated anterior chamber lens dislocation. The images documented the optical effect and visual consequences of IOL decentration and subsequent recentration.

Estimating Variability in Placido-Based Topographic Systems

To describe a new software tool for the detailed presentation of corneal topography measurements variability by means of color-coded maps. Software was developed in Visual Basic to analyze and process a series of 10 consecutive measurements obtained by a topographic system on calibration spheres, and individuals with emmetropic, low, high, and irregular astigmatic corneas. Corneal surface was segmented into 1200 segments and the coefficient of variance of each segment's keratometric dioptric power was used as the measure of variability. The results were presented graphically in color-coded maps (Variability Maps). Two topographic systems, the TechnoMed C-Scan and the TOMEY Topographic Modeling System (TMS-2N), were examined to demonstrate our method. Graphic representation of coefficient of variance offered a detailed representation of examination variability both in calibration surfaces and human corneas. It was easy to recognize an increase in variability, as the irregularity of examination surfaces increased. In individuals with high and irregular astigmatism, a variability pattern correlated with the pattern of corneal topography: steeper corneal areas possessed higher variability values compared with flatter areas of the same cornea. Numerical data permitted direct comparisons and statistical analysis. We propose a method that permits a detailed evaluation of the variability of corneal topography measurements. The representation of the results both graphically and quantitatively improves interpretability and facilitates a spatial correlation of variability maps with original topography maps. Given the popularity of topography based custom refractive ablations of the cornea, it is possible that variability maps may assist clinicians in the evaluation of corneal topography maps of patients with very irregular corneas, before custom ablation procedures.

Endothelial Keratoplasty Using Donor Tissue Not Suitable for Full-Thickness Penetrating Keratoplasty

To evaluate the use of corneal donor tissue deemed unsuitable for full-thickness penetrating keratoplasty (PK) for use in deep lamellar endothelial keratoplasty (DLEK) and to compare postoperative results to those of DLEK surgery using donor tissue that is suitable for PK. Small-incision DLEK surgery was performed using 39 donor corneas unsuitable for PK. Thirty-five donors had anterior scars or opacities, 3 donors had pterygia within the 8-mm zone, and 1 had prior LASIK. All donor preparation was completed by manual stromal dissection. The DLEK surgical and postoperative courses were reviewed. Preoperative and 6-month postoperative results of this study group were compared with a control group consisting of the first 55 consecutive small-incision DLEK patients receiving donor corneas that had no criteria excluding them from use in PK. Four eyes in the study group and 1 eye in the control group had the confounding variables of the presence of an anterior-chamber lens or surgical vitrectomy with macular disease in the recipient eye. There was no significant difference in preoperative measurements of best spectacle-corrected visual acuity (BSCVA; P = 0.372), donor endothelial cell density (ECD; P = 0.749), or corneal topography [surface regularity index (SRI), P = 0.485; or surface asymmetry index (SAI), P = 0.154] between the 2 groups. For the patients receiving corneas deemed unacceptable for PK, at 6 months after surgery, the vision (P = 0.002) and corneal topography measurements improved significantly from before surgery (SRI, P < 0.001; SAI, P < 0.001), and there was no significant change in refractive astigmatism (P = 0.240). There was a significant difference in the vision at 6 months postoperatively between the overall study group and the control group, with the mean vision of the study group at 20/56 and the control group at 20/43 (P = 0.015). If eyes with known cystoid macular edema (CME) and vitrectomy are removed from each group, there is no significant difference in vision at 6 months between the study group and the control group (P = 0.110), with the average BSCVA of those receiving donor corneas unsuitable for PK equal to 20/48 (range, 20/25-20/200) and the average vision for those receiving PK-acceptable donor tissue equal to 20/43 (range, 20/20-20/80). The 6-month average refractive astigmatism of the study group was 1.12 +/- 0.99 D (range, 0.00-4.00 D), and the average endothelial cell count was 2064 +/- 396 cells/mm(2) (range, 1208-2957 cells/mm(2)). There was no significant difference in 6-month postoperative endothelial cell count (P = 0.443), refractive astigmatism (P = 0.567), or corneal topography (SRI, P = 0.332; SAI, P = 0.110) in study patients who received corneas unsuitable for PK compared with control patients who received corneas suitable for PK. Endothelial keratoplasty such as DLEK surgery with manual donor preparation broadens the donor pool by enabling corneas that cannot be used for PK to be used for selective endothelial transplantation without deleterious postoperative results.

External Factors Affecting Data Acquisition During Corneal Topography Examination

To analyze the factors affecting data acquisition during corneal topography examination with the Medmont E-300 videokeratoscope and to provide strategies to minimize their effects. Sixty eyes from thirty young adults were examined. A second observer registered incidences with the potential to affect data acquisition. Those factors were correlated with the difficulty of measurements as judged subjectively by the practitioner who performed the examination. Measurements of axial curvature were analyzed to evaluate the variability expressed as intrasession and intersession coefficient of variation and the standard error of the mean (SEM). The level of difficulty rated by the practitioner was in general low, with 70% of the eyes being easy or very easy to measure. For the remaining 30% of the eyes, corneal topography measurements were considered to be difficult (27%) or very difficult (3%). Of the external parameters investigated, only fixation instability (P<0.001, chi2) and the need for head repositioning (P=0.024, chi2) were associated significantly with a higher level of difficulty, as rated subjectively by the practitioner. Further analysis showed that some external factors, including those previously mentioned and others related to tear instability, affect the variability of measurements at certain corneal locations, particularly in the vertical meridian when related to tear instability and in the horizontal meridian when related to the need for head repositioning on the chin rest owing to physiognomy interferences with the keratoscope cone. Intersession SEM improved when three readings from each session were considered. The level of subjective difficulty found during videokeratoscopy examination is correlated strongly with fixation instability and the need for head reorientation in the chin rest, whereas tear-related events seem to be less relevant in the practitioner perception of test ease or difficulty. Those factors have relevance in measurement variability.

Uncorrectable Oblique Astigmatism and Impaired Binocular Vision in Case of Orbital Cherubism

A 22-year-old Japanese woman presented with diplopia of 12 years duration. She had a history of cherubism and had had repeated osteoplasties of the mandible. A left hypertropia with “A” pattern exotropia, bilateral superior oblique overaction, inferior oblique underaction, and incyclotorsion were present. The bilateral field of fusion determined by Goldmann perimetry showed diplopia in all fields except 10–20 degrees of the central field. Corneal topographic measurements showed characteristic oblique astigmatism due to the incyclotorsion. Titmus stereoscopic test showed depressed stereoacuity. The astigmatism was too large and axes too variable during eye movements to be corrected with spectacles, and the retraction of the lower lid prevented a proper fitting of contact lens. Thus, we conclude that orbital cherubism has many ophthalmological difficulties that are not easily correctable or treatable.

The effect of Gonioscopy on keratometry and corneal surface topography

BackgroundBiometric procedures such as keratometry performed shortly after contact procedures like gonioscopy and applanation tonometry could affect the validity of the measurement. This study was conducted to understand the short-term effect of gonioscopy on corneal curvature measurements and surface topography based Simulated Keratometry and whether this would alter the power of an intraocular lens implant calculated using post-gonioscopy measurements. We further compared the effect of the 2-mirror (Goldmann) and the 4-mirror (Sussman) Gonioscopes.MethodsA prospective clinic-based self-controlled comparative study. 198 eyes of 99 patients, above 50 years of age, were studied. Exclusion criteria included documented dry eye, history of ocular surgery or trauma, diabetes mellitus and connective tissue disorders. Auto-Keratometry and corneal topography measurements were obtained at baseline and at three follow-up times – within the first 5 minutes, between the 10th-15th minute and between the 20th-25th minute after intervention. One eye was randomized for intervention with the 2-mirror gonioscope and the other underwent the 4-mirror after baseline measurements. t-tests were used to examine differences between interventions and between the measurement methods. The sample size was calculated using an estimate of clinically significant lens implant power changes based on the SRK-II formula.ResultsClinically and statistically significant steepening was observed in the first 5 minutes and in the 10–15 minute interval using topography-based Sim K. These changes were not present with the Auto-Keratometer measurements. Although changes from baseline were noted between 20 and 25 minutes topographically, these were not clinically or statistically significant. There was no significant difference between the two types of gonioscopes. There was greater variability in the changes from baseline using the topography-based Sim K readings.ConclusionReversible steepening of the central corneal surface is produced by the act of gonioscopy as measured by Sim K, whereas no significant differences were present with Auto-K measurements. The type of Gonioscope used does not appear to influence these results. If topographically derived Sim K is used to calculate the power of the intraocular lens implant, we recommend waiting a minimum of 20 minutes before measuring the corneal curvature after gonioscopy with either Goldmann or Sussman contact lenses.

Corneal and total wavefront aberrations in phakic and pseudophakic eyes after implantation of monofocal foldable intraocular lenses

To compare the correlation between corneal and total wavefront aberrations in normal phakic and pseudophakic eyes after implantation of foldable monofocal intraocular lenses (IOLs). University Hospital, Eye Clinic, Zurich, Switzerland. Wavefront aberrations and corneal topography of 29 eyes that had cataract surgery with implantation of hydrophobic monofocal foldable IOL (AcrySof, Alcon Labs) were measured at least 2 months postoperatively and compared with wavefront measurements performed in 33 normal young phakic eyes. The total wavefront aberrations were measured by means of a Tscherning wavefront sensor at a wavelength of 660 nm (Allegro Wave Analyzer, WaveLight Laser Technology). The corneal aberrations were derived from corneal topography measurements ascertained with a Placido-based topography system (Keratograph 70600, Oculus). The correlations between corneal and total wavefront aberrations were calculated for all Zernike coefficients from 2nd up to 6th order. There was a significant correlation between corneal and total wavefront aberrations in astigmatism C3 and C5 as well as for all 3rd-order Zernike coefficient in both groups (except C8 in the pseudophakic group). The correlation between corneal and total astigmatism (C3 and C5) was higher in the pseudophakic than in the phakic eyes. In contrast, the correlation for the coma-like aberrations was weaker in the pseudophakic eyes (R>0.18) than in the group of phakic eyes (R>0.58). In both groups, there was no significant correlation between spherical aberration C12 of the cornea and the C12 of the total eye. After cataract surgery with an IOL implantation, both vertical and horizontal coma, as well as spherical aberration, were of higher value than in normal eyes. The compensation effect for corneal aberrations of the natural lens is absent in the IOL and explains these findings. The corneal aberrations in pseudophakic eyes reflect better the optical quality of the total eye than the phakic eyes. Nevertheless, the missing correlation in some specific aberrations, such as C8 and C10, shows the inability of corneal topography to provide suitable information on the optical quality of the total eye after cataract surgery. Thus, both corneal and total wavefront measurements are relevant for the assessment of outcomes after cataract surgery.

Intraocular lens power calculation using ray tracing following excimer laser surgery

To evaluate intraocular lens (IOL) power calculation using ray tracing in patients presenting with cataract after excimer laser surgery. Ten eyes of seven consecutive patients who presented for cataract surgery following excimer laser treatment without any pre-refractive biometry data were enrolled in this prospective clinical study. Preoperatively, IOL power calculation was performed using a ray tracing software called OKULIX. Keratometry data (C-Scan) were imported and axial length (IOLMaster) was entered manually. Accuracy of IOL power calculation was investigated by subtracting attempted and achieved spherical equivalent. Mean spherical equivalent was -3.51+/-2.77 D (range -10.38 to -0.5 D) preoperatively and -1.01+/-1.08 D (range -2.5 to +0.75 D) postoperatively. Mean error was 0.31+/-0.84 D, mean absolute error was 0.74+/-0.46 D, and IOL calculation errors ranged from -1.39 to +1.47 D. A total of 40% of eyes were within +/-0.5 D, 70% within +/-1.0 D, and 100% within +/-1.5 D. Three eyes with corneal radii over 10 mm showed calculation errors exceeding +/-1.0 D. Mean best-corrected visual acuity increased from 20/60 to 20/30 postoperatively. IOL power calculation after excimer laser surgery can be difficult, especially when pre-refractive keratometry values are not available. In these cases, ray tracing combined with corneal topography measurements provides reliable and satisfactory postoperative results. However, it is advisable to be careful when calculating IOL power for eyes with corneal radii exceeding 10 mm because of slightly higher prediction errors.

Early and Late Effects of Pterygium Surgery on Corneal Topography

The aim of this study was to compare the corneal topographic changes at the early and late postoperative period after pterygium surgery. Sixteen eyes of 15 patients with primary pterygium were included in the study. All of the pterygia were excised primarily. Computerized corneal topography measurements were taken before surgery and at the second week (postoperative early period) and third month (postoperative late period) after the operation. Keratometric astigmatism, total mean corneal refractive power, and flattened or steepened areas of the cornea were detected at each visit. The topographical changes were evaluated using the analysis of variance. Multiple comparison analyses were made with the Tukey HSD test. Keratometric astigmatism was 2.58 +/- 2.33 diopters (D) at the preoperative period, 1.30 +/- 1.24 D at the postoperative early period, and 0.61 +/- 0.38 D at the postoperative late period (P < .001). The surgically induced corneal astigmatism was 3.23 +/- 2.33 D (range, 0.52 to 8.32 D) at the postoperative early period and 2.70 +/- 2.24 D (range, 0.47 to 7.72 D) at the postoperative late period (P = .287). Astigmatism was 81.25% with-the-rule at the preoperative period, 43.75% oblique and 37.50% against-the-rule at the postoperative early period, and 62.50% with-the-rule at the postoperative late period. Total mean corneal refractive power was 42.51 +/- 1.99 D at the preoperative period, 43.95 +/- 1.58 D at the postoperative early period, and 43.89 +/- 1.80 D at the postoperative late period (P = .014). Corneal topographic changes induced by pterygium greatly improve at the postoperative early period. However, normal corneal topography patterns are formed at the postoperative late period.

Clinical and Molecular Characterization of a Family With Autosomal Recessive Cornea Plana

Autosomal recessive cornea plana is characterized by a flattened corneal surface associated with hyperopia and various anterior segment abnormalities. Mutations have been detected in the keratocan gene (KERA), a member of the small leucine-rich proteoglycan family. To clinically and molecularly characterize a consanguineous family of Hispanic origin in which 3 individuals are affected with cornea plana. Clinical ophthalmic examination, including corneal topography and axial eye length measurement, was performed on 7 family members. Molecular analysis of KERA was performed on DNA from each family member who had been examined. All 3 affected individuals showed extreme flattening of the cornea (< 36 diopters [D]), normal axial eye lengths, and hyperopia greater than 6.25 D (spherical equivalent). Anterior segment abnormalities included scleralization of the cornea and central iris strands to the corneal endothelium. Affected individuals were homozygous for a novel mutation in KERA. The sequence change was found in exon 2, which results in an asparagine to aspartic acid change at codon 131. This amino acid change occurs within a highly conserved leucine-rich repeat of keratocan. The cause of disease in this family is likely to be a mutation in exon 2 of KERA. Other mutations in KERA known to cause cornea plana also fall within the region encoding the leucine-rich repeat motifs and are predicted to affect the tertiary structure of the protein. This is the first report of the identification of a mutation within KERA in a family of Hispanic origin with autosomal recessive cornea plana. Although the vast majority of cases of cornea plana are in individuals of Finnish descent, this report demonstrates the occurrence of the disease in other populations.

Reliability of Orbscan II topography measurements in relation to refractive status

To investigate the reliability of corneal topography measurements using the Orbscan II topography system (Bausch & Lomb) not only referring to a normal cohort but also to different refractive conditions. Department of Ophthalmology, Ruprecht-Karls-University of Heidelberg, Heidelberg, Germany. Eighty patients (mean age 46.4 +/- 19.0 years) were assigned to 4 refractive groups (Group A: emmetropia (n=20); Group B: astigmatism (n=20) (-1.98 +/- 1.77 diopters [D]); Group C: hyperopia (n=20) (+4.84 +/- 1.6 D); Group D: myopia (n=20) (-9.64 +/- 3.79 D). Three measurements were performed in a series. Thirteen defined, standardized points of the entire cornea (apex, 3.0 mm, 5.0 mm, and 7.0 mm zone) were evaluated for 3 different maps (anterior elevation, pachymetry, keratometry). In all 80 patients, the following mean values were found in relation to analyzed zones: with regard to the anterior elevation map, the values decreased from 5.53 microm (center) to -6.52 microm (7.0 mm zone), the corneal thickness increased from 549.41 microm to 638.63 microm peripheral and the keratometry from 43.86 D (apex) to 45.4 D (7.0 mm zone), respectively. Analysis of the 3 different maps in all 4 refraction groups showed a tendency toward an increase in SD from the center to the 7.0 mm zone for anterior elevation and pachymetry maps. For keratometry, however, the lowest SD was found in the 7.0 mm zone. In particular, patients with hyperopia showed significant differences (P<.01), compared with emmetropic patients; the SD was higher with regard to anterior elevation as well as keratometry in peripheral zones. Group B (astigmatism) showed significantly higher SD for anterior elevation in zones 3.0, 5.0, and 7.0 mm when compared with emmetropic patients. The repeated Orbscan II measurements showed SD in the micrometer range for anterior elevation and pachymetry but values between 0.48 D and 0.97 D for keratometric data. Patients with astigmatism and especially hyperopia showed significantly higher SD values in peripheral zones for anterior elevation and keratometry, indicating a lower reliability, compared with the emmetropic cohort. However, the different ages of the patients could also be a possible explanation for these findings. Therefore, the Orbscan II seems to be a predictable and useful device for measuring corneal topography.

Effect of Glycemic Control on Refractive Changes in Diabetic Patients With Hyperglycemia

To investigate the effect of intensive glycemic control on hyperglycemia- induced changes in refraction, corneal topography, lenticular and corneal thickness in diabetic patients. Eighteen diabetic patients with plasma glucose >300 mg/dl were enrolled in the study consecutively. Autorefraction, C-Scan corneal topography, Javal keratometry, corneal pachymetry and ultrasonic biometric measurements were carried out at presentation and after metabolic control of blood sugar (plasma glucose profile <200 mg/dl). One eye of each patient was selected randomly for statistical analysis. Mean plasma glucose levels were 356.00 mg/dl before and 133.78 mg/dl after treatment. The median values of the autorefractometric measurements were - 0.125 D on admission and + 0.375 D at the second examination. The difference in the refraction was statistically significant (P = 0.022). Nine of 18 patients became hyperopic, 2 became myopic and 7 showed no change after treatment. There was a significant change in the corneal topographic keratometric measurements at the flattest corneal meridian after treatment (P = 0.037). After treatment no statistically significant changes were observed in the pachymetric corneal thickness, anterior chamber depth, biometric dioptric lens calculations and Javal keratometric data. Hyperglycemia is the major cause of the transient refractive changes in diabetic patients. Following intensive medical treatment, a considerable number of patients tend to become more hyperopic compared with the hyperglycemic state. During the treatment period, changes in the corneal topographic parameters might be a potential source of error for keratorefractive and cataract surgery.

Repeatability of Corneal Topography Measurement in Keratoconus with the TMS-1

The purpose of this study was to report the test-retest variability of simulated indices derived from the TMS-1 topography instrument (Tomey Technology, Waltham, MA) in keratoconus subjects enrolled in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. Four images were taken at an initial visit and at a repeat visit several weeks later. From these images, 17 indices were simulated from published formulas. Mixed-model analysis was used on test-retest data from the TMS-1 videokeratography instrument during the baseline year. This analysis yields estimates of within- and between-visit variability. Repeatability analysis revealed that within-visit standard errors were 1.0 to 5.9 times greater in keratoconus eyes than in normal controls when two images were analyzed from each visit. These values changed only slightly when more images were used. The ratio of between-visit standard errors of the indices were nearly equally greater than normal controls for (0.9-4.6 and 0.9-4.3) two images per eye and all images per eye, respectively. These results suggest that the repeatability of simulated indices derived from TMS-1 topography in keratoconus subjects is poorer than in normal controls.

Curvature sensor for the measurement of the static corneal topography and the dynamic tear film topography in the human eye

A system to measure the topography of the first optical surface of the human eye noninvasively by using a curvature sensor is described. The static corneal topography and the dynamic topography of the tear film can both be measured, and the topographies obtained are presented. The system makes possible the study of the dynamic aberrations introduced by the tear film to determine their contribution to the overall ocular aberrations in healthy eyes, eyes with corneal pathologies, and eyes wearing contact lenses.

Alcon CustomCornea Wavefront-guided Retreatments After Laser in situ Keratomileusis

To evaluate the outcome of wavefront-guided ablations for the correction of residual myopia and astigmatism after standard laser in situ keratomileusis (LASIK). Twenty nine eyes of 26 patients who underwent wavefront-guided LASIK retreatment with Alcon CustomCornea (Alcon Laboratories Inc, Fort Worth, Tex) were evaluated. Complete ophthalmologic examination, corneal topography, and wavefront measurements were performed. Uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), manifest refraction, and wavefront analysis were evaluated preoperatively, 1 week, 3 months, and 6 months after surgery. Wavefront measurements were assessed using the LADARWave device. Statistical analysis was performed using the McNemar test and percentages of success. One week postoperatively, UCVA was > or =20/40 in 100% of eyes (> or =20/20 in 31%) and BSCVA was > or =20/40 in 100% (> or =20/20 in 73%). Wavefront analysis showed a decrease in total aberrations, high order aberrations, defocus, coma, spherical aberration, and other terms of higher order aberrations at 1-week follow-up. Three months postoperatively, UCVA was > or =20/40 in 100% of eyes (> or =20/20 in 38%) and BSCVA was > or =20/40 in 100% (> or =20/20 in 81%). Six months postoperatively, UCVA was > or =20/40 in 100% of eyes (> or =20/20 in 60%) and BSCVA was > or =20/40 in 100% (> or =20/20 in 90%). Wavefront analysis showed decrease in total aberration, high order aberration, defocus, coma, and spherical aberration. Wavefront-guided LASIK retreatment in post-LASIK eyes represents a good option for laser vision correction. All eyes showed reduction in pre-existing total aberrations. Some high order aberration components decreased in this initial series. Further follow-up is necessary to assess the initial predictability of wavefront-guided LASIK upgrade.