EyeSys corneal topography measurement applied to calibrated ellipsoidal convex surfaces

Corneal topography with an aberrometry-topography system

PURPOSE

To investigate the agreement between the central corneal radii and corneal eccentricity measurements generated by the new Wave Analyzer 700 Medica (WAV) compared to the Keratograph 4 (KER) and to test the repeatability of the instruments.

METHODS

20 subjects (10 male, mean age 29.1 years, range 21-50 years) were recruited from the students and staff of the Cologne School of Optometry. Central corneal radii for the flat (r_c/fl) and steep (r_c/st) meridian as well as corneal eccentricity for the nasal (e_nas), temporal (e_temp), inferior (e_inf) and superior (e_sup) directions were measured using WAV and KER by one examiner in a randomized order.

RESULTS

Central radii of the flat (r_c/fl) and steep (r_c/st) meridian measured with both instruments were statically significantly correlated (r = 0.945 and r = 0.951; p < 0.001). Comparison between the WAV and KER showed that r_c/fl and r_c/st measured with WAV were significantly steeper than those measured with KER (p < 0.001). Corneal eccentricities were statistically significantly correlated in all meridians (p < 0.05). Compared to KER, e_temp and e_sup measured with WAV were greater (p < 0.05), while there were no statistically significant differences for e_nas and e_inf (p = 0.350 and p = 0.083). For the central radii, repeated measurements were not significantly different for the KER or WAV (p > 0.05). Limits of agreement (LoA) indicate a better repeatability for the KER compared to WAV.

CONCLUSIONS

Corneal topography measurements captured with the WAV were strongly correlated with the KER. However, due to the differences in measured corneal radii and eccentricities, the devices cannot be used interchangeably. For corneal topography the KER demonstrated better repeatability.

Validity of autorefractor based screening method for irregular astigmatism compared to the corneal topography- a cross sectional study

AIM

To present a method of screening for irregular astigmatism with an autorefractor and its determinants compared to corneal topography.

METHODS

This cross-sectional validity study was conducted in 2013 at an eye hospital in Spain. A tabletop autorefractor (test 1) was used to measure the refractive status of the anterior surface of the cornea at two corneal meridians of each eye. Then corneal topography (test 2) and Bogan's classification was used to group eyes into those with regular or no astigmatism (GRI) and irregular astigmatism (GRII). Test 1 provided a single absolute value for the greatest cylinder difference (Vr). The receiver operating characteristic (ROC) were plotted for the Vr values measured by test 1 for GRI and GRII eyes. On the basis a Vr value of 1.25 D as cut off, sensitivity, specificity were also calculated.

RESULTS

The study sample was comprised of 260 eyes (135 patients). The prevalence of irregular astigmatism was 42% [95% confidence interval (CI): 36, 48]. Based on test 2, there were 151 eyes in GRI and 109 eyes in GRII. The median Vr was 0.75 D (25% quartile, 0.5 D) for GRI and 1.75 D (25% quartile, 1.25 D) for GRII. The area under curve was 0.171 for GRI and 0.83 for GRII. The sensitivity of test I was 78.1% and the specificity was 76.1%.

CONCLUSION

A conventional autorefractor can be effective as a first level screening method to detect irregular corneal astigmatism in places where corneal topography facilities are not available.

Anterior corneal asphericity calculated by the tangential radius of curvature

We propose a method of calculating the corneal asphericity (Q) and analyze the characteristics of the anterior corneal shape using the tangential radius. Fifty-eight right eyes of 58 subjects were evaluated using the Orbscan II corneal topographer. The Q-values of the flat principal semi-meridians calculated by the sagittal radius were compared to those by the tangential radius. Variation in the Q-value with semi-meridian in the nasal and temporal cornea calculated by the tangential radius was analyzed. There were significant differences in Q-values (P<0.001) between the two methods. The mean Q-values of the flat principal semi-meridians calculated by tangential radius with -0.33 ± 0.10 in the nasal and -0.22 ± 0.12 in the temporal showed more negative than the corresponding Q-values calculated by the sagittal radius. The Q-values calculated by tangential radius became less negative gradually from horizontal semi-meridians to oblique semi-meridians in both nasal and temporal cornea. Variation in Q-value with semi-meridian was more obvious in the nasal cornea. The method of calculating corneal Q using the tangential radius could provide more reasonable and complete Q-value than that by the sagittal radius. The model of a whole anterior corneal surface could be reconstructed on the basis of the above method.

Accuracy of cornea and lens biometry using anterior segment optical coherence tomography

We assess the accuracy of the Visante anterior segment optical coherence tomographer (AS-OCT) and present improved formulas for measurement of surface curvature and axial separation. Measurements are made in physical model eyes. Accuracy is compared for measurements of corneal thickness (d(1)) and anterior chamber depth (d(2)) using-built-in AS-OCT software versus the improved scheme. The improved scheme enables measurements of lens thickness (d(3)) and surface curvature, in the form of conic sections specified by vertex radii and conic constants. These parameters are converted to surface coordinates for error analysis. The built-in AS-OCT software typically overestimates [mean+/-standard deviation(SD)]d(1) by +62+/-4 mum and d(2) by + 4 +/- 88 microm. The improved scheme reduces d(1) (-0.4 +/- 4 microm) and d(2) (0 +/- 49 microm) errors while also reducing d(3) errors from +218 +/- 90 (uncorrected) to +14 +/- 123 microm (corrected). Surface x coordinate errors gradually increase toward the periphery. Considering the central 6-mm zone of each surface, the x coordinate errors for anterior and posterior corneal surfaces reached +3 +/- 10 and 0 +/- 23 microm, respectively, with the improved scheme. Those of the anterior and posterior lens surfaces reached +2 +/- 22 and +11 +/- 71 microm, respectively. Our improved scheme reduced AS-OCT errors and could, therefore, enhance pre- and postoperative assessments of keratorefractive or cataract surgery, including measurement of accommodating intraocular lenses.

Measuring non-spherical optical surfaces

INTRODUCTION

More lens designs for the presbyopic market are emerging and the use of novel constructions to generate varifocal optics in contact lenses is increasing. A number of designs incorporate aspheric shapes, of known p-values on the front and back surface of contact lenses. Frequently, the back optic zone radius quoted by the manufacturer is a nominal reference making verification of these lenses difficult. The purpose of this study was to determine whether a videokeratoscope or an optical microspherometer could verify these complex surfaces.

METHODS

Fifty-four concave test surfaces of known vertex curvatures (7.30 to 8.10 mm, in 0.1 mm steps) and known p-values (1.0 to 0.0, in 0.1 steps) were produced and measured with two different systems.

RESULTS

The optical microspherometer was able to verify the vertex radius to a high degree of accuracy (+/- 0.072[2SD]) without bias (mean difference = 0.008 +/- 0.01(2SE]). However, verification of the p-value by utilising Baker's equation demonstrated a poor degree of accuracy (+/-0.574) and bias (mean difference=0.134, +/-0.078). The degree of accuracy for the measurement of the vertex radius of the concave surface, when using the videokeratoscope, was also very high (+/-0.09) but with a demonstration of bias (mean difference=-0.017, +/-0.012). The videokeratoscope was able to describe the p-value for the surface to a high degree of accuracy (+/-0.136) but did demonstrate bias (-0.052, +/-0.018). These results for the measurement of the p-value for the optical microspherometry were statistically significantly different from that of the control measurement (p = 0.0379) and that for the videokeratoscope (p = 0.0042).

CONCLUSION

To determine the degree of asphericity of a concave optical surface to an acceptable degree of accuracy a videokeratoscope can be used in preference to an optical microspherometer. Both instruments demonstrated a high degree of accuracy when measuring the vertex radius.

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

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

These results suggest that the repeatability of simulated indices derived from TMS-1 topography in keratoconus subjects is poorer than in normal controls.

Validation of a combined corneal topographer and aberrometer based on Shack-Hartmann wave-front sensing

A corneal aberrometer based on Shack-Hartmann wave-front sensing was developed and validated by using calibrated aspheric surfaces. The aberrometer was found to accurately measure corneal reflective aberrations, from which corneal topography and corneal refractive aberrations were derived. Measurements of reflective aberrations correlated well with theory (R2 = 0.964 to 0.994). The sag error root mean square (RMS) was small, ranging from 0.1 to 0.17 microm for four of the five calibrated surfaces with the fifth at 0.36 microm as a result of residual defocus. Measured refractive aberrations matched with theory and whole-eye aberrometry to within a small fraction of a wavelength. Measurements on three human corneas revealed very large refractive astigmatism (0.65-1.2 microm) and appreciable levels of trefoil (0.08-0.47 microm), coma (0.14-0.19 microm), and spherical aberration (0.18-0.25 microm). The mean values of these aberrations were significantly larger than the RMS in repeated measurements.

The asphericity, curvature and tilt of the human cornea measured using a videokeratoscope

The EyeSys videokeratoscope (VK) measurements of the principal corneal meridians of 98 subjects already analysed by Douthwaite et al. [Ophthal. Physiol. Opt. (1999)19:467-474] were re-analysed in order to revise the assessment of asphericity, to derive information on corneal tilt and to assess the degree to which the corneal section approximates to that of a conic section. The range of normality for the revised p-value (asphericity) was from 0.57 to 0.97 for the near horizontal and from 0.56 to 1.08 in the near vertical principal meridians. The approximate corneal tilt angles ranged from -3.95 to +8.13 degrees in the horizontal and from -8.99 to +9.33 degrees in the vertical meridian. A tilted conicoidal surface will display a linear relationship (r = 1) when a scatterplot is drawn of the perpendicular distance squared vs radius squared, after first averaging the two semimeridian results for each VK ring. Analysing the results from the human cornea in the same way allows an assessment of the degree to which the corneal section approximates to that of the conic section.

Corneal asphericity after hyperopic laser in situ keratomileusis

PURPOSE

To analyze corneal asphericity after hyperopic laser in situ keratomileusis (LASIK) and its relationship to the clinical outcomes.

SETTING

Corneal and Refractive Surgery Service, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, USA.

METHODS

In a retrospective case series, 23 patients (33 eyes) with hyperopia or hyperopic astigmatism who had LASIK were evaluated. A computer program (Holladay Diagnostic Summary, EyeSys Laboratories) was used to analyze corneal asphericity (Q) before and after LASIK. Corneal asphericity was evaluated to determine the association with the postoperative refractive error, best spectacle-corrected visual acuity (BSCVA), uncorrected visual acuity (UCVA), achieved refractive correction, mean corneal power (K), refractive yield (achieved/attempted correction), and keratometric yield (change in keratometry/attempted correction).

RESULTS

After hyperopic LASIK, all corneas exhibited increased negative central Q. The postoperative corneal radius of curvature, BSCVA, and refractive and keratometric yields were not significantly correlated with the preoperative Q values. The asphericity change, Delta Q, was highly correlated with the achieved correction (r = 0.747, P <.0001). The postoperative Q value correlated well with the preoperative value (r = 0.534, P <.05) and the achieved correction (r = 0.601, P <.05) but not with the Delta Q. Neither the postoperative Q nor the Delta Q was correlated with the spherical equivalent, K, BSCVA, or UCVA.

CONCLUSIONS

Asphericity may be a useful quantitative descriptor of the corneal optical contour after hyperopic LASIK. Negative central Q increased after hyperopic LASIK, especially when greater degrees of refractive correction were attempted.

Videokeratoscope-line-of-sight misalignment and its effect on measurements of corneal and internal ocular aberrations

A growing number of research laboratories are using the new technologies of videokeratoscopy and Shack-Hartmann aberrometry, in combination, to study the optical structure of the human eye. A potential source of error arises, however, because the two instruments are designed to measure the human eye along different reference axes. The Shack-Hartmann aberrometer is usually aligned coaxially with the line of sight, but videokeratoscopes usually are not. Thus far, corneal optics research has not adequately addressed the problem of videokeratoscope-line-of-sight misalignment and its effect on the computation of corneal and internal ocular aberrations. We measured corneal, ocular, and internal aberrations for three normal human eyes, developed a method to compensate for videokeratoscope-line-of-sight misalignment, and analyzed the importance of compensating for the misalignment. Our results show that when the value of angle lambda (the angle between the line of sight and the pupillary axis) is larger than 2 degrees-3 degrees, the misalignment, if ignored, can lead to incorrect estimates of corneal and internal aberrations as well as the corneal/internal aberration balance.

Application of linear regression to videokeratoscope data for tilted surfaces

PURPOSE

To investigate the use of linear regression analysis performed on the tabular data display of the EyeSys videokeratoscope (VK). When a radius squared vs distance squared scatterplot is produced from aspheric surface data the equivalent conic section can be deduced from the intercept and slope of the linear regression line. Non-linear plots are often produced. Linear regression may then be applied in a number of ways.

METHOD

Topographical data derived from both the EyeSys VK and a computer model of the instrument were analysed by three methods of linear regression. The resultant apical radii, p-values and predicted surface tilts were compared with known values.

RESULTS

The three methods predict different surface characteristics whose errors were found to vary depending upon the asphericity of the surface and its tilt.

CONCLUSIONS

Apical radius is most accurately predicted by linear regression method (1). Both p-value and tilt are best predicted by averaging the radius and position data for corresponding points in each semi-meridian before squaring the resultant points and performing linear regression (method 3).

Precision of three topography instruments in keratoconus subjects

PURPOSE

To determine the test-retest reliability of three popular and commercially available Placido-ring videokeratography instruments in subjects with keratoconus.

METHODS

Nine subjects (16 eyes) with keratoconus of varying degrees of severity had up to four images per eye generated, in random order, from the EyeSys Model II, Dicon CT 200 and the Keratron Corneal Analyzer. Test-retest analyses for the images were sampled at four locations: 1.5 mm nasal, inferior, temporal, and superior from center. The average standard deviation of all points was used to determine the short-term variability of the measurements.

RESULTS

The short-term variability (in diopters) of the Dicon, EyeSys, and Keratron was 0.61 to 3.31 D, 0.94 to 1.51 D, and 0.58 to 2.85 D, respectively, for axial distance maps and 1.07 to 6.82 D, 0.79 to 1.77 D, and 1.23 to 3.03 D for tangential curvature maps.

CONCLUSION

Results support the notion of a loss in repeatability for all three instruments when corneal irregularity is present, which reduces test-retest reliability.

Developments in corneal topographic analysis following contact lens wear and refractive surgery

There is now a wide range of devices available for corneal topographic analysis. Although most devices use the Placido disk approach, fluorescein profilometry, laser holography and scanning slit technology have also been employed. The colour-coded topographical maps have been designed for ease of clinical interpretation. The application of this technology to further our understanding of the effects of contact lens wear and various forms of refractive surgery is demonstrated. Current developments include the merging of corneal topographic analysis and ocular wavefront sensing technology to create the capability of etching sophisticated corneal shapes in the course of refractive surgery so as to provide optimal aberration control.

Corneal topography by keratometry

AIMS

To investigate the ability of a telecentric keratometer to describe the asphericity and curvature of convex ellipsoidal surfaces and human corneas.

METHODS

22 conicoidal convex surfaces and 30 human corneas were examined by conventional keratometry. Additional keratometric measurements were made when the surface was tilted in the horizontal plane relative to the instrument optical axis. This resulted in a series of radius measurements derived from different regions of the surface. These measurements were used to determine the apical radius and the p value of the horizontal meridian of each surface. The results were compared with those derived from measurements using the EyeSys videokeratoscope and form Talysurf analysis. The method was repeated on 30 human corneas and the results compared with those of a videokeratoscope.

RESULTS

For the aspheric buttons, the keratometric and the EyeSys results tended to give higher values for both apical radius and the p values than those of the Talysurf analysis. The best agreement was between the Talysurf and the keratometer where the results were not significantly different. For the human corneas, the apical radii were significantly different comparing the keratometer with the videokeratoscope but the p values were not significantly different.

CONCLUSION

The keratometric method for assessing curvature and asphericity appears to hold promise as a method for quantifying the corneal topography.

The EyeSys videokeratoscopic assessment of apical radius and p-value in the normal human cornea

The EyeSys videokeratoscope was used to assess the corneal topography in 98 subjects. Scatterplots of distance squared versus radius squared were plotted for the near horizontal and near vertical principal meridians of the two eyes. The regression lines allowed calculation of the surface apical radius and the p-value. The group average apical radius was 7.93 mm (horizontal) and 7.78 mm (vertical). The group average p-value was 0.76 (horizontal) and 0.82 (vertical). Both apical radius and p-value were similar when comparing the two eyes for both the horizontal and the vertical meridians. The two meridians in a single eye, however, had different values for both apical radius and p-value. Male apical radii were longer than those of females but the p-values were the same. There is no apparent association between age and either apical radius or p-value for the subjects used in this study. The asphericity of the cornea does not show any apparent association with corneal curvature in this group of subjects.

The reproducibility of videokeratoscope measurements as applied to the human cornea

To be useful in contact lens fitting, the videokeratoscope (VK) must be able to provide the practitioner with credible data on the vertex radius and central topography, or 'shape, of the patient's cornea. For this purpose, it is desirable that the measured value of the former should be credible at the level of 0.05 mm. In order to examine the accuracy, repeatability and reproducibility of the corneal dimensions provided by the videokeratoscope, eight eyes were measured on four different EyeSys systems at four independent sites. The order of measurement was random at each site. This amounted to a balanced uniform-level precision experiment ('ring test') of the VK instruments as defined in International Standard ISO 5725. The resulting data were analysed using the statistical procedures given in this Standard to provide a formal statement of the measurement precision of the EyeSys VK system. Based on this study, the estimated measurement precision of the EyeSys videokeratoscope as applied to the latter principal meridian of the normal human cornea is: Vertex radius: repeatability standard deviation, S(R)=0.0805, reproducibility standard deviation, S(R)=0.1041. p value: repeatability standard deviation, S(r)=0.0473, reproducibility standard deviation, S(R)=0.0574. The estimated measurement precision for the steeper principal meridian is: Vertex radius: repeatability standard deviation, S(r)=0.0771; reproducibility standard deviation, S(R)=0.1015. p value: repeatability standard deviation, S(R)=0.0698; reproducibility standard deviation, S(R)=0.0749.

Regular and irregular refractive powers of the front and back surfaces of the cornea

The refractive status of the posterior corneal surface has not been well studied. The purpose of this study is to quantitatively evaluate the regular and irregular astigmatism of the posterior corneal surface. In 50 normal human eyes, topography of the anterior and posterior corneal surface was measured with the scanning-slit videokeratoscope. Using Fourier series harmonic analysis, dioptric data on a mire ring were decomposed into spherical component, regular astigmatism, asymmetry (tilt or decentration), and higher order irregularity. The obtained values for the central 3.0 mm of the posterior corneal surface were -6.55+/-0.32 D (spherical component), 0.18+/-0.16 D (regular astigmatism), 0.40+/-0.22 D (asymmetry), and 0.02+/-0.02 D (higher order irregularity). The posterior to anterior ratios of these parameters were 13.6+/-0.6%, 35.0+/-41.3%, 45.8+/-56.9%, and 39.9+/-39.8%, respectively. The ratio of the spherical component was significantly lower than the other three parameters (P<0.001, Wilcoxon signed-rank test), indicating that non-spherical components (regular and irregular astigmatism) of the posterior corneal surface are not negligible in the optical quality of the cornea. The current results can serve as the control data and reference for the future clinical studies of optical characteristics of the cornea as a whole.

Comparison of Videokeratoscope and Autokeratometer Measurements on Ellipsoid Surfaces and Human Corneas

BACKGROUND

Various studies have compared the accuracy and repeatability of autokeratometers and videokeratoscopes using calibrated convex surfaces. We investigate the agreement between the Topcon KR-3500 autokeratometer and the EyeSys videokeratoscope on human corneas and calibrated convex surfaces.

METHODS

Measurements were obtained from 30 convex ellipsoidal buttons and 20 right eyes of 20 young normal human subjects. Vertex radius and p-values were compared for the two instruments.

RESULTS

The two instruments showed excellent agreement on convex buttons. The human data showed no such relationship. For vertex radius, a good level of agreement was obtained only for surfaces whose p-values were near unity. Repeatability was also calculated and was shown to be better with the Topcon autokeratometer than with the EyeSys videokeratoscope. Editing the EyeSys data to encompass the same corneal area as that of the Topcon improved its repeatability, although it did not reach the level of the Topcon autokeratometer.

CONCLUSIONS

The Topcon autokeratometer and the EyeSys videokeratoscope showed reasonable agreement for surface topography on convex conicoidal plastic test buttons but not for human corneas. Alterations in the data-capture mechanisms of videokeratoscopes could improve their ability to accurately image paraboloidal surfaces.

Effect of incision location on preoperative oblique astigmatism after scleral tunnel incision

PURPOSE

To evaluate the effect of incision location or clinically relevant preoperative oblique astigmatism.

SETTING

Department of Ophthalmology, Virchow Medical Center, Humboldt-University, Berlin, Germany.

METHODS

This prospective study included 68 patients who had phacoemulsification and posterior chamber lens implantation using a standardized 7.0 mm self-sealing trapezoidal scleral tunnel incision. Each patient was randomly assigned to one of three incision locations: Group A, conventional superior incision; Group B, temporal incision; Group C, oblique incision centered on the steeper meridian (modified BENT incision). Astigmatism analysis was performed by manual keratometry and corneal topography.

RESULTS

A significant mean reduction in astigmatism of 0.58 diopter (D) (P < .01) was achieved in only the modified BENT incision group. Postoperatively, significant flattening of 0.27 D (P < .01) in the steeper meridian as well as steepening of 0.29 D (P < .01) in the flatter meridian occurred. No decrease in astigmatism was noted in the superior or temporal incision groups. Five months postoperatively, vector analysis showed that surgically induced astigmatism was significantly higher in the superior incision group (1.16 D +/- 0.44 [SD]) than in the temporal incision group (0.66 +/- 0.32 D) or modified BENT incision group (0.82 +/- 0.50 D). Corneal topographic analysis confirmed these results within +/- 0.3 D.

CONCLUSIONS

Only the oblique incision centered on the steeper meridian (modified BENT incision) effectively and predictably reduced preoperative oblique astigmatism. In eyes with clinically relevant oblique astigmatism, we recommend using a modified BENT incision.

Extent and effect of surface tilt on the data display of the EyeSys videokeratoscope

AIMS

This study examined the occurrence, magnitude, and the consequences of a possible tilt between the corneal surface and optical axis of the EyeSys videokeratoscope.

METHODS

Initially, a theoretical model was developed to calculate the angle of tilt. The predictions of the model were verified empirically using a convex conicoid surface and were found to predict the tilt to within 0.5 degree of the actual tilt. The likely effects of the tilt on the corneal power were also examined. The angle of tilt was then measured on the human cornea and the effect of neutralising the tilt on the videokeratoscopic data display was observed.

RESULTS

The angle of tilt was found to lie between 1 degree and 6 degrees in a temporal direction.

CONCLUSION

When the corneal tilt on the human subjects was neutralised, then a reduction in the nasal/temporal asymmetry was observed.