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.

Contact lens back surface specification derived from the EyeSys videokeratoscope.

Rigid corneal lenses were designed for 36 subjects using the software available with the EyeSys Corneal Analysis System 2000 (software version 2.00W) and a program from the textbook 'Contact lens Optics and Lens Design' Both alignment and apical clearance designs were investigated. It was found that the EyeSys lenses produced tear layer thickness and axial edge clearance values that were excessive in some cases, especially for flatter corneas. In the case of an alignment mode of fitting, the designs were relatively steep in the back optic zone and flat in the periphery when compared with designs derived from the textbook.

Acuity predictions from visually evoked potential to checkerboard pattern reversal stimuli: the effect of reversal rate.

INTRODUCTION: A pattern reversal stimulus (check size 5.5 minutes of arc) was used to elicit the visually evoked potential (VEP). METHODS: The peak to trough amplitude of the VEP wave was measured and compared to subjective visual acuity (Landolt C). The recordings were made at three, six and 12 pattern reversals per second. RESULTS: The correlation between VEP amplitude and visual acuity was found to decrease at higher temporal frequencies. The decrease is not confined to the use of a television system reversing checkerboard pattern stimulus as suggested by previous workers. DISCUSSION: It may well be concerned with a change in processing of the visual information as could be expected at these higher frequencies.

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.

Surface tilt measured with the EyeSys videokeratoscope: influence on corneal asymmetry.

PURPOSE: To investigate whether the apparent corneal asymmetry in the EyeSys videokeratoscope (VK EyeSys Laboratories, Houston, TX) image is a result of the cornea being tilted in relation to the instrument and to examine the possibility of deducing this tilt from a single captured image. METHODS: Videokeratoscopic images were captured with and without a front surface conicoidal contact lens (experiment 1). An image was captured with central fixation followed by image capture with fixation 10 degrees off center. These two images were used to calculate the angle of tilt with central fixation. The approximate tilt of the cornea derived from a single captured image was determined by the use of a mathematical model applied to some of the ring images (experiment 2). Twenty-four subjects were used in each of the above experiments. RESULTS: The mean tilt for the first group of subjects with the contact lens on the cornea was 2.8 degrees, whereas the tilt for the cornea alone was 3.2 degrees (experiment 1). The corneal tilt for the second group of subjects was 3.3 degrees, and the approximate tilt derived from a single captured VK image, using the equation, was 3.2 degrees (experiment 2). CONCLUSIONS: The similarity of the tilt angle with and without the contact lens in place suggests that the apparent asymmetry seen in the VK images of human corneas may be largely an artifact arising from corneal tilt and not nasal/temporal asymmetry. The agreement of the corneal tilt derived from two VK images and the approximate tilt derived from a single image indicates that the latter may offer a quick and convenient way to determine the fixation adjustment required to eliminate corneal tilt.

The corneal-thickness profile in Hong Kong Chinese.

PURPOSE: This study investigated the topographic corneal thickness in four groups of Hong Kong Chinese. METHODS: The corneal thickness was determined with an ultrasound pachometer mounted on a X-Y plate. A head rest was used to reduce further any movement from the subject during the measurement. After the central cornea was measured, the peripheral cornea was determined on nasal, temporal, superior, and inferior regions, 2 and 5 mm from the corneal center. RESULTS: The corneal thickness was not significantly different between the right and left eyes. For the right eye, there was no significant thickness difference for the four quadrants at a midperipheral and peripheral region, respectively. There was a general thinning of the corneal thickness at all regions from aging but no difference between the genders. CONCLUSION: The corneal thickness in our subjects was similar to that in other studies. The mean central corneal thickness varied from 541.7 (m to 560.8 microm, dependent on age. This study provides some information for the future studies of Chinese corneal thickness.

Validating a "pillar and collar" technique for measuring the edge lift of rigid contact lenses.

PURPOSE: The clinical importance of the edge lift of rigid contact lenses is often neglected, possibly due to previous difficulties in its measurement. A new method of measuring axial edge lift (AEL) and radial edge lift (REL) using standard contact lens verification equipment, such as an optical spherometer, a thickness gauge, and contact lens V gauge, is described. METHODS: The technique was validated for trueness (accuracy) and precision (repeatability) by measuring the edge lift of a number of monocurve lenses, manufactured both with and without a normal edge finish. RESULTS: Edge lift was measured to an accuracy of 0.01 mm. CONCLUSIONS: As long as a mean of eight independent measurements of back optic zone radius (BOZR), sagitta, and one measurement of center thickness are taken, the pillar and collar technique is capable of producing accurate and repeatable measurements of the edge lift of a rigid contact lens.

"Pillar and collar" technique for measuring the axial edge lift of multicurve rigid lenses.

BACKGROUND: The method of using an optical spherometer and a lens thickness gauge, in combination with a pillar and collar, has previously been shown to be repeatable and accurate (to 0.01 mm) in measuring the edge lift of rigid monocurve lenses. PURPOSE: This paper goes on to validate the technique for measurements taken on edge-finished multicurve rigid lenses. METHODS: The axial edge lift (AEL) of a series of multicurve rigid lenses having known values of AEL was measured using an optical spherometer, a thickness gauge, and contact lens V gauge in a similar way as was described in the companion paper. RESULTS: The results show that as long as a correction factor of 0.01 mm is applied, as found in the previous paper, the technique will allow satisfactory verification of the edge lift. CONCLUSIONS: The technique is accurate and reproducible, even in multicurve lens designs, provided that a calibrating correction factor of 0.01 mm is applied to account for methodological error. It will also indirectly allow the accuracy of manufacture of the peripheral curves and diameters of any rigid contact lens to be verified.

The effect of an artificially-elevated intraocular pressure on corneal thickness in Chinese eye.

We measured the central corneal thickness and the applanation intraocular pressure (IOP) on 45 Hong Kong Chinese. There was no obvious relationship between these two parameters, as different from other literatures. It could be due to either a limited number of subjects with a high IOP level (only six subjects with IOP > or = 22 mmHg), or Chinese has a thicker central cornea in general. The mean central cornea of our subjects was thicker (566 +/- 36 microns) than some previous findings. Thirty subjects had their intraocular pressure further increased by adopting a 40 degrees head-down posture. Their IOP and topographic corneal thickness were measured again. There was no significant change in the central corneal thickness even though the IOP was elevated by 11.7 mmHg. However the nasal cornea demonstrated a thinning effect (by some 18 microns) during the IOP elevation but it returned to the pre-inverted level after returning to a sitting posture for 5 min. Further investigation with more corneal regions being measured would be valuable to evaluate the in vivo effect of IOP elevation from glaucoma attack on corneal thickness.

Does the change of anterior chamber depth or/and episcleral venous pressure cause intraocular pressure change in postural variation?

PURPOSE: Previous studies have found that the intraocular pressure (IOP) variation from postural change is due to the obstruction of aqueous outflow by an increase in episcleral venous pressure. This study investigated if any shift of anterior lens position from postural variation would be another contributing factor. METHODS: Thirty-three Chinese subjects were recruited with their IOP and anterior chamber depth (ACD) measured in the sitting, supine, and prone postures. The IOP was measured using a Pulsair 2000 noncontact tonometer and ACD with a Nidek US-2000 EchoScan unit. RESULTS: The highest IOP was obtained in the prone position and this value was significantly different from the IOP obtained in other postures, whereas there was no significant difference in ACD. CONCLUSIONS: Because no significant variation in ACD was demonstrated, the prone and supine IOP variation could be due to something other than the change in lens position. However, a higher IOP in the prone position rather than in the supine position also suggests that it is not merely the episcleral venous pressure causing the IOP change. Investigation of the entire iris profile at different postures would be more informative in future studies.

Measurement of posterior corneal asphericity on Hong Kong Chinese: a pilot study.

The posterior corneal p-value and apical radius of 60 Hong Kong Chinese were assessed. The values were derived based on the information of the anterior corneal topography and the corneal thickness in different regions. The mean posterior corneal apical radius along the horizontal meridian was 6.51 mm (SD +/- 0.40 mm) and the p-value was 0.34 (SD +/- 0.38). The apical radius is greater while the p-value is smaller than a previous study using a similar principle. This may indicate a flatter posterior cornea and greater peripheral flattening in Hong Kong Chinese. No significant difference between the nasal and temporal corneal thickness, nasal and temporal posterior p-value and apical radius was demonstrated. The right and left eyes were also similar in different ocular parameters apart from a smaller anterior corneal p-value on the right eye (R eye: 0.70 +/- 0.13; L eye: 0.67 +/- 0.12), but the difference may not be significant clinically. The method used here is simple and the generation of posterior corneal topography is informative.

The effect of an artificially elevated intraocular pressure on the central corneal curvature.

Twenty-three optometry students with normal corneal condition were recruited. The intraocular pressure and central corneal curvature of the right eye were measured in a sitting and a 30 degrees head-down posture. The mean (standard deviation) IOPs before and during posture change were 15.6 (2.4) mmHg and 22.1 (2.3) mmHg respectively. This 6.5 mmHg mean rise in IOP was found to be statistically significant which is similar to the results from previous studies. The mean changes in radius of corneal curvature and the orientation of the axis of the vertical principal meridian were 0.02 mm (SD 0.025 mm) and 2.4 degrees (SD 10.4 degrees) respectively. No significant variation was demonstrated on these keratometric results due to the 30 degrees head-down posture. The maximum change in radius of curvature was only 0.055 mm for one subject. Perhaps this amount of pressure rise was not sufficient enough to distort the corneal surface centrally. Another possibility could be an even distribution of the elevated pressure around the cornea, or the distribution of pressure is not even but could not be revealed by a conventional keratometer.

A pilot study on the measurement of central posterior corneal radius in Hong Kong Chinese using Purkinje image technique.

The central posterior corneal radius was measured by assessing the first and second Purkinje images produced from a modified keratometer. This method was initially assessed by measuring five PMMA lens buttons with back surface radii from 6.2 mm to 7.0 mm. It was reasonably accurate when compared with the measurements made using a conventional radiuscope. The technique was found to be repeatable for human subjects measured on three separate occasions. The mean central posterior corneal radii from a group of 30 Hong Kong Chinese were 6.64 mm (SE 0.05 mm) and 6.39 mm (SE 0.05 mm) along the horizontal and vertical meridians respectively. These results are similar to previous studies using a similar technique. The posterior corneal radii were found to be similar for right and left eyes and there were no gender differences.

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.

Application of a modified keratometer in the study of corneal topography on Chinese subjects.

A corneal topography study was carried out on Hong Kong Chinese subjects using a modified keratometer. The Chinese corneal topography measured by this modified keratometer was similar to other studies using the Wesley-Jessen Photo Electronic Keratoscopy (PEK) System 2000. The mean p-values along the horizontal meridian and the vertical meridian were 0.82 and 0.87, respectively. A negative correlation was found between the p-value and the central corneal radius along the horizontal meridian which may indicate that a steeper central cornea will have a greater p-value. However, the correlation coefficient of 0.56 was not too certain for this conclusion to be drawn. A difference of 0.04 was found between the horizontal p-value and the vertical p-value, which may be a result of the tight lid tension along the vertical meridian. This modified keratometer can be considered as an inexpensive instrument for the study of corneal topography.

The TMS-1 corneal topography measurement applied to calibrated ellipsoidal convex surfaces.

The purpose of this report is to assess the accuracy of the TMS-1 videokeratoscope (Computed Anatomy Inc.) by using convex ellipsoidal surfaces. The ellipsoids were calibrated using Form Talysurf analysis, which allowed for subsequent calculation of the vertex radius and p value. The videokeratoscope was used to examine the same ellipsoids. The data provided by the instrument software were used to plot a graph of r2 verses y2, where r is the measured radius at y, the distance from the corneal point being measured to the optical axis of the instrument. The intercept on the ordinate of this graph gives the vertex radius, and the slope give the p value. The results arising from the Talysurf and the TMS-1 techniques were compared. The TMS-1 videokeratoscope gave readings for the vertex radius that were generally higher than those of the Talysurf analysis. The vertex radius was up to 0.09 mm greater. The p value results were similar by the two methods for p values of approximately 0.8; however, the TMS-1 results were higher, and the discrepancy increased as the p value approached that of a paraboloid. Although the videokeratoscope may be useful in comparative studies of the cornea, there must be some doubt about the absolute values displayed as the surface becomes increasingly aspheric.

EyeSys corneal topography measurement applied to calibrated ellipsoidal convex surfaces.

AIMS/BACKGROUND: This study was carried out to assess the accuracy of the EyeSys videokeratoscope by using convex ellipsoidal surfaces of known form. METHODS: PMMA convex ellipsoidal buttons were calibrated using Form Talysurf analysis which allowed subsequent calculation of the vertex radius and p value of the surface. The EyeSys videokeratoscope was used to examine the same ellipsoids. The tabular data provided by the instrument software were used to plot a graph of r2 versus y2 where r is the measured radius at y, the distance from the corneal point being measured to the surface vertex. The intercept on the ordinate of this graph gives the vertex radius and the slope the p value. The results arising from the Talysurf and the EyeSys techniques were compared. RESULTS: The EyeSys videokeratoscope gave readings for both vertex radius and p value that were higher than those of the Talysurf analysis. The vertex radius was around 0.1 mm greater. The p value results were similar by the two methods for p values around unity but the EyeSys results were higher and the discrepancy increased as the p value approached that of a paraboloid. CONCLUSIONS: Although the videokeratoscope may be useful in comparative studies of the cornea, there must be some doubt about the absolute values displayed. The disagreement is sufficiently large to suggest that the instrument may not be accurate enough for contact lens fitting purposes.

The Bausch and Lomb keratometer does not measure the tangential radius of curvature.

The purpose of this research was to determine if the conventional keratometer measures tangential or sagittal radius. A Bausch and Lomb keratometer was modified to enable an assessment of vertex radius and p-value to be made on convex conicoidal buttons of known surface form. Calculation of these two parameters was achieved assuming that the keratometer was measuring tangential radius and then repeated assuming a sagittal measurement. The resulting values could then be compared with those obtained by Form Talysurf analysis and by the use of an autokeratometer designed to measure sagittal radius. There was good agreement between the three instruments when the modified Bausch and Lomb keratometer was assumed to measure sagittal radius. We conclude that the Bausch and Lomb keratometer makes a radius measurement closer to the sagittal than the tangential radius of a conicoid of revolution.

Derivation of corneal flattening factor, p-value.

A mathematical equation was derived to calculate the corneal p-value based on the central and peripheral corneal radii and the corresponding semichord diameters. The calculation was simplified by a computer program written using GWBASIC. The accuracy of corneal p-value generated from this program was checked with Douthwaite's modified keratometer with good correlation. This program could be used to find out the corneal p-value and apical radius ro for any topographic keratometer which only provides central and peripheral radii without any information of the p-value and apical radius.