Leo G. Carney

The Mean Shape of the Human Cornea

The anterior surface of the cornea is the major refracting element of the human eye. Knowledge of its shape and the variation of this shape throughout the population are important in calculations of the corneal contribution to ocular aberrations. Since the cornea is the surface on which a contact lens rests, the corneal shape is also of importance in contact lens design. This paper presents results of the fitting of a conicoid equation to corneal shape data derived from a precision photokeratoscope. Both rotationally symmetric and non-rotationally symmetric forms of the conicoid equation were developed. The results show that if the cornea is regarded as a rotationally symmetric conicoid, the conicoid is ellipsoidal but with insufficient asphericity to eliminate spherical aberration for distance vision. More importantly, the results show that the cornea is significantly asymmetric in both radius of curvature and asphericity.

Diurnal variations of corneal topography and thickness.

ABSTRACT Measurements of central and peripheral corneal thickness, corneal topography, and central corneal curvature were made for 21 subjects at hourly intervals for 12 hr of 1 day for each subject. Values for corneal radius and asphericity were obtained from keratometry and photokeratoscopy, and their stability throughout the day was evaluated. Corneal radius was observed to steepen during the day, whereas asphericity remained constant. Corneal thickness was measured by pachometry and was found to be at its thickest on first awakening. Corneal curvature and central corneal thickness were highly correlated; low correlation was found between corneal asphericity and central corneal thickness. Some subjects exhibited large fluctuations in one or all of the parameters considered.

Corneal aberrations and reading

Purpose. To investigate the effects of eyelid pressure on corneal shape and corneal aberrations during reading. Methods. Twenty young subjects with normal ocular health were recruited for the study. The experiment was conducted early in the morning, with subjects instructed not to perform any prolonged reading before the experiment. Corneal topography of one eye was measured with a videokeratoscope before reading and then again after a 60-min reading task. The natural position of the eyelids was photographed in primary gaze and during the reading task. Results. Twelve of the 20 corneas showed significant changes in central topography immediately after reading. The location of the changes corresponded closely to the position and angle of the subject’s eyelids during reading. The change in shape was best described as a wave-like distortion that significantly altered some corneal wavefront Zernike coefficients. There was a significant correlation between the changes in primary vertical coma and trefoil (along 30°). Within the central 6 mm of the cornea, there were significant changes in the root mean square error, overall refractive power, and astigmatism. Conclusions. The changes we observed in corneal topography appear to be directly related to the force exerted by the eyelids during reading. Because the cornea is the major light-refracting surface of the eye, the optical characteristics of some eyes can be significantly changed during reading by the force of the eyelids. These findings may have important implications for the definition of refractive status and may also aid in the understanding of the relationship between reading and the development of refractive errors.

THE NATURE OF NORMAL BLINKING PATTERNS

The blinking patterns of 20 normal subjects were examined under controlled experimental conditions. While the mean blink rate was 12.55 blinks/min, there were marked variations between individuals. As well, each subject displayed a characteristic pattern of blinking, mixing inter‐blink periods of shorter and longer durations in a regular fashion.

The accuracy and precision performance of four videokeratoscopes in measuring test surfaces.

In this study we evaluated the accuracy and precision of three placido-disk videokeratoscopes (the Keratron, Medmont and TMS) and one videokeratoscope that uses the raster-stereogrammetry technique (PAR-CTS) in elevation topography with six test surfaces. The test surfaces were a sphere, an asphere, a multicurve, and three bicurve surfaces. Each instrument performed well on certain test surfaces, but none of the instruments excelled on all of the surfaces. The results showed high accuracy of the Keratron and Medmont instruments in measuring the sphere, asphere, and multicurve surfaces, but not the bicurve surfaces. The precision of the Keratron and Medmont instruments were high. The TMS and PAR-CTS instruments showed poorer accuracy than the Keratron and Medmont instruments for the multicurve test surface but showed better performance for the bicurve surfaces. The PAR-CTS had the poorest performance in precision of the four instruments. The use of the Noryl spherical test surface instead of polymethyl methacrylate (PMMA) resulted in small differences in the accuracy performance of the placido-disk videokeratoscopes only.

Menstrual cycle variations of corneal topography and thickness.

ABSTRACT Two studies were performed to investigate dimensional stability of the cornea throughout the female menstrual cycle. In the first study, changes of corneal curvature, topography, and thickness were measured for six women for one complete menstrual cycle. Steepening of horizontal and vertical curvatures occurred at the beginning of the cycle, and flattening occurred after ovulation. Corneal thickening occurred on the second day of the cycle and around the time of ovulation, then thinning and another slight thickening on day 21. In the second study, more detailed information was sought on the timing of corneal thickness changes throughout any cycle. Corneal thickness was studied for two subjects throughout three consecutive cycles. Both subjects showed a slight decrease in thickness toward the end of the menses; thickening occurred at ovulation followed by thinning. Thickening also occurred 4 days after ovulation. Consideration of changes in urine levels of estrogen and pregnanediol suggests that rises in estrogen are accompanied by increases in corneal thickness.

Meridional variations of corneal shape.

ABSTRACT The corneal shape of 196 eyes was determined by photokeratoscopy and subsequent fitting of a conicoid equation to four individual corneal meridians. This conicoid was found in almost all instances to be ellipsoidal, with the asphericity not varying between meridians. The corneal radius of curvature did vary as expected between meridians. The radii of curvature of these corneal meridians also varied with the age of the subjects, resulting in reduced corneal astigmatism, while the asphericity of each meridian did not vary substantially with age.

Hyperopia is predominantly axial in nature.

PURPOSE Myopia has been found to be predominantly axial in nature, i.e. myopic eyes have longer than normal axial lengths, with corneal radius variations having only a small influence on the magnitude of the refractive error. In this study we assess whether a similar relationship exists for hyperopia. METHODS Biometric data were collected on 57 subjects with either emmetropic or hyperopic refractive errors ranging in magnitude from -0.37 D to +17.25 D. Our main analysis concentrated on subjects with less than +10 D of hyperopia (group 1, n = 53), as subjects with +10 D of hyperopia or more (group 2, n = 4) exhibited marked differences in their biometric characteristics. RESULTS Analysis of group 1 data revealed a significant relationship (r2 = 0.611, p = 0.0001) between the degree of hyperopia and the measured axial lengths. A weak but statistically significant relationship (r2 = 0.128, p = 0.009) was also found between mean corneal radius measures and mean spherical refractive errors, with the mean corneal radius flattening with increasing hyperopia. In group 2, three of the four subjects exhibited much steeper corneal characteristics than predicted from the group 1 data. CONCLUSIONS Our results suggest that hyperopia, like myopia, is predominantly axial in nature, although the corneal radius also plays a role in determining refractive error magnitude. These results have implications for refractive surgery and visual performance in hyperopic eyes.

Corneal topography and accommodation

Purpose. To investigate whether there are significant changes in corneal topography during accommodation in normal corneas and corneas that are pathologically thinner due to keratoconus. Methods. A videokeratoscope was modified to present an accommodation stimulus that was coaxial with the instruments measurement axis. Six subjects with normal corneas and four subjects with keratoconus were studied. Eighteen videokeratoscope measurements of one eye of each subject were taken at 0 diopter (D) accommodation demand and six for both of 4 D and 9 D accommodation demand. The effects of ocular micromovements on multiple topography maps were minimized using software algorithms. Average maps for the 4 D and 9 D accommodation demands were calculated and subtracted from the average map of the 0-D accommodation demand. A t test was applied at each point location within the topography maps to analyze the statistical significance of change (p < 0.001) within the difference maps. Results. In the initial analysis, we found that a number of the subjects showed significant changes in corneal topography as accommodation changed. However, further analysis showed a significant group mean excyclotorsion of the topography maps of 1.6 ± 1.1° (p < 0.03) for the 4-D stimulus and 2.0 ± 1.3° (p < 0.01) for the 9-D stimulus compared with the 0-D stimulus. When we accounted for the excyclotorsion, we did not find clear evidence of statistically significant changes in corneal topography as a result of accommodation, either for the normal corneas or the keratoconic corneas. Conclusions. It appears unlikely that changes occur in central corneal shape during accommodation up to a level of 9 D in normal or keratoconic corneas. A small ocular excyclotorsion typically accompanies accommodation, and this changes the relative orientation of the topography of the cornea. This has significant implications for the interpretation of the optical characteristics of eyes during near viewing conditions.

The relation between corneal and total astigmatism

Using computer-assisted videokeratoscopy we measured corneal astigmatism and compared these results over a range of corneal zone diameters with total ocular astigmatism derived by subjective refraction. Videokeratoscopes permit a more detailed analysis of the power distribution within a given corneal surface area, enabling comparison to the total astigmatism for equivalent aperture sizes. Although there were significant individual variations, the group average data supports the traditional view of a linear relation between corneal and total astigmatism. This was true across the range of apertures tested from 2 to 7 mm, with the coordinates of the relation being consistent with that of the modified Javals rule; namely a slope of 1 and an intercept of approximately 0.50 D against-the-rule residual astigmatism.