Tobias F. Buehren

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.

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.

Retinal image quality, reading and myopia.

Analysis was undertaken of the retinal image characteristics of the best-spectacle corrected eyes of progressing myopes (n = 20, mean age = 22 years; mean spherical equivalent = -3.84 D) and a control group of emmetropes (n = 20, mean age = 23 years; mean spherical equivalent = 0.00 D) before and after a 2h reading task. Retinal image quality was calculated based upon wavefront measurements taken with a Hartmann-Shack sensor with fixation on both a far (5.5 m) and near (individual reading distance) target. The visual Strehl ratio based on the optical transfer function (VSOTF) was significantly worse for the myopes prior to reading for both the far (p = 0.01) and near (p = 0.03) conditions. The myopic group showed significant reductions in various aspects of retinal image quality compared with the emmetropes, involving components of the modulation transfer function, phase transfer function and point spread function, often along the vertical meridian of the eye. The depth of focus of the myopes (0.54 D) was larger (p = 0.02) than the emmetropes (0.42 D) and the distribution of refractive power (away from optimal sphero-cylinder) was greater in the myopic eyes (variance of distributions p < 0.05). We found evidence that the lead and lag of accommodation are influenced by the higher order aberrations of the eye (e.g. significant correlations between lead/lag and the peak of the visual Strehl ratio based on the MTF). This could indicate that the higher accommodation lags seen in myopes are providing optimized retinal image characteristics. The interaction between low and high order aberrations of the eye play a significant role in reducing the retinal image quality of myopic eyes compared with emmetropes.

Repeatability and agreement in optical biometry of a new swept-source optical coherence tomography–based biometer versus partial coherence interferometry and optical low-coherence reflectometry

Purpose To estimate the repeatability of biometric parameters obtained with a new swept‐source biometer and to compare the agreement with that of partial coherence interferometry (PCI) and optical low‐coherence reflectometry (OLCR). Setting Department of Ophthalmology, Helios Hospital Erfurt, Erfurt, Julius‐Maximilians University, Würzburg, and Philipps University, Marburg, Germany. Design Prospective comparative multicenter clinical study. Methods Biometry was taken with the use of 3 different biometers: the IOLMaster 700 swept‐source biometer, the PCI‐based IOLMaster 500, and the OCLR‐based Lenstar LS 900. Axial length (AL), anterior chamber depth (ACD), and spherical equivalent (SE) were compared between swept‐source and PCI biometry and central corneal thickness (CCT) and lens thickness (LT) between swept‐source and OLCR biometry. The repeatability of swept‐source biometry was evaluated on the basis of 3 measurements captured for each patient. Results One hundred twenty cataract eyes were included in the study. The mean difference between swept‐source and PCI biometry for AL, ACD, and SE measurements was 4 &mgr;m ± 25 (SD), 17 ± 122 &mgr;m, and −0.001 ± 0.19 diopter (D), respectively. The mean difference between swept‐source and OLCR biometry for LT and CCT measurements was 21 ± 122 &mgr;m and 0.15 ± 4.51 &mgr;m, respectively. Differences between swept‐source biometry and the other devices distributed around zero without statistical significance. The standard deviation of repeatability for AL, ACD, LT, CCT, and SE was 8.8 &mgr;m, 9.8 &mgr;m, 2.3 &mgr;m, 19.5 &mgr;m, and 0.1 D, respectively. Conclusions Swept‐source biometry showed high repeatability performance for all biometric parameters. The agreement of AL, ACD, and SE between swept‐source and PCI biometry as well as that of LT and CCT between swept‐source and OLCR biometry was excellent. It remains to be validated whether high repeatability shown by swept‐source biometry will result in better postoperative refractive outcomes. Financial Disclosure Drs. Blum and Sekundo are members of the Scientific Advisory Board of Carl Zeiss Meditec AG. Drs. Peter and Bühren are employees of Carl Zeiss Meditec AG.

Regression of lid-induced corneal topography changes after reading.

Purpose. The purpose of this article is to investigate the magnitude of lid-induced corneal topography changes as a function of time spent reading and the subsequent time course of regression of these changes. Methods. Six young subjects, five myopes and one emmetrope with normal ocular health, participated in the study. Corneal topography of one eye was measured with videokeratoscopy before four reading sessions of 10, 30, 60, and 120 min duration performed on four separate mornings. Corneal topography was again measured at fixed intervals up to 180 min after each reading session. A control trial without reading was also performed on a separate morning. Results. All six subjects showed significant changes in corneal topography directly after each of the reading sessions. Longer reading periods caused larger corneal changes. Local instantaneous power changes reached up to 5.95 D (± 2.80 D), whereas refractive power changes in the superior cornea ranged up to 1.26 D (± 0.44) for the 120-min reading trial. The duration of regression of corneal changes showed slower recovery times after longer reading periods. The pattern of regression was similar for all reading times, showing a rapid recovery within the first 10 minutes followed by a slower regression period. Conclusions. The length of time spent reading has a cumulative effect on the period over which corneal topography remains altered as a result of lid forces.

Factors influencing lid pressure on the cornea.

Purpose. To investigate the effects of eyelid pressure on corneal topography in different angles of gaze and under different eye movement conditions. Methods. Ten young subjects with healthy eyes were recruited for the study. Corneal topography of the right eye was measured with a videokeratoscope before and after four 15-minute visual tasks (downward gaze of 25° or 45°, both with no eye movements or 1 Hz eye movements). The four tasks were conducted on four separate mornings. Results. All subjects showed significant regions of topographic change after each of the four tasks. In general, eye movements were found to cause greater topographic changes than tasks without eye movements, and the larger angle of downward gaze led to greater changes in topography. The topographic changes were located close to the lid margin position during each task. Conclusions. The pressure exerted by the eyelid is capable of altering corneal topography. These changes are enhanced when the eye looks downward and when the eye moves laterally.

A refined bootstrap method for estimating the Zernike polynomial model order for corneal surfaces

Following our previous work on optimal modeling of corneal surfaces with Zernike polynomials, we have developed a refined bootstrap-based procedure which improves the accuracy of the previous method. We show that for normal corneas, the optimal number of Zernike terms usually corresponds to the fourth or fifth radial order expansion of Zernike polynomials. On the other hand, for distorted corneas such as those encountered in keratoconus or in surgically altered cases, the estimated model was found to be up to three radial orders higher than for normal corneas.

Potential higher-order aberration cues for sphero-cylindrical refractive error development.

Purpose. To investigate analytically whether higher-order wavefront errors comprising combinations of trefoil along 30° (trefoil30), vertical coma, and spherical aberration could provide cues to sphero-cylindrical refractive error development. Methods. A total of 25 test wavefronts, subdivided into five different types and five levels of higher-order root mean square errors (HO-RMS), were created for the study. One type contained spherical aberration only, producing HO-RMS levels between 0.1 and 0.5 &mgr;m. Four wavefront types contained coma, trefoil, and spherical aberration of various sign combinations also producing HO-RMS levels between 0.1 and 0.5 &mgr;m. From the 25 wavefronts, refractive power maps were created and 2025 different sphero-cylindrical combinations were added to each refractive power map. For each sphero-cylinder combination, the visual Strehl ratio based on the modulation transfer function (VSMTF) was calculated. Retinal images and refractive power histograms were calculated for the refractive power maps corresponding to the peak of the VSMTF. Results. Spherical aberration affected the best focal plane thereby inducing spherical or defocus cues. The VSMTF produced by vertical coma and trefoil30, in combination with spherical aberration, could be improved with sphero-cylinders of various magnitudes and directions (i.e., with-the-rule, against-the rule, myopic astigmatism, or hyperopic astigmatism). Clinical significance of sphero-cylinders (i.e., ≥0.25 D) was reached at HO-RMS levels between 0.2 and 0.3 &mgr;m for a 5-mm pupil zone. Conclusions. In the context of compensatory blur driven eye growth, commonly occurring combinations of the three considered higher-order aberrations have the potential to produce cues to eye growth resulting in myopia and with-the-rule astigmatism.