Tobias Buehren - Academia.edu (original) (raw)

Papers by Tobias Buehren

Investigative Ophthalmology & Visual Science, 2016

Centre for Health Research; Faculty of Health; Institute of Health and Biomedical Innovation, 2006

Aim: A modified wavefront sensor was used to investigate the effect of accommodative errors on re... more Aim: A modified wavefront sensor was used to investigate the effect of accommodative errors on retinal image quality. Methods: Ten subjects were presented with six vergence stimuli between 0.17 D and 5.00 D. For each vergence distance, subjective visual acuity and ocular wavefronts were measured. Wavefronts were analysed for natural pupil sizes and for a fixed threemillimetre pupil diameter. Visual Strehl ratios computed in the frequency domain (VSOTF) and retinal images were calculated for each condition. Results: Subjective visual acuity was significantly improved at intermediate vergence distances (1.00 D and 2.00 D; p < 0.01), and decreased significantly at 5.00 D compared with 0.17 D (p < 0.05). VSOTF magnitude was associated with subjective visual acuity and VSOTF peak location correlated with accommodation error. Apparent accommodative errors due to spherical aberration were highly correlated with accommodative lead and lag for natural pupils (R2 = 0.80) and accounted f...

Cornea, 2003

Purpose. To investigate whether there are significant changes in corneal topography during accomm... more 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 instrument's 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.

Cornea, 2001

Purpose. Videokeratoscopes provide a wealth of information about the topography of the ocular sur... more Purpose. Videokeratoscopes provide a wealth of information about the topography of the ocular surface. Although there have been numerous studies of the accuracy and precision of videokeratoscopes with inanimate test objects, little information exists on their precision (repeatability) for real eyes. Methods. To investigate the stability of the ocular surface in the inter-blink period, 10 patients were recruited for videokeratoscopy. Tear break-up time was measured and videokeratographs were acquired immediately post-blink and again at 4, 8, and 12 seconds post-blink. To permit statistical inferences to be drawn from the data, we acquired 24 videokeratographs for each of the four post-blink intervals. The videokeratograph data were interpolated (bilinear) to a common grid, and average and standard deviation (SD) maps were derived for each post-blink condition. t Tests were used to test the significance of changes observed in the topography. Results. The instantaneous power SD maps showed increasing variation toward the periphery, with most maps showing less than ±0.5 diopters (D) of SD in the central 4 to 5 mm and variation in the periphery often reaching more than ±1 D SD at the edge of an 8-mm diameter. When the 4-, 8-, and 12-second average maps were subtracted from the average map acquired immediately after blinking, regions of statistically significant (p < 0.001) change were apparent in the upper and lower regions of the maps. The upper and lower bands of change were found to correlate with the natural position of the patients' lid margins. Conclusions. For normal eyes, the central regions of videokeratographs show high stability in the inter-blink period. However, the upper and lower edges of 8-mm diameter maps show statistically significant variability, which appears to be related to the effects of eyelid pressure.

Acta Ophthalmologica Scandinavica, 2005

To compare lid-induced changes in corneal optics following reading, microscopy and computer work.... more To compare lid-induced changes in corneal optics following reading, microscopy and computer work. Methods: Nine subjects with normal ocular health were recruited for the study. Five subjects were myopic, two were emmetropic, one was astigmatic and one was hyperopic. Corneal topography was measured before and after 60 mins of reading a novel, performing a blood cell counting task on a microscope and Internet searching. Corneal topography data were used to derive the corneal wavefront Zernike coefficients up to the fourth order. A meridian analysis of instantaneous corneal power along the upper 90-degree semi-meridian was performed to examine local changes caused by eyelid pressure. Digital photography was used to capture body posture and eyelid position during the tasks. Results: Each of the three tasks showed systematically different effects on both the characteristics and location of corneal topography changes. Reading and microscopy generally exhibited larger and more centrally located changes compared with the computer task. Differences in wavefront aberration characteristics between the three tasks were apparent in both lower and higher order aberrations. The location of corneal distortions differed significantly between microscopy and computer work, with microscopy causing distortions to occur closer to the videokeratoscope measurement axis compared with computer work (p = 0.015). Conclusions: Reading, microscopy and computer work have different effects on corneal aberrations. The results are in agreement with the hypothesis that lidinduced corneal aberrations may play a role in myopia development.

Journal of Cataract and Refractive Surgery, 2013

To establish repeatability and validity of a partial coherence interferometry (PCI) device in pat... more To establish repeatability and validity of a partial coherence interferometry (PCI) device in patients with astigmatism. SETTING: Industry and university. DESIGN: Observational studies. METHODS: A prospective study determined the agreement between the IOLMaster 500 PCI device and the Marco 1-position manual keratometer for measurement of mean corneal power, astigmatic power, and axis. Interinstrument and interoperator reproducibility was also assessed. A retrospective study determined the agreement between the PCI device and the Javal 2-position manual keratometer. Repeatability was also assessed. The 95% limits of agreement (LoA) were determined for all comparisons. RESULTS: Comparing the PCI device and the 1-position manual keratometer, the axis was within 5 degrees in 72.1% of eyes; the cylinder power differed by 0.18 diopter (D) (95% LoA, G0.45 D) and the mean power by 0.33 D (95% LoA, G0.25 D). Comparing the PCI device and the 2-position manual keratometer, the axis was within 5 degrees in 77.1% of eyes. The mean difference in cylinder power between the 2 instruments was close to zero (95% LoA, G0.37 D); however, the mean power differed by 0.34 D (95% LoA, G0.25 D). The repeatability of the PCI device was better than that of both manual keratometers. CONCLUSIONS: Agreement between the PCI device and the 2-position manual keratometer for measures of corneal astigmatism and axis was excellent. The 1-position manual keratometer gave slightly lower estimates of corneal astigmatism, although the agreement for axis was excellent. Thus, the PCI device is an appropriate choice for the calculation of toric intraocular lenses. Financial Disclosure: Dr. Bullimore is a consultant to Carl Zeiss Meditec AG and Alcon Surgical, Inc. Drs. Buehren and Bissmann are employees of Carl Zeiss Meditec AG.

Vision research, 2006

Analysis was undertaken of the retinal image characteristics of the best-spectacle corrected eyes... more 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 ...

Investigative Ophthalmology & Visual Science, 2011

$Research paper thumbnail of Pilot evaluation of refractive prediction errors associated with a new method for ray-tracing–based intraocular lens power calculation$

Journal of Cataract & Refractive Surgery

Journal of Cataract & Refractive Surgery, 2016

To estimate the repeatability of biometric parameters obtained with a new swept-source biometer a... more 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). Department of Ophthalmology, Helios Hospital Erfurt, Erfurt, Julius-Maximilians University, Würzburg, and Philipps University, Marburg, Germany. Prospective comparative multicenter clinical study. 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. 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 μm ± 25 (SD), 17 ± 122 μm, 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 μm and 0.15 ± 4.51 μm, 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 μm, 9.8 μm, 2.3 μm, 19.5 μm, and 0.1 D, respectively. 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. 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.

$Research paper thumbnail of Interactions between higher-order aberrations and sphero-cylinder refraction$

Centre For Health Research Faculty of Health Institute of Health and Biomedical Innovation, 2006

Centre For Health Research Faculty of Health Institute of Health and Biomedical Innovation, Jan 11, 2007

To evaluate whether significant change occurs in the topography of the anterior or posterior corn... more To evaluate whether significant change occurs in the topography of the anterior or posterior cornea or in corneal thickness with accommodation. Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Queensland, Australia. Corneal data were acquired for 12 young, healthy emmetropic subjects under 2 accommodation levels (relaxed and approximately 5.0 diopters) using a rotating Scheimpflug camera. Anterior and posterior corneal elevation and sagittal curvature data as well as pachymetry data were analyzed to evaluate changes associated with accommodation. Corneal elevation data were also analyzed using a recentering procedure designed to minimize the effect of ocular micromovements between measurements. Changes in corneal parameters resulting from accommodation were small and generally insignificant. The mean change in pachymetry was 0.49 microm +/- 2.62 (SD) across an 8.0 mm pachymetry map. No significant change was found in the best-fit corneal spherocylinder (anterior and posterior cornea) (P&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;.05). Small changes were found in the corneal cylinder axis, indicative of small cyclotorsional eye movements accompanying accommodation. The mean changes in corneal elevation were 0.09 +/- 0.41 microm (anterior cornea) and 0.17 +/- 0.65 microm (posterior cornea). Only 0.50% (anterior cornea) and 0.27% (posterior cornea) of corneal elevation data points exhibited a statistically significant change with accommodation. Recentering the corneal elevation data led to a reduction in the magnitude of change in corneal elevation with accommodation. Small cyclotorsional eye movements occurred with accommodation. When these eye movements were accounted for in data analysis, no consistent significant change in the topography of the cornea was found with accommodation across an 8.0 to 9.0 mm corneal diameter.

Centre For Health Research Faculty of Health Institute of Health and Biomedical Innovation, 2004

ABSTRACT Purpose:Reading causes the corneal shape to change. We studied the time course of recove... more ABSTRACT Purpose:Reading causes the corneal shape to change. We studied the time course of recovery of corneal topography changes following various periods of reading. Methods:Six young subjects with normal ocular health were recruited for the study. Experiments were conducted early in the morning on four separate days and the subjects were instructed to perform no significant reading tasks prior to the experiment. At each session, baseline corneal shape was measured by taking six videokeratographs and averaging the maps. The subject then read a novel for 10, 30, 60 or 120 mins. Following the reading task, videokeratoscopy was measured at 17 pre–defined time intervals up to 3 hrs following the reading task. Six videokeratographs were acquired at each of these time points during the recovery phase and later averaged. During the recovery period the subjects were not permitted to perform any intensive reading or near work. Analysis took the form of comparison of the corneal topography pre–reading versus post–reading during the recovery phase. Results:All six subjects showed statistically significant changes in the cornea immediately following all four reading conditions (10 to 120 mins). In the corneal region corrsponding to the position of the upper lid margin, the group average maximum change in local refractive power was 0.94 D (+/–0.44) for the 120 min reading condition, 0.64 D (+/–0.31) for the 60 min condition, 0.60 D (+/–0.28) for the 30 min condition and 0.44 D (+/–0.42) for the 10 min reading condition. A one–way repeated measures ANOVA showed that the regression of corneal power over time was highly significant for all conditions (p=0.001). Plotting the regression of corneal power following reading showed a rapid recovery of shape following all 4 reading conditions during the initial 10 mins, with a subsequent slow recovery phase. As a generalization of the regression trends, we found that the topography was almost completely recovered to pre–reading shape after as many minutes as the subject had previously been reading. For example, 120 mins of reading required approximately 120 mins for full corneal shape recovery. Conclusions:Corneal topography and optics change following reading and the magnitude of the changes are related to the length of time spent reading. The regression of these changes have an initial rapid phase of approximately 10 mins followed by a slower recovery phase.

Centre For Health Research Faculty of Health Institute of Health and Biomedical Innovation, Mar 1, 2006

ABSTRACT Aim: A modified wavefront sensor was used to investigate the effect of accommodative err... more ABSTRACT Aim: A modified wavefront sensor was used to investigate the effect of accommodative errors on retinal image quality. Methods: Ten subjects were presented with six vergence stimuli between 0.17 D and 5.00 D. For each vergence distance, subjective visual acuity and ocular wavefronts were measured. Wavefronts were analysed for natural pupil sizes and for a fixed threemillimetre pupil diameter. Visual Strehl ratios computed in the frequency domain (VSOTF) and retinal images were calculated for each condition. Results: Subjective visual acuity was significantly improved at intermediate vergence distances (1.00 D and 2.00 D; p < 0.01), and decreased significantly at 5.00 D compared with 0.17 D (p < 0.05). VSOTF magnitude was associated with subjective visual acuity and VSOTF peak location correlated with accommodation error. Apparent accommodative errors due to spherical aberration were highly correlated with accommodative lead and lag for natural pupils (R2 = 0.80) and accounted for 93 per cent of the total accommodative error. Conclusions: The combination of higher-order aberrations and accommodative errors improved retinal image quality compared with accommodative errors or higher order aberrations alone. The one to one stimulus/response slope should not be considered as ideal in the presence of higher-order aberrations. Pupil size and higher order aberrations play an important role in ASR.

$Research paper thumbnail of Potential Higher-Order Aberration Cues for Sphero-Cylindrical Refractive Error Development$

Centre For Health Research Faculty of Health Institute of Health and Biomedical Innovation, Mar 1, 2007

To investigate analytically whether higher-order wavefront errors comprising combinations of tref... more To investigate analytically whether higher-order wavefront errors comprising combinations of trefoil along 30 degrees (trefoil30), vertical coma, and spherical aberration could provide cues to sphero-cylindrical refractive error development. 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 microm. Four wavefront types contained coma, trefoil, and spherical aberration of various sign combinations also producing HO-RMS levels between 0.1 and 0.5 microm. 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. 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., &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;or=0.25 D) was reached at HO-RMS levels between 0.2 and 0.3 microm for a 5-mm pupil zone. 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.

Faculty of Health Institute of Health and Biomedical Innovation, May 1, 2002

We aimed to minimize the effect of ocular microfluctuations during videokeratoscopy to improve th... more We aimed to minimize the effect of ocular microfluctuations during videokeratoscopy to improve the qualitative and quantitative analysis of corneal topography. A technique was developed to measure the tilt, displacement, and cyclotorsion in multiple videokeratographs from the same cornea. This information was used to reposition each videokeratograph according to the average position of a sample of multiple measurements. The corneal topography of 10 subjects was measured 20 times each, using videokeratoscopy. The root mean square error calculated from difference between single videokeratographs and the average videokeratograph decreased by an average of 24.6% for the 10 subjects&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; data. The standard deviation of some terms of Zernike polynomials fitted to the cornea was significantly reduced after the procedure. The method that we propose can improve the precision performance of videokeratoscopy in multiple measurements of corneal topography.