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Biomedical subjects

W Drexler

Publications and source records attributed to W Drexler.

26 records · Page 2Linked to original sources
PubMed1997

Biometric investigation of changes in the anterior eye segment during accommodation.

Non-invasive biometry of the anterior structures of the human eye can be performed with unprecedented precision of 8-10 microns and a resolution of approximately 9 microns by partial coherence interferometry, which has the potential to assess the effect of cycloplegia on the ocular components of the anterior eye segment, to further improve the precision to 1-2 microns by the use of these agents and to quantify the amount of residual accommodations in different states of cycloplegia. In addition, the anterior chamber depth, the thickness of the crystalline lens, their changes during accommodation, as well as the movement of the anterior and posterior lens pole during accommodation can be quantified objectively and accurately to investigate the mechanism of accommodation.

Accommodation, Ocular↗
PubMed1997

Submicrometer precision biometry of the anterior segment of the human eye.

PURPOSE: To demonstrate the feasibility of measuring the anterior structures of the human eye by partial coherence interferometry and to determine its precision for eyes under normal and cycloplegic conditions. METHODS: The dual-beam version of partial coherence interferometry, a recently developed noninvasive optical ranging technique, enables high resolution measurements of several intraocular distances with unprecedented precision. A modified, more sensitive scanning version of this technique was used to assess the central and peripheral corneal thickness, the anterior chamber depth, and the lens thickness of 20 healthy, emmetropic to moderately myopic eyes. Furthermore the anterior structures of three eyes were measured under cycloplegia (1% cyclopentolate) to investigate the influence on the precision of this technique after suppression of residual accommodations. RESULTS: The mean geometric precision (standard deviation) of the measurement of the central corneal thickness was 0.29 micron (range, 0.22 micron to 0.38 micron) and 0.43 micron (range, 0.27 micron to 0.56 micron) for the peripheral corneal thickness at a distance 2 mm from its apex. The precision for measuring the anterior chamber depth and the lens thickness for fixation at infinity was 8.7 microns (range, 3.9 microns to 16.8 microns) and 8.9 microns (rang, 2.9 microns to 14.4 microns) for noncycloplegic eyes and 1.9 microns (range, 1.7 microns to 2 microns) and 1.4 microns (range, 0.7 micron to 1.8 microns) for cycloplegic eyes, respectively. CONCLUSIONS: The dual-beam partial coherence interferometry enables fast, noninvasive, submicrometer precision biometry of the anterior segment of the eye. The precision of determining the anterior chamber depth and the lens thickness is more than one order of magnitude better than that of the currently used ultrasound and optical techniques, and it can be improved by a factor of 5 by using cycloplegia.

Adult↗
PubMed1994

Interferometric measurement of corneal thickness with micrometer precision.

The recently developed partial coherence laser Doppler interferometry technique was improved to measure central and peripheral corneal thickness with high precision. Corneal thickness profiles were measured on 18 eyes of health, volunteer subjects. All of these eyes were measurable at angles (between visual axis and measuring direction) ranging from 20 degrees nasal to 25 degrees temporal. At larger angles (up to 35 degrees) only part of the eyes was measurable. The thickness profiles of the 18 corneas have a nearly perfectly parabolic shape within the measured region. The precision (standard deviation) was 1.6 microns for central measurements and decreased somewhat to about 3.5 microns at measuring angles in the range of 25 to 30 degrees. No significant interobserver variability was found on 14 eyes measured by three different observers. This study indicates that the new technique is likely to be superior to currently used ultrasound and conventional optical pachymetry techniques, especially for refractive procedures.

Cornea↗
PubMed1993

Measurement of the axial length of cataract eyes by laser Doppler interferometry.

PURPOSE: To examine the applicability of the recently developed laser Doppler interferometry technique for measuring the axial length of cataract eyes in a realistic clinical situation. To determine the performance of the instrument as a function of cataract grade. To compare the results to those of ultrasound methods. METHODS: A total of 196 cataract eyes of 100 patients were examined. The axial eye length was determined by laser Doppler interferometry and by two different ultrasound techniques, the applanation technique and the immersion technique. The cataract grade was determined by a commercial instrument that measures backscattered light. RESULTS: Laser Doppler interferometry worked very well except in the cases of the highest cataract grades (4% of the eyes of this study were not measurable because of a too-high lens density). Only 3.5% of the other eyes were not measurable because of fixation problems of the patients. The precision of laser Doppler interferometry is not influenced by the cataract grade (except the highest grade). The standard deviation of the geometric eye length is approximately 20 microns. Linear regression analysis revealed a very good correlation of laser Doppler interferometry and ultrasonic measurements, but a systematic difference was found. The eye lengths measured by laser Doppler interferometry were about 0.18 mm longer than those measured by the immersion technique and about 0.47 mm longer than those measured by the applanation technique. CONCLUSION: These differences are attributed to the laser Doppler interferometry results including the retinal thickness and indentation of the cornea by the applanation technique. The main advantages of the laser Doppler interferometry technique are high precision, high accuracy, and more comfort for the patient because it is a noncontact method, anesthesia is unnecessary, and the risk of corneal infection is avoided.

Adult↗
PubMed1992

Measurement of corneal thickness by laser Doppler interferometry.

The laser Doppler interferometry (LDI) technique, which was recently developed for axial eye length measurement, has been modified to measure the corneal thickness of the human eye in vivo. High accuracy is achieved. The standard deviation of the technique is about 7 microns, and improvement by a factor of 5 is possible. First comparisons with a usual slit lamp pachometer show a general agreement but a systematic difference of about 20 microns. Possible reasons for this discrepancy are discussed. Finally, the new method is compared to standard optical and ultrasound pachometry from a theoretical point of view, and advantages and drawbacks of the various techniques are discussed.

Cornea↗