Peter Mierdel

Increased higher-order optical aberrations after laser refractive surgery: A problem of subclinical decentration

Purpose: To study the clinical and theoretical effects of subclinical decentrations on the optical performance of the eye after photorefractive laser surgery. Setting: Department of Ophthalmology, University of Dresden, Dresden, Germany. Methods: Ocular aberrations were determined before and 1 month after uneventful photorefractive keratectomy (PRK) with the Multiscan laser (Schwind) in 10 eyes of 8 patients. The corrections ranged from –2.5 to –6.0 diopters, and ablation zones of 6.0 mm and larger were used. The measured wavefront errors were compared to numerical simulations using the individually determined decentrations and currently used ablation profiles. Results: The PRK‐induced aberrations were significantly greater than the preoperative aberrations. The numerically calculated increase in the higher‐order optical aberrations correlated with the clinical results, demonstrating a major increase in coma‐ and spherical‐like aberrations. Subclinical decentration (less than 1.0 mm) was found to be a major factor in increased coma‐like and spherical‐like aberrations after corneal laser surgery. Conclusion. To minimize higher‐order optical errors, special efforts to center the ablation zone are necessary; for example, by eye‐tracking systems that consider the visual axis.

Principles of Tscherning Aberrometry

Higher-order optical errors of the human eye are often responsible for reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations on the eye involving optical structures. The presented wavefront analyzer is based on Tschernings aberroscope. A collimated laser beam illuminates a mask with regular matrix pin holes which forms a bundle of thin parallel rays. These rays form a retinal spot pattern on the retina that is more or less distorted according to the optical errors of the eye. This retinal spot pattern is imaged onto the sensor of a low-light CCD camera by indirect ophthalmoscopy. The deviations of all spots from their ideal regular positions are measured by means of a personal computer, and from these values the optical aberrations are computed in the form of Zernike polynomials up to the 8th order.

Effects of photorefractive keratectomy and cataract surgery on ocular optical errors of higher order.

Abstractu2005u2005· Background: This pilot study was carried out to assess the effects of photorefractive keratectomy (PRK) for myopia and myopic astigmatism and cataract surgery on the ocular optical aberrations of higher degrees.u2005u2005· Methods: The optical aberrations were measured in 12 patients before and after PRK and in 10 patients after cataract surgery with a video aberroscope for clinical use (based on Tscherning’s aberroscope) designed by the authors. To characterize the optical performance of the eye the deviation of the wavefront of a foveal image point from its ideal (spherical) shape (wavefront aberration) was determined. The wavefront aberration is represented mathematically in Zernike polynomials. The first 14 Zernike coefficients Ki were determined and compared with data from normal eyes with full visual acuity. · Results: Most Zernike coefficients were considerably greater after PRK than before surgery. These changes differed significantly from the variability of repeated individual measurements. In particular, coefficients corresponding to astigmatism, spherical aberration or coma were highly significantly increased (P<0.001). After cataract surgery, the averaged Zernike coefficients exhibited no significant differences from normal values, except the coefficient K5 (astigmatism at 0° or 90°). However, coefficients showed a significant high variability, especially the coefficients for spherical aberration or astigmatism.u2005u2005· Conclusion: Both PRK and cataract surgery are operations which may considerably increase the ocular optical errors of higher order. These aberrations are not predictable and can affect the visual acuity despite optimal sphero-cylindrical correction, in particular under mesopic conditions.

Meßplatz zur Bestimmung der monochromatischen Aberration des menschlichen Auges

Fragestellung: Nach refraktiven Eingriffen an der Hornhaut und nach Kataraktoperationen können okulare Bildfehler (Aberrationen) auftreten, die nicht mit sphärischen oder astigmatischen Linsen korrigierbar und wahrscheinlich der Grund dafür sind, daß der maximale retinal bedingte Visus trotz optimaler Korrektur häufig nicht erreicht wird. Die Erfassung dieser Bildfehler unter klinischen Bedingungen wäre ein erster wichtiger Schritt zur Dokumentation und Korrektion dieser Aberrationen mittels moderner photorefraktiver Verfahren.Meßprinzip: Die Erfassung der Bildfehler erfolgt unter Beachtung des Wellencharakters des Lichts als Aberration der realen Wellenfront eines zentralen retinalen Punkts von der idealen sphärischen Form (Wellenfrontaberration). Die Messung erfolgt analog dem Prinzip des Aberroskops nach Tscherning.Ergebnis: Das Meßproblem wird mit einem Geräteverbund gelöst, der aus einem Lasersystem, einer CCD-Funduskamera und einem PC besteht. Ein Bündel paralleler äquidistanter Einzelstrahlen wird mit einer Linse vor dem Auge so gebrochen, daß auf der Netzhaut ein entsprechendes Lichtpunktmuster entsteht, das gemäß der okularen Aberration mehr oder weniger verzerrt ist. Mit dem PC wird die Abweichung der einzelnen Punkte von ihrer idealen (äquidistanten) Position bestimmt und daraus die Wellenfrontaberration in Form von Zernike- bzw. Taylor-Polynomen formuliert. Erste Ergebnisse von gesunden emmetropen Augen werden vorgestellt.Schlußfolgerung: Das Verfahren erlaubt eine ausreichend genaue Bestimmung der Wellenfrontaberration und dürfte damit nach einigen technischen Verbesserungen für klinische Anwendungen geeignet sein.Background: After refractive or cataract surgery, ocular optical errors can occur that are not correctable with spherical or astigmatic lenses and are probably responsible for the fact that in many cases the best possible (retinal) acuity is not achieved in spite of an optimum refraction. Assessment of these aberrations in the clinical routine is an important first step towards documentation and correction of these errors with modern photorefractive methods.Principle of measuring: Ocular optical errors are assessed from the viewpoint of the wave property of light as an aberration of the real wavefront of a central retinal image point from the ideal spherical form (wavefront aberration). The measurement is based on the principle of the Tscherning aberroscope.Result: A test setup is presented consisting of a laser system, a CCD fundus camera and a PC. A bundle of parallel equidistant rays is refracted by means of a lens in front of the eye producing an equivalent pattern of light spots on the retina. This pattern is more or less distorted according to the ocular aberrations. The deviations of all spots from their ideal (equidistant) positions are measured by means of the PC and from these values the wavefront aberration is computed in the form of Zernike and Taylor polynomials. The first results of emmetropic eyes are presented.Conclusions: The method allows sufficiently accurate assessment of the ocular wavefront aberration and might be on principle suitable for clinical use, provided that some technical improvements are installed.

Ocular optical aberrometer for clinical use

Higher-order optical errors of the human eye are often responsible for reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations in the eye that involve optical structures. The ocular aberrometer presented is based on Tschernings aberroscope. A collimated laser beam (532 nm, 10 mW) illuminates a mask with a regular matrix of holes which forms a bundle of thin parallel rays of 0.3 mm diameter. These rays are focused by a lens in front of the eye so that their intraocular focus point is located a certain distance in front of the retina, generating a corresponding pattern of light spots on it. According to the existing ocular optical errors, this spot pattern is more or less distorted in comparison to the mask matrix. For a 6 mm pupil diameter 68 retinal spots are plottable for assessment of the optical aberrations. The retinal spot pattern is imaged onto the sensor of a low-light charge coupled device video camera by indirect ophthalmoscopy. Deviations of all spots from their ideal regular positions are measured by means of a PC, and from these values the intraocular wave front aberration is computed in the form of the sum of Zernike polynomials up to sixth order.

Diurnal Fluctuation of Higher Order Ocular Aberrations: Correlation With Intraocular Pressure and Corneal Thickness

PURPOSEnOptimal wavefront-guided refractive corneal laser surgery requires sufficiently exact data of optical higher order aberrations. We investigated whether these aberrations had a systematic during-the-day variation, studied the range of variation, and changes in intraocular pressure and central corneal thickness.nnnMETHODSnIn 22 eyes of 22 young volunteers the optical aberrations of higher order were measured by means of a Tscherning-type ocular aberrometer three times during one day (7 AM, 12 noon, 4 PM). In addition, in 12 of these eyes the intraocular pressure and central corneal thickness were measured. The intraocular wavefront aberration was computed using Zernike polynomials up to the sixth order, and Zernike coefficients of third and fourth order were analyzed.nnnRESULTSnOnly the coefficient Z 2/4 (C13) showed a significant increase during the day by a mean 0.016 microm. A significant regression could be detected between changes of coefficients Z3/3, Z-2/4, Z0/4, Z4/4, and changes of intraocular pressure or central corneal thickness during the day.nnnCONCLUSIONSnDue to the small values, the measured during-the-day changes of higher order aberrations had no direct practical consequences for the aberrometry-guided corneal laser surgery. Alterations of some Zernike coefficients during the day may be explained by the biomechanical behavior of the cornea.

Clinical Experience With the Tscherning Aberrometer

PURPOSEnWith the aberrometer based on Tschernings principle, measurements of wavefront aberrations of human eyes with high accuracy and reproducibility are available for standard diagnostic investigations.nnnMETHODSnDuring investigational and clinical trials, wavefront-aberrations of about 300 human eyes were measured and evaluated within the last few years.nnnRESULTSnmeasurements are presented in terms of Zernike coefficients and as height maps that can be converted directly to ablation profiles for wavefront-guided laser treatments.nnnCONCLUSIONnThe Tscherning aberrometer is a simple optical device with high accuracy appropriate for routine clinical investigations on optical aberrations of the human eye.

Automated ocular wavefront analyzer for clinical use

Higher-order optical errors of the human eye are often responsible for a reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations in the eye involving optical structures. The presented wavefront analyzer bases on Tschernings aberroscope. A collimated laser beam (532 nm, 10 mW) illuminates a mask with a regular matrix of 0.3 mm diameter holes which forms a bundle of thin parallel rays. These rays are focused by a lens in front of the eye that their intraocular focus point is located in a certain distance in front of the retina generating a corresponding pattern of light spots on it. According to the existing ocular optical errors, this spot pattern is more or less distorted in comparison to the mask matrix. For a 6 mm pupil diameter 68 retinal spots are plottable for the assessment of optical aberrations. The retinal spot pattern is imaged onto the sensor of a low-light CCD video camera by indirect ophthalmoscopy. The deviations of all spots from their ideal regular positions are measured by means of a PC, and from these values the intraocular wave-front aberration is computed in the form of Zernike polynomials up to the 6th order.

Laserinduzierte Druckwellen am Auge Ausbreitungscharakteristik

Background: The characteristics of shock waves during photoablation were investigated for an IR and a UV laser. These stress waves may be harmful to ocular structures.Material and methods: The amplitude of shock waves was measured by a needle-shaped hydrophone in enucleated porcine eyes during excimer laser (193 nm, 23 ns, diameter of ablation 1.5 – 7.5 mm) and Er:YAG laser photoablation (2.94 µm, 200 µs, 1.2 mJ/cm2, diameter of ablation 4 mm).Results: With the excimer laser at ablation zones larger than 4.5 mm, a pressure focus occurs at a distance of 4 – 6 mm behind the cornea. The pressure amplitudes are smaller than 80 bar for a fluence of 180 mJ/cm2 and decrease steadily to values below 10 bar towards the retinal level. Higher fluences produce higher pressure values; in the range of 60 to 220 mJ/cm2 the relation is linear. For the Er:YAG laser, pressure amplitudes are smaller than 0.5 bar.Conclusions: Mechanical damage of the retina is unlikely during excimer- or Er:YAG-laser ablation. The existence of a pressure focus may result in mechanical damages of the posterior lens or anterior vitreous at large ablation diameters. During Er:YAG laser ablation, shock waves could not be detected with our measurements. Theoretical estimations yield values of less than 700 mbar at a fluence of 1.2 J/cm2. The pressure load of the endothelium is independent of diameter but dependent on fluence.Hintergrund: Die Ausbreitungscharakteristik der Druckwelle nach Photoablation der Hornhaut soll für einen IR- und einen UV-Laser in Abhängigkeit vom Ablationsdurchmesser, vom Abstand hinter der Hornhaut und von der Energiedichte untersucht werden. Diese Druckwellen können schädlich für manche intraokularen Strukturen sein.Material und Methode: An enukleierten Schweineaugen wurde bei der Photoablation mit einem Excimerlaser (193 nm, 23 ns, Ablationsdurchmesser 1,5 – 7,5 mm) und mit einem Er:YAG-Laser (2,94 µm, 200 µs, 1,2 mJ/cm2, Ablationsdurchmesser 4 mm) die Amplitude der Druckwelle mit einem piezoelektrischen Nadelhydrofon gemessen.Ergebnisse: Beim Excimerlaser bildet sich bei einem Ablationsdurchmesser >4,5 mm ein Druckfokus im Abstand von 4 – 6 mm hinter der Hornhaut aus. Bei einer Energiedichte von 180 mJ/cm2 liegen die Druckwerte im Mittel alle <8 ⋅ 106 Pa. Sie verringern sich stetig in Richtung Netzhaut bis unter 106 Pa. Höhere Energiedichten ergeben höhere Druckwerte, wobei der Zusammenhang im Bereich von 60 – 220 mJ/cm2 linear ist. Die Druckamplituden beim frei laufenden Er:YAG-Laser liegen <5 ⋅ 104 Pa.Schlußfolgerungen: Eine mechanische Schädigung an der Netzhaut ist beim Excimerlaser und Er:YAG-Laser unwahrscheinlich. Das Vorhandensein des Druckfokus bei großen Ablationsdurchmessern beim Excimerlaser könnte zur mechanischen Schädigung der Linsenkapsel führen. Beim Einsatz des Er:YAG-Lasers ließen sich mit der vorhandenen Meßempfindlichkeit keine Drucksignale detektieren. Theoretische Abschätzungen ergeben Werte <7 ⋅ 104 Pa bei einer Energiedichte von 1,2 J/cm2. Die Wirkung der Druckamplitude auf das Endothel ist unabhängig vom Durchmesser, aber abhängig von der Energiedichte.

Is the human eye a perfect optic

The aim of this work was to study the optical aberrations of higher order in a normal population and to answer the question on the optical quality of the human eye. Therefore, the optical aberrations of 130 eyes (90 individuals) have been measured by means of a wavefront measuring devices of Tschernings-type. The pupil of each measured eye was dilated to at least 7 mm in diameter and wavefront sensing was performed with respect to the line of sight. The optical aberrations are expressed in terms of Zernike coefficients up to the 6th order, root-means-squared wavefront errors and critical pupil size. The main finding of this paper is that the `average eye has only minimal wavefront errors indicating that the construction of the human eye, in principle, provides excellent optics exceeding the Marechal- criterion only by a factor of 2.1 (pupil diameter 5 mm). However, such minimal aberrations are achieved in only 6% of the individual eyes examined in this study. In conclusion, the `averaged human eye has nearly a perfect optic, but the individual eye provides poor optical quality.