Wavefronts and the limits of human vision Part 2: applications

The mean values for all the Zernike coefficients are approximately zero, while the individual variability is very high, what means that humans as a species in general have a very good optical system, though individually imperfect. Some temporal instability of higher-order aberrations, whose largest source is accommodation has been described. There arises a new question: will the correction of all the aberrations for the non-accommodated state of the eye be as beneficial for the near sight also? It is possible to modify and decrease the optical aberrations by a process of photo-ablation carried out by a scanning spot LASER with the wavefront data serving as a basis for the customized ablation profile. Perfect alignment of the LASER beam on the eye by means of high frequency eye trackers is fundamental. The theoretical visual benefit by correcting the higher order aberrations is up to 12 times. The main goal of the wavefront-guided treatments is not to allow the decrease in visual acuity after refractive surgical treatments, as may occur nowadays with the traditional LASER treatments; the next step would be treatments aiming at the improvement of visual acuity, as well as the treatments of highly irregular corneas (primary irregular, small or decentered ablations, central islands or irregularities after corneal transplantation). Understanding the new terminology and the bases of the new technological evolution, together with the continuous and critical follow-up of the new results in the literature, is the key of success of the modern approach to the problems related to refraction and refractive surgery.Os valores médios de todos os coeficientes de Zernike são de aproximadamente zero, ao passo que a variabilidade individual é muito grande, o que significa que os seres humanos, como espécie em geral, têm sistema óptico muito bom, mas individualmente imperfeito. Certa instabilidade temporal das aberrações de alta ordem foi descrita em função da acomodação. Este fato leva a uma nova pergunta: a correção de todas as aberrações para visão em estado não acomodado será benéfica para visão de perto também? É possível modificar e diminuir as aberrações ópticas por meio de foto-ablação por scanning spot LASER tendo os dados das frentes de ondas como a base para perfil de ablação individualizado. É fundamental a fixação perfeita do feixe de LASER em relação ao olho, obtida com os eye-trackers de alta freqüência. O benefício visual teórico da correção das aberrações de alta ordem seria de até 12 vezes. A meta principal dos tratamentos guiados pelas frentes de onda é de não permitir a piora da visão depois do tratamento cirúrgico refrativo, como pode ocorrer hoje em dia com os tratamentos tradicionais a LASER; em seguida, a meta se es-tenderia aos melhoramentos visuais, bem como ao tratamento dos casos de córneas extremamente irregulares (ablações primárias irregulares, pequenas ou descentradas, a presença das ilhas centrais ou irregularidades após transplante de córnea). O entendimento da terminologia e das bases da nova evolução tecnológica, bem como o constante acompanhamento crítico dos resultados publicados é fundamental para a abordagem moderna dos problemas relativos à refração e à cirurgia refrativa.UniversitätsSpital ZürichUniversidade Federal de São Paulo (UNIFESP) EPM Departamento de OftalmologiaInstituto de Física AplicadaUNIFESP, EPM, Depto. de OftalmologiaSciEL

Acute hydrops followed by corneal perforation five years after corneal cross-linking for keratoconus.

We report a case of acute corneal hydrops followed by corneal perforation five years after corneal cross-linking for keratoconus. A healthy 24-year-old female patient underwent Dresden protocol cross-linking in her left eye due to advanced keratoconus. After five years of a stable cornea, she returned with epiphora, blurred vision, and a soft left eye. Acute hydrops and corneal perforation were diagnosed. There was no history of pregnancy, atopy, eye rubbing, trauma, or contact lens use. Local antibiotic and eye patching were applied. Three months after the resolution of the acute episode, she retained useful visual acuity with no need for further surgery. Although cross-linking efficiently halts keratoconus, progression can occur, leading to corneal hydrops and perforation, even in the absence of any risk factors

Experimental Results of Preparing Laser-shaped Stromal Implants for Laser-assisted Intrastromal Keratophakia in Extremely Complicated Laser in situ Keratomileusis Cases

ABSTRACT PURPOSE: To evaluate the feasibility of lasershaped stromal implants from a donor eye for correcting extreme high hyperopia with irregular astigmatism and an exceptionally thin corneal bed. METHODS: Thirty-one fresh enucleated porcine eyes were used. The procedure was initiated with mechanical de-epithelialization followed by a lamellar cut with a microkeratome, resulting in a hinged flap (thickness 150 µm, diameter 9.5 mm). The cornea was photoablated with PRK treatment for +8.00 D sphere (hyperopic lenticule group) and -5.00 D sphere (myopic group) by a scanning spot excimer laser. A customized scanning software algorithm was used to create a circumferential cut with a 6.5-mm internal diameter. The lenticule was removed from the stromal bed and measured by a surface profiling system. A clinical case was performed on a patient with previous LASIK and highly irregular hyperopic astigmatism to verify the clinical utility of the experimental setup. RESULTS: Under the microscope, lenticules seemed round, regular, and transparent. The average surface profile of porcine corneas demonstrated good parabolic shape with individual variations as large as 30 µm, probably due to the corneal size and curvature differences between the porcine eye and the human eye-for which the microkeratome is designed. The patient underwent a topographyguided treatment after laser-assisted intrastromal keratophakia (LAIK) and 1-month follow-up showed an increase of UCVA, BSCVA, and central corneal thickness. CONCLUSIONS: The use of modern scanningspot excimer lasers and microkeratomes enabled us to produce stromal lenticules of good quality, which might be acceptable to implant into a human eye. [J Refract Surg 2002;18:S639-S643] W e have witnessed the rapid development of surgical methods for the correction of myopia, hyperopia, and astigmatism. Although most patients who undergo refractive surgery are elective first time patients, there is also an emergent group of secondary iatrogenic hyperopic and astigmatic patients after complicated laser treatments with a decreased residual corneal thickness. The first techniques of adding more corneal tissue for refractive purposes were keratophakia, epikeratophakia, and keratomileusis. They were introduced by Barraquer in the 1950s and were used during the 1980s, but due to the complexity of the procedure and equipment, the inaccuracy of refractive results, postoperative complications, and delayed visual rehabilitation 1 , they are rarely performed today, even after recent studies of the laser assisted lenticule lathing. 2 Another alternative with promising but inconclusive results is provided by artificial lenses made of different materials (flint glass, polymethylmethacryate, polysulfone, and hydrogel) that are implanted in the stroma

Patient-specific finite element analysis of human corneal lenticules: An experimental and numerical study.

The number of elective refractive surgeries is constantly increasing due to the drastic increase in myopia prevalence. Since corneal biomechanics are critical to human vision, accurate modeling is essential to improve surgical planning and optimize the results of laser vision correction. In this study, we present a numerical model of the anterior cornea of young patients who are candidates for laser vision correction. Model parameters were determined from uniaxial tests performed on lenticules of patients undergoing refractive surgery by means of lenticule extraction, using patient-specific models of the lenticules. The models also took into account the known orientation of collagen fibers in the tissue, which have an isotropic distribution in the corneal plane, while they are aligned along the corneal curvature and have a low dispersion outside the corneal plane. The model was able to reproduce the experimental data well with only three parameters. These parameters, determined using a realistic fiber distribution, yielded lower values than those reported in the literature. Accurate characterization and modeling of the cornea of young patients is essential to study better refractive surgery for the population undergoing these treatments, to develop in silico models that take corneal biomechanics into account when planning refractive surgery, and to provide a basis for improving visual outcomes in the rapidly growing population undergoing these treatments

Acute corneal melt and perforation - A possible complication after riboflavin/UV-A crosslinking (CXL) in keratoconus.

Purpose To report two cases of acute corneal melting and perforation requiring emergency penetrating keratoplasty after corneal crosslinking (CXL) in advanced keratoconus. Observations Case 1 was a 34 and case 2 was a 16-year old male, both with progressive keratoconus, who underwent CXL (Dresden protocol). After riboflavin imbibition, patients had a minimal pachymetry of 337 μm and 347 μm, and therefore required stromal swelling by hypoosmolar riboflavin resulting in pachymetries of 470 μm and 422 μm, prior to the 30 minute UV-irradiation with 3mW/cm2. In case 1, on the 7th postoperative day a 4mm linear perforation occurred. Extensive post-hoc examinations revealed no infectious cause. In case 2, a corneal melting developed within 24 hours, from which Staphylococcus aureus was cultured. Conclusions and importance Acute corneal melting and perforation may occur after CXL. Dysfunctional collagen metabolism, atopia, thin preoperative pachymetry and the use of hypoosmolar substances may have initiated this complication in our cases

Konzeptionierung einer lösbaren Zellkontaktierung für Akku-Packs zur Erhöhung der Demontage- und Recyclingfreundlichkeit

Die Verwendung von Batteriesystemen im Mobilitätssektor ist ein wichtiger Faktor zur Reduzierung von Treibhausgasemissionen und somit für den Schutz der Umwelt. Allerdings ist die Demontage, die Wiederverwendung und das Recycling der Akkus derzeit noch eine Herausforderung. Batteriezellen werden in der Regel durch Schweißverbindungen kontaktiert. Dies macht es unmöglich, einzelne Zellen aus dem Pack zerstörungsfrei zu entfernen. Bisher gibt es bereits Kenntnisse über die Eigenschaften von lösbaren elektrischen Kontakten. Es gibt jedoch noch keine Ansätze, die eine lösbare Kontaktierung ermöglichen, wobei die Herausforderungen bei Batteriepacks mit mehreren Zellen und deren Design berücksichtigt werden. Die Demontage des Packs ist das Hauptproblem im Hinblick auf ein kreislaufwirtschaftskonformes Design. Lösbare Zellkontakte sind hierfür eine mögliche Lösung. Die größte Herausforderung bei der Umsetzung lösbarer Zellkontakte besteht darin, einen möglichst geringen und gleichmäßigen Kontaktwiderstand zu erreichen. Dazu ist ebenfalls ein Verständnis über die Abhängigkeiten zwischen dem Kontaktwiderstand und verschiedenen Einflussfaktoren notwendig. Daher stellen die Autoren eine Analyse relevanter Wechselwirkungen in der lösbaren Kontaktierung von Batteriezellen im Format 18650 vor. Diese umfasst unter anderem eine Untersuchung der Kontaktflächen verschiedener Zellen und die Ermittlung deren Kontaktwiderstände unter Verwendung verschiedener Kontaktpaarungen. Abschließend werden, basierend auf Anforderungen an die Kontaktierung der Zellen, verschiedene Lösungsprinzipien für eine lösbare Kontaktierung definiert und ein initiales Konzept daraus abgeleitet

Depth-dependent mechanical properties of the human cornea by uniaxial extension.

The purpose of this study was to investigate the depth-dependent biomechanical properties of the human corneal stroma under uniaxial tensile loading. Human stroma samples were obtained after the removal of Descemet's membrane in the course of Descemet's membrane endothelial keratoplasty (DMEK) transplantation. Uniaxial tensile tests were performed at three different depths: anterior, central, and posterior on 2 x 6 × 0.15 mm strips taken from the central DMEK graft. The measured force-displacement data were used to calculate stress-strain curves and to derive the tangent modulus. The study showed that mechanical strength decreased significantly with depth. The anterior cornea appeared to be the stiffest, with a stiffness approximately 18% higher than that of the central cornea and approximately 38% higher than that of the posterior layer. Larger variations in mechanical response were observed in the posterior group, probably due to the higher degree of alignment of the collagen fibers in the posterior sections of the cornea. This study contributes to a better understanding of the biomechanical tensile properties of the cornea, which has important implications for the development of new treatment strategies for corneal diseases. Accurate quantification of tensile strength as a function of depth is critical information that is lacking in human corneal biomechanics to develop numerical models and new treatment methods