Room-temperature high-frequency transport of dirac fermions in epitaxially grown Sb2Te3- and Bi2Te3-based topological insulators.

We report on the observation of photogalvanic effects in epitaxially grown Sb2Te3 and Bi2Te3 three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac fermions driven back and forth by the terahertz electric field results in a dc electric current. Because of the "symmetry filtration" the dc current is generated by the surface electrons only and provides an optoelectronic access to probe the electron transport in TI, surface domains orientation, and details of electron scattering in 3D TI even at room temperature.

[Adoption of OSCE Examination Procedures in Ophthalmology]. / Etablierung der OSCE-Prüfungsmethode in der Ophthalmologie.

BACKGROUND: The department of ophthalmology at the medical faculty of the University of Ulm provides additionally for interested students in clinical semesters an optional activity. In small groups the students deepen their theoretical knowledge and learn more about ophthalmological diagnostic standard procedures. The "objective structured clinical examination" (OSCE) is known as a useful tool in the assessment of clinical skills. The OSCE is a well-established and valid examination method, but also time-consuming and costly. In form of a trial the OSCE was launched during the optional activity "Look into my eyes, baby"; in the summer semester 2009. METHODS: By means of four examination areas established diagnostic methods in ophthalmology were tested. During the tests two simulation patients as well as two phantom heads for ophthalmological examinations were assessed. The preparation of the examination materials occurred in close collaboration with the department of ophthalmology and the division exam development of our university. In the assessment of the examinations a high value was set on good communication skills between students and simulation patients as well as on the professional handling of the student tasks. After the examinations the acceptance of the test methods was evaluated using a focused group interview between the students of the optional activity and the participating examiners. RESULTS: We performed two OSCEs involving three students each in the last two semesters. The OSCE was to a great extent time- and resource-consuming, due to the intensive pre- and post-reviewing and the time students needed to pass the various examination areas. Students and examiners as well confirmed the validity of the assessment and acknowledged a positive effect on the students learning behaviour. CONCLUSIONS: The teaching staff members are willing to accept the OSCE, especially when the assessment procedures are thoroughly planned and well structured. The acceptance of the students can be achieved by providing valid assessment and reviewed teaching conditions. The high input in personnel and instrumental resources for the assessment and the student's individual supervision should be critically discussed in the light of the efficacy of the additional ongoing hospital and outpatient services and the personnel-relevant education budget assigned to the department of ophthalmology of a university.

Growth, characterization, and transport properties of ternary (Bi1-x Sb x )2Te3 topological insulator layers.

Ternary (Bi1-x Sb x )2Te3 films with an Sb content between 0 and 100% were deposited on a Si(1 1 1) substrate by means of molecular beam epitaxy. X-ray diffraction measurements confirm single crystal growth in all cases. The Sb content is determined by x-ray photoelectron spectroscopy. Consistent values of the Sb content are obtained from Raman spectroscopy. Scanning Raman spectroscopy reveals that the (Bi1-x Sb x )2Te3 layers with an intermediate Sb content show spatial composition inhomogeneities. The observed spectra broadening in angular-resolved photoemission spectroscopy (ARPES) is also attributed to this phenomena. Upon increasing the Sb content from x = 0 to 1 the ARPES measurements show a shift of the Fermi level from the conduction band to the valence band. This shift is also confirmed by corresponding magnetotransport measurements where the conductance changes from n- to p-type. In this transition region, an increase of the resistivity is found, indicating a location of the Fermi level within the band gap region. More detailed measurements in the transition region reveals that the transport takes place in two independent channels. By means of a gate electrode the transport can be changed from n- to p-type, thus allowing a tuning of the Fermi level within the topologically protected surface states.

Coherent ultrafast spin-dynamics probed in three dimensional topological insulators.

Topological insulators are candidates to open up a novel route in spin based electronics. Different to traditional ferromagnetic materials, where the carrier spin-polarization and magnetization are based on the exchange interaction, the spin properties in topological insulators are based on the coupling of spin- and orbit interaction connected to its momentum. Specific ways to control the spin-polarization with light have been demonstrated: the energy momentum landscape of the Dirac cone provides spin-momentum locking of the charge current and its spin. We investigate a spin-related signal present only during the laser excitation studying real and imaginary part of the complex Kerr angle by disentangling spin and lattice contributions. This coherent signal is only present at the time of the pump-pulses' light field and can be described in terms of a Raman coherence time. The Raman transition involves states at the bottom edge of the conduction band. We demonstrate a coherent femtosecond control of spin-polarization for electronic states at around the Dirac cone.

Inhibition of rabbit keratocyte and human fetal lens epithelial cell proliferation by retrovirus-mediated transfer of antisense cyclin G1 and antisense MAT1 constructs.

The aim of this study is to evaluate the potential of gene transfer of cell cycle control genes as treatment of corneal haze or secondary cataract formation. The guiding hypothesis is that strategic modulation of the cyclin G1 or MAT1 gene by retrovirus-mediated gene transfer will inhibit proliferation of rabbit keratocytes (RabK) and fetal human lens epithelial (FHLEpi) cells in vitro. RabK and FHLEpi cell cultures were transduced in triplicate with retroviral vectors bearing either a nuclear-targeted beta-galactosidase, an antisense cyclin G1 (aG1), an antisense MAT1 (aMAT1) construct, or the neo(r) gene. The presence of beta-galactosidase activity in the transduced cultures was detected by immunohistochemical X-Gal staining, while cyclin G1 and MAT1 protein expression levels were evaluated by Western analysis. Proliferation of RabKs and FHLEpi cells was analyzed by counting the number of cells in the aG1 and aMAT1 vector-transduced cultures over 5 days. The mean transduction efficiency was 34.4% (SD 1.41) for RabKs and 19.7% (SD 1.83) for FHLEpi cells. Downregulation of cyclin G1 and MAT1 protein expression was noted 24 hr after transduction of RabK cultures with the respective vectors. Cytostatic effects of the aG1 and aMAT1 vectors in both RabKs and FHLEpi cells were most pronounced on the fifth day (RabKs, p < 0.0007; FHEpi cells, p < 0.001). An increased incidence of apoptosis was identified in both aG1 and MAT1-transduced FHLEpi cells. Taken together, these data suggest the potential utility of developing aG1 and aMAT1 retroviral vectors in gene therapy protocols for corneal haze and secondary cataract formation.

Complementation of a Toxoplasma gondii ROP1 knock-out mutant using phleomycin selection.

The ROP1 gene of Toxoplasma gondii encodes a rhoptry protein that has been implicated in host cell invasion by this obligate intracellular protozoan. To further explore the function of this protein, we created a ROP1 deletion mutant by transfection with a plasmid encoding the bacterial chloramphenicol acetyltransferase (cat) gene flanked by ROP1 genomic sequences. Selection for chloramphenicol resistance yielded the desired ROP1-deleted or 'knock-out' mutant. Analysis of this mutant both in vitro and in vivo shows no significant alterations in growth rate, host specificity, invasiveness or virulence and thus the ROP1 gene is not obligatory for the RH strain, at least under the conditions tested. However, electron microscopy reveals that the mutant strain's rhoptries are altered in ultrastructure; they are thinner and homogeneously electron-dense compared with the thicker and normally mottled or honeycombed appearance of wild-type rhoptries. The knock-out mutant was rescued using co-transfection of a cosmid carrying the ROP1 gene together with a plasmid encoding a new selectable marker for T. gondii, the bleomycin resistance gene (ble) from Streptoalloteichus. Southern blot analysis showed that both DNAs were stably integrated into the Toxoplasma genome, although not into the ROPI locus. The resulting strain showed wild-type levels of ROP1 expression and rescue of the ultrastructural phenotype (i.e., the rhoptries returned to their normal, mottled appearance), thus establishing a cause/effect relationship between the absence of ROP1 and the electron-opacity. These results demonstrate the utility of the reverse genetic approach in the study of Toxoplasma gene function and provide a further selectable marker for such manipulations.

Octreotide prevents growth factor-induced proliferation of bovine retinal endothelial cells under hypoxia.

Ocular diseases such as proliferative diabetic retinopathy are the major cause of blindness in industrialized countries. The main pathologic features of these diseases are hypoxia and overproduction of growth factors resulting in pathologic proliferation of endothelial cells and new vessel formation. Particularly, hypoxia and growth factors, such as VEGF, IGF-1, bFGF and TGF beta(2), show a complex interaction in the onset and progression of the diseases. Therefore, to date, most therapeutic strategies for proliferative retinopathies have targeted proliferation of endothelial cells evoked by growth factors. Recently, a synthetic analog of somatostatin, octreotide, has been shown to inhibit the proliferation of various cells in vitro, including endothelial cells. In this study, we have investigated the proliferative response of bovine retinal endothelial cells (BREC) to growth factors under hypoxic conditions and the modulation by octreotide. We found a dose-dependent increase of cell proliferation with VEGF, IGF-1 and bFGF, and inhibition of hypoxia-induced cell proliferation with TGF beta(2). Moreover, growth factor-induced, but not hypoxia-induced, cell proliferation was attenuated in the presence of octreotide. In contrast, TGF beta(2) abolished hypoxia-induced BREC proliferation. Similar to octreotide BIM23027, a somatastatin receptor subtype 2 (SSTR2) receptor agonist was able to attenuate the growth factor-induced proliferation of BREC expressing mRNA and protein for SSTR2. Therefore, we postulate that octreotide exerts its effects mainly through binding to the SSTR2. Taken together, our findings point to octreotide as a promising candidate for the treatment of eye disorders involving growth factor-dependent proliferation of endothelial cells.

The effectiveness of a topical antibiotic irrigating solution in a model of staphylococcal keratitis after lamellar keratectomy.

PURPOSE: To create a model of Staphylococcus aureus keratitis after lamellar keratectomy; to assess the toxicity of an antibiotic irrigating solution on the corneal stromal bed; and to test the chemotherapeutic effectiveness of a topical antibiotic, both alone and with an antibiotic-containing irrigating solution in preventing S. aureus keratitis after lamellar keratectomy. METHODS: The right eye of each of 38 rabbits were used in this study. In 18 eyes, a lamellar flap was created with a microkeratome, and an inoculum of S. aureus (either 1,000, 5,000, or 50,000 CFUs) was instilled under each flap; the eyes were examined for signs of infection and inflammation at 24 and 48 hours. In another five eyes, a lamellar flap was created in the same manner and the stromal bed was irrigated with 0.3% ofloxacin; the eyes were assessed for ocular inflammatory changes and evidence of crystalline deposits. Finally, in each of 15 additional eyes, 1,000 CFUs of S. aureus were instilled under a lamellar flap to create experimental infectious keratitis. The keratitis was treated according to three regimens: irrigation of the stromal bed with sterile balanced salt solution; irrigation of the stromal bed with 0.3% ofloxacin, followed by application of topical ofloxacin four times a day; application of topical ofloxacin only, four times a day. Eyes were examined for infection and ocular inflammatory changes at 24 and 48 hours. RESULTS: Staphylococcus aureus keratitis can consistently be produced under the stromal flap by inoculation of relatively few organisms. Irrigation of the stromal bed with commercial-strength topical ofloxacin does not appear to be toxic to the stromal bed, with no evidence of crystalline precipitates of the antibiotic. In our model of infectious keratitis after lamellar keratectomy, both topical ofloxacin alone and the combination of topical ofloxacin and irrigation of the stromal bed with 0.3% ofloxacin were effective at preventing S. aureus keratitis. However, the combined treatment of antibiotic irrigation plus topical antibiotic was more effective at preventing inflammation than topical ofloxacin alone. CONCLUSIONS: In this model of S. aureus keratitis after lamellar keratectomy, irrigation of the stromal bed with antibiotic plus topical antibiotic appears to be both safe and effective for preventing infection.

Feasibility of intraoperative corneal topography monitoring during photorefractive keratectomy.

PURPOSE: We propose a feasibility study of new corneal topography technology with the aim of monitoring intraoperative corneal topography during excimer laser photorefractive keratectomy. The PAR system measures corneal topography with single grid projection and triangulation but requires fluorescent fluid to be deposited on the corneal surface for shape extraction. We propose and demonstrate a novel corneal topography system based on structured incoherent visible light projection and triangulation that does not require addition of fluorescent fluid. METHODS: We used a binary liquid crystal spatial light modulator to display multiple fringe patterns onto the cornea. The depth accuracy of the corneal topography system was measured using a white reflected test sphere mounted on a micrometer translation stage. The performance of the corneal topography system was tested on 5 de-epithelialized swine eyes in vitro ablated with a VISX excimer laser. RESULTS: Depth accuracy on the test sphere was 0.5+/-0.75 microm over an area of 17.6 mm2. On de-epithelialized swine corneas, ablation at the apex of the cornea treated with an excimer laser was measured without addition of fluorescein. CONCLUSIONS: This new corneal topography system achieved an adequate level of accuracy on a test sphere.

Comparison of free-running vs. Q-switched Er:YAG laser photorefractive keratectomy (scanning mode) in swine eyes.

PURPOSE: To examine morphology in plane and incisional corneal ablation (in vitro) induced by an Er:YAG laser (2.94 microm) in two modes: free-running and q-switched. METHODS: Sequences of different fluences in each mode were applied to freshly enucleated pig eyes. Parameters of free-running mode were: pulse length 50 micros, fluences 1.21 to 4.77 J/cm2, frequency 80 Hz, spot size 500 microm FWHM, hexagonal spot shape. Parameters of q-switched mode were: pulse length 200 ns, fluences 0.79 to 2.33 J/cm2, frequency 20 Hz, spot size 500 microm FWHM, round spot shape. RESULTS: Histology showed thermal damage of 10 to 25 microm in depth caused by the free-running mode compared with 4.5 to 7.5 microm by the q-switched mode. In both gross photography and scanning electron microscopic examination, the surface was more homogeneous and smoother in the q-switched mode. CONCLUSIONS: Depending on the different application modes, both laser systems could be used for a defined corneal ablation in photorefractive keratectomy. At present, results using the Er:YAG laser are not as favorable as with the excimer laser.

Corneal morphology in vitro after superficial keratectomy with q-switched Er:YSGG and free-running Er:YAG lasers.

PURPOSE: Examination of morphology in corneal ablation induced by a q-switched Er:YSGG (2.79 microm) laser and a free-running Er:YAG laser (2.94 microm). METHODS: Defined ablation of 6-mm diameter and 15, 30, 55, 90, and 120-microm depth was performed on freshly enucleated swine eyes. Er:YSGG laser parameters: fluence 1.6 J/cm2, frequency 6 Hz, spot-size 465 microm FWHM, scanning-mode, pulse number 1520 to 6210. Er:YAG laser parameters: fluence 3 J/cm2, frequency 1.5 Hz, spotsize 6 mm, wide area ablation, pulse number 2 to 13. Corneal morphology was analyzed by gross photography, histology, scanning electron microscopy, and scanning nearfield acoustic microscopy. RESULTS: Histology showed thermal damage of 5 to 15 microm in depth caused by the Er:YSGG laser in comparison with 10 to 20 microm by the Er:YAG laser. Average roughness of the ablated surface measured with scanning nearfield acoustic microscopy was 20 to 40 microm for the Er:YSGG laser and 5 to 15 microm for the Er:YAG laser. These data confirm the subjective impression of images created by scanning electron microscopy and gross photography. CONCLUSIONS: Although the Er:YAG laser system appeared to demonstrate a smoother corneal surface than the q-switched Er:YSGG laser, the thermal damage in either case poses a potential limitation for clinical use in lamellar refractive surgery.

Diode laser thermokeratoplasty: application strategy and dosimetry.

PURPOSE: To investigate suitable application parameters for efficient hyperopic correction by laser thermokeratoplasty (LTK) using mid-infrared laser diodes. SETTING: Medical Laser Center Lübeck, Lübeck, Germany. METHOD: A tunable continuous-wave laser diode in the spectral range between 1.845 and 1.871 microns was used. Transmitted by waveguides, the laser energy was used to induce coagulations on freshly enucleated porcine eyes to increase corneal curvature. The coagulations were equidistantly applied by a fiber-cornea contact and a noncontact focusing device that were adjusted on a ring concentric to the corneal apex. Different laser parameters and application geometries were evaluated. Refractive changes were measured by computer-assisted corneal topography before and after treatment. Polarization light microscopy and temperature calculations were used to analyze the coagulations. RESULTS: Because of the tunability of the laser diode, the influence of the corneal absorption coefficient (between 0.9 and 1.6 mm-1) on the refractive change could be measured. A laser power between 125 and 200 mW was adequate to achieve refractive changes up to 10.0 diopters. In the preferable focusing device, the refractive change increased almost logarithmically with the irradiation time up to 15 seconds. The number of coagulations on a fixed application ring showed no significant influence on refractive change; however, it showed an almost linear decrease with increasing ring diameter from 5.0 to 10.0 mm. Histological analysis revealed 3 stages of thermal damage. CONCLUSION: Diode LTK provided defined and uniform coagulations when using a well-adapted focusing device, resulting in sufficient refractive change. The results indicate that diode LTK is superior to pulsed holmium LTK.

Influence of temperature and time on thermally induced forces in corneal collagen and the effect on laser thermokeratoplasty.

PURPOSE: To investigate thermomechanical aspects of corneal collagen denaturation as a function of temperature and time and the effect of the induced forces on refractive changes with laser thermokeratoplasty (LTK). SETTING: Medical Laser Center Lübeck, Lübeck, Germany. METHODS: In a material-test setup, porcine corneal strips were denatured in paraffin oil at various constant temperatures for 10 and 500 seconds, and the temporal course of the contractive forces was studied under isometric conditions. Typical LTK lesions were performed in porcine eyes in vitro with a continuous-wave infrared laser diode at a wavelength of 1.87 microm for 10 and 60 seconds. The laser power was chosen to achieve comparable denatured volumes at both irradiation times. The refractive changes were measured and analyzed by histologic evaluations and temperature calculations. RESULTS: The time course of the induced forces was characterized by a maximal force, which increased almost linearly with temperature, and a residual lower force. After 500 seconds of heating, the highest force was achieved with a temperature of 75 degrees C. With a limited heating period of only 10 seconds, the forces steadily increased with temperature over the entire observation period. Laser thermokeratoplasty produced less refractive change after 10 seconds of irradiation than after 60 seconds, although the laser power was 25% higher in the short heating period. Polarization light microscopy of LTK lesions revealed different stages of thermal damage. CONCLUSION: The course of the contractive forces during and after heating is a complicated function of the spatial time/temperature profile. Laser thermokeratoplasty lesions produced with 2 irradiation times showed different stages of denaturation and induced refractive change.

Significance of corneal topography in predicting patient complaints after photorefractive keratectomy.

PURPOSE: To evaluate the sensitivity and specificity of postoperative corneal topography to predict potential patient complaints after photorefractive keratectomy (PRK). SETTING: Doheny Eye Institute, Los Angeles, California, USA. METHODS: Postoperative tangential corneal topographic maps, in 0.5 and 1.0 diopter (D) relative scales, were obtained from patients (n = 34) at least 4 months after PRK. Topographies of complaining (n = 18) and noncomplaining patients (n = 16) were analyzed by 6 masked examiners with 2 different experience levels in PRK (experts, n = 2; beginners, n = 4), who assigned the topographies to 1 of the 2 groups. RESULTS: Topographies of complainers (sensitivity) and noncomplainers (specificity) were correctly classified in 53.2% overall and in 44.0% and 63.5% (P = .06) in complainers and noncomplainers, respectively. Experienced examiners were not significantly more accurate than inexperienced examiners (46.3% and 56.6%, respectively; P = .09). Images of 1.0 D scales received significantly more correct responses than those of 0.5 D scales (56.4% and 50.0% respectively; P = .03). The reproducibility between images for the same patient in both scales was significantly better for the experienced examiners than the inexperienced examiners (kappa coefficient 0.73 and 0.51, respectively; P = .05). CONCLUSIONS: Subjective analysis of postoperative corneal topography alone is not sufficient to predict potential patient complaints after PRK. Topographic findings should be interpreted only in the context of a complete clinical examination.

Thermal and biomechanical parameters of porcine cornea.

PURPOSE: New methods in refractive surgery require a considerable understanding of the material "cornea" and are often studied by theoretical modeling in order to gain insight into the procedure and an optimized approach to the technique. The quality of these models is highly dependent on the preciseness of its input parameters. Porcine cornea often is used as a model in preclinical studies because of its similarity to man and its availability. METHODS: The important physical parameters for biomechanical deformation, heat conduction, and collagen denaturation kinetics have been determined for porcine cornea. Experimental methods include densitometry, calorimetry, turbidimetry, tensile tests, stress relaxation, and hydrothermal isometric tension measurements. RESULTS: The density of porcine cornea was measured as p = 1062+/-5 kg/m3, the heat capacity gave c = 3.74+/-0.05 J/gK. The stress-strain relation for corneal strips is represented by a third order approximation where the secant modulus yields about Esec approximately equal to 0.4 MPa for small strains less than 2%. The normalized stress relaxation is described by an exponential fit over time. The denaturation process of cornea is characterized by specific temperatures which can be related to the change of the mechanical properties. Denaturation kinetics are described according to the model of Arrhenius yielding the activation energy deltaEa = 106 kJ/mol and the phase transition entropy deltaS = 39 J/(mol x K). CONCLUSIONS: The established set of parameters characterizes the porcine cornea in a reliable way that creates a basis for corneal models. It furthermore gives direct hints of how to treat cornea in certain refractive techniques.

Effect of thalidomide, octreotide, and prednisolone on the migration and proliferation of RPE cells in vitro.

PURPOSE: The retinal pigment epithelium (RPE) is involved in the pathogenesis of age-related macular degeneration. The purpose of this study was to assess the effect of thalidomide, octreotide, and prednisolone on the proliferation and migration of bovine RPE cells in vitro. METHODS: The migration assay was performed in double-chamber-wells separated by a membrane filter with 8 microm pores. Cells were allowed to migrate vertically for 7 hr, afterwards the cells on both filtersides were fixed, stained, and the migrated cells were counted. To examine RPE proliferation, bovine RPE cells were seeded subconfluently followed by an incubation with octreotide, thalidomide or prednisolone in a concentration gradient for 24 hr. Stimulation or inhibition of DNA synthesis was measured by [(3)H]-thymidine incorporation. Statistical analysis was performed with the paired student's t-test. RESULTS: Statistically significant (p < 0.05) inhibition of RPE cell proliferation was measured for thalidomide at a concentration of 10-50 microg/ml, for octreotide at a concentration of 5 x 10(-4) and 5 x 10(-5) M, and for prednisolone at a concentration of 250 and 500 microg/ml as compared to the negative control. RPE cell migration was significantly (p < 0.05) inhibited by thalidomide at a concentration of 10 microg/ml, by octreotide at a concentration of 5 x 10(-5) M, and also by prednisolone at a concentration of 500 microg/ml as compared to the negative control. CONCLUSIONS: Although the main effect of thalidomide, octreotide, and prednisolone when treating patients with choroidal neovascular membranes is probably related to the inhibition of angiogenesis it should be kept in mind that these substances may additionally inhibit RPE proliferation and migration.

[Excimer laser sclerostomy of secondary glaucoma]. / Excimer-Laser Sklerostomie des Sekundärglaukoms.

Initial results after nine ab externo sclerotomies on seven patients awaiting enucleation because of intensive pain caused by secondary glaucoma have encouraged us to present the methods used for this technique of fistula construction and the course in these patients. A special applicator permits precise manual positioning of the excimer laser beam (lambda = 308 nm) subconjunctivally at the limbus using a quartz-quartz fiber (diameter: 0.32 mm). The energy applied at the fibertip is 4 mJ per pulse, with a repetition rate of 80 Hz. All patients undergoing the operation, which lasted only a few minutes, were treated as out-patients without a retrobulbar anaesthesia. Because of the minimal irritation of the anterior chamber, only antibiotic treatment was required after the operation. The intraocular pressure immediately decreased to normal levels after the operation without any further glaucoma treatment in five cases. In the other two cases reduced, but still minimally elevated, pressure was recorded. In the case of one heavily inflamed eye, which had been subjected to two operations, pain felt by the patient was relieved when the pressure was reduced. The maximum period of post-operative follow-up so far is over 10 months and the decrease of intraocular pressure has remained adequate without any additional antiglaucoma therapy. In all cases enucleation was avoided.

[Photorefractive keratectomy. Free-running vs. q-switched Er:YAG laser (scanning mode)]. / Photorefraktive Keratektomie. Freilaufender vs. gütegeschalteter Er:YAG-Laser (Scanning Modus).

BACKGROUND: Examination of morphology in plane corneal ablation (in vitro) induced by an Er:YAG-laser (2.94 microns) in two modes: free-running (50 microseconds) and q-switched (200 ns). METHODS: Sequences of different fluences in each mode were applied to freshly enucleated swine eyes. Parameters of free-running mode: fluences 1.21-4.77 J/cm2, frequency 80 Hz, spot size 500 microns FWHM, hexagonal spot shape. Parameters of q-switched mode: fluences 0.79-2.33 J/cm2, frequency 20 Hz, spot size 500 microns FWHM, round spot shape. RESULTS: Histology showed thermal damage of 10-25 microns in depth caused by the free-running mode compared with 4.5-7.5 microns by the q-switched mode. In both gross photography and scanning electron microscopic examination, the surface was found to be more homogeneous and smoother in the q-switched mode. CONCLUSIONS: Depending on the different application modes, both laser systems could be used for a defined corneal ablation in photorefractive keratectomy. However, at the moment, results using the Er:YAG laser are not as favorable as with the excimer laser.

[Comparative study of ab-externo sclerostomy with the excimer and Er:YAG laser]. / Vergleichende Studie zur Ab-externo-Sklerostomie mit dem Excimer- und Er:YAG-Laser.

The specific ablative effect of pulsed 308 nm XeCl-excimer laser radiation (4 mJ 80 Hz) on the tissue has proved its worth in clinical use in glaucoma patients. However, the cytotoxic and mutagenic photochemical reactions induced by intraocular ultraviolet irradiation could theoretically be cataractogenic and retinotoxic. Unlike the excimer laser technique, ab externo sclerostomy with the Er:YAG laser (2940 nm/11 mJ, 7 Hz) excludes these risks. Histological and scanning electron microscope analysis of pig eyes showed thin, and smoothly limited zones of necrosis and only minimal irritation of the adjacent tissue and a slightly wavy surface. A newly developed handpiece for the Er:YAG laser enables energy transport via zirconium fluoride fiber and coupling to a quartz fiber tip with a core diameter of 320 microns. Er:YAG laser sclerostomy has so far been performed on 16 eyes in which the average preoperative IOP was 29 mmHg. The procedure took only a few minutes and the patients reported feeling no pain although retrobulbar anesthesia was not induced. The postoperative average IOP was less than 20 mmHg after 6 weeks and for the rest of a maximum observation period of 12 weeks. Reoperation was necessary in 2 cases. Er:YAG laser ab externo sclerostomy could be another alternative for the operative therapy of glaucoma.