[Use of far UV light in photoablation of the cornea].
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Animals↗
Biomedical subjects
T Bende
Publications and source records attributed to T Bende.
Subdermal implants of different steroidal compounds for treatment of endocrinological disorders.
Various types of steroid-containing implants were used in the treatment of benign prostatic hypertrophy, congestive prostatitis, and hypogonadism. 2 Silastic capsules, each containing 22-23 mg of ethinyl estradiol, were inserted subcutaneously in 5 men with benign prostatic hypertrophy. A decline in testosterone levels to less than 1 ng/ml was attained within 6 months following insertion. 6-8 Silastic capsules containing 21-23 mg each of testosterone were implanted in 15 men: 2 castrates and 13 hypogonadals. Testosterone levels increased initially, reaching the highest level at 3 months. Thereafter, the testosterone concentrations declined but remained slightly above pretreatment levels at 9 months. In a group of 9 subjects, 7 men with hypogonadism and 2 women with hypernephroma were treated with 2 rods, each containing approximately 200 mg testosterone propionate. Testosterone concentrations showed initial rises but did not remain elevated. 12 male subjects, 6 with congestive prostatis and 6 with benign prostatic hypertrophy, were treated with a combination of 3 levonorgestrel rods and 3 estrone rods. Each rod contained 40 mg of steroid. This regimen had a therapeutic effect but was only partially successful in inhibiting spermatogenesis.
Adult↗
Thermal collateral damage in porcine corneas after photoablation with free electron laser.
BACKGROUND: The study describes a quantitative and systematic investigation of the collateral thermal damage during infrared photoablation as a function of wavelength between 2.7 and 6.7 microns. METHODS: Using the tunable Free Electron Laser at Vanderbilt University, Nashville, Tenn, 60 freshly removed porcine cadaver eyes were irradiated at wavelengths between 2.7 and 6.7 microns, at a fluence of 1.3 J/cm2. For wavelengths where no photoablation occurred, fluence was increased to 3.5 J/cm2; pulse length (macropulse) was 4 microseconds, consisting of a train of micropulses (pulse duration 2 ps at a 2.9 GHz repetition rate). The corneal buttons were removed and stained with hematoxylin and eosin (H&E) and analyzed by histologic micrometry. RESULTS: Two thermal damage zones in the remaining tissue were observed: zone 1 showed superficial carbonization and measured between 2 and 4 microns; beyond, the eosinophilic zone 2 measured between 10 and 100 microns. The extent of zone 2 was inversely related to the absorption spectra of the cornea; it was minimal at the 3- and 6-micrometer water absorption bands and maximal at minimal target absorption. CONCLUSION: The results correlated well with a model of the ablation process. The study provides a systematic and predictive element for the determination of collateral thermal adverse effects; it does not yet include pulse length variation as a determining factor.
Animals↗
Correction of myopia with Er:YAG laser fundamental mode photorefractive keratectomy.
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Aged↗
Laser thermal keratoplasty using a continuous wave diode laser.
BACKGROUND: Laser thermal keratoplasty is currently performed with a pulsed Ho:YAG laser at 2.07 microns wavelength. Long-term stability depends critically on the coagulation depth of each cone and thus on emission wavelength (absorption in corneal tissue) and focusing, all contributing to controlled stable collagen shrinkage. To achieve this, a temperature range of 65 degrees to 90 degrees C is needed. A continuous wave laser source meets the coagulation requirements more effectively by avoiding tissue cooling by thermal diffusion as well as the peak temperatures of pulsed lasers, which counteracts the intended central corneal steepening. METHODS: A continuous wave diode laser was developed, emitting at 1.885 microns with a maximal energy output of 450 mW. In a contact focusing application, the absorption depth in water as a function of wavelength was measured. Using laser parameters, comparable to those used for a pulsed Ho:YAG laser in contact mode, coagulation spots in human cornea were applied for the continuous wave diode laser. RESULTS: The macroscopic and microscopic effects of the diode laser coagulation on corneas in vitro and in situ were comparable to those of the Ho:YAG laser, if a comparable amount of total energy per spot was applied. CONCLUSION: Due to better optimized laser-collagen interaction, higher corrections and more stable clinical refractive effects appear achievable using the continuous wave diode laser.
Cadaver↗
Silicon cast method for quantification of photoablation.
BACKGROUND: Topometry and measurement of photoablation patterns are key questions for keratorefractive photoablation. Ablation rates have been determined previously by either tissue perforation or by micrometry performed on histologic sections. METHODS: A three-dimensional cast of cornea after irradiation was made by using a two-component silicon gel that polymerizes within minutes, thus preserving the corneal topography immediately after photoablation. Polymerization is athermal and nontoxic. The resulting silicon blocks were cut perpendicularly to the anterior surface and measured by calibrated light microscopy. RESULTS: The silicon surface is extremely smooth and the accuracy of the cast is better than 0.25 micron. Reproducibility and long-term stability were demonstrated for casts of photoablated polymethylmethacrylate. Thus, ablation rates and profile, volumetry, and topometry can be determined following laser ablation. The method has been applied for 193-nanometer excimer laser in vitro irradiation of the human cornea. Ablation rates in Bowman's layer and stroma for various radiant energies and distinct pulse numbers were found to be in agreement with published data, and an incubation effect for the first laser pulses could be demonstrated. CONCLUSIONS: The method is nondestructive, accurate, inexpensive, practical, and reduces requirements for laboratory animals.
Animals↗
Does Bowman's layer determine the biomechanical properties of the cornea?
BACKGROUND: Bowman's layer is believed to be the stabilizing element of corneal curvature due to its assumed mechanical stiffness. METHODS: Uniaxial stress-strain analysis was performed in paired corneal strips to compare the contribution from Bowman's layer. Two central strips were taken from each cornea and Bowman's layer was removed from one of them with the excimer laser. RESULTS: Pairwise comparison yielded no statistical difference in elastic or viscoelastic properties according to presence or absence of Bowman's layer. At a strain of 2%, the stress was measured to be (5.06 +/- 2.01) x 10(3) N/m2 with Bowman's layer and (4.72 +/- 1.3) x 10(3) N/m2 without Bowman's layer. Also, the two relaxation times did not differ significantly. CONCLUSIONS: These findings imply that Bowman's layer does not contribute significantly to mechanical stability within the cornea.
Aged↗
Ablation rate of human corneal epithelium and Bowman's layer with the excimer laser (193 nm).
Laser keratomileusis is a laser-specific procedure whereby a layer of corneal tissue as thin as 10 microns or more is removed from the anterior surface. In most cases, the laser ablates not only Bowman's layer but also portions of the anterior stroma. The histologic evaluation presented shows that the ablation behavior of these two layers is not uniform: at a fluence of 205 mJ/cm2 in Bowman's layer, the ablation rate was 0.38 +/- 0.05 microns per pulse, whereas in stroma it amounted to 0.55 +/- 0.1 microns per pulse. In epithelium, the ablation rate was 0.68 +/- 0.15 microns per pulse, but decreased with deeper excisions. We discuss the consequences of these different ablation rates on the procedure of laser keratomileusis.
Cornea↗
Laser thermokeratoplasty by means of a pulsed holmium:YAG laser for hyperopic correction.
A new technique of hyperopic correction similar to radial thermokeratoplasty is presented. A pulsed Holmium:YAG laser was used, emitting a wavelength of 2.06 microns. The laser light was guided by a quartz fiber and focused by means of a handpiece. Eight of 16 coagulations located on rings concentric to the pupil resulted in central corneal steepening. The refractive change increased with the applied pulse energy above a threshold of about 10 mJ per pulse and was constant between 15 and 35 mJ per pulse. The effect decreased linearly with greater distance from the center. Four blind human eyes demonstrated that the parameters evaluated in the human cadaver eyes can be transferred to the living eye. Immediately after surgery, folds in Descemet's membrane parallel to the limbus appeared. With time, they gradually diminished but were still persistent after 4 months. Hyperopic changes of up to 5.00 diopters were obtained, remaining stable for 4 months.
Blindness↗
Photoablation of gelatin with the free-electron laser between 2.7 and 6.7 microns.
BACKGROUND: Photoablation in the infrared (IR) is an option for future refractive and corneal surgery; its basic principles have not yet been investigated systematically. For the first time, the free electron laser allows the dynamic study of photoablation over a wide range of wavelengths with variable combinations of pulselength and energy. The goal of this study is to use the free electron laser as a tool to describe photoablation in the IR quantitatively. We studied the function of wavelength as it is related to target material spectroscopy and the effects of corneal hydration and the pulse repetition rate. METHODS: Surface absorption spectroscopy of the human cornea and of gelatin as a proven model of the cornea was performed between 2.7 and 6.7 microns. Gelatin probes of well-defined thickness (140 +/- 5 microns) and controlled hydration (wet/dry weight 1 to 4.5) served as target material. Photoablation was performed with the Vanderbilt University free electron laser (Nashville, Tenn) in September 1992 at a fluence of 1.27 J/cm2, and a macropulse of 4 microseconds, composed of 2 ps micropulses at a 2.9 GHz pulse repetition rate. Wavelength was tunable between 2.7 and 6.7 microns at stable beam profiles. Ablation experiments were performed as a function of energy, hydration, and pulse repetition rate. Ablation rates were assessed by a) perforation experiments, and b) direct measurements using confocal laser topometry (UBM, Ettlingen, FRG). RESULTS: Ablation rate, assessed by perforation experiments and topometry, correlated well with the corresponding measured absorbencies of the target material: maximal ablation rate at maximal target absorption, around the 3- and 6-micrometer water absorption bands. The ablation threshold at 6.2 microns was 0.7 +/- 0.05 J/cm2 (perforation) and 0.55 +/- 0.08 J/cm2 for depth measurements. Ablation rate as a function of hydration increased to 2.3 (wet/dry weight) with a decrease for higher hydrations. Ablation rate as a function of the pulse repetition rate showed an increase of up to 20 Hz, where it was found to be 60% higher. CONCLUSION: The first systematic use of free electron laser technology positively correlated ablation efficiency with target material absorption, thus identifying a "new" promising wavelength at around 6.2 microns for materials with a high water content such as corneal tissue.
Absorption↗