Per G. Soederberg

Angular dependence of the intensity of backscattered light from human lenses with nuclear cataract: implications for measurement

It is concluded that there is no angular dependence of back scattering of light from a nuclear cataract. This implicates that reading one meridian, e.g. the 90 degree, is enough for measurement of nuclear cataract with slit lamp microscopy with Scheimpflug projection. Altogether, ten patients with nuclear cataract were measured with the NIDEK EAS-1000 system in 6 meridians. There was a slight angular dependence fitting a second order polynomial with a maximum at 90 degrees. In total, three different concentrations of standard scattering solution were measured with the same system at the same meridians. Also for these measurements, there was a slight angular dependence fitting a second order polynomial with the maximum at 90 degrees. Thus, there is a system related factor that causes a slight change of intensity of back scattered light as function of meridian.

Temperature-induced corneal shrinkage

Thermokeratoplasty is a procedure utilizing heat to reshape the surface of the cornea through shrinkage of collagen within the stroma. We have constructed an apparatus for measurements of thermally induced corneal shrinkage. Using this apparatus, we have defined the cornea shrinkage rate and examined the relationships between age, shock temperature, and shock temperature duration to the amount of shrinkage on fifteen corneal strips from Eye-Bank eyes. The results show that less heat energy was required to induce shrinkage in older corneas, that acute shrinkage increases with increasing shock temperature and that shrinkage increases with increasing shock temperature duration.

Transmittance of the lens capsule

The transmittance of the lens capsule decreases towards shorter wavelengths from around 300 nm. There is a minimum in the transmittance of around 70% at 280 nm followed by a peak of around 75% at 250 nm and then a quick decrease to 2 - 4% at 225 nm. These results were obtained from in vitro measurement of rabbit lens capsules. It is believed that the cyclic amino acids tryptophan, tyrosine and phenylalanine are responsible for the minimum transmittance at 280 nm. It is concluded that the lens capsule efficiently protects the interior of the lens from short wave ultraviolet radiation.

External Standard for Measurements with The Scheimpflug Slitlamp Microscope

Scheimpflug slitlamp microscopy is the outstanding method to record nuclear light scattering from the crystalline lens. Until yet, calibration of measurements with this device has not been possible. In the current work, an external standard for Scheimpflug slitlamp microscopy has been developed. The standard makes it possible to calibrate measurements within a research center and between research centers. The standard consists of an artificial anterior eye chamber that is filled with an aqueous solution of polystyrene spheres. The standard is placed at a position corresponding to a measured eye in the Scheimpflug slitlamp camera. A recording is taken. The higher the concentration of the scatterer, the more light scattering is recorded. A calibration curve relating concentration of light scatterer in the phantom to units of internal gray scale in the Scheimpflug microscope, is created. The scattering of a patient lens is recorded in units of internal gray scale and transformed to concentration of standard scatterer. The developed external standard opens the possibility to make comparative measurements of nuclear lens scattering in cataract for anti cataract drug development and cataract epidemiology studies.

Three month follow-up of changes in the rabbit cornea after photoablation with a pulsed scanning beam at 213 nm

A scanning beam of nano-second pulses at 213 nm flattens the cornea as predicted. However, there is a considerable variability in the flattening and the ablation is not safe. Ablation for 16 D flattening with an active spot overlap of 50% induced 8.9 +/- 5.3 D (n equals 7) as measured by the TMS topography system (ring 7 average) and 5.8 +/- 4.1 D (n equals 5) as measured with the SK-1 (2 mm zone) system. Ablation for 6 D flattening with an active spot overlap of 70% induced approximately 2 D flattening to 2 D steepening (n equals 3) as measured with the TMS (ring 7) and 6.6 +/- 4.33 D (n equals 7) flattening as measured by SK-1 (2 mm zone) keratometry. There was no change in IOP at 12 weeks after as compared to before ablation. There was a net increase of central and peripheral corneal thickness at 12 weeks after the ablation as compared to preoperatively. Epithelial defects remained up to 4 weeks after ablation. After four weeks, vessels had invaded the cornea in 30% of the cases and remained throughout the three months observed. It is concluded that 213 nm nano-second pulses can be used for flattening the cornea but the system should not be used for clinical trials in humans until the adverse effects can be avoided.

Thermoelectrically controlled device for studies of temperature-induced corneal shrinkage

The purpose of this study was to design and calibrate a device to measure the dynamics of thermal shrinkage in corneal and scleral strips. The apparatus consists of a thermoelectric cell controlled by a temperature controller designed to generate temperatures up to 90 degree(s)C in rectangular corneal strips; a copper cuvette filled with Dextran solution that holds the corneal strip and a displacement sensor that measures the change in length of the tissue during heat-induced shrinkage. The device was tested on corneal tissue from Florida Eye-Bank eyes that were cut into 2x4mm rectangular strips. Preliminary results indicate that our system can reproducibly create and accurately measure thermally induced corneal shrinkage. Shrinkage experiments will be used to optimize laser parameters for corneal shrinkage during laser thermokeratoplasty and laser scleral buckling.

Evaluation of the smoothness and accuracy of scanning photorefractive keratectomy on PMMA by optical profilometry

The smoothness and accuracy of PMMA ablations with a prototype scanning photorefractive keratectomy (SPRK) system were evaluated by optical profilometry. A prototype frequency- quintupled Nd:YAG laser (Laser Harmonic, LaserSight, Orlando, FL) was used (wavelength: 213 nm, pulse duration: 15 ns, repetition rate: 10 Hz). The laser energy was delivered through two computer-controlled galvanometer scanners that were controlled with our own hardware and software. The system was programmed to create on a block of PMMA the ablations corresponding to the correction of 6 diopters of myopia with 60%, 70%, and 80% spot overlap. The energy was 1.25 mJ. After ablation, the topography of the samples was measured with an optical profilometer (UBM Messtechnik, Ettlingen, Germany). The ablation depth was 10 to 15 micrometer larger than expected. The surfaces created with 50% to 70% overlap exhibited large saw-tooth like variations, with a maximum peak to peak variation of approximately 20 micrometer. With 80% overlap, the rms roughness was 1.3 micrometer and the central flattening was 7 diopters. This study shows that scanning PRK can produce smooth and accurate ablations.

Corneal wound healing after photoablation with a 213-nm scanning solid state laser: histological and ultrastructural study

To investigate wound healing in rabbits after corneal photoablation with a 213 nm UV scanning solid state laser. We used a frequency quintupled Nd:YAG laser to photoablate the cornea of 32 rabbits (5 mm ablation zone, 6 D myopic correction). The contralateral eyes and the eyes of 3 untreated animals served as controls. Light and electron microscopy analyses were performed on postoperative days 0, 7, 28, 90. Abnormal, light basal epithelial cells were observed during the first postoperative month, but normal epithelial maturation was evident at 3 months. Basement membrane duplication was noted. Keratocyte proliferation occurred in the anterior stroma and in a few cases, in the posterior stroma, where keratocyte activation was present. Degenerative endothelial changes were observed immediately after ablation with intracellular disorganization and junction alteration. Surface profiles with variable regularity led to a variable wound healing. Apart from differences in epithelial basal cell appearance, photoablation with a 213 nm solid state laser induced wound healing processes similar to those observed previously with prototype ArF excimer lasers. It is hoped that laser refinements will improve the surface regularity and lead to more consistent wound healing responses. However, several studies are required to assess mutagenicity, penetration depth and ablation rate of the 213 nm wavelength on ocular tissues, as well as the effects of hydration on the clinical outcome.

Calculation of laser pulse distribution maps for corneal reshaping with a scanning beam

A method for calculating pulse distribution maps for scanning laser corneal surgery is presented. The accuracy, the smoothness of the corneal shape, and the duration of surgery were evaluated for corrections of myopia by using computer simulations. The accuracy and the number of pulses were computed as a function of the beam diameter, the diameter of the treatment zone, and the amount of attempted flattening. The ablation is smooth when the spot overlap is 80% or more. The accuracy does not depend on the beam diameter or on the diameter of the ablation zone when the ablation zone is larger than 5 mm. With an overlap of 80% and an ablation zone larger than 5 mm, the error is 5% of the attempted flattening, and 610 pulses are needed per Diopter of correction with a beam diameter of 1 mm. Pulse maps for the correction of astigmatism were computed and evaluated. The simulations show that with 60% overlap, a beam diameter of 1 mm, and a 5 mm treatment zone, 6 D of astigmatism can be corrected with an accuracy better than 1.8 D. This study shows that smooth and accurate ablations can be produced with a scanning spot.

Model for photorefractive keratectomy with a scanning beam

The optimal values of the parameters of a scanning system for laser corneal surgery are determined by using a computer model. The correction of myopia is simulated. The accuracy of the correction, the smoothness of the corneal surface after ablation, and the time needed for surgery are computed. With a beam diameter of 0.5 mm, an overlap of 60% to 80%, and a 5 mm diameter treatment zone, the simulated error is less than 0.1 Diopter for a correction of 6 Diopters or less; the roughness is less than 7% of the central ablation depth; the number of pulses per Diopter of correction is 2500 if the beam intensity distribution is Gaussian and 580 if the beam intensity distribution is uniform.