Biomedical subjects

T Seiler

Publications and source records attributed to T Seiler.

170 records · Page 10Linked to original sources

PubMedDate not supplied

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↗

PubMedDate not supplied

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↗

PubMedDate not supplied

Photorefractive keratectomy as a second attempt to correct myopia after radial keratotomy.

BACKGROUND: Undercorrection after radial keratotomy is not uncommon. To correct the residual refractive error, several techniques are available including photorefractive keratectomy. METHODS: We report five eyes of four patients, which remained undercorrected following radial keratotomy and underwent photorefractive keratectomy with an excimer laser (193 nm) to correct the residual myopia, with a follow up of 6 months to 1 year. RESULTS: The corneal-healing response was similar to that of corneas treated by photorefractive keratectomy alone. No permanent corneal scarring occurred. The average preoperative refraction before photorefractive keratectomy was -2.25 +/- 0.55 diopters; it was close to plano (+0.12 +/- 0.26 D) after 6 months. One eye regressed 0.50 D between 6 months and 1 year. Best-spectacle corrected visual acuity did not change. CONCLUSIONS: Excimer laser photorefractive keratectomy seems to be effective as a second procedure in eyes undercorrected following radial keratotomy.

Adult↗

PubMedDate not supplied

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↗

PubMedDate not supplied

Excimer laser (193 nm) myopic keratomileusis in sighted and blind human eyes.

Ten blind and 13 sighted human eyes underwent excimer laser (193 nm) keratomileusis. The radiant exposure was 180 mJ/cm2 with an ablation zone diameter of 3.5 mm. The follow-up of the blind eyes ranged from 9 to 11 months, whereas that of the sighted eyes was at least 6 months. The intended refraction change was -3.0 and -5.0 D in the blind eyes and ranged from -1.5 to -7.0 D in the sighted eyes. After 3 months, 12 of 13 sighted eyes (92%) achieved a refraction between +/- 1.0 D, whereas after 6 months 77% of the refractions were in this range. Temporary subepithelial haze occurred in all eyes except some of the -3.0 D blind eyes. After 6 months, the subepithelial haze had resolved to a clinical non-significant level (trace to grade 0.5 haze), except in one eye that showed a focal scar.

Adult↗

PubMedDate not supplied

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↗

PubMedDate not supplied

Aspheric photorefractive keratectomy with excimer laser.

BACKGROUND: Increased spherical aberration of the cornea is a common finding after excimer laser keratorefractive keratectomy. The shape of the paracentral cornea is probably related to the incidence of side effects like glare and halos at night depending on ablation zone size and pupil diameter. Aspheric curvature changes of the paracentral cornea at the edge of the ablation may decrease those side effects. MATERIALS AND METHODS: Fifteen patients were treated with photorefractive keratectomy for myopia correction with a Summit excimer laser (Summit Technology, Waltham, Mass), one eye using a standard spherical algorithm and the fellow eye using a new aspherical algorithm. Eyes were followed for 6 to 18 months. RESULTS: None of 15 patients reported halos in the aspherically corrected eye stronger than in the fellow eye, whereas five patients claimed stronger halos in the spherically corrected eye. The effective clear zone size was 3.16 +/- 0.35 mm in the spherical cases compared to 3.43 +/- 0.31 in the aspherical cases, although the same ablation zone diameters of 5 mm were used. This difference is highly statistically significant. Effective spherical aberration based on raytracing analysis of the central 5 mm of the corneal topographic maps was significantly smaller in the corneas with aspheric correction than in those with spheric corrections. CONCLUSION: These results indicate better optical homogeneity in eyes after aspheric photorefractive keratectomy for myopia compared to standard spherical corrections.

Adult↗

PubMedDate not supplied

Holmium:YAG laser thermokeratoplasty for hyperopia and astigmatism: an overview.

The holmium:YAG laser can be used to perform corneal collagen shrinkage and treat refractive errors. Studies are underway for the treatment of hyperopia by shrinking collagen in the peripheral cornea, which produces central and paracentral steepening. There is initial overcorrection followed by a regression of effect postoperatively, which appears to stabilize 4 to 6 months postoperatively. Refinement of current nomograms and definition of the amount of expected regression will enhance its accuracy. The holmium:YAG laser may be superior to radial thermokeratoplasty with a hot needle. Nearly 100 years ago, Lans was the first to report that heating of the cornea could induce collagen shrinkage with resultant corneal curvature changes. Various collagen shrinkage procedures have evolved since then. To date, the most well-known form of collagen shrinkage involves the hot needle and a technique developed by Fyodorov to do radial thermokeratoplasty for hyperopia. This article will review the current state of holmium:YAG laser technology and the advantages it may provide over radial thermokeratoplasty.

Astigmatism↗