PubMed Health⌕ Search

Biomedical subjects

T Juhasz

Publications and source records attributed to T Juhasz.

At least 19 recordsLinked to original sources
PubMed2006

A finite element model for ultrafast laser-lamellar keratoplasty.

A biomechanical model of the human cornea is employed in a finite element formulation for simulating the effects of Ultrafast Laser-Lamellar Keratoplasty. Several computer simulations were conducted to study curvature changes of the central corneal zone under various physiological and surgical factors. These factors included the combined effect of corneal flap and residual stromal bed thickness on corneal curvature; the effect of the shape of the lenticle on the surgical procedure outcomes and the effect of flap thickness on stress distribution in the cornea. The results were validated by comparing computed refractive power changes with clinical results. The effect of flap thickness on the amount of central flattening indicates that for flap thickness values 28% over the corneal thickness, central corneal flattening decreases. Moreover, the change in corneal curvature induced by subtraction of a plano-convex lenticle under a uniform flap, naturally imply a smaller change in the structure of the anterior layers of the cornea, but a bigger deformation in the structure of the posterior layers that are left behind the resection of the lenticle. In addition, the model also verified that the corneal curvature increased peripherally with simultaneous thinning centrally after subtraction of corneal tissue. This result shows that not only the treated zone is affected by the surgery, indicating the important role of the biomechanical response of the corneal tissue to refractive surgery, which is unaccounted for in current ablation algorithms. The results illustrate the potentialities of finite element modeling as an aid to the surgeon in evaluating variables.

Computer Simulation↗
PubMed2006

Biomechanical model of corneal transplantation.

PURPOSE: Refractive consequences of corneal transplants are analyzed using corneal biomechanical models assuming homogeneous and inhomogeneous stiffness distributions across the cornea. Additionally, refractive effects of grafts combined with volume removal procedures are also evaluated to develop methods to reduce postoperative refractive management of patients. METHODS: Refinements of a two-dimensional finite element model are applied to simulate the biomechanical and refractive effects of different corneal transplant procedures: anterior lamellar keratoplasty, posterior lamellar keratoplasty, and penetrating keratoplasty. The models are based on a nonlinearly elastic, isotropic formulation. Predictions are compared with published clinical data. RESULTS: The model simulating the penetrating keratoplasty procedure predicts more change in the postoperative corneal curvature than models simulating anterior lamellar keratoplasty or posterior lamellar keratoplasty procedures. When a lenticle-shaped tissue with a central thickness of 50 microns and a diameter of 4 mm is removed from the anterior corneal surface along with the anterior lamellar keratoplasty or posterior lamellar keratoplasty, the models predict a refractive correction of -8.6 and -8.9 diopters, respectively. CONCLUSIONS: Simulations indicate that a posterior lamellar keratoplasty procedure is preferable for obtaining a better corneal curvature profile, eliminating the need for specific secondary treatments.

Biomechanical Phenomena↗
PubMed2001

An analytically solvable model for biomechanical response of the cornea to refractive surgery.

An analttically solvable model that considers the elasticity of the cornea is developed for use in the current and novel corneal refractive surgery procedures. The model assumes that the cornea is a thin spheroid shell with an elastic response to intraocular pressure. The value of the Young's modulus of the post-operative cornea and its dependence on the geometric parameters of the ablation zone are estimated employing "best-fit" approach to nomograms currently used in corneal refractive surgery. These elasticity parameters are applied for quantitative modeling of different types of refractive surgery for myopia.

Biomechanical Phenomena↗
PubMed2001

Ultra-short pulse (femtosecond) laser surgery: initial use in LASIK flap creation.

The highly localized tissue effects of low energy femtosecond duration (ultrashort) laser pulses may be used to create three-dimensional intrastromal resections with micron precision and minimized collateral tissue damage. A surgical laser system that produces and delivers such pulses has been developed and tested clinically for creation of a corneal flap in LASIK. Expanded evaluation of this technology in this and additional keratorefractive applications is currently underway.

Corneal Stroma↗
PubMed1998

Clinical analysis of the neodymium:YLF picosecond laser as a microkeratome for laser in situ keratomileusis. Partially Sighted Eye Study.

PURPOSE: To evaluate the use of a picosecond neodymium:YLF (Nd:YLF) laser as a nonmechanical intrastromal microkeratome. SETTING: Universita Cattolica del Sacro Cuore, Rome, Italy. METHODS: An intrastromal spiral disc pattern of picosecond laser pulses was used to create a corneal flap for laser in situ keratomileusis (LASIK) in 14 partially sighted eyes. RESULTS: Flaps with a 6.0 mm diameter and 180 to 200 microns depth were successfully created in most cases. The underlying stroma was treated with a Lambda Physik excimer laser using a 3.5 to 4.5 mm optical zone. Patients were divided into 3 groups for target corrections of 5.0, 10.0, and 15.0 diopters of myopia. Good corneal clarity and refractive undercorrection were recorded in each group 6 months postoperatively. The undercorrection was due in part to the limited optical zone of the laser's delivery system. Some flap decentration was noted. CONCLUSION: This pilot study indicates that the Nd:YLF picosecond laser may be clinically applied for creating corneal flaps for LASIK. Further refinements of the laser delivery system will include enlargement of the flap diameter and improvements in flap centration. The use of a femtosecond laser may expand the capabilities and precision of this technology.

Adult↗
PubMed1996

Study of corneal ablation with picosecond laser pulses at 211 nm and 263 nm.

BACKGROUND AND OBJECTIVE: Corneal ablation has been studied by picosecond laser pulses in the far-UV region. STUDY DESIGN/MATERIALS AND METHODS: Laser pulses of 25 ps duration at 211 nm and 263 nm wavelengths and a 1 kHz repetition rate have been used to ablate human and rabbit corneas. The dependence of the etch rate on laser fluence has been measured at both wavelengths. The collateral tissue damage has been investigated by electron microscopy. RESULTS: The ablation threshold for human cornea is determined to be about 3.0 mJ/cm2 at 211 nm, while the thresholds for rabbit cornea are about 2.3 mJ/cm2 at 211 nm and 8.0 mJ/cm2 at 263 nm. The slopes of the ablation curves and the dimensions of the damage zones have also been determined. CONCLUSION: We compare these results to the existing data on corneal ablation by nanosecond UV pulses and discuss the deficiency of the photochemical model. Experimental results are analyzed in terms of a model that features plasma ablation assisted by chromophore absorption.

Animals↗
PubMed1996

Time-resolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in corneal tissue and water.

BACKGROUND AND OBJECTIVE: Photodisruption in ocular media with high power pulsed lasers working at non-absorbing frequencies have become a well established surgical tool since the late seventies. Shock waves and cavitation bubbles generated by the optical breakdown may strongly influence the surgical effect of photodisruptive lasers. We have investigated the shock wave and cavitation bubble effects of femtosecond laser pulses generated during photodisruption in corneal tissue and water. The results are compared to those obtained with longer laser pulses. STUDY DESIGN/MATERIALS AND METHODS: Laser pulses with 150 fs duration at approximately 620 nm wavelength have been focused into corneal tissue and water to create optical breakdown. Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles. RESULTS: A rapid decay of the shock waves is observed in both materials with similar temporal characteristics and with a spatial range considerably smaller than that of shock waves induced by picosecond (or nanosecond) optical breakdown. Cavitation bubbles are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses. In corneal tissue, single intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses. CONCLUSIONS: The reduced shock wave and cavitation bubble effects of the femtosecond laser result in more localized tissue damage. Therefore, a more confined surgical effect should be expected from a femtosecond laser than that from picosecond (or nanosecond) lasers. This indicates a potential benefit from the applications of femtosecond laser technology to intraocular microsurgery.

Absorption↗
PubMed1995

Myopic intrastromal photorefractive keratectomy with the neodymium-yttrium lithium fluoride picosecond laser in the cat cornea.

OBJECTIVE: To evaluate the refractive effects of myopic intrastromal photorefractive keratectomy (IPRK) with the neodymium-yttrium lithium fluoride (Nd-YLF) picosecond laser. METHODS: A 1053-nm Nd-YLF picosecond laser was used to produce myopic IPRK in cat corneas at threshold energy densities. Preoperative corneal topography and ultrasonic pachymetry were performed followed by IPRK in one eye each of 21 cats. No anti-inflammatory medications were administered. Postoperative corneal topography, ultrasonic pachymetry, and slit-lamp examination were performed at scheduled intervals. RESULTS: An average topographic flattening of 11.4 diopters and pachymetric thinning of 50 microns at the center of the treatment zone were observed by 6 weeks following treatment. The average diameter of the zone of flattening was 5.1 mm at 6 weeks and 6.0 mm at 12 weeks postoperatively. At 6 months postoperatively, the average central corneal power was unchanged, while the average central thickness had returned to the preoperative value. A mild subepithelial haze was detected at 1 month following IPRK, which was barely perceptible at the second through sixth months. Regression of the topographic effects of PRK was not observed. CONCLUSION: Intrastromal PRK with the Nd-YLF picosecond laser produces central flattening of the cat cornea with barely perceptible stromal haze and no regression of topographic effect after 6 months in the absence of treatment with anti-inflammatory agents.

Animals↗
PubMed1994

Dynamics of shock waves and cavitation bubbles generated by picosecond laser pulses in corneal tissue and water.

Time-resolved flash photography was used to investigate the dynamics of shock waves and cavitation bubbles generated by picosecond optical breakdown in bovine corneal tissue and water. A picosecond Nd:YLF laser was employed. A rapid decay of the shock waves was observed in both materials, with similar temporal characteristics, indicating that water serves as a good model for shock wave studies. In contrast, differences in the cavitation bubble dynamics were found between cornea and water, which are related to differences in the mechanical and thermal properties of the two media, suggesting that water should not be used to model cavitation dynamics in cornea. The experimental results also suggest that the efficiency of intrastromal ablation may be increased by using short pulses and moderate pulse energies in order to avoid the creation of large cavitation bubbles. The experiment indicates that the optimum laser repetition rate for intrastromal ablation is between 1 and 5 kHz.

Animals↗