Corneal photocoagulation with continuous wave and pulsed holmium: YAG radiation

Very High-frequency Ultrasound Analysis of Non-contact Holmium Laser Thermal Keratoplasty Treatment Spots

PURPOSE

To objectively measure the corneal treatment spots in vivo using very high-frequency ultrasound (VHFU) after non-contact laser thermal keratoplasty (LTK) to better understand the variability and regression of refractive outcomes.

METHODS

In an institutional setting, VHFU was performed on 128 spots (8 eyes of 4 patients) using an immersion scanning technique 1 to 2 years after LTK with a single element focused transducer (50 MHz arc scanning ultrasound). Biometric techniques were used to evaluate the treatment spot depth, corresponding corneal thickness, and spot profile between patients, eyes (left/right), and by location on the cornea. The identical technique was used in a rabbit immediately after LTK to compare ultrasound versus histologic findings.

RESULTS

The mean treatment spot penetration depth ranged between 0.373 and 0.533 mm, representing 64% to 78% of the corneal thickness compared to previous reports of 80% to 90%. Treatment spot depth, the corresponding corneal thickness, and percentage of overall cornea penetrated differed significantly across patients. Treatment spot depth was not significantly related to the level of applied laser energy (230 to 258 mJ) (0.082 Pearson sign). Spot profiles were not uniformly cone-shaped; W- and wedge-shaped were also identified. Ultrasound findings in the rabbit were similar to histology results and confirmed evidence of epithelial remodeling.

CONCLUSIONS

Very high-frequency ultrasound of 128 treatment spots after non-contact LTK demonstrates epithelial remodeling and inconsistencies in penetration depth and profile.

Semianalytical thermal model for subablative laser heating of homogeneous nonperfused biological tissue: application to laser thermokeratoplasty

We present a semianalytical technique to calculate the temperature in homogeneous nonperfused tissue during subablative laser heating. Analytical expressions of the temperature distribution in time and space are provided for collimated beams with Gaussian and top-hat intensity distributions perpendicularly incident on a finite tissue slab. The temperature distribution produced with a collimated Gaussian beam is the triple sum of the product of four functions of separate variables. The semianalytical technique can be used to rapidly calculate the temperature in laser-irradiated tissue at any point in time and space. The model was used to estimate the corneal temperature during pulsed holmium:YAG laser thermokeratoplasty with various boundary conditions at the anterior and posterior corneal surface. The model demonstrates that the corneal temperature during laser thermokeratoplasty (LTK) with a pulsed Ho:YAG laser may be sufficient to induce superficial vaporization of epithelial cells and local thermal damage to the endothelium. The calculations show that convection at the anterior corneal surface does not have a significant effect on the corneal temperature distribution, but that a better knowledge of the cooling effect of the aqueous is required to better estimate the corneal temperature distribution during LTK.

Dynamics of pulsed holmium:YAG laser photocoagulation of albumen

The pulsed holmium:YAG laser (lambda = 2.12 microm, tau(p) = 250 micros) has been investigated as a method for inducing localized coagulation for medical procedures, yet the dynamics of this process are not well understood. In this study, photocoagulation of albumen (egg white) was analysed experimentally and results compared with optical-thermal simulations to investigate a rate process approach to thermal damage and the role of heat conduction and dynamic changes in absorption. The coagulation threshold was determined using probit analysis, and coagulum dynamics were documented with fast flash photography. The nonlinear computational model, which included a Beer's law optical component, a finite difference heat transfer component and an Arrhenius equation-based damage calculation, was verified against data from the literature. Moderate discrepancies between simulation results and our experimental data probably resulted from the use of a laser beam with an irregular spatial profile. This profile produced a lower than expected coagulation threshold and an irregular damage distribution within a millisecond after laser onset. After 1 ms, heat conduction led to smoothing of the coagulum. Simulations indicated that dynamic changes in absorption led to a reduction in surface temperatures. The Arrhenius equation was shown to be effective for simulating transient albumen coagulation during pulsed holmium:YAG laser irradiation. Greater understanding of pulsed laser-tissue interactions may lead to improved treatment outcome and optimization of laser parameters for a variety of medical procedures.

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.

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.