Simulation of the temperature increase in human cadaver retina during direct illumination by 150-kHz femtosecond laser pulses

Dependence of the heating effect on tissue absorption coefficient during corneal reshaping using different UV lasers: A numerical study

Corneal reshaping using laser ablation effect is a well-known ophthalmologic operation implemented to correct many vision disorders. UV pulsed lasers are utilized in this surgery due to their high absorption in corneal tissue without affecting the other parts of the eye. Ideally ablation may not be associated with heat transport, since high temperature is the enemy of soft tissues. However, the thermal effect of this procedure can cause tissue damage if the temperature rises above the safety level without appropriate handling. In the present study, we try to find the trade-off between efficient ablation and minimal temperature rise. So the thermal effect on the corneal tissue after applying five UV wavelengths; 193, 210, 213, 223 and 248 nm has been simulated and investigated by solving Penne's bio-heat transfer equation using the finite element method. A 3D model of the human cornea is constructed using COMSOL Multiphysics computer software and the temperature rise is determined at the end of the laser pulse. The same spot size and pulse duration are used for each wavelength. While the absorption coefficient of the corneal tissue is the only variable as it is a wavelength dependent. The proposed results show that, the heating effect is directly proportional to the absorption coefficient. The highest recorded temperature was 259 °C at 193 nm, while, the minimum value was (70.1 °C) recorded at 248 nm at which the cornea has the lowest absorption coefficient.

Effects of femtosecond and excimer lasers on implanted KAMRA corneal inlay in animal models

PURPOSE

To evaluate the effect of femtosecond laser and excimer laser on an intracorneal inlay (KAMRA^®) implanted in animal models.

METHODS

Femtosecond laser was used to create corneal intrastromal pockets at 250μm depth in five porcine eyes. Four intact KAMRA inlays, examined by slit-lamp biomicroscopy and light microscopy, were implanted in the pocket of four eyes. A standard LASIK flap was created above each implanted inlay in the four eyes using a femtosecond laser with flap thicknesses of 150μm, 130μm, 110μm and 90μm. In the fifth porcine eye, a LASIK flap was created using femtosecond laser at 110μm depth, and a fifth inlay was then implanted in the 250μm pocket. Excimer laser ablation was performed under the flap targeting a -3.00 refraction. The inlay was then explanted, examined and reimplanted in the same pocket followed by a second similar excimer laser ablation.

RESULTS

Significant burn, shrinkage and distortion of microholes were noted in all the first four inlays following the femtosecond laser flap creation at all the various flap thicknesses. The damage was noted to be more prominent as the distance between the flap and inlay decreased. No apparent effect was noted on the fifth inlay following repeated excimer laser ablations.

CONCLUSION

Unlike excimer laser, femtosecond laser appears to be hazardous and damaging to the intracorneal KAMRA inlay when applied above it.

Neural Probes with Integrated Temperature Sensors for Monitoring Retina and Brain Implantation and Stimulation

Gold (Au) resistive temperature sensors were integrated on flexible polyimide-based neural probes to monitor temperature changes during neural probe implantation and stimulation. Temperature changes were measured as neural probes were implanted to infer the positions of the neural probes, and as the retina or the deep brain region was stimulated electrically. The temperature sensor consisted of a serpentine Au resistor and surrounded by four Au electrodes with 200 and [Formula: see text] diameter (dia.). The Au temperature sensors had temperature coefficient of 0.32%, and they were biocompatible and small in size. In vivo measurements of temperature changes during implantation and stimulation were carried out in the retina and deep brain region in rats. The desired implantation position was reached when temperature measured by the sensor increased to the calibrated level and became stable. There was no temperature increase when low level stimulation current of 8 and [Formula: see text] each for the two 200- and 400- [Formula: see text]-dia. electrodes, respectively, were applied. When higher level stimulation current of 100 and [Formula: see text] each were applied to the two 200- and 400- [Formula: see text]-dia. electrodes, respectively, maximum temperature increases of 1.2 °C in retina and 1 °C in deep brain region were found.

Investigating the ocular temperature rise during femtosecond laser lens fragmentation: an in vitro study

PURPOSE

To investigate the trend of temperature variation during lens fragmentation simulated by a femtosecond laser on an in vitro eye model.

METHODS

In our experimental study, a convex cylinder of gelatinous material, usually employed in femtosecond laser calibration, was used to simulate both an anterior segment and a crystalline lens during fragmentation performed with the Victus femtosecond laser (Technolas Perfect Vision GmbH, Germany; Bausch + Lomb Incorporated, USA). Two radiated energies (7000 nJ and 9000 nJ) and three cutting patterns (crosses, circles and cross + circle) were applied. Trends of temperature variation as a function of time were obtained using a T-type thermocouple.

RESULTS

The maximum value of temperature rise during lens fragmentation ranged from 3.53 to 5.13 °C; the rise was directly proportional to the intensity of the radiated energy (7000 nJ or 9000 nJ) and the cutting pattern performed. This behavior was experimentally represented by an asymmetric function with a characteristic bell curve shape, whereas it was mathematically described by a transport diffusive model.

CONCLUSIONS

Since the temperature rise at the fragmentation volume base resulted to be around 5 °C in our in vitro study, lens fragmentation performed using the Victus femtosecond laser might be considered safe form a thermal point of view.

Bilateral macular injury caused by a femtosecond laser

CASE REPORT

We describe the case of a 35-year-old man who arrived in the Emergency Department with bilateral macular injury caused by accidental exposure to an industrial femtosecond laser.

DISCUSSION

Workers operating industrial lasers must protect their eyes properly when handling these devices. Otherwise, retina damage may occur which usually is recoverable. However, sometimes this damage causes permanent visual loss.

Finite element model of the temperature increase in excised porcine cadaver iris during direct illumination by femtosecond laser pulses

In order to model the thermal effect of laser exposure of the iris during laser corneal surgery, we simulated the temperature increase in porcine cadaver iris. The simulation data for the 60 kHz FS60 Laser showed that the temperature increased up to 1.23°C and 2.45°C (at laser pulse energy 1 and 2 [micro sign]J, respectively) by the 24 second procedure time. Calculated temperature profiles show good agreement with data obtained from ex vivo experiments using porcine cadaver iris. Simulation results of different types of femtosecond lasers indicate that the Laser in situ keratomileusis procedure does not present a safety hazard to the iris.