Radio-frequency heating of the cornea: theoretical model and in vitro experiments

Abstract

We present a theoretical model for the study of cornea heating with radiofrequency currents. This technique is used to reshape the cornea to correct refractive disorders. Our numerical model has allowed the study of the temperature distributions in the cornea and to estimate the dimensions of the lesion. The model incorporates a fragment of cornea, aqueous humor and the active electrode placed on the cornea surface. The finite element method has been used to calculate the temperature distribution in the cornea by solving a coupled electric-thermal problem. We analyzed by means of computer simulations the effect of: a) temperature influence on the tissue electrical conductivity, b) the dispersion of the biological characteristics, c) the anisotropy of the cornea thermal conductivity, d) the presence of the tear film, and e) the insertion depth of the active electrode in the cornea, and the results suggest that these effects have a significant influence on the temperature distributions and thereby on the lesion dimensions. However, the cooling of the aqueous humor in the endothelium or the realistic value of the cornea curvature did not have a significant effect on the temperature distributions.An experimental model based on the lesions created in rabbit eyes has been used in order to compare the theoretical and experimental results. There is a tendency towards the agreement between experimental and theoretical results, although we have observed that the theoretical model overestimates the lesion dimension.

Applicator for RF Thermokeratoplasty: Feasibility Study Using Theoretical Modeling and Ex Vivo Experiments

“…Also interesting is the low dispersion found in Group 4 lesion width. Similar behavior had been observed in a previous study (Berjano et al 2002), in which two groups of corneas were heated by RF currents for 1 s using two levels of applied voltage (16 and 21 V). In that study, lesion depth was very different for the 2 groups (21±1% vs. 30±4% of corneal thickness), whereas width was similar (106±7% vs. 108±9%).…”

“…In contrast, this did not occur in Groups 5 and 6, in which the longer heating time caused thermal conduction to affect the thermal lesions. Additionally, these results confirm that intrinsic dispersion of tissue characteristics has a greater influence on lesion size in longer heating periods (Berjano et al 2002). Also interesting is the low dispersion found in Group 4 lesion width.…”

“…In that study, lesion depth was very different for the 2 groups (21±1% vs. 30±4% of corneal thickness), whereas width was similar (106±7% vs. 108±9%). By taking all these results into account, we can conclude that for short heating periods (<5 s), lesion depth is highly related to initial impedance and applied voltage (Berjano et al 2002), whereas lesion width could be less dependent on those factors and perhaps more closely related to electrode size.…”

“…We considered a flat geometry of the cornea. Although the inclusion of curvature of the cornea in the theoretical modeling could modify the computed temperature distributions (Berjano et al 2002), the suction pressure in the new applicator keeps the cornea totally flat, and hence the model geometry is suitable.…”

Review of the mathematical functions used to model the temperature dependence of electrical and thermal conductivities of biological tissue in radiofrequency ablation

“…A rise in k likewise involved a drop in the maximum temperature reached at the end of a preset time [34].…”