Fundus disease has been listed as one of the top three blinding eye diseases by the World Health Organization, accounting for 54.7% of all blinding eye diseases, and is the leading cause of blindness in the elderly. Traditional surgical methods are difficult to implement because the fundus is located behind the eyeball. Laser hyperthermia is the first choice for many retinal diseases. However, the probability of photoreceptor cell damage due to improper selection of laser parameters can be over 50%. Constructing an accurate three-dimensional heat transfer model of the entire eye will allow clinicians to select laser parameters. Therefore, a macroscopic heat transfer model of the entire eye was established on the basis of the real structure, and the effects of the external environment of the eye, anterior tissue absorption, and choroidal hemoperfusion on the fundus temperature distribution during laser surgery were analyzed using numerical simulation.
A whole eye numerical model based on the Pennes' biological heat transfer equation was developed to calculate the thermal response process in the eyeball under steady state and laser heating in this paper. First, the fundamental properties of each region were strictly defined, including the external environment of the cornea, the initial temperature of the eyeball, and the setting of boundary conditions. The steady-state temperature distribution of the entire eye was solved by utilizing Pennes' biological heat transfer equation. The steady-state calculation results were then employed as the initial conditions to further solve the transient eye temperature distribution by changing the wavelength, pulse width, and energy density of the laser.
In this study, the whole eye heat transfer model was established on the basis of real structure, the heat transfer process of fundus laser surgery was calculated, and the effects of ambient temperature, the light absorption of the anterior tissue of the eyeball, and choroid hemoperfusion on the temperature distribution in the eyeball were studied. The simulation findings showed that: 1) the heat transfer between the cornea and the external environment can change the temperature of the eyeball. The temperature distribution of the anterior eyeball is more sensitive to environmental factors, whereas the fundus tissue is less affected; thus, fundus laser surgery will not be affected (Fig. 5). 2) If the absorption and scattering of laser energy by the four layers of anterior eyeball tissue are not considered, the maximum errors of fundus temperature rise were 24% and 56%, respectively, in the common clinical wavelength of 450-900 nm, which significantly affects the accuracy of numerical calculation (Fig. 6). 3) The effect of choroid perfusion term in ocular temperature distribution in laser surgery mainly depends on pulse width. Because of the short laser action time, the peak temperatures of the eyeball were 48.58 ℃ and 48.63 ℃ when the choroidal blood perfusion effect was considered or ignored, respectively, with little difference. However, when the effect of choroid perfusion was considered or ignored, the peak values of eyeball temperatures were 48.54 ℃ and 58.15 ℃ (Fig. 7 and Fig. 8), respectively, and the temperature rise was 11.54 ℃ and 21.15 ℃, respectively, with an error of 83%.
Through theoretical research, the heat transfer process of fundus surgery under laser irradiation was analyzed from the macroscopic viewpoint in this report. The results indicated that the heat transfer between the cornea and the external environment could change intraocular temperature. The temperature distribution in the anterior part of the eyeball was more sensitive to the environmental factors, whereas the fundus tissue was less affected; thus, the fundus laser surgery would not be affected. The absorption and scattering of laser energy by the anterior tissue of the eyeball should be considered in the simulation to ensure the accuracy of the simulation of laser fundus surgery. The effect of choroidal blood perfusion on ocular temperature distribution in laser surgery was primarily determined using the laser pulse width. When the pulse width time was short (tp=0.1 s), hemoperfusion had no obvious cooling effect on fundus tissues, and the influence of perfusion factors could be ignored. Flow perfusion had a significant cooling effect on fundus tissues and significantly affected fundus temperature when the pulse width was long (tp=60 s). Therefore, the influence of perfusion factors must be considered. The results of this study provide important theoretical guidance for the clinical laser treatment of fundus diseases.
