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Using the finite element method to determine the temperature increase during phacoemulsification
Session Title: Phaco Techniques
Session Date/Time: Saturday 05/10/2013 | 08:30-10:30
Paper Time: 09:22
Venue: Main Lecture Hall (Ground Floor)
First Author: : S.Buschschlüter GERMANY
Co Author(s): : C. Koch J. von Eicken H. Höh
Temperature increase represents a risk during phacoemulsification. To assess possible adverse effects by thermal exposure on the corneal endothelium the temperature rise was calculated using a theoretical model based on finite element method (FEM) taking into account different sources of heat production. On the basis of the model various conclusions of mitigation of thermal damage during treatment can be drawn.
Physikalisch-Technische Bundesanstalt, Braunschweig, Germany
A mathematical model was developed providing the amount of thermal energy transferred into the eye during phacoemulsification. As heat sources the emitted acoustic power, the self-heating of the tip and the viscous friction between phacoemulsification tip and the fluid inside the anterior chamber were identified and described. The model permits the calculation of heat production during surgery in dependence on important parameters such as longitudinal stroke and operating frequency. The theoretical results were compared with calorimetric measurements covering different system settings (Geuder Megatron, Germany). In an additional step, the geometry of an eye was constructed and introduced into a FEM simulation package. The temperature increase within the anterior chamber was computed and the results were compared with temperature measurements in pig eyes using thermocouples with an outer diameter of 0.7 mm.
The total power input into the eye raises if the stroke or the operating frequency increases. With an output power setting of 100 % the amount of produced acoustic power can be estimated to be around 14 mW and the self-heating of the phacoemulsification tip yields a heat production of 16 mW. The calculation of the dissipated power due to viscous friction obtains 4 W. Depending on flow volume the FEM model calculates in this case a temperature increase between 5 °C (35 ml/min) and 7 °C (25 ml/min). The maximum temperature rise occurs in a cylindrical layer around the phacoemulsification tip with a thickness of about 1 mm. Outside of this region heating of the aqueous humor is very uniform, the differences in temperature are below 1 °C. With an output power setting of 50 %, a duty-cycle of 50 % and a flow volume of 30 ml/min the computed maximum temperature rise remains below 1.5 °C. These results are in good agreement with the experimental observations in pig eyes.
The mathematical model showed that energy dissipation due to friction is the main heat source during surgery. In contrast the input by absorption of sound energy or self-heating is negligible. The knowledge of the produced heat enables the computation of the local temperature elevation during surgery depending on different system settings as flow volume. Thus the FEM model can provide significant contributions to the evaluation of the thermal exposure on the cornea during phacoemulsification. On thermal grounds usage of the pulse-mode is recommended. Measurements and calculations showed no harmful temperature rise in this case. When performing surgery with higher output power setting high flow volume can reduce thermal exposure. A risk of thermal hazard of the endothelium remains if aspiration is blocked for a longer period of time or if the distance of the phacoemulsification tip to the vulnerable eye structure is too small.