Investigation of corneal/scleral burning during cataract surgery

The use of ultrasound (US) probes in the emulsification andremoval of cataracts has the side effect of corneal and scleralburning. In order to quantify the effects of burning on thesetissues, experiments were conducted using bovine corneas todetermine the activation energy (E) and frequency factor (A)from an Arrhenius damage model. Bovine corneas were heatedon a constant temperature copper stage set at the desiredtemperature (ranging between 40-70°C). The damage criterionset was the loss of transparency of the tissue and the timerequired to reach the opacity was used to construct an Arrheniusinjury model. Results yielded an activation energy of 98.56kJ/mole and a frequency factor of 2 9 x 10¹³s^-1. Afterdetermining the parameters for the Arrhenius damage integral,a numerical model was developed to simulate the damage basedon variations of the temperature at the ultrasound probe tipwith time. Results indicate that as much as 39% of the cornea(in area) can be damaged irreversibly (Q>1) for temperaturesreaching 65°C for a time period of 3 minutes. An intermittentprocedure which does not allow temperatures to reach beyond60°C for more than a minute was found to be optimal fortreatment purposes. These results show that the sensitivity ofthe corneal tissue during US heating can be quantitativelymeasured and predicted. It is therefore important to optimizethe treatment procedure to minimize thermal injury during theprocedure.