[In Vitro Study of Subablative Er : YAG Laser and Diode Laser Therapy vs. Thermal Cautery in Microbial Keratitis]

In microbial keratitis, thermal cautery leads to thermal necrosis associated with germ reduction. Subablative heating of the cornea using an Er : YAG laser or diode laser is an alternative option. Thermal effects and reproducibility of both procedures were investigated.

Thermal effects were monitored with an infrared imaging system. Temperature of stromal tissue of enucleated porcine eyes was measured. Effects of thermal injury were rated by microscopic photography and histopathologic examination. Reduction of living bacteria was visualized by laser scanning microscopy and fluorescent stain on stromal tissue models containing E. coli bacteria.

Corneal samples showed thermal damage depths of 110 - 250 µm after thermal cautery at contact times of 1 - 3 s. Superficial temperature ranged from 70 to 80 °C. Thickness of thermal necrosis after laser irradiation was between 50 and 270 µm. Temperature ranged from 60 to 120 °C. Laser scanning microscopy of a stromal tissue model, comprising Escherichia coli bacteria in soft agar, showed a mortification of bacteria over 40% up to 100 µm depth after treatment for 25 s. Treatment with diode laser showed an even better effect: more than 60% of bacteria were dead up to 250 µm depth and more than 50% up to 450 µm depth in the stromal tissue model.

All procedures are able to induce high temperatures in tissue to achieve effective germ reduction in corneal stroma. Effect and reproducibility of the thermal cautery procedure depends on the surgeon's experience. Energy dose and thermal effect can be calculated for the Er : YAG and diode laser procedure to affect the infected stromal layer selectively.