Pulsed infrared laser ablation rates and characteristics in otic capsule

Abstract

Holmium:YAG (2.1 micrometers ) and Erbium:YAG (2.9 micrometers ) lasers have been suggested for use in neurotologic operations. The relatively high absorption in biologic tissues and minimal thermal diffusion associated with pulsed lasers make them attractive alternatives to CO<SUB>2</SUB> and visible wavelength lasers, which currently enjoy widespread clinical use. There has been no systematic investigation of the properties of these lasers in a tissue model relevant to temporal bone operations, though there are numerous clinically oriented studies using these wavelengths. The microarchitecture of otic capsule (bone tissue of the inner ear) differs dramatically from bone found elsewhere in the body. Porcine otic capsule was machined into 0.8 mm thick specimens. The ablation rates of Ho:YAG and Erbium:YAG lasers were determined with incident flux varying from 6 - 20 J/cm<SUP>2</SUP>/pulse at 4 Hz PPR. Otic capsule specimens were ablated under both wet (physiologic) and dry (chemical dehydration) conditions. With Ho:YAG irradiation, dry otic capsule ablated with multiple episodes of stalling out. The relative number of specimens where stalling out occurred decreased with increasing laser flux. Wet tissue demonstrated similar characteristics, but with a lower ablation rate for a given fluence. In Erbium:YAG treated tissue, similar observations were noted, but the dependence on fluence is less clear in both wet and dry tissues. Water is shown to play a key role in the ablation process. The threshold of ablation is lower in wet tissue than in dehydrated tissue. The homogeneous microarchitecture of otic tissue also strongly influences the ablation process. Possible mechanisms are discussed.