Laser ablation of skull tissue using transverse excited 9.6-μm CO2 lasers with pulse durations of 1-100 μs

Craniotomy using a drill and saw frequently results in fragmentation of the skull plate. Lasers have the potential to remove the skull plate intact. TE CO₂ lasers operating at the peak absorption wavelength of bone (λ=9.6 μm) and with pulse durations of 5-10 μs, approximately the thermal relaxation time in hard tissue, produced high ablation rates and minimal peripheral thermal damage. Both thick (2 mm) and thin (250 μm) bovine skull samples were perforated and the ablation rates calculated. Results were compared with Q-switched and free-running Er:YAG lasers (λ=2.94 μm, τ_p = 150 ns and 150 μs). The CO₂ laser perforated thick sections at ablation rates of 10-15 μm per pulse and fluences of approx. 6 J/cm². There was no discernible thermal damage and no need for water irrigation during ablation. Pulse durations ≥ 20 μs resulted in significant tissue charring which increased with the pulse duration. Although the Er:YAG laser produced ablation rates of approx. 100 μm per pulse, fluences > 30 J/cm² were required to perforate thick samples, and thermal damage measured 25-40 μm. In summary, the novel 5-10 μs pulse length of the TE CO₂ laser is long enough to avoid a marked reduction in the ablation rate due to plasma formation and short enough to avoid peripheral thermal damage through thermal diffusion during the laser pulse. Further studies with the TE CO₂ laser are warranted for potential clinical application in craniotomy procedures.