Minimization of thermomechanical side effects and increase of ablation efficiency in IR ablation by use of multiply Q-switched laser pulses

Large thermal damage zones have been observed after application of free-running holmium laser pulses inside the human body as, for example, for arthroscopic surgery. The aim of our study is to reduce thermal damage by increasing the ablation efficiency, and to achieve a smooth surface of the ablated tissue. For that purpose we use a multiply Q-switched thulium laser (λ = 2.0 μm, acousto-optical QS) that emits pulse series consisting of a pre-pulse of 40 mJ energy and up to 6 ablation pulses of 100 mJ each, separated by time intervals of 60 μs. Q-switched laser pulses explosively ablate the target material. In a liquid environment, this leads to the formation of cavitation bubbles and to mechanical damage of the surrounding tissue. The pre-pulse of 40 mJ serves to minimize the cavitation effects, as it produces a small cavity that is then filled by the ablation products created by the burst of 100-mJ pulses. The pre-pulse creates, furthermore, a channel between fiber tip and target that reduces absorption losses in the liquid. Reduction of cavitation effects and channel formation are demonstrated by time-resolved photography. The use of a thulium laser instead of a holmium laser contributed to the desired reduction of thermal damage, because the penetration depth of the thulium laser light in cartilage (≈170 μ m) is only half as large as with the holmium laser. The main reduction of thermal damage was, however, achieved through the use of Q-switched pulses. The short laser pulse duration of 70-150 ns created stress confinement conditions leading to a 2-3 times more efficient ablation than with free running pulses and thus to less residual heat in the tissue. The mass ablated by one burst of 6 Q-switched pulses equaled the mass ablated by a 1.5-J free-running pulse. The thermal damage zone of multiply Q-switched thulium pulses was only 1/3 as large as with free-running holmium pulses. The damage was, furthermore, less severe and the surface of the ablation craters was smoother than with free-running pulses.