Characterization of handpieces to control tissue ablation with pulsed CO₂ laser

Focusing handpieces used for CO₂ beam delivery allow large variation of the power density in the spot depending on the distance to the tissue and hence the effect on the tissue. In contrast to the cw CO₂ laser, the pulsed CO₂ laser vaporizes tissue water instantly (:=ablation threshold), leaving a charless crater in the tissue surface. Only if the fluence is below or near threshold, the tissue effects are comparable with the cw laser. The threshold and tissue effects were studied for focusing (f119 mm) and collimating (0 3 mm) handpieces coupled to an ultrapulsed CO₂ laser. Using a special thermal imaging setup based on Schuieren techniques, the ablation threshold was determined depending on spotsize and pulse energy (1-200 mJ). In the focus of the handpiece, the threshold was already exceeded at 1 mJ, creating holes that were larger than the theoretical expected spotsize. The ablation threshold (J/cm2) increased for larger spotsizes. Below threshold, there is heating of the tissue resulting in coagulation. Above threshold, the exploding water vapor consumed thermal energy suppressing heating of the surrounding tissue. The gaussian shape of the collimated beam results in relatively more thermal effects. Focusing handpieces provide a wide range in power density and thus require experience from the surgeon. Collimated handpieces might be more easy to handle but offer less flexibility in tissue effect and a larger thermal zone.