Effect of water-layer modulation on Er:YAG laser ablation of hard bone tissue using an off-axis water-air jet

Traditional water-assisted Er:YAG laser ablation of hard bone tissue is limited by the difficulty of controlling the water-layer thickness. Because water strongly absorbs 2.94 μm Er:YAG laser radiation, an excessively thick or stagnant water layer attenuates the incident laser energy and can reduce ablation efficiency. In this study, an off-axis water-air-jet-assisted Er:YAG laser ablation method is proposed to regulate the water layer on the surface of hard bone tissue. The air jet impinges on the water layer in the ablation region, and the water-layer characteristics are modulated by water pressure, air pressure, and air-nozzle diameter. A computational fluid dynamics model was established to simulate air-jet modulation of the water layer and to analyze the resulting water-layer thickness, water volume fraction, and flow velocity. Comparative ablation experiments were then performed under air-jet, water-jet, and off-axis water-air-jet assistance. Relative to air-jet assistance, the off-axis water-air jet markedly reduced visible carbonization and the optically observed heat-affected region. Relative to water-jet assistance, the ablation depth and hole diameter increased by 5.8% and 13.5%, respectively. Single-factor experiments further showed that water pressure, air pressure, and air-nozzle diameter affected the ablation morphology and depth in a nonlinear manner through changes in water-layer thickness and water volume fraction. These findings demonstrate that off-axis water-air-jet modulation can balance cooling and laser-energy transmission during Er:YAG laser ablation of hard bone tissue. The assessment of thermal effects was limited to optically visible carbonization and heat-affected morphology; histological evaluation will be required to quantify microscopic thermal injury.