Correlation of thermal and mechanical effects of the holmium laser for various clinical applications

The Holmium laser has become established in orthopedic surgery and urology due to its unique combination of mechanical and thermal properties induced by explosive vapor bubbles. In a specialized setup, real-time high-speed and thermal images of dynamic vapor bubbles and thermal relaxation at a water tissue interface were obtained simultaneously. The thermal effects in the tissue model were correlated to the characteristics of the bubbles dependent on pulse energy (0.2-4 J), pulse repetition frequency (5-40 Hz), distance and angle of fiber delivery system (diameter 365 μm) to the tissue surface. Up to a fiber-to-tissue distance of 50% of the radius of the bubble, only a superficial tissue layer was heated. During bubble implosion, the tissue surface was attracted to the fiber, ripping of irregularities, and was effectively cooled by turbulence. In case of hard tissues, the bubble detached from the fiber imploding towards the hard surface. At closer distances (<50% of bubble radius), the tissue itself was vaporized resulting in mechanical damage and thermal relaxation into the tissue, especially above repetition rates of 5 Hz. There is a strong correlation between the path length of the free beam within the bubble and the degree of mechanical and thermal damage in the tissue directly irradiated by this beam. During clinical applications the surgeon should be aware of the size of the vapor bubble in relation to the distance and angle with the tissue for safe and optimal use of the mechanical and thermal properties of the Holmium laser.