A Biophysical Framework for High-Intensity Laser Therapy Based on Photoacoustic Pressure Thresholds

High-Intensity Laser Therapy (HILT) represents a mechanistic subset of High-Power Laser Therapy (HPLT), distinguished by the addition of a photoacoustic component to established photochemical and photothermal effects. High-peak (kW), short-pulse emission generates pressure waves exceeding 10 kPa in water (27 °C) and approximately 100 kPa in vivo, levels that are compatible with the activation of mechanotransductive processes relevant to cellular differentiation. These pressure waves propagate several centimeters into biological tissues, extending beyond the optical penetration depth of light. We introduce Pulse Energy Dose (PED), a physically grounded and clinically oriented dose metric, to determine whether a laser system meets the photoacoustic threshold while remaining within the thermoelastic regime. Only systems combining kilowatt-range peak power, microsecond pulses, high pulse energy, and very low duty cycles (<1%) consistently induce pressure waves within the therapeutic thermoelastic regime. PED was validated against the Margheri equation, showing a strong linear correlation with calculated pressure wave amplitude (Pearson r > 0.9, p < 0.0001). Based on these results, we define operational bounds that identify high-power laser systems capable of producing reproducible photoacoustic effects within thermoelastic conditions. This framework shifts classification from average power to mechanism of action, providing guidance for safe parameter selection and supporting a mechanism-based clinical use of high-power lasers, particularly in musculoskeletal disorders, cartilage regeneration, bone healing, and deep-tissue repair.