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

High-Intensity Laser Therapy (HILT) differs from Low-Level (LLLT) and High-Power Laser Therapy (HPLT) by adding a photoacoustic component to photochemical and photothermal effects. High-peak, short-pulse emission generates pressure waves > 10 kPa in water (27 °C) and ~100 kPa in vivo—sufficient to trigger mechanotransduction and differentiation. These waves propagate like ultrasound, enabling regenerative effects centimeters deep, beyond optical limits. We introduce Pulse Energy Dose (PED) as a practical metric for determining whether a laser exceeds the photomechanical threshold while remaining in the thermoelastic regime. Only systems with kilowatt-range peak power, microsecond pulses, high pulse energy (hundreds of mJ/cm² to several J/cm²), and very low duty cycles (<1%) consistently produce therapeutic pressure waves. PED is validated against the Margheri equation, showing strong correlation across lasers. We define lower and upper operating bounds that separate true HILT from HPLT devices that rely on heat and cannot elicit meaningful photoacoustic stimulation. This classification corrects misleading nomenclature based on average power and guides parameter selection to preserve thermoelastic safety while maximizing mechanotransductive efficacy. The findings have practical implications for musculoskeletal and dental indications, including cartilage regeneration, bone healing, temporomandibular joint therapy, and deep-tissue repair, and provide a basis for protocol optimization and device evaluation.