Therapeutic ultrasound mechanisms and parameters: 1) Thermal effects - acoustic energy converted to heat increasing tissue temperature; 1 MHz frequency penetrates 3-5 cm (deeper tissues, hip, low back), 3 MHz penetrates 1-2 cm (superficial tissues, hand, foot); intensity 1-2 W/cm² for thermal effects, 0.5-1 W/cm² for milder; continuous mode for thermal, pulsed (20%, 50%) for non-thermal; 2) Non-thermal effects - cavitation (microbubble formation/oscillation), acoustic streaming (fluid movement), microstreaming around oscillating cells; promotes membrane permeability, fibroblast activity, calcium influx, protein synthesis; 3) Phonophoresis - drives topical medication (corticosteroid, NSAID gel) into tissue; modest evidence. Parameters - frequency, intensity, duty cycle, ERA (effective radiating area), treatment area (ERA/4 = max area), duration (5-10 min). Treatment area should be ≤4× ERA.
Therapeutic ultrasound clinical applications and evidence: 1) Tendinopathy - moderate evidence for chronic tendinopathy (lateral epicondylitis, supraspinatus, Achilles, patellar) when combined with eccentric exercise; pulsed parameters (20%, 0.5 W/cm²); 2) Soft tissue injury - acute phase pulsed (anti-inflammatory), subacute thermal (tissue extensibility), used for hematoma resolution, ligament sprain; 3) Bone fracture healing - LIPUS (low-intensity pulsed ultrasound) shows evidence for delayed/non-union; 4) Carpal tunnel syndrome - pulsed for nerve healing; 5) Calcific tendinitis - high-intensity ECSWT (extracorporeal shockwave therapy) - related modality; 6) Scar mobility and adhesions - thermal effect; 7) Joint stiffness - increases collagen extensibility before stretching. Contraindications - over malignancy, pregnancy abdomen, growth plates in children, eyes, gonads, infected tissue, anesthetic/insensate skin, vascular disease, hemorrhage, recent radiation, cardiac pacemaker over chest. Recent meta-analyses show effects often modest; integrate with active exercise.
Low-level laser therapy / photobiomodulation: 1) Mechanisms - photons absorbed by mitochondrial cytochrome c oxidase increase ATP production, modulate ROS, reduce inflammation, increase nitric oxide release, modulate gene expression, neuromodulation; 2) Parameters - wavelength 600-700 nm (superficial 5 mm penetration) and 800-1100 nm (deeper 3-5 cm), power output 5-500 mW, energy density (fluence) 1-20 J/cm², total dose 1-5 J/point, treatment time and frequency depending on condition. Pulsed and continuous modes. Class 3B (LLLT) and Class 4 (high-power, photobiomodulation therapy) lasers; 3) Indications - chronic neck and low back pain (WALT recommendations), lateral epicondylitis, knee osteoarthritis, plantar fasciitis, frozen shoulder, oral mucositis (chemotherapy/radiation), wound healing, scar treatment, post-stroke recovery (transcranial PBM emerging), peripheral neuropathy; 4) Contraindications - over malignancy, eyes (laser glasses mandatory), thyroid (relative), pregnancy abdomen, photosensitivity, recent corticosteroid injection. Eye safety is paramount - all personnel and patient wear wavelength-appropriate protective glasses. Treatment frequency 2-3 sessions/week for 4-12 weeks.