Therapeutic Ultrasound Overview

Study Notes

Therapeutic ultrasound (US) is widely utilized for treating various conditions and is often misapplied.
Key applications include reducing soft tissue inflammation and pain, enhancing tissue extensibility, remodeling scar tissue, and aiding recovery from acute soft tissue injuries.
Common conditions treated with US include back, shoulder, knee, and neck pain, as well as walking difficulties and gait abnormalities.
A study found that 82.4% of physical therapists incorporate US into their treatments, with 36.4% using it daily and averaging treatment on 40% of their patients.
Despite the prevalence of US in therapy, its effectiveness is poorly documented; only 13% of therapists rely on research evidence for clinical decisions, while 40% depend on personal experience.

US involves high-frequency mechanical waves produced through acoustic energy that exert pressure on vibrating molecules, initiating a chain reaction of vibration.
Transmission of sound energy occurs best in substances with higher molecular densities, facilitating more efficient US transfer.
Dense tissues exhibit greater acoustic impedance, causing quicker dissipation of ultrasonic energy, which necessitates a coupling agent due to poor sound wave transmission through air.
Sound waves travel in a medium via longitudinal motions, characterized by compressions (increased molecular density) and rarefactions (decreased molecular density), with the wave's frequency dictating the duration of these phases.
Dispersion occurs as waves radiate from the emitter in multiple directions, with interactions leading to reflection, refraction, or absorption of waves.

Absorption of ultrasonic energy converts kinetic energy into thermal energy, enhancing tissue temperature.
Wave interactions can yield standing waves, amplifying intensity when synchronized, or reducing intensity when asynchronous.
Most body tissues, behaving like liquids of varying densities, transmit longitudinal waves, while bone presents a solid structure capable of both longitudinal and transverse wave transmission.
In denser tissues (e.g., ligaments and tendons), US energy is more rapidly absorbed, leading to higher temperatures compared to less dense tissues like muscle and adipose.
Applying US near high-density tissues can cause reflected waves to generate standing waves, intensifying energy and heat, particularly at junctions between high and low-density regions, such as near bone.

Ultrasound is generated through the piezoelectric effect, where a piezoelectric crystal is compressed and expanded by alternating electric current.
Absence of current maintains the crystal's shape, while alternating current reverses polarity, causing cyclical compression (concavity) and expansion (convexity).
This rapid shape change of the crystal produces acoustic waves that are transmitted through the attached sound head.