Therapeutic ultrasound is one of the most widely used and least well supported treatments in musculoskeletal practice, and the single most useful thing a sport and exercise medicine (SEM) clinician can know about it is that the phrase covers two entirely different things. Conventional therapeutic ultrasound, delivered by a physiotherapy machine through a handheld transducer and coupling gel, is applied to soft tissue for pain and healing, and its evidence base is weak. Low-intensity pulsed ultrasound (LIPUS) is a distinct application delivered by a dedicated device at specific fixed parameters to stimulate bone healing, and it has a defined UK position for fracture non-union. Conflating the two is a common error. This page covers how ultrasound energy affects tissue, what each application is used for, and what the evidence supports.
How does therapeutic ultrasound affect tissue?
Ultrasound is a mechanical pressure wave transmitted into tissue through coupling gel, and its effects are described as thermal and mechanical. Continuous output generally produces greater heating, which is used with the intention of increasing local blood flow, increasing the extensibility of collagen before stretching, and reducing pain and muscle spasm. Absorption is greatest in tissues with high collagen content such as tendon, ligament and joint capsule, and frequency influences depth: around three megahertz is absorbed more superficially, while around one megahertz penetrates more deeply, although depth is not exact or uniform across tissues. Pulsed output reduces average heating because the beam is interrupted, and any remaining effect is attributed to proposed mechanical mechanisms such as acoustic streaming and stable cavitation, which are suggested to influence cell membrane permeability and cellular activity. Thermal and mechanical effects can coexist, and the balance depends on intensity and duty cycle rather than on pulsing alone.
Continuous output generally produces greater heating, while pulsed output reduces average thermal exposure and is associated with proposed mechanical effects. The two can coexist depending on intensity and duty cycle.
Dose is described by frequency, intensity in watts per square centimetre, duty cycle for pulsed modes, and treatment time, and the transducer is moved continuously to avoid standing waves and local overheating. Two practical points follow. First, much of what determines the delivered dose in clinical practice is operator-dependent and poorly standardised, which is one reason trial results are so inconsistent. Second, the parameters used for conventional therapy differ completely from those used for low-intensity pulsed ultrasound, which delivers a fixed low-intensity signal for a set period daily from a dedicated device. Conventional physiotherapy ultrasound should not be assumed to reproduce the effects reported for that device.
What does the evidence support?
For soft tissue musculoskeletal conditions the honest position is that the evidence does not support routine use. Trials of conventional therapeutic ultrasound in tendinopathy, shoulder pain, low back pain and soft tissue injury are numerous but small, heterogeneous in dose and frequently poorly blinded, and systematic reviews have repeatedly concluded that any benefit over sham is small, inconsistent or absent. UK guidance is explicit across several conditions: therapeutic ultrasound should not be offered for chronic primary pain, for osteoarthritis, or for low back pain and sciatica. The supporting evidence review of electrical physical modalities found considerable uncertainty in the data and little long-term evidence. Where ultrasound continues to be used it is generally as a low-cost adjunct within a package of care, and the clinician should be clear that the active ingredient in that package is the exercise programme rather than the machine.
Conventional therapeutic ultrasound is operator-applied to soft tissue with variable parameters, while low-intensity pulsed ultrasound is a dedicated device applied daily at fixed parameters to a fracture site. The guidance positions for one do not transfer to the other.
Low-intensity pulsed ultrasound is a different case, and the UK picture involves two distinct publications that are frequently conflated. Device-specific technology guidance supports adoption of one commercially available ultrasound bone healing system for long bone fractures with non-union, defined as failure to heal after nine months, where the fracture is stable and well aligned; that appraisal reported high observed healing rates and cost savings through avoiding surgery, while identifying uncertain benefits and costs in delayed healing. Separately, general procedure guidance on low-intensity pulsed ultrasound to promote healing of delayed-union and non-union fractures concludes that safety raises no major concerns but that the efficacy evidence is inadequate in quality, so the procedure should be used only with special arrangements for clinical governance, consent and audit or research, informing governance leads, explaining the uncertainty in writing and auditing outcomes. That guidance also states the procedure should be used alongside other treatments for these fractures and managed by specialists in treating them. Both positions are current and should be quoted together rather than one alone. For the exam, the distinction to hold is that a device-specific appraisal supports a named system for selected stable non-union, that general procedure guidance still requires special arrangements because efficacy evidence is judged inadequate, and that conventional therapeutic ultrasound for soft tissue lacks convincing evidence, with none of these positions transferring to the others.
Exam Tips
•Continuous output generally produces greater heating, while pulsed output reduces average heating; thermal and mechanical effects can coexist depending on intensity and duty cycle, and acoustic streaming and stable cavitation are proposed rather than established mechanisms.
•Absorption is greatest in collagen-rich tissue; a higher frequency of around three megahertz treats superficially and a lower frequency of around one megahertz penetrates more deeply.
•Dose depends on frequency, intensity, duty cycle and time, and the transducer is moved continuously to avoid standing waves and local overheating.
•UK guidance advises against offering therapeutic ultrasound for chronic primary pain, for osteoarthritis, and for low back pain and sciatica.
•Low-intensity pulsed ultrasound (LIPUS) is a distinct application using a dedicated device at fixed low-intensity parameters, applied for around twenty minutes daily.
•Two UK positions on low-intensity pulsed ultrasound coexist: device-specific technology guidance supports one ultrasound bone healing system for stable, well-aligned long bone non-union after nine months, while general procedure guidance judges efficacy evidence inadequate and requires special arrangements for governance, consent and audit or research.