Therapeutic ultrasound is a physical modality that uses high-frequency acoustic energy to produce thermal and non-thermal effects in human tissue for therapeutic purposes. As a deep-heating agent, it has been a staple in physical therapy clinics for much of the profession's history, used with the intention of managing pain, reducing inflammation, and facilitating tissue repair 1. The modality's application is predicated on the transmission of mechanical sound waves, typically at frequencies between 1.0 and 3.0 megahertz (MHz), from a transducer head into the body's soft tissues.
The biophysical effects of ultrasound are broadly categorized as thermal or non-thermal, determined primarily by the selected duty cycle. Continuous ultrasound, which uses a 100% duty cycle, delivers uninterrupted energy to maximize thermal effects 2. The therapeutic goal is to elevate tissue temperature to a range of 40-45°C for at least five minutes, a threshold believed necessary to increase collagen extensibility, alter nerve conduction velocity, and increase local blood flow 3. This heating is not uniform; tissues with high acoustic impedance (Z), such as collagen-rich tendons, ligaments, and bone, absorb more energy and heat more rapidly than tissues with low impedance like muscle and fat 4. Conversely, pulsed ultrasound, using duty cycles of 20% or 50%, minimizes thermal accumulation and is thought to produce therapeutic benefits through non-thermal mechanical mechanisms, including stable cavitation (the formation and oscillation of microscopic gas bubbles) and acoustic streaming (the movement of fluids along cell membrane boundaries) 2.
Despite its widespread and longstanding use, the clinical efficacy of therapeutic ultrasound is one of the most contentious debates in physical therapy. Proponents point to a body of evidence suggesting benefits for specific conditions. A notable example is the 1994 randomized controlled trial by Heckman et al., which demonstrated that low-intensity pulsed ultrasound (LIPUS) significantly accelerated the healing of tibial fractures, reducing the time to radiographic and clinical union from 154 days in the placebo group to 96 days in the treatment group, a reduction of nearly 38% 5. Such findings support the continued use of ultrasound, particularly for promoting bone and soft tissue healing.
However, a substantial volume of research has failed to demonstrate the superiority of ultrasound over placebo or other interventions. An influential 2001 systematic review by Robertson and Baker, published in Physical Therapy, concluded that the evidence for using ultrasound to treat musculoskeletal injuries and pain was weak and lacked scientific support 6. Similarly, a 2002 Cochrane Review on shoulder pain found no evidence from placebo-controlled trials to support the use of ultrasound for rotator cuff disease, concluding it offered no additional benefit when combined with exercise 7. This persistent lack of high-quality supporting evidence has led to a re-evaluation of its role in clinical practice. Modern clinical practice guidelines often reflect this ambivalence. The 2019 clinical practice guideline for carpal tunnel syndrome from the American Physical Therapy Association's Academies of Orthopaedic and Hand and Upper Extremity Physical Therapy gives thermal ultrasound a "D" grade, reflecting conflicting evidence, and recommends that clinicians should not use it for mild to moderate cases. 8. This encapsulates its current status: a modality with plausible mechanisms but an inconsistent and often disappointing clinical trial record.
Historical development
The origins of therapeutic ultrasound lie not in medicine, but in early 20th-century physics. Following the 1880 discovery of the piezoelectric effect by Jacques and Pierre Curie, the first major application of high-frequency sound waves was developed by French physicist Paul Langevin during World War I for the detection of submarines (SONAR). The first explorations of ultrasound's biological effects began in the 1920s and 1930s, a period when physical modalities were an area of growing interest in medicine, as evidenced by journals such as Physical Therapeutics, first published in 1927 9. In a foundational 1927 study, American physicist Alfred Lee Loomis and R.W. Wood documented the destructive capabilities of high-intensity ultrasound, observing its lethal effects on fish and frogs and its ability to rupture cells. These early findings, while crude, established that acoustic energy could produce potent biological changes.
The modality's transition into clinical practice accelerated after World War II. The physical therapy profession, which had grown significantly in response to the polio epidemics and the needs of wounded veterans, was receptive to new technologies that promised enhanced therapeutic outcomes 1. Commercial ultrasound generators became widely available in the early 1950s and were rapidly adopted by physical therapists 10. Ultrasound was promoted as a form of "deep heat," a logical and appealing extension of existing thermal modalities like shortwave diathermy. Its key advantage was the ability to selectively heat deep, collagen-rich tissues that were not easily reached by superficial agents. The rationale, now understood through modern physics, was that tissues with high acoustic impedance, such as tendon, ligament, and joint capsules, absorb and reflect more acoustic energy, resulting in a more significant temperature increase 4.
For decades, this thermal model dominated clinical thinking and application, with continuous-wave ultrasound used to achieve deep heating and increase tissue extensibility. However, by the late 20th century, a more nuanced understanding emerged that differentiated thermal effects from non-thermal, or mechanical, effects. Researchers began to investigate the therapeutic potential of acoustic streaming and stable cavitation, which could be produced without a significant rise in tissue temperature 2. This led to the refinement of equipment to allow for pulsed duty cycles, which deliver acoustic energy in short bursts to minimize thermal accumulation. The clinical significance of this non-thermal paradigm was powerfully validated by research into bone healing. In a 1994 randomized controlled trial, Heckman et al. found that low-intensity pulsed ultrasound (LIPUS) significantly accelerated the healing of tibial fractures. The treatment group achieved radiographic and clinical healing in an average of 96 days, a 37.7% reduction compared to the 154 days for the placebo control group 5. This landmark study provided high-level evidence for a primarily non-thermal biological effect, establishing a distinct, evidence-supported niche for the modality.
Scientific basis and mechanism of action
The therapeutic application of ultrasound is governed by fundamental physical principles that determine how acoustic energy is delivered to and absorbed by biological tissues. A primary parameter controlled by the clinician is the duty cycle, which dictates the temporal pattern of energy delivery. Using a 100% duty cycle, known as continuous ultrasound, provides an uninterrupted stream of acoustic energy, maximizing the generation of heat within the tissues 2. This thermal effect is the goal when treating conditions requiring increased tissue extensibility or blood flow. Foundational research established that a tissue temperature elevation to the range of 40-45°C must be sustained for at least five minutes to achieve these therapeutic outcomes 3. In contrast, pulsed ultrasound utilizes lower duty cycles, such as 20% or 50%, where the energy is delivered in discrete bursts. This approach minimizes the accumulation of heat and instead emphasizes the non-thermal, or mechanical, effects of the sound waves, which include stable cavitation (the formation and oscillation of microscopic gas bubbles) and acoustic streaming (the unidirectional movement of fluid along cell membranes) 2.
The degree to which different tissues absorb ultrasound energy and convert it to heat is determined by their acoustic impedance (Z). Acoustic impedance is an intrinsic property of a medium, defined as the product of the tissue's density (ρ) and the speed at which sound travels through it (c) 4. Tissues with high acoustic impedance, which are typically dense and rich in collagen, absorb and reflect a greater proportion of the ultrasound energy. Consequently, structures like bone, tendons, ligaments, and joint capsules are heated more effectively than tissues with lower acoustic impedance, such as muscle and adipose tissue, which are more transparent to the sound waves 4. This principle of differential heating allows for the selective targeting of specific connective tissues, a key concept taught in physical rehabilitation programs.9 This mechanism underpins the rationale for using thermal ultrasound in conditions like carpal tunnel syndrome, where the goal may be to heat the transverse carpal ligament, as noted in clinical practice guidelines 8.
Proper application technique is essential to ensure safe and effective energy delivery while avoiding adverse events. A significant risk arises from the creation of standing waves, which occur if the ultrasound transducer is held stationary over a specific point. This causes the reflected waves to superimpose on the incident waves, creating fixed points of intense energy concentration that can lead to hot spots and damage to temperature-sensitive tissues like the endothelium of blood vessels 11. Standard clinical protocol explicitly requires the practitioner to keep the transducer head in continuous, slow motion over the treatment area to distribute the energy evenly and prevent the formation of these potentially harmful standing waves 11. Understanding these mechanisms also informs contraindications; for instance, ultrasound is not used over a known malignancy because its mechanical and thermal effects can increase cell activity and blood flow, which may promote tumor growth and metastasis 12.
Physiological effects: thermal and non-thermal
The physiological impact of therapeutic ultrasound is primarily attributed to two distinct mechanisms: thermal and non-thermal effects. The dominance of one mechanism over the other is determined by the selected application parameters, most notably the duty cycle. A continuous duty cycle (100%) maximizes the delivery of acoustic energy, leading to significant tissue heating, while a pulsed duty cycle (e.g., 20% or 50%) minimizes thermal accumulation and emphasizes mechanical, non-thermal effects 2.
Thermal effects are achieved through the conversion of acoustic energy into heat as the ultrasound wave is absorbed by tissues. Foundational research established that to achieve therapeutic outcomes like increased collagen extensibility, reduced joint stiffness, and increased blood flow, the target tissue temperature must be elevated to a range of 40-45°C and maintained for at least 5 minutes 3. The degree of heating is not uniform across all tissues; it depends on the tissue's acoustic impedance (Z), which is the product of its density and the speed of sound within it 4. Tissues with high acoustic impedance, such as those rich in collagen like tendons, ligaments, joint capsules, and the periosteal surfaces of bone, absorb more energy and thus heat more readily than tissues with lower impedance like muscle or adipose tissue 4. This selective heating makes continuous ultrasound a targeted modality for preparing stiff, collagenous structures for stretching or manual therapy, as described in texts such as Physical Rehabilitation: Evidence-based Examination, Evaluation, and Intervention.
In contrast, non-thermal effects result from the mechanical forces generated by the sound waves. These include stable cavitation, which is the cyclical expansion and compression of microscopic gas bubbles within tissue fluids, and acoustic streaming, the unidirectional movement of fluid along cell membranes and around the oscillating bubbles. These mechanical perturbations are thought to alter cell membrane permeability and stimulate cellular activity, contributing to tissue repair. To isolate these effects, clinicians use pulsed ultrasound, where the intermittent delivery of energy prevents significant temperature rise 2. The most prominent clinical application of this principle is low-intensity pulsed ultrasound (LIPUS) for accelerating fracture healing. A landmark 1994 randomized controlled trial by Heckman et al. demonstrated that LIPUS significantly reduced the time to radiographic and clinical healing for tibial fractures to an average of 96 days, compared to 154 days in the placebo group, representing a 37.7% acceleration in healing 5.
Despite these well-defined biophysical mechanisms, the clinical effectiveness of ultrasound remains a subject of considerable debate. A 2001 systematic review by Robertson and Baker, published in Physical Therapy, concluded that the scientific support for using ultrasound to treat pain and various soft tissue lesions was questionable 6. Similarly, a 2002 Cochrane Review by Green et al. found no evidence from placebo-controlled trials to support ultrasound for shoulder pain and noted it provided no additional benefit when combined with exercise 7. Nevertheless, the modality retains a place in some clinical guidelines. For instance, the 2019 Clinical Practice Guideline for Carpal Tunnel Syndrome from the Academy of Orthopaedic Physical Therapy gives thermal ultrasound a "C" grade, stating it "may be used" for short-term symptom relief 8. This mixed evidence base requires clinicians to apply ultrasound judiciously, weighing the proposed physiological effects against the quality of evidence for specific conditions.
Evidence and research on effectiveness
The clinical effectiveness of therapeutic ultrasound is a topic of considerable debate, reflecting a history where its adoption often preceded rigorous scientific validation 10. While the modality remains common in physical therapy clinics, its evidence base is highly variable depending on the condition being treated and the specific parameters used. Several high-quality systematic reviews have challenged its widespread use for general musculoskeletal pain. A landmark 2001 systematic review by Robertson and Baker, published in Physical Therapy, analyzed the existing literature and concluded that the effectiveness of ultrasound for treating people with pain and soft tissue injuries was questionable and lacked strong scientific support 6. This finding was echoed in a 2002 Cochrane Review focused on shoulder pain. The review, by Green et al., found no evidence from placebo-controlled trials to support the use of ultrasound for patients with rotator cuff disease or other shoulder pathologies and determined that it offered no additional benefit when combined with an exercise program 7.
The evidence for other common applications is often weak or equivocal. For instance, the 2019 Clinical Practice Guideline for Carpal Tunnel Syndrome, issued by the Academy of Orthopaedic Physical Therapy, assigns thermal ultrasound a "C" grade recommendation, indicating that it "may be used" for achieving short-term pain and symptom relief but is not strongly supported 8. The evidence for phonophoresis, the use of ultrasound to enhance the transdermal delivery of medication, is also limited. A 1993 study by Byl et al. investigated the use of ultrasound with hydrocortisone and dexamethasone for influencing tissue healing. The researchers found no evidence that a single application of sonated hydrocortisone produced a measurable effect on cellular activity after seven days of healing, casting doubt on the efficacy of this specific application and technique 13.
In stark contrast to the largely negative or weak findings for soft tissue pain, the use of low-intensity pulsed ultrasound (LIPUS) for fracture healing stands as the most well-supported application of this technology. A significant 1994 randomized controlled trial by Heckman et al. demonstrated a notable acceleration of healing in fresh tibial fractures. In the study, patients treated with LIPUS achieved overall radiographic and clinical healing in an average of 96 days, compared to 154 days for the placebo control group, representing a 37.7% reduction in healing time 5. The mechanism for this effect is attributed to the non-thermal, mechanical stimulation provided by a pulsed duty cycle, which minimizes thermal accumulation and instead promotes biological processes like osteogenesis through acoustic streaming and stable cavitation 2. This specific application highlights how outcomes are critically dependent on matching the correct ultrasound parameters to the targeted physiological process, a concept central to modern evidence-based practice. The success of LIPUS illustrates a critical distinction: while continuous, thermal ultrasound aimed at increasing tissue temperature to 40-45°C for extensibility 3 has a questionable evidence base for many pain conditions, the non-thermal effects of pulsed ultrasound have proven effective for a very specific pathology.
Variants and related approaches
Beyond general thermal applications, several distinct approaches utilize ultrasound's mechanical and thermal properties for specific therapeutic goals. One of the most evidence-supported variants is low-intensity pulsed ultrasound (LIPUS), which is applied in a non-thermal, pulsed mode to stimulate bone growth 2. Its efficacy was demonstrated in a landmark 1994 randomized controlled trial by Heckman et al. on tibial fractures. The study found that daily LIPUS treatment significantly accelerated healing, reducing the time to radiographic and clinical union to 96 days, a 37.7% reduction compared to the 154 days required for the placebo control group 5. This application relies on the mechanical stimulation of osteogenesis rather than on generating the level of heat required for increasing tissue extensibility 3.
Another variant is phonophoresis, which involves the use of ultrasound to theoretically enhance the transdermal delivery of topical medications. The proposed mechanism suggests that acoustic energy increases the permeability of the stratum corneum, allowing drug molecules to penetrate deeper into underlying tissues. However, the clinical evidence for this approach is inconsistent. A 1993 study by Byl et al. published in the Journal of Orthopaedic & Sports Physical Therapy investigated the effect of phonophoresis with corticosteroids on tissue healing. While the study noted some effect on collagen deposition with sonated dexamethasone, it found no measurable impact on cellular activity after a single application of sonated hydrocortisone at 7 days post-injury 13. The effectiveness of phonophoresis remains a subject of debate, with outcomes appearing highly dependent on the drug used, the ultrasound parameters, and the target tissue.
In contrast to the targeted success of LIPUS, the widespread use of conventional thermal ultrasound for general soft tissue injuries and pain conditions has faced significant scrutiny. A 2001 systematic review by Robertson and Baker in Physical Therapy concluded that evidence for the effectiveness of ultrasound in treating musculoskeletal injuries and pain was questionable and lacked robust scientific backing 6. Similarly, a 2002 Cochrane Review by Green et al. found no evidence from placebo-controlled trials to support ultrasound for shoulder pain and determined it offered no additional benefit when combined with exercise 7. More recent guidelines offer a nuanced view. For example, the 2019 Clinical Practice Guideline for Carpal Tunnel Syndrome from the Academy of Orthopaedic Physical Therapy gives thermal ultrasound a "C" grade, suggesting it "may be used" for short-term symptom relief but stopping short of a strong recommendation 8.
Proper application and an understanding of contraindications are critical for safety. A key technical principle is the need for continuous movement of the transducer to avoid the formation of standing waves, which can concentrate energy and cause damage to tissues, particularly the endothelium of blood vessels 11. Furthermore, ultrasound is strictly contraindicated over known or suspected malignancies. The modality can increase cellular activity and promote angiogenesis, potentially accelerating tumor growth and metastasis, a critical safety warning emphasized in clinical textbooks such as Michlovitz's "Modalities for Therapeutic Intervention" 12. These considerations are fundamental to responsible clinical practice.
Clinical applications and parameters
The clinical application of therapeutic ultrasound requires the practitioner to select specific parameters to achieve a desired physiological effect, which can be broadly categorized as thermal or non-thermal. This selection is primarily controlled by the duty cycle, which dictates the temporal profile of the energy delivery 2. The choice between a thermal or non-thermal treatment goal depends on the target tissue, the stage of healing, and the strength of the clinical evidence for the specific pathology.
For thermal effects, the therapeutic goal is to elevate tissue temperature to a range of 40-45°C for a minimum of five minutes, a benchmark established to increase collagen extensibility and local blood flow 3. This is achieved using a continuous, 100% duty cycle that delivers an uninterrupted stream of acoustic energy 2. The heating effect is most pronounced in dense, collagen-rich tissues like tendons, ligaments, and joint capsules, which have a high acoustic impedance (Z) and thus absorb more energy compared to muscle or fat 4. One application is in the management of carpal tunnel syndrome, for which the 2019 Clinical Practice Guideline from the Academy of Orthopaedic Physical Therapy provides a "C" grade recommendation, stating that continuous ultrasound "may be used" for short-term pain and symptom reduction 8.
In contrast, non-thermal effects are pursued by using a pulsed duty cycle, typically at 20% or 50%, which minimizes the accumulation of heat. This approach emphasizes the mechanical effects of acoustic streaming and stable cavitation to alter cell membrane permeability and stimulate cellular activity 2. The strongest evidence for a non-thermal application exists for fracture healing. A 1994 randomized controlled trial by Heckman et al. on tibial fractures found that low-intensity pulsed ultrasound (LIPUS) reduced the median time to radiographic and clinical healing to 96 days, a 37.7% improvement over the 154 days observed in the placebo control group 5.
Despite these specific applications, the general efficacy of therapeutic ultrasound for many common musculoskeletal conditions remains a subject of considerable debate. A 2001 systematic review by Robertson and Baker published in Physical Therapy concluded that the scientific support for its effectiveness in treating people with pain and soft tissue lesions was questionable 6. This is exemplified by the evidence regarding shoulder pathology. A 2002 Cochrane Review conducted by Green et al. analyzed placebo-controlled trials and determined there was no evidence to support the use of ultrasound for patients with shoulder pain, including rotator cuff disease, and that it provided no additional benefit when combined with exercise 7. Similarly, the use of ultrasound to deliver medication transdermally (phonophoresis) has shown limited success; a 1993 study by Byl et al. reported that a single application of sonated hydrocortisone produced no measurable effect on cellular activity during tissue healing 13. This conflicting body of evidence necessitates a discerning, evidence-based approach to the modality's use in clinical practice.
Contraindications, precautions, and limitations
The clinical application of therapeutic ultrasound is governed by a strict set of contraindications designed to prevent harm. Chief among these is the application of ultrasound over a known or suspected malignancy. The biophysical effects of ultrasound, particularly its ability to increase cellular activity and blood flow through angiogenesis, could theoretically accelerate tumor growth and promote metastasis 12. As detailed in foundational texts like Michlovitz's Modalities for Therapeutic Intervention, this risk is considered absolute 12. Other absolute contraindications include applying ultrasound over the gravid uterus, over areas of active thrombophlebitis due to the risk of dislodging a thrombus, directly over the eyes where cavitation could damage the lens or retina, and over active infections. Caution is also warranted over metallic or plastic implants; the significant difference in acoustic impedance (Z) between the implant and surrounding soft tissue can cause the ultrasound energy to be reflected at the interface, leading to rapid, concentrated heating and potential tissue damage 4. Similarly, areas with compromised circulation or impaired sensation are generally contraindicated for thermal ultrasound, as the patient cannot provide accurate feedback to prevent a burn.
Beyond absolute contraindications, specific procedural precautions are necessary to avoid iatrogenic tissue injury. A primary risk is the formation of a standing wave, which occurs if the transducer is held stationary over the treatment area. This phenomenon concentrates acoustic energy into fixed points of high intensity, which can damage delicate tissues such as the endothelium of blood vessels 11. To prevent this, clinicians must keep the transducer head in continuous, slow motion throughout the application 11. Another critical precaution involves treatment over bony prominences. Bone has a much higher acoustic impedance than adjacent soft tissues, causing it to absorb a greater amount of energy and heat rapidly 4. This can lead to periosteal overheating and significant pain, requiring the clinician to use a lower intensity or maintain faster transducer movement over such areas. Careful management of the duty cycle is also essential. While continuous ultrasound (100% duty cycle) is used to achieve the thermal effects necessary to increase tissue extensibility, it also carries the highest risk of causing burns if intensity and duration are not properly controlled 2.
Perhaps the most significant limitation to the widespread use of therapeutic ultrasound is the questionable and often contradictory body of evidence supporting its efficacy. A landmark systematic review by Robertson and Baker, published in Physical Therapy in 2001, analyzed the existing literature and concluded that for many common applications, including the treatment of pain and various musculoskeletal injuries, the effectiveness of ultrasound remains scientifically unproven 6. This conclusion was reinforced by influential subsequent analyses, such as the 2002 Cochrane Review by Green et al. 7. This review found no evidence from placebo-controlled trials to support using ultrasound for shoulder pain and determined it offered no additional benefit over exercise alone 7. More recent evidence continues to reflect this uncertainty. For example, the 2019 Clinical Practice Guideline for Carpal Tunnel Syndrome from the Academy of Orthopaedic Physical Therapy assigned thermal ultrasound a "C" grade, indicating that while it "may be used" for short-term symptom relief, the supporting evidence is weak 8. This persistent lack of high-quality evidence forces clinicians to weigh the modality's theoretical benefits against a well-documented history of inconclusive scientific support, a central tenet of modern evidence-based practice.
Sources
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Healing the Generations: A History of Physical Therapy and the American Physical Therapy Association by Wendy B. Murphy (1995, Amer Physical Therapy Assn) - https://books.google.com/books/about/Healing_the_Generations.html?hl=&id=NJSTQgAACAAJ
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Using a 100% duty cycle (continuous ultrasound) maximizes thermal effects by delivering uninterrupted energy, whereas a pulsed duty cycle of 20% or 50% minimizes thermal accumulation and emphasizes non-thermal (mechanical) effects like cavitation and acoustic streaming. - https://www.scribd.com/document/511977759/Physical-Agents-in-Rehabilitation-From-Research-to-Practice-5th-Edition-Michelle-H-Cameron
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According to foundational research, a tissue temperature of 40-45°C must be reached for at least 5 minutes to produce therapeutic effects such as increased extensibility. - https://www.inchem.org/documents/ehc/ehc/ehc22.htm
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Acoustic impedance (Z), the product of tissue density (ρ) and the speed of sound in the tissue (c), is higher in dense, collagen-rich tissues like bone and tendon, causing more of the ultrasound wave to be reflected and absorbed as energy (heat) compared to tissues with lower impedance like muscle o - https://radiopaedia.org/articles/acoustic-impedance-1
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In the 1994 randomized controlled trial by Heckman et al., low-intensity pulsed ultrasound (LIPUS) reduced the time to overall radiographic and clinical healing for tibial fractures to 96 days, compared to 154 days in the placebo control group, a 37.7% reduction. - https://pubmed.ncbi.nlm.nih.gov/8282704/
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The systematic review by Robertson and Baker (2001) in Physical Therapy concluded that the effectiveness of therapeutic ultrasound for treating people with pain, musculoskeletal injuries, and soft tissue lesions remains questionable and lacks scientific support. - https://academic.oup.com/ptj/article/81/7/1339/2857619
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The 2002 Cochrane Review by Green et al. concluded there was no evidence from placebo-controlled trials to support the use of ultrasound for patients with shoulder pain (rotator cuff disease or mixed diagnosis) and that ultrasound provided no additional benefit over exercise alone. - https://pubmed.ncbi.nlm.nih.gov/12804491/
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Erickson M, Lawrence M, Stegink Jansen CW, et al., 2019 - https://www.jospt.org/doi/10.2519/jospt.2019.0301
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Bernadette Hecox, Joseph Weisberg, Integrating physical agents in rehabilitation (2005) - https://openlibrary.org/works/OL18836384W
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Professionalism in Physical Therapy: History, Practice, and Development by Laura Lee (Dolly) Swisher, Catherine G. Page (2005, Elsevier Health Sciences) - https://books.google.com/books/about/Professionalism_in_Physical_Therapy.html?hl=&id=sRepgakWGf0C
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A standing wave, which can be created if the ultrasound transducer is held stationary, concentrates energy in fixed locations and can cause damage to temperature-sensitive tissues like the endothelium of blood vessels; this is prevented by keeping the transducer in continuous motion. - https://academic.oup.com/ptj/article/81/7/1351/2857618
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According to Michlovitz's "Modalities for Therapeutic Intervention", ultrasound is contraindicated over a known malignancy because it can increase cell activity and proliferation, and promote angiogenesis, which could fuel tumor growth and metastasis. - https://dokumen.pub/modalities-for-therapeutic-intervention-4th-ed-0803611382-9780803611383.html
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A 1993 study by Byl et al. in JOSPT found that while phonophoresis with dexamethasone showed some effect on collagen deposition, there was no evidence that a single application of sonated hydrocortisone had a measurable effect on cellular activity after 7 days of healing. - https://www.jospt.org/doi/pdf/10.2519/jospt.1993.18.5.590