Ultrasound therapy: ultrasound in physiotherapy, indications

Ultrasound therapy is the use of high-frequency sound waves to locally affect tissue to reduce pain, modulate inflammation, and influence healing processes. In physiotherapy, low-intensity modes are used, different from diagnostic ultrasound and high-intensity focused ablation, used in oncology and fibroid therapy. The principle is based on the piezoelectric effect: a crystal in the transducer head converts an electrical signal into mechanical vibrations, which are transmitted to the tissue through a coupling gel. [1]

Despite widespread use in practice, the evidence base for the clinical efficacy of ultrasound therapy for most common musculoskeletal conditions remains mixed. Several reviews and guidelines show limited or no clinical benefit for many indications, particularly low back pain and many tendinopathies. This does not preclude certain niche scenarios, but it does require careful consideration. [2]

Ultrasound therapy utilizes two key mechanisms: thermal and non-thermal. Thermal mechanisms involve microvibration and heating of protein-rich tissues, which can increase local blood flow and elasticity. Non-thermal effects include acoustic jets and cavitation, which theoretically influence cellular processes. In practice, clinical outcomes depend on dose, frequency, and mode. [3]

Modern devices allow for a frequency of 1 MHz for deeper tissues and 3 MHz for superficial structures, as well as the use of continuous mode for thermal exposure or pulsed mode to reduce thermal load. However, even proper parameter settings do not guarantee clinically significant improvement, as emphasized by systematic reviews. [4]

Physical principles and mechanisms

The thermal effect occurs due to energy absorption by the protein structures of muscles and tendons, which may help reduce muscle spasm and increase tissue compliance. This effect is more often expected with continuous exercise. However, the translation of these physiological changes to sustained clinical outcomes is not always confirmed in randomized trials. [5]

Non-thermal effects are described as acoustic jet and stable cavitation. They can modulate cellular responses and remodeling processes, but fundamental biophysical evidence fully supporting the clinical significance of these effects at typical physiotherapeutic doses is lacking. A classic review by Baker et al. points to the inadequacy of the biophysical basis for widespread clinical application. [6]

In ultrasound therapy, the dose is a combination of frequency, intensity, mode, time, and area of application. For example, at 3 MHz, energy is absorbed more strongly by superficial tissues, while at 1 MHz, it penetrates deeper. Clinically, it makes sense to select a higher frequency for superficial structures and a lower frequency for deeper structures, while simultaneously controlling the thermal load and session duration. [7]

The practical technique involves continuous movement of the head over the treatment area using a contact gel to prevent wave reflection and localized overheating. A typical guideline from training materials is a range of 1-3 MHz, an intensity of approximately 1 W/cm², and a duration of approximately 10 minutes per area. However, such templates are not a substitute for individual selection and do not guarantee clinical benefit. [8]

Table 1. Key mechanisms of action and expected effects

Mechanism	What's happening in the tissue?	Expected physiological effect	Clinical note
Thermal	Energy absorption by protein tissues, temperature increase	Increased blood flow, extensibility, decreased spasm	The effect is more frequent with continuous mode
Acoustic jet	Microflows in the intercellular environment	Potential stimulation of metabolism	The clinical significance is controversial.
Cavitation is stable	Microbubble oscillations without collapse	Effect on membranes and receptors	Requires controlled doses
Inertial cavitation	Bubble collapse with shock waves	Risk of damage due to excess energy	Not used in standard physical therapy

Source for the table: summary of review materials. [9]

Indications and evidence base: where ultrasound is appropriate and where it is not

For chronic non-specific low back pain, leading guidelines advise against the use of ultrasound due to a lack of proven benefit. The UK National Institute of Low Back Pain guidelines specifically state: "Do not offer ultrasound to patients with low back pain, with or without sciatica." A Cochrane review of chronic low back pain also found no convincing clinical benefit. [10]

For knee osteoarthritis, the evidence is mixed. Early and some recent studies reported a modest short-term reduction in pain, but high-quality osteoarthritis management guidelines emphasize lifestyle, exercise, weight loss, and pharmacotherapy, with the role of ultrasound being low or uncertain. Systematic reviews of guidelines note a lack of consensus in favor of the routine use of ultrasound. [11]

For shoulder and rotator cuff tendinopathies, the pooled evidence generally does not support the superiority of ultrasound over placebo, exercise, or laser therapy. Recent clinical guidelines on the rotator cuff emphasize active rehabilitation, education, and graded exercise; physical modalities such as ultrasound are not leading. [12]

Some areas of potential efficacy include mild improvements in carpal tunnel syndrome and myofascial pain, but the quality of evidence is low and the effects are short-lived. The choice in such cases should be individualized, with priority given to conservative first-line strategies. [13]

Table 2. Common clinical situations and the position of the evidence base

State	Summary of the evidence	Practical position
Chronic low back pain	There are no convincing advantages	Not recommended for routine
Osteoarthritis of the knee joint	Mixed data, mostly weak effect	Priority is exercise and body weight
Shoulder tendinopathies	More often than not without benefits	The main focus is active rehabilitation
Carpal tunnel syndrome	Small short-term effect, low data quality	Acceptable as a supplement, not the basis of treatment

Sources for the table: manuals and reviews. [14]

Parameters, dosage and technique

Frequency selection depends on tissue depth: 1 MHz is used for deep structures, and 3 MHz for superficial structures. Intensity is specified as power per unit area of the emitter; textbooks for musculoskeletal pathology often indicate a target of approximately 1 W per cm² with an exposure time of approximately 10 minutes per area. In any case, parameters should be selected individually, taking into account the patient's sensations and thermal load. [15]

The modes are divided into continuous and pulsed. Continuous mode generates a more pronounced thermal component, while pulsed mode reduces overall heating while maintaining mechanical effects. Pulsed modes are more often considered in cases where overheating is undesirable, such as in subacute conditions. Selecting the pulse duty cycle affects the dose, but there are no universal protocols for all cases. [16]

The application technique involves constant movement of the emitter, uniform coverage of the area, and the use of a contact gel for reliable energy transfer. The treatment area is typically correlated with the effective radiation area of the head, and the time is chosen to prevent localized overheating and to remain within the patient's safe perception thresholds. Patient monitoring during the procedure is mandatory. [17]

The total treatment dose is determined not by a single parameter, but by a combination of them. When planning a course, it is advisable to evaluate the goal of the intervention, the stage of the process, the depth, and the type of tissue, and also to combine ultrasound with active rehabilitation, if chosen. A hardware session alone without exercises rarely provides a sustainable clinical result. [18]

Table 3. Selection of basic parameters by depth and purpose

Target	Fabric depth	Frequency	Mode	Typical intensity	Time for the zone
Thermal effect	Deep muscles and tendons	1 MHz	Continuous	about 1 W per cm²	about 10 minutes
Thermal effect	Superficial tissues	3 MHz	Continuous	lower with equal area	5-8 minutes
Non-thermal accent	Different	1 or 3 MHz	Pulse	below thermal conditions	5-10 minutes

Source for table: tutorials and technique reviews. [19]

Safety and contraindications

Key safety principles include avoiding local overheating and adhering to contraindications. Ultrasound is not used in areas of active infection, localized malignancy, recent bleeding, severe ischemia, the eye area, the thyroid gland, or the carotid sinus. It is not used in areas of thrombosis or recent irradiation. [20]

During pregnancy, avoid exposure to areas that could lead to fetal exposure, especially in the first trimester. This principle is reflected in regulatory guidelines for the safe use of ultrasound. Even at low power levels, overheating the fetus is unacceptable. [21]

Metal implants were long considered contraindicated, but modern data for low-intensity regimens indicate that heating in the area of the endoprostheses, when administered at the correct dose, does not reach dangerous levels. However, direct application to the implant or in its immediate vicinity is not required unless indicated. The type of material and location are always taken into account. [22]

Side effects are rare and typically limited to local discomfort or a superficial burn when the head is stationary and there is excess heat. Prevention includes constant movement, monitoring sensations, and proper dosage adjustment. If any unusual pain, dizziness, or worsening condition occurs, discontinue the session. [23]

Table 4. Contraindications and precautions

Category	Examples	Comment
Absolute local	Pregnancy in the treatment area, active malignant tumor locally	Avoid exposure to the fetal and tumor areas
Absolute local	Active infection, thrombosis, fresh wound with bleeding	Risk of spread and complications
Relative	Endoprosthesis area, impaired sensitivity	Requires reduced dosage and monitoring
Prohibited areas	Eyes, thyroid gland, carotid sinus	Risk of unsafe effects

Sources for table: safety manuals. [24]

Special options: phonophoresis and wearable systems

Phonophoresis is an attempt to enhance transdermal drug delivery using ultrasound. Recent meta-analyses of knee osteoarthritis show short-term pain reduction with gels containing nonsteroidal anti-inflammatory drugs or glucocorticosteroids; however, the quality of evidence and long-term effectiveness are limited, and the method is not included in core guidelines. The decision to use requires consideration of risk and cost. [25]

For non-specific neck pain and myofascial pain, the addition of ultrasound or phonophoresis to exercises shows very low quality of evidence and inconsistent results. Exercises and patient education remain first-line treatment. [26]

Wearable, long-lasting, low-intensity, continuous ultrasound systems for home use have become available. Pilot and early clinical studies report pain reduction in tendinopathies and myofascial pain, but the sample sizes are small and the designs are often limited. These are promising but unstandardized technologies that require confirmation in larger trials. [27]

A few multicenter studies have been published for low back pain, with varying conclusions, but given the persistent guideline position against routine ultrasound, these cannot be considered grounds for reconsidering practice. Active rehabilitation remains the priority. [28]

Table 5. Phonophoresis and wearable devices: what is known

Approach	What is it aimed at?	What the research shows	Restrictions
Phonophoresis for knee osteoarthritis	Short-term pain relief	There is a moderate effect on pain in the short term	Low quality evidence, no long-term outcome data
Phonophoresis for neck pain	Pain and Function	Very low data quality, ambiguous	High risk of systematic errors
Wearable long-lasting ultrasound	Tendinopathy, myofascial pain	Early work shows pain reduction	Small samples, confirmation required

Sources for the table: contemporary reviews and pilot studies. [29]

Comparison with alternatives and place in complex therapy

For low back pain and non-specific musculoskeletal pain, high-quality guidelines prioritize physical therapy, education, weight loss if overweight, and behavioral approaches. Passive modalities, including ultrasound, are not considered a mainstay of treatment, especially in chronic cases. [30]

For the shoulder girdle, the primary strategy is graduated activity, scapular strength and control exercises, and short-term analgesia. Ultrasound, however, does not provide consistent benefits compared to active methods and does not improve long-term pain or function outcomes. [31]

For knee osteoarthritis, the priorities are exercise, weight management, orthotics as indicated, and drug therapy. If ultrasound is considered as an adjunct, it should be alongside an active program and with a clear understanding of the short-term nature of the potential effect. [32]

Phonophoresis and wearable devices may be an option for highly selected patients when basic strategies have been implemented but the effect is limited. Any addition should be assessed based on clinically relevant outcomes and discontinued if there is no beneficial response. [33]

Table 6. Comparison with alternative approaches in typical conditions

State	First line	The role of ultrasound	Comment
Chronic low back pain	Exercises, training, cognitive-behavioral approaches	Not recommended	Lack of proven benefit
Shoulder tendinopathies	Exercises, measured load	The benefits have not been proven.	Laser and exercise show better results in a number of studies
Osteoarthritis of the knee joint	Exercise, weight, drug therapy	Supplement by individual decision	The effect, if any, is short-term.

Sources for the table: manuals and reviews. [34]

How to perform safely: a procedure checklist

Before the session, the indications are assessed, contraindications are ruled out, and expected goals and success metrics are agreed upon. The patient is informed of the likely short-term effect and the need for active rehabilitation. [35]

The parameters are selected: frequency of 1 or 3 MHz depending on the depth, continuous or pulsed mode, intensity at a comfortable level without overheating, and exposure time taking into account the area of the zone. Constant movement of the emitter and uniform distribution of the gel are controlled. [36]

During the session, subjective sensations and skin are assessed, and the treatment is discontinued if pain, burning, or dizziness occurs. For areas with decreased sensitivity, more gentle modes are selected or the procedure is discontinued. [37]

After the session, the response is recorded, and continuation of the course is discussed only if there is clinically significant improvement, not based on a template. The home exercise program is clarified, as it is the one that makes the biggest contribution to outcomes. [38]

Table 7. Minimum set of settings and quality control

Stage	Action	Target
Screening	Exclude contraindications	Safety
Setting up	Frequency, mode, intensity, time	Adequate dose
Technique	Continuous motion, contact gel	Preventing overheating
Monitoring	Patient feedback, skin	Early detection of side effects
Evaluation of the result	Function and pain scales	Decision to continue

Source for table: clinical guidelines and educational materials. [39]

Frequently asked questions

Does ultrasound help with chronic low back pain?
High-level recommendations indicate no routine use. The focus is on exercise and education. [40]

Does it make sense for knee osteoarthritis?
The evidence is mixed, and the effect, if any, is usually short-lived. Basic measures remain the priority. [41]

Is phonophoresis worth trying?
Some studies have shown a short-term effect on pain, but long-term evidence is scarce, and the quality of the evidence is limited. The decision is individual. [42]

Is ultrasound dangerous around implants?
Low-intensity modes, when used with the correct dose, do not produce dangerous heating around implants, but exposure near the implant should be performed with caution and as indicated. [43]

Table 8. Brief decision-making algorithm

Question	Yes	No
Basic first-line measures have been implemented	Consider ultrasound as a supplement	First implement the first line
There are local contraindications	Do not conduct	You can schedule a session
There are measurable goals and metrics	Start a course with revaluation	Set goals before you start
There is an answer after 3-4 visits	Continue as planned	Stop and reconsider tactics

Sources for the table: clinical guidelines. [44]

Conclusions

Ultrasound therapy is a technically convenient and generally safe procedure, but for most popular indications, its clinical efficacy is limited or controversial. It is not a basis for routine use for chronic low back pain and many tendinopathies. A short-term effect is possible in osteoarthritis, but there is no uniform, strong recommendation. The method's justified place is as an adjunct option within a multidisciplinary program with a focus on active rehabilitation. [45]