Efficacy and safety of low-intensity ultrasound therapy for myofascial pain syndrome: a systematic review and meta-analysis

Abstract

Background

Myofascial Pain Syndrome (MPS) is a common pain disorder characterized by the presence of trigger points within the muscles or fascia. Low-intensity ultrasound therapy, as a noninvasive modality, has indeed found application in the management of MPS, but its efficacy for myofascial pain syndrome has still been controversial. The objective of this systematic review was to assess the safety and efficacy of low-intensity ultrasound therapy for MPS.

Methods

We searched PubMed, Embase, PEDro, Web of Science, and CENTRAL for RCTs on ultrasound therapy in MPS patients. We included RCTs comparing ultrasound with other therapies or placebo-sham ultrasound. Clinical outcomes included pain scores and physical functional performance. Risk of bias and heterogeneity were assessed. Two authors of the review independently evaluated the risk of bias of each trial and extracted the data.

Results

This systematic review included sixteen RCTs involving a total of 1063 participants with MPS. None of the included studies reported adverse events. Comparing with sham or no treatment, the application of low-intensity ultrasound yielded additional benefits for pain (SMD [CI] = − 1.04 [− 1.72, − 0.36], P < 0.0003), with high heterogeneity (χ² = 116.63, P < 0.00001, I² = 91%). Patients receiving low-intensity ultrasound had improved on pressure pain threshold. Compared with other treatments, there were no differences in outcomes functional scores.

Conclusions

The current study indicates that low-intensity ultrasound effectively reduces pain intensity in MPS patients. The heterogeneity regarding the parameters of ultrasound, including frequency, intensity, time was found to be high among the included studies. Each therapeutic modality works differently in various situations and may lead to multitudinous effects. The positive impact of low-intensity ultrasound on functional improvement needs to be further analyzed through more high-quality clinical trials with large sample sizes in the future.

Trial registration

This study was registered on the following website: https://www.crd.york.ac.uk/PROSPERO/. The PROSPERO registered ID is CRD42023472032.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-024-08174-7.

Background

Myofascial Pain Syndrome (MPS) is defined by a constellation of sensory, motor, and autonomic symptoms that are rooted in muscle stiffness [1]. This stiffness arises due to hypersensitive nodules, termed myofascial trigger points (MTrPs), as well as fascial constrictions within the musculoskeletal fibers [2]. MTrPs can be clinically classified as active or latent [3]. MPS is an extremely prevalent cause of persistent pain disorders in all parts of the body leading to pain, loss of function, and a compromised range of motion [4]. Myofascial pain syndrome is a common disorder in patients with nonspecific chronic neck pain. MPS frequently occurs in the trapezius muscle [5, 6]. The exact prevalence of MPS in the general population has rarely been mentioned in the existing literature. It suggests that between 30 to 85% of patients presenting with musculoskeletal pain are diagnosed with MPS [7]. The prevalence of MPS among patients visiting clinics for pain treatment varies between 30 and 93%. [8, 9]. MPS is usually found in the population aged 27 to 50 years [10].

The regional pain of MPS is characterized by the presence of one or more myofascial trigger points (MTrPs) [8, 11]. Currently, the exact pathophysiology of MPS remains elusive. Numerous scientific inquiries and hypotheses have been explored, with one widely accepted theory suggesting an energy crisis within muscle fibers [8, 10]. At present, there are various treatment methods for MPS in clinical practice, most of which relieve tension zones and pain points, alleviate local pain, and restore muscle tissue elasticity and performance, thus improving clinical symptoms [12]. There are Multiple treatment options for MPS. Education on stretching exercises and ergonomic modifications. Nonsteroidal anti-inflammatory drugs (NSAIDs) and muscle relaxants, however, their efficacy is not definitively supported by current evidence. [12]. Physical modalities play a major role in the management of MPS. Many studies have demonstrated the significant reduction of pain in MPS patients through the application of extracorporeal shockwaves and low-power lasers [13]. Transcutaneous electrical nerve stimulation provides short-term relief from pain but lacks long-term efficacy. Low-intensity ultrasound, while commonly employed in MPS treatment, has inconclusive evidence supporting its beneficial impact [14]. There exist invasive methods for treating MPS. Dry needling is a useful technique in which clinicians use a thin filiform needle to release MTrPs. The injection of a local anesthetic directly into MTrPs can provide optimal pain relief. Systematic reviews suggested that dry needling and local anesthetic injection have therapeutic effects on MPS [15]. Acupuncture can also be utilized as a treatment option for MPS [16].

Therapeutic ultrasound, a noninvasive modality, is often used for treating musculoskeletal conditions. Nearly 94, 65, and 50% of physical therapists in Canada, the United States, and the United Kingdom, respectively, commonly use ultrasound therapy, a noninvasive therapeutic method, in the physical therapy field [17]. Ultrasound probes consist of piezoelectric crystals which convert electrical energy into mechanical oscillation energy through high-frequency alternating currents. This energy is applied directly to the patient's skin via a transducer or applicator [18]. The ultrasound waves are primarily absorbed by connective tissues such as ligaments, tendons, fascia, and scar tissue [19]. Biological responses, including muscle relaxation, tissue regeneration, and a decrease in inflammation, are induced by the thermal and nonthermal effects of therapeutic ultrasound [20]. It has been suggested that ultrasound improves local metabolism, circulation, and tissue repair by imparting thermal and mechanical properties to deep tissue through the provision of ultrasonic energy [21, 22].

A consensus regarding to the most appropriate therapeutic approach for MPS has still not been reached. The results of previous systematic research evaluating the effectiveness of low-intensity ultrasound treatment for MPS and the characteristics of clinical studies are different. Some studies have demonstrated that the use of ultrasound for MPS considerably relieves pain intensity [23–26]. Other studies have not shown any obvious effect on pain or superiority over placebos or other therapies [27, 28]. A systematic review by Xia et al. revealed that the evidence is not clear enough to support ultrasound as an effective method for treating MPS [14]. The review by Xia et al. included 10 studies. That review compared ultrasound therapy with placebo also involved combined therapies with other treatments. A systematic review published in 2006 indicates that conventional ultrasound is no more effective than placebo or no treatment for MTrPs [29]. A systematic review and meta-analysis by Qing et al., therapeutic ultrasound may be more effective in alleviating neck pain intensity than sham ultrasound or no treatment [30]. Therefore, it is important to include recent high-quality clinical trials in meta-analyses and review existing evidence to determine the results.

Objectives

The objective of this review was to determine the efficacy and safety of low-intensity ultrasound in the management of MPS.

Methods

The protocol for this systematic review and meta-analysis was according to the Cochrane Handbook for Systematic Reviews of Interventions (https://training.cochrane.org/handbook/current) and reported in accordance with the PRISMA statement for reporting systematic reviews and meta-analyses of randomized controlled trials. Two authors searched the following electronic databases separately for articles published before December 1, 2023: PubMed, EMBASE (via Ovid), and CENTRAL (via The Cochrane Library). Using medical subject headings(MeSH) terms. These terms were combined with the Boolean operator “OR”. The search words used in PubMed, EMBASE, and CENTRAL were as follows: ultrasound therapy, therapeutic ultrasound, ultrasonic therapy, myofascial pain, myofascial pain syndromes. To evaluate the safety of the treatment, the number of patients encountering adverse reactions during treatment and follow-up was documented, and the frequency of these reactions was computed.

Inclusion and exclusion criteria

Types of studies

Randomized controlled studies (RCTs) on low-intensity ultrasound for MPS were included in the current review. We excluded nonrandomized studies, quasitrials, animal studies, letters and reviews.

Types of participants

Studies were included if they involved adult participants with MPS and a minimum of one trigger point, either active or latent, in the trapezius muscle, neck, shoulder and back. However, studies including patients diagnosed without MTrPs were excluded.

Types of interventions

RCTs that compared low-intensity ultrasound with other interventions, a sham intervention, or no treatment for MPS were included. Studies that did not specify parameters for low-intensity ultrasound were excluded. The mode, frequency, intensity, and treatment time of therapeutic ultrasound were not limited.

Types of outcome measures

In this systematic review, the effects of therapeutic ultrasound were evaluated through the analysis of pain intensity and functional improvement. The pressure pain threshold (PPT) and visual analog scale (VAS) or numerical rating scale (NRS), which are similar methods and have the same maximum score, were used as pain intensity assessments. The neck disability index (NDI) was used to evaluate the impact on neck function. Furthermore, adverse events or side effects resulting from therapeutic ultrasound were also analyzed.

Data extraction

Two reviewers independently extracted the data from identified studies using a standardized data extraction form to extract the following data: basic information of the studies (first author and publication date), study population, low-intensity ultrasound intervention, combined intervention, therapeutic parameters, outcome measures, and follow-up duration, number of included studies reporting adverse events. Any divergence was settled through discussion or consultation with another author (Peijue He).

Assessment of risk of bias and methodological quality

Two reviewers independently evaluated the risk of bias of the included studies. Risk of bias were assessed according to the Cochrane risk of bias table. Figures generated in RevMan 5.3 (https://training.cochrane.org/online-learning/core-software/revman) were created to provide summary assessments of the risk of bias. The Grading of Recommendations, Assessment, Development and Evaluation (GRADE) was used to rate the quality of evidence, and the GRADE evidence summary of the findings are shown in tables using standardized terms.

Statistical analysis

All studies were grouped and analyzed by the outcome variables reported. Two comparisons were made: low-intensity ultrasound plus other treatments vs other treatments and therapeutic ultrasound vs a sham or no treatment. For measurement of VAS, PPT and NDI, the mean change from baseline to the end of the study in each group was calculated using the standardized mean difference (SMD) with the 95% confidence interval (CI).

Review Manager software (RevMan, version 5.3) was used to perform the meta-analyses. Continuous data were presented as the standardized mean difference (SMD) with 95% confidence interval (CI). Heterogeneity between included studies was assessed using the I² statistic. We analyzed the minimal clinically important difference (MCID) by the value of the mean difference (MD) for an 11-point scale for pain. According to the previous literature, the MCID of VAS and Numerical Rating Scale (NRS) was determined to be1 − 2 [31].

Results

Description of included studies

The electronic database search revealed a total of 492 studies: 87 from PubMed, 135 from EMBASE, 270 from CENTRAL. After combining the results, removing duplicates, and screening the titles and abstracts, we included 19 full-text articles. The 19 studies were analyzed and approved by both reviewers, and the data were extracted. In the end, 16 studies were included in the quantitative synthesis because two studies did not provide data on effect size [32, 33]. One study was excluded because it did not include VAS or PPT results [34]. The flow of the search and selection process is presented in Fig. 1.

Fig. 1.

Preferred reporting items for systematic reviews and meta-analyses (PRISMA) flow chart of study selection

Characteristics of the included studies are summarized in Table 1 The ultrasound parameters used were pulsed mode (two studies), continuous mode (eight studies), and unclear mode (six studies). Nine studies used an ultrasound frequency of 1 MHz, two used 3 MHz, one used 1 to 3 MHz. The ultrasound mode and frequency used in 4 studies need to be clarified. The intensity of the ultrasound ranged from 0.5 to 3 W/cm², and the specific intensity used in 1 study was 0.132 W/cm². The follow-up periods in these studies ranged from 0–6 months. The course of the treatment ranged between 1 and 15 sessions.

Table 1.

Summary of included randomized controlled trials

Author, year	Country		Sample Size (n)	Age (y) US/ Control	Therapeutic ultrasound Group	Control Group	Follow-Up	Outcomes
Aguilera et al. [35]	Spain	66	22 22 22	38.0 ± 8.7 39.0 ± 7.9 34.7 ± 5.7	group2: US, pulse mode, 1 W/cm2, 1 MHz, 2 min	group 3: Sham US group 1: ischemic compression (IC)	0 week	PPT, ROM, BEA
Akturk et al. [36]	Turkey	60	20 20 20	33.45 ± 8.02 35.45 ± 8.07 35.65 ± 11.03	group 3: US, continuous mode, 1.5 w/cm2, 5 min,5times/week, 2 weeks	group 1: ESWT, 1.6–3.0 bar, 200–400 shocks/trigger point, total of 2,000–3,000 shock/session, maximum 3 min/session, with at most 3 day intervals between sessions, 4 sessions; group 2: Sham ESWT	4 weeks	PPT, TPS, VAS, SF-36, HADS
Aridici et al. [37]	Turkey	61	30 31	38.1 ± 11.39 40.5 ± 10.1	group 1: US, 1.5 ~ 2 W/cm2, 4 times, with a 3-day interval	group 2: Dry needling (DN)	1,4 weeks	VAS, NPDS, number of trigger points, ROM, SF-36, BDI (the Beck Anxiety Inventory, sonoelastographic tests)
Ay et al. [25]	Turkey	60	20 20 20	48.80 ± 10.94 37.90 ± 12.29 49.45 ± 13.61	group 2: US, 1.5 W/cm2, 1 MHz, 10 min,5times/week, 3 weeks, 15 sessions	group 1: diclofenac phonophoresis group 3: Sham US	3 week	VAS, PPT, NPDS, ROM, numbers of trigger points
Baltazar et al. [38]	Brazil	19	9 10	39.89 ± 6.36 42.20 ± 9.37	group 1: US, 1 MHz, 1 W/cm2, 5 min,1time/weeks, 10 weeks,	group 2: IA	1 week,1,3,6 month	VAS, NCS, McGill Pain Questionnaire, SF-36
Dibai-Filho et al. [39]	Brazil	60	20 20 20	25.75 ± 5.02 23.95 ± 4.74 23.45 ± 4.07	group 2: continuous mode, 1 MHz, 1.5 W/cm2, 1.5 min, 10 treatment sessions, twice/weeks for 5 weeks	group 1: Manual therapy; group 3: Manual therapy + diadynamic currents	4 weeks	NRS, NDI, PRSSS
Dündar et al. [24]	Turkey	55	28 27	36.6 ± 11.9 35.8 ± 12.5	group 1: US, continuous mode, 1.5 W/cm2, 1 MHz, 8 min, 15 sessions (within 3 weeks)	group 2: Sham US	0, 4,12 weeks	VAS, ROM, NDI, NHP
Esenyel et al. [40]	Turkey	108	36 36 36	32 ± 5.5 30 ± 7.7 31 ± 6.7	group 1: US: 1.5 W/cm2, 6 min, 10 sessions + neck-stretching exercises	group 2: Trigger point injections (1% lidocaine) + neck-stretching exercises group 3: neck-stretching exercises	3 month	BDI, TMAS, PI, VAS, PPT, ROM
Kavadar et al. [26]	Turkey	59	30 29	37.43 ± 9.07 y 35.83 ± 5.68 y	group 1: US, continuous mode, 1.5 W/cm2,1 MHz, 6 min, 15 sessions	group 2: Sham US	0, 3 months	VAS, PPT, 0–5 scale, BDS
Lee et al [41]	USA	50	17 11 10 12	42.1 ± 16.4	group 2: US: 0.5 W/cm2, 6 min	group 1: Sham US: group 3: Electrotherapy: group 4: US + electrotherapy:	0 week	VAS, PPT, cervical joint ROM
Manca et al. [42]	Italy	24	12 12	24.5 ± 1.44 y	continuous mode: 1.5 W/cm2, 3 MHz, 12 min,5 times/week, 2 weeks 10 sessions	Sham US Active LLLT Sham LLLT Only evaluations	0, 12 weeks	NRS, PPT, cervical joint ROM
Rigby et al. [27]	USA	51	32 19	31.8 ± 12.5 y	continuous mode, 3 MHz, 0.132 W/cm2, 4 h, 3 times/weeks, 4 weeks	Sham US	4 weeks	NRS, GROC, PPT
Sarrafzadeh et al. [28]	USA	60	15 15 15 15	34.5 ± 13.35 28.7 ± 10.6	pulse mode: 1.2 W/cm2, 1 MHz, 5 min, six sessions	Group 4 (n = 15): blank Group 1 (n = 15): PhH 1% pulse mode, 1.2 W/cm2, 1 MHz, 5 min Group 2 (n = 15): Pressure release 90 s	0 week	VAS, PPT, cervical joint ROM
Srbely et al. [43]	Canada	50	25 25	28–65 y	US: 0.52 W/cm2, 1 MHz, 10 min	Sham US	0 week	PPT
Unalan et al. [44]	Turkey	49	25 24	41.0 ± 12.4 42.6 ± 13.8	group 1: continuous mode, 1- 3 MHz, 0.5- 2.0 W/cm2	group 2: IA: injection of 1 mL of 0.5% local anesthetic (lidocaine)	1,4 weeks	VAS, ROM, therapy sessions, side effects
Yildirim et al. [45]	Turkey	54	27 27	29.8 ± 5.2 31.1 ± 5.7	group 1: continuous mode, 1 MHz, 1.5 W/cm2, 5 min, 10 sessions every weekday + Stretching exercises and range of neck joint movement exercises	group 2: Sham US + Stretching exercises and range of neck joint movement exercises	0 week	VAS, PPT, PMSD, BDI

Risk of bias of included studies

The results of the risk of bias assessment are shown in Fig. 2. Five studies provided unclear information on randomization [28, 40, 41, 44, 45]. Unclear information on allocation concealment was observed in 7 studies [27, 28, 35, 40, 41, 43]. The patients were not blinded in 3 studies [28, 37, 40]. Five studies did not mention whether blinding of the outcome assessor was performed [28, 35, 36, 40, 43]. Two studies had a high risk of incomplete outcome data [36, 40]. One study had a high risk of selective reporting [36]. Two studies had an unclear risk of selection bias because of a 0-week follow-up period, insufficient information about the baseline, and a significant difference in the number of participants between the experimental group and the control group [28, 41].

Fig. 2.

Risk of bias assessment. Notes: A Risk of bias graph: review authors’ judgments about each risk of bias item presented as percentages across all included studies. B Risk of bias graph: review authors’ s judgments about each risk of bias item for each included study

The quality of the included studies is presented in Table 2.

Table 2.

Summary of findings

(A) Summary of findings: Low-intensity ultrasound compared to sham group or no treatment for MPS. (B) Summary of findings: Low-intensity ultrasound compared to other treatments for MPS

Effect of interventions

Low-intensity ultrasound vs sham or no treatment for MPS

The effect of low-intensity ultrasound on pain intensity was evaluated by the VAS or NRS in eight studies [15, 24, 26, 27, 41, 42, 45]. Overall, the application of low-intensity ultrasound had a significant impact on pain relief (SMD [CI] = − 1.04 [− 1.72, − 0.36], P < 0.0003), with high heterogeneity (χ² = 116.63, P < 0.00001, I² = 91%; Fig. 3). Nine studies used the PPT to evaluate patient pain intensity [25, 26, 28, 35, 40–43, 45]. Both studies showed that PPT improved in the control group. The results showed that SMD [CI] = 1.26 [1.03, 1.49] (P < 0.00001) with significant heterogeneity (χ² = 80.62, P < 0.00001, I² = 90%; Fig. 4).

Fig. 3.

Patients with MPS in the low-intensity ultrasound group vs sham group or no treatment group. The results of the VAS

Fig. 4.

Patients with MPS in the low-intensity ultrasound group vs sham group or no treatment group. The results of PPT

Low-intensity ultrasound vs other treatments for MPS

Four studies compared low-intensity ultrasound with other treatments for MPS [36–38, 44]. There is no significant difference in the VAS score between patients who underwent extracorporeal shock wave therapy (ESWT) and those who underwent low-intensity ultrasound [36]. There is low-quality evidence that injection of local anesthetic (IA) results in a significantly greater reduction in pain intensity [38, 44]. There is no significant difference in the VAS score between DN patients and patients receiving therapeutic ultrasound [37]. The results showed that SMD [CI] = 0.43 [0.11, 0.75] (P = 0.008) with significant heterogeneity (χ² = 59.13, P < 0.00001, I² = 95%; Fig. 5).

Fig. 5.

Patients with MPS in the low-intensity ultrasound group vs other treatment group. The results of the VAS

For the outcome of function, two studies showed that low-intensity ultrasound has different influences on the NDI [24, 39]. There is unclear evidence that low-intensity ultrasound results in a significantly greater improvement in neck function. The results showed that SMD [CI] = −0.15 [−0.19, −0.11] (P < 0.00001) (χ² = 0.16, P = 0.69, I² = 0%; Fig. 6).

Fig. 6.

Patients with MPS in the low-intensity ultrasound group vs the sham or other treatment group. The results of the NDI

Adverse events

Of the 16 included studies, none reported adverse events or side effects observed during low-intensity ultrasound treatment. The remaining studies included did not mention any adverse events or side effects. These results suggest that low-intensity ultrasound is a safe treatment.

Discussion

This systematic review analyzed 16 RCTs that used low-intensity ultrasound to treat MPS. The primary findings from this systematic review and meta-analysis reveal that low-intensity ultrasound treatment provides greater pain relief for MPS patients compared to sham low-intensity ultrasound or no treatment.

The results of this meta-analysis suggested that low-intensity ultrasound yielded additional significant benefits for relieving pain. According to the preliminary findings of this systematic review and meta-analysis, the effectiveness of low-intensity ultrasound therapy on function improvement may not always be significantly better than other treatment methods or placebo treatments. No adverse events caused by low-intensity ultrasound were found in the included studies, demonstrating the safety of this treatment.

The inconsistency and heterogeneity between studies might be attributed to differences in the parameters used for low-intensity ultrasound, treatment course, and pain duration. In two studies conducted by Rigby et al. and Sarrafzadeh et al., two distinct intensity parameters of 0.132 W/cm² and 1.2 W/cm² were utilized [27, 28]. The primary outcomes of our study diverge from the conclusions drawn in a prior systematic review, which concluded that ultrasound was not effective at reducing pain and was not suggested for use in managing MPS. The main result of the systematic review and meta-analysis by Xia et al. needs to be more evident to support ultrasound as an effective method to treat MPS [14]. The review conducted by Xia et al. included only 10 studies on ultrasound. Our study included 16 studies that were published since the publication of the prior review, and the criteria for inclusion and exclusion were different. Another limitation is that the findings from the articles comparing ultrasound therapy and placebo also encompassed combined therapy with other treatments A systematic review published in 2006 studied five types of treatments for MPS: laser therapies, electrotherapies, ultrasound, magnet therapies, and physical/manual therapies. According to the results of this review, there is moderate evidence indicating that conventional ultrasound is no more effective than placebo or no treatment for MTrPs in the neck or upper back [29]. Another systematic review and meta-analysis by Qing et al. showed that therapeutic ultrasound may reduce the intensity of pain more than sham ultrasound or no treatment [46]. This review focused on the impact of ultrasound on neck pain. However, this meta-analysis assessed the treatment of MTrPs in both the neck and upper trapezius regions.

As reported in these studies, the results of two studies on improving cervical spine function are opposite. One study suggested that improvements in the NDI score and pain and physical abilities of Nottingham health profile (NHP) patients were greater in the low-intensity ultrasound group than in the sham group [24]. There is low-quality evidence that low-intensity ultrasound results in a significantly greater improvement in neck function. However, Dibai-Filho's study revealed that applying static ultrasound or dynamic currents to myofascial trigger points in the upper trapezius did not offer greater benefits compared to manual therapy alone [39]. Another systematic review and meta-analysis by Qing et al. showed that therapeutic ultrasound combined with other conventional treatments did not clearly improve disability or quality of life [46]. Another systematic review and meta-analysis of ultrasound focused on the pain-relieving effects of ultrasound on MPS, without mentioning any functional outcome measures [47]. Future research needs to include more functional clinical outcome indicators to assess the effectiveness of ultrasound in improving MPS function.

In clinical there are multiple therapeutic interventions available for pain relief, such as drug therapy, exercise, physical therapy, acupuncture, and needling therapy, which includes dry needling and trigger point injection. However, the most appropriate, proper, and effective methods are still under debate, and MPS remains among the most challenging diseases contributing to musculoskeletal pain conditions [48]. Massage, acupuncture, and ultrasonography can effectively alleviate MTrPs noninvasively through mechanical effect. Treatments that provide skin and muscle temperature changes provide a form of counter stimulation to disrupt MTrPs. Electric currents generated by transcutaneous electrical nerve stimulation (TENS) or electroacupuncture are other ways to stimulate muscles and MTrPs. Other treatments that cause direct mechanical or chemical stimulation of muscles include acupuncture as well as MTrPs injection of local anesthetic, corticosteroids, botulinum toxin, or saline [49]. A meta-analysis in 2017 showed that most acupuncture therapies, either alone or in combination with other treatments, demonstrate efficacy in reducing pain and enhancing physical function for patients suffering from MPS [50]. According to the results of this study, there was no significant difference between acupuncture and ultrasound in the treatment of MPS, and both can effectively improve pain intensity. Shockwave therapy is another form of therapy that is being explored for MPS. This study revealed no significant difference between ESWT and ultrasound in the treatment of MPS. The shock wave uses a compressed air power source. However, there is a review, containing ten related studies, has a similar conclusion. It reported that ESWT relieves pain significantly better than sham ESWT or ultrasound therapy. But it did not show a significantly different outcome compared to dry needling, trigger point injection(TPI), or laser therapy [51]. There is low-quality evidence suggesting a significantly greater reduction in pain intensity with IA [38, 44]. These treatment options are not exhaustive, and further work needs to be done to compare the efficacy of the treatment options.

Low-intensity ultrasound is a commonly-used noninvasive modality for musculoskeletal disfunction. These ultrasonic waves are predominantly absorbed by connective tissues, such as ligaments, tendons, fascia, and scar tissue [19]. The biological responses including muscle relaxation, tissue regeneration, and a decrease in inflammation was induced by the thermal and nonthermal effects of low-intensity ultrasound [20]. It has been suggested that ultrasound transmits ultrasonic energy to deep tissue, enhancing metabolism, circulation, and tissue repair through its thermal and mechanical properties. In one study, Rigby et al. [27] used low-intensity ultrasound with the following parameters: continuous mode, 3 MHz, 0.132 W/cm², 4 h, 3 times/week, and 4 weeks [27]. One study concluded that continuous ultrasound therapy was more effective at reducing pain at rest in myofascial pain syndrome patients than was sham or pulsed ultrasound therapy [33]. Usually, the treatment depth of 3 MHz ultrasound is shallower, so it may not have a significant therapeutic effect under low-intensity conditions. Another study by Sarrafzadeh et al. [28] showed a small number of sample sizes and a high risk of bias [28]. The small number of studies included may also limit the findings.

Standardization, ultrasound dosimetry, benefit assurance and side effect risk minimization must be carefully considered to ensure an optimal benefit-to-risk ratio for the patient. The main parameters of ultrasound include frequency, intensity, time, and continuous or pulse mode [52]. From the included studies in Table 1, it was found that the frequency of ultrasound is mainly 1–3 MHz [53]. In general, the treatment depth of 3 MHz ultrasound is shallower. MPS is often treated using 1 MHz waves for deep thermal therapy, a continuous wave for a heating effect and waves with an intensity as high as 1.5 W/cm². These parameters varied greatly among the 19 studies included in this review, and the results also differed. Previously published studies used different parameters. These include pulsed mode ultrasound or continued mode ultrasound, different intensities, different locations, and different materials of the coupling agent.

One study comparing the effectiveness of low-, moderate- and high-dose ultrasound therapy showed that high-power-pain threshold (HPPT) therapy was significantly superior to medium-dose ultrasound, and medium-dose ultrasound was determined to be more effective than low-dose ultrasound, in terms of VAS score [54]. These findings indicated that parameters should be selected carefully when researching therapeutic agents. Each therapeutic modality works differently in various situations and may lead to multitudinous effects. Despite the wide usage of this treatment, supporting scientific evidence is insufficient. Thus, we conducted this study to investigate the effect of low-intensity ultrasound on the MPS. We should strive to explore and develop standardized treatment parameters, even guideline recommendations.

This study has important clinical guidance for the treatment of MPS with low-intensity ultrasound. Our study revealed that the parameters of ultrasound treatment are key to the effectiveness of treating MPS. Reasonable and appropriate treatment parameters will have better therapeutic effects, whether functional improvement or pain improvement. Most of the studies we included used treatment parameters of 1.5 watts, 1 MHz, and 5 min. These results revealed improvements in pain and joint range of motion indicators. Therefore, we can use this effective parameter in clinical practice to treat MPS. Moreover, physical therapists must explore more effective treatment parameters during the clinical treatment process.

Study limitations

The present study has several potential limitations. First, there were 2 modes of low-intensity ultrasound used, and we did not perform subgroup analysis for the different modes. Moreover, other parameters included in the study, such as treatment duration, number of treatment courses, output intensity, and pain duration, may also have led to heterogeneity and limited the certainty of the evidence. Second, most of the articles we included were from the same country or region (Turkey), which may have led to some errors. Third, due to the risk of bias and the small amount of low-quality research, the accuracy of the evidence is limited. Because of the small number of sample sizes included in the studies, more sufficient statistical power is needed to support our findings.

Authors’ conclusions

Our study suggested that low-intensity ultrasound yields additional benefits in terms of pain intensity and pressure pain threshold when combined with other conventional treatments. Low-intensity ultrasound significantly decreased pain intensity compared with sham ultrasound or no treatment in patients with MPS. However, no studies have reported the side effects of low-intensity ultrasound. Therefore, low-intensity ultrasound has been demonstrated to be a safe and effective treatment for MPS. However, the included randomized trials had different levels of quality and high heterogeneity. The number of included trials was small. This study was unable to determine the long-term impact of low-intensity ultrasound on pain relief or its effect on functionality. Therefore, high-quality and large sample size studies with effective methods may have a significant impact on our conclusions, and there is a need to determine optimal ultrasound parameters for the treatment of MPS.

It appears that low-intensity ultrasound may not be an ideal therapeutic method to replace conventional therapies but that it could serve as an adjunct therapeutic method to those treatments. In the future, high-quality clinical trials with large sample sizes are needed to analyze the effect of low-intensity ultrasound.

Abbreviations

CI

Confidence interval

GRADE

Grading of Recommendations, Assessment, Development and Evaluation

MPS

Myofascial pain syndrome

MTrP

Myofascial trigger point

NDI

Neck disability index

NHP

Nottingham health profile

NRS

Numerical rating scale

PEDro

Physiotherapy Evidence Database

QoL

Quality of life

RCT

Randomized controlled trial

SMD

Standardized mean difference

VAS

Visual analog scale

PPT

Pressure pain threshold

NSAIDs

Nonsteroidal anti-inflammatory drugs

HPPT

High-power-pain threshold

ESWT

Extracorporeal shock wave therapy

NSAIDs

Nonsteroidal anti-inflammatory drugs

IA

Injection of local anesthetic

Authors’ contributions

Xize Li: Data analysis and Writing. Peijue He: Formal analysis. Qian Wang: Methodology. Yijun Lin: Validation.

Funding

No funding.

Data availability

All data generated or analysed during this study are included in this published article [and its supplementary information files].

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.