Predictive factors of response in radial Extracorporeal Shock-waves Therapy for Myofascial and Articular Pain: A retrospective cohort study

Abstract

BACKGROUND:

Myofascial Pain Syndrome causes disability in daily life activities and despite all efforts, it continues to be a challenge, perpetuating suffering, overloading services and costs. New treatment options need to be tested.

OBJECTIVE:

We aimed to quantify the rESWT short-term analgesic effect and identify the predictors of success through comparing results achieved in MPS and Articular Pain (AP).

METHOD:

Retrospective cohort study of 1,580 patients with Myofascial Pain Syndrome or Articular Pain underwent two weekly radial Extracorporeal Shock-wave Therapy sessions. The pain intensity was measured by Visual Analog Scale before and one week after the end of the treatment (3 weeks).

RESULTS:

The therapy decreases pain by 62.50% ( $p<$ 0.0001), with a high success rate (91.59%) and a low worsening of baseline conditions rate (2.1%). The best recommendation is for patients with intense myofascial pain (Visual Analog Scale $\geqslant$ 70 mm), using high shock-wave frequency ( $\geqslant$ 15 Hz).

CONCLUSIONS:

Two rESWT sessions promote pain relief, with a high success rate and low rates of treatment abandonment and worsening. The best results are obtained in myofascial pain patients with high pain intensity, treated with high-frequency dosage.

Keywords

Myofascial Pain Syndrome musculoskeletal pain pain management rehabilitation Extracorporeal Shock-wave Therapy

1. Introduction

Myofascial Pain Syndrome (MPS) is a muscular benign condition, associated with disability for daily life activities and absence from work, frequently overwhelming health services [1, 2]. The diagnosis is clinical, considering localized symptoms with typical referred pain, active trigger point in a taut band, associated with localized muscle weakness and twitch response when inactivated [2]. Interventions results can be measured by Visual Analog Scale (VAS), a sensitive, simple, reproducible and universal tool for pain intensity assessment, recommended to follow-up patients until a maximum of 4 weeks after interventions [3].

Although there are different treatments for MPS (stretching, dry-needling, ischemic compression, and trigger point injection) [2], it continues to be a challenge, perpetuating suffering and overloading services [4]. Recently, radial Extracorporeal Shock-waves Therapy (rESWT) has been applied in a different musculoskeletal range conditions as lymphedema, burn wound, pressure ulcer, bursitis and tendinitis [5], although the strongest scientific evidence is for plantar fasciitis [6, 7]. There is a lack of dosimetry studies on rESWT for MPS [8].

A therapeutic shock wave is a biphasic, intense, short-lived mechanical pulse that propagates following the acoustics laws, in cycles of compression (positive phase) and decompression (negative phase) [9]. Reflecting at interfaces (skin, subcutaneous, fascia, muscle and bone) and more intensely in areas of greater structural density (trigger point, motor end plate, areas of inflammation or calcifications) [6, 10, 11].

It reaches a maximum depth of 4 cm and reflects as it hits solid obstacles [12]. The reflected wave collides with the next emitted, generating a secondary wave. By the laws of physics, these waves form bubbles, which collide and burst, creating jet streams capable of activating the inflammatory system [6]. The alternation between the compression and decompression cycles triggers biological processes, which result in analgesia, neovascularization and repair [13, 14, 15].

This study aims to quantify the rESWT short-term analgesic effect, identifying the predictors of success through comparing results achieved in MPS and Articular Pain (AP).

2. Objective

The objectives were quantifying the rESWT short-term analgesic effect, by VAS, and secondarily identifying the success predictors.

3. Methods

3.1 Study design and participants

This was a retrospective cohort analysis based on the medical records of patients treated in the Department of Physical and Rehabilitation Medicine of Hospital do Servidor Publico Estadual, Sao Paulo, Brazil, between October 2015 and July 2018. Two researchers reviewed the cases to identify inclusion criteria: (1) Patients with myofascial or articular pain, diagnosed by two independent physiatrists, through clinical assessment and additional tests (imaging and laboratory) when necessary to elucidate differentials diagnosis, (2) Patients with VAS $\geqslant$ 40 mm after at least 8 sessions of conventional treatment like stretching exercises, postural and ergonomic counseling, and (3) Patients treated with to 2 sessions of rESWT. Patients reporting pain in upper and lower limbs, including hip and knee pain, were included. Vertebral pain was not included and incomplete records were excluded.

Data about gender; age; duration of symptoms, pain intensity and treatment dosimetry (number of pulses, frequency and pressure applied) were collected. The sample was divided in 2 groups: Myofascial Pain Syndrome (MPS) or Articular Pain (AP), according to anatomical structure involved (muscle or joint, respectively).

3.2 Treatments

Radial Extracorporeal Shock-wave Therapy was applied on muscular trigger points in the MPS or in the most painful articular bone points for AP group, in two sessions with an interval of one week between each session, with the same 15 mm ${}^{2}$ applicator and generator (Swiss DolorClast; EMS Electro Medical Systems, Nyon Switzerland). The physician determined the dosimetry, according with his clinical experience on trigger point inactivation or joint analgesia: number of pulses (n), frequency (Hz), and pressure intensity (bar).

The VAS was assessed immediately before the first session, initial VAS (VAS-I), and one week after the second session, final VAS (VAS-F), totaling a 3 weeks follow-up. A successful result was defined as a reduction of more than 30% in VAS-I, and a worsening result when VAS-F was higher than VAS-I. VAS is a unidimensional, continuous, self-administered tool, free of charge (public domain), sensitive to detect changes in pain assessed for up to 4 weeks after interventions [3], with a minimal clinically significant change of 13.7 mm [16].

To assess possible predictors of a better analgesic response to rESWT defined as pain reduction $>$ 30%, some variables were grouped as High dose, when frequency was $>$ 15 Hz, and Low dose, when $<$ 13 Hz; High pulses dose, when $>$ 2,000 pulses/session, and Low pulses dose, when $<$ 1,800 pulses/session; High pressure, when $>$ 3 bar/session, and Low pressure, when $<$ 2 bar/session; pain was categorized as absent/mild (VAS $<$ 40 mm), moderate (VAS $>$ 40 and $<$ 70 mm) or severe (VAS $>$ 70 mm); duration of symptoms in $<$ 6 months and $>$ 24 months. Every one million pulses, the equipment underwent preventive maintenance, ensuring its durability, reliability, and smooth operation.

3.3 Statistical analysis

The statistical analysis was performed with Stata 15.0 (Stata Corp., College Station, TX, USA). Categorical variables are presented as absolute and relative frequencies, and the continuous as median with interquartile range (IQR) or as mean and standard deviation, according to the Shapiro-Wilk test. VAS-I and VAS-F are presented as medians and as means, in order to facilitate comparisons with the results of other studies.

The comparison between quantitative variables (before and after treatment) was made by two-tailed Wilcoxon test, with a type II error of 0.05. Odds Ratios were calculated, with 95% confidence intervals, after Fisher’s exact tests or chi-square tests, depending on the magnitude of the reduction in pain, to reveal the influence of each variable on treatment success. Clinical, demographic, and dosimetric variables were tested by stepwise multiple linear regression to identify the analgesic predictors.

3.4 Ethics

The study was approved by the Research Ethics Committee (Reference no. 51289115.2.0000.5463).

4. Results

The study analyzed 1,800 medical records, with 220 excluded due to insufficient data, resulting in 1,580 participants. The sample was mostly (80.38%) of women, elderly, with chronic pain (more than 3 months) as shown in Table 1. Means of dosimetry were 1983 $\pm$ 406.5 pulses/session, 14.00 $\pm$ 2.05 Hz and 2.5 $\pm$ 0.5 bar.

Table 1
General demographic and clinical data

Variable	( $N=$ 1,580)
Age (years), median (IQR)	60 (51–69)
Women, $n$ (%)	1,270 (80.38)
Men, $n$ (%)	310 (19.62)
Duration of symptoms (months), median (IQR)	12 (6–30)
VAS-I (mm), median (IQR)	80.00 (60–90)
VAS-F (mm), median (IQR)	30.00 (0–50)
Myofascial pain $n$ (%)	1,342 (84.9)
Articular pain $n$ (%)	239 (15.1)

N, number of observations; SD, standard deviation; IQR, interquartile range; VAS, visual analog scale; I, Initial; F, Final.

The median VAS-I score in all patients was 80 mm (IQR, 60–90), and the median VAS-F score was 30 mm (IQR, 0–50), corresponding to a post-treatment reduction of 50 mm, which translates to a 62.5% reduction in relation to the initial pain score, with statistically significant difference ( $P<$ 0.0001), as shown in Fig. 1. Analgesia was successfully achieved in 1,448 patients (91.59%), the rate of treatment abandonment was 5.02% (79 patients), and worsened in 33 (2.1%) of the cases in the sample.

Figure 1.

Comparative analysis of the initial and final visual analog scale: VAS-I (initial) and VAS-F (final), respectively. ${}^{*}$ Wilcoxon test.

MPS and AP groups started the treatment with similar pain intensity, 80 (65–90) and 80 (70–90) mm, respectively, without statistical difference. After the treatment, both groups showed pain reduction with VAS-F of 30 (10–50) and 40 (20–60) mm, respectively, which are statiscally significant ( $p<$ 0.001 for both). However, VAS-F was statistically lower in MPS group, Table 2.

Table 2

VAS comparisons pre and post-treatment between myofascial and articular pain groups

Variable	Articular pain	Myofascial pain	$P$ -value
	Median (IQR)	Median (IQR)
VAS-I	80 (70–90)	80 (65–90)	0.903
VAS-F	40 (20–60)	30 (10–50)	$<$ 0.001
P (Wilcoxon test)	$<$ 0.001	$<$ 0.001

Abbreviations: IQR, interquartile range; VAS, visual analog scale; I, Initial; F, Final.

Table 3

Univariate analysis of the predictors of success of rESWT

Variable	Categorization	OR ${}^{*}$	95% CI	$p^{*}$
Initial pain intensity	Severe vs. moderate/mild	2.00	1.40–2.80	$<$ 0.001
	Moderate vs. mild	1.60	0.70–3.20	0.152
Gender	Men vs. women	1.10	0.70–1.70	0.565
Age group	Elderly vs. younger adults	0.60	0.30–1.00	0.038
Duration of symptoms	$\leqslant$ 6 months vs. $>$ 24 months	1.30	0.80–2.10	0.218
Dosimetry	High vs. low frequency	1.70	1.20–2.50	$<$ 0.001
	High vs. low number of pulses	1.20	0.80–1.80	0.013
	High vs. low pressure	1.50	1.00–2.50	$<$ 0.001
Pain	Myofascial vs. articular	1.5	1.04–2.40	0.04

Table 4

Multiple regression analysis among initial pain intensity, pulses frequency and articular pain, predicting analgesia by radial Extracorporeal Shock-Wave therapy

Variable	C $\beta$	SE	$P$	95% CI	Result
VAS-I	$-$ 0.67	0.04	$<$ 0.001	$-$ 0.74; $-$ 0.59	$r^{2}=$ 0.22
Frequency	$-$ 0.13	0.04	$<$ 0.001	$-$ 0.21; $-$ 0.06	$r^{2}$ adj $=$ 0.22
Joint pain	0.71	0.21	0.001	0.31; 1.12	$P^{*}<$ 0.001

C $\beta$ , coefficient $\beta$ ; SE, standard error; CI, confidence interval; VAS-I, Visual Analog Scale-initial; $r^{2}$ , coefficient of determination; adj, adjusted; $P^{*}$ , model $P$ -value.

Univariate analysis showed that elderly patients of both genders had less successful outcome. However, severe pain independently of duration of symptoms had twice more chance to achieve significant analgesia than those with lower initial pain intensity, Table 3. Patients who were treated with high-frequency rESWT had 1.7 more chances to improve than those receiving low-frequency treatment. Similarly, patients treated with higher pressures had 1.5 more chances of success compared with lower pressures. No difference was found in number of pulses, Table 3. The possibility of achieving statistically significant analgesia was 1.5 times greater in the MPS than AP group, Table 3.

The stepwise multiple regression revealed higher initial VAS score and a higher pulse frequency as positive predictors of response to rESWT, but Articular Pain was a negative predictor, Table 4.

5. Discussion

In the present study, we identified severe Myofascial Pain Syndrome (VAS score $\geqslant$ 70 mm) and the use of high-frequency shock waves, values $\geqslant$ 15 Hz, as positive predictors of rESWT response. To our knowledge, this is the first study to address the predictors of a response to rESWT.

The literature reveals that 4 rESWT sessions with 2,000–3,000 pulses, 1.60–3.00 bars, and 10 Hz reduced 15.3 mm in VAS [17]. Using lower frequency (5 Hz), with 1.70 bar, and 2,175 pulses, 7 rESWT sessions are needed to achieve 26 mm reduction on VAS [18]. According to our study, higher frequency is the key to greater pain reduction, without having to extend the number of sessions or pulses in dosimetry.

As introduced before, therapeutic shockwaves propagate triggering dose-dependent reactions though mechanotransduction phenomena (mechanical stimuli conversion into biological signals) capable of activating the inflammatory processes [21]. Higher frequencies increase compression/decompression cycles, resulting in greater effect, precisely in pain related areas (trigger point), explaining how higher frequency increases analgesia in comparison with low frequencies [23, 24].

Pain intensity is related to the number of active nociceptors, which act as a reflective surface (of higher density), providing more bubbles and greater therapeutic effect. In addition, rESWT mechanically inhibits the active nociceptors, suppressing them directly, generating fast analgesia [24, 25], explaining how higher initial pain is a positive predictor of success. Therefore, patients with higher pain levels are excellent candidates for the procedure, especially in case of MPS. Severe pain causes activity limitations and fast and effective pain relief is important to resume daily activities sooner, contributing to solve overwhelmed services.

Comparing our results with a radial ballistic generator with that available in the literature for focal equipment, ranging from 26.3 mm of pain reduction with 4 sessions to 48.0 mm with 3 sessions for MPS [7, 26], our study reveals superiority, because it decreased the mean VAS by 43 mm with 2 sessions. The radial generator must be serviced every one million pulses, and our dosimetry (2 sessions, higher frequency, $\geqslant$ 15 Hz, and mean 1983 pulses/session) saves dosimetric charge, allowing to treat more patients. As the literature shows conflicting results about the best shockwave generator [7, 27], the present study contributes to choose, for affordability reasons, pneumatic radial ballistic equipment.

The follow-up time used in this study (3 weeks) was proper to the VAS sensitivity to detect changes in pain: hourly for a maximum of 4 hours and weekly for up to 4 weeks [3], with a minimal clinically significant change of 13.7 mm [16]. MPS is managed, regardless of cause or location in the same way: analgesics, anti-inflammatory or opioids for up to 7 days [4, 28]. Shockwaves are not a curative treatment, and can be considered as a non-pharmacological, short duration, analgesic intervention, showing differences in age, gender and duration of symptoms do not influences results. This paper proposes the use rESWT for fast analgesia, in which two doses alleviate pain for up to 1 week after the end of treatment (3 weeks since the first session), more than any placebo effect reported by literature (25–35% for pain reduction) [27, 28, 29, 30], representing a solution to manage the overload of services.

5.1 Limitations

This was a retrospective analysis without an active control, placebo, or randomization, reducing its power. The existence of a control group would require a “placebo” shock wave applicator, which would make this study impossible, because we would need to provide pain relief specifically where the other treatments failed (inclusion criteria). But the comparison with conventional treatment could contribute with the power of this analysis.

5.2 Strengths

This was a real-world study on the science of everyday clinical practice, which uses retrospective data on the care provided at a facility revealing a potential for improving healthcare and management processes. Our findings categorize this nonpharmacological treatment as effective, providing data to guide the recommendation and dosimetry, demonstrating the importance of frequency for analgesia, something previously linked to patient comfort and tolerance during the application.

6. Conclusions

Footnotes

Conflict of interest

None to report.

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