The Effectiveness of Auricular Acupressure on Chronic Musculoskeletal Pain: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Abstract

Objectives:

To assess the effectiveness of auricular acupressure (AA) in managing pain and disability in individuals with chronic musculoskeletal pain.

Materials and Methods:

A systematic search on six electronic databases was performed from their inception to May 7, 2023, to identified relevant randomized controlled trials (RCTs). Two independent reviewers screened the abstracts and full texts, extracted data, and assessed risk of bias using RoB 2. The primary outcomes were pain intensity and disability. The secondary outcomes were pain pressure thresholds, pain catastrophizing level, and fear avoidance beliefs. A random-effects model was used for meta-analyses. The certainty of evidence was evaluated using the Grading of Recommendations Assessment, Development, and Evaluation. Sensitivity analyses were conducted after removing low-quality papers.

Results:

Of 633 identified records, six studies involving 496 participants were included. All included studies compared the effectiveness of AA with sham controls in treating various chronic musculoskeletal pain. Four meta-analyses were conducted to compare the effectiveness of AA with sham controls. Low-quality evidence supported that AA had a large effect size on postintervention subjective pain reduction (standardized mean difference [SMD] = −0.95; 95% confidence interval [CI]: −1.36 to −0.54; p = 0.00; I ² = 52.61%); moderate-quality evidence substantiated that AA had a large effect size on enhancing postintervention pressure pain threshold (SMD = −0.55; 95% CI: −0.88 to −0.23; p = 0.00; I ² = 0%). There was low-quality evidence that AA had a large effect on reducing postintervention disability (SMD = −0.68; 95% CI: −1.24 to −0.12; p = 0.02; I ² = 51.33%). Our sensitivity analysis reaffirmed the same conclusion regarding pain reduction immediately after the intervention. Fourteen participants reported minimal adverse events, including soreness, tenderness, irritation, and redness, which disappeared within 1–7 days.

Discussion:

Our systematic review revealed that AA significantly improved pain, pressure pain thresholds, and disability in individuals with various chronic musculoskeletal pain conditions immediately post-treatment compared with sham treatment. Given the paucity of studies and inconsistent protocols, future RCTs are warranted to evaluate the effectiveness of AA in people with chronic musculoskeletal pain at a longer follow-up with detailed protocols, which allows researchers and clinicians to optimize AA intervention.

Conclusion

: AA has immediate post-treatment benefits for chronic musculoskeletal pain, whereas its effects at the 1- or 6-month follow-up remain uncertain.

Introduction

According to the World Health Organization, chronic musculoskeletal pain affected 1.71 billion people worldwide and was the biggest contributor to years lived with disability in 2019.¹ It poses a significant challenge on individuals’ life and health care systems worldwide.^2,3 Therefore, effective, safe, and affordable pain management strategies are needed for this population. Although pharmacological interventions (e.g., paracetamol or opioid) are common for pain relief,^4
–6 they are associated with undesirable side effects (e.g., sleep disorder, hyperalgesia, nausea, menstrual irregularities etc.),^5,6 small or questionable effectiveness,^4
–6 and the potential risk of addiction.⁷ Therefore, nonpharmacologic therapies (e.g., acupuncture, yoga, or Tai-Chi) have gained attention as potential promising alternatives.^8,9

Auriculotherapy is a treatment using ear-point stimulation, which is deeply rooted in traditional Chinese medicine (TCM) and European medical tradition.¹⁰ The potential mechanism of auricular acupressure (AA) may involve the stimulation of trigeminal and vagus nerves located on the external ear, which induces effects on the functional connectivity of the brain and subsequently modifies the pain processing pathway within the central nervous system.¹¹ The mapping of the ear’s microsystem was first established in 1956, where specific points on the auricle correspond to different anatomical regions of the body.¹⁰ Compared with various types of auriculotherapy, AA is a less time consuming and safe approach for attaining beneficial clinical effects.^12,13 Previous studies have demonstrated that AA could relieve pain,^{14

–21} normalize pressure pain thresholds,^14
–16 and improve functional disability.^{14,15,18,19,22} Although five systematic reviews and meta-analyses summarized the effects of AA on pain management,^13,23

–26 they had several limitations that required the conduction of an updated systematic review to summarize the latest evidence. In particular, two previous systematic reviews summarized the effects of AA on pain management without any meta-analysis.^25,26 The remaining three reviews included individuals with acute pain whose conditions largely differed from chronic pain.^23,25,26 More importantly, previous systematic reviews and meta-analyses included combined interventions (e.g., exercise, mobilization, interactive internet-based intervention), diverse diagnoses (e.g., postoperative pain, dysmenorrhea, migraine etc.), and different forms of auriculotherapy (e.g., auricular acupuncture, electroacupuncture stimulation, and transcutaneous electrical nerve stimulation), which provided limited information regarding the effectiveness of AA in individuals with chronic musculoskeletal pain.^13,23,24 Furthermore, all included studies in previous reviews were published on or before March 2016, and multiple new relevant studies have been published since then.^14
–16,19 Therefore, it is imperative to perform an updated systematic review and meta-analysis to summarize the evidence on this topic.

Given the above, the current systematic review and meta-analysis aimed to assess the effectiveness of AA in managing pain, pain-related disability, and other clinical outcomes among individuals with chronic musculoskeletal pain.

Materials and Methods

The study protocol was registered with PROSPERO (CRD42023423123). The study was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.²⁷

Search strategy

PubMed, Embase, CINAHL, SPORTDiscus, Web of Science, and the Cochrane Central Register of Controlled Trials (CENTRAL) were searched from their inspection to May 7, 2023. The keywords included medical subject headings of “chronic pain” and free text terms of “auricular acupressure.” Detailed search strategies are presented in Supplementary Table S1. In addition, forward citation tracking was conducted using Web of Science to identify potential articles that cited the included studies. Backward citation tracking was also conducted by screening the reference lists of the included studies to identify potential publications that might not have been identified through the databases.

Study selection and eligibility criteria

Two independent reviewers (K.W. and R.C.) screened the titles and abstracts to determine the eligibility for the full-text screening. Any disagreements were addressed through consensus with or without a third reviewer (A.W.). Full-text screening adhered to the same procedures.

The eligible criteria were formulated according to population, intervention, comparison, outcomes, and study design (PICOS).²⁸ (1) Population: adults (≥18 years old) with chronic musculoskeletal pain that was defined by the ICD-11 for chronic musculoskeletal pain or the ACTTION-APS Pain Taxonomy (AAPT)^29
–31; (2) intervention: only involved AA; (3) comparison: sham treatment, no intervention, or usual care; (4) outcomes: primary outcomes were subjective pain intensity (e.g., visual analog scale [VAS], numeric rating scale, or brief pain inventory—short form [BPI-sf]), and pain-related disability (e.g., Roland–Morris disability questionnaire [RMDQ], the Oswestry Disability Index [ODI], SF-12) as measured by validated scales and secondary outcomes included objective pain measurements (e.g., pain pressure threshold [PPT]), pain catastrophizing (e.g., pain catastrophizing scale [PCS]), and fear avoidance beliefs (e.g., fear avoidance beliefs questionnaire [FABQ]); and (5) study design: randomized controlled trials (RCTs). Exclusion criteria were (1) acute pain or other types of pain [e.g., cancer pain and dysmenorrhea), (2) other forms of alternative therapy (e.g., acupuncture] or combined interventions, or (3) nonRCT, observational studies, case series, case reports, conference abstracts, comments, animal studies, or reviews.

Data extraction

One reviewer (K.W.) extracted data from the included studies using a standardized form. Another reviewer (R.C.) verified the results independently. Any disagreements were resolved by consulting a third reviewer (A.W.). The extracted data included (1) bibliometric data (e.g., authors, years of publication, and country); (2) participants’ characteristics (e.g., age, sex, type of chronic musculoskeletal pain, and sample size); (3) study design; (4) details of the AA protocol (e.g., acupoints selected, auricle map, operators’ experiences, method of taping, stimulators, retaining time, and intervention doses); (5) outcome measures (e.g., pain intensity, disability, and/or pressure pain threshold); and (6) results at baseline, postintervention, and other follow-ups.

For studies using multiple outcome measures for assessing the same outcome domain, data were extracted from the most frequently used assessment measure across studies to reduce heterogeneity. Moreover, when data reported in a given study were incomplete or unclear, attempts were made to contact the corresponding author by emails. Three emails were sent spanning 1 week to request for relevant information. If they did not respond, the study would be excluded.

Risk of bias assessments

The methodological quality of the included studies was assessed independently by two reviewers (K.W. and R.C.) according to version 2 of the Cochrane tool for assessing risk of bias in randomized trials (RoB 2).^32,33 Disagreement was resolved by a discussion with a third reviewer (A.W.). The RoB 2 tool comprises six domains: randomization process, deviation from intended interventions, missing outcomes data, measurement of the outcome, selection of the reported result, and overall bias. The overall risk of bias was rated as “low,” “some concerns” or “high” according to the results of each domain.

Meta-analysis

All meta-analyses were conducted using Comprehensive Meta-Analysis (Version 3.3.070 Biostat, Inc.). Random-effects models were performed to synthesize data as standardized mean difference (SMD), which was interpreted as small (<0.2), medium (0.2–0.5), or large (>0.5) effect size.³⁴ Forest plots were plotted to present the effect sizes and 95% confidence intervals (95% CIs). A negative SMD indicated a reduction in subjective pain intensity, improvements in disability, and increased PPT.

Q-Statistic was adopted to assess the degree of homogeneity, which was quantified by I ². The results were interpreted as low (25%–50%), moderate (51%–75%), and high (>75%) heterogeneity.³⁵ The sensitivity analysis was performed by removing low-quality studies to determine the findings from moderate and/or high-quality papers. The significant level was set at 0.05 for all meta-analyses. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach was also used to evaluate the certainty of evidence for each outcome measure (Supplementary Table S2).³⁶

Results

Literature searches

A total of 633 citations were identified (Fig. 1), in which 495 publications were identified from citation tracking and additional search. After removing 254 duplicates, 379 were screened for the titles and abstracts. Twenty studies were retrieved for full-text screening, and six RCTs were included in this review.^{14
–16,18
–20,22}

FIG. 1.

PRISMA flowchart for the study selection process. RCT, randomized controlled trial.

Study characteristics

The characteristics of seven included studies and treatment parameters are presented in Tables 1 and Table 2, respectively. These studies involved 496 participants (389 females, 78%). The sample sizes ranged from 21 to 265. The types of chronic musculoskeletal pain conditions were diverse, encompassing chronic low back pain, chronic nonspecific spinal pain, osteoarthritis, and chronic neck pain. Regarding the primary outcomes, four and two studies used VAS and BPI-sf to assess pain, respectively. For disability, one study used RMDQ, whereas another three studies used ODI, NDI, and SF-12, respectively. Three studies used PPT test for assessing objective pain threshold. One study adopted PCS and FABQ to assess pain catastrophizing and fear avoidance beliefs, respectively. All included studies used a placebo control group, which the acupoints unrelated to the pain conditions were selected. All studies measured clinical outcomes at baseline. During the intervention, two studies measured the outcomes on a weekly basis, whereas one study assessed the effects daily. In addition, another study measured the outcomes 2 weeks after the initiation of the treatment. After the intervention, five studies evaluated the parameters immediately post-treatment, whereas the remaining one assessed the outcomes of 1 week post-treatment. Two studies reassessed participants at post-treatment follow-ups, with one study measuring at the 1-month follow-up and another one assessing the outcomes at the 6-month follow-up. A total of four meta-analyses were conducted to compare the effectiveness of AA with sham treatments on various clinical outcomes.

Table 1.

Characteristics of the Included Studies

References, year	Country	Study design	Type of pain	Measurement timepoint	Sample size	Age (years)	Sex (intervention)	Control	Intervention (intervention)	Control	Average pain intensity (out of 10)	Dropout rate
Lee & Park,¹⁶	South Korea	RCT (randomized, single-blinded, placebo-controlled experimental study)	Osteoarthritis	Baseline, weekly for 7 weeks, immediately postintervention	Intervention: 26 Control: 26	Intervention: 78.38 ± 4.547 Control: 79.27 ± 4.50	Male: 11 (42.3%); Female: 15 (57.7%)	Male: 11 (42.3%); Female: 15 (57.7%)	AA	Placebo	Intervention: 5.04 ± 1.68 Control: 4.42 ± 1.81	10.3%
Kim & Park,¹⁴	South Korea	RCT (randomized, single-blinded, and placebo-controlled study)	Chronic low back pain	Baseline, weekly for 5 times, immediately postintervention	Intervention: 26 Control: 25	Intervention: 77.77 ± 6.76 Control: 77.20 ± 7.48	Male: 0; Female: 26 (100%)	Male: 0; Female: 25	AA	Placebo	Intervention: 6.27 ± 0.92 Control: 5.76 ± 0.93	16.9%
Lee & Park,¹⁵	South Korea	RCT (single-blind, randomized, sham-controlled study)	Chronic neck pain	Baseline, 2 weeks into intervention, immediately postintervention	Intervention: 25 Control: 23	Intervention: 44.68 ± 9.16 Control: 40.26 ± 13.55	Male: 8 (32%); Female: 17 (68%)	Male: 9 (39.1%); Female: 14 (60.9%)	AA	Placebo	Intervention: 4.87 ± 1.95 Control: 4.28 ± 1.69	4%
Yeh et al,¹⁷	USA	RCT (prospective, randomized controlled trials)	Chronic low back pain	Baseline, daily, immediately postintervention	Intervention: 30 Control: 31	Intervention: 60.97 ± 17.44 Control: 65.61 ± 16.04	Male: 10 (33.3%); Female: 20 (66.7%)	Male: 10 (32.3%); Female: 21 (67.7%)	AA	Placebo	Intervention: 6.87 ± 1.82 Control: 7.00 ± 1.82	Intervention: 17% Control: 32%
Vas et al,²²	Spain	RCT (multicentre randomized controlled study with two parallel arms)	Chronic nonspecific spinal pain (Cervical, thoracic, or lumbar)	Baseline, 1 week after the end of treatment, 6 months follow-up	Intervention: 130 Control: 135	Intervention: 50.7 ± 15.2 Control: 51.5 ± 14.9	Male: 19 (14.6%); Female: 111 (84.5%)	Male: 25 (18.5%); Female: 110 (81.5%)	AA	Placebo	Intervention: 6.94 ± 1.58 Control: 6.97 ± 1.56	5%
Yeh et al,¹⁸	USA	RCT (prospective, randomized clinical trial feasibility study)	Chronic low back pain	Baseline, immediate postintervention, 1-month follow-up	Intervention: 10 Control: 9	Intervention: 45.4 ± 21.8 Control: 49.8 ± 14.4	Male: 2 (20%); Female: 8 (80%)	Male: 2 (22%); Female: 7 (78%)	AA	Placebo	Intervention: 3.40 ± 0.70 Control: 5.13 ± 1.36	Intervention: 9.09% Control: 10%

AA, auricular acupressure; RCT, randomized controlled trial.

Table 2. Treatment parameters of the included studies.

References, year	Acupoints selected	Map	Operator	Method of taping	Stimulator	Retaining time	Intervention dose	Outcome measures	Adverse events
Lee & Park,¹⁶	Shenmen, Liver, Heart, Subcortex, Sympathesis	NM	Researcher	Alternative single ear	Vaccaria seeds	5 days/week	4 times, 2 mins, 8 weeks	VAS, PPT, PSQI-K, PSG, actigraphy, levels of melatonin in the blood	No
Kim & Park,¹⁴	Shenmen, Hip, Lumbosacral, Kidney, Liver	NM	Researcher	Alternative single ear	Vaccaria seeds	5 days/week	3 times, 3 mins, 6 weeks	VAS, PPT, ODI	Soreness, skin irritation
Lee & Park,¹⁵	Shenmen (TF4), Kidney (CO10), Liver (CO12), Shoulder (SF5), Cervical vertebra (AH13), Occiput (AT3)	NM	Researcher (completed AA education program)	Single ear	S. alba seeds	5 days/week	Whenever they felt neck pain, 4 weeks	VAS, PPT, NDI, Cervical ROM	NM
Yeh et al,²⁰	2 corresponding ear points (low back), shenmen, sympathetic, nervous subcortex	Chinese Standard Ear-Acupoints Chart	Therapist	Bilateral ears	Vaccaria seeds	5 days/week	3 times, 3 mins, 4 weeks	AA practice, BPI-sf, MQS III	Minimal
Vas et al,²²	Shenmen (triangular fossa), thalamus (inner wall of the antitragus) was obligatory, other were added according to the sensitivity of the auricular areas	Chinese map	Doctors and nurses	Alternative single ear	Vaccaria seeds	7 days	3 times, 10 times each ear plants, 8 weeks	VAS, SF −12, Lattinen index, MPQ	No
Yeh et al,¹⁸	Shenmen, sympathetic, nervous subcortex, low back	Chinese ear map	Therapist	NM	Vaccaria seeds	5 days/week	3 times and whenever they felt pain, 3 mins, 4 weeks	BPI, RMDQ, ODI, FABQ, PCS, WHOQOL-BREF, treatment satisfaction, Daily diary	Yes

AA, auricular acupressure; BPI, brief pain inventory; BPI-sf, brief pain inventory-short form; Cervical ROM, cervical range of motion; FABQ, fear avoidance beliefs questionnaire; MPQ, McGill pain questionnaire; MQS III, the medication quantification score version III; NDI, neck disability index; NM, not mentioned; ODI, Oswestry disability index; PCS, pain catastrophizing scale; PGIC, patient global impression of change; PSG, polysomnography; PSQI-K, Pittsburgh Sleep Quality Index-Korean; PT, pressure threshold; RMDQ, Roland–Morris disability questionnaire; VAS, visual analog scale; WHOQOL-BREF, world health organization quality of life instruments.

The number of acupoints used varied across studies. Three studies applied 10, 6, and 4 acupoints on their participants, respectively. The remaining three studies used five acupoints. All included studies selected shenmen as the acupoint, with three studies selected subcortex, sympathetic, liver, and kidney as their acupoints. Four studies used acupoints related to the pain sites (low back, hip, cervical vertebra, shoulder, lumbosacral, and occiput). There was only one study incorporated the heart acupoint. Five included studies used Vaccaria seeds as the stimulator, while the remaining one used S. alba seeds. The auricle map was reported in only three studies, with those studies using the Chinese ear map. Three included studies implemented a 4-week AA protocol, one study extended it to 6 weeks, and two studies used an 8-week treatment. For frequency, four included studies instructed the participants to apply pressure three times daily, whereas four times daily and “whenever they felt pain” were used by the remaining two studies, respectively. Three studies mandated a treatment duration of 3 min per session, whereas another study required only 2 min. The remaining two studies did not provide a specific time frame. Instead, instructions of “10 times for each acupoints” and “whenever they felt pain” were given to the participants. Three included studies reported no treatment-related adverse events. However, adverse events were reported by two included studies, including symptoms such as soreness, tenderness, irritation, and redness, which resolved within 1–7 days. One included study reported the occurrence of adverse effects without specifying the details, whereas the remaining study did not mention any adverse effects.

Risk of bias of the included studies

The methodology quality of included studies is summarized in Supplementary Figures S1a and Supplementary Figures S1b. Four included studies demonstrated a moderate risk of bias,^{15,16,19,20,22} and two included studies showed a high risk of bias.^14,18 The common sources of bias in these RCTs were (1) unclear randomization process, (2) deviations from the intended interventions, and (3) selective reporting of results. For (1), four studies did not disclose whether the allocation sequence was concealed until the participants were enrolled and assigned to interventions.^14,15,18,20 For (2), four studies deviated from the original intention-to-treat approach and exclusively used a per-protocol approach. They excluded participants who dropped out during the data analysis, which increased the risk of bias.^14
–16,18 For (3), all the included RCTs did not provide a prespecified analysis plan.^{14
–16,18,20,22}

Effectiveness of AA in pain reduction immediately post-treatment

Five included studies involving 231 participants investigated the effectiveness of AA in reducing subjective pain. Our meta-analyses revealed that AA significantly reduced subjective pain with a large effect size (SMD = −0.95; 95% CI: −1.36 to −0.54; p = 0.00; I ² = 52.61%; Fig. 2). Three included studies involving 151 participants investigated pressure pain thresholds. The meta-analysis revealed that AA significantly increased PPT with a large effect size (SMD = −0.55; 95% CI: −0.88 to −0.23; p = 0.00; I ² = 0%; Fig. 3). The GRADE assessment revealed very low-quality evidence for subjective pain reduction but moderate-quality evidence for PPT increment.

FIG. 2.

A meta-analysis of the effects of auricular acupressure on subjective pain immediately posttreatment. SD, standard deviation.

FIG. 3.

A meta-analysis of the effects of auricular acupressure on pressure pain threshold immediately post-treatment. SD, standard deviation.

Effectiveness of AA in improving disability immediately post-treatment

Three included studies involving 118 participants examined the effectiveness of AA in improving pain-related disability. Low-quality evidence from our meta-analysis showed that AA significantly improved pain-related disability with a large effect size (SMD = −0.68; 95% CI: −1.24 to −0.12; p = 0.02; I ² = 51.33%; Fig. 4).

FIG. 4.

A meta-analysis of the effects of auricular acupressure on pain-related disability immediately post-treatment. SD, standard deviation.

Effectiveness of AA in improving pain catastrophizing immediately post-treatment

One included study involving 19 participants investigated the effectiveness of AA in improving pain catastrophizing. Low-quality evidence showed that AA did not significantly improve pain catastrophizing.

Effectiveness of AA in improving fear avoidance beliefs immediately post-treatment

One included study involving 19 participants examined the effectiveness of AA in improving fear avoidance beliefs. Low-quality evidence showed that AA did not significantly improve fear avoidance beliefs.

Effectiveness of AA in reducing subjective pain and disability at 1-week postintervention

Only one included study involving 265 participants investigated the effects of AA on subjective pain and disability at 1 week postintervention. There was high-quality evidence that AA significantly reduced subjective pain and disability at 1 week postintervention.

Effectiveness of AA in reducing subjective pain and disability at the 1-month follow-up

One included study involving 19 participants investigated the effects of AA on subjective pain intensity and disability at the 1-month follow-up. Low-quality evidence displayed that AA significantly decreases subjective pain intensity but not disability at 1 month after treatment.

Effectiveness of AA in improving pain catastrophizing and fear avoidance beliefs at the 1-month follow-up

One included study involving 19 participants investigated the effectiveness of AA in improving pain catastrophizing and fear avoidance beliefs at 1 month post-treatment. Low-quality evidence revealed that AA did not significantly improve either pain catastrophizing or fear avoidance beliefs at the 1-month follow-up.

Effectiveness of AA in reducing subjective pain and disability at the 6-month follow-up

Only one included study investigated the effects of AA on subjective pain and disability of 265 participants at the 6-month follow-up. The GRADE assessment revealed high-quality evidence in reducing subjective pain and disability at the 6-month follow-up.

Sensitivity analysis on subjective pain immediately post-treatment

By removing low-quality studies, the heterogeneity remained low with large effect size in reducing pain immediately post-treatment (SMD = −0.70; 95% CI: −1.04 to −0.36; p = 0.00; I ² = 10.91%; Supplementary Fig S2), which concurred with the original analyses.

Discussion

This systematic review and meta-analysis comprehensively reviewed and updated the evidence regarding the effects of AA on subjective pain intensity, objective pain thresholds, pain-related disability, pain catastrophizing, fear avoidance beliefs, and other clinical outcomes among individuals with chronic musculoskeletal pain. Very low- to low-quality evidence supported that AA significantly improved subjective pain and pain-related disability. Moreover, moderate-quality evidence indicated the significant effects of AA on increasing PPT.

Our results regarding subjective pain reduction concurred with two previous meta-analyses, which concluded that AA had large effects on reducing pain in individuals with chronic pain.^13,24 Although a prior systematic review and meta-analysis concluded favorable effects of AA on decreasing subjective pain intensity, their conclusions were based on a wide range of pain conditions (e.g., postsurgery pain, dysmenorrhea, or migraine attack). Therefore, the specific effects of AA on chronic musculoskeletal pain remained unclear. Our review addressed this gap by incorporating the latest definition of pain constructed by the International Association for the Study of Pain and the American Pain Society.^29
–31

Our primary meta-analysis and sensitivity analysis found significant effects of AA on improving pain-related disability immediately post-treatment, which was different from a previous systematic review. The discrepancy may be ascribed to the relatively small sample size in the previous meta-analysis due to scarce studies investigating pain-related disability.¹³ Given that pain intensity and disability are interrelated in individuals with chronic musculoskeletal pain,³⁷ functional disability is one of the criteria in diagnosing chronic musculoskeletal pain as it covers a wide range of daily interference, such as working, sleeping, or engaging in social activities.²⁹ Different types of chronic musculoskeletal pain may affect individuals to different extents. Future RCTs should evaluate the short- and long-term effects of AA on pain-related disability in individuals with different types of chronic musculoskeletal pain.

Given that there was only one study investigating the post-treatment effects at the 1- or 6-month follow-up, meta-analyses were not conducted. Future large-scale RCTs are warranted to investigate the long-term effects of AA on various clinical outcomes.

The selection of proper acupoints is the key for effective AA treatment.³⁸ It is recommended that the choice of acupoints should be based on several principles: relating to the corresponding body parts according to Zang-Fu dialectics and meridian theory, aligning with modern scientific understanding, considering the functions of the acupoints, and drawing from clinical experiences.³⁹ Among the included studies, shenmen, sympathetic, subcortex, liver, kidney, and the corresponding pain sites were the most frequently selected acupoints, known for their effects on the nervous system.³⁸ According to TCM theory, shenmen exhibits anti-inflammatory and calming effects, sympathetic promotes blood circulation, and subcortex is responsible for pain management with analgesic effect.³⁸ Our results concurred with a previous systematic review focused on pain reduction with AA, which also highlighted the widespread usage of shenmen, sympathetic, and subcortex in participants with pain.²⁶ In addition, it is noted that the liver and kidney acupoints are commonly used in AA on the management of chronic musculoskeletal pain, which aligned with the principles guiding the selection of acupoints.³⁹

The treatment parameters (treatment duration, methods of taping, doses) of the included studies were heterogeneous. It is recommended that AA treatment should be at least 4 weeks, which all our included studies fulfilled the minimum requirements of treatment duration.³⁸ Two included studies compared the effects of unilateral and bilateral auricular transcutaneous electrical nerve stimulation on pain thresholds in healthy adults. Both methods significantly increased the pain thresholds without significant differences between groups.^40,41 However, it remains unclear whether unilateral and bilateral AA would yield similar results. Interestingly, one included study, which instructed participants to apply pressure whenever they felt pain, did not find significant improvements in perceived pain.¹⁵ This may indicate that it is necessary to apply AA regularly for three times, 3 min daily, which were commonly used in our included studies.^14,18,20

Although the potential mechanisms underlying the effectiveness of AA on chronic musculoskeletal pain remains uncertain, it could be related to alteration in pain processing pathways. A resting state functional magnetic resonance imaging (fMRI) study on individuals with chemotherapy-induced neuropathy after AA revealed altered functional connectivity in brain regions related to pain processing (e.g., decreased functional connectivity between salience and basal ganglia networks and increased functional connectivity between the executive network and salience network),¹¹ which indicates possible normalization of maladaptive brain activity.¹¹ In addition, AA has the potential to restore conditioned pain modulation, reflecting enhanced pain processing through the descending pain pathway.¹¹ Furthermore, this is the first meta-analysis to summarize evidence regarding the significant effect of AA on increasing PPT. Peripheral sensitization is probably the main mechanism that accounts for hyperalgesia to pressure.⁴² Our review supported the notion that AA could increase PPT by lowering peripheral pain sensitivity.

In addition, the auricular branch of the vagus nerve (ABVN) is the only peripheral branch of the vagus nerve that establishes a mutual connection between the brain and major body structures. It contains 75% of all parasympathetic fibers and involves various functions of the body’s autonomic nervous system (ANS).^43

–46 By inducing pressure at the ABVN, it functions to carry both nociceptive fibers for pain and sensory fibers for maintaining homeostasis and generating appropriate responses across the brain and body. Specifically, AA could increase anti-inflammatory cytokine interleukin (IL)-4 and IL-13 following a 4-week AA protocol.^17,47 Moreover, AA was found to decrease proinflammatory cytokines IL-1β, IL-12, IL-6, and tumor necrosis factor - alpha (TNF-α).^12,17,47 These results suggest that AA could contribute to pain relief by influencing pain-related immune responses. These findings highlight that future studies should consider assessing the roles of brain networks, quantitative sensory testing, and immune biomarkers to determine the relationship between AA and chronic musculoskeletal pain.

Limitations

Several limitations should be noted in this review. First, the methodological quality of all included studies was low or moderate, and some meta-analyses were pooled from only three studies (e.g., PPT, subjective pain, and disability immediately post-treatment). In addition, two meta-analyses (i.e., post-treatment pain and disability) showed moderate I ² due to the inclusion of studies with high risk of bias. Future large-scale high-quality RCTs should validate our findings to inform clinical practice. Second, because the number of included studies was less than 10, meta-regression could not be performed to explore the effects of potential modifiers (e.g., age and gender) on our results. Third, the included studies showed diverse AA treatment protocols (e.g., choices of auricle map and methods of taping). Future RCTs should explore the effects of different treatment parameters or dosages on individuals with different chronic musculoskeletal pain conditions.

Conclusions

This updated systematic review and meta-analysis highlights the benefits of AA on managing chronic musculoskeletal pain. Our results indicated that AA significantly improved pain, peripheral pain sensitivity, and pain-related disability immediately post-treatment, whereas its effects at the 1- or 6-month follow-up remain uncertain. Our findings help clinicians and researchers make informed decisions about the short-term effects of AA for chronic pain management. Given the heterogenous AA protocols of the included studies, future high-quality RCTs should compare the effectiveness of different AA parameters in managing individuals with different types of chronic musculoskeletal pain conditions.

Footnotes

Authors’ Contributions

Each author has made significant contributions throughout the research process. They have contributed to the study’s conception, design, data acquisition, analysis, interpretation, and article writing. Furthermore, all reviewers have made substantive revisions and given their approval for the final article. We also confirm that the order of authors listed in the article has been agreed upon by all of us.

Author Disclosure Statement

There were no financial or competing conflicts of interest in this work.

Funding Information

Research Institute in Sports Sciences and Technology—The Hong Kong Polytechnic University. Grant number :1-CD6C

Supplementary Material

Supplementary Figures S1

Supplementary Figures S2

Supplementary Table S1

Supplementary Tables S2

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