Visual abstract

Introduction

The concept of pain, revised in 2020, is defined as an “unpleasant sensory and emotional experience, associated with or similar to that associated with actual or potential tissue damage” [1]. Pain is a subjective experience directly perceived by the sufferer. It is a multidimensional phenomenon characterized by pain location, intensity, temporal characteristics, quality, impact, and significance. Pain does not exist in isolation, but rather in a specific human being among psychosocial, economic, and cultural factors that influence the meaning, experience, and verbal and nonverbal expressions of pain [2].

Acupuncture is recommended by the World Health Organization as a treatment for acute and chronic pain caused by different pathologies, to provide a better quality of life to patients [3]. Studies demonstrate the use of acupuncture to relieve cancer pain [4,5], pain caused by myofascial syndrome [6], pain from bone wear caused by osteoporosis [7], cephalalgia [8], and pain caused by chronic pancreatitis [9].

Chronic pain is a sensation that lasts long after the primary lesion has healed, and may or may not be caused by trauma or disease. Its location, features, and frequency are less specific than acute pain [10]. Chronic pain is more than just a symptom; it is a sickness that lasts, does not go away after the damage has healed, and is associated with chronic pathological processes. Chronic pain might manifest as temporomandibular dysfunction (TMD). Patients with TMD commonly experience discomfort in the masticatory muscles, preauricular region, or temporomandibular joint (TMJ) [11]. It is estimated that 40%–75% of the population has at least 1 TMD indication, such as TMJ noise, and 33% have at least 1 symptom, such as facial discomfort or TMJ pain [12]. TMDs can be muscular or articular in character; nevertheless, muscular types are the most common [13]. Furthermore, whereas TMD affects both sexes, new research has indicated there is a higher frequency experienced by women than men [14]. Acupuncture therapy can offer alleviation and/or complete reduction of pain intensity in the muscles affected by TMD, enhance jaw movements, and reduce masticatory muscle hyperactivity. In most situations, patients with TMD can use acupuncture as a supplemental or primary treatment to alleviate unpleasant symptoms and enhance oral function [15].

Acute pain is defined as a sensation of severe, uncomfortable, or painful discomfort caused by trauma or disease, with a specific location, time, and characteristics [10]. It begins with an injury or lesion, after which algogenic chemicals are generated and released at the site, activating nerve endings (nociceptors) in thin myelinated or unmyelinated fibers. Acute pain usually resolves spontaneously. However, continuous activation of multiple neural pathways can modify the character of pain, causing acute pain to become chronic pain [16].

Irreversible acute pulpitis is a pulpal disease that indicates a severe level of inflammation. Clinically, the patient has sudden, spontaneous, intense, pulsating pain that may take seconds to hours to subside. Patients frequently claim that the pain is worse at night. This is because lying down raises internal pulpal pressure, due to increased blood flow to the head, which causes the discomfort to worsen. Even strong analgesics may not relieve pain. In this scenario, endodontic therapy with biopulpectomy, or pulpal tissue removal, is recommended [17]. Acupuncture, with its analgesic effect, has been shown in studies to be a useful pain control therapy in patients with acute dental pain, such as pulpitis [18,19]. Research in cognitive processes has focused on the impact of expectation on health care as well as the placebo effect [20]. Although some studies have found a substantial relationship between patient expectations and responsiveness to acupuncture therapy, there are some inconsistencies [21].

Expectation can be defined as a mental construct that does not need to be explicitly stated but whose reality is a form of self-talk. Expectation simply suggests, never affirms, and hence has a subjective quality. Expectation is always the product of a hypothesis. The word expectancy conveys the notion of desire [22]. Satisfaction is commonly defined as an evaluation of outcomes, related to the efficacy of care or a specific benefit from a specific intervention [23].

There are few studies that assess the relationship between expectations and acupuncture outcomes. It has been reported that there appears to be a strong relationship between patient expectations and treatment responses following acupuncture, however, the findings are not consistent [22].

Unlike previous studies that have primarily focused on expectation as an isolated predictor of acupuncture outcomes, the present study was a secondary analysis of data from 2 randomized controlled trials which were conducted in distinct clinical contexts (chronic and acute pain). This approach allowed the simultaneous evaluation of expectation, satisfaction, and pain outcomes across different conditions and treatment types (real and sham acupuncture), to provide a broader perspective on the psychological and contextual influences in acupuncture related analgesia.

Materials and Methods

This was a secondary analysis study of 2 clinical trials which were conducted in accordance with the Consolidated Standards of Reporting Trials and STandards for Reporting Interventions in Clinical Trials of Acupuncture (STRICTA) guidelines and all participants gave informed written consent. Study 1: A clinical, controlled, randomized, double-blind study was conducted at the Specialization Clinic of the Faculdade de Odontologia de Piracicaba in Piracicaba, São Paulo, Brazil, on patients suffering from chronic TMD pain (Brazilian Clinical Trials Registry – ReBEC no.: RBR-77y2sp) [24]. Study 2: A clinical, controlled, randomized, double-blind study was conducted at the Specialization Clinic of the Faculdade de Odontologia de Piracicaba and the Centro de Especialidades Odontológicas (Dental Specialties Center) in Piracicaba, São Paulo, Brazil, on patients with acute dental pain who had been diagnosed with pulpitis (Brazilian Clinical Trials Registry – ReBEC no.: RBR-8sxw2r) [25].

1. Inclusion criteria

For Study 1, adult patients of both sexes, aged 20 to 50 years, with chronic pain due to muscular or mixed TMD, with or without opening limitation, as described by the Research Diagnostic Criteria for Temporomandibular Disorders [26].

For Study 2, patients older than 18 years with odontalgia of pulp origin with pain scale score greater than 4 and no pain medication in the last 12 hours (contraceptive medication allowed).

2. Exclusion criteria

For Study 1, patients with severe trauma or TMJ infections, under medication with analgesics and / or anti-inflammatory drugs, pregnant women, patients who reported being afraid of needles or who were undergoing any other treatment for TMD, edentulous patients and patients with total dentures.

For Study 2, patients with pain of non-pulpal origin, use of medication for pain in the last 12 hours, and pregnant women.

3. Volunteers and needling

There were 40 patients included in Study 1 (20 from the treatment group and 20 from the sham group) and 28 patients in Study 2 (14 from the treatment group and 14 from the sham group).

3.1. Study 1

A total of 77 volunteers with TMD were recruited from Faculdade de Odontologia de Piracicaba (students, patients, and staff) and the Dental Specialties Center in Piracicaba. From the 77 volunteers, 43 were randomly assigned to either treatment (real acupuncture) or sham (nonpenetrating sham acupuncture) as reported previously [24]. The allocation was determined by 2 acupuncturists who took part in the research. The randomization was performed using software to generate numbers and allocate the volunteers randomly. The researcher and volunteers were not informed of the allocation.

3.2. Study 2

The study included 28 patients with acute tooth pain from the Faculdade de Odontologia de Piracicaba and the Dental Specialties Center in Piracicaba, as reported previously [25]. The randomization was performed using codified closed letters. The researcher and volunteers were not informed of the allocation.

Both randomized controlled trials were conducted and originally reported as double-blind studies [24,25]. In both trials, participants and outcome assessors / statistical analysts were blinded to group allocation. Acupuncture interventions were administered by trained professionals who were not part of the research team and were not involved in outcome assessment or data analysis; however, blinding of the intervention provider was not feasible due to the nature of the procedure (the Consolidated Standards of Reporting Trials flow diagrams of the original trials are provided in the Supplementary Materials).

4. Real acupuncture and nonpenetrating sham acupuncture

Two calibrated acupuncturists (8 hours of theoretical and practical instruction) inserted the needles to minimize treatment bias. The volunteers were not informed about which acupuncture group they had been allocated to (treatment or sham), and the researcher was not involved in the acupuncture procedure.

The identical acupoints were used in the nonpenetrating sham acupuncture technique (sham), which employed a sham needle (Streitberger Placebo Needle Set; 0.30 × 30 mm. Asia-Med brand, Germany). Although the blunt tip of this retractable needle prevents it from penetrating the skin, the patient experiences a puncture-like sensation when it connects with the skin [27]. Following skin asepsis, with 70% alcohol (Plastic Rings for Placebo Needles. Asiamed, Suhl, Germany), an intermediary circular device, with a diameter of 1 cm, was used to secure the sham needle to the acupuncture site. Adhesive tape (Steri-StripTM Reinforced Skin Closures, 12 × 50 mm; R1549. 3M Health Care, St. Paul, MN, United States) was used to adhere this device to the skin. The sham needle goes through the adhesive and just touches the skin without penetrating it. This type of acupuncture has been appropriately termed as “nonpenetrating sham” or “noninvasive sham” acupuncture [28].

For real acupuncture, after skin asepsis using 70% alcohol at the intended needle penetration site, the sterile and disposable needles (Qizhou; Wujiang 0.30 × 0.25 mm) were placed unilaterally on the right side. The anatomical differences between each volunteer’s application sites determined the depth of needle penetration. Using “Deqi” acupuncture, the needle was rotated clockwise and counterclockwise until the desired needling sensation was obtained [29]. To prevent bias in the study, the needles were fixed using the same procedure as the sham group, which involved utilizing a plastic ring and fixation adhesive.

The sham procedure followed established methodological recommendations for placebo-controlled acupuncture trials, aiming to maintain participant blinding through sensory simulation of needling without skin penetration.

5. Acupoints

The ST6, ST7, SI18, GV20, GB20, BL10, and LI4 acupoints used in Study 1 were selected for the treatment of TMD [29]. In Study 2, the LI4, ST44, CV23, and EX-HN3 (YinTang) acupoints were used as part of a dental analgesia scheme [18]. Once a week, volunteers received 4 sessions of either nonpenetrating sham acupuncture or real acupuncture. Using the same acupoints, the sham and treatment groups received nonpenetrating sham acupuncture and real acupuncture, respectively (Section 2.4). After reaching the Deqi, the needles were kept in place for 20 minutes before being removed.

6. Assessments

6.1. Patient expectation assessment

Patient expectation regarding acupuncture treatment was measured in the first session before any procedure, with the following question for Study 1: “Do you think acupuncture can solve or improve your TMD problem?” and for Study 2: “Do you think acupuncture can solve or improve your toothache problem you are experiencing right now?” In both studies, volunteer response was recorded using a Likert scale with the following options: (0) No; (1) I do not think so; (2) Maybe; (3) I think so; (4) Yes (Figure 1).

6.2. Patient satisfaction assessment

In both studies the patient’s perception regarding satisfaction with the acupuncture treatment was assessed using the 5-point Likert scale. For Study 1, this was at the end of the 4^th session, and in Study 2, it was assessed at the end of the single session. The Likert scale ranges from 0 to 4 where (0) indicates No; (1) A little; (2) Moderately; (3) A lot; (4) Totally.

In Study 1 patients were asked, “Do you think the acupuncture treatment improved your TMD problem?” and Study 2 patients were asked, “Do you think the acupuncture treatment improved your toothache problem?” (Figure 1). The expectation and satisfaction measures were based on single-item Likert-type questions developed for the specific clinical context of each trial and were not derived from previously validated psychometric instruments.

6.3. Pain intensity assessment

In Studies 1 and 2, self-assessed pain was reported using the numerical Visual Analogue Scale (VAS) ranging from a score of 0 (indicating no pain) to 10 (worst possible pain). The scores were recorded before acupuncture (initial VAS score) and after acupuncture (final VAS score). In Study 1, pain was assessed at the beginning and at the end of the 4 sessions, while, in Study 2, it was assessed at the beginning and at the end of the single session.

7. Statistical analysis

The data were analyzed using IBM SPSS Statistics for Windows, Version 27 (IBM Corp., Armonk, NY, USA) and graphs were made using GraphPad Prism 8.0 for Windows (GraphPad Software, Boston, Massachusetts USA). Descriptive statistics were used to summarize the data, and statistical significance was set at p < 0.05.

The variables expectation and satisfaction were measured on an ordinal Likert-type scale, while pain intensity was assessed using the VAS, recorded at baseline (initial VAS score) and after the intervention (final VAS score). Pain reduction was defined as change in VAS score and the difference between the initial and final VAS score was calculated.

For ordinal variables (expectation and satisfaction), results were expressed as medians with minimum and maximum values, and comparisons between chronic and acute pain groups were performed using the Mann-Whitney U test. The association between expectation and satisfaction was evaluated using Spearman’s correlation.

For continuous variables (initial VAS score, final VAS score, and change in VAS score), data were summarized as medians with minimum and maximum values. Between-group comparisons were performed using the Mann-Whitney U test. Within-group comparisons between initial and final VAS scores were assessed using the Wilcoxon signed-rank test. Effect sizes Cohen’s d were calculated for Wilcoxon tests to estimate the magnitude of differences. In addition to p values, effect sizes and 95% confidence intervals were considered to improve the interpretation of the magnitude and precision of the observed effects.

This secondary analysis study integrated data from 2 randomized controlled trials with distinct designs, patient populations, intervention schedules, and outcome assessment time points, therefore, a mixed-effects analytical strategy was adopted to explicitly address methodological heterogeneity to avoid inappropriate direct comparisons between trials. A 2-way mixed Analysis of Variance model was specified, with time (baseline and post-intervention) treated as a within-subject factor, and group (chronic pain vs. acute pain) as a between-subject factor. This allowed the evaluation of temporal changes in pain intensity and satisfaction whilst formally testing whether outcome trajectories differed between clinical contexts. Sphericity assumptions were assessed using Mauchly’s test, and when violated, Greenhouse-Geisser and Huynh-Feldt corrections were applied. Effect sizes were estimated using partial eta squared (ηp²), and statistical power was calculated for main effects and interactions. When significant interactions were detected, post hoc pairwise comparisons were performed using the Bonferroni adjustment. This analytical framework was selected to prioritize within-trial changes over time, while treating any between-group contrasts as exploratory. Analyses were conducted using SPSS (Version 22.0).

The key characteristics and primary findings of the 2 randomized controlled trials included in this secondary analysis are summarized in Table 1. The table provides an overview of baseline participant characteristics, treatment duration, number of sessions, and main outcomes, as originally reported, allowing readers to contextualize methodological differences between the trials. This summary facilitates interpretation of the present analyses and clarifies why contrasts between clinical contexts are considered exploratory.

No formal a priori sample size calculation was performed for the present secondary analysis, as it is based on previously conducted randomized controlled trials. Therefore, the sample size may be insufficient to detect moderate effect sizes, and the results should be interpreted with caution, particularly for between-group comparisons.

Results

1. Treatment groups

At baseline, patient expectations regarding acupuncture treatment were high in both Study 1 (chronic pain) and Study 2 (acute pain), with no statistically significant difference between the chronic pain and acute pain [median (min-max): 3 (2–4) vs. 3 (1–4), respectively; p = 0.097]. Following the intervention, satisfaction levels were also high in both contexts. Exploratory analyses indicated higher post-treatment satisfaction in the acute pain context [median (min-max): 3 (2–4)] compared with the chronic pain context [3 (0–4), p = 0.029]. However, these contrasts are presented descriptively and should be interpreted with caution due to differences in study design and treatment schedules. In this sense, the estimated mean difference for expectation (Final-Initial) was −0.493; 95% CI (−0.967; −0.019); t (27) = 2.22, p = 0.042; ηp²= 0.123, with an observed power of 0.54.

Regarding pain intensity, baseline VAS scores differed across clinical contexts, with higher initial pain reported in the acute pain [median (min-max): 7.3 (5–10)] compared with the chronic pain [5.5 (1–9), p = 0.020]. After acupuncture treatment, pain intensity was substantially reduced in both contexts, and no significant difference was observed in post-treatment VAS scores [1.5 (0–8) vs. 0.5 (0–10), p = 0.959]. Within Study 1 and Study 2, Wilcoxon signed-rank tests confirmed significant reductions in pain intensity from baseline to post-intervention. The mean difference between VAS scores (Final-Initial) was −2.707; 95% CI (−4.012; −1.402); t (27) = −4.22, p < 0.001; and ηp² = 0.358. The Cohen’s d effect size was 1.57, with observed power = 0.984.

Exploratory analyses of pain reduction (change VAS scores) suggested differences in the magnitude of improvement across clinical contexts. Study 1 involving chronic pain showed a median reduction of 3 points (0–7), whereas Study 2 involving acute pain showed a median reduction of 5.3 points (0.6–10; p = 0.043). These findings reflect distinct clinical trajectories and baseline characteristics and should not be interpreted as direct comparative effectiveness estimates between studies.

Correlation analyses conducted within each clinical context revealed distinct association patterns. In the chronic pain study, satisfaction was positively correlated with expectation (r = 0.673, p = 0.001) and baseline pain intensity (r = 0.554, p = 0.011), and negatively correlated with post-treatment pain intensity (r = −0.610, p = 0.021). In descriptive terms, the acute pain context was associated with an approximate 89% reduction in pain intensity, whereas the chronic pain context showed an approximate 64% reduction (Figure 2). These percentages are provided for descriptive purposes and do not imply direct equivalence between studies. The means for final and initial VAS scores, expectation and satisfaction of the study groups treated with real acupuncture are shown in Figure 3.

2. Sham groups

In the sham acupuncture condition, baseline expectations differed between clinical contexts, with higher expectations observed in the chronic pain study [median (min-max): 3.5 (2–4)] compared with the acute pain study [3 (2–4), p = 0.034]. Post-treatment satisfaction levels, however, did not differ significantly between contexts [3 (0–4) vs. 2.5 (1–4), p = 0.559]. When comparing chronic pain vs. acute pain over time, expectation showed no statistically significant difference at Time 1 [mean difference (Final-Initial) = 0.557; 95% CI (−0.037; 1.151); p = 0.065, Bonferroni], although with a marginal trend. At Time 2, the difference was significant (mean difference (Final-Initial) = −0.693; 95% CI (−1.290; −0.096); p = 0.024, Bonferroni), with ηp² = 0.149 and observed power = 0.630, indicating comparable subjective evaluations of treatment outcomes within each study.

Baseline pain intensity was significantly higher in the acute pain study sham group [median (min-max): 6.1 (4–10)] than in the chronic pain study sham group [3 (0–8), p < 0.0001]. Following the intervention, post-treatment VAS scores were low in both contexts, with no statistically significant difference observed [1 (0–3) vs. 1.35 (0–6.2), p = 0.183]. For VAS scores, at baseline, a significant reduction was observed and associated with the contrast between chronic pain and acute pain, with a mean difference (Final-Initial) = −2.186; 95% CI (−3.783; −0.588); p = 0.009, Bonferroni, and ηp² = 0.195 (moderate effect), with observed power = 0.771. After the intervention, there was no significant difference [mean difference (Final-Initial) = 0.029; 95% CI (−1.619; 1.676); p = 0.972], with ηp² ≈ 0.001 and observed power = 0.050. Within-context analyses confirmed significant reductions in pain intensity over time in both sham trials.

Exploratory analyses of pain reduction indicated a median decrease of 3 points (−1–7) in the chronic pain study sham group and 4.3 points (1.5–6.2) in the acute pain study sham group (p = 0.024). Within-context correlation analyses revealed a negative association between satisfaction and post-treatment pain intensity in the acute pain study (r = −0.620, p = 0.018), as well as a strong positive correlation between baseline and post-treatment pain intensity (r = 0.837, p = 0.00019). In the chronic pain study sham group, satisfaction positively correlated with expectation (r = 0.507, p = 0.023). Descriptively, pain reduction corresponded to approximately 76% in the chronic pain context and 66% in the acute pain context (Figure 2). The means for final and initial VAS scores, expectation and satisfaction in the study groups treated with sham acupuncture, are shown in Figure 3.

The mixed-effects analysis revealed significant temporal effects (time) for both satisfaction and pain intensity, indicating changes over the course of treatment within clinical contexts. Significant time × group interactions were observed for both outcomes, suggesting that temporal trajectories differed across contexts. These interactions were examined to explore differential patterns of change over time rather than to establish direct equivalence between trials. Detailed results, including effect sizes and sphericity corrections, are presented in Tables 2 and 3.

Overall, the results consistently demonstrate significant improvements over time within each clinical context, while contrasts between acute and chronic pain trials are presented as exploratory and interpreted with caution due to differences in study design, baseline characteristics, and intervention schedules.

Discussion

Expectation and satisfaction across 2 distinct clinical pain (acute and chronic) contexts were jointly examined in the present study whilst considering both real and sham acupuncture conditions. This approach contributes to a more comprehensive understanding of contextual and placebo-related mechanisms in pain modulation.

It has been shown that patient expectations can affect outcome results as much as the intervention [31], but few studies have looked at the relationship between expectations and responses to acupuncture therapy. Patient expectations are not regularly evaluated during clinical trials or taken into consideration in the analysis. The current research showed that participants in both chronic and acute pain studies had high initial expectations before receiving acupuncture therapy, and that both study contexts had a considerable decrease in pain. From a clinical perspective, the magnitude of pain reduction observed in both clinical contexts exceeds the minimum clinically important difference, which is commonly reported to range from approximately 1.3 to 2.0 points on a 0–10 pain scale, depending on the clinical context [32,33]. This indicates that the observed changes are not only statistically significant but also clinically meaningful.

The analgesic efficacy of acupuncture treatments can therefore be enhanced by high positive expectations [34]. Endogenous opioids are thought to be released as a result of a neurochemical reaction triggered by the expectation of pain reduction linked with the application of a therapy [35]. Investigating the relationship between high expectations and motivators is essential. Patient expectations, whether they are influenced by culture, prior experiences in the clinical setting, verbal communication, conditioning, or a combination of these, have a discernible impact on treatment outcomes [34]. These findings suggest that patient expectations may have contributed to pain reduction, although causal inferences should be interpreted cautiously given the study design.

High expectations on the part of volunteers receiving acupuncture treatments are common. In a systematic review on the effect of expectancy on treatment responses to acupuncture, the authors found 9 articles addressing this, 5 of these studies found statistically significant effects of expectation on at least 1 outcome, and 3 studies found evidence suggesting an interaction between expectation and the type of acupuncture used (real or nonpenetrating sham acupuncture) [21]. In this current study, both chronic pain and acute pain treated with real acupuncture had similar pain reduction for equally high expectations.

The acupuncture services grew substantially in the Sistema Único de Saúde, Brazilian Unified Health System in the period from December 2008 to December 2016 [36]. Despite that, few Brazilians had access to acupuncture. A study carried out in Rio Claro and Cerquilho (localities close to the city where this study was carried out) concluded that few users of the public health system had access to the technique (less than 10%), but most had already heard of acupuncture and were interested in its availability [37].

In the National Health Interview Survey of Brazil, many respondents (44%) stated that they sought acupuncture treatment because they believed that conventional medicine would not be able to treat their health problems [38]. The 5 health problems most treated by acupuncture amongst those who responded to the survey were related to some type of pain: back pain (34%), joint pain (16%), neck pain (13.6%), strong headache (9.9%), and recurrent pain (8.1%)[37]. Thus, it is conceivable that, in the present study, the hope of finding a solution to the problem of pain may have generated high patient expectation for treatment with acupuncture, as this is an unconventional treatment. Within each clinical context, substantial reductions in pain intensity were observed following real acupuncture treatment, despite differences in baseline pain severity and clinical characteristics.

As for the nonpenetrating sham acupuncture, it was determined in this current study that the groups were different in terms of pain. The chronic pain study had greater pain reduction (76%) than the acute pain study (66%). It has been reported that a placebo effect is significantly influenced by the nature and severity of the pain under study, and the mean level of analgesia reported by participants with higher initial levels of pain was considerably higher than that reported by participants with lower initial pain [39]. However, even patients who report high initial levels of pain had only a 50% chance of obtaining placebo analgesia; therefore, there are other factors that significantly influence the placebo effect. In the present study greater pain reduction was observed in the chronic pain group, in which the mean initial pain was lower. Probably, pain reduction was greater in the chronic pain group due to the higher expectation that this group had at the beginning of the study. This emphasizes the significance of considering patient expectation as a factor influencing the outcomes, as was performed in the current study.

It has been shown that expectation plays an important role in the subjective and behavioral effects of drugs that affect the Central Nervous System and the expectation of receiving caffeine was shown to result in the production of dopamine in the thalamus as measured by tomography, and after placebo administration [40].

Note that, comparing chronic pain treated with real acupuncture or nonpenetrating sham acupuncture and considering that the number of sessions was the same for both groups, and that both groups were blinded, the only difference was the higher expectation in the chronic pain group treated with sham acupuncture. Thus, it can be inferred that expectation influenced the outcomes of chronic pain reduction. The performance of a placebo treatment is related to patient expectation. Therefore, creating a positive thinking context in a group of patients can lead to a placebo treatment resulting in better responses [35].

Positive satisfaction with treatment in the 2 sham subgroups is associated with a reduction in pain in both subgroups, although the reduction in chronic pain was greater, once again suggests the influence of patient expectation on the outcomes of a treatment. In one study, patients classified as “high expectation” had an improvement compared with those classified as “low expectation,” and this improvement was significantly more pronounced in patients who received real acupuncture compared with nonpenetrating sham acupuncture [35]. These findings suggest an interaction between expectation and type of acupuncture, which should be considered in randomized clinical studies. In the current study, the treatment group had high expectations for both the acute pain study and the chronic pain study, resulting in pain reductions of 89% and 64%, respectively but, more markedly than the sham group for acute pain which resulted in a 66% reduction. In the present study, a lower reduction in the level of pain in the group who received nonpenetrating sham acupuncture may be due to 2 reasons: (1) because expectation was lower than in the chronic pain group; or (2) because of the low effect for being sham. Therefore real acupuncture and high expectation may have influenced the outcomes.

Thus, observing and using the expectation effect is important to optimize people’s health. However, when designing randomized clinical studies, this effect should be considered. Improving the study design and the interpretation of clinical trial results, a better understanding and characterization of nonspecific responses that comprise the placebo effect, potentially will ensure that intervention groups are more compatible with placebo responsiveness.

This study has limitations. The present analysis integrates data from 2 randomized controlled trials conducted in different clinical contexts, with differences in study design, treatment duration, baseline pain severity, and outcome assessment schedules which limit direct comparability between trials. A mixed-effects analytical approach was applied to address methodological heterogeneity. However, residual confounding cannot be excluded. Another important limitation is the relatively small sample size of both included trials, which may reduce statistical power, particularly for detecting moderate effects. Both trials were originally reported as double-blind. In practical terms, participants and outcome assessors / statistical analysts were blinded to group allocation, whereas practitioner blinding was not feasible due to the nature of acupuncture interventions. This design reduces detection and analysis bias, but performance-related effects should be considered when interpreting exploratory between-context findings. In addition, no formal assessment of blinding effectiveness (e.g., credibility or expectancy questionnaire after intervention) was conducted. Therefore, its success cannot be confirmed although procedures were designed to maintain participant blinding.

Conclusion

It is important to take patient expectations into consideration when studying acupuncture since pain outcomes were associated with different levels of expectation, whether they were high or low. Nevertheless, they do not appear to have an impact on treatment satisfaction. Both chronic pain and acute pain, led to similarly high expectations, and had equally significant pain reduction in the real acupuncture therapy group. At the conclusion of treatment, patients with acute pain reported greater treatment satisfaction. Chronic pain led to higher expectations, pain relief was greater in the sham group receiving nonpenetrating sham acupuncture. Both groups, however, indicated equal satisfaction.

Supplementary Materials

Article information

Author Contributions

Conceptualization: VLRZ and MLRS. Methodology: VLRZ and TBA. Validation: MILM and MLRS. Formal analysis: MILM and VABS. Investigation: VLRZ and TBA. Resources: MLRS. Writing - Original Draft: TBA and UTM. Writing - Review & Editing: TBA and VABS. Visualization: TBA and MLRS. Supervision: MLRS. Project administration: MLRS, VLRZ, and TBA. Funding acquisition: MLRS.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

Author Use of AI Tools Statement

The authors declare that Generative AI Gemini was used solely for grammar and language improvement.

Ethics Statement

This research was reviewed and approved by the institutional review board of Research Ethics Committee of the Piracicaba Dental School (registration no.: 109/2014 and 78633317.1.0000.5418). Informed consent was obtained from all participants.

Funding

This research was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) - no.: 133752/2017-8; Fundo de Apoio ao Ensino, à Pesquisa e Extensão (FAEPEX) - no.: 1032/2017; Fundo de Apoio ao Ensino, à Pesquisa e Extensão (FAEPEX) - no.: 129414.

Data Availability

Figure 1

Assessment scales for (A) patient expectations and (B) treatment satisfaction.

Figure 2

Spearman’s correlation analyses between expectations, satisfaction, initial VAS, and final VAS.

* p < 0.05.

** p < 0.01.

VAS = visual analog scale.

Figure 3

Mean and standard error of the mean of initial and final VAS scores, for expectation, and satisfaction in the study groups receiving real or sham acupuncture.

VAS = visual analog scale.

The bar graph represents pain intensity on the left Y-axis (VAS), the dashed line on right Y-axis represents expectation, and the continuous line on the right Y-axis represents satisfaction.

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