Influence of Control Group on Effect Size in Trials of Acupuncture for Chronic Pain: A Secondary Analysis of an Individual Patient Data Meta-Analysis

This study represents a fundamental secondary analysis of the Acupuncture Trialists' Collaboration individual patient data meta-analysis, investigating how different types of control groups influence the effect size observed in acupuncture trials for chronic pain. The researchers analyzed data from 29 high-quality randomized controlled trials, including 19,827 patients with chronic painful conditions such as low back pain, neck pain, shoulder pain, osteoarthritis, and headache. The study was motivated by the need to better understand variations in acupuncture trial results, which can be influenced by the choice of control group. The methodology involved detailed categorization of the types of controls used in the original studies.

For sham controls (simulated acupuncture), the researchers classified whether needles or noninvasive methods were used, whether the needles penetrated the skin, and whether they were applied at true acupuncture points. For non-sham controls, they distinguished between 'routine care' (unspecified treatment as needed) and 'protocol-guided care' (standardized treatment specified in the study protocol). The results revealed that acupuncture was significantly superior to all types of controls. However, important differences in effect size were found depending on the type of control used.

In trials with sham controls, acupuncture showed less superiority when compared with penetrating needles (effect size: 0.17) than when compared with non-penetrating needles (0.43) or no-needle methods (0.38). The difference between penetrating and non-penetrating needles was statistically significant (-0.45; p = 0.007). For non-sham controls, although the difference did not reach statistical significance, there was a trend toward a greater acupuncture effect versus routine care (0.55) compared with protocol-guided care (0.29). These findings have important implications for the interpretation of clinical trials and for the design of future studies.

The results suggest that penetrating needles, even when inserted superficially at non-acupuncture points, possess physiologic activity of their own, calling into question their suitability as a true placebo control. This is consistent with theories about acupuncture mechanisms, which may include stimulation of nerve endings and release of neurotransmitters even with superficial insertion. The study also demonstrates that the intensity of treatment in the control group influences the observed effect size, with direct implications for sample size calculations in future trials. Limitations include the limited number of trials using some specific control types and heterogeneity in the practical implementation of sham controls.

In addition, incomplete understanding of acupuncture mechanisms limits definitive conclusions about which controls can be considered true placebos. For researchers, this study recommends avoiding penetrating needles as a sham control when the goal is to control for nonspecific effects, favoring non-penetrating needles or no-needle methods. For clinicians and patients, the results reinforce the efficacy of acupuncture for chronic pain regardless of the type of control, but suggest that the benefits may be even greater than previous estimates indicated.