Because the mechanisms underlying the clinical and physiological responses to acupuncture remain unclear, various types of controls have been implemented in acupuncture research. For the purposes of this review, any type of placebo control for acupuncture has been classified as sham acupuncture. Many recent studies have used a non-penetrating sham control, referred to as the placebo needle [7
]. Other placebo controls use minimal acupuncture and stimulation at inactive acupoints or non-acupoints involving skin penetration.
Minimal acupuncture utilizes needles inserted only 1-2 mm below the surface of the skin [10
]. Needles are applied with no stimulation or light stimulation either mechanically by twirling the needles or electrically by applying a small pulsating current to the needles [10
]. Despite the popular use of minimal acupuncture as a sham control, many clinical trials have suggested that this type of control has the potential to evoke analgesia through stimulation of cutaneous afferent nerves [15
]. However, a recent study in mice reported that although superficial needle insertion caused analgesic effects, the effect was local and distinct from the acupuncture responses that rely on the afferent innervation of the skin [16
]. In addition, the effects of electroacupuncture (EA) on blood pressure have been shown to result from stimulation of deeper nerves such as the median nerve located below Jianshi-Neiguan
(P5-6) acupoints rather than from stimulation of superficial (i.e., cutaneous) branches such as the superficial radial nerve at Panli-Wenliu
(LI6-LI7) acupoints [17
]. In fact, investigation of reflex-induced pressor responses in cats showed that minimal or superficial acupuncture can serve as a valid control, because it does not alter the cardiovascular responses [19
Needling at either inactive acupoints or non-acupoints are also common controls used in studies of acupuncture [20
]. Non-acupoints are points located several millimeters or centimeters from verum acupoints, halfway between two parallel meridians or arbitrarily on the side of the trunk or the shoulder region away from most meridians [25
and non-acupoints are thought to have no therapeutic influence and thus may serve as inert placebo controls. Stimulation of inactive acupoints also is believed to be ineffective, so they can be used as sham controls. Inactive acupuncture points are chosen according to their anatomical locations, underlying neural pathways, corresponding Chinese meridians, proximity to verum acupoints and role in treating diseases [18
]. With adequate blinding, the effects of stimulating either inactive acupoints or non-acupoints have been hypothesized to represent placebo responses.
The difference between sham and verum acupuncture remains unclear. Recent systematic reviews have suggested that stimulation of non-acupoints and inactive acupoints can elicit effects similar to those of stimulation of verum acupoints [26
]. Thus, Langevin et al. [5
] noted similar responses in sham and verum acupoint stimulation and attributed the nonspecific needling effects to either the summation of stimulation of multiple superficial sensory nerve endings or to the therapeutic effect related to practitioner-patient interactions. Despite these findings, some experts believe that it is illogical to conclude that stimulating all points on the body will elicit the same effect, because there is an uneven distribution of nerve endings throughout the body, suggesting differential sensory input at different points on the body [25
]. In fact, many well-controlled studies do support the principle of point specificity. For example, recent studies in cardiovascular disease have shown that stimulation of verum acupoints elicits significantly greater responses than stimulation of both non-acupoints and inactive acupoints [18
]. In addition, brain imaging studies have documented differential patterns of activation resulting from stimulation of inactive acupoints and non-acupoints [21
]. Furthermore, experimental investigations have found that stimulation of different acupoints produces differential input to regions of the brain that regulate sympathetic outflow and cardiovascular function [18
Because of contradictory results from many studies, some investigators have questioned the use of sham acupuncture as a placebo control [29
]. Furthermore, some reviews argue that the principle of point specificity would be invalidated if stimulation at non-acupuncture points and inactive acupuncture points produced the same effects as verum acupuncture [26
]. However, because the studies have used a wide variety of acupuncture techniques, controls and measureable outcomes, it is difficult to assess the validity of point specificity by evaluating this literature.